Studies of the adsorption of acetaldehyde, methyl acetate, ethyl acetate, and methyl trifluoroacetate on silica

Abstract

Microcalorimetric, infrared spectroscopic, and temperature-programmed-desorption studies are combined with quantum-chemical calculations based on density-functional theory (DFT) to study the adsorption of acetaldehyde, methyl acetate, ethyl acetate, and methyl trifluoroacetate on amorphous silica. Adsorption of these molecules on silica proceeds primarily through formation of two hydrogen bonds per adsorbate, involving the donation of electron density from the lone-pair orbital on the carbonyl oxygen to hydrogen atoms in surface hydroxyl groups. The formation of these hydrogen bonds causes shifts to lower wavenumbers of infrared bands associated with the stretching of CO and O–H bonds. On the basis of microcalorimetric and thermal-desorption measurements, hydrogen bonds are estimated to have an average energy of 34±4 kJ/mol for methyl or ethyl acetate adsorption, and an energy of 27±4 kJ/mol per bond for the adsorption of acetaldehyde or methyl trifluoroacetate. The initial heats of adsorption of methyl acetate, ethyl acetate, and methyl trifluoroacetate are 95, 96, and 92 (±5) kJ/mol, respectively. These high heats of adsorption at low coverages are assigned, based on DFT calculations and spectroscopic measurements, to the formation of more than two hydrogen bonds with the oxide surface, thereby involving the alkoxy oxygen of the esters. The high initial heat of acetaldehyde adsorption (86 kJ/mol) may be caused by oligomerization processes.