Interpretation of CO2 adsorption isotherms on metal-organic frameworks Cu3(BTC)2, MOF-2 and MOF-74
Abstract
The cluster adsorption model is applied to interpret the adsorption isotherms of carbon dioxide on the metal-organic frameworks MOF-2, MOF-74 and Cu3(BTC)2. According to this model, monomers, dimers, and clusters of n-molecules from monomeric molecules of the sorbate are formed on the sorbent. The physicochemical justification for adsorption in the form of clusters is that the formation of sorbate clusters provides a higher adsorption energy compared to the fixation of individual molecules due to a decrease in the energy of the system due to the contribution of sorbate-sorbate bonds to the adsorption energy. The equation of the cluster adsorption isotherm was obtained analytically in previous works; its parameters are the equilibrium coefficients of the reactions of formation of monomers and sorbate clusters, the capacity of the monolayer and the structural characteristics of the sorbate clusters; the latter allows one to estimate the structure of the sorbate. For single-layer cluster adsorption, a graphical method for determining the parameters has been developed and a criterion has been introduced that allows one to determine, based on experimental sorption data, whether the monolayer adsorption is cluster adsorption or adsorption of individual molecules (Langmuir). It has been shown that for the Cu3(BTC)2 - CO2 system, the sorbent surface area is sufficient for a single-layer coating and the adsorption is single-layer. The clustering criterion that confirmed the formation of sorbate clusters on the sorbent has been used to analyze the adsorption nature of this system. It has been shown that during adsorption on Cu3(BTC)2, the sorbate clusters consist of 3-4 molecules. For the Cu3(BTC)2 - CO2 system, the isotherm equation parameters have been determined graphically and by the least-squares method. The CO2 adsorption isotherm parameters on MOF-2 and MOF-74 have been found by the least-squares method. It has been shown that in this case the adsorption is two-layer. On MOF-2, clusters of ~2-3 carbon dioxide molecules are formed, of which ~2 molecules are in the first layer and ~1 molecule in the second. On the surface of MOF-74, there are mainly clusters of 2 molecules with 1-2 molecules in the first layer.
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References
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