BIFUNCTIONAL REACTIONS OF ALKANES ON TUNGSTEN CARBIDES MODIFIED BY CHEMISORBED OXYGEN

Tungsten carbides modified by chemisorbed oxygen catalyze n-heptane isomerization with high selectivity. Kinetic, isotopic tracer, and deuterium-exchange measurements show that the reaction proceeds via sequential n-heptane dehydrogenation and heptene isomerization steps. At low temperatures, isomerization rates are limited by heptene rearrangements but dehydrogenation steps become increasingly rate-limiting as temperature increases. The isomer distribution in n-heptane and 3,3-dimethylpentane reaction products and the 13C distribution in isoheptanes formed from n-heptane-1-13C show that isomerization occurs predominantly by methyl migration steps typical of carbenium-ion rearrangements on acid sites. WOx species on carbide surfaces appear to introduce acid sites similar to those present in supported tungsten oxides. n-Heptane dehydrocyclization and hydrogenolysis reactions also require heptene intermediates. Dehydrocyclization occurs predominantly by (1, 6) ring closure while hydrogenolysis leads to random cleavage of carbon-carbon bonds in n-heptane. © 1991.