Metal-free bi-functional cooperative catalysis: amine and quaternary amine-functionalized dendritic fibrous nanosilica as heterogeneous catalysts for the Henry reaction and CO2 conversion

Abstract

This study explores amine-functionalized dendritic fibrous nanosilica (DFNS) as a highly effective, base-free heterogeneous catalyst for nitro-aldol (Henry) condensation and additive-free CO₂ utilization. Initially, high surface area DFNS was synthesized using a novel template in an emulsion biphasic system and chemically processed to create DFNS@NH₂ for the Henry reaction and further quaternized to form DFNS@TBAB for CO₂ utilization. DFNS is expected to outperform conventional mesoporous materials (SBA-15 and MCM-41) due to its dendritic morphology, higher surface area, superior mass transfer, ease of functionalization, and greater active site exposure, enhancing its catalytic efficiency and versatility. In addition, the total basicity values for DFNS@NH₂ and DFNS@TBAB were determined to be 0.48 and 0.124 mmol g⁻¹, respectively. Furthermore, FeSEM, TEM, and HR-TEM analyses evidenced the dendritic morphology of the solid substrate with a fibrous structure termed DFNS. Amine-functionalized DFNS achieved complete conversion of benzaldehyde to β-nitrostyrene with over 99% selectivity at 50 °C in 12 h using nitromethane. It successfully catalyzed a range of substrates in the Henry reaction, including electron-donating and electron-withdrawing aromatic aldehydes and biologically derived aldehydes such as furfural and 5-methyl furfural. Electron-withdrawing groups on aromatic rings showed reduced catalytic activity. Furthermore, DFNS functionalized with n-butyl bromide produced a quaternary amine catalyst capable of converting CO₂ and styrene oxide to styrene carbonate without external additives, achieving 94% conversion and 98% selectivity. This dual functionality highlights DFNS's potential in carbon–carbon bond formation and CO₂ utilization, offering a promising route for CO₂ emission mitigation through chemical conversion.