低硅铝比ZSM-5分子筛的合成及其正庚烷裂化反应性能

以氨水为矿化剂，通过添加NH₄⁺离子水热合成了具有较低骨架硅铝比的ZSM-5分子筛。通过X射线衍射(XRD)、扫描电镜(SEM)、固体核磁共振(MAS-NMR)等表征手段，研究了硅源、铝源、矿化剂、阳离子等对ZSM-5分子筛的结晶度、形貌尺寸和骨架硅铝比等的影响，研究了ZSM-5分子筛的骨架硅铝比对正庚烷催化裂化反应的影响。研究表明，投料硅铝比越低，铝原子越难进入到分子筛骨架中；当氨水为矿化剂、正硅酸四乙酯为硅源时可以合成骨架硅铝比较低的氢型ZSM-5分子筛，添加NH₄⁺离子可以增强骨架铝的嵌入，进一步降低分子筛的骨架硅铝比(24.2)。正庚烷裂化反应结果表明，降低分子筛的骨架硅铝比可以提高正庚烷裂化反应的活性，但会降低低碳烯烃的选择性。

Synthesis of ZSM-5 zeolites with low silica-to-alumina ratio and its performance in the cracking of n-heptane

ZSM-5 zeolites with low silica-to-alumina ratios were hydrothermally synthesized by adding NH₄⁺ cation and using ammonia as mineralizer. The effects of Si source, Al source, mineralizer, and cation on the physiochemical properties of the prepared ZSM-5 zeolites were investigated. The crystallinity, crystal size, and silica-to-alumina ratio of zeolites were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM), solid state nuclear magnetic resonance (MAS-NMR), etc. The effect of silica-to-alumina ratio on the catalytic cracking of n-heptane was also studied. The results demonstrate that the lower the silica-to-alumina ratio feeding, the harder it is for Al atoms to insert into zeolite framework. Hydrogen-form ZSM-5 zeolites with low framework silica-to-alumina ratios could be synthesized with ammonia as mineralizer and tetraethyl orthosilicate as Si source. The addition of NH₄⁺ cation enhanced the insertion of Al atom and the framework silica-to-alumina ratio of ZSM-5 further decreased to 24.2. The results of n-heptane cracking showed that the conversion of n-heptane was improved with decreasing the framework silica-to-alumina ratio of ZSM-5 zeolites, however, the selectivity of light olefins was decreased.