The retention of heterocyclics by siliceous frameworks - Part I - The role of the heterocyclic

Flow microcalorimetry has been used to probe acid-base interactions between five-membered-ring heterocyclics and thermally pre-treated, porous silica. The adsorbates (1-methylpyrrolidin-2-one, pyridine, pyrrolidine, pyrrole, 2-methylthiophene, 2-octyl-4-isothiazolin-3-one, 4,5-dichloro-2-octyl-4-isothiazolin-3-one and 2-cyclopentenone,) varied in basicity, polarity and pi -character. The amounts of the adsorbates retained by the silica were determined, along with enthalpy of adsorption (ranging from -5.5 kJ mol(-1) to -57.8 kJ mol(-1)) and enthalpy of desorption (ranging from 5.6 kJ mol(-1) to 26.1 kJ mol(-1)). For the majority of the adsorbates the enthalpy of adsorption is consistent with hydrogen bonding to isolated silanols. Although increasing basicity enhanced the adsorption enthalpy and hence the strength of associations, desorption was inhibited when a carbonyl, or unsaturated carbonyl, group was adjacent to the active basic centre. Bulky electron-withdrawing agents (chlorine atoms) substituted at the double bond of the unsaturated carbonyl reduced the adsorption considerably. This was attributed to steric hinderance restricting the proximity of the basic groups with the active silanol sites. (C) 2001 Kluwer Academic Publishers.