Molecular understanding of the impacts of structural characteristics on ethanol adsorption performance for adsorption heat pumps†

Abstract

Adsorption heat pumps (AHPs) powered by low-grade waste heat or renewable energy can reduce electricity consumption and carbon emission. The exploration of the high-performing adsorbents of AHPs is the key to improving their coefficient of performance (COP) by tuning their adsorption capacity and step location. The structure–property relationship of adsorbents can provide useful guidance for developing and designing potential adsorbents for AHPs. However, given the complexity of the chemical composition and structural diversity of adsorbents, it is extremely challenging to extract the structure–property relationship from high-throughput computational screening based on molecular simulations of existing adsorbents. In this study, ideal nanoporous crystal structures comprising Lennard-Jones (LJ) spheres were generated to simplify this process. The effects of pore size and LJ interaction parameters (σ and ε) on the adsorption performance of the structures, including the saturation uptake (W_s), step location of adsorption isotherms (α) and the uptake change at step location (W_α), were investigated by grand canonical Monte Carlo (GCMC) simulations. It was demonstrated that large σ, ε and cell length or pore size are favorable for W_s and W_α. 0 < α < 0.4 is favorable for W_s and W_α for small-pore structures, and 0.6 < α < 1 is preferential for large-pore structures, which can be attributed to the strong interaction strength of small-pore structures and the relatively weak interaction in large-pore structures. Given the various optimal pore sizes of W_s and W_α, developing an effective strategy to simultaneously improve W_s and W_α by tuning the structural properties of adsorbents is key in the future.