Mofs Materials For Adsorption Separation Of C1-C3 And Its Water Vapor Stability Enhancement By Fluorine Modification

The separation of light hydrocarbons is an important chemical separation process in the petroleum industry,which mainly relies on cryogenic distillation technology.To date,huge energy consumption is the biggest problems facing the industry.Finding energy-efficient alternative methods for separation at ambient condition is the development direction in future.Metal organic framework materials?MOFs?,which is the key of the adsorption and separation technology,has been widely concerned in recent years.MOFs materials have shown great potential application prospects in the field of light hydrocarbon gases adsorption separation,due to their high specific surface areas,high porosity,diversified structure,adjustable pore sizes and chemical functions.At present,there is a common problem:some divalent MOFs which show excellent properties for light hydrocarbons separation display a generally poor chemical and steam stability,and their pores will happen to collapse once encounter water vaper.Therefore,highly chemical-stability high-valence Zr?IV?and Fe?III?-based MOFs were synthesized in this paper,their properties of C1-C3 adsorption separation and chemical/water vapor stabilities were studied.The unsaturated porphyrin Zr-based PCN-224 is fluorine-modified to enhance its water vapor stability.The research contents and results are summarized as follows:1)We prepared a highly crystalline PCN-224 material,and its chemical/steam stabilities and adsorption separation performance of C1-C3 light hydrocarbons were investigated.The results show that PCN-224 has excellent stability after soaking in water for 48 h,because the highly-charged Zr cation and porphyrin anion belong to hard lewis acid and hard base,respectively,which forms strong coordination bond through combination of strong electrostatic interaction,resulting a high resistance to be hydrolyzed in an aqueous environment;at 298 K and 100 kPa,the saturated adsorption capacities of CH₄,C₂H₆ and C₃H₈ are 0.48,2.93 and 8.25 mmol·g^-1,and the IAST selectivities for C2/C1 and C3/C1 are12 and 609,respectively.The PCN-224 porous material can achieve high-efficiency adsorption separation of C1/C2/C3 under ambient conditions.The separation performance of C3/C1 is at the international advanced level compared with the reported data.2)We have prepared a Fe-based MIL-142A and systematically investigate its chemical/steam stabilities and adsorption separation performance of C1-C3.Resulting that MIL-142A has ultra-high chemical and water vapor stabilities,which can maintain structure stable after immersing in aqueous solution for 7 days and exposing at 80%high humidity for15 days;MIL-142A also shows good performance of adsorption separation C1/C2/C3.The separation of these three gases can be achieved at room temperature and pressure by MIL-142A and its IAST selectivity of C3/C1 exceeds 1000.3)We have synthesized Fe-based MIL-143 porous material,and its adsorption separation performance of ethylene/ethane and stabilities were studied.The results show that MIL-143has the property of preferentially adsorbing ethane.At 298 K and 100 kPa,the adsorption capacities of C₂H₆ and C₂H₄ are 2.6 and 2.2 mmol·g^-1,respectively,and the corresponding selectivity of C₂H₆/C₂H₄ achieves 1.5;in addition,MIL-143 displays excellent water stability.4)PCN-224-F_n porous material was synthesized by fluorine post-modification method,and its water vapor stability was systematically studied.As results,perfluoroalkyl acids are attached to PCN-224 through esterification reaction,which significantly improves the water vapor stability of PCN-224 material.After being exposed in 55%humidity air for 20 days,the BET specific surface area of PCN-224 is only 9%of the original,PCN-224-F₅ still maintains the original 90%,PCN-224-F₇ has almost no BET decline.After the water vapor adsorption under 0-90%moisture humidity,the BET of PCN-224 is basically zero,PCN-224-F₅ remains the original 31%,PCN-224-F₇ can still maintain up the original 85%,indicating that the material's humidity resistance range is gradually increasing.Measuring ethylene/ethane adsorption after exposure in 55%moisture humidity for 20 days,PCN-224 is reduced by 61%,and PCN-224-F₅ is reduced by 13%,while PCN-224-F₇ has no capacity loss.With fluorine precursor varying from TFA to HFBA,the water contact angle of PCN-224 and PCN-224-F_n increase from 60°to 76°,indicating that fluorine modification can improve the hydrophobic surface of PCN-224 and significantly enhance its water vapor stability.