ON THE REACTION-MECHANISM FOR HYDROCARBON FORMATION FROM METHANOL OVER SAPO-34 .1. ISOTOPIC LABELING STUDIES OF THE CO-REACTION OF ETHENE AND METHANOL

C-13-Methanol and C-12 ethene (fed as ethanol) have been co-reacted ov er SAPO-34 in a flow system at 400 degrees C using argon as a carrier (diluent) gas. The feed contained an equal number of C-13 and C-12 ato ms. The products were analyzed by CC-MS, allowing the determination of the isotopic composition of the reactor effluent. The ethanol was imm ediately converted to ethene, so the reaction system was equivalent to a feed consisting of methanol/ethene/water. While the methanol was co mpletely or almost completely converted to hydrocarbons, the larger pa rt of the ethene emerged unreacted. The products propene and butenes w ere mostly formed from methanol and contained a large excess of C-13 a toms. The ethene effluent consisted mainly of all C-12 or all C-13 ato ms, and only to a small extent of C-12-C-13 molecules. The reaction sy stem was followed from an initially very active catalyst until the cat alyst was sufficiently deactivated that C-1 was not completely convert ed to hydrocarbons. The tendency for ethene to emerge unreacted, and f or all new hydrocarbons to be formed from methanol became more pronoun ced with progressing catalyst deactivation. The results show clearly t hat the higher hydrocarbons are, over this catalyst, not formed by suc cessive methylations of ethene. A previously proposed ''carbon pool'' mechanism can explain the gross effects seen in the product and isotop ic distribution. (C) 1994 Academic Press, Inc.