Schiff base modified starch: A promising biosupport for palladium in Suzuki cross-coupling reactions

Starch can be used in diverse fields because it is a readily available, non-toxic polysaccharide with adaptable functionality and biodegradability. In this study, taking the aforementioned characteristics into consideration, we designed a modified starch (Starch-SB), which serves as supporting material for palladium stabilization. This new air and moisture-stable robust palladium composite [Starch-SB-Pd(II)] was characterized by FT-IR, XRD, TGA, XPS, SEM, EDX, TEM, CP/MAS ¹³C NMR, and ICP-MS analytical techniques. The catalytic studies exhibit high activity (up to 99 %) and stability in Suzuki cross-coupling reactions for this starch supported catalytic system under mild conditions (lower reaction temperature and green solvents) because of the cooperative interactions of multifunctional capturing sites on starch (Schiff base, hydroxy and amine groups) with palladium species. The experiments on reusability demonstrate that Starch-SB-Pd(II), which was prepared from functionalized starch, could be readily recycled several cycles through centrifugation. Moreover, we proposed a possibly multifunctional complex structure. This work presents an appealing and intriguing pathway for the utilization of polysaccharide as crucial support in green chemical transformations.

TBAX/Oxone-Mediated Halogenation of Pyrazoles and Other Heterocycles: An Entry to Important Cross-Coupling Reactions

A facile, sustainable and eco-friendly protocol has been developed for the halogenation of various heterocycles using TBAX (TBAI/TBAB/TBACl) as halogenating agent, which afforded the products in 90-95% isolated yields. The reaction proceeds with low-cost TBAX and uses greener conditions like EtOH as a solvent and microwave as an alternative energy source for reaction. This protocol has been applied on pyrazoles and extended to different heterocycles like 7-azaindole, indazole, indole and 2-phenylimidazo[1,2-α]pyridines. The gram-scale iodination reaction has also been successfully performed by optimizing conventional heating conditions, which demonstrates its potential applicability in organic synthesis. Further these halogenated pyrazoles have been utilized for different coupling reactions including formation of arylated, alkynylated and sulfenated pyrazoles. However, TBAF mediated fluorination did not work.

Controlled Synthesis of Palladium Nanoparticles with Size-Dependent Catalytic Activities Enabled by Organic Molecular Cages

Particle size plays a key role in the performance of metal nanoparticles (MNPs). However, the size-controlled synthesis of MNPs still represents a challenging task. In this work, we revealed a strong solvent effect on the growth of palladium nanoparticles (PdNPs), which was directed by a porous [2 + 3] organic molecular cage (OMC, Phos-cage) containing triphenylphosphine moieties. PdNPs with different average diameters of 0.8, 1.2, and 3.3 nm supported by Phos-cage were obtained by simply varying the reaction media. The catalytic performance of such ultrafine PdNPs in the reduction of p-nitrophenol and a Suzuki-Miyaura coupling reaction has been studied, which clearly shows size-dependent catalytic activity and stability. The knowledge gained in this study, controlling the size of PdNPs supported by the OMC template in different solvents, will open new possibilities for size-controlled synthesis of ultrafine MNPs with high catalytic activity and stability.

Tannic Acid: A green and efficient stabilizer of Au, Ag, Cu and Pd nanoparticles for the 4-Nitrophenol Reduction, Suzuki-Miyaura coupling reactions and click reactions in aqueous solution

Due to the good electrical, optical, magnetic, catalytic properties, transition metal nanoparticles (TMNPs) have been becoming more and more interesting in the fileds of environment, material, biomedicine, catalysis, and so on. Here, tannic acid (TA) is used as a green and efficient stabilizer to fabricate all kinds of TMNPs including AuNPs, AgNPs, CuNPs and PdNPs. These TMNPs possess small sizes ranging from 1 nm to 6 nm, which is conducive to several catalytic reactions in aqueous solution, such as 4-nitrophenol (4-NP) reduction, CuAAC reactions and Suzuki-Miyaura coupling reactions. AuNPs and PdNPs are found to have distinctly higher catalytic activities than AgNPs and CuNPs in the 4-NP reduction process. Especially, PdNPs show the highest catalytic activities with TOF up to 7200 h^-1 in the 4-NP reduction. Furthermore, PdNPs also exhibit satisfying catalytic performance in the Suzuki-Miyaura coupling process, and CuNPs are catalytically active in the copper-catalyzed azide alkyne cycloaddition (CuAAC) reactions. The applicability and generality of PdNPs and CuNPs are respectively confirmed via the reaction between different substrates in the Suzuki-Miyaura coupling reactions and the CuAAC reactions. This work present a simple, fast, green and efficient strategy to synthesize TMNPs for multiple catalysis.

Rational Design of Pt-Pd-Ni Trimetallic Nanocatalysts for Room-Temperature Benzaldehyde and Styrene Hydrogenation

Nanostructures of the multimetallic catalysts offer great scope for fine tuning of heterogeneous catalysis, but clear understanding of the surface chemistry and structures is important to enhance their selectivity and efficiency. Focussing on a typical Pt-Pd-Ni trimetallic system, we comparatively examined the Ni/C, Pt/Ni/C, Pd/Ni/C and Pt-Pd/Ni/C catalysts synthesized by impregnation and galvanic replacement reaction. To clarify surface chemical/structural effect, the Pt-Pd/Ni/C catalyst was thermally treated at X=200, 400 or 600 °C in a H₂ reducing atmosphere, respectively termed as Pt-Pd/Ni/C-X. The as-prepared catalysts were characterized complementarily by XRD, XPS, TEM, HRTEM, HS-LEIS and STEM-EDS elemental mapping and line-scanning. All the catalysts were comparatively evaluated for benzaldehyde and styrene hydrogenation. It is shown that the "PtPd alloy nanoclusters on Ni nanoparticles" (PtPd/Ni) and the synergistic effect of the trimetallic Pt-Pd-Ni, lead to much improved catalytic performance, compared with the mono- or bi- metallic counterparts. However, with the increase of the treatment temperature of the Pt-Pd/Ni/C, the catalytic performance was gradually degraded, which was likely due to that the favourable nanostructure of fine "PtPd/Ni" was gradually transformed to relatively large "PtPdNi alloy on Ni" (PtPdNi/Ni) particles, thus decreasing the number of noble metal (Pt and Pd) active sites on the surface of the catalyst. The optimum trimetallic structure is thus the as synthesised Pt-Pd/Ni/C. This work provides a novel strategy for the design and development of highly efficient and low-cost multimetallic catalysts, e. g. for hydrogenation reactions.

Construction of stable bio-Pd catalysts for environmental pollutant remediation

It has been reported that Pd nanoparticles were a little weak to bind to the dried microbial (yeast) surface, leading to the poor stability of the bio-supported catalysts. The objectives of the study are to construct stable Pd nanocatalysts supported on the dried yeast surface with the help of a tiny amount (<0.1 wt%) of reduced graphene oxide (Pd/yeast/rGO) and apply the catalysts in environmental pollutant remediation. The characterizations of the as-obtained Pd/yeast/rGO catalysts showed that reduced GO could cover Pd/yeast materials and prepare 15–21 nm Pd nanoparticles under acid and base media. The catalytic performance of the Pd/yeast/rGO catalyst was compared with that of control Pd/yeast catalysts without GO. The results revealed the kinetic constant K_app in the reduction of 4-nitrophenol of Pd/yeast/rGO catalysts could reach 3.6 × 10⁻² s⁻¹ without stirring during the reaction, which was 2.4 times higher than that of Pd/yeast catalysts, and the Pd/yeast/rGO catalysts kept a good stability even after being reused in seven cycles. Furthermore, the catalysts also showed quite good catalytic activities on CO oxidation and decolorization of dye methylene blue (MB). Thus, Pd/yeast/rGO catalysts were proven to be highly active and stable for environmental remediation and have the advantage that they can prevent the loss of noble metals and be prepared conveniently from discarded microorganisms.

Stable bio-supported Pd/yeast/rGO catalysts were prepared by covering with a tiny amount (<0.1 wt%) of GO based on a non-enzyme reduction method.