Reactions of methylpropene-2- 13C on H-mordenite : I. Reaction mechanisms

Abstract

The reactions of isobutene on a freshly prepared and outgassed H-mordenite have been investigated at 120 and 170 °C. Initially most of the reactant is retained on the surface and the gaseous products are mainly isopentane and isopentenes. Increasing reaction temperature increases the amounts of saturated hydrocarbons. After aging the catalyst, a stationary regime is reached where the formation of the hydrocarbon residues is no longer apparent. The products, mostly olefinic, include C 8 hydrocarbons with trimethylpentane or dimethylhexane structure and C 5C 7 hydrocarbons, mostly with dimentylbutane and dimethylpentane skeletons. The use of methylpropene-2- 13C demonstrates the existence of two types of products. Trimethylpentenes and 2,5-dimethylhexenes are doubly labeled on carbons 2 and 4 or carbons 2 and 5, respectively. In the other gaseous products C n+4 H 2 n+8 (2 ≤ n ≤ 4: 2,4-dimethylhexenes, 2,3-dimethylbutenes, 2,4-dimethylpentenes), four carbon atoms retain the structure and the label of the reacting isobutene while the n additional carbon atoms are randomly labeled, with an overall 13C content of n 4 . The specific labeling of these compounds is best explained by the complete randomization of all the carbon atoms in isobutene via transient formation of a cyclopropylcarbinyl-cyclobutyl cation. Addition of the resulting scrambled C 4 unit as α-methylallylic cation to isobutene yields partially scrambled 2,4-dimethylhexene- 13C 2. Attachment of the C 4 scrambled unit as a homoallylic cation or α-methylallylic cation to the zeolite framework provides an adsorbed intermediate species which yields, after protonation, olefin addition, possibly hydride and methyl shifts, and β-fission the expected C 5C 7 hydrocarbons. It is suggested that two types of sites are involved in the reactions of isobutene on aged mordenite at 120 °C. Weak Brønsted acid sites account for the formation of trimethylpentenes. Strong acid sites, resulting most probably from the conjunction of Brønsted and Lewis sites, are responsible for the generation of allylic cations by hydride abstraction, yielding the dimethylhexenes and the C 5C 7 olefins.