Alkylation of toluene on HSAPO-5 with various Si concentrations

HSAPO-5 with various Si concentrations was used as a catalyst for the alkylation of toluene with methanol to investigate weak acid sites. On HSAPO-5 with Si/(Al + P + Si) ≦ 0.026, the conversion of toluene increased with time on stream for a certain period, whereas the conversion decreased monotonously on HSAPO-5 with higher Si concentrations. Pretreatment with methanol on HSAPO-5 with Si/(Al + P + Si) = 0.018 increased the initial conversion of toluene. From the measurement of the amount of coke deposited on HSAPO-5, it is suggested that during the initial stage of the reaction coke is mainly produced by the reaction of methanol by itself at the acid sites on the external surface of HSAPO-5 with low Si concentrations. A small amount of coke thus formed would gradually retard the further self-conversion of methanol into coke. Therefore, the fraction of methanol used for the alkylation could increase, resulting in the increase in toluene conversion. It is suggested that the acid sites on the external surface are stronger than those formed by the substitution of Si for P but are weaker than those formed in aluminosilicate regions existing in HSAPO-5 with high Si concentrations.