ARTICLE pubs.acs.org/IECR
Kinetic Model of the Dehydrogenation of Methylcyclohexane over Monometallic and Bimetallic Pt Catalysts Faisal Alhumaidan,*,† David Cresswell,‡ and Arthur Garforth‡ † ‡
Petroleum Research and Studies Center, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait Environmental Technology Center, School of Chemical Engineering and Analytical Science, The University of Manchester, P.O. Box 88, Sackville St., Manchester M60 1QD, U.K. ABSTRACT: Various kinetic models were developed for methylcyclohexane (MCH) dehydrogenation over supported Pt catalysts. The best fitting mechanistic model is of the non-Langmuirian/noncompetitive Horiuti-Polanyi type. In this model, the Horiuti-Polanyi aromatic hydrogenation mechanism, which assumes an atomic hydrogen addition to aromatics on the catalyst surface, is applied in reverse to MCH dehydrogenation. The model also assumes that hydrogen and MCH molecules adsorb noncompetitively on two different types of sites to accommodate the observed near zero-order dependence of reaction rate on MCH and the negative order dependence upon hydrogen. To account for the increase in the hydrogen inhibition effect with pressure, a non-Langmuirian adsorption isotherm is adopted, which assumes a nonlinear dependency between the adsorption equilibrium constant for hydrogen and the system pressure. The reversible and irreversible deactivation kinetics are satisfactorily included in the kinetic model.
1. INTRODUCTION Hydrogen storage for stationary and mobile applications is an expanding research topic. One of the more promising hydrogen storage techniques relies on the reversibility, high selectivity, and high hydrogen density of liquid organic hydrides, in particular, methylcyclohexane.1-10 Liquid organic hydrides are largely compatible with the current transport infrastructure, whereas alternatives such as liquid and gaseous hydrogen and metal hydrides would require a completely new infrastructure. The relative hydrogen supply cost from liquid organic hydrides is 20% lower than compressed hydrogen and 30% lower than liquefied hydrogen.1 In spite of its technical, economical, and environmental advantages, the concept of hydrogen storage in liquid organic carriers has not been commercially established due to technical limitations, such as the amount of energy required to extract the hydrogen from methylcyclohexane and the development of a dehydrogenation catalyst that combines high dehydrogenation activity, high selectivity toward aromatic products, and low deactivation rate. The first step in overcoming these technical limitations is the development of a mechanistic kinetic model for the methylcyclohexane (MCH) dehydrogenation reaction that provides insight into the reaction mechanism and can be accurately extrapolated beyond the range of experimental data. Previous research indicated that the Pt/Al2O3 catalyst and its Pt-Re/Al2O3 successor are the best catalysts for MCH dehydrogenation in terms of activity, selectivity, and stability.1-22 The good dehydrogenation ability of the Pt catalyst is attributed to the rapid elimination of hydrogen from the reaction system, which shifts the chemical equilibrium in favor of the products. The fast evolution of hydrogen on a Pt catalyst is mainly credited to the effects of hydrogen spillover and hydrogen recombination.2 r 2010 American Chemical Society
Many research groups have investigated the kinetics of MCH dehydrogenation to toluene over Pt/Al2O3 catalysts over the past decades.11-17 These research groups derived different rate expressions because they assumed either different reaction mechanisms or different rate-determining steps. Sinfelt and coworkers11,12 reported that the dehydrogenation kinetics of MCH over Pt/Al2O3 exhibited zero reaction order with respect to both MCH and hydrogen. The zero reaction order with respect to MCH clearly suggests that the active platinum sites are completely covered by adsorbed MCH molecules, which makes the reaction rate independent of further MCH additions. Jossens and Petersen13 and Van Trimpont et al.14 extended the result initially observed by Sinfelt and co-worker11 by indicating that MCH dehydrogenation appears to be first order with respect to MCH at low MCH partial pressure (
