Schiff Base Structured Acid–Base Cooperative Dual Sites in an Ionic Solid Catalyst Lead to Efficient Heterogeneous Knoevenagel Condensations

Abstract

An acid-base bifunctional ionic solid catalyst [PySaIm](3)PW 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, [PySaIm](3)PW was evaluated in Knoevenagel condensation under solvent and solvent-free conditions. The Schiff base structure attached to the IL cation of [PySaIm](3)PW 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 [PySaIm](3)PW 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 [PySaIm](3)PW 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 [PySaIm](3)PW, but also the lower activities of various control catalysts.

Easy Scale‐Up Synthesis of Mo8O26(C5H6N)4.H2O Hybrid with a Rectangular Prism Morphology and Its Application as an Efficient and Highly Recyclable Bi‐functional Catalyst for Knoevenagel Condensations

“…It is established that acids, bases, or compounds involving both acidic and basic sites can catalyze Knoevenagel condensation reactions. According to reported mechanisms,,,− we suggested a possible mechanism for the Knoevenagel reaction of malononitrile with benzaldehyde over Mo 8 O 26 (C 5 H 6 N) 4 .H 2 O hybrid, as shown in Scheme . It is proposed that the Mo 8 O 26 (C 5 H 6 N) 4 .H 2 O hybrid has played a dual role and activated both reactants.…”

Synthesis and conductive performance of quaternary molybdotungstovanadophosphoric heteropoly acid with Keggin structure

“…The data of the XRD are listed in Table 1. Its intense peaks mainly show in the regions of 7-101, 16-221, 25-301 and 33-381, which indicates that the acid has crystal structure [13]. The peaks in the region of 2θ¼ 7-101 are the characteristic peaks of HPAs with Keggin structure.…”

Hydrophobic Mesoporous Poly(ionic liquid)s towards Highly Efficient and Contamination‐Resistant Solid‐Base Catalysts

“…[52,57,58] In previous works, acid-base bifunctional catalysts were designed and exhibited superior performance in solvent-free Knoevenagel reaction. [59,60] In this work, PAMB 1 was evidenced as a medium strong solid base with the base strength of H_ = 18.4~22.5 rather than an acid-base bifunctional catalyst, and at the solvent-free condition it exhibits a much faster catalytic reaction rate than PAMB 1(Br) with only weak amino Lewis basic sites. The reason for the high activity of PAMB 1 can be attributed to the coexistance of Lewis and Brønsted basic sites that are respectively provided by ÀNH 2 and OH À groups.…”