Molecularly Imprinted MOFs:Novel Materials For Removal Of Environmental Organic Pollutants

In recent years,water pollution has become a major global problem.Among the organic pollutants,waste drugs were one of the main sources of water pollution.Metal-organic frameworks?MOFs?are a class of porous coordination compounds,which constructed by self-assembly of metal ion clusters and organic linkers.Due to the advantages including large specific surface area,designable tailored structure,controllable pore property and open metal active sites,MOFs have been widely used in gas storage,adsorption separation and catalysis.However,the adsorption selectivity of MOFs is limited.Molecularly imprinted polymers?MIPs?with high specific recognition ability for target molecules are generally prepared by molecular imprinting technique.Due to the excellent specific affinity and selectivity towards the template molecules,MIPs have great potential to remove target organic pollutants in the environment,but their adsorption capacity is relative low,especially in the aqueous phase.Advanced oxidation technology is capable of generating strong oxidizing free radicals for catalytic oxidative degradation of organic pollutants,which are widely used in current water treatment systems.However,the affinity between the catalyst and the pollutant is weak,and the degradation efficiency of the target organic pollutant in the complex system is susceptible to other coexisting organic pollutants.In view of the above deficiencies and based on the advantages of MOFs and MIPs,this study for the first time reported novel molecularly imprinted MOFs with specific recognition ability and catalytic oxidation activity for selective removal of environmental organic pollutants.Part 1 Preparation and Characterization of molecularly imprinted MIL-101Objective:Metal-organic frameworks?MOFs?can be used for separation and removal of environmental pollutants from waste water due to their designable tailored structures and controllable pore sizes.However,the adsorption selectivity of MOFs is limited,since most of the matrix components in waste water could compete with the target for adsorption sites on MOFs.As artificial antibodies,molecularly imprinted polymers?MIPs?with highly specific recognition capability are widely used for selective removal of organic pollutants from the complex samples.However,traditional molecular imprinting often produces particles with low adsorption capacity.In order to improve the selectivity of MOFs,the present work prepared the molecularly imprinted metal-organic frameworks?MOF-MIPs?by integrating the high adsorption capacity of MOFs and high selectivity of MIPs.Methods:In the present work,MMOF-CMIPs were prepared by an in-situ self-assembly template strategy for the first time.Fe???,2-aminoterephthalic acid?NH₂-BDC?and caffeine?CAF?were selected as the metal ion,organic ligand and model template molecule,respectively.The crystalline structures,morphologies and surface chemical groups of MMOF-CMIPs were characterized by various modern physical and chemical methods,including XRD,FT-IR,SEM,and TEM.The adsorption selectivity of MMOF-CMIPs was evaluated by investigating the adsorption performance of MMOF-CMIPs for CAF in single target systems?only one substance in water?and binary systems?the template with a coexisting substance in water?.Moreover,the adsorption kinetics and thermodynamic behaviors of MMOF-CMIPs were also studied.Results:SEM and TEM images showed that the shapes of both MMOF-CMIP and MMOF-NIP were regular octahedrons.XRD patterns showed that no noticeable difference was observed between the main diffraction peaks of MMOF-CMIP and MMOF-NIP,implying that the introduction of CAF induced no significant change in the crystal structure of MOFs.Batch binding experiments showed that CAF could be easily adsorbed on the MMOF-CMIP and only 10 min was required to reach the adsorption/desorption equilibrium on MMOF-CMIP,while 60 min was needed to reach the adsorption/desorption equilibrium on MMOF-NIP.According to the Langmuir isotherm model,the calculated maximum adsorption capacity of MMOF-CMIP(671.1mg g^-1)was 20 times higher than that of MMOF-NIP(33.7 mg g^-1).In the presence of structural analogues of CAF and small molecule interferents,MMOF-CMIPs also provided excellent specific recognition capability.The above results indicated that the imprinted sites in MMOF-CMIPs effectively enhanced the adsorption selectivity of MOFs.Conclusion:Based on the high selectivity of MIPs,molecularly imprinted MOFs have been developed by an in-situ self-assembly template strategy for the first time.The presence of molecularly imprinted sites effectively improved the specificity of MOFs.Compared with traditional MOFs and MIPs,MMOF-CMIPs not only provided good recognition performance for the CAF in water,but also exhibited an ultra-high adsorption capacity.This system offers a new idea for the construction of adsorbents to selectively remove the environmental organic pollutants from waste water.Part 2 Preparation and Characterization of molecularly imprinted UiO-66Objective:Ketoprofen?KET?,acting as an anti-inflammatory drug,is generally used to relieve pain and reduce inflammation.Due to its wide application and unreasonable discharge,KET has been detected frequently in many surface waters and wastewaters around the world.Previous works showed that KET has a potential threat to humans,animals and plants.In order to effectively remove KET from waste water,in this work we prepared KET imprinted MOFs using a similar synthesis strategy in Part 1.Methods:The NH₂-UiO-66-based molecularly imprinted MOFs?UMOF-KMIPs?were synthesized through hydrothermal method by using KET,Zr?IV?and 2-aminoterephthalic acid?NH₂-BDC?as the template molecule,central metal ion,and organic ligand,respectively.The crystal structures,morphologies,surface chemical groups,specific surface areas and surface charges of UMOF-KMIPs were investigated by various modern physical and chemical methods,respectively.The adsorption performances of the UMOF-KMIPs were also evaluated.Results:KET imprinted UMOF-KMIPs were prepared by the in situ self-assembly template strategy under hydrothermal conditions.By optimizing the KET amount in the synthesis process,it was found that the KET amount significantly affected the specific adsorption performance of UMOF-KMIP.When the molar ratio of Zr???,NH₂-BDC and KET was 2:1:2,UMOF-KMIP-2 provided the best specific recognition capability.SEM results showed that UMOF-KMIP-2 displayed a spherical structure with diameters of 30-50 nm,while lamellar stacking particle with particle size distribution of 0.3-1.5?m was dominative for UMOF-NIP.XRD results showed that the characteristic diffraction peaks of UMOF-KMIP-2 were consistent with those of UMOF-NIP,while the crystallinity of UMOF-KMIP-2 was better than that of UMOF-NIP.The pore size distribution curves showed that UMOF-KMIP-2 has a mesoporous structure with larger pore size than UMOF-NIP.Batch binding experiments showed that the maximum adsorption capacity of UMOF-KMIP-2(295.2 mg g^-1)was about 1.5 times higher than that of UMOF-NIP(196.6 mg g^-1),and the adsorption equilibrium time was about 30 min.In addition,the pH of solvent showed no significant effect on the specific adsorption?imprinting factor?of UMOF-KMIP-2 over a wider pH range?3.4-9.4?.Selective adsorption experiments showed that UMOF-KMIP-2 provided excellent recognition performance on a class of acidic drugs like KET.Conclusion:UMOF-KMIP-2 was prepared by an in-situ self-assembly template strategy,and exhibited good specific adsorption capability for the KET template and its structural analogues,which can be used to remove a class of acid drugs like KET from the waste water.Based on the results in both Part 1 and Part 2,it can be concluded that the in-situ self-assembly template strategy was an efficient and universal method for the preparation of MOF-MIPs to remove organic pollutants from the complex environmental samples.Part 3 Selective photo-catalyzed Fenton degradation of sulfonamide antibiotics by molecularly imprinted MOFsObjective:Antibiotics are powerful drugs for the treatment of infectious diseases.However,their excessive application and unreasonable effluent have caused serious environmental pollution.Antibiotics were detected in most of surface water,groundwater and drinking water,indicating that existing wastewater treatment technologies are difficult to remove antibiotics completely.In recent years,antibiotic pollution has become a special threat to the public and environmental health.Therefore,it is urgent to develop a new method and technique for deeply removal of antibiotic organic pollutants from wastewater.According to Part 1 and Part 2,MOF-MIPs prepared by the in-situ self-assembly template strategy exhibit promising applications in the field of highly selective removal of antibiotic.However,the adsorption/desorption equilibrium on MOF-MIPs is easily reached,and the target pollutant is difficult to be completely removed.Advanced oxidation technology can be used for the degradation of organic pollutants through the generation of reactive species with strong oxidation ability.Therefore,this work aimed to achieve the completely and selectively removal of antibiotics by combining the molecular imprinting and advanced oxidation techniques.Methods:Based on the in-situ self-assembly template strategy,sulfamethazine?SM₂?imprinted MOFs photocatalysts were prepared by using Fe?III?and NH₂-BDC as metal ion and organic ligand,respectively.The crystal structures,morphologies and surface chemical groups of MMOF-SMIPs were characterized by various modern physical and chemical methods.The adsorption performances of MMOF-SMIPs were also evaluated.Since MMOF-SMIPs have the ability of photocatalytic activation of H₂O₂,the photo-catalyzed degradation of MMOF-SMIPs for SM₂ in the presence of H₂O₂ under visible light was further investigated.Results:XRD results showed that the characteristic diffraction peaks of MMOF-SMIP were consistent with those of MMOF-NIP,indicating that the introduction of template molecules did not change the crystal structure of MOFs.Batch binding experiments showed that the maximum adsorption capacity of MMOF-SMIP(192.3 mg g^-1)was about 4.7 times higher than that of MMOF-NIP(40.7 mg g^-1).Selective adsorption experiments showed that MMOF-SMIP provided excellent recognition performance on a class of sulfonamides similar with SM₂.Under the irradiation of visible light,SM₂ could be efficiently degraded by MMOF-SMIP in the presence of H₂O₂.Under the optimal conditions,SM₂ could be completely removed in 30 min.The degradation rate constant over MMOF-SMIPs was 0.294 min^-1,which was much higher than that over MMOF-NIP(0.223 min^-1).All the above results indicated that the presence of imprinted sites in MMOF-SMIPs could not only enhance the adsorption capacity of MMOF-SMIPs,but also improve the photo-catalyzed Fenton ability of MMOF-SMIPs.Conclusion:Based on the highly selective recognition performance of MIPs and excellent photo-Fenton catalytic ability of iron-based MOFs,molecularly imprinted MOFs involved photo-catalyzed Fenton system was constructed for the efficient degradation of SM₂.The present work solved the problem that traditional MOF photocatalyst is easily affected by the sample matrix in sewage system,and provided a new method for selective photocatalytic degradation of sulfonamide antibiotics in water environment.