Partial oxidation of C5 and C6 alkanes over monolith catalysts at short contact times

Olefins can be produced with high selectivity and yield by oxidative dehydrogenation of C-2 to C-4 alkanes autothermally over Pt coated foam monolith catalysts at contact times of similar to 5 ms at atmospheric pressure and 700-900 degrees C. In this paper we extend these reactions to higher alkanes by reacting pentane, isopentane, n-hexane, or cyclohexane vapor mixed with air or O-2 in a nearly adiabatic tubular reactor consisting of an alpha-Al2O3 foam monolith catalyst coated with a Pt film. Pentane and n-hexane form primarily ethylene and propylene at low fuel/O-2 ratios and form primarily C-5 and C-6 linear olefins at high fuel/O-2 ratios. Isopentane produces primarily isobutylene over a wide range of fuel/O-2 ratios and never favors production of isopentene. Cyclohexane ring opening reactions dominate at low fuel/O-2 ratios with ethylene and 1,3-butadiene production favored, while dehydrogenation reactions occur at high fuel/O-2 ratios with cyclohexene, cyclohexadiene, and benzene becoming the dominant products. In contrast to the C-2 to C-4 alkanes, 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 O-2 and fuel breakthrough somewhat, indicating that some reactions with unreacted O-2 are 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 CO2 production), especially at lower fuel/O-2 ratios. beta-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 l-olefins at high fuel/O-2 ratios. (C) 1998 Academic Press.