Study On The Construction Of Adsorbent-Catalyst Bifunctional Materials For The Treatment Of Spray-Painting VOCs Exhaust

In recent years,air pollution has become a serious social problem.As important precursors of ozone,photochemical smog and fine particulate pollution,volatile organic compounds（VOCs）caused serious harm to both ecological environment and human health due to their easy diffusibility,toxicity and volatility.It was estimated that the emission of spray-painting VOCs exhaust accounted for more than 20%of the entire industrial source,accounting for about 12%of the total VOCs emission source.Elimination of spary-painting VOCs exhaust was recognized as one of the key objects for VOCs prevention and control.The adsorbent-catalyst bifunctional materials provided a new idea for the development of VOCs treatment technology,i.e.,combining adsorbent materials with high adsorption capacity and catalytic materials with high catalytic activity organically.On one hand,it could utilize adsorbent materials to enrich VOCs so as to provide the required concentration for the thermocatalytic process;On the other hand,it could achieve the in situ desorption regeneration of the adsorbent by utilizing the heat in the thermos-catalytic process.In this study,the non-precious metals were supported on zeolite molecular sieves to prepare a series of adsorbent-catalyst bifunctional materials.Firstly,the types of zeolite molecular sieves,the types of metal active centers and the construction methods of bifunctional materials were filtered,and it was found that the Mn O_x/ZSM-5 constructed by an ethanol-citric acid complexing method presented the best performance.Secondly,the Mn O_x/ZSM-5 was modified by doping with Cu,Co and Ni to screen out the Cu Mn O_x/ZSM-5,followed by the optimization of copper-manganese compound ratio to obtain the Cu₇Mn₃O_x/ZSM-5 with excellent adsorption and catalytic oxidation performance.On this basis,in view of the weakness of the material’s poor water resistance during the adsorption process,the Cu₇Mn₃O_x/ZSM-5 was modified with hydrochloric acid,nitric acid,citric acid and oxalic acid,and Cu₇Mn₃O_x/ZSM-5-N-0.50（citric acid modification）with excellent thermal stability and water resistance was obtained.Afterwards,the materials were characterized by N₂ adsorption-desorption,XRD,SEM,H₂-TPR,XPS,NH₃-TPD,Py-IR and TG to establish the structure-activity relationship between the adsorption-catalytic oxidation performance and physicochemical properties.The reaction mechanism of toluene on adsorbent-catalyst bifunctional materials was further elucidated by in situ FTIR.Finally,the spray-painting VOCs exhaust of a certain enterprise was collected and analyzed to verify the typical VOCs components as the target pollutants.The adsorption and catalytic oxidation performance of Cu₇Mn₃O_x/ZSM-5-N-0.50 for the above single and mixed pollutants were investigated,followed by the regeneration and stability test of Cu₇Mn₃O_x/ZSM-5-N-0.50 in simulated spray-painting VOCs exhaust.The results showed that the prepared Cu₇Mn₃O_x/ZSM-5-N-0.50 had high adsorption capacity and excellent purification effect for typical VOCs components in spray-painting exhaust,and had good stability and regeneration under high humidity environment,multi-component VOCs,long-term operation and other complex working conditions.The main research results obtained in this paper were as follows:（1）Materials design and optimization:ZSM-5 zeolite molecular sieve and Mn O_x were designed as adsorption unit and catalytic oxidation unit respectively,the Mn O_x/ZSM-5adsorbent-catalyst bifunctional material was constructed by an ethanol-citric acid complexing method.Further,the Mn O_x/ZSM-5 was modified by doping with transition metals to obtain the Cu Mn O_x/ZSM-5 with excellent adsorption and catalytic oxidation performance.Its equilibrium adsorption capacity for toluene could reach 25.90 mg/g,and the T₉₀ for toluene oxidation was268℃.Then,on one hand,the copper-manganese component loading ratio was optimized to obtain Cu₇Mn₃O_x/ZSM-5,and it was determined that Cu/Mn=7:3 was the optimal metal compound ratio of Cu_yMn_10-yO_x/ZSM-5.On the other hand,in view of the weakness of the material’s poor water resistance during the adsorption process,the citric acid modification increased the adsorption capacity of Cu₇Mn₃O_x/ZSM-5 by 24%,and decreased the T₉₀ for toluene oxidation by 15℃.Its adsorption capacity of toluene in high humidity environment was also increased by 51.1%compared with Cu Mn O_x/ZSM-5.Finally,the Cu₇Mn₃O_x/ZSM-5-N-0.50 with large adsorption capacity,good catalytic activity,water resistance,stability and regeneration was obtained.（2）Materials characterization and reaction process:The N₂ adsorption-desorption results showed that the larger specific surface area and pore capacity were beneficial to the adsorption of toluene molecules.The SEM characterization showed that the metal oxide particles distribution on the surface of the acid-modified materials was more uniform and the particles size was smaller,providing more reactive sites for the materials.The H₂-TPR and XPS tests showed that the abundant Cu²⁺and Mn⁴⁺in Cu₇Mn₃O_x/ZSM-5-N-0.50 improved its redox performance,which was beneficial to the catalytic oxidation of toluene.In the analysis of NH₃-TPD and Py-IR,it was found that acid modification could adjust the acid content,acid distribution and types of acid sites of Cu₇Mn₃O_x/ZSM-5,and Lewis acid sites were favorable for the adsorption of toluene molecules.The in situ FTIR test results showed that the reaction process of toluene on Cu₇Mn₃O_x/ZSM-5-N-0.50 was consistent with the reaction mechanism described by the Mv K mechanism.After being adsorbed on the materials surface,toluene was first oxidized into benzoate under the combined action of surface active oxygen,lattice oxygen and O₂,and then oxidized to CO₂ and H₂O completely with the increase of temperature.（3）Practical application verification:The sampling analysis of spray-painting VOCs exhaust in a certain enterprise showed that toluene,xylene,acetone and butyl acetate were typical spray-painting VOCs components.The constructed Cu₇Mn₃O_x/ZSM-5-N-0.50 had high adsorption capacity and excellent purification effect for the above typical components in spray-painting VOCs exhaust.The total adsorption capacity of Cu₇Mn₃O_x/ZSM-5-N-0.50 for VOCs was as high as 55.55 mg/g,which could completely convert the exhaust into CO₂ and H₂O at300℃.Besides,it had good stability and regeneration under high humidity environment,multi-component VOCs,long-term operation and other complex working conditions.