Hydrocarbons adsorption on templated mesoporous materials: effect of the pore size, geometry and surface chemistry

Adsorption-desorption isotherms of aromatic (mesitylene and toluene) and aliphatic (methylcyclohexane, neopentane and n-pentane) hydrocarbons were measured on ordered mesoporous materials, including MCM-41, MCM-48, SBA-15, SBA-16 and MCF silicas, a periodic mesoporous organosilica and a CMK-3 carbon, in order to evaluate the effect of the adsorbent characteristics on the organic compounds adsorption behaviour. A clear separation between aliphatic and aromatic hydrocarbons is observed at low p/p(o) for the materials having pores accessible by narrow openings. The presence of narrow pore openings causes an increase in the volume adsorbed of mesitylene, toluene, methylcyclohexane and n-pentane prior to capillary condensation that does not occur for neopentane. The increase of the hydrophobicity and change of the surface structure, resulting from the incorporation of chloromethyl groups on the silica walls, causes the p/p(o) at which the aromatic hydrocarbons condense to increase while the introduction of aromatic rings into the pore walls has a less significant effect on the condensation pressures. However, at low p/p(o), all the hydrocarbons have higher affinity for the periodic mesoporous organosilica surface than for the pure silica or silica with chloromethyl groups surfaces, while these lower the affinity of the aromatic hydrocarbons. Good estimates of pore size (D-p) are obtained using the classical Kelvin equation from the mesitylene, toluene and methylcyclohexane adsorption data, for spheroidal pores of at least similar to 20 nm. With n-pentane this occurs for pores of similar to 30 nm while neopentane underestimates D-p values even for pores as large as these, although to a lesser extent than nitrogen.