Controlled Synthesis Of Microporous Organic Frameworks And Their Applications In Gas Separations

Separation is an important industrial step with a critical role in the chemical,petrochemical,pharmaceutical and nuclear industries,as well as in many other fields.A highlight was published on Nature in 2016,stating that seven chemical separations are changing the world;namely,hydrocarbons from crude oil,uranium from seawater,alkenes from alkanes,rare-earth metals from ores,benzene derivatives from each other,trace contaminants from water and greenhouse gases from dilute emissions.Despite much progress in past decade,there are still significant challenges in separation science and technology.For example,many separation processes are energy intensive,such as distillation-based separation process alone accounts for 10-15% of the world's energy consumption.It is therefore desirable to explore alternative methods that can allow separation with better economy and less adverse impact on environment.Herein,in this work,we focus on microporous organic frameworks,centering on target synthesis of metal organic frameworks?MOFs?and porous organic frameworks?POFs?,understanding gas separation mechanism and exploring the applications of materials in adsorption and membrane separations.The main contents are in the following:1.Nanosizing crystals can bring superior properties into materials,particularly for facilitating mass transport in porous materials.Herein,we report the synthesis of CaSDB metal-organic framework?MOF?nanocrystals using additives;two types of additives?ammonia and lauric acid?are adopted to effectively control the crystal size down to 400-700 nm.In individual synthesis systems,ammonia acts as a nucleation promoter and lauric acid serves as a growth inhibiter during crystallization.The CaSDB crystals can be easily engineered from nanoscale?600 nm?to macroscale?35 ?m?by adjusting the ammonia content in the synthesis solutions.The CaSDB nanocrystals prepared using both additives have the same porosity including the same pore size?5.0 ??,similar surface areas(140-157 m² g^-1),and ultramicropore volumes(0.02-0.027 cm³g^-1),probed by N₂ sorption measurements.Xe and Kr adsorptions show that the CaSDB nanomaterials exhibit a good thermodynamic selectivity of 20.5 for Xe over Kr owing to the optimized interactions between Xe molecules?4.1 ??and CaSDB pores?5.0 ??.Breakthrough measurements show that the dynamic selectivity for Xe over Kr is close to the thermodynamic selectivity.More importantly,nanosized CaSDB MOF crystals exhibit a very fast rate for xenon adsorption that is demonstrated by a significantly higher rate constant of nanocrystals(1.8 × 10^-2min^-1)than those of micro-(5.53 × 10^-3min^-1)and macrocrystals(1.65 × 10^-3min^-1).The enhanced adsorption rate is due to the high external surface area of CaSDB nanocrystals.Repetitive tests reveal that the CaSDB nanomaterials are robust and reproducible in terms of constant Xe/Kr selectivity and Xe uptake,holding it great promise for application in adsorption-based xenon separation and capture.2.Most metal organic frameworks?MOFs?based hybrid membranes face the challenge of low gas permeability in CO₂ separation.This study presents a new strategy of interweaving UiO-66 and PIM-1 to build freeways in UiO-66-CN@sPIM-1 membranes for fast CO₂transport.In this strategy,sPIM-1 is rigidified via thermal treatment to make polymer voids permanent,and concurrently polymer chains are mutually linked onto UiO-66-CN crystals to minimize interfacial defects.XRD and N₂ adsorption results show that the pore chemistry of UiO-66-CN is kept intact in hybrid membranes,allowing full utilization of MOF pores and selective adsorption for CO₂.Separation results show that UiO-66-CN@sPIM-1 membranes possess exceptionally high CO₂ permeability?15433.4-22665 Barrer?,approaching to that of UiO-66-NH₂ crystal?65-75% of crystal-derived permeability?.Additionally,the CO₂/N₂ permeation selectivity for a representative membrane?23.9-28.6?moves toward that of single crystal?24.6-29.6?.The product of permeability and selectivity for the UiO-66-CN@sPIM-1 membrane is far beyond most of MOFs based hybrid membranes,ranking this membrane in the top class of membrane materials.The unique structure and superior CO₂/N₂ separation performance make UiO-66-CN@sPIM-1 membranes promising in practical CO₂ separations.3.Efficient membrane CO₂ separation requires fast and selective permeation for CO₂ over other gas species through crack-free mixed matrix membranes.We have been developing porous organic frameworks?POFs?as a new class of nanofillers for fabrication of mixed matrix membranes.A triangle monomer of melamine?MA?reacts with another linear monomer of 1,4-piperazinedicarboxaldehyde?PDA?in a microwave system to form the MAPDA POF material.As-synthesized MAPDA possesses pure organic framework featured with high surface area of 548.3 m² g^-1and a large pore of ¹.0 nm.SEM shows that MAPDA particles are in size of ⁴2 nm,and adsorption measurement reveals that MAPDA is favorable for adsorbing CO₂ with an uptake of 47.0 cm³ g^-1at 298 K and 101 kPa.Single gas permeations demonstrate that CO₂ permeability is increased dramatically from 3694.5 to 7861.9 Barrer,meanwhile the CO₂/N₂ selectivity is enhanced from 18.9 to 23.9 for pure PIM-1 and for a representative membrane of MAPDA/PIM-1 with 15 wt% MAPDA.High porosity and molecular affinity make the predominant contributions to the enhancements of CO₂ permeability and CO₂/N₂ selectivity.MAPDA/PIM-1 membranes are also very selective for capturing CO₂ from gas mixtures of 50CO₂:50N₂and 15CO₂:85N₂ with separation performances exceeding the latest upper bound.The good separation property and the high stability of MAPDA/PIM-1 have shed a light on next-generation membrane for CO₂ separation development.