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Silica-alumina gel; Silanol groups; Heat of adsorption; Calorimetry; NMR
Abstract:
The surface chemistry of silica-alumina gels is largely determined by number and type of silanol groups present. However, until now the influence of different silanol groups on adsorption processes is largely unknown. The aim of this study is to develop a method for an energetic characterization of silica-alumina gels by combining data measured by nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA) and adsorption calorimetry. The silica-alumina gels used for the studies are systematically modified by pretreating at temperatures between 200 °C and 1000 °C. To quantify number and distribution of silanol groups, complementary data from 29Si-NMR and 1H NMR and thermogravimetric measurements is combined. Adsorption measurements with the adsorptives cyclopentane (nonpolar) and pyrrolidine (polar molecule capable of hydrogen bonding) provide further information. Based on the adsorption isotherms and load-dependent heats of adsorption, it is shown that number and distribution of silanol groups have little effect on the adsorption of cyclopentane. In contrast, different energy levels are found for the heat of adsorption of pyrrolidine at ∼106 kJ mol−1, ∼95 kJ mol−1, ∼87 kJ mol−1, and ∼80 kJ mol−1 depending on pretreatment temperature. Based on the combination of all data, a model is proposed to assign the identified energy levels of adsorption to different silanol groups.
