# Mitigating Global Methane Emissions Using Metal-Organic Framework Adsorbents
## 4 Discussion  
### Enhancing Volumetric Energy Density and Cyclability in MOFs for Global Methane Emissions Mitigation  
In the case of global methane emissions, entropy is important to determine how much can adsorbed and how fast it can be adsorbed in the MOF. Van der Waals interaction forces between methane molecules and atoms of MOFs enable the physisorption of methane to yield a density greater than that of the gaseous methane at the same conditions. There is an enormous amount of research going on to improve the methane adsorption in MOFs by altering the frameworks which tunes the interaction between atoms and methane to achieve higher stored energy densities. The results combined together of the entropy of adsorption of methane as well as the pre-exponential for desorption of methane provide a guideline with which to design MOFs for methane emissions abatement. One key component from both results is that the pore volume and pore structure of the MOF is key in determining the driving force for adsorption and how fast the desorption can occur. The larger the occupiable volume, the less the rotational degrees of freedom are decreased. This leads to a decrease in the magnitude of the entropy change of adsorption. With a decreased magnitude, the maximum equilibrium adsorption uptake of the MOF decreases. The more rotational degrees of freedom that are lost, the larger the pre-exponential factor for desorption. This is associated with an increased rate of desorption. The most effective MOF for enhanced methane capture at 1 bar and 298 K possessed a nano-trap. It was designed by arranging coordinatively unsaturated metal centers oppositely adjacent to one another. This was coupled with the inclusion of a high density of alkyl groups. This increased the energetic favorability to have methane in the pore. The results in Tables 1 and 2 demonstrate that MOFs exhibited the best methane uptakes at 1 bar and 298 K as compared to other materials investigated such as carbonaceous materials and zeolites and are effective adsorbents. ATC-Cu, Cu-**tbo**-MOF-5, and HKUST-1 were found to be most effective for enhanced methane capture at 298 K and 1 bar. The found differences between the MOFs can be related to the arrangement of coordinatively unsaturated metal centers and functional groups. In addition, a high heat of adsorption is also favorable for this application under these conditions. Interestingly, the best performing MOF for enhanced methane capture at 298 K and 1 bar contain copper. MOFs which have an inadequate overlap of the potential fields from opposite pore walls are not as effective at enhanced methane capture [35]. Ultra-high surface area is not as critical a property for enhanced methane capture as it is for MOFs designed for methane storage at 35-100 bar. MOFs that possess a nano-trap with high Qst, coordinatively unsaturated metal centers with a high density of functional groups are most promising for enhanced methane capture under these conditions. It is also important to note that MOFs designed for methane storage for NGV do not always perform as well for methane emission mitigation. This is in part due to the fact that they have been designed to have low uptakes at pressures lower than 5 bar [60]. Therefore, design approach has to be altered for this application. Polarizability of methane, and functionality of MOF, and pore size need to be determined. There are a wide range of structures that can be made and the results here give an idea of the features for global methane emissions abatement. The results shown here assist in the design of MOFs for this application. From the pore structure the entropy change of adsorption can be calculated and related to Qst and desired pore structure of the MOF. From there, reticular chemistry can be used to design the MOF and test it at these conditions with some confidence and in a methodical manner.