物理化学学报 >> 2019, Vol. 35 >> Issue (11): 1248-1258.doi: 10.3866/PKU.WHXB201901062

论文 上一篇    下一篇

不同B,Al分布对ZSM-5分子筛的甲醇制丙烯反应性能的影响

翟岩亮,张少龙,张络明,尚蕴山,王文轩,宋宇,姜彩彤,巩雁军*()   

  • 收稿日期:2019-01-24 录用日期:2019-03-04 发布日期:2019-03-08
  • 通讯作者: 巩雁军 E-mail:gongyj@cup.edu.cn
  • 基金资助:
    国家自然科学基金(U1662116);国家自然科学基金(21276278)

Effect of B and Al Distribution in ZSM-5 Zeolite on Methanol to Propylene Reaction Performance

Yanliang ZHAI,Shaolong ZHANG,Luoming ZHANG,Yunshan SHANG,Wenxuan WANG,Yu SONG,Caitong JIANG,Yanjun GONG*()   

  • Received:2019-01-24 Accepted:2019-03-04 Published:2019-03-08
  • Contact: Yanjun GONG E-mail:gongyj@cup.edu.cn
  • Supported by:
    the National Natural Science Foundation of China(U1662116);the National Natural Science Foundation of China(21276278)

摘要:

本文设计了两个系列的硼改性ZSM-5分子筛:一步法合成的B-Al-ZSM-5系列分子筛和两步法合成的Al-ZSM-5@B-ZSM-5核壳分子筛。通过X射线衍射(XRD)、扫描电子显微镜(SEM)、扫描透射电子显微镜面扫(STEM mapping)、N2物理吸附脱附、氨气程序升温脱附(NH3-TPD)、1, 3, 5-三异丙基苯(TIPB)裂解等表征发现,两个系列的样品中B1-Al-ZSM-5和Al@B1-ZSM-5,B2-Al-ZSM-5和Al@B2-ZSM-5以及B3-Al-ZSM-5和Al@B3-ZSM-5分别具有相似的织构性质、强弱酸量、酸强度和比例,以及不同的B、Al元素分布和酸分布。我们用这两个系列样品对比研究不同的强弱酸分布-强弱酸均匀分布和梯度分布对甲醇制丙烯(MTP)反应性能的影响。通过研究发现,强弱酸均匀分布的样品具有更高的丙烯选择性,归因于更低的整体强弱酸密度;而强弱酸梯度分布的样品具有更长的MTP反应寿命,归因于外表面上更低的强酸密度和更高的弱酸密度。

关键词: B-Al-ZSM-5, Al-ZSM-5@B-ZSM-5, 核壳分子筛, 酸分布, 甲醇制丙烯

Abstract:

Propylene is widely used as a raw material for producing polypropylene, acrylonitrile, propylene oxide, etc. Typical manufacturing processes for propylene (steam cracking and FCC process) are over-reliant on petroleum resources and cannot meet the rapidly growing global demands. New routes for producing propylene from non-oil resources, particularly methanol-to-propylene (MTP) technology, have attracted increasingly more attention, where a fixed-bed reactor is used and ZSM-5 zeolite is the best alternative catalyst. However, structural optimization of ZSM-5 to enhance the lifetime and propylene selectivity and a deep understanding of the mechanism of the MTP reaction are still considerable challenges. For the conventional ZSM-5 zeolite, carbon deposition preferentially occurs near the outer surface of the zeolite particles because of the high acid density on the external surface, which accelerates the deactivation by blocking the outer pore openings, especially in a long-term MTP reaction. Large amounts of external strong acids also promote secondary reactions, such as hydrogen transfer reactions, resulting in a decrease in propylene selectivity. To study the effects of strong and weak acid distributions of ZSM-5 zeolite on the MTP reaction, two series of boron-modified ZSM-5 zeolites were designed: B-Al-ZSM-5 zeolites by one-step synthesis and Al-ZSM-5@B-ZSM-5 core-shell zeolites by two-step synthesis. These were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) mapping, N2 physical adsorption-desorption, temperature-programmed desorption of ammonia (NH3-TPD) and 1, 3, 5-triisopropylbenzene (TIPB) cracking, and B1-Al-ZSM-5 and Al@B1-ZSM-5, B2-Al-ZSM-5 and Al@B2-ZSM-5, and B3-Al-ZSM-5 and Al@B3-ZSM-5 samples in the two series were found to have similar texture properties, acid amounts and acid strengths, but different B and Al elemental distributions and acid distributions. We used these two sets of samples to compare the effect of different strong and weak acid distributions—a uniform distribution and a gradient distribution of strong and weak acids on the performance of the MTP reaction. The results showed that samples with a uniform distribution of strong and weak acids have higher propylene selectivity due to lower strong and weak acid densities, whereas samples with a gradient acid distribution have a longer catalytic lifetime in the MTP reaction due to the absence of strong acid density and higher weak acid density on the outer surface. The different acid distributions lead to two different carbon deposition modes. Carbon deposition of the sample with the uniform acid distribution preferentially formed on the outer surface, resulting in rapid deactivation by blocking external micropores and leaving the internal active centers not fully utilized. However, for the sample with the gradient acid distribution, the carbon-blocking rate of the external surface considerably decreased, which increased the time that the reactant molecules had to enter the internal micropores. Thus, the utilization rate of the active centers and the catalytic lifetime of the Al-ZSM-5@B-ZSM-5 core-shell sample considerably increased.

Key words: B-Al-ZSM-5, Al-ZSM-5@B-ZSM-5, Core-shell zeolite, Acid distribution, Methanol to propylene

MSC2000: 

  • O643