Molecular orbital analysis on the dissolution of nano-ball allophane under alkaline condition

Abstract

Reaction mechanism between hydroxide ion (OH - ) and pore region of nano-ball allophane was analyzed by molecular orbital method (MOPAC-PM3) with a model cluster including the pore. Many Si atoms with silanol groups (Si-OH) are exposed as broken edge at the pore region, and nucleophilic OH - may attack the positively charged Si atoms. When Na + , as a counter ion of OH - , was held on the silanol group by cation exchange reaction (Si-ONa), calculated positive charge of the Si atom increased from that of Si-O - . This may promote nucleophilic attack and adsorption of OH - toward the Si atom. In model cluster with an OH - adsorbed on the Si atom to form five-fold coordination, decreases in calculated bond orders, bond strengths, of some Si-O and Al-O bondings were observed near the OH - adsorption site. This bond weakening will lead to dissolution of one Si and three Al atoms from the pore region of fundamental allophane structure, which is consistent with experimental observation. The increase in positive charge on Si atom and decrease in bond order of Si-O and Al-O bondings were also observed in calculations with one to six water molecules added to simulate reactions in aqueous system.