ON THE REACTION-MECHANISM FOR HYDROCARBON FORMATION FROM METHANOL OVER SAPO-34 .2. ISOTOPIC LABELING STUDIES OF THE CO-REACTION OF PROPENE AND METHANOL

[C-13]Methanol and [C-12]propene (fed as isopropanol, which is immedia tely converted to propene) have been co-reacted over SAPO-34 in a flow system at 400 degrees C using argon as a carrier gas. The feed was eq uimolar in C-13 and C-12 atoms, The products were analyzed by gas chro matography-mass spectrometry, allowing determination of the isotopic c omposition. While the methanol was completely or almost completely con verted to hydrocarbons, the larger part of the propene emerged unreact ed. The products ethene and butenes were mostly formed from methanol a nd contained a large excess of C-13 atoms, The propene effluent consis ted mainly of all-C-12 or all-C-13 molecules and, only to a small exte nt, isotopically mixed molecules. The tendency for propene to emerge u nreacted and all new hydrocarbons to be formed from methanol became mo re pronounced with progressing catalyst deactivation. The results show that the higher hydrocarbons are, over this catalyst, not formed by s uccessive methylations of bulk gas-phase propene. A previously propose d ''carbon pool'' mechanism can explain the gross effects seen in the product and isotopic distribution, but it is pointed out that the nonr eactivity of propene in SAPO-34 may be caused by slow diffusion of pro pene in the pores. (C) 1996 Academic Press, Inc.