Adsorption of acetone on nonporous and mesoporous silica

The ambient temperature solid/gas interaction of acetone with a nonporous silica (Aerosil 200) and two mesoporous silicas (a spherical MCM-41 preparation, with highly uniform and very small meso-pores, and a reference conventional MCM-41 preparation), vacuum activated at different temperatures, has been investigated by the combined use of gas-volumetry/adsorption-microcalorimetry and in situ FTIR spectroscopy. In the early stages of acetone uptake, when no physical adsorption occurs yet, two specifically adsorbed (chemisorbed) acetone species form, characterized by the formation of two H-bonds with suitably spaced silanol couples, and of one (stronger) H-bond with single silanols, respectively. Both types of chemisorbed acetone are fully reversible, upon evacuation, in the case of nonporous silica, whereas they are partly irreversible, though to a rather different extent, in the case of mesoporous silicas. The energy involved in the acetone adsorption process, expressed in the form of (differential) molar adsorption heats evolved for admission of small doses of adsorptive, depends on activation (dehydration) conditions and is definitely larger in the case of the mesoporous silica characterized by very small-bore meso-porosity. In particular, on small-pores spherical MCM-41, somewhat stronger H-bondings are formed, as indicated by lower stretching frequencies of the acetone carbonyl group, but also other forces are likely to be active in the adsorption process occurring inside the narrow meso-pores network. The comparison between overall (i.e., nonresolved) gas-volumetric quantitative uptake data and band-resolved IR spectroscopic data allows, on one side, an approximate though realistic evaluation of the number of hydroxyl groups involved in the two types of specific acetone adsorption and, on the other side, yields clear indication that also the hydroxyl groups, that were initially involved in mutual H-bonding interactions, become involved in H-bonding adsorption of acetone molecules.