Solvent-free Selective Oxidation Research Articles

Mononuclear Cu-complex embedded ordered nano-porous silica based hybrid catalyst for oxidation of benzyl alcohol

A novel mixed ligand copper (׀׀) complex having 2,9-dimethyl-1,10-phenanthroline and nitrate has been successfully synthesized and characterized by single crystal XRD. Following synthesis, this complex was anchored on acid functionalized nano-porous materials such as MCM-41-SO3H and SBA-15-SO3H, and these hybrid materials were characterized using a variety of techniques such as FT-IR, PXRD, BET, SEM, TG, and so on. These modified mesoporous materials (MCM-41-SO3HCu and SBA-15-SO3HCu) were subjected to solvent-free selective benzyl alcohol oxidation with TBHP (Tert-Butyl Hydroperoxide) as an oxidant. Various reaction parameters were investigated to optimize the reaction conditions. MCM-41-SO3HCu was discovered to be an excellent catalyst in terms of benzyl alcohol conversion and benzaldehyde selectivity.

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 selective oxidation of benzyl alcohol using Ru loaded ceria-zirconia catalysts

Selective oxidation of alcohols to aldehydes or ketones is a fundamental process in organic chemistry and the development of green and sustainable transformations are strongly required. Herein, we focused on the development of Ru-based catalysts deposited over ceria-zirconia supports (CZRu) for the solvent free oxidation of benzyl alcohol using air as oxidant, which is a cheap and safe process and meets to the atom economy and environment requirements. XPS characterization of the material reveals that the formation of RuO2 on the surface is necessary to achieve higher catalytic activity. The activity is strongly related to the reducibility of the material and to the close interaction between ceria-zirconia and RuO2, with the likely formation of Ru-O-Ce arrangements. The greater mobility of oxygen due to the formation of bridging oxygens in Ru-O-Ce and to the formation of superoxide species (O2−) over Ce3+ sites, significantly boosts the selective oxidation of alcohols. CZRu is a promising material with a remarkable 61% alcohol conversion at 90 °C without solvents; it also exhibits high activity (55% conversion) and complete selectivity (100%) for the selective oxidation of 1-phenylethanol to acetophenone. Overall, the formulation is among the most active reported so far under solvent free conditions and molecular oxygen. In addition, the reaction over CZRu exhibits an environmental factor (E-factor) lower than 1 (0.95) confirming the sustainability of the proposed process.

Solvent-free selective oxidation of cyclohexane to KA oil in air over CoWO4@W18O49 catalyst

The challenge for researchers in heterogeneous catalysis is to develop robust catalysts while using benign reaction conditions, without producing unwanted by-products and preventing leaching of metal during the processes. We have synthesized nanosized CoWO4@W18O49 catalyst with oxygen vacancy sites for molecular oxygen activation from air, and this catalyst selectively oxidized cyclohexane to KA oil (KA oil referred to ketone-alcohol oil) in solvent-free conditions, where under optimum conditions, ∼24 % cyclohexane conversion with ∼93 % KA oil selectivity was achieved. The catalyst has been synthesized by the co-precipitation method and studied with the help of various analytical tools. DFT studies were also performed to calculate the energy needed for the oxygen vacancy generation over the CoWO4 (111), W18O49 (010) and CoWO4@W18O49 (010) surfaces. The energy required for the removal of oxygen atom at the CoWO4@W18O49 (010) surface was found to be − 15.8 kJ/mol, which is 96.2 kJ/mol and 46.9 kJ/mol lower than the CoWO4 (111) and W18O49 (010) surfaces respectively. The exceptional catalytic activity of CoWO4@W18O49 is an outcome of the combined contribution of the active CoWO4 and W18O49 interfaces, which have redox sites and a lot of vacancies of oxygen that benefit from the effective interaction.

Ultrasonic irradiation-assisted MnFe2O4 nanoparticles catalyzed solvent-free selective oxidation of benzyl alcohol to benzaldehyde at room temperature.

Magnetic nanoparticles (NPs) play a vital role in heterogeneous catalysis because of their easy separation, and effective recyclability. Herein, we report the synthesis of MnFe2O4 NPs for use as catalysts in the selective oxidation of benzyl alcohol to benzaldehyde under mild conditions. The MnFe2O4 NPs have been synthesized by precipitation method followed by hydrothermal ageing at 180 °C for 4.0 h. We have investigated the effect of chitosan and carboxymethyl cellulose on the size or morphology of the formed MnFe2O4 NPs. The X-ray diffraction study confirms the formation of pure and crystalline MnFe2O4 with varying average crystallite sizes ranging from 18 to 28 nm based on the type of additive used. The electron microscopy study reveals that the additive plays a significant role in controlling the morphology of the formed MnFe2O4 NPs. These MnFe2O4 NPs exhibit superparamagnetic behaviour at room temperature and can effectively catalyze the solvent-free selective oxidation of benzyl alcohol to benzaldehyde in the presence of tert-butyl hydroperoxide at room temperature under ultrasonic irradiation. The developed protocol can be extended to various substituted benzyl alcohols having both the electron withdrawing and electron donating groups to afford moderate to excellent yield of the products. The catalyst is magnetically retrievable, highly stable, and can be reused up to the sixth run without significant loss of catalytic activity.

High performance BaCO3-CeO2 composite catalyst for solvent-free selective oxidation of cyclohexane with molecular oxygen

BaCO3-CeO2 composite catalysts were prepared by the hydrothermal synthesis method, and the influence of Ba/Ce on the structural and catalytic performance for selective oxidation of cyclohexane with molecule oxygen was studied. The characterization results exhibited that the doping amount of BaCO3 would influence the morphology, the content of surface Ce3+ and surface oxygen species (Oβ), and alkalinity. Meanwhile, the amount of alkali is the key to the decomposition of cyclohexyl hydrogen peroxide (CHHP). 10.5% conversion of cyclohexane and 85.0% selectivity of KA oil were achieved under the reaction conditions of 1.0 MPa O2 and 150 °C for 2 h. The excellent catalytic activity was attributed to its high atomic ratios of Ce3+/Cetotal and Oβ/Ototal on the surface and optimal redox capacity.

Interface engineered Ca–Co mixed oxide heterostructures for boosting the aerobic and solvent-free selective oxidation of ethybenzene

Exploration of a high-performance, low-cost and easy-to-prepare solid catalyst is a permanent goal to advance the selective oxidation of hydrocarbons to high-value-added oxygenated products. Herein, we demonstrated for the first time that Ca–Co mixed oxide with abundant strongly interacting heterointerfaces for advanced ethylbenzene oxidation performance under solvent-free conditions and using molecular oxygen as the only oxidant. The interface engineering strategy contributes to the formation of weakened Co–O bonds, enhanced oxygen vacancy concentration and boosted reactivity of lattice oxygen, facilitating the generation of superoxide radical anions for driving the ethylbenzene (EB) oxidation catalysis. In addition, the CaxCoyOz catalyst exhibits good recyclability and high stability. This study provides a paradigm for the rational construction of metal oxides with hybrid interfaces towards a lab as well as a large-scale green oxidation of EB and beyond.

Solvent-free selective oxidation of alcohols with tert-butyl hydroperoxide catalyzed by dioxo-molybdenum (VI) unsymmetrical Schiff base complex immobilized on CoFe2O4@SiO2 nanoparticles

Solvent-free selective oxidation of alcohols with tert-butyl hydroperoxide catalyzed by dioxo-molybdenum (VI) unsymmetrical Schiff base complex immobilized on CoFe2O4@SiO2 nanoparticles

Synthesis of single-atom dispersed Co-NC catalytic materials in supercritical CO2 environment with inorganic salt precursor

Supported single-atom catalytic materials (SACMs) have aroused extensive interest due to their remarkable properties. Herein, a novel method was proffered to prepare Co-NC SACMs using custom-designed N-doped carbon (NC) as support, CoCl2 as the precursor in supercritical CO2 with appropriate co-solvent. It was found that when ethanol was used as co-solvent, nanoparticles were obtained while only Co single-atom was obtained when N, N-Dimethylformamide (DMF) was used as co-solvent. Both the theoretical loading and the deposition time influenced the Co loading a lot, and the maximum Co loading was as high as 1.71 wt%. Strong acid, strong alkali, corrosive solvent, and high-temperature treatment were avoided in the whole process. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM), X-ray photoelectron spectroscopy (XPS), Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES), and Energy Dispersive X-ray spectroscopy (EDX) were conducted to confirm the existence and uniform distribution of Co atoms over the N-doped carbon materials. The conversion rate of 90.0% and selectivity of 86.0% were obtained for the solvent-free selective oxidation of benzyl alcohol to benzaldehyde at 70 °C and 1 h using the Co-NC SACMs.

Solvent-free liquid-phase selective catalytic oxidation of toluene to benzyl alcohol and benzaldehyde over CeO2–MnOx composite oxides

A novel CeO2–MnOx composite oxide was prepared by the improved sol–gel method. The synergistic catalysis of Mn3+/Mn2+ and Ce4+/Ce3+ was responsible for the good catalytic performance in the liquid phase solvent-free selective oxidation of toluene.

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 alcohols with tert-butyl hydroperoxide catalyzed by palladium(II) isatin Schiff base complex supported into three-dimensional mesoporous silica KIT-6

In this work, the catalytic activity of a palladium(II) isatin Schiff base complex immobilized into mesoporous silica KIT-6 (Pd-isatin Schiff base@KIT-6) was studied for the oxidation of different alcohols with tert-butyl hydroperoxide (TBHP, 70% aqueous solution) as an oxidant under solvent-free conditions. To find the suitable reaction conditions, the effect of essential factors including the solvent, temperature, catalyst amount and kind of oxidant on the oxidation of benzyl alcohol was explored. The results showed that in this catalytic system, the corresponding aldehydes and ketones were obtained with high to excellent yields at 50 °C without the formation of carboxylic acids as by-products. The catalyst was easily recovered by simple filtration and reused in five subsequent reaction cycles without any significant loss in the catalytic activity. Moreover, the comparison of the Fourier transform infrared (FT-IR) spectrum, X-ray diffraction (XRD) pattern, scanning and transmission electron microscopy (SEM and TEM) images of the used catalyst with a fresh one showed that the structure of the Pd-isatin Schiff base@KIT-6 catalyst remained intact after five times of reuse.

Enhanced stability and activity for solvent-free selective oxidation of cyclohexane over Cu2O/CuO fabricated by facile alkali etching method

The easy deactivation of cuprous oxide (Cu2O) have significantly limited their practical applications in catalysis. Formation of composites with other semiconductors including CuO is an crucial strategy for stability improvement of Cu2O. However, this strategy developed so far has often focused on only Cu2O thin film systems. Herein, Cu2O/CuO composites were fabricated via a simple alkali etching method, for the first time, to stabilize the powdered Cu2O in liquid-phase selective oxidation of cyclohexane under solvent-free conditions. The etching method presented remarkable high dispersion of copper species and an appropriate Cu+/Cu2+ ratio on catalyst surface, which leads to superior catalytic activity and stability. Hot filtration experiment confirmed that Cu2O/CuO composite was a heterogeneous catalyst which could be reused at least five times without considerable loss of catalytic activity, whereas the pure Cu2O suffered from severe deactivation caused by oxidative decomposition of Cu2O and copper leaching. Moreover, the amount of Cu+ and Cu2+ sites on catalyst surface are dramatically affected by the amount of alkali during etching process and the catalytic performance can be further tuned by regulating the Cu+/Cu2+ ratio. The Cu2O/CuO with a molar ratio of NaOH/Cu2O = 1:1 exhibited the highest catalytic activity with a cyclohexane conversion of 84.3 % and cyclohexanone selectivity of 77.0 %. The present work provides a promising strategy to stabilize Cu2O catalysts in catalytic oxidation systems.

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.

Site-specific deposition creates electron-rich Pd atoms for unprecedented C−H activation in aerobic alcohol oxidation

Here, we demonstrate a photochemical strategy to site-specifically deposit Pd atoms on Au nanoparticles. The high-sensitivity low-energy ion scattering spectra combined with the X-ray photoelectron spectra reveal that the surface electronic structure of Pd can be continuously regulated by tailoring the Pd-to-Au molar ratio and the location of Pd atoms in AuPd nanoparticles. It is revealed that electron-rich Pd atoms are considerably more active than the net Pd atoms in aerobic alcohol oxidation. Remarkably, the catalyst with the most electron-rich Pd sites (binding energy downshift: 1.0 eV) exhibits an extremely high turnover frequency (˜500000 h−1 vs 12000 h−1 for that with net Pd atoms) for solvent-free selective oxidation of benzyl alcohol, which is, to the best of our knowledge, the highest value ever reported. Kinetic studies reveal that electron-rich Pd atoms can accelerate the oxidation of benzyl alcohol by facilitating C-H cleavage, as indicated by the significant reduction in the activation energy as compared to net Pd atoms.

Nitrogen-Dopped Ordered Mesoporous Carbon Anchored Pd Nanoparticles for Solvent Free Selective Oxidation of Benzyl Alcohol to Benzaldehyde by Using O2.

Introducing electron-rich nitrogen atoms to ordered mesoporous carbons (OMC) as supports for noble metal catalysts, not only improves the hydrophilic properties of a mesoporous carbon surface, but also enhances the coordination and binding abilities of metal ion. In the present work, nitrogen-doped ordered mesoporous carbons (NOMCs) were successfully fabricated via a facile hydrothermal self-assembly. The prepared NOMCs were characterized through powder X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption-desorption isotherm. The analyses demonstrated that the NOMCs prepared at a pyrolysis temperature of 750°C possessed an ordered 2D hexagonal mesoporous structure, a high graphitization degree, large surface area, and a well-distributed pore size. In particular, NOMCs could anchor Pd nanoparticles uniformly because of the introducing N atoms with strong electronegativity, which were selected as efficient catalysts for the partial oxidation of benzyl alcohol to benzaldehyde. Approximately 24.63% conversion with 85.71% selectivity to benzaldehyde was obtained without using any solvent by molecular O2 oxidation. Most importantly, the TOF value of the catalyst in the reaction system was up to 8698 h−1. After five runs reaction, TOF and selectivity of the catalyst remained essentially same. Hence, the proposed catalyst has a potential engineering application value.

Hydrotalcite-derived Co-MgAlO mixed metal oxides as efficient and stable catalyst for the solvent-free selective oxidation of cyclohexane with molecular oxygen

Developing a highly efficient and stable catalyst for the selective aerobic oxidation of saturated hydrocarbons to functional compounds is highly desirable in chemical industry. In our present work, hydrotalcite-derived Co-MgAlO mixed metal oxides as efficient and stable heterogeneous catalyst for the selective aerobic oxidation of cyclohexane to KA-oil has been developed. The results indicate that hydrotalcite-derived Co-MgAlO mixed metal oxides catalysts are active and selective; the synergistic catalysis of Co3+ and Co2+ can effectively promote the decomposition of cyclohexyl hydroperoxide (CHHP) intermediate to KA-oil. 9.1% of cyclohexane conversion and 82.0% of KA-oil selectivity over 2%Co-MgAlO catalyst with high turnover numbers (3800) was obtained at 423 K and 0.6 MPa for 2 h in this oxidation reaction. Moreover, the hydrotalcite-derived Co-MgAlO catalyst is easy to be separated and recycled, and its catalytic performance was still stable after five runs. The schematic reaction pathway for the oxidation of cyclohexane to KA-oil over Co-MgAlO mixed metal oxides catalyst was also suggested. Maybe this work employing inexpensive hydrotalcite-derived Co-MgAlO as highly efficient and environmentally friendly catalyst for the preparation of KA-oil has potential industrial application prospects.

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.

Solvent-free selective oxidation of toluene over metal-doped MCM-22

The synthesis of MCM-22, subsequent metal doping, and physicochemical characterization of the products are reported. The solvent-free catalytic oxidation of toluene using hydrogen peroxide is explored, and the reaction parameters are optimized. A possible reaction mechanism is also described.

Supported TiO2/MCM-41 as an efficient and eco-friendly catalyst for highly selective preparation of cyclohexanone oxime from solvent-free liquid phase oxidation of cyclohexylamine with molecular oxygen

A green and efficient approach for solvent-free selective catalytic oxidation of cyclohexylamine to cyclohexanone oxime in liquid phase has been developed. The results showed that the supported 30%TiO2/MCM-41 as efficient and eco-friendly catalyst gave 78.4% of cyclohexylamine conversion with 89.1% of selectivity to cyclohexanone oxime. It has been demonstrated that the catalytic active sites are the surface titanium hydroxyl groups of catalyst. Moreover, the present catalyst can be easily separated, recycled, regenerated and reused. The probable catalytic reaction mechanism of cyclohexylamine with molecular oxygen over supported TiO2/MCM-41 was suggested. This work affords a new and environmentally friendly route for the synthesis of cyclohexanone oxime, and has important industrial application prospects.