Effect of external surface of HZSM-5 zeolite on product distribution in the conversion of methanol to hydrocarbons

The external surface of HZSM-5 zeolite was passivated by liquid siliceous deposition and by acidic sites poisoning with lepidine, respectively.Then methanol-to-hydrocarbons(MTH) reaction was investigated over the above as-prepared catalysts and the dissoluble coke on these used catalysts was analyzed by GC-MS, to study the role of the external surface of HZSM-5 in the catalytic reaction. Comparison with the experimental results based on parent ZSM-5 showed that the product distribution of MTH reaction was obviously influenced by the external surface.Evidences were listed as follows:(1) the final product on parent HZSM-5 showed higher aromatic selectivity, lower olefin selectivity, lower ratio of C2/C3+aliphatics and higher ratio of C3/C4+aliphatics than the reaction mixture produced by the sole catalysis of acidic sites in HZSM-5channel;(2) a little of pentamethylbenzene and hexamethylbenzene in the product on parent HZSM-5, was produced via multi-methylation of methylbenzene on the external surface. The above conclusion may also be suitable for MTH reaction over other zeolites with 10-ring channel.