Study On The Catalytic Oxidation Of Co-based Metal Oxides Derived From Co-MOF For Toluene

VOCs are volatile organic pollutants that can be involved in atmospheric photochemical reactions.As an important precursor of ozone and PM_2.5,VOCs seriously threatens and damages the environment and human health,and has potential harm to China’s economic development.With the further development of industrialization and human yearning for a higher quality life,the governance of VOCs is imperative.With strong photochemical reactivity and high ozone generation potential,toluene is a typical and highly toxic benzene ring VOCs,thus it is often regarded as a target pollutant for VOCs treatment.Among the many treatment methods,catalytic oxidation has the advantages of low temperature operation,high efficiency and environmental friendliness,so it has a very high market share both at home and abroad.The key to this technology is to develop a low-cost catalyst which is efficient at low temperature and stable at high temperature.The high price of noble metal catalysts limits their large-scale industrial application.Among the inexpensive and readily available non-noble metal catalysts,cobalt oxide is one of the most effective oxides to toluene catalytic oxidation.The catalytic activity of toluene oxidation by Co₃O₄ catalyst is closely related to its physical and chemical properties such as pore structure,active site and low temperature reducibility.The conventional preparation route of Co₃O₄ is a hydrothermal method or precipitation followed by direct thermal decomposition.However,in the conventional way,it is difficult to control the physical and chemical properties such as pore structure of the as-synthesized catalysts.Based on the excitation,we selected the porous Co-MOF as the precursor,the pore structure,lattice defects,low temperature reducibility and specific surface area of the MOF derived Co₃O₄ catalysts were all successfully controlled by regulating the calcination conditions.We also picked ZSA-1 as a precursor of Co based metal oxide for doping modification and preparing cobalt based composite metal oxides for toluene catalytic oxidation.The physicochemical properties of the catalysts were explored,and the intermediate products and reaction mechanisms of toluene combustion were also investigated by means of various characterization techniques.The main research conclusions are as follows:（1）Different morphologic ZIF-67,MOF-74 and ZSA-1 with N-donor,O-donor and N-O-donor linkers were employed as precursors for acquiring Co₃O₄catalysts,respectively.Notebly,the different morphologies and ligands of Co-MOF affected the peculiarities of the as-synthesized Co₃O₄ to a great degree.The Co₃O₄ derived from diverse parent Co-MOFs possess differential characteristics such as Co³⁺/Co²⁺,O_ads/O_latt,specific surface area,low temperature reducibility.Furthermore,the Co₃O₄ catalysts exhibited discrepant catalytic activity and stability for toluene oxidation,which can be ordered as follow:ZSA-1-Co₃O₄（N-O-ligand with octahedra shape）>MOF-74-Co₃O₄（O-ligand with rod shape）>ZIF-67-Co₃O₄（N-ligand with dodecahedral shape）.These differences can be attributed to the shape of the parent Co-MOFs and content of C,N and O in the linkers.In a word,it is a promising way for designing desired Co₃O₄catalyst for toluene combustion by choosing parent Co-MOFs with appropriate ligands under the same calcination condition.（2）Co₃O₄ catalysts with different Co³⁺/Co²⁺and O_ads/O_latt were obtained bycontrolling the heating rate,calcination time and temperature of calcining ZSA-1.It’s worth noting that the calcination temperatures played a crucial role in the formation of structure and peculiarities of the as-synthesized samples.Compared with other obtained catalysts,the ZSA-1-Co₃O₄-350 catalyst with largest specific surface area,occupied the highest Co³⁺/Co²⁺and O_ads/O_latt.During the evaluation of catalytic activity and stability of toluene oxidation,ZSA-1-Co₃O₄-350 catalyst displayed the highest catalytic property.The T_90%（the temperature at which toluene conversion achieved 90%）was 239°C,attributting not only to the above superior characters,but also to the smaller nanosized crystal size,more defect sites,richer pore volume and proper average pore size as well as the stronger low temperature reducibility.Moreover,the degradation process of toluene was revealed by the in situ infrared spectrum,the finally CO₂ and H₂O were dated in sequence from benzyl alcohol,benzoic acid,and maleate species.（3）The activated ZSA-1were impregnated by 50%aqueous Mn（NO₃）₂dissolved into anhydrous ethanol,which is calculated based on the Co₃O₄ derived from ZSA-1,to adjust the original Co:Mn（molar ratio）to be 1,2 and 3,respectively.The as-synthesized Mn doped ZSA-1were calcined under the same condition of the M-Co₃O₄ catalysts.A series of cobalt-based composite metal oxide catalysts M-Co_aMn_bO_x were successfully obtained.The catalytic activities of M-Co_aMn_bO_x were better than that of M-Co₃O₄,certifying the promoting effect to toluene oxidation of Mn.The M-Co₁Mn₁O_x in which the Co:Mn=1:1projected a enhanced catalytic performance with by comparison.According to the analysis of the characterization results,the M-Co₁Mn₁O_x presented more plentiful Co³⁺/Co²⁺,Mn³⁺/Mn⁴⁺and O_ads/O_latt as well as lattice defects and stronger low temperature reducibility,which were all benifical to toluene combusition.The tracking results of the in situ infrared spectrum indicate that benzaldehyde,benzoic acid,maleate species and anhydride are the main intermediate products of toluene catalytic oxidation over M-Co₁Mn₁O_x.（4）A series of Co-Mn composite metal oxides catalysts were successfully constructed via calcination of M-doped（M=Cu,Mn,Fe and Ni）Co-MOF.M-Co₁M₁O_x（the molar ratios of Co/M were 1:1）with different mental dopants owned different physicochemical properties.The synergistic effect of Co-Cu and Co-Mn displayed promoting effect to toluene combustion,while the doping of Fe and Ni did not play a positive role in catalytic oxidation of toluene.The as-synthesized M-Co₁Cu₁O_x with strong interaction between Co and Cu exhibited the highest Co³⁺/Co²⁺,O_ads/O_latt,specific surface area and the strongest low temperature redox property,displaying the most outstanding performance for toluene degradation.The toluene conversion of 90%over M-Co₁Cu₁O_x was achieved at 208°C,which was 31°C lower than that of M-Co₃O₄ derived from no-doped ZSA-1.According to the results of the In Situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy,toluene could be more easily degraded on M-Co₁Cu₁O_x with lower activation energy than the no-doped M-Co₃O₄.The main intermediate products were benzaldehyde,benzoic acid,anhydride,maleate species.The degradation mechanism conformed to Langmuir-Hinshelwood（L-H）model.