Abstract
A pulse-flow reaction system has been used to investigate the conversions of C₂-C₇ alkanes over SiO₂-,La₂O₃-, and charcoal-supported nickel catalysts. The effects of nickel loading level, reaction temperature, and structure of the reactant were examined in order to obtain a better understanding of the processes involved in the conversion of a variety of alkanes over these catalysts. The observed catalytic behaviors appear to be related to the size of the nickel particles present on the catalyst after reduction. At 400°C, small nickel particles (<1 nm), such as were present on charcoal-supported catalysts with loading levels of 0.5 and 2.0 wt. percent nickel, catalyzed the unimolecular dehydrocyclization of alkanes possessing linear units of six or more carbon atoms, but displayed virtually no activity for the conversion of alkanes having continuous units of fewer than six carbon atoms. Homologation reactions did not occur over these catalysts, and the formation of surface carbonaceous materials typically accounted for less than 10 percent of converted reactant. Over catalysts with nickel particles predominantly in the size range 1-3nm, such as were present on silica-supported catalysts with loading levels of 0.5, 2.0 and 53.0 wt. percent nickel and a 5.0 wt. percent Ni/charcoal catalyst, the formation of gas phase products by hydrogenolysis and dehydrocyclization reactions at 400°C occurs by multimolecular pathways that involve the fragmentation of reactant molecules, followed by reassembly of the fragments. The formation of gas phase products over these catalysts was accompanied by the deposition of surface carbonaceous materials in quantities comparable to or greater than the sum of the gas phase products. These catalysts were active for homologation reactions that yield product molecules possessing higher molecular weights than the initial reactant. Lanthanum oxide-supported catalysts with loading levels of 0.5, 2.0, and 5.0 wt. percent nickel contained predominantly large metal particles (>3 nm) and rapidly deactivated at 400°C due to the formation of surface carbonaceous materials. The only gas phase product observed in the initial stages of the reaction was methane.
Palke, Dale Robert (1989). Characterization of alkane conversions over supported nickel catalysts. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -1027882.