Preparation And Liquid Separation Performance Of UiO-66 Series Membranes

The fzamily of UiO-66 MOFs compounds consists of zirconium tetrachloride?ZrC14?linked with 1,4-benzenedicarboxylate?H₂BDC?or its derivatives.As a typital Zr-based MOFs material,UiO-66 shows advantages over most of other MOFs materials in terms of stability and features for excellent thermal stability?up to 500??,and remains unaltered for a long time towards water,DMF,acetone and other organic solvents.Besides,it also has a strong acid resistance and a certain degree of alkali resistance.These high thermal stability and chemical stability make UiO-66 and its isostructural MOFs promising candidates for membrane preparation.Nevertheless,as far as we are concerned,it is more challenging to fabricate Zr-based UiO-66 series MOFs membranes than Zn-ZIFs membranes.On the one hand,the growth characteristics of these MOFs materials itself make it difficult to grow to a continuous membrane.On the other hand,the strict requirements on the membrane preparation increase the probability of failure in fabricating Zr-based MOFs membranes.Consequently,there have been few reports on continuous UiO-66 series polycrystalline membranes.Here,we focus on the preparation and application of the family of UiO-66 membranes.According to the growth characteristics of UiO-66 and its isostructural MOFs materials,different strategies are designed to fabricate stable UiO-66 series membranes on alumina tubular substrates.And these UiO-66 membranes are attempted to be applied in liquid separation such as pervaporation and nanofiltration based on the membrane polarity and the pore sizes of the MOFs crystals.The main contents are as follows:?1?Continuous and stable UiO-66 membranes were successfully fabricated on macroporous ?-Al₂O₃ tubes by employing the seeded growth method,in which two key factors affecting the growth of the UiO-66 membranes were investigated,and were applied to the separation of methanol/methyl tert-butyl ether?MTBE?mixtures by pervaporation for the first time.It has been found that the seed layer plays an important role in the formation of UiO-66 membrane and that water and acetic acid have great impacts on the membrane growth.Water can promote the intergrowth of UiO-66 crystals and acetic acid plays significant roles in promoting crystallization,leading to the formation of high-performance UiO-66 membranes.Under the optimum synthesis conditions,a dense UiO-66 membrane with a thickness of 6 ?m can be obtained.In the pervaporation process,due to the hydrophilic property of the membrane and molecular sieving effect of UiO-66,UiO-66 membrane can effectively separate methanol from methanol/MTBE mixtures and shows excellent PV performance for separating 5 wt.%methanol/MTBE mixtures with a high flux of 1.2 kg·m-2 ·h-1 and a separation factor of nearly 600.What's more,the UiO-66 membrane exhibits highly stable PV performance due to the excellent chemical stability of UiO-66 structure,showing great potential for practical applications.?2?As the metal oxides contribute to inducing the growth of MOFs membranes,a thin zirconia layer was introduced on the inner side of a-Al₂O₃ tubular substrate by sol-gel technique.A more stable UiO-66-NH₂ membrane could be obtained on the zirconia layer through solvothermal growth,and then the UiO-66-NH₂ membrane was explored for the deep desulfurization of model gasoline via pervaporation.The deliberate modification of the a-Al₂O₃ substrates with ZrO₂ greatly promotes the growth of continuous UiO-66-NH₂ membranes.ZrO₂ layer introduced not only serves as fillers to modify the support and creates a graded layer of finer pore size suitable for the growth of UiO-66-NH₂ membrane,but also provides nucleation and anchoring sites for the deposition and growth of UiO-66-NH₂ membrane by producing a Zr source for the formation of the UiO-66-NH₂ structure,favoring the formation of high quality UiO-66-NH₂ membranes.Therefore,the reproucibility and gas separation performance are improved compared with the UiO-66-NH₂ membraInes from seeded growth.Besides,UiO-66 membrane and mixed-ligand UiO-66 membrane have been also triumphantly prepared employing the same "zirconia-induced" strategy,demonstrating its universality for the synthesis of Zr-based MOFs membranes.UiO-66-NH₂ membranes with amino group have preferential adsorption for thiophene because thiophene has a relatively stronger polarity than n-octane.Furthermore,zirconium can form a chemical bond with the thiophene molecules through a ?-complexation,therefore,UiO66-NH₂ membranes are potential membrane materials for pervaporation desulfurization.When employed in pervaporation desulfurization,the UiO-66-NH₂ membrane shows an optimum desulfurization performance with a permeation flux of 2.2 kg m-2 h-1 and an enrichment factor of 17.9 under 40?for 1312 ppm thiophene/n-octane mixtures,which displays higher enrichment factor than polymer-based membranes in literatures.By comparing the PV results of UiO-66 membrane,mixed-ligand UiO-66 membrane and UiO-66-NH₂ membrane,it has been found that membrane polarity plays a dominant role in the PV process.More importantly,as the zirconia modification layer can act as strong anchoring sites between the membrane and substrate,UiO-66-NH₂ membranes demonstrate excellent stability in model gasoline,thus showing the potential for the application in practical gasoline desulfurization.?3?A dense and thin UiO-66 membrane was successfully prepared on alumina hollow fiber tubes by a continuous flow synthesis method and employed to remove p-nitrophenol from contaminated water.During the synthetic process,the effect of flow rate and synthesis time are investigated,and the result shows that the continued supply of fresh mother solution can accelerate the formation of UiO-66 grains,promoting the formation of UiO-66 membrane.Under the optimum synthesis conditions,a thin UiO-66 membrane with 2 ?m in thickness can be achieved after flow synthesis for 30 h.However,the p-nitrophenol serarating ability of UiO-66 membrane is unsatisfactory due to the membrane defects caused by coordinative defects during the membrane formation.Therefore,it is necessary to modify the UiO-66 membrane employing the bridging ligands.After post-modified treatments,the capacity of p-nitrophenol removal is greatly improved compared with pure UiO-66 membranes with a high rejection?>95%?.What's more,owing to the outstanding stability of UiO-66 framework,the post-modified UiO-66 membranes exhibit outstanding long-term operational stability.