van Voorthuizen Thijmen A, van der Veen Monique A, de Smet Louis C P M, Smulders Maarten M J
Laboratory of Organic Chemistry, Wageningen University & Research, Wageningen, 6708 WE, The Netherlands.
Department of Chemical Engineering, Delft University of Technology, Delft, 2629 HZ, The Netherlands.
Chemistry. 2025 Jul 11;31(39):e202501512. doi: 10.1002/chem.202501512. Epub 2025 Jun 18.
Due to the irreversible tautomerization of imine linkages to their corresponding ketoenamines, β-ketoenamine-linked Covalent Organic Frameworks (COFs) are a stable type of COF that displays high surface areas. In the solvothermal synthesis of such COFs, the use of (acetic) acid is ubiquitous. However, the effect of the added acid on the COF properties (notably their surface area) has never been investigated. Building on an extensive literature overview, we systematically studied the effect of the pK of several added acids on COF performance characteristics and extended the investigation by including a series of (organo-)bases with varying pK. Interestingly, the highest BET surface areas, above 1400 m/g, were found in the alkaline region of the pK window, with a maximum near pK ∼10.8 for triethylamine (TEA) and N,N-diisopropylethylamine (DIPEA). Considering the pK values related to the three phenolic hydroxyl groups of 2,4,6-triformylphloroglucinol, one of the COF building blocks, these organobases fully deprotonate two of these hydroxyl groups and partly deprotonate the third one, which optimizes the reaction rate of the β-ketoenamine bond formation, explaining the improved COF crystallinity and associated microporosity. The largely overlooked use of organobases in the synthesis of β-ketoenamine-linked COFs thus offers a promising approach to improve the COF performance.
由于亚胺键会不可逆地互变异构为相应的酮烯胺，β-酮烯胺连接的共价有机框架（COF）是一种稳定的COF类型，具有较高的比表面积。在这种COF的溶剂热合成中，（乙酸）的使用很普遍。然而，添加的酸对COF性能（尤其是其比表面积）的影响从未被研究过。基于广泛的文献综述，我们系统地研究了几种添加酸的pK对COF性能特征的影响，并通过纳入一系列具有不同pK的（有机）碱来扩展研究。有趣的是，在pK窗口的碱性区域发现了最高的BET比表面积，高于1400 m²/g，三乙胺（TEA）和N，N-二异丙基乙胺（DIPEA）在pK ∼10.8附近达到最大值。考虑到与COF构建块之一2,4,6-三(甲酰基)间苯三酚的三个酚羟基相关的pK值，这些有机碱使其中两个羟基完全去质子化，并使第三个羟基部分去质子化，这优化了β-酮烯胺键形成的反应速率，解释了COF结晶度和相关微孔率的提高。因此，在β-酮烯胺连接的COF合成中很大程度上被忽视的有机碱的使用为改善COF性能提供了一种有前景的方法。
