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)3AlOSiOn(OH)(3-n)(n+1)- (n = 2 or 3) at 0 degree C. These rate constants are more than 10(4)-fold faster than those extrapolated from 60 to 90 degrees 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.

The longevity of metastable, highly alkaline Na and K aluminosilicate solutions has been examined over a wide range of Si/Al concentration ratios R (0.04 [Formula: see text] R [Formula: see text] 20) at 24°C and, in selected cases, at 50 and 75°C. In general, the longest solution lifetimes (tppt) (several weeks) were found at 24°C and for the highest alkalinities and (or) large R, but long tppt were also found for R = 1.0 at the lower alkalinities. Solution lifetimes generally shortened when the temperature was raised, but the opposite effect was observed with the smaller Al concentrations when 2 [Formula: see text] R [Formula: see text] 6. These results are interpreted in terms of the requirement for =Si-OH groups for precipitate growth by condensation and of the relatively high solubility (few nuclei for gel growth) of the aluminosilicate solutes when R = 1.0.Key Words: aluminosilicates, precipitation kinetics, aluminates, aqueous silicates, gelation, Bayer process, zeolite synthesis.

No abstract

Three 1,3,5-trialkyl-1,3,5-triazacyclohexane chromium tricarbonyl complexes fac-[Cr(CO) 3 (C 3 H 6 N 3 R 3 )] (R = Me, Et or Bu t ) have been prepared and their reactivities investigated. Both the kinetic and the thermodynamic stabilities of the complexes increase as the size of the R group increases. When R = Me or Et, the ligand is susceptible to displacement to produce [Cr(CO) 3 L 3 ] [L = pyridine or P(OMe) 3 ]. Room-temperature single crystal X-ray studies were carried out on the R = Me and Bu t complexes. Both molecules adopt the expected 'piano-stool' configuration with putative 3m symmetry; a crystallographic mirror plane passes through the methyl adduct. The Cr᎐N distances in the R = Me complex [2.153(3), 2 × 2.181(2) Å] are shorter than in the Bu t case [2.202(5) Ϫ 2.216(4) Å]; in both the substituents are obligate equatorial relative to the triazacyclohexane ring.

No abstract