**2.4. Novel adsorbent materials: metal-organic frameworks (MOFs)**

Metal-organic frameworks (MOFs) are highly crystalline porous material that are widely regarded as promising materials for various applications such as catalysis [19], gas separation [20] and gas storage [21]. The high crystallinity of MOFs can be highlighted through the description of MOF-5 structure which was once described as "The zinc carboxylate cluster with the six carboxylate carbons forming a regular octahedron but with tetrahedral symmetry was elegantly beautiful especially when linked in such regular arrays like terracotta warriors" [22]. Usually, the approach of assembling new frameworks out of molecular building blocks or secondary building blocks held together by strong bonding has been considerably used in designing new materials even though it is a challenge to control the assembly of the basic building blocks in the solid state and thus predicting of the resulting structure. Based on the same concept or what is called the reticular synthesis, MOFs are designed based on the assembly of organic units and metal clusters as secondary building units (SBUs) to build the robust complex structures (**Figure 5**) [23]. Compared to conventional microporous inorganic materials such as zeolites and silica gel, MOFs were found to be flexible regarding controlling their architecture and functionalization of the pores [24]. Such tunable properties have given the lead to MOFs over conventional adsorbents as they offer high stability and porosity as shown in **Figure 6** . As mentioned above, these exceptional properties made this class of materials very interesting in a number of applications. Adsorption heat pumping for cooling applications has attracted a massive research over the past few years. For decades, the adsorption cooling application was mainly based on silica gel, activated carbon and zeolites which suffer from the limited adsorption capacity. The next section will discuss the different MOF materials with different refrigerants for cooling applications.

#### *2.4.1. MOFs-water pair*

Water is an environment friendly refrigerant with high latent heat of evaporation and high heat and mass transfer properties. Water-based adsorption systems use adsorbents like silica gel and zeolites which have limited water uptake capabilities (up to 0.3 gw/gads) leading to low specific cooling power. The introduction of metal-organic frameworks (MOFs) material for adsorption cooling application allowed an improvement in the performance of the systems due to the high-water uptake that can reach up to 1 gw/gads and the potential of using low temperature waste heat or solar collectors as primary energy sources. Shi et al. [25] showed that using CPO-27(Ni) MOF material (a max water uptake of 0.45 gw/gads) for automotive air conditioning can outperform SAPO 34 zeolite material in terms of specific cooling power. They showed that CPO-27(Ni) produced specific cooling power of 440 W kg−1 at a desorption temperature of 130°C and a cycle time of 900 s compared to 310 W kg−1 for SAPO-34 at the same operating conditions. Numerous metal-organic framework materials have been studied to investigate their water adsorption capacity, **Figure 7** shows maximum water uptake of a number of MOFs that were investigated for adsorption cooling applications at 25°C.

Ehrenmann et al. [26] showed that MIL-101Cr can adsorb up to 1 gw/gads with high performance stability, also the heat of adsorption value was near the evaporation enthalpy of water, meaning that the interaction energy with the framework was believed to be very low compared to other materials used so far, like zeolites and hence the material do not require high regeneration temperature. A further modification was investigated by Khutia et al. [27] as

**Figure 7.** Maximum water uptake comparison between some reported MOFs, silica gel and zeolite.

**Figure 6.** BET surface area comparison between some reported MOFs and zeolite [48, 49].

Adsorption Refrigeration Technologies http://dx.doi.org/10.5772/intechopen.73167 81

**Figure 5.** Schematic representation of how the framework is formed [47].

**Figure 6.** BET surface area comparison between some reported MOFs and zeolite [48, 49].

on the same concept or what is called the reticular synthesis, MOFs are designed based on the assembly of organic units and metal clusters as secondary building units (SBUs) to build the robust complex structures (**Figure 5**) [23]. Compared to conventional microporous inorganic materials such as zeolites and silica gel, MOFs were found to be flexible regarding controlling their architecture and functionalization of the pores [24]. Such tunable properties have given the lead to MOFs over conventional adsorbents as they offer high stability and porosity as shown in **Figure 6** . As mentioned above, these exceptional properties made this class of materials very interesting in a number of applications. Adsorption heat pumping for cooling applications has attracted a massive research over the past few years. For decades, the adsorption cooling application was mainly based on silica gel, activated carbon and zeolites which suffer from the limited adsorption capacity. The next section will discuss the different MOF

Water is an environment friendly refrigerant with high latent heat of evaporation and high heat and mass transfer properties. Water-based adsorption systems use adsorbents like silica gel and zeolites which have limited water uptake capabilities (up to 0.3 gw/gads) leading to low specific cooling power. The introduction of metal-organic frameworks (MOFs) material for adsorption cooling application allowed an improvement in the performance of the systems due to the high-water uptake that can reach up to 1 gw/gads and the potential of using low temperature waste heat or solar collectors as primary energy sources. Shi et al. [25] showed that using CPO-27(Ni) MOF material (a max water uptake of 0.45 gw/gads) for automotive air conditioning can outperform SAPO 34 zeolite material in terms of specific cooling power. They showed that CPO-27(Ni) produced specific cooling power of 440 W kg−1 at a desorption temperature of 130°C and a cycle time of 900 s compared to 310 W kg−1 for SAPO-34 at the same operating conditions. Numerous metal-organic framework materials have been studied to investigate their water adsorption capacity, **Figure 7** shows maximum water uptake of a

number of MOFs that were investigated for adsorption cooling applications at 25°C.

materials with different refrigerants for cooling applications.

**Figure 5.** Schematic representation of how the framework is formed [47].

*2.4.1. MOFs-water pair*

80 Sustainable Air Conditioning Systems

**Figure 7.** Maximum water uptake comparison between some reported MOFs, silica gel and zeolite.

Ehrenmann et al. [26] showed that MIL-101Cr can adsorb up to 1 gw/gads with high performance stability, also the heat of adsorption value was near the evaporation enthalpy of water, meaning that the interaction energy with the framework was believed to be very low compared to other materials used so far, like zeolites and hence the material do not require high regeneration temperature. A further modification was investigated by Khutia et al. [27] as the water loading capacity of four nitro or amino-functionalized MIL-101Cr materials (fully and partially functionalized) was assessed for heat transformation applications. The fully aminated MIL-101Cr-NH<sup>2</sup> , and partially aminated MIL-101Cr-pNH<sup>2</sup> , showed the best water loadings (about 1.0 gw/gads) and proving the weak host-guest interactions and hence a lower regeneration temperature is required. Elsayed et al. [25] further improved the thermal conductivity and the water vapor capacity of MIL-101(Cr) to be used in adsorption heat pump application through using hydrophilic graphene oxide. Two methods have been used to develop MIL-101(Cr)/GrO composites. It was shown that introducing low amounts of GrO (2%) to the neat MIL-101(Cr) enhanced the water adsorption characteristics at high relative pressure but enhanced the heat transfer properties by 20–30% while using more than 2% of GrO reduced the water adsorption uptake but significantly enhanced the thermal conductivity by more than 2.5 times. Yan et al. [28] managed to improve the performance of the material through developing another composite (MIL-101@GO) of MIL-101(Cr) and graphite oxide (GO) with high-water vapor capacity for adsorption heat pumps (AHPs). It showed that MIL-101@GO possessed a super-high adsorption capacity for water vapor up to 1.58 gw/gads. This superior water vapor adsorption/desorption performance make MIL-101@GO a promising candidate as the water vapor adsorbent for adsorption heat pumps (AHPs) process. Another factor that was studied was the effect molding on the water adsorption properties of MIL-101(Cr) after pressing the prepared powder into a desired shape which was investigated by Rui et al. [29] . It showed that the forming pressure has a large influence on pore structure of shaped MIL-101, as the forming pressure increases from 3 to 5 MPa, the equilibrium adsorption capacity of water is up to 0.95 gw/gads at the forming pressure of 3 MPa. Other types of MOFs such as Al fumarate was investigated by Jeremias et al. [30] in the form of coating on a metal substrate via the thermal gradient approach. It was concluded that Al fumarate is a promising adsorbent for heat pumping applications as it can be regenerated at low temperature as low as 60°C with a water loading difference higher than 0.5 gw/gads. Fadhel et al. [31–33], generated cooling effect from using aluminum fumarate and MIL-101(Cr) in different multi-bed water adsorption systems. The performance was compared to other adsorbent materials such as AQSOA-Z02 and conventional silica gel. The isostructural CPO-27(Ni) was compared to aluminum fumarate by Elsayed et al. [34]. It was highlighted that the CPO-27(Ni) outperformed the aluminum fumarate at low evaporation temperatures, while the aluminum fumarate was more suitable for applications requiring high evaporation temperature. It was also mentioned that CPO-27(Ni) is suitable for systems operated with high desorption temperature while on the contrary aluminum fumarate can be regenerated at low desorption temperatures.

Birm-1(K) and Birm-1(Li) showed water uptake of 0.14–0.35 gw/gads which is lower than the water adsorption capacity of HKUST-1, proving that HKUST-1 regarding to the water capacity outperform conventional porous materials such as silica gel and other MOF materials [36], comparing HKUST-1 with other zeolite materials like SAPO-34 and AlPO-18 showed that the best SAPO-34 samples had a water uptake of 0.253 gw/gads which is a factor of 4.9 larger compared to the reference silica gel. Those results were only exceeded by the best AlPO-18 sample with a measured water uptake of 0.254 gw/gads for the low driving temperatures. This equaled an improvement by a factor of 6.2. For driving temperature of 140°C, the highest water uptake was found for the metal-organic framework HKUST-1 [37].Other MOFs such as MIL-100 (Fe and Al) with a water uptake of 0.76 and 0.5 gw/gads were found to be also very interesting candidates for thermally driven, sorption-based chilling or heat pump systems [38, 39]. A 3D MOF material (ISE-1) was found to have water loading of 0.210 gw/gads which was found to be larger than other five zeolites in that study and of the reference silica gel demonstrating the potential of MOF materials for use in adsorption heat pumping processes [40]. MIL-53(Al), MIL-100(Fe) and ZIF-8 were compared with the previous materials and were found to have a water uptake higher than 0.3 gw/gads proving that MOFs are a very promising class of materials for the use in adsorption heat pumping/cooling processes [41,

ity with a water load of ≈0.4 gw/gads and were considered to be especially beneficial for the

Saha et al. [44] presented experimental and theoretical investigations of adsorption characteristics of ethanol onto metal-organic framework namely MIL-101(Cr). The experiments have been conducted within relative pressures between 0.1 and 0.9 and adsorption temperatures ranging from 30 to 70°C, which are suitable for adsorption cooling applications. Adsorption isotherm data exhibit that 1 g of MIL-101(Cr) can adsorb as high as 1.1 g of ethanol at adsorption temperature of 30°C. The experimental results showed that the studied pair would be a promising candidate for developing high performance cooling device. Rezk et al. [45] experimentally investigated the ethanol adsorption characteristics of six MOF materials namely CPO-27(Ni), MIL-101(Cr), HKUST-1, MIL-100(Fe), MIL-53(Cr) and MIL-100(Cr) compared to that of silica gel as a conventional adsorbent material that is widely used in commercial adsorption systems. The results revealed that MIL-101(Cr) have shown superior performance with uptake value of 1.2 gw/gads. Also, MIL-101(Cr) proved to be stable through 20 successive cycles at 25°C. The results from theoretical modeling of a two-bed adsorption system with heat and mass recovery have shown that using MIL-101(Cr)/ethanol pair has remarkable

Jeremias et al. [46] showed that the use of alcohols (methanol) as working fluids turned be a good prospect for the application of otherwise promising, but hydrothermally unstable or

N-UiO-66 and H<sup>2</sup>

Adsorption Refrigeration Technologies http://dx.doi.org/10.5772/intechopen.73167 83

O adsorption isotherms due to their enhanced hydrophilic-

N-MIL-125)

42]. The amino-functionalized MOFs UiO-66 and MIL-125 (H<sup>2</sup>

featured also very promising H2

*2.4.2. MOFs-ethanol pair*

*2.4.3. MOFs-methanol pair*

intended heat pump application [43].

potential in low temperature cooling applications.

The performance of a number of MOFs such as HKUST-1 and MIL-100(Fe) was investigated and compared to silica gel RD-2060 by Rezk et al. [35]. They showed that HKUST-1 performed better than silica gel RD-2060 with an increase of water uptake of 93.2%, which could lead to a considerable increase in refrigerant flow rate, cooling capacity and/or reducing the size of the adsorption system. However, MIL-100(Fe) MOF showed reduced water uptake comparable to silica gel RD-2060 for water chilling applications with evaporation at 5°C. These results highlight the potential of using MOF materials to improve the efficiency of water adsorption cooling systems. Other MOFs such as MIL-53(Cr), MIL-53(Fe), Birm-1, Birm-1(K) and Birm-1(Li) showed water uptake of 0.14–0.35 gw/gads which is lower than the water adsorption capacity of HKUST-1, proving that HKUST-1 regarding to the water capacity outperform conventional porous materials such as silica gel and other MOF materials [36], comparing HKUST-1 with other zeolite materials like SAPO-34 and AlPO-18 showed that the best SAPO-34 samples had a water uptake of 0.253 gw/gads which is a factor of 4.9 larger compared to the reference silica gel. Those results were only exceeded by the best AlPO-18 sample with a measured water uptake of 0.254 gw/gads for the low driving temperatures. This equaled an improvement by a factor of 6.2. For driving temperature of 140°C, the highest water uptake was found for the metal-organic framework HKUST-1 [37].Other MOFs such as MIL-100 (Fe and Al) with a water uptake of 0.76 and 0.5 gw/gads were found to be also very interesting candidates for thermally driven, sorption-based chilling or heat pump systems [38, 39]. A 3D MOF material (ISE-1) was found to have water loading of 0.210 gw/gads which was found to be larger than other five zeolites in that study and of the reference silica gel demonstrating the potential of MOF materials for use in adsorption heat pumping processes [40]. MIL-53(Al), MIL-100(Fe) and ZIF-8 were compared with the previous materials and were found to have a water uptake higher than 0.3 gw/gads proving that MOFs are a very promising class of materials for the use in adsorption heat pumping/cooling processes [41, 42]. The amino-functionalized MOFs UiO-66 and MIL-125 (H<sup>2</sup> N-UiO-66 and H<sup>2</sup> N-MIL-125) featured also very promising H2 O adsorption isotherms due to their enhanced hydrophilicity with a water load of ≈0.4 gw/gads and were considered to be especially beneficial for the intended heat pump application [43].
