Turke Kevin, Meinusch Rafael, Cop Pascal, Prates da Costa Eric, Brand Raoul D, Henss Anja, Schreiner Peter R, Smarsly Bernd M
Institute of Physical Chemistry, Justus Liebig University Giessen, Heinrich-Buff-Ring 17, D-35392 Giessen, Germany.
Center for Materials Research, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany.
ACS Omega. 2020 Dec 21;6(1):425-437. doi: 10.1021/acsomega.0c04857. eCollection 2021 Jan 12.
Porous carrier materials functionalized with organocatalysts offer substantial advantages compared to homogeneous catalysts, e.g., easy separation of the catalyst, scalability, and an improved implementation in continuous operations. Here, we report the immobilization of (3-aminopropyl)trimethoxysilane (APTMS) onto self-prepared silica monoliths and its application as a heterogeneous catalyst in the Knoevenagel condensation between cyano ethylacetate and various aromatic aldehydes under continuous-flow conditions. The meso-macroporous silica monoliths (6-7 cm in length) were optimized to be used in flow taking advantage of their hierarchical meso- and macroporosity. The monoliths were cladded with a poly(ether ether ketone) (PEEK) tube by a refined procedure to guarantee tight connection between the carrier material and PEEK. Functionalization of the bare silica monoliths consisting of APTMS can be efficiently performed in flow in ethanol and toluene. While a large grafting gradient is obtained for toluene, the grafting in ethanol proceeds homogenously throughout the monolith, as evidenced by elemental analysis and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The silica monoliths exhibit high conversion up to 95% with concurrent low back pressures, which is of importance in flow catalysis. By connecting two monoliths, high conversions can be maintained for several flow rates. Two types of monoliths were synthesized, possessing different mesopore sizes. The monolith bearing the larger mesopore size showed an enhanced turnover frequency (TOF), while the monolith with the smaller mesopores allowed for larger quantities of the product to be synthesized, due to the higher surface area. A long-term stability test showed that the functionalized monoliths were still active after 66 h of continuous usage, while the overall yield decreased over time.
与均相催化剂相比，用有机催化剂功能化的多孔载体材料具有显著优势，例如催化剂易于分离、可扩展性强以及在连续操作中实施效果更佳。在此，我们报告了将（3-氨基丙基）三甲氧基硅烷（APTMS）固定在自制的硅胶整体柱上，并将其作为多相催化剂用于氰基乙酸乙酯与各种芳香醛在连续流动条件下的Knoevenagel缩合反应。优化后的介孔-大孔硅胶整体柱（长度为6 - 7厘米）利用其分级的介孔和大孔结构用于流动反应。通过精细工艺将整体柱包覆在聚醚醚酮（PEEK）管中，以确保载体材料与PEEK之间紧密连接。由APTMS组成的裸硅胶整体柱的功能化可以在乙醇和甲苯中通过流动高效进行。虽然甲苯获得了较大的接枝梯度，但乙醇中的接枝在整个整体柱中均匀进行，这通过元素分析和飞行时间二次离子质谱（ToF-SIMS）得到了证明。硅胶整体柱在高达95%的转化率下同时具有低背压，这在流动催化中很重要。通过连接两个整体柱，在几种流速下都能保持高转化率。合成了两种具有不同介孔尺寸的整体柱。具有较大介孔尺寸的整体柱显示出更高的周转频率（TOF），而具有较小介孔的整体柱由于表面积较大，能够合成更多量的产物。长期稳定性测试表明，功能化的整体柱在连续使用66小时后仍具有活性，而总产率随时间下降。
