4. Conclusions
A straightforward synthesis procedure for cysteine functionalized (organo)silicas with an aliphatic, aromatic or silica character has been developed. Catalytic testing of the synthesized materials in the aldol condensation of 4-nitrobenzaldehyde and acetone was indicative of a crucial interplay between the reactant, solvent, traces and support properties in achieving the desired catalyst performance. An organosilica with aromatic linkers exhibited a low catalytic activity, which was attributed to a pronounced 4-nitrobenzaldehyde enrichment enhancing the formation of inhibiting species. Of the two other catalysts, i.e., an organosilica with an aliphatic character and a hydrophilic silica material, the former exhibits the highest catalytic activity. This suggests that a hydrophobic support might lead to a better acetone to 4-nitrobenzaldehyde ratio inside its pores and, hence, would be beneficial for the catalytic activity. Additionally, the hydrophilic silica material loses about 40% of its activity in the presence of a small amount of water due to poisoning of the active sites and suppression of the formation of the reactive enamine intermediate. The aromatic organosilica exhibits an activity decrease of 28% in agreement with its hydrophobic character and the suppression of the formation of inhibiting species while the aliphatic organosilica apparently fully repels water from its pores and retains its activity.
