**2.3 Covalent organic frameworks**

Another exciting avenue for reversible storage of CH4 is the covalent organic frameworks (COFs). The COFs are nanoporous materials formed by the covalent crosslinking of organic functional groups. These are lightweight materials with large pore volume, low density, and presence of hydrocarbon frames that enhances the interaction towards CH4 and improve the uptake capacity. Importantly, these possess high thermal stability and low molecular weight. For example, the COFs based on boronic acid linkage exhibited high thermal stability till 500°C and surface area up to 1590 cm2 /g [53]. The surface area of boronic acid-based COFs increased further by restructuring the pore structure through the incorporation of two nodes, i.e. triangular (ctn) and tetrahedral (bor). The surface area of the COFs was measured to be 4210 cm<sup>2</sup> /g [54]. Earlier studies have proposed to minimize the methane COF interaction and increase the heat of adsorption to enhance the delivery efficiency of methane. These studies have further proposed that substitution on the phenyl ring does not alter the binding ability in the samples [55].

A study based on aromatic imine networks revealed that sub-stoichiometric construction of COFs hexagonal building blocks with four connecting sites, where the two unreacted sites can be used to enhance the selectivity of hydrocarbon adsorption [56]. The COF displayed a high BET surface area up to 3478 m<sup>2</sup> /g and methane adsorption capacity of 11.2 cm3 /g. Similarly, another sub-stoichiometric COFs based on imines displaying bex net topology synthesized using tri- and tetratopic linkers are reported in the literature [57]. Topology control in COF is another important aspect to control the uptake capacity. For example, recently an N,N-dimethyl acetamide and 1,3,5-trimethylbenzene based imine-COF was synthesized that displayed two different types of triangular micropores of different pore dimensions. The diameter of the pores was 11.3 and 15.2 Å respectively [58]. The COFs with tbo topology have been reported in the literature by using planar porphyrin with four coordinating sites and 3 coordinating trigonal aldehydes of a triphenylamine. The COFs are arranged into a *Pm*3 space group and constitute a non-interpenetrating framework [59].

COFs possessing pcu topology have been synthesized using distorted aromatic compounds serving as triangular antiprismatic nodes [60]. Imine-based COFs possessing fjh topology was synthesized by coupling a triangular linker with a square building block [61]. Triptycene-based COFs displaying a non-interpenetrated ceq or acs topology was synthesized using a combination of a triangular prism and

planar triangle nodes [44]. The sample displayed methane adsorption capacity up to 48 cm3 /g at 0°C and 1 bar. The COF exhibited a BET surface area value of 2650 m2 /g. Overall, studies have shown that the organic building block structure and functionality can be utilized as a control to develop COFs with controlled pore size, pore structure, and topology. The functionality in the COF system may be introduced to enhance the selectivity of such systems towards methane adsorption compared to that of the polar gases. The stoichiometry also serves as one of the handles to control the topology of the COFs. Several studies have shown that these materials possessing adequate thermal stability and binding ability towards methane may be a useful option for methane storage applications.
