镁改性对HZSM-5分子筛催化乙烯制丙烯的影响
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摘要
采用等体积浸渍法制备了一系列不同Mg含量(0–1.0%)的HZSM-5分子筛。利用X射线衍射(XRD)、N_2吸附/脱附、铝魔角旋转固体核磁共振(~(27)AlMASNMR)、~(29)SiMASNMR、氨-程序升温脱附(NH_3-TPD)和吡啶吸附傅里叶变换红外(Pyridine-IR)光谱等技术对改性前后样品的结构和酸性进行了详细表征,在常压连续流动固定床反应器上考察其对乙烯转化制丙烯(ETP)反应的催化性能,评价了反应条件和Mg改性的影响。结果表明,在温度为550°C、乙烯体积空速GHSV=3000 h~(-1)的适宜反应条件下,0.5%适量镁改性HZSM-5导致乙烯转化率有所下降,但丙烯选择性增加到45%以上,而副产物芳烃的选择性降到8%以下。反应前样品的酸性表征和反应后积碳样品的TPO及~(13)CCP/MASNMR谱图分析表明适量镁改性使HZSM-5分子筛的总酸量和强B酸量减少而中强酸量增加,从而提高了丙烯的选择性,但是过量的镁改性使分子筛的总酸量明显减少,导致催化剂的活性显著下降。
The ever-increasing demand for propene has driven some new strategies to produce propene, such as propane dehydrogenation, metathesis of ethene and 2-butene, catalytic cracking of C_4 alkenes, conversion of methanol and ethanol, and direct conversion of ethene, instead of the conventional naphtha-cracking process. The transformation of ethene to propene(ETP) is of great interest owing to the abundant supply of ethylene from ethane crackers and shale gas recently. HZSM-5 zeolite is an effective ETP catalyst and the acid properties of HZSM-5 significantly affect ethene conversion and propeneselectivity. A series of HZSM-5 zeolites modified with different amounts of Mg(0–1.0%) were prepared by an incipient impregnation method. Their structures and acidity were systematically characterized by X-ray diffraction(XRD), N_2–adsorption/desorption analysis, ~(27) Al magic-angle spinning nuclear-magnetic-resonance(27 Al MAS NMR), ~(29) Si MAS NMR, temperature-programmed desorption of NH_3(NH_3-TPD), and Fourier transform infrared(FT-IR) of pyridine adsorption techniques. Their catalytic performances in the direct conversion of ethene to propene using a continuous-flow fixed-bed micro-reactor were evaluated. The effects of reaction conditions and Mg contents were thoroughly investigated. Ethene conversion is found to decrease with increasing reaction temperature and gaseous hourly space velocity(GHSV) of HZSM-5 zeolite, while the selectivities of propene show the opposite trend. The optimized reaction temperature is 550 °C and GHSV is 3000 h~(-1) for the maximum propene yield. After Mg modification, ethene conversion decreases, while propene selectivity increases for HZSM-5 with increasing Mg loading under the optimized reaction conditions. HZSM-5 modified with an appropriate amount of 0.5% Mg shows enhanced selectivity of propene exceeding 45%, and the propene yield is maintained above 20%. Moreover, the selectivity of by-product aromatics is less than 8%. ~(27) Al and ~(29) Si MAS NMR results indicate that the introduction of Mg will result in dealumination in HZSM-5. NH_3-TPD and pyridine-IR results indicate that the addition of Mg will reduce both the total acid amount and strong Br?nsted acid amount and that the proper amount of Mg leads to increase in the amount of medium-strong acid. By-products such as alkanes and aromatics are mainly formed at these strong Br?nsted acid sites by oligomerization and hydrogen transfer reactions. Therefore, the introduction of Mg decreases the number of strong Br?nsted acid sites and further enhances the selectivity of propene. Temperatureprogrammed oxidation(TPO) and ~(13)C CP/MAS NMR analysis of the coked catalysts indicate that the addition of Mg not only inhibits coke deposition, but also changes the type of coke. However, excessive Mg modification results in a remarkable reduction of HZSM-5 activity owing to the significant decrease in the number of total acid and strong Br?nsted acid sites.
The ever-increasing demand for propene has driven some new strategies to produce propene, such as propane dehydrogenation, metathesis of ethene and 2-butene, catalytic cracking of C_4 alkenes, conversion of methanol and ethanol, and direct conversion of ethene, instead of the conventional naphtha-cracking process. The transformation of ethene to propene(ETP) is of great interest owing to the abundant supply of ethylene from ethane crackers and shale gas recently. HZSM-5 zeolite is an effective ETP catalyst and the acid properties of HZSM-5 significantly affect ethene conversion and propeneselectivity. A series of HZSM-5 zeolites modified with different amounts of Mg(0–1.0%) were prepared by an incipient impregnation method. Their structures and acidity were systematically characterized by X-ray diffraction(XRD), N_2–adsorption/desorption analysis, ~(27) Al magic-angle spinning nuclear-magnetic-resonance(27 Al MAS NMR), ~(29) Si MAS NMR, temperature-programmed desorption of NH_3(NH_3-TPD), and Fourier transform infrared(FT-IR) of pyridine adsorption techniques. Their catalytic performances in the direct conversion of ethene to propene using a continuous-flow fixed-bed micro-reactor were evaluated. The effects of reaction conditions and Mg contents were thoroughly investigated. Ethene conversion is found to decrease with increasing reaction temperature and gaseous hourly space velocity(GHSV) of HZSM-5 zeolite, while the selectivities of propene show the opposite trend. The optimized reaction temperature is 550 °C and GHSV is 3000 h~(-1) for the maximum propene yield. After Mg modification, ethene conversion decreases, while propene selectivity increases for HZSM-5 with increasing Mg loading under the optimized reaction conditions. HZSM-5 modified with an appropriate amount of 0.5% Mg shows enhanced selectivity of propene exceeding 45%, and the propene yield is maintained above 20%. Moreover, the selectivity of by-product aromatics is less than 8%. ~(27) Al and ~(29) Si MAS NMR results indicate that the introduction of Mg will result in dealumination in HZSM-5. NH_3-TPD and pyridine-IR results indicate that the addition of Mg will reduce both the total acid amount and strong Br?nsted acid amount and that the proper amount of Mg leads to increase in the amount of medium-strong acid. By-products such as alkanes and aromatics are mainly formed at these strong Br?nsted acid sites by oligomerization and hydrogen transfer reactions. Therefore, the introduction of Mg decreases the number of strong Br?nsted acid sites and further enhances the selectivity of propene. Temperatureprogrammed oxidation(TPO) and ~(13)C CP/MAS NMR analysis of the coked catalysts indicate that the addition of Mg not only inhibits coke deposition, but also changes the type of coke. However, excessive Mg modification results in a remarkable reduction of HZSM-5 activity owing to the significant decrease in the number of total acid and strong Br?nsted acid sites.
引文
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