Modification Of ZSM-5 Zeolite And Its Catalysis In Methanol Aromatization

Aromatics,especially benzene,toluene and xylene,are important chemical intermediates widely used in many processes of modern chemical industry,China's petroleum is highly dependent on foreign countries,aromatic production capacity cannot meet the demand of the polyester industry of China.Therefore,it is very important to find sustainable new technologies for the aromatics production from alternative petroleum routes.Methanol shows a great potential as energy and a basic raw material of chemical industry in the future.Its synthesis technology has been matured and can be produced from a wide range of feedstock,such as coal,natural gas and biomass.The rapid advances in the technology of coal chemistry and renewable biomass application in recent years make the production capacity of methanol showing a trend of surplus.Therefore,conversion of methanol into high added-value chemicals is important and urgent.The conversion of methanol to hydrocarbons(MTH)over HZSM-5 zeolites was discovered by the scientists at Mobil Oil in 1970S,which can be used as an alternative route for traditional aromatic production that relies on petroleum.Recently,with the rapid increase in the demand for aromatics,the conversion of methanol to aromatics(MTA)has become a new hot spot in the research of methanol.conversion.The catalysts for MTA are mainly ZSM-5 zeolite based materials;their catalytic performance can be improved via metal cation modification as well as post-treatment.In this paper,research related to the improvement in aromatic selectivity and catalyst lifetime as well as the understanding of the aromatic formation mechanism in MTA process has been carried out.(1)For MTA reaction,HZSM-5 zeolites with low SiO2/Al2O3 ratio(23,30,50,80)were selected.Their crystallinity,pore structure and acidity were characterized.Their catalytic performance in MTA reaction was evaluated in terms of a continuous flow fixed-bed reactor.It has been found that the influence of the SiO2/Al2O3 ratio of ZSM-5 zeolites on the product selectivity is mainly resulted from the difference of the acidity.With increase in the SiO2/Al2O3 ratio,the selectivity of the aromatics and low carbon paraffins in the products increases while the selectivity of C5-C9 non-aromatics and low carbon olefins decreases.The zeolites with SiO2/Al2O3 ratio of 30-50 show a high selectivity of aromatics.(2)The performance of three HZSM-5 zeolites with SiO2/Al2O3 ratio of 50 but different crystal size was evaluated.These zeolites have been characterized in terms of XRD,SEM,NH3-TPD as well as N2 adsorption-desorption techniques.The performance evaluation results indicate that the crystal size of ZSM-5 zeolites significantly influences the product distribution and the catalyst lifetime.The total aromatic selectivity over these three catalysts is essentially the same,being about 36.0wt%.However,the selectivity of various aromatics and C5-C9 non-aromatics is obviously different.The HZSM-5 zeolite with large crystal size shows higher selectivity of benzene and toluene but lower selectivity of xylenes and trimethylbenzene,which can be attributed to the longer residence time of the reaction products in the pores of larger crystal size HZSM-5.Smaller crystal size of HZSM-5 zeolite is in favor of the fast diffusion of reaction products in the pores,with the synergy of larger specific surface area and pore volume,results in significantly longer catalyst lifetime.(3)A HZSM-5 zeolites was modified with Ce,Mo,Mn,Zn metal ions and modification effect on MTA reaction was evaluated.Characterization of the catalysts in terms of XRD,NH3-TPD,FTIR and pyridine-adsorption FTIR indicates that the introduction of these metals ions results in a decrease of the crystallinity of HZSM-5 and a change of the acidity distribution.The amount of the Br(?)nsted acid sites was reduced while that of the Lewis acid was increased.Results of the catalyst evaluation show that only Zn modification can increase the selectivity of aromatics.Although the modification with Mn gives a similar distribution in acid strength and amount,the selectivity of aromatics on Mn modified ZSM-5 decreases.This indicates that the metal modification effect on the aromatic selectivity is closely related to the nature of the metals.The promotional effect of Zn on the aromatic formation in MTA is resulted from the synergetic interaction between newly formed active sites and acid sites.(4)Cadmium modified HZSM-5 catalyst(Cd/ZSM-5)prepared by incipient wetness impregnation shows markedly high selectivity of aromatics,the aromatic selectivity of Cd/ZSM-5 under the optical conditions increases by 27wt%.The Cd/ZSM-5 catalysts have been characterized with XRD,UV-Vis spectra,XPS,FTIR and pyridine-adsorption FTIR techniques.Majority of the introduced Cd cations can exchange with the protons of the bridged hydroxyl groups in HZSM-5,leading to reduced number of Br(?)nsted acid sites while generating new Lewis acid sites on the catalysts.The selectivity of aromatics with the Cd/ZSM-5 is found to be linearly correlated with the concentration of the strong Lewis acid sites.Compared to Zn/ZSM-5,the most extensively investigated catalyst thus far for the MTA,Cd/ZSM-5 is found to be more effective in promoting the aromatics formation.The different catalytic behavior observed for Cd/ZSM-5 and Zn/ZSM-5 catalysts is most likely due to the difference in ionic radius of Cd and Zn.(5)Hierarchical micro-mesoporous HZSM-5 was prepared by combined multi-step post-treatment,including alkaline treatment,alkaline treatment followed by steam treatment,and alkaline treatment followed by steam and citric acid treatment.The alkaline treatment of the microporous HZSM-5 generates hierarchical micro-mesoporous structure.This hierarchical micro-mesoporous structure is remained in the followed steam and steam +citric acid treatment.The presence of the mesoporous in HZSM-5 zeolites prolongs the lifetime of the catalysts but does not noticeably influences the aromatics selectivity.Modification of the hierarchical micro-mesoporous HZSM-5 zeolites with Cd enhances the aromatics selectivity and the presence of the mesoporous in these catalysts also prolongs the lifetime as compared with the Cd modified microporous HZSM-5.The various post-treatments of HZSM-5 not only creates hierarchical micro-mesoporous structures but also results in redistribution of acid strength and concentration.The enhanced aromatics selectivity of Cd modified hierarchical micro-mesoporous HZSM-5 can be ascribed to the generation of new active sites from the cation exchange of Cd ions with the residual strong Br(?)nsted acid sites.The increased lifetime can be ascribed to the increased coke capacity toleration from the hierarchical mesoporous structure.(6)In order to understand the mechanism and the promotional effect of metal ions on aromatics formation in MTA,The product distribution with long chain alkenes(isobutylene,1-hexene,1-heptene,1-octene),cycloolefin(cyclohexene),cycloparaffin(methylcyclohexane)and the mixture of methanol with those chemicals as feeds has been investigated over HZSM-5 and Zn/ZSM-5 catalysts.The results indicate that the long chain alkenes cannot directly converted to aromatic via cyclodehydrgenation but via a series of cracking-oligomerization steps over HZSM-5 and Zn/ZSM-5.The significantly higher aromatic selectivity observed with cyclohexene as the feed indicates that the formation of unsaturated cyclo-species is the controlling step for the formation of aromatics in MTA.Over the Zn/ZSM-5,the aromatics selectivity(mainly toluene)with methylcyclohexane as the feed is also significantly high,indicating that Zn/ZSM-5 can promote the direct dehydrogenation of methylcyclohexane to toluene.