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°C 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-5 with 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°C for 4 h under 100% steam atmosphere.