Studies On Conversion Of Methanol To Aromatics Over Modified Zeolite And Composite Zeolite Catalysts

As the growing demand for BTX and depletion of petroleum resources,conversion of methanol to aromatics(MTA)has attracted great attention.The difficulty of MTA is the low aromatics yield and fast deactivation because of coke.In this thesis,the modified zeolites and composite zeolites were introduced,with the target to transform methanol into BTX with high selectivity.A detailed study of the acid and pore structure of zeolites for selectivity and stability in MTA reaction was conducted.The catalytic performances of conversion of methanol over various zeolites were studied.The results show that H-ZSM-5 exhibited highest BTX yield.Studies over H-ZSM-5 with different Si/Al ratios showed that,with decreasing Si/Al ratio,the selectivity to BTX increases,but the stability decreases.The catalytic performance of conversion of propane、propene and methanol with and without zinc modification show that,the modification of zinc could promote the production of BTX in the conversion of methanol.The methods of fluoride treatment was used to modify H-ZSM-5,and the catalytic performance was studied in detail.We demonstrated that the treatment of H-ZSM-5 by NH4F followed by calcination afforded a high yield of 74%of BTX from methanol.The characterization suggested that F treatment led desilication of the H-ZSM-5 zeolite.The strong Bronsted acidity of H-ZSM-5 has been weakened and new micropores with sizes about 0.75 nm were generated.The change of pores led to the change of"hydrocarbon pool species",which was proposed to increase the production of BTX from methanol.Mesoporous H-ZSM-5 was prepared by treating with NaOH,and the catalytic performance was studied.The catalysts stability were enhanced significantly.At a NaOH concentration of 0.4 M,the catalyst lifetime increased by ten times.The characterizations suggested that alkali treatment led to the decrease of strong Br(?)nsted acidity.And also generated mesopores,which greatly enhanced the diffusion and suppressed the formation of coke,so the catalyst lifetime increased significantly.The methods of F treatment and alkali treatment were used together to study the MTA reaction of catalysts with different pore structures.The characterization suggested that four catalysts own four different pore structures but with the same Br(?)nsted acidity.The MTA reaction over various pore structures showed that,the more advanced the pore structure,the better performance of methanol aromatization.That is the better yield of BTX and the longer lifetime of catalyst.The characterizations suggested that,the more advanced the pore structure,the more generation of lager hydrocarbon pool species,the less formation of coke after the same reaction time and the greater proportion of surface coke during the same time.The reaction route of methanol conversion over trimodal hierarchical porous structure catalyst was:at first light olefins were the main products,then BTX became main products.The synthesis of core-shell like composite zeolites and their catalytic performance of methanol conversion were also studied.Our characterizations suggested that the composite zeolite were successfully hydrothermally synthesized in one step.The characterizations including XRD、TEM、SEM and Ar physical adsorption showed the coexistence of two different zeolites.The MTA reaction showed that the composite zeolite exhibited the best catalytic performance.Higher yield of BTX and the much better lifetime were obtained over this zeolite.It is mostly likely to be the combination of the useful properties of different zeolites,and the synergism in acidity,topology and pore structure.