Research Of Acid-base Bifunctional Organic-inorganic Hybrid Materials Andacid-base Cooperative Catalysis

Aldol condensation which is usually catalyzed by either acid or base is widely regarded as one of the most important C-C bond-forming reactions used in organic synthesis. Under the acidic conditions, an enol behaves as nucleophile and attacks an activated carbonyl group which has been protonated by acid. Nevertheless, under basic conditions, the aldol condensation involves the nucleophilic addition of an enolate ion to another carbonyl group. The product of aldol condensation could dehydrate to anα,β-unsaturated carbonyl compound in either acidic or basic circumstance. Hence, both the aldol condensation and the dehydration are capable of being catalyzed by conventional monofunctional catalyst which possess either acidic or basic centers.Acid-base cooperative-activation is first enlightened from enzyme, which is a prevalent concept in perception of the bifunctional materials with acid and base recently. By anchoring both acidic and basic centers onto the surface of solid support with spatial proximity between each other, these incompatible groups can coexist and probably catalyze the substrates simultaneously as enzyme, although acidic and basic molecules induce neutralization promptly affording inactive salts in homogeneous system.Acid–base organic-inorganic hybrid materials have drawn much attention from researchers, presumably because of the apparent paradox that two antagonistic species that induce neutralisation and create inactive salts in a homogeneous system can coexist and catalyse the substrates simultaneously. The high efficiency and selectivity of these materials that are unreachable by monofunctional heterogeneous catalysts or homogeneous catalysis are also reasons for why this field has drawn so much interest. Many efficient acid-base bifunctional materials with a random distribution of acids and bases that catalyse C-C bond-forming reactions, together with proposed mechanisms, have been reported.The conclusion obtained from catalytic results and other characterisations is that cooperative catalysis relies on the two functional groups being close enough to each other on the surface. However, the mechanism of cooperative catalysis for aldol condensation is still unclear. Thus, in-depth exploration of the influence of the acid-base geometry and the match between acid and base on the cooperativity has become an urgent necessity, as understanding this behaviour is essential to designing superior multifunctional catalysts.Under this context, a series of acid-base bifunctional organic-inorganic hybrid materials have been prepared. The structure and acid-base properties of catalysts were studied by various characterization means of XRD, N₂-sorption, EA, NMR, titration, XRF and TEM, etc. The influence of acid-base geometry and the matching between acid and base on the cooperative catalysis have been investigated. The main conclusions are as follows:1. In-depth exploration of acid-base geometric influence on cooperative activation mechanism for aldol condensation reactionAcid–base organic-inorganic hybrid materials with a fixed isolation distance of two carbon atoms between the acid and the base was prepared. No remarkable decrement was observed after the protection of the carboxylic acid, indicating that efficient cooperative catalysis relies on the approximate extent of the appropriate geometry for these two antagonistic functional groups. Moreover, the effective distance for acid-base cooperativity in the aldol condensation reaction was quantitatively measured with the aid of a Gaussian calculation.2. Optimizing the matching between the acid and base of cooperative catalysis to inhibit dehydration in the aldol condensationA series of acid–base organic-inorganic hybrid materials were prepared, high yield and excellent selectivity of aldol condensation has been achieved only through adjustment of the matching between acid and base. Furthermore, we observed a fascinating phenomenon that the modification of acid performs double roles in the cooperative catalysis: one is to facilitate cooperative-activation markedly; the other is to restrain the subsequent dehydration evidently without perturbation on yield.3. The influence of acid-base strength matching on cooperative activation in aldol condensation reactionA series of acid-base bifunctionalized materials containing pyrrolidine or primary amine paired with various acidic supports were prepared with post-grafted approach. These materials were used as catalysts in the aldol condensation reaction between 4-nitrobenzaldehyde and acetone, aiming to investigate the influence of the acid-base strength matching on the acid-base cooperative catalysis. The catalytic results showed that the acid-base strength matching has a great impact on the catalytic behavior of acid–base functionalized materials in the aldol condensation reaction. High level of the aldol product conversion was obtained when Al-SBA-15 containing primary amine was used as the catalyst (the combination of medium acid and medium base).