Decoration of chitosan microspheres with inorganic oxide clusters: Rational design of hierarchically porous, stable and cooperative acid–base nanoreactors

One of the fundamental enzymatic catalyst assets, which is the most difficult to engineer in synthetic systems, is the coexistence of multifunctional sites and their synergetic cooperation. In this work, an efficient approach toward cooperative acid–base materials using natural matrices is proposed. Taking advantages from chitosan polysaccharide as nano-assembling system and on the supercritical drying technique to preserve their porosity, the mutual interactions between different glucosamine units and the Lewis acidic precursors (Ti, Zr, Al, Sn) allowed the preparation of hierarchically porous microspheres in which well-separated amino groups from chitosan are replicated with highly dispersed acidic inorganic oxides. This decoration at the nano-scale entails a notable improvement on the hydrothermal stability of the resulting organic–inorganic hybrid materials. The resulting acid–base hybrid materials are assessed for three carbon–carbon forming reactions (Henry condensation, Michael addition and jasminaldehyde synthesis) and systematically compared to the pure acidic inorganic oxide and basic chitosan microspheres. The bifunctional materials displayed interesting catalytic activity and selectivity, with respect to monofunctional ones, witnessing thus on the cooperative effect attainable in chitosan@inorganic oxide microspheres.