Bifunctional supported catalysts for fine chemical synthesis

The objective was to prepare and optimise solid acid and solid base catalysts for liquid phase reactions. The approach has been to functionalize porous silica support materials with acid and base catalytic groups. Solid acid, solid base and bifunctional solid acid/base catalysts were studied. Evidence for acid-base cooperative catalytic mechanisms was found, suggesting that these bifunctional catalysts could show significant advantages over singly functionalized materials of mixtures thereof.

Silicas functionalized with tethered aminopropyl groups were prepared by both a grafting method and a sol–gel method. The solids were fully characterized and were tested in the nitroaldol condensation between nitromethane and benzaldehyde to afford nitrostyrene and the aldol reaction between 4-nitrobenzaldehyde and acetone to afford 4-(4-nitrophenyl)-4-hydroxy-2-butanone. The catalytic activities of these materials were found to be dependent on the dispersion and accessibilities of the active sites which, in turn, depend on the methods utilized for the catalyst preparation.

Solid acid catalysts were prepared by grafting silica with mercaptopropyl-trimethoxysilane (MPTS) followed by oxidation. The influence of the oxidation procedure on the acidity of the catalyst is described. The use of concentrated HNO3 optimizes the oxidation process and increases the concentration of active sites in comparison to H2O2. The activities of these catalysts were tested in the deacetalization of benzaldehyde dimethyl acetal to benzaldehyde.

The use of solid acid and solid base catalysts in the same system was examined, in a two-stage acid-catalyzed deacetalization and base-catalyzed Henry reaction.

Solid bifunctional acid-base catalysts were prepared by grafting on amorphous silica in two ways: 1) by grafting propylsulfonic acid and aminopropyl groups to the silica surface
(NH2-SiO2-SO3H) and 2) by grafting aminopropyl groups and then partially neutralizing with phosphotungstic acid, relying on the H2PW12O40- ion for surface acidity (NH2-SiO2-NH3+[H2PW12O40-]. These two bifunctional catalysts were compared with each other and with the singly functionalised catalysts described above. Surface acidities and basicities were characterized by adsorption calorimetry, using SO2 as a probe for surface basicity and NH3 for surface acidity. Catalytic activities were measured in the tandem deacetalization/Henry reaction described above, and in an aldol reaction in which a cooperative acid-base catalytic mechanism is thought to be effective. Overall NH2-SiO2-SO3H catalysts showed higher concentrations and strengths of both acid and base sites, and higher activities. Both catalysts showed evidence of cooperative acid-base catalytic sites. Even in the deacetalization/Henry reaction, the bifunctional catalysts exhibited a catalytic advantage over physical mixtures of singly functionalized catalysts.

A further bifunctional acid-base catalyst was prepared and studied by tethering proline to silica. In this case, the catalyst was chiral and was tested in the asymmetric aldol reaction between acetone and 4-nitrobenzaldehyde. Grafting methods with and without protecting groups for the active sites on proline were investigated. Remarkably the optimised supported proline catalysts showed higher activities and higher enantioselectivities than proline in homogeneous solution, and showed minimal loss in activity with time. Both activity and enantioselectivity depended strongly on the nature of the reaction solvent.