Study On The Adsorptive Separation Of Light Hydrocarbons On Ultra-Microporous Metal-Organic Frameworks

As one of the most important processes in the petrochemical industry,the separation and purification of the light hydrocarbon mixtures are almost realized by the method of distillation.However,it has the disadvantages including high energy cost and huge instrument investment.In this paper,taking consideration of the eventful separation requirements,we propose new adsorptive separation methods applied in the purification of ethylene or 1,3-butadiene,capture of carbon dioxide and quality upgrade of natural gas with novel gallate-based metal-organic frameworks(M-gallate,M= Ni,Mg,Co,Mn)and ionic liquids supported metal-organic frameworks(ILs@MOFs).The adsorption performance and mechanism are deeply investigated to evaluate their potential in practical industrial application.We hope our research could have some contribution to the innovation of the conventional separation technology for light hydrocarbons.we for the first time evaluated the adsorptive separation performance of gallate-based MOFs for C2H2/C2H4/C2H6 mixtures,systematically studied the influence of fine-tuning of the pore size on gas uptakes and adsorption selectivity.Moreover,we also analyzed the adsorption mechanism of C2H2 and C2H4 molecules on the frameworks at molecular level.The results indicated that the family of M-gallate had excellent C2H4/C2H6 and C2H2/C2H4 separation performance.Especially,Co-gallate kept the highest C2H4 uptake and C2H4/C2H6 selectivity.Its unprecedented adsorption selectivity for the C2H4/C2H6 mixture exceeds all reported MOFs materials.Moreover,Ni-gallate with the smallest pore size possessed the highest C2H2/C2H4 selectivity among the family of M-gallate.We confirmed the recognition mechanism and binding sites of C2H2/C2H4 in M-gallate framework through the neutron diffraction experiments.It revealed that the gas molecules are tightly bound within the cavities of branch channels by synergistic effect of supramolecular and hydrogen-bond interactions.According to the transient breakthrough simulation,Co-gallate set the benchmark of the productivities of polymer-grade C2H4 and C2H6.And the productivity of polymer-grade C2H4 in Ni-gallate for C2H2/C2H4 mixtures surpassed all MOF-based adsorbents except UTSA-200.The dynamic fixed-bed breakthrough tests were employed to verify the state-of-the-art separation performance of M-gallate for binary and ternary mixtures.The breakthrough flow diagram based on a single Co-gallate fixed bed was designed to simulate the practical industrial process for the C2H4/C2H6 mixture.Then,the feasibility was confirmed by obtaining polymer-grade C2H4(>99.95%).Finally,the stability in humid atmosphere,process conditions in scale-up synthesis,and feasibility of molding to millimeter size were also verified.The adsorption performance of C4 components on gallate-based MOFs was also studied,and they all showed excellent separation performance.Ni-gallate exhibited the best separation performance with the separation factors of 8863 and 1382 for C4H6/iso-C4H8 and n-C4H8/iso-C4H8 respectively.Furthermore,its separation factor for trans-C4H8/cis-C4H8 reached up to 118 as well.The results revealed M-gallate not only had satisfactory C4H6 uptake,but also enabled the efficient purification of C4H6 from the C4 mixtures.The multiple C?-—H?+O-supramolecular interactions between C4H6 or trans-C4H8 and the framework was confirmed by DFT calculations.The experimental breakthrough tests also indicated the efficient separation of C4H6 from C4 mixtures.The CO2/N2 separation performance was investigated through static adsorption experiments.The family of M-gallate(M= Mg,Co,Ni)all showed S-shaped CO2 adsorption isotherms.It has extraordinary advantages in a temperature swing adsorption process in comparison with conventional adsorbents with Langmuir-type isotherms.The huge energy saving and high productivity are simultaneously achieved because M-gallate can reach an extremely high working capacity at relatively low desorption temperature.The IAST adsorption selectivity of CO2/N2 exceeded ten thousand even hundred thousand,better than all reported MOFs materials.Meanwhile Mg-gallate maintained remarkable CO2 uptakes at high temperature of 40 ? and 70?,indicating it suitable for carbon capture from low-or high-temperature flue gas.Moreover,M-gallate exhibited excellent separation performance for the CO2/CH4 mixture.The adsorption selectivity of Ni-gallate was up to 3171,comparable with the benchmark material Qc-5-Cu-sql-p.As a result,it also has broad prospect in the quality upgrade of the natural gas.The great separation ability was further confirmed via breakthrough tests in M-gallate.Based on the high gas uptakes in MOFs with open metal sites and high selectivity of ionic liquids,ionic liquid supported metal-organic framework[Bmim][OAc]@MIL-101 was synthesized by the diffusion method and applied in the separation of C2H2 and C2H4.Compared with original MIL-101,[Bmim][OAc]@MIL-101 exhibited much higher C2H2/C2H4 selectivity and better separation performance in fixed-bed breakthrough tests.Besides,the isosteric heat of adsorption for C2H2 and C2H4 is much lower in composites than in MIL-101,illustrating the lower energy consumption in the regeneration process.The stability and renewability of[Bmim][OAc]@MIL-101 were verified by cyclic breakthrough tests as well.