Microcalorimetric, FTIR, and DFT studies of the adsorption of methanol, ethanol, and 2,2,2-trifluoroethanol on silica

摘要

The adsorption and reaction of methanol, ethanol, and 2,2,2-trifluoroethanol on silica at 300 K were studied by combining microcalorimetric and infrared spectroscopic (FTIR) measurements with quantum-chemical calculations based on density-functional theory (DFT). Methanol, ethanol, and 2,2,2-trifluoroethanol adsorb molecularly on silica via formation of hydrogen bonds, with initial heats of interaction of 78, 100, and 90 (+/-2) kJ/mol, respectively. Methanol and 2,2,2-trifluoroethanol adsorbed on silica can be removed by evacuation at 473 K, whereas small amounts (similar to 5 mu mol/g) of ethoxy species remain on silica after evacuation at 573 K. Two pathways are considered for the alkoxylation of silica: one pathway involving protonation of the adsorbed alcohol by surface hydroxyls and the other involving protonation and subsequent cleavage of Si-OH bonds or SiO-Si bridges by the adsorbed alcohol. The activation energies for the formation of methoxy, ethoxy, and 2,2,2-trifluoroethoxy species via the first pathway are estimated to be 309, 285, and 321 kJ/mol, respectively, whereas activation energies for the second pathway are estimated to be 117, 117, 138 kJ/mol. The high activation barrier for the first pathway is caused by the localization of positive charge in the alkyl group of the transition state, which is made difficult by the weak acidity of silica and the instability of methyl, ethyl, and 2,2,2-trifluoroethyl carbenium ions. The second proposed mechanism is controlled mainly by the acid strength of the alcohols and the extent of delocalization of electron density in the four-member ring present in the transition states. (C) 1998 Academic Press. [References: 61]