Yongjian Ai

Porous silica-encapsulated and magnetically recoverable Rh NPs: a highly efficient, stable and green catalyst for catalytic transfer hydrogenation with “slow-release” of stoichiometric hydrazine in water

A core–shell structured nanocatalyst (Fe3O4@SiO2-NH2-RhNPs@mSiO2) that is encapsulated with porous silica has been designed and prepared for catalyzing the transfer hydrogenation of nitro compounds into corresponding amines. Rh nanoparticles serve as the activity center, and the porous silica shell plays an important role in the “slow-release” of the hydrogen source hydrazine. This reaction can be carried out smoothly in the green solvent water, and the atom economy can be improved by decreasing the amount of hydrazine hydrate used to a stoichiometric 1.5 equivalent of the substrate. Significantly, high catalytic efficiency is obtained and the turnover frequency (TOF) can be up to 4373 h−1 in the reduction of p-nitrophenol (4-NP). A kinetics study shows that the order of reaction is ∼0.5 towards 4-NP, and the apparent active energy Ea is 58.18 kJ mol−1, which also gives evidence of the high catalytic efficiency. Additionally, the excellent stability of the catalyst has been verified after 15 cycles without any loss of catalytic activity, and it is easily recovered by a magnet after reaction due to the Fe3O4 nucleus.

Egg-like magnetically immobilized nanospheres: A long-lived catalyst model for the hydrogen transfer reaction in a continuous-flow reactor

A novel egg-like nanosphere was designed as a long-lived catalyst and is described as Fe3O4@nSiO2-NH2-Fe2O3•xBi2O3@mSiO2. The catalyst was prepared using a modified Stöber method with template-free surface-protected etching. The catalyst particle consists of a magnetic Fe3O4 core as the “yolk”, an inner silica shell bearing active Fe2O3•xBi2O3 species as the “egg white”, and outer mesoporous silica as the “egg shell”. It exhibits an excellent performance in the catalytic reduction of nitro aromatics to corresponding anilines in a fixed-bed continuous-flow reactor. The reaction could be performed at 80 °C and could reach complete conversion in less than 1 min with only a 7% excess of hydrazine hydrate. The catalyst bed could be easily shifted between different substrates without cross-contamination because of the uniformity of the catalyst particles. This catalyst exhibited very good stability in the continuous-flow protocol. In the long-term reduction of p-nitrophenol with 0.5 mmol·min−1 productivity, it worked for more than 1,500 cycles without any catalytic activity loss.

3D Porous Carbon Framework Stabilized Ultra‐Uniform Nano γ‐Fe2O3: A Useful Catalyst System

We present a novel strategy for the scalable fabrication of γ-Fe2 O3 @3DPCF, a three-dimensional porous carbon framework (PCF) anchored ultra-uniform and ultra-stable γ-Fe2 O3 nanocatalyst. The γ-Fe2 O3 @3DPCF nanocomposites were facilely prepared with the following route: condensation of iron(III) acetylacetonate with acetylacetonate at room temperature to form the polymer precursor (PPr), which was carbonized subsequently at 800 °C. The homogeneous aldol condensation offered an ultra-uniform distribution of iron, so that the γ-Fe2 O3 nanoparticles (NPs) were uniformly distributed in the 3D carbon architecture with the average size of approximate 20 nm. The Fe2 O3 NPs were capped with carbon, so that the iron oxide maintained its γ-phase instead of the more stable α-phase. The nanocomposite was an excellent catalyst for the reduction of nitroarene; it gave >99 % conversion and 100 % selectivity for the reduction of nitroarenes to the corresponding anilines at 100 °C. The fabrication of the γ-Fe2 O3 @3DPCF nanocatalyst represents a green and scalable method for the synthesis of novel carbon-based metal oxide nanostructures.

Recyclable Acid-Base Bifunctional Core-Shell-Shell Nanosphere Catalyzed Synthesis of 5-Aryl-1H-1,2,3-triazoles through the “One-Pot” Cyclization of Aldehydes, Nitromethane, and Sodium Azide

A magnetically separable core–shell–shell nanosphere, Fe3O4@nSiO2‐SO3H@MS‐NHCOCH3 (n=nonporous, MS=microporous SiO2), was fabricated as an acid–base collaborative catalyst for the three‐component cyclization of aromatic aldehydes, nitroalkane, and sodium azide to afford 1H‐1,2,3‐triazoles. The bifunctional heterogeneous catalyst showed high activity for this transformation and good chemoselectivity, and toxic HN3 was not released during the course of the reaction. A variety of aldehydes were transformed into the corresponding 5‐aryl‐1H‐1,2,3‐triazoles in up to 98 % yield. Furthermore, the catalyst could be recovered by using an external magnetic field and reused many times without any loss in activity. In contrast, a homogeneous catalyst system comprising ammonium acetate/acetic acid also worked in this three‐component cyclization to afford 1H‐1,2,3‐triazoles.

Amorphous Flowerlike Goethite FeOOH Hierarchical Supraparticles: Superior Capability for Catalytic Hydrogenation of Nitroaromatics in Water

Fabrication of anilines from the corresponding nitroaromatics is a hot topic both for academia and for industry; however, conducting this protocol in water over a noble-metal-free catalytic system is still a great challenge. Continuous efforts are being made on exploiting novel catalysts for this transformation. In this work, we developed a scalable method for synthesizing the uniform flowerlike amorphous α-FeOOH hierarchical supraparticles. The well-defined amorphous α-FeOOH was prepared through an environmentally benign method, which is hydrolysis of the self-assembled iron glycolate at room temperature. Compared with other iron-only catalysts, this flowerlike amorphous α-FeOOH hierarchical supraparticle catalyst exhibits the best performance in the catalytic reduction of nitroaromatics to corresponding anilines by using water as the reaction solvent (turn over frequency is 106 h-1 for 4-nitrophenol in water). The further results indicated that the amorphous structure, special nanostructures, and adsorption-desorption synergy offered excellent activity. The kinetics study shows that the reduction of 4-nitrophenol is first order for α-FeOOH, and the apparent active energy Ea is 75.9 kJ mol-1. Furthermore, this catalyst can be used for eight times without obvious catalytic activity loss. We believe that this novel flowerlike amorphous α-FeOOH hierarchical supraparticle catalyst is a milestone in the reduction of nitro compounds.