A Systematic Study of Methylation from Benzene to Hexamethylbenzene in H-SSZ-13 Using Density Functional Theory and Ab Initio Calculations

Methylation of aromatic molecules in H-SSZ-13 is studied from benzene to hexamethylbenzene via all possible isomers. Both methylations with methanol (MeOH) and dimethyl ether (DME) are investigated via both stepwise and concerted mechanisms. Calculations are carried out using periodic density functional theory corrected by high-level DLPNO-CCSD(T) calculations on cluster models. While we find little selectivity between the methylations of different aromatics, our calculations indicate that isomerization via methyl shifts between different isomers is a viable mechanism with barriers comparable to methylation. The generally observed trend is that MeOH methylation barriers are smaller than those using DME, with the difference between MeOH and DME increasing with the level of methylation of the aromatic molecule.