Kinetics of hydrolysis of carbon tetrachloride by acidic solids

Carbon tetrachloride (CCl4) has been catalytically hydrolysed to CO2 + HCl over acidic solids (1, TiO2 (rutile); 2, TiO2/SiO2; 3, γ-Al2O3; 4, Al2O3/SiO2; 5, 13% Al2O3–SiO2) between 520 and 610 K without loss of activity. Rates have been measured as a function of water vapour pressure (PH) at various pressures of CCl4 (PC) and temperatures, and the results modelled by simple Langmuir–Hinshelwood rate expressions. Over 1, 2 and 4 CCl4 adsorption is insignificant, and a satisfactory expression is r = kPCθH(1 − θH) where θH = fractional coverage by water. Water adsorption is defined by a Langmuir equation, and regression analysis of the linearised equation provides values for its adsorption coefficient bH and for k, the temperature dependence of which leads respectively to the heat of water adsorption qH and the true activation energy Et. Apparent activation energies are markedly dependent on PH, except on 1 where water is very strongly adsorbed. On 3 and 5, adsorption of CCl4 cannot be ignored, and a bimolecular rate expression: r = kθCθH (θC = coverage by CCl4) works well; multilinear regression analysis of the linearised expression then gives values of k, bH and bC (the adsorption coefficient of CCl4). Extensive structural and chemical characterisation has helped to explain certain aspects of catalytic performance. Dichloromethane (CH2Cl2) and chloroform (CHCl3) have also been hydrolysed over 1 and 3; rates increase with the number of chlorine atoms, and Arrhenius parameters are reported.

CCl4 has been hydrolysed by titania and alumina either unsupported or supported on silica to CO2 + HCl between 520 and 610 K. Dependence of rate on H2O and CCl4 pressures has been fitted to one or other Langmuir–Hinshelwood rate expressions, depending on whether the CCl4 is chemisorbed or not. Water was strongly chemisorbed in all cases, especially on TiO2. Figure optionsDownload as PowerPoint slide