不同形貌ZSM-5分子筛的制备及其催化甲醇转化制烯烃性能

采用廉价的原料,通过添加尿素及调节碱度,在水热条件下合成了具有不同形貌的ZSM-5分子筛。研究了分子筛形貌对甲醇转化制丙烯催化性能的影响。结果表明,通过控制尿素含量及碱度,可以对ZSM-5分子筛晶体的生长方向及形貌进行调控。当urea/SiO₂(mol ratio)=0.28,Na₂O/SiO₂(mol ratio)=0.035时,所得到的分子筛沿b轴方向生长最慢,产品形貌呈薄片状,厚度为130 nm;在一定范围内提高碱度,会使分子筛形貌逐渐变为纳米颗粒聚集体。ICP、NH₃-TPD和N₂吸附表征结果表明,所合成的分子筛SiO₂/Al₂O₃物质的量比、酸性质及孔道性质接近。催化反应评价结果表明,薄片状样品HZ-1由于在b轴方向上具有更短的扩散路径,且结晶度高,在催化甲醇转化反应中不仅表现出良好的选择性,双烯(乙烯+丙烯)收率可达60%以上;P/E比(丙烯/乙烯)最高可达8.4,在连续反应200 h后,甲醇转化率仍保持在95%以上,表现出优异的催化稳定性。

Synthesis of ZSM-5 zeolites with different morphology and their catalytic performance in methanol to propylene reactions

Using cheap raw materials, ZSM-5 zeolites with different morphology were hydrothermally synthesized by adding urea and adjusting alkalinity in initial solutions. The effects of zeolite morphology on the catalytic performance in methanol to propylene reactions were studied. The results show that the contents of urea and alkalinity have a great effect on the crystal morphology and growth directions. Specifically, ZSM-5 crystal shows the slowest growth rate along b-axis direction and the crystals of product appear as flakes of about 130 nm in thickness when urea/SiO₂(mol ratio)=0.28 and Na₂O/SiO₂(mol ratio)=0.035. As the content of alkalinity increases within a certain range, the products gradually convert to the congeries of nano-size particles. The characterizations of ICP, NH₃-TPD, and N₂-adsorption show that the SiO₂/Al₂O₃ mol ratios, acidity, and pore structures for all the products are much near to each other. The products of flake shape show many advantages in catalytic reactions: good selectivity in methanol to propylene conversions, high over 60% of olefin (ethylene + propylene), about 8.4 of propylene/ethylene ratio, and superior catalytic stability, being over 95% of methanol conversion for 200 h continuous reaction. The excellent performance may be attributed to the short diffusion path length along b-axis direction and high crystallinity.