Claude Universite

Bernard -Lyon

M Le Professeur

Frédéric Fleury

Hamda Ben

Hadid M Le Professeur Didier

Revel M Le Professeur

Philippe Chevalier

M Fabrice

Vallée Mme

Dominique Marchand

Composantes Sante

G Rode

La Mme

C Professeure

Burillon

D Bourgeois

Vinciguerra

Catalytic activity of catalysts synthesized by SOMC protocol ..........

COMPOSANTES ET DEPARTEMENTS DE SCIENCES ET TECHNOLOGIE

Acknowledgement

The pursuit of this PhD has as much been a journey in scientific exploration as it has been about self-discovery. In this I have been immensely fortunate to have had people around me, who both steered and supported me through this adventure.

My first and foremost thanks must go to my supervisor, Dr. Elsje Alessandra Quadrelli, whose scientific fervor and insight have been the guiding force behind this work. Her guidance has been instrumental not only in the pursuit of this research, but in the development of a sound rational to conduct scientific inquiry. Our many interactions have shaped my work ethic and will continue to guide me in future. I take this opportunity to thank her also on personal note for always believing in me and helping me believe in myself during the tougher times.

I am most grateful to Prof. Fabrizio Cavani whose scientific expertise has shaped every aspect of this research. His insightful questions helped me explore new directions to conduct experiments. My mobility in Bologna will always remind me of his warm presence and concern for my well being in the new lab and city.

My deepest words of gratitude go to Dr. Mostafa Taoufik whose ideas and scientific inputs form the backbone of this work. I appreciate the intense discussions that boosted my scientific appetite to keep learning more about the field of surface organometallic chemistry and chemistry as whole.

I must also extend my thanks to Dr. Aimery DeMallmann who has been responsible for conducting and helping me interpret all of the EXAFS experiments that are a part of this thesis. I thank Dr. Kai Szeto for always extending a helpful hand whether it be in conducting experiments or in discussing the principle and workings of any and every instrument in our laboratory. I am am also grateful to Dr. Clement Camp, Mrs. Christine Lucas, Dr. Nesrine Cherni and Mr. Laurent Veyre from CPE, Lyon for their timely help. I must thank Mr. Ruben Vera from University of Lyon 1 for XRD characterizations of the material, Prof. Silvia Bordiga and Dr. Alessandro Damin from University of Torino for UV-vis characterization and Dr. Mimoun Aouine from IRCELYON for the HRSTEM analysis.

SINCHEM is an administratively intense program. I must thank Mrs. Emmanuelle Fouilhe (CPE, Lyon) and Mrs. Angela Maritato (UniBo) for being extremely kind and helpful in keeping me administratively up to date.

It is difficult for me to imagine the completion of this thesis without the help and support from Dr. Juliana Velasquez Ochoa. During the short time that was available in Bologna, it was her constant help and guidance that enabled successful completion and interpretation of experiments related to catalytic testing. I also thank my colleagues from Bologna-Francesco, Tomaso, Aisha, Valeria, Danilo and Giada for their complete support during the period of my mobility.

In Lyon I was found myself extremely lucky to have been surrounded by some extremely astute and insightful colleagues. I would specially like to mention Cherif and Marc for valuable discussions at a crucial time. A very special thanks to Vittoria and Iurii for their help regarding translations and more. I extend a warm thanks to all my other colleagues from Lyon-Jessica, Nicolas, Dominique, Marc, Pascal, Giuliana, Debora, Frederic, Anthony, Matthieu, Clement and Ravi for being such a marvelous support at all times. I extend a very special thanks to Tapish for being my go-to person throughout the three years in Lyon. To say the least it would have been a very different tenure without his constant support and assurance.

Being a part of the SINCHEM program has been an experience in being part of a global family. I will forever cherish fond memories from each of our interactions. I would like to express my warm regards to Prof. Stefania Albonetti for incredible effort on her part in establishing the program. I wish to give a special mention to Tin and Matilde for being a source of constant strength and support. I was told every PhD is a different struggle and I realized nothing helps more but to seek help in times of distress. I am glad I found such a fantastic mentor in Ms. Shruti Chakravarti who lifted me up and steered me back into the arena every such time.

My thanks will never be complete without mentioning my friends from the other part of the globe who kept loving and nourishing me despite the time zones. I thank Puneet, Adwait, Mihir, Nikita, Radha, Anagha, Noopur, Rajashree, Divya, Priya, for being rock solid pillars of support and to Surahit for being a constant in this otherwise tumultuous journey.

My final words of gratitude are reserved for my parents. I must thank them for their continual love and support, but even more for inculcating the spirit of constant learning and unlearning, which has been the foundation for all my scientific endeavors. I am humbly aware that this thesis stands on the shoulders of all of the above and many more people. I extend my heartfelt gratitude to all of them.

Abbreviations

Summary

In the recent years, on-purpose synthesis of butadiene using bioethanol has gained unprecedented attention owing to rise in interest of bio-based building blocks along with steeply increasing demand for butadiene. 1 A relevant process in this context is the Ostromyslensky process, which proceeds via two steps, involving dehydrogenation of ethanol to acetaldehyde in a separate step (see eq. i), followed by butadiene production in the second stage by co-feeding ethanol and acetaldehyde (see eq. ii). 2 Although the economic viability and feasibility of this ethanol to butadiene (ETB) process is well-established, there is a room for better catalytic performances and selectivity. 3 The productivity of this transformation is marred by several parallel reactions generating a large number of side products. Additionally the process is known to have short catalyst regeneration cycle due the rapid deactivation of catalyst by coke formation. 4 Following the original catalyst developed by Ostromyslensky, there has been a constant upsurge in the study of Ta/SiO2 systems as catalysts for two step ETB transformations. A notable study is that by Jeong et al. 5 who prepared a series of ordered mesoporous silica (OMS) supported Tantalum oxide samples by wetness impregnation technique followed by calcination at high temperature. The catalysts showed better catalytic performances towards butadiene production compared to the earlier systems, indicating need for better dispersed tantalum on silica catalysts. The study primarily focused on effect of surface morphology on catalytic activity while no conclusive comment was made on the surface active species. A remarkable attempt to synthesize a well-defined active specie was recently made by Dzwigaj et al 6,7 . who reported Ta incorporation into vacant T-atom sites of the SiBEA zeolites framework as mononuclear Ta(V) allowing for the preparation of highly selective catalysts for 1,3-butadiene synthesis. Though both these studies point towards the usefulness of well-dispersed tantalum oxide species on silica, there is no clear evidence on the nature of the species populating the surface of the catalyst or on that of the active species taking part in catalysis.

In general, the surface of Ta(V)/SiO2 systems can be complex. It could include isolated tetrahedral moieties [(≡SiO)3Ta=O], oligo-and polymeric TaxOy species presenting simultaneously bridging O atoms, tantalate (Ta=O) and Ta-O-support bonds, and particles of bulk Ta2O5 formed at much higher Ta loading. 8 Then to be able to successfully develop structure activity relationship for these catalysts it will be imperative to be able to selectively tailor various model catalysts with isolated and oligomeric/polymeric species on the surface of the support. To this end, one of the powerful approaches can be surface organometallic chemistry (SOMC), which permits the preparation of well-defined surface species.

Surface Organometallic Chemistry is a technique developed to tailor well-defined heterogeneous catalysts by grafting organometallic complexes on surface sites of a support in a controlled fashion. [9][10][11] This approach is based on controlling the concentration of grafting sites per unit area of the support, which in turn controls the final density of organometallics grafted on the surface. Due to its ability to perform molecularly controlled grafting, SOMC produces surface sites with a known coordination sphere. This attribute is essential in developing a structure activity relationship for a catalyst and in turn for rational design of ideal catalysts for a process. 12 Scheme 1-Schematic representation of preparation of a well-defined catalyst using SOMC

The main requirements for applying SOMC approach is the presence of a wellcharacterized support, and an organometallic complex -MLnXx (L= functional ligand, X= ancillary ligand). 13 Upon grafting the complex on the support, it can be used as a catalyst by itself or be subjected to post-treatments (calcination, hydrogenation, sulfidation) to obtain a more stable structure or to attain a new functionality. Figure 1 schematically represents an ideal SOMC catalyst with its key components.

The primary aim of this work is to synthesize a family of well-defined silica-supported TaOx catalysts with varying tantalum nuclearity and to study its effect on the second step of the Ostromyslensky process, i.e. transformation of EtOH/AA to butadiene. Secondly our aim is to use the above knowledge to build a meaningful structure activity relationship for the Ta (V)/SiO2 catalytic systems in the synthesis of BD from EtOH/AA.

As mentioned before, the grafting of tantalum alkyl complexes on partially dehydroxylated silica to obtain 'single site' catalysts with isolated tantalum centers has been well documented in the literature. 14,15 Thus, an evident strategy to synthesize single site TaOx catalysts was via mild oxidation of these formerly studied well-defined silica supported tantalum species.

The first contribution of this thesis was to develop isolated single site TaOx species was via mild oxidation of existing silica-supported tantalum hydrides [(≡SiO)2Ta(H)x=1,3] with N2O. Well-definedtantalum hydride [(≡SiO)2Ta(H)x=1,3] species and their precursing tantalum alkyl [≡SiOTa(=CH t Bu)(CH2 t Bu)2] species have been a model study in surface organometallic chemistry. 9,10,16 Materials with these species were synthesized according to the literature and their formation was monitored via IR, NMR spectroscopy and Upon oxidative treatment of [(≡SiO)2Ta(H)x=1,3] with N2O at room temperature, the material (SOMC-I) was characterized by IR, NMR spectroscopies, as well as elemental analysis. Formation of mostly well-defined isolated mono-tantalate specie [(≡SiO)2Ta(=O)(OH)] was established via EXAFS analysis. HRSTEM analysis of the material found largely isolated dispersion of Ta (V) center, with some degree of agglomeration.

Scheme 1

The second approach pursued in this thesis was to generate well-defined surface TaOx species was via calcination of silica supported peralkyl species

[≡SiOTa(=CH t Bu)(CH2 t Bu)2] at 500°C for 10 hrs under dry oxygen. The material so formed (SOMC-II), was characterized by by IR, NMR spectroscopies, EXAFS analysis and HRSTEM microscopy. Scheme 3 represents the scheme for synthesis of SOMC-II along with the representative HRSTEM micrographs of the surface species involved. The EXAFS spectroscopy confirmed the absence of Ta2O5, while suggesting the possibility of Ta-Ta interaction. The HRSTEM microscopy evidenced co-existence of isolated Ta (V) centers along with Ta species in a string like arrangement. This data together with the information from literature suggested the presence of a mixture of mononuclear, dinuclear and oligomer like species on the surface of SOMC-II.

Scheme 2

The family of catalysts synthesized by SOMC protocols was tested in the production of butadiene from ethanol and acetaldehyde in a lab scale continuous flow reactor under atmospheric pressure. A catalyst amount of 0.25 g was loaded in the reactor in the form of powder. Inlet feed molar ratios were always constant and were set at 2 mol% ethanol and acetaldehyde mixture with 98% nitrogen. Downstream products were fed to an automatic sampling system for gas-chromatography. Figure 2 shows the schematic diagram for the lab scale reactor used in the catalytic testing. This decrease in the concentration of isolated Ta (V) sites on the surface of SOMC-I also resulted in decrease in the overall conversion of EtOH/AA. Based on the combined information from catalytic activity tests and HRSTEM analysis it can be regarded probable that isolated Ta (V) centers act as the primary active sites in the conversion of EtOH/AA to BD while the Ta atoms in a string like organization show considerable but reduced activity in the same transformation.

Assuming isolated [(≡SiO)2Ta(=O)(OH)] to be the active site in the transformation of EtOH/AA to BD, an attempt was made to understand the mechanism of the reaction on the surface of SOMC-I. Based on the widely accepted Kagan mechanism, initially two molecules of acetaldehyde combine to form crotonaldehyde, which is then reduced by ethanol to form crotyl alcohol. This crotyl alcohol is ultimately dehydrated to butadiene.

The Meerwein-Ponndorf-Verley reduction of crotonaldehyde is thought to be catalyzed by tantalum. 17,18 Based on in-situ DRIFT experiments for adsorption of (i) ethanol and (ii) a mixture of ethanol and acetaldehyde on the surface of the catalyst at 350°C, the following mechanism was hypothesized for the reduction step on the isolated Ta (V) center (Figure 5).

Figure 5-Proposed and hypothesized reaction mechanism for MPV reduction over the Ta (V) site of SOMC-I Therefore, SOMC was found to be a successful approach in developing highly performing catalysts for the transformation of EtOH/AA to BD. Moreover the findings from this project illustrate the potential of SOMC technique in developing material with tunable surface properties via adjusting the nature and the number of surface active sites.

This property can prove highly beneficial in developing multifunctional catalysts for various industrial processes.

Résumé

Au cours des dernières années, la synthèse ciblée du butadiène en utilisant le bioéthanol a suscité une attention sans précédent en raison de l'intérêt croissant aux matières premières biosourcées ainsi que de la demande croissante en butadiène. 1 Un processus pertinent dans ce contexte est le processus d'Ostromyslensky, qui s'effectue en deux étapes, comprenant la déshydrogénation de l'éthanol en acétaldéhyde en une étape séparée (eq.

1), suivie de la production de butadiène dans la deuxième étape par réaction d'acétaldéhyde avec de l'éthanol supplémentaire. (eq. 2) 2

Bien que la viabilité économique et la faisabilité de cette transformation d'éthanol en butadiène(ETB) soient bien établies, de meilleures performances catalytiques et une meilleure sélectivité sont possibles. 3 La productivité de cette transformation est affectée par plusieurs réactions parallèles générant un grand nombre de produits secondaires. De plus, le procédé a un cycle de régénération du catalyseur court en raison de la désactivation rapide du catalyseur par formation de coke. 4 À la suite du catalyseur original mis au point par Ostromyslensky, on observe une recrudescence constante dans l'étude des systèmes Ta/SiO2 en tant que catalyseurs de transformations ETB en deux étapes. Une étude remarquable est celle de Jeong et al. 5 La chimie organométallique de surface est une technique développée pour préparer des catalyseurs hétérogènes bien définis en greffant des complexes organométalliques sur des sites de surface d'un support de manière contrôlée. [9][10][11] Cette approche repose sur le contrôle de la concentration des sites de greffage par unité de surface du support, ce qui permet de contrôler la densité finale des espèces organométalliques greffées à la surface.

Riassunto

Negli ultimi anni, la sintesi specifica del butadiene a partire dal bio-etanolo ha ottenuto un'attenzione mai riscontrata precedentemene a causa dell'aumento dell'interesse sia verso building blocks a base biologica, che al rapido aumento della domanda per il butadiene stesso. 1 Un processo rilevante in questo ambito è quello Ostromysensky, costituito da due step: la deidrogenazione dell'etanolo ad acetaldeide in una prima fase, seguita dalla produzione del butadiene in un secondo stadio della reazione dove è previsto un co-feeding di etanolo ed acetaldeide. 2 Nonostante questo processo di produzione da etanolo a butadiene (ETB) sia attualmente assestato sia da un punto di vista economico che operativo, c'è ancora spazio di miglioramento sia dal punto di vista delle performance catalitiche che da quello della selettività dei prodotti ottenuti. 3 Infatti la selettività del processo è inficiata dalla presenza di numerose reazioni parallele che danno vita a un gran numero di sottoprodotti di reazione. Inoltre è noto che il processo è caratterizzato da un ciclo di rigenerazione catalitica corto, a causa della rapida deattivazione per formazione di carbone. 4 In seguito al primo sviluppo del catalizzatore di Ostromyslenky, c'è stato un costante aumento nello studio di sistemi Ta/SiO2 come catalizzatori per le trasformazioni ETB in due step catalitici. Un importante studio è stato svolto da Jeon et al. 5 i quali hanno preparato una serie ossidi di Tantalo supportati su silica mesoporosa ordinata (OMS) tramite impregnazione liquida seguita da calcinazione ad alta temperatura. I catalizzatori hanno mostrato performance catalitiche verso la produzione di butadiene migliori rispetto ai precedenti sisemi, indicando la necessità di avere una migliore dispersione delle specie di tantalo sulla superficie dei catalizzatori. Lo studio da loro condotto si focalizza principalmente sull'effetto della morfologia della superficie sull'attività catalitica, mentre non sono riportati commenti conclusivi sul tipo di specie attive presenti sulla superficie.

Un tentativo rimarchevole verso la sintesi di specie attive ben definite è stato recentemente effettuato da Dzwagaj et al., i quali hanno riportato l'incorporazione del Ta nelle vacanze di ipo T-atom nella struttura di zeoliti SiBEA come Ta(V) mononucleare, permettendo la preparazione di catalizzatori altamente selettivi per la produzione di 1,3butadiene. Nonostante entrambi questi studi dimostrino l'utilità di una buona dispersione degli ossidi di tantalo sulla superficie della silica, non ci sono chiare prove della natura delle specie che popolano la superficie del catalizzatore o di quella delle specie attive che prendono parte al processo catalitico.

Figura 1 -Sistemi Ta/SiO2 riportati in letteratura per la trasformazione di EtOH/AA a BD.

In generale, la superficie dei sistemi Ta(V)/SiO2 può essere complessa. Può includere frammenti tetraedrici isolati [(≡SiO)3Ta=O], specie del tipo TaxOy di natura oligomerica e polimerica che presentino allo stesso tempo atomi di ossigeno a ponte, tantalati (Ta=O) e legami il Ta e l'O del supporto, e particelle bulk di Ta2O5 formatisi a loading di Ta molto più elevati. 6 È quindi necessario, per poter sviluppare con successo una relazione struttura-attività per questi catalizzatori, essere in grado di sintetizzare selettivamente catalizzatori modello per le specie isolate e quelle oligo/polimeriche sulla superficie del supporto. A questo fine, un approccio efficace è la chimica organometallica di superficie (SOMC), che permette la preparazione di specie di superficie strutturalmente ben definite.

La chimica organometallica di superficie è una tecnica sviluppata per la sintesi catalizzatori eterogenei ben definiti da un punto di vista strutturale tramite grafting in maniera controllata di complessi organometallici su siti attivi di un supporto. Questo apporccio è basato sul controllo della concentrazione dei siti di grafting per unità di area del supporto, che a sua volta conrolla la densità finale delle specie organometalliche graffate sulla superficie. Grazie all'abilità di effettuare un grafting contrallato da un punto di vista molecolare, la SOMC produce siti di superficie con una sfera di coordinazione nota. Questo attributo è essenziale nello sviluppo di una relazione struttura-attività per un catalizzatore e in seguito per il design razionale di un catalizzatore ideale per un determinato processo.

Figura 2 -Rappresentazione schematica della preparazione di un catalizzatore definite da un punto di vista strutturale tramite SOMC.

I requisiti principali per l'applicazione dell'approccio SOMC è la presenza di un supporto ben caratterizzato, e di un complesso organometallico -MLnXx (L= legando funzuionale, X=legando ancillare). Una volta che il complesso oranometallico è stato graffato sul supporto, può essere utilizzato direttamente come catalizzatore o essere sotoposto a trattamenti successivi (calcinazione, idrogenazione, sulfidazione) per ottenere una struttura più stabile o per ottenere una nuova funzionalità. La Figura 2 rappresenta schematicamente un ideale catalizzatore SOMC con le sue componenti chiave. Lo scopo principale di questo lavoro è sintetizzare una famiglia di catalizzatori ben definiti basati su specie TaOx supportate su silica con il tantalo in forma sia monomerica che oligo/polimerica e di studiare l'efetto dell'intorno del tantalo sul secondo step del processo Ostromyslensky, ossia la trasformazione del EtOH/AA a butadiene. Abbiamo anche investigato l'effetto del grafting di differenti precursori di tantalo e l'uso di diversi supporti per il grafting stesso. In secondo luogo il nostro scopo è quello di utilizzare le conoscenze acquisite per costruire una relazione struttura-attività per i sistemi catlitici Ta(V)/SiO2 nella sintesi del BD da EtOH/AA.

Schema 2

La spettroscopia EXAFS ha confermato l'assenza di Ta2O5, mentre suggerisce la presenza di possibili interazioni Ta-Ta. La spettroscopia HRSTEM ha evidenziato la coesistenza di centri di Ta(V) isolati ed atomi di Ta in disposizione di tipo stringa. Questi risultati insieme alle informazioni di letteratura suggeristono la presenza sulla superficie di SOMC-II di un misto di spece mononucmeari, dinucleari e oligomeriche.

La famiglia di catalizzatori sintetizzata tramite protocollo SOMC è stata testata nella produzione di butadiene da etanolo e acetaldeide a pressione atmosferica in un reattore da laboratorio a flusso continuo. 0.25 g di catalizzatore sono stati caricati all'interno del reattore sotto forma di polvere. I rapporti molari di alimentazione in ingresso del reattore sono stati mantenuti sempre costanti e la miscela utilizzata era costituita al 2% di etanolo e acetaldeide ed al 98% azoto. I prodotti in uscita dal reattore sono stati analizzati tramite un gascromatografo munito di autocampionatore. La Figura 4 mostra un diagramma schematico del reattore da laboratorio utilizzato nelle prove catalitiche.

Figura 1 -Apparato di reazione per la sintesi di butadiene a partire da etanolo e acetalaldeide.

La Figura 5 mostra un confronto tra i catalizzatori più performanti ottenuti tramite l'approccio SOMC e i sistemi a base di tantalo più performanti studiati in letteratura.

Quando testati in condizioni catalitiche comparabili, i catalizzatori ottenuti tralite SOMC mostrano un'eccellente selettività e resa nei confronti del BD rispetto ai migliori catalizzatori a base di Ta presenti in letteratura. SOMC-I, con siti Ta(V) isolati si è dimostrato il più performante, con una selettività del 86.9% e una resa del 48.2% nei confronti del BD.

Chapter 1 Introduction

The intent of this chapter is to summarize the developments in on-purpose production of butadiene, both from an industrial and academic point of view and to understand the limitations of the process with respect to the catalysis.

The second part of the chapter introduces the concept of surface organometallic chemistry (SOMC) as a technique to tailor well-defined heterogeneous catalysts and tries to explore its relevance in the production of butadiene from ethanol in Ostromyslensky's two-step

process.

The last section delineates the objectives of this thesis

Contents

A1.2. Present industrial processes for production of butadiene

More than 95% of butadiene is currently obtained as a byproduct of steam cracking to produce ethylene 4 . This makes butadiene production highly dependent on the feedstock used to obtain ethylene.

With the recent developments in Shale gas technology, vast reservoirs of natural gas have opened up for chemical industry at cost effective prices 5,6 . In the wake of these changes, the production of ethylene has shifted from steam cracking of heavier feedstock like naptha or gas oil to lighter starting material like the ethane obtained from shale gas. 2 However, the quantity of crude C4 hydrocarbons (and consequently the amount of This is leading to restricted butadiene production and subsequent rise in its price. 4 Until now the most established alternative route to produce BD has been via catalytic dehydrogenation of butanes obtained via petroleum based feedstock. This process is known to produce good yields of BD with selectivity up to 90% at competitive market prices. Though the process is already industrially viable, the high reaction temperatures (600°C-700°C) and issues relating to the catalyst stability leave much room for improvement. Also its inherent dependence on fossil based resources continues to be of concern.

In light of this growing environmental conscientiousness, both industry and academia are merging their competences in search of newer and more environmentally benign routes to produce BD [1][2][3]7 . In a comprehensive review, Sels et al. 2 summarize the routes to obtain BD from both fossil based and bio-based feedstocks. (Figure 1.3)

The most prominent amongst the new routes is the one from bio-ethanol. BD can be obtained from ethanol either by (i) one-step conversion of ethanol to BD employing mixed metal oxide catalysts (Lebedev Process) or by a (ii) two-step process involving dehydrogenation of ethanol to acetaldehyde in a separate step, followed by Butadiene production in the second stage by co-feeding acetaldehyde along with ethanol (Ostromyslensky process) Both these processes have been the pioneering approaches to BD production from the time of WWII. 8,9 Their proven industrial viability and easy and cheap access to their starting material in form of bioethanol make them the most likely 'green' candidates for on-purpose BD production in near future. whose conversion to BD is well known to the industry by Reppe's process. 12 The route from syngas (as waste gas from industries or biomass gasification) proceeds either via butanediols or via ethanol formation. Direct gas fermentation of CO to BD is also being studied. [13][14][15] Table 1.1 summarizes in detail the specific advantages and limitations of the processes discussed above.

A1.4. Emergence of bio-ethanol as a building block

With the renewed interest in butadiene production routes from Ethanol, Bio-ethanol, obtained from the fermentation of sugars, emerges to be the most relevant source of Biocarbon. Today, Bioethanol is an important building block for synthesis of a variety of industrial chemicals. 16,17 Its annual production, which is an order of a magnitude higher than the global Butadiene demand, only corroborates its potential to be used as a raw material for Butadiene synthesis on an industrial scale in near future. 2 The most commonly used plants to produce ethanol are sugar cane and sugar beet since the sucrose present in them is easily accessible for fermentation. Other plants used for fermentation are the corn, wheat potatoes or barley. The ethanol so produced is known as first generation ethanol and is presently the main route to bioethanol. However, there are two major concerns surrounding the first generation bioethanol and biofuels in general.

The production of ethanol competes with the use of crops as food. Secondly, it excludes the use of most of the herbaceous biomass since the sugars in it are in the form of cellulose and hemicellulose which are more robust sugar polymers and thus less easily accessible. In fact, real success will lie in producing second generation bio-ethanol via efficient breakdown of cellulose to sugar monomers and its subsequent fermentation. Nevertheless, efforts are being made in this area to obtain ethanol from several lignocellulosic sources such as straw, nut shells, bagasse and so on. 16,17 Also the integration of algae (microalgae and macro-algae) as a sustainable feedstock for bioethanol is being evaluated. Though this approach can be useful considering food security and environmental impact, there are significant technological hurdles in its feasible commercialization.

Biomass Advantages Limitations

Bio-Ethanol

. Routes in consideration for on purpose butadiene production

Interestingly, the search for newer route has led to the rediscovery of synthesis routes from back in history, during the time of World War II. In the wartime effort, Ostromyslensky had proposed a catalytic route from a mixture of ethanol and acetaldehyde over alumina or clay catalysts at 440-460 °C, yielding 18% BD. 18 It was essentially a two-step process from ethanol to butadiene, involving dehydrogenation of ethanol to acetaldehyde in a separate step, followed by butadiene production in the second stage by co-feeding acetaldehyde along with ethanol. (See scheme 1. Carbon Chemicals Corporation started performing the industrial manufacturing of BD from ethanol in the US over 2%Ta/SiO2 using the two-step process. 20 After the war however, both these routes died out in the US and most of Europe, with the only surviving plants in India and China.

Though in the literature addition of acetaldehyde to ethanol has been reported to increase the overall BD yield by some authors, there is no evidence to support the preferentiality of one technological process over the other. However the following points are often cited in favor of the two-step process-

The two-step process allows for an enhanced flexibility in the selection of appropriate reaction conditions for each reaction separately, thereby increasing the overall BD yield. 8 For example the ideal temperatures for conversion of ethanol to acetaldehyde over a copper oxide catalyst is considered to be 250°C whereas for the second step (conversion of ethanol and acetaldehyde to BD) 325-350°C are known to be thermodynamically favorable.

Decoupling the dehydrogenation of ethanol from the steps of aldol condensation,

MPV reduction and dehydration result in decreased catalyst complexity

The ideal temperature range reported for the two-step process is around 325-350°C compared to 400-425°C reported for the one-step process

The acetaldehyde which is produced in the first step of the transformation in a separate reactor can be treated as a value added product with its own market. This can in turn help in economizing the overall process.

A2.2. Butadiene synthesis by Ostromyslensky's two-step process

Proposed mechanism-Generally, the one-step and the two-step process for BD synthesis are only considered two technological options for BD production, essentially going through the same mechanisms [START_REF] Angelici | Chemocatalytic conversion of ethanol into butadiene and other bulk chemicals[END_REF]22 . However this has not yet been proven and pathway for this reaction remains a point of contention.

The Kagan mechanism, which was later validated by the thermodynamic studies by Bhattacharya et. al. 23 and Natta and Rigamonti, is the generally accepted mechanism. Though the Kagan mechanism has been generally accepted, some issues have been identified with the proposed five step mechanism. The first issue concerns the assumption of acetaldol as an intermediate which has seldom been detected amongst the products of the reaction. The second issue is that the engineers at Union Carbide Corporation have reported that acetaldol was converted back to acetaldehyde when fed together with ethanol into a reactor packed with a 2% Ta/SiO2 catalyst without producing butadiene.

Together this might suggest that the reaction proceeds through a different pathway.

Based on this and their observation on purely basic MgO catalysts, Cavani et al. 25,26 have recently proposed a new, self-consistent mechanism featuring an intermediate carbanion species in the formation of C4 molecules.

Scheme 1.6 -Novel reaction pathway for the formation of butadiene from ethanol proposed by Cavani et al. 26,[START_REF] Chieregato | On the chemistry of ethanol on basic oxides: Revising mechanisms and intermediates in the lebedev and guerbet reactions[END_REF] Though these results are recent and need to be proven on catalytic systems other than MgO, they indicate the need to study the reaction pathway from ethanol to butadiene in further detail.

Though the mechanism of the reaction has not been unequivocally stated, several authors have described the thermodynamics for the conversion of ethanol to BD, mostly based on the Kagan mechanism. [START_REF] Angelici | Chemocatalytic conversion of ethanol into butadiene and other bulk chemicals[END_REF] ΔG calculations for equilibrium conditions for both one step and two step processes indicate that there is no thermodynamic advantage when performing EtOH to BD in two steps. The increase in BD yield and selectivity of the two-stage process can therefore only be explained by kinetic factors, related to the catalyst properties.

A2.3. Catalyst design

Due to the variety of elementary steps involvedbetween conversion of ethanol and acetaldehyde to butadiene, it is certain that the catalyst responsible for this transformation must be multifunctional. It is widely accepted that the catalytic systems should possess a mixture of acidic, basic and redox properties. While the redox and basic sites can participate in dehydrogenation of ethanol to acetaldehyde, the acidic and basic sites can be thought to be active in the condensation and dehydration reactions. However, we know that transformation of ethanol and acetaldehyde to BD is marred by numerous side reactions which can be favoured or suppressed based on the kinetic and thermodynamic conditions. For example, excessive acidity in the catalytic system can lead to dehydration of ethanol to ethylene and diethyl ether. Therefore an optimal balance between the acidic, basic and redox sites is crucial for ensuring good BD yields. As stated earlier by Weckhuysen et al. [START_REF] Angelici | Chemocatalytic conversion of ethanol into butadiene and other bulk chemicals[END_REF] and Ivanova et al., [START_REF] Sushkevich | With Open Arms: Open Sites of ZrBEA Zeolite Facilitate Selective Synthesis of Butadiene from Ethanol[END_REF][START_REF] Ordomsky | Study of acetaldehyde condensation chemistry over magnesia and zirconia supported on silica[END_REF] the key issues in the designing good catalysts are-understanding the optimal catalytic functions (acid/base/redox) and establishing structure-function relationship for the catalyst.

A2.4. State-of-the-art catalytic systems for Ostromyslensky's two-step process

Due to the long history of the ETB process, a variety of catalytic systems have been investigated for both the one step and the two-step process. Both Corson et. al. 8 and Toussaint et al. [START_REF] Toussaint | Production of Butadiene from Alcohol[END_REF] investigated lewis acid catalysts consisting of silica supported Group IV and Group V metal oxides for the Ostromyslensky process. It was found that these metal oxides were active in aldol condensation and MPV reduction. These catalysts could later be modified for the Lebedev process by addition of metal (Ag, Cu, Ni) promoters with redox sites which could aid transformation of ethanol to acetaldehyde. [START_REF] Sushkevich | Design of a Metal-Promoted Oxide Catalyst for the Selective Synthesis of Butadiene from Ethanol[END_REF] Zirconia based systems Han et al. [START_REF] Han | SI for Sol-gel synthesis of ZrO2-SiO2 catalysts for the transformation of bioethanol and acetaldehyde into 1,3butadiene[END_REF] Zhang et. al [START_REF] Xu | Ethanol/acetaldehyde conversion into butadiene over sol-gel ZrO 2 -SiO 2 catalysts doped with ZnO[END_REF] . Investigated ZnO promoted ZrO2-SiO2 catalysts synthesized by a sol-gel method for the two-step ethanol transformation to butadiene. Surface acidity studies showed that the addition of ZnO into ZrO2-SiO2 system decreased quantity of acidity yet did not change the strength of the acid sites on the catalysts. This is thought to lower the selectivity to dehydration products while boosting the BD selectivity. The best performance with ethanol/acetaldehyde conversion of 40.7% and BD selectivity of 83.3% was reached at 310°C, ethanol/acetaldehyde mole ratio of 3.5 and WHSV of 1.4 h -1 using a 0.5 wt% ZnO doped ZrO2-SiO2 catalyst.

Tantalum based systems

Tantalum is considered to be amongst the most active catalyst in ETB transformation from the time of WWII. Corson et.al. 8 showed copper-doped silica-supported Tantalum oxide to be amongst the most active catalysts for the one-step conversion of ethanol.

A notable study is that by Chae et al. [START_REF] Chae | Butadiene production from bioethanol and acetaldehyde over tantalum oxide-supported ordered mesoporous silica catalysts[END_REF] who prepared a series of ordered mesoporous silica (OMS) supported Tantalum oxide samples by wetness impregnation technique followed by calcination at high temperature. The catalysts showed better catalytic performances towards butadiene production compared to the earlier systems, indicating need for better dispersed tantalum on silica catalysts. The study primarily focused on effect of surface morphology on catalytic activity while no conclusive comment was made on the surface active species.

A remarkable attempt to synthesize a well-defined active specie was recently made by Dzwigaj et al. who reported Ta incorporation into vacant T-atom sites of the SiBEA zeolites framework as mononuclear Ta(V) allowing for the preparation of highly selective catalysts for 1,3-butadiene synthesis. In the study higher Ta(V) content increased the formation of ethylene, which the authors relate to the formation of closed tantalum sites bound to four silicon atoms. Accordingly, these sites were said to be less active towards butadiene formation.

Niobium based systems

Niobium oxide was found to be active for the two-step process by Toussaint et al. 24 in 1947.Ivanova et al. 36 reiterated this claim by adding niobium to the list of metals that could be used to produce butadiene from ethanol when appropriately promoted.

Kyriienko et al. [START_REF] Kyriienko | Effect of the niobium state on the properties of NbSiBEA as bifunctional catalysts for gas-and liquid-phase tandem processes[END_REF] investigated niobium-modified zeolite BEA for the ETB reaction. The catalysts were synthesized by a two-step post-synthesis method similar to the Ta/SiBEA system mentioned above. The performance of the Nb/SiBEA system was reported to be poorer than the Ta/SiBEA system. At 350°C, with an EtOH/AA ratio of 2.7 and for a WHSV of 0.8 h-1, butadiene yield over 0.7% Nb/BEA was of 23.6, for a productivity of 0.11 gBD•g -1 cat•h -1 .Combining the observations from this study together with the observations made earlier by Toussaint et. al 24 ., Niobium is an unlikely candidate for industrial application in two step synthesis of BD.

Other systems

Palkovits et al. [START_REF] Klein | Formation of 1,3-butadiene from ethanol in a two-step process using modified zeolite catalysts[END_REF] tested a set of zeolite based catalysts, designed for variation of the concentration and relative ratio of acidic and basic sites for materials of comparable textural properties by varying the zeolite's module, adding magnesium oxide and exchanging the proton by alkaline and earth alkaline metals, respectively. MgO added zeolite material could facilitate the highest BD selectivity of 72%.

Tan et al [START_REF] Zhu | Conversion of Ethanol and Acetaldehyde to Butadiene over MgO-SiO2 Catalysts: Effect of Reaction Parameters and Interaction between MgO and SiO2 on Catalytic Performance[END_REF] . Studied MgO-SiO2catalysts with different surface properties, synthesized by simple tuning of the calcination temperature. The best MgO-SiO2 catalyst with a significant amount of amorphous magnesium silicates and few crystalline magnesium silicates, showed the highest BD selectivity of 80.7%.

A2.5. Towards better catalysis

Whereas a series of catalyst compositions have been tested, most of the above studies are incomplete in their thorough physico-chemical characterization. The studies lack a molecular insight into the species populating the catalyst surface. As a result, definitive identification of the active sites on these catalytic systems remains an open question.Indeed the usual preparation of heterogeneous catalysts by-wetness impregnation, sol-gel technique, wetness kneading, produces catalysts with a broad distribution of metal coordination environments [START_REF] Pelletier | Catalysis by Design: Well-Defined Single-Site Heterogeneous Catalysts[END_REF][START_REF] Copéret | Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities[END_REF] . This makes it difficult to characterize them at the molecular level.

In addition to insufficient understanding of the active sites, the inherent heterogeneous nature of the surface species mars the overall BD yield and catalyst productivity. This can be witnessed by a broad product distribution and to some extent catalyst deactivation due to formation of higher alkanes.

In this context Surface organometallic chemistry (SOMC) can provide invaluable tools in bridging the gap betweenhomogeneous and heterogeneous catalysts by designing welldefined single sitesurface complexes. In the next section we discuss the principle and practicalities of the SOMC protocol to explore the possibility of designing well-defined catalysts for on-purpose synthesis of butadiene by Ostromyslensky's process.

Chapter 1

B. Surface Organometallic Chemistry

B1. General Introduction

Catalysis represents an important driver for our chemical industries and thereby our economy. While heterogeneous catalysts are often thought to be more industrially viable due to their robust nature, easy recovery and regeneration, they barely possess the high degree of molecular uniformity inherent to their homogeneous counterparts. [START_REF] Takehira | Heterogeneous Catalysis in the Liquidphase Oxidation of Olefins. IV. The Activity of a Supported Vanadium or Chromium Oxide Catalyst in the Decomposition of t -Butyl Hydroperoxide[END_REF][START_REF] Dalton | Heterogeneous Catalysis in Solution. Part 18.l The Catalysis by Carbons of Oxidation-Reduction Reactions[END_REF][START_REF] Soignier | Tantalum hydrides supported on MCM-41 mesoporous silica: Activation of methane and thermal evolution of the tantalum-methyl species[END_REF] As a result of their numerous, heterogeneous surface sites these catalysts lack selectivity towards a desired end product. Difficulty also lies in characterization of their active sites with molecular precision. This confines the development of new heterogeneous catalysts to an empirical approach rather than a rational one. To combine the advantages of both homogeneous and heterogeneous catalysis, efforts have been directed towards the generation of heterogeneous catalysts with well-defined active sites.In this respect, one of the powerful approach is that of Surface Organometallic Chemistry (SOMC) [START_REF] Copéret | Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities[END_REF][START_REF] Candy | Analogy between Surface and Molecular Organometallic Chemistry[END_REF] Surface Organometallic Chemistry is a technique developed to tailor well-defined heterogeneous catalysts by grafting organometallic complexes on surface sites of a support in a controlled fashion. [START_REF] Copéret | Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities[END_REF] This approach is based on controlling the concentration of grafting sites per unit area of the support, which in turn controls the final density of organometallics grafted on the surface. Due to its ability to perform molecularly controlled grafting, SOMC produces surface sites with a known co-ordination sphere.

This attribute is essential in developing a structure activity relationship for a catalyst and in turn for rational design of ideal catalysts for a process. Keeping this in mind, the main requirements for applying SOMC approach would be the presence of a well-characterized support, and an organometallic complex -MLnXx (L= functional ligand, X= ancillary ligand). [START_REF] Mcdaniel | The activation of the phillips polymerization catalyst: I. Influence of the hydroxyl population[END_REF][START_REF] Hoveyda | Catalytic Asymmetric Olefin Metathesis[END_REF] Upon grafting the complex on the support, it can be used as a catalyst by itself or be subjected to post-treatments (calcination, hydrogenation, sulfidation) to obtain a more stable structure or to attain a new functionality. Figure 1 schematically represents an ideal SOMC catalyst with its key components.

B1.1. Preparation and characterization of catalyst supports

Various supports such as silica [START_REF] Bilhou | Surface-linked metal cluster carbonyls: fixation, behaviour and reactivity of Rh6(CO)16 bonded to a phosphinated silica[END_REF] , alumina [START_REF] Dufour | Surface organometallic chemistry on oxides: Reaction of hydrogen with bis(allyl)rhodium grafted to silica, titania and alumina[END_REF] , silica-alumina [START_REF] Le Roux | Silica-Alumina-Supported, Tungsten-Based Heterogeneous Alkane Metathesis Catalyst: Is it Closer to a Silica-or an Alumina-Supported System?[END_REF] , zeolites etc have been investigated for grafting organometallic moeties. Every support is unique in the grafting site it carries on its surface. Grafting sites for various SOMC supports have been listed in Mesostructured materials such as MCM-41 [START_REF] Wang | Synthesis and characterization of zirconium containing mesoporous silicas: I. Hydrothermal synthesis of Zr-MCM-41-type materials[END_REF] and SBA-15 [START_REF] Pelletier | Catalysis by Design: Well-Defined Single-Site Heterogeneous Catalysts[END_REF] are prepared using sol-gel synthesis methods in the presence of a surfactant template to form ordered mesopores of desired shape and size. These are high surface area material with surface areas up to 1000

m 2 g -1 .
Silica is made up of SiO4 tetrahedra linked by siloxane (≡Si-O-Si≡) bridges and surface silanols (≡Si-OH). Heating under dyanamic vacuum can affect the concentration and nature of these surface silanols. Under UHV, at 150°C the physisorbed water is removed from the surface of silica. Heating at higher temperatures leads to condensation of adjecent (≡Si-OH) goups and formation of (≡Si-O-Si≡) bridges with simultaneous evolution of water. Thus different dehydroxylation temperatures produce silica with varying topology. The transformation of silica surface can be monitored by IR spectroscopy where the stretching of isolated, vicinal and geminal silanols can be identifiend at 3747cm -1 , 3650 cm -1 and 3750 cm -1 respectively.

Quantitative estimation of these hydroxyl surface sites has also been performed by their chemical titration against metal alkyls (n-BuLi) or metal alkyl halides (MeMgBr). From this, the silanol density of SiO2-200, SiO2-500 and SiO2-700has been calculated as 2.6, 1.4 and 0.7 OH/nm 2 respectively. These values were later verified by quantitative solid-state

B1.2. Grafting of organometallic complexes on catalyst support

Grafting of a metal precursor onto the support is carried out under strict anhydrous and anaerobic conditions. This is due to the general sensitivity of organometallic complexes Calcination, which can be defined as heat treatment (at temperatures typically above 400°C) performed under air, is a commonly applied treatment in both preparation and regeneration of SOMC catalysts. During catalyst preparation the aim is to remove the organic template and stabilization of the catalyst. The surface sites are largely reported to remain isolated (more so for early metals) when low surface loadings are used. However, calcination at very high temperatures can lead to incorporation of the metal into the support matrix and/or to sintering to form metal oxide clusters.

Thermolysis under vacuum-Supported organometallics with -N(SiMe3)2,

-OSi(OtBu)3,-OtBu, or -Cl as ligands, readily lose their ligands when treated at high temperatures (>150-400°C) under vacuum, while remaining mostly isolated. Thermolysis involves reaction of the metal site with an adjacent siloxane bridges and generating more stable silicate species.

Thermal treatment under H2-The reaction of silica, alumina, and silica-alumina supported early transition metal alkyl complexes with dry hydrogen at 100-200 °C typically forms metal hydrides. The formation of supported metal hydrides is proposed to occur through hydrogenolysis of metal-carbon bonds to form highly reactive metal hydride species. For silica supported complexes, the hydride species further react with the adjacent siloxane bridges creating new metal oxygen bonds and transferring hydrides to the surface.

B1.4. Characterization tools for studying the surface sites

One of the defining features of catalysts synthesized by SOMC technique is the detailed knowledge of the nature of species populating the surface of the catalyst. Such precise understanding of the surface is a result of advanced characterization techniques, which together provide the complete picture of the surface of the catalyst. that mimic the surfaces of inorganic oxide supports and aid understanding their interaction with the grafted organometallic fragments. Additionally, crystallographic parameters of molecular complexes can be used for building models for the interpretation of XAS data of the supported species.

Computational chemistry is being increasingly employed as a complementary tool to obtain energy profiles that can relate to structural preferences and particular spectroscopic signatures of surface species. Computational methods can confirm or rule out possible structures and allow a deeper understanding of the relation between structure and reactivity. This understanding can be useful in thereby establishing the catalytic sites and mechanism of a reaction.

B2. Well defined silica supported Tantalum species B2.1. Grafting of Tantalum-alkyls on silica

In 1995, Dufaud et al. [START_REF] Dufaud | Surface Organometallic Chemistry of Inorganic Oxides: The Synthesis and Characterization of (≡SiO)Ta(=CHC(CH3)3)(CH2C(CH3)3)2 and (≡SiO)2Ta(=CHC(CH3)3)(CH2C(CH3)3)[END_REF] firstsucceeded in grafting tantalum onto silica by reacting Ta(CH t Bu)(CH2 t Bu)3 with the surface hydroxyls of a partially dehydroxylated silica.

Based on IR spectroscopy, solid state NMR spectroscopies, elemental analyses, and quantitative chemical reactivity toward ethanol and H2, it was shown that (≡SiO)Ta(CH t Bu)(CH2 t Bu)2 is obtained as the sole surface species (> 95 %) when grafted on silica dehydroxylated at 700°C.This study was followed by other studies that showed that the nature and in the concentration of the surface silanol groups (by varying the temperature of dehydroxylation) can affect the nature of the resulting surface species.

(Scheme 1.7)In 2001 Chabanas et al. [START_REF] Chabanas | Molecular insight into surface organometallic chemistry through the combined use of 2D HETCOR solid-state NMR spectroscopy and silsesquioxane analogues[END_REF] used high-resolution solid-state one-dimensional (1D) and two-dimensional (2D) NMR spectroscopy to study to define the structure of (≡SiO)Ta(CH t Bu)(CH2 t Bu)2 at a molecular level and to investigate the reaction pathway leading to the grafted species.

Scheme 1.7-Formation of different well-defined Tantala species depending on the dehydroxylation temperature of SiO2

B2.2. Grafting of Tantalum-alkoxys on silica

Petroff et al. [START_REF] Saint-Arroman | Deperoxidation of cyclohexyl hydroperoxide by silica-supported alkoxo-tantalum complexes[END_REF] demonstrated the grafting of tantalum alkoxy complex as shown in scheme 1.8. The appeal of grafting an alkoxy precursor compared to the organometallic Ta(CH t Bu)(CH2 t Bu)3 was its easy and comparatively cheap availability. Based on IR spectroscopy, elemental analysis and NMR studies it was found that by grafting tantalum methoxide Ta(OMe)5 on SiO2-700 a monopodal ( SiO)Ta(OMe)4 surface specie was obtained. One of the primary concerns of the study was the inherent dimeric nature of tantalum alkoxy in solution. However upon EXAFS analysis, no Ta-Ta interaction was reported and the presence of an isolated monopodal tantalum methoxides supported on silica was confirmed.

Scheme 1.8-Grafting of Ta(OMe)5 onto SiO2-(700) according to Petroff et al.

B2.3. Ta-H species supported on silica

Silica supported tantalum alkyls lead to a mixture of supported tantalum hydrides 3), by treatment under hydrogen at 150 °C. [START_REF] Vidal | Characterization, and Reactivity, in the C-H Bond Activation of Cycloalkanes, of a Silica-Supported Tantalum(III) Monohydride Complex: (≡SiO)2TaIII-H[END_REF][START_REF] Chow | Ammonia and dinitrogen activation by surface organometallic chemistry on silica-grafted tantalum hydrides[END_REF] Soignier et al. [START_REF] Soignier | Tantalum hydrides supported on MCM-41 mesoporous silica: Activation of methane and thermal evolution of the tantalum-methyl species[END_REF] characterizedthese surface complexes by the combined use of several techniques such as IR and EXAFS spectroscopies as well as 1H MAS, 13C CP/MAS, 2D 1H-13C HETCOR, and J-resolved solid-state NMR and mass balance analysis.

[(≡SiO)2Ta(H)x](x=1,
The proposed mechanism for the formation of supported tantalum hydrides involves a reaction of a siloxane bridge of silica with one hydrogen atom from the coordination sphere of tantalum generating tantalum hydrides and surface (≡Si-H) species.

Scheme 1.9-Proposed mechanism for the formation of supported tantalum hydrides from tantalum alkyls upon treatment under hydrogen at 150 °C44

C. Aim of the project

The most distinguishing advantage of surface organometallic chemistry is that, via a molecular approach, the coordination sphere of a grafted metal can be designed based on molecular considerations drawn from homogeneous organometallic chemistry. In this manner, catalysts with homogeneous and uniformly distributed active sites can be synthesized.

We have discussed both-(i) the current limitations in butadiene production from ethanol and acetaldehyde from the point of view of catalysis and (ii) the scope of SOMC in synthesizing novel catalysts with a molecular precision. Thus the aim of this work is to utilize the expertize of SOMC in developing to new tantalum-based catalysts for the transformation of ethanol/acetaldehyde to butadiene.

Towards this endeavor, the aim of this project is-

To develop a family of well-defined TaOx/SiO2 catalysts through SOMC protocol with varying Tantalum nuclearity

To compare their performances to other classic Ta(V)/SiO2 heterogeneous catalysts discussed in literature for the conversion of ethanol and acetaldehyde to butadiene.

The study will provide new insights the structure activity relationship and study the role of support and metal oxide in the conversion of ethanol and acetaldehyde to butadiene. This understanding will be immensely useful in designing catalytic systems with better catalytic performances (Conversion, Selectivity and Catalyst Lifetime)

A. Introduction:

In our endeavor to prepare well-defined catalysts for butadiene synthesis from ethanol and acetaldehyde, our first strategy was to explore the possibility of designing 'single site' TaOx/SiO2catalysts for this transformation. The tools to tailor such catalysts were borrowed from surface organometallic chemistry (SOMC).

Synthesis of Group IV MOx species has been studied in the past. Petroff et al. 1 studied the synthesis (≡SiO)3M(OH) by addition of O2 to the corresponding metal hydrides,(≡SiO)3MH as "single site" heterogeneous catalysts for the selective deperoxidation of cyclohexyl hydro peroxide. It was found that the action of molecular oxygen at 25 °C onto supported metal hydrides is, however, not as clean.

In 2007, Rataboul et al. Indeed gas phase interaction of both N2O and O2 with metal atoms has been studied extensively using a variety of experimental and theoretical methods. Theoretical studies predominantly support end-on O-coordination of N2O to the metal atom (η1-O) with subsequent charge transfer (metal-to-N2O) and consequent M-O-NN bending occuring along the N-O bond scission pathway. 4 Whereas in the case of oxygen interaction with a metal, both end-on and side-on coordination is possible forming superoxo and peroxo type of complexes respectively. These complexes can further interact with other neighboring metal atoms. 5 The predominance of linear M-O-NN binding in N2O complexes can then provide a cleaner pathway to oxidation of supported metal complexes. Building upon this, the reaction of [(≡SiO)2Ta(H)x=1,3] with N2O is a novel attempt to generate well defined 'single site' Group V MOx moiety on the surface of silica. model study in multi-technique surface organometallic chemistry. [6][7][8][9] Extensive characterization and mechanistic investigations by the combined use of IR and EXAFS spectroscopies as well as 1H MAS, 13C CP/MAS, 2D 1H-13C HETCOR 9 , and solid-state NMR and mass balance analysis have allowed for a detailed molecular description of these silica-grafted complexes.

The first step of this strategy was to synthesize [≡SiOTa(=CH t Bu)(CH2 t Bu)2] species. For this the grafting procedure from the literature was reproduced. 6 A known quantity of the silica dehydroxylated at 500°C was reacted with a pentane solution of molecular [Ta(=CH t Bu)(CH2 t Bu)3], containing a stoichiometric amount of tantalum with respect to the full coverage of the starting silanols. This complex was named complex 1.1. Analysis of the lightly orange solid by 1H NMR, 13C NMR, DRIFT, quantitative titration of evolved neopentane and elemental analysis of the final solids confirmed that the expected species were obtained with full coverage of the surface silanols. In IR spectroscopy, the ν(O-H) band at 3747 cm -1 assigned to isolated silanol groups disappears, indicating complete consumption of hydroxyl groups. In addition to the sample, with full coverage of surface silanols, another sample with 2 wt% Ta loading was prepared by the same procedure with the purpose of catalytic testing.

The next step was synthesis of their corresponding hydrides [(≡SiO)2Ta(H)x=1,3]. Their

formation has already been reported in the literature by reaction of [(SiO)Ta(=CH t Bu)(CH2 t Bu)2] species with H2. 6,10 The reaction yields light brown powder. These tantalum hydride species supported on silica have been extensively

characterized by IR and EXAFS spectroscopies, by chemical reactivity and by quantification of the evolved gases during its formation.

Consistent with the literature, when [(≡SiO)Ta(=CH t Bu)(CH2 t Bu)2]was treated with hydrogen at 150°C for 16hrs, the IR bands in the range 2700-3000 cm -1 ѵ(C-H) and 1300-1500 cm-1 δ(C-H) disappeared and a new set of bands appeared in the 1800 cm-1 region corresponding to the Ta-H stretching modes with two shoulders at 1815 and 1855 cm-1.Also the ѵ(Si-H) bands increase in intensity. This can be explained by the fact that the proposed mechanism for the formation of the supported hydride species involves a reaction of an adjacent siloxane bridge of silica with an hydrogen from the coordination sphere of tantalum, giving rise to new Si-H species on the surface. The In recent years newer characterization possibilities for these complexes have emerged due to advances in transmission electron microscopy (TEM) techniques. 11 These techniques can be useful in the visualization of isolated metal atoms anchored on supports and thus in direct observation of singly dispersed catalytic sites on the substrate material, necessary to understand its catalytic performance and reaction mechanism.

High-resolution scanning transmission electron microscopy (HRSTEM) represents a new

avenue in characterization for the study of immobilized inorganic or organometallic centers. This technique was never before used in the characterization of well-defined surface organometallic species. We herein report the first attempt to directly visualize silica-supported tantalum alkyls [≡SiOTa(=CH t Bu)(CH2 t Bu)2] and tantalum hydrides[(≡SiO)2Ta(H)x=1,3] species by HRSTEM. Micrographs of the two samples can be seen in Figure 2.1-a and 2.2-a respectively. For both the samples homogeneously distributed tantalum atoms are visible without appearance of larger clusters. Software assisted counting of the tantalum atoms showed that for [≡SiOTa(=CH t Bu)(CH2 t Bu)2] more than 90% of the tantalum atoms were isolated whereas for [(≡SiO)2Ta(H)x=1,3] around 82% of tantalum centers are isolated with a few examples di-nuclear and trinuclear moieties.

For both the samples software assisted analysis of the images afforded a statistical distribution of the sizes of the tantalum centers as shown in Figure 1b and 2b as a histogram. For [≡SiOTa(=CH t Bu)(CH2 t Bu)2] the FWHM (Full width at half maximum) of the lonrentzian fit of the histogram is considerably narrow with an average size of the tantalum center around 0.251 nm. For [(≡SiO)2Ta(H)x=1,3], which has been heated to 150°C during hydrogenation, the fit is slightly less narrow. The presence of an oxygen atom at a distance of 1.75 Å from tantalum indicates the presence of a Ta=O bond. The distance of 1.91 Å for the other three oxygen atoms as the first shell around tantalum is in good agreement with the values found in the literature. 12 In the second coordination sphere, the presence of two silicium atoms at 3.26 Å from tantalum indicates the presence of bi-podal specie on the surface of silica. It is to be noted that no Ta-Ta interactions were found.

From the previous results from IR spectroscopy and 1 H solid state MAS NMR, we expect the presence of a Ta-OH bond. This can be explained by the proposition that of the three oxygen neighbors in the first co-ordination sphere of Ta, two oxygens correspond to the Si-O-Ta bonds, while the remaining oxygen corresponds to the Ta-OH bond as witnessed by the spectroscopies.

HRSTEM analysis:

After analyzing the molecular structure of the TaOx species with EXAFS spectroscopy, an attempt was made to visualize the spatial arrangement of the same on the surface of the precatalyst with HR-STEM. The distribution was fitted by a lorentzian function, for which the FWHM was still narrow but now greater than that of the distribution for [(≡SiO)2Ta(H)x=1,3].

Scheme 2.2 B3. Evolution of surface TaOx species with temperature

The above spectroscopic and microscopic analyses inform us of the molecular structure and spatial arrangement of surface species as evolved at room temperature. However, since the ultimate aim of this study is to evaluate the performance of these catalysts in synthesis of butadiene from ethanol and acetaldehyde by Ostromyslensky process, the evolution of these surface species at the target reaction temperature of 325°C was studied.

SOMC-I was heated to 325°C under UHV for 6hrs. The material was labeled SOMC-I (∆325°C). A DRIFT spectrum for the same was then collected under argon (Figure 6). It was observed that upon heating, the peak at 3721cm -1 attributed to Ta-OH stretching vibration disappears and while only the Si-OH stretching persists. After grafting the material was given three washing cycles to remove the excess physisorbed molecular complex. Post washing, the volatiles were condensed into a reactor of known volume in order to quantify the neopentane released during the reaction.

The released gas was quantified by gas chromatography. Analysis of the gas phase during this grafting indicates the release of 1.6 equiv. of neopentane per grafted Ta atom.

Elemental analysis on the resulting solid shows a 8.59 wt% Ta and 3.13 wt% C. Firstly, considering that the Ta wt% of SOMC-I FC was 5.6%, it can be said that about 54% grafting was achieved in this step. Also the ratio of carbon to the second Tantalum can be calculated as 15C/Ta. However, formation of 54% dimeric Ta-O-Ta species on the surface cannot be assumed as we know from the DRIFT that the tantalum alkyl precursor also reacted with the surface Si-OH functionality.

This suggests that the grafting of second molecule on the surface of SOMC-I FC is not as straightforward. A monopodal grafting of second tantalum grafting (as hypothesized)

should have resulted in release of 1 equivalent of neopentane ligand per grafted tantalum.

Release of approximately 1.5 equiv. of ligand, suggests a mixture of monopodal and bipodal grafting of second Ta.

Thorough characterization of the material by EXAFS and other spectroscopic techniques will give better insight into the grafting. However, we can propose this as a potential strategy to selectively tailor Ta-O-Ta dimeric species on silica surface. The sample was tested for catalytic activity under the code SOMC-I Ta-O-Ta.

C. Conclusion

Upon oxidative treatment of [(≡SiO)2Ta(H)x=1,3] with N2O at room temperature, the material (SOMC-I FC) was characterized by IR, NMR spectroscopies, as well as elemental analysis. Formation of well-defined isolated mono-tantalate specie[(≡SiO)2Ta(=O)(OH)] was established via EXAFS analysis. HRSTEM analysis of SOMC-I found largely isolated dispersion of Ta (V) centre, with some degree of agglomeration.

Evolution of these species at high temperature was studied via DRIFT and HRSTEM analysis. Upon heat treatment under vacuum, these isolated tantalum species were observed to aggregate into string-like and cluster

formations.

An attempt to yield well-defined di-nuclear tantalum species was performed

D. Experimental Details: D1. General procedure for preparation of starting material

All experiments were carried out under controlled atmosphere, using schlenk and glovebox techniques. For the synthesis and treatment ofthe surface species, reactions were carried out using high-vacuum lines (1.34 × 10 -5 torr) and glovebox techniques.

Pentane and diethyl ether were distilled on NaK alloy and benzophenone/Na, respectively, followed by degassing by freeze-pump-thaw cycles. Hydrogen was dried over freshly regenerated molecular sieves (3 Å) and deoxo traps; N2O was purchased from Linde and used as received (purity of N2O: 5.0)

D2. Synthesis and characterization of catalyst support

Typically SiO2 (Aerosil® Degussa, 200 m 2 .g-1) was compacted with distilled water calcined at 200 °C in air for 1 h and partially dehydroxylated at 500°C and 700°C under high vacuum (10 -5 torr) for 16 h at 3°C/min to give SiO2-500 and SiO2-700 respectively. 13

D3. Synthesis and characterization of molecular precursor

Synthesis of Neopentylmagnesiumchloride, NpMgCl. 7.4 g of magnesium (300 mmol, 1.3 equiv) with 30 ml of distilled ether, 30 ml of distilled THF, 25 g of neopentyl chloride (235 mmol) and 0.4 ml of 1,2-dichbromoethane (4.6 10 -3 mmol) were introduced, under argon, in a 250 ml glass two neck reactor equipped with a cooler. The mixture was then refluxing at 60°C for 15 h. After filtration and dilution with 60 ml of distilled ether, a pale yellow solution was stored under argon. The titration was done following the literature procedure. 14 Synthesis of [Ta(=CH t Bu)(CH2 t Bu)3] The molecular precursor was prepared following the literature procedure with 4 g of TaCl4 (18.65 mmol) and 34 ml of t BuCH2MgCl at

D5. Catalyst characterization by HRSTEM

HR-STEM experiments were performed at IRCELYON on TITAN ETEM from. The microscope has a STEM mode allowing guaranteed atomic imaging at 0.136 nm in HAADF, ADF and bright field mode. The aberration of sphericity is adjustable thanks to a "Cs" corrector of "objective" type (for high resolution imaging).

All the analysis was performed using an electron beam of 300kV, with a dose of 83.2e/A 2 s at a magnification of 5.1Mx. The refinements were performed by fitting the structural parameters Ni, Ri, i and the energy shift, E0 (the same for all shells). The fit residue, was calculated by the following formula: where is the number of degrees of freedom in the signal, was calculated and its minimization considered in order to control the number of variable parameters in the fits.

D6. Catalyst characterization by EXAFS

= [k 3 (k) -k 3 (k)] exp cal 2 [k 3 (k)]

A. Introduction

One of the common approaches employed in the synthesis of state-of-the-art Ta/SiO2 catalysts for Ostromyslensky process has been via impregnation of a tantalum precursor (TaCl5 1,2 , Ta(OEt2)5 3 ) on the surface of the oxide support followed by calcination underdyanamic flow of air at high temperatures. Catalysts synthesized in this way have been most prominently characterized by Wachs et al. [4][5][6] Interaction of molecular with silica-supported tantalum alkys synthesized by SOMC has been studied in the past. It is known that the peralkyl tantalum complexes are highly sensitive towards oxygenated molecules like alcohol, water or molecular oxygen. The reactivity of molecular oxygen with supported peralkyl tantalum complex ( SiO)Ta(CH t Bu)(CH2 t Bu)2 was first studied by Petroff. et. al. 7 in 2002. The volumetric titration of oxygen consumed per supported tantalum species gave 1.8 molecules of O2 per ( SiO)Ta(CH t Bu)(CH2 t Bu)2. The reaction was exothermic with the solid changing its colour from light orange to white.

The 13 C CP MAS NMR of the partially labeled 13 C complex confirmed the transformation of the alkyl and alkylidene ligands and the formation of alkoxo species on the surface.

The spectra also confirmed presence of carboxylates on the surface.

Based on the IR and NMR studies, a mechanism was proposed that suggested that the addition of molecular oxygen onto ( SiO)Ta(CH t Bu)(CH2 t Bu)2 produces different supported tantalum complexes with alkoxo, oxo and carboxylate ligands in their coordination sphere, the major product being probably ( SiO)Ta(O)(OCH2 The reactivity of grafted tantalum organometallic complex with oxygen at room temperature was further studied at an electronic level by Jacinto et. al. 8,9 The changes in the local Ta electronic structure were followed by in situ high-energy resolution offresonant spectroscopy (HEROS). The study revealed that the reaction of SiOTa(CH t Bu)(CH2 t Bu)2, with molecular oxygen results predominantly in formation of Ta dimers, in contrast with the isolated tantalum oxo alkoxo complexes proposed earlier.

Indeed it is difficult to elucidate the exact structure of the oxidized complex considering the tantalum atom can be present on the surface in different coordination environments, as isolated or oligomeric species. In this scenario, even X-ray absorption spectropies, like HEROS, only allow access to the information on heavy nuclei, namely metal centers, indicating the nature and number of ligands surrounding the metal within its first (and, with lesser accuracy, second) coordination sphere. Also, Raman spectroscopy which is highly powerful in probing the presence of specific groups in a given material, and is

O Si O O O O Ta t Bu t Bu t Bu H O 2 O t Bu H t Bu H H H t Bu O O Si O O O O Ta O O t Bu O O Si O O O O Ta O t Bu O O t
Bu + maj a or minor highly sensitive to detect subtle changes at the molecular level cannot not be adapted to these systems since at such low surface loadings, the weak Raman scattering of the dehydrated surface TaOx species do not give rise to any distinct vibrations against the stronger SiO2 support vibrations. 10 With this in mind, our primary aim was to synthesize and study the surface species generated upon dynamic oxidation of tantalum alkyls grafted on the surface of silica by SOMC protocol.

In addition to the family of catalysts synthesized from [Ta(=CH t Bu)(CH2 t Bu)3] another set of catalysts were synthesized by grafting commercially available tantalum ethoxide on the surface of partially dehydroxylated silica. The advantage of synthesizing supported alkoxo tantalum species starting from alkoxo tantalum complexes, in comparison to the starting complex Ta(CH t Bu)(CH2 t Bu)3, is the important reduction of the number of synthesis steps. Indeed alkoxo tantalum are commercially available complexes in the contrary to the alkyl tantalum species. But a disadvantage of grafting molecular alkoxo tantalum onto silica comes from thermodynamics. Indeed, the enthalpy of this reaction is almost equal to zero. The grafting reaction may then not be as clean as when the starting material is tantalum alkyl. 11 EtOH/AA to BD, to ensure compatibility while comparing with the state-of-the-art catalysts, which are reported at 2 wt% Ta loading. The same grafting, to ensure 2 wt% of tantalum was repeated on SBA-15 dehydroxylated at 700°C, to study the effect of catalyst surface area on catalytic activity (later discussed in chapter 4). Table 3.2 reports the results from elemental analysis, gas analysis and DRIFT experiments for the samples mentioned above. The materials obtained by the above procedure were calcined under the flow of dry oxygen at 500°C for 10 hrs under the experimental conditions earlier reported by Wachs et al. 5 Upon contacting with oxygen, the light orange colour of the samples immediately turned white. DRIFT spectroscopy showed disappearance of IR bands attributed to ν (C-H) and δ(C-H) at 2800-3000cm -1 and 1500-1300 cm -1 respectively which are diagnostic to the neopentyl ligands. However a concomitant appearance of ν (O-H) band at 3747 cm -1 was also observed.

It is to be noted that detailed spectroscopic characterizations were carried out only for the material based on SiO2-700. The material obtained after calcination of sample obtained by 1:1 grafting of [Ta(=CH t Bu)(CH2 t Bu)3] to surface silanols was named SOMC-II FC (to signify full coverage of silanols). Whereas the material obtained from tantalum loading of 2 wt% was named SOMC-II. The data was in agreement with XRD characterization of SOMC-II FC where crystalline Ta2O5 phase could not be observed.

Sample Metal Loading

Neopentane

B3.Synthesis of TaOx/SiO2 catalysts via oxidation with molecular O2

With the view of synthesizing pre-catalysts with a tantalum precursor which is more industrially viable and economically favorable compared to the organometallic [Ta(=CH t Bu)(CH2 t Bu)3], silica dehydroxylated at 700°C (SiO2-700) and 500°C (SiO2-500)

was grafted with commercially available tantalum ethoxide. The impregnation technique typically consisted of stirring at 25°C a mixture of SiO2-700 and tantalum ethoxide in toluene. The calculations were made to graft 1 equivalent of Ta per equivalent of surface silanol. The material obtained from grafting performed on SiO2-700 was coded Complex-2.1' while that obtained via grafting on SiO2-500 was coded Complex-2.2'. and 1500-1300 cm -1 respectively, disappeared and appearance of a band at 3747 cm -1 was also observed.

C. Conclusions:

The reactivity of previously known [≡SiOTa(=CH t Bu)(CH2 t Bu)2] species with oxygen was investigated under continuous flow conditions at 500°C for 10 hrs. The resulting material was completely characterized by IR and NMR spectroscopy, UV-DRS spectroscopy and XRD. The latter two analysis confirmed absence of Ta2O5 microcrystals on the surface. EXAFS analysis corroborated the absence of Ta2O5 phase while indicating the possibility of some Ta-Ta interaction. The HRSTEM microscopy evidenced co-existence of isolated Ta (V) centers along with Ta species in a string like arrangement.

This data together with the information from literature suggested the presence of a mixture of mononuclear, di-nuclear and oligomer like species on the surface of the material.

An attempt was made to reproduce the above synthesis strategy with cheaper and commercially available [Ta(OEt)5]2 . The formation of these catalysts was monitored by DRIFT, SSNMR and XRD.

(cm -1 ): 1300-1500 s, δ(CH2, CH3)), 2800-3000 (s, ν(C-H)). Gas-phase analysis: 1 equiv/ ppm (s, CH2, CH3). Infrared (cm -1 ): 1300-1500 s, δ (CH2, CH3)), 2600-2900 (s, ν(C-H)).

D4. In operando FTIR spectroscopy experiments

A self-supporting disk of silica of known weight was dehydroxylated at 700°C for 12hrs under UHV. [Ta(=CH t Bu)(CH2 t Bu)3] was sublimed on this disk at 80 •C, 10 -5 torr. This was followed by a reverse sublimation to remove the excess of the organometallic precursor. During this process, the pellet turned from white to orange. Following this the pellet was calcinated under flow of dry O2. Transmission FTIR spectroscopy was used to study pellet at every 100°C from 50 to 500°C.

D5. X-ray diffraction

The crystallographic information of all samples was investigated at CDHL, University of , where is the number of degrees of freedom in the signal, was calculated and its minimization considered in order to control the number of variable parameters in the fits.

A. Introduction:

The transformation of ethanol and acetaldehyde to butadiene is a complex process involving a network of reactions such as dehydration, dehydrogenation and aldol condensation. 1 Such process demands for a multifunctional catalyst, with an appropriate balance in number, strength and ideal proximity of acidic, basic and redox sites in order to maximise the productivity of the reaction. 2,3 In recent years there has been a growing upsurge in the study of catalytic systems for the two step process. Though these studies are helpful in pointing towards the usefulness of certain metals or supports in a catalyst to improve the overall BD yield, there is no clearevidence on the nature of the species populating the surface of the catalyst. In the absence of this understanding, it is difficult to generate or balance the acid/base or redox sites on the catalyst surface in a way that provides for the ideal reaction kinetics. 4 To this end we synthesized two families of TaOx/SiO2 pre-catalysts with well-defined surface species (as reported in Chapter 2 and 3). Upon extensive characterization it could be concluded that these pre-catalysts differ in their dominant surface species in terms of tantalum nuclearity and the tantalum co-ordination sphere. In this chapter, we evaluate the activity of these catalysts in the transformation of ethanol and acetaldehyde to BD. With the reliable knowledge of the chemical and structural identity of this catalyst we would like to better appreciate its correlation to the catalyst performance (detailed discussion in Chapter 5)

Furthermore, the aim of this chapter is to compare the catalytic performances of these two families of SOMC pre-catalysts with the state of the art catalysts studied in literature.

Following the original catalyst developed by Ostromyslensky, ample study on Ta/SiO2 systems as catalysts for two step ETB transformations is available. Of these, a notable study is that by Jeonget al. 5 who prepared a series of ordered mesoporous silica (OMS)

supported Tantalum oxide samples. The catalysts showed better catalytic performances towards butadiene production compared to the earlier systems, indicating need for better dispersed tantalum on silica catalysts. (We refer to this catalytic system as WI-i). Another remarkable attempt to synthesize a well-defined catalyst was recently made by Dzwigaj et Chapter 4 92 al. 6 , who tailored isolated mononuclear Tantalum (V) centers in the zeolite framework.

(Zeo-i) Resulting catalysts were highly selective in the transformation of ethanol and acetaldehyde to butadiene. Both these systems represent two of the best performing catalysts in the second step of Ostromyslensky process (i.e. transformation of ethanol and acetaldehyde to BD).

Table 4.1-Reported catalysts and reaction conditions for the two step ETB process 4 It is worthy to note that there is a limitation in comparing the catalytic activity of the catalysts mentioned in literature. This is due to the differences in both, the catalytic systems as well as the conditions of catalytic testing. Even while comparing only the silica supported tantalum oxide systems, the catalysts may differ in metal loadings, surface areas and surface morphologies. Also the testing conditions vary slightly in reaction temperatures, flow rates, molar ratios (EtOH/AA) and time on stream. Table 4.1

provides the summary of these details for the catalysts in comparison. We have tried to maintain as much uniformity possible in terms of reaction conditions while drawing these comparisons.

B. Results and Discussions: B1. General description of catalytic tests-

The catalytic tests for production of butadiene from ethanol and acetaldehyde were performed in a lab scale continuous flow reactor under atmospheric pressure. A catalyst amount of 0.25 g was loaded in the reactor in the form of powder. Inlet feed molar ratios were always constant and were set at 2 mol% ethanol and acetaldehyde mixture with 98% nitrogen. Downstream products were fed to an automatic sampling system for gaschromatography. From the thermodynamic study, it is known that, reaction temperatures between 325 and 430°C are preferred for this transformation, since lower temperatures restrict acetaldehyde formation, whereas higher temperatures limit the aldol condensation. 7,8 Toussaint et al. 9 investigated the influence of the reaction temperature, the molar ratio of EtOH to AA and the space velocity on the catalyst activity of a 2 wt% Ta2O5 on silica.

Highest BD selectivity was obtained for reaction temperatures 325-350°C for a feed mixture with a EtOH: AA ratio of 2-2.5.

Chapter 4

94

Keeping this in mind, the catalysts were tested at a reaction temperature of 325°C with the molar ratio of the feed being maintained at EtOH: AA=2.5. The weight hour space velocity (WHSV)of the feed was maintained at 0.5gEtOH/AA• g -1 cat•h -1 . Independent study of the effect of these parameters (contact times and EtOH: AA ratio) on the catalytic activity will be discussed separately in chapter 5, with focus on mechanistic study.

The percentage values of conversion, selectivity and yield were determined from the gas chromatograms, using the Eq. (i)-(iii)-

B2. Catalytic activity of SOMC catalysts

Table 4.2 shows the total conversion of ethanol (EtOH) and acetaldehyde (AA) and the BD selectivity of all the TaOx/SiO2pre-catalysts synthesized via the aforementioned two routes by the SOMC protocol. However, detailed discussion on the catalytic behavior has been made only for the pre-catalysts with tantalum loading ≈2wt%. Catalytic activity results for the pre-catalysts with tantalum loading >2 wt% can be seen in Table 4.2 from

(5)-( 10). (The reported data has been normalized in order to be comparable with the catalytic results reported in literature. In general a carbon loss of ≈25% has been reported

for each of the catalyst) This evolution of surface species can be better visualized through HRSTEM. To test this hypothesis, we tested the catalytic activity of SOMC-I (∆325°C). (See chapter 2). This catalyst, (which is SOMC-I heated at 325°C for 6hrs) showed catalysis extremely similar to SOMC-II. It is to be noted that HRSTEM micrographs of SOMC-I (∆325°C) evidenced aggregation of isolated species into string like and cluster like formations. These results lie in line with the above finding.

Catalytic performance of SOMC-II-SBA was also investigated. SOMC-II-SBA synthesized by the same strategy as that of SOMC-II, represents a catalytic system with better dispersed tantalum active sites by the virtue of larger surface area of SBA-15. The catalyst showed slightly better BD yield of 35.5% as compared to SOMC-II. However notably, the catalytic activity was seen to be maximum at the start of the reaction while progressively dropping and reaching a stable value after TOS=5hrs. From preliminary evidence such a drop in activity could be attributed tocoalescence of isolated Ta active sites.

B2.2. Comparison of the

B3. Butadiene selectivity and product distribution

As mentioned earlier, the reaction from ethanol to BD generates several by-products. The product distribution is affected by several factors such as the reaction conditions and the catalytic system. A difference in temperature alters the thermodynamics and kinetics of the process, possibly favoring the formation of byproducts. 4,10,11 (Scheme 4.1) However taking into consideration the overall thermodynamics of ETB process, in the temperature range of 325-430°C, BD yield and selectivity are kinetically determined. 12,13 Thus, it is the choice of catalyst that plays a major role in determining the product distribution and BD selectivity. Both SOMC-I and SOMC-II exhibit extremely narrow product distributions. These catalysts represent systems with relatively well-defined and homogeneous surface species. The presence of such surface species can limit the number of alternate reaction pathways thereby suppressing the number of side products. This feature is significant for an industrial catalyst since it prevents wasting carbon in undesirable byproducts while also reducing the steps of separation.

The first observation is that for WI-i, a wide range of by-products is generated compared to the other three catalysts. This can be explained by considering the surface species populating the surface of the catalyst. By typical wetness impregnation technique of catalyst synthesis, dispersed metal ions with a variable number of M-O bonds between the support and the metal sites are formed. Such broad distribution of metal coordination environments leads to active sites with varying acid/base properties. 15 These sites can then catalyze numerous side reactions leading to a broad spectrum of reaction products.

B4. Post-catalysis analysis

Coke formation is believed to be the major culprit for catalyst deactivation in the ETB processes. 5,[16][17][18] Few systematic studies for catalyst deactivation by coke formation could be found for the two step ETB process.

The amounts of coke formation for both spent SOMC-I and SOMC-II catalysts after 8 h reaction was also observed by TGA analysis (Figure 4.7). The weight losses measured by TGA analysis in the temperature range from 250 to 700 °C, which is temperature range for decoking 5 , were 1.53wt% and 1.38wt% for SOMC-I and SOMC-II, respectively. The lower propensity towards coke formation can be related to the fact that the catalytic reaction does not produce many heavy compounds and would mainly cause deactivation of the catalyst by creation of coke. 16 Compared to the aforementioned catalysts, the TGA analysis of the spent SOMC-II-SBA after 8 hrs time stream evidenced 6.2% coke formation. Chae et al. 5 attributed such coking to the blockage of pores of the mesoporous material by the heavier byproducts produced in the reaction.

D2. Analysis of coke by Thermo-gravimetric analysis (TGA)

The thermogravimetric analysis used in this laboratory is a Q50 TGA from TA instruments. The temperature control system includes a circular environment chamber, a sample pan furnace, and a purge gas supply system. In the testing, polymer sample was heated up in the environment chamber which provides a stable temperature environment for a consistent measurement. Purging gas can be provided from both horizontal and vertical directions. The horizontal, 60ml/min flow can be either air or nitrogen depending on the type of decomposition and the specific testing purposes. The vertical, 40ml/min flow always supplies nitrogen to prevent oxidative degradation of the balance elements.

In this lab, nitrogen gas was selected for the horizontal flow to prevent additional mass loss due to oxidization of the sample material. The mass loss of the sample was measured with a photodiode balance using a null-balance principle which measures the current required to return the meter to the null position.

A. Introduction

The long history of ETB process has resulted in numerous intensive studies and discussions on the reaction mechanism. [1][2][3][4][5] Early discussions were mostly based on reaction network studies based on the experimental results of catalytic tests using various feed combinations, temperatures and contact times as well as thermodynamic calculations of the proposed mechanisms. 4,6 Most of these studies did not take into consideration the structure of the catalyst in the proposition. 7,8 In the recent years with development in computational tools and spectroscopic characterization techniques there have been significant achievements in explaining the reaction mechanism by a employing a combination of in situ DRIFT spectroscopy and DFT calculations 1,2,9,10 . Herein the surface of the catalyst has been taken into account to explain the adsorption and desorption of intermediate species involved in the transformation of ethanol to butadiene.

The one step process predominantly utilizes mixed oxide catalysts with a significant basic character and redox sites (for dehydrogenation of ethanol to acetaldehyde). Chieregato et al. 1,2 recently studied the one-step process over basic MgO suggesting a carbanion based mechanism for BD formation.

On the contrary, the two-step process is mostly performed over Lewis acid based catalysts 7,8,[11][12][13][14] . These catalysts are mainly Group IV and Group V metal oxides supported on silica or zeolites. Though their functionality is not very efficient in dehydrogenation of ethanol to acetaldehyde, they show high activity and selectivity towards BD from mixture of ethanol and acetaldehyde. Lately, significant progress has been made in studying the reaction mechanism of Lewis acid catalyzed coupling of EtOH/AA mixture and more importantly, explaining the catalyst's structure activity relationship.

In 2016, Ivanova et al. 15 The same concept was extended to Ta/SiBEA system in EtOH/AA transformation to BD by Dzwigaj et al. 13,16 These catalysts which were prepared by tantalum incorporation into vacant T-atom sites of the SiBEA zeolites framework 16 , showed better selectivity to BD than conventional catalysts synthesized by wetness impregnation. 14,17 The high selectivity was attributed to the presence of Ta 'open' sites in higher concentration. However, DFT calculations on the structural and energy properties of these catalysts found that the substitution of tantalum into the zeolite structure results in numerous Ta reactive sites. 13,16,18 Using the expertise of SOMC, in Chapter 2 we reported the synthesis of well-defined selectively single site Ta(V) pre-catalysts supported on silica, while in Chapter 3 we synthesized well-defined TaOx catalysts with mononuclear, di-nuclear and oligomer-like species. Their catalytic performances in EtOH/AA transformation to BD were evaluated and compared in Chapter 4. Based on this information in this chapter, we attempt to build a meaningful structure activity relationship for TaOx/SiO2 catalysts in EtOH/AA transformation to BD.

B. Results and Discussions

B1. Evaluation of surface acidity of SOMC catalysts and its effect on catalytic performance

The spectroscopic study of adsorbed pyridine is an efficient way to determine a catalyst's surface acidity. [19][20][START_REF] Angelici | Chemocatalytic conversion of ethanol into butadiene and other bulk chemicals[END_REF] The type of interaction between the adsorbed pyridine and the acid sites on the catalyst surface produces several characteristic IR bands. The frequency of these bands enables one to distinguish between Bronsted (BPy) and Lewis acid sites (LPy).Typical IR absorption bands of pyridine adsorbed on Bronsted acid sites (BPy) are found at around 1491, 1545, 1578, and 1640 cm -1 , while Lewis acidity gives rise to bands (LPy) around 1450, 1491, 1578, and 1612 cm -1 . 19,22,23 The LPy bands at 1450 and 1612 cm -1 are sensitive to the strength of the Lewis acidity.

For pyridine adsorption experiments, samples were pretreated at 150°C under the flow of helium for 30 mins and then a pyridine pulse (0.01 mL) was injected at 50 °C. The temperature was raised till 400 °C. DRIFT spectra were collected for every 100°C rise in temperature. which can be attributed to the adsorption of pyridine on stronger Lewis sites. 19 In addition to the signals corresponding to lewis acid sites, a small peak can be observed at 1490 cm -1 for SOMC-I and SOMC-II'. This peak can be attributed either Lewis or Bronsted acid sites.

This band is conspicuously absent in case of SOMC-II.

To establish the quantity of acid sites for SOMC-I compared to SOMC-II a thermal programmed desorption (TPD) using NH3 as probe molecule was performed. As shown in For SOMC-II a broad desorption band is observed around 100°C. Instead, for SOMC in addition to a more intense peak at 100°C, a peak at 302°C is observed. This peak can be indicative of the Bronsted Ta-OH sites on the surface of the catalyst. It is notable that for all the above mentioned catalysts surface acidity can be directly correlated to the overall BD yield. SOMC-I and SOMC-II' which represent the catalytic systems with highest surface acidity perform significantly better compared to SOMC-II.

These findings illustrate the potential of SOMC technique in developing material with tunable surface properties via adjusting the nature and the number of surface active sites.

B2. Studying the influence of reaction parameters on catalytic activity B2.1. Effect of feed composition on catalytic activity

The effect of molar ratio of EtOH/AA on the BD selectivity and yield was studied. (Figure For both the catalysts, increasing the EtOH/AA ratio, mostly led to decrease in BD selectivity. This can be explained by the fact that increasing the amount of ethanol in the feed accelerates its dehydration to ethylene and diethyl ether, thereby suppressing the overall selectivity towards butadiene.

By feeding only EtOH, both the pre-catalysts SOMC-1 and SOMC-2 do not show significant conversion to BD. This is not surprising since the catalysts are predominantly acidic and lack sufficient basic character or redox sites to promote AA formation from EtOH.

Reducing the EtOH/AA ration from 2.5:1 to 1:1exhibited a slight decrease in BD selectivity. This observation is in tune with the studies in literature. 24 At EtOH:AA ratio of 1:1, the amount of EtOH in the feed is not enough to complete the MPV reduction of crotonaldehyde to crotyl alcohol (which will subsequently dehydrate to BD) The effect of residence time of EtOH/AA mixture was studied by changing the reactant flow rate. Generally stating, the change of residence time mostly led to a great difference of the conversion. 24 This law was consistent with our experiment. As showed in Figure 5.7, despite the increase of residence time, the selectivity of BD, ethylene, diethyl ether and ethyl acetate had little change, while the total conversion had a decreasing trend.

B2.2. Effect of residence time on catalytic activity

B4. Proposed reaction mechanism for transformation of ethanol and acetaldehyde to butadiene

The nature and distribution of active sites on the surface of TaOx/SiO2 catalysts is not straightforward. However with the SOMC protocol we could tailor catalysts with well defined surface TaOxspecies. In Chapter 4 from the study of structure activity relationship of the catalysts, we proposed that isolated Ta (V) centres act as the primary active sites in conversion of EtOH/AA to BD by Ostromyslensky process. This conclusion was in line with the findings of Ivanova et al. 15 With increase in temperature, the spectra of the catalyst after ethanol adsorption clearly showed two negative peaks around 3747cm -1 and 3721cm -1 . These peaks can be attributed Si-OH and Ta-OH sites, signaling their consumption. Unfortunately, due to a low signal-tonoise ratio the peaks corresponding to the intermediates formed due to interaction of ethanol with the catalyst could not be observed.

Following this experiment, the same DRIFT study was repeated by adsorbing a EtOH/AA mixture (2.5:1 molar ratio), was adsorbed on the surface of the catalyst and then the catalyst was heated up to 350°C. As in the case of ethanol, two negative peaks appear in the OH region indicating consumption of both Si-OH and Ta-OH sites. Additionally in this case a broad peak is observed in the region of 3200cm -1 to 3600cm -1 suggesting formation of water.

Upon desorption of the adsorbed ethanol and acetaldehyde at 350°C, two peaks became apparent at 1562cm -1 and 1461cm -1 . These vibrations could be from the reaction products of adsorbed ethanol and acetaldehyde. As previously assigned by Taifan et al. 10 , these two peaks could be attributed to C=C stretch (1575 cm -1 ) and the bending modes of CH2 or CH3 was studied over the catalyst at 325°C under batch conditions. After two hours, the gas phase was analysed by GC-MS. The mass spectra revealed the formation of mono-deuterated butadiene, showing that the C4 molecule is formed via coupling of two molecules of acetaldehyde. This is consistent with the generally accepted Kagan mechanism.

As discussed earlier the Kagan mechanism proceeds via five preliminary steps-(i) acetaldehyde formation from Ethanol (ii) aldol condensation of acetaldehyde to acetaldol (iii) dehydration of acetaldol to crotonaldehyde (iv) Meerwein-Ponndorf-Verley Reaction between crotonaldehyde and ethanol to obtain crotyl alcohol and acetaldehyde (v) dehydration of crotyl alcohol to butadiene.

Meerwein-Ponndorf-Verley Reaction between crotonaldehyde and ethanol is thought to be catalysed by tantalum. 25,26 It is known from the point of view of organometallic chemistry that ethanolysis of tantalum oxo hydroxo specie will generate ethoxy species on tantalum (1) and producing water as a byproduct. This could be seen by DRIFT study of ethanol

C. General conclusion

In this chapter we focused on studying the effect of catalyst properties on catalytic activity. Studying the effect of type and amount of acidic sites on the catalyst surface, it was concluded that a small proportion of Bronsted acid sites along with the lewis acid sites were beneficial in the conversion of EtOH/AA to BD. The most performing catalyst SOMC-I showed the highest acidity. The higher surface acidity could be a result of better steric availability of the isolated Ta sites.

The effect of molar ratio of EtOH/AA on the BD selectivity and yield was studied for SOMC-I and SOMC-II. For both the catalysts, increasing the EtOH/AA ratio, mostly led to decrease in BD selectivity. The catalysts did not show significant activity in transformation of only EtOH to BD.

Also, assuming isolated [(≡SiO)2Ta(=O)(H)] to be the active site in the transformation of EtOH/AA to BD, an attempt was made to understand the mechanism of the reaction on the surface of SOMC-I. The Meerwein-Ponndorf-Verley Reaction between crotonaldehyde and ethanol is thought to be catalysed by tantalum. Based on in-situ DRIFT experiments for adsorption of (i) ethanol and (ii) a mixture of ethanol and acetaldehyde on the surface of the catalyst at 350°C, a mechanism was hypothesized for the reduction of adsorbed crotonaldehyde by the hydrogen from ethanol on the isolated Ta (V) center.

presence of mostly isolated dispersion of Ta (V) centers, with a small degree of agglomeration. A part of the study also involved study of evolution of these isolated species upon heating at the target reaction temperature i.e 325°C. The HRSTEM analysis indicated evolution of single site species into string like arrangements and cluster formations. One of the most important conclusions of Chapter 4 was that isolated Ta (V) centers act as the primary active sites in the conversion of EtOH/AA to BD while the Ta atoms in a

Faculté

  Techniques des Activités Physiques et Sportives Observatoire des Sciences de l'Univers de Lyon Polytech Lyon Ecole Supérieure de Chimie Physique Electronique Institut Universitaire de Technologie de Lyon 1 Ecole Supérieure du Professorat et de l'Education Institut de Science Financière et d'Assurances Directeur : M. F. DE MARCHI Directeur : M. le Professeur F. THEVENARD Directeur : Mme C. FELIX Directeur : M. Hassan HAMMOURI Directeur : M. le Professeur S. AKKOUCHE Directeur : M. le Professeur G. TOMANOV Directeur : M. le Professeur H. BEN HADID Directeur : M. le Professeur J-C PLENET Directeur : M. Y.VANPOULLE Directeur : M. B. GUIDERDONI Directeur : M. le Professeur E.PERRIN Directeur : M. G. PIGNAULT Directeur : M. le Professeur C. VITON Directeur : M. le Professeur A. MOUGNIOTTE Directeur : M. N. LEBOISNE "The world always seems a little brighter when you've just made something that was not there before." -Neil Gaiman

Figure 1 -

 1 Figure 1-The general structure of surface organometallic compounds and the possibility of bearing different possible supports, transition metals, functional and spectator ligands

Figure 2 -Figure 3

 23 Figure 2-Reaction apparatus for synthesis of Butadiene from Ethanol and Aldehyde

Figure 3 -Figure 4 -

 34 Figure 3-Catalytic activity of SOMC based catalysts compared to state-of-the-art tantalum based catalysts in conversion of EtOH/AA to BD

  qui ont préparé une série d'échantillons d'oxyde de tantale supportés sur de la silice mésoporeuse ordonnée (OMS) par une technique d'imprégnation par voie humide suivie d'une calcination à haute température. Les catalyseurs ont montré de meilleures performances catalytiques vis-à-vis de la production de butadiène par rapport aux systèmes précédents, indiquant la nécessité de catalyseurs au tantale mieux dispersés sur de la silice. Ces études portaient principalement sur l'effet de la morphologie de surface sur l'activité catalytique, mais aucun commentaire concluant n'a été fait sur les espèces actives de surface. Une tentative remarquable de synthèse des espèces actives bien définies a récemment été réalisée par Dzwigaj et al.6,7 Ils ont rapporté l'incorporation de Ta dans des lacunes d'atomes T du réseau de zéolithes SiBEA en tant que Ta(V) mononucléaire,donnant lieu à descatalyseurs hautement sélectifs pour la synthèse de 1,3butadiène. Bien que ces deux études mettent en évidence l'utilité d'espèces d'oxyde de tantale bien dispersées sur la silice, il n'existe aucunaperçu de la nature des espèces àla surface du catalyseur ou des espèces actives participant dans la catalyse.

Figure 1 -

 1 Figure 1 -Etat de l'art des systèmes Ta/SiO2 pour la transformation d'EtOH/AA en BD.

  Grâce à sa capacité à effectuer un greffage moléculairecontrôlé, SOMC permet obtenir des sites superficiels avec une sphère de coordination connue. Cet attribut est essentiel pour établir une relation entre la structure et l'activité d'un catalyseur et pour concevoir de manière rationnelle des catalyseurs idéaux pour un certain processus.12

Figure 2 -

 2 Figure 2-Représentation schématique de la préparation d'un catalyseur bien défini par SOMC

Figure 3 - 1 2

 312 Figure 3-Structure générale des composés organométalliques de surface représentant différents supports, métaux de transition et ligands fonctionnels possibles

Figure 4 -

 4 Figure 4-Représentation schématique du réacteur pour la synthèse de butadiène à partir d'éthanol et acétaldéhyde

Figure 5 -

 5 Figure 5-Activité catalytique de SOMC-I et SOMC-II comparée aux catalyseurs au tantale de l'état de l'art pour la conversion d'EtOH/AA en BD

Figure 6 -

 6 Figure 6 -Répartition des produits de réactionpour chaque catalyseur : (i) WI-I (ii) Zeo-I (iii) SOMC-I (iv) SOMC-II

Figura 3 -

 3 Figura 3 -Tipica struttura dei composti organometallici di superficie mostrante la possibilità di utilizzo di diversi supporti, metalli di transizione e legandi funzionali e spettatori.

Figura 2 -

 2 Figura 2 -Attività catalitica di SOMC-I and SOMC-II paragonata allo stato dell'arte dei catalizzatori a base di tantalo nella conversion di EtOH/AA a BD.

Figura 3 -

 3 Figura 3 -Distribuzione dei prodotti dei catalizzatori considerati (i) WI-I (ii) Zeo-I (iii) SOMC-I (iv)SOMC-II .

Figure 4 -

 4 Figure 4 -Meccanismo di reazione proposto ed ipotizzato per la riduzione MPVsui siti di Ta(V) di SOMC-I.

Part A - A1. 1 .A2. 1 .B1. 1 . 1 , 3 -Figure 1 . 1 )

 -1111311 Catalytic advances in on-purpose production of butadiene A1. General Introduction .................................................................................................. Importance of Butadiene ........................................................................................ A1.2. Present industrial processes for production of butadiene ....................................... A1.3. Emergence of bio-ethanol as a building block .................................................... A2. Towards on purpose butadiene production ........................................................... Routes in consideration for on-purpose butadiene production ............................ A2.2. Butadiene synthesis by Ostromyslensky's two-step process .............................. A2.3. Catalyst design .................................................................................................... A2.4. State of the art catalytic systems ......................................................................... A3. Towards better catalysis .......................................................................................... Part B-Surface Organometallic Chemistry B1. General Introduction ............................................................................................... Preparation and characterization of catalyst supports .......................................... B1.2. Grafting of organometallic complexes on catalyst support ................................. B1.3. Evolution of well-defined surface sites upon post-treatment .............................. B1.4. Characterization tools for studying the surface sites ............................................ B2. Well defined silica supported Tantalum species .................................................... B2.1.Grafting of Tantalum-alkyls on silica ................................................................... B2.2.Grafting of Tantalum-alkoxys on silica ................................................................ B2.3. Ta-H species supported on silica ......................................................................... C. Aim of this work ......................................................................................................... D. References .................................................................................................................... Butadiene is one of the most versatile and hence one of the most useful chemical building block 1 . Its largest use is in the production of synthetic elastomers such as Styrene-Butadiene Rubber (SBR), Styrene-Butadiene latex, Polybutadiene rubber, Nitrile rubber, and Chloroprene rubber 2-4 . (It also has some non-elastomeric uses, such as a raw material for making hexamethylenediamine, which is the precursor of nylon 6,6, and in the production of largest volume engineering thermoplastic resin, Acrylonitrile-Butadiene-Styrene (ABS). The global butadiene production for 2020 is predicted to be 15.91Mtonnes with an expected growth of 3%per annum from 2017-2024.

Figure 1 . 1 -

 11 Figure 1.1-Global butadiene production forecast by application in billion USD (left) Major applications of butadiene (right)

Figure 1 . 2 -

 12 Figure 1.2-Butadiene produced from different cracker feedstock 4

Figure 1 . 3 -

 13 Figure 1.3-Possible synthesis routes for 1,3-butadiene from fossil based and bio derived feedstocks

Figure 1 . 4 -

 14 Figure 1.4-Schematic representation of first generation and second generation bioethanol synthesis

(Scheme 1 . 3 )Scheme 1 . 3 -Scheme 1 . 5 -

 131315 Scheme 1.3-Generally accepted reaction pathway for ETB conversion based on Kagan mechanism

Figure 1 . 5 -

 15 Figure 1.5 -Scheme of EtOH transformation considering the role of surface acid, basic and redox sites 22

Figure 1 . 6 -

 16 Figure 1.6-Schematic representation of preparation of a well-defined catalyst using SOMC

Figure 1 . 7

 17 Figure 1.7 Schematic representation of a well-defined catalyst using SOMC

1 H

 1 MAS NMR by Lefebvre et. al.[START_REF] Bartram | A Reexamination of the Chemisorption of Trimethylaluminum on Silica[END_REF][START_REF] Millot | Surface organometallic chemistry: a route to well-defined boron heterogeneous co-catalyst for olefin polymerisation[END_REF]

Magnesia

  Hydroxyl groups; lacunar magnesium sites Zeolites Protons with different locations which can be more or less accessible; extra-framework aluminium sites; silanol groups on the external surface of the crystallites Carbon All chemical functions can be found: alcohols, amines, ethers, thiols, ketones, aldehydes, carboxylic acid, etc to air and water along with the increased sensitivity of surface species supported on the surface of the oxides. A typical grafting reaction is carried out in a double-schlenk with a sintered glass filter between the two arms of the flask. (Figure) A solution ofthe organometallic precursor is placed in one half of the double-schlenk while the support is added to the other half. The solution is then transferred by a freeze-pump-thaw cycle to the support through the filter. The resulting suspension is slowly stirred. After the reaction is complete, the supernatant is filtered into the other half of the double-Schlenk.To wash the reactedmaterial, the solvent is transferred back into the other chamber of the double-Schlenk by vacuum distillation. The material is washed to remove the excess organometallic precursor adsorbed on its surface washing step 2-3 times.

Figure 1 . 8 -

 18 Figure 1.8-Schematic representation of grafting in a double-Schlenk flask by SOMC protocol

For the catalysts synthesized

  by SOMC techniques the evolution of surface species during various post-treatments has been monitored via various spectroscopic and analytical techniques.

Figure 2 . 1 -Figure 2 . 2 -

 2122 Figure 2.1-Representative HR-STEM micrographs of (a) [≡SiOTa(=CH t Bu)(CH2 t Bu)2] and statistical evaluation of the STEM micrographs in the form of (b) size distribution histogram of the Ta centres (c) area distribution histogram of the Ta centres

Figure 2 . 4 -Table 2 . 2 -

 2422 Figure 2.4-TantalumLIII-edge k3-weighted EXAFS (left) and corresponding Fourier transform (right, modulus and imaginary part; R uncorrected from phase shift) with comparison to simulated curves for SOMC-I

Figure 2 . 5 -Figure 2 . 5

 2525 Figure 2.5-Representative HR-STEM micrographs of (a) [(≡SiO)2Ta(=O)(OH)] and statistical evaluation of the STEM micrographs in the form of (b) size distribution histogram of the Ta centres (c) area distribution histogram of the Ta centres

Figure 1 . 6 -Scheme 2 . 3 HRFigure 2 . 7 -

 162327 Figure 1.6-DRIFT spectra of a) [(≡SiO)2Ta(H)x=1,3]+N2O at room temperature b) [(≡SiO)2Ta(H)x=1,3]+N2O after heating at 325°C for 6hrs

Figure 2 . 8 -

 28 Figure 2.8-IR spectra for the formation of SOMC-I Ta-O-Ta under inert conditions

  by reacting the [(≡SiO)2Ta(=O)(OH)] species with another molecule of [Ta(=CH t Bu)(CH2 t Bu)3]. Preliminary results from DRIFT, elemental analysis and gas analysis suggest a mixture of monopodal and bi-podal tantalum species formed on the surface of SOMC-IFC.

1. 46 M

 46 . Reddish orange crystals were obtained by sublimation : 2.8 g, 54% yield. 1H NMR (C6D6) δ ppm : (8, [CH2C(CH3)3]), 1.15 (12, [CH2C(CH3)3]).

  spectroscopy X-ray absorption spectra were acquired at ESRF on BM23 beam-line (experiment code IN-986), at room temperature at the tantalum LIII edge, with a double crystal Si (111) monochromator detuned 70% to reduce the higher harmonics of the beam. The spectra were recorded in the transmission mode between 9.65 and 10.9 keV. The supported Ta samples were packaged within an argon filled dry box in a double air-tight sample holder equipped with kapton windows. The program FEFF8 was used to calculate theoretical files for phases and amplitudes based on model clusters of atoms. (Ankudinov, A. L.; Ravel, B.; Rehr, J. J.; Conradson, S. D. Phys. Rev. B:1998, 58, 7565-7576.) The value of the scale factor was determined from the k 2 and k 3 .χ(k) spectra of a reference compound, a sample of LiTaO3 diluted in BN and carefully mixed and pressed as a pellet.

  the Standards and Criteria Committee of the International XAFS Society,(Reports of the Standards and Criteria Committee of the International XAFS Society 2000: http://ixs.iit.edu/subcommittee_reports/sc/) the quality factor, ( ) 2 / ,

Figure 3 . 1

 31 Figure 3.1 presents the in situ IR spectroscopy of the reaction of molecular oxygen onto ( SiO)Ta(CH t Bu)(CH2 t Bu)2. The study indicated the presence of both carboxylate and alkoxo ligands in the coordination sphere of organometallic complex.

Figure 3 . 1 -

 31 Figure 3.1-IR studies of the action of O2 on SiOTa(CH t Bu)(CH2 t Bu)2 (a) : SiOTa(CH t Bu)(CH2 t Bu)2 (b) : SiOTa(CH t Bu)(CH2 t Bu)2 + O2, 2 hours at 25 °C. (c) : SiOTa(CH t Bu)(CH2 t Bu)2 + O2, 24 hours at 70 °C

Figure 3 . 2 - 3 . 6 .Scheme 3 . 3 B2. 6 Figure 3 . 4 -Figure 3 . 3 -

 32363363433 Figure 3.2-DRIFT spectroscopy of SiO2-700, Complex 2.1 and SOMC IIFC

Figure 3 . 5 -

 35 Figure 3.5-XRD spectrum of SOMC-IIFC

Figure 3 . 6 -

 36 Figure 3.6-Representative HR-STEM micrographs of (a) SOMC-II and statistical evaluation of the STEM micrographs in the form of (b) size distribution histogram of the Ta centres (c) area distribution histogram of the Ta centres

Figure 3 . 7 :

 37 Figure 3.7: Tantalum LIII-edge k 3 -weighted EXAFS (left) and corresponding Fourier transform (right, modulus and imaginary part; R uncorrected from phase shift) with comparison to simulated curves for a Ta-Schrock complex grafted onto SiO2-500 and then calcined by O2 at 500°C. Solid lines: experimental; dashed lines: spherical wave theory.

Figure 3 . 8

 38 Figure 3.8 shows the DRIFT spectra for the formation of SOMC-II'FC by calcination of Complex 2.1'. Similar to the samples grafted with tantalum alkyls, upon calcination the IR bands attributed to ν (C-H) and δ(C-H) at 2800-3000cm -1

Figure 1 . 8 -

 18 Figure 1.8-DRIFT spectroscopy of SiO2-700, Complex 2.1' and SOMC II'FC

Figure 3 . 9 -

 39 Figure 3.9-XRD spectrum of SOMC-II'FC

  Lyon (Villeurbanne) by powder X-ray diffraction (XRD, Bruker D8 Advance, Cu, Kα radiation, λ =1.5406 Ǻ, 40 kV, 40 mA). The samples were prepared in the form of fine homogeneous powder. A thin smooth layer of the samples mounted on a non-crystalline substrate such as PMMA (Poly (methylmethacrylate)) was held in the path of X-rays. The diffracted X-rays correspond to all sets of planes in the crystal powder which could be orientated in every possible direction relative to the X-ray beam. The XRD patterns with diffraction intensity versus 2θ were recorded, usually from 10° to 70° at a scanning speed of 2° per min. A LYNXEYE XE detector with an opening of 3° was used to detect the diffracted beam. D5. Catalyst characterization by EXAFS spectroscopy X-ray absorption spectra were acquired at ESRF on BM23 beam-line (experiment code IN-986), at room temperature at the tantalum LIII edge, with a double crystal Si(111) monochromator detuned 70% to reduce the higher harmonics of the beam. The spectra were recorded in the transmission mode between 9.65 and 10.9 keV. The supported Ta samples were packaged within an argon filled dry box in a double air-tight sample holder equipped with kapton windows. The program FEFF8 was used to calculate theoretical files for phases and amplitudes based on model clusters of atoms.(Ankudinov, A. L.; Ravel, B.; Rehr, J. J.; Conradson, S. D. Phys. Rev. B:1998, 58, 7565-7576.) The value of the scale factor was determined from the k 2 and k 3 .χ(k) spectra of a reference compound, a sample of LiTaO3 diluted in BN and carefully mixed and pressed as a pellet. The refinements were performed by fitting the structural parameters Ni, Ri, i and the energy shift, E0 (the same for all shells). The fit residue, was calculated by the following formula: the Standards and Criteria Committee of the International XAFS Society,(Reports of the Standards and Criteria Committee of the International XAFS Society 2000: http://ixs.iit.edu/subcommittee_reports/sc/) the quality factor, ( ) 2 /

Figure 4 . 1

 41 shows the schematic diagram for the lab scale reactor used in the catalytic testing.

Figure 4 . 1 -

 41 Figure 4.1-Reaction apparatus for synthesis of Butadiene from Ethanol and Aldehyde

Figure 4 . 2

 42 Figure 4.2 shows the total conversion of EtOH/AA for [Ta(=CH t Bu)(CH2 t Bu)3] based SOMC catalysts over 8hrs time on stream.

Figure 4 . 2 -

 42 Figure 4.2-Total conversion of EtOH/AA for [Ta(=CH t Bu)(CH2 t Bu)3] based SOMC catalysts over 8hrs time on stream

Figure 4 . 3

 43 showsthe representative HRSTEM micrographs of SOMC-I before catalysis and after 8 hrs time on stream. Post-catalysis the isolated Ta(V) sites were seen to align in a string like arrangement, similar to the organization witnessed on the surface of SOMC-II. For SOMC-II, which has been subjected to calcination at 500°C during its synthesis, the surface species remain mostly consistent during the entire run of the reaction. (Figure4.4).

Figure 4 . 3 -Figure 4 . 4 -

 4344 Figure 4.3-Representative HRSTEM micrographs of SOMC-I before catalysis and post 8 hrs time on stream

  =CH t Bu)(CH2 t Bu)3]. This was followed by calcination under dry O2 at 500°C for 10hrs.

Figure 4 . 5 -

 45 Figure 4.5-Total conversion of EtOH/AA for SOMC-II and SOMC-II' 8hrs time on stream

Figure 4 . 5

 45 Figure 4.5 shows the conversion of EtOH/AA over SOMC-II and SOMC-II'. Since the EXAFS analysis of the catalysts (discussed in Chapter 3) did not point towards markedlydifferent surface species, SOMC-II' showed significant improvement in the total conversion. However it is notable that similar to SOMC-II, the conversions remain stable throughout the time on stream for SOMC-II'.

Figure 4 . 6

 46 Figure 4.6 summarizes the catalytic activity of the SOMC catalysts in comparison with the best performing state-of-the-art TaOx systems.

Figure 4 . 6 -

 46 Figure 4.6-Catalytic activity of SOMC based catalysts compared to state-of-the-art tantalum based catalysts in conversion of EtOH/AA to BD

Scheme 4 . 1 -

 41 Scheme 4.1 -Reaction by-products for EtOH/AA to BD as presented by Quattlebaum et. al14

Figure 4 . 7

 47 Figure 4.7 shows the selectivity of SOMC-I and SOMC-II towards BD monitored over 8hrs time on stream. As it has been discussed before, the two catalysts have different distribution of surface species-SOMC-I has predominantly isolated [(≡SiO)2Ta(=O)(H)]species whereas SOMC-II has a co-existing isolated Ta(V) species along with oligomer like linear arrangement of Ta(V) atoms on its surface. Despite this both the catalysts show extremely similar trends in BD selectivity.

Figure 4 . 7 -

 47 Figure 4.7 -BD selectivity of SOMC-I and SOMC-II for 8 hrs time on stream

Figure 4 . 8 -

 48 Figure 4.8 -Product distribution of catalyst in consideration (i) WI-I (ii) Zeo-I (iii) SOMC-I (iv) SOMC-II

1 )Figure 5 . 1 -

 151 Figure 5.1-Relative amount of open and closed Lewis sites determined by FTIR spectroscopy of adsorbed CO versus initial rates of ethanol conversion

(Figure 5 Figure 5 . 2 -

 552 Figure 5.2-Environments of the reactive sites studied in on Ta/SiBEA system developed by Dzwigaj et al.13,16,18

Figure 5 . 3 -Figure 5 . 3

 5353 Figure 5.3-DRIFT spectra of adsorbed pyridine over SOMC-I, SOMC-II and SOMC-II' catalysts after desorption at 200°C

Figure 5 . 4 ,

 54 Figure 5.4, NH3 desorption signal was recorded in the range of 50°C to 350°C.

Figure 5 . 4 -

 54 Figure 5.4-NH3 TPD profiles for SOMC-I (blue) and SOMC-II (green)

Chapter 1 118

 1 The higher Lewis acidity can be attributed to the isolated nature of active sites on the surface of N2O-oxidized SOMC-I. Indeed while studying a zirconium based system for ETB conversion, Ivanova et al. correlated the strength of lewis acidity to the presence of isolated Zr atoms with one open hydroxyl functionality. It can be argued that the higher surface acidity can also be a result of better steric availability of the isolated Ta sites.The difference in surface acidity of all the above mentioned catalysts was clearly reflected in the distribution of reaction by-products. SOMC-I and SOMC-II' both show markedly higher propensity towards formation of ethylene and diethylether when compared to SOMC-II.

Figure 5 . 5

 55 Figure 5.5 shows the selectivity towards the different reaction by-products for 8 hrs time on stream in the EtOH/AA conversion to BD.

Figure 5 . 5 -

 55 Figure 5.5-FTIR spectra of adsorbed pyridine over SOMC-I, SOMC-II and SOMC-II' catalysts after desorption at 200°C

5. 6 )Figure 5 . 6 -

 656 Figure 5.6-The effect of molar ratio of EtOH/AA on the BD selectivity and yield was studied

Figure 5 . 7 -

 57 Figure 5.7-Effect of WHSV on EtOH/AA conversion to BD

8 ) 1 122Figure 5 . 8 -

 8158 Figure 5.8-In situ DRIFTS spectra acquired of ethanol adsorbed on SOMC-I catalyst from 50-350°C in 3200 to 1000 cm-1 spectral region

Chapter 5 123( 9 )Figure 5 . 9 -

 5959 Figure 5.9-DRIFTS spectrum acquired of EtOH/AA (2.5:1) adsorbed on SOMC-I catalyst at 350°C

Chapter 1 124 2 .Scheme 5 . 1 -

 1251 Scheme 5.1-Proposed and hypothesized reaction mechanism for MPV reduction over the Ta(V) site of SOMC-I

Chapter 3

 3 examined the effect of calcination on well-defined peralkyl tantalum species supported on silica. A systematic investigation was carried out on the reactivity of O2 with silica supported tantalum peralkyl [≡SiOTa(=CH t Bu)(CH2 t Bu)2]and tantalum hydride [(≡SiO)2Ta(H)x=1,3] species at room temperature. Combined with the previous studies from literature, it was concluded that reaction of oxygen with these well-defined tantalum species resulted in formation of some dimers and clusters. Further the reactivity of [≡SiOTa(=CH t Bu)(CH2 t Bu)2] (synthesized according to procedure subscribed in literature) with oxygen was investigated under continuous flow conditions at 500°C for 10 hrs. The resulting material was completely characterized by IR and NMR spectroscopy, UV-DRS spectroscopy and XRD. The latter two analysis confirmed absence of Ta2O5 microcrystals on the surface. EXAFS analysis corroborated the absence of Ta2O5 phase while indicating the possibility of some Ta-Ta interaction. The HRSTEM microscopy evidenced co-existence of isolated Ta (V) centers along with Ta species in a string like arrangement. This data together with the information from literature suggested the presence of a mixture of mononuclear, di-nuclear and oligomer like species on the surface of the material.The catalytic activity of the aforementioned systems synthesized by SOMC protocol was evaluated in the conversion of EtOH/AA to BD in a fixed bed continuous flow reactor. In Chapter 4 the catalytic performance of the SOMC-I and SOMC-II are discussed in terms of BD selectivity, catalyst stability and coke formation with respect to the state-ofthe-art catalysts mentioned in literature. Notably both the catalysts exhibit extremely narrow product distribution when compared with catalysts synthesized by conventional wetness impregnation techniques. This was attributed to the presence of homogeneous, well defined sites on the surface of the catalyst. TGA analysis of the spent catalysts reported negligible coke formation.

Table 1 . 1 -

 11 Advantages and limitations of the proposed bio-based routes to BD synthesis

  acid sites were detected. While the Zr loading did not affect the Bronsted acidity, the Lewis acidity increased with increase in Zr%. Citing the amphoteric nature of Zirconium, the authors hypothesize that varying ZrO2 loading can affect the acid/base properties of the catalysts. The optimal catalysts, recorded at 593 K for a WHSV of 1.8 h -1 and EtOH/AA ratio of 3.5 had a butadiene yield of 31.6% for 3 hrs on stream.

	Lee et al 33 . UsedZrO2/SiO2catalysts synthesized from zirconium oxynitrate dispersed on
	mesocellular siliceous foam (MCF) by urea hydrolysis method. With optimization,
	butadiene selectivity of 73% for a WHSV of 3.7 h -1 after 15 h was obtained. The
	deactivation of catalysts was attributed to coke formation. The activity is attributed to the
	high dispersion of the metal oxides, while the large pores of the support are thought of
	preventing mass transfer issues and coking.

examined ZrO2/SiO2 catalysts for the Ostromyslensky process. The catalysts were prepared by sol-gel technique from zirconium oxynitrate and tetraethyl orthosilicate with ZrO2 loading from 1.4 to 8.4 wt. % followed by calcination at 550°C. Both Lewis and Bronsted

Table 1 .2. Depending

 1

on the nature, density and homogeneity of these reactive sites, different behaviors are observed, many a times leading to completely different catalytic applications.

Table 1 . 2 -

 12 Grafting sites present in some supports used for SOMC[START_REF] Copéret | Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities[END_REF] Silica represents one of the most studied supports. It is typically an amorphous solid with relatively high surface areas between 50 and 1000 m 2 g -1 . Its texture and shape depends mostly upon the method of preparation. Flame pyrolysis of SiCl4 and H2/O2 above 1000 °C produces high purity, nonporous silica with surface areas between 50-380 m 2 g -1 .

	Support	Grafting site
	Silica	Hydroxyl groups called silanols (≡Si-O-(H))
	Silica-Alumina	Silanol groups; Si-O(H)-Al bridges

Alumina

Hydroxyl groups (at least five different types); Al-O-Al bridges (where the bond is not covalent); Lewis acid sites

Table 1

 1

.3 enlists the commonly used characterization techniques and the information they provide

Table 1 . 3 -

 13 Characterization techniques usedfor catalysts synthesized by SOMC protocolThese spectroscopic and analytical techniques can also be accompanied by the preparation of soluble molecular analogues of the surface. These analogues can be more easily and precisely characterized by solution techniques. Such an approach is particularly powerful with silica-supported systems since numerous molecular analogues are already available. Quadrelli et. al. have published a comprehensive review on molecular models

Table 2 . 1 -

 21 Grafting of [Ta(=CH t Bu)(CH2 t Bu)3] on SiO2-500 by impregnation in pentane at 25°C

	1 H solid state

Table 3 .

 3 1 -Grafting of [Ta(=CH t Bu)(CH2 t Bu)3] on SiO2-700 by impregnation in pentane at 25°C

Table 3 .

 3 1 -EXAFS parameters. (a) The errors generated by the EXAFS fitting program "RoundMidnight" are indicated between parentheses

		Type of neighbour	Number of neighbours	Distance (Å)	2 (Å 2 )
		Ta-Oa	4.9(4)	1.905(5)	0.0040(5)
		Ta--O	1.2(9)	2.98(4)	0.0076(54)
		Ta--Si	2.2(11)	3.29(3)	0.0059(31)
	MS 3l & 4l O'-Ta-O (b)	2 x 1.9(10)	3.88(4)	0.007(5)
	(	Ta--Ta	0.1(3)	3.44(5)	0.002(7)	)

  Ta of CH3-t Bu evolved. Elemental analysis: % wt Ta 5.1% C 4.86%

	Complex 2.1'-A solution of [Ta(OEt)5]2 (0.11 mL) in Toluene (20 mL) and SiO2-(500) (1
	g) was stirred at 25 °C for 2 h. After filtration, the solid was washed three times with
	Toluene. The solvent was then removed, and the white solid was dried under dynamic
	vacuum at 25 °C. 1 H MAS solid-state NMR (300 MHz, 25 °C): δ (ppm) 0.8 (s, CH2, CH3)
	13 C MAS solid-state NMR (300 MHz, 25 °C): δ (ppm) 17 ppm(s, CH2, CH3), 67.91 ppm
	(s, CH2, CH3). Infrared (cm -1 ): 1300-1500 s, δ(CH2, CH3)), 2600-2900 (s, ν(C-H))
	Complex 2.2'-A solution of [Ta(OEt)5]2 (0.06 mL) in toluene (20 mL) and SiO2-(700) (1
	g) was stirred at 25 °C for 2 h. After filtration, the solid was washed three times with
	Toluene. The solvent was then removed, and the white solid was dried under dynamic
	vacuum at 25 °C. 1 H MAS solid-state NMR (300 MHz, 25 °C): δ (ppm) 0.8 (s, CH2,
	CH3). 13 C MAS solid-state NMR (300 MHz, 25 °C): δ (ppm) 17 ppm(s, CH2, CH3), 68

Table 4 . 2 -

 42 Catalytic activity of SOMC based catalysts in conversion of EtOH/AA to BD

	and SOMC-II'-500-FC for the purpose of catalytic testing
	Note-Complex 1.2 and Complex 2.2' were calcined at 500°C for 10 hrs to give SOMC-II-500-FC

. Comparison of the [Ta(=CH t Bu)(CH2 t Bu)3] based catalysts

Part B:

Surface Organometallic Chemistry Chapter 2

Tailoring 'single site' TaO x species on silica support via mild oxidation with N 2 O

The aim of this chapter is to develop well-defined, 'single site' TaO x species on the surface of silica by reaction with N 2 O, a mild oxidant using the surface organometallic chemistry protocol.

Contents B2. Tailoring single site TaOx species on the surface of silica

The reaction of complex 1.2 with N2O is a novel attempt to generate well-defined Ta-OH species supported on silica. Upon treating complex 1.2 with dried nitrous oxide at room temperature for 2 hrs, a pale white powder was obtained. This material was named SOMC-IFC. The DRIFT spectroscopy shows complete disappearance of the peak at 1815 and 1830 cm -1 (ν Ta-H). At the same time a new band appears at 3721cm -1 . This band can be assigned to the ν Ta-OH. The same was confirmed by 1 H solid state MAS NMR (500MHz) spectroscopy. Resonances were observed at 1.1 ppm (Si-C(CH3)3), 2.0 ppm (Si-OH). The signals at 4.33 ppm and 6.9 ppm could be attributed to the new hydroxyl groups formed on tantalum. The sample prepared by the same procedure with 2 wt% Ta loading was prepared by the same procedure for the purpose of catalytic testing was coded SOMC-I

D4. Preparation of surface complexes by SOMC protocol

Complex 1.1: A solution of [Ta(=CH t Bu)(CH2 t Bu)3] (0.43 g, 0.9 mmol) in pentane (15 mL) and SiO2-(500) (2 g, 0.9 mmol OH/g ) was stirred at 25 °C for 2 h. After filtration, the solid was washed three times with pentane. The solvent was then removed, and the light orange solid was dried under dynamic vacuum at 25 °C. 1 H MAS solid-state NMR (300 MHz, 25 °C): δ (ppm) 1 (s, CH2,CH3). 13C CP-MAS solid-state NMR (300 MHz, 25 °C):

δ (ppm) 34 (s, C(CH3)3), 95 (s, CH2-t Bu). Infrared (cm -1 ): 1450-1500 s, δ(C-H)), 2800-3000 (s, ν(C-H)). Gas-phase analysis: 1.5 equiv/Ta of CH3-t Bu evolved. Elemental analysis: % wt Ta 5.56% C 5.52% After three washing cycles, the volatiles were condensed into a reactor of known volume in order to quantify the neopentane (NpH) released during the reaction. The released gas was quantified by gas chromatography. The resultant solid was dried under UHV for an hour to obtain a light orange powder which was characterized by DRIFT, SSNMR and elemental analysis.

Scheme 3.2

The grafting was done for different loadings of tantalum. Further another grafting was performed to obtain a pre-catalyst with tantalum loading of 2 wt% on SiO2-700. This sample was prepared from the point of view of catalytic testing in transformation of Another grafting was performed to obtain a pre-catalyst with 2 wt% of tantalum. This sample was prepared from the point of view of catalytic testing in transformation of EtOH/AA to BD, to ensure compatibility while comparing with the state-of-the-art catalysts, which are reported at 2 wt% Ta loading. (later discussed in chapter 4).

After washings the resultant solid was dried under UHV for an hour to obtain a white powder which was characterized by DRIFT, SSNMR and elemental analysis. To the best of our knowledge, the grafting of tantalum ethoxide on dehydroxylated silica by SOMC protocol has not been reported in the literature. Table 3. 4 shows the preliminary results obtained from the DRIFT and SSNMR characterization of the resulting solid. or tantalum (Ta-OEt). The resonance at 68 ppm can be attributed to the carbon of methylene O-CH2-CH3 in the ethoxy group coordinated to tantalum.

Commercial [Ta(OEt)5]2 is available in the form of a dimer. Therefore a critical point when grafting [Ta(OMe)5]2 on a silica surface is to know whether the surface tantalum complex is mono-or di-nuclear. No clear conclusion could be drawn based on SS NMR and UV-vis-DRS spectroscopy. Further spectroscopic investigation will be needed to confirm the exact nature of the supported species.

Nonetheless, the materials obtained by the above procedure were calcined under the flow of dry oxygen at 500°C for 10 hrs. The material obtained after calcination of SOMC-2.1' was named SOMC-II'FC. Whereas the material obtained from tantalum loading of 2 wt% was named SOMC-II'.

Sample %Metal Loading DRIFT

D. Experimental Details: D1. General procedure for preparation of starting material

All experiments were carried out under controlled atmosphere, using schlenk and glovebox techniques for the organometallic synthesis. For the synthesis and treatments ofthe surface species, reactions were carried out using high-vacuum lines (1.34 × 10-3 kPa) and glovebox techniques.

Pentane and diethyl ether were distilled on NaK alloy and benzophenone/Na, respectively, followed by degassing by freeze-pump-thaw cycles.

D2. Synthesis of catalyst supports

SiO2 (Aerosil Degussa, 200 m2 g -1 ) was compacted with distilled water and partially dehydroxylated at 500°C and 700 °C under vacuum (10-5torr) for 12 h. SBA-15 was synthesized according to the procedure reported in literature. 14 The BET surface area calculated by N2 adsorption experiments was 981.42 m 2 /g.

D3. Preparation of surface complexes by SOMC protocol

Complex 1.1: A solution of [Ta(=CH t Bu)(CH2 t Bu)3] (0.43 g, 0.9 mmol) in pentane (15 mL) and SiO2-(500) (2 g, 0.9 mmol OH/g ) was stirred at 25 °C for 2 h. After filtration, the solid was washed three times with pentane. The solvent was then removed, and the light orange solid was dried under dynamic vacuum at 25 °C. 1 H MAS solid-state NMR (300 MHz, 25 °C): δ (ppm) 1 (s, CH2,CH3). 13 C CP-MAS solid-state NMR (300 MHz, 25 °C): δ (ppm) 34 (s, C(CH3)3), 95 (s, CH2-t Bu). Infrared (cm -1 ): 1450-1500 s, δ(C-H)), 2800-3000 (s, ν(C-H)). Gas-phase analysis: 1.5 equiv/Ta of CH3-t Bu evolved. Elemental analysis: % wt Ta 5.56% C 5.52% Complex 2.1-A solution of [Ta(=CH t Bu)(CH2 t Bu)3 ] (0.12 g) in pentane (15 mL) and SiO2-(700) (1 g) was stirred at 25 °C for 2 h. After filtration, the solid was washed three times with pentane. The solvent was then removed, and the white solid was dried under dynamic vacuum at 25 °C. 1 H MAS solid-state NMR (300 MHz, 25 °C): δ (ppm) 0.8 (s, CH2, CH3). 13

D. Experimental details D1. Evaluation of surface acidity

D1.1. By Pyridine adsorption desorption

For pyridine adsorption experiments, samples were pretreated and the pyridine pulse (0.01 mL) was injected at 50 °C, then temperature was raised to 400 °C. The IR apparatus used was a Bruker Vertex 70 with a Pike Diffuse IR cell attachment. Spectra were recorded using a MCT detector after 128 scans and 2 cm-1 resolution. The mass spectrometer used was an EcoSys-P from European Spectrometry Systems.

D1.2. By NH3 Temperature programmed desorption (NH3-TPD)

NH3-temperature-programmed desorption (TPD) measurements were obtained with a TPD/TPR/TPO Micromeritics instrument. 50-100 mg of catalysts were pre-treated at the calcination temperature for 1 h under a He flow. After cooling down to 80 °C, NH3 was adsorbed by flowing of a 10% NH3 in He gas mixture for 30 min (30 mL min-1, NTP),with subsequent He treatment at 80 °C for 15 min to remove physisorbed molecules.

Catalysts were then heated under He flow (50 mL min-1, NTP) at a heating rate of 10 °Cmin-1 up to the calcination temperature.

D2. Surface area by N2 adsorption desorption isotherms

The specific surface area was measured applying the single point BET method. The instrument used for this analysis was a Carlo Erba Sorpty 1700. The BET method (Brunauer Emmet Teller) calculates the surface area of the sample from the volume of the gas corresponding to the monolayer adsorption. The single-point approximation consists in the measurement of the pressure of adsorption and the corresponding gas volume adsorbed. In the analysis around 0.5g of the sample was placed inside the sample holder and then heated at 150°C under vacuum (4 Pa) in order for it release the water, air or other molecules adsorbed. Afterwards the sample was put in liquid nitrogen and the adsorption of the gaseous N2 was carried out.

General Conclusions

Surface organometallic chemistry (SOMC) has provided valuable tools in bridging the gap between homogeneous and heterogeneous catalysts by designing well-defined single site heterogeneous surface complexes. In the past these well-defined catalysts have been known to catalyze various reactions like alkane metathesis, olefin metathesis, alkene polymerization etc with excellent results. The aim of this project was to utililize this expertise of SOMC to develop catalysts for the on-purpose synthesis of butadiene by Ostromyslensky's two step process.

Ta (V)/SiO2 represents one of the most studied systems for this transformation i.e conversion of EtOH/AA to BD. Most of the prior catalytic systems studied in literature employ the conventional technique of solvent impregnation followed by calcination to generate TaOx/SiO2 catalysts. By their nature such systems face the major drawback of having numerous kinds of surface species and in many cases low concentration of the active species. Therefore need was felt for a higher control over the surface structures that can lead to a more selective and active catalysis. Such study was anticipated to provide insights into identification of active specie in the transformation as well as in establishing the structure activity relationship for TaOx/SiO2 systems.

In this endeavor our strategy was to synthesize and thoroughly characterize a family of well-defined Ta-based silica-supported catalysts through SOMC with varying tantalum nuclearity and to evaluate and analyze their performance in the EtOH/AA to BD conversion.

In Chapter 2 a novel route to selectively tailor isolated single site Ta (V) species on the surface of silica was discussed. In this strategy, previously studied [(≡SiO)2Ta(H)x=1,3] were synthesized on the surface of silica dehydroxylated at 500°C according to the procedure discussed in literature. The formation of these species could be verified by IR and NMR spectroscopies. Also for the first time HRSTEM of microscopy of this material was conducted which provided an additional proof of the tantalum moieties being isolated and single site. These species were then subjected to mild oxidation with excess N2O at room temperature. The following material was characterized by IR and NMR spectroscopy as well as by EXAFS analysis and HRSTEM microscopy. EXAFS analysis established the exclusive formation of well-defined isolated mono-tantalate specie [(≡SiO)2Ta(=O)(OH)] on the surface of the material. HRSTEM analysis corroborated the string like organization show considerable but reduced activity in the same transformation. Based on the HRSTEM analysis of SOMC-I, pre-catalysis and post catalysis, it was indicated that the isolated Ta(V) sites evolved into string like arrangements after 8 hrs time on stream, similar to the surface of SOMC-II. This decrease in the concentration of isolated Ta (V) sites on the surface of SOMC-I also resulted in decrease in the overall conversion of EtOH/AA. Chapter 5 focused on studying the effect of catalyst properties on catalytic activity.

Studying the effect of type and amount of acidic sites on the catalyst surface, it was concluded that a small proportion of Bronsted acid sites along with the lewis acid sites were beneficial in the conversion of EtOH/AA to BD. Changing the molecular precursor from [≡SiOTa(=CH t Bu)(CH2 t Bu)2] to Ta(OC2H5)5 showed perceptible effect on the BD yield from 33% to 43%. This effect could be linked to the increase in surface acidity for the latter. Changing the support from non-porous silica to mesoporous SBA-15 also showed increase in BD yield while exhibiting similar BD selectivity. This could be explained by considering that the larger surface area in case of SBA-15 resulted in better dispersed Ta (V) sites, which have been earlier proposed to be the active sites in this conversion. Also, assuming isolated [(≡SiO)2Ta(=O)(H)] to be the active site in the transformation of EtOH/AA to BD, an attempt was made to understand the mechanism of the reaction on the surface of SOMC-I. The Meerwein-Ponndorf-Verley Reaction between crotonaldehyde and ethanol is thought to be catalysed by tantalum. Based on in-situ DRIFT experiments for adsorption of (i) ethanol and (ii) a mixture of ethanol and acetaldehyde on the surface of the catalyst at 350°C, a mechanism was hypothesized for the reduction of adsorbed crotonaldehyde by the hydrogen from ethanol on the isolated Ta (V) center.

In conclusion, surface organometallic chemistry was seen to be a fruitful technique for synthesizing catalysts for on purpose production of butadiene from ethanol and acetaldehyde. Amongst the family of Ta based catalyst synthesized, most showed improved catalytic activity in terms of BD selectivity, yield and resistance to coke formation. More notably the selective tailoring of desired Ta sites on the surface of catalyst helped in identification of active sites in the transformation while also discussing their structure activity relationship at a molecular level.

B1. Infrared spectroscopy

B1.1 Fourier transform infrared spectroscopy in transmission

A Nicolet 5700-FT spectrometer and infrared cell equipped with ZnSe windows was used to perform FT-IR spectroscopy. Typically, 64 scans were accumulated for each spectrum (resolution 1 cm -1 ).

B1.2 Diffused reflectance infrared spectroscopy (DRIFTS)

Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) is an infrared spectroscopy technique used on powder samples with no preparation. The samples are placed in a cell. Some of the infrared radiation is absorbed by the sample at specific wavenumber; these wavenumbers are helpful for the identifications of the sample chemical functional groups. DRIFT spectra were recorded on a Nicolet 6700-FT spectrometer using a cell equipped with CaF2 window. Typically, 64 scans were accumulated for each spectrum (resolution 4 cm -1 ).

B1.3 Attenuated total reflectance infrared spectroscopy (ATR)

Attenuated total reflectance spectra of the materials were recorded at room temperature with an ALPHA-FTIR instrument at a resolution of 2 cm -1 . First a background was taken to eliminate the contribution of atmospheric water and carbon dioxide. Later on, the powder was put in intimate contact with the crystal to perform the measurement. Figure 16 illustrates the principle of this technique also known as Multiple Internal Reflection Spectroscopy (MIR). Here the IR beam is passed through a thin, IR transmitting crystal in a manner such that it alternately undergoes total internal reflection from the interface with the sample. At each reflection, some of the IR radiation may be absorbed by species adsorbed on the solid surface. After one or several internal reflections, the IR beam exits the ATR crystal and is directed to the IR-detector.

Appendix-I

Common instrumental details

B2. Nuclear Magnetic Resonance (NMR) spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy is a non-destructive analytical chemistry technique used to determine the content and purity of a sample as well as its molecular structure. The NMR of the sample can be recorded either in solution phase or as a solid powder.

B2.1 Nuclear Magnetic Resonance (NMR) spectroscopy in liquid medium

Solution NMR spectra were recorded on BRUKER AVANCE 300 spectrometer ( 1 H: 300.1 MHz, 13 C: 75.4 MHz). Chemical shifts are given in ppm (h) relative to TMS (tetramethylsilane). Spectra were recorded using the residual peak of the deuterated solvent as internal standard.

B2.2 1 H and 13 C Solid state nuclear magnetic resonance spectroscopy

Solid state NMR spectra were collected on BRUKER AVANCE III 500 spectrometer operating at 125 MHz for 13 C. The zirconia impeller of 4mm is filled with the desired product inside glove box under Argon and sealed with a kel-f stopper. It was then transferred into the probe Bruker CP 4 mm spectrometer allowing rotation of the rotor at a speed of 10 kHz. The time between two acquisitions was always optimized to allow complete relaxation of the protons.

B3. Elementary microanalysis

B4. Gas evolution analysis by Gas Chromatography

Gas phase analysis was performed on a HP 5890 gas chromatograph, equipped with a flame ionization detector (FID) and a KCl/Al2O3 on fused silica column (50 m × 0.32 mm). Gaseous alkanes from the reaction were filled into a glass balloon of known volume. A spherical syringe joint was used to extract 300μL of gaseous product into a gas syringe and was injected into GC.