Vanadium Based Catalyst in Alcohol Oxidation Reaction

Abstract

Selective oxidation of benzyl alcohol is considered as a challenging process due to the formation of various side products formed during the reaction. Different process has been developed to control the formation side by controlling the free radical generation in the reaction. This chapter deal with the oxidation of benzyl alcohol with various vanadium based catalyst and controlling the side was discussed.

Introduction

Oxide compounds are formed by combination between an electropositive atom such as metal and oxygen atom. Transition metal oxides are recognized as excellent material due to its versatile use in different field such as magnetic, optical, catalysis, energy sector etc.

These metal oxide also used in photocatalytic reaction due to their suitable band gap. The activity of the metal oxides were depends on the structure of the material.

1 Recently, one of the fastenings fields in heterogeneous catalysis in the designing of nanomaterial in the range of 1-100 nm in size. Different types of nanomaterial was designed such zero, one or two dimension for various application.

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As the physical and chemical properties were changes by tuning the size as well as dimension of the nanomaterial.2 The morphology of the material can be regulating by changing the different reaction parameter such as pressure, temperature, time etc. Shape of the material was also control by introducing the surfactants or capping agent into the reaction system.3

Vanadium complex or vanadium oxides are widely used in different application and it can be synthesized using different ligands and morphology to improving activity of the material. Recently, these types of materials are extensively applied as gas sensors, Li batteries, electrochemical, water splitting and as catalyst in various organic transformations.4 Different forms of vanadium oxides are synthesized such as V2O5, VO2, V2O3 and VO and some of the mixed valance vanadium is also synthesize with general formula VnO2n-1. Thermodynamically most stable phase of vanadium oxide is the V2O5. The VO2 phase usually transform into V6O13 and V2O5.

Both the pure and mixed phase of vanadium oxide has great potential in different industrial scale fine chemical synthesis. This unique feature of vanadium based compounds to form variety of compounds due to the phase transition ability at 340 K. Synthesis of one phase or control oxidation state vanadium oxides was a challenging task for the researchers. Among all the unit of vanadium oxides, the reversible phase transition temperature of vanadium dioxide (VO2) was observed at 68 °C between monoclinic insulator to rutile metallic, VO2(M) and VO2(R), respectively. The phase transition of VO2 leads to change in the electrical and optical properties. The transition between VO2(M) and VO2(R) was took place within 100 fs and it will depends on the purity and size of the material as well as presence of defect.5,6

V2O5 material has high specific capacity, low toxicity and easy synthesis procedure which promote it as an excellent cathode material. The commercial V2O5 limits its application as a cathode material due to its low ionic diffusivity, intrinsic structural instability as well as its electrical conductivity decreases after intercalation. The drawbacks of commercial V2O5 will overcome by synthesis of various nanomaterials such as nanorods, nanowire, nanobelts etc. Vanadium oxides catalysts were sometimes also difficult to recycle and separate from the reaction. To overcome this problem, researchers are hybrid the metal oxide catalyst with different support like carbon nanotube, silica, alumina, graphene etc. Recently, carbon material such as multiwalled carbon nanotubes (MWCNTs) was recognized as excellent support materials due to its high surface area and electrical conductivity, remarkable mechanical and thermal stability, decreases the poisoning effect in reaction and various morphology of metal oxide were synthesize over the MWCNTs. Another advantage of MWCNTs is the synthesis of different functional group attached to the carbon atoms which alter its chemical properties.

The functional groups such as phenol, ketone, carboxylic acid, acid anhydride which contain oxygen atom are in general coordinate to CNT via control oxidation process. The functional group which contain oxygen atom enhances the hydrophilicity of CNT in polar solvents. The carboxylic acid functionalizedCNT promotes the electrochemical reaction as it will decreases the activation energy and boosts the electronic transfer of CNT of surface. MWCNT was first synthesized by Iijima during the preparation of fullerenes. Within a very short period of time it shows its application in different fields like energy and gas storage material, electronic, sensing and catalysis. MWCNTs was synthesized both industry and laboratory using different process such as arc discharge, laser ablation, flame synthesis, chemical vapor deposition, electrolysis etc. As the diameter of the CNT increases with increasing the temperature which indicates that it can encapsulated more metal particle inside the tube.

The oxygen containing functional group such as carboxylic acids are also used for coordination of metal particle in the CNT surfaces. The CNT material was hybridized by encapsulation or deposition of various NPs or metal oxides for wide range of application such as catalysis, gas sensing, electrochemical reaction etc. The COOHfunctionalizedMWCNT was extensively used for anchoring of metal particles and absences of functional group the metal particles are formed on the surface of the MWCNT via impregnation process. In this process of hybridized material only physical interaction between particle and support was present as a result the catalytic activity of the material and leaching were usually observed. The V2O5 supported over MWCNTs composite shows high surface area and good conductive effect.7

Benzaldehydes are considered as an important derivatives precursor in different reaction such as Schiff base synthesis, pharmaceutical important product synthesis.8 Benzaldehyde can be synthesized via oxidation as well as reduction process from different substrate. In the reduction process different costly metal catalysts are used in presence of reducing agent. Some of the reported starting materials for production of benzaldehyde via reduction process are benzoic acid or acid chloride, Scheme 1. The reduction of benzoic acid was performed by reduction of benzoic acid to benzyl alcohol and further reduced to its corresponding aldehyde. Another two steps process is the conversion of benzoic acid to it’s reactive from and reduced to benzaldehyde. One step reduction of benzoic acid was carried out in presence of triethylsilane or its derivatives are used as reducing agent, Scheme 2. Benzaldehyde can synthesize via oxidation of benzyl alcohol, toluene, chlorobenzene, styrene etc.

Selective oxidation of benzyl alcohol to its corresponding benzaldehyde in presence of catalyst is a desirable task. Benzaldehydes usually synthesized via oxidation of benzyl alcohol using some oxidizing agent like molecular oxygen (O2), hydrogen peroxide (H2O2) or TBHP etc. in presence of some metal catalyst. Oxidation of alcohols in presence of H2O2 requires less time than that of O2 but the main problem associated with H2O2 is the controlling of the side products during the reaction. The major side products in alcohol oxidations are benzoic acid, benzyl benzoate etc. occurred via the free radical mechanism and controlling the side reaction is the major challenge for the researchers, Scheme 3. In presence of H2O2, the recyclability of the metal complex was usually difficult as it from peroxo-complex. Vanadium oxide based catalysts are recognized as active catalyst in benzyl alcohol oxidation. In this chapter, oxidation of benzyl alcohol is discussed using various oxidizing agent.

Homogeneous vs Heterogeneous Catalyst

Alcohol oxidation reaction was performed with both homogeneous as well as heterogeneous catalyst. In case of homogeneous catalysis, the reactant and catalyst are in in liquid phase. Different types of metal complex are used in alcohol oxidation which readily in the solvent of the reaction mixture. In most of the cases H2O2 which use as oxidant in the oxidation process dissolved catalyst and make the reaction in homogeneous condition. These types of complex show excellent activity in alcohol oxidation reaction in presence of H2O2. The some of the vanadium complex employed in oxidation of benzyl alcohol are depicted in Figure 1. Vanadium oxides such as V2O5 or different vanadium oxide unit are also employed in alcohol oxidation in presence of molecular oxygen as well as O2 as an oxidant. Similar to metal complex, V2O5 also dissolved in presence of H2O2 during the oxidation reaction. The recyclable of the catalyst was control in presence of molecular oxygen. The heterogeneous catalysts are also synthesized via supporting metal catalyst in different support.

The advantages associated with homogeneous reaction are

1. In most of cases, activity of homogeneous catalyst was more in comparison to heterogeneous catalyst.
2. The catalyst and reactant exist as homogeneous mixture and which eliminate the pore diffusion.
3. To study the mechanism of the reaction in homogeneous reaction was comparatively easier than the heterogeneous catalyst.

The disadvantage of homogeneous catalyst

1. In homogeneous catalysis, the catalyst and reactant mixture are in same phase i.e. liquid. The separation of catalyst after reaction was very difficult and most of the cases the activity of the recycle catalyst was very low.
2.  The stability of the catalyst was very low than the heterogeneous catalyst. The reaction was usually performed in at mild condition.

Advantage of heterogeneous catalyst

1. The stability of the catalyst was very high.
2. Catalyst can be reusable upto large number of cycle.
3. Catalyst can be separated from reaction via filtration.

Disadvantage of heterogeneous catalyst

1. It is difficult to know the mechanism of the reaction.

Oxidation of benzyl alcohol as a homogeneous reaction

Oxidation of benzyl alcohol to benzaldehyde was performed in presence of V2O5 catalyst using atmospheric oxygen as an oxidant at 100 ° C. Upto 93% yield of benzaldehyde was obtained within 25 h. The reaction benzyl alcohol oxidation was performed in presence of 0.5 equiv. of K2CO3. The presence of K2CO3 increases the pH of reaction which restricts the formation of hemiacetal.9 Li etal. reported that oxidation of benzyl alcohol in presence of V2O5 and H2O2 as an oxidant under acidic condition. The reaction was performed in presence benzyltriethylammonium bromide (BTEAB) which acts as a phase transfer agent in the reaction. The product of benzaldehyde was not observed at room temperature or refluxing condition in presence of V2O5, BTEAB and H2O2 which used as oxidizing agent. High yield of benzaldehyde was obtained after maintain the reaction pH at 4 using HCl, H2SO4, HBr and H3PO4 at 60 ° C. If the pH of the reaction further increases above 5, the formation of benzaldehyde was not detected. It is well known that V2O5 in presence of H2O2 can be oxidized to vanadium peroxide species. They propose that VO(O2)2ˉ species behave as an oxidant as well as nucleophile in the oxidation reaction.

The oxidation of benzyl alcohol was also performed using VO(acac)2 supported in polyaniline using molecular oxygen as an oxidant. The reaction was proceeding with high yield without using any base during the reaction.11 Using H2O2 in different oxidation reaction was considered as a green oxidant. Presence of peroxide with metal complex or oxide usually form peroxo complex and the progress of the reaction was took place within a very short period of time. The oxidation reaction carried out in presence of molecular oxidant was selective process but it requires higher time in comparison to H2O2. In addition, the benzyl alcohol oxidation in presence of molecular oxygen in usually required some co-catalyst such as DABCO, TEMPO, NHP etc. Some of the vanadium based catalysts are in Table 1.

The oxidation reaction in presence of H2O2 leads to the decreases the stability of material. Therefore photocatalytic oxidation benzyl alcohol was considered as useful process for the production of benzaldehyde. Semiconductor materials are best candidate to act as photocatalyst. As the oxide material can serve as photocatalyst, among them TiO2 was an ideal candidate as photocatalyst due to its high stability and reactivity. The main drawback associated with TiO2 is the separation between photoinduced electron and holes are very low and its high band gap i.e. 3.2 eV which makes it low response to visible light. Therefore, there is an urgent need to find a suitable semiconductor material which can trap the solar energy and used in different photocatalytic reaction. Among all the metal oxides, vanadium oxide considered as an excellent semiconductor materials in photocatalytic reaction due to its low band gap i.e. 2.6 eV.

The narrow band gap of the material makes it as an active catalyst for utilization of both UV and visible spectrum of solar light. The main problem associated with the vanadium oxides material is the rapid recombination of electron and hole pair as a result decreases the activity in UV as well as visible region. The activity of the material in photocatalytic reaction was enhances by adopting two approaches. First approach was the synthesis of nanoscaled material as a photocatalyst. As the migration distance of electron and hole to the reaction active sites decreases from the bulk in case of nano material. Secondly, separate the photogenerate electron and hole generate during the photocatalytic reaction by doping some co-catalyst into the metal oxides.

Bismuth vanadate material can be used in different types of photocatalytic reaction due to its ability to absorb visible light of the solar spectrum. Moreover, the BiVO4 as photocatalyst has several advantages such as high stability, low cost and low toxicity. Unsworthetal. synthesized bismuth vanadatenanoparticles via hydrothermal process. The material was used as a photocatalyst in oxidation of benzyl alcohol using oxygen as an oxidant with 99% selectivity. The reaction was under LED light with wavelength 470 nm. The nano-BiVO4 shows higher activity than the bulk BiVO4.12

The photocatalytic oxidation of benzyl alcohol was performed in presence of Pt and MnO2 doped co-catalyst deposited on BiVO4 material. The presence of Pt and MnO2 decreases the reverse reaction as well as recombination or electron and hole. The recombination reaction is the main drawback in the photocatalytic reactions.13

When the light falls on the surface of the catalyst, the photogenerated electron and hole generated from the metal oxide which react with O2 and H+ to form OH and superoxide radical. O2 and H+ are generated from the catalyst via water oxidation as the reaction was carried out in water. The OH and superoxide radical are the active species for alcohol oxidation.

Vermaetal. synthesized VO@g‑C3N4 for oxidation of benzyl alcohol under light. Graphitic carbon nitrides act as support for the metal which controls the leaching of the metal during the reaction.14 Carbon material has high conductivity and mobility of electron which makes it as excellent support for various nanomaterials. In VO@g‑C3N4, the deposition of metal in C3N4 leads to enhances in the chemical as well as thermal stability of the material. in addition to various advantage, activity of the material was monitored by varying the composition of metal.

Conclusion

In conclusion, the oxidation of benzyl alcohol was discussed in various vanadium based catalyst. The oxidant such as H2O2 and O2 greatly impact on the activity of the catalyst. The presence of H2O2 decreases the reaction time but regeneration of the catalyst was difficult. Further oxidation of benzyl alcohol via photocatalytic process was found an excellent yield than the other oxidation process.

References

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