Catalytic hydrogenation of propylene and isobutylene over platinum. Effect of noncompetitive adsorption

Abstract

A mathematical and kinetics model is presented for reactions catalyzed by solid surfaces where both competitive and noncompetitive odsorption take place, a situation that may be general in reactions between large and small molecules catalyzed by solid surfaces.In the adsorption of large molecules steric hindrance or multiple site adsorption cause maximum surface coverage of such molecules before all sites are occupied. This does not, however, preclude further adsorption of small molecules on the remaining isolated vacant sites nor does it preclude further surface reoction. The overall reaction model for such conditions may be expressed as This particular model has been evaluated for the catalytic hydrogenation of propylene and isobutylene with the expectation that it may be useful in reactions that are of industrial importance. A reaction model based on either competitive adsorption or noncompetitive adsorption alone fails to correlate the experimental data.In these two reactions maximum rates occur a t olefin concentrations below 5 mole % and the catalytic adsorption constants for the olefins are a hundredfold greater than for hydrogen.This reaction model is similar to that proposed by Bond and Turkevich (151, who have further demonstrated through the deutetation of propylene that the actual mechanism of the reaction is much more complicated.In this paper are presented the results of a study of the kinetics of the hydrogenation of propylene and isobutylene gases over a commercial supported platinum catalyst. This study was undertaken to obtain reliable kinetic data on the hydrogenation of olefins and from these to select a mathematical model of use in correlating the rates of similar reactions of commercial importance. These studies are apparently the first ublished on the hydrogenation flow type of reactor.of propylene and isobuty P ene catalyzed by platinum in a PREVIOUS WORKAlthough the catalytic hydrogenation of light olefins. ethylene in articular, has been widely studied, there still for the reaction. The desired kinetic model is a mathematical expression of the reaction rate suitable for design purposes, even though it represents an oversimplification of the actual molecular mechanism involved.Part of the uncertainty in obtaining a reliable model is due to the difEculty of obtaining rate data under isothermal conditions. The hydrogenation of olefins is highly exothermic and when catalyzed by the Group VIII metals (nickel, platinum, palladium, rhodium, and ruthenium) the reaction rate becomes exceedingly rapid. As a result mass and heat transfer resistances at the catalytic surface obscure and complicate the interpretation of the experimental data. For the hydrogenation of ethylene gas over nickel in a stirred fixed volume reactor it has been shown exists consi B erable uncertainty as to the best kinetic model ( 1 ) that the reaction rate depends strongly on the degree of agitation of the gaseous system. An analysis of heat and mass transfer effects in the hydrogenation of propylene over nickel in a fked-b...

Kinetic Study of Propylene Hydrogenation over Pt/Al2O3 by Parahydrogen-Induced Polarization

“…The adsorption constants for both hydrogen and propylene over Pt/Al 2 O 3 were calculated at the temperature of 100°C using the pre-exponential factors and activation energies estimated by Rogers et al [23]. Thus, the values of adsorption constants K H = 0.087 bar -1 and K P = 1.6 bar -1 were obtained for hydrogen and propylene, respectively.…”

“…Computer modeling of propylene hydrogenation kinetics was carried out to explain our experimental observations. Several equations from references [22,23] and also some equations based on the Langmuir-Hinshelwood mechanisms derived by us were used in the calculations.…”

“…For the non-pairwise hydrogen addition, several kinetic equations presented in [22,23] were used for the calculations. However, none of them gave satisfactory results that would fit our experimental data.…”

“…Using the kinetic model described in Ref. [23], the reaction order of propylene hydrogenation over Pt/Al 2 O 3 was investigated by Galvagno et al [24]. It was shown that the reaction order is 0.94 with respect to H 2 and 0.16 with respect to propylene.…”

“…2* $ 2H*, where * denotes the active site of the catalyst. Rogers et al [23] showed that both competitive and non-competitive adsorption of hydrogen and substrate may take place on the metal surface. Using the kinetic model described in Ref.…”