Gasoline is the current fuel of choice for personal transportation, but it generates pollutants by combustion and evaporation, including nitrogen oxides, sulfur oxide, carbon monoxide, and traces of carcinogens chemicals. This has motivated the search for alternative routes toward new energy sources. Natural gas, a natural resource, whose main component is methane (>95% methane, with some ethane, nitrogen, higher hydrocarbons, and carbon dioxide), becomes a good candidate for an alternative fuel because it is inexpensive, and highest H/C ratio among all fossil fuels for reduced carbon dioxide emission. Moreover, the huge reserves of natural gas around the world are comparable to the energy content of the world's petroleum reserves. However, to utilize this methane, inexpensive means of transporting and storing are required. Since methane has a critical temperature of 191 K and critical pressure of 46.6 bar, it cannot be liquefied at room temperature, increasing the cost of its transportation. Thus, for utilization of natural gas, storing is very crucial.
To overcome the disadvantage of methane hard to store, many strategies have been developed and the most common three ways are as follows:
Among these alternatives, CNG requires costly multi-stage compressors and heavy high-pressure tanks for storage, while LNG can only be stored with complicated cryogenic cooling systems. So, it is believed that storing methane via porous materials such as MOFs and COFs is the most promising near-term route because it allows operation at reasonable pressure (1-300 bar) and temperature (7-298 K) and does not require extra energy input for conversion to higher hydrocarbons or methanol.
The interactions between MOFs and methane are moderate, storing methane in MOFs can be realized at room temperature and reasonably high pressure. In 2015, Eddaoudi and coworkers reported a MOF named Alsoc-MOF-1 for methane storage, its gravimetric total methane uptake and working capacity at 298 K are 0.42 g/g (65 bar) and 0.37 g/g (5-65 bar), respectively. Usually, developing MOFs with suitable pore sizes, incorporating functional groups/sites into MOFs can enhance their volumetric methane capacities. And MOFs with higher pore volumes and surface areas are more likely to have higher gravimetric methane capacities.
Fig.1 Methane adsorption isotherms of Al-soc-MOF-1 at different temperatures
COFs are held together by strong covalent bonds between light elements such as B, C, O, H, and Si. They have high surface areas, large pore volumes, and the lowest densities for any known crystalline material (as low as 0.17 g/cm3 ), all of which are prerequisites for high uptake of methane. Usually, 3D COFs are superior to 2D COFs with respect to the methane adsorption. For example, the 3D COF-102 with a pore volume of 1.55 cm3/g and COF-103 with a pore volume of 1.54 cm3/g exhibit remarkable high-pressure methane uptake capacities of 187 mg/g (18.7 wt %) and 175 mg/g (17.5 wt %), respectively, at 35 bar and 298 K, which are the highest among COFs. On the contrary, 2D COF-5 with a pore volume of 1.07 cm3/g shows a methane uptake capacity of 89 mg/g (8.9 wt %) under otherwise identical conditions, which is the highest among 2D COFs.
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