Liquid Phase Oxidation Of Alcohol Research Articles

Deactivation behavior and aggregation mechanism of supported Au nanoparticles in the oxidation of monoethanolamine to glycine

Deactivation behaviors of gold catalysts in the liquid-phase oxidation of monoethanolamine to glycine were revealed. Properties of the catalysts including Au loading, the valence distribution, and particle size of Au nanoparticles before and after batch reactions were investigated by BET, ICP, XRD, XPS, and TEM. It was found that the main reason for the catalytic deactivation was aggregation of Au nanoparticles. The Au aggregation mechanism of oxidative dissolution-reductive deposition in the reaction was proposed and this mechanism could help to understand the common aggregation of Au nanoparticles in the liquid-phase oxidation of alcohols.

Metal-Support Cooperative Effects in Au/VPO for the Aerobic Oxidation of Benzyl Alcohol to Benzyl Benzoate.

This paper studies the cooperative effect of Au nanoparticles deposited on vanadyl pyrophosphate oxide (VPO) in the liquid phase oxidation of benzyl alcohol. VPO was prepared using the classical method by thermally treating VOHPO4·0.5H2O precursor in reacting atmosphere at 420 °C for a period of 72 h. Au nanoparticles were deposited by incipient wetness method. The catalysts were characterized by means of XRD, TEM, XPS and Raman. The bulk VPO catalyst contains vanadyl pyrophosphate phase ((VO)2P2O7), and a small amount of VOPO4. The catalytic system exhibits a high activity in the base-free liquid phase oxidation of alcohols compared to Au on activated carbon, classic catalyst used for this type of reaction. Au/VPO showed a high peculiar selectivity to benzyl benzoate (76%), an important product used in the pharmaceutical and perfume industries. This behavior might be ascribed to the presence of strong acid sites of VPO, as determined by liquid phase titration. Stability tests performed on Au/VPO showed a deactivation of 10% after the first run, but a constant conversion along the following five cycles. This phenomenon can be attributed to the increase of mean Au particle size (from 19.1 to 23.4 nm) after recycling tests as well as the partial leaching of Au and V in the reaction media. Moreover, XRD evidenced a modification in the VPO structure with the partial formation of VOHPO4·0.5H2O phase.

Ag-Based Catalysts in Heterogeneous Selective Oxidation of Alcohols: A Review

Alcohols (bioalcohols) is a class of chemicals that are used as a feedstock for the manufacturing of a large number of valuable intermediates in industrially important processes. Currently, sustainable technologies for selective conversion of alcohols utilize “green” oxidants, mainly, ambient air or oxygen. Due to the high affinity of oxygen towards silver, the latter serves as an active component of supported heterogeneous catalysts. In this review, we consider Ag-based catalysts that participate in gas- or liquid-phase oxidation of alcohols. Oxidation of methanol, ethanol, ethylene glycol, propylene glycol, glycerol, benzyl and allyl alcohols is mostly considered. A particular attention is paid to selective photooxidation of alcohols over Ag-based catalysts. We discuss the catalyst composition in terms of (1) the state of the active component, (2) the nature of the substrate, (3) support nature, and (4) the strength of the metal–support interactions.

Selective Benzyl Alcohol Oxidation over Pd Catalysts

In the last decades, the selective liquid phase oxidation of alcohols to the corresponding carbonyl compounds has been a subject of growing interest. Research has focused on green methods that use “clean” oxidants such as O2 in combination with supported metal nanoparticles as the catalyst. Among the alcohols, benzyl alcohol is one of the most studied substrates. Indeed, benzyl alcohol can be converted to benzaldehyde, largely for use in the pharmaceutical and agricultural industries. This conversion serves as model reaction in testing new potential catalysts, that can then be applied to other systems. Pd based catalysts have been extensively studied as active catalytic metals for alcohol oxidation for their high activity and selectivity to the corresponding aldehyde. Several catalytic materials obtained by careful control of the morphology of Pd nanoparticles, (including bimetallic systems) and by tuning the support properties have been developed. Moreover, reaction conditions, including solvent, temperature, pressure and alcohol concentration have been investigated to tune the selectivity to the desired products. Different reaction mechanisms and microkinetic models have been proposed. The aim of this review is to provide a critical description of the recent advances on Pd catalyzed benzyl alcohol oxidation.

Understanding Heteroatom-Mediated Metal–Support Interactions in Functionalized Carbons: A Perspective Review

Carbon-based materials show unique chemicophysical properties, and they have been successfully used in many catalytic processes, including the production of chemicals and energy. The introduction of heteroatoms (N, B, P, S) alters the electronic properties, often increasing the reactivity of the surface of nanocarbons. The functional groups on the carbons have been reported to be effective for anchoring metal nanoparticles. Although the interaction between functional groups and metal has been studied by various characterization techniques, theoretical models, and catalytic results, the role and nature of heteroatoms is still an object of discussion. The aim of this review is to elucidate the metal–heteroatoms interaction, providing an overview of the main experimental and theoretical outcomes about heteroatom-mediated metal–support interactions. Selected studies showing the effect of heteroatom–metal interaction in the liquid-phase alcohol oxidation will be also presented.

Modified Ag/TiO2 systems: Promising catalysts for liquid-phase oxidation of alcohols

The current work is the first study concerning the liquid-phase oxidation of n-octanol and betulin over modified and unmodified Ag/TiO2 catalysts. Catalytic activity of nanosilver catalysts supported on titania for alcohols selective oxidation can be enhanced by modification of the support with Ce, Fe or Mg oxides. In most cases reductive or oxidative pretreatments of these catalysts were detrimental for their activity. The main reason for performance of Ag/MxOy/TiO2 catalysts is changes of the electronic state of the supported Ag, and especially changes in the surface concentration of Ag+ ions. Monovalent Ag+ ions are active sites in silver-containing catalysts for n-octanol, as well as for betulin oxidation. The obtained results show a potential of silver-containing catalysts for liquid phase oxidation of alcohols, and by selecting the optimum modifier and the support as well as, pretreatment conditions the catalytic properties and stabilization of the active sites can be optimized.

Silver catalysts for liquid-phase oxidation of alcohols in green chemistry: Challenges and outlook

Selective oxidation of alcohols allow using them as a key resource for obtaining of aldehydes, ketones, esters and carboxylic acids, which are valuable intermediates and high value products for the fine chemical, pharmaceutical and agrochemical industries. The high environmental impact of current technologies is driving the research of greener processes in the liquid phase. Most of this research is devoted to study catalytic systems based on platinum group metals and gold. However, despite their high levels of activity, these catalyst systems are quite expensive and suffer of deactivation and stability problems. Silver-containing catalysts may be their excellent alternative. They are the basis for a number of important industrial oxidation processes in the gas-phase. However, research on its use for liquid-phase processes is quite limited, especially for selective oxidation of alcohols.Therefore, this short review focuses on the systematization and analysis of the available literature data on the liquid phase oxidation of alcohols using silver-containing catalysts with the aim of identifying the problems and limitations of the state of the art and the prospects of further studies and practical implementation of such systems for the target processes.

Thiol Treatment Creates Selective Palladium Catalysts for Semihydrogenation of Internal Alkynes

Summary Surface and interfacial engineering of heterogeneous metal catalysts is effective and critical for optimizing selective hydrogenation for fine chemicals. By using thiol-treated ultrathin Pd nanosheets as a model catalyst, we demonstrate the development of stable, efficient, and selective Pd catalysts for semihydrogenation of internal alkynes. In the hydrogenation of 1-phenyl-1-propyne, the thiol-treated Pd nanosheets exhibited excellent catalytic selectivity (>97%) toward the semihydrogenation product (1-phenyl-1-propene). The catalyst was highly stable and showed no obvious decay in either activity or selectivity for over ten cycles. Systematic studies demonstrated that a unique Pd-sulfide/thiolate interface created by the thiol treatment was crucial to the semihydrogenation. The high catalytic selectivity and activity benefited from the combined steric and electronic effects that inhibited the deeper hydrogenation of C=C bonds. More importantly, this thiol treatment strategy is applicable to creating highly active and selective practical catalysts from commercial Pd/C catalysts for semihydrogenation of internal alkynes.

Nanoporous montmorillonite clay stabilized copper nanoparticles: Efficient and reusable catalyst for oxidation of alcohols

Stable and spherical copper nanoparticles of size less than 10 nm were generated into nanopores of activated montmorillonite clay and evaluated their catalytic activity for liquid phase alcohol oxidation to corresponding carbonyl compounds. Synthesis of copper nanoparticles was accomplished by incipient wetness impregnation of Cu(CH3COO)2 into the nanopores of the acid activated montmorillonite followed by reduction with NaBH4. The activation of montmorillonite was carried out by treating with mineral acid such as H2SO4 under controlled conditions at 80 °C to generate nanopores and high surface area. Powder-XRD, SEM-EDX, HRTEM, N2 adsorption, XPS etc. analysis were carried out to characterize the stabilized copper nanoparticles as well as the support. The synthesized copper nanoparticles may serve as a new alternative to ligand and additive free green catalyst for broad range of oxidation of alcohols with molecular O2 to corresponding carbonyl compounds with excellent yields (up to 97%) and selectivity (100%) under mild reaction conditions. The metal nanocatalysts exhibit good reusability, simple workup procedure and straightforward approach to carbonyl compound formation.

Fe(CrO2)2 - Photocatalyst of Liquid Phase Oxidation of Alcohols

Fe(CrO2)2 - Photocatalyst of Liquid Phase Oxidation of Alcohols

Carbon Quantum Dots-TiO2 Nanocomposites with Enhanced Catalytic Activities for Selective Liquid Phase Oxidation of Alcohols

Carbon quantum dots-titanium dioxide composite (CQDs-TiO2) has been synthesized by a simple hydrothermal process. It was used as a heterogeneous catalyst for oxidation of alcohols using water as a solvent. The morphologies and composition of the resulting catalysts were characterized to show that the CQDs were uniformly dispersed on the surface of the TiO2 with well combination with each other. The obtained catalyst showed enhanced catalytic activities compared to the pure CQDs catalyst. The enhanced catalytic activities were mainly attributed to the highly dispersive CQDs with more active centers which caused by the TiO2 support. The CQDs-TiO2 catalyst can also be reused without any significant loss in their catalytic activities after five times.

Au-Based Catalysts: Electrochemical Characterization for Structural Insights.

Au-based catalysts are widely used in important processes because of their peculiar characteristics. The catalyst performance depends strongly on the nature and structure of the metal nanoparticles, especially in the case of bimetallic catalysts where synergistic effects between the two metals can be occasionally seen. In this paper, it is shown that electrochemical characterisation (cyclovoltammetry CV and electrochemical impedance spectroscopy EIS) of AuPd systems can be used to determine the presence of an electronic interaction between the two metals, thus providing a strong support in the determination of the nature of the synergy between Au and Pd in the liquid phase oxidation of alcohols. However, it seems likely that the strong difference in the catalytic behavior between the single metals and the bimetallic system is connected not only to the redox behaviour, but also to the energetic balance between the different elementary steps of the reaction.

Transition Metal-Free Carbon Quantum Dots for Selective Liquid Phase Oxidation of Alcohols Using Water as an Only Solvent

Transition metal-free oxidation of alcohols catalyzed by carbon quantum dots (CQDs) with NaClO oxidant using water as an only solvent was developed. High quality CQDs were synthesized via a simple one-pot electrochemistry method using cheap graphite rod as substrate materials, without the assistance of any chemicals but only pure water. The morphologies and composition of the resulting catalyst were characterized in detail. The results indicated that the CQDs catalyst showed higher activity and selectivity comparing to other carbon based transition metal-free catalyst (such as carbon nanotubes, graphene et al.) in the selective oxidation of alcohols to aldehydes. The high performance of the CQDs can be mainly attributed to its small particle size and large amounts of surface oxygen containing groups. The CQDs catalyst can be reused five times without any significant loss in their catalytic activities.

Zirconia Supported Iron as an Efficient Green Catalyst for the Selective Liquid Phase Solvent Free Oxidation of Alcohol with Molecular Oxygen

Abstract Liquid phase oxidation of alcohol to corresponding aldehyde and ketone in solvent free condition with molecular oxygen, at mild temperature and pressure was studied in a self-designed three necked batch reactor, using iron supported on zirconia as a catalyst. The catalyst was prepared by incipient wetness technique, reduced in hydrogen flow and characterized by X-ray Diffractometry, Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, Thermo Gravimetric and Differential Thermal Analysis, Fourier Transform Infrared Spectroscopy, particle size analyzer, surface area and pore size analyzer. Effects of different parameters such as temperature and time on the rate of reaction were studied. It was observed that the reduced catalyst showed higher catalytic activity and selectivity than the unreduced catalyst for the liquid phase oxidation of alcohols to corresponding aldehydes and ketones.

Gold Stabilized by Nanostructured Ceria Supports: Nature of the Active Sites and Catalytic Performance

The interaction of gold atoms with CeO2 nanocrystals having rod and cube shapes has been examined by cyanide leaching, TEM, TPR, CO IR and X-ray absorption spectroscopy. After deposition–precipitation and calcination of gold, these surfaces contain gold nanoparticles in the range 2–6 nm. For the ceria nanorods, a substantial amount of gold is present as cations that replace Ce ions in the surface as follows from their first and second coordination shells of oxygen and cerium by EXAFS analysis. These cations are stable against cyanide leaching in contrast to gold nanoparticles. Upon reduction the isolated Au atoms form finely dispersed metal clusters with a high activity in CO oxidation, the WGS reaction and 1,3-butadiene hydrogenation. By analogy with the very low activity of reduced gold nanoparticles on ceria nanocubes exposing the {100} surface plane, it is inferred that the gold nanoparticles on the ceria nanorod surface also have a low activity in such reactions. Although the finely dispersed Au clusters are thermally stable up to quite high temperature in line with earlier findings (Y. Guan and E. J. M. Hensen, Phys Chem Chem Phys 11:9578, 2009), the presence of gold nanoparticles results in their more facile agglomeration, especially in the presence of water (e.g., WGS conditions). For liquid phase alcohol oxidation, metallic gold nanoparticles are the active sites. In the absence of a base, the O–H bond cleavage appears to be rate limiting, while this shifts to C–H bond activation after addition of NaOH. In the latter case, the gold nanoparticles on the surface of ceria nanocubes are much more active than those on the surface of nanorod ceria.

Gold catalyzed liquid phase oxidation of alcohol: the issue of selectivity

Commercial carbon nanotubes (CNTs) and carbon nanofibers (CNFs) modified in various ways at the surface have been used as supports for gold nanoparticles (AuNPs) in order to study their influence on the activity/selectivity of catalysts in the aqueous oxidation of alcohol. Particularly oxidative treatment was used to introduce carboxylic functionalities, whereas subsequent treatment with NH3 at different temperatures (473 K, 673 K and 873 K) produced N-containing groups leading to an enhancement of basic properties as the NH3 treatment temperature was increased. The nature of the N-containing groups changed as the temperature increased, leading to an increase in the hydrophobicity of the support surface. Similar Au particle size and similar textural properties of the supports allowed the role of chemical surface groups in both the activity and the selectivity of the reaction of glycerol oxidation to be highlighted. An increase of basic functionalities produced a consistent increase in the activity of the catalyst, which was correlated to the promoting effect of the basic support in the alcoholate formation and the subsequent C-H bond cleavage. The selectivity towards primary oxidation products (C3 compounds) was the highest for the catalysts treated with NH3 at 873 K, which presented the most hydrophobic surface. The same trend in the catalyst activity has been obtained in the aqueous benzyl alcohol base-free oxidation. As in the case of glycerol, the increasing of basicity and/or hydrophobicity increased the consecutive reactions.

Metallic short fibers for liquid-phase oxidation reactions

The application of metallic short fibers (MSF) for the liquid phase oxidation of alcohols and the epoxidation of alkenes has been investigated. Screening of various MSF lead to the conclusion that Cu 3Sn- and Cu64Ni28Mn7Fe1-fibers are the most promising candidates for oxidation reactions. Due to increased decomposition of H 2O 2 in presence of MSF containing Pt, Pd or Rh, lower yields were obtained with these catalysts. Oxidation reactions with the two above mentioned MSF are characterized by high selectivities for the oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones, respectively. Reactions are independent from reaction atmosphere and H 2O 2-dosage protocol, whereas parameters like solvent, temperature, and oxidant concentration had a significant influence.

In Situ Formation of Au–Pd Bimetallic Active Sites Promoting the Physically Mixed Monometallic Catalysts in the Liquid‐Phase Oxidation of Alcohols

The catalytic oxidation of alcohols with molecular oxygen on supported nanometallic catalysts represents one of the green methods in a crucial process for the synthesis of fine chemicals. We have designed an experiment using physically mixed Au/AC and Pd/AC (AC=activated carbon) as the catalyst in the liquid-phase oxidation of benzyl alcohol by aerobic oxygen. The evolution of the physically mixed catalyst structures at different stages in the catalytic reaction was investigated by aberration-corrected high-resolution transmission electron microscopy and spatially resolved element mapping techniques at the nanometre scale, and they were also compared with the structure of the bimetallic alloy. For the first time we show the formation of surface Au-Pd bimetallic sites by reprecipitation of Pd onto Au nanoparticles. Negligible Au leaching was observed. The in situ structural evolution can be directly correlated to the great enhancement of the catalyst activity. Moreover, we distinguish the different behaviours of Au and Pd, thus suggesting an oxygen differentiating mechanism for Au and Pd sites. The findings are of great importance to both the understanding of the structure-activity correlation and the design of highly active catalysts in green chemistry.

Nitrogen functionalized carbon nanostructures supported Pd and Au–Pd NPs as catalyst for alcohols oxidation

Carbon nanofibers (CNFs) PR24-PS and carbon nanotubes (CNTs) Baytubes were functionalized by oxidation with nitric acid and further amination with gaseous NH 3. Thus Au and Au–Pd nanoparticles were prepared by PVA/NaBH 4 system and anchored on the surface of pristine carbon nanostructures (CNS) and N-CNS (nitrogen functionalized carbon nanostructures). TEM analysis revealed that the introduction of nitrogen functionalities improves the dispersion of the metal nanoparticles on the surface of the support. This phenomena leads to an improved activity of N-CNS based catalysts with respect to pristine CNS when tested in the liquid phase oxidation of alcohols.

Pd on carbon nanotubes for liquid phase alcohol oxidation

Pd nanoparticles supported on carbon nanotubes (CNTs) showed a higher selectivity than Pd nanoparticles supported on activated carbon (AC) in the liquid phase oxidation of benzylic alcohol to benzaldehyde. Under solventless conditions a significant improvement in selectivity was observed for Pd/CNTs, whereas using Pd/AC a considerable over-oxidation of benzaldehyde was observed. Differently from other solvents cyclohexane improved significantly the selectivity to benzaldehyde for both catalysts. Characterisation by means of transmission electron microscopy revealed differences in metal dispersion between Pd/AC and Pd/CNTs that can be ascribed to textural, chemical and physical differences between active carbon and carbon nanotubes. The higher activity in the case of Pd on AC than on CNTs can be attributed to the improved Pd dispersion in the first case. On recycling Pd/CNTs resulted more stable (activity loss 50% in 7 runs) than Pd/AC (activity loss 70% in 7 runs) even a structural change of catalysts after reaction is observed. The Pd leaching and particle coalescence are the main reasons for the loss of activity. An extraordinary improving of catalyst life has been observed by alloying the Pd nanoparticles with Au, When CNTs are used as support the strong Pd leaching can be greatly limited and the activity/selectivity maintained at least for 8 runs.