State Key Laboratory of Materials-Oriented, Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Jiangsu, PR China.
Chemistry. 2012 Oct 1;18(40):12773-82. doi: 10.1002/chem.201201338. Epub 2012 Aug 21.
An acid-base bifunctional ionic solid catalyst PySaImPW was synthesized by the anion exchange of the ionic-liquid (IL) precursor 1-(2-salicylaldimine)pyridinium bromide ([PySaIm]Br) with the Keggin-structured sodium phosphotungstate (Na(3) PW). The catalyst was characterized by FTIR, UV/Vis, XRD, SEM, Brunauer-Emmett-Teller (BET) theory, thermogravimetric analysis, (1)H NMR spectroscopy, ESI-MS, elemental analysis, and melting points. Together with various counterparts, PySaImPW was evaluated in Knoevenagel condensation under solvent and solvent-free conditions. The Schiff base structure attached to the IL cation of PySaImPW involves acidic salicyl hydroxyl and basic imine, and provides a controlled nearby position for the acid-base dual sites. The high melting and insoluble properties of PySaImPW are relative to the large volume and high valence of PW anions, as well as the intermolecular hydrogen-bonding networks among inorganic anions and IL cations. The ionic solid catalyst PySaImPW leads to heterogeneous Knoevenagel condensations. In solvent-free condensation of benzaldehyde with ethyl cyanoacetate, it exhibits a conversion of 95.8 % and a selectivity of 100 %; the conversion is even much higher than that (78.2 %) with ethanol as a solvent. The solid catalyst has a convenient recoverability with only a slight decrease in conversion following subsequent recyclings. Furthermore, the new catalyst is highly applicable to many substrates of aromatic aldehydes with activated methylene compounds. On the basis of the characterization and reaction results, a unique acid-base cooperative mechanism within a Schiff base structure is proposed and discussed, which thoroughly explains not only the highly efficient catalytic performance of PySaImPW, but also the lower activities of various control catalysts.
一种酸碱双功能离子型固体催化剂PySaImPW 通过离子液体（IL）前体 1-(2-水杨醛亚胺基)吡啶溴化物([PySaIm]Br)与 Keggin 结构的磷酸氢钠（Na(3)PW）的阴离子交换合成。通过傅里叶变换红外光谱（FTIR）、紫外可见光谱（UV/Vis）、X 射线衍射（XRD）、扫描电子显微镜（SEM）、BET 理论、热重分析（TGA）、（1）H 核磁共振波谱（NMR）、电喷雾质谱（ESI-MS）、元素分析和熔点等手段对催化剂进行了表征。与各种对照物一起，在溶剂和无溶剂条件下，PySaImPW 被用于 Knoevenagel 缩合反应。PySaImPW 中 IL 阳离子上的席夫碱结构包含酸性水杨醛羟基和碱性亚胺，为酸碱双位提供了一个可控的邻近位置。PySaImPW 的高熔点和不溶性与 PW 阴离子的大体积和高化合价以及无机阴离子和 IL 阳离子之间的分子间氢键网络有关。离子型固体催化剂PySaImPW 导致非均相 Knoevenagel 缩合反应。在苯甲醛与氰基乙酸乙酯的无溶剂缩合反应中，转化率为 95.8%，选择性为 100%；转化率甚至高于乙醇作为溶剂时的转化率（78.2%）。固体催化剂具有良好的可回收性，在随后的循环中转化率只有轻微下降。此外，该新型催化剂对许多具有活泼亚甲基的芳香醛底物具有高度适用性。基于表征和反应结果，提出并讨论了一种独特的席夫碱结构内的酸碱协同作用机制，该机制不仅彻底解释了PySaImPW 的高效催化性能，还解释了各种对照催化剂的较低活性。
