Density functional investigation of the water exchange reaction on the gibbsite surface

The water exchange reactions on the gibbsite surface have been investigated by density functional calculations (B3LYP/6-31G(d) level) combining the supermolecular model and PCM model in this paper, and the water exchange rate constants on the gibbsite surface have also been predicted. In the proposed reaction pathways, the clusters Al6(OH)18(H2O)6(0) and Al6(OH)12(H2O)12(6+) are used as the models of gibbsite surface and protonated gibbsite surface respectively to examine the effect of protonation of gibbsite surface on the water exchange rate constants. The activation energy barriers DeltaE(s) not equal to (aq) for Al6(OH)18(H2O)6(0) and Al6(OH)12(H2O)12(6+) are 28.6 and 27.2 kJ*mol-1, respectively. The reaction energies DeltaE(s) (aq) for Al6(OH)18(H2O)6(0) and Al6(OH)12(H2O)12(6+) are 2.9 and 14.4 kJ mol-1, respectively, indicating that hexacoordinate aluminum in the gibbsite surface is more stable. The log k(TST) for Al6(OH)18(H2O)6(0) and Al6(OH)12(H2O)12(6+) are 6.5 and 7.5 respectively, and the log k(ex) calculated by the given transmission coefficient for Al6(OH)18-(H2O)6(0) and Al6(OH)12(H2O)12(6+) are 2.4 and 3.4 respectively, indicating that the protonation of gibbsite surface promotes the water exchange reaction of gibbsite surface and accelerates the dissolution rate of gibbsite. The relationship between the calculated free energy and experimental rate constants was explored, and according to this relationship, the log k(ex) for Al6(OH)18(H2O)6(0) and Al6(OH)12(H2O)12(6+) are 2.5 and 3.1 respectively, close to the corresponding values calculated by the given transmission coefficient. The water exchange rate constant of gibbsite surface is close to those of K-MAl(12)(M = Al, Ga, and Ge) polyoxocations, but deviates from that of Al(H2O)6(3+), implying that the same reactions with similar structure have similar water exchange rate constants.