DETERMINATION OF THE ACID STRENGTH OF SOLID CATALYSTS IN WATER BY MEANS OF A KINETIC TRACER

The kinetic analysis of the hydrolysis of acetals in the presence of v arious solid acid catalysts, dealuminated II-form mordenites, H-montmo rillonite and strong ion exchange resins has been carried out. Experim ents performed in water as the solvent at different agitation speeds, catalyst weights and acetal concentrations show that the reaction is n ot controlled by external diffusion, nor by internal diffusion as resu lting from the calculation of the Thiele modulus. From the standard pl ots of the logarithms of the rate constants against the Hammett acidit y function H-0 established in homogeneous catalyzed reactions, it is p ossible to calculate such an acidity function for solid catalysts from the measurement of the reaction rates and extrapolation on the previo usly established standardization plot. Kinetic analysis of the experim ental results allows the determination of the maximum reaction rates i n the absence of diffusional limitations and, as a consequence, the de termination of the maximum acidity of solids under working reaction co nditions. The influence of the dealumination of mordenites is as expec ted, a maximum of acidity being found for Si/Al ratios around 10-15. I n a similar way, the perfluorinated sulfonic resin referred to as Nafi on is by far the most acidic catalyst. Nevertheless, a leveling effect of the acidity of solids is observed in water as the solvent, the str ongest acid being the hydroxonium ion H,Of with a Ho limit value of - 1.74. For dealuminated mordenites, the relationship found between the measured maximum acidity function and the protonic acidity calculated from the number of protons in the lattice would mean that only 20% of the protons present on the solid are efficient for the catalytic react ion. As in homogeneous catalysis, it is then possible to establish com pared acidity scales of solid acids in solvents, water in this work.