Synthesis Of Hierarchical ZSM-5 And Its Catalytic Performance In Methanol-to-Aromatics Reaction

The aromatics widely used in industry are mainly dependent on the conventional gasoline derivative technology.As the conditions of rich coal less oil and the serious shortage of aromatics production,our aromatics mainly rely on imports.Based on the above reasons,the supply and demand of aromatics is serious imbalance,which leads to expensive aromatics.Looking for a new aromatics non-oil production line is imminent.With the rapid development of methanol industry,methanol production is also rapid developing.However,the development of methanol downstream products is far behind the pace of methanol development.The current operating rate of methanol production equipment and the price of methanol are at a low level,the development of methanol downstream production is also imminent.In recent years,the process of methanol to aromatics is a good solution to the supply and demand contradiction of methanol and aromatics,which is also a great strategic significance to our country.At present,there are some problems such as low selectivity of BTX（benzene,toluene,xylene）and poor stability of catalyst in the methanol to aromatics.The loading metal method is mainly used to solve the problem of low BTX selectivity and the best is Zn.Due to the unclear reaction mechanism,the research on BTX selectivity has been a bottleneck.For the problem of thestability of the catalyst,the method of introducing mesoporous was mainly used by the researchers.However,there are still some problems,which need further research.In this paper,we start from the direction of mesoporous introduction.The glycogen or phenyltriethoxysilaneis used as the template to preparethe hierarchical ZSM-5 and the excellent performance in methanol to aromatics was expected.Firstly,the glycogen tried to be used as the template to prepare hierarchical ZSM-5 and its performance in methanol to aromatics was also investigated.The XRD results showed that the crystallinity of the zeolite was not affected by the presence of glycogen.The results of N₂ adsorption desorption showed that glycogen could promote the formation of mesoporous.The SEM results showed that the existence of glycogen hindered the dimension of crystal growth and changed the morphology of the crystals,which may be the underlying reason for the formation of mesoporous.NH3-TPD results showed that the glycogen also affected the formation of acid sites and its strength distribution.We also compared the performance of the catalysts in the methanol to aromatics.Results showed that the glycogen improvement for the catalyst stability is not very obvious.it is probably because that the hard template glycogen,which is poor water solubility,is not very well near the precursor in the crystallization process and causes thatthe mesoporous generation can not be well near the crystal surface.It is difficult to play a very good to improve mass transfer based on these reason.The BTX selectivity was also increased from 19.55 % to 23.84 % and its reasons may be that the changes of acid properties and specific surface area caused that.The glycogen is a very good role in the introduction of mesoporous and the catalytic performance of the catalyst.In order to avoid the problem of easy phase separation caused by the poor water solubility,the phenyltriethoxysilane,which is not easy to be separated from the precursor,was used as the template.Based on that,the hierarchical ZSM-5 was synthesized and the good catalytic performance was showed in the methanol to aromatics reaction.We also studied the relationship between the amount of the phenyltriethoxysilane and the catalytic performance.We found that the stability of the catalyst showed the trend of increased first and then decreased and up to 126 h.In addition,the BTX selectivity increasing to 33.01 % with the increase of the amount of the phenyltriethoxysilane.In order to explore the reason of different catalytic performance,we do a series of characterization.N₂ adsorption desorption characterization results showed that mesoporous volume or surface area and catalyst stability exhibited the same trend.The thermogravimetric results also showed that the amount of the phenyltriethoxysilane can obviously improve the carbon deposition resistance of the catalyst.The SEM and NH3-TPD results showed that the morphology and acid properties are affected by that.The XRD results showed that the amount of that had no significant effect on the crystallinity of zeolite but that was obviously affected when the amount was too much.Due to the better water solubility of the phenyltriethoxysilane and the participation of the zeolite framework,we have grafted the benzene,an organic group,onto the surface of the molecular sieve by this material,which avoids the phase separation.The mesopores we introduce in this way can improve the mass transfer of the catalyst well and enhance the stability of the catalyst.