Methylcyclohexane transformation over HMCM22 zeolite: Mechanism and location of the reactions

At 350 degrees C, over a HMWW zeolite (Si/Al = 14.5), methylcyclohexane (mch) transforms into four types of products: isomers (I), C-1-C-8 aliphatic hydrocarbons (C), benzenic hydrocarbons (A), and trapped carbonaceous compounds (coke). The role played by the various micropores was established through a two-step method: deactivation of supercages by coking and poisoning of the outer hemicage sites by 2,4-dimethylquinoline. More than 85% of mch transformation occurs in the supercages, similar to 11% occurs in the outer hemicages, and only <2% occurs in the sinusoidal channels, which, considering the protonic site distribution, corresponds to turnover frequency values of 33, 82, and 2 h(-1). The product distributions are very different: I, C-3-C-5 branched alkanes, and coke in similar amounts; essentially I and C-3-C-8 branched alkanes; and mainly C-1-C-8 and A products. The carbenium ion chain mechanism can account for the products formed in supercages and hemicages, whereas protolytic mechanisms play an important role in the narrow sinusoidal channels. Deactivation by coking is very fast in supercages and negligible in the other locations. (C) 2008 Elsevier Inc. All rights reserved.