MFI zeolite coating with intrazeolitic aluminum (acidic) gradient supported on SiC foams to improve the methanol-to-propylene (MTP) reaction

To hinder the deactivation and improve the propylene selectivity in the methanol-to-propylene (MTP) reaction, MFI coating with the intrazeolitic aluminum (acidic) gradient supported on SiC foam support (G-MFI/SiC foam) was proposed. The solid polycrystalline silicon was used in the synthesis of G-MFI/SiC foam catalyst provided a prolonged release of silica nutrient in the liquid phase and suppressed the precipitation phenomena. The resulting MFI coating showed the aluminum gradient along the surface normal direction of SiC foams with ZSM-5 layer (about 20 μm) near the SiC surface followed by the silicalite-1 layer (about 10 μm). The alumina (acidic) gradient in the MFI coating renders a passive outer layer of silicalite-1 with fairly large amount of weak and medium acid sites prevented the coke formation as well as promoted the selectivity to propylene in the MTP reaction. Compared to the conventional ZSM-5/SiC foam catalyst, the G-MFI/SiC foam catalyst showed excellent performance in the MTP reaction with good catalytic longevity (8 h vs. 76 h for >95% methanol conversion) and low coke deposition (6.7 × 10−3 wt.% h−1vs. 0.26 wt.% h−1), as well as high propylene selectivity (ca. 36% vs. 46%).