# Mitigating Global Methane Emissions Using Metal-Organic Framework Adsorbents
## 1 Introduction  
Global emission of methane reached a record high in 2020 [1; 2]. It is expected that this rise in methane concentrations will continue in the coming years despite the economic slowdown stemming from the coronavirus pandemic [3]. In 2020, an active leak of sea-bed methane was discovered in Antarctica for the first time [4]. This is concerning because if warming continues other parts of the seafloor may also begin to leak methane and microbes may not quickly move in to prevent methane from rising to the atmosphere. In addition, in 2020, 300 tons of methane were released in the state of Florida in the United States of America (USA) [5]. Methane release is problematic because methane is a major greenhouse gas and a cause of global warming [6; 7; 8]. Methane is 100 times more potent than carbon dioxide as a greenhouse gas when it first goes into the atmosphere. Therefore, effective strategies to curb methane emissions are needed. To tackle this problem, the environmental protection agency (EPA) in the USA initiated a voluntary methane program for the oil and natural gas industry [9]. In 2019, a consumers energy methane reduction plan was initiated to achieve net zero methane emissions by 2030 from a natural gas delivery system [10]. Recently it was reported that the European Union's greenhouse gas strategy failed to plug the hole associated with methane emissions [11]. What has worked to abate carbon dioxide emission is the use of the use of aqueous amine solutions as a technology to capture carbon dioxide as well as adsorption, which has been viewed as an alternative technology [12]. One adsorbent that was proposed to mitigate carbon dioxide emissions is metal-organic frameworks (MOFs) which were shown to be effective at adsorbing carbon dioxide inthe presence of water [13; 14; 15; 16]. MOFs are compounds comprised of metal nodes coordinated to organic ligands. The metal ions form a cluster known as the secondary building unit (SBU) which can be linked by organic compounds through reticular synthesis to form designed three-dimensional structures that can also be used for methane adsorption (Figure 1) [17; 18]. Reticular synthesis allows for the molecular assembly of MOFs with a structure tailored for methane adsorption [19; 20; 21; 22; 23]. MOFs can be used as an adsorbent for methane abatement emissions technology. MOFs are a promising platform because their porosity can be adjusted and their chemistry can be tailored [24; 25]. For gaseous fuel storage for natural gas vehicles (NGV), some MOFs have exhibited methane storage capabilities of more than 170 cm\({}^{3}\) (STP = standard temperature and pressure equivalent volume of methane per volume of the adsorbent material: T = 273.15 K, P = 101.325 kPa) cm\({}^{-3}\)[19; 26; 27; 28]. This is due in part to the high surface areas of MOFs used for gaseous fuel storage [21; 29; 30]. However, for methane emission mitigation the process conditions are different as well as the material design approach as opposed to gaseous fuel storage for NGVs which necessitates the need for information regarding the adsorption capabilities of MOFs for this technology under different conditions. In this work, the efficacy of metal-organic frameworks (MOFs) for methane capture at 1 bar and 298 K was assessed from experiments and compared to experimental results from other adsorbents such as carbonaceous materials, polymers, and zeolites. The entropy of adsorption can be used to determine the adsorption capacity of these materials as well as how fast they adsorb methane. Few measurements of the entropies of adsorption of methane on MOFs have been reported. Here, the entropy of adsorption of methane is determined in well-defined MOFs with nanoporosity and high surface area to assess the efficacy of MOFs to help curb global methane emissions. MOFs have shown exceptional performance for gas storage of key molecules such as methane and hydrogen which will power tomorrow's energy future [32; 33; 34].