Dr Julien A Christmann

Ahmad Ibrahim

Vincent Charlot

Celine Croutxé-Barghorn

Christian Ley

Xavier Allonas
email: x.allonas@uha.fr

Prof X Ibrahim

Dr V Allonas

Prof. C Charlot

Prof C Croutxé-Barghorn

Ley

Elucidation of the Key Role of [Ru(bpy)3]2+ in Photocatalyzed RAFT Polymerization

scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Organometallic complexes, typified by [Ru II (bpy)3]Cl2 (bpy = 2,2'bipyridine), are well-known compounds since the 1970's. They exhibit interesting photochemical properties, such as absorption in the visible region and photoreaction through photoinduced single-electron oxidation or reduction. [1][2][3][4][5] However, since these pioneering works, few studies have been reported until 2008. From this date, several groups started to exploit the photocatalytic ability of such complexes for a broad range of visible light induced organic reactions under mild conditions: alkylation, dehalogenation, cyclization, cycloaddition... [6][7][8][9][10][11][12][13][14][15] Photoinduced electron transfer (PeT) has been proposed to explain most of these reactions while triplet energy transfer (P3T) has only scarcely been reported to be involved. [14][15] In parallel, groups have begun to use organometallic complexes to initiate Free Radical Photopolymerization (FRP). [16][17] One of its major limitations is a lack of control of the macromolecular mass and chain distribution due to transfer and termination reactions. Controlled photopolymerization reactions have been subsequently proposed. [18][19][20][21] Among them, Reversible Addition-Fragmentation chain Transfer (RAFT) photopolymerization have been initially based on the dissociation of RAFT agents under UV light. [22][23][24][25] Generated radicals further initiate and control the radical polymerization. Detrimental UV lamps were afterwards replaced by visible light sources thank to the introduction of organometallic complexes. [26][27][28][29][30] Excited complexes act then as photosensitizers for the dissociation of the RAFT agents. Most recent studies have reported direct dissociation of RAFT molecules under blue and green light irradiation. [31][32][START_REF] Xu | Controlled Radical Polymerization: Mechanisms[END_REF] Although that the key-role of organometallic complexes has been highlighted in photosensitized systems, no detailed investigation was made to clarify the mechanism. [26][27][28][29][30] The aim of this paper is to understand the photochemical mechanism implied in RAFT polymerization photocatalyzed by [Ru II (bpy)3]Cl2. Photopolymerization experiments of methyl acrylate will be discussed in the light of the involved mechanism.

As mentioned above, two different mechanisms can be proposed to describe the photocatalysis of the RAFT process by [Ru II (bpy)3] 2+ : photoinduced electron and energy transfer. For electron transfer, initiating and transfer radicals are separately produced, as proposed in Scheme 1a. [26] Initiating radicals are generated by the bimolecular electron transfer from the excited Ru complex to a RAFT (macro)molecule. Transfer species result from the bimolecular reaction of the oxidized Ru complex with the reduced form of the RAFT agent. However, the involvement of two consecutive bimolecular reactions tends to lower the probability of the radical production. In energy transfer mechanism, both radicals are produced through a dissociation of the C -S bond of the excited RAFT (macro)molecule (Scheme 1 b). In both cases, addition-fragmentation controls the macromolecular mass distribution (Scheme 1 c). In order to elucidate the actual photochemical mechanism, eight commercial RAFT agents were selected. Structures of [Ru II (bpy)3]Cl2 (1) and the RAFT agents (2 -9) are shown in Figure 1. The Gibbs free-energy change of the photoinduced electron transfer reaction GPeT between an electron donor D and an acceptor A is calculated according to the Rehm-Weller equation: Quenching rate constants (kq) of ( 1) by the RAFT agents were deduced from Stern-Volmer plots, using Laser Flash Photolysis (LFP) experiments. [START_REF] Turro | Modern Molecular Photochemistry of Organic Molecules[END_REF] All these data are collected into Table 1.

Electron transfer reaction

Log(kq) is plotted as a function of GPeT with (1) taken either as an oxidant or a reductant (Figure 2). With (1) as an oxidant, no clear relationship appears between log(kq) and GPeT. By contrast, the kq values increase with increasing GPeT values.

When (1) acts as a reductant, experimental values would better follow a Rhem-Weller behaviour. However, high values of kq, as found for ( 4) to ( 7) are not consistent with the corresponding positive values of Gibbs free-energy. An electron transfer mechanism thus appears unlikely. In order to confirm this contention, attempt was made to detect the formation of oxidized Ru complex by laser flash photolysis. First, the transient absorption spectrum of (1) was recorded in acetonitrile after laser flash (Figure 3a). As known, the triplet state of Ru complex absorbs at 360 nm, and the photobleaching of the ground state appears around 450 nm. In the presence of iodonium salt (Irgacure 250, I250) as strong oxidant (Figure 3b), a new absorption band appears at 420 nm which is attributed to [Ru III (bpy)3] 3+ . [2] The ground state of (1) at 440 nm also exhibits a partial photobleaching as a consequence of the long-lived oxidized form of Ru complex.

Compound

Eox / SCE (V) [a] GPeT Ru=ox (kJ.mol -1 )

Ered / SCE (V) [a] GPeT Ru=red (kJ.mol -1 )

ET rel (kJ.mol -1 )

GP3T

(kJ.mol -1 ) [a] "> 2" or "< -2" refers to values out of the analysis range offered by our setup. [b] From [1-2].

kq (M -1 .s -1 ) 1
(

) (6) (7) (2) (3) (4) (8) (9) 5
Figure 2. Plot of log(kq) as a function of GPeT for (1) as an oxidant (•) and a reductant (▲).

However, when replacing the iodonium salt by RAFT agents, no such band appears at 420 nm (Figure 3c, also see SI for other RAFT agents), evidencing that that no oxidation of (1) occurs. Moreover, transient kinetics at 440nm, corresponding to the Ru ground state photobleaching, clearly shows that no long-lived transient is formed after triplet quenching by the RAFT agents. It should be noticed that direct excitation of RAFT agents hardly leads to detectable transient. All these results evidence that photoinduced electron transfer is not involved. This, in turn, suggests that energy transfer would be favoured in this reaction.

Energy transfer reaction

The lack of experimental evidence of electron transfer in laser flash photolysis experiments suggests that energy transfer is involved. In order to evaluate the feasibility of such energy transfer reaction from the triplet excited Ru complex to the RAFT agents, the triplet state energy of these latter were calculated using density functional theory method (Table 1). All relaxed triplet energies ET rel of the RAFT agents were found to be lower than that of (1). This enables exergonic energy transfer reaction to occur. A plot of log(kq) as a function of GP3T highlights the monotonous change in the rate constant with the free energy of the reaction, a plateau value close to the diffusion limit being found for (3). Therefore triplet energy transfer clearly explains the sensitization reaction of RAFT agents by Ru complex. The kq values can be successfully fitted by the Balzani's model, assuming kdiff = k-diff ≈ 2.10 10 M -1 .s -1 in MeCN (Figure 4): [START_REF] Balzani | [END_REF] ΔGP3T # (0) represents the activation free energy when ΔGP3T = 0 and is related to the reorganization energy of the reactants after the reaction. k 0 en is the pre-exponential factor of the rate constant, i.e. when ΔGP3T # = 0.

Values of 24.4 kJ.mol -1 and 3.1.10 10 M -1 .s -1 were respectively found for ΔGP3T # (0) and k 0 en. The high value of k 0 en simply indicates that the energy transfer preexponential factor is higher than the diffusion rate constant. The relatively high value of ΔGP3T # (0) highlights a large molecular reorganization of the compounds after the reaction. As the structure of the Ru complex is quite rigid, this would indicate that the triplet states of the RAFT agents are distorted with respect to their ground states. [37][38][39] This may somehow be related to a predissociative triplet state.

Therefore, energy transfer process is in agreement with the transient absorption experiments described above.

Photosensitized controlled polymerization

Polymerization of poly(methylacrylate) (PMA) was performed in presence of [Ru II (bpy)3]Cl2 and the different RAFT agents with a 455 nm LED in tetrahydrofuran. Polymer conversions were measured for two different irradiation times (10 and 40 min) by FTIR spectroscopy. Mean-average molecular mass ( 𝑀 ̅ 𝑛 ) and PolyDispersity Index (PDI) of the obtained polymers were characterized by steric exclusion chromatography (Table 2 and Figure 5). No inhibition time could be noticed for these experiments performed under Ar atmosphere.

As seen on Figure 5, only two RAFT agents lead to efficient polymerization (7 and 9). 𝑀 ̅ n are about 10,000-16,000 g.mol -1 and PDI 1.9. The effectiveness of the polymerization is partly governed by the dissociation free energy (Gdiss) of the RAFT agent:

Gdiss = BDE -ET rel
Eq. 4 BDE stands for the bond dissociation energy of the RAFT agent as estimated by DFT. These values are collected in Table 2. Figure 5 shows that there is a clear relationship between the monomer conversion and the photodissociation reaction. No photosensitized bond-breaking reaction is expected for compounds (4), ( 5) and ( 6) due to unfavorable thermodynamics (BDE > ET rel ). For compounds (2), ( 3) and (7), Gdiss values are close to zero and consequently the conversion are low. Differences in conversion values merely arise from differences in

Conv at 40 min (%)

G diss / kJ.mol chemical structures. A detailed investigation of these effects is out of the scope of this paper. By contrast, the thermodynamics of ( 9) is highly favorable. Consequently, it leads to the best conversion of MA. It can be concluded that the triplet state of ( 9) is sufficiently energetic to dissociate and to create the initiating radicals. SEC results confirm that a polymer was obtained which exhibits PDI = 1.9, corresponding to a well-controlled polymerization for such a free radical process.

In conclusion, the key role of [Ru II (bpy)3]Cl2 as photocatalyst for the RAFT photopolymerization of methyl acrylate has been elucidated. Mechanistic studies by LFP and calculations of ΔG changes indicated that photoinduced electron transfer (PeT) is disfavored. On the contrary, triplet photoinduced energy transfer (P3T) mechanism appears to match quite well the experimental data using the computed triplet energies of the RAFT agents. Balzani's model has been successfully applied to the experimental data. In addition, photosensitized controlled polymerization of MA has also emphasized the importance of the bond dissociation energy (BDE) of the RAFT agent. The better understanding of the photochemical mechanisms involved will offer new perspectives for the development of controlled radical photopolymerization photosensitized with visible light.

Methods

Complete name, origin and purity of the chemical compounds are detailed in the SI. Redox potentials were determined by cyclic voltammetry, as described elsewhere. [40] Quantum mechanical calculations were performed at (U)B3LYP/6-31G* (for geometry optimization) and (U)B3LYP/6-311++G(d,p) (for single point calculations) levels, using Gaussian 03. Description of the complete methodology is presented in the SI. Description of the setup and the experimental conditions for laser flash photolysis experiments is also given in the SI. A typical formulation for the polymerization of MA was composed of 6.7 g of MA (3.15 mol.L -1 ), ruthenium complex (0.04 wt% -0.13 mmol.L -1 ) and RAFT agent (0.4 wt%), completed to 25 mL with tetrahydrofuran (THF). Irradiation was ensured by a 455-nm blue LED (Phoseon -Firejet FJ200) set at 8W and located at 5 cm from the cuvette containing the formulation. Monomer conversion was studied with a Real-Time Fourier Transform InfraRed (RT-FTIR) spectrometer (Brucker Optics -Vertex 70) equipped with a DTGS detector and used in stationary mode. The conversion was determined by the decrease of the area of the C=C vibration band of MA in near-infrared (4,739 cm -1 ) and calculated with the following formula:

Conv (%) = (A 0 -A t ) A 0 x 100
Eq. 5 where A0 and At represent respectively the absorption band area before exposure and at a time t. Complementary details are set in the SI. Mean-average molecular mass (𝑀 ̅ n) and PolyDispersity Index (PDI) of the formed PMA were determined using Size Exclusion Chromatography (SEC). Analyses were performed on a Shimadzu LC-20AD liquid chromatograph. Molecular masses are given in polystyrene equivalents. Experimental conditions are further described in the SI.

Keywords: photocatalysis • energy transfer • computational chemistry • RAFT photopolymerization

Scheme 1 .

 1 Scheme 1. a) Photoinduced electron transfer (PeT) mechanism (adapted from [26]). b) Triplet photoinduced energy transfer (P3T) mechanism c) Reversible addition-fragmentation transfer mechanism.

Figure 1 .

 1 Figure 1. Structure of the compounds studied: [Ru II (bpy)3]Cl2 (1) and RAFT agents (2 -9).

Eq. ( 1 )

 1 Eox(D) -Ered(A) -E* + C Eox and Ered refer to oxidation and reduction potentials, respectively. E* stands for the triplet energy of Ru complex and C is a Coulombic term which could be neglected in polar solvents. Relaxed triplet energies of the RAFT agents (2 -9) were calculated by Quantum Mechanics (QM) at the Density Functional Theory (DFT) level. The possibility of an energy transfer is estimated through calculation of the Gibbs free-energy variation GP3T:  GP3T = E*(A) -E*(D) Eq. (2) E*(A) and E*(D) are the triplet state energies of the Acceptor and Donor, respectively.

Figure 3 .

 3 Figure 3. Transient absorption spectrum in MeCN of a) [Ru II (bpy)3] 2+ . b) [Ru II (bpy)3] 2+ + I250. c) [Ru II (bpy)3] 2+ + (5).

Figure 4 .

 4 Figure 4. Plot of log(kq) as a function of GP3T and corresponding fit with Balzani's energy transfer model.

Figure 5 .

 5 Figure 5. Plot of conv at 40 min (%) as a function of ΔGdiss.

Table 1 :

 1 Redox potentials, relaxed triplet energy, corresponding Gibbs free-energies and quenching rate constant of the studied compounds.

Table 2 :

 2 Conversions, mean-average molecular mass, polydispersity index of poly(methylacrylate) after 10 and 40 min irradiation, bond-dissociation and relaxed triplet energies of RAFT agents and associated dissociation free energy from their triplet state.

	Compound	Conv (%) -10 / 40 min	𝑀 ̅ n (g.mol -1 ) -10 / 40 min	PDI -10 / 40 min	BDE (kJ.mol -1 )	ET rel (kJ.mol -1 )	ΔGdiss (kJ.mol -1 )
	2	≈ 0 / 0	/	/	143.0	137.2	5.8
	3	≈ 0 / 0	/	/	90.0	≥ 86.1	3.9
	4	≈ 0 / 0	/	/	140.9	120.1	20.8
	5	≈ 0 / 0	/	/	179.8	159.2	20.7
	6	≈ 0 / 0	/	/	186.5	162.7	23.8
	7	5.8 / 17.3	24,000 / 16,000	1.93 / 1.92	170.7	≥ 165.1	5.6
	8 [a]	/	/	/	191.2	187.8	3.5
	9	30.2 / 52.0	10,600 / 10,000	1.85 / 1.92	173.0	201.2	-28.2
	[a] No experiment was possible with this compound.

Acknowledgements: ANR and Mäder are fully acknowledged for financial support of the DeepCure project #ANR-13-CHIN-0004-01.


Kristoffer H Møller

Maxime Debost

Louwanda Lakiss

Søren Kegnaes

Svetlana Mintova

Interzeolite conversion of micronsized FAU to nanosized CHA zeolite free of organic structure directing agent with a high CO 2 capacity

Interzeolite conversion of a micronsized FAU to a nanosized CHA zeolite free of organic structure directing agent with a high CO 2 capacity

Introduction

Zeolites are unique ordered microporous aluminosilicates which have found many uses in the chemical industry. [START_REF] Maesen | Introduction to Zeolite Science and Practice[END_REF] Zeolites have pores, channels, and cages of molecular dimensions and this connected porous network can be used for molecular sieving. Molecular sieving is a very useful property in both catalysis [START_REF] Liu | [END_REF][3][4][5][6] and molecules separation in gas and liquid phases 7- 9 . With more than 240 [START_REF]International Zeolite Association Structure Commission, Database Zeolite Struct[END_REF] different zeolite framework structures, a key parameter is finding the zeolite structure that fits the right needs for the desired application. One zeolite structure that has received much attention lately is the small pore chabazite (CHA). [START_REF] Li | [END_REF][12][13][14][15][16] The chabazite zeolite consists of double six ring (d6r) rings and cha composite building units (cages). The cha composite building units are connected through 8 ring openings making the channels of about 3.8 Å that run in three dimensions. [START_REF] Baerlocher | Atlas of Zeolite Framework Types[END_REF] One of the applications where the chabazite zeolite has shown to be useful is in the automotive industry where it acts as a catalyst support for the selective catalytic reduction (SCR) reaction in diesel vehicles due to its high stability and activity 14,[START_REF] Korhonen | [END_REF][19][20] . Other studies have shown that chabazite can be useful in gas adsorption and separation, where it can be used for natural gas or flue gas purification and for carbon capture due to the size selectivity for gasses. 7,21,22 Chabazite can be synthesized using organic structure directing agents (OSDAs) which are typically costly and require calcination of the zeolite to remove the organic template. [23][24][25][26][27] Another way to obtain the CHA zeolite is through interzeolite transformation where only inorganic structure directing agents are used. 13,[28][START_REF] Robson | Verified Syntheses of Zeolitic Materials[END_REF][START_REF] Van Tendeloo | [END_REF][31] CHA obtained by an interzeolite transformation of faujasite (FAU) has been reported, however, only limited research has been dedicated on the effect of the starting material on the size and morphology of the final CHA crystals.

In the application of zeolites, mass transfer and diffusion of molecules through zeolite channels are very important and size of both zeolite crystals and pores have an effect on the utilization of the zeolite. 32 Higher utilization of the zeolite can be accomplished by introducing an additional porosity in the zeolite or by decreasing the size to reduce the mean diffusion path-length for molecules. [33][34][START_REF] Van Den Bergh | Zeolites and Catalysis[END_REF] Herein, we present the successful interzeolite transformation of micronsized FAU zeolite to nanosized CHA zeolite via alkali treatment. The composition, size, morphology, and adsorption characteristics of the produced nanosized CHA zeolites made from different FAU zeolites were studied and showed that the starting FAU zeolite has a large impact on the size, composition, and morphology of the final CHA zeolite product.

Experimental Materials

Commercially available chemical were used as received. NaOH (99%, VWR chemicals), KOH (90%, flakes, Sigma Aldrich), water (double distilled), FAU(2.6) (CBV400, Zeolyst) zeolite, FAU(15) (CBV720, Zeolyst) zeolite, H3BO3 (99.8%, Sigma Aldrich), HF (40%, Fisher chemical), HCl (37 %, VWR), HNO3 (65%, carlo erba) CO2 (99.9999, air liquide), CO(99.9999, air liquide) Synthesis 36 with modifications to the synthesis composition. NaOH (40 wt% in water), KOH (45 wt% in water), and water were mixed in a polypropylene container (PP, Nalgene) before FAU(2.6) was added to the container. The mixture was shaken for 30 s before it was put in a preheated oven at 95 °C under static conditions. After heating, the solid was isolated by centrifugation (20.000 rpm, 20 min), washed with water until neutral pH of the wash water, and dried at 50 °C overnight. See Table S1 in supporting information for details on amounts. Synthesis of CHA from high silica FAU Synthesis of CHA from FAU (15) zeolite was performed in a similar way to the as described above. KOH (45 wt% in H2O) was mixed with water in a PP container before CBV720 was added to the container. The mixture was shaken for 30 s before it was put in a preheated oven at 95 °C under static conditions. The solids were isolated by centrifugation (20.000 rpm for 20 min). The solid were washed with water by first dispersing the solid in 90 °C water followed by sonicated for 20 min and isolation by centrifugation (20.000 rpm, 20 min). The washing procedure was repeated until neutral pH of the wash water. The solid was dried at 50 °C overnight. See Table S1 in supporting information for details on amounts. Characterization X-ray powder diffraction was performed using a PANalytical X'Pert Pro diffractometer with CuKα monochromatized radiation (λ = 1.5418 Å). Nitrogen physisorption was performed on a Micromeretics 3 flex at liquid nitrogen temperature (77 K). The samples were degassed at 350 °C under vacuum prior to analysis. The surface area was determined by the Brunauer-Emmett-Teller (BET) method and mircropore volume by the tplot method. The total pore volume was determined from a single point at P/Po=0.97 on the adsorption branch. CO2 adsorption was performed on a Micromeretics ASAP 2020 at 273.15 K. The sample was degassed at 350 °C under vacuum prior to analysis. Scanning electron microscopy (SEM) was performed using a MIRA-LMH (TESCAN) fitted with a field emission gun using an accelerating voltage of 30.0 kV. The samples were dispersed on carbon tape and coated with Pt and Pd prior to analysis. Transmission electron microscopy (TEM) was performed on a FEI Tecnai T20 G2 microscope operating at 200 kV. The samples were dispersed on lacey carbon grid prior to analysis. Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) was performed using a Varian ICP-OES 720-ES instrument. 50 mg of solid samples was dissolved in 0.5 mL aqua regia and 3 mL HF at 110 °C for 1 h in a PTFE beaker and cooled to rt. After that, boric acid (1.8 g) was added to the beaker along with 96.5 g of water and shaken overnight for homogenization. In-situ infrared spectroscopy with CO2 as probe molecule was performed using a Nicolet 6700 IR spectrometer (Thermo Scientific, Villebon sur Yvette, France) equipped with a mercury cadmium telluride (MCT) detector and an extended KBr beam splitter. The spectra were recorded with 128 scans and a resolution of 4 cm -1 . For analysis, the solid was pressed as a self-supported wafer (about 20 mg for a disc of 2.01 cm 2 ). Prior to CO2 adsorption, the solid was activated by heating the pellet at 350 °C for 2 h with a 3 h heating ramp under vacuum (~10 -5 torr) which was then allowed to cool to room temperature. A spectrum with the activated pellet was used as a background and subtracted from the obtained spectra recorded at different concentrations of CO2. Additionally, in-situ IR spectroscopy study using CO as probe molecule was performed; the sample was activated at 350 °C for 2 h under vacuum (~10 -5 torr). The IR spectra under CO delivery were recorded at 77 K.

Results and Discussion

Materials and Characterization

Interzeolite transformation of micronsized FAU zeolites with an initial Si/Al ratios of 15 (FAU( 15)) and 2.6 (FAU(2.6)) to CHA was performed under treatment at 95 °C for 4-7 days in solution with the following chemical composition: vSi/xK/yNa/zH2O ratio of v=1, x=0.76-1, y=0-0.25, z=30-40. The preparation of nanosized CHA zeolites is based on a modified version reported by Bourgogne et al. 36 using a purely inorganic synthesis approach. In general, the choice of reactions conditions, silica/alumina source and Si/Al ratio strongly affects the size and morphology of resulting crystal. [START_REF] Takata | [END_REF] In this study, the Si/Al ratio of FAU (2.6 to 15 from Zeolyst) and the concentration of cations are varied to study the effect on the physicochemical properties of formed nanosized CHA materials under mild conditions (95 °C). In comparison, J. Kim and D. H. Kim previously produced CHA crystals of 300-500 nm by an interzeolite transformation of FAU (from Zeolyst) with Si/Al ratio of 6 at 140 °C. 39 Moreover from previous studies on interzeolite transformation of FAU to CHA via an OSDA-free synthesis it has been only successfully shown for FAU with a Si/Al = 6 or below, while studies using higher Si/Al ratio FAU are reported only via synthesis with OSDA and higher temperature. 31 T. Takata et al has previously shown that it is possible to form nanosized CHA crystals from FAU with Si/Al =15 and higher in the presence of a OSDA 37 . Our study is exclusively focusing on the OSDA-free synthesis. Details of selected experiments are presented in Table S1, supporting information (SI).

X-ray Powder diffraction

The X-ray powder diffraction (XRPD) patterns for the starting materials FAU (15) and FAU(2.6) are shown in Figure 1. The XRPD patterns of corresponding products (CHA(2.3) and CHA(1.9)) after 7 and 4 days interzeolite transformations, respectively, are also shown in Figure 1. The diffraction patterns clearly show intensive peaks corresponding to the CHA type framework structure. Figure 1 clearly indicate successful interzeolite transformations since no traces of FAU zeolite are observed. A minor impurity peak likely corresponding to GME type zeolite in sample CHA(1.9) is observed (Fig. 1D). The presence of GME peak and the combination of broad and narrow peaks indicates the presence of stacking fault in CHA(1.9) as described in literature. 38,39 For CHA(2.3) there is no GME peak, however, Please do not adjust margins Please do not adjust margins there is a combination of broad and narrow peaks, which could indicate minor faulting or direction oriented growth. Interzeolite transformation of micronsized FAU (15) to CHA (Table S1, entry 4) results in CHA with a Si/Al ratio of 2.3 (CHA(2.3)), whereas the interzeolite transformation of low silica FAU(2.6) (Table S1, entry 1) results in CHA with a Si/Al ratio of 1.9. During this interzeolite transformation, only a part of the Si is retained in the CHA product from the starting FAU, however, the Si/Al of the starting materials has an influence on the chemical composition of the product. As evidenced, a higher Si/Al ratio in the starting FAU material results in a higher Si/Al in the product CHA. Considering the relative amount of Si going into the CHA zeolite, less of the initial Si is retained in the CHA(2.3) from FAU(15) compared to CHA(1.9) from FAU(2.6). The remaining Si is likely present as unreacted species in the reaction medium, lowering the yield of the zeolite. The difference in composition between samples CHA(2.3) and CHA(1.9) could potentially affect the application of the materials, since a higher Si/Al normally results in a higher thermal stability [START_REF] Mccusker | Introduction to Zeolite Science and Practice[END_REF] , whereas a lower Si/Al has shown a stronger effect in selectivity for different gasses 22 .

Electron microscopy

The size and morphology of the zeolites were assessed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM image of FAU(15) (Figure S1 A) shows that it consists of zeolite crystals of varying sizes (200-1000 nm) with a little rough surface. After interzeolite transformation to the product CHA(2.3) the crystals appearance changed. CHA(2.3) consists of agglomerates (~500 nm in size) of smaller zeolite crystals of 50-100 nm (Figures S1 B andS2). The shaped of the agglomerated indicates some oriented growth of the nano-sized crystallites. The mechanism for interzeolite transformation has been suggested to proceed through a dissolution of the zeolite crystals that breaks up into smaller building units which then are used to construct new zeolite form. [START_REF] Li | [END_REF][42][43][44] These nanoparts are then directed by the K + to form CHA rather than other zeolite structures which are directed by different cations, as described by Van Tendeloo et al. [START_REF] Van Tendeloo | [END_REF] It is speculated if this is the case and that the mesopores in the FAU(15) could facilitate a dissolution of the crystal into smaller crystals which would result in the formation of smaller CHA zeolite crystals. It is unknown whether the zeolites dissolves and recrystallize in the solution or on the surface of the dissolving FAU zeolites. For the interzeolite transformation of FAU(2.6), the parent zeolite consists of both larger zeolite crystals of 200-1000 nm but also has smaller crystals of 100-150 nm (Figure S1 C). Based on SEM analysis, the CHA(1.9) crystals compared to CHA(2.3) is much different (Figures S1 D andS3). The CHA(1.9) zeolite contains more plate/disc-like structured particles consisting of smaller zeolite crystals with its rough looking surface, however, it is not possible to see individual small zeolite crystals making up the plate. It is possible that the presence of small FAU crystals in the FAU(2.6) zeolite aids the synthesis of smaller plate-like CHA zeolite crystals. The plate-like morphology is a typical shape of faulted CHA, which is consistent with the observations in the XRD pattern. 39 For both CHA(1.9) and CHA(2.3) there is some oriented growth of the zeolite. It is speculated if this could arise from faulting, when purely inorganic SDAs are used rather than when OSDAs are used for synthesis.

Figure 2 shows the TEM images of both CHA(1.9) and CHA(2.3) samples. The CHA(1.9) contains crystals with a size of 300-500 nm (Figure 2 A) consisting of densely packed smaller crystallites. Figure S4 shows lower magnification TEM images of CHA(1.9) which indicates the plate-like morphology with smaller crystallites growing from the plate. This is consistent with the SEM observations made for the CHA(1.9) sample. The CHA(2.3) sample contains very small crystallites in discrete form (Figure 2 B). The size of the individual zeolite crystals is about 50 nm, and the crystalline fringes are clearly observed in the TEM images. Figure S5 shows TEM images of agglomerates of CHA(2.3). Here, it appears that smaller zeolite crystals are overlapping, which is consistent with the agglomerates observed in SEM, however, it is difficult to see any direction oriented growth. The Si/Al ratio of the starting FAU materials and the concentration of both cations and hydroxide are different for the synthesis of two CHA zeolites. Those are the This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins main factors that affect the morphology and the size of the final product. Based on those two samples the larger Si/Al ratio in FAU results in the smallest crystal size (FAU (15) to CHA(2.3) gives crystals of 50-100 nm). This correspond well with the previous studies where Si/Al = 15 and higher (and using a OSDA at 125-170 °C) also results in nanosized crystals. [START_REF] Takata | [END_REF] However, J. Kim and D. H. Kim 39 showed a Si/Al = 6 (OSDA-free at 140 °C) results in larger crystal size similar to the once obtained here for CHA(1.9) produced from FAU(2.6). By studying different Si/Al ratios and silica/alumina sources, T. Takata concluded that the starting reaction mixture with common composite building unit and suitable Si/Al ratio causes abundant and uniform nucleation, resulting in the formation of nanosized CHA zeolites. [START_REF] Takata | [END_REF] Figure 2. TEM images of samples CHA(1.9) (A) and CHA(2.3) (B).

Porosity and CO2 capacity

The porosity of CHA(2.3) and CHA(1.9) zeolite samples was assessed by nitrogen physisorption measurements. The N2 physisorption isotherm of FAU(2.6), FAU (15), CHA(1.9), and CHA(2.3) are shown in Figures S6-9. The N2 isotherm for CHA(2.3) reveals the presence of both micro-and mesoporosity from the uptake at low pressure (<0.01 P/P0) and from the uptake of N2 at higher pressure, respectively (Figure S9). The calculated porosity for FAU and CHA zeolites are listed in Table S3. CHA(2.3) exhibits a high BET surface area of 251 m 2 /g compared to other CHA zeolites in the K-form obtained from interzeolite conversion [45][46][START_REF] Nedyalkova | Top. Please do not adjust margins Please do not adjust margins[END_REF] . The total pore volume of 0.39 cm 3 /g has only a limited contribution from the micropores with 0.04 cm 3 /g, hence the total porosity of the material is primarily mesopores. The hysteresis loop at high P/Po for nanosized CHA(2.3) is due the presence of textural (interparticle) porosity, which is typical for nanosized zeolites. 34 CHA(1.9) on the other hand has significantly lower surface area of 43 m 2 /g and a total pore volume of 0.07 cm 3 /g of which 0.004 cm 3 /g is the micropore volume. This is typically seen for the K form of the CHA zeolite obtained from the transformation in the potassium form [45][46][START_REF] Nedyalkova | Top. Please do not adjust margins Please do not adjust margins[END_REF] . CHA(1.9) is almost exclusively in the K form determined by ICP analysis (93% K and 6% Na relative to Al) which is the reason for this low porosity. It's reported that the K + extra-framework cations sit in the channels of CHA with low Si/Al ratio and limits the access of N2 to the channels, thus, lowering the surface area determined by N2 physisorption at 77 K. 12,45 CHA(1.9) does not show the same interparticle porosity which suggest that smaller crystals are grown together and are not accessible for the N2 molecules at 77 K. Considering the higher micropore volume for CHA(2.3) compared to CHA(1.9), it could be a result of the higher mesoporosity which provides access to more pore mouth openings of the CHA zeolite. The interparticle mesoporosity of CHA(2.3) and the lack of this in CHA(1.9) is consistent with the SEM and TEM observations for the two samples. This illustrates the large effect that the starting zeolite has on the porosity of the obtained CHA with the agglomerates of discrete nanosized CHA zeolite crystals (CHA(2.3)) rather than the thin plates of intergrown CHA zeolite (CHA(1.9)). Changing the cation in CHA greatly affects the adsorption properties of CHA. This was nicely demonstrated by Shang et al. 12 , who showed that changing to a smaller cation than K + increases the uptake of N2. However, both Si/Al, type of cation and degree of ion-exchange affects the adsorption properties of CHA. One of the applications of CHA is the selective capture of CO2 7,21,22 hence, the capacity of CO2 was measured to evaluate the potential use of the newly synthesized materials. The accessibility of CO2 and not N2 in the K-CHA zeolite has been shown before and this is related to the different polarizability of the two gas molecules with CO2 being much more polarizable. 45 CHA(2.3) shows a large uptake of CO2 at low relative pressure and then levels off at higher relative pressure (Figure 3). Interestingly, the difference of the two CHA zeolites with either interparticle porosity (CHA(2.3)) or intergrown CHA (CHA(1.9)) does not have a large impact on the adsorption profile and capacity of CO2. At p/p0=0.030, the CO2 capacity of CHA(2.3) is 4.26 mmol/g, which is similar to the CHA(1.9) of 4.25 mmol/g. The CO2 capacity is close to other similar reported materials 7,12 (see reference points in Figure 3), however, it is hard to make a profound comparison, since both Si/Al ratio together with the location of cations affect the capacity. Additionally, the amount and location of cations also affect the selectivity of CHA zeolite for different gasses. 7 Figure 3. CO2 adsorption of samples CHA(2.3) (red) and CHA(1.9) (black) at 273.15 K. CO2 capacity from Zhang et al. 7 (K-CHA, Li-CHA, and Ca-CHA) and Shang et al. 12 (r1KNaCHA, r2KCHA, and r2CsKCHA) reported as single points at relative pressure of 0.030 (104 kPa).

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FT-IR CO2 adsorption

The CO2 adsorption on the CHA zeolites was studied by in situ FTIR (Figure 4 and Figures S10-13). Two modes of CO2 adsorption, i.e., physisorbed CO2 at 2348 cm -1,48 and the chemisorbed CO2 giving rise to carbonates such as mono, bi, polydentate carbonates are measured. 21,[49][50][51] The nature of the formed carbonates depends on the zeolite type structure and the polarizing effect of the compensating cations present in the porosity. 52 The IR spectra of the activated samples before CO2 adsorption reveal two distinct bands in the OH region (Figure 4 A). The first band at 3740 cm -1 is assigned to external terminal silanol sites, while the band at 3726 cm -1 is attributed to internal silanol nests or silanol defects. 53 As shown in Figure 4 A, CHA(2.3) displays a higher amount of silanol sites, both external and internal silanol sites. The height ratio of the external silanol band at 3740 cm -1 , between the two samples after normalization to the weight of the pellet is equal to 6. This implies that sample CHA(2.3) displays an external surface area 6 times higher than sample CHA(1.9). The ratio is in reasonable agreement with external surface area ratio obtained from the N2 physisorption measurements which is 4.9. This is mainly due to the formation of nanosized crystals that are improving the adsorption of nitrogen. CHA(2.3) has also a higher amount of internal silanol sites as confirmed by the high intensity of the band at 3726 cm -1 (the ratio is 2.6). This result is in a good agreement with the nitrogen sorption analyses that showed structured mesoporosity for sample CHA(2.3). Figures 4 B,C reports the difference spectra obtained at adsorption of a specific dose of CO2 after subtraction of the background (the IR spectrum recorded after activation of the samples). After the first CO2 dose delivered, two broad bands are observed at 1324 cm -1 and 1680 cm -1 . After several doses, two additional distinct bands at 1665 cm -1 and 1345 cm -1 appeared. The bands at 1324 cm -1 and 1680 cm -1 are attributed to carbonate species formed during CO2 adsorption (Figure 4 B). As shown in Figure 4 D, these bands increase by increasing CO2 doses to reach saturation at 40-60 µmol/g CO2 (~1 μmol CO2 for 20 mg sample). It is already reported in the literature that the double degenerated v3 vibration of the carbonates is usually split into two bands. 54 Busca et al. 55 used the shift between these two bands to identify the carbonate species. In our case, the D v3 for 1680-1324 cm -1 and 1665-1354 cm -1 is higher than 300 cm -1 , which corresponds to bidentate carbonates. Such a result is in a good agreement with the low Si/Al ratio of the samples, which gives rise to strong basic zeolite sites. In addition, a new band assigned to physisorbed CO2 appears at 2348 cm -1 due to the polarization of CO2 by the compensating cations present in the CHA crystals. 48 The intensity of the band at 2348 cm -1 increases with an increase of the CO2 pressure (Figure 4 C). Moreover, a new shoulder at 1380 cm -1 appeared. This shoulder is also attributed to physisorbed CO2 molecules. 48 At high loading, more than 3000 µmol/g of CO2, a shift of the silanol bands at 3740-3725 cm -1 is observed, which is due to the perturbation of silanol sites by CO2 molecules (see Figure S10).

The evolution of the integrated band area of the carbonates at 1325 cm -1 and 1681 cm -1 and the polarized CO2 at 2348 cm -1 as a function of the amount of CO2 introduced in the cell is depicted in Figure 4D. Three distinct phenomena are observed (i) First at a low CO2 loading, CO2 is interacting with strong basic sites of zeolite leading to the formation of carbonates. This is traduced by the rising of the bands in the region 1300-1400 cm - 1 and 1600-1700 cm -1 ; (ii) After introduction of 4-5 µmol of CO2, the strongest basic sites of zeolite are completely neutralized with carbonates. No more carbonates are formed; the IR bands at 1325 cm -1 and 1681 cm -1 corresponding to carbonates reached saturation that correspond to 200 µmol CO2/g of zeolite. Only the band at 2348 cm -1 related to the interaction of CO2 with the extra-framework cations continues to increase linearly with the amount of CO2 introduced; (iii) At a higher amount of CO2, above 3500-4000 µmol/g of zeolite, after a complete consumption of the cationic sites, a shift of the silanol bands in the region 3700-3750 cm -1 is observed. This is due to the perturbation of the external silanol sites by CO2. This amount is in a good agreement with the amount of extraframework cations as determined by ICP (about 3600 µmol of potassium/g of zeolite)

The amount of carbonates formed on the two samples is estimated based on the integrated areas of the bands at 1325 cm -1 and 1681 cm -1 . Since the absorption coefficient is unknown, we calculated the height ratio of the two bands (A1325 CHA(1.9)/A1325 CHA(2.3) = 1.56 and A1681 CHA(1.9)/A1681 CHA(2.3) = 1.6). This means that the amount of strong basic sites of CHA(1.9) compared to CHA(2.3) is 1.6 times higher. For physisorbed species, the integrated band area is almost the same for the two samples with a ratio equal to 0.9.

The desorption properties of CHA(2.3) was tested by desorption of CO2 until a pressure of 10 -5 torr. Figure S14 shows the IR bands for the chemisorbed and physisorbed species at 2800 μmol/g and at 10 -5 torr. This clearly shows that the physisorbed species are absent after desorption whereas the chemisorbed species are much more strongly bound at present and still are present at 10 -5 torr in similar quantities. Thus, the physisorbed species can reversibly be adsorbed and desorbed to CHA(2.3). A similar trend is observed for CHA(1.9) (not shown). This journal is © The Royal Society of Chemistry 20xx

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CO adsorption

To further investigate the type and strength of interaction between adsorbates and cationic sites, CO adsorption was performed on CHA(1.9). The CO probe molecule is commonly used to distinguish between the different cationic sites present in zeolites. CO molecule is very sensitive to its environment and can easily reveal the types of cationic sites located in different confined space. [56][57][58] The experiment was performed with pulses of CO at 77 K and monitored by in situ IR. The IR spectra recorded at different doses of CO the sample are shown in Figure S14. Two main IR absorption bands at 2147 cm - 1 and 2160 cm -1 are observed. These bands arise from the polarization of CO by cationic sites, in our case the potassium present in the CHA zeolite. [56][57][58] This leads to a blue shift of the fundamental CO vibrational mode as compared to the band at 2143 cm -1 corresponding to free CO molecules. Thus, the IR absorption band at 2160 cm -1 corresponds to strong cationic acid sites of CHA zeolite. Indeed, the band at 2147 cm -1 might result from the interaction of CO with potassium cations located in a more confined space. In this case, the positive electric field of potassium cations are highly perturbed and weakened by neighbouring negative oxygen atoms.

Conclusions

The morphology, size and porosity of the CHA zeolite was found to be highly dependent by the type of the initial FAU zeolite samples. Using appropriate conditions, the transformation from two types of FAU zeolites to types of CHA zeolites was accomplished by alkali treatment without the use of an organic structure directing agent. The CHA(2.3) sample consisted of 50-100 nm discrete zeolite crystals while the CHA(1.9) sample consisted of big agglomerated crystals. Both zeolite samples CHA(1.9) and CHA(2.3) showed an extremely high CO2 adsorption capacity. The amount of chemisorbed CO2 species on the CHA zeolite is strongly related to the Si/Al ratio of the material. The sample CHA(1.9) with low Si/Al ratio showed higher amount of carbonate species which is proportional to the amount of compensating cations in zeolite structure.

Please do not adjust margins Please do not adjust margins from Haldor Topsøe A/S, Independent Research Fund Denmark (grant no. 6111-00237), and Villum fonden (Grant No. 13158).

Notes and references

Figure 1 .

 1 Figure 1. XRPD patterns of (A) FAU(15), (B) CHA(2.3), (C) FAU(2.6), and (D) CHA(1.9); trace of impurity marked with circle.

Figure 4 .

 4 Figure 4. IR spectra of CHA(1.9) and CHA(2.3) after activation at 350 °C under vacuum in the region 1500-4000 cm -1 (the spectra were normalized to 20 mg (A). IR spectra under pulsed CO2 adsorption on CHA(2.3) in the regions of 2000-1250 cm -1 (B), 2500-2200 cm -1 (C) and integrated area of bands in FTIR spectra corresponding to chemisorbed CO2 at 1330 cm -1 and 1681 cm -1 and physisorbed CO2 at 2348 cm -1: (D

CHA from low silica FAU The

  synthesis of CHA from FAU(2.6) zeolite is based on the synthesis reported by Bourgogne et al.

a. Technical University of Denmark, Department of Chemistry, Kemitorvet 207, 2800 Kongens Lyngby, Denmark b. ENSICAEN, Laboratoire Catalyse & Spectrochimie, 6 Boulevard Maréchal Juin, 14050 Caen Cedex 4, France Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5

This journal is © The Royal Society of Chemistry 20xx

Acknowledgements

Dr. Farnoosh Goodarzi is thanked for doing TEM analysis on the zeolites. Engineer Valérie Ruaux and Technician Marie Desmurs is thanked for their help with obtaining IR, ICP, N2 physisorption and CO2 adsorption data. The authors are grateful for funding

Conflicts of interest

There are no conflicts to declare


Edgar Muhumuza

Pingping Wu

Tian Nan

Lianming Zhao

Peng Bai
email: baipeng@upc.edu.cn

Svetlana Mintova

Zifeng Yan

Perovskite-Type LaCoO 3 as an Efficient and Green Catalyst for Sustainable Partial Oxidation of Cyclohexane

Keywords: Perovskite, Cyclohexane partial oxidation, LaCoO3, DFT, Structure-activity relationship

come

INTRODUCTION

The liquid phase partial oxidation of hydrocarbons into oxidized products is of great commercial importance in the chemical industry, especially, in the polymer and petrochemical industries. [START_REF] Liu | Liquid-phase oxidation of cyclohexane to cyclohexanone over cobalt-doped SBA-3[END_REF] Cyclohexane partial oxidation is a commercially desirable reaction for the oxidation of petroleumbased feedstock processes, for which the target products, cyclohexanone (C6H10O) and cyclohexanol (C6H11OH) also referred to as K/A oil, [START_REF] Alshaheri | Catalytic oxidation of cyclohexane using transition metal complexes of dithiocarbazate Schiff base[END_REF] are crucial intermediates for the production of Nylon-6 and Nylon-66.

For cyclohexane partial oxidation, heterogeneous transition metal catalysts have been widely studied, among which cobalt-based catalysts are the foremost investigated, due to the proven performance of cobalt salts in the industrial homogeneous process,, where the conversion of cyclohexane is controlled to be around 4% in order to inhibit the side-reactions, commonly having a K/A oil selectivity of about 80%. [START_REF] Alshaheri | Catalytic oxidation of cyclohexane using transition metal complexes of dithiocarbazate Schiff base[END_REF] Cobalt has been applied either as oxides or immobilized on different supports, such as oxides [START_REF] Zhou | Catalytic oxidation of cyclohexane to cyclohexanol and cyclohexanone over Co3O4 nanocrystals with molecular oxygen[END_REF] , molecular sieves, [START_REF] Kondo | Mechanisms of reactions of methoxy species with benzene and cyclohexane over H-ZSM-5 zeolites[END_REF] MOFs, [START_REF] Shi | A novel photosensitizing decatungstate-based MOF: Synthesis and photocatalytic oxidation of cyclohexane with molecular oxygen[END_REF][START_REF] Li | Boosting selective oxidation of cyclohexane over a metal-organic framework by hydrophobicity engineering of pore walls[END_REF] and carbon-based supports. [START_REF] Andrade | The role of nanoporous carbon materials in catalytic cyclohexane oxidation[END_REF] These catalysts, however, typically necessitate the use of an initiator, additional organic solvent, as well as tert-butylhydroperoxide (TBHP) as an oxidizing agent, leading to not only product contamination and the tedious separation procedure but also critical environmental issues.

For environmental and economic reasons, solvent-free systems that utilize molecular oxygen as an oxidizing agent (no extra solvent besides cyclohexane) are attractive for modern commercial applications. In addition, a more environmentally-friendly process would be achieved if a low-cost heterogeneous catalyst was developed to replace the homogeneous one. Co-TUD catalysts with different Si/Co ratios (10-100) have been prepared and applied for the aerobic cyclohexane oxidation, yielding a selectivity of 90.4% to mono-oxygenates with a low cyclohexane conversion (3.8%) under 15 bars. [START_REF] Ramanathan | Co-TUD-1 catalysed aerobic oxidation of cyclohexane[END_REF] Previous studies indicate that a combination of two or more elements can significantly improve the catalyst activity. [START_REF] Zhang | Structural modification of LaCoO3 perovskite for oxidation reactions: The synergistic effect of Ca 2+ and Mg 2+ co-substitution on phase formation and catalytic performance[END_REF][START_REF] Xie | Hierarchical flower-like Co-Cu mixed metal oxide microspheres as highly efficient catalysts for selective oxidation of ethylbenzene[END_REF][START_REF] Konsolakis | Effect of cobalt loading on the solid state properties and ethyl acetate oxidation performance of cobalt-cerium mixed oxides[END_REF][START_REF] Jiao | Effect of La2O3 doping on syntheses of C1-C18 mixed linear α-alcohols from syngas over the Co/AC catalysts[END_REF] For example, Xie et al. incorporated cobalt into mixed metal oxide catalysts to enhance the catalytic performance in the selective oxidation of ethylbenzene. [START_REF] Xie | Hierarchical flower-like Co-Cu mixed metal oxide microspheres as highly efficient catalysts for selective oxidation of ethylbenzene[END_REF] As an interesting catalytic material, transition-metal oxides with perovskite-type structures bear intrinsic advantages, such as structural simplicity and flexibility, high hydrothermal stability, high poisoning resistance, cost-effectiveness, and excellent oxygen mobility, which received wide attention and has been considered as green catalyst, especially in oxidation reactions. [START_REF] Liu | Preparation and evaluation of copper-manganese oxide as a high-efficiency catalyst for CO oxidation and NO reduction by CO[END_REF][START_REF] Chagas | Alumina-supported LaCoO3 perovskite for selective CO oxidation (SELOX)[END_REF][START_REF] Phadtare | Crystalline LaCoO3 perovskite as a novel catalyst for glycerol transesterification[END_REF] The redox behavior of these ABO3 type perovskites is essentially governed by B-site transition metal cation properties and the role of A-site cations is usually secondary. For example, Futai and colleagues [START_REF] Futai | Characterization of perovskite-type oxide catalysts RECoO3 by TPR[END_REF] conducted an H2-TPR analysis on the A-site substitution of ACoO3 compounds by rare earth metals (A = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy). All compounds exhibited two reduction peaks assigned to the reduction of cobalt ions. The oxygen mobility studies by O2-TPD indicate that the surface of Lanthanum-based perovskites is capable of adsorbing oxygen and desorbing active oxygen species. [START_REF] Baiker | Influence of the A-site cation in ACoO3 (A = La, Pr, Nd, and Gd) perovskite-type oxides on catalytic activity for methane combustion[END_REF] The liberated active oxygen species have a fundamental role in fueling the reaction. Zhang et al. [START_REF] Zhang | H2S selective catalytic oxidation over Ce substituted La1-xCexFeO3 perovskite oxides catalyst[END_REF] synthesised a series of La1-xCexFeO3 catalysts which exhibited a high sulphur selectivity (almost 100%) and low activation energy in the selective oxidation of H2S. It was found that with cerium incorporation, more Fe 3+ and Ce 4+ ions participated in the oxidation reaction, improving the reducibility and lattice oxygen mobility of the catalyst. A nonstoichiometric LaMnO3 perovskite-type catalyst was tested in NO oxidation and results indicated that a lower La content in A site induced the conversion of Mn(III) into Mn(IV) for charge balance and structural stability. Compared to the stoichiometric LaMnO3, the nonstoichiometric La0.9MnO3 catalyst exhibited a higher activity in NO oxidation due to the easily regenerated oxygen associated with Mn(IV). [START_REF] Chen | The influence of nonstoichiometry on LaMnO3 perovskite for catalytic NO oxidation[END_REF] In another report, [START_REF] Yoon | Catalytic activity of perovskite-type doped La0.08Sr0.92Ti1-xMxO3-δ (M = Mn, Fe, and Co) oxides for methane oxidation[END_REF] the effect of M-doped (M = Mn, Fe, and Co) single-phased La0.08Sr0.92Ti1-xMxO3-δ on the activity of methane oxidation was investigated. Fe and Co-doped catalysts demonstrated excellent performance associated with the high atomic ratios of Fe 3+ /Fe 4+ and Co 3+ /Co 4+ in comparison to Mn 3+ /Mn 4+ in the Mn-doped sample. Although the perovskite materials exhibit a high potential in the oxidation reaction, there is no report on the application of such materials in partial oxidation of cyclohexane so far.

Intrigued by the inherent advantages of perovskite in oxidation reactions, a perovskite-type Lanthanum Cobalt oxide (LaCoO3) was prepared and tested in the solvent-free partial oxidation of cyclohexane with molecular oxygen as an oxidant in this work. LaCoO3 perovskites with different La to Co molar ratios were synthesized and characterized by a variety of techniques. The nature of active sites in the perovskites for cyclohexane oxidation were clarified by combining the experimental and theoretical calculation results.

EXPERIMENTAL SECTION

Chemicals

La(NO3)3• 6H2O , Co(NO3)2• 6H2O , NaOH , C6H8O7•H 2O , cyclohexane (99.99%) and absolute ethanol (99.98%), all obtained from Sinopharm Chemical company, were used as received without further purification.

Catalyst Preparation

The catalysts were prepared by a modified citric acid method using citric acid as the chelating agent. Lanthanum nitrate and cobalt nitrate were dissolved in water with certain molar ratios. The above solution was added dropwise to a sodium hydroxide solution. After two hours of constant stirring to ensure complete precipitation, the sample was filtered and washed to remove the residue ions. The obtained wet cake was added to a citric acid solution and stirred at room temperature for acid to total metal ions was 1.2 to 1. Then, the mixture was dried at 110 o C for 24 h. Finally, the obtained powder was annealed in air at 750 o C for 4 h. According to the molar ratio of La: Co, the resultant samples were named as La1Co0.8O3, La1Co1O3, and La0.8Co1O3, respectively. The pure metal oxides were prepared through the same procedure and named as La2O3 and Co3O4 for lanthanum and cobalt oxides, respectively.

where K is a unit cell geometry dependent constant, which usually takes a value of about 0.9, λ is the wavelength of the X-ray source used in XRD, β is the width of the observed diffraction line at its half-intensity maximum in radian and θ is the Bragg peak angle [START_REF] Chagas | Alumina-supported LaCoO3 perovskite for selective CO oxidation (SELOX)[END_REF] . A Bruker Vertex 70V Tensor 27 spectrometer at 4 cm -1 resolution with 32 scans was utilized to perform Fourier transform infrared (FT-IR) spectroscopy in KBr media at room temperature. The FT-IR spectra were measured in the 4000-400 cm -1 region. Nitrogen physisorption isotherms were recorded at 77 K using a Micromeritics ASAP2020 gas adsorption analyser. Each sample was degassed at 300 °C for 3 h prior to the adsorption. According to the Brunauer-Emmett-Teller (BET) formula, the specific surface area was determined from the adsorption branch in the P/Po range of 0.05 to 0.30.

The pore size distribution was determined from the desorption branch analysis of the N2 isotherm using the BJH (Barrett-Joyner-Halenda) method. Surface morphology and elemental composition were determined by scanning electron microscopy (SEM, JSM-7600F; JEOL, USA) equipped with an energy-dispersive X-ray spectroscopy (EDX). A JEOL JEM-1210 microscope working at 113-115 kV and fitted with an Olympus Megaview G2 camera was used to obtain transmission electron microscopy (TEM) images. In order to examine the reduction behavior of the catalysts, H2temperature programmed reduction (H2-TPR) experiments were performed on a chemisorption apparatus (Micromeritics AutoChem II 2920). The sample (100 mg) was typically pre-treated for 1 h under an Ar stream (30 mL / min) at 300 °C to eliminate moisture and impurities. The system was then cooled to 50 °C, and reduced by introducing a 10 vol% H2/Ar stream (30 mL/min).

Afterwards, the temperature was increased linearly from 100 °C to 900 °C at 10 °C/min, and the thermal conductivity detector (TCD) signal was recorded. The evaluation of X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Fisher Scientific spectrometer fitted with an Al electrode as an anode at an electron kinetic energy of 1486.6 eV. The spectra were calibrated with C 1s binding energy (284.5 eV) as an internal standard. The results were analysed by CasaXPS software using Shirley and tougaard backgrounds for oxygen and cobalt, respectively. The room temperature X-band electronic paramagnetic resonance (EPR) spectra were collected on a JEOL JESFA200 ESR spectrometer.

Catalytic Evaluation

The catalysts' performance in the cyclohexane partial oxidation was evaluated in a 100 mL stainless steel reactor furnished with a magnetic stirrer. In a standard reaction, 7. were utilized for all calculations. After convergence assessment, a 3 × 3 × 1 Monkhorst-Pack kpoint mesh for the Brillouin zone sampling at a global orbital cutoff of 5.0 Å was employed. The convergence specifications for optimal geometry concerning energy, force, and displacement were set as 2 × 10 -5 Ha, 4 × 10 -3 Ha/Å, and 5 × 10 -3 Å, respectively. The rhombohedral LaCoO3 unit cell and its primitive cell structure were extracted from the Materials Project database. Based on XRD results, LaCoO3 (1 1 0) slab was cleaved and a vacuum region of 15 Å applied to lessen interactions between periodic images. The slab-bottom three layers were fixed to describe the bulk.

For oxygen and cyclohexane adsorption studies, the adsorption energy (Eads) was computed from the following formula:

Eads = Esurf-ads -Esurf -Em (2)
where Esurf-ads refers to the total energy of the surface with adsorbed oxygen/cyclohexane, Esub and Em are the total energy of clean surface and free molecular oxygen/cyclohexane, respectively.

RESULTS AND DISCUSSION

Crystalline Structure

The XRD patterns of La2O3, La1Co0.8O3, La1Co1O3, La0.8Co1O3, and Co3O4 catalysts are presented in Figure 1 and their structural compositions generated by the Rietveld refinement method are summarized in Table 1. In Figure 1 S1).

Furthermore, the refined atomic positions, bond-angle, and bond length of La1Co1O3 are shown in Table S2. For this sample, the calculated Co-O-Co bond angle were found to be 165 o , and the six Co-O bonds have average lengths of 1.931 A o , which are in agreement with literature results. 21- 22 No obvious lattice parameter variation of the perovskite phase was observed with the different La/Co molar ratios. The average crystallite sizes of different catalysts were calculated by Debye-Scherrer equation and the results are shown in Table 1. The mean crystallite sizes of La1Co1O3, La2O3, and Co3O4 were estimated as 21.8, 40.4, and 56.0 nm, respectively. However, the mean crystallite sizes of catalysts La1Co0.8O3 and La0.8Co1O3 were between those of LaCoO3 and their respective metal oxide, showing consistency with refinement results. It was noted that for the pure La2O3 sample, La2O3 phase was unstable and easily converted into La(OH)3 phase within 2 hours of exposure to air as illustrated by XRD patterns (Figure S1). The newly generated XRD peaks at 2θ = 23.5 o and 24.5 o are due to the La(OH)3 phase. [START_REF] Lee | Luminescent Eu(III) and Tb(III) activator ions in La(OH)3 and La2O3 nanowire matrices[END_REF][START_REF] Kang | Synthesis and physicochemical properties of La(OH)3 and La2O3 nanostructures[END_REF] However, this phenomenon disappeared after formation of LaCoO3 phase, indicating the formation of perovskite rhombohedral structure led to a higher stability.

The FT-IR spectra of La2O3, La1Co0.8O3, La1Co1O3, La0.8Co1O3, and Co3O4 are displayed in Figure 2. The presence of different functional groups originating from the hydroxide and carbonates [START_REF] Gangwar | Combustion synthesized La2O3 and La(OH)3: Recyclable catalytic activity towards Knoevenagel and Hantzsch reactions[END_REF] was observed on sample La2O3, which was attributed to its hygroscopicity and basicity. Water and atmospheric CO2 are easily adsorbed on the surface of La2O3 under ambient conditions, leading to the formation of hydroxides, chemisorbed surface carbonates, and bicarbonates species. [START_REF] Lee | Luminescent Eu(III) and Tb(III) activator ions in La(OH)3 and La2O3 nanowire matrices[END_REF][START_REF] Kang | Synthesis and physicochemical properties of La(OH)3 and La2O3 nanostructures[END_REF] The sharp peak observed at 3607 cm -1 was ascribed to the bulk hydroxyl groups from La(OH)3. [START_REF] Lee | Luminescent Eu(III) and Tb(III) activator ions in La(OH)3 and La2O3 nanowire matrices[END_REF] Generally, the bands appearing around 3435 cm -1 and 1629 cm -1 corresponded to O-H stretching and bending vibration of adsorbed water molecules. [START_REF] Chumakova | Nanocrystalline LaCoO3 modified by Ag nanoparticles with improved sensitivity to H2S[END_REF] Two bands around 1467 cm -1 and 1381 cm -1 were detected on sample La2O3 which are attributed to the presence of carbonate or bicarbonate species. [START_REF] Kang | Synthesis and physicochemical properties of La(OH)3 and La2O3 nanostructures[END_REF] The intensity of these two peaks reduced with the decrease of La/Co ratio on catalyst La1Co0.8O3 and La1Co1O3, and eventually diminished on sample La0.8Co1O3. The fingerprint band around 423 cm -1 was assigned to La-O stretching vibration while the bands at 555 and 595 cm -1 were due to the Co-O stretching vibration in CoO6 octahedra of perovskite LaCoO3. The presence of these three peaks confirmed the formation of perovskite phase LaCoO3 in samples La1Co0.8O3, La1Co1O3, and La0.8Co1O3. [START_REF] Phadtare | Crystalline LaCoO3 perovskite as a novel catalyst for glycerol transesterification[END_REF][START_REF] Zhou | Enhanced catalytic activity for NO oxidation over Ba doped LaCoO3 catalyst[END_REF] The shoulder peak (555 cm -1 ) signifies a lower symmetry structure such as the rhombohedra due to the asymmetrical lengthening of the Co-O bond in the structure's CoO6 octahedra. [START_REF] Phadtare | Crystalline LaCoO3 perovskite as a novel catalyst for glycerol transesterification[END_REF] For the Co3O4 material, the distinguished sharp peaks at 571 and 669 cm -1 are associated with the OB3 and the ABO3 vibrations in the spinel lattice, respectively, where B denotes Co 3+ in an octahedral coordination and A denotes the Co 2+ in a tetrahedral coordination, [START_REF] Adhikari | Synthesis and electrochemical performance of hydrothermally synthesized Co3O4 nanostructured particles in presence of urea[END_REF] indicating the presence of both Co 2+ and Co 3+ species on sample Co3O4. The peak intensity at 669 cm -1 corresponding to Co 2+ species gradually reduced with the increase of La/Co ratio and disappeared on catalyst La1Co1O3.

Furthermore, no other characteristic bands between 1000 cm -1 and 3500 cm -1 , suggesting the absence of impurities such as CoOOH and Co(OH)2 in the sample. [START_REF] Fu | Pine needle β-Co(OH)2 grown on Ni foam substrate for high specific capacitance and good cycling stability as advanced electrochemical pseudocapacitor materials[END_REF] Textural Properties and Microstructures

The N2 adsorption-desorption isotherms and their corresponding BJH pore size distribution curves of the samples are presented in Figure S2(a) and Figure S2(b), respectively. All samples show a type-II sorption isotherm with a macroporous or nonporous structure. [START_REF] Maity | Engineered spinel-perovskite composite sensor for selective carbon monoxide gas sensing[END_REF] Distinct H-3 hysteresis loops in the high relative pressure range of 0.8 ∼ 1.0 were observed. [START_REF] Thommes | Physical adsorption characterization of nanoporous materials[END_REF] The absence of adsorption plateaus at relative pressures close to unity could be related to the existence of gaps created by pile-up of the nanoparticles, an indication that the pore size distribution extends to the macropore range clearly evidenced in the pore size distribution plots. The specific surface area, pore size, and pore volume are summarized in Table 1. The specific surface areas for the samples are low due to the high calcination temperature (750 o C) and no significant difference was observed on these samples.

The morphology and elemental distribution of catalysts La2O3, La1Co0.8O3, La1Co1O3, La0.8Co1O3, and Co3O4 were revealed by SEM with EDX mapping as shown in Figure 3 and Figure

Reduction Properties

In order to examine the redox ability of La2O3, La1Co0.8O3, La1Co1O3, La0.8Co1O3, and Co3O4, H2-TPR experiments were performed as shown in Figure 5. For catalyst La1Co1O3, the hydrogen consumption involved two reduction regions located between 250 -450 °C and 480 -650 °C, which correspond to the reduction of Co 3+ to Co 2+ and Co 2+ to Co 0 , respectively. [START_REF] Li | Comparison of the nickel addition patterns on the catalytic performances of LaCoO3 for low-temperature CO oxidation[END_REF][START_REF] Luo | Enhanced activity for total benzene oxidation over SBA-15 assisted electrospun LaCoO3[END_REF] In the lower temperature region, the main peak at 360 °C was ascribed to the reduction of surface Co 3+ to Co 2+ and a shoulder peak at 393 °C to the reduction of bulk Co 3+ to Co 2+ ions. [START_REF] Luo | Enhanced activity for total benzene oxidation over SBA-15 assisted electrospun LaCoO3[END_REF] With the decrease of La to Co ratio, the first two reduction peaks of La0.8Co1O3 shifted to lower temperatures. The variation was associated with the presence of dispersed Co3O4 particles in the perovskite phase which could be easily reduced. This is confirmed by the reduction peak of pure Co3O4 sample showing a complete reduction of Co 3+ to Co 0 within the lowest temperature range of 250 -480 °C among all catalysts. The influence of lanthanum on the reduction property of catalysts is explicitly noticeable as the ratio of lanthanum to cobalt increases, that is, the reduction temperature increased with the increase of La to Co ratio. This is probably due to the strong interaction between cobaltlanthanum ions and the segregation of La2O3 species on catalyst surface which inhibit the reduction of Co species. [START_REF] Jiao | Effect of La2O3 doping on syntheses of C1-C18 mixed linear α-alcohols from syngas over the Co/AC catalysts[END_REF][START_REF] Brabant | Promotion of lanthanum-supported cobalt-based catalysts for the Fischer-Tropsch reaction[END_REF] No noticeable reduction peaks were detected on sample La2O3 since La 3+ ions are non-reducible under this temperature. [START_REF] Pandey | Nonreducible, basic La2O3 to reducible, acidic La2-xSbxO3 with significant oxygen storage capacity, lower band gap, and effect on the catalytic activity[END_REF] Surface Composition and Valence States XPS analysis was conducted to explore the surface properties of the catalysts. The detailed surface atomic concentrations are listed in Table S3 and the calculated surface La/Co ratios are summarized in Table 1. As shown in Table 1, the bulk La/Co ratios measured by EDX characterization are consistent with nominal atomic ratio. However, the surface La/Co ratios on all catalysts are higher than the bulk ones, indicating the enrichment of lanthanum at the surface of perovskites which have been considered as a common observation. [START_REF] Gupta | Nickel and titanium doubly doped lanthanum strontium chromite for high temperature electrochemical devices[END_REF][START_REF] Haynes | Characterization of calcination temperature on a Ni-substituted lanthanum-strontium-zirconate pyrochlore[END_REF] In La 3d spectra, the binding energies (BE) of La 3d5/2 are located at 833.9 and 837.6 eV, the BEs of La 3d3/2 are located at 850.4 and 854.4 eV, respectively, as shown in Figure 6. In this work, the difference between the spin-orbit splitting of the La 3d core-level for the perovskite containing samples are found to be ~16.7 eV identical to those of La2O3 (16.8 eV), which suggests that the La manifests as La 3+ species for all samples. [START_REF] Haynes | Characterization of calcination temperature on a Ni-substituted lanthanum-strontium-zirconate pyrochlore[END_REF] However, the peaks of La 3d shift to higher BE as the lanthanum loading increases, suggesting La loses electrons, which indicated the presence of La(OH)3 species. [START_REF] Sunding | XPS characterisation of in situ treated lanthanum oxide and hydroxide using tailored charge referencing and peak fitting procedures[END_REF] Correlating with the surface metal composition results (Table 1), it can be concluded that more La species enriched on the catalyst surface with the increase of La/Co ratio, which tends to form surface hydroxide species when exposed to water/moisture.

Figure 7 shows the XPS spectra of Co 2p, two major observed peaks with BE of 779. [START_REF] Anke | Selective 2-Propanol Oxidation over Unsupported Co3O4 Spinel Nanoparticles: Mechanistic Insights into Aerobic Oxidation of Alcohols[END_REF] The summarized energy differences results (ΔE in Table 2) indicate that La1Co1O3 displayed the lowest ΔE compared to other catalysts while the highest value was found in sample Co3O4. This emphasizes the enrichment of Co 2+ on the surface of sample Co3O4. As indicated in Table 2, the highest surface Co 3+ / Cototal ratio was observed on catalyst La1Co1O3 and it decreases with the reduction of perovskite phase. It has been widely accepted that the valence state of the cobalt species is crucial for the catalytic activity. [START_REF] Rodionova | Binuclear cobalt complex with Schiff base ligand: Synthesis, characterization and catalytic properties in partial oxidation of cyclohexane[END_REF][START_REF] Wang | Polymer nanofilm-coated Co-Mn oxide nanoparticle catalysts for trans-membrane CO oxidation at 50° C under moisture-rich conditions[END_REF] The O 1s spectra of catalysts La2O3, La1Co0.8O3, La1Co1O3, La0.8Co1O3, and Co3O4 are presented in Figure 8 and their relative oxygen atomic concentrations are displayed in Table 2. A satisfactory deconvolution yielded three components located at 529.2 ± 0.5, 530.8 ± 0.5 and 532.7 ± 0.6 eV, which correspond to surface lattice oxygen (Oα), surface adsorbed oxygen (Oβ) and oxygencontaining group species on the surface (Oγ) such as CO3 2-or OH -. [START_REF] Wu | Method for estimating ionicities of oxides using O1s photoelectron spectra[END_REF][START_REF] Liu | Highly active Mn3-xFexO4 spinel with defects for toluene mineralization: Insights into regulation of the oxygen vacancy and active metals[END_REF][START_REF] Liu | In situ fabrication of highly active γ-MnO2/SmMnO3 catalyst for deep catalytic oxidation of gaseous benzene, ethylbenzene, toluene, and o-xylene[END_REF] Generally, the results show an increase of surface oxygen species (Oβ and Oγ) at the expense of the lattice oxygen Oα with the increase of lanthanum ratio in the samples, which is consistent with those reported by Bae et al. [START_REF] Bae | Highly water-resistant Ladoped Co3O4 catalyst for CO oxidation[END_REF] The shift in binding energy positions is due to the incorporation of different elements which alter the electron density around the base element. The Oβ species play a vital role in promoting the surface reactivity of an oxidation reaction since they have higher mobilities relative to the Oα species. [START_REF] Chen | Catalytic oxidation of NO over MnO2 with different crystal structures[END_REF] A higher Oβ/Oα value usually indicates a higher oxygen vacancy density, which is believed to act as the adsorption site for the active oxygen species. [START_REF] Zhao | XPS study of surface absorbed oxygen of ABO3 mixed oxides[END_REF] The EPR spectra of La1Co1O3 and La0.8Co1O3 samples shown in Figure 9, were recorded to further understand the valence state of cobalt ion. Cobalt (III) oxidation state is diamagnetic in nature (EPR silent) while the cobalt (II) ion is paramagnetic (EPR active). [START_REF] Shit | Azide/thiocyanate incorporated cobalt(III)-Schiff base complexes: Characterizations and catalytic activity in aerobic epoxidation of olefins[END_REF][START_REF] Varma | Efficient photocatalytic hydrogen production in water using a cobalt (III) tetraaza-macrocyclic catalyst: electrochemical generation of the low-valent Co(I) species and its reactivity toward proton reduction[END_REF] Our results demonstrate the presence of paramagnetic ions on both catalyst surfaces with a stronger peak intensity for La0.8Co1O3 confirming more Co 2+ ions [START_REF] Varma | Efficient photocatalytic hydrogen production in water using a cobalt (III) tetraaza-macrocyclic catalyst: electrochemical generation of the low-valent Co(I) species and its reactivity toward proton reduction[END_REF] than in the La1Co1O3 sample. These results are in agreement with the XPS analysis.

DFT Calculations

Oxygen and cyclohexane adsorption on the perovskite surface were investigated using density Since adsorbates prefer association with cations over oxygen anions, we studied the effect of La and Co ions on O2 and cyclohexane adsorption. [START_REF] Penninger | DFT analysis of NO oxidation intermediates on undoped and doped LaCoO3 perovskite[END_REF] Free molecular oxygen and cyclohexane ring were placed in various orientations on both surfaces as adsorbents and optimized to identify the lowest energy structures, which are shown in Figure 11 and the structural properties summarized in Table 3. As is shown, the O2 adsorption prefers the Co-atop and hollow-atop sites for CoO2(110)

and LaO(110)-terminations, respectively.

The computed O-O bond length of free molecular oxygen was 1.225 Å, which is slightly larger than the experimental value of 1.208 Å 52 but in agreement with the previously reported value. [START_REF] Liu | First-principles study of O2 adsorption on the LaFeO3(010) surface[END_REF] Adsorption on the CoO2(110) and LaO(110)-terminated facets lengthens it by 0.053 and 0.169 Å, respectively. La2O3(001) surface also shows a higher elongation (0.089 Å) than that depicted by the cobalt perovskite surface, which suggests that molecular oxygen continuously withdraws more electrons from the La surface to the O2-2π* orbitals creating the larger O-O bond elongation.

Furthermore, the O2 adsorption is much more energetically stable on the LaO(110)-termination (-2.04 eV) than on CoO2(110)-termination (-0.56 eV). These results may be attributed to the difference in electronegativity between the La (1.1) and Co (1.88) ions which implies that La ions can strongly attract O2 compared to the Co ions. The preference for La over Co surface indicates that molecular O2 is easily activated upon adsorption on the La ion to yield O-O bond breakage which is key for the progress of the oxidation process. These computational results are in good agreement with the oxygen XPS results and that of reported literature. [START_REF] Choi | Experimental and computational investigation of effect of Sr on NO oxidation and oxygen Exchange for La1-xSrxCoO3 perovskite catalysts[END_REF] From the orientations of the different adsorption sites considered for cyclohexane on perovskite surfaces, the obtained results showed that cyclohexane molecule adsorbed predominantly in the chair conformation [START_REF] Saeys | Adsorption of cyclohexadiene, cyclohexene and cyclohexane on Pt(111)[END_REF] for both terminations (Figure 11). The adsorption on the CoO2-termination recorded the most stable configuration with an energy of -0.99 eV compared to LaO-termination (-0.82 eV) and the reference La2O3 surface (-0.73 eV). Additionally, the most significant increase for cyclohexane adsorption was from 1.104 to 1.113 Å in the longest C-H bond on the CoO2-termination. This change was larger than the increase from 1.104 to 1.107 Å and 1.104 to 1.106 Å for the longest C-H bond on LaO-termination and La2O3 surfaces, respectively. The preference and elongation of cyclohexane on cobalt-perovskite surface show that the cleavage of inert C-H bond is more likely on CoO2-termination.

Catalytic Performance

The catalytic performance of samples prepared in this work and employed for cyclohexane oxidation at 150 o C for 1 hour were summarised in Table 4. For the single metal oxide catalysts, Co3O4 exhibited a slightly higher conversion (3.9%) than its counterpart La2O3 (2.6%).

Nevertheless, its activity was quite lower than that of perovskite containing catalysts. La1Co1O3

was the most active among the studied catalysts achieving a conversion of 8.3%, which suggests a synergistic action between the two metals. However, as the predominant crystal phase changes from perovskite to a mixture of phases, the conversion decreases i.e. to 6.1% and 6.3% for La1Co0.8O3 and La0.8Co1O3, respectively, which signified that the purity of the perovskite influences the reaction. All catalysts showed good K/A oil selectivity (≥ 90%) and favored cyclohexanone formation except catalyst La2O3 which yielded more cyclohexanol as clearly reflected by the K/A ratio.

In order to understand the stability of the perovskite catalyst during the reaction-regeneration cycles, the La1Co1O3 catalyst was retrieved and regenerated. Figure 12 shows the conversion/selectivity results of the regenerated La1Co1O3 catalyst over five cycles. The regenerated La1Co1O3 catalysts' performance was quite similar to that of the fresh catalyst and the achieved conversions are above 8.3% with target product selectivity above 86.5%. No evident deterioration in activity/selectivity of the regenerated La1Co1O3 catalyst was observed after the five cycles. The results show that the spent catalyst was able to recover its activity after the simple calcination treatment. The XRD patterns of catalyst La1Co1O3 before and after reaction revealed no obvious structure change (Figure S4), certified the good stability of catalyst. The relation between Co 3+ /Cototal, Oβ/Oα, and the conversion attained on different catalysts was displayed in Figure 13. For the cobalt-containing catalysts, a volcano-shape profile was observed in the cyclohexane conversion, which increased with increase in the Co 3+ /Cototal ratio. La1Co1O3 catalyst with the highest Co 3+ /Cototal ratio (0.53) recorded the best conversion (8.3%) whereas Co3O4 (0.32)

gave the least conversion (3.9%). Generally, with incorporation of La ions, the Oβ/Oα increased with increment in the lanthanum loading which could be due to the difference in electronegativity between the La (1.1) and Co (1.88) ions, hence La ion possesses the ability to attract more oxygen compared to the cobalt ions. A comparison of the attained conversion results between the singlephase and mixed-phase catalysts confirmed that the presence of the perovskite phase influenced the catalyst activity. DFT calculations suggest that La 3+ sites are more favorable for molecular oxygen adsorption/activation than the Co 3+ sites, whereas the Co 3+ ions prefer the adsorption of cyclohexane to foster the C-H bond activation. This trait is in good agreement with the data from the literature, where Co 2+ is commonly recognized inactive while Co 3+ as the active species for heterogeneous oxidation reactions. [START_REF] Ding | Enhanced CO catalytic oxidation of flower-like Co3O4 composed of small nanoparticles[END_REF][START_REF] Yan | Shape-controlled fabrication of the porous Co3O4 nanoflower clusters for efficient catalytic oxidation of gaseous toluene[END_REF] Therefore, it can be concluded that the perovskite catalyst (LaCoO3) favors the activation of C-H bond which is believed to be the rate-controlling step in cyclohexane partial oxidation [START_REF] Liang | Synergistic hydrogen atom transfer with the active role of solvent: Preferred one-step aerobic oxidation of cyclohexane to adipic acid by N-hydroxyphthalimide[END_REF] and the catalytic performance can be enhanced through the catalyst design. The formation of a single-phase perovskite, leading to a prevalence of Co 3+ and the inherent surface oxygen of lanthanum containing perovskites are responsible for the enhanced cyclohexane oxidation.

A reaction mechanism for partial oxidation of cyclohexane over the LaCoO3 perovskite catalyst was proposed on the basis of experimental and theoretical studies, as shown in Scheme 1.

Molecular oxygen (O2 gas) is activated on LaCoO3 surface by chemisorption on the lanthanum surface, accompanied by the formation of activated oxygen species (O2 - ads) while cyclohexane is activated on Co 3+ on the catalyst surface. The activated species interact to form the cyclohexylperoxyl radical which further transforms to cyclohexyl hydroperoxide (CHHP) intermediate. Then, the CHHP decomposes to cyclohexanol and cyclohexanone which are the main products of the reaction.

CONCLUSIONS

A series of catalysts with varying lanthanum to cobalt ratios were successfully prepared using a modified citric acid approach and evaluated in cyclohexane oxidation. The obtained LaCoO3 perovskite material was identified as an excellent catalyst for cyclohexane oxidation reaction, which was interpreted by correlating the catalytic activity with a variety of characterization techniques and DFT studies. Based on the XRD and FT-IR characterization data, La1Co1O3, and Co3O4 samples show the presence of a single-phase while La2O3, La1Co0.8O3, and La0.8Co1O3 samples contain mixed phases. Cobalt-XPS and EPR analysis confirmed that the surface Co 3+ species in the material increased with the purity of the perovskite phase. DFT calculations coupled with oxygen-XPS studies reveal that La ion sites are more favorable for molecular oxygen adsorption than the Co ion sites whereas cyclohexane preferred Co sites. The single-phase rhombohedral La1Co1O3 sample exhibited the highest activity. The excellent catalytic performance of LaCoO3 perovskite under mild reaction conditions could be attributed to the dominance of Co 3+ ions which aid in cyclohexane C-H bond activation and the presence of relatively abundant adsorbed oxygen species on the catalyst due to the amplified O2 adsorption lanthanum surface.

These findings could shed light on the design of an environmentally-friendly, low-cost, and highly active perovskite catalyst for cyclohexane oxidation.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge at XRD patterns for La2O3, N2 adsorption-desorption isotherms and pore size distribution, EDX mappings, surface atomic concentrations measured by XPS, structural parameters and refined unit cell lattice parameters.

List of Figures

Scheme 1. Reaction mechanism of cyclohexane partial oxidation on LaCoO3

For Table of Content use only

  Characterization X-ray diffraction (XRD) patterns of the samples were reported on a Bruker AXS D8 Advance X-ray diffractometer fitted by a LynxEye detector with 40 kV and 40 mA working Cu-Kα radiation (λ=1.5406 Å). Diffraction patterns were collected in a 2θ range of 15° to 75° with a 0.02° step size at a scan rate of 2°/min. The average crystallite size was calculated by Debye-Scherrer's formula. Diffrac Eva v4.3 software was employed for phase identification and Topas v6 package for the Rietveld refinement of the crystalline structures. A fifth-order Chebychev background model and Thompson-Cox-Hastings pseudo-Voigt peak model were used to fit the diffraction patterns. The atomic positions were determined for each material and the thermal parameter was constrained for all atoms. From the diffractograms, the crystallite size (D) of crystals were calculated using the Debye-Scherrer equation; D= cos K 

  56 mL of cyclohexane and 50 mg of catalyst were added into the reactor. The reactor was heated to 150 o C after oxygen purging and charged with 0.8 MPa of O2 for 1 hour under stirring. After the reaction, the mixture was diluted with a certain amount of ethanol to dissolve the by-products, and then the catalyst was removed by a micro-filtration. The products were identified by GC-MS (Agilent 7890A-5975C) with a Restek Stabilwax-DA capillary column. The liquid products were analyzed by a Perkin-ElmerClarus 500 gas chromatograph equipped with a capillary column packed with cross-bond PEG treated with nitro-terephthalic acid (30*0.32*0.25) and a FID detector. Pentanol was used as the internal standard. The regeneration was carried out by separating the spent catalyst from the reaction mixture through centrifugation, dried at 100 o C in an oven for 5 hours, calcined at 550 o C for 4 hours, and then charged for the further run under the same reaction conditions. Due to the loss of catalyst, the amount of cyclohexane fed for the following cycles was reduced accordingly.Theoretical CalculationDensity functional theory (DFT) calculations were implemented using the DMol 3 program of Materials Studio 2018 software package. For geometry optimization steps, spin unrestricted calculations were performed using the Perdew-Burke-Ernzerhof (PBE) within the generalized gradient approximation (GGA) method to describe the exchange-correlation functional. Double numerical basis sets with polarization functions (DNP) and DFT semi-core pseudopotential (DSSP)

  , no other phase was observed on sample La1Co1O3, except rhombohedral perovskite phase (R-3C) of LaCoO3, whereas obvious diffraction peaks of La2O3 and Co3O4 phases were detected on La1Co0.8O3 and La0.8Co1O3, respectively. The refinement results confirmed a pure rhombohedral perovskite LaCoO3 phase on catalyst La1Co1O3, while hexagonal La2O3 phase (P63/mmc) and cubic Co3O4 phase (Fd-3m) were observed on catalysts La1Co0.8O3 and La0.8Co1O3, respectively, besides the R-3C perovskite phase. The proportion of different phases on catalysts La1Co0.8O3 and La0.8Co1O3 were quantified as La2O3/LaCoO3 (P63/mmc: R-3C) =23: 77 and Co3O4/LaCoO3 (Fd-3m: R-3C) = 20: 80, respectively (as listed in Table

S3.

  Nanosized particle aggregates were observed on all catalysts. For catalyst La1Co1O3, the nearly spherical nanoparticles can be clearly observed with the sizes ranging from 60 to 180 nm. The color-coded EDX mapping (Inset of Figure3) revealed a homogeneous distribution of La: Co: O elements in sample La1Co1O3 with an atomic ratio of nearly 1:1:2.7, indicating the formation of pure perovskite phase. FigureS3shows the EDX composition scans for La2O3, La1Co0.8O3, La0.8Co1O3, and Co3O4. The atomic ratios are in agreement with nominal ratios, i.e. 2:3.1, 1:0.7:2.3, 0.7:1:2.8 and 3:4.1 for La2O3, La1Co0.8O3, La0.8Co1O3 and Co3O4, respectively. TEM technique was used to further characterize the morphology and microstructure of catalyst La1Co1O3. As shown in Figure4, the lattice fringes could be clearly observed from the highmagnification image with a spacing of d = 0.268 ± 0.01 nm, corresponding to the (104) plane of the rhombohedral LaCoO3 structure. The selected area electron diffraction (SAED) pattern of La1Co1O3 sample (Inset of Figure4) shows spotty ring patterns revealing a polycrystalline structure. The diffraction rings from SAED patterns were measured to be the (012), (104), and (024) planes, indicating the rhombohedral LaCoO3 structure as well. These results are consistent with the XRD analysis.

  functional theory (DFT) calculations. The optimized primitive cell with lattice parameters of a = b = c = 5.393 Å and α= β = γ= 61.2 o was adopted for the studies. The dominant XRD perovskite phase (110), was chosen as the surface of adsorption. The 2 x 2 x 1 supercell of LaCoO3 slab model along the (110) direction, consisting of six alternating layers of LaO and CoO2 planes was used to study the adsorption behavior of molecular oxygen and cyclohexane. The CoO2(110)-termination with the corresponding Co-atop, O-atop, and hollow-atop sites are illustrated in Figure 10(A) and (C), respectively. Similarly, Figure 10(B) shows the LaO(110)-terminated surface. The top view exposes both La and O atoms as well as La-La bridge sites shown in Figure 10(D).

Figure 1 .

 1 Figure 1. XRD patterns of La2O3, La1Co0.8O3, La1Co1O3, La0.8Co1O3, and Co3O4.

4 Figure 2 .

 42 Figure 2. The FTIR spectra of catalysts La2O3, La1Co0.8O3, La1Co1O3, La0.8Co1O3, and Co3O4.

Figure 3 .

 3 Figure 3. SEM images of La2O3, La1Co0.8O3, La1Co1O3, La0.8Co1O3, and Co3O4. Inset: EDX of

Figure 4 . 4 Figure 5 .

 445 Figure 4. TEM micrographs of La1Co1O3 nanoparticle. Inset: Selected area electron diffraction

  865

Figure 9 .

 9 Figure 9. Room temperature EPR spectra for La1Co1O3 and La0.8Co1O3.

Figure 10 .

 10 Figure 10. Six-layer slab model of CoO2(110) (A) and LaO(110) (B) terminations and their

Figure 11 .

 11 Figure 11. The most stable adsorbate positions of the optimized oxygen and cyclohexane

Figure 12 .Figure 13 .

 1213 Figure 12. Cyclohexane partial oxidation with recycled La1Co1O3 catalyst.

  7 eV and 794.6 eV were assigned to Co 2p3/2 and Co 2p1/2, respectively. Deconvolution of the asymmetrical Co 2p2/3 spectrum of each catalyst yields five components at BEs of 779.3, 780.1, 781.9, 784.2,

	and 789.6 eV, which correspond to surface metal Co 0 , Co 3+ , Co 2+, and two shake-up satellite peak
	(Sat 1 and Sat 2) species, respectively. 39-40 Recent studies on oxidation state variations of the cobalt
	species reveal that the shake-up peak at ~ 784.2 eV (Sat 1) show the pronounced intensity
	characteristic of high spin cobalt (II) species, whereas the low spin cobalt (III) states show a very

weak shake-up peak at about 789.6 eV (Sat 2). As the Co content increases, a noticeable change in the high spin satellite peak can clearly be seen. By conducting an area comparison of the two fitted shakeup satellites, changes in the oxidation state of cobalt species are noted. The ratio between Sat 1 to Sat 2 generally increases with the increase of cobalt content as shown in Table 2, indicating a decrease in Co 3+ content on the surface. The energy difference (ΔE) between the 2p3/2 and 2p1/2 peaks can be used, in addition to the satellite comparison, to analyze the changes in the state of cobalt oxidation. An energy difference of 15-15.4 eV suggests a dominant contribution of Co 3+ , as proposed by Anantharamaiah and Joy while the ΔE ranges from 15.7 to 16.1 eV for samples rich in Co 2+ .

Table 3 .

 3 The Structural Properties of Adsorbed Oxygen and Cyclohexane Molecules.

	Sites	Ads Energy (eV) Bond length (Å) O2 C6H12 O-O C-H
	CoO2(1 1 0)	-0.57	-0.99	1.278	1.113
	LaO(1 1 0)	-2.04	-0.82	1.394	0.107
	La2O3(0 0 1) -1.82	-0.73	1.314	1.106

Table 4 .

 4 The Catalytic Performance of Catalysts in Cyclohexane Oxidation a Reaction conditions: cyclohexane, 7.56 ml; reaction time, 1 h; reaction temperature, 423 K; catalyst, 50 mg; K/A, cyclohexanone to cyclohexane ratio; b By-products are mainly ring-opened acids such as n-butric, succinic, glutaric, adipic acid and their corresponding esters.

					Selectivity (%)
		Conversion	K/A	Cyclohexanone	Cyclohexanol	K/A	By-products b
	Catalysts	(%)	ratio	(K)	(A)	oil
	Blank	2.6	1.5	55.7	36.4	92.1	7.9
	La2O3	2.7	0.6	38.3	60.6	98.9	1.1
	La1Co0.8O3	6.1	2.3	64.2	27.7	91.9	8.1
	La1Co1O3	8.3	1.8	58.2	31.8	90.0	10.0
	La0.8Co1O3	6.3	1.9	59.6	30.7	90.3	9.7
	Co3O4	3.9	1.4	54.5	40.1	94.6	5.4
	a

  Figure 6. La3d spectra for La2O3, La1Co0.8O3, La1Co1O3, La0.8Co1O3, and Co3O4.

				La 3d 3/2 Co 2p 1/2 O 	851.9		O 	La 3d 5/2 834.9 Co 2p 3/2 O 
			La 2 O 3			La 1 Co 0.8 O 3		La 1 La 0.8 Co 1 Co 1 O 3	O 3
	Intensity, a.u Intensity, a.u Intensity, a.u	Intensity, a.u	La 0.8 Co 1 O 3 La 1 Co 0.8 O 3 La 1 Co 1 O 3	850.4 La 1 Co 1 O 3 La 0.8 Co 1 O 3 854.4 Satellite peaks La 2 O 3 La 1 Co 0.8 O 3 La 1 Co 1 O 3 La 0.8 Co 1 O 3 Shakeup	833.9 Co 3+ Co 2+ 837.6	Co o
								(Sat 2)	(Sat 1)
			Co 3 O 4			Co 3 O 4
		800 536	860 2800	855 795 534 3000	3200	850 790 532 3400	845 3600 785 840 530 3800	835 4000 780 528	830 4200	825 775 526
							Binding energy, eV Binding energy, eV Binding energy, eV Magnetic Field, G
	Figure 7. Co 2p spectra for La2O3, La1Co0.8O3, La1Co1O3, La0.8Co1O3, and Co3O4. Figure 8. O 1s spectra of catalysts La2O3, La1Co0.8O3, La1Co1O3, La0.8Co1O3, and Co3O4.

ACKNOWLEDGEMENTS

This work was supported by National Natural Science Foundation of China (51601223, 21991091), the National Key Technologies R & D program of China (2018YFE0118200), Shandong Provincial Natural Science Foundation (ZR2017MB003), PetroChina Innovation Foundation (2018D-5007-0504), the Fundamental Research Funds for the Central Universities (17CX05018, 17CX02056, 19CX05001A) and the Sino-French International Laboratory (LIA) "Zeolites". E. M. would like to appreciate the financial support from the China Scholarship Council.

Authors Pingping Wu-State Key Laboratory of Heavy Oil Processing, CNPC Key Laboratory of Catalysis, College of Chemical Engineering, China University of Petroleum (East China), Qingdao 266580, China; Edgar Muhumuza-State Key Laboratory of Heavy Oil Processing, CNPC Key Laboratory of Catalysis, College of Chemical Engineering, China University of Petroleum (East China), Qingdao 266580, China; Tian Nan-State Key Laboratory of Heavy Oil Processing, CNPC Key Laboratory of Catalysis, College of Chemical Engineering, China University of Petroleum (East China), Qingdao 266580, China; Lianming Zhao-School of Materials Science and Engineering, Institute of Advanced Materials, China University of Petroleum (East China), Qingdao 266580, China; Svetlana Mintova-State Key Laboratory of Heavy Oil Processing, CNPC Key Laboratory of Catalysis, College of Chemical Engineering, China University of Petroleum (East China), Qingdao 266580, China; Normandie University, ENSICAEN, UNICAEN, CNRS, Laboratory of Catalysis and Spectrochemistry, 14000 Caen, France; Zifeng Yan-State Key Laboratory of Heavy Oil Processing, CNPC Key Laboratory of Catalysis, College of Chemical Engineering, China University of Petroleum (East China), Qingdao 266580, China;

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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