Temperature programmed desorption of toluene, p-xylene, mesitylene and naphthalene on mesoporous high silica MCM-41 for characterizing its surface properties and measuring heats of adsorption

Abstract

Temperature programmed desorption (TPD) of aromatic hydrocarbons (viz. toluene, p-xylene, mesitylene and naphthalene) on mesoporous high silica MCM-41 from 323 to 673 K at different linear heating rates (5, 10, 15 and 25 K min^-1) has been investigated. TPD of toluene at different adsorbate loading (0.5 to 16.3 μmol g^-1) has also been investigated. All the TPD curves have a single asymmetric peak. The heats of adsorption of aromatic hydrocarbons on the mesoporous material, estimated from the TPD peak maximum temperatures measured at different heating rates, was found to occur in the following order: toluene<p-xylene<mesitylene<naphthalene. This ΔH_a increases with the decrease in the ionization potential of the hydrocarbons. The adsorption results from weak interactions between the π-electrons of aromatic hydrocarbons and the terminal poorly acidic silanol (Si-OH) groups. In the case of toluene TPD, the TPD peak maximum temperature increases with decreasing the adsorbate loading, indicating the presence of site energy distribution on the high silica MCM-41.