Diffusion of Trimethylbenzenes, Toluene, and Xylenes in UWY Zeolite as a Catalyst for Transalkylation of Trimethylbenzenes with Toluene

A molecular dynamics
study has been carried out on the diffusion of trimethylbenzene (TMB),
toluene, and xylene molecules in transalkylation of TMB and toluene
in UWY zeolite, containing crossing 10-ring and 12-ring channels.
Two models of UWY have been employed, the pure silica and an acidic
form of UWY including Al and Brønsted sites, using a recently
parameterized general force field for zeotypes, which reproduce pore
diameters within ca. ±0.2 Å. Molecular traffic has been
observed from the result of TMBs using almost exclusively the 12-ring
channels and the preferential location of p-xylene
and toluene in the 10-ring channels at high loading (reaction conditions).
From the three different 10-ring channels, only p-xylene and toluene can fit in the two smallest channels,
whereas m-xylene and 1,2,4-TMB can also diffuse in
the largest 10-ring channel. An in-depth analysis of transition-state
shape selectivity has been performed, showing that all transition
states of transalkylation of toluene and TMBs can be formed in the
12-ring channels. Although this is a disadvantage for the selective
production of p-xylene, the previous factor of molecular
traffic will contribute to a selectivity of p-xylene
over the other xylene isomers. Overall, UWY is suggested as a promising
catalyst for transalkylation of toluene and TMBs.