Explaining the role of vanadium in homogeneous glucose transformation reactions using NMR and EPR spectroscopy

Article abstract of DOI:10.1016/j.apcata.2018.10.030

Our contribution investigates the formation of various catalytically active vanadium species for different glucose transformation reactions. By applying both ⁵¹V-NMR (nuclear magnetic resonance) and continuous wave EPR (electron paramagnetic resonance) spectroscopy, we were able to identify the different vanadium species that catalyse glucose transformation to several organic acids in aqueous solution depending on the reaction atmosphere and type of vanadium precursor. Under aerobic conditions (20 bar oxygen atmosphere) we could prove that only the higher-substituted vanadium containing polyoxometalate HPA-5 (H₈PV₅Mo₇O₄₀) catalyses the desired glucose oxidation to formic acid at 90 °C. Moreover, our results suggest that substituted V⁵⁺ species are the predominantly catalytic active species in the production of formic acid. Using anaerobic conditions (20 bar nitrogen atmosphere), we could show that different vanadium species are formed depending on the nature of the precursor. Hereby, paramagnetic acid-bound vanadyl species formed from the VOSO₄ and NH₄VO₃ as well as the HPA-5 precursor seem to be predominantly responsible for lactic acid formation from glucose under anaerobic conditions.

Full text of DOI:10.1016/j.apcata.2018.10.030

Applied Catalysis A, General 570 (2019) 262–270
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Explaining the role of vanadium in homogeneous glucose transformation
reactions using NMR and EPR spectroscopy
a,⁎
b
a
a
Jakob Albert , Matthias Mendt , Michael Mozer , Dorothea Voß
b
a
Lehrstuhl für Chemische Reaktionstechnik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058, Erlangen, Germany
Felix-Bloch-Institut für Festkörperphysik, Universität Leipzig, Linnéstraße 5, 04103, Leipzig, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords:
Our contribution investigates the formation of various catalytically active vanadium species for different glucose
5
1
Keggin polyoxometalates
Biomass transformation
EPR spectroscopy
transformation reactions. By applying both V-NMR (nuclear magnetic resonance) and continuous wave EPR
(
electron paramagnetic resonance) spectroscopy, we were able to identify the different vanadium species that
catalyse glucose transformation to several organic acids in aqueous solution depending on the reaction atmo-
sphere and type of vanadium precursor. Under aerobic conditions (20 bar oxygen atmosphere) we could prove
NMR spectroscopy
Vanadium redox chemistry
that only the higher-substituted vanadium containing polyoxometalate HPA-5 (H
8
PV
5
Mo O40) catalyses the
7
5+
desired glucose oxidation to formic acid at 90 °C. Moreover, our results suggest that substituted V species are
the predominantly catalytic active species in the production of formic acid. Using anaerobic conditions (20 bar
nitrogen atmosphere), we could show that different vanadium species are formed depending on the nature of the
precursor. Hereby, paramagnetic acid-bound vanadyl species formed from the VOSO
4
and NH
4
VO as well as the
3
HPA-5 precursor seem to be predominantly responsible for lactic acid formation from glucose under anaerobic
conditions.
1
. Introduction
platform chemicals like levulinic acid (LevA), formic acid (FA), gamma-
valerolactone (GVL) and derived products like several oxygenates
Catalytic oxidation chemistry of biomass is gaining both scientific
[8–10].
and industrial interest. Hereby, several pretreatment steps are required
before direct chemical valorisation is possible. The main effort is to
overcome the recalcitrant nature of the water-insoluble feedstock by
acid-catalysed depolymerisation followed by oxidative cleavage of the
carbon bonds in the biomass framework [1,2].
The reaction sequence for valorisation of lignocellulose starts with
the fractionation of the main components followed by acid-catalysed
hydrolysis of cellulose and hemicellulose into water-soluble mono-
saccharides such as glucose, fructose, or xylose and the subsequent
oxidative cleavage of CeC bonds in monosaccharides into low-mole-
cular carboxylic acids like FA [11], acetic acid (AA) [12,13] or lactic
acid (LA) [14].
Lignocellulosic biomass as the most abundant class of biogenic
materials typically contains more than 50 wt% sugars that can be up-
graded to valuable platform chemicals [3]. Lignocellulose consists of
the three main components cellulose (ca. 50%), hemicellulose
Over-oxidation of the monosaccharides and intermediates results in
the thermodynamically favored complete combustion to carbon dioxide
and water [15]. Consequently, it is of critical importance that the ap-
(
25–30%) and lignin (15–25%) [4]. The aromatic lignin is the most
underutilized fraction. It has been traditionally employed for heat and
power purposes through combustion in the pulp and paper industry due
plied catalyst systems prevent total oxidation leading to CO but cata-
2
lyze partial oxidation leading to FA [16]. Moreover, in the context of a
sustainable chemical transformation of biomass, only environmentally
friendly reagents such as water (solvent) or molecular oxygen (oxi-
dizing agent) should be used [17].
to its high caloric value [5]. Hemicellulose mainly contains C sugars
5
like xylose or arabinose having important applications for biofuel
production (e.g. bioethanol) and for the generation of valuable che-
mical intermediates (e.g. furfural) [6]. Cellulose is comparably con-
sidered as one of the most abundant biopolymers on earth comprising
The required oxidative carbon-carbon bond cleavage by using mo-
lecular oxygen (O ) in aqueous media can be accomplished by several
2
linear β(1,4) glucose C
6
-chain links [7]. Furthermore, the latter can be
metal catalysts [18–20]. In particular, V-containing catalysts such as
polyoxometalates (POMs) [21–23] and water-soluble vanadium
converted into valuable products such as biofuels (e.g. bioethanol) and
⁎
Corresponding author.
E-mail address: jakob.albert@fau.de (J. Albert).
https://doi.org/10.1016/j.apcata.2018.10.030
Received 29 August 2018; Received in revised form 17 October 2018; Accepted 23 October 2018
Available online 30 November 2018
0926-860X/ © 2018 Elsevier B.V. All rights reserved.

J. Albert et al.
Applied Catalysis A, General 570 (2019) 262–270
precursors such as NaVO
3
[24] or VOSO
4
[14,25] can effectively cat-
line widths of each hfi transition [39]. Here least square fitting routines
alyse this transformation.
are very helpful for reproducing their exact line shapes. Unfortunately,
POMs are well-defined metal-oxyanions linked with oxygen bridges
the effects of τ and the hfi-anisotropy stand in such a certain mutual
c
6
+
of early transition metals at their highest oxidation state (e.g. Mo
,
relationship, that a unique determination of both from fast motion EPR
6
+
5+
4+
W
or V ). They can also contain a multitude of hetero atoms to
spectra of V
species alone is not possible without knowing at least
improve their chemical and thermal stability [26]. POMs reveal unique
physical and chemical properties like tunable acid-base properties,
great redox activity based on the fast and reversible multielectron
transfer, high thermal stability and excellent solubility and stability in
water [27,28].
one of both quantities. Therefore one has to set one parameter to a
certain but arbitrary value, using the second as a fitting parameter.
Nevertheless, the isotropic g-value and isotropic hfi parameter alone
provide valuable information since they likewise are characteristic for
4
+
specific V
species.
Mostly used in homogeneous catalyzed oxidation reactions are
In this contribution, we wanted to clarify the role of different va-
nadium species from several precursors (simple vanadium salts as well
n−
POMs from the so called Keggin-type [XM12
O
40
]
. They contain a
template of various coordinating anions, e.g. oxoanions, oxometalates
or halides, together with a framework metal which is typically an early,
high-valent transition metal [29,30]. The catalytic activity is mostly
introduced by substituting some of the framework metals (W, Mo) with
easily reducible heterometals like vanadium that results in shifting their
reactivity from acidic to redox-dominance [31]. The generated com-
as heteropolyacids) for the transformation of glucose under aerobic (O -
2
atmosphere) or anaerobic (N -atmosphere) conditions in aqueous
2
media by applying both NMR and EPR spectroscopy. This allows for a
deeper understanding of biomass transformation processes. Herein, we
used two commercially available water-soluble vanadium sources, one
4+
5
+
containing initially V
(NH
4
VO ) and one containing initially V
3
pounds have the composition H3+n[PV
n
Mo12-n
O
40] and are called
(VOSO ) in order to identify the different catalytically active vanadium
4
heteropolyacids, abbreviated as HPA-n depending on the content of
vanadium atoms (n) and have proven to be the most efficient biomass
oxidation catalysts in aqueous solution [32,33].
species under reaction conditions. Thereby, it is known that formation
of various carboxylic acids like FA or LA from glucose is dependent on
the reaction atmosphere [14,34]. Moreover, two heteropolyacids,
In 2011, some of us developed the selective oxidation of biomass to
formic acid (OxFA process) using the above mentioned homogeneous
POM catalysts [34,35]. By modifying both the process conditions and
the chemical composition of the catalyst, we were able to expand the
raw material base on complex, water-insoluble biomass of different
origin and composition. Furthermore, a systematic study of the re-
oxidation behavior of the homogeneous POM catalyst in aqueous so-
lution together with an optimisation of the catalyst synthesis lead to a
superior performance concerning lower oxygen pressures and faster
reaction kinetics [36].
namely H
4
1
PV Mo11
O
40 (HPA-1) and H
PV
8 5
Mo O40 (HPA-5) containing
7
different amounts of vanadium were also tested for the catalytic
transformation of glucose under aerobic or anaerobic conditions.
2. Experimental
2.1. Materials
All reagents and substrates were commercially available and used as
received. The model substrate glucose was supplied by Merck KGaA
Nuclear magnetic resonance (NMR) and electron paramagnetic re-
sonance (EPR) spectroscopy methods are very powerful tools to identify
with a purity of 99.5%. The vanadium precursor NH
4
VO
3
was pur-
was ob-
tained from Alfa Aesar with a purity of 99.9%. The catalysts HPA-1
chased from Acros Organics with a purity of 99.5%. VOSO
4
5
+
4+
51
various V as well as V species in solution. The V nuclear isotope
occurs in a natural abundance of almost 100% and has a nuclear spin of
(H
4
PVMo11
O
40) and HPA-5 (H
8
PV
5
Mo O40) were both synthesized ac-
7
5
+
I = 7/2. Furthermore, the V
ion is diamagnetic since it has a closed
cording to the literature [16,40]. The characterization of the catalysts
has been carried out using a Perkin Elmer Plasma 400 ICP-OES device
resulting in a P/V/Mo ratio of 1/0.82/11.14 for HPA-1 and 1/4.80/6.93
for HPA-5, respectively. Oxygen (4.5 GA 201) and Nitrogen (5.0) were
bought from Linde AG. Demineralized water was used as a solvent for
all experiments.
6
shell 3p electron configuration. Therefore, NMR is an ideal tool for the
5
+
detection of various V
species in aqueous solution. It resolves only
the isotropic chemical shift, whereas the chemical shift anisotropy as
well as the first-order nuclear quadrupole coupling average to zero due
to the rapid tumbling of the molecules [37]. Nevertheless, the isotropic
5
+
chemical shift of V species is highly informative since it significantly
depends on the local electronic environment and is therefore a finger-
2.2. Catalytic transformation reactions (results of Tables 1 and 2)
5
+
print for a certain V
species.
5+
EPR cannot detect V
measure characteristic signals of V
electron configuration and therefore an electron spin S = 1/2. Thus,
species due to their diamagnetism but can
The catalytic transformation reactions were carried out in a tenfold
screening plant with a batch mode reactor setup. It consists of ten 20 mL
autoclaves out of Hastelloy C276. All pipes, valves and fittings were
made of stainless steel 1.4571. The gaskets used were made of Teflon.
The autoclaves were connected in parallel to a single oxygen supply line
via individual couplings and placed inside a heating plate in order to
adjust the required temperature. The heating plate was equipped with a
magnetic stirrer whereby magnetic stirrer bars could be used for stir-
ring. Additionally, each reactor was connected to a rupture disk with a
burst pressure maximum of 90 bar.
4
+
4+
1
species, since V
has a 3d
4
+
one usually cannot detect NMR signals of V
species due to the fast
magnetically induced nuclear relaxation [38]. In this sense, NMR and
EPR complement each other. Two magnetic interactions define pre-
4
+
dominantly the signal of a V species, measured by EPR in continuous
wave (CW) mode. The electron Zeeman interaction describes the in-
teraction between the unpaired electron and the external magnetic field
and is described by the so called dimensionless g-tensor, which is the
EPR equivalent of the chemical shift tensor in NMR. In liquids the g-
tensor anisotropy is likewise averaged out so that liquid EPR usually
2.3. Typical work-up procedure
4
+
resolves only the isotropic g-value giso which is for V near but slightly
4
+
smaller than the free electron g-value g
e
= 2.0023. Liquid EPR of V
For the catalytic transformation reactions, each autoclave was filled
with 1 mmol glucose (0.18 g), 0.1 mmol catalyst and 10 g water as the
solvent. The system was purged with 10 bar pure oxygen or nitrogen in
order to remove the residual air out of the reactors. Afterwards, the
reactors were pre-pressurized with about 16 bar, the stirrer was set to
300 rpm and the heating was switched on. When the desired tempera-
species also resolves the isotropic hyperfine interaction (hfi) between
5
1
the unpaired electron and the V nucleus spin I = 7/2, which leads to a
characteristic splitting of the EPR spectrum into eight hfi transitions.
Again, the hfi anisotropy is almost averaged out by the fast tumbling of
the molecule. Nevertheless, in the fast motion regime, which applies for
4
+
the present V species in aqueous solution, the hfi anisotropy as well
ture (90 °C under O
2
-pressure respectively 90 °C or 160 °C under N -
2
as the rotational correlation time τ
c
still determine to some extend the
pressure) was reached, the pressure was increased to the required
263

J. Albert et al.
Applied Catalysis A, General 570 (2019) 262–270
pressure of 20 bar and the stirring speed was set to 1000 rpm in order to
start the gas entrainment. This moment was set as starting time of the
experiment.
least square routine implemented by the MatLab R2017a optimization
toolbox, allowing fits with high precision. The fitting parameters for the
simulations of each species were the isotropic g-value
g
, the isotropic
iso
5
1
V hfi parameter Aiso, two phenomenological isotropic convolutional
2.4. Determination of quantitative reaction parameters
Gaussian and Lorentzian line width parameters, the base-10 logarithm
of the rotational correlation time for isotropic rotational diffusion, the
total intensity of the simulated signal as well as the relative amount of
the second species, if considered. The anisotropies of the g- and hfi
After the reactions all products were quantitatively determined by
HPLC- and GC-analysis. The yields of FA, AA, LA, LevA and HMF were
determined by means of HPLC measurements using a HPLC from Jasco
equipped with a 300 mm x 8 mm SH1011 Shodex column and calcu-
g
g
|
|
tensors are not resolved but where set to typical values
=
0.014
3
A
||
A
[
48] and
= 111.5 MHz [50] assuming axial symmetric g- and hfi
3
lated as n(product)/n(C-atoms glucose). The yields of CO were de-
2
tensors. Here g and
g
are the principle values of the g-tensor in z-
||
termined by means of GC-analysis using a Varian GC 450 equipped with
a 2 m x 0.75 mm ID ShinCarbon ST column and calculated as n(CO )/n
C-atoms glucose). No other gaseous products could be detected by the
used GC.
51
direction and in the x,y-plane, as
A
|| and A are for the V hfi tensor.
2
Note that a large range of such g- and hfi-tensor anisotropy is consistent
to the experimental signals depending on the correlation time. There-
fore in the present study the latter was used only as a fitting parameter.
Corresponding values are given in the ESI, part 4. The errors of the
parameters were estimated by varying the corresponding parameter
around the best fit of the simulated signal.
(
2.5. Nuclear magnetic resonance (NMR) spectroscopy
The NMR spectra were recorded on a Jeol ECX-400 MHz spectro-
The experimental intensities of the normalized EPR spectra were
calculated by the double integration of the spectra, after third order
polynomial base line corrections before each integration step.
5
1
meter (9.4 T) at 293 K. The V NMR spectra were measured with 2024
scans in a range of -580 to -460 ppm with an excitation frequency of
1
05.25 MHz and a resolution of 0.77 Hz. The field frequency stabili-
zation was locked to deuterium by placing a coaxial inner tube with
O into a 10 mm tube containing the sample.
D
2
3. Results and discussion
2
.6. Electron paramagnetic resonance (EPR) spectroscopy
3.1. Catalytic glucose oxidation using various V-sources under aerobic
conditions
Continuous wave EPR spectra of the liquid samples were measured
at room temperature at X-band frequency (9.5 GHz) using a rectangular
Varian resonator and special flat quartz glass tubes to avoid dielectric
losses by the water molecules enabling the coupling of the microwave
In the first set of experiments, we chose the well-known glucose
oxidation reaction under O atmosphere (based on former studies)
2
[8,56] to investigate the effects of free vanadium in solution. Therefore,
4
+
(
mw) to the resonator. The liquid samples were pipetted into such a
we chose VOSO
4
as a source for paramagnetic V
and NH
4
VO as a
3
5
+
cell, filling the same volume during the EPR measurements in all cases
with an estimated error of 5%. All experimental EPR spectra where
normalized with respect to the number of scans and where otherwise
conducted with the same experimental parameters, including a micro-
wace power of 20 mW, a modulation frequency of 100 kHz and a
modulation amplitude of 1 m T. Before each measurement of a single
sample, the quality (Q) factor was measured and the presented signals
were also normalized with respect to it. The spectra were measured on
two different days. The first day the Q-factor was about 2000 and the
source for diamagnetic V in comparison to vanadium-ions bound in a
three-dimensional network like the Keggin-POM structures HPA-1 and
HPA-5 for their oxidation ability under aerobic conditions. The chosen
reaction conditions were comparable to those of the classical OxFA
process [11,34].
Scheme 1 shows the main reaction pathway from glucose to formic
acid suggested by Wölfet et al. [8] and Li et al. [56] under aerobic
conditions catalyzed by a HPA-2 catalyst.
The experiments were carried out in a tenfold screening plant with a
batch mode reactor setup consisting of ten 20 mL autoclaves. Each re-
actor was filled with 1 mmol glucose (0.18 g), 0.1 mmol catalyst and
10 g demineralized water as the solvent. The catalytic transformation
reactions were performed under 20 bar oxygen pressure at 90 °C using
1000 rpm for 3 h reaction time in order to study the initial kinetic re-
gime.
second day about 1000. But a same VOSO sample before the reaction
4
was measured on both days as a reference. Its normalized EPR signal
intensities, considering the Q-factor, were exactly the same for mea-
surements on both days, verifying that the measurement conditions
were reproducible and that the experimental derived EPR intensities
are comparable.
4
+
EPR signals of the V
species were analyzed using the MatLab
Table 1 shows the results of the screening of the four different va-
nadium sources under aerobic conditions.
toolbox EasySpin version 5.2.20 [53]. Before their simulations they
were base line corrected by third order polynoms. All signals were fitted
Using a reaction time of 3 h, we could demonstrate that only the
HPA-5 catalyst shows a good activity for glucose oxidation (X = 42.9%)
under aerobic conditions without the help of any additives like acidity
4
+
by simulated EPR signals of one or the sum of two V species using the
EasySpin function garlic for isotropic fast motional EPR spectra and a
Scheme 1. Main reaction steps for vanadium-catalyzed glucose oxidation in aqueous solution under aerobic conditions [56].
264

J. Albert et al.
Applied Catalysis A, General 570 (2019) 262–270
Table 1
confirmed neither by NMR nor EPR [45].
Screening of different vanadium precursors for the reaction with glucose under
Experimental and simulated room temperature EPR signals of the
5
1
O -atmosphere in aqueous solution.
2
VOSO
4
sample before the reaction in combination with V-NMR
4+
Yielda
measurements confirmed that exclusively one paramagnetic V spe-
species is present
within the sensitivity of NMR (see ESI, Figure S2). More specifically,
Entry
Catalyst
Conversion
Yield
CO
%]
pH-value
before [-]
pH-value
after [-]
5
+
[
%]
FA [%]
2
cies is present (see ESI, Figure S1) whereas no V
[
4
+
EPR detects before the reaction a single V
species AVOSO4 with an
51
1
2
3
4
VOSO
4
VO
3.4
2.8
2.7
0.7
0.7
0.2
8.6
3.3
6.7
1.5
1.8
2.1
3.0
1.5
1.6
isotropic g-value giso = 1.9638 ± 0.001 and an isotropic V hyperfine
interaction (hfi) value of Aiso = 317.5 ± 2 MHz (see ESI, Table S1),
NH
4
3
2.0
HPA-1
HPA-5
0.2
< 0.1
34.3
2
+
4+
2
42.9
which can be assigned to vanadyl (VO ) in the form of [V O(H
5
O) ]
2
[
48,49]. After the reaction under aerobic conditions at 90 °C for 3 h, the
Reaction conditions: 1 mmol glucose, 0.1 mmol catalyst dissolved in 10 g H O,
2
same vanadyl species AVOSO4 was predominantly found using EPR
spectroscopy (see ESI, Figure S9 and Table S1). However, its EPR de-
rived amount has decreased to about 22% of its amount before the
reaction (see ESI, Table S1). Moreover, NMR detects a new peak with a
2
0 bar O , 90 °C, 3 h, 1000 rpm; Conversion, yields and selectivities determined
2
a
as described in the corresponding section of the experimental part. Neither
AA, LA, LevA or HMF could be detected under these conditions.
+
−
chemical shift at -543 ppm, which is attributable to a VO
2
species in
promotors (sulfuric acid [22], para-toluenesulfonic acid [34]) or co-
aqueous solution (see ESI, Figure S10) leading to a decreasing pH level
oxidants like FeCl [15]. Under mild oxidation temperatures of 90 °C, a
3
4
+
5+
[
14,46]. Obviously, the oxidation of some V -ions to V in the form
moderate conversion of glucose resulting in a FA-yield of 34% with an
+
of VO
2
did not lead to a significant catalytic oxidation of glucose to FA
outstanding liquid-phase selectivity of > 99% (proven by HPLC mea-
under the applied reaction conditions. Therefore, we claim that VOSO
is not a suitable catalyst for low-temperature biomass oxidation below
00 °C.
Using NH
4
surements) was reached. CO was the only byproduct in the gas phase
2
(
Y
CO2 = 8.6%) resulting from the competing total oxidation pathway of
glucose to carbon dioxide and water [8]. Interestingly, former pub-
lications claimed that simple vanadium precursors like NaVO and
VOSO should also be suitable oxidation catalysts for carbohydrates like
glucose under oxidative conditions [12,24]. However, the simple V
1
4
VO as vanadium source did also not lead to a noteworthy
3
3
51
catalytic oxidation activity (see Table 1), although V-NMR and EPR
4
5
+
measurements confirm that all initial vanadium before the reaction is
5
+
−
4
+
present as V in the form of VO (see ESI, Figure S3 and Figure S4).
3
source NH
4
VO
3
as well as the commercial V
source VOSO both
4
5
1
According to Hayashi et al. [47], the dominant signal in the V-NMR
showed the same poor activity with only around 3% glucose conversion
after 3 h reaction time. One possible explanation for this diverse be-
havior could be found in literature, where the presence of different
vanadium species is linked to the pH value in aqueous solution [40,41].
Interestingly, the pH using the HPA-5 drops only slightly from 1.8 to 1.6
although FA was formed. However, the same effect can be observed
spectrum at -578 ppm can be assigned to α-NH
minor peaks at -558 ppm and-572 ppm refer to impurities like KVO
NaVO , respectively (see ESI, Figure S4). After the reaction, three peaks
4
VO
3
whereby the two
3
and
3
5
1
are observable in the V-NMR spectra corresponding to the dec-
5
+
4-
avanadate species [H
2
V
10
O
28
]
(see ESI, Figure S12) [42,46]. Ob-
- precursor does not undergo a trans-
5
+
viously, vanadium in the V
O
3
using VOSO where the pH drops from 3.3 to 2.1 without significant FA
4
5
+
+
formation to V
O
2
under the applied oxidative conditions which is
formation indicating a change in the nature of the vanadium species.
supposed to happen at the pH level of 3.3 [41]. Additionally, con-
Regarding NH VO , there was a very drastic change from almost neu-
4
3
4
+
tributions of two different V
species ANH4VO3 and BNH4VO3 can be
tral (6.7) before reaction down to 3.3 after reaction without significant
FA formation. Finally, the HPA-1 heteropolyacid did not show any
catalytic activity under the applied reaction conditions as only traces of
FA could be detected by the used HPLC analysis. The pH also remains
constant indicating no change in the nature of the vanadium species.
Therefore, we exclude pH effects being the only reason for forming a
catalytic active vanadium species under the applied reaction condi-
tions.
resolved by EPR after the reaction (see ESI, Figure S11). Within the
spectral resolution, the isotropic spin Hamiltonian parameters of spe-
cies ANH4VO3 almost equals those of species AVOSO4 (see ESI, Tables S1
4
+
and S2). We therefore attribute tentatively this species to [V
O
2
+
(
H
2
O)
simulations
iso = 296.5 ± 2 MHz (see ESI, Table S2). This species can probably be
5
]
, too [48,49]. For species BNH4VO3 one derives by spectral
the parameters iso = 1.9657 ± 0.002 and
g
A
4
+
attributed to an acid-bound vanadyl complex, presumably [V
O
In order to clarify these findings and to investigate the nature of the
5
1
(COOH) ] [49,52]. The small amounts of both vanadyl species (see ESI,
2
applied vanadium species, we performed both V-NMR and EPR-
spectroscopy measurements before and after the reaction, as it is well-
known that V-O species in aqueous solution undergo certain dissocia-
tion reactions depending on the pH-level of the solution [40–42]. In
redox catalysis, the vanadium–oxygen (V–O) species are firstly reduced
by the substrate and then oxidized by an oxidant under aerobic con-
ditions to complete a catalytic cycle, or the vanadium–oxygen species
activates the oxidant to form an intermediate that oxidizes the reactants
Table S2) indicate that only a small fraction of VO - is reduced. Relying
3
on these results, one can exclude that VO - leads to a significant cata-
3
lytic oxidation of glucose to FA under the applied reaction conditions.
Regarding the HPA-1 heteropolyacid, small contributions of two
4
+
different V
species AHPA1 and BHPA1 are resolved by EPR before the
reaction (see ESI, Figure S5). Due to its isotropic spin Hamiltonian
parameters
g
iso = 1.9639 ± 0.001 and
A
iso = 318.1 ± 2 MHz the
4
+
2+
former can be attributed again to [V O(H
2
O)
5
]
[48,49]. For the
[
42]. The selectivity of the products is highly dependent on the V–O
latter, parameters giso = 1.9635 ± 0.002 and Aiso = 250.4 ± 8 MHz
species present in these reactions depending on the oxidation state of
vanadium in aqueous solution.
have been derived (see ESI, Table S3). They almost equal those of the
4
+
V
species in ammonium [50] and cesium [54] salts of
4− 4+
First, we want to admit that for all respective samples we cannot
3
+
3+
2
[PVMo
1 40
O
]
. As it was indicated by EPR, the corresponding V
1
exclude the presence of distinct V
species [43]. V
has a 3d elec-
species are not incorporated at a Mo site [54]. Structures have been
tron configuration and has therefore an electron spin S = 1. Since it is
paramagnetic, its detection by NMR should fail. Moreover, since it has
an integer electron spin, no allowed EPR transition can be excited by
the applied microwave frequency (≈9.5 GHz), if the zero field splitting
2
+
proposed, where a VO
species rather coordinates to three or four
oxygens at the outer surface of the Keggin molecule [54,55]. Conse-
2
+
quently, we tentatively assign species BHPA1 to VO , coordinating to
5
+
4-
3
+
the outer surface of a [PV Mo O ] heteropolyanion. Moreover, the
1
1 40
is larger than 30 GHz, as spectral simulations suggest. For a V
hex-
51 5+
V-NMR spectrum shows V
present as only one distinct peak refer-
aaqua cation, a zero field splitting of about 143 GHz was determined by
high-field multifrequency EPR [44]. Thus we expect that a significant
5
+
4-
ring to the single isomer [PV Mo11
O
40
]
according to the literature
presence of V3+ in the various aqueous solutions cannot ruled out or
(see ESI, Figure S6) [43]. After the applied reaction time of 3 h under
oxidative conditions, the same two paramagnetic V4+ species can be
265

J. Albert et al.
Applied Catalysis A, General 570 (2019) 262–270
5
1
Fig. 2. V-NMR spectra of the 0.1 mmol HPA-5 solution before (top) and after
bottom) the reaction with glucose under aerobic conditions.
(
present as V5+ before and after the reaction, as can be seen in the
5
1
corresponding V-NMR spectra (see Fig. 2). As already known from the
literature, aqueous HPA-5 solution is a complex mixture of different
species and isomers because of different equilibrium reactions of V-Mo-
P-substituted heteropolyacids in aqueous solution (see Scheme 2) [51].
Fig. 1. Experimental (black) and simulated (red) EPR spectra of the 0.1 mmol
HPA-5 solution before (a) and after (b) the reaction with glucose under aerobic
conditions. The EPR signal before the reaction (a) can be simulated by a single
species AHPA5 as indicated in the figure. After the reaction (b) the simulated EPR
signal (sum) is a superposition of the simulated signals of species AHPA5 and
species BHPA5 as indicated in the figure (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this
article).
acid-base equilibrium for all HPA-x (x = 1–6):
•
H
3+x [PV
x
Mo12
x
O
40
]
H
2+x [PV
x x 40
Mo12 O ]
+
40]2 + 2H+
+
H
H
1+X [PV
x x
Mo12 O
detected by EPR (see ESI, Figure S13, and Table S3) as well as the same
protolysis in strong acidic media for HPA-x (x > 1):
5
+
4-
51
•
[
PV Mo11
O
40
]
diamagnetic species was visible using
V-NMR
(13 x)Hx 1 [PVx Mo12 x O40]4 + 16H
+
(12
spectroscopy (see ESI, Figure S14). Interestingly, the amount of species
x)Hx 2 [PVx 1Mo13 x O40]4 + 12VO+
B
HPA1 slightly increases during the reaction (see ESI, Table S3), in-
2
5
+
4-
4+
5-
dicating the reduction of some [PV Mo11
O
40
]
to [PV Mo11
O
40] .
+
H3PO4 + 12H2O
However, the results clearly demonstrate that the HPA-1 precursor is
not catalytically active under the applied reaction conditions.
For the active HPA-5 catalyst, interestingly a comparably large
disproportionation reaction for HPA-x (x > 2):
•
4
+
amount of vanadium is present as paramagnetic V
(29% relative to
Hx 1[PVx Mo12 x O40]4
Hx [PVx+1Mo11 x O40]4
2
4
+
the amount of V
in the VOSO sample before the reaction, see ESI,
4
+
Hx 2 [PVx 1Mo13 x O40]4
Table S4), whereby one species AHPA5 could be detected by EPR prior to
the reaction identified again by its isotropic spin Hamiltonian para-
Consequently, aqueous solutions of HPA-5 contain not only
4
+
2+
meters as [V O(H
2
O)
5
]
(see Fig. 1 and ESI, Table S4) [48,49]. After
H
8
PV
5
Mo
7
O
40 itself but also lower and higher V-substituted Keggin
the reaction, the amount of this species has been decreased significantly
species such as
H
9
PV
6
Mo
6
2
O
40 (HPA-6),
H
7
PV
4
Mo
8
O40 (HPA-4),
4
+
to 8% (see ESI, Table S4), indicating the oxidation of most V
to
H
6
PV
3
Mo
9
O
40 (HPA-3), H
5
PO
PV
Mo10
O
40 (HPA-2), H
4
PV
1
Mo11O40 (HPA-
5
+
+
various V species. In addition, EPR resolves the minor presence of a
1), as well as VO
2
, H
3
4
and H
3
PMo12
O
40 (HPA-0). Therefore, dif-
4
+
second V
attributed again to acid-bound vanadyl, presumably [V O(COOH)
see ESI, Table S4) [49,52]. However, a large amount of vanadium is
species BHPA5 after the reaction (see Fig.1), which can be
ferent isomers depending on the degree of V-substitution in the Keggin
structure are possible [42,51].
4
+
2
]
5
1
(
Figs. 1 and 2 show a comparison of the EPR and V-NMR spectra
before and after the reaction with glucose under aerobic conditions for
266

J. Albert et al.
Applied Catalysis A, General 570 (2019) 262–270
Scheme 2. Different equilibrium reactions of HPA-x in aqueous solution [51].
Table 2
Summary of the different vanadium species resulting from the reaction with glucose under aerobic conditions determined by EPR and NMR spectroscopy.
4
+
a
4+
a
5+
b
5+
Species after reaction under O b
Entry Catalyst
V
Species before reaction under O
2
V
Species after reaction under O
2
V
Species before reaction under O
2
V
2
4+
2+
2+
2+
5+
+
1
2
3
VOSO
4
VO
[V4+O(H
–
2
O)
5
5
]2+
[V O(H
2
2
2
O)
O)
O)
5
5
5
]
]
]
–
[V
O
2
]
4
+
[V4+O(COOH)
5+
−
5+
4−
NH
4
3
[V O(H
2
]
]
[V
[H
O
3
]
[H
2
1
V
10
28
O ]
4
+
2+
4+
40]3−
40]3−
HPA-1
HPA-5
[V O(H
2
O)
]
[V O(H
1
PVMo11
O
[H
PVMo11
O
2
+
5+
40]4−
2+
5+
4−
VO @ [PV Mo11
O
VO @ [PV Mo11
O
40
]
4
+
2+
4+
2+
4+
4
[V O(H
2
O)
5
]
[V O(H
2
O)
5
]
[V O(COOH)
2
H
H
4
5
PV
1
2
Mo11
Mo10
O
O
40
40
H
4
5
PV
1
2
Mo11
O
40
40
PV
H
PV
Mo10
O
+
+
VO
2
VO
2
H
H
H
H
6
PV
7
PV
8
PV
9
PV
3
4
5
6
Mo
9
Mo
8
Mo
7
Mo
6
O
40
H
H
H
H
6
7
8
9
PV
PV
PV
PV
3
4
5
6
Mo
Mo
Mo
Mo
9
8
7
6
O
40
O
40
O
40
O
40
O
O
O
40
40
40
Reaction conditions: 1 mmol glucose, 0.1 mmol catalyst dissolved in 10 g H
2
O, 20 bar O , 90 °C, 3 h, 1000 rpm; Conversion, yields and selectivities determined as
2
described in the corresponding section of the experimental part. a determined by EPR spectroscopy; determined by V-NMR spectroscopy.
b 51
the most active HPA-5 catalyst.
containing precursors in similar concentrations like mentioned in
5
1
The V NMR spectra (see Fig. 2) are showing the typical peaks for
higher V-substituted species such as HPA 4, HPA-5 and HPA 6 (-560 to
Table 1. In the beginning, the same reaction conditions despite the
reaction atmosphere (nitrogen instead of oxygen) as in the first study
were used. However, there was no remarkable catalytic activity de-
tectable for all different vanadium precursors using 90 °C reaction
temperature for 3 h reaction time under anaerobic conditions (see ESI,
Table S5). Additionally, no changes in the NMR or EPR spectra could be
observed. Consequently, we increased the reaction temperature to
160 °C based on a previous study carried out by Tang et al. [14] in order
to compare the catalytic performance of diamagnetic and paramagnetic
vanadium species under literature-known conditions with a known
catalytic reaction pathway. These observations lead us to the assump-
tion that the activation barrier for V catalyzed glucose transformation
under anaerobic conditions is much higher than under aerobic condi-
tions that may affect the formation of catalytic active species sig-
nificantly.
-
620 ppm) as well as the peaks of lower V-substituted species such as
+
HPA-1 (-533 ppm) and HPA-2 (-537 ppm) as well as VO
2
/HPA-3
(
broad peak at -540 to -560 ppm) [40,51] indicating that higher sub-
stituted V-POM-species are the catalytic active species for the glucose
oxidation reaction under the applied reaction conditions.
These observations clearly indicate the unique nature of the HPA-5
catalyst leading to an unrivaled activity under the applied mild oxi-
dizing conditions.
For a better overview on the different vanadium species observed in
the experiments, Table 2 shows a summary of the different vanadium
species resulting from the reaction with glucose under aerobic condi-
tions in aqueous solution.
Scheme 3 shows the main reaction pathway from glucose to lactic
acid suggested by Tang et al. [14] under anaerobic conditions catalyzed
3
.2. Catalytic glucose transformation using various V-sources under
anaerobic conditions
by a VOSO catalyst.
4
For the glucose transformation reactions under inert atmosphere,
we again dissolved 0.1 mmol catalyst in 10 g H O and added 1 mmol
glucose (0.18 g). The experiments were carried out in the same tenfold
In the next set of experiments, we wanted to investigate the beha-
vior of different vanadium sources under anaerobic conditions at 20 bar
2
nitrogen atmosphere. Therefore, we used the same V4 and V
+
5+
Scheme 3. Main reaction steps for vanadium-catalyzed glucose transformation in aqueous solution under anaerobic conditions [14].
267

J. Albert et al.
Applied Catalysis A, General 570 (2019) 262–270
Table 3
Screening of catalysts with different vanadium species for reaction with glucose under N
2
-atmosphere at 160 °C.
Yield LevA [%]
Entry
Catalyst
Conversion
Yield FA [%]
Yield AA [%]
Yield LA [%]
Yield HMF
[%]
Yield
CO
%]
pH-value
before [-]
pH-value
after [-]
[
%]
2
[
1
2
3
4
VOSO
4
VO
47.7
36.2
12.5
27.8
–
1.6
2.5
3.2
0.8
30.2
20.5
–
3.5
0.7
2.8
–
12.5
7.6
–
–
–
–
3.3
6.7
1.5
1.8
1.9
1.8
1.5
1.8
NH
4
3
5.0
4.1
9.7
HPA-1
HPA-5
2.4
12.8
4.5
Reaction conditions: 1 mmol glucose, 0.1 mmol catalyst dissolved in 10 g H
2
O, 20 bar N , 160 °C, 3 h, 1000 rpm; Conversion, yields and selectivities determined as
2
described in the corresponding section of the experimental part.
screening plant with a batch mode reactor setup consisting of ten 20 mL
autoclaves in order to neglect influences of different reaction material
or up-scaling effects. The reactions were carried out applying increased
temperature (160 °C) by keeping all other parameters like stirrer speed,
reaction volume and time constant. The results are presented in Table 3.
Contrary to the results obtained under aerobic conditions, all cata-
lysts show higher conversion of glucose due to the higher reaction
temperature under 20 bar nitrogen atmosphere and no CO could be
2
detected in all samples. Regarding the product selectivity, several dif-
ferences could be observed.
The VOSO
4
shows the highest glucose conversion with 47.7%
leading predominantly to lactic acid (LA) with a yield of 30% and the
intermediate 5-Hydroxymethylfurfural (HMF) with a yield of 12.5%.
Moreover, small amounts with yields < 5% of levulinic acid (LevA), as
well as acetic acid (AA) could be detected. The thermally induced
glucose isomerisation and dehydration reaction via HMF lead to the
obtained reaction products following the mechanisms introduced by
Niu et al. [12] and Tang et al. [14] Regarding the spectroscopic in-
vestigations, EPR measurements showed again the presence of the same
4
+
4+
2+
V
species [V O(H
2
O)
5
]
(AVOSO4) as before the reaction (Fig. 3
and ESI, Table S1). Surprisingly, its amount has decreased to about 16%
of the amount before the reaction (see ESI, Table S1). However, NMR
5
+
does not resolve any V signal after the reaction (see ESI, Figure S18),
which is reasonable since no oxidising agent was present. In addition to
species AVOSO4 also an acid-bound vanadyl species [49] BVOSO4 could be
detected by EPR in very low amounts (Fig. 3 and ESI, Table S1), which
4
+
might be attributed to a [V O(COOH) ] species [49,52]. Moreover,
2
4+
these findings suggest that parts of the V species have been reduced
3
+
to a V
species [43], which is expected to be neither NMR nor EPR
4+
active (see comment above). Here, at least two V
species are neces-
sary to explain not only these small line splittings, but also the relative
intensities of all eight hfi transitions, as it is proven unambiguously by
the presented spectral simulations in Fig. 3. Moreover, the pH level
after the reaction with 1.9 is slightly lower compared to the one ob-
tained under oxidative conditions (2.1). This is caused by the formation
of carboxylic acids (mainly LA) that decrease the pH level of the re-
action system.
5
+
The V
precursor NH
4
VO showed also a medium conversion of
3
3
6.2% giving mostly LA (20.5%), HMF (7.6%), FA (5.0%), AA (2.5%)
and traces of LevA (0.7%). Interestingly, the EPR spectra showed the
same ([V O(H O) ) and acid-bound vanadyl species [49,52] than
VOSO
4
+
2+
2
5
]
4
after the reaction under anaerobic conditions (see ESI, Figure
4+
S19, Tables S1 and S2). Furthermore, the total amount of V has been
increased by one order of magnitude (see ESI, Table S2), which might
be explained by the increased reaction temperature, leading pre-
5
+
−
dominantly to a reduction of the vanadate (V
O
3
) to the vanadyl
4
+ 2+
(
V
O
) species. This large EPR derived increase of the amount of
acid-bound vanadyl is consistent with the lower pH value after the
reaction under anaerobic (1.8) than under aerobic conditions (3.0),
5
1
which is the largest difference among all vanadium precursors. The V-
NMR spectrum did not change qualitatively (see ESI, Figure S20) con-
5
+
firming no change in the V
precursor structure.
HPA-1 shows the lowest conversion rate (X = 12.5%) and a com-
plete different product selectivity as, compared to all other samples, no
LA could be detected. Only few amounts of the thermally-induced in-
termediate HMF (2.4%), its consecutive products FA (4.1%) and LevA
Fig. 3. Experimental (black) and simulated (red) room temperature EPR spectra
before (a) and after (b) the reaction of 0.1 mmol VOSO solution with glucose
4
under anaerobic conditions. The experimental signal before the reaction (a) can
be simulated by a single species AVOSO4 as indicated in the figure. The simulated
signal of the spectrum after the reaction (b, sum) is a superposition of two
(
2.4%) as well as a small amount of AA (3.2%) were observed. The EPR
4
+
spectra (see ESI, Figure S21) confirmed that the same paramagnetic
vanadium species as under aerobic conditions are present (see ESI,
different V species AVOSO4 and species BVOSO4 as indicated in the figure. (For
interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article).
Figure S13 and Table S3). Like for the NH
4
VO precursor, their amounts
3
268

J. Albert et al.
Applied Catalysis A, General 570 (2019) 262–270
Table 4
Summary of the different vanadium species resulting from the reaction with glucose under anaerobic conditions determined by EPR and NMR spectroscopy.
4
+
a
4+
a
5+
b
5+
b
Entry Catalyst
V
Species before reaction under
V
Species after reaction under N
2
V
Species before reaction under
V
Species after reaction under
N
2
N
2
N
2
4+
2+
1
2
3
4
VOSO
NH VO
4
[V4+O(H
–
2
O)
5
]2+
[V O(H
2
O)
5
]
–
–
4
4
+
+
+
[
V
O(CH
3
CH
2
OHCOO)
2
]
]
2+
[V5+
]−
[V5+
O
3
]−
4
3
[V O(H
2
O)
CH
O)
5
]
O
3
4
[
V
O(CH
3
2
OHCOO)
2
4
2
+
+
2+
4+
2+
40]3−
40]3−
HPA-1
HPA-5
[V O(H
2
O)
5
]
[V O(H
2
5
]
[H
1
PVMo11
O
[H
1
PVMo11
O
5+
40]4−
2+
5+
4−
VO @ [PV Mo11
O
VO @ [PV Mo11
O
40
]
4
+
2+
4+
2+
4+
[V O(H
2
O)
5
]
[V O(H
2
O)
5
]
[V O(COOH)
2
] or
H
H
4
5
PV
1
2
Mo11
Mo10
O
O
40
40
H
4
5
PV
1
2
Mo11
O
40
40
4
+
[
V
O(CH
3
CH
2
OHCOO)
2
]
PV
H
PV
Mo10
O
+
+
VO
2
VO
2
H
H
H
H
6
7
8
9
PV
3
PV
4
PV
5
PV
6
Mo
Mo
Mo
Mo
9
8
7
6
O
40
6 3 9 40
H PV Mo O
O
40
O
40
O
40
Reaction conditions: 1 mmol glucose, 0.1 mmol catalyst dissolved in 10 g H
2
O, 20 bar N , 160 °C, 3 h, 1000 rpm; Conversion, yields and selectivities determined as
2
described in the corresponding section of the experimental part. a determined by EPR spectroscopy; determined by V-NMR spectroscopy.
b 51
are significantly larger as before the anaerobic reaction and after the
reaction under aerobic conditions (see ESI, Table S3), indicating an
NMR and EPR spectroscopy, we could clearly prove that the pre-
dominant catalytic active species for this transformation are higher
5
+
5+
increased reduction of the V
species due to the larger reaction tem-
substituted heteropolyacids containing V . For glucose transformation
5
+
4−
perature. This leads us to the assumption that [PV Mo11
O
40
]
could
under anaerobic conditions (20 bar nitrogen atmosphere) at 160 °C we
5
+
4+
not be the active species neither for FA formation under aerobic (see
Table 1) nor for LA under anaerobic conditions (see Table 3) as it could
be resolved only for HPA-1 with this precursor being the only one that
did not give both products.
could show by quantitative EPR that a reduction of V to V species
takes place for the NH
4
VO
3
as well as both heteropolyacid (HPA-1 and
HPA-5) precursors independent on the structure of the compounds.
4
+
Interestingly, the total amount of V
species decreased for VOSO
4
3
+
Finally, we investigated the behavior of HPA-5 under anaerobic
conditions (Entry 4 in Table 3). The conversion rate for glucose
assuming a further reduction to V species that are neither detectable
by EPR nor by NMR. Hereby, paramagnetic acid-bound vanadyl species
(
X = 27.8%) under the applied reaction conditions was somewhat
formed from the VOSO
4
and NH
4
VO as well as the HPA-5 precursor
3
higher than that of HPA-1 but lower than the other precursors VOSO
4
seem to be predominantly responsible for lactic acid formation from
glucose under anaerobic conditions. By combining highly efficient de-
tection tools like NMR and EPR for various redox active metals and
applying them to complex chemical transformations like biomass oxi-
dation in aqueous media, we hopefully pave the way for more biomass
transformation technologies being commercialized in the near future.
and NH
4
VO , respectively. Hereby, mostly LA (12.8%) and FA (9.7%)
3
were formed. Again, also thermal-induced reaction products (HMF,
LevA and AA) could be detected. Regarding the detected species, NMR
(
see ESI, Figure S24) gives similar results compared to the aerobic
5+
conditions suggesting no qualitative changes of the V
species. As
under aerobic conditions, EPR (see ESI, Figure S23) shows the presence
4
+
2+
of [V O(H
HPA5), but with a significant higher amount due to the absence of any
oxygen (see ESI, Table S4). The amount of [V O(H O) seems to
2
O)
5
]
(species AHPA5) and acid-bound vanadyl (species
Acknowledgments
B
4
+
2+
2
5
]
J. A. and D. V. thank Dr. Nicola Taccardi for performing the ICP
measurements. M. M. gratefully acknowledges financial support by the
DFG (Deutsche Forschungsgemeinschaft) within the research unit 2433.
JA and DV thank the DFG Cluster of Excellence „Engineering of
Advanced Materials “within the EAM Starting Grant SG6. The authors
declare no competing financial interests.
remain constant during the reaction under anaerobic conditions (see
5
+
ESI, Table S4). Consequently, a partial reduction of the V
an acid-bound vanadyl species is indicated.
species to
Obviously, HPA-1 is the only vanadium precursor, which shows
under anaerobic conditions no conversion to LA as well as no presence
of acid-bound vanadyl after the reaction, whereas the presence of
4
+
2+
[
V
O(H
2
O)
5
]
after the reaction under anaerobic conditions could
Appendix A. Supplementary data
be verified for all four vanadium precursors by EPR. This strongly
suggests that acid-bound vanadyl might be the predominant active
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2018.10.030.
4
+
2+
species for the conversion to LA, whereas [V O(H
2
O)
5
]
alone does
51
not provide significant catalytic activity. Regarding the
V-NMR
5
+
measurements, no V species seems to occur significantly when LA is
formed.
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spectroscopy for the conversion of glucose under anaerobic conditions.
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