Adsorption of benzene, toluene, and p-xylene on microporous SiO2

Adsorption isotherms of aromatic BTX (benzene, toluene, and p-xylene) hydrocarbons on pure and Ag-doped SiO2 microporous substrates synthesized by the sol-gel procedure have been measured by the gas chromatographic technique at several temperatures in the range between 423 and 573 K. Both Langmuir and Dubinin adsorption models can reasonably fit every set of BTX adsorption data at all temperatures. The uptake degree of these hydrocarbon adsorptives on both SiO2 and Ag/SiO2 microporous substrates is temperature-dependent. Additionally, the degrees of interaction of BTX molecules with SiO2 and Ag/SiO2 microporous walls have been investigated by evaluation of the corresponding isosteric heats of adsorption (q(st)) by means of a Clausius-Clapeyron-type equation. The isosteric heats of adsorption of BTX hydrocarbons at different adsorbate loadings have been inferred from the adsorption isotherms and found to follow the decreasing sequence q(st)(benzene) < q(st)(toluene) < q(st)(p-xylene). Isosteric heats of adsorption of BTX compounds on SiO2 microporous substrates have been found, in general, to increase with an increase in the adsorbate loading as a consequence of the cohesive interactions between adsorbate molecules. Addition of Ag atoms to the SiO2 structure helps to homogenize the magnitudes of the isosteric heats of adsorption of the BTX compounds.