# Evaluation of Metal-Organic Frameworks as Potential Adsorbents for Solar Cooling Applications
## 6 Comparison of MOFs with Other Adsorbents  
Figure 9 provides a comparison of conventional adsorbents and MOFs in terms of their surface area and pore volume [4]. In general, MOFs showed exceptionally high porosity, well-defined molecular adsorption sites, and a large surface area (up to 5500 m\({}^{2}\)/g) in comparison to currently used adsorbents like silica gel (1000 m\({}^{2}\)/g), zeolite (900 m\({}^{2}\)/g), and activated carbon (1300 m\({}^{2}\)/g), which suffer from low water uptake. The HKUST-1 has 93.2% better uptake as compared to silica gel RD-2060. This is \begin{table}
\begin{tabular}{c c c} \hline \hline
**Name of MOF** & **Water Uptake Capacity** & **Remarks** \\ \hline \multirow{2}{*}{CU-BTC} & 6\% by weight & \\  & 0.324 kgref/kgads & Not stable \\ \hline \multirow{2}{*}{MIL-101 Cr} & 1.01 kgref/kgads & \multirow{2}{*}{Stable} \\  & 0.3 kgref/kgads & \\ \hline \multirow{2}{*}{CPO-27} & 0.1 to 0.5 kgref/kgads & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characteristics of CU-BTC, MIL-101 Cr, and CPO-27. due to the higher surface area of HKUST-1 as can also be observed from Figure 10[115]. This could lead to a considerable increase in refrigerant flow rate, cooling capacity, and/or reduction of the size of the adsorption cooling system. This also highlights that the use of MOFs will improve the efficiency of adsorption-based cooling systems. It can also be noted that the advanced MOFs are being investigated with time which is providing a possibility to increase the thermal performance of thermal cooling technologies. The ratio between water uptake at equilibrium and maximum water uptake capacity concerning partial pressure for three materials shows that HKUST-1 is more effective than silica gel RD-2060 in adsorbing water vapors closer to its maximum water uptake, in particular at low partial pressure values [115]. Figure 11 reports the comparison between dehumidification efficiency (\(\eta_{\text{deh}}\)) predicted by the model developed by [PERSON] et al. [94] for silica gel and MILGO (MIL101@GO-6)-based desiccant wheel. The simulations were carried in the same operating conditions for both desiccant wheels. It can be observed that a MILGO-based desiccant wheel has better dehumidification performance as compared to the silica gel desiccant wheel. The water adsorption isotherms of conventional adsorbents and MOFs are presented in Figures 12 and 13, respectively [4]. It can be noticed from the adsorption isotherms that the water uptake performance of MOFs is better with MIL-101(Cr) showing the highest capacity to adsorb water vapors. Figure 10: Physical properties of three different MOFs [115]. Figure 9: Comparison of conventional adsorbents and MOFs [4]. The type of isotherm further reveals that the MIL group of MOFs are more suitable for applications at high relative pressure [116], while CPO will be better at a low relative pressure range [117; 118; 119]. The enthalpy of adsorption which is the heat released during the process of adsorption is crucial for the selection of an adsorbent as it determines the heat required for regeneration. This directly influences the efficiency of an adsorption cooling or heating system; the lower the adsorption heat of a material, the less energy will be required to regenerate the material and the higher the efficiency will be. The calculated enthalpy of adsorption for different MOF materials is presented in Figure 14 [77]. The CAU family of MOFs has the enthalpy of adsorption in the range of 44-52 kJ/mol, whereas the MIL group has it in the range of 40-72 kJ/mol. The value of adsorption enthalpy for the UiO group of MOFs is found to be between 48 and 53 kJ/mol. In general, most of the MOFs have a lower enthalpy of adsorption in comparison to conventional adsorbent materials. This further indicates that the MOF materials have the potential to increase the efficiency of adsorption-based cooling technologies due to the lower requirement of energy for the regeneration process [77].