Research On The Properties And Extensions Of Flexible Metal-organic Frameworks Based On CH₄/N₂Separation

In human’s history, coal has created industrial civilization, oil has createdthe modern civilization, and the gas energy will boost the Low-carboncivilization in future. Natural gas is an important part of gas energy, and there isa serious supply shortage of natural gas in China, which requirement reaches90billion cubic meters.Coal bed methane (CBM) is a significant component of unconventional gas,which has attracted extensive attention as a complement of natural gas. CBMcan be divided into two classes: surface mine methane (SMM), which can beused directly with the concentration of methane is higher than90%, andunderground mining methane (UMM) with a concentration of methane lowerthan30%(low-concentration CBM). Because UMM cannot be used effectivelytill now, it has caused a serious waste of resources and environment pollution.Therefore it is a key technology for the efficient utilization of low-concentrationCBM to separate CH4and N2. However it is difficult for the traditionaladsorbent with low-adsorption selectivity to separate methane and nitrogen. Inthis work, the CH4/N2separation properties of the flexible MOFs materials used for low-concentration CBM were studied deeply based on their high adsorptionselectivity for CH4and N2. The main contents and conclusions are as follow:1. Flexible MOFs are good candidates for gas separation based on its highCH4/N2adsorption selectivity, and it should be noted that such expectationshave commonly been based solely on inspection of their pure-componentadsorption isotherms. For a further study, the two flexible MOFs[Cu(dhbc)2(4,4’-bipy)] and [Cu(4,4’-bipy)2(BF4)2] were synthesized and the CH4and N2adsorption properties were also studied. It is found that the volume of theflexible MOFs was expanded obviously after gas adsorption. Based on which,methane was separated from the binary mixtures (CH4/N2) by adjusting pressureand temperature under the non-confinement gas separation apparatus.Furthermore, MOFs structural stability and the water effect were investigated.The results show that the flexible MOFs have potential in industrial separationprocess.2. Lower the gate-opening pressure of flexible MOFs is the key point tosolve its industrial application. In this part, the two flexible MOFs such asCu(dhbc)2(4,4’-bipy) and Cu(dhbpc)2(4,4’-bipy) with the same layer asCu(4,4’-bipy)2were synthesized, the gate-opening behaviors of flexible MOFsby introducing C2-C3hydrocarbons as adsorbate were also investigated. Theresults show that C2-C3hydrocarbons were well adsorbed on the flexible MOFsand they exhibit a step adsorption behavior with gate-opening pressure lowerthan0.1MPa. The gate-opening pressure of flexible MOFs for C2-C3 hydrocarbons adsorption was correlated inversely with their boiling point. Andthe GCMC simulation results indicate that condensation is the main reason ofstep adsorption of C2-C3hydrocarbons on flexible MOFs. The separations ofseveral binary C2-C3hydrocarbon mixtures were studied by using self-madeseparation apparatus based on flexible MOFs. The results show that C2-C3hydrocarbon mixtures can be separated effectively by flexible MOFs under lowgate-opening pressure.3. The two2D interpenetrated MOFs Cu(1,3-BDC)(PY)2was synthesizedby introducing pyridine in [Cu(1,3-BDC)(H2O)]2H2O structure and adsorptionproperties of CO2, CH4and N2were further studied. The results show thatCu(1,3-BDC)(PY)2has higher CH4adsorption capacities than[Cu(1,3-BDC)(H2O)]2H2O, which because of the introduced pyridine ligandsand suitable CH4adsorption sites. The CO2and CH4adsorption sites of the twosamples were calculated by using DFT theory. It is found thatCu(1,3-BDC)(PY)2has high CH4adsorption ability by introducing the pyridineligands and the CH4adsorption enthalpies increase from20.67to39.38kJ/mol.The results reveal that the adsorption capacity of CH4and CH4/N2adsorptionselectivity can be improved by introducing organic ligands between the MOFlayers.4. For a further structure modulation, two1D flexible MOFs[Cu(2,2’-bipy)(adipate)]nand [Cu(2,2’-bipy)(ipa)]n, were synthesized and theCO2and CH4adsorption properties were also studied.[Cu(2,2’-bipy)(adipate)]n showed no adsorption at low pressure, but the adsorption amount fast increase at1MPa and the adsorption isotherm show a steep rise at298K. The CO2adsorption isotherm of [Cu(2,2’-bipy)(ipa)]nshowed a Langmuir I type in therange of0–0.6MPa, and the isotherm showed a similar adsorption stepphenomenon when the pressure is up to0.6MPa, which was confirmed by thedensity functional theory (DFT). In addition, the CH4adsorptions on the twosamples were measured. The adsorption volume ratios of CO2-CH4on twoMOFs are22and10respectively at298K. It is expected that1D coordinationpolymers have potential applications in CO2/CH4separation.