Olefin Pretreatment of ZSM-5: Formation of Catalytically Active and Inactive Species toward the Methanol-to-Olefins Process

The structure and nature of active sites are key aspects in determining the product selectivity of heterogeneously catalyzed reactions. In this work, we reveal that among a variety of hydrocarbon species formed within the pores of ZSM-5 by an olefin pretreatment, only a small fraction is accessible and catalytically active in the methanol-to-olefin (MTO) reaction. Pretreatment of ZSM-5 with C6–C7 olefins in a temperature program generated adsorbed aliphatic oligomers as well as acyclic and cyclic allylic carbenium ions, except for benzenium ions. Despite a different hydrocarbon pool composition, the pristine and olefin-pretreated ZSM-5 exhibited an almost identical product selectivity, which we ascribe to two causes: (i) Oligomers that could actively shifting the product selectivity toward higher hydrocarbons are instead converted to allylic carbenium ions by reaction with methanol. This is the origin of polymethylbenzenium ions and other allylic ions. (ii) Most of the deposited hydrocarbon species are a priori not catalytically active. Deuterium exchange experiments demonstrated that only ca. 10% of the adsorbed oligomers and ca. 35% of the cyclopentenyl cations are accessible to methanol and catalytically active. The catalytic function of adsorbed hydrocarbons is therefore closely related to their accessibility and the local reaction environment within the zeolite.