Trends in the condensation/evaporation and adsorption enthalpies of volatile organic compounds on mesoporous silica materials

Abstract

The adsorption–desorption of toluene, methylcyclohexane, neopentane and n-pentane was measured at temperatures between 258 and 308 K on mesoporous silica materials with different pore structures and pore sizes (MCM-41, SBA-15, SBA-16, LPC and MCF). It was found that the change in the width of the hysteresis cycles with temperature is similar for MCM-41, SBA-15 and MCF but distinct for the materials having pores accessible through very narrow openings (SBA-16 and LPC). A comparison between the plots of V ads vs. potential for SBA-16, LPC and MCF suggested that the differences are a consequence of the non equilibrium evaporation mechanism of the organics from the pores of SBA-16 and LPC. The isosteric enthalpies of adsorption indicated that the strength of the gas–solid interactions for toluene is superior to those corresponding to neopentane, methylcyclohexane or n-pentane. The isosteric enthalpies associated with capillary condensation are higher than the molar condensation enthalpies for the materials with narrower mesopores. As the size of the pores increases, the difference gradually decreases and the isosteric enthalpies are identical to the molar condensation enthalpies for the materials with pores larger than ∼20 nm. The apparent evaporation enthalpies were found to be higher than the apparent condensation enthalpies when the hydrocarbons isotherms exhibited developed hysteresis, except in the case of the MCF silica, for which the values of both enthalpies were similar.