**2.2 Metal organic frameworks**

In this regard, the MOFs have gained strong interest in their ability to reversibly store natural gas for vehicular application. MOFs are a wide family of reticular and highly porous coordination polymers formed by the coordination self-assembly of the metal center with multidentate organic building blocks [32, 33]. The MOFs are reported to exhibit ultra-high surface area up to 10,000 m<sup>2</sup> /g, and tunable pore sizes *Adsorbed Natural Gas Storage for Vehicular Applications DOI: http://dx.doi.org/10.5772/intechopen.101216*

#### **Figure 2.**

*(a) The crystal structure of Zn and Co-based bimetallic MOF, (b) the photographs of solutions possessing different molar ratios of Co2+ to Zn2+, (c and d) the TEM and (e) SEM images of the MOF and produced nanoporous carbon., Reproduced with permission from 30. Copyright 2016 Nature Publishing Group. The elemental mapping images show the presence of Co and Zn in the parent system.*

#### **Figure 3.**

*Structure of some of the rod MOFs synthesized in recent literature. Reproduced with permission from [31], Copyright 2021, American Chemical Society.*

with the possibility to functionalize them [34]. A number of MOF materials such as MIL-74, CAU-8, Zn6(H2O)3(BTP)4, Zn-MOF-74, 3W-ROD-1, and Ni-MOF-74 are studied towards their ability to store natural gas (**Figure 3**) [31]. Similarly, Pore size optimization is one of the important targets in the field of MOF materials. The effective way to address the same is by gradually changing the length of the organic linker and introducing functionality into the linker. For example, the introduction of sulfone and carbonyl groups notably improved the CO2 absorption capacity compared to that of the un-functionalized one (**Figure 4**) [35].

For an adsorbent to exhibit adequate adsorption ability, the system should maintain a balance between gravimetric (SAG) and volumetric surface area (SAV)

**Figure 4.**

*Effect of ligand size on the pore structure of the resulting MOFs. Reproduced with permission from [35], Copyright 2014, American Chemical Society.*

along with high porosity. Often the normalized value of the product of SAG and SAV is utilized to suggest the above. It is known that the helium void fraction displays a volcano relationship with the largest pore diameter. The NU-1500; Al and PETbased MOF systems exhibit void fraction and largest pore diameter values of 0.76 and 12.7 Å respectively and display high CH4 storage capacity. A similar system with Fe at the coordinating center, the NU-1501 series displayed increased VF values of 0.87 and an LPD value of 18.8 Å [36]. The MOF series based on extended PET system and Al metal displayed SAG and SAV of 7310 m2 /g and 2060 m2 /cm3 respectively. The corresponding Fe-based system showed a somewhat lower SAG value (7140 m<sup>2</sup> /g) and comparable SAV value (2130 m<sup>2</sup> /cm3 ). The study showed that the SAG and SAV values can be controlled further to significantly improve the gravimetric uptake capacity while retaining the volumetric storage capacity.

The gas adsorption capacity and separation performance of the MOFs may be optimized further by suitably modifying the organic building blocks with functional groups using simple organic transformation. The presence of polar functional groups in the mainframe of MOFs effectively improves the CO2 capture and separation abilities. For example, MOF is based on flexible hexadentate ligands containing amide groups, N-tris-isophthalic acid-1,3,5-benzenetricarboxamide (TPBTM) such as [Cu24(TPBTM6−)8(H2O)24](Cu-TPBTM) with high surface area [37]. The pore structure of the resulting MOFs depended on the size, structure, and functionality of the organic building blocks as shown in **Figure 5** for representative purposes.

These class of materials are reported to exhibit adsorption capacity up to 180 V/V. For example, the HKUST-1 based on Cu metal and 1,3,5-benzenetricarboxylate organic linker exhibits surface area up to 1800 m2 /g with adequate stability [37]. Similarly, a MOF system based on Cu and a hexadentate linker 3,3′,3″,5,5′,5″-benzene-1,3,5-triyl-hexabenzoic acid exhibited BET surface area up to 6240 m2 /g [39]. The open metal site in these MOFs serves as the site to bind to the methane molecule. At 100 bar pressure and 25°C, these MOFs display

*Adsorbed Natural Gas Storage for Vehicular Applications DOI: http://dx.doi.org/10.5772/intechopen.101216*

**Figure 5.**

*The Cu-TPNTM MOFs and their pore structure is depicted above. Reproduced with permission from [38]. Copyright 2014, Royal Society of Chemistry.*

storage capacity up to 330 V/V. The issue with this MOF system towards practical applications is that higher hydrocarbons, such as ethane and propane adsorb more strongly compared to that of the methane and tend to block the pore site [40]. Seki reported a MOF system, based on copper and triethylenediamine (TED), [Cu-(O2CRCO2)·1/2TED}n] [R = 4,4′-C6H4C6H4 (1)] which recorded a volumetric storage capacity for methane up to 225 V/V [41]. For the synthesis of MOFs for said application, key factors such as the number of coordinating sites, size, shape, and geometric configuration of organic linkers require careful attention, which determines the self-assembled structure and gas adsorption performance of MOFs. Typically, the coordinating sites of the organic building blocks consist of electrondonating elements such as O, S, and N. Among the corresponding coordinating functionalities, the carboxylic acid groups tend to form stable MOFs. A set of crystalline MOFs based on 4,4′,4″-[benzene-1,3,5-triyl-tris(ethyne-2,1-diyl)] tribenzoate organic linker and Zn4O(CO2)6 was synthesized that exhibited noninterpenetrating 3D crystal structures with pores of ~48 Å sizes [42]. The MOF displayed an extremely high BET surface area of 6240 m2 /g. A table summarizing the adsorption capacity under given temperature and pressure conditions for various MOFs is included for reference (**Table 1**) [43].

Considering the gas density in the pores of the adsorbent to be considerably higher compared to that of the bulk density, the optimization of void space becomes important to maximize the adsorption capacity. Therefore, monoliths of MOFs are prepared using compression, binder, and polymeric additives [45]. However, the use of additives though known to improve the mechanical strength has their drawback as these tend to block the pore space and decrease the uptake capacity [46]. Therefore, preparation of pure monolith of MOF using compression has been pursued in literature. Even mechanical compression notably decreased the uptake capacity possibly due to the collapse of the pore structure. For example, the uptake capacity of Ni2(1,4-dioxido-2,5-benzenedicarboxylate) based MOF decreased from 230 to 100 V/V at 34 bar and 30°C after palletization [47]. Another way to handle the issue is to use a sol-gel technology in which gradual evaporation of solvent from the monoliths without destroying the pore structure. Recently, HKUST-1 monolith prepared using sol-gel technique exhibited uptake capacity of 259 V/V at 65 bar and 25°C [48].

The methane binding sites were studied in the literature on Zn-based MOF samples using low-temperature synchrotron analysis. The data revealed two primary adsorption sites and one secondary adsorption site. The primary adsorption sites

#### *Natural Gas - New Perspectives and Future Developments*


#### **Table 1.**

*The methane gas adsorption capacity of various MOFs under different pressures conditions [38, 43, 44].*

were located near the Zn complex paddlewheel and the center of the small windows was recognized as the second primary adsorption site. At these sites, the methane interacted with the phenyl units and paddlewheel units. The secondary adsorption site was recognized as the center of the cavity [49]. However, still, the interaction of functional groups with methane is not very clear. More studies with support from spectroscopic tools may be necessary for future to completely ascertain the mechanism of methane storage. Using flexible MOFs is attractive to improve desorption capacity and minimize the loss during desorption. Considering the desorption pressure of the working engine is fixed at 4.8 bar by DoE, the reported flexible

## *Adsorbed Natural Gas Storage for Vehicular Applications DOI: http://dx.doi.org/10.5772/intechopen.101216*

MOFs are known to absorb ANG at between 35 and 65 bar and release most of the gas at ~5 bar pressure [50, 51]. The type of metal center in the MOF also controls the uptake capacity and thermal management of the system. For example, Co(bdp) and Fe(bdp) based MOFs through exhibit comparable methane uptake capacity, Fe-based system has desorption steps at higher pressures of 10 bar compared to that of the Co-based system [52]. Similarly, in terms of intrinsic thermal management, the amount of heat released by Fe(bdp) based system (64.3 kJ/L) is 12% compared to that of the Co(bdp) (73.4 kJ/L) based system. Overall, the challenge with these systems lies with the fact that the 3D structure to be produced in such a way that the aromatic rings are exposed for the CH4 interaction. However, expansion of the organic linkers leads to fragile materials and allows for the self-interpenetration of lattices. Furthermore, the thermal stability of the coordination linkage, low heat of adsorption, and high packing density are also important from the perspective of commercial viability. Keeping in view of the above, suitable MOFs with high surface area and porosity may be designed and synthesized for the ANG storage application.
