Thermodynamics and kinetics of adsorption of bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (1,5-cyclooctadiene) ruthenium (II) on carbon aerogel from supercritical CO2 solution

The thermodynamics and kinetics of adsorption of bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (1,5-cyclooctadiene) ruthenium (II) (Ru(cod)(tmhd)2) on carbon aerogel particles from supercritical carbon dioxide was investigated. The particles had an approximate radius of 1 mm and average pore size of 22 nm. The adsorption isotherms were measured at different temperatures and pressures, thus at different supercritical fluid densities. It was observed that at constant temperature, adsorbed amount (q) decreased with the increasing scCO2 density or pressure at a particular concentration in the fluid phase. The adsorption isotherms were best represented by the Modified Langmuir Model. The maximum uptakes were reached at concentrations considerably less than the solubility of Ru(cod)(tmhd)2 in scCO2 and were found to correspond to monolayer coverage of all the accessible surface of the carbon aerogels as determined by BET measurements. The kinetics of adsorption could be modeled using a model consisting of coupled ODEs based on diffusion in the pore volume and assuming local equilibrium at the adsorbent—fluid interface within the pores. Various simulations of the model were made in order to investigate the effect of isotherm parameters, particle size and pore size of carbon aerogel particles on the kinetics of adsorption. It was observed that in order to prepare Ru(cod)(tmhd)2 impregnated carbon aerogel particles larger than 5 mm with a radially uniform metal distribution which are used as catalysts in industrial applications, one has to take into consideration the long diffusion time. The pore size of the carbon aerogel particles is also very important for the adsorption process. As the pore size starts to approach the solute size, the time to reach equilibrium starts to increase significantly.