Ab initio calculations on the five-membered alumino-silicate framework rings model: implications for dissolution in alkaline solutions

Abstract

An ab initio restricted Hartree–Fock calculation utilising the standard 6-31G basis set was used to calculate total energies after PM3 calculations of energy-optimised geometries for the five-membered alumino-silicate framework rings cluster for a total of ten T sites. Calculations have shown that in the absence of protons or other ions, the most favourable sites for 1Al, 2Al and 3Al substitution of Si are the T 6, T 1 and T 9 sites respectively. With more Al atoms replacing Si in a cluster, TO bond lengths and TOT angles show lengthening and sharpening trends respectively, which indicates that the structure is distorted to a more relaxed symmetry with O br atoms moving outwards. The calculated bond lengths and angles have been shown to match the values observed in previous studies, including those for a four-membered alumino-silicate single ring cluster. Based on the optimised five-membered alumino-silicate framework rings model, a further ab initio HF calculation has been conducted on ring breakage for releasing Al(Q 3) and Si(Q 3) centres to form T(OH) 4 and HOT(OM) 3 tetrahedra under local and highly alkaline environment. The obtained results suggest that Al(Q 3) compared with Si(Q 3) breaks more readily with the exothermal reaction enthalpy being in excess of −244.4 kJ/mol, while the most reactive Si(Q 3) centre shows an exothermal reaction enthalpy of only −33.8 kJ/mol. This indicates that Al dissolves in preference to Si in local environment. The dissolution mechanism of the five-membered Al–Si framework rings model in highly alkaline solutions has been suggested to be composed of an ion-pairing reaction and an interaction between the remaining broken ring cluster ≡TOH and MOH.