Modeling of Adsorption Thermodynamics of Linear and Branched Alkanes in the Aluminum Fumarate Metal Organic Framework

Aluminum fumarate is one of the most stable metal-organic frameworks (MOFs), showing good cycling performance in water adsorption and desorption. Because of its rather small pore size, this MOF shows shape selectivity in the adsorption of linear and branched alkanes. In this work, the interaction of a broad series of alkanes with this MOF was studied through molecular simulations. We expand the transferability of a periodic density functional theory (DFT)-derived force field previously reported by Kulkarni and Sholl to the case of alkane adsorption on this aluminum fumarate MOF. With this force field and using configurational bias Monte Carlo simulations (CBMC), low coverage adsorption enthalpies, adsorption entropies, and Henry's adsorption constants were calculated. Experimental enthalpies of adsorption (-ΔH ₀) of C5-C8 n- and iso-alkanes are accurately reproduced by our calculations, e.g., within 5% relative error for n-alkanes. Interestingly, a compensation effect between adsorption enthalpy and adsorption entropy is found in the simulations, with a calculated slope almost identical to the experimental value. This indicates that the force field is very well capable of predicting tendencies with respect to the energetic interactions between the confined molecules and the MOF pore walls. Our calculations also predict separation between linear and branched alkanes with very good accuracy.