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Aminated poly(ethylene glycol) methacrylate resins as stable heterogeneous catalysts for the aldol reaction in water

Anton De Vylder, Jeroen Lauwaert, Jeriffa De Clercq, Pascal Van Der Voort, Christopher W. Jones, Joris W. Thybaut
Journal of catalysis 2020 v.381 pp. 540-546
Fourier transform infrared spectroscopy, Lewis bases, acetone, active sites, batch systems, carbon, catalysts, catalytic activity, chlorination, ethylenediamines, hexane, hydrogels, hydrolysis, leaching, methylamine, moieties, nuclear magnetic resonance spectroscopy, polyethylene glycol, polymerization, resins, secondary amines, silica, solvents, stable isotopes
Amine functionalized silicas have frequently been investigated as potential aldol reaction catalysts. However, active site leaching due to hydrolysis cannot be avoided, limiting the long-term stability of these catalysts in aqueous aldol reactions. Therefore, novel catalysts based on an organic resin have been developed starting from a suspension polymerized poly(ethylene glycol) methacrylate (PEGMA) hydrogel with poly(ethylene glycol) dimethacrylate (PEGDMA) as cross-linker. Amine functionalization was performed by chlorination of the terminal hydroxyl groups in the resulting PEGMA resin and subsequent nucleophilic substitution with an amine precursor, i.e., ethylenediamine (EDA), N,N′-dimethylethylenediamine (DED), or methylamine (MA). The successful synthesis of the catalysts was confirmed by ¹³C NMR, FT-IR, and elemental CHN analysis. Performance evaluation in a batch reactor for the aqueous aldol reaction of acetone with 4-nitrobenzaldehyde resulted in a turnover frequency (TOF) of the PEGMA-EDA catalyst amounting to 6.3 ± 0.4 · 10⁻⁴ s⁻¹, which is of the same order of magnitude as that of the corresponding state-of-the-art amine functionalized silica evaluated using hexane as solvent. The PEGMA-DED catalyst exhibited a somewhat lower TOF of 3.1 ± 0.2 · 10⁻⁴ s⁻¹, while the PEGMA-MA catalyst did not exhibit any turnover, indicating that the secondary amine in the backbone of the active site in the PEGMA-EDA catalyst is inactive. Continuous-flow evaluation of the PEGMA-EDA catalyst in a packed-bed reactor indicated that, as opposed to a primary amine functionalized silica catalyst, a stable catalytic activity as a function of time on stream could be achieved for at least 8 h and, hence, that no deactivation has occurred in this timeframe.