Density Functional Theory Study of Gas Phase Hydrolysis of Titanium Tetrachloride

Abstract

Thermodynamic parameters of four elementary steps of hydrolysis of TiCl4, viz. formation of (i) TiCl3(OH), (ii) TiCl2(OH)2, (iii) TiCl(OH)3, and (iv) Ti(OH)4 as well as condensation of the hydroxides giving rise to titanoxanes (Cl4−x(OH)x−1Ti–O–TiCl4−x(OH)x−1 (x = 1–4)) have been studied using B3LYP/6-311++G(d,p) level of DFT. The first elementary step required the highest free energy of activation (ΔG# = 22.8 kcal mol−1) as well as the highest enthalpy of activation (ΔH# = 16.9 kcal mol−1), while subsequent steps showed a decrease in both ΔG# and ΔH#, reaching the lowest value of ΔG# = 20.4 kcal mol−1 and ΔH# = 11.9 kcal mol−1 for the fourth step. Condensation reactions were triggered by O–H···O–H hydrogen bonding in pre-reactant complexes. The ΔG# of the condensation of mono-, di-, tri-, and tetrahydroxides are 23.0, 17.1, 20.8, and 14.5 kcal mol−1, respectively. Thus gas phase hydrolysis of TiCl4 and subsequent condensation of TiCl4−x(OH)x are feasible at low temperature and higher order condensation would lead to the formation of TiO2. TiCl4 showed higher Lewis acid character to a water dimer than a water molecule as their respetive interaction energies were −4.8 and −14.2 kcal mol−1. Coordination of the water dimer to titanium has significant effect on lowering the activation bariers of the hydrolysis reactions and therefore the amount of water vapor in the gas phase reaction is critical to the overall rate of the reaction.