Adsorption equilibria of rhodium acetylacetonate with MCM-41, MSU-H, and HMS silica substrates in supercritical carbon dioxide for preparing catalytic mesoporous materials

Abstract

Adsorption isotherms of rhodium(III) acetylacetonate (Rh(acac)3) in supercritical carbon dioxide (scCO2) with methanol co-solvent for three types of mesoporous silica (MCM-41, MSU-H, and HMS) at 313–353K and at 15.0MPa were measured using a fixed bed method. The measured adsorption equilibria varied among the adsorbent substrates and depended on the specific surface area and pore structure of the adsorbents. The amount of adsorbed Rh(acac)3 increased with increasing temperature owing to strong effects of the CO2 density. Fitting parameters determined from correlation of the data with the Dubinin-Astakhov (DA) equation demonstrated that the interaction of the precursor for the adsorbents, CO2 density and pore structure of adsorbents were dominant factors in the adsorption equilibria of metal precursors in scCO2. The impregnation of Rh on the silica supports by a batch method in scCO2 could be characterized by the DA equation for which Eprec showed correlation with impregnated Rh particle size and W0,prec showed correlation with Rh loading amount. The DA equation is useful for characterizing adsorption equilibria in mesoporous materials with scCO2 systems and allows estimation of conditions required for impregnating metals into mesoporous silica substrates.