Effect of catalyst modification on the conversion of methanol to light olefins over SAPO-34.

The catalytic activity and selectivity of as-prepared and modified samples of SAPO-34 and Me-APSO-34 (Me = Co, Ni) for the conversion of methanol to olefins has been investigated. The catalytic performance for the conversion of methanol to light olefins of all the catalyst samples prepared was found to be closely related to the number of strong acid sites present. Mild steaming, encountered during deep-bed calcination, increased the lifetime of SAPO-34 due to the formation of stronger acid sites probably on the external surface of the crystallites. Selectivities to light olefins were typical of those previously reported and was essentially constant for all the catalysts investigated. The absence of C5+ olefins is ascribed to the ‘cage effect’. Dilution of the methanol with water as opposed to nitrogen increased the catalyst utilization value threefold and reduced the rate of coke formation during reaction. Treatments such as steaming, silanization and poisoning of strong sites by ammonia all reduced the number of strong acid sites and, thus, reduced catalytic performance