Adipic acid formation from cyclohexanediol using platinum and vanadium catalysts: Elucidating the role of homogeneous vanadium species

Article abstract of DOI:10.1039/d0cy00914h

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.

Full text of DOI:10.1039/d0cy00914h

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Adipic acid formation from cyclohexanediol using
platinum and vanadium catalysts: elucidating the
role of homogeneous vanadium species†
a
Cite this: DOI: 10.1039/d0cy00914h
Owen Rogers,a Samuel Pattisson, a Rebecca V. Engel,
Robert L. Jenkins,a
Keith Whiston, b Stuart H. Taylor a and Graham J. Hutchings
a
*
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.
Received 5th May 2020,
Accepted 2nd June 2020
DOI: 10.1039/d0cy00914h
rsc.li/catalysis
associated N2O release. However the prospect of completely
eliminating N2O from the process would be beneficial.7–9 This
Introduction
The production of chemicals using the principles of green
chemistry is becoming increasingly important.1 Drives to cut
emissions are at the forefront of government policy and as
such the incentives for industry to follow suit are much
greater. Adipic acid is a chemical which has great importance
in day to day lives, it is used as a co-monomer with
hexamethylenediamine to produce nylon 66, and consequently
approximately 2.5 million tonnes is produced annually.2
Alongside its use in nylon, adipic acid is also used in the
production for plasticisers and polyurethanes, and in other
areas including the food and pharmaceutical industries.3
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
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
a Cardiff 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
b INVISTA Performance Technologies, The Wilton Centre, Wilton, Redcar, TS10 4RF,
UK
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cy00914h
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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
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.
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
Experimental
Chemicals
The following chemicals were used in this investigation without
further purification. Vanadium oxide (V2O5, 99.95%), sodium
sulfate (NaSO4, 99.99%), copperIJI) acetate (CuOAc, 97%),
hydrogen peroxide (H2O2, 30 wt%), ammonium metavanadate
(NH4VO3, 99.9%), vanadium carbide (99%), urea (99%), trans-
1,2-cyclohexanediol (98%), 5 wt% Pt/C, 2-hydroxycyclohexanone
dimer (97%) were all sourced from Sigma-Aldrich.
Catalyst preparation
NaV6O15 with NaSO4. Anhydrous Na2SO4 (3.6 mmol, 511
mg) and V2O5 (1.8 mmol, 328 mg) were dissolved in
deionised water (30 ml). The mixture was then loaded into a
70 ml Teflon-lined stainless-steel autoclave. The autoclave
was then heated to 200 °C under autogenous pressure for 24
h and allowed to cool to room temperature. The precipitate
was collected by vacuum filtration, washed with distilled
water and ethanol. The solid was then dried in a vacuum
oven at 60 °C for 16 h.
CuV2O6. CuOAc (0.5 mmol, 61.3 mg) and V2O5 (2.0 mmol,
364 mg) were dissolved in deionised water (30 ml), Cu/V
molar ratios were varied from 0.25–0.5. H2O2 (5 ml, 30%) was
then added to this mixture and stirred at room temperature
for 2 h. This mixture was then transferred to a 70 ml Teflon-
lined stainless-steel autoclave and placed in an oven for 72 h
at 175 °C under autogenous pressure. The resulting solid was
washed with deionised water and dried at 40 °C for 16 h in a
vacuum oven. The dried solid was then heat-treated at 500 °C
in N2 for 2 h at 10 °C min−1.
AgVO3. NH4VO3 (0.5 mmol, 58.5 mg) was dissolved in
deionised water (30 ml). A solution of AgNO3 (0.5 mmol, 84.9
mg, 10 ml) was added dropwise to the NH4VO3 solution. This
mixture was stirred for 15 min and then transferred to a 70
ml Teflon-lined stainless-steel autoclave and heated at 180 °C
under autogenous pressure for 18 h. The resulting precipitate
was collected via vacuum filtration and rinsed with boiled
deionised water and ethanol. The solid was then dried under
vacuum at 60 °C.
observed catalysis. Nevertheless,
a
truly heterogeneous
vanadium catalyst would mark a major advance in this field.
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 H5ijPV2Mo10O40], 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,
Scheme 1 Reaction scheme for oxidation of cyclohexanediol to adipic acid as proposed by Obara et al.
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V–C3N4. Urea (0.2 mol, 12.0 g) was dissolved in water (100
ml). Ammonium metavanadate (NH4VO3, 4.36 mmol, 510 mg)
was then added to this mixture. The solution was then
heated to 100 °C to remove water. The dried catalyst
underwent a heat treatment at 300 °C for 2 h at 3 °C min−1
under N2. The temperature was subsequently increased to
550 °C for 4 h at 3 °C min−1 under N2.
100 ppm then samples were diluted as required using
deionised water and then filtered using PTFE syringe filters
(0.45 μm). For solid samples, 10 mg of catalyst was dissolved
in freshly prepared aqua regia (5 ml) and left for 24 h. The
solution was then diluted up to 50 ml. Elemental
composition was determined using an Agilent MP-AES 4100,
quantification was achieved by calibration of the instrument
using commercial calibration standards.
NMR spectroscopy. NMR spectroscopy was conducted on a
Bruker Avance III 500 MHz (11.7 Tesla) spectrometer
equipped with a Prodigy cryoprobe, chemical shifts are
reported in ppm. When required, reaction samples were
diluted by 10% with D2O and 51V NMR measurements
acquired over 512 scans at a frequency of 131.55 MHz over
the spectral range −100 ppm to −900 ppm.
XRD. Powder X-ray diffraction was carried out using a
Panalytical X'Pert diffractometer equipped with a Cu X-ray
source operating at 40 kV and 40 mA. The ICDD standard
database was used for phase identification where possible or
checked against examples in the literature.
Catalytic testing
Oxidation of cyclohexanediol to adipic acid under O2. A
solution of cyclohexanediol (5 ml, 10 000 ppm) was loaded
into a low-pressure glass reactor. When required, Pt/C (10
mg) and the vanadium co-catalyst (2 mg) were then added to
the vessel. The reactor was purged with O2 and then charged
with 3 bar O2. The reactor was then stirred at 80 °C for 4 h.
Samples were centrifuged and filtered using PTFE syringe
filters (0.45 μm) before HPLC analysis.
Oxidation of 2-hydroxycyclohexanone. A stock solution of
vanadium was prepared by stirring V2O5 (2.20 mmol, 400 mg)
in water (50 ml) for 16 h. The resulting solution was then
centrifuged, and the supernatant was collected. After
filtration using PTFE syringe filters (0.45 μm) an accurate
concentration was then determined by using MP-AES. A
solution of 2-hydroxycyclohexanone (5 ml, 4000 ppm) was
loaded into a low-pressure glass reactor. The required volume
of vanadium solution was then added to the vessel. The
reactor was purged with O2 and then charged with 3 bar O2.
The reactor was then stirred at 80 °C for 4 h. Samples were
centrifuged and filtered using PTFE syringe filters (0.45 μm)
before HPLC analysis.
HPLC analysis. Analysis was carried out using an Agilent
1260 Infinity HPLC equipped with ultraviolet and refractive
index detectors. Reactants and products were separated using
a Metacarb 67H column. The mobile phase was an aqueous
solution of H3PO4 (0.01 M) and the flow rate was 0.5 ml
min−1. For the quantification of the amounts of reactants
consumed and products generated, an external calibration
method was used. The conversion, selectivity and mass
balance were calculated as follows:
Results and discussion
Obara et al.24 have demonstrated the potential for Pt/C and
vanadium to operate catalytically in tandem offering a highly
selective route to adipic acid from cyclohexanediol. Firstly,
the results (Table 1 – entry 1) show that the blank reaction
displayed no activity. The use of V2O5 (Table 1 – entry 2) as
the sole catalyst resulted in little activity, with only a small
amount of conversion observed. This is followed by Pt/C
(Table 1 – entry 3), which showed a conversion of 39.4% and
a selectivity of 59.4% towards 2-HCO. This is accompanied by
low selectivity to other oxidation products, such as glutaric
acid and succinic acid at 2.7% and 1.4%, respectively. When
a vanadium catalyst is added to the Pt/C catalyst, selectivity
towards adipic acid was observed, as shown in entry 4. In this
system selectivity to adipic acid increases to 76.1% and
conversion remains at a similar level of 49.5%. This reaction
is likely a largely homogeneous reaction, as most of the
vanadium oxide catalyst has been measured by MP-AES to be
dissolved in the reaction media. Obara et al. suggested that
despite losing approximately half the solid vanadium on each
use, the remaining catalyst is responsible for the activity
observed.24 In either case, this leached homogeneous
vanadium makes the system far less industrially viable due to
the poor stability, and therefore lifetime, of the vanadium
oxide under reaction conditions in addition to the possible
contamination of the product with toxic vanadium species. If
a stable heterogeneous vanadium catalyst could be prepared,
then this would be more practical industrially. For this
purpose, we therefore prepared a range of vanadium bronzes
incorporating Na, Cu or Ag to increase stability, as they have
commonly been used previously as heterogeneous catalysts
in aqueous reactions, albeit with V leaching not being
reported.34–37
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
Catalyst characterisation
MP-AES. Samples of the catalyst testing solution after
reaction were tested without dilution. If samples exceeded
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Table 1 Reproduction of work by Obara et al.24 for the oxidation of cyclohexanediol. Reaction conditions: 80 °C, 3 bar O2, 4 h, 10 000 ppm
cyclohexanediol in water (5 ml), 10 mg 5% Pt/C, 2 mg V2O5
Selectivity/%
Adipic
Conversion/% acid
Glutaric Succinic
Vanadium
Vanadium
Mass
Entry Catalyst
2-HCO acid
acid
Unknowns leaching/% concentration/ppm balance/%
1
2
3
4
Blank
V2O5
Pt/C
0.0
3.0
39.4
0.0
0.0
0.0
0.0
0.0
59.4
1.2
0.0
0.0
2.1
4.5
0.0
0.0
1.3
1.4
0.0
0.4
9.2
5.4
—
—
100
97.5
90.4
96.2
—
88.4
—
168
Pt/C + V2O5 49.5
76.1
All bronze catalysts in this study were synthesised using a
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.
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.
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
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
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.
Fig. 1 XRD patterns for synthesised bronzes: NaV6O15 (black), CuV2O5
(red), AgVO3 (blue).
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Table 2 Reaction data for the oxidation of cyclohexanediol. Reaction conditions: 80 °C, 3 bar O2, 4 h, 10 000 ppm cyclohexanediol in water (5 ml), 10
mg 5% Pt/C, 2 mg vanadium catalyst
Selectivity/%
Adipic
Conversion/% acid
Glutaric Succinic
Vanadium
Vanadium
Mass
Entry Catalyst
2-HCO acid
acid
Unknowns leaching/% concentration/ppm balance/%
1
2
3
4
Pt/C + Na2V6O16
36.7
Pt/C + Cu0.25V2O6 32.0
Pt/C + Cu0.33V2O6 27.2
70.1
64.0
60.4
63.7
4.0
4.9
9.0
9.5
6.5
0.0
0.0
0.0
1.6
0.0
0.0
2.2
1.5
4.3
0.0
5.2
41.8
16.6
5.8
67
35
11
33
96.0
91.2
91.2
95.5
Pt/C + Cu0.5V2O6
25.1
19.8
Table 3 Reaction data for the oxidation of cyclohexanediol. Reaction conditions: 80 °C, 3 bar O2, 4 h, 10 000 ppm cyclohexanediol in water (5 ml), 10
mg 5% Pt/C, 2 mg vanadium catalyst
Selectivity/%
Adipic
acid
Glutaric
acid
Succinic
acid
Vanadium
leaching/%
Vanadium
concentration/ppm
Mass
balance/%
Entry
Catalyst
Conversion/%
2-HCO
Unknowns
1
2
3
AgVO3
V–C3N4
VCa
18.4
24.7
19.4
41.6
59.3
55.0
21.3
5.3
—
0.0
0.0
0.0
2.1
1.7
0.0
0.0
33.7
16.0
5.0
—
1.2
9
33
17
92.8
91.4
91.3
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.
The final bronze of the series of vanadium catalysts was
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
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.
Efforts to probe the heterogeneity of this reaction
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.
Effect of homogeneous vanadium on the oxidation of
cyclohexanediol
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
To elucidate the role of homogenous vanadium, a series of
reactions using low concentrations of vanadium in solution
were conducted. A time online study was also undertaken to
assess the effect of conversion on selectivity, as conversion is
largely controlled by Pt/C. As there is a slight correlation
between conversion and selectivity (Fig. 2a), 2-HCO was used
as the starting material for these reactions, and therefore the
addition of Pt/C was unnecessary.
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
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Fig. 2 a) Effect of conversion on selectivity taken over reactions from 2 h to 22 h. Reaction conditions: 80 °C, 3 bar O2, 5 ml 10 000 ppm
cyclohexanediol in water, 10 mg Pt/C, 2 mg V2O5 b) oxidation of 2-HCO to adipic acid using solutions of vanadium between 0 and 45 ppm.
Reaction conditions: 80 °C, 3 bar O2, 4 h, 4000 ppm 2-hydroxycyclohexanone in water (5 ml), vanadium added as a stock solution of 200 ppm
V2O5 in water. Vanadium concentrations are obtained using MP-AES analysis which are run in triplicate and averaged.
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
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 VIJIII) or VIJIV) so the catalytic VIJV) 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,
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 10 000 ppm
cyclohexanediol. The main reason for this could be the
polarity of the different substrates affecting the solubility of
vanadium.
Fig. 3 Leaching of vanadium induced by differing sets of reaction
conditions. Reaction conditions: 80 °C, 3 bar O2, 4 h, 5 ml reaction
solution (as specified), 10 mg Pt/C (when required), 5 mg V2O5 (when
required).
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
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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.
Conclusions
A range of vanadium bronzes were assessed for the oxidation
of cyclohexanediol to adipic acid. Initial sodium containing
bronzes demonstrate promising activities, however this is
accompanied by high degrees of leaching. This leaching can
be significantly mitigated by the incorporation of Cu or Ag.
However, even the low levels of leaching, observed with the
Cu0.33V2O6 and AgVO3, enable the conversion of 2-HCO to
adipic acid. In addition, efforts to reduce leaching using a
carbon nitride as a support proved unsuccessful despite
strong acid–base interactions being reported as stabilising
the vanadium onto the surface of the support. In subsequent
studies it was shown that even trace levels of vanadium
leached from these materials was enough to catalyse the
reaction homogeneously. 51V NMR studies have revealed that
Fig.
4 UV-vis spectra of reaction filtrate from bronze reactions
showing presence of only vanadium +5. V2O5 stock solution (pink),
post reaction mixture of V2O5 (blue), post reaction filtrate of NaV6O15
(green).
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
Fig. 5 51V NMR of a) vanadium decavanadate which is present at low pH with presence of monovanadate also b) reaction sample of Pt/C and
V2O5 with 4000 ppm 2-HCO solution c) V10 compound with labelled vanadium sites corresponding to the shifts shown in the NMR spectra.
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the speciation in these homogeneous reactions to be in the
form of a decavanadate.
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Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
The authors acknowledge INVISTA and the EPSRC Centre for
Doctoral Training in Catalysis for funding, project reference
EP/L016443/1. RVE acknowledges funding from the European
Union's Horizon 2020 research and innovation programme
under the Marie Skłodowska-Curie grant agreement no.
663830. Information on the data underpinning the results
presented here can be found in the Cardiff University data
catalogue at: http://doi.org/10.17035/d.2020.0108567396.
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