KINETIC METHOD FOR THE CHARACTERIZATION OF BRONSTED SITES ON OXIDE SURFACES .1. TRIMETHYLORTHOBENZOATE HYDROLYSIS OVER A SERIES OF AL2O3 SIO2 MIXED OXIDES/

The hydrolysis of trimethylorthobenzoate was studied over the pH range 4-7. In homogeneous experiments, overall first-order kinetics was fou nd for both constant and variable proton concentrations, with an avera ge intrinsic rate constant of 6 . 10(3) l/mol . min. Within the above pH range, it was also found that a series of alumina/silica oxides cou ld act as general catalysts for the same reaction. Proton binding on t he oxides determined from potentiometric titration data revealed two t ypes of potentially active, proton-donor, sites with acid strengths pK (a)(I) approximate to 4 and pK(a)(II) approximate to 5.5. A detailed k inetic analysis is presented which accounts for the time dependent mas s, charge and site balance in the system. Kinetic scanning, which invo lves a pH perturbation and subsequent analysis of the heterogeneous sy stem relaxation, is proposed as a general method for evaluation of the concentration of surface sites active for general acid catalysis. Act ive site densities, determined by means of this approach, increased fr om 0.085 to 0.13 mmol/g as the aluminum content in the mixed oxides in creased from 3 to 10% by weight. The proton transfer rate constant's v alue, essentially independent of oxide composition, was estimated at a pproximate to 1500 g/mol . min.