Linear free energy relationships in heterogeneous catalysis: I. Dealkylation of alkylbenzenes on cracking catalysts

Abstract

It was attempted to survey the applicability of LFER (Linear Free Energy Relationships) in heterogeneous catalysis to dealkylation of alkylbenzenes. The principle of LFER in heterogeneous catalysis is to find linear relationships between the rate data and some numerical variables which represent either reactivity of reactants or catalytic activity of heterogeneous catalysts. The microcatalytic gas chromatographic technique was used to measure the reaction rates at 400 °C on cracking catalysts. As for monoalkyl-benzenes, the logarithms of the rate constants hold a linear relationship with Δ H c +(R 1) for about two and one-half orders of magnitude in the rate constants. This relationship can be formulated by the following equation: log k i( R 1) = log k i(0) − γ′ i ΔH c + ( R 1) 2.303RT where k i(0) is a characteristic value dependent upon both the catalyst i and the kind of reaction (dealkylation in this case), but not upon the reactants, and Δ H c +(R 1) is the enthalpy change for the hydride abstraction from corresponding paraffins. By this equation it is meant that LFER is applicable to solid acid catalysis. Therefore, the reaction rate constants can be estimated from the values of Δ H c +(R 1) and a few measured values such as k i(0) and γ′ i . The effects of the second substituent group on dealkylation rate are also discussed by the Hammett law. Linear relationships between the logarithms of the rate constants and Δ H c +(R 1) or σ(R 2) are applied to the work previously published. In these cases, fine linear relationships are also obtained.