Design,Fabrication And Application Of Metal-Organic Framework Nanofibrous Membranes Based On Electrospinning

Metal organic frameworks?MOFs?represent a class of porous materials which are formed by strong bonds between metal ions and organic linkers.MOFs have diverse architecture and high surface area.Compared with traditional porous materials such as zeolites and carbon materials,MOFs are unique in terms of their extraordinarily high porosities,tunable pores,and diverse functional sites.These advantages make MOFs widely investigated as functional materials in gas adsorption,separation,catalysis and sensing detection.However,most of the reports focused on the powdered MOFs.In some applications,powdered MOFs crystals will cause some problems.For example,pipes may be blocked in the process of flue gas adsorption and separation,particle aggregation may lead to the decline of catalytic activity,and more importantly,powdered MOFs are not easy to be separated,recovered and reused.From the standpoint of separation and recovery of MOFs materials from reaction system,MOFs films are more predominant compared with powdered MOFs because no separation process is needed for the films.In view of this,more and more attention has been paid to the design and preparation of MOFs films with certain functions.Electrospinning is a convenient and effective technique to fabricate functional nanofibrous membranes?NFMs?with high porosity,large specific surface area,tunable structure and ease of surface modification Compared to thin films,electrospun NFMs have greater surface area approximately 1-2 orders of magnitude.Recently,more attention has been paid to electrospinning because this versatile technique is able to convert MOFs particles into self-supported and flexible MOFs NFMs.The preparation of electrospun MOFs NFMs and their applications,such as air filter,gases capture,heavy/radioactive metal adsorption and sensing detection have been reported.Most of MOFs NFMs were fabricated via co-electrospinning of MOFs crystals and polymers.In practical application,the active sites of the composite NFMs may be significantly reduced because MOFs particles were encapsulated by polymers.It has been reported that MOFs crystals were grown directly on electrospun polyacrylonitrile NFM using microwave irradiation technology,but owing to the weak interaction of MOFs particles and polymer matrix,the MOFs crystals were unstable and easily fall off the membrane.Therefore,it is of great significance to design and prepare stable MOFs NFMs,and study its application in separation,sensing and catalysis.In this paper,three kinds of MOFs NFMs with different functions were prepared by means of electrospinning.The as-prepared MOFs NFMs were characterized by SEM,TEM,IR,PXRD and XPS.In addition the properties of separation,sensing and photocatalysis of the MOFs NFMs have been investigated.The main results of this thesis are as follows:1.A nano fibrous filter was successfully fabricated via electrospinning by selecting polyacrylonitrile?PAN?as template and encapsulating anionic bio-MOF-1 into the nanofibrous matrix.The as-prepared bio-MOF/PAN filter can be used for adsorption and selective separation of cationic dye from aqueous solution.It was reported that bio-MOF-1 can maintain its crystallinity for several weeks in water.PAN has been employed as matrix due to its robust mechanical strength,excellent water insolubility,low cost and good hydrophilicity.More importantly,because C?N in PAN was a nucleophilic group,which was capable of adsorbing cation,there was a synergistic effect between PAN and bio-MOF-1 in the adsorption process.Due to the free NH₂?CH₃?_<sup>+cations in the channel of bio-MOF-1 and nucleophilic growp C?N in PAN,both bio-MOF-1 crystals and PAN nanofibers were only capable of adsorbing cationic dyes and could not adsorb neutral and anionic dyes.Benefiting from the integrated merits of electrospun NFM,PAN and bio-MOF-1 crystals,the bio-MOF/PAN filter not only presented rapid adsorption rate towards methylene blue?MB⁺?,but also realized the fast selective separation of MB⁺from mixed dyes aqueous solution.More importantly,the bio-MOF/PAN filter can be effectively regenerated through ion-exchange process.The adsorption capacities of the filter towards MB⁺ kept relatively stable values within the five cycles of filtration-regeneration,indicating that bio-MOF/PAN filter can meet the requirement of long service life.This work may provide a new insight into the development of electrospun MOFs nanofibrous membrane as a filter for rapid separation and purification of organic dyes from practical wastewater.2.A stable fluorescent NFM was fabricated by in situ growing Eu-MOF crystals onto electrospun polyacrylonitrile?PAN?nano fibers modificated with?-aminobutyric acid?GABA?.The NFM has been used as a convenient fluorescent test paper for rapid and cyclic detection of nitrobenzene?NB?.The abundant active nitrile groups of PAN can be used for surface modification.GABA performed well as a grafting agent,because it not only can introduce a great number of coordination sites?carboxyl groups?to the nanofibrous matrix for anchoring Eu-MOF particles,but its relatively long carbon chains can reduce the steric hindrance of both grafting reaction and crystals growing process.Because a great number of carboxyl groups were introduced to the nanofibrous matrix for anchoring Eu-MOF particles,the as-prepared PAN-GABA@Eu-MOF NFM was stable in the fluorescence experiments and thus can be reused multiple times with constant quenching efficiency.Benefiting from the merits of electrospun NFM,the fluorescent test paper needed no separation process before and after recovering,hence,the "off-on" fluorescence switching process can be conducted more conveniently and rapidly compared with powdered sensing materials.This work may provide a new insight into the development of rapid,stable and reusable test paper based on electrospun nano fibers and MOFs crystals for effective sensing detection.3.A MOFs NFM photocatalyst was designed and fabricated by in situ growing phosp ho tungstic acid PW₁₂@UiO-66 crystals onto cross-linked polyacrylic acid-Poly?vinyl alcohol??PAA-PVA?NFM.The as-prepared PAA-PVA/PW₁₂@UiO-66 NFM can simultaneously degrade methyl orange?MO?and formaldehyde?FA?in aqueous solution under ultraviolet irradiationPAA contains a large number of carboxyl groups,which can be used as sites for in situ growth of MOFs crystals.In order to solve the water-solubility problem of PAA,PVA was intrduced by co-electrospinning,and then PAA and PVA were partially crosslinked.UiO-66 has good tolerance in water and acid solution.Moreover,UiO-66 has certain photocatalytic activity,which can promote the photocatalytic reaction.In the presence of FA,PAA-PVA/PW₁₂@UiO-66 NFM exhibited excellent degradation efficiency towords MO.The degradation efficency can reach 97.35%in 120 min The enhanced degradation rate may be mainly attributed to the reduction path of MO degradation in the presence of FA.H₃PW₁₂O₄₀ absorbed light and converted itself into excited state,and then was reduced to H₄PW₁₂O₄₀ via accepting electron donated by FA.At the same time,FA was oxidative degradated.MO was degraded by reductive pathway with H₄PW₁₂O₄₀ which had strong reducibility.The electron donating from FA to H₃PW₁₂O₄₀ may be the rate-limiting step of the photo catalytic reaction,so H₄PW₁₂O₄₀ production may play an important role in the degradation rate of MO.The PAA-PVA/PW₁₂@UiO-66 NFM can be easily separated from photocatalytic system and reused.The NFM still maintained high photocatalytic activity after being reused for 5 times.