Molecular Dynamics Kinetic Study on the Zeolite-Catalyzed Benzene Methylation in ZSM-5

Abstract

The methylation of arenes is a key step in the production of hydrocarbons from methanol over acidic zeolites. We performed ab initio static and molecular dynamics free energy simulations of benzene methylation in H-ZSM-5 to determine the factors that influence the reaction kinetics. Special emphasis is given to the effect of the surrounding methanol molecules on the methylation kinetics. It is found that for higher methanol loadings, methylation may also occur from a protonated methanol cluster, indicating that the exact location of the Brønsted acid site is not essential for the zeolite-catalyzed methylation reaction. However, methylations from a protonated methanol cluster exhibit higher free energy barriers than a methylation from a single methanol molecule. Finally, comparison with a pure methanol solvent reaction environment indicates that the main role of the zeolite during the methylation of benzene is to provide the acidic proton and to create a polar environment for the reaction. The metadynamics approach, which is specifically designed to sample rare events, allows exploring new reaction pathways, which take into account the flexibility of the framework and additional guest molecules in the pores and channels of the zeolite framework. This approach goes beyond the often applied static calculations to determine reaction kinetics.

Complex Reaction Environments and Competing Reaction Mechanisms in Zeolite Catalysis: Insights from Advanced Molecular Dynamics

“…[27][28][29][30][31][32][33][34][35][36][37] Only a limited number of studies have applied techniques that go beyond the static approach to describe chemical transformation in zeolites. [19,20,[38][39][40][41][42][43][44] The strength of a dynamical approach was earlier demonstrated by some of the present authors.We reported that the influence of the presence of multiple protic molecules on the methylation of benzene [44] and methoxide formation [42] in H-ZSM-5 has to be studied with molecular dynamics. These studies demonstrated that formation of protonated clusters of protic molecules has to be taken into account when modeling reactions under conditions of high reactant loadings, in particular in case of protic reactants that can play a solvent-like role.…”

“…The effect of this phenomenon on activation barriers was recently demonstrated by some of the present authors. [42,44] For the methylation of benzene in H-ZSM-5 it was found that after framework deprotonation, protonated hydrogen bonded methanol clusters are formed, which are less reactive towards methylation compared to single protonated methanol molecules. These protonated clusters of protic molecules could be observed as well during our simulations (Table S5.1).…”

“…[6] Consequently, numerous studies focused on the methylation by methanol, dimethyl ether or methoxides of aromatics and alkenes in the framework of the MTO process. [16,17,26,42,44,46,82,83] It should be mentioned that the static approach with finite cluster models as applied in most of these studies has been very successful in reproducing and predicting experimentally measured kinetic data. [16,17] However, a similar approach is not applicable for the study of methylation reactions in the large pore AFI structured H-SSZ-24 as will be demonstrated in this case study.…”

“…As an example we mention the role of assisting water molecules on a recently proposed reaction cycle for olefin elimination during methanol conversion in H-SAPO-34. [74] That protic molecules affect proton transfer and possibly also reaction mechanisms, can be well understood by the application of dynamical methods, as was already proven for chemical reactions in aqueous solution [75] and zeolites [42,44] . Whether methanol and water forms ion pairs upon adsorption in H-SAPO-34 is a point of discussion in literature and seems to depend on the applied conditions.…”

“…We reported that the influence of the presence of multiple protic molecules on the methylation of benzene [44] and methoxide formation [42] in H-ZSM-5 has to be studied with molecular dynamics. These studies demonstrated that formation of protonated clusters of protic molecules has to be taken into account when modeling reactions under conditions of high reactant loadings, in particular in case of protic reactants that can play a solvent-like role.…”