Solvent-free Oxidation Of Benzyl Alcohol Research Articles

Precise tuning of silica pore length and pore diameter on silica-encapsulated gold core–shell nanoparticles and catalytic impact

The ability to individually tune the pore length and pore diameter of silica-encapsulated gold core–shell nanoparticles (CSNPs) via a seeded encapsulation method is demonstrated and their impact on catalysis determined. Reducing the solvent volume during the shell growth phase resulted in CSNPs with shells of increasing thickness from 75 nm to 202 nm, following a linear relationship. The addition of 1, 3, 5-triisopropylbenzene (TIB) during synthesis increased the average pore diameter from 25 Å to 32 Å by swelling the pore template, swelling further to 38 Å when adding both TIB and n-decane. By decoupling the gold core reduction and silica condensation stages, gold core diameters were largely unaffected by silica growth reaction conditions. This was achieved by encapsulating CTAB-stabilized nanoparticles rather than reducing and stabilizing the nanoparticles during silica condensation. When used to catalyze solvent-free benzyl alcohol oxidation, it was found that CSNPs with increasing shell thicknesses demonstrated increased formation of the desirable aldehyde product, holding the undesired ester product yield constant. In contrast, CSNPs with increasing pore diameter exhibit increased formation of both aldehyde and ester products unselectively. These results—uniquely observed through the parametric isolation of pore length and diameter—suggest the reactant concentration at the pore wall due to hydrogen bonding, forming concentric “microphases” within the pores.

Sustainable Solvent-Free Selective Oxidation of Benzyl Alcohol Using Ru(0) Supported on Alumina

The selective oxidation of primary alcohols into their corresponding carbonyl compounds is challenging because of the easy over oxidization to acids and esters. The traditional reaction requires large amounts of solvent and oxidant, causing serious environmental issues. Recently, several efforts have been made to transform the reaction into a more sustainable process. Here, we investigated the solvent-free oxidation of benzyl alcohol using air as a green oxidant in the presence of ruthenium supported on alumina and zirconia, thereby meeting atom economy and environmental requirements. The materials were extensively characterized and, in addition to their activity, selectivity, and reusability, the environmental sustainability of the process was assessed according to green chemistry metrics. XRD, TEM, and XPS analyses suggest that the formation of metallic Ru on the support plays a key role in the catalytic activity. Ru supported on alumina, after a reduction treatment, achieves good activity (62% conversion) and a complete selectivity in a very sustainable process (without a solvent and with air as oxidant), as indicated by the very low E-factor value. The formulation is very stable and maintains high activity after recycling.

Solvent-free efficient oxidation of benzyl alcohol on nano-Pd/Al2O3: Effect of palladium source and pH value

PA-Na-x and PA-H-1 catalysts were prepared by a cation-assisted polyol method using different Pd sources (Na2PdCl4 or H2PdCl4) and pH values (x = 1, 2, 3 or 5), and applied to solvent-free oxidation of benzyl alcohol (BzOH). As confirmed by multiple analytical characterizations, the base number, Pd0 content and abundance of oxygen vacancies for PA-Na-x prepared were higher than those on PA-H-1, and their synergistic effect made PA-Na-x possess the higher BzOH conversion and benzaldehyde selectivity. The contents of Pd0 and oxygen vacancies on PA-Na-x showed an inverted volcano trend with the increase of the pH value. PA-Na-x also followed the inverted volcano order for the catalytic activity, where PA-Na-1 prepared from Na2PdCl4 at pH 1 exhibited the best catalytic efficiency with a turnover frequency of 29.5 s−1 and 94.8% selectivity of benzaldehyde, and excellent stability. This study provided an efficient method of preparing nano-Pd catalysts.

Nanosheet array-like NixMg3-xAl-LDH/rGO hybrids loaded atomically precise Aun nanoclusters for the solvent-free oxidation of benzyl alcohol

A series of hierarchically structured hybrids NixMg3-xAl-LDH/rGO (LDH: layered double hydroxides, x = 1, 1.5 and 2, rGO: reduced graphene oxide) supported atomically precise Aun (n = 25, 38 and 127) nanoclusters (NCs) catalysts Aun/NixMg3-xAl-280 were firstly prepared via a double pH-controlled electrostatic adsorption method using captopril-capped Aun NCs as precursors followed proper calcination. Detailed characterizations show that the as-obtained hierarchically structured Aun NCs catalysts possess the nanosheet array-like structure formed by the LDHs nanosheets (∼55–70 × 10 nm) interdigitated vertically grown on the both sides of rGO layers, and Aun NCs with size of 1.0–1.9 nm are mainly dispersed on the edge and cross-linking sites of LDH/rGO hybrid. All the hierarchically structured Aun NCs catalysts exhibit excellent catalytic activity for the solvent-free oxidation of benzyl alcohol without additional base, and the activities show a volcanic trend with increasing Ni/Mg ratios and gradually decrease with the increased Au atomicities in Aun clusters. The highest TOF value of 5118 h−1 for the Au25/Ni1.5Mg1.5Al-280 can be ascribed to the smallest Au25 NCs containing the Au0/Auδ+-Ni2+/Mg2+–OH interface with the highest Auδ+/(Au0 + Auδ+) ratio, and the strongest Au25−LDH/rGO synergistic interaction along with hierarchical nanosheet array-like structure facilitating the transmission and diffusion of the reactants. The present work provides a new idea for preparing the hierarchically structured LDH/rGO hybrid supported metal nanoclusters catalysts with high-performance for various catalytic applications.

Highly Dispersed Pd Nanoclusters on Layered Double Hydroxides with Proper Calcination Improving Solvent-Free Oxidation of Benzyl Alcohol

Highly Dispersed Pd Nanoclusters on Layered Double Hydroxides with Proper Calcination Improving Solvent-Free Oxidation of Benzyl Alcohol

Transforming Waste Clamshell into Highly Selective Nanostructured Catalysts for Solvent Free Liquid Phase Oxidation of Benzyl Alcohol

High yield production of benzaldehyde in the solvent-free oxidation of benzyl alcohol by using green catalysts is highly desirable. In this work, calcium hydroxide derived from waste clamshell was used as low-cost and environmentally friendly catalyst support (CaSUP) for Pd and V nanoparticles. The physicochemical properties of the catalysts were analyzed using X-ray diffraction spectroscopy (XRD), Fourier transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET) technique, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The catalytic oxidation of benzyl alcohol to benzaldehyde was studied in a liquid phase reaction by using H2O2 as an oxidizing agent. The effects of catalyst loading, the molar ratio of hydrogen peroxide to benzyl alcohol, temperature and reaction duration were investigated. In the optimized conditions, Pd nanoparticles supported on clamshell-derived supports displayed excellent catalytic conversion (88%) and selectivity to benzaldehyde (89%). Furthermore, the catalyst can be effectively reused without a significant loss in its activity and selectivity. The high yield and stability can be related to the structural and basic properties of the catalyst. These results provide important insights into the benzyl alcohol oxidation process for industrial applications.

P,N co-doped biomass carbon as a remarkable metal-free catalyst for solvent-free oxidation of benzyl alcohol with ambient air: The key promoting role of N co-doping

Benzaldehyde is an important chemical intermediate, and its production via the oxidation of benzyl alcohol with O2 is highly pursued as an alternative for the existing industrial process to meet the demand of green chemistry. Most of the reported catalytic systems are dependent on the noble/toxic metal catalysts. Herein, a biomass carbon catalyst doped with P and N heteroatoms is prepared by calcinating the phosphoric acid-treated N-containing biomass of soybean flour in N2. In the absence of solvent or additive, the obtained P,N co-doped carbon catalyst exhibits aremarkablecatalyticactivity for oxidation of benzyl alcohol to benzaldehyde with air (99.4% yield, TOF = 62.1 × 10-4 mol BAD/(g catalyst × h reaction time)). The present turnover frequency surpasses all the previously reported ones of similar metal-free carbon catalysts for this reaction. The N co-doping facilitates the P doping into the carbon matrix, favors the formation the catalytic active species and has a remarkable electronic effect on them, which are identified via experiments and DFT calculations.

Solvent-Free Oxidation of Benzyl Alcohol to Benzaldehyde Catalyzed by Magnesium Oxide Supported Bimetallic Gold-Palladium

Solvent-Free Oxidation of Benzyl Alcohol to Benzaldehyde Catalyzed by Magnesium Oxide Supported Bimetallic Gold-Palladium

Au-Pd nanoparticles immobilized on TiO2 nanosheet as an active and durable catalyst for solvent-free selective oxidation of benzyl alcohol

TiO2nanocrystals with controlled facets have been extensively investigated due to their excellent photocatalytic performance in sustainable and green energy field. However, the applications in thermal catalysis without applying UV irradiation are comparably less and the identification of their intrinsic roles, especially the different catalytic behaviors of each crystal facet, remains not fully recognized. In this study, bimetallic AuPd nanoparticles supported on anatase TiO2 nanosheets exposing {001} facets or TiO2 nanospindles exposing {101} as a catalyst were prepared by sol-immobilization method and used for solvent-free benzyl alcohol oxidation. The experimental results indicated that the exposed facet of the support has a significant effect on the catalytic performance. AuPd/TiO2-001 catalyst exhibited a higher benzyl alcohol conversion than that of the AuPd/TiO2-101. Meanwhile, all the prepared AuPd/TiO2 catalysts were characterized by XRD, ICP-AES, XPS, BET, TEM, and HRTEM. The results revealed that the higher number of oxygen vacancies in TiO2-sheets with the exposed {001} facets of higher surface energy could be responsible for the observed enhancement in the catalytic performance of benzyl alcohol oxidation. The present study displays that it is plausible to enhance the catalytic performance for the benzyl alcohol oxidation by tailoring the exposed facet of the TiO2 as a catalyst support.

Solvent-free selective oxidation of aromatic alcohol with O2 over MgAl-LDH supported Pd nanoparticles: Effects of preparation methods and solvents

In this work, layered double hydroxide (LDH) supported Pd samples were prepared by sol-immobilization and impregnation methods in the different solvents. As confirmed by XRD, N2 adsorption, TEM results and catalytic reaction, the methods and solvents used in the preparing process greatly influenced the dispersions of the supported Pd NPs on LDH support and the textural structures as well as the catalytic performances of the prepared Pd/LDH samples. The samples prepared with ethanol as the colloidal or impregnation solvent (Sol-Pd/LDH-Et or IM-Pd/LDH-Et) had higher Pd dispersions and surface areas than the corresponding samples prepared with water as the colloidal or impregnation solvent (Sol-Pd/LDH-Aq or IM-Pd/LDH-Aq). These differences were ascribed to different interaction of solvent with stabilizer PVP and LDH support. Sol-Pd/LDH-Et exhibited a better catalytic activity and selectivity in solvent-free oxidation of benzyl alcohol with O2 than Sol-Pd/LDH-Aq and the samples prepared by impregnation. This work provides an alternative and efficient route to improve the catalytic performances of the LDH supported catalysts via adjusting the solvents and methods of supporting metal NPs.

Thermally stable Pd/reduced graphene oxide aerogel catalysts for solvent-free oxidation of benzyl alcohol

In this work, Pd/reduced graphene oxide (Pd/RGO) aerogels, with large surface areas and wrinkles, are obtained through the lyophilisation, vacuum and reduction treatments. Pd/RGO aerogels exhibit higher catalytic performances and thermal stability in comparison to the reference catalyst Pd/activated carbon (Pd/AC) in the oxidation of benzyl alcohol. The surface analyses indicate that Pd/RGO aerogels possess a higher proportion of Pd2+ than the Pd/AC catalyst. Meanwhile, the oxygen-containing functional groups can modulate the interaction between palladium and the support to form active phases for the reaction. Moreover, Pd nanoparticles (PdNPs) remain well dispersed in nanoscale on the reacted Pd/RGO aerogels.

Screening of the Au:Pt Atomic Ratio Supported in SrCO3: Effects on the Performance of the Solvent-Free Oxidation of Benzyl Alcohol

We have prepared, by a sol-immobilization method, bimetallic catalysts with different Au:Pt atomic ratios supported on commercial SrCO3. The catalytic performance for the oxidation reaction of benzyl alcohol of such materials was compared to the monometallic counterparts, aiming at the obtaining of the best composition of the material. It was found that the Au:Pt atomic ratio presents a remarkable effect on the system performance, i.e., Pt-rich systems are more selective; however, less active. Thus, an equilibrium related to the activity and selectivity of the system was obtained by considering the yield of the system. Also, some density functional theory (DFT) insights were obtained by using a cluster of 14 atoms of Au and 1 or 2 atoms of Pt. X-ray photoelectron spectroscopy, elemental mapping in scanning transmission electron microscopy before and after catalyst usage, flame atomic absorption spectroscopy, Rietveld refinement, among other techniques, were used and associated to the experimental data, which allowed us to propose a catalytic mechanism for the system, which was important since SrCO3 has not been considered before as catalyst support for alcohol oxidation reactions.

Mesoporous silica-encapsulated gold core–shell nanoparticles for active solvent-free benzyl alcohol oxidation

Silica-encapsulated gold core@shell nanoparticles (Au@SiO2 CSNPs) were synthesized via a tunable bottom-up procedure to catalyze the aerobic oxidation of benzyl alcohol.

Investigations of supported Au-Pd nanoparticles on synthesized CeO2 with different morphologies and application in solvent-free benzyl alcohol oxidation

Au-Pd bimetallic nanoparticles immobilized on series of CeO2 supports with different morphologies, e.g., rod, cube, and polyhedrons were prepared through the deposition-precipitation method with a consequent investigation on their catalytic performances for benzyl alcohol oxidation in the absence of solvent. The experimental results exhibited that the morphology of CeO2 has a markedly impact on the catalytic performance of Au-Pd/CeO2. In which Au-Pd supported on CeO2 rod could achieve higher benzyl alcohol conversion than that supported on CeO2 polyhedrons and CeO2 cube, however, CeO2 cube supported Au-Pd showed the highest selectivity towards the production of benzaldehyde. ICP-AES, XRD, Raman, N2-BET, TEM, HAADF-STEM, and XPS were conducted to characterize the catalysts. The results revealed that the excellent behavior of Au-Pd/CeO2-rod in benzyl alcohol oxidation was closely related with the smaller size of CeO2 particle, the higher concentration of oxygen defects in support and the higher number of Ce3+ and Pd2+ species on the catalyst surface. The present study on the morphologies of CeO2 support in solvent-free benzyl alcohol oxidation would offer a notable approach for the future catalyst design.

Engineering polyoxometalate anions on porous ionic network towards highly catalytic active noble metal clusters

High surface energy of noble metal clusters (NMCs) with sub-nanometer size (<2 nm) endows their remarkable catalytic activity but naturally increases the risk of deactivation due to aggregation or oxidation. As a result, it is challengeable to construct efficient and stable naked NMCs. Herein, we reported the solvothermal synthesis of polyhedral oligomeric silsesquioxane (POSS) based porous ionic polymers with large surface areas, tunable pore structures and abundant ionic sites that resemble the structure of traditional ionic liquids. A series of highly dispersive and stable NMCs (Pd, Pt, Ru and Rh) were generated on these polymers by modulating their polyoxometalate (POM) anions, attributable to their robust framework and ion-exchange property. These organic cations paired POM anions are found to provide versatile strong interaction towards noble metal species, responsive for the formation and stabilization of NMCs. The obtained Pd clusters effectively catalyzed the solvent-free oxidation of benzyl alcohol with atmospheric dioxygen (O2), affording a high yield of 97% and a huge turnover number (TON) that exceeded 639 times of commercial Pd/C catalyst. The catalyst also exhibited pleasurable reusability and broad substrate compatibility.

Preparation of cyclonic Co3O4/Au/mesoporous SiO2 catalysts with core–shell structure for solvent-free oxidation of benzyl alcohol

• A core–shell catalyst was prepared by cyclonic Co 3 O 4 as core and m-SiO 2 as shell for selective oxidation of benzyl alcohol. • 58% conversion and 82% selectivity were obtained together with the excellent reusability and stability. • A plausible oxidation reaction mechanism over SiO@Co 3 O 4 /Au@m-SiO 2 catalysts was tentatively proposed. As one of the ideal catalysts, noble metal materials can realize the conversion from benzyl alcohol to benzaldehyde. However, in previous reports, the loss of surface noble metal is one of the important reasons for the decrease in reaction performance. Here, a simple method was reported for stepwise fabrication of SiO 2 @Co 3 O 4 /Au@m-SiO 2 catalysts with core–shell structure. On the one hand, the core–shell structure provided abundant reaction sites for the oxidation of benzyl alcohol to benzaldehyde. On the other hand, the outer layer of m-SiO 2 effectively prevents the loss of Au nanoparticles. In this process, we studied the effects of multiple factors on the target reaction by controlling a single variable. The experimental results show that under the optimal conditions (the catalyst dosage, reaction temperature, reaction time and O 2 flow rate are 40 mg, 160 °C, 6 h, 60 ml/min, respectively), the conversion rate of the target reaction reaches 58% and the selectivity is as high as 82%. In the end, a mechanism was put forward to illustrate the underlying reaction pathway in benzyl alcohol oxidation.

Three-dimensional N-doped graphene aerogel-supported Pd nanoparticles as efficient catalysts for solvent-free oxidation of benzyl alcohol.

Herein, three-dimensional (3D) nitrogen-doped graphene with large surface areas and abundant porous structures was prepared by a hydrothermal synthesis method, which served as a novel support to enhance the catalytic properties of noble metal catalysts for the solvent-free selective oxidation of benzyl alcohol. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and Brunauer–Emmett–Teller (BET) method. The results clearly showed that the introduced N-containing group prevented the aggregation of graphene sheets and provided more structural defects to maximize the number of exposed active sites. The three-dimensional structure can provide a unique porous structure and large specific surface area. Moreover, the three-dimensional structure makes the recycling and reuse of the catalyst easier. The combination of these properties results in the reduction of the average particle size of metal palladium to 3.2 nm; this significantly increases the catalytic activity of the catalyst. The three-dimensional N-doped graphene aerogel-supported Pd nanoparticle (3D Pd/NRGO) composites exhibit excellent catalytic activity for the solvent-free selective oxidation of benzyl alcohol to benzaldehyde by molecular oxygen at 90 °C for 3 hours under atmospheric pressure, resulting in a 72.2% conversion of benzyl alcohol with 94.5% selectivity for benzaldehyde. In addition, the catalytic efficiency shows no obvious loss even after six repeated cycles. Thus, 3D Pd/NRGO can be used as an efficient, easily separable, recyclable, and stable catalyst for the solvent-free selective oxidation of benzyl alcohol under relatively mild conditions.

Rational design of ordered Pd–Pb nanocubes as highly active, selective and durable catalysts for solvent-free benzyl alcohol oxidation

The selective oxidation of alcohols is an important chemical process with many potential applications in fine-chemical synthesis, however, the catalysts reported so far generally suffer from low activity and poor selectivity. We present here a class of ordered Pd3Pb nanocubes (NCs) with an intermetallic phase and well-defined surface as unique catalysts for selective oxidation of benzyl alcohol to benzaldehyde. The optimized Pd3Pb NCs exhibit both enhanced activity with the turnover frequency of 191 000 h-1 and excellent selectivity up to 91.0%, outperforming the Pd3Pb/Pd NCs and Pd3Pb nanoparticles (NPs) as well as Pd NPs. The Pd3Pb NCs show increased activity in the initially consecutive reaction cycles and do not suffer from distinct deactivation thereafter. X-ray photoelectron spectroscopy reveals that the high ratio of Pd2+ to Pd0 in the surface of Pd-Pb NPs enhances the benzaldehyde productivity, and the electronic modification of the Pd3Pb NCs boosts the benzaldehyde selectivity. Attenuated total reflectance infrared spectra further confirms that the weak benzaldehyde absorption on the Pd3Pb surface results in high selectivity of benzaldehyde.

Synthesis of highly dispersed gold nanoparticles on Al2O3, SiO2, and TiO2 for the solvent-free oxidation of benzyl alcohol under low metal loadings

We reported the organic template-free synthesis of gold (Au) nanoparticles (NPs) supported on TiO2, SiO2, and Al2O3 displaying uniform Au sizes and high dispersions over the supports. The Au-based catalysts were prepared by a deposition–precipitation method using urea as the precipitating agent. In the next step, the solvent-free oxidation of benzyl alcohol was investigated as model reaction using only 0.08–0.05 mol% of Au loadings and oxygen (O2) as the oxidant. Very high catalytic performances (TOF up to 443,624 h−1) could be achieved. Specifically, we investigated their catalytic activities, selectivity, and stabilities as well as the role of metal–support interactions over the performances. The conversion of the substrate was found to be associated with the nature of the employed support as the Au NPs presented similar sizes in all materials. A sub-stoichiometric amount of base was sufficient for the catalyst activation and the observation of the catalysts profile over the time enable insights on their recyclability performances. We believe this reported method represents a facile approach for the synthesis of uniform Au-supported catalysts displaying high performances.

Suzuki–Miyaura reaction and solventfree oxidation of benzyl alcohol by Pd/nitrogen-doped CNTs catalyst

Suzuki–Miyaura C–C coupling reactions were investigated with Pd/nitrogen-doped carbon nanotubes (Pd/N-CNTs) as a catalyst. Also, the same catalyst was examined for the solventfree oxidation of benzyl alcohol to benzaldehyde. Nitrogen-doped carbon nanotubes (N-CNTs) were synthesized from 1-ferrocenylmethyl(2-methylimidazole) and benzophenone via a chemical vapour deposition technique. Acetonitrile was used as a solvent and source of both carbon and nitrogen constituents of N-CNTs. Pd nanoparticles (Pd NPs) were successfully dispersed on N-CNTs via a metal organic chemical vapour deposition method. SEM, TEM, XRD, elemental analysis and ICP-OES measurements were used to characterize the nanomaterials. From the TEM analysis, it was observed that Pd NPs were spherical and with particle sizes ranging from 3 to 8 nm. For Suzuki C–C coupling reactions, phenylboronic acid, aryl halide, Pd/N-CNTs catalyst and a base (NaOAc, K2PO4, K2CO3, NaOH, Et3N and Na2CO3) were used. The optimized experiments indicate that K2CO3, as the base, and ethanol/water (1:1 v/v, 10 mL) mixture, as a solvent, are the best reaction conditions. The solventfree oxidation reactions of benzyl alcohol were also done with Pd/N-CNTs catalyst and benzyl alcohol as a substrate. In both sets of reactions, C–C coupling and oxidation, the increase in pyrrolic nitrogen species was found to be responsible for higher catalytic activities of Pd/N-CNT catalysts, and this was attributed to the ease of Pd NP dispersion on N-CNTs, relative to pristine CNTs. Also, the higher catalytic activity of Pd/N-CNTs could be ascribed not only to the smaller Pd NP size or surface area, but to also the surface properties and the nature of the support when compared with the undoped counterpart, Pd/CNTs.