Stijn Van

Daele ( Conceptualization

Stijn Van Daele

Delphine Minoux

Nikolai Nesterenko

Sylvie Maury

Vincent Coupard

Valentin Valtchev

Arnaud Travert
email: arnaud.travert@ensicaen.fr

Jean-Pierre Gilson

A highly selective FER-based catalyst to produce n-butenes from isobutanol

Keywords: isobutanol, linear butenes, zeolite, Ferrierite, operando FT-IR, chemometrics, MCR-ALS

A highly selective FER-based catalyst to produce n-butenes from isobutanol

Introduction

The search for sustainable pathways to the major building blocks of the petrochemical industry is a lively research topic. The adverse effects of climate change, a C4-shortage and recent progress in bio-chemical isobutanol synthesis made isobutanol an attractive fuel component and platform molecule for petrochemistry. [START_REF] Bruijnincx | Shale Gas Revolution : An Opportunity for the Production of Biobased Chemicals ?[END_REF][START_REF] Amghizar | New Trends in Olefin Production[END_REF][START_REF] Chen | Frontiers in microbial 1-butanol and isobutanol production[END_REF] Its dehydration provides a renewable source of C4 molecules, precursors of higher value products such as butyl rubber (automotive market), polyisobutylene (lubricant), and ethyl tert-butyl ether (fuel additive).

The dehydration of butanol isomers is a well-established reaction catalyzed by many solid acids, such as -alumina. Generally, the isomer produced at low temperatures (< 673 K) is the direct dehydration product. [START_REF] Knözinger | The dehydration of alcohols over alumina: VII. The dependence of reaction direction on the substrate structure[END_REF][START_REF] Knözinger | The Dehydration of Alcohols over Alumina : I. The Reaction Scheme[END_REF][START_REF] Knözinger | The dehydration of alcohols on alumina: XIV. Reactivity and mechanism[END_REF] Moreover, isobutene is thermodynamically favored over linear butenes (see e.g. ref. [START_REF] Taylor | Dehydration of Fermented Isobutanol for the Production of Renewable Chemicals and Fuels[END_REF] and Table S1 of the SI) and generally prevails during the isobutanol dehydration as it is often both the kinetic and thermodynamic product. When the dehydration is accompanied by simultaneous or consecutive skeletal isomerization, the selectivity to linear butene isomers is altered. [START_REF] Macho | Dehydration of C4 alkanols conjugated with a positional and skeletal isomerisation of the formed C4 alkenes[END_REF]9] Such a cascade reaction produces all linear butene isomers, regardless of the initial butanol isomer used as a reactant. However, when using a -alumina catalyst, the second step is slow and requires high contact times, therefore, limiting its industrial interest for linear butene production. [START_REF] Zhang | N-Butanol to iso-butene in one-step over zeolite catalysts[END_REF] More recently, a technology for isobutanol dehydration to linear butenes, ATOL-C4 TM was developed in a joint collaboration between IFPEN, Total Research & Technology Feluy and Axens. The dehydration reaction takes place in the gas phase with a low water dilution of the alcohol feedstock in a fixed-bed multi-tubular reactor on a ferrierite (FER)-based catalyst. [START_REF] Adams | Simultaneous dehydration and skeletal isomerisation of isobutanol on acid catalysts[END_REF] The process is very attractive since in this case the isobutanol conversion is almost complete in a single pass, the reaction-regeneration cycles enable to maintain the catalyst performances for several months, and the selectivity to linear butenes is very high.

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However, an improved fundamental understanding of the separate reactions (dehydration and isomerization) is required to design even better performing catalysts. Extensive research on the skeletal isomerization of 1-butene to isobutene started more than two decades ago and highlighted the high activity and selectivity of zeolites with medium pore openings (10 MR: FER-and TON-type). [START_REF] Guisnet | Selective isomerization of n-butenes into isobutene over aged H-ferrierite catalyst: nature of the active species[END_REF][START_REF] O'young | Skeletal isomerization of 1-butene on 10-member ring zeolite catalysts[END_REF][START_REF] Domokos | The importance of acid site locations for n-butene skeletal isomerization on ferrierite[END_REF][START_REF] Van Donk | Probing the Accessible Sites for n-Butene Skeletal Isomerization over Aged and Selective H-Ferrierite with d3-Acetonitrile[END_REF][START_REF] Meriaudeau | Isomerization of butene in isobutene J o u r n a l P r e -p r o o f on ferrierite catalyst: A mono-or a bimolecular process?[END_REF] Although some details are still debated, it is generally accepted that both mono-and bimolecular mechanisms occur on the fresh catalyst, while the monomolecular prevails on an aged catalyst. Moreover, the participation of carbonaceous species, formed at the pore mouths/external surface of the zeolite, was proposed in a pseudomonomolecular reaction mechanism. This led Zhang et al. to use Ferrierite for the one-step conversion of n-butanol to isobutene between 673 -773 K. [START_REF] Zhang | N-Butanol to iso-butene in one-step over zeolite catalysts[END_REF] . However, they concluded that Ferrierite is not stable under these operating conditions as its framework is dealuminated.

This study explores the direct conversion of isobutanol into n-butenes on Ferrierite, a cascade reaction where isobutanol dehydration is followed by the skeletal isomerization of the primary products, at low temperature (< 573 K) to alleviate the deactivation. [START_REF] Adams | Simultaneous dehydration and skeletal isomerisation of isobutanol on acid catalysts[END_REF][START_REF] Buniazet | Unexpected selectivity of FER for the conversion of isobutanol to linear butenes and water effects[END_REF][START_REF] Minoux | Process to make olefins from isobutanol[END_REF] We focus in particular on the fine-tuning of the acidic and textural properties of Ferrierite and use IR operando spectroscopy to gain more insight into the global reaction mechanism. Due to the complex nature of such IR spectra, spectral interpretations are assisted by chemometric tools to provide relevant information on the surface species, including those leading to deactivation.

Our study highlights an unexpectedly high n-butenes selectivity, shed some light on the reaction mechanism, and relates performance with structural and textural features of the FERtype zeolite.

Materials and methods

Samples

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Commercial ferrierites (CP 914C and CP 914) were supplied by Zeolyst International and hereinafter referred to as FERx (x=Si/Al molar ratio), namely FER9 and FER28. FER28 was used as the reference unless otherwise specified. A third home-made sample, FER46 was evaluated. Two other samples were prepared according to recently published reports [START_REF] Lee | Synthesis and catalytic behavior of ferrierite zeolite nanoneedles[END_REF][START_REF] Catizzone | Catalytic application of ferrierite nanocrystals in vapour-phase dehydration of methanol to dimethyl ether[END_REF]; both are nanosized crystals with different morphologies, namely needle and plate-like ferrierites (respectively FER NN and FER NP). These two samples are referred to in graphs as FER Nano. Both MFI and gamma-alumina were commercial samples provided by Zeolyst International (CBV 8014) and Sigma-Aldrich, respectively. An ERI (Erionite) and a TON zeolite (ZSM-22) were prepared according to published procedures. [START_REF] Jamil | Facile control of nanosized ZSM-22 crystals using dynamic crystallization technique[END_REF] The ammonium form of the zeolites was converted to their protonic form by calcination at 773 K for 5 h, (ramp rate of 1 K min -1 under dry air flow). Table 1 summarizes an overview of all characterization results of the samples, which will be reported upon later in this work.

Post-synthetic modifications

Na-exchanged zeolites

To evaluate the influence of the number of acid sites on the reaction, the ammonium forms of the zeolites were partially ion-exchanged with sodium. Sodium nitrate solutions of different concentrations (0.1 M, 0.01 M, 0.001 M) were mixed with the zeolites (1 g zeolite / 25 mL solution) at 353 K for 10, 60 and 180 min, filtered and washed with distilled water prior to collecting the zeolite. The process was repeated one to three times to achieve a wide range of ion-exchange (40-96 %). The samples were then dried overnight at 373 K prior to calcination (vide supra). The percentage of sodium introduced was determined by in-situ FT-IR of deuterated acetonitrile. These samples are referred to in graphs as FER Na.

Modification of the external acidity

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The external surface of the FER zeolite was passivated according to the procedure reported by Losch et al. [START_REF] Losch | Impact of external surface passivation of nano-ZSM-5 zeolites in the methanol-to-olefins reaction[END_REF] -0.5 g of zeolite was suspended in 12.5 mL of n-hexane (Sigma-Aldrich > 99 % purity) and refluxed at 353 K under vigorous stirring (500 rpm). Next, 0.150 mL TEOS (4 wt. % of SiO2 with respect to the parent zeolite) was added, and the mixture stirred for 1 h.

The n-hexane was then removed by evaporation, and the zeolite calcined at 823 K for 4 h (FER 1SP). This procedure was repeated twice to ensure complete surface coverage (FER 2SP). The ratio of Brønsted/Lewis acidity on the external surface was adjusted by aluminum chemical deposition with aluminum isopropoxide (Al(O-iPr)3). A procedure similar to the surface silylation was developed, albeit with a lower amount, 1.5 wt. % of aluminum, to avoid clustering of the Lewis acid sites (FER ESA). These three samples are referred to in graphs as FER SP (SP stands for surface passivation).

Steaming

FER28 was first calcined under air (dry air flow, 25 cc min -1 ) at 723 K (ramp rate of 1 K min -1 ) for 6 hours prior to steaming (523, 623 and 823 K for 4 hours under a 20 vol. % H2O / N2 flow, 25 cc min -1 ) and cooling down to room temperature under a flow of dry N2 (25 cc min -1 ). These three samples are referred to in graphs as FER steamed.

Catalyst characterization

A Micromeritics 2020 ASAP was used for nitrogen sorption measurements. The samples were first outgassed at 373 K for 1 h and 623 K for 10 h prior measurements. Scanning Electron Microscopy (SEM) pictures were recorded on a Tescan (MIRA/LMH) microscope equipped with a field emission gun. The nanometer-sized zeolites were further characterized by Transmission Electron Microscopy (TEM) on a Philips CM 120 microscope equipped with a LaB6 filament. The catalysts regeneration by combustion of the carbonaceous residue J o u r n a l P r e -p r o o f ("coke") was monitored with a Setaram SDT Q600 TA thermogravimetric analyzer coupled by heating up to 1173 K (5 K min -1 ) under a 100 ml min -1 (10 % O2 in He) flow.

Solid-state magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR) was used to characterize the local 27 Al environments in the zeolite samples. All data were recorded on a Bruker Advance III -500 MHZ spectrometer using 4 mm rotors. The 27 Al MAS NMR spectra were recorded at 130.29 MHz with a p/12 pulse length of 0.77 ns, a spinning rate of 14 kHz and a recycle delay of 1 s. Al(NO3)3 (1 M) was used as a reference for chemical shifts for 27 Al. Samples were left overnight in a desiccator with a saturated NaCl salt solution to ensure an equilibrated and steady water content. { 1 H}-13 C cross-polarization (CP) MAS experiments were performed using a spinning frequency of 12 kHz. A contact time of 1.5 ms and a recycle time of 2 s were used. The number of scans was varied between 5000 and 60000 depending on the amount of carbon remaining after regeneration. Tetramethylsilane (TMS) was selected as reference to measure 13 C chemical shifts.

In-situ FT-IR spectroscopy was used to quantify the acidity of the zeolites on a Nicolet Magna 550-FTIR spectrometer at 4 cm -1 optical resolution equipped with a DTGS detector.

The samples were pressed under 1 ton into self-supported wafers (diameter 2 cm 2 , 18 mg) prior to loading in the IR cell. Before spectra acquisition (298 K), each sample was activated in the IR cell connected to a vacuum line (393 K reached at 1 K min -1 for 4 h followed by 5 h at 723 K reached at 1 K min -1 under a vacuum of 10 -6 torr). Perdeuterated acetonitrile was used to quantify the total acidity and its nature (Brønsted/Lewis) as this molecule probes both the 8-and 10-membered rings of the FER porosity. [START_REF] Van Donk | Probing the Accessible Sites for n-Butene Skeletal Isomerization over Aged and Selective H-Ferrierite with d3-Acetonitrile[END_REF][START_REF] Wichterlová | Determination and properties of acid J o u r n a l P r e -p r o o f sites in H-ferrierite: A comparison of ferrierite and MFI structures[END_REF] 2,6-dimethyl pyridine and tertbutyl nitrile were used to quantify respectively the Brønsted and Lewis acid sites located on the external crystal surface. [START_REF] Montanari | Use of nitriles as probe molecules for the accessibility of the active sites and the detection of complex interactions in zeolites through IR spectroscopy[END_REF][START_REF] Sadowska | Accessibility of acid sites in hierarchical zeolites: Quantitative IR studies of pivalonitrile adsorption[END_REF] n-hexane (4.3 Å), too bulky to reach the 8 MR cavities (3.5 x 4.8 Å), was selected to quantify the Brønsted acidity in the 10 MR (4.5 x 5.4 Å) J o u r n a l P r e -p r o o f channels [START_REF] Eder | On the role of the pore size and tortuosity for sorption of alkanes in molecular sieves[END_REF][START_REF] Magnoux | Coking, aging, and regeneration of zeolites. II. Deactivation of HY zeolite during n-heptane cracking[END_REF] (see Figure S1 of the SI). All probe molecules were adsorbed at ambient temperature (295 K) and an equilibrium pressure of 0.5 torr. Difference spectra were reported by subtracting a reference spectrum (recorded after activation and prior to adsorption of any probe molecule) from the spectra of interest. The characteristics of all samples investigated are summarized in Table 1.

Catalytic tests

The catalytic performances were evaluated in a fixed-bed plug-flow reactor (PFR) at 523 K under conditions free from external diffusion limitation (see Figure S2 of the SI). The zeolite powders were pressed, crushed and sieved to a size between 100 and 200 µm and diluted in silicon carbide (100 -200 µm and a Zeolite/SiC4 ratio of 1/20) to obtain a 1 cm reactor bed height. The powders were activated in the reactor at 393 K (1 K min -1 ) for 4 h, the temperature raised to 723 K (1 K min -1 ) and kept for 5 h under a 25 cc min -1 flow of lean air (95 % N2 -5 % O2) before cooling to the reaction temperature. Trans-2-butene and isobutene, (Air Liquid) were continuously fed to the reactor using mass flow controllers, while a saturator was used to supply isobutanol (Sigma Aldrich) vapors. The reaction products were analyzed online with a Bruker 450 gas chromatograph fitted with an FID and a Thermo TGbond Q capillary column. The reaction rates are reported as pseudo-first order rate constants 𝑘, calculated from the following formula:

𝑘 = 𝐹 𝑊 ln 1 1 -𝑋 𝑡=4ℎ
where 𝑊 is the catalyst mass (g), 𝐹 is the reactant flow rate (µmol min -1 ) and 𝑋 𝑡=4ℎ is the fractional conversion after four hours on stream.

The n-butene selectivities were interpolated to the same conversion (50 %, standard deviation of 0.8 %, Figure S3 of the SI) and collected after 4 h time-on-stream. The octenes J o u r n a l P r e -p r o o f selectivities were calculated by interpolating the sum of the octenes and its cracking product, propene, to the same conversion (50 %, standard deviation of 0.3 %).

Operando IR measurements

The transmission operando IR cell, its validation and use were reported earlier by Li et al. [START_REF] Li | Effective bulk and surface temperatures of the catalyst bed of FT-IR cells used for in-situ and operando studies[END_REF] To avoid external diffusion limitations, a self-supporting wafer with a surface of 2 cm 2 obtained by pressing a mixture of approximately 5 mg of zeolite and 15 mg of inert silica was loaded in the reactor and activated as discussed above. The IR measurements were carried out on a Nicolet 6700 FT-IR instrument equipped with an MCT detector at 4 cm -1 resolution.

After activation, a spectrum of the clean sample was recorded and used as the reference, subtracted from the spectra recorded during the reaction. These resulting difference spectra were corrected for the water present in the gas phase and in the optical path. After reaction, the reactant flow was stopped and a spectrum recorded after a 20 minutes nitrogen purge.

Chemometrics

The chemometric analysis was performed with SpectroChemPy, a python API. [START_REF] Travert | SpectroChemPy (Version 0.1)[END_REF][30] The Multivariate-Curve Regression by Alternating Least Squares (MCR-ALS) algorithm was selected to extract more information on the nature of the species reacting on the surface of the catalysts and their kinetics. [START_REF] De Juan | Multivariate Curve Resolution (MCR). Solving the mixture analysis problem[END_REF] A Principal Component Analysis (PCA) was first used to determine the optimum number of pure species to adequately describe the experimental spectra X. Once the number of principal components was known, a first estimate of the concentration profile C of these species was calculated via an Evolving Factor Analysis J o u r n a l P r e -p r o o f (EFA) algorithm. The spectra and concentration profile of pure species could then be estimated from the matrix of experimental spectra (X) by solving the linear matrix equation:

X = C S T + E
Where 𝐶 is the MCR-ALS concentration profile (dimensionless), S T is the matrix of spectral profiles (absorbance units) and E the matrix of residuals (absorbance units) unaccounted by the model. These concentration and spectral profiles were further optimized by an alternate least square algorithm (MCR-ALS), including hard constraints such as the non-negativity of concentration and spectral profiles, as reported by Vilmin et al. [START_REF] Vilmin | Reactivity of bis[3-(triethoxysilyl)propyl] tetrasulfide (TESPT) silane coupling agent over hydrated silica: Operando IR spectroscopy and chemometrics study[END_REF] Our MCR-ALS optimization was performed on a limited spectral region, 1700 -1300 cm -1 , allowing to impose the non-negativity of spectral profiles and improve the optimization robustness. We assumed that the concentration profile C1700-1300 for this region provided sufficient information to reconstruct the whole spectral region. The validity of this hypothesis was tested by comparing the original spectra with the reconstructed dataset 𝑋 ̂ = C S T . The global algorithm is summarized in Table S2, which can be found in the supporting information J o u r n a l P r e -p r o o f

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Results and discussion

Preliminary catalyst screening

Five catalysts including -Al2O3 and four zeolites of different structures (ERI, FER, TON, and MFI) were first evaluated in the conversion of isobutanol (Table 2). Both activity and n-butene selectivity of all zeolites are significantly higher than those of -alumina, suggesting that confinement in the zeolite porosity or strong Brønsted acidity promotes n-butene selectivity.

Among the zeolites, Ferrierite (FER28) displays an excellent activity and the highest n-butenes selectivity (81.4 % at 50 % conversion), confirming earlier results. [START_REF] Buniazet | Unexpected selectivity of FER for the conversion of isobutanol to linear butenes and water effects[END_REF] It is worth noticing that such a ranking seems paradoxical as the FER-type zeolite is well-known to efficiently catalyze the skeletal isomerization of n-butene to iso-butene. [START_REF] Grandvallet | Process for the conversion of a feedstock comprising linear olefins[END_REF] Table S1 (SI) compares the experimental distribution of butene isomers with their thermodynamic equilibrium and shows that the experimental ratio of linear butenes to isobutene (4.89) is much larger than at thermodynamic equilibrium (0.70). On the other hand, the internal distribution between linear butenes is close to thermodynamic equilibrium. This suggests that isobutene and linear butenes are formed from distinct routes and that thermodynamic equilibrium is rapidly reached between the linear butenes. Finally, it is worth pointing out that at our conditions isobutanol dehydration to butenes is not thermodynamically limited.

J o u r n a l P r e -p r o o f Figure 1a shows the evolution of the reaction rate constant on FER28 with time on stream and indicates a continuous deactivation; this could be the result of acid site poisoning or pore plugging. The deactivation is accompanied by a change of the n-butene selectivity showing a maximum after approximately two hours. It is worth noting that such a change (+ 4 %) remains small and is not as pronounced as those reported for 1-butene to isobutene isomerization (typically ~ 40 % increase with time on stream). [START_REF] Hu | Designing ferrierite-based catalysts with improved properties for skeletal isomerization of n -butene to isobutene[END_REF] Because of the similarities between the two reactions, however, such selectivity change deserves further investigation. Minor amounts of side products (propene, pentenes and octenes) were also formed. Figure 2 illustrates the possible reaction pathways for the reaction. Butenes are the major products together with some propene, pentenes and octenes. n-butenes are either primary products or secondary products obtained by isobutene isomerization. As shown in the bottom of Figure 2, Minor products are probably formed by butene dimerization followed by cracking to propene and pentenes. [START_REF] Mooiweer | Skeletal isomerisation of olefins with the zeolite Ferrierite as catalyst[END_REF][START_REF] Houzvicka | The role of deposits in butene isomerisation[END_REF]

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Influence of Si/Al ratio

Three FER-type zeolites with increasing Si/Al (9, 28 and 46), FER9, FER28, FER46 have been compared in the conversion of isobutanol to n-butenes; their characteristics, catalytic activities (pseudo first-order rate constants) and n-butene selectivities are gathered in Table 3 and Table 4. While FER28 and FER46 are similar but for their Si/Al, FER9 differs by a high Lewis/Brønsted acidity ratio and a low fraction of Brønsted acid sites in its 10 MR channel.

FER9 displays the highest pseudo-first order rate constant while FER46 is the most selective towards n-butene. However, no correlation was found between the aluminum content and the dehydration rate constant or n-butene selectivity. Increasing the aluminum content leads to an J o u r n a l P r e -p r o o f increased total Lewis acidity, as shown by in-situ IR (795 > 70 > 30 µmol g -1 for FER9, FER28 and FER46, respectively). On the other side, the Lewis acidity of the external surface does not increase proportionally with the total aluminum content. Moreover, the catalytic results on FER46 show that the absence of Lewis acidity on the external surface has no impact on the dehydration rate and n-butene selectivity. This finding suggests that the external Lewis acidity does not play an essential role in the reaction. On the other hand, the high activity of FER9 appears to be correlated with the external Brønsted acid sites density: the latter is twice as high as the other FER zeolites and its activity is also doubled.

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Impact of water on the catalyst structure and activity

As water is formed during the reaction, its potential impact on the stability of the FER catalysts needs to be investigated. To this end, the reference sample (FER28) is steamed ex-situ at three different temperatures: two near the reaction temperature (523 K and 623 K) and one higher (823 K). The key characteristics of the steamed FER28 samples are summarized in Table 5. Their XRD patterns and 27 Al NMR spectra are gathered in Figure S4. While steaming near the reaction ( a in-situ FT-IR, b at 50 % isobutanol conversion in a tubular PFR, reaction conditions: T = 523 K, WHSV = 100 h -1 , P = 1 bar, Pisobutanol = 45 mbar, TOS = 4 h)

Isobutanol accessibility to the FER microporosity

The isobutanol accessibility to the ferrierite microporosity can best be studied by monitoring isobutanol adsorption by in-situ IR spectroscopy at various temperatures. Figure 3a shows difference IR spectra (spectrum of the pristine zeolite subtracted from the spectra recorded after isobutanol adsorption) at room temperature after adsorption of 0.5 torr of isobutanol at a J o u r n a l P r e -p r o o f temperature between 298 K -573 K. Between 298 K to 373 K, isobutanol is only adsorbed on silanol groups as indicated by the negative band at 3748 cm -1 . These silanol groups are mainly located on the external surface of the crystals. The IR band characteristic of Brønsted acid sites, 3590 cm -1 , is unaffected suggesting that isobutanol does not enter the microporosity. [START_REF] Van Donk | Probing the Accessible Sites for n-Butene Skeletal Isomerization over Aged and Selective H-Ferrierite with d3-Acetonitrile[END_REF] However, at a temperature of 398 K, the intensity of this band starts decreasing. At the same time, three broad bands appear at 2922, 2405 and 1672 cm -1 , characteristic of an ABC structure, indicating that above 398 K some internal Brønsted acid sites are involved in strong hydrogen bond. Such ABC structures result from Fermi resonances between the (OH) stretching mode of the bridged OH groups extending to low frequencies due to the formation of a strong hydrogen bonds (with isobutanol in the present case) and harmonic or combinations of the (OH) and (OH) bending modes [START_REF] Bordiga | Probing zeolites by vibrational spectroscopies[END_REF]. As isobutanol dehydration starts at 398 K (Figure 3b), the disappearance of Brønsted acid sites during in-situ IR experiments is due to their interactions with the dehydration products. An IR spectrum after water adsorption at room temperature on FER (Figure 3a, dotted line) confirms that the ABC structure is characteristic of water adsorption. Therefore, during the reaction, isobutanol does not significantly enter the microporosity; it reacts mostly on the external surface or at the pore mouths and only its dehydration products, water in particular, diffuse inside the zeolite pores.

Impact of external surface acidity modifications

As isobutanol does not penetrate the FER microporosity, external acid sites should be active in isobutanol dehydration and responsive to modifications of this acidity. To verify this, the external acid site density of FER28 was modified by two post-synthesis treatments to either decrease its overall external (Brønsted and Lewis) acidity or increase the Lewis/Brønsted acidity ratio. Decreasing the external acid site density was performed by chemical deposition of TEOS (tetra-ortho-ethylsilicate); the treatment was done once (1x TEOS) and repeated (2x TEOS).

Increasing the Lewis/Brønsted acidity ratio was performed by chemical deposition of Al(O-iPr)3 (aluminum tri-isopropoxide).

The main features of the resulting FER zeolites are summarized in Table 6. They indicate that the external acidity can be significantly changed without impacting other properties of interest (micropore volume, total Brønsted and Lewis acidity). The slight changes in Btot and Ltot could be due to the extra calcination steps undergone by the modified samples. ( a in-situ FT-IR, b at 50 % isobutanol conversion in a tubular PFR, reaction conditions: T = 523 K, WHSV = 100 h -1 , Patm = 1 bar, Pisobutanol = 45 mbar, TOS = 4 h) These three post-synthesis treatments result in lower rate constants in isobutanol dehydration with respect to the parent FER28. The external Brønsted acid sites are, therefore, those active for isobutanol dehydration. In addition, the negative impact on the n-butene selectivity indicates a crucial role of the external surface acidity on the isomerization reaction.

Role of the location of internal Brønsted acid sites

As internal Brønsted acid sites in FER can be located either in 8 and 10 membered rings (MR), a selective poisoning could shed some light on their respective roles on n-butene selectivity during isobutanol conversion. A selective ion-exchange with sodium is commonly used to investigate that effect. [START_REF] Terlouw | Catalyst composition[END_REF][START_REF] Makarova | Quantification of Bronsted acidity in mordenites[END_REF] Table 7 shows the main characteristics, catalytic properties of the parent zeolite (FER28) and its sodium exchanged derivatives. ( a in-situ FT-IR, b at 50 % isobutanol conversion in a tubular PFR, reaction conditions: T = 523 K, WHSV = 100 h -1 , Patm = 1 bar, Pisobutanol = 45 mbar, TOS = 4 h) A partial ion-exchange alters the ratio between the Brønsted acid sites located in the 8 and 10 MR. It confirms also a preferential location of sodium in the 8 MR channels, in agreement with Lercher et al. 4 This results in a decrease of the reaction rate without changing much n-butene selectivity, indicating that neither the density nor the location of internal Brønsted acid sites is critical in the isomerization step. However, as the water produced during isobutanol conversion increases sodium mobility, it may not remain in the 8 MR of FER under the reaction conditions, and no definitive conclusion can be drawn at this stage. [START_REF] Pissis | Dielectric Studies of Molecular Hydrated Mobility in Hydrated Zeolites[END_REF]

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Role of the internal micropores

The role of the micropore volume of Ferrierite can be studied by plugging it with carbonaceous deposits. Such a sample was prepared by deactivating a physical mixture of 80 % FER28 -20 % silica in the conversion of isobutanol under industrial conditions. Table 8 indicates that while total and external Brønsted acid site density decrease, the micropore volume drops by 98 %, i.e.

an almost complete plugging. The rate constant decreases, but to a smaller extent (75 %) while n-J o u r n a l P r e -p r o o f butene selectivity is moderately affected (-10 %). Therefore, a possible role of carbonaceous species cannot be excluded. Table 8. Selected properties of spent FER28.

Catalyst

FER28 Si Spent

Btot -Ltot a (µmol g -

Identification of surface species

FER-based catalysts are known for their excellent isobutene selectivity in the n-butene skeletal isomerization reaction, vide supra. For n-butene to isobutene conversion, one hypothesis is that carbonaceous residues ("coke") contain active sites. [START_REF] Guisnet | Selective isomerization of n-butenes into isobutene over aged H-ferrierite catalyst: nature of the active species[END_REF][START_REF] De Menorval | Mechanism of n-butene skeletal isomerization over HFER zeolites: a new proposal[END_REF][START_REF] Andy | Skeletal Isomerization of n-Butenes II. Composition, Mode of Formation, and Influence of Coke Deposits on the Reaction Mechanism[END_REF] Due to the similarities between the two reactions, assessing the contribution of such carbonaceous species to the activity could bring valuable insight into the reaction mechanisms. Operando IR spectroscopy is ideal for monitoring the formation of surface species at working conditions. Some difference IR spectra recorded during the first 4 hours of reactionwhile deactivation with time on stream is continuous (Figure 1a) -are presented in Figure 4. Chemometric analysis was used to extract valuable information from such a large number of spectra (c.a. 250). Principal component analysis (PCA) and multivariate curve resolution analysis (MCR-ALS) have been used for this purpose. PCA indicates that the first component captures a major part of the variance of the spectra (99.53 %), which is typical in the case of closely related spectra [START_REF] Vilmin | Reactivity of bis[3-(triethoxysilyl)propyl] tetrasulfide (TESPT) silane coupling agent over hydrated silica: Operando IR spectroscopy and chemometrics study[END_REF]. The following components captured 0.30%, 0.12% and 0.03% of the variance, suggesting that the number of components n required to describe adequately the experimental data lies between 1 and 4. This number was chosen on the basis of the residuals 𝐸 between the matrix 𝑋 1700-1400 of experimental spectra and the matrix 𝑋 ̂1700-1400 of their estimates by MCR-ALS. As shown in Figure S5 of the supplementary information section, the use of two components vs. one yields a significant improvement of the modeling of the spectra, while the addition of a third or fourth component does not improve much the model. In particular the sum of residuals was of the same order of magnitude: (50 and 100 for two and three components vs. 10 3 for a single component). Further, as shown in Figure S6, the evolution of the and 1392 cm -1 belong to isobutanol. The first component represents therefore the interaction of isobutanol and water with the zeolite. As water is a primary product of the reaction, we cannot exclude that other products (butenes) appear alongside water and contribute also to this component. However, as shown in Figure 6 below, isobutene does not enter the FER microporosity. Moreover, when iso-and n-butenes are fed to the catalyst at the same temperature and pressure, their peak intensities are substantially lower than those observed with isobutanol (Figure 6), suggesting that these butenes promptly desorb after their formation and do not contribute significantly to the difference spectral profiles. The second component shows the silanol and Brønsted acid site consumption at 3749 and 3590 cm -1 , and the formation of new bands at 3483, 1505, 1473, 1392, 1367 and 1326 cm -1 . The 3483 cm -1 band is characteristic of H-bonded bridged hydroxyl groups and can be formed due to the interaction of Brønsted acid sites with a single or a set of species with absorptions at 1505, 1473, 1392, 1367 and 1326 cm -1 . These bands belong either to a single adsorbed species or to a set of adsorbed species whose relative concentrations remain constant during the reaction. The appearance of the band at 3483 cm -1 , characteristic of H-bonded bridged OH groups and the simultaneous disappearance of the ABC structure suggests that the interaction between the Brønsted acid sites and water is systematically replaced with the new set of species. This is confirmed by the evolution of the MCR-ALS concentration profiles of pure species (Figure 5c). At the timescale and under the reaction conditions of the operando IR experiments (one spectrum recorded each 42 s over 4 hours of reaction), no significant change of the nature of this/these species is evidenced. The absence of bands in the 3000 -3100 cm -1 range allows to conclude that the corresponding species are most probably not aromatic, in agreement with Paze et al. who studied 1-butene isomerization at similar reaction temperatures. [START_REF] Paze | FTIR and UV-Vis Spectroscopic Study of Interaction of 1-Butene on H-Ferrierite Zeolite[END_REF] The extent of the (OH) shift between the frequency of the free Brønsted acid sites and the band at 3483 cm -1 provides additional information on the nature of these interacting species. This shift (118 cm -1 ) is similar to that observed upon adsorption of n-hexane while the shift observed upon adsorption of other classes of compounds are much larger (water : 712 cm -1 and ABC structure, alkane : 118 cm -1 , alkene : ~ 700 cm -1 , alkyne : 220 cm -1 ). [START_REF] Zholobenko | Ferrierite and SUZ-4 Zeolite: Characterization of Acid Sites[END_REF][START_REF] Castellanos | An operando FT-IR study of the NOx SCR over Co-HFER and Fe-HFER using acetylene as a reducing agent[END_REF] Quantification of the frequency shift J o u r n a l P r e -p r o o f resulting from the interaction with an aromatic carbon ring was not successful since the smallest aromatic compound, benzene, cannot access the microporous volume. The absence of a (OH) band characteristic of alkene adsorption ((OH) ~ 700 cm -1 ) under such reaction conditions, might be explained by i) the small number of light alkenes adsorbed, ii) a proton transfer to the alkene moieties of strongly adsorbed species. In order to determine if such species have a direct impact on catalytic activity, the reaction rates and the product yields have been compared to the evolution of the MCR-ALS concentration profiles of these species. As shown in figure 7a, a weak and negative correlation was found between the reaction rate of isobutanol and the MCR-ALS concentration of the second pure species. This suggests that most of these species are not directly involved in the deactivation. Moreover, no correlation was observed between the MCR-ALS concentration of these species and the product yields or selectivities. In order to determine the nature of the sites involved in the formation of these species, their initial rate of formationas determined by the variation of the MCR-ALS concentration over the first 30 minutes of reaction was compared to the concentrations of acidic sites.

J o u r n a l P r e -p r o o f

Figure 7 shows that a significant linear correlation holds between the rate of formation of the second pure species and the total Brønsted acid site concentration of the samples. This is a strong indication that these carbonaceous species are produced by reactions involving the products of isobutanol dehydration on Brønsted acid sites.

Impact of morphology

As the external Brønsted acidity of FER appears to play an important role in the isomerization reaction, the effect of particle size and shape (needle and plate-like shape) on catalytic performances should shed further insight in mechanisms. [START_REF] Lee | Synthesis and catalytic behavior of ferrierite zeolite nanoneedles[END_REF][START_REF] Catizzone | Catalytic application of ferrierite nanocrystals in vapour-phase dehydration of methanol to dimethyl ether[END_REF] While decreasing crystal size increases the specific external surface area, modifying crystal morphology can affect the ratio between the different types of pore mouths, 8 vs. 10 MR. Two FER zeolites with a nanoplate and a nanoneedle morphology were prepared and tested in isobutanol conversion; their relevant properties are reminded in Table 9. As the nanosized samples have a high aluminum content, they are best compared with FER9. Table 9 shows that n-butenes selectivity changes for both nanocrystals. Despite the increased external surface (more pore mouths), there is no correlation with the n-butenes selectivity.

Moreover, this selectivity does not follow the same trend as the external Brønsted acidity. This indicates that the superior n-butenes selectivity of Ferrierite cannot just be explained by the number of external Brønsted acid sites.

J o u r n a l P r e -p r o o f

Study of butene isomerization

Next, the isomerization reactions involved in the one-step conversion were studied to gain a better comprehension of this multistep reaction. The conversions of iso-and n-butenes were compared on the FER28 catalyst under identical conditions. As shown in Figure 9, two possible reaction mechanisms for the one-step dehydration of isobutanol over Ferrierite can be considered, namely a direct (red) or indirect (blue) formation of n-butenes.

J o u r n a l P r e -p r o o f and indirect (Blue) formation of n-butenes from isobutanol. Isomerization of n-butenes to isobutene (Green) and isobutene to n-butenes (Orange) Since a stable isobutene intermediate is formed in the indirect pathway, a rake-type mechanism could be considered (blue arrows, Figure 9). The reaction starts with isobutanol dehydration and the formation of a primary carbocation. It must then undergo a 1,2 -rearrangement to form a more stable ion. While both a proton and methyl shifts are valid options, the proton shift is expected to occur more rapidly [9]. However, this will form a more stable tertiary carbocation yielding isobutene after deprotonation. To obtain n-butenes, a consecutive isomerization step is required. On the other hand, in the direct mechanism (red arrow), the slower methyl shift does not require any further isomerization. The zeolite external surface can provide some additional stabilization of the carbocation favoring the methyl shift and the direct formation of n-butenes.

The latter can then isomerize into isobutene as a side reaction (green arrow); it is thus worthwhile to compare the kinetics of isobutanol conversion with that of butene isomerization.

The rate constants of each reactions were measured on the reference ferrierite (FER28) sample at the same temperature used for isobutanol conversion and presented in Table 10. The rate constants for isomerization of branched and linear butenes are about two orders of magnitude lower than for the transformation of isobutanol to n-butenes. This indicates that these steps do not participate in the transformation of isobutanol to n-butene and a rake-type mechanism can be discarded. Hence, isomerization and dehydration must either occur simultaneously on a single site of the surface or occur sequentially. In the latter case, isomerization should occur before formation of isobutene from the iso-and tert-butyl cations (or their corresponding alkoxides) formed after the dehydration step. This implies that the alkoxide intermediate is stabilized on the surface to prevent its desorption before its isomerization, or that dehydration and isomerization sitesif distinct -are in close proximity. Figure 10 shows the difference spectra obtained in the operando IR cell during the transformations of trans-2-butene and isobutene over FER. For the isomerization of n-butenes, the spectra display two negative bands at 3742 cm -1 and 3590 cm -1 , evidence of the progressive and irreversible consumption of silanol groups and Brønsted acid sites.

J o u r n a l P r e -p r o o f The spectra recorded during isobutene isomerization show the absence of a negative band at 3590 cm -1 , clearly visible during n-butene isomerization. The absence of this band suggests that isobutene does not enter the ferrierite microporosity under our reaction conditions. This hypothesis can be confirmed by the spectra obtained after isobutene adsorption at various temperatures (Figure 11). In all cases, a negligible amount of isobutene reacts (≈ 1 %). For comparative purpose, difference spectra obtained after consumption of the external acid sites by 2,6-dimethylpyridine adsorption [START_REF] Armaroli | An FT-IR study of the adsorption of aromatic hydrocarbons and of 2,6-lutidine on H-FER and H-ZSM-5 zeolites[END_REF] and after consumption of the 10 MR Brønsted acid sites by n-hexane are added.

Structure-activity relationship in isobutanol conversion to n-butenes

The above results identify unambiguously the external Brønsted acid sites as active sites for the dehydration of isobutanol as this reactant cannot access the Ferrierite microporosity and their subsequent involvement in the consecutive isomerization step. Error! Reference source not found.a shows that the initial dehydration rate constants of the various FER investigated in this work are strongly correlated to their external Brønsted acid site concentration. The latter encompass the Brønsted acid sites which can be probed by 2,6-lutidine i.e. those located on the external surface and at the pore mouth. A direct relation between the quantity of external Brønsted acid sites concentration and the nbutene selectivity however, is not found. The possible mechanisms discussed in the previous section suggest that the proximity between external Brønsted acid sites can play a key role in explaining the high n-butene selectivity. Error! Reference source not found.b highlights the relationship between the external Brønsted acid sites density and n-butene selectivity for all the FER-type zeolites investigated in this work. There is a steep variation of n-butene selectivity (from ~ 20 % to ~ 80 %) for the zeolites with the lowest external Brønsted acid site density (< 0.20 µmol m -2 ), while beyond a threshold value of about 0.20 µmol m -2 , the n-butene selectivity reaches a plateau (~ 80 %) with no further increase with the Brønsted acid sites density.

Conclusions

In the newly developed ATOL-C4 TM process, bio-sourced linear butenes are generated from isobutanol with high selectivity after a one-step dehydration and skeletal isomerization. A more fundamental understanding of the separate reactions at play (dehydration and isomerization) was required to design better performing catalysts. Due to the similarities with the isomerization of nbutenes to isobutene, the possible involvement of the formed carbon pool could not be excluded.

By combining state-of-the-art operando spectroscopy with a chemometric analysis (PCA and MCR-ALS), time-resolved information on the carbonaceous species formation was extracted out of the complex data mixture. Since these data are not correlated with the evolving n-butene selectivity, this strongly suggests that carbonaceous species do not participate in the isomerization step as for pseudo-monomolecular mechanism proposed for n-butene isomerization. Next, the reactions involved in the one-step isobutanol conversion were studied to gain a better comprehension of this multistep reaction. By fine-tuning acidity (Lewis, Bronsted, external, 10 or 8MR channel…), morphological and textural properties of FER samples, a library of modified Ferrierites was built in order to evaluate the main factors influencing the catalytic performances. The impact of the modifications was carefully quantified using a systematic insitu IR analysis with specific probe molecules. The combination with catalytic testing allowed establishing that neither isobutanol nor isobutene enter the ferrierite internal structure. However, a linear relationship was found between the Brønsted acid sites located on (or close to) the J o u r n a l P r e -p r o o f external surface and the dehydration rate constant. We suggest those acid sites are the isobutanol dehydration active sites. Afterward, a comparison of pseudo-first-order rate constant for isobutene isomerization and isobutanol to n-butene transformation allowed to exclude isobutene as an intermediate (which would lead to a much slower rate) and the initially imagined rake-type mechanism. Therefore, it was proposed that proximity between the dehydration and isomerization active site is required to avoid desorption of intermediate carbocation prior to isomerization occurring. Finally, proximity between external Brønsted acid sites was found to have a major impact on the selectivity toward n-butene and it can be concluded that a threshold surface density of external Brønsted acid sites is required to obtain a high selectivity in n-butene selectivity. The existence of this threshold explains the exceptional n-butene selectivity of FERtype zeolites in comparison with other 10 MR zeolites such as MFI-or TON-type. In conclusion, this study allowed identifying the sites responsible for both the dehydration of isobutanol and the consecutive isomerization step and proposing a mechanism for this one-step reaction. The information gathered in this study offers insights to optimize the next generation of industrial catalysts for the dehydration of isobutanol to linear butenes.

(

  a t-plot, b at 50 % conversion, reaction conditions: T = 523 K, WHSV = 100 h -1 , Patm = 1 bar, Pisobutanol = 45 mbar, TOS = 4 h)

Figure 1 .

 1 Figure 1. Evolution of (a) the pseudo-first order rate constant, (b) n-butene selectivity with timeon-stream (h) for FER28. (reaction conditions: operando reactor, T = 525 K, WHSV = 100 h -1 ,

Figure 2 .

 2 Figure 2. Top: dehydration and isomerization of isobutanol on FER, Bottom: Side products

J o u r

  n a l P r e -p r o o f temperature does not result in any significant loss of crystallinity or structural changes, steaming at 823 K results in the loss of 20 % of tetrahedral Al. However, almost no octahedral Al is detected by NMR, suggesting that the extra-framework Al atoms are in a distorted environment preventing their detection.[START_REF] Grobet | Correct determination of aluminium in Y zeolites by magic angle spinning NMR spectroscopy[END_REF] Upon steaming, both external Brønsted acid site density and isobutanol dehydration rate constant decrease, indicating that these acid sites play an important role in the reaction. Finally, the absence of a significant impact of steaming at 523 and 623 K, i.e. close to the reaction temperature, on the total amount of acid sites highlights that overall the FER crystals are stable under our reaction conditions (T = 523 K).

J o u rFigure 3 .

 3 Figure 3. (a) Difference spectra after isobutanol adsorption and gas phase correction (Psat = 0.5

  r n a l P r e -p r o o f

(

  a in-situ FT-IR, b TG, c at 50 % isobutanol conversion in a tubular PFR, reaction conditions: T = 523 K, WHSV = 100 h -1 , Patm = 1 bar, Pisobutanol = 45 mbar, TOS = 4 h)

Figure 4 .

 4 Figure 4. A selection (∆t = 30 min) of difference IR spectra obtained during isobutanol conversion on

  r n a l P r e -p r o o f

Figure 5 .

 5 Figure 5. PCA and MCR-ALS on the isobutanol conversion over FER28 (T = 525 K, WHSV = 100 h -1 , Patm = 1 bar, Pisobutanol = 45 mbar, TOS = 4 h) a) Overview of the percentage of variance

Figure 6 Figure 6 .

 66 Figure 6 compares the IR spectrum obtained under operando conditions after four hours of

Figure 7 .

 7 Figure 7. (a) Evolution of the rate constant versus the MCR-ALS concentration of the second

  r n a l P r e -p r o o f

Figure 8 .

 8 Figure 8. SEM picture of FER nanoplate (left), TEM picture of FER nanoneedle (right)

Figure 9 .

 9 Figure 9. Reaction scheme for the one-step conversion of isobutanol into butenes. Direct (Red)

  r n a l P r e -p r o o f

Figure 10 .

 10 Figure 10. Difference IR spectra (saturation -reference) of FER28 during the skeletal

22

 22

Figure 11 .

 11 Figure 11. Difference in-situ IR spectra after adsorption on FER of 0.5 torr of isobutene (blue

  Figure 12 (a) Effect of the external Brønsted acid site concentration on the initial dehydration rate

  r n a l P r e -p r o o f Remarkably, the three zeolites presenting other pore topologies, namely ERI (8 MR), MFI (10 MR) and TON (10 MR) follow the same general trend observed for the FER zeolites. This suggests that the excellent selectivity of Ferrierite is a result of its high external Brønsted acid sites density. Close proximity between Brønsted acid sites could prevent fast desorption of adsorbed iso-and tert-butyl cations (or their corresponding alkoxides) and therefore the formation of isobutene. Another possibility is that a high external surface density of Brønsted acid sites could statistically favor the formation of vicinal, paired Brønsted acid sites acting as single active sites.

Figure 13 .

 13 Figure 13. Effect of external Brønsted acid density on the conversion drop. (reaction conditions:

Table 1 .

 1 Physicochemical characteristics of the employed Ferrierite samples.

		parents FER		Steamed FER28		Surface modified FER28
	Catalyst	FER 9	FER 28 FER 46 523 K	623 K	823 K	1x TEOS 2x TEOS 1 x Al(O-iPr)3
	Si/Al	9	28	46	28	28	28	28	28	28
	Vmicro (cm 3 /g)	0.15	0.14	0.10	0.13	0.13	0.13	0.12	0.12	0.13
	Sext (m 2 g -1 )	25	18	14	46	46	48	42	39	44
	Btot -Ltot (µmol g -1 )	770 -795 540 -70 307 -30 580 -46 570 -69 420 -142 530 -82 460 -96 550 -89
	B10MR / Btot (%)	16	53	56	48	56	51	51	50	49
	Bext -Lext (µmol g -1 ) 26 -6	17 -11	16 -0	13 -12	8 -11	7 -5	12 -6	5 -1	13 -19
	k (mol h -1 gcat -1 )	3.13	1.50	1.57	1.36	0.95	0.61	0.70	0.37	0.68
	Sn-C4 (%)	80.4	81.4	85.0	80.2	81.0	81.2	76.3	74.7	76.8

J o u r n a l P r e -p r o o f

Table 2 .

 2 Characteristics and performances of catalysts in one-step isobutanol transformation

	Catalyst	-alumina ERI4 FER28 TON36 MFI40
	Si/Al	-	4	28	36	40
	Vmicro a (cm 3 /g)	0.08	0.19 0.14	0.11	0.10
	Sext a (m 2 g -1 )	208	69	18	70	100
	k (mol h -1 gcat -1 )	0.08	0.33 1.50	2.08	1.97
	Sn-C4 b (%)	1.36	30.5 81.4	62.1	23.3

Table 3 .

 3 Characteristics of FER catalysts with different Si/Al ratios

	Catalyst	FER9	FER28	FER46
	Commercial name	CP 914C	CP 914	-
	Btot -Ltot (µmol g -1 )	770 -795	540 -70	307 -30
	B10MR / Btot (%)	16	53	56
	Bext -Lext (µmol g -1 )	26 -6	17 -11	16 -0
	k (mol h -1 gcat -1 )	3.13	1.50	1.57
	Sn-C4 a (%)	80.4	81.4	85.0
	( a at 50 % isobutanol conversion in a tubular PFR, reaction
	conditions: T = 523 K, WHSV = 100 h -1 , Patm = 1 bar, Pisobutanol = 45
	mbar, TOS = 4 h)

Table 4 .

 4 Isobutanol dehydration/isomerization on FER zeolites

		Yields (%)				Ratio (%)	Selectivity (%)
	Catalyst	i-C4 = n-C4 = C3 =	C5 =	C8 =	i-C4 = /total C4 = n-C4 =
	FER9	9.1	38.2	0.1	0.0	0.1	19.1	80.4
	FER28	4.3	17.5	0.1	0.0	0.2	20.0	81.4
	FER46	5.6	28.8	0.1	0.0	0.1	15.9	85.0
	(

a at 50 % isobutanol conversion in a tubular PFR, reaction conditions: T = 523 K, WHSV = 100 h -1 , Patm = 1 bar, Pisobutanol = 45 mbar, TOS = 4 h)

Table 5 .

 5 Characteristics of FER28 steamed at 523, 623 and 823 K.

	Catalyst	FER28	523 K	623 K	823 K
	Btot -Ltot a (µmol g -1 )	540 -70 580 -46 570 -69 420 -142
	B10MR / Btot a (%)	53	48	56	51
	Bext -Lext a (µmol g -1 ) 17 -11	13 -12	8 -11	7 -5
	k (mol h -1 gcat -1 )	1.50	1.36	0.95	0.61
	Sn-C4 b (%)	81.4	80.2	81.0	81.2

Table 6 .

 6 Characteristics of external surface modified ferrierites.

	Catalyst	FER28	1x TEOS 2x TEOS 1 x Al(O-iPr)3
	Btot -Ltot a (µmol g -1 ) 540 -70 530 -82	460 -96	550 -89
	B10MR / Btot a (%)	53	51	50	49
	Bext -Lext a (µmol g -1 ) 17 -11	12 -6	5 -1	13 -19
	k (mol h -1 gcat -1 )	1.50	0.70	0.37	0.68
	Sn-C4 b (%)	81.4	76.3	74.7	76.8

Table 7 .

 7 Characteristics of ferrierites with internal acidity modifications.

	Catalyst	FER28 30 %

Na 60 % Na 86 % Na 96 % Na

	Btot -Ltot a (µmol g -1 ) 540 -70 380 -57	220 -42	75 -48	22 -53
	B10MR / Btot a (%)	53	75	98	88	100
	Bext -Lext a (µmol g -1 ) 17 -11	15 -11	16 -14	18 -11	17 -14
	k (mol h -1 gcat -1 )	1.50	1.48	1.24	1.12	1.00
	Sn-C4 b (%)	81.4	80.6	79	80.7	80.8

Table 9 .

 9 Characteristics of nanosized Ferrierites.

	Catalyst	FER 9	Nanoneedle	Nanoplate
	Sext (m 2 g -1 )	25	240	42
	Btot -Ltot a (µmol g -1 )	770 -795	688 -275	697 -937
	B10MR / Btot a (%)	16	5	28
	Bext -Lext a (µmol g -1 )	26 -6	36 -85	17 -13
	k (mol h -1 gcat -1 )	3.13	5.21	3.99
	Sn-C4 b (%)	80.4	75.4	84.1
	( a in-situ FT-IR, b at 50 % isobutanol conversion in a tubular PFR,
	reaction conditions: T = 523 K, WHSV = 100 h -1 , P = 1 bar, Pisobutanol =
	45 mbar, TOS = 4 h)

Table 10 .

 10 Comparison of pseudo-first order kinetic rate constants at 525 K

		Isobutanol	Isobutanol	Isobutene	n-butene
	Reaction
		to all products	to n-butene	to n-butene	to isobutene
	WHSV (h -1 )	100	100	0.01	0.01
	P (bar)	0.05	0.05	1.00	1.00
	rate constant
	(mol.h -1 . gcat -1 .bar -1 )	21.28	15.55	0.01	0.02
	(operando reactor, FER28, Patm = 1 bar, TOS = 4 h)

Acknowledgments

This work was made possible by a CIFRE PhD grant (S. van Daele) from Total Research and Technology Feluy (TRTF) and Agence Nationale de la Recherche Technologique (France, 2015/0177). J o u r n a l P r e -p r o o f

Nikolai Nesterenko (Conceptualization) (Funding acquisition) (Project administration) (Supervision) (Writing -review and editing), Sylvie Maury

residuals with time on stream was similar for 2 and 3 components with a maximum located at the beginning of the reaction (0-5 minutes time on stream). This suggests that the use of two components is optimal for the modeling of these experimental spectra by MCR-ALS.

The MCR-ALS modeling of the spectra in the 1700-1400 cm -1 range, yielded the matrix of concentration profiles C (Figure 5c) which in turn, was used to compute the matrix of pure spectral profiles S T over the whole spectral range (Figure 5d). The matrix of reconstructed spectra X ̂ was finally computed using: X ̂ = C S T . Figure 5b compares the experimental and reconstructed spectra from the concentration and spectral profiles shown in Figure 5c andd.

Such a comparison allows a visual inspection of the model quality. Here, the residuals are significantly higher than the experimental noise and are mostly observed for the three spectra in the OH, CH and CC range, which could be accounted by the non-steady flow conditions that prevail at short reaction time (< 5 mins). While this transitional regime would certainly be worth being studied, the time resolution of the spectra acquisition (> 1 min) and, above all, the products analysis (> 5 min) did not allow us to carry out such an analysis. On the other hand, an excellent agreement between experimental and reconstructed spectra was found above 10 minutes of reaction and no significant improvement was obtained when using a third component to describe the experimental spectra.

Examination of the first pure spectral profile of S T shows two negative bands (3746 and 3590 cm -1 ) indicating the consumption of terminal silanols and Brønsted acid sites, respectively.

Additionally, an ABC structure is visible at 2935, 2447, and 1631 cm -1 , indicating that Brønsted acid sites are hydrogen-bonded with water. [START_REF] Bordiga | Probing zeolites by vibrational spectroscopies[END_REF] The positive bands at 3672, 3029 -2806, 1473

J o u r n a l P r e -p r o o f

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


Yusran Yusran

Hui Li

Dr Xinyu Guan

Daohao Li

Prof Lingxue M Tang

Ming Xue

Zhongbin Zhuang

Yushan Yan

Valentin Valtchev

Shilun Qiu

Qianrong Fang
email: qrfang@jlu.edu.cn

Prof S Xue

Prof Q Qiu

Fang

Prof Z Zhuang

Prof V Valtchev

Prof Y Yan

Exfoliated Mesoporous 2D Covalent Organic Frameworks for High-Rate Electrochemical Double-Layer Capacitors

Keywords: Covalent organic frameworks, chemical exfoliation, double-layer capacitor, high charge-discharge rate

Abstract:

The electrochemical double-layer capacitors (EDLCs) are highly demanded electrical energy storage devices due to their high power density with thousands of cycle life compared with pseudocapacitors and batteries. Herein, we report a series of capacitor cells composed of exfoliated mesoporous 2D covalent organic frameworks (e-COFs) that are able to perform excellent double-layer charge storage. The selected mesoporous 2D COFs possess eclipsed AA layer-stacking mode with 3.4 nm square-like open channels, favorable BET surface areas (up to 1170 m 2 /g), and high thermal and chemical stabilities. We further exfoliate the COFs via the facile, scalable and mild chemical exfoliation method to produce thin-layer structure with average thickness of about 22 nm. The e-COF-based capacitor cells achieve high areal capacitance (5.46 mF cm -2 at 1,000 mV s -1 ), high gravimetric power (55 kW kg -1 ), and relatively low τ 0 value (121 ms). More importantly, they performed nearly an ideal DL charge storage at high charge-discharge rate (up to 30,000 mV s -1 ) and maintained almost 100% capacitance stability even after 10,000 cycles. This study thus provides insights into the potential utilization of COF materials for EDLCs.

In the field of electrochemical energy storage (EES), electrochemical double-layer capacitors (EDLCs) are highly demanded electrical energy storage devices due to their higher power density with thousands of cycle life compared with pseudocapacitors and batteries. [1] The EDLCs involve rapid adsorption of electrolyte ions onto the electrode-electrolyte interfaces. [2] This charge storage system enables EDLCs to endure high charge-discharge rates and longlife cycles without deteriorating the electrode. Notably, a micro-patterned design, namely microcapacitor, can store and deliver charges at relatively high charge-discharge rate (>1,000 mV s -1 ), in which it is designed by shortening and aligning the ion diffusion path within graphitic-carbon electrode. [3,4] However, the only way to establish microcapacitor architecture relies on the laser-writing [5] and/or lithography techniques [6] that are complex and costly. Thus, it is highly desired to develop non-graphitic carbon capacitors with high charge-discharge rate performance.

Covalent organic frameworks (COFs) are a fascinating class of crystalline porous polymers, where their organic building units are precisely integrated into an extended structure with periodical skeleton and ordered pores. [7] The synthesis of COFs follows the reticular chemistry principle, which provides judicious control of two-or three-dimensional networks, porosity and physicochemical properties (stability, optical, conductivities and so on). [8] Specifically, 2D COFs generally possess eclipsed-layer stacking with typical 1Daligned perpendicular channel mimicking the structural backbone of graphene. [9] This structural feature provides an aligned diffusion pathway for ions or charges mobility and ensures efficient mass transport. In fact, 2D COFs have been extensively studied for proton [10] electrical [11] conductionand as charge carriers. [12] The results show that the 2D COFs with high surface areas are attractive for EES applications, especially for EDLC electrode construction. In the previous attempts, several 2D COFs have been tested as active materials for pseudocapacitor electrode construction. For instance, Dichtel et al. mixed DAAQ-TFP COF (35 wt%) and carbon black (60 wt%) to fabricate pseudocapacitor electrode for the first time, [13] while Jiang et al. utilized NiP-COF (20 wt%) to do so. [14] Furthermore, other 2D COFs have been designed and utilized for similar missions. [15] However, COF-based EDLCs have rarely been explored so far.

Herein, we prepared EDLC electrodes composed of exfoliated mesoporous 2D COFs (e-COFs) with a pore size of 3.4 nm and high specific surface areas. These capacitor cells performed nearly an ideal DL charge storage at high charge-discharge rate (up to 30,000 mV s -1 ), and achieved high areal capacitance (5.46 mF cm -2 at 1,000 mVs -1 ), relatively low τ 0 value (121 ms) and high gravimetric power (55 kW kg -1 ). In addition, they also maintained almost 100% capacitance stability even after 10,000 cycles. To the best of our knowledge, this is the first example of COF-based EDLCs that can demonstrate DL charge storage at high scan rate with superior areal capacitance and gravimetric power.

We designed mesoporous 2D COF series with high surface areas (termed as JUC-510, JUC-511, and JUC-512; JUC = Jilin University China) and further exfoliated them to obtain few-layer COFs for fabricating EDLC electrodes (Figure 1). These COFs were synthesized by employing a linear building block of hydroxyl containing 4,4'-(1,2-ethynediyl)bis-2hydroxybenzaldehyde (1) as the basic backbone (Figure 1a). [16] Condensing (1) with the square-shape 5,10,15,20-tetrakis[(4-aminophenyl)porphyrin ( 2) into an anhydrous ethanol/odichlorobenzene mixed solvent, the crystalline solid of JUC-510 was produced with a high yield (85%). Meanwhile, by using 5,10,15,20-tetrakis[(4-aminophenyl)porphyrinato]nickel(II)

(3) or 5,10,15,20-tetrakis[(4-aminophenyl)porphyrinato]copper(II) (4), both JUC-511 and JUC-512 were exclusively obtained with very convincing yields (~ 83%). The advantages of utilizing building block (1) can not only drive the formation of large pores but also possibly establish hydrogen bonding within the framework of targeted COFs, which has been reported to enhance the chemical stability of COFs to high extent. [17] The successful condensation among the building blocks was systematically studied by Fourier-transform infrared (FT-IR) and solid-state 13 C cross-polarization magic angle spinning (CP-MAS) NMR spectroscopies. The new stretching band emerged at 1590 cm -1 on the FT-IR spectrum of JUC-510 corresponds to the formation of imine (-C=N-) linkages (Figure S1, Supporting Information). This result was concomitant with the significant diminishment of stretching band at 1665 cm -1 assignable to the absorption band for carbonyl (C=O) group of (1) and at 3370 cm -1 corresponding to the stretching band of amine group of (2). A similar phenomenon was observed on the FT-IR spectra of both JUC-511 and JUC-512, in which the establishment of imine linkages was recorded at 1589 cm -1 and 1586 cm -1 , respectively (Figures S2 andS3, Supporting Information). Meanwhile, the solid-state 13 The crystallinity and unit cell parameters of COFs were resolved by powder X-ray diffraction (PXRD) analysis and further corroborated with the structural simulation using Materials Studio software package (version 7.0). [18] The structural models of COFs after a geometrical energy minimization reveal that they adopt P 4 space group with unit cell These refinement results offer unit cell parameters that are very close to the prediction with ideal agreement factors (R ωp = 1.29% and R p = 0.29% for JUC-510; R ωp = 2.10% and R p = 1.39% for JUC-511; and R ωp = 1.59% and R p = 1.02% for JUC-512, respectively). Indeed, these structural simulations further confirm that all COFs possess eclipsed AA layer-stacking mode with 3.3 nm predicted 1D square-like open channel (Figure 2b; Figures S7b,S8b and Tables S4-6, Supporting Information). We also anticipated another alternative space group (I 4 /m) for their structural models in which the proposed structures are in staggered AB layerstacking arrangement and the unit of adjacent sheets are horizontally offset by a distance of a/2 and b/2 (Figures S9-11 and Tables S7-9, Supporting Information). However, the PXRD profiles between the simulated and experimental patterns are not matched. On the basis of these results, the COFs are proposed as eclipsed-layer architectures with aligned mesoporous pores. We assume that the tendency of forming eclipsed layer stacking COFs is due to the presence of hydrogen bonding effects and the π-π interactions between the adjacent sheets.

Furthermore, the COFs are optic-active materials under solid-state UV-Vis spectroscopy (Figures S12-14, Supporting Information). JUC-511 absorbs a wide range of visible light (λ = 400-750 nm) with maximum adsorption at λ = 445 nm, 550 nm and 600 nm. Essentially, it exhibits broad Soret band centered at λ = 445 nm which is red-shifted 5 nm relative to (3), indicating the present of conjugated structure. The similar red-shifted phenomenon is observed in JUC-512 and JUC-510 samples, confirming the presence of similar conjugated structures in both COFs.

The porosity of COFs was evaluated by nitrogen (N 2 ) adsorption-desorption measurement at 77 K (Figure 2c; Figures S15 andS16, Supporting Information). For example, the adsorption isotherm of JUC-511 belongs to typical type-IV isotherm, indicating the mesoporous character. Specifically, the sharp increase of N 2 uptake amount at low pressure (P/P 0 < 0.05) indicates that the N 2 molecules feeding into the square-like pores rapidly. Meanwhile, the isotherm's inclination between P/P 0 = 0.2 and 0.4 is observed, which further confirms the presence of textural mesopores. The Brunauer-Emmett-Teller (BET) surface area of JUC-511 is about 842 m 2 /g. Meanwhile, the pore size distribution based on the non-local density functional theory (NL-DFT) confirms the presence of 3.4 nm open channel, which is very close with the predicted structure (3.3 nm). Furthermore, both JUC-510 and JUC-512 exhibit similar porosity character (BET surface area of 1170 m 2 /g and pore size of 3.4 nm for JUC-510 as well as BET surface area of 704 m 2 /g and pore size of 3.4 nm for JUC-512). JUC-510 achieves slightly higher BET surface area due to the absence of metal species on porphyrin moiety of JUC-510. The total pore volume determined at P/P 0 = 0.9 is 0.80 cm 3 /g, 0.71 cm 3 /g and 0.54 cm 3 /g for JUC-510, JUC-511 and JUC-512, respectively.

It is known that the stability of active materials is an essential issue for EDLC electrodes; hence these COFs' stabilities were further studied. Notably, the isolated COFs were chemically stable in common organic solvents (THF, MeOH, hexane, acetonitrile, acetone, and DMF for 24 h) and water (at 25 °C, 24 h). Furthermore, they maintained structural integrity in both 1 N HCl and 1 N NaOH at 25 °C overnight (Figures S17-19, Supporting Information). This remarkable structural stability may originate from the presence of intralayer hydrogen bonding, which interlocks the tetragonal 2D sheets in planar conformation. [19] Besides, the AA layer-stacking mode further amplifies the interaction to enhance structural integrity. On the other hand, the COFs exhibit high thermal stability up to 380 °C under thermal gravimetric analysis (TGA) (Figures S20-22, Supporting Information).

We further exfoliated above COFs via the facile, scalable and mild chemical exfoliation method to produce e-COFsOur approach is milder than the ball-milling method, [20] and it is able to to exfoliate the 2D COF thus providing the required thin sheets, which are The surface area reduction was clearly seen on e-COFs samples (Figures S38-40, Supporting Information). For instance, e-JUC-511 exhibits a BET surface area of 416.6 m 2 /g, which is 50% lower than that of pristine JUC-511. Meanwhile, both e-JUC-512 and e-JUC-510 manage a BET surface area of 336 m 2 /g and 666 m 2 /g, respectively. Indeed, those surface areas are much higher than many other reported covalent organic nanosheets (CONs). [21] In addition, the pore size of e-COFs remains essentially unchanged relative to their parent COFs.

All these results mentioned above indicate that the exfoliation treatment does not lead to serious structural deformation. In addition, e-COFs demonstrate notable thermal stability up to 350 °C (Figures S41-43, Supporting Information). Due to their high surface area and defined layer-structure with spacious pore size, e-COFs are promising materials for EDLC electrode.

In the prior analysis, CV tests were performed on both COF and e-COF capacitor cells in the full 1.3 V working potential window to access their charge storage behaviour (Figures S44-46, Supporting Information). The CV curves of e-COF capacitor cells exhibited almost ideal rectangular curves when cycled at a scan rate of 50 mV s -1 . These voltammogram features indicated the DL charge storage took place. In addition, no noticeable redox waves were observed, confirming that the DL charge storage was dominant over possible Faradicstorage. In contrast, the CV curves of COF capacitor cells demonstrated typical distorted rectangular shape when cycled at similar working potential window and scan rate. Their voltammograms features revealed the sluggish ion adsorption and inefficient charge storage.

These results emphazise that layer exfolation enables DL charge storage and allows for fast ion transport. We assume that it enhances the accessible surface area for charge occupation and shortens the ion diffusion path (Figure 1b). To prove this assumption, we further performed CV tests on e-COF capacitor cells at higher scan rate (100-1,000 mV s -1 ). Notably, retained rectangular voltammogram was demonstrated even at 1,000 mV s -1 , which reveals the high charge-discharge rate capability of e-COF capacitor cells (Figure 3d; Figure S47, Supporting Information). It is worth noting that this rate capability is rarely reported among conventional graphitic-carbon capacitors and other reported COF-based capacitors (commonly operated at scan rate of ≤ 100-500 mV s -1 ).

We further performed CV tests on e-COF capacitor cells at higher scan rates (3,000-10,000 mV s -1 ) to evaluate their rate capabilities and charge storage stabilities (Figure 4a-b).

Remarkably, the CV profiles of both e-JUC-511 and e-JUC-512 capacitor cells can retain rectangular voltammograms. These CV results reveal the significantly high rate capabilities and charge storage stablities of both capacitor cells in delivering and storing charges simultaneously. Noteworthy, this performance is maintained even at high scan rate (up to 30,000 mV s -1 ), which is two orders of magnitude higher than CV of conventional graphiticcarbon capacitors and comparable with microcapacitors (Figures S48 andS49, Supporting Information). The ability to store charges at high charge-dischage rate implies the fully accesbile surface area and the efficient mobility of the charges and ions within the entire electrode. Meanwhile, e-JUC-510 capacitor cell can also be cycled up to 7,000 mV s -1 , which is relatively lower rate performance compared to those of e-JUC-511 and e-JUC-512 capacitor cells. However, it is still relatively significant compared to conventional graphiticcarbon capacitors (Figure S50, Supporting Information). This result is fairly reasonable since both e-JUC-511 and e-JUC-512 contain electroactive metal (Ni and Cu), which enhance the ionic conductivity of corresponding e-COFs and the possible faradic-storage contribution. The redox peaks are not apparent on their CV curves, clarifying that DL storage dominates the storage mechanism. Since e-COF capacitor cells contained a minor amount of carbon-black as conductive agent (having microporous pore), CV test was further performed on conductive carbon capacitor cell to analize its contribution to the high rate performance of e-COF capacitor cells (Figure S51, Supporting Information). As clearly seen, this capacitor cell could only perform efficient DLs charge storage at low scan rate (100 mV s -1 ) and started to diminish at more higher scan rate (500-1,000 mV s -1 ). Once again, this result clearly emphasizes the high rate capability and efficient DL charge storage in e-COF capacitor cells. Indeed, the ability to sustain at a high rate is representative of high instantaneous power corresponding to high specific capacitance. [3] Notably, both e-JUC-511 and e-JUC-512 capacitor cells manage a high areal capacitance of 5.46 mF cm -2 and 5.85 mF cm -2 at 1,000 mVs -1 , respectively. Meanwhile, e-JUC-510 capacitor cell achieves areal capacitance of 4.17 mF cm -2 at similar scan rate (Table S1). Comparatively, those values are three-times higher than that from the state-of-the-art onion-like carbon (OLC) microcapacitor (1.7 mF cm -2 at 1,000 mVs -1 ), [3] much higher than other graphitic carbon micro-patterned capacitors (at a scan rate of 1-100 mVs -1 ) and laser-scribed graphene based capacitor (3.67 mF cm -2 , Table S2). [22] Considering their simple fabrication technique (without micro-patterned approach) and high charge-discharge rate capability, the areal capacitances of e-COF capacitor cells are superior to graphitic-carbon microcapacitors. Furthermore, as the manifestation of their high rate performance, both e-JUC-511 and e-JUC-512 capacitor cells show significant maximum power density (P max , per volummetric stack device) compared to several graphitic-carbon microcapacitors. For instance, power densities of 5.32 W cm -3 and 4.08 W cm -3 are successfully achieved by e-JUC-511 and e-JUC-512 capacitor cells, respectively. Again, those values are much higher than those of OLC microcapacitors (~ 1.0 W cm -3 ) and hydrated GO microcapacitors (1.7 W cm -3 ). Meanwhile, they also show moderate energy (E max , per volummetric stack device) of ~ 0.26 mWh cm -3 . Furthermore, considering the mass of the active material, both e-JUC-511 and e-JUC-512 capacitor cells manifest maximum gravimetric power (P gra ) of 55.0 kW kg -1 and 42.2 kW kg -1 , respectively. It is an exciting result that their gravimetrric powers are superior to graphitic-carbon based capacitors, including curved-graphene capacitors, nitrogen-doped mesoporous carbon capacitors and reduced graphene-oxide (rGO) capacitors (Table S3). Nevertheless, up to 50% of the areal capacitances is maintained by both capacitor cells, even at high scan rate (10,000 mV s -1 , Table S1). On the other hand, e-JUC-510 capacitor cell manages maximum power density of 1.02 W cm -3 , energy density of 0.23 mWh cm -3 , and gravimetric power of 9.93 kW kg -1 , respectively.

To further analyze the high rate capability of e-COF capacitor cells, the electrochemical impedance spectroscopy (EIS) method was applied. The analysis is conducted at a frequency range of 10 mHz-10 kHz with amplitude of 10 mV (Figure 4c-d; Figure S52, Supporting Information). The Nyquist plots of both e-JUC-511 and e-JUC-512 capacitor cells exhibit pure capacitive behavior even at high frequencies (0.38 kHz for e-JUC-511 and 0.32 kHz for e-JUC-512 capacitor cells, respectively). The results indicate the efficient charge mobility within the entire electrodes of both cells (Figure 4c). Nevertheless, they demonstrate small equivalent series resistance (ESR, obtained from the intercept of the real axis) of 10.8 Ω for e-JUC-511 and 15.5 Ω for e-JUC-512 capacitor cells. These small ESR values mean the excellent ionic conductivity and very low internal electrolyte-electrode resistance within the electrodes. Indeed, they also prove the efficient ion propagation and efficient formation of DL charge storage within electrodes of both cells. The high charge-dicharge rate performance of both capacitor cells can be attributed to the small ESR values. Interestingly, e-JUC-510 capacitor cell exhibits a little higher ESR value (60 Ω) with relatively lower capacitance response (detected at frequency of 68 Hz), exemplifying that the presence of electroactive metal plays the pivotal role in enhancing the ionic conductivity of the electrodes (Figure S52a, Supporting Information). Furthermore, the Bode plots of e-COF capacitor cells further amplify their high charge-dicharge rate performance (Figure 4d). Notably, both e-JUC-511 and e-JUC-512 capacitor cells exhibit high characteristic frequency constants (f o , obtained atphase = 45 o ) of 8.25 Hz and 5.62 Hz respectively, which indicates the fast and robust ion transport within the electrodes. Depending on these values, the relaxation time constants (τ 0 , a minimum time needed to discharge all the energy from electrode with an efficiency more than 50%) [3] are further determined. Remarkably, the τ 0 value is about 121 ms for e-JUC-511 and 178 ms for e-JUC-512, which represents the fast discharging process. Significantly, those τ 0 values are much lower than those of reported activated carbon microcapacitor (700 ms) and chemically reduced graphene oxide capacitor (188 ms). [3,24] Meanwhile, e-JUC-510 capacitor cell exhibits capacitive response of 68 Hz with relatively high τ 0 value (464 ms) (Figure S52b, Supporting Information). Nevertheless, those small τ 0 values further emphasize the high charge-discharge rate and superior capacitive response in e-COF capacitor cells.

The galvanostatic charge-discharge (GCD) method is also employed to explore the charge-discharge behaviors of e-COF capacitor cells. Interestingly, e-COF capacitor cells demonstrate an efficient charge-discharge process within second domain of time (Figures S53-55, Supporting Information). Specifically, they exhibit nearly ideal triangular curves without obvious voltage drop, indicating the simultaneous and rapid charge-discharge within each electrode over potential applied. This result further reveals the effcient charge storage and fast ion adsorption behaviors within e-COF capacitor cells, which is much faster than batteries, conventional capacitors and in-line with those of microcapacitors. In brief, the CV, EIS and GCD analyses provide clear evidence of the high charge-discharge rate and superior capacitive performance of e-COF capacitor cells. These results, however, corroborate that layer exfoliation method provides fast ion mobility by providing shorter ion transport path within the entire active surface of the electrodes. Thus, it allows for efficient DL charge storage and enhances power performance.

From the commercial and industrial point of view, the robustness of the capacitor cell is a crucial aspect. The stability and reliability issues are essential parameters for electrical energy storage materials. In view of these characteristics, the stability and reliability of both e-JUC-511 and e-JUC-512 capacitor cells were evaluated due to their significant power performance (Figure 5; Figures S56 andS57, Supporting Information). Notably, both e-JUC-511 and e-JUC-512 capacitor cells maintained nearly 100% capacitance retention even after 10,000 consecutive cycles (Figure 5a). In consideration of practical applications, capacitors are usually assembled either in parallel or series configurations to reach certain operating voltage or output current. The adaptability of e-COF capacitor cells for serial and parallel configurations was demonstrated by connecting three coins (packaged in alligator clip, Figure 5b-e; Figures S56 andS57, Supporting Information). As clearly depicted, the tandem cells exhibited good control over the operating voltage window and capacity when the voltage range was increased in parallel of series combination. Essentially, in both configurations, e-COF capacitor cells established almost ideal rectangular in shape voltammograms and triangular charge-discharge curves without significant voltage drop, which again proves the robust and reliability of e-COF capacitor cells. All these results reveal the reliable performance of e-COF capacitor cells as prospective EES material.

In addition, the capacitive performances of e-COF capacitor cells were compared with other energy-storage devices (commercial battery and capacitor) presented in the Ragone plot (Figure 6). The plot presents the power and energy densities of all devices being compared.

To this comparison, both power and energy densities of e-COF capacitor cells were calculated based on the volummetric stack of the assembled coin-cell, including the micron thickness of the active electrode, current collector and separator as well. Notably, e-COF capacitor cells exhibited maximum power density of 4.1-5.4 W cm -3 , which are two-order of magnitude higher than Li thin-film battery, one-order of magnitude higher than commercial activated carbon and comparable with Al electrolytic capacitor. Meanwhile, they showed a moderate maximum energy density (~ 0.27 mWh cm -3 ), which is two-order of magnitude higher than Al electrolytic capacitor and comparable with activated carbon electrochemical capacitor. Indeed, this plot placed e-COF capacitor cells as potential power supply. Finally, we expected that the energy and power densities could be potentially increased with improved processing and device optimization by extending the working potential window. Based on these promissing results, optimization studies will be performed in order to further improve the electrochemical double-layer capacitator performance.

In summary, we have demonstrated that COFs, as highly crystalline and porous materials are prospective active candidates for construction EDLCs. We prepared capacitor cells composed of exfoliated 2D mesoporous COFs, which showed an ideal DL charge storage and high charge-discharge rate, a rare phenomenon in conventional graphitic-carbon and COFsbased capacitors. Moreover, compared to graphitic-carbon microcapacitors and conventional capacitors, they exhibited superior areal capacitance (5.46 mF cm -2 at 1,000 mV s -1 ) and gravimetric power (55 kW kg -1 ). Indeed, these preliminary results could provide an early view for further researches in employing COFs as potential active materials for ELDC electrodes.

We also forecast that COFs hold great promise for microcapacitors that we expect to show more sophisticated performance. commercial AC-EC, and Al electrolytic capacitor are interpolated from ref. [22].

The table of contents entry:

Exfoliated mesoporous 2D COFs (e-COFs) have been prepared and utilized for EDLC electrode construction. The-e-COF capacitor cells perform double-layer charge storage at a high charge-discharge rate (up to 30,000 mVs -1 ) and exhibit low τ 0 value. The e-COF capacitor cells achieve areal capacitance and gravimetric power that are superior to graphiticcarbon microcapacitors and conventional capacitors and also maintain almost 100% capacitance retention after 10,000 cycles.

  C CP-MAS NMR spectroscopy further confirmed the formation of carbon-nitrogen species in JUC-510, 511, and 512, where clear resonance signals at δ = 160 ppm corresponding to the carbon atom of the imine linkage were observed (Figures S4-6, Supporting Information). The crystal morphology was examined by scanning electron microscopy (SEM, Figures S23-25, Supporting Information) and transmission electron microscopy (TEM, Figure 2d; Figures S26-28, Supporting Information), which showed isometric crystals with highly aggregated rod-like morphology.

  parameters of a = b = 45.95 Å, c = 3.65 Å and α = β = γ = 90° for JUC-510; a = b = 45.91 Å, c = 3.66 Å and α = β = γ = 90° for JUC-511; and a = b = 45.60 Å, c = 3.71 Å and α = β = γ = 90° for JUC-512, respectively. The PXRD patterns of the isolated COFs are in good agreement with the simulated patterns as further proved by full profile pattern matching (Pawley) refinements (Figure 2a; Figures S7a and S8a, Supporting Information). The intense diffraction peak at 2θ = 2.76° corresponds to (110) Bragg peak and additional moderate peaks at 2θ = 3.90°, 5.82° and 8.21° are assignable to (200), (300) and (330) Bragg peaks, respectively. Meanwhile, the broad peaks observed at 2θ > 20° (corresponding to (001) Bragg peak) indicate the formation of layered structure strengthened by intralayer π-π interactions.

  indispensible to reach high capacitance and relatively low τ 0 values. The SEM images of e-COFs display fined-layer like morphologies, which fundamentally contrast with their parent COFs (Figures S28-30, Supporting Information). Furthermore, the atomic force microscopy (AFM) analysis reveals that e-COFs establish thin-layer structure with average thickness of ~ 22 nm (Figure 3a; Figures S31-33, Supporting Information). On the contrary, their parent COFs possess typical aggregate particles and are much thicker (215-389 nm). TEM images of e-COFs shown typical exfoliated layer-like structures which corroborate with their SEM images (Figure 3b; Figures S34 and S35, Supporting Information). Since COFs are electronbeam sensitive materials, it is challenging to obtain the diffraction pattern under selective area electron diffraction (SAED)-TEM method. However, additional live-FFT capture of certain areas of e-COFs confirms the presence of ordered lattice. Furthermore, their PXRD patterns reveal the preservation of the major peaks relative to pristine COFs (Figure 3c; Figures S36 and S37, Supporting Information).

FiguresFigure 1

 1 Figures

Figure 3 .

 3 Figure 3. Microstructural and electrochemical analyses of e-JUC-511: a) AFM image of e-

Figure 4 .

 4 Figure 4. Capacitive performances of e-COF capacitor cells: a) CV curves of e-JUC-511capacitor cell at scan rate 3,000-10,000 mV s -1 with preservation of rectangular shape. b) CV curves of e-JUC-512 capacitor cell at scan rate 3,000-10,000 mV s -1 with preservation of

Figure 5 .

 5 Figure 5. Stability and reliability analyses of e-COF capacitor cells: a) Capacitance retention

Figure 6 .

 6 Figure 6. Ragone plot of e-COF capacitor cells where the power density and energy density

Keyword:

  Covalent organic frameworks, chemical exfoliation, high rate, double-layer capacitor Y. Yusran, H. Li, X. Guan, D. Li, L. Tang, M. Xue, Z. Zhuang, V. Valtchev, Y. Yan, S. Qiu, and Q. Fang* Exfoliated Mesoporous 2D Covalent Organic Frameworks for High-Rate Electrochemical Double-Layer Capacitors

Acknowledgements

This work was supported by National Natural Science Foundation of China (21571079, 21621001, 21390394, 21571076, and 21571078), 111 project (B07016 and B17020), and the program for JLU Science and Technology Innovative Research Team. V.V. and Q.F. acknowledge the Thousand Talents program (China). V.V., S.Q. and Q.F. acknowledge funding from the French-Sino International Laboratory (LIA) Zeolites.

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Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.