The rational design of heterogeneous base catalysts with a high active site concentration and stability in aqueous media remains a significant challenge. In this experimental study, ethylenediamine (EDA)-functionalized polymeric catalysts were synthesized using two different monomers, 2-hydroxyethyl methacrylate (HEMA) and glycerol 1,3-diglycerolate diacrylate (GDGDA), to investigate the effect of monomer structure on amine loading, active-site concentration, and catalytic performance. The produced catalysts were examined in the aqueous aldol condensation of acetone with 4-nitrobenzaldehyde as a model reaction. Elemental analysis reveals distinct nitrogen contents for produced catalysts, with HEMA-EDA exhibiting a nitrogen content of 2.8 wt%, corresponding to an active-site concentration of 1.03 mmol g-1, while GDGDA-EDA showed a higher nitrogen content of 10.2 wt% and an active-site concentration of 3.64 mmol g-1. The enhanced amine loading in GDGDA-EDA is attributed to the higher hydroxyl group of GDGDA, which contains three -OH groups per monomer unit compared to one in HEMA. During synthesis, these hydroxyl groups act as anchoring sites for substitution reactions, enabling more extensive grafting of EDA and a higher density of accessible basic sites. Catalytic evaluation demonstrated that GDGDA-EDA outperformed HEMA-EDA, achieving a turnover frequency (TOF) of ~ 4.6 × 10-4 s-1, compared to ~3.0 × 10-4 s-1 for HEMA-EDA under identical reaction conditions. The significantly higher TOF of GDGDA-EDA correlates directly with its increased nitrogen content and active-site concentration in aqueous media. Towards more industrial applications, catalyst loading vs. conversion studies (55°C, 4 h reaction time), HEMA-EDA required 0.15 g loading for 22% conversion, while GDGDA-EDA achieved the same 22% conversion with < 50% loading (~ 0.03 g), demonstrating the same catalytic efficiency. GDGDA-EDA showed stable conversion (TOFavg = 2.5 × 10-4 s-1) over 12 h time-on-stream in a continuous-flow setup, confirming operational stability in aqueous media.
The results highlight the critical role of monomer selection in controlling the amine functionalization and demonstrate that multifunctional monomers such as GDGDA are highly beneficial for developing efficient heterogeneous base catalysts for aqueous aldol reactions. In addition, this highly active site-density platform converts aldol catalysis from lab-scale demonstrations to economically competitive industrial processes for successful complex chemicals production.