| Abstract
| - Silicon-29 selective inversion−recovery shows that small, acyclic aluminosilicate species in highly alkaline aqueous solution exchange silicate monomer much more quickly than do either comparable all-silicate anions or aluminosilicates that contain Al in ring or cage structures. This may explain why zeolites grown hydrothermally often show no obvious structural relationship to cyclic silicate or aluminosilicate anions known to be present at equilibrium in the supernatant solution.
- In strongly alkaline aqueous KOH solutions containing SiIV in large excess over AlIII, the kinetics of exchange ofmonomeric silicate with small acyclic aluminosilicate solute species is much more rapid than with either cyclicaluminosilicates or any all-silicate anions. Selective inversion recovery 29Si NMR studies of homogeneous solutionsof 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 ofmonomeric silicate with (HO)3AlOSiOn(OH)(3-n)(n+1)- (n = 2 or 3) at 0 °C. These rate constants are more than104-fold faster than those extrapolated from 60 to 90 °C for comparable reactions of silicate anions. The greaterlability of acyclic aluminate centers relative to silicate is ascribed partly to the availability of HO− groups forcondensation 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 tothe mechanism of formation of zeolites from alkaline aqueous media, it is suggested that small, labile AlOSiunits add rapidly to growing zeolitic structures “on demand”, whereas the more kinetically inert cage or ringstructures cannot. This would explain why a silicate or aluminosilicate structure that is dominant among solutespecies at equilibrium in the presence of a particular cation may bear little or no geometric relation to the zeoliticframework promoted kinetically by that same cation.
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