Amine functionalization of cellulose nanocrystals for acid–base organocatalysis: surface chemistry, cross-linking, and solvent effects

Sulfuric acid-derived cellulose nanocrystals (CNCs) are investigated as an alternative support for bifunctional heterogeneous organocatalyst materials. CNC catalysts are developed through direct surface functionalization by TEMPO-mediated oxidation and subsequent EDC amide coupling with alkyl diamines, used to impart both acid and base character to CNC surfaces. Sulfate ester content is controlled by partial acid hydrolysis prior to chemical functionalization, optionally reducing the initial sulfur content by approximately 50%. The resultant acidic and basic catalytic moieties are analyzed qualitatively by FT-IR spectroscopy and quantitatively by conductometric titration and elemental analysis. Relatively high degrees of substitution with amine functionality are achieved with diamine loadings around 0.9 mmol/g. This synthesis partially cross-links the CNCs, the extent of which is analyzed by dynamic light scattering of materials functionalized with 2–6 carbon spacer alkyl diamines. These CNC materials are demonstrated to be effective acid–base catalysts for an aldol condensation test reaction in batch liquid phase kinetic experiments. Relatively slow reaction rates were achieved in water:acetone reaction mixtures, in which decreased sulfate ester content is shown to decrease catalytic activity. Significant rate improvements are achieved by elimination of rate limiting transport and miscibility issues by solvent exchange to organic-only acetonitrile:acetone co-solvent reaction mixtures. Cross-linking and limited acid–base cooperativity are shown to decrease activity in CNCs functionalized with too long and too short diamines, respectively. This study demonstrates CNCs as alternative supports for bifunctional heterogeneous organocatalysis with site-specific activity on the order of that of well-studied aminosilica organocatalysts.