Evaluation Of Methane Storage Performance In Wet MOFs/Carbon-based Composite Nanoporous Materials

Natural gas,as a low-carbon energy source,will play an important role in future energy transformation and upgrading.Improving the storage density of natural gas is an important measure to expand its application.Nanoporous materials have rich porous structures,huge specific surface area,and pore volume,which are conducive to methane storage.However,their low methane volume adsorption capacity and the contradiction between weight adsorption capacity limit their application in different scenarios.Based on this,two composite nanoporous materials,ZIF-8@AC and MIL-101(Cr)@AC,were prepared by material composite method and combined with adsorption-hydration wet gas storage technology to further enhance methane storage density in this paper.The details of the research are as follows:Firstly,two nanoporous materials,ZIF-8 and MIL-101(Cr),were prepared by ambient stirring method and high temperature hydrothermal method,and the wet methane storage performance of ZIF-8,MIL-101(Cr)and AC was investigated by low temperature isothermal adsorption experiments,and the effects of water content,water absorption mode and pressurization step were evaluated.The results show that when the water content is low,the nanoporous materials are more effective for methane wet storage,and the methane wet storage capacity of AC,ZIF-8 and MIL-101(Cr)is increased by 9.69%,18.03%and 32.76%at R_w=0.1,respectively,compared with dry storage.In addition,the storage in the adsorption section of the low water content sample depends on the humidification method and the storage in the adsorption section of the higher water content sample depends on the drip method,and the humidification method distributes water deeper in the pore,resulting in a higher driving force required for methane hydrate generation.As the boosting step increases,the AC becomes larger for methane wet storage,and the optimal boosting step is 1.4 MPa,which increases the methane storage by 30.12%compared to 0.7 MPa boosting step,while the optimal boosting step of ZIF-8 is 1.1 MPa.Secondly,ZIF-8@AC composite nanoporous material was prepared,which combined with adsorption-hydration method enhanced the weight and volume adsorption performance of methane.A large number of additional mesopores and macropores were generated in the ZIF-8@AC-100 mg composite,leading to an increase of 6.78%and 30.34%in the weight adsorption of methane compared with the parent material AC and ZIF-8,respectively.In addition,the formation of hydrates increased the storage density of methane in mesopores,macropores and intergranular pores,producing a larger volume adsorption of methane,which increased by 12.65%and 53.24%compared to the parent material AC and ZIF-8,respectively.The hydration behavior in the confined space was investigated using low temperature in situ differential scanning calorimetry.At low water content(R_w=0.1),the effect of preadsorbed water on the physical adsorption of methane was small,and the enhanced mass transfer ensured the formation of hydrate in the mesopores,and the accumulation of hydrate in the mesopores discrete the pore space into some micropores,further enhancing the adsorption of methane.At higher water content(R_w=0.3),the presorbed water completely blocked the micropores and hydrate was formed preferentially in the macropores and intergranular pores,which acted as nano-reactors to promote hydrate formation,the formation of hydrate in these pore spaces clogs the internal pores,resulting in a large amount of pre-adsorbed water in the internal pores that cannot be converted to hydrat,which in turn reduced the intrapore hydration conversion.Finally,MIL-101(Cr)@AC composite nanoporous material was prepared by high temperature hydrothermal reaction.The composite produced a large number of additional micropores and mesopores,which increased the specific surface area and pore capacity and greatly improved the adsorption performance and hydration space.The results of methane low-temperature isothermal adsorption experiments showed that the pre-wetted MIL-101(Cr)@AC could reach 0.2580 g/g and 0.2522 g/g for methane storage when the carbon doping amount was 50 mg and 100 mg,which were 33%and18%higher than AC and MIL-101(Cr),respectively,while the 500 mg carbon doping amount was smaller for methane storage.In contrast,the volumetric storage capacity of the three types of carbon doped nanoporous materials for methane was 146.8 V/V,158.9V/V and 179.3 V/V,respectively,which were 16.88%,24.33%and 38.03%higher than the corresponding dry materials.In addition,the hydrate generation under MIL-101(Cr)@AC-100 mg solid loading was explored using low-temperature in situ Raman generation technique,and the hydrate growth tended to be complete with increasing experimental equilibrium pressure,and the ratio of large cage to small cage area is close to the theoretical value of 3:1.