Formation of alkenyl carbenium ions by adsorption of cyclic precursors on zeolites

The adsorption of cyclopentene (CPE), 1-methylcyclopentene (MCPE), methylenecyclopentane (MECP), cyclohexene (CHE) and 1,3-cyclohexadiene (diene) on faujasite (Y), ZSM-5, mordenite and beta zeolites as well as on silica-alumina, was studied by in situ FT-IR and UV-VIS spectroscopies at 150-573 K. The formation of alkenyl carbenium ions, characterized by an IR band at ca. 1490-1530 cm(-1), were observed in a wide temperature range. UV-VIS results also confirm the formation of alkenyl carbenium ions. The ability to generate alkenyl carbenium ions on zeolites follows the sequence: 1, 3-diene greater than or equal to 1-MCPE = MECP > CPE approximate to CHE. All the cyclic olefins have undergone oligomerization on zeolite Y at low temperatures, giving dimerized alkenyl carbenium ions. The formation of alkenyl carbenium ions on zeolites with small pores (like ZSM-5) was confined at low temperatures due to the inaccessibility of cyclic molecules to acidic OH groups, while the acid strength of zeolites was found to be important for stabilizing the formed carbenium ions and for producing some other species. Based on the experimental results, alkenyl carbenium ions are proposed to be formed by direct protonation of 1,3-diene, or by hydrogen transfer between alkyl carbenium ion and cyclic olefins or their corresponding oligomers. (C) 2002 Elsevier Science B.V. All rights reserved.