| With continuous acceleration of industrialization,environmental pollution caused by the volatile organic compounds(VOCs)is urgently needed to be solved.In recent years,VOCs emission standards have become increasingly strict,which puts forward higher requirements for VOCs control.Adsorption and catalytic oxidation are the mainstream technologies for VOCs treatment.However,conventional adsorbents and catalysts have problems such as low adsorption capacity,insufficient low-temperature catalytic activity and poor stability.Therefore,the development of efficient,stable,economical and practical VOCs adsorbent and catalyst is an urgent problem to be solved in basic research and industrial application.Metal organic frameworks(MOFs)have the virtue of ultra-specific surface area,tunable pore size and rich unsaturated metal coordination.MOFs derivatives have highly dispersed active sites and excellent redox properties,which have great application potential in the treatment of VOCs.Aiming at the bottleneck problem of VOCs purification materials,Fe-based MOFs and their derivatives were innovatively developed for efficient and stable adsorption and lowtemperature catalytic oxidation of VOCs by using the pore characteristics and surface metal active sites of MOFs.The micro-mesoporous structure of the adsorbent was designed and screened,the morphology of the catalyst was adjusted and the lattice defects were constructed,and the related performance and mechanism were studied.In this paper,hierarchical porous adsorbent with excellent adsorption performance and good stability has been developed,the contribution of micro-mesoporous structure to the enhancement of adsorption capacity has been explored,and Fe-based MOFs derivative catalysts with uniformly dispersed active sites have been prepared,bimetallic MOFs derivatives have been constructed to form lattice defects,enhance the low-temperature catalytic activity,and analyze the structure-activity relationship between the micro-nano structure,active species and catalytic activity of MOFs derivatives.Combined with in-situ DRIFTS and DFT calculation,the reaction mechanism of VOCs catalytic oxidation was explained.Based on MOFs derivatives,the "in-situ impregnation and derivative" preparation strategy for monolithic catalyst was developed to explore its industrial application potential.The main research content of this study is listed below:Firstly,the adsorption performance and mechanism of Fe-based MOFs for VOCs were studied.Three Fe-based MOFs adsorbents,MIL-101(Fe),MIL-100(Fe)and MIL-53(Fe),were synthesized by solvothermal method.Toluene was used to assess the static and dynamic adsorption performance.The relationship between the physicochemical property(pore parameters,morphology,crystal structure,etc.)and adsorption capacity were analyzed by means of characterizations.Furthermore,the adsorption kinetic parameters were calculated for Fe-based MOFs,and the adsorbent regeneration was realized via microwave irradiation.Results show the static adsorption capacity of MIL-100(Fe)for toluene is up to 663 mg/g,which is 35 times higher than those of MIL-101(Fe)and MIL-53(Fe).According to the static adsorption isotherms and pore parameters,it reveals the micropores supply the primary adsorption sites for toluene,while the mesopores provide higher diffusion coefficients.The kinetic calculation results show the adsorption process could be described by Langmuir model,the theoretical maximum capacity(qm)of the monolayer adsorption is 587.1 mg/g.The pseudo-first order(PFO)kinetic model proves the toluene adsorption of MIL-100(Fe)is physical adsorption,and the mechanism is π-π stacking interaction between the MIL-100(Fe)ligand and benzene ring of toluene.At the same time,MIL-100(Fe)exhibits favorable stability,wherein the dynamic adsorption capacity of toluene still reach 90%after three cycles.Microwave irradiation could completely regenerate the saturated MIL-100(Fe)within 20 min.Secondly,catalytic performance and mechanism of Fe-based MOFs derivatives for toluene oxidation were studied.Fe-based MOFs derivative catalysts(FeOx-n)were constructed by pyrolysis strategy,in which MIL-101(Fe),MIL-100(Fe),MIL-88(Fe),and MIL-53(Fe)were used as sacrificial templates.Effect of space velocity on catalytic performance of FeOx-n for toluene oxidation was investigated.The reaction rate and apparent activation energy of FeOx-n for toluene oxidation were calculated.At the same time,the relationship between FeOx-n surface chemical properties,pore structure and catalytic activity was analyzed by various characterizations.In addition,the degradation path and reaction mechanism of toluene on FeOxn were investigated by the in-situ DRIFTS.Results show that FeOx-100 exhibits the optimal catalytic performance for toluene oxidation,wherein the T50,T90 is 223℃ and 249℃,respectively,and the apparent activation energy is 45.29 kJ/mol.The sequence of other three FeOx-n is FeOx-88>FeOx-101>FeOx-53.Combined with catalyst characterization,it is found that FeOx-100 inherits the developed pore structure of MIL-100(Fe),and its specific surface area and pore volume are 137.8m2/g and 0.414 cm3/g,respectively,which provides abundant adsorption sites for toluene.In the meantime,FeOx-100 possesses the excellent catalytic activity due to the high concentration of lattice oxygen(Oiatt).Furthermore,toluene adsorbed on FeOx-100 surface is activated by active oxygen species and reacts with Olatt to generate intermediate products,then it is subsequently oxidized with gaseous oxygen and transformed into CO2 and H2O.In addition,the performance and mechanism of doped metal enhancing MOFs derivatives for VOCs oxidation at low temperature were studied.Based on MIL-100(Fe),the bimetallic MOFs derivatives were prepared by introducing Mn,Co and Ce,respectively.The catalytic performance of MnFeOx,CoFeOx and CeFeOx for toluene and acetone was investigated.The influences of different metals on physicochemical property were investigated by means of characterization.The mechanism of doped metal on enhancing low-temperature activity of MOFs derivatives was clarified.Experimental results show Mn,Co,Ce are able to enhance the low-temperature catalytic activity of FeOx-100.T90 of toluene catalyzed by MnFeOx,CoFeOx and CeFeOx is 158℃,240℃ and 228℃,thereinto,MnFeOx exhibits the best catalytic performance.Meanwhile,MnFeOx also has the excellent catalytic oxidation on acetone,in which the T50,T90 are 116℃ and 140℃,respectively.MnFeOx not only has the highest specific surface area of 163.9 m2/g,also possesses the highest surface oxygen vacancy concentration of 7.50 μmol/g,this is conducive to VOCs adsorption on catalyst surface,and then oxidized with active oxygen species.Besides,no catalytic activity reduction is observed after four cycles,indicating MnFeOx owns good stability.Finally,the catalytic performance of MOFs derivative monolithic catalysts for VOCs oxidation was studied.Two monolithic catalysts(FeOx/cordierite and MnFeOx/cordierite)were developed by using iron based MOFs derivatives and cordierite honeycomb ceramics.With that,catalytic performance and service span of the developed catalysts for oxidizing toluene and acetone were investigated.With space velocity of 500 h-1,the T50 and T90 of FeOx/cordierite catalyzing toluene is 329℃ and 360℃.Bimetallic MOFs derivatives significantly improves the low-temperature activity of monolithic catalyst,wherein the corresponding T50 and T90 are decreased to 168℃ and 204℃,respectively.Even though the space velocity is increased to 2000 h-1,MnFeOx/cordierite could still realize the complete oxidation of toluene at 290℃.Furthermore,MnFeOx/cordierite exhibits excellent low-temperature catalytic performance for acetone oxidization,in which the T50,T90 is 161℃ and 185℃ respectively.More structural defects are formed on catalyst surface with Mn introduction and the concentration of adsorbed oxygen is increased,thus accelerating the adsorption of oxygen molecules and promoting the redox reaction.In addition,the service span of MnFeOx/cordierite monolithic catalyst exceeds 10 h for toluene and acetone oxidization. |
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