Diffusion mechanisms and preferential dynamics of promoter molecules in ZSM-5 zeolite

The diffusion in ZSM-5 zeolite of methanol and of two series of promoters of the methanol to dimethyl ether reaction (linear methyl esters, benzaldehyde, 4-n-alkyl benzaldehydes) has been studied using classical molecular dynamics in the NVT ensemble. Whereas promoter diffusion coefficients decrease with increasing alkyl chain length in methyl esters, the aromatic aldehyde promoters all have similar diffusion coefficients. The lowest diffusion coefficient is that of benzaldehyde. All the promoters exhibit a preference for moving in the straight pore, a preference that is most pronounced for the 4-n-alkylbenzaldehydes and least for the longest aliphatic esters. A novel diffusion mechanism, a molecular ‘3-point turn’, is observed. This likely plays an important role in allowing the most potent promoters, with longer linear alkyl chains, to access all of the Brønsted acid reaction sites. The diffusion coefficient of methanol is larger than that of all the promoters. The more catalytically active aromatic aldehyde promoters limit methanol diffusion less than the aliphatic esters.