Selective Benzyl Alcohol Oxidation over Pd Catalysts

Homo- and Heterogeneous Benzyl Alcohol Catalytic Oxidation Promoted by Mononuclear Copper(II) Complexes: The Influence of the Ligand upon Product Conversion

The catalytic properties of three copper complexes, [Cu(en)2](ClO4)2 (1), [Cu(amp)2](ClO4)2, (2) and [Cu(bpy)2](ClO4)2 (3) (where en = ethylenediamine, amp = 2-aminomethylpyridine and bpy = 2,2'-bipyridine), were explored upon the oxidation of benzyl alcohol (BnOH). Maximized conversions of the substrates to their respective products were obtained using a multivariate analysis approach, a powerful tool that allowed multiple variables to be optimized simultaneously, thus creating a more economical, fast and effective technique. Considering the studies in a fluid solution (homogeneous), all complexes strongly depended on the amount of the oxidizing agent (H2O2), followed by the catalyst load. In contrast, time seemed to be statistically less relevant for complexes 1 and 3 and not relevant for 2. All complexes showed high selectivity in their optimized conditions, and only benzaldehyde (BA) was obtained as a viable product. Quantitatively, the catalytic activity observed was 3 > 2 > 1, which is related to the π-acceptor character of the ligands employed in the study. Density functional theory (DFT) studies could corroborate this feature by correlating the geometric index for square pyramid Cu(II)-OOH species, which should be generated in the solution during the catalytic process. Complex 3 was successfully immobilized in silica-coated magnetic nanoparticles (Fe3O4@SiO2), and its oxidative activity was evaluated through heterogenous catalysis assays. Substrate conversion promoted by 3-Fe3O4@SiO2 generated only BA as a viable product, and the supported catalyst's recyclability was proven. Reduced catalytic conversions in the presence of the radical scavenger (2,2,6,6-tetrametil-piperidi-1-nil)oxil (TEMPO) indicate that radical and non-radical mechanisms are involved.

Catalytic, Antifungal, and Antiproliferative Activity Studies of a New Family of Mononuclear [VIVO]/[VVO2] Complexes

Ligands derived from 2-(1-phenylhydrazinyl)pyridine and salicylaldehyde (HL1), 3-methoxysalicylaldehyde (HL2), 5-bromosalicylaldehyde (HL3), and 3,5-di-tert-butylsalicylaldehyde (HL4) react with [VIVO(acac)2] in MeOH followed by aerial oxidation to give [VVO2(L1)] (1), [VVO2(L2)] (2), [VVO2(L3)] (3), and [VVO2(L4)] (4). Complex [VIVO(acac)(L1)] (5) is also isolable from [VIVO(acac)2] and HL1 in dry MeOH. Structures of all complexes were confirmed by single-crystal X-ray and spectroscopic studies. They efficiently catalyze benzyl alcohol and its derivatives' oxidation in the presence of H2O2 to their corresponding aldehydes. Under optimized reaction conditions using 1 as a catalyst precursor, conversion of benzyl alcohol follows the order: 4 (93%) > 2 (90%) > 1 (86%) > 3 (84%) ≈ 5 (84%). These complexes were also evaluated for antifungal and antiproliferative activities. Complex 3 with MIC50 = 16 μg/mL, 4 with MIC50 = 12 μg/mL, and 5 with MIC50 = 16 μg/mL are efficient toward planktonic cells of Candida albicans and Candida tropicalis. On Michigan cancer foundation-7 (MCF-7) cells, they show comparable cytotoxic effects and exhibit IC50 in the 27.3-33.5 μg/mL range, and among these, 4 exhibits the highest cytotoxicity. A similar study on human embryonic kidney cells (HEK293) confirms their less toxicity at lower concentrations (4 to 16 μg/mL) compared to MCF-7.

A DFT investigation of the catalytic oxidation of benzyl alcohol using graphene oxide

CONTEXT

Metal-free heterogeneous materials have attracted great interest due to their potential to facilitate various organic transformations in line with circular economy and green chemistry principles. Among various 2D materials, graphene oxide (GO) is considered an attractive material for numerous applications in physics, chemistry, biology, material sciences, and catalysis. Furthermore, graphene-based catalysts exhibit good catalytic activity toward the selective oxidation of benzyl alcohol to benzaldehyde or benzoic acid under eco-friendly conditions. In this regard, a theoretical investigation was carried out to study both catalytic oxidation reaction pathways (i.e., benzyl alcohols to aldehyde and to benzoic acid) using GO as an eco-friendly and metal-free catalyst.

METHODS

In this study, we report a theoretical investigation at the B3LYP/6-31G level to better understand the oxidation of benzyl alcohol using GO as a metal-free catalyst. The possible bond formation was investigated using the global and local reactivity indexes derived from Fukui functions. Furthermore, we performed a non-covalent interaction (NCI) analysis to unveil the stability and the interaction nature between both reagents and GO surface. The effect of the solvent on the oxidation efficiency was also performed and the results indicate that the solvent significantly affects the decrease of reactivity by increasing the activation barriers through oxidation reactions of benzyl alcohol. Additionally, the electron localization function (ELF) analysis was performed for all intermediates showing the ionic nature of the studied epoxide structure of GO and rules out any type of covalent interaction during the oxidation reaction of benzyl alcohol. All these obtained results are in good agreement with experimental observations and reveal that the epoxide functions on the graphene surface promote an excellent catalyst turnover.

Metallic nanoparticles for catalytic reduction of toxic hexavalent chromium from aqueous medium: A state-of-the-art review

The ever increasing concentration of toxic and carcinogenic hexavalent chromium (Cr (VI)) in various environmental mediums including water-bodies due to anthropogenic activities with rapid civilization and industrialization have become the major issue throughout the globe during last few decades. Therefore, developing new strategies for the treatment of Cr(VI) contaminated wastewaters are in great demand and have become a topical issue in academia and industry. To date, various techniques have been used for the remediation of Cr(VI) contaminated wastewaters including solvent extraction, adsorption, catalytic reduction, membrane filtration, biological treatment, coagulation, ion exchange and photo-catalytic reduction. Among these methods, the transformation of highly toxic Cr(VI) to benign Cr(III) catalyzed by metallic nanoparticles (M-NPs) with reductant has gained increasing attention in the past few years, and is considered to be an effective approach due to the superior catalytic performance of M-NPs. Thus, it is a timely topic to review this emerging technique for Cr(VI) reduction. Herein, recent development in synthesis of M-NPs based non-supported, supported, mono-, bi- and ternary M-NPs catalysts, their characterization and performance for the reduction of Cr(VI) to Cr(III) are reviewed. The role of supporting host to stabilize the M-NPs and leading to enhance the reduction of Cr(VI) are discussed. The Cr(VI) reduction mechanism, kinetics, and factors affecting the kinetics are overviewed to collect the wealthy kinetics data. Finally, the challenges and perspective in Cr(VI) reduction catalyzed by M-NPs are proposed. We believe that this review will assist the researchers who are working to develop novel M-NPs catalysts for the reduction of Cr(VI).

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.

A Novel Pd-P Nano-Alloy Supported on Functionalized Silica for Catalytic Aerobic Oxidation of Benzyl Alcohol

Catalytic aerobic oxidation of benzyl alcohol (BnOH) to benzaldehyde (PhCHO) over supported noble metal catalysts has grabbed the attention of researchers due to the critical role of PhCHO in numerous industrial syntheses. In the present study, a novel catalyst, Pd-P alloy supported on aminopropyl-functionalized mesoporous silica (NH2-SiO2), was prepared through in situ reduction and characterized by BET-BJH analysis, SEM, TEM, XRD, FTIR, TG-DTA, and XPS. Chemical properties and catalytic performance of Pd-P/NH2-SiO2 were compared with those of Pd° nanoparticles (NPs) deposited on the same support. Over Pd-P/NH2-SiO2, the BnOH conversion to PhCHO was much higher than over Pd°/NH2-SiO2, and significantly influenced by the nature of solvent, reaching 57% in toluene at 111 °C, with 63% selectivity. Using pure oxygen as an oxidant in the same conditions, the BnOH conversion increased up to 78%, with 66% selectivity. The role of phosphorous in improving the activity may consist of the strong interaction with Pd that favours metal dispersion and lowers Pd electron density.

A Perspective on Heterogeneous Catalysts for the Selective Oxidation of Alcohols

Selective oxidation of higher alcohols using heterogeneous catalysts is an important reaction in the synthesis of fine chemicals with added value. Though the process for primary alcohol oxidation is industrially established, there is still a lack of fundamental understanding considering the complexity of the catalysts and their dynamics under reaction conditions, especially when higher alcohols and liquid‐phase reaction media are involved. Additionally, new materials should be developed offering higher activity, selectivity, and stability. This can be achieved by unraveling the structure–performance correlations of these catalysts under reaction conditions. In this regard, researchers are encouraged to develop more advanced characterization techniques to address the complex interplay between the solid surface, the dissolved reactants, and the solvent. In this mini‐review, we report some of the most important approaches taken in the field and give a perspective on how to tackle the complex challenges for different approaches in alcohol oxidation while providing insight into the remaining challenges.

Synthesis of Palladium and Copper Nanoparticles Supported on TiO2 for Oxidation Solvent-Free Aerobic Oxidation of Benzyl Alcohol

The use of metal oxides as supports for palladium and copper (Pd–Cu) nanoalloys constitutes a new horizon for improving new active catalysts in very important reactions. From the literatures, Pd-based bimetallic nanostructures have great properties and active catalytic performance. In this work, nanostructures of titanium dioxide (TiO2) were used as supports for Pd–Cu nanoparticles catalysts. Palladium and copper were deposited on these supports using the sol-immobilisation method. The composite nanoalloys were characterized using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The catalyst was evaluated for the oxidation of benzyl alcohol. The effect of the Cu–Pd ratio using sol-immobilization methods supported on TiO2 was investigated. The results show that monometallic Cu/TiO2 was observed to have a low activity. However, as soon as the catalyst contained any palladium, the activity increased with a significant increase in the selectivity towards isomerization products. The influence of support and temperature were investigated. Furthermore, the catalyst reusability was also tested for oxidation of benzyl alcohol reactions, by repeatedly performing the same reaction using the recovered catalyst. The Pd–Cu/TiO2 catalyst displayed better reusability even after several reactions

Bimetallic PdAu Catalysts within Hierarchically Porous Architectures for Aerobic Oxidation of Benzyl Alcohol

Hierarchically porous (HP) zeotype materials (possessing both micropores and mesopores) offer improved diffusional access to intra-framework active sites, analogous to mesoporous materials, yet retain the high selectivity of the microporous (MP) bulk. We have recently designed crystalline hierarchically porous silicoaluminophosphates (SAPOs) with enhanced mass-transport characteristics, which can lead to significant improvement in catalytic activity and catalyst lifetime. In this study, we have prepared PdAu bimetallic nanostructures supported on HP-SAPO frameworks by an incipient impregnation of metal precursors followed by H₂ reduction at 300 °C, for the aerobic oxidation of benzyl alcohol to benzaldehyde. PdAu NPs supported on HP framework displayed significantly enhanced catalytic activities, when compared with their MP analogues, clearly highlighting the benefits of introducing hierarchical porosity in the SAPO support matrix.

Pd Nanoparticles-Loaded Vinyl Polymer Gels: Preparation, Structure and Catalysis

Four vinyl polymer gels (VPGs) were synthesized by free radical polymerization of divinylbenzene, ethane-1,2-diyl dimethacrylate, and copolymerization of divinylbenzene with styrene, and ethane-1,2-diyl dimethacrylate with methyl methacrylate, as supports for palladium nanoparticles. VPGs obtained from divinylbenzene and from divinylbenzene with styrene had spherical shapes while those obtained from ethane-1,2-diyl dimethacrylate and from ethane-1,2-diyl dimethacrylate with methyl methacrylate did not have any specific shapes. Pd(OAc)2 was impregnated onto VPGs and reduced to form Pd0 nanoparticles within VPGs. The structures of Pd0-loaded VPGs were analyzed by XRD, TEM, and nitrogen gas adsorption. Pd0-loaded VPGs had nanocrystals of Pd0 within and on the surface of the polymeric supports. Pd0/VPGs efficiently catalyzed the oxidation/disproportionation of benzyl alcohol into benzaldehyde/toluene, where activity and selectivity between benzaldehyde and toluene varied, depending on the structure of VPG and the weight percentage loading of Pd0. The catalysts were stable and Pd leaching to liquid phase did not occur. The catalysts were separated and reused for five times without any significant decrease in the catalytic activity.

A new porous Co(ii)-metal–organic framework for high sorption selectivity and affinity to CO2 and efficient catalytic oxidation of benzyl alcohols to benzaldehydes

A new porous Co(ii)-MOF I has been prepared. Gas sorption and catalytic experiments show that the guest-free sample I′ shows the high capacity and selectivity to CO₂ over CH₄ and catalytically oxidize benzyl alcohols efficiently into benzaldehydes.

An overview of palladium supported on carbon-based materials: Synthesis, characterization, and its catalytic activity for reduction of hexavalent chromium

Palladium plays a pivotal role in most of the industrial heterogeneous catalysts, because of its unique properties such as well-defined structure, great intrinsic carrier, outstanding electronic, mechanical and thermal stability. The combination of palladium and various porous carbons (PCs) can widen the use of heterogeneous catalysts. This review highlights the advantages and limitations of carbon supported palladium-based heterogeneous catalyst in reduction of toxic hexavalent chromium (Cr^(VI)). In addition, we address recent progress on synthesis routes for mono and bimetallic palladium nanoparticles supported by various carbon composites including graphene-based materials, carbon nanotubes, mesoporous carbons, and activated carbons. The related reaction mechanisms for the Cr^(VI) reduction are also suggested. Finally, the challenge and perspective are proposed.

Oxidized Palladium Supported on Ceria Nanorods for Catalytic Aerobic Oxidation of Benzyl Alcohol to Benzaldehyde in Protic Solvents

In the present study, the catalytic activity of palladium oxide (PdOx) supported on ceria nanorods (CeO2-NR) for aerobic selective oxidation of benzyl alcohol (BnOH) to benzaldehyde (PhCHO) was evaluated. The CeO2-NR was synthesized hydrothermally and the Pd(NO3)2 was deposited by a wet impregnation method, followed by calcination to acquire PdOx/CeO2-NR. The catalysts were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), transmission electron microscopy (TEM), Brunauer–Emmet–Teller (BET) surface area analysis, and X-ray photoelectron spectroscopy (XPS). In addition, the TPR-reduced PdOx/CeO2-NR (PdOx/CeO2-NR-Red) was studied by XRD, BET, and XPS. Characterizations showed the formation of CeO2-NR with (111) exposed plane and relatively high BET surface area. PdOx (x > 1) was detected to be the major oxide species on the PdOx/CeO2-NR. The activities of the catalysts in BnOH oxidation were evaluated using air, as an environmentally friendly oxidant, and various solvents. Effects of temperature, solvent nature and palladium oxidation state were investigated. The PdOx/CeO2-NR showed remarkable activity when protic solvents were utilized. The best result was achieved using PdOx/CeO2-NR and boiling ethanol as solvent, leading to 93% BnOH conversion and 96% selectivity toward PhCHO. A mechanistic hypothesis for BnOH oxidation with PdOx/CeO2-NR in ethanol is presented.

Benzyl alcohol oxidation with Pd-Zn/TiO2: computational and experimental studies

Pd-Zn/TiO2 catalysts containing 1 wt% total metal loading, but with different Pd to Zn ratios, were prepared using a modified impregnation method and tested in the solvent-free aerobic oxidation of benzyl alcohol. The catalyst with the higher Pd content exhibited an enhanced activity for benzyl alcohol oxidation. However, the selectivity to benzaldehyde was significantly improved with increasing presence of Zn. The effect of reduction temperature on catalyst activity was investigated for the catalyst having a Pd to Zn metal molar ratio of 9:1. It was found that lower reduction temperature leads to the formation of PdZn nanoparticles with a wide particle size distribution. In contrast, smaller PdZn particles were formed upon catalyst reduction at higher temperatures. Computational studies were performed to compare the adsorption energies of benzyl alcohol and the reaction products (benzaldehyde and toluene) on PdZn surfaces to understand the oxidation mechanism and further explain the correlation between the catalyst composition and its activity.

High-Throughput Production of Heterogeneous RuO2/Graphene Catalyst in a Hydrodynamic Reactor for Selective Alcohol Oxidation

We report on the high-throughput production of heterogeneous catalysts of RuO2-deposited graphene using a hydrodynamic process for selective alcohol oxidation. The fluid mechanics of a hydrodynamic process based on a Taylor–Couette flow provide a high shear stress field and fast mixing process. The unique fluidic behavior efficiently exfoliates graphite into defect-free graphene sheets dispersed in water solution, in which ionic liquid is used as the stabilizing reagent to prevent the restacking of the graphene sheets. The deposition of RuO2 on a graphene surface is performed using a hydrodynamic process, resulting in the uniform coating of RuO2 nanoparticles. The as synthesized RuO2/IL–graphene catalyst has been applied efficiently for the oxidation of a wide variety of alcohol substrates, including biomass-derived 5-hydroxymethylfurfural (HMF) under environmentally benign conditions. The catalyst is mechanically stable and recyclable, confirming its heterogeneous nature.