Methanol-to-Olefin Conversion over Small-Pore DDR Zeolites: Tuning the Propylene Selectivity via the Olefin-Based Catalytic Cycle,ACS Catalysis

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Methanol-to-Olefin Conversion over Small-Pore DDR Zeolites: Tuning the Propylene Selectivity via the Olefin-Based Catalytic Cycle
ACS Catalysis ( IF 11.3 ) Pub Date : 2020-02-17 , DOI: 10.1021/acscatal.9b05521
Jia Hua ₁ , Xinglong Dong ₂ , Jianjian Wang ₃ , Cailing Chen ₂ , Zhan Shi ₁ , Zhaohui Liu _{2,

3} , Yu Han ₂

Affiliation

Zeolites that have ultrasmall pore sizes usually exhibit high selectivity toward light olefins when used as catalysts for the conversion of methanol to hydrocarbons (MTH). However, tuning the selectivity of a desired product in a controllable manner remains difficult. In this study, we performed MTH reactions on DDR zeolites at 400 °C and subcomplete methanol conversions to investigate how the highly constrained pore structure of a DDR zeolite impacts on the reaction pathways and the product selectivity. We find that the propylene selectivity monotonically increases as the aluminum content and crystallize size of the DDR zeolite decreases. With this understanding, we are able to maximize the propylene selectivity up to 50.6% by modifying the DDR zeolite. We performed isotopic tracing experiments and analysis of residual species in the catalysts to validate the dual-cycle (aromatic- and olefin-based cycle) mechanism in DDR zeolite-catalyzed MTH and demonstrate that propylene is primarily produced from the olefin-based cycle. We attribute the unique behavior of DDR zeolites to that their ultrasmall pore sizes impose restriction on the molecular diffusion of higher olefins, leading to a more important role of the olefin-based catalytic cycle in the reaction, in comparison with other zeolites with larger pores.

中文翻译：

小孔DDR沸石上的甲醇制烯烃转化：通过基于烯烃的催化循环调节丙烯的选择性

当用作甲醇转化为碳氢化合物（MTH）的催化剂时，具有超小孔径的沸石通常表现出对轻质烯烃的高选择性。但是，以可控的方式调节所需产物的选择性仍然很困难。在这项研究中，我们在400°C和不完全甲醇转化率下对DDR沸石进行了MTH反应，以研究DDR沸石高度受限的孔结构如何影响反应路径和产物选择性。我们发现，丙烯选择性随铝含量和DDR沸石结晶尺寸的降低而单调增加。有了这种认识，我们就可以通过改性DDR沸石使丙烯选择性最大化，达到50.6％。我们进行了同位素示踪实验并分析了催化剂中的残留物种，以验证DDR沸石催化的MTH中的双循环（芳烃和烯烃基循环）机理，并证明了丙烯主要来自基于烯烃的循环。我们将DDR沸石的独特行为归因于其超小孔径限制了高级烯烃的分子扩散，与其他具有较大孔径的沸石相比，它们导致了基于烯烃的催化循环在反应中的重要作用。

更新日期：2020-02-17

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