Kinetics of Silicate Exchange in Alkaline Aluminosilicate Solutions

In strongly alkaline aqueous KOH solutions containing SiIV in large excess over AlIII, the kinetics of exchange of monomeric silicate with small acyclic aluminosilicate solute species is much more rapid than with either cyclic aluminosilicates or any all-silicate anions. Selective inversion recovery 29Si NMR studies of homogeneous solutions of stoichiometric composition 3.0 mol kg-1 of SiO2, 0.1 mol kg-1 of Al2O3, and 8.0 mol kg-1 of K2O in 60−75% D2O gave rate constants of 2.0 ± 0.2 kg mol-1 s-1 and 17 ± 4 s-1 for the forward and reverse reactions of monomeric silicate with (HO)3AlOSiO n (OH)(3 - n ) ( n +1)- (n = 2 or 3) at 0 °C. These rate constants are more than 104-fold faster than those extrapolated from 60 to 90 °C for comparable reactions of silicate anions. The greater lability of acyclic aluminate centers relative to silicate is ascribed partly to the availability of HO− groups for condensation reactions on Al and mainly to the ease of expansion of the coordination number of AlIII beyond 4. The latter attribute is diminished when AlIII is constrained to be tetrahedral in cyclic structures. With respect to the mechanism of formation of zeolites from alkaline aqueous media, it is suggested that small, labile AlOSi units add rapidly to growing zeolitic structures “on demand”, whereas the more kinetically inert cage or ring structures cannot. This would explain why a silicate or aluminosilicate structure that is dominant among solute species at equilibrium in the presence of a particular cation may bear little or no geometric relation to the zeolitic framework promoted kinetically by that same cation.