Study On The Adsorption-catalytic Performance Of Modified MFI Molecular Sieves For Benzene

In recent years,ozone（O₃）has gradually become the primary pollutant in spring and summer in China.As the main precursor of ozone,volatile organic compounds（VOCs）have attracted more and more attention because of their numerous types,complex sources,large emissions and their negative impact on human health and ecological environment.How to develop non noble metal catalysts to effectively treat VOCs with high activity,hydrophobicity and low price,especially benzene series with high toxicity and difficult to deal with,has become a research hotspot at home and abroad.In this paper,benzene was selected as the probe molecule of VOCs,and microporous ZSM-5 zeolite was used as the carrier.A series of modified catalysts with nitrogen and titanium sources were prepared successively by different preparation methods.The physical and chemical properties of the modified catalysts were analyzed by various characterization techniques.Then the performance of benzene adsorption,catalytic combustion and photocatalytic degradation were evaluated.Finally,in-situ FTIR diffuse reflectance technique was used to study the catalytic mechanism of benzene on various catalysts.（1）A series of N-ZSM-5 catalysts with different N content were prepared by adding urea into ZSM-5 zeolites in situ by one-step solid-state method.The addition of urea had no effect on the microporous structure of the carrier.With the increase of urea content,the specific surface area of N-ZSM-5 catalysts increased first and then decreased,and reached the maximum value（418.43 m²/g）at 2 N-ZSM-5 catalyst.After modification,a large number of nitrogen-containing groups appeared in the catalysts,and urea combined with hydroxyl groups on the zeolite.At room temperature,the adsorption capacity of N-ZSM-5 catalysts for benzene increased first and then decreased.Among them,the adsorption capacity of 2 N-ZSM-5 catalyst was the largest,reaching 47.56 mg/g,which is nearly 5 times of that of pure ZSM-5（9.70 mg/g）.In addition,the hydrophobicity of N-ZSM-5 catalysts was improved.The T₅₀ and T₉₀ of 2N-ZSM-5 catalyst were 144℃and 184℃,respectively.The catalytic combustion performance of the modified catalysts for benzene were significantly improved.（2）A series of Ti-ZSM-5 catalysts with different Ti content were prepared by hydrothermal synthesis with tetrabutyl titanate（TBOT）as titanium source.The addition of titanium did not change the original microporous structure of the support.Compared with the support,the specific surface area of Ti-ZSM-5 catalysts decreased to a certain extent.The intensity and width of the infrared absorption peak corresponding to the stretching vibration of the surface adsorbed water and hydroxyl group were stronger than those of the unmodified ZSM-5 catalyst,and the shoulder peak(970 cm^-1)appeared with Ti entering the zeolites’framework.After the introduction of Ti,the combustion range of Ti-ZSM-5 catalysts shifted to lower temperature,which was favorable for low temperature adsorption.The interaction between titanium containing functional groups and supports improved the affinity of benzene molecules on the surface of the material and enhanced the adsorption capacity of benzene,which may be related to the formation of some active oxygen or hydroxyl groups on the surface of the catalysts.Ti-ZSM-5 catalysts had a certain photocatalytic effect on benzene,and the highest degradation efficiency was 29.7%.（3）A series of Ti-N-ZSM-5 catalysts were prepared by introducing N into Ti-ZSM-5 precursor by one-step solid-state method.The addition of urea did not destroy the microporous structure of Ti-ZSM-5 catalysts,but effectively increased the specific surface area.Lattice oxygen and adsorbed oxygen appeared in the catalysts,and the peak of adsorbed oxygen was obvious,which was helpful to improve the catalytic oxidation ability of benzene.With the increase of Ti content,the adsorption capacity of Ti-N-ZSM-5 for benzene first increased and then decreased.The adsorption capacity reached the highest value（63.72 mg/g）at 0.1 Ti-N-ZSM-5 catalyst which was much higher than that of N-ZSM-5,Ti-ZSM-5 and even other modified ZSM-5 zeolites.The photodegradation rate of benzene by Ti-N-ZSM-5 catalysts was as high as 29.2%.（4）The catalytic combustion mechanism of benzene was deduced.In the catalytic combustion of benzene over N-ZSM-5 catalysts,benzene was adsorbed on N-ZSM-5to form hydroxyl group.With the increase of temperature,nitrogen-containing functional groups promoted the formation of lattice oxygen.Hydroxyl substances rearranged on the catalyst surface and continued to react with surface oxygen to form a monoatomic ligand adsorbed on the catalyst surface.C-H bond was broken to form olefins,and C-C and C=C bonds broke successively,species appeared in the form of alkanes,and finally desorbed in the form of CO₂ and H₂O.In the process of photocatalysis,the synergism of N and Ti could further increase the surface oxygen and adsorbed oxygen.The generated holes and electrons reacted with OH-and O₂ on the catalyst surface respectively to generate.OH and O₂.^-.Finally,.OH reacted with gaseous benzene to generate CO₂ and H₂O.