Removal Of Toluene By Non Thermal Plasma Combined With Molecular Sieves Of Different Pore Structure

Volatile organic compounds?VOCs?pollution is one of the most serious problems in our country which are in need of control.Non-thermal plasma?NTP?combined catalysis technology is generally regarded as one of the most effective routes for eliminating VOCs with lots of distinguished advantages,such as high destructive efficiency,high selectivity and rapid reaction at room temperature and atmospheric pressure.In the technology,is the key factor to ensure high removal efficiency and minerization of VOCs oxidation.It is necessary to study the action mechanism of catalysts.In this work,the molecular sieves were chosen as the research object,and the effect of pore dimension on the removal of toluene was studied.Finally,the reaction path of the degradation of toluene in plasma was discussed.A series of HZSM-5 catalysts with various pore size were prepared by chemical liquidphase deposition method.The structure and adsorption performance were measured by X-ray diffraction?XRD?,N2 adsorption?BET?,shape-selective adsorption test and dynamic adsorption,respectively.These results demonstrated that the catalysts possessed same MFI structure,similar specific surface area,pore volume but the different pore size.Moreover,the toluene decomposition activity and the concentration of ozone in a dielectric barrier discharge system were also investigated,and the results show that the HZSM-5 with largest pore size showed the best toluene adsorption ability,highest carbon balance and the lowest ozone concentration.In addition,products in outlet-gas and on the surface of catalysts were analyzed by TOF-MS and GC-MS,respectively.With the increase of pore size,both GC-MS and TOFMS detected less organic byproducts.The MZ catalyst with multi-level-pore was prepared,and the pore structure of MZ were characterized by XRD and Nitrogen adsorption-desorption experiments.The results showed that the MZ had ordered and multi-pore structure,and the mesoporous pore were between 2-5nm and microporous pore were centered on 0.58 nm.The catalytic activity for toluene decomposition was investigated,for comparison,the typical microporous molecular sieve material HZSM-5 and mesoporous molecular sieve material MCM-41 were chosen.The carbon balance of the catalysts follows the sequence: MZ > HZSM-5 > MCM-41,while the order of CO2 selectivity is MZ > HZSM-5 > MCM-41.MCM-41 performed larger adsorptive capacity than HZSM-5,which could adsorp more toluene on the surface.But its adsorption strength was the lowest among the three catalyst,and the intermediates would easily escape from the surface of the catalyst,leading to low CO2 selectivity.The micropore structure of HZSM-5 provided stronger adsorption strength,prolonging the retention time of the intermediates,which contibuted to the thorogh oxidation of the intermediates and improved CO2 selectivity.Moreover,the micropore structure made shape-selective of the intermediates,inhibiting the hard-degradable isomers.The evolution of the intermediate products in plasma catalytic process were analysed by TOF-MS,GC-MS and in situ infrared spectroscopic.Several kinds of benzene compounds and small organic,such as benzaldehyde,benzyl alcohol,fomic,biformyl and methyl-glyoxal,had been detected by TOF-MS.The result of in situ infrared spectroscopic showed that benzaldehyde,benzyl alcohol and some aldehydes formed on the surface of the catalyst in the plasma.Moreover,the spent catalysts were extracted by CS₂ and ethanol.The result was in accord with the datum of TOF-MS and in situ infrared spectroscopic.In addition,cisbutenedioic anhydride and many ethanols like ethyl pyruvate had been detected.Finally,the degradation mechanism of toluene was deduced based on the analysis of byproducts.