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

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 313–353 K and at 15.0 MPa 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 E_prec showed correlation with impregnated Rh particle size and W_0,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.