Partial Oxidation of C5and C6Alkanes over Monolith Catalysts at Short Contact Times

Abstract

Olefins can be produced with high selectivity and yield by oxidative dehydrogenation of C2to C4alkanes autothermally over Pt coated foam monolith catalysts at contact times of ∼5 ms at atmospheric pressure and 700–900°C. In this paper we extend these reactions to higher alkanes by reacting pentane, isopentane, n-hexane, or cyclohexane vapor mixed with air or O2in a nearly adiabatic tubular reactor consisting of an α-Al2O3foam monolith catalyst coated with a Pt film. Pentane and n-hexane form primarily ethylene and propylene at low fuel/O2ratios and form primarily C5and C6linear olefins at high fuel/O2ratios. Isopentane produces primarily isobutylene over a wide range of fuel/O2ratios and never favors production of isopentene. Cyclohexane ring opening reactions dominate at low fuel/O2ratios with ethylene and 1,3-butadiene production favored, while dehydrogenation reactions occur at high fuel/O2ratios with cyclohexene, cyclohexadiene, and benzene becoming the dominant products. In contrast to the C2to C4alkanes, some oxygen breakthrough is seen at all operating conditions for higher alkanes, and breakthrough can be reduced only slightly by preheating the feed gases. Quenching downstream gases increases O2and fuel breakthrough somewhat, indicating that some reactions with unreacted O2are continuing homogeneously downstream of the catalyst. A comparison of alkane oxidation over monolith catalysts with thermal pyrolysis of alkanes shows similar product distribution (ignoring CO and CO2production), especially at lower fuel/O2ratios. β-elimination of H atoms from adsorbed surface species can also explain the distribution of products for the different fuels, and surface reactions explain dehydrogenated products and 1-olefins at high fuel/O2ratios.