# Evaluation of Metal-Organic Frameworks as Potential Adsorbents for Solar Cooling Applications
## 5 Historical Background of MOFs as Adsorbents  
[PERSON] [84] in the year 2007 initially suggested the idea of MOF-based heat pumps and since then several MOFs had been studied and investigated for their adsorption and water uptake capabilities. Cu-BTC, MIL-101 Cr, and CPO-27 are some of the widely researched MOFs for their potential use as adsorbent materials. The structural details of Cu-BTC (HUST-1) can be found in references [85; 86]. The process of water adsorption and uptake capacity of MOF materials are investigated by [87; 88]. The water uptake capacity of CU-BTC was found to be 6% by weight at 22 \({}^{\circ}\)C and 2.7 mbar. [PERSON] et al. [15] concluded that, among different MOFs, the water uptake capacity of Cu-BTC and FeBTC is found to be the highest. This large uptake capacity is because of their high porosity and surface area. In another study related to Cu-BTC, [PERSON] et al. [87] showed that the Cu-BTC network can be decomposed because of the presence of water in its structure. The adsorption characteristics of MIL-101 Cr have been investigated widely [14; 89]. [PERSON] et al. [89] found that MIL-101 Cr has a high-water uptake capacity at low operating pressure and there will be only a slight decrease in its adsorption capacity over time. [PERSON] et al. [14; 15] concluded that, with ethanol as a working pair, the adsorption behavior of MIL-101 Cr is better as compared to MIL-100 Cr, MIL-53 Cr,  
Figure 8: Adsorption isotherms of MIL-101(Cr) and MIL-100(Cr) compounds [83]. CPO-27 Ni, Cu-BTC, and Fe-BTC. The results also showed that MIL-101 Cr is stable after 20 cycles at 25 \({}^{\circ}\)C. CPO-27 (MOF-74) is another MOF family which is attractive for use in adsorption applications. CPO-27 has four types: CPO-27 Zn [90], CPO-27 Co [91], CPO-27 Ni [92], and CPO-27 Mg [93]. [PERSON] et al. [93] stated that all these four types of CPO-27 show a competitive water adsorption behavior. A summary of three main MOFs which can be potentially employed for adsorption-based cooling application is presented in Table 2.  
A comparative numerical analysis carried out by [PERSON] et al. [94] showed that the desiccant wheel emended with MIL101@GO-6 (MILGO) has performed better in comparison to conventionally available silica gel-based desiccant wheels. [PERSON] et al. [95] tested several MOF materials and concluded that MIL-101 (Cr) has promising characteristics for use in adsorption-based cooling. Similarly, [PERSON] et al. [96] found that MIL-101 (Cr) has a high water uptake capacity which is desirable for their use in adsorption chillers. [PERSON] et al. [97] employed the MIL-101 (Cr) structure and developed a novel structure, MIL101@GO-6, which has a water uptake capacity six times higher than silica gel's. Moreover, other different MOFs have also been investigated for their potential use in adsorption-based applications such as ISE-1 [98], 3D pillared-layer [99], MOF-5 [100], MOF-177 [100], copper-based MOF [101], MOF-801 [102], and MOF-841 [102]. [PERSON] et al. [103] reported desirable water uptake characteristics and thermal stability of that Al-based metal-organic framework MIL-53(Al)-TDC which has an average heat of adsorption of only 2.6 kJ g\({}^{-1}\). This indicates the low heat requirement of these MOFs which can be provided by the residual heat from the condenser [104]. In another study, [PERSON] et al. [105] concluded that MOF-177, NH2-MIL-125(Ti), and MIL-160 show excellent adsorption characteristics and they are stable under both humid and dry conditions. [PERSON] et al. [106] also highlighted the use of MOFs to achieve higher COP of adsorption-based cooling and identified the 10 best MOFs. MOFs have the potential to be ideal adsorbent materials for this application, as they can be tuned to have desired adsorption behavior for different refrigerants. Different MOFs have also shown good characteristics for adsorption of water, methane, or other gases. [PERSON] et al.