Reactions of methylpropene-2- 13C on H-mordenite : II. Influence of aging and reaction temperature

Abstract

Catalyzed reactions of methylpropene-2- 13C on H-mordenite have been carried out at 50, 120, 170 (before and after catalyst aging) and 400 °C. At low temperature (50 °C) no hydrocarbon residues are formed, and simple protonation takes place leading to trimethylpentenes. At 120 °C during aging and at 170 °C the carbon-13 distribution shows that the gaseous C 5C 7 products are deeply rearranged. The distributions cannot be accounted for by the same mechanism of partial scrambling—i.e., addition of scrambled monolabeled C 1, C 2, and C 3 units to an isobutene molecule (mechanism D)—observed on aged mordenite at 120 °C (Part I) and are best explained by combining linearly a distribution associated with mechanism D and a distribution involving complete redistribution of all carbon atoms (statistical). At 400 °C isobutene yields saturated hydrocarbons and aromatics, all of which have a statistical distribution of the carbon-13. These results and the mass spectrometric analysis of the hydrocarbon residues allowed us to propose the following sequence of reactions. At 120 °C, during aging, and at 170 °C polymeric cations attached to the zeolite framework are deactivated by β-fission producing carbenium ions and adsorbed olefln radicals. These radicals may then act as hydride and proton donors leading to saturated hydrocarbons and adsorbed alkadiene radicals. Such adsorbed olefinic and dienic radicals, which represent the strongly adsorbed residues left on the zeolite at 120 and 170 °C, yield at 400 °C paraffins by hydride transfer and triene precursors of the aromatics by β-fission. In this reaction scheme, the formation of deeply rearranged hydrocarbons is explained by successive adsorption-desorption steps of the primary olefins and cations.