Effects of boron modification on the activity of HZSM-5 toward MTP

BHZSM-5 zeolite was synthesized using one-step hydrothermal crystallization by changing SiO₂/Al₂O₃ (100 and 200) with B₂O₃/Al₂O₃=1. The catalytic activity for methanol to propylene (MTP) was studied. Boron modification lead to increased propylene selectivity and improved stability. Boron modification reduced the amount of the strong Brønsted (B) acid sites. Subjected to hydrothermal treatment at 480 ℃, BHZSM-5 remained 50% of the amount of the strong B acid sites, higher than that of HZSM-5, showing enhanced hydrothermal stability. The distribution of the framework Al also changed. The Al located in the straight and the sinusoidal channels of the ZSM-5 crystal was stable whereas those at the channel intersections was easy to be removed, favoring the MTP activity via the olefin cycle mechanism. As the hydrothermal treatment velocity was increased from 1 h^-1 to 9 h^-1, the B acid sites amount of the BHZSM-5 further decreased and more Al at the intersections was removed.