The Contribution of Gas-Phase Reactions in the Pt-Catalyzed Conversion of Ethane–Oxygen Mixtures

Abstract

This paper presents an analysis of the oxidative dehydrogenation of ethane on platinum-containing monoliths. The purpose of the work is to make a quantitative assessment of the extent to which homogeneous gas-phase reactions contribute to the overall conversion of the ethane. In making the analysis, extensive use is made of kinetic information obtained and compiled by A. M. Dean and associates for elementary homogeneous reaction steps and by L. D. Schmidt and associates for elementary surface reactions. A critical part of the analysis is concerned with accounting for the heat effects and for the reactor temperature gradient resulting therefrom. This is absolutely essential for meeting the objective of this investigation. The rise in temperature as the gases proceed through the reactor is responsible for a very substantial contribution of homogeneous gas-phase reactions in the chemical transformation occurring. One can view the process as a sequential one in which ethane is first oxidized on the platinum surface to CO, CO2, and H2O in the front region of the monolith. The formation of these products causes a substantial temperature increase that drives the dehydrogenation of ethane to ethylene (and acetylene) in the gas phase. The heat required to sustain these endothermic reactions in the tail end of the reaction zone is supplied by exothermic gas-phase oxidation reactions that form additional H2O and CO. Overall, the system can be viewed as one in which the catalyst initiates gas-phase chemistry through the acceleration of exothermic reactions at the front of the reactor which increase the downstream temperature to the point where gas-phase reactions occur readily.