Modification of the acidic and textural properties of ZSM-5 zeolite by using double mineralizers in synthesis and its catalytic performance in the conversion of methanol to propene
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摘要: 采用静态水热法在F--OH-体系中，以四丙基氢氧化铵为模板剂、偏铝酸钠为铝源、正硅酸乙酯为硅源，合成了纳米SiO2-ZSM-5分子筛，考察了F-/Al2O3物质的量比对所合成的ZSM-5分子筛织构性质和甲醇转化制丙烯催化性能的影响。结果发现，随着初始溶胶F-/Al2O3物质的量比的增大，产物中SiO2的含量增大，ZSM-5分子筛的相对结晶度有所降低；同时，分子筛的比表面积和孔容减小、酸强度降低、酸量减少。对于甲醇转化制丙烯，最佳F-/Al2O3物质的量比为12；此时，丙烯选择性高于45%，丙烯/乙烯（P/E）比值大于10。反应机理分析表明，过渡态择形选择性是控制烯烃选择性的重要因素。Abstract: A static hydrothermal approach was adopted to synthesize nanosized SiO2-ZSM-5 zeolite in the media of F--OH- with double mineralizers, using tetraethoxysilane, sodium aluminate, and tetrapropylammonium hydroxide as the silicon source, aluminum source, and template agent, respectively. The physical and chemical properties of the synthesized ZSM-5 zeolites were characterized and their catalytic performance was evaluated in the conversion methanol to propene (MTP); the effect of F-/Al2O3 molar ratio on the catalytic performance of synthesized H-ZSM-5 was investigated. The results indicate that an increase in the F-/Al2O3 molar ratio of the synthesis mixture leads to an increase in the surface content of microcrystalline SiO2, accompanying with a decrease in the relative crystallinity, surface area, pore volume, and acid strength and density. With a F-/Al2O3 molar ratio of 12, the SiO2-ZSM-5 zeolite exhibits the best catalytic performance in MTP, with a selectivity of 45% to propene and a propene/ethene (P/E) ratio of greater than 10. It is further hypothesized that the transition state shape selectivity plays an important role in determining the product selectivity in MTP.
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Key words:
- mineralizer /
- ZSM-5 /
- methanol conversion /
- propene /
- ethene
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Figure 5 Methanol conversion (a), selectivity to ethene (b), selectivity to propene (c), and the P/E ratio (d) for MTP over various H-ZSM-5 zeolites: S-1, S-2, S-3, and S-4
Table 1 Relative crystallinity, SiO2/Al2O3 ratio, and textural properties of H-ZSM-5 prepared with different F-/Al2O3 ratios
Catalyst Relative crystallinitya/% SiO2/Al2O3b Surface area c A/(m2·g-1) Pore volume v/(cm3·g-1) total micro meso totalf microg S-1 100 285 388.0 356.0 32.0 0.2422 0.1774 S-2 64 295 386.7 348.7 37.9 0.2277 0.1740 S-3 63 359 154.8 126.8 27.9 0.1238 0.0657 S-4 23 395 109.7 92.6 17.1 0.0797 0.0476 note: a: the relative crystallinity of ZSM-5 was calculated based on the intensity of the diffraction peaks at 2θ of 22°-25°, using S-1 as the reference; b: the SiO2/Al2O3 ratio was determined by ICP-AES; c: total BET surface area was obtained by the BET method using adsorption data in p/p0 ranging from 0.05 to 0.25, whereas micro and meso-surface areas were determined by the t-plot method; d: the total pore volume was estimated from the adsorbed amount at p/p0 = 0.99, whereas the micro pore volume was determined by the t-plot method 
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