Synthesis,Metal Modification And Methanol Aromatization Properties Of Nano ZSM-5 Molecular Sieves

In this paper,a series of nano-scale ZSM-5 molecular sieves with micro-mesoporous structure were prepared by hydrothermal synthesis.Methanol aromatization(MTA)was used as a model reaction.The effects of the synthesis conditions on the physicochemical properties and catalytic properties of molecular sieves were systematically investigated by SEM,XRD,BET,ICP and NH3-TPD spectroscopy.With the aromatics mass yield as the objective function,the ZSM-5 molecular sieves synthesized with a ratio of Si:Al:TPAPH:H2O=13:1:3.4:575 and crystalized at 180? for 72 h demonstrated the optimal properties.The ZSM-5 molecular sieve synthesized under this condition has a micro mesoporous structure with a grain size of 400-600 nm and a specific surface area of 309.28 m2g-1.Based on the optimal conditions,the metal ion modification of ZSM-5 sieves were also studied.Firstly,the properties of the molecular sieves were investigated by in-situ synthesis,ion exchange,excess impregnation and equal volume impregnation on the basis of metal Zn loading.According to these investigations,the metal modified ZSM-5 molecular sieve prepared by the excess impregnation method contained the best MTA catalytic performance.The mass yield of aromatic hydrocarbons increased compared with the metal was unloaded(from 17.6%to 20.1%).Moreover,the more than 95%of the added metal was loaded through the excess impregnation method.As comparison,the actual Zn loading by the in-situ synthesis method and the ion exchange method was only 18.45%and 23%,respectively.Based on the excess impregnation method,the effects of metal species and metal content on the performance of molecular sieves were further investigated.The results were followed:(1)Zinc with suitable extranuclear electron arrangement and dehydrogenation activity contained the best modification effect.Zinc could chelate with L acid and form a weak Zn-L acid,which reduces the amount of strong acid.(2)Even the stability is high,the aromatics yield of the molecular sieve loaded by metal such as Ag and Ga were decreased.The possible reason is that part of the acid active center was covered by the metal oxides such as Ga2O3 and AgO.Besides,a second metal component was also added to the 2%Zn-HZSM-5 molecular sieve to prepare a bimetallic molecular sieve catalyst(2%Zn-1%Cd/HZSM-5 molecular sieve),which demonstrated an increased aromatics mass yield from 17.6%to 20.97%.The addition of the second metal component barely promoted the aromatics yield and declined the life-time of the molecular sieve.Based on 2%Zn-HZSM-5 molecular sieve,the effects of the reaction temperature(425-500?)and methanol mass space velocity(1.69 h-1-6.76 h-1)on the reaction process of MTA were detailly investigated.The results indicated that with the increase of the reaction temperature,the aromatics yield barely increased,while the life of molecular sieves decreased.Meanwhile,the decreases of the methanol mass space velocity could significantly increase the yield of aromatics and prolong the effective service life of the molecular sieves.The optimal process conditions were determined as 425? with the methanol mass space velocity of 1.69 h-1,which demonstrated the highest aromatic mass yield of 23.1%.