Gold nanoparticles supported on mesoporous silica: origin of high activity and role of Au NPs in selective oxidation of cyclohexane

Synthesis of novel hybrid mesoporous gold iron oxide nanoconstructs for enhanced catalytic reduction and remediation of toxic organic pollutants

The development of highly efficient, rapid, and recyclable nanocatalysts for effective elimination of toxic environmental contaminants remains a high priority in various industrial applications. Herein, we report the preparation of hybrid mesoporous gold-iron oxide nanoparticles (Au-IO NPs) via the nanocasting "inverse hard-templated replication" approach. Dispersed Au NPs were anchored on amine-functionalized iron oxide incorporated APMS (IO@APMS-amine), followed by etching of the silica template to afford hybrid mesoporous Au-IO NPs. The obtained nanoconstructs were fully characterized using electron microscopy, N₂ physisorption, and various spectroscopic techniques. Owing to their magnetic properties, high surface areas, large pore volumes, and mesoporous nature (S _BET = 124 m² g^-1, V _pore = 0.33 cm³ g^-1, and d _pore = 4.5 nm), the resulting Au-IO mesostructures were employed for catalytic reduction of nitroarenes (i.e. nitrophenol and nitroaniline), two of the most common toxic organic pollutants. It was found that these Au-IO NPs act as highly efficient nanocatalysts showing exceptional stabilities (>3 months), enhanced catalytic efficiencies in very short times (∼100% conversions within only 25-60 s), and excellent recyclabilities (up to 8 cycles). The kinetic pseudo-first-order apparent reaction rate constants (k _app) were calculated to be equal to 8.8 × 10^-3 and 23.5 × 10^-3 s^-1 for 2-nitrophenol and 2-nitroaniline reduction, respectively. To our knowledge, this is considered one of the best and fastest Au-based nanocatalysts reported for the catalytic reduction of nitroarenes, promoted mainly by the synergistic cooperation of their high surface area, large pore volume, mesoporous nature, and enhanced Au-NP dispersions. The unique mesoporous hybrid Au-IO nanoconstructs synthesized here make them novel, stable, and approachable nanocatalyst platform for various catalytic industrial processes.

Synthesis and biological activities of alcohol extract of black cumin seeds (Bunium persicum)-based gold nanoparticles and their catalytic applications

Fast, simple, and environmentally friendly gold nanoparticles (Au-NPs) capped and stabilized with black cumin (Bunium persicum) seed alcohol extract are reported. The aqueous gold ions (Au3+) were treated with B. persicum (BP) seed extract, which resulted in a rapid color change to red, indicating the synthesis of Au-NPs. UV-Vis spectroscopy, FTIR, SEM, energy dispersive X-ray (EDX), and X-ray diffraction (XRD) techniques were used to further characterize the Au-NPs. Its stability was assessed against various pH levels and sodium chloride levels (NaCl), different salts of same concentration as well as at a range of temperature (30–100°C). The UV-Vis spectrum in the Au-NPs produced a 540 nm plasmon surface resonance, and a 25–50 nm range of particulates was shown in the SEM analysis. In addition, the FTIR spectra confirmed the inclusion in the capping and decrease of Au-NPs of amines, amide groups, and alcohols. The EDX analysis confirmed the presence of element Au. Furthermore, Au-NPs were tested for enzyme inhibition and antibacterial and antifungal activities and showed remarkable response. These findings have concluded that BP seed extract is an effective bio-reductant of gold nanoparticle synthesis, which can be further applied in different biomedical and pharmaceutical industries.

Selective and leaching-resistant palladium catalyst on a porous polymer support for phenol hydrogenation

Selective hydrogenation of phenol is promising for the utilization of renewable lignocellulose and production of cyclohexanone that usually relies on petroleum, but it is challenging to simultaneously achieve high activity and selectivity. Herein, we report an amino-functionalized nanoporous polymer stabilized palladium nanoparticle catalyst, which is prepared via a one-pot co-polymerization method, as highly active and selective catalysts for the phenol hydrogenation, giving cyclohexanone selectivity over 99.5% with full conversion of phenol under mild reaction conditions without any soluble additives. Importantly, the palladium leaching was efficiently hindered, maintaining the catalytic performances in continuously recycle tests. In contrast, the commercial palladium catalysts exhibit much lower selectivity and obvious deactivation because of the palladium leaching.

A comprehensive and updated review of studies on the oxidation of cyclohexane to produce ketone-alcohol (KA) oil

Oxidation of cyclohexane is an essential chemical reaction for the industrial manufacture of cyclohexanol and cyclohexanone. These two compounds, together known as ketone–alcohol (KA) oil, are the main feedstock for nylon 6 and nylon 6,6 productions. Several types of catalysts and reaction conditions have been used for cyclohexane oxidation. This paper presents a thorough literature review of catalytic materials used for cyclohexane oxidation to produce KA oil using oxygen, air and other oxidizing agents as well as utilizing different solvents. This review covers research and development reported over the years 2014–2020. This review aims to comprehend the type of catalysts, solvents, oxidants and other reaction parameters used for the oxidation of cyclohexane. Three types of cyclohexane oxidation processes namely thermocatalytic, photocatalytic and microwave-assisted catalytic have been reported. The results of the review showed that metal and metal oxide loaded silica catalysts performed excellently and provided high selectivity of KA oil and cyclohexane conversion. The use of peroxides is not feasible due to their high price compared to air and oxygen. Gold nanoparticles supported on silica performed with high selectivity and good conversion. The use of hydrochloric acid as an additive was found very effective to enhance the photocatalytic oxidation of cyclohexane. Water on the catalyst surface enhanced the reactivity of the photocatalysts since it helps in the generation of hydroxyl radicals.

Regulating Heterogeneous Catalysis of Gold Nanoparticles with Polymer Mechanochemistry

Polymer mechanochemistry has emerged as a unique approach to regulate homogeneous catalysis in chemical transformations. The utilization of polymer mechanochemistry to regulate heterogeneous catalysis, however, still remains to be investigated. In this study, using polymer-grafted gold nanoparticles as the model heterogeneous catalysts, we show that polymer chains can be mechanically ruptured from the surface of gold nanoparticles, and thus, the catalytic activity of gold nanoparticles can be accelerated under sonication. The mechanical activation of polymer-grafted gold nanoparticles only occurs when the grafted polymer chains exceed a threshold molecular weight. This mechanical behavior is similar to those mechanophore-linked polymers. More importantly, further characterizations reveal that the Au-Au bonds instead of the Au-S bonds are broken at the heterointerfaces of polymer chains and gold nanoparticles. Our study unveils an unprecedented characteristic of polymer-grafted metallic nanoparticles in response to external mechanical stress.

Role of the Support in Gold-Containing Nanoparticles as Heterogeneous Catalysts

In this review, we discuss selected examples from recent literature on the role of the support on directing the nanostructures of Au-based monometallic and bimetallic nanoparticles. The role of support is then discussed in relation to the catalytic properties of Au-based monometallic and bimetallic nanoparticles using different gas phase and liquid phase reactions. The reactions discussed include CO oxidation, aerobic oxidation of monohydric and polyhydric alcohols, selective hydrogenation of alkynes, hydrogenation of nitroaromatics, CO₂ hydrogenation, C–C coupling, and methane oxidation. Only studies where the role of support has been explicitly studied in detail have been selected for discussion. However, the role of support is also examined using examples of reactions involving unsupported metal nanoparticles (i.e., colloidal nanoparticles). It is clear that the support functionality can play a crucial role in tuning the catalytic activity that is observed and that advanced theory and characterization add greatly to our understanding of these fascinating catalysts.

Ru Nanoparticles Embedded in Cubic Mesoporous Silica SBA-1 as Highly Efficient Catalysts for Hydrogen Generation from Ammonia Borane

Cubic mesoporous silica SBA-1 functionalized with carboxylic acid (-COOH), namely S1B-C10, is used as a support to fabricate and confine Ru nanoparticles (NPs). The uniformly dispersed organic functional groups in SBA-1 are beneficial in attracting Ru cations, and as a result, homogenously distributed small sized Ru NPs are formed within the mesopores. The prepared Ru@S1B-C10 is utilized as a catalyst for H2 generation from the hydrolysis of ammonia borane (AB). The Ru@S1B-C10 catalyst demonstrates high catalytic activity for H2 generation (202 mol H2 molRu min−1) and lower activation energy (24.13 kJ mol−1) due to the small sized Ru NPs with high dispersion and the support’s interconnected mesoporous structure. The nanosized Ru particles provide abundant active sites for the catalytic reaction to take place, while the interconnected porous support facilitates homogenous transference and easy dispersal of AB molecules to the active sites. The catalyst demonstrates good recycle ability since the accumulation and leaking of NPs throughout catalysis can be effectively prevented by the support.

Supported Gold Nanoparticles as Catalysts for the Oxidation of Alcohols and Alkanes

Supporting gold nanoparticles have shown to be extremely active for many industrially important reactions, including oxidations. Two representative examples are the oxidation of alcohols and alkanes, that are substrates of industrial interest, but whose oxidation is still challenging. This review deals with these reactions, giving an insight of the first studies performed by gold based catalysts in these reactions and the most recent developments in the field.

Highly sensitive detection of nitrite by using gold nanoparticle-decorated α-Fe2O3 nanorod arrays as self-supporting photo-electrodes

Gold nanoparticle (Au NP)-decorated-Fe₂O₃ nanorod arrays (AuNPs-Fe₂O₃) as a photoelectrode are applied to the detection of nitrite solution with a low limit of detection and high sensitivity.

Interfacial CoOx Layers on TiO2 as an Efficient Catalyst for Solvent-Free Aerobic Oxidation of Hydrocarbons

Construction of efficient interfaces to improve the performance of supported metal catalysts is a challenging but effective technique. A newly synthesized catalyst with layered cobalt oxide on the surface of titania (layer-CoO_x /TiO₂ ) is highly selective towards the aerobic oxidation of C-H bonds in a series of hydrocarbons under sustainable conditions. The layer-CoO_x /TiO₂ easily outperforms the state-of-the-art noble metal catalysts and homogeneous cobalt salts used in industry. In-depth structural and functional characterization reveal that the layer-CoO_x /TiO₂ readily reacts with O₂ for the adsorption and activation of C-H bonds. The layered structure of CoO_x can maximize the interfacial effect of CoO_x /TiO₂ leading to a good performance for the oxidation of C-H bonds.

One pot ligand exchange method for a highly stable Au-SBA-15 catalyst and its room temperature CO oxidation

A modified deposition precipitation (DP) method has been developed to address a fundamental issue of supporting well dispersed Au nanoparticles on silica. Ammonium chloride (NH4Cl) plays an important role in in situ modifying the gold precursor (HAuCl4·3H2O) solution allowing facile deposition of gold NPs in the channels of SBA-15. The Au-SBA-15 catalyst (2.8 wt%) synthesized by this procedure showed 100% conversion for CO oxidation at room temperature with excellent stability at room temperature and high temperature.

Cucurbit[7]uril-based high-performance catalytic microreactors

Catalytic microreactors manufactured using microfluidic devices have received significant research interest in recent years. However, little attention has been paid to immobilising metallic nanoparticles (NPs) onto microchannel walls for high efficiency catalytic reactions. We demonstrate a facile preparation of cucurbit[7]uril-based catalytic microreactors, where metallic NPs are immobilised onto microchannels via supramolecular complexation with methyl viologen@cucurbit[7]uril (CB[7]). These microreactors exhibit a remarkable catalytic activity owing to the substantially high surface area to volume ratio of the microchannels and metallic NPs. Superior to most conventional heterogeneous catalytic reactions, separation post reaction and complicated recycling steps of the catalysts are not required. Moreover, CB[7] can complex a variety of metallic NPs to its portal, providing a multifunctional high-performance in situ catalytic platform.

Gold/Periodic Mesoporous Organosilicas with Controllable Mesostructure by Using Compressed CO2

Gold nanoparticles confined into the walls of periodic mesoporous organosilicas (PMOs) with controllable morphology have been successfully fabricated through a one-pot method by using different CO₂ pressures. The synthesis can be easily conducted in a mixed aqueous solution by using HAuCl₄ as gold source and bis[3-(triethoxysilyl)propyl] tetrasulfide and tetramethoxysilane as the organosilica precursor. P123 and compressed CO₂ served as the template and catalytic/regulative agent, respectively. Transmission electron microscopy, N₂ adsorption, and X-ray diffraction were employed to characterize the structure of the obtained composite materials. To further investigate the formation mechanism, a series of ordered PMOs with one-dimensional nanotube, two-dimensional hexagonal, vesicle-like, and cellular foam structures were obtained by using different CO₂ pressures without the gold source. The mechanism for mesostructure evolution of PMOs with different CO₂ pressures was proposed and discussed in detail. The catalytic performance of Au-based PMOs was evaluated for the reduction of 4-nitrophenol (4-NP). These obtained composites with different mesostructures not only exhibit excellent catalytic activity, high conversion rate, and remarkable thermal stability, but they also exhibit morphology-dependent reaction properties in the reduction of 4-NP. The possible reaction pathway of the reactants to embedded Au active sites was proposed and schemed.

Stable and solubilized active Au atom clusters for selective epoxidation of cis-cyclooctene with molecular oxygen

The ability of Au catalysts to effect the challenging task of utilizing molecular oxygen for the selective epoxidation of cyclooctene is fascinating. Although supported nanometre-size Au particles are poorly active, here we show that solubilized atomic Au clusters, present in ng ml⁻¹ concentrations and stabilized by ligands derived from the oxidized hydrocarbon products, are active. They can be formed from various Au sources. They generate initiators and propagators to trigger the onset of the auto-oxidation reaction with an apparent turnover frequency of 440 s⁻¹, and continue to generate additional initiators throughout the auto-oxidation cycle without direct participation in the cycle. Spectroscopic characterization suggests that 7–8 atom clusters are effective catalytically. Extension of work based on these understandings leads to the demonstration that these Au clusters are also effective in selective oxidation of cyclohexene, and that solubilized Pt clusters are also capable of generating initiators for cyclooctene epoxidation.

Gold catalysts have previously been reported for the epoxidation of alkenes with molecular oxygen. Here the authors show that, rather than the gold nanoparticles, the active species for this reaction are actually small, soluble gold species stabilized by the oxidised organic products.

Silver metal nano-matrixes as high efficiency and versatile catalytic reactors for environmental remediation

Nano-porous metallic matrixes (NMMs) offer superior surface to volume ratios as well as enhanced optical, photonic, and electronic properties to bulk metallic materials. Such behaviours are correlated to the nano-scale inter-grain metal domains that favour the presence of electronic vacancies. In this work, continuous 3D NMMs were synthesized for the first time through a simple diffusion-reduction process whereby the aerogel matrix was functionalized with (3-Mercaptopropyl)trimethoxysilane. The surface energy of the silica monolith templates was tuned to improve the homogeneity of the reduction process while thiol functionalization facilitated the formation of a high density of seeding points for metal ions to reduce. The diameter of NMMs was between 2 and 1000 nm, corresponding to a silver loading between 1.23 and 41.16 at.%. A rates of catalytic degradation kinetics of these NMMS which is three orders of magnitude higher than those of the non-functionalized silver-silica structures. Furthermore, the enhancement in mechanical stability at nanoscale which was evaluated by Atomic Force Microscopy force measurements, electronic density and chemical inertness was assessed and critically correlated to their catalytic potential. This strategy opens up new avenues for design of complex architectures of either single or multi-metal alloy NMMs with enhanced surface properties for various applications.

ZSM-5 functionalized in situ with manganese ions for the catalytic oxidation of cyclohexane

We report the preparation of transition metal-containing ZSM-5 catalysts, which were active and selective for cyclohexane oxidation.

Green synthesis of gold nanoparticles reduced and stabilized by squaric acid and supported on cellulose fibers for the catalytic reduction of 4-nitrophenol in water

Gold nanoparticles reduced and stabilized by sodium squarate in water that attach to cellulose fibers and catalyse the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with sodium borohydride.