Understanding the relationship between the geometry of metal-organic frameworks (MOFs) and the carbon dioxide enthalpy of adsorption is of utmost importance to control CO₂ adsorption/desorption on MOFs, associated heat in/out and to realize their applications in heat pump and energy storage systems. In this study, the adsorption/desorption characteristics and enthalpy of adsorption of R744 (CO₂) refrigerant on MOFs were investigated using volumetric adsorptions and calorimetric measurements. To understand the effect of MOF pore size, three MOFs – MIL-101, MOF-177, and UiO-66 – were selected in the decreasing order of pore diameter as shown in Figs. 1(a), (b), and (c). MIL-101, with a large pore (34 Å), has a higher adsorption capacity, but the differential enthalpy of adsorption decreases as the amount of CO₂adsorbed increases (Fig. 1(d)). On the other hand, MOF-177 (10 Å) and UiO-66 (6 Å) with smaller pores have lower adsorption capacity, but the differential enthalpy of adsorption increases with CO₂ adsorption (Figs. 1(e) and (f)). Grand Canonical Monte Carlo simulations were used to determine the contribution of the interaction energy and configuration of CO₂ adsorbed in the frameworks and based on this, the effect of pore size on the differential heat of adsorption could be explained. Compared to MOFs with large pores, MOFs with small pore sizes have multiple metal atoms interacting with the incoming adsorbent resulting in a higher enthalpy of adsorption. Moreover, the adsorption phase of CO₂ exists in a ‘liquid-like’ state in MIL-101 and MOF-177, while a ‘solid-like’ adsorption phase is observed in UiO-66 due to a higher degree of confinement effect. These results indicate that there exists a trade-off between adsorption capacity and enthalpy of adsorption depending on the pore size of the MOF. This opens new possibilities for the design of MOFs by considering pore size specifically for heat pump applications.

Figure 1: Structure and pore distributions in (a) MIL-101, (b) MOF-177, and (c) UiO-66. The differential enthalpy of adsorption per mol (ΔH<sub>ads</sub>) vs relative uptake (n/n<sub>sat</sub>) curves for (d) MIL-101, (e) MOF-177, and (f) UiO-66. ΔH<sub>ads</sub> decreases with CO<sub>2</sub> uptake in MIL-101, while in MOF-177 and UiO-66 ΔH<sub>ads</sub> increases with CO<sub>2</sub> uptake.

Figure 1: Structure and pore distributions in (a) MIL-101, (b) MOF-177, and (c) UiO-66. The differential enthalpy of adsorption per mol (ΔH_ads) vs relative uptake (n/n_sat) curves for (d) MIL-101, (e) MOF-177, and (f) UiO-66. ΔH_ads decreases with CO₂ uptake in MIL-101, while in MOF-177 and UiO-66 ΔH_ads increases with CO₂ uptake.

Hibiki Kimura

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