Alkane Transformations on Supported Platinum Catalysts

ISSN： 0021-9517

Source： Journal of Catalysis, Vol.166, Iss.2, 1997-03, pp. : 172-185

Disclaimer: Any content in publications that violate the sovereignty, the constitution or regulations of the PRC is not accepted or approved by CNPIEC.

Previous Menu Next

Abstract

The dependence of the rate of hydrogenolysis of C₂H₆, C₃H₈, andn-C₄H₁₀on 0.3% Pt/Al₂O₃(AKZO CK303, EUROPT-3) has been determined at several temperatures, using short reaction pulses to minimise deactivation by carbon deposition. Rate maxima, which with C₂H₆occur at very low H₂pressures, become broader and move to higher H₂pressures as the chain-length or temperature increases. Criteria for selecting an appropriate rate expression with which to model the results is discussed in detail: one is selected that is based on the competitive chemisorption of the reactants, that of the alkane requiring the loss of more than one H atom to activate it. For each data set, optimum values of the constants of the rate equation are obtained by computation. Changes in the form of the kinetic curves are described by (i) values of the rate constantk₁and the equilibrium constant for alkane chemisorptionK_Athat increase with alkane chain length and with temperature, (ii) values ofK_H, the adsorption coefficient for H₂, that decrease with alkane chain-length but are not very temperature-dependent, and (iii) a decreasing degree of the needful alkane dehydrogenation as the chain-length increases. For C₃H₈andn-C₄H₁₀, true activation energies are respectively 82 and 76 kJ mol^-1, and enthalpy changes for alkane chemisorption are 88 and 79 kJ mol^-1. Apparent activation energies increase with H₂pressure in consequence of the Temkin equation and show compensation effects. Rate dependences on H₂pressure were measured at a single temperature using 0.3% Re-0.3% Pt/Al₂O₃(AKZO CK433, EUROPT-4): rates were faster than for Pt/Al₂O₃, and values ofk₁,K_A, andK_Hwere larger. Measurements were also made with catalysts partially deactivated by carbon deposition; slower rates were associated with lower values ofk₁andK_A. Changes in product selectivities with operating variables are recorded, but are only significant for PtRe/Al₂O₃, and forn-C₄H₁₀on Pt/Al₂O₃, where at 547 K the extent of internal C–C bond fission decreases as H₂pressure increases. The difficulty of devising a simple rate expression to embrace all the experimental observations is discussed.