Density Functional Theory Calculations of the Activation of Methanol by a Broensted Zeolitic Proton

Trimethyloxonium ion – a zeolite confined mobile and efficient methyl carrier at low temperatures: a DFT study coupled with microkinetic analysis

The reaction network of ethene methylation over H-ZSM-5, including methanol dehydration, ethene methylation, and C3H7+ conversion, is investigated by employing a multiscale approach combining DFT calculations and microkinetic modeling.

Influence of Topology and Brønsted Acid Site Presence on Methanol Diffusion in Zeolites Beta and MFI

Detailed insight into molecular diffusion in zeolite frameworks is crucial for the analysis of the factors governing their catalytic performance in methanol-to-hydrocarbons (MTH) reactions. In this work, we present a molecular dynamics study of the diffusion of methanol in all-silica and acidic zeolite MFI and Beta frameworks over the range of temperatures 373–473 K. Owing to the difference in pore dimensions, methanol diffusion is more hindered in H-MFI, with diffusion coefficients that do not exceed 10 × 10−10 m2s−1. In comparison, H-Beta shows diffusivities that are one to two orders of magnitude larger. Consequently, the activation energy of translational diffusion can reach 16 kJ·mol−1 in H-MFI, depending on the molecular loading, against a value for H-Beta that remains between 6 and 8 kJ·mol−1. The analysis of the radial distribution functions and the residence time at the Brønsted acid sites shows a greater probability for methylation of the framework in the MFI structure compared to zeolite Beta, with the latter displaying a higher prevalence for methanol clustering. These results contribute to the understanding of the differences in catalytic performance of zeolites with varying micropore dimensions in MTH reactions.

Mechanistic Insight into the Framework Methylation of H-ZSM-5 for Varying Methanol Loadings and Si/Al Ratios Using First-Principles Molecular Dynamics Simulations

The methanol-to-hydrocarbon process is known to proceed autocatalytically in H-ZSM-5 after an induction period where framework methoxy species are formed. In this work, we provide mechanistic insight into the framework methylation within H-ZSM-5 at high methanol loadings and varying acid site densities by means of first-principles molecular dynamics simulations. The molecular dynamics simulations show that stable methanol clusters form in the zeolite pores, and these clusters commonly deprotonate the active site; however, the cluster size is dependent on the temperature and acid site density. Enhanced sampling molecular dynamics simulations give evidence that the barrier for methanol conversion is significantly affected by the neighborhood of an additional acid site, suggesting that cooperative effects influence methanol clustering and reactivity. The insights obtained are important steps in optimizing the catalyst and engineering the induction period of the methanol-to-hydrocarbon process.

Computational QM/MM investigation of the adsorption of MTH active species in H-Y and H-ZSM-5

The transformation of methanol-to-hydrocarbons (MTH) has significant potential as a route to synthesise low-cost fuels; however, the initial stages of the zeolite catalysed MTH process are not well understood.

Modelling metal centres, acid sites and reaction mechanisms in microporous catalysts

We discuss the role of QM/MM (embedded cluster) computational techniques in catalytic science, in particular their application to microporous catalysis. We describe the methodologies employed and illustrate their utility by briefly summarising work on metal centres in zeolites. We then report a detailed investigation into the behaviour of methanol at acidic sites in zeolites H-ZSM-5 and H-Y in the context of the methanol-to-hydrocarbons/olefins process. Studying key initial steps of the reaction (the adsorption and subsequent methoxylation), we probe the effect of framework topology and Brønsted acid site location on the energetics of these initial processes. We find that although methoxylation is endothermic with respect to the adsorbed system (by 17-56 kJ mol(-1) depending on the location), there are intriguing correlations between the adsorption/reaction energies and the geometries of the adsorbed species, of particular significance being the coordination of methyl hydrogens. These observations emphasise the importance of adsorbate coordination with the framework in zeolite catalysed conversions, and how this may vary with framework topology and site location, particularly suited to investigation by QM/MM techniques.

Room temperature methoxylation in zeolites: insight into a key step of the methanol-to-hydrocarbons process

Neutron scattering methods studying mobility and vibrational spectra observed complete room temperature methoxylation in a commercial sample of methanol-to-hydrocarbons (MTH) catalyst H-ZSM-5.

Influence of acid site density on the three-staged MTH induction reaction over HZSM-5 zeolite

High acid site density can accelerate formation of active species and shorten the initial two stages of MTH induction reaction.

Methanol diffusion in zeolite HY: a combined quasielastic neutron scattering and molecular dynamics simulation study

The diffusion of methanol in zeolite HY is studied using tandem quasielastic neutron scattering (QENS) experiments and molecular dynamics (MD) simulations at 300–400 K.

Methane to acetic acid over Cu-exchanged zeolites: mechanistic insights from a site-specific carbonylation reaction

The selective low temperature oxidation of methane is an attractive yet challenging pathway to convert abundant natural gas into value added chemicals. Copper-exchanged ZSM-5 and mordenite (MOR) zeolites have received attention due to their ability to oxidize methane into methanol using molecular oxygen. In this work, the conversion of methane into acetic acid is demonstrated using Cu-MOR by coupling oxidation with carbonylation reactions. The carbonylation reaction, known to occur predominantly in the 8-membered ring (8MR) pockets of MOR, is used as a site-specific probe to gain insight into important mechanistic differences existing between Cu-MOR and Cu-ZSM-5 during methane oxidation. For the tandem reaction sequence, Cu-MOR generated drastically higher amounts of acetic acid when compared to Cu-ZSM-5 (22 vs 4 μmol/g). Preferential titration with sodium showed a direct correlation between the number of acid sites in the 8MR pockets in MOR and acetic acid yield, indicating that methoxy species present in the MOR side pockets undergo carbonylation. Coupled spectroscopic and reactivity measurements were used to identify the genesis of the oxidation sites and to validate the migration of methoxy species from the oxidation site to the carbonylation site. Our results indicate that the Cu(II)-O-Cu(II) sites previously associated with methane oxidation in both Cu-MOR and Cu-ZSM-5 are oxidation active but carbonylation inactive. In turn, combined UV-vis and EPR spectroscopic studies showed that a novel Cu(2+) site is formed at Cu/Al <0.2 in MOR. These sites oxidize methane and promote the migration of the product to a Brønsted acid site in the 8MR to undergo carbonylation.

First principle chemical kinetics in zeolites: the methanol-to-olefin process as a case study

To optimally design next generation catalysts a thorough understanding of the chemical phenomena at the molecular scale is a prerequisite. Apart from qualitative knowledge on the reaction mechanism, it is also essential to be able to predict accurate rate constants. Molecular modeling has become a ubiquitous tool within the field of heterogeneous catalysis. Herein, we review current computational procedures to determine chemical kinetics from first principles, thus by using no experimental input and by modeling the catalyst and reacting species at the molecular level. Therefore, we use the methanol-to-olefin (MTO) process as a case study to illustrate the various theoretical concepts. This process is a showcase example where rational design of the catalyst was for a long time performed on the basis of trial and error, due to insufficient knowledge of the mechanism. For theoreticians the MTO process is particularly challenging as the catalyst has an inherent supramolecular nature, for which not only the Brønsted acidic site is important but also organic species, trapped in the zeolite pores, must be essentially present during active catalyst operation. All these aspects give rise to specific challenges for theoretical modeling. It is shown that present computational techniques have matured to a level where accurate enthalpy barriers and rate constants can be predicted for reactions occurring at a single active site. The comparison with experimental data such as apparent kinetic data for well-defined elementary reactions has become feasible as current computational techniques also allow predicting adsorption enthalpies with reasonable accuracy. Real catalysts are truly heterogeneous in a space- and time-like manner. Future theory developments should focus on extending our view towards phenomena occurring at longer length and time scales and integrating information from various scales towards a unified understanding of the catalyst. Within this respect molecular dynamics methods complemented with additional techniques to simulate rare events are now gradually making their entrance within zeolite catalysis. Recent applications have already given a flavor of the benefit of such techniques to simulate chemical reactions in complex molecular environments.

Unraveling the reaction mechanisms governing methanol-to-olefins catalysis by theory and experiment

The conversion of methanol to olefins (MTO) over a heterogeneous nanoporous catalyst material is a highly complex process involving a cascade of elementary reactions. The elucidation of the reaction mechanisms leading to either the desired production of ethene and/or propene or undesired deactivation has challenged researchers for many decades. Clearly, catalyst choice, in particular topology and acidity, as well as the specific process conditions determine the overall MTO activity and selectivity; however, the subtle balances between these factors remain not fully understood. In this review, an overview of proposed reaction mechanisms for the MTO process is given, focusing on the archetypal MTO catalysts, H-ZSM-5 and H-SAPO-34. The presence of organic species, that is, the so-called hydrocarbon pool, in the inorganic framework forms the starting point for the majority of the mechanistic routes. The combination of theory and experiment enables a detailed description of reaction mechanisms and corresponding reaction intermediates. The identification of such intermediates occurs by different spectroscopic techniques, for which theory and experiment also complement each other. Depending on the catalyst topology, reaction mechanisms proposed thus far involve aromatic or aliphatic intermediates. Ab initio simulations taking into account the zeolitic environment can nowadays be used to obtain reliable reaction barriers and chemical kinetics of individual reactions. As a result, computational chemistry and by extension computational spectroscopy have matured to the level at which reliable theoretical data can be obtained, supplying information that is very hard to acquire experimentally. Special emphasis is given to theoretical developments that open new perspectives and possibilities that aid to unravel a process as complex as methanol conversion over an acidic porous material.

Conversion of methanol to hydrocarbons: how zeolite cavity and pore size controls product selectivity

Liquid hydrocarbon fuels play an essential part in the global energy chain, owing to their high energy density and easy transportability. Olefins play a similar role in the production of consumer goods. In a post-oil society, fuel and olefin production will rely on alternative carbon sources, such as biomass, coal, natural gas, and CO(2). The methanol-to-hydrocarbons (MTH) process is a key step in such routes, and can be tuned into production of gasoline-rich (methanol to gasoline; MTG) or olefin-rich (methanol to olefins; MTO) product mixtures by proper choice of catalyst and reaction conditions. This Review presents several commercial MTH projects that have recently been realized, and also fundamental research into the synthesis of microporous materials for the targeted variation of selectivity and lifetime of the catalysts.

Theoretical simulations elucidate the role of naphthalenic species during methanol conversion within H-SAPO-34

The role of naphthalenic species during the methanol-to-olefins (MTO) process in a silicoaluminophosphate zeolitic material exhibiting the chabazite topology (H-SAPO-34) has been studied from first principles. These species could either act as active olefin-eliminating compounds or as precursors for deactivating species. Results incorporating van der Waals contributions for finite large clusters point out that successive methylation steps of naphthalenic compounds are feasible. The calculated intrinsic activation barrier is relatively independent of the number of methyl groups already attached on the aromatic compound and is approximately 140 kJ mol(-1). The influence of the composition of the catalyst and hence the acidic strength on the intrinsic chemical kinetics was investigated in detail through comparison with the isostructural high-silicon material. Apparent chemical kinetics, starting from adsorbed methanol on the acid site, were also computed. The initiation steps of the side-chain route starting from a trimethylated naphthalenium ion were also examined. The actual side-chain methylation exhibits a high barrier and hence this mechanism involving methylated naphthalenes is not expected to be an active ethene-eliminating route in H-SAPO-34.

Selective decomposition of alkyl hydroperoxides on H-type zeolite via a concerted approach

Catalyzed by H-type zeolite, the selective decomposition of alkyl hydroperoxides (HP), including 1-phenylethyl hydroperoxide (PEHP), phenylmethyl hydroperoxide (PMHP), and methyl hydroperoxide (MHP), to their corresponding carbonyl compounds and H2O have been investigated by DFT calculations. In this calculation, a 19-atom cluster containing four tetrahedral atoms (denoted as 4T) was used to model the H-type zeolite catalyst, and two competitive reaction channels were evaluated. Calculations predict that the concerted reaction channel is favorable for selective decomposition of alkyl hydroperoxides on the H-type zeolite from both kinetics and thermodynamics points of view. We found that the rate-determining step of the stepwise reaction channel proceeds via a one-hydrogen-bond transition state followed by dehydration. In the concerted reaction channel, whose transition state is an eight-membered ring with two hydrogen bonds, the decomposition of hydroperoxides occurs synchronously to form carbonyl compounds and H2O, involving the proton exchange between hydroperoxides and the zeolite.

Reactivity enhancement of 2-propanol photocatalysis on SO4(2-)/TiO2: insights from solid-state NMR spectroscopy

Solid-state NMR techniques have been employed to investigate the surface acidic properties of TiO2 and sulfated TiO2, as well as their photocatalytic activities toward 2-propanol. The multinuclear MAS NMR experiments clearly revealed that three different types of Brønsted acid sites with much stronger acid strength were generated after the sulfation of TiO2. Due to the enhanced Brønsted acidity, the protonation of 2-propanol can occur more easily, preferentially leading to the formation of Ti-bound 2-propoxy species on the SO42-/TiO2 catalyst The 2-propoxy species can be directly converted to CO2 and thus the photocatalytic activity of sulfated TiO2 catalyst is remarkably enhanced. For comparison, both hydrogen-bonded 2-propanol and Ti-bound 2-propoxy species are present on the TiO2 catalyst with the former being predominant The hydrogen-bonded 2-propanol species are oxidated into acetone molecules that are difficult to further convert into CO2, and the conversion of 2-propoxy species to 2-propanol hampers the direct mineralization of 2-propoxy species on the TiO2 catalyst.

Photo-induced proton-transfer cycle of 2-naphthol in faujasite zeolitic nanocavities

The excited-state deprotonation and ground-state reprotonation of a 2-naphthol molecule encapsulated in the zeolitic nanocavity of NaX have been studied by measuring static and time-resolved spectra of fluorescence and reflectance. The excited molecule undergoes enol dissociation within 300 ps to form an isolated ion pair, which undergoes geminate recombination in 1200 ps or separation to produce the anionic species of 2-naphtholate on the time scale of 2500 ps. Ground-state reprotonation, controlled by the diffusion rate of a proton, is then followed in 0.8 ms with an activation energy of 13 kJ mol(-1).

Hydrated metal ions in the gas phase

Studying metal ion solvation, especially hydration, in the gas phase has developed into a field that is dominated by a tight interaction between experiment and theory. Since the studied species carry charge, mass spectrometry is an indispensable tool in all experiments. Whereas gas-phase coordination chemistry and reactions of bare metal ions are reasonably well understood, systems containing a larger number of solvent molecules are still difficult to understand. This review focuses on the rich chemistry of hydrated metal ions in the gas phase, covering coordination chemistry, charge separation in multiply charged systems, as well as intracluster and ion-molecule reactions. Key ideas of metal ion solvation in the gas phase are illustrated with rare-gas solvated metal ions.

New all-atom force field for molecular dynamics simulation of an AlPO4-34 molecular sieve

A force field of the triclinic framework of AlPO(4)-34, important in methanol-hydrocarbon conversion reactions, was developed using an empirical potential function. Molecular dynamics simulation of an AlPO(4)-34 triclinic framework segment of 1216 atoms, containing the template molecules isopropylamine and water, was performed with explicit consideration of atomic charges. The average RMS difference between instantaneous positions of the framework atoms during 1 ns simulation and their positions in the structure determined from single crystal X-ray diffraction was calculated, and the average structure of the flexible framework was determined. The computed Debye-Waller factors and simulated FTIR spectra are in good agreement with the experimental data. The new force field permits detailed molecular dynamics simulations of flexible, charged aluminophosphate molecular sieves which should lead to a better understanding of the catalytic processes and the crucial role played by templating molecules.

Methylation of Alkenes and Methylbenzenes by Dimethyl Ether or Methanol on Acidic Zeolites

The methylation of propene and toluene with dimethyl ether has been studied using both experimental and theoretical methods, and the results are compared with results obtained for methylation with methanol. The results indicate that the nature of the methylation reaction mechanisms is very similar for both methylation agents. Both experiment and theory show that dimethyl ether is a slightly more reactive methylating agent than methanol.

DFT Investigation of Alkoxide vs Alkylammonium Formation in Amine-Substituted Zeolites

Density functional theory (DFT) cluster calculations were used to describe bifunctional acid-base properties of amine-substituted zeolites containing a Brønsted acid site. Preliminary results (J. Am. Chem. Soc. 2004, 126, 9162) indicated that efficient use of both functional groups might lead to a substantial lowering of activation barriers. In this paper, comparison is made between the alkoxide formation in zeolites containing only oxygen bridges and alkylammonium formation on the bridging NH groups in amine-functionalized zeolites for various guest species, such as methanol, ethene, and chloromethane. The amine functionalization only lowers barriers for SN2 type reactions with otherwise highly strained transition states, as is the case for chloromethane. In these new materials more basic sites are introduced into the zeolite framework, enabling optimal linear SN2 type transition states incorporating various T sites.

Modeling the local structure and energetics of protozeolitic nanoclusters in hydrothermally stable aluminosilicate mesostructures

The density functional theory (DFT) method is used to investigate the structure and bonding of silica and aluminosilicate nanoclusters containing five- and six-membered oxygen rings. The clusters, which are derived from the BEA zeolite structure, are considered as models of the protozeolitic clusters that are incorporated into the pore walls of steam stable aluminosilicate mesostructures assembled from zeolite seeds. Two locally different Brønsted acid sites in the aluminosilicate structure are identified for the adsorption of a water molecule. The sterically more open acid site is favored for water binding. The stability of the aluminosilicate structure in the presence of H2O molecule is studied by breaking an Al-O bond and inserting a water molecule into the five-membered ring structure. We find that an excitation energy at least 18 times larger than the room-temperature thermal energy is needed to break the stable five-membered ring structure, implying a high hydrothermal stability and acidity for this aluminosilicate structure.

Electron solvation by highly polar molecules: Density functional theory study of atomic sodium interaction with water, ammonia, and methanol

This study further extends the scope of a previous paper [Y. Ferro and A. Allouche, J. Chem. Phys. 118, 10461 (2003)] on the reactivity of atomic Na with water to some other highly polar molecules known for their solvation properties connected to efficient hydrogen bonding. The solvation mechanisms of ammonia and methanol are compared to the hydration mechanism. It is shown that in the case of ammonia, the stability of the solvated system is only ensured by electrostatic interactions, whereas the methanol action is more similar to that of water. More specific attention is given to the solvation process of the valence 3s Na electron. The consequences on the chemical reactivity are analyzed: Whereas ammonia is nonreactive when interacting with atomic sodium, two chemical reactions are proposed for methanol. The first process is dehydrogenation and yields methoxy species and hydrogen. The other one is dehydration and the final products are methoxy species, but also methyl radical and water. The respective roles of electron solvation and hydrogen bonds network are analyzed in detail in view of the density of states of the reactive systems.

Computational elucidation of the transition state shape selectivity phenomenon

The most commonly cited example of a transition state shape selective reaction, m-xylene disproportionation in zeolites, is examined to determine if the local spatial environment of a reaction can significantly alter selectivity. In the studied reaction, ZPE-corrected rate limiting energy barriers are 136 kJ/mol for the methoxide-mediated pathway and 109 to 145 kJ/mol for the diphenylmethane-mediated pathway. Both pathways are likely to contribute to selectivity and disfavor one product isomer (1,3,5-trimethylbenzene), but relative selectivity to the other two isomers varies with pore geometry, mechanistic pathway, and inclusion of entropic effects. Most importantly, study of one pathway in three different common zeolite framework types (FAU, MFI, and MOR) allows explicit and practically oriented consideration of pore shape. Variation of the environment shape at the critical transition states is thus shown to affect the course of reaction. Barrier height shifts on the order of 10-20 kJ/mol are achievable. Observed selectivities do not agree with the transition state characteristics calculated here and, hence, are most likely due to product shape selectivity. Further examination of the pathways highlights the importance of mechanistic steps that do not result in isomer-defining bonds and leads to a more robust definition of transition state shape selectivity.

Evidence for an initiation of the methanol-to-olefin process by reactive surface methoxy groups on acidic zeolite catalysts

Recent progress reveals that, in the methanol-to-olefin (MTO) process on acidic zeolites, the conversion of an equilibrium mixture of methanol and DME is dominated by a "hydrocarbon pool" mechanism. However, the initial C-C bond formation, that is, the chemistry during the kinetic "induction period" leading to the reactive hydrocarbon pool, still remains unclear. With the application of a stopped-flow protocol, in the present work, pure surface methoxy groups [SiO(CH(3))Al] were prepared on various acidic zeolite catalysts (H-Y, H-ZSM-5, H-SAPO-34) at temperatures lower than 473 K, and the further reaction of these methoxy species was investigated by in situ (13)C MAS NMR spectroscopy. By using toluene and cyclohexane as probe molecules which are possibly involved in the MTO process, we show the high reactivity of surface methoxy species. Most importantly, the formation of hydrocarbons from pure methoxy species alone is demonstrated for the first time. It was found that (i) surface methoxy species react at room temperature with water to methanol, indicating the occurrence of a chemical equilibrium between these species at low temperatures. In the presence of aromatics and alkanes, (ii) the reactivity of surface methoxy groups allows a methylation of these organic compounds at reaction temperatures of ca. 433 and 493 K, respectively. In the absence of water and other organic species, that is, under flow conditions and on partially methylated catalysts, (iii) a conversion of pure methoxy groups alone to hydrocarbons was observed at temperatures of T >/= 523 K. This finding indicates a possible formation of the first hydrocarbons during the kinetic induction period of the MTO process via the conversion of pure surface methoxy species (case iii). After the first hydrocarbons are formed, or in the presence of a small amount of organic impurities, surface methoxy groups contribute to a further methylation of these organic compounds (case ii), leading to the formation of a reactive hydrocarbon pool which eventually plays an active role in the steady state of the MTO process at reaction temperatures of T >/= 573 K.

Bridging Methoxy Groups in NaY, NaX and NaLSX Zeolites

Distribution of chemisorbed methyl groups and sodium cations in the structure of NaY, NaX and NaLSX zeolites was estimated by neutron diffraction. Chemisorbed methyl groups were prepared in the structure by reaction of methyl iodide with reactive sodium cations available in SII and SIII positions of faujasites. Methyl cations CH3+, formed in the reaction, react immediately with the lattice oxygen forming surface bonded methyl groups in bridging configuration. 13C NMR signals of chemisorbed surface species and their linear dependence on the intermediate electronegativity of the zeolite lie in the interval from 53 ppm for most basic CsLSX to 58 ppm TMS for stabilized and acid leached sample of H,NaY-St. Changes in the distribution of structural sodium cations in the lattice after chemisorption of methyl cations have been detected. C-O distances in surface methoxy groups in void cavities were longer than in ordinary crystalline organometallic compounds with bridging methoxy groups. The location of chemisorbed methyl groups at the O1 lattice oxygen type was most probable for NaY. Nuclear densities of chemisorbed methyl groups were detected in NaX at O1 and at O4 lattice oxygens. The origin of the split signal at 58 ppm on NaX and NaLSX samples has been discussed.