Thermodynamic studies of methanol adsorption on metal impregnated γ-alumina samples

A series of metal impregnated alumina samples has been prepared by impregnation of the host oxide (γ-alumina) with different additives (Cr, Mn, Fe and Co). In the present paper, a study has been made of the adsorption isotherms of methanol (MeOH) on parent and metal impregnated γ-alumina samples in the temperature range 273–313 K as a function of temperature and coverage, using CAHN 1000 electrobalance. Furthermore, the effect of different concentrations of the metals loaded on γ-alumina has also been investigated by means of X-ray diffraction (XRD), low temperature nitrogen adsorption, and thermal analysis techniques (thermogravimetry (TG) and differential thermogravimetry (DTG)). From the adsorption data, thermodynamic parameters such as free energy (ΔG°), differential enthalpy (ΔH) and molar entropy (ΔS°) of adsorption have been determined as a function of temperatures and coverage. From the data, it is noted that values of the enthalpy (ΔH) are higher for metal impregnated alumina as compared to parent alumina indicating that strong adsorbate–adsorbent interaction is found after impregnation. It is also evident from the results that values of the free energy of adsorption (ΔG°) for the adsorption of MeOH on alumina systems are negative, showing that adsorption processes are spontaneous in nature These values decrease with increase in temperature indicating that alumina samples have higher adsorption affinity for methanol at low temperature. The low values of entropy (ΔS°) for metal impregnated alumina indicate more constraint on the mobility of the adsorbate molecules. It is noted from the result of X-ray diffraction that surface properties of γ-alumina change with metal impregnation. X-ray diffraction analysis on the impregnated alumina also reveals the existence of crystalline material. Furthermore, analysis of the Dubinin–Radushkevich (D–R) plot of nitrogen adsorption indicates that microporosity of the alumina decreases due to progressive closure of the micropores as the metals are loaded on the surface of the alumina. This decrease in microporosity further supports the idea that these impregnated metals residues distribute uniformly over the surface of the alumina. It is also found from TG curves that mass loss for metal impregnated alumina systems in all cases is greater than for parent alumina which means that transition metals used for impregnation are able to increase the percentage loss in mass. The DTG analysis of dehydration of water process confirms the results of TG, showing endothermic peaks which correspond to mass loss on TG curve.