C. Moreau et al., DETERMINATION OF THE ACID STRENGTH OF SOLID CATALYSTS IN WATER BY MEANS OF A KINETIC TRACER, Journal of molecular catalysis. A, Chemical, 112(1), 1996, pp. 133-141
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.