The sorption of hydrogen sulphide, dimethyl sulphide, methanethiol, and ethanethiol on silica gel

The adsorption of hydrogen sulphide, methanethiol, dimethyl sulphide, and ethanethiol has been studied on a range of silica xerogels with various surface characteristics, including materials modified by heat treatment and/or methylation. Complete adsorption–desorption isotherms were determined at the boiling points of the various gases on xerogels previously heat treated at 500°. Monolayer region isotherms were determined within ±30° of the respective boiling points of the compounds, and from the adsorption isosteres the isosteric heats of adsorption (qst) were calculated. On 500° gel, the qst values determined at θ= 0·2, for example, were 9·8, 16·2, and 13·6 kcal. mole–1 for methanethiol, ethanethiol, and dimethyl sulphide respectively. These values decrease in step with decreasing concentration of surface hydroxy-groups and are explained in terms of interactions involving weak hydrogen-bonds. For hydrogen sulphide adsorptionon the same gel, the variation of qst with surface coverage shows a maximum value of 5·8 and a minimum of 5·0 kcal. mole–1 at θ= 0·3 and 0·8 respectively. These data are interpreted on the basis of purely physical adsorption. All adsorbents used were characterised by low-temperature (77°K) nitrogen adsorption–desorption isotherms from which B.E.T. and pore-size distribution data were calculated. Lump densities of the gels were measured by immersion in mercury, and hydroxy- and methoxy-group concentrations determined thermogravimetrically.