Combined computational and neutron scattering studies of hydrocarbons confined in mesoporous materials

Abstract

Molecular Dynamics (MD) simulations and Quasi Elastic Neutron Scattering (QENS) experiments are used to investigate the dynamics of industrially relevant catalysis reactions. Despite the widespread industrial applications of mesoporous materials there is often little understanding of the dynamical processes within the catalytic pores. In fact the rate limiting step is often the diffusion of the reactants within the pores and so therefore to be able to quantify the diffusion behaviour within these pores, and as bulk liquids to provide a comparison, is important. The dynamics and associated diffusion behaviour of benzene, cyclohexane and cyclohexene both as unconfined liquids and confined in MCM-41 and the catalyst Pt/MCM-41 were obtained. Similar combined MD and QENS studies were carried out on toluene and methylcyclohexane as well as on the three xylene isomers, mesitylene and trifluorobenzene. The time and length scales investigated was on the order of ps to ns and Å in both the MD simulations and QENS experiments. According the QENS experiments the effect of confining a liquid is a decrease in mobility as evidenced by a lower diffusion coefficient for confined liquids. This is true for all five liquids. All confined liquids have similar diffusion coefficients suggesting MCM-41 has the same effect on all liquids. MD simulations were carried out using a three different framework models of MCM-41; a rigid, a semi-flexible and a novel fully flexible to test the effect of framework flexibility on the dynamics of confined sorbates. Both MD and QENS found decreases in the mobility of confined sorbates compared to unconfined. Comparing the bulk diffusion coefficients obtained through QENS experiments and MD simulations shows it is possible to achieve acceptable agreement. The MD simulations underestimate the QENS results for most of the liquids and have larger errors between theory and experiment then the confined systems. The confined systems show much better agreement than the bulk liquids between the MD and QENS studies. In addition the fully flexible MCM-41 framework proved superior in matching the experimental QENS translational diffusion coefficient results. In addition to the dynamical behaviour studied some structural information is obtained through the MD simulations namely distribution functions and probability densities. Through this the effect of confinement on each sorbate is studied. In conclusion the combination of neutron scattering and molecular simulations has provided insight into the dynamics of hydrocarbons both as unconfined bulk liquids and confined in mesoporous materials; namely the mobility of confined liquids is reduced. It is also possible to conclude that framework flexibility is necessary for MD simulations to match experimental results.