Methanol diffusion and dynamics in zeolite H-ZSM-5 probed by quasi-elastic neutron scattering and classical molecular dynamics simulations.

Zeolite ZSM-5 is a key catalyst in commercially relevant processes including the widely studied methanol to hydrocarbon reaction, and molecular diffusion in zeolite pores is known to be a crucial factor in controlling catalytic reactions. Here, we present critical analyses of recent quasi-elastic neutron scattering (QENS) data and complementary molecular dynamics (MD) simulations. The QENS experiments show that the nature of methanol diffusion dynamics in ZSM-5 pores is dependent both on the Si/Al ratio (11, 25, 36, 40 and 140), which determines the Brønsted acid site density of the zeolite, and that the nature of the type of motion observed may vary qualitatively over a relatively small temperature range. At 373 K, on increasing the ratio from 11 to 140, the observed mobile methanol fraction increases and the nature of methanol dynamics changes from rotational (in ZSM-5 with Si/Al of 11) to translational diffusion. The latter is either confined localized diffusion within a pore in zeolites with ratios up to 40 or non-localized, longer-range diffusion in zeolite samples with the ratio of 140. The complementary MD simulations conducted over long time scales (1 ns), which are longer than those measured in the present study by QENS (≈1-440 ps), at 373 K predict the occurrence of long-range translational diffusion of methanol in ZSM-5, independent of the Si/Al ratios (15, 47, 95, 191 and siliceous MFI). The rate of diffusion increases slightly by increasing the ratio from 15 to 95 and thereafter does not depend on zeolite composition. Discrepancies in the observed mobile methanol fraction between the MD simulations (100% methanol mobility in ZSM-5 pores across all Si/Al ratios) and QENS experiments (for example, ≈80% immobile methanol in ZSM-5 with Si/Al of 11) are attributed to the differences in time resolutions of the techniques. This perspective provides comprehensive information on the effect of acid site density on methanol dynamics in ZSM-5 pores and highlights the complementarity of QENS and MD, and their advantages and limitations. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Experimental and Modeling Studies of Local and Nanoscale para-Cresol Behavior: A Comparison of Classical Force Fields.

The dynamics of bulk liquid para-cresol from 340-390 K was probed using a tandem quasielastic neutron scattering (QENS) and molecular dynamics (MD) approach, due to its relevance as a simple model component of lignin pyrolysis oil. QENS experiments observed both translational jump diffusion and isotropic rotation, with diffusion coefficients ranging from 10.1 to 28.6 × 10-10 m2s-1 and rotational rates from 5.7 to 9.2 × 1010 s-1. The associated activation energies were 22.7 ± 0.6 and 10.1 ± 1.2 kJmol-1 for the two different dynamics. MD simulations applying two different force field models (OPLS3 and OPLS2005) gave values close to the experimental diffusion coefficients and rotational rates obtained upon calculating the incoherent dynamic structure factor from the simulations over the same time scale probed by the QENS spectrometer. The simulations gave resulting jump diffusion coefficients that were slower by factors of 2.0 and 3.8 and rates of rotation that were slower by factors of 1.2 and 1.6. Comparing the two force field sets, the OPLS3 model showed slower rates of dynamics likely due to a higher molecular polarity, leading to greater quantities and strengths of hydrogen bonding.

Octane isomer dynamics in H-ZSM-5 as a function of Si/Al ratio: a quasi-elastic neutron scattering study.

Dynamical behaviour of n-octane and 2,5-dimethylhexane in H-ZSM-5 zeolite catalysts of differing Si/Al ratios (15 and 140) was probed using quasi-elastic neutron scattering, to understand molecular shape and Brønsted acid site density effects on the behaviour of common species in the fluid catalytic cracking (FCC) process, where H-ZSM-5 is an additive catalyst. Between 300 and 400 K, n-octane displayed uniaxial rotation around its long axis. However, the population of mobile molecules was larger in H-ZSM-5(140), suggesting that the lower acid site concentration allows for more molecules to undergo rotation. The rotational diffusion coefficients were higher in H-ZSM-5(140), reflecting this increase in freedom. 2,5-dimethylhexane showed qualitative differences in behaviour to n-octane, with no full molecule rotation, probably due to steric hindrance in the constrictive channels. However, methyl group rotation in the static 2,5-dimethylhexane molecules was observed, with lower mobile fractions in H-ZSM-5(15), suggesting that this rotation is less hindered when fewer Brønsted sites are present. This was further illustrated by the lower activation barrier calculated for methyl rotation in H-ZSM-5(140). We highlight the significant immobilizing effect of isomeric branching in this important industrial catalyst and show how compositional changes of the zeolite can affect a range of dynamical behaviours of common FCC species upon adsorption. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

Integrated Theoretical and Empirical Studies for Probing Substrate-Framework Interactions in Hierarchical Catalysts.

Soft templating with siliceous surfactant is an established protocol for the synthesis of hierarchically porous silicoaluminophosphates (HP SAPOs) with improved mass transport properties. Motivated by the enhanced performance of HP SAPOs in the Beckmann rearrangement of cyclohexanone oxime to the nylon 6 precursor ϵ-caprolactam, an integrated theoretical and empirical study was carried out to investigate the catalytic potential of the siliceous mesopore network. Inelastic neutron scattering (INS) studies, in particular, provided unique insight into the substrate-framework interactions in HP (Si)AlPOs and allowed reactive species to be studied independent of the catalyst matrix. The spectroscopic (INS, FTIR spectroscopy, MAS NMR spectroscopy) and computational analyses revealed that in the organosilane-templated SAPO, the interconnectivity of micro- and mesopores permits cooperativity between their respective silanol and Brønsted acid sites that facilitates the protonation of cyclohexanone oxime in a physical mixture at ambient temperature.

Comprehensive Vibrational Spectroscopic Characterization of Nylon-6 Precursors for Precise Tracking of the Beckmann Rearrangement.

As a key step in nylon-6 synthesis, the Beckmann rearrangement is an ongoing target of catalytic studies that seek to improve the sustainability of polymer manufacture. Whilst solid-acid catalysts (predominantly zeotypes) have proven effective for this transformation, the development of more active and selective systems demands an understanding of fundamental catalytic mechanisms. In this undertaking, in situ and operando characterization techniques can be informative, provided rigorous spectroscopic groundwork is in place. Thus, to facilitate mechanistic studies we present a detailed investigation of the vibrational spectra of cyclohexanone, cyclohexanone oxime, ϵ-caprolactam and their D10-isotopomers, in the solid state. Variable-temperature infrared (150-300 K) and Raman (10-300 K) spectra are reported alongside inelastic neutron scattering data. Moreover, where key vibrational modes have been assigned with the aid of periodic density functional theory calculations, it has been possible to include hydrogen-bonding interactions explicitly.

Neutron spectroscopy as a tool in catalytic science.

Catalytic science currently has access to a range of advanced experimental methods for the study of molecular behaviour in chemical processes. Neutron spectroscopy, however, is uniquely placed to gain detailed insight into such systems, particularly through techniques such as vibrational spectroscopy with neutrons (INS) which gives access to vibrational modes unavailable to conventional spectroscopy techniques, and quasielastic neutron scattering (QENS) which studies molecular motion on a range of timescales. The present article illustrates the role of these techniques in advancing the field of catalysis. We first provide a brief introduction to the basic principles of the techniques, and then discuss their use in the study of three key catalytic systems: the behaviour of hydrocarbons confined in zeolite catalysts; the methanol-to-hydrocarbons process; and methane reforming. We demonstrate the importance of neutron spectroscopy in understanding established catalytic processes, but also consider its role in the design of future catalytic systems.

Ammonia mobility in chabazite: insight into the diffusion component of the NH3-SCR process.

The diffusion of ammonia in commercial NH3-SCR catalyst Cu-CHA was measured and compared with H-CHA using quasielastic neutron scattering (QENS) and molecular dynamics (MD) simulations to assess the effect of counterion presence on NH3 mobility in automotive emission control relevant zeolite catalysts. QENS experiments observed jump diffusion with a jump distance of 3 Å, giving similar self-diffusion coefficient measurements for both Cu- and H-CHA samples, in the range of ca. 5-10 × 10(-10) m(2) s(-1) over the measured temperature range. Self-diffusivities calculated by MD were within a factor of 6 of those measured experimentally at each temperature. The activation energies of diffusion were also similar for both studied systems: 3.7 and 4.4 kJ mol(-1) for the H- and Cu-chabazite respectively, suggesting that counterion presence has little impact on ammonia diffusivity on the timescale of the QENS experiment. An explanation is given by the MD simulations, which showed the strong coordination of NH3 with Cu(2+) counterions in the centre of the chabazite cage, shielding other molecules from interaction with the ion, and allowing for intercage diffusion through the 8-ring windows (consistent with the experimentally observed jump length) to carry on unhindered.

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. The experimental diffusion coefficients were measured in the range 2-5 × 10(-10) m(2) s(-1) and simulated diffusion coefficients calculated in the range of 1.6-3.2 × 10(-9) m(2) s(-1). Activation energies were measured as 8.8 and 6.9 kJ mol(-1) using QENS and MD respectively. Differences may be attributed predominantly to the experimental use of a dealuminated HY sample, containing significant defects such as strongly adsorbing silanol nests, compared to a perfect simulated crystal containing only evenly distributed Brønsted acid sites. Experimental and simulated diffusivities measured in this study are lower than those obtained from those previously calculated in siliceous faujasite, due to methanol H-bonding to Brønsted acid sites as observed in the MD simulations. However, both experimental and simulated diffusivities were significantly higher than those obtained in NaX, due to the higher concentration of extraframework cations present in the previously studied structures.

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 observed complete room temperature conversion of methanol to framework methoxy in a commercial sample of methanol-to-hydrocarbons (MTH) catalyst H-ZSM-5, evidenced by methanol immobility and vibrational spectra matched by ab initio calculations. No methoxylation was observed in a commercial HY sample, attributed to the dealumination involved in high silica HY synthesis.

Molecular dynamics simulations of longer n-alkanes in silicalite: a comparison of framework and hydrocarbon models.

The diffusion of n-alkanes ranging from length n-C8 to n-C20 in the zeolite silicalite is studied using classical molecular dynamics simulations. Different simulations were performed using a united-atom hydrocarbon model with a rigid zeolite framework, an all-atom hydrocarbon model with a rigid zeolite framework, and an all-atom hydrocarbon model with a flexible zeolite framework, all at 300 K. The latter two models have never previously been used to simulate longer alkanes in silicalite. Diffusion coefficients measured using a rigid zeolite framework exhibited a periodic dependence on chain length in the [010] direction in line with the previously observed phenomenon of resonant diffusion, regardless of the hydrocarbon model used. Explanations are considered in terms of the location of low energy traps within the silicalite structure, presenting a diffusion barrier. A monotonic dependence on diffusivity with chain length was observed however, on using an all-atom hydrocarbon model and a flexible framework, which was attributed to the occurrence of pore 'breathing' assisting diffusion. It was also noted that the calculated diffusion coefficients were up to an order of magnitude lower, and experimental diffusion coefficients are in much closer agreement when the latter model is used.