PLS vs. zeolites as sorbents and catalysts 4. Effects of acidity and porosity on alkylation of benzene by primary alcohols Original Research Article

The alkylation of benzene by MeOH, EtOH, iso-PrOH and n-octanol has been investigated in H-ZSM-5, mordenite, USY and Theta-1 and the acidity/porosity relations obtained compared with those in an alumina-pillared montmorillonite (BP-PILC) and an alumina-pillared saponite (ATOS). Pseudo-first order alkylation rates follow the order: H-ZSM-5 > mordenite > USY > Theta-1 > ATOS > BP-PILC which for the zeolites is not that expected on the basis of acidity. Conversely, EtOH conversion in H-ZSM-5s with Si/Al ratios 5/35→5/400 do follow expected acidity trends and MeOH consumption during alkylation can be used to construct an acidity scale. It is concluded that the reaction occurs via electrophilic attack of intermediate matrix-attached oxonium ion at incoming benzene molecules and that pore size, rather than acidity, is the determining factor. Thus, the smaller channels of H-ZSM-5 are the most favourable because they allow close approach between oxonium ion and benzene, whereas both pore sizes in both PILCs are too large, which hinders efficient alkylation.