ALKANE TRANSFORMATIONS ON SUPPORTED PLATINUM CATALYSTS .4. KINETICS OF HYDROGENOLYSIS OF ETHANE, PROPANE, AND N-BUTANE ON PT AL2O3 (EUROPT-3) AND PTRE/AL2O3 (EUROPT-4)/

The dependence of the rate of hydrogenolysis of C2H6, C3H8, and n-C4H1 0 on 0.3% Pt/Al2O3 (AKZO CK303, EUROPT-3) has been determined at sever al temperatures, using short reaction pulses to minimise deactivation by carbon deposition. Rate maxima, which with C2H6 occur at very low H -2 pressures, become broader and move to higher H-2 pressures as the c hain-length or temperature increases. Criteria for selecting an approp riate 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 co nstants of the rate equation are obtained by computation. Changes in t he form of the kinetic curves are described by (i) values of the rate constant k(1) and the equilibrium constant for alkane chemisorption K- A that increase with alkane chain length and with temperature, (ii) va lues of K-H, the adsorption coefficient for H-2, that decrease with al kane chain-length but are not very temperature-dependent, and (iii) a decreasing degree of the needful alkane dehydrogenation as the chain-l ength increases. For C3H8 and n-C4H10, true activation energies are re spectively 82 and 76 kJ mol(-1), and enthalpy changes for alkane chemi sorption are 88 and 79 kJ mol(-1). Apparent activation energies increa se with Hz pressure in consequence of the Temkin equation and show com pensation effects. Rate dependences on H-2 pressure 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 of k(1), K-A, and K- H were larger. Measurements were also made with catalysts partially de activated by carbon deposition; slower rates were associated with lowe r values of k(1) and K-A. Changes in product selectivities with operat ing variables are recorded, but are only significant for PtRe/Al2O3, a nd for n-C4H10 on Pt/Al2O3, where at 547 K the extent of internal C-C bond fission decreases as Hz pressure increases. The difficulty of dev ising a simple rate expression to embrace all the experimental observa tions is discussed. (C) 1997 Academic Press.