Preparation And Application Of Metal-Organic Framework Ni-LAB Crystal And Membrane

Metal-organic frameworks(MOFs)are a class of novel porous crystalline materials which have been widely used in gas storage,catalysis,sensing,membrane-based separation and so on,owing to their fascinating features including large surface area,adjustable pore size and structure and easy functionalization.MOF membranes have shown excellent potential in gas and liquid separation applications in recent research progress.However,so far,the research on MOF materials for membrane-based separation mainly focus on several series of stable MOFs such as ZIFs,MILs and UiO-66 series due to the high requirement on stability of MOF materials.Therefore,designing or seeking stable MOFs as membrane materials for separation application is an effective way to improve the development of MOF membrane.Ni-LAB[Ni2(L-asp)2bipy],a kind of homochiral MOF,is an ideal membrane material due to its good stability,simple synthesis process,as well as cheap and widely available raw materials.It is assembled by coordination of NiCO3,chiral ligand L-aspartic acid(H2L-asp)and 4,4'-bipyridine(bipy),giving a 1 D channel along b axis about 3.8×4.7 A in size.So far,reports on Ni-LAB crystal and membrane separation application are scarce,which are limited on the separation of enantiomers.However,according to the structural properties of Ni-LAB crystal,besides the property for enantiomers separation,it also shows separating potential for H2/CO2 and CH3OH/MTBE mixtures.Therefore,in this work,we selected Ni-LAB as an objective for research on controllable crystal growth and membrane fabrication on porous ?-Al2O3 tubes from the perspective of crystal growth kinetics,as well as application exploration of Ni-LAB membrane for separation of H2/CO2 and CH3OH/MTBE mixtures.The major research contents are as following:(1)Firstly,growth characteristics of Ni-LAB crystal were investigated in terms of crystal growth kinetics by regulating the crystal growth rate through adjusting the properties of precursors.The usage of soluble metal precursor together with addition of deprotonation agent(DA)improved the supersaturation of synthetic solution,further accelerated the crystal growth rate,promoting the formation of Ni-LAB crystal.Gas and vapor adsorption experiments showed that Ni-LAB exhibits potentials for H2 separation and purification as well as separation of CH3OH/MTBE mixtures.These results provided experimental and theoretical foundation for further fabrication and separation application of Ni-LAB membranes.(2)Based on the research results for Ni-LAB crystal,the simpler "linker salt method" was employed for efficient fabrication of Ni-LAB membrane.The utilization of soluble Ni(NO3)2·6H2O and organic linker salt KHL-asp instead of NiCO3 and the protonated H2L-asp provided a higher supersaturation of synthetic solution which enhanced the growth rate of Ni-LAB crystal,leading to an efficient preparation of Ni-LAB membrane on porous ?-Al2O3 tube.Continuous and compact Ni-LAB membranes were obtained by crystallizing from synthetic solution with a concentration of 0.0067 mol/L at 150? for only 1 h or even at 70? for 4 h with H2 permeances of 4.25×10-6 and 1.51×10-6 mol·m-2·s-1·Pa-1,as well as ideal selectivities of 5.31 and 5.21 for H2/CO2,respectively,which shows a potential of Ni-LAB membrane for separation of H2/CO2.In addition,it was found that the concentration of synthetic solution has large effect on membrane orientation.The membrane synthesized from a low concentration solution showed a preferential(200)orientation,while with a relative high concentration the membrane showed a preferential(101)&(002)orientation.(3)Further employing the DA assisted preparation strategy to the fabrication of Ni-LAB membrane and post-modifying the as-synthesized membrane with polyethylene glycol(PEG)to improve the selectivity for H2/CO2.A continuous and compact Ni-LAB membrane was prepared on porous ?-Al2O3 tube via secondary growth through optimizing the synthetic conditions containing additive amount of deprotonation agent TEA and crystallization temperature,exhibiting a H2 permeance of 3.56×10-6 mol·m-2·s-1·Pa-1 with an ideal separation factor of 7.91 for H2/CO2.PEG was used to post-modify the as-synthesized Ni-LAB membrane as it can not only reduce the size of grain boundary defects but also suppress the diffusion of CO2 molecule,improving the selectivity for H2/CO2.After PEG modification,.the H2 permeances of Ni-LAB membrane decreased to 0.79×10-6 mol·m-2·s-1·Pa-1 while the ideal selectivity for H2/CO2 increased to 15.80,indicating the potential of Ni-LAB membrane for H2 separation and purification.(4)A continuous Ni-LAB membrane was successfully fabricated on porous ?-Al2O3 tube via the secondary growth method using soluble Ni(NO3)2·6H2O as metal precursor,and futher applied for PV separation of CH3OH/MTBE mixtures for the first time.It was found that the Ni-LAB membrane prepared by crystallizing at 150? for 4 h from a synthetic solution with a reactant molar ratio of 1 Ni(NO3)2·6H2O:1 H2L-asp:0.5 bipy,a solvent volume ratio of 10 CH3OH:1 H2O and a concentration of 0.0100 mol/L showed best single gas permeation properties and it was used for PV separation of CH3OH/MTBE mixtures.When operating at 50?,the fluxes of Ni-LAB membrane for CH3OH/MTBE mixtures with a CH3OH content of 15 wt%and 5 wt%were 234.06 g·m-2·h-1 and 187.25 g·m-2·h-1,and the corresponding separation factors were 36.93 and 67.24 respectively,expanding the application range of Ni-LAB membrane.These results further indicated the potential of Ni-LAB membrane for liquid separation.