High-silica nanocrystalline ZSM-5 zeolites with various amounts of boron have been synthesized by one-step hydrothermal method (denoted as Bx-ZSM-5, x = 0, 1, 2, 3 corresponding to the boron content of 0, 0.5%, 1.0% and 1.5%, respectively), which have similar cylindrical morphology and crystal size (ca. 150nm). NH3-TPD results show that, with the increase of boron content, both the strength and concentration of strong acid sites of the four samples are close, but the strength of weak acid sites gradually decreases, the concentration of weak acid sites gradually increases. This is due to the incorporation of Si-OH-B groups with weaker acid strength. The catalytic performance was investigated in methanol to propylene (MTP) reaction under conditions (WHSV = 3.0 h−1, T = 743K, n (CH3OH) : n (H2O) = 1 : 1). Among the samples, propylene selectivity are the lowest for B0-ZSM-5 without boron. With increasing boron content, the catalytic lifetime and the highest propylene selectivity of B-Al-ZSM-5 catalysts increase firstly and then decrease, the butylene selectivity gradually increases, while the C1-C4 alkanes, ethylene and C5+ selectivities gradually decrease. This is due to the increasing weak acidity in a proper range promotes the olefin cycle and suppresses the hydrogen transfer reaction and aromatic cycle, resulting in the decrease of C1-C4 alkanes, ethylene and C5+ selectivities and increase of propylene, butylene selectivities and catalytic lifetime. The optimal sample B2-ZSM-5 has the longest lifetime of 216 h and the highest propylene selectivity of 50.4%, compared with the values of parent B0-ZSM-5 (87 h and 46.1%). The significant improvement of catalytic performance in MTP reaction mainly attributes to the proper strong and weak acid properties of ZSM-5 zeolite with optimized boron content.

Effect of various amounts of boron on the catalytic performance of nanocrystalline ZSM-5 zeolites in methanol to propylene reaction

2019 Spring National Meeting

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