Owen
Rogers
a,
Samuel
Pattisson
a,
Rebecca V.
Engel
a,
Robert L.
Jenkins
a,
Keith
Whiston
b,
Stuart H.
Taylor
a and
Graham J.
Hutchings
*a
aCardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, UK. E-mail: hutch@cf.ac.uk; Tel: +44 (0)2920 874059
bINVISTA Performance Technologies, The Wilton Centre, Wilton, Redcar, TS10 4RF, UK
First published on 2nd June 2020
Vanadium compounds have shown great potential alongside Pt/C for the oxidation of cyclohexanediol to adipic acid. However, the low stability of these materials often leads to ambiguity when considering the homogeneous or heterogeneous nature of the active species. In this article we describe our attempts to synthesise stable vanadium catalysts through the utilisation of vanadium bronze structures. By the addition of sodium, copper or silver into these structures, leaching could be decreased to 5% for AgVO3, compared to 88.4% with V2O5. These reactions were run in aqueous conditions under 3 bar O2. However, despite significant stabilisation of vanadium in the bronze structures, we show that as little as 7.6 ppm of a homogeneous vanadium species in the reaction solution can cause the selective oxidation of 2-hydroxycyclohexanone to adipic acid. Analysis of the speciation by 51V NMR and UV-vis has revealed the active species to be in the +5 oxidation state in the form of a decavanadate compound with the presence of small amounts of monovanadate.
Currently, adipic acid is produced predominantly from a cyclohexane-based feedstock.3–5 In this process the use of nitric acid as the oxidant results in the release of N2O, which has a global warming potential of 300 relative to CO2.6 The environmental impact of this adipic acid process has been diminished due to recent catalytic and thermal abatement of the N2O emissions, resulting in significant reduction of associated N2O release. However the prospect of completely eliminating N2O from the process would be beneficial.7–9 This is gradually becoming more feasible due to the possibility of using cyclohexene as a substrate instead of cyclohexane. Cyclohexene is more easily oxidised and therefore, greener oxidants such as oxygen and hydrogen peroxide can be used, which can offer better atom economy.1,10–12
Oxidation starting from cyclohexene may need to be run in two steps. The first step would be an oxidation of cyclohexene to cyclohexanediol, which can be achieved using O2, albeit with a maximum selectivity of 50%.13 It is for this reason that a process which uses H2O2 in a first step, would be the most economically viable. However, this would likely only be economic at commercial scale if a stoichiometric amount of oxidant relative to substrate could be used.14 This could then be followed by a second step to convert cyclohexanediol to adipic acid, which can be achieved using O2.
The oxidative cleavage of vicinal diols is an important process in organic synthetic chemistry and is mostly achieved using expensive noble metal-based homogeneous catalysts. However, these catalysts suffer from a poor substrate range and do not offer reusability.15–17 The Malaprade reaction18 and the Criegee oxidation19 offer classical examples of the cleavage of 1,2-diols, however, they require the use of stoichiometric oxidants, such as high-valent iodine or lead, resulting in large amounts of toxic waste.20,21 Heterogeneous catalysts have also been used for vicinal diol cleavage, particularly, noble metals such as Pt, Ru, and Au have been reported but these usually suffer from low activity.22–24 It is rare that a non-noble metal shows activity for this reaction. However, Escande et al. found that a sodium–manganese oxide was active at 100 °C, albeit with a minimal substrate scope and, in particular, being inactive for the oxidation of cyclohexanediol.25
Brégeault and co-workers demonstrated the ability of various vanadium compounds to perform the aerobic oxidative cleavage of 2-hydroxycyclohexanone (2-HCO) to form adipic acid.26 Rozhko et al. subsequently studied the ability of Keggin type P/Mo/V polyoxometalates for the oxidation of cyclohexanediol under acidic conditions. However, it was suggested that despite high initial selectivity, the adipic acid formed underwent further reaction with cyclohexane diol to form the corresponding ester, resulting in low overall yields of the acid.27 More recently, Obara et al.24 demonstrated the potential for a system utilising Pt/C and V2O5 that can selectively cleave the vicinal diol to adipic acid (Scheme 1). Using O2 as oxidant and H2O as solvent, a yield of over 90% can be reached when both catalysts were combined in a one-pot reaction over 48 h. Cyclohexanediol was converted to 2-HCO over Pt/C alone, whereas this reaction did not proceed with only V2O5. By using a V2O5 co-catalyst, the 2-HCO was then converted to cyclohexanediol. The main problem with such a system is the solubility of the V2O5 catalyst. The researchers highlighted that over each re-use approximately half of the vanadium leached into solution, although it was suggested that the remaining solid was responsible and sufficient for the observed catalysis. Nevertheless, a truly heterogeneous vanadium catalyst would mark a major advance in this field.
Scheme 1 Reaction scheme for oxidation of cyclohexanediol to adipic acid as proposed by Obara et al. |
Supported vanadium oxides have been extensively used in oxidation reactions, notably in the selective oxidation of alkenes and alkanes and the oxidation of H2S.28–30 However, heterogeneous vanadium catalysts are well-known to leach in aqueous solutions. This can be alleviated to some extent by changing the support and reaction conditions as shown in work by Masumoto et al.31 However, even the most successful attempt at mitigating leaching, a V/Al2O3 catalyst, still showed 37.6% leaching of vanadium into the solvent. The effect of leaching of vanadium catalysts in heterogeneous liquid phase reactions is also stated in studies by Ziolek et al., whereby V/MCM-41 leaches 71 wt% of vanadium into the reaction solution.32 Interestingly, work by Tiwari et al. on polyoxometalate structures, specifically H5[PV2Mo10O40], have shown that the dissolution of two reactive pervanadyl ions by H2SO4 is a reversible process, which can be controlled by the pH.33
Vanadium bronzes, which when combined with other metals such as sodium, copper or silver, can show stability in aqueous media and have various applications in areas such as, photocatalysts and in electrochemical energy storage, however elemental analysis of aqueous media is not always reported.34,35 In this report we show the synthesis of several vanadium bronzes with the aim of achieving stabilisation in aqueous media. However, we also demonstrate that even small amounts of leaching from these catalysts can have a great effect on reactivity. The filtrate of these reactions is also analysed to determine the likely active vanadium speciation.
Conversion (%) = [(mol of consumed substrate)/(mol of initial substrate)] × 100 |
Selectivity (%) = [(mol of product)/(mol of consumed substrate)] × 100 |
Mass balance (%) = [(mol of product final + mol of final substrate)/(mol of initial substrate)] × 100 |
Entry | Catalyst | Conversion/% | Selectivity/% | Vanadium leaching/% | Vanadium concentration/ppm | Mass balance/% | ||||
---|---|---|---|---|---|---|---|---|---|---|
Adipic acid | 2-HCO | Glutaric acid | Succinic acid | Unknowns | ||||||
1 | Blank | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | — | — | 100 |
2 | V2O5 | 3.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.4 | 97.5 | ||
3 | Pt/C | 39.4 | 0.0 | 59.4 | 2.1 | 1.3 | 9.2 | — | — | 90.4 |
4 | Pt/C + V2O5 | 49.5 | 76.1 | 1.2 | 4.5 | 1.4 | 5.4 | 88.4 | 168 | 96.2 |
All bronze catalysts in this study were synthesised using a hydrothermal technique and the XRD patterns are shown in Fig. 1. The Na2V6O16 bronze was synthesised according to a previous method described by Xu et al.,38 using V2O5 and NaSO4 as precursors. The XRD pattern (black) can be referenced to the monoclinic Na2V6O16 phase with a P21/m space group with reflections at 2θ = 11.70, 25.93, 28.35, 29.77, 40.06, 46.26 and 50.83° all indicative of this phase.39,40 This is accompanied by a high degree of crystallinity. Included in Fig. 1 is the XRD pattern for Cu0.5V2O6 (red), which shows reflections that would be indicative of a CuV2O6 phase with peaks at 2θ = 20.7, 26.7, 33.6, 42.1, 42.2 and 44.8°. However, there also appear to be reflections which can be identified as related to the monoclinic β-Cu0.261V2O5 phase, which is characterised by reflections at 2θ = 29.16, 12.15, 26.39, 31.16, 33.09, 37.25, 39.73, 40.93 and 46.23°.29,36 The XRD pattern for AgVO3 (blue) shows strong similarities to that of the β-AgVO3 phase with the monoclinic structure and I2/m space group.35,41 There were no other detectable reflections in the sample suggesting the sample was mainly composed of the β-AgVO3 phase.
When used for the oxidation of cyclohexanediol, the Na2V6O16 catalyst (Table 2 – entry 1) demonstrates a similar conversion and adipic acid selectivity to the Pt/C + V2O5 system, achieving 36.7% and 70.1% respectively. However, the main drawback of this catalyst was the amount of leaching observed. MP-AES analysis of the vanadium bronze post reaction solution revealed a total of 67 ppm V which amounts to 41.8% of the overall vanadium present in the bronze. This level of leaching was still distinctly lower than that for a Pt/C + V2O5 catalytic reaction which leached 88.4%, however it is still an unsustainable amount. This level of leaching was consistent even after catalysts were washed with 1 L boiling water prior to the catalytic testing and also after multiple reuse experiments. These results show that sodium is not a suitable metal substitute to stabilise the leaching of vanadium species in the mixed metal oxide.
Entry | Catalyst | Conversion/% | Selectivity/% | Vanadium leaching/% | Vanadium concentration/ppm | Mass balance/% | ||||
---|---|---|---|---|---|---|---|---|---|---|
Adipic acid | 2-HCO | Glutaric acid | Succinic acid | Unknowns | ||||||
1 | Pt/C + Na2V6O16 | 36.7 | 70.1 | 4.0 | 6.5 | 1.6 | 1.5 | 41.8 | 67 | 96.0 |
2 | Pt/C + Cu0.25V2O6 | 32.0 | 64.0 | 4.9 | 0.0 | 0.0 | 4.3 | 16.6 | 35 | 91.2 |
3 | Pt/C + Cu0.33V2O6 | 27.2 | 60.4 | 9.0 | 0.0 | 0.0 | 0.0 | 5.8 | 11 | 91.2 |
4 | Pt/C + Cu0.5V2O6 | 25.1 | 63.7 | 9.5 | 0.0 | 2.2 | 5.2 | 19.8 | 33 | 95.5 |
As an attempt to further decrease the vanadium leaching, the effect of using copper or silver as stabilising metals in the vanadium bronze structure was investigated. Metals such as Cu and Ag are commonly substituted in these materials, due to the resulting enhanced electrochemical properties and increased activity in the oxidation of H2S.29,41–43 For the copper bronzes, shown in Table 2 entries 2–4, different molar ratios of copper to vanadium were prepared to investigate any effect that copper had on the stability of the vanadium bronze. The molar ratios of copper to vanadium studied were 0.25, 0.33, and 0.5. From Table 2 it can also be seen that vanadium leaching is reduced dramatically on inclusion of copper to 5.8% for Cu0.33V2O6. This contrasted with Cu0.25V2O6 and Cu0.5V2O6, which showed vanadium leaching of 16.6% and 19.8% respectively. This is a drastically lower leaching level than that observed with the Na2V6O16 bronzes, demonstrating that the copper successfully increases the stability of the bronze. As seen in Table 2, the copper bronzes achieve lower conversions than observed with Na2V6O16. However, this level of conversion is mostly influenced by Pt/C and therefore it is more useful to compare selectivity to adipic acid. It is evident from the data that CuxV2O6 showed lower selectivity to adipic acid. This may be due to copper atoms occupying active sites at the surface of the catalyst, or due to the additional copper affecting the electronic structure of vanadium at the surface, making it more stable but potentially less reactive. Another explanation for this change in selectivity may also be a result of less leaching in the reaction solution highlighting the role of homogeneous vanadium. These bronzes show similar selectivities towards adipic acid, despite there being a lower vanadium concentration present in the Cu0.33V2O6 reaction solution. However, this may be due to only low concentrations of homogeneous vanadium being required to have this marked effect on selectivity.
The final bronze of the series of vanadium catalysts was AgVO3, which is shown in Table 3 – entry 1. This bronze catalyst shows the highest stability with regards to vanadium leaching with only 5.0% V leached into solution. This corresponds to a total of 9 ppm V present in the reaction solution, the lowest levels of vanadium leaching from the bonzes. Unfortunately, this catalyst also showed a noticeably lower cyclohexanediol conversion than Na2V6O16 and CuxV2O6. It can be observed (Table 3 entry 1) that there is still 21.3% of 2-HCO in the solution so this had not been completely converted to adipic acid. This may be due to less active vanadium sites or also it could be attributed to the lower concentration of homogeneous vanadium in solution.
Entry | Catalyst | Conversion/% | Selectivity/% | Vanadium leaching/% | Vanadium concentration/ppm | Mass balance/% | ||||
---|---|---|---|---|---|---|---|---|---|---|
Adipic acid | 2-HCO | Glutaric acid | Succinic acid | Unknowns | ||||||
a Reaction conditions: 80 °C, 3 bar O2, 4 h, 4000 ppm 2-hydroxycyclohexanone in water (5 ml), 5 mg vanadium carbide after washing with 5 L water. | ||||||||||
1 | AgVO3 | 18.4 | 41.6 | 21.3 | 0.0 | 2.1 | 0.0 | 5.0 | 9 | 92.8 |
2 | V–C3N4 | 24.7 | 59.3 | 5.3 | 0.0 | 1.7 | 33.7 | — | 33 | 91.4 |
3 | VCa | 19.4 | 55.0 | — | 0.0 | 0.0 | 16.0 | 1.2 | 17 | 91.3 |
Efforts to probe the heterogeneity of this reaction extended to synthesising a support that had a strong interaction with vanadium, to minimise leached vanadium in solution. In previous studies Ding et al.44 had shown that vanadium supported on carbon nitride can be a promising catalyst in liquid phase reactions for the synthesis of phenol from benzene. The purpose of carbon nitride as a support to stabilise vanadium was predominantly due to basic NH and NH2 groups on the surface which help to bind to the acidic vanadium species. These acid–base interactions have also been shown to decrease leaching to a negligible amount in vanadium-substituted molybdophosphoric acid, due to the strong interactions between the heteropolyacid and amine groups on SBA-15.45 The V–C3N4 material was successfully synthesised and tested in the oxidation reaction (Table 3 – entry 2). However, after a hot wash prior to the catalytic testing, 32.6 ppm V was still leached into the reaction solution. An accurate measure of vanadium leaching from V–C3N4 was not obtained, due to the difficulty of dissolving graphite and carbon nitride in aqua regia. In addition to the synthesised vanadium catalysts, a commercial vanadium carbide, which is insoluble in aqueous solutions,46 was tested (Table 3 – entry 3). However, under reaction conditions leaching was observed for this material as well. The leaching continued even after a pre-wash of the catalyst with 5 L of boiling water. However, after this treatment only 1.2% of total vanadium leached from the catalyst, giving a total concentration of 17.0 ppm vanadium in the reaction mixture. To observe whether this level of vanadium leaching was significant for cyclohexanediol oxidation a further study using solutions with known quantities of homogeneous vanadium was conducted.
To study the effect of homogeneous vanadium, a solution of 4000 ppm 2-HCO in water was used as the starting solution. A stock solution of V2O5 in water was prepared with a concentration of 200 ppm. This was then inserted into the reaction as required to obtain vanadium concentrations within the range of 0 to 45 ppm. The final concentration post reaction was then analysed by MP-AES. The results can be seen in Fig. 2b. These solutions contained only soluble vanadium species so any reaction observed would be completely homogeneous. Fig. 2b demonstrates that at these low concentrations, a linear trend can be observed between vanadium in solution and yield to adipic acid. Interestingly, it is only very minimal amounts of vanadium that are required to promote conversion of 2-HCO to adipic acid. A concentration of 7.6 ppm is sufficient to observe an effect on the reaction and yield 1.5% adipic acid. This suggests that even a small amount of leaching from the above vanadium materials would likely result in catalytic activity. These vanadium species can also be determined to be acting catalytically and not as stoichiometric oxidants (SI). Higher concentrations above 45 ppm proved difficult to prepare under these reaction conditions, despite seeing concentrations above 150 ppm previously when using the bronzes as catalysts. High concentrations above 50 ppm seems to be achievable only under certain reaction conditions, which were not present in the study of 4000 ppm 2-HCO. As such, a study was undertaken to probe which conditions would induce leaching of vanadium in reaction solution and the results are illustrated in Fig. 3.
Fig. 3 shows the effect of reaction conditions on vanadium leaching, specifically the effect of the presence of Pt/C and the effect of different reaction mixtures. The reactions containing both Pt/C and V2O5 showed consistently more leaching than their counterpart which only included V2O5. The effect of the Pt/C on the leaching may have been due to the Pt activating the oxygen causing it to destabilise the vanadium structure and promote leaching of the V2O5 more readily. The most drastic decrease in leaching of V2O5 is observed in 4000 ppm 2-HCO where only 4.7% was homogeneous. This is compared to 38.8% in 10000 ppm cyclohexanediol. The main reason for this could be the polarity of the different substrates affecting the solubility of vanadium.
Since we have shown that adipic acid selectivity was mainly reliant on homogeneous vanadium, UV-vis and 51V NMR studies were undertaken to determine the speciation of vanadium present in the reactions. The UV-vis is shown in Fig. 4, and shows the reaction filtrate from Na2V6O16 (green) and the V2O5 reaction mixture (blue) as well as the V2O5 stock solution (pink) used for the vanadium homogeneous reactions (Fig. 2). From the UV-vis spectra of these mixtures it can be concluded that the vanadium oxidation state is +5 with no other oxidation states present.47 From the UV-vis spectra we do not see V(III) or V(IV) so the catalytic V(V) must have a closed catalytic cycle. The spectra for 51V NMR studies are shown in Fig. 5. By comparing the NMR spectrum with data published by Andersson et al.48 the vanadium species can be identified as a vanadium decavanadate, which has 3 chemical environments as shown by the spectra. These shifts can be observed at −424 ppm, −508 ppm, −526 ppm for Vc, Vb and Va respectively. An additional signal, which can be attributed to the monovanadate species, can also be seen in the NMR spectrum.49 To confirm that the spectrum observed for the reaction mixture in Fig. 5a was as a result of a V10 species (Fig. 5c), a sample of the decavanadate was prepared from an acidified solution of sodium orthovanadate.49–51 As can be seen in the spectrum the signals overlay perfectly, with the only exception of the vanadium environment signified by Va. For this environment there is a small shift from −526 ppm to −529 ppm for the reaction filtrate and the acidified sodium orthovanadate V10 species, respectively. This shift may result from a binding of Va with reaction products. There also appears to be a stronger monovanadate peak in the 51V NMR spectrum of V10, which may be a consequence of the lower pH of this sample.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0cy00914h |
This journal is © The Royal Society of Chemistry 2020 |