Catalytic Cracking Of Post-Mtbe C₄LPG Over Modified Nano-sized ZSM-5 Zeolite

The effects of nano-sized ZSM-5 zeolite acidity and reaction conditions such as reaction temperature, dilution ratio of water vapor and space velocity on the catalytic cracking performance of butenes in post-MTBE C4LPG were studied in a small fixed bed reactor. By using catalyst characterization means such as ammonia temperature-programmed desorption (NH3-TPD) and pyridine-adsorption infrared spectroscopy (Py-IR), the following results are obtained:(1) The propylene selectivity of the butenes catalytic cracking is mainly limited by hydrogen-transfer and dehydrogenation reactions. These C-H bond related side-reactions produce by-products such as alkanes and aromatics, and are responsible for the coking of the catalyst. The weakening of catalyst acidity can suppress these side-reactions, and even completely stop dehydrogenation, but it is impossible to avoid hydrogen-transfer under the conditions of butene catalytic cracking and even decreases butenes conversion. High reaction temperature favors to limit hydrogen-transfer, but it encourages dehydrogenation reactions to produce aromatics meanwhile.(2) When the catalytic cracking was carried out at 450℃and above, the decrease of butenes conversion occurs because of the disproportionation of propylene to isobutene and ethylene. On the other hand, the chemical equilibrium between propylene and butene, or among butene isomers is another constraint for the low conversion rate of butenes.(3) Suitable dilution ratio of water vapor, high weight hourly space velocity (WHSV) and catalyst with proper acidity and high ratio of B-acid and L-acid (B/L) can suppress the side-reactions effectively during butene catalytic cracking process. As a result, they promote the yield and selectivity of propylene due to diminution of alkanes and C5+ liquid. But the conversion rate of butenes decreases simultaneously because of the mismatching of dimerization and cracking reactions.(4) The best catalytic cracking performance of post-MTBE C4LPG over modified nano-sized ZSM-5 zeolite, which includes propylene selectivity and conversion rate of butenes were 38.73% and 82.02%, respectively, was gained under optimum experiment conditions. Considering the lower propylene selectivity is difficult to improve, we propose that the integrated utilization of the cracking products can make a better profit.