Montmorillonite-stabilized gold nanoparticles for nitrophenol reduction

Generation of sub-5 nm AuNPs in the special space of the loop-cluster corona of a polymer vesicle: preparation and its unique catalytic performance in the reduction of 4-nitrophenol

The hybrid vesicle AuNP@LCCV, in which a large number of AuNPs with an average size of about 2.8 nm were densely and uniformly distributed in an isolated state throughout the corona of the unusual polymer vesicle, was prepared via in situ reduction of Au³⁺ ions, which were encapsulated in advance in the unique polymer vesicle (LCCV) consisting of a hydrophobic membrane of poly(2-phenyl-2-oxazoline) and a hydrophilic loop-cluster corona of polyethyleneimine. The vesicle was formed via self-assembly from a comb-like block copolymer in which a polystyrenic main chain was grafted densely with diblock polyethyleneimine-b-poly(2-phenyl-2-oxazoline) and acted as a reactor for the reduction of Au³⁺. The hybrid vesicle AuNP@LCCV showed powerful catalytic ability in the reduction of nitrophenols (NPs). Interestingly, the reduction reactions of NPs showed a remarkably long induction time, which could be shortened dramatically from 60 min to 1-2 min by greatly increasing the concentration of NaBH₄. It is revealed that the oxygen adsorbed on the AuNPs significantly inhibited the reduction, causing the induction time. Once the oxygen is chemically cleaned from the surface of the AuNPs, the reduction of 4-NP proceeds gradually for a while and then completes suddenly. The reduction mechanism accompanying the oxygen-dependent induction time is proposed from the view of the strong oxygen affinity of the catalyst AuNP@LCCV.

Montmorillonite-based Heterogeneous Catalysts for Efficient Organic Reactions

In this review, we give a brief overview of recently developed montmorillonite-based heterogeneous catalysts used for efficient organic reactions. Cation-exchanged montmorillonite catalysts, metal catalysts supported on montmorillonite, and an interlayer design used for selective catalysis are introduced and discussed. In traditional syntheses, homogeneous acids and metal salts were used as catalysts, but the difficulty in separation of catalysts from products was a bottleneck when considering industrialization. The use of solid heterogeneous catalysts is one of the major solutions to overcome this problem. Montmorillonite can be used as a heterogeneous catalyst and/or catalyst support. This clay material exhibits strong acidity and a stabilizing effect on active species, such as metal nanoparticles, due to its unique layered structure. These advantages have led to the development of montmorillonite-based heterogeneous catalysts. Acidic montmorillonite, such as proton-exchanged montmorillonite, exhibits a high catalytic activity for the activation of electrophiles, such as alcohols, alkenes, and even alkanes. The montmorillonite interlayer/surface also functions as a good support for various metal species used for oxidation and carbon-carbon bond forming reactions. The use of an interlayer structure enables selective reactions and the stabilization of catalytically active species.