ZSM-5分子筛的磷改性作用

选取硅铝比为45、150和250的H-ZSM-5分子筛，通过磷酸氢二铵水溶液饱和浸渍、干燥，以及在550℃的100%水蒸气气氛下水热处理2 h，制备出一系列不同磷负载量的磷改性ZSM-5分子筛样品。采用XRD、XRF、BET、NH₃-TPD、²⁷Al NMR、³¹P NMR等表征方法研究这些样品的磷氧化物分布状态及对骨架铝配位、物相、孔结构和酸性的影响。通过脉冲微反实验评价分析磷改性对ZSM-5分子筛催化甲醇制丙烯（MTP）反应活性和烃类产物选择性的影响。结果表明，ZSM-5分子筛的硅铝比或骨架铝密度对磷氧化物分布状态及对磷改性作用有显著影响。对低硅铝比的ZSM-5分子筛进行磷改性，磷氧化物的聚合程度高，磷改性作用强，磷与骨架铝形成P-O-Al化学配位，导致四配位骨架铝（Al(OSi)₄)的数量减少；磷氧化物不仅分布在分子筛外表面，并且深入到分子筛孔道内部；比表面积和孔体积也随磷负载量增加而大幅度下降，酸量和酸强度同样也有较大幅度降低。对高硅铝比的ZSM-5分子筛进行磷改性，磷氧化物多以单分子磷氧化物或二聚磷氧化物形式分布在分子筛的外表面，比表面积和孔体积受磷负载量的影响极小，酸量和酸强度变化也不明显，磷改性作用较弱。不同硅铝比的磷改性ZSM-5分子筛的甲醇转化催化活性随磷负载量增加到某一数值后均出现突然降低现象，表明活性降低与磷氧化物对分子筛的孔道修饰作用或堵孔有关。通过提高硅铝比或磷改性降低ZSM-5分子筛的酸密度和酸强度，降低了甲醇转化过程的氢转移反应和烯烃裂解反应的活性，有利于提高产物中丙烯和C₄烃的选择性，而乙烯和烷烃选择性降低。

Effect of Phosphorus Modification on ZSM-5 Molecular Sieves

P-modified ZSM-5 samples with n(SiO₂)/n(Al₂O₃) of 45, 150 and 250 were prepared by saturating impregnation with aqueous solutions of (NH₄)₂HPO₄ according to different n(P)/n(Al), followed by drying and hydrothermal treatment at 550℃ for 2 h in 100% steam. Those samples were characterized by means of XRD, XRF, BET, NH₃-TPD and ²⁷Al NMR, ³¹P NMR to analyze the existence of phosphorus oxides and their effects on framework Al coordination, phase, pore structure and acidity parameter. The catalytic performances of those P-modified ZSM-5 samples in methanol to propylene (MTP) reaction were evaluated by pulse-feed micro-reactor experiments. The results showed that framework Al density or n(SiO₂)/n(Al₂O₃)of P-modified ZSM-5 determined the existence and effect of phosphorus oxides. P-modified ZSM-5 with low n(SiO₂)/n(Al₂O₃) lost more tetrahedral framework Al as Al(OSi)₄, which transformed into several sorts of Al_dis-f and Al_non-f by coordinated with phosphorus oxides mainly as polyphosphates due to strong interaction of the zeolite framework with phosphorus loading rising, and also lost more acid strength and capacity and more specific surface area and pore volume, because those phosphorus oxides not only distribute on the outer surface but also penetrate into the depth of channels of zeolite. While P-modified ZSM-5 with high n(SiO₂)/n(Al₂O₃) lost less tetrahedral framework Al as Al(OSi)₄ and lost less acid strength and capacity and less specific surface area and pore volume owing to weak interaction of phosphorus oxides with the zeolite framework, majority of which as free orthophosphate or diphosphate mainly exposed on the outer surface of zeolite with the phosphorus loading rising. The phenomena that the catalytic activity of P-modified ZSM-5 with different n(SiO₂)/n(Al₂O₃) dropped abruptly in methanol conversion when phosphorus loading reached some critical value mainly attributed to the blockage or narrow down of channel or opening of zeolite with phosphorus oxides. It was also found that catalyzed by the ZSM-5 with high n(SiO₂)/n(Al₂O₃) or modified with phosphorus oxides having less acid strength and capacity, the selectivities of propylene and C₄ hydrocarbon were increased and the amounts of ethylene and alkanes were decreased in methanol conversion due to their low catalytic activity of hydrogen transfer and cracking of alkenes.