Evaluation of the sorbent surface homogeneity of mesoporous MCM-41 and its modified analogues by inverse gas chromatography

Abstract

Inverse gas chromatography is one of the most informative methods to study the surface heterogeneity. This method allows to calculate the values of a distribution function of adsorption energy from an experimental data (the heterogeneity function, χ(ε)). The asymptotically correct condensation approximation method is the most accessible practical way of calculating χ (ε) from the values of the specific retention volumes. It is based on the description of the local adsorption isotherm through condensation isotherm. Today the heterogeneity functions studied for a wide range of solids by the method of inverse gas chromatography. However, data about of the heterogeneity functions of adsorbents such as MCM-41 are absent. In the present work, the heterogeneity functions of surface of the original MCM-41 and its modified analogues has been investigated by the method of inverse gas chromatography.
From the experimentally obtained values of the specific retention volumes for two test-sorbates (nhexane and n-butanol) at the original MCM-41 and modified with methyl and phenyl groups, MMet and MDC respectively, a distribution function of adsorption energy (χ and χ2) has been calculated. Using adsorbents that capable of nonspecific (n-hexane) and specific (n-butanol) interactions allows to differentiate of sorption sites and to distinguish geometric and chemical heterogeneity of the sorbents surface.
The occurrence of the heterogeneity function in the case of measurements with n-butanol and its deficiency in the case of measurements with n-hexane for MCM-41 sorbent has been found out. This implies that the chemical energy heterogeneity is characterized for the MCM-41 because of the charges with different energies on the surface presence. At the same time the geometric surface heterogeneity as the occurrence differing surface topography for the MCM-41 sorbent is absents. It has been found that its homogeneous structure slightly damaged as result grafting trimethylsilane to the surface of MCM-41, and after modification of the surface MCM-41 by phenyl groups its becomes unhomogeneous.