Synthesis Of M/HZSM-5 Catalysts And Their Application In The Methanol-to-Propylene Reaction

Propylene serves as an essential feedstock in petrochemical industry,which is widely used to produce polypropylene,acrylonitrile,propylene oxide and many other substitutes for non-plastic materials such as paper,steel and wood.As the byproduct of ethane in the traditional process,supply-demand relationship of propylene becomes on edge because of the rapidly increasing demand of propylene. To avoid using the non-renewable petroleum resources as raw materials,the Methanol-to-Propylene（MTP） technology provides a new route to convert coal or natural gas into propylene,which is a new way to solve the energy problem when petroleum resources dry up in the future.Due to the above reasons,the MTP process has attracted much interest.Though the MTP process is based on the Methanol-to-Gasoline process which has been proposed for 36 years,the process is still facing many problems such as heat transfer,mass transfer,coking and other problems caused by the nature of the reaction itself,which limited its application in industry.The mechanism of the MTP reaction has been under dispute till now.The reaction conditions and rules are necessary to be disscussed.The stability and propylene selectivity has a large potential to be enhanced.Hence it is necessary to explore the mechanism of reaction process and the catalytic behaviors of catalysts. It is still a challenge to enhance the selectivity of propylene and the stability in the MTP reaction process.Our work focuses on the following field:（1） study of catalytic performance of HZSM-5 catalysts in the MTP reaction,and optimization of the reaction factors and the Si/Al ratio;（2） Synthesis of P modified HZSM-5 catalysts by impregnation method and evaluate their activities in the MTP reaction;（3） investigation on the properties of W,Ce and Ga modified HZSM-5 catalysts and their performance in the MTP reaction;（4） study on controlling of crystal size for the effect on propylene selectivity and catalytic stability.1.Investigation on the performance of HZSM-5 catalyst in the MTP reaction and the catalytic process.Chemical thermodynamic analysis shows that the MTP process is a strong exothermic reaction.So a large amount steam was used to remove reaction heat. The influence of Si/Al ratio,temperature and space velocity（LHSV） on the catalytic performance was investigated in the MTP reaction over HZSM-5 catalyst synthesized by tradition method.It is shown that the proper Si/Al ratio for propylene formation is around 220 and the optimized reaction conditions are 460℃and 0.75 h^-1.Under the above reaction conditions,the conversion of methanol/DME is 100%, while propylene selectivity is up to 46.4%with total C₂-C₄ alkene selectivity up to 72.1%.2.Synthesis of modified HZSM-5 catalysts and their performance in the MTP reaction.HZSM-5（220） catalyst was modified by impregnation method using a series of different promoters.Catalytic tests under the optimized conditions show that 0.1% P modification led to the highest propylene selectivity.Propylene selectivity was enhanced from 46.4%to 55.6%with methanol/DME completely converted.And C₂-C₄ alkene selectivity reached 79.4%.Compared with the results before P modification,the stability was remarkably enhanced.Selectivity of aromatics were less than 0.5%for 100h.Characterization results indicated that P modification led to partial elimination of the strong acid sites on the HZSM-5 catalyst,pore size and pore volume were increased without obvious damage of the MFI structure,which resulted enhancement of shape selectivity for propylene and remarkably prolonged catalytic life.TGA results revealed that coking amount on HZSM-5 catalyst decreased from 1.05 wt%to 0.61 wt%after 100h testing. 3.Preparation,characterization and catalytic performance of supported XHZSM-5（X=W,Ce and Ga） catalysts.Elements of W,Ce or Ga were selected as promoters to modify HZSM-5 catalyst. W modification diminished the micropore volume and effectively enhanced the shape selectivity for propylene.After 1%W modification,propylene selectivity was increased from 46.4%to 50.8%with the P/E ratio increasing from 6.2 to 9.3.C₅ products were suppressed and C₂⁼-C₄⁼ products were enhanced.After 1%Ce modification,packing pore size was decreased with decreasing of the amount of BrΦnsted acid sites.Propylene selectivity increased from 46.4%to 51.4%with decreasing of C₅ product selectivity.Ga modification can remarkably enhance the amount of BrΦnsted acid sites and introduce the dehydrogenation activity.The MTP results show that propylene,butylene（including isomers） and C₅ products were suppressed with the increasing of aromatics.The dehydrogenation activity of Ga₂O₃ exhibited as the increasing of ethene in the final products.4.Synthesis of nano HZSM-5 materials and its application in the MTP reaction.Nano HZSM-5 zeolites with different Si/Al ratios（200nm） were synthesized and applied in the MTP reaction.The effect of Si/Al ratio of nano HZSM-5 catalyst is similar with that of traditional HZSM-5 catalyst.However,the aromatic selectivity is much higher than nomal HZSM-5 catalyst and P/E ratio decrease greatly.The conversion is 98%at rather a high space velocity of 2.24h^-1 LHSV over nano HZSM-5 ctalyst with Si/Al ratio of 316.HZSM-5 zeolites with different sizes（200nm and 1μm） and nearly the same Si/Al ratio（93-95） were synthesized and studied for catalytic test.It is shown that HZSM-5 catalyst with relative smaller crystal size exhibited higher activity and stability.HZSM-5 catalyst with relative larger crystal size showed better selectivity of C₂⁼-C₄⁼ products with a rapid deactivation rate at high space velocity.Besides,uniform size of catalyst is important to achieve high stability in the MTP reaction.