The contribution of gas-phase reactions in the Pt-catalyzed conversion of ethane-oxygen mixtures

This paper presents an analysis of the oxidative dehydrogenation of ethane on platinum-containing monoliths. The purpose of the work is to make a quan titative 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 t he reactor temperature gradient resulting therefrom. This is absolutely ess ential for meeting the objective of this investigation. The rise in tempera ture as the gases proceed through the reactor is responsible for a very sub stantial contribution of homogeneous gasphase reactions in the chemical tra nsformation occurring. One can view the process as a sequential one in whic h ethane is first oxidized on the platinum surface to CO, CO2, and H2O in t he front region of the monolith. The formation of these products causes a s ubstantial temperature increase that drives the dehydrogenation of ethane t o ethylene (and acetylene) in the gas phase. The heat required to sustain t hese endothermic reactions in the tail end of the reaction zone is supplied by exothermic gas-phase oxidation reactions that form additional H2O and C O. Overall, the system can be viewed as one in which the catalyst initiates gas-phase chemistry through the acceleration of exothermic reactions at th e front of the reactor which increase the downstream temperature to the poi nt where gas-phase reactions occur readily. (C) 2000 Academic Press.