Mechanistic studies of the reactions of 3,3-dimethylbut-1-ene with deuterium over supported-metal catalysts

Abstract

The reactions of 3,3-dimethylbut-1-ene with deuterium have been followed over a number of silica-supported metal catalysts at 254 K and the products analysed by mass spectrometry (MS) and deuterium NMR spectroscopy. The main reaction was the formation of the alkane, 2,2-dimethylbutane, in which extensive redistribution of H and D atoms had occurred in the ethyl group. It was possible to make a complete analysis of all 12 isotopic alkanes, i.e.(CH₃)₃CCX₂CX₃ where X represents H or D, in the products formed over supported Pd and Rh using both the NMR and MS results. Relatively little exchange of alkene occurred but it was most noticeable over Pd/SiO₂.

The results were interpreted in terms of a mechanistic model involving three types of adsorbed species: alkene, 1-alkyl and 2-alkyl. Any adsorbed alkene may either be desorbed or form one of the alkyls, both alkyls may either be desorbed as alkane product or revert to alkene. By choosing suitable parameters and solving 23 simultaneous equations, it was possible to obtain calculated distributions for the 12 isotopic products in good agreement with the experimental results. The presence of the tertiary butyl group has an influence on the relative stability of the adsorbed intermediates and also on the relative ease of activation (addition or removal) of primary and secondary H or D atoms.