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

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 1A1, 2A1 and 3A1 substitution of Si are the T6, T1 and T9 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 Obr 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(Q3) and Si(Q3) centres to form T(OH)4 and HOT(OM)3 tetrahedra under local and highly alkaline environment. The obtained results suggest that Al(Q3) compared with Si(Q3) breaks more readily with the exothermal reaction enthalpy being in excess of -244.4 kJ/mol, while the most reactive Si(Q3) 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 triple bond TOH and MOH.