磷改性ZSM-5分子筛的水热稳定性

选取n(SiO₂)/n(Al₂O₃)为45、150、250的磷改性ZSM-5分子筛样品，在800℃和100%水蒸气气氛下分别水热处理4、8和17 h，得到一系列高温水热老化后的磷改性ZSM 5分子筛样品。采用XRD、XRF、BET、NH3-TPD、³¹P NMR、²⁷Al NMR表征方法研究这些老化样品的磷氧化物状态变化及骨架铝配位、物相、孔结构和酸性变化情况。通过脉冲微反实验考察这些老化样品的甲醇制丙烯（MTP）反应催化性能稳定性。结果表明，ZSM 5的n(SiO₂)/n(Al₂O₃)对磷改性作用和水热稳定性有重要影响，提高水热老化温度和延长老化时间会明显加剧磷氧化物的迁移、缩合及与骨架铝的配位成键作用。低n(SiO₂)/n(Al₂O₃)时，四配位骨架铝容易转变为与磷有配位关系的各类缺陷位骨架铝和六配位非骨架铝；高n(SiO₂)/n(Al₂O₃)时，尤其在n(SiO₂)/n(Al₂O₃)为150时，存在大量骨架铝既具有很高的抗磷氧化物配位成键能力，又具有很好的抗水蒸气水解能力，具有更高的反应催化活性和水热稳定性。提高水热处理温度还具有疏通磷改性ZSM-5分子筛孔道的作用，低n(SiO₂)/n(Al₂O₃)的磷改性ZSM-5分子筛的比表面积和孔体积有部分恢复，高n(SiO₂)/n(Al₂O₃)的微孔部分比表面积有所增加。不同n(SiO₂)/n(Al₂O₃)磷改性ZSM-5分子筛的相对结晶度均随水热老化温度提高和老化时间延长而下降；酸量和酸强度均随水热老化温度升高而急剧下降，但随水热老化时间延长保持相对稳定，n(SiO₂)/n(Al₂O₃)为150的磷改性ZSM 5分子筛的酸性最稳定。n(SiO₂)/n(Al₂O₃)为150的磷改性ZSM-5分子筛经过800℃和100%水蒸气老化4 h后具有优异的MTP催化性能，如高反应活性、高丙烯选择性和烃产物组成稳定性。

Hydrothermal Stability of P-modified ZSM-5 Molecular Sieves

Hydrothermally aged P-modified ZSM-5 samples with n(SiO₂)/n(Al₂O₃) of 45, 150 and 250 and different n(P)/n(Al) were prepared by hydrothermal treatment at 800℃ for 4 h, 8 h and 17 h under 100% steam atmosphere, respectively, and characterized by means of XRD, XRF, BET, NH₃-TPD, and ³¹P NMR, ²⁷Al NMR to analyze the existence and effect of phosphorus oxides on framework aluminum coordination, phase, pore structure and acidity parameters. Catalytic performances of those samples in methanol to propylene (MTP) reaction, such as hydrothermal stability concerned on catalytic activity and hydrocarbon selectivity and composition, were evaluated by pulse-feed micro-reactor experiments. The results showed that n(SiO₂)/n(Al₂O₃) was a critical factor in improving the effects of phosphorus modification and the hydrothermal stability of P-modified ZSM-5. High temperature and long time in hydrothermal treatment strengthened the migration, condensation of phosphorus oxides and their interaction with framework aluminum. P-modified  ZSM-5 with low n(SiO₂)/n(Al₂O₃)  tended to lose most tetrahedral framework aluminum, which transformed into several sorts of dislodged aluminum and non-framework aluminum by strong coordinated with phosphorus oxides mainly as polyphosphates in its hydrothermal treatment of elevated temperature and prolonged time, while those with high n(SiO₂)/n(Al₂O₃) , especially with n(SiO₂)/n(Al₂O₃)  of 150, kept high ratio of tetrahedral framework aluminum which had both resistance of hydrolysis by steam and coordination with phosphorus oxides. P-modified ZSM-5 samples were unclogged with elevated temperature steam, so that part of specific surface area and pore volume picked up with low n(SiO₂)/n(Al₂O₃) while specific micropore surface area increased slightly with high n(SiO₂)/n(Al₂O₃) . The relative crystallinity of P-modified ZSM-5 with different n(SiO₂)/n(Al₂O₃)  declined with the elevated temperature and prolonged time of the hydrothermal treatment, and their acid strength and capacity declined abruptly with the elevated temperature, but kept relatively stable with the prolonged time in hydrothermal treatment, especially for the P-modified ZSM-5 with n(SiO₂)/n(Al₂O₃)  of 150. The excellent MTP performance of P-modified ZSM-5with n(SiO₂)/n(Al₂O₃)  of 150 was obtained, such as high catalytic activity and propylene selectivity, and stable hydrocarbon composition, after its hydrothermally aged at 800℃ for 4 h under 100% steam atmosphere.