Surface Methoxy Species Research Articles

Role of H2O2 as Oxidant and H2O as Co-catalyst over Anatase-TiO2 (101) for Conversion of Methane to Methanol.

Computations based on density functional theory are carried out to examine the mechanism of photocatalytic oxidation of methane to methanol with H2O2 as oxidant and water as co-catalyst over anatase TiO2 (101) surface. The reaction proceeds with hydrogen abstraction from methane followed by the formation of surface methoxy species, which is reduced to methanol. We compare the reaction energetics for C-H dissociation in the presence and absence of surface defect, but find no discernible impact of O-vacancy on methane oxidation. In comparison, hydroxyl produced as a result of H2O2 or H2O photo-decomposition dramatically reduces the barrier for CH3-H bond cleavage. The reaction proceeds further by the reduction of surface methoxy group and is the rate-limiting step for methanol formation. Additionally, we find that methyl can also react with water to form methanol with a considerably lower barrier, suggesting an active involvement of water for methanol formation. We also study the role of water as a co-catalyst and observe significant reduction in barriers, facilitated by alternate pathway for proton transfer. The reaction pathways presented provide valuable in- sights into the mechanism of methane oxidation in the presence of H2O2 as oxidant and demonstrate the rate-enhancing role of water for these steps.

Confinement-Driven Dimethyl Ether Carbonylation in Mordenite Zeolite as an Ultramicroscopic Reactor.

ConspectusThe conversion of C1 molecules to methyl acetate through the carbonylation of dimethyl ether in mordenite zeolite is an appealing reaction and a crucial step in the industrial coal-to-ethanol process. Mordenite zeolite has large 12-membered-ring (12MR) channels (7.0 × 6.5 Å2) and small 8MR channels (5.7 × 2.6 Å2) connected by a side pocket (4.8 × 3.4 Å2), and this unique pore architecture supplies its high catalytic activity to the key step of carbonylation. However, the reaction mechanism of carbonylation in mordenite zeolite is not thoroughly established in that it is able to explain all experimental phenomena and improve its industrial applications, and the classical potential energy surface exerted by static density function theory calculations cannot reflect the reaction kinetics under realistic conditions because the diffusion kinetics of bulk DME (kinetic dimeter: 4.5 Å) and methyl acetate (MA, kinetic dimeter: 5.5 Å) were not well considered and their restricted diffusion in the narrow side pocket and 8MR channels may greatly alter the integrated kinetics of DME carbonylation in mordenite zeolite. Moreover, the precise illustration of the dynamic behaviors of the ketene intermediate and its derivatives (surface acetate and acylium ion) confined within various voids in mordenite has not been effectively portrayed.Advanced ab initio molecular dynamics (AIMD) simulations with or without the acceleration of enhanced sampling methods provide tremendous opportunities for operando modeling of both reaction and diffusion processes and further identify the geometrical structure and chemical properties of the reactants, intermediates, and products in the different confined voids of mordenite under realistic reaction conditions, which enables high consistency between computations and experiments.In this Account, the carbonylation process in mordenite is comprehensively described by the results of decades of continuous research and newly acquired knowledge from both multiscale simulations and in-(ex-)situ spectroscopic experiments. Three primary steps (DME demethylation to surface methoxy species (SMS), carbon-carbon bond coupling between SMS and CO to acetyl species, and methyl acetate formation by acetyl species and methanol/DME) have been respectively studied with a careful consideration of different molecular factors (reactant distribution, concentration, and attack mode). By utilizing the free-energy surface of diffusion and reaction obtained from AIMD simulations, a comprehensive reaction/diffusion kinetic model was formulated for the first time, illustrating the entire zeolite catalytic process. In this context, a comprehensive and informative analysis of the reaction kinetics of carbonylation in mordenite, including the function of the 12MR channels, 8MR channels, and side pockets in the adsorption, diffusion, and reaction of DME carbonylation, was performed. The different channels of mordenite play different roles in all ordered reaction steps, illustrating a highly organized ultramicroscopic reactor that is encompassed.

Mechanistic investigation of methanol-to-olefins conversion catalyzed by H-ZSM-5 zeolite: a DFT study.

The mechanisms for the formation of the first C - C bond and lower olefins on methanol to olefins (MTO) conversion on H-ZSM-5 had been focused in dispute. In this paper, density functional theory has been used to study the reaction mechanisms of methanol to olefins on ZSM-5. The configurations of reactants, intermediates, products and transition state of the numerous reactions involved in such a process have been optimized, as well as the elementary reactions related to these configurations were determined by the calculation of corresponding activation energy barriers and reaction heats. Here, two different kinds of the mechanisms were proposed for the formation of dimethylether (DME), one involving an associative interaction of two methanol molecules with the zeolite Brønsted acid sites and the other occurring via a surface methoxy species and a methanol molecule. A critical intermediate of the methoxy methyl cation was theoretically verified by the reaction of the methoxy species and dimethylether. Besides, it was found that the first intermediates containing a C - C bond were 1,2-dimethoxyethane and 2-methoxy-ethanolare, in which the former was formed from methoxy species with dimethyl ether and the latter was formed from methanol by onium ions((CH3)2O+CH2CH2OCH3), respectively. For the whole reaction mechanism, the results in this paper indicated that the ethene formation is more favorable than propylene formation due to the low activation energy barrier for ethene formation (123.49 vs. 162.09kJ.mol-1). From these calculations, it would be concluded that ethene is the first alkene product that induces the occurrence of the hydrocarbon pool mechanism. All the periodic density function theory (DFT) calculations were performed by the Vienna Ab Initio Simulation package (VASP). The interaction between nucleus and valence electron was described using the pseudopotentials found in the projector augmented wave (PAW) method. PBE-D3 was used in the whole DFT calculations and CI-NEB was used to locate transition state.

Confinement driving mechanism of surface methoxy species formation in mordenite zeolite: An interplay of different molecular factors

Surface methoxy species (SMS) are key intermediates in methanol conversion within zeolites. We propose a reaction mechanism for SMS formation using methanol or dimethyl ether in mordenite zeolites, based on ab initio molecular dynamics (AIMD) simulations and validated by in situ experiments. Key factors such as adsorption, diffusion, distribution, and attacking modes of reactants were considered. The forward attacking mode of both methanol and dimethyl ether on the acid site is more feasible for SMS formation than the backward mode in the 8 member-ring of mordenite (MOR-8MR). For methanol, two end-to-end molecules in MOR-8MR promote SMS formation via a stepwise pathway, avoiding the rotation of protonated methanol, while two methanol molecules on different sides of acid site hinder SMS formation. Dimethyl ether, however, converts to SMS in MOR-8MR regardless of concentration and distribution. The kinetics align with in situ Fourier transform infrared experiments and are confirmed by two-dimensional correlation analysis of methanol dehydration.

Water‐Induced Micro‐Hydrophobic Effect Regulates Benzene Methylation in Zeolite

AbstractWater is a ubiquitous component in heterogeneous catalysis over zeolites and can significantly influence the catalyst performance. However, the detailed mechanism insights into zeolite‐catalyzed reactions under microscale aqueous environment remain elusive. Here, using multiple dimensional solid‐state NMR experiments coupled with ultrahigh magic angle spinning technique and theoretical simulations, we establish a fundamental understanding of the role of water in benzene methylation over ZSM‐5 zeolite under water vapor conditions. We show that water competes with benzene for the active sites of zeolite and facilitates the bimolecular reaction mechanism. The growth of water clusters induces a micro‐hydrophobic effect in zeolite pores, which reorients benzene molecules and drives their interactions with surface methoxy species (SMS) on zeolite. We identify the formation and evolution of active SMS‐Benzene complexes in a microscale aqueous environment and demonstrate that their accumulation in zeolite pores boosts benzene conversion and methylation.

Water-Induced Micro-Hydrophobic Effect Regulates Benzene Methylation in Zeolite.

Water is a ubiquitous component in heterogeneous catalysis over zeolites and can significantly influence the catalyst performance. However, the detailed mechanism insights into zeolite-catalyzed reactions under microscale aqueous environment remain elusive. Here, using multiple dimensional solid-state NMR experiments coupled with ultrahigh magic angle spinning technique and theoretical simulations, we establish a fundamental understanding of the role of water in benzene methylation over ZSM-5 zeolite under water vapor conditions. We show that water competes with benzene for the active sites of zeolite and facilitates the bimolecular reaction mechanism. The growth of water clusters induces a micro-hydrophobic effect in zeolite pores, which reorients benzene molecules and drives their interactions with surface methoxy species (SMS) on zeolite. We identify the formation and evolution of active SMS-Benzene complexes in a microscale aqueous environment and demonstrate that their accumulation in zeolite pores boosts benzene conversion and methylation.

Selective Oxidative Dehydrogenation of Ethane and Propane over Copper‐Containing Mordenite: Insights into Reaction Mechanism and Product Protection

AbstractCopper(II)‐containing mordenite (CuMOR) is capable of activation of C−H bonds in C1‐C3 alkanes, albeit there are remarkable differences between the functionalization of ethane and propane compared to methane. The reaction of ethane and propane with CuMOR results in the formation of ethylene and propylene, while the reaction of methane predominantly yields methanol and dimethyl ether. By combining in situ FTIR and MAS NMR spectroscopies as well as time‐resolved Cu K‐edge X‐ray absorption spectroscopy, the reaction mechanism was derived, which differs significantly for each alkane. The formation of ethylene and propylene proceeds via oxidative dehydrogenation of the corresponding alkanes with selectivity above 95 % for ethane and above 85 % for propane. The formation of stable π‐complexes of olefins with CuI sites, formed upon reduction of CuII‐oxo species, protects olefins from further oxidation and/or oligomerization. This is different from methane, the activation of which proceeds via oxidative hydroxylation leading to the formation of surface methoxy species bonded to the zeolite framework. Our findings constitute one of the major steps in the direct conversion of alkanes to important commodities and open a novel research direction aiming at the selective synthesis of olefins.

Selective Oxidative Dehydrogenation of Ethane and Propane over Copper-Containing Mordenite: Insights into Reaction Mechanism and Product Protection.

Copper(II)-containing mordenite (CuMOR) is capable of activation of C-H bonds in C1 -C3 alkanes, albeit there are remarkable differences between the functionalization of ethane and propane compared to methane. The reaction of ethane and propane with CuMOR results in the formation of ethylene and propylene, while the reaction of methane predominantly yields methanol and dimethyl ether. By combining in situ FTIR and MAS NMR spectroscopies as well as time-resolved Cu K-edge X-ray absorption spectroscopy, the reaction mechanism was derived, which differs significantly for each alkane. The formation of ethylene and propylene proceeds via oxidative dehydrogenation of the corresponding alkanes with selectivity above 95 % for ethane and above 85 % for propane. The formation of stable π-complexes of olefins with CuI sites, formed upon reduction of CuII -oxo species, protects olefins from further oxidation and/or oligomerization. This is different from methane, the activation of which proceeds via oxidative hydroxylation leading to the formation of surface methoxy species bonded to the zeolite framework. Our findings constitute one of the major steps in the direct conversion of alkanes to important commodities and open a novel research direction aiming at the selective synthesis of olefins.

Reaction mechanism of co-coupling conversion of propane and methanol over H-ZSM-5 zeolite

Co-coupling conversion of propane and methanol is an effective method to promote the activation of propane; however, the reaction mechanism has not been fully understood due to the complex reaction network. In this study, the reaction kinetics and thermodynamics of various elementary steps for propane activation under the assistance of methanol over H-ZSM-5 zeolite were systemically investigated by density functional theory (DFT) calculations combining with model experiments. The results show that direct dehydrogenation of propane on the acid sites of zeolite is very difficult, while introduction of methanol considerably decreases the free energy barrier for propane activation. This is because large amounts of active intermediates, such as surface methoxy and alkene-based species, are generated in methanol conversion that promote the propane dehydrogenation through hydrogen transfer reactions. Among these active intermediates, alkene-based species (including adsorbed alkene and tertiary carbocations) show higher activity towards C–H bond activation of propane.

Cerium coordination-dependent surface intermediates regulate activity in dimethyl carbonate synthesis from CO2 and methanol

The direct synthesis of dimethyl carbonate (DMC) from CO2 and methanol is a potential green alternative to the current industrial methods using toxic chemicals. Although many CeO2-based catalysts have been explored, the key structural factor affecting their DMC activity is still unclear. Herein, single-crystalline CeO2 catalysts bearing distinct but well-defined Ce coordination structures were examined. We found that methanol is the key species being activated by surface Ce sites (cf. CO2) to produce methyl carbonate (MC) and later DMC. The reactivity of surface methoxy species towards CO2 varies considerably with its configuration determined by Ce coordination structures. The head-to-head terminal methoxy species converts CO2 to MC faster than its atilt counterparts, resulting in a higher DMC activity. The bridging methoxy species is, however, too stable to react with CO2. The established structure-activity relation here should also guide the design of CeO2-based catalysts in other reactions involving methanol activation.

Kinetic and Spectroscopic Studies of Methyl Ester Promoted Methanol Dehydration to Dimethyl Ether on ZSM-5 Zeolite

Methyl carboxylate esters have been shown to be potent promoters of low-temperature methanol dehydration to dimethyl ether (DME) using various zeolite catalysts. In the present work, catalytic kinetic studies, in-situ Fourier-transform infrared spectroscopy (FT-IR) and solid-state nuclear magnetic resonance spectroscopy (NMR) techniques were used to elucidate the promotional mechanism of methyl carboxylate esters on methanol dehydration to DME, using the medium pore zeolite H-ZSM-5 (MFI) as the catalyst. Kinetic studies were performed using the very potent methyl n-hexanoate promoter. The DME yield was dependent on both the methanol and methyl n-hexanoate partial pressures across the temperature ranges used in this study (110 to 130 °C). This is consistent with the promoted reaction being a bimolecular reaction between methanol and ester species adsorbed at the catalyst active sites, via an SN2 type reaction, as previously postulated. The in-situ FT-IR studies reveal that the Brønsted acid (BA) sites on H-ZSM-5 were very rapidly titrated by ester carbonyl group adsorption and bonded more strongly with esters than with methanol. Upon methanol addition, an even lower DME formation temperature (30 °C) was observed with methyl n-hexanoate pretreated H-ZSM-5 samples in the in-situ NMR studies, further confirming the strong promotion of this methyl ester on methanol dehydration to DME. The adsorption and reactivity of different methyl esters on H-ZSM-5 indicates that while methyl formate more easily dissociates into a surface methoxy species, [Si(OMe)Al], and carboxylic acid, it is a less potent promoter than alkyl-chain-containing methyl esters in methanol dehydration to DME, which in turn did not show this dissociative behavior in the low-temperature NMR studies. This indicates that methyl alkyl carboxylates do not need to be dissociated to a surface methoxy species to promote the methanol dehydration reaction and that a bimolecular associative mechanism plays an important role in promoting DME formation.

Site-specific scaling relations observed during methanol-to-olefin conversion over ZSM-5 catalysts

The conversion of methanol over ZSM-5 catalysts was studied using step response and temperature-programmed desorption and surface reaction analyses in a temporal analysis of products reactor, as well as quasi-elastic neutron scattering and Fourier transform infrared investigations, buttressed by archived 13C magic angle spinning nuclear magnetic resonance studies. The results were combined with micro-kinetic models that simulated the formation of the first C–C bond and primary olefin(s) from methanol. Dimethyl ether was the major surface oxygenate and a source of surface methoxy species. Propylene, the major olefin produced from dimethyl ether, was formed with a reaction barrier of ∼ 200 kJ mol−1, in agreement with archived density functional theory calculations. Propylene could form from dimethyl ether via a methoxymethyl mechanism under intrinsic kinetic conditions. Site-specific scaling relations between the barriers to methyl propenyl ether and methoxy methyl species formation and dimethyl ether desorption were observed. The active sites of the ZSM-5 catalysts can be locally optimised and selectively tuned to improve their activity during the conversion of methanol to olefin(s).

Operando Spectroscopic Monitoring of Active Species in CO2 Hydrogenation at Elevated Pressure and Temperature: Steady-State versus Transient Analysis

Operando infrared spectroscopy is an invaluable tool to provide a deep insight into underlying mechanisms in heterogeneous catalysis. Recent advances in spectroscopic techniques enabled operando analysis under elevated pressure and temperature. In this study we compared a conventional steady-state IR analysis with a transient IR analysis by modulation excitation spectroscopy (MES) using a model reaction, CO2 hydrogenation to methanol at 30 bar and 250 °C. The steady-state IR spectra provide information about surface properties of catalysts while the transient IR spectra are a powerful tool to unveil the kinetics of surface species involved in catalytic cycles and therefore to determine rate-limiting step. Operando MES-IR spectroscopy evidenced that under the reaction conditions (30 bar, 250 °C) hydrogenation of surface methoxy species (CH3O−) is the rate-limiting step for the Cu–Zn based catalyst. We herein demonstrate that comparative analysis of both steady-state and transient spectra at elevated pressure provides a solid understanding of surface processes, allowing a rational catalyst design.

Progressive steps and catalytic cycles in methanol-to-hydrocarbons reaction over acidic zeolites

Understanding the complete reaction network and mechanism of methanol-to-hydrocarbons remains a key challenge in the field of zeolite catalysis and C1 chemistry. Inspired by the identification of the reactive surface methoxy species on solid acids, several direct mechanisms associated with the formation of the first C-C bond in methanol conversion have been recently disclosed. Identifying the stepwise involvement of the initial intermediates containing the first C-C bond in the whole reaction process of methanol-to-hydrocarbons conversion becomes possible and attractive for the further development of this important reaction. Herein, several initial unsaturated aldehydes/ketones containing the C-C bond are identified via complementary spectroscopic techniques. With the combination of kinetic and spectroscopic analyses, a complete roadmap of the zeolite-catalyzed methanol-to-hydrocarbons conversion from the initial C-C bonds to the hydrocarbon pool species via the oxygen-containing unsaturated intermediates is clearly illustrated. With the participation of both Brønsted and Lewis acid sites in H-ZSM-5 zeolite, an initial aldol-cycle is proposed, which can be closely connected to the well-known dual-cycle mechanism in the methanol-to-hydrocarbons conversion.

Molecular Routes of Dynamic Autocatalysis for Methanol-to-Hydrocarbons Reaction

The industrially important methanol-to-hydrocarbons (MTH) reaction is driven and sustained by autocatalysis in a dynamic and complex manner. Hitherto, the entire molecular routes and chemical nature of the autocatalytic network have not been well understood. Herein, with a multitechnique approach and multiscale analysis, we have obtained a full theoretical picture of the domino cascade of autocatalytic reaction network taking place on HZSM-5 zeolite. The autocatalytic reaction is demonstrated to be plausibly initiated by reacting dimethyl ether (DME) with the surface methoxy species (SMS) to generate the initial olefins, as evidenced by combining the kinetic analysis, in situ DRIFT spectroscopy, 2D 13C-13C MAS NMR, electronic states, and projected density of state (PDOS) analysis. This process is operando tracked and visualized at the picosecond time scale by advanced ab initio molecular dynamics (AIMD) simulations. The initial olefins ignite autocatalysis by building the first autocatalytic cycle-olefins-based cycle-followed by the speciation of methylcyclopentenyl (MCP) and aromatic cyclic active species. In doing so, the active sites accomplish the dynamic evolution from proton acid sites to supramolecular active centers that are experimentally identified with an ever-evolving and fluid feature. The olefins-guided and cyclic-species-guided catalytic cycles are interdependently linked to forge a previously unidentified hypercycle, being composed of one "selfish" autocatalytic cycle (i.e., olefins-based cycle with lighter olefins as autocatalysts for catalyzing the formation of olefins) and three cross-catalysis cycles (with olefinic, MCP, and aromatic species as autocatalysts for catalyzing each other's formation). The unraveled dynamic autocatalytic cycles/network would facilitate the catalyst design and process control for MTH technology.

Thermal Decomposition of Dimethyl Methylphosphonate on Size-Selected Clusters: A Comparative Study between Copper Metal and Cupric Oxide Clusters

Room temperature decomposition and thermal decomposition of dimethyl methylphosphonate (DMMP), a chemical warfare agent (CWA) simulant, on size-selected copper clusters have been studied via combined X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). Cu100 and (CuO)80, which have the same nominal masses, were chosen to present a direct comparison between the reactivity of metallic copper and that of cupric oxide with DMMP. Room temperature XPS results have shown that most of the DMMP molecules decompose completely and reductively into atomic phosphorus on Cu100, while almost all the DMMP molecules are only dissociatively adsorbed on (CuO)80 as methyl methylphosphonate (MMP). XPS and TPD have been carried out to analyze the thermal decomposition of adsorbed DMMP by identifying the surface species after annealing to certain temperatures and the gaseous products evolved during linear temperature ramps, respectively. Methanol, formaldehyde, and methane are the three most significant gaseous products for DMMP decomposition on both Cu100 and (CuO)80. Methanol and formaldehyde, which evolve in the low temperature region, are believed to originate from surface methoxy species. Methanol, formaldehyde, and methane evolved in the high temperature region are related to further decomposition of the phosphorus-containing surface species. A set of methanol-probed TPD experiments have also been carried out, which suggest that methane evolution originates from the methyl group within DMMP instead of the surface methoxy species.

A temporal analysis of products (TAP) study of C2-C4 alkene reactions with a well-defined pool of methylating species on ZSM-22 zeolite

Reactions between alkenes and methanol or dimethyl ether (DME) on zeolite catalysts are involved in industrial processes that are highly relevant for the transition to renewable carbon sources, such as the Methanol-To-Hydrocarbons (MTH) process. In MTH chemistry, alkene methylation increases the length of product carbon chains, and its relative rate with respect to other reactions largely controls the overall selectivity. Experimental studies of alkene methylation present a considerable challenge because they are typically accompanied by cracking, hydrogen transfer, and aromatization reactions. Herein, the pulse-response Temporal Analysis of Products (TAP) methodology and complementary FTIR measurements were employed to isolate a well-defined population of surface species, consistent with Surface Methoxy Species (SMS) on Bronsted acid sites that are reactive in alkene methylation on a ZSM-22 (TON) zeolite. Their coverage was determined by TAP titration to be ca. 5% of the total amount of Brønsted acid sties, which was also indirectly suggested by FTIR data. C2-C4 alkenes were quantitatively reacted with SMS to precisely measure the intrinsic kinetic parameters of isolated alkene methylation steps. The rate constants increased and the activation energies decreased as functions of the carbon number (EC2H4 = 52 kJ/mol > EC3H6 = 32 kJ/mol > Ec-C4H8 = 16 kJ/mol). However, the rate constant for iso-butene was comparable to propene, despite its activation energy (Ei-C4H8 = 19 kJ/mol) being much lower than propene’s. This effect is in agreement with the increased steric hindrance predicted by DFT for isobutene adsorption and methylation in TON zeolites. Our results considerably extend previously available TAP data on alkene methylation reactions and furthermore validate ab initio models of these crucial steps in the complex MTH chemistry on acidic zeolites.

Kinetics of Dimethyl Methylphosphonate Adsorption and Decomposition on Zirconium Hydroxide Using Variable Temperature In Situ Attenuated Total Reflection Infrared Spectroscopy.

The decomposition mechanisms of dimethyl methylphosphonate (DMMP), a widely used simulant for organophosphorus chemical warfare agents (CWAs), are relatively well understood from previous studies. However, there still lacks a quantitative description of DMMP decomposition kinetics under ambient conditions that is relevant for sequestration applications. We investigated adsorption and decomposition kinetics of DMMP on amorphous zirconium hydroxide (ZH) using variable-temperature in situ attenuated total reflection (ATR) infrared spectroscopy. We demonstrate that quantifying DMMP decomposition kinetics using conventional methods, where the integrated absorbance of P-O vibrational modes is monitored, can be inaccurate because these spectra are also convoluted with C-O vibrational modes from transient surface methoxy species that are not proportional to DMMP decomposition due to methanol desorption. Here, we propose to use the ρ(PCH3) modes as an alternative way to track DMMP adsorption and decomposition reactions. On the basis of density functional theory (DFT) simulations and comparisons to relatively unreactive monoclinic zirconia (m-ZrO2), we assign the deconvoluted components of the ρ(PCH3) region and use it to monitor decomposition products over time at various temperatures. Because the PCH3 group is present in many toxic organophosphorus compounds, tracking the PCH3 bands in time-dependent IR spectra is useful for measuring surface kinetics of CWAs and their simulants on various decontamination materials.

CO2 Hydrogenation to Methanol over La2O3-Promoted CuO/ZnO/Al2O3 Catalysts: A Kinetic and Mechanistic Study

The hydrogenation of CO2 to methanol has been investigated over CuO/ZnO/Al2O3 (CZA) catalysts, where a part of the Al2O3 (0, 25, 50, 75, or 100%) was substituted by La2O3. Results of catalytic performance tests obtained at atmospheric pressure showed that the addition of La2O3 generally resulted in a decrease of CO2 conversion and in an increase of methanol selectivity. Optimal results were obtained for the CZA-La50 catalyst, which exhibited a 30% higher yield of methanol, compared to the un-promoted sample. This was attributed to the relatively high specific surface area and porosity of this material, the creation of basic sites of moderate strength, which enhance adsorption of CO2 and intermediates that favor hydrogenation steps, and the ability of the catalyst to maintain a large part of the copper in its metallic form under reaction conditions. The reaction mechanism was studied with the use of in situ infrared spectroscopy (DRIFTS). It was found that the reaction proceeded with the intermediate formation of surface formate and methoxy species and that both methanol and CO were mainly produced via a common formate intermediate species. The kinetic behavior of the best performing CZA-La50 catalyst was investigated in the temperature range 190–230 °C as a function of the partial pressures of H2 (0.3–0.9 atm) and CO2 (0.05–0.20 atm), and a kinetic model was developed, which described the measured reaction rates satisfactorily.

Methane Activation on H-ZSM-5 Zeolite with Low Copper Loading. The Nature of Active Sites and Intermediates Identified with the Combination of Spectroscopic Methods.

Cu-modified zeolites have enormous potential as the catalysts facilitating the conversion of methane to methanol. It becomes important to investigate the active sites and the reaction mechanisms involved. In this paper, several spectroscopic methods such as UV-vis diffuse reflectance spectroscopy (UV-visDRS), pulse electron paramagnetic resonance (EPR), diffuse reflectance Fourier transform infrared spectroscopy, and solid-state (13C MAS) NMR have been employed to characterize the state of the Cu sites and the intermediates formed during the catalyst activation and methane-to-methanol transformation on Cu/H-ZSM-5 zeolite with low (0.10 wt %) Cu content. UV-vis DRS and EPR data imply the presence of two types of Cu2+ cations bound to the zeolite framework Si-O--Al sites (Z). One of them is a species of the type Z[Cu(II)O] or Z[Cu(II)(OH)] with extra-framework O- or OH- ligands. The other one refers to Z2Cu(II) species without extra-framework O-containing ligands. CW EPR studies reveal that the Z2Cu(II) species are the major part of the Cu(II) sites present in the zeolite. 1H HYSCORE and DRIFTS data are supportive of the formation of a molecular complex of methane and Z2Cu(II) species, with a strongly polarized C-H bond and a 3.3 Å separation between the hydrogen atom of methane and Cu. 13C MAS NMR provides evidence for the formation of both the surface methoxy intermediate and physisorbed methanol. It is suggested that experimentally identified Z[Cu(II)O] or Z[Cu(II)(OH)] are those sites that provide a homolytic cleavage of the methane C-H bond to yield surface bound methoxy species and/or methanol molecule, the possibility that has been recently justified with density functional theory ( Kulkarni et al. Catal. Sci. Technol. 2018 , 8 , 114 ). The comparison of the amount of the surface methoxy intermediates formed and the number of different Cu(II) sites present in the zeolite allowed us to conclude the involvement of Z2Cu(II) sites in methane C-H bond activation. The mechanism of methane activation on Z2Cu(II) sites has been proposed. It includes two steps: (1) the formation of the molecular complex of methane with Z2Cu(II); (2) heterolytic dissociation of the polarized C-H bond affording surface copper(II) hydride and methoxy species, both bound to zeolite framework Si-O--Al sites.