Combining autoclave and LCWM reactor studies to shed light on the kinetics of glucose oxidation catalyzed by doped molybdenum-based heteropoly acids

Article abstract of DOI:10.1039/c9ra05544d

In this work we combined kinetic studies for aqueous-phase glucose oxidation in a high-pressure autoclave setup with catalyst reoxidation studies in a liquid-core waveguide membrane reactor. Hereby, we investigated the influence of Nb- and Ta-doping on Mo-based Keggin-polyoxometalates for both reaction steps independently. Most importantly, we could demonstrate a significant increase of glucose oxidation kinetics by Ta- and especially Nb-doping by factors of 1.1 and 1.5 compared to the classical HPA-Mo. Moreover, activation energies for the substrate oxidation step could be significantly reduced from around 80 kJ mol^-1 for the classical HPA-Mo to 61 kJ mol^-1 for the Ta- and 55 kJ mol^-1 for the Nb-doped species, respectively. Regarding catalyst reoxidation kinetics, the doping did not show significant differences between the different catalysts.

Full text of DOI:10.1039/c9ra05544d

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Combining autoclave and LCWM reactor studies to
shed light on the kinetics of glucose oxidation
catalyzed by doped molybdenum-based
heteropoly acids†
Cite this: RSC Adv., 2019, 9, 29347
Dorothea Voß,a Sebastian Ponce,b Stefanie Wesinger,a Bastian J. M. Etzoldb
a
*
and Jakob Albert
In this work we combined kinetic studies for aqueous-phase glucose oxidation in a high-pressure autoclave
setup with catalyst reoxidation studies in a liquid-core waveguide membrane reactor. Hereby, we
investigated the influence of Nb- and Ta-doping on Mo-based Keggin-polyoxometalates for both
reaction steps independently. Most importantly, we could demonstrate a significant increase of glucose
oxidation kinetics by Ta- and especially Nb-doping by factors of 1.1 and 1.5 compared to the classical
HPA-Mo. Moreover, activation energies for the substrate oxidation step could be significantly reduced
from around 80 kJ molꢀ1 for the classical HPA-Mo to 61 kJ molꢀ1 for the Ta- and 55 kJ molꢀ1 for the
Nb-doped species, respectively. Regarding catalyst reoxidation kinetics, the doping did not show
significant differences between the different catalysts.
Received 18th July 2019
Accepted 11th September 2019
DOI: 10.1039/c9ra05544d
rsc.li/rsc-advances
the past two decades. Several groups have shown that Nb can
Introduction
also be introduced into Keggin-type structures, i.e. vanado-
9ꢀ
niobate clusters like {VNb12O40(VO)2},16 [PM2Nb12O40
]
17 or
Multiphase chemical reactions, especially gaseous–liquid
reactions, are oen applied to make valuable chemicals from
biomass. One intensively studied approach is the selective
partial oxidation of carbohydrates to carboxylic acids, espe-
cially formic acid1,2 and acetic acid.3,4 Oxygen is usually
activated by metal catalysts. Hereby, mostly vanadium-
containing catalysts like polyoxometalates (POMs),5–7 or
water-soluble vanadium salts are used.8,9 In particular POMs
are highly promising catalysts for these kinds of reaction
due to their strong Brønsted acidity combined with a high
redox activity and high stability in aqueous solution.10,11
Within the redox-cycle, shown schematically in Fig. 1, POM
catalysts allow the use of molecular oxygen as oxidant.
Mostly used in homogeneously catalyzed reactions are POMs
12ꢀ
18
[SiNb12O40
]
.
Recently, also a few Ta-containing POMs
could be synthesized as well.19 Nevertheless, up to now the
inuence of Nb or Ta on the catalytic properties of Keggin-
POMs for biomass oxidation to formic acid is not studied.
Traditionally, kinetic studies of such gaseous–liquid reac-
tions are carried out in stirred tank reactors.20 E.g. for the two
vanadium-substituted POMs H5PV2Mo10O40 (HPA-2) and
H8PV5Mo7O40 (HPA-5) the oxidation kinetics of mono-
saccharides like glucose, fructose, sorbitol, and gluconic acid as
well as disaccharides like cellobiose and sucrose, up to water-
insoluble biomass like beech and spruce wood were deter-
mined.21 Moreover, Yokoyama et al. performed kinetic studies
on the oxidation of non-phenolic lignin subunits by tungsten-
based Keggin-POMs in water.22 Besides, these autoclave
studies, which allow to operate close to a later on technical
system, employing micro-scale reactors for kinetic studies can
show signicant advantages.23,24 These reactors enable studies
with minimal sample consumption under precise control of
reaction parameters in combination with enhanced mass- and
heat-transfer, either to suppress transfer limitations or to
enable kinetic transient response studies.25 Recently, some of us
introduced a new optouidic microreactor which combines for
gas/liquid reactions very fast and continuous gas/liquid mass
transfer with in situ spectroscopy and the general advantages of
microreactors.26 These so called liquid core waveguide
membrane (LCWM) microreactor was used to study
nꢀ
12–14
from the so-called Keggin-type [XM12O40
]
.
Nyman
summarized that studies on polyoxometalates containing W,
Mo and V are abundant as these structures could be easily
obtained through simple acidication while investigations
on the POM chemistry of Nb and Ta are poor.15 However,
especially polyoxoniobates have received certain attention in
aLehrstuhl fur Chemische Reaktionstechnik, Friedrich-Alexander-Universitat
¨
¨
¨
Erlangen-Nurnberg, Egerlandstraße 3, 91058 Erlangen, Germany. E-mail: jakob.
albert@fau.de
b
¨
¨
Lehrstuhl fur Technische Chemie, Technische Universitat Darmstadt, Alarich-Weiss-
Straße 8, 64287 Darmstadt, Germany
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra05544d
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Fig. 1 Schematic illustration of the catalyst redox cycle.
independently the kinetics of the reduction and reoxidation speed was set to 1000 rpm in order to start the gas entrainment.
step (see Fig. 1) of four different Keggin-type POM-catalysts with This moment was set as starting time of the experiment.
varying vanadium substitution degree.27
The kinetic experiments were performed as described above.
In this work we prepared two Nb- and Ta-doped Keggin-type As soon as the desired temperature and pressure were reached,
POMs as also a non-substituted one (HPA-Mo) as reference. a liquid sample was taken for the determination of the initial
Combining autoclave and LCWM-reactor studies we deduce the substrate and product concentrations. Then, the stirrer speed
inuence of the Nb and Ta doping on the oxidation as also was set to 1000 rpm in order to start the reaction. Liquid
reoxidation kinetics and resulting product distribution with samples were taken each 15 minutes during the rst hour, each
glucose as a model substrate.
30 minutes during the second hour, as well as aer three and
four hours, respectively. During sampling, the gas entrainment
was stopped by reduction of the stirrer speed to 300 rpm. Aer
the total reaction time of four hours, a gas sample was taken in
order to analyze the gas composition and to close the carbon
mass balance.
Experimental details
Chemicals
All chemicals were obtained commercially and used as received
without further purication. The Keggin-type polyoxometalate
catalyst H3PMo12O40 (HPA-Mo) was synthesized according to
the literature.6,28 The tantalum and niobium doped HPA-Mo
species (HPA-Ta and HPA-Nb) were synthesized according to
a modied synthesis procedure by Nyman.29 Oxygen, (4.5 GA
201, Linde AG) was used as the oxidizing agent. Demineralized
water was used as a solvent. D(+)-glucose (99.9%, Merck KGaA)
was used as a substrate.
Liquid core waveguide membrane microreactor setup
Fig. 2 shows a scheme of the reactor apparatus. For a detailed
explanation on the set-up and working principle please refer to
ref. 26. Briey, the samples are pumped through the inlet
uidic cell, which also connects the light source (DH-2000,
Ocean Optics, Tungsten-Halogen Lamps) through a standard
optical ber. At the outlet, a second uidic cell delivers the
liquid stream to the waste, and transmitted light to the spec-
trometer (Spectro 320 (D) R5, Instrument Systems) through
a second optical ber. Before and aer the inlet and outlet
Experimental setup for the high pressure autoclave
experiments
The oxidation reactions were carried out in a 600 mL autoclave uidic cells, respectively, two valves control the liquid-phase
made of Hastelloy C276 and equipped with a gas entrainment pressure. Additionally, between the uidic cells, the Teon
impeller. A at Teon sealing located at the reactor head was tube (AF-2400, Biogeneral, 1.0 mm i.d., 1.6 mm o.d) is
used. All pipes, valves and ttings were made of stainless steel enclosed in a steel tube, in which gas ows (2 L hꢀ1) at
1.4571. The reactor was equipped with a cooling coil made of constant pressure. Finally, a heating wire surrounds the outer
Hastelloy C276 and a heating jacket to control the reactor tube, enabꢁling temperature control during the experiments
temperature. The oxygen was fed into the system by a mass ow (up to 150 C).
controller to keep up the required pressure. A pressure control
For the reduction experiments, a constant temperature was
valve was installed to prevent higher pressures than 65 bar. set on the LCWM reactor (130 ꢁC). The steel outer tube was
Additionally, the reactor was connected to a rupture disk with ushed with nitrogen and the pressure was set to 2 bar.
a burst pressure maximum of 100 bar.
Subsequently, the system was lled with double-distilled water
For the batch experiments, the autoclave was lled with and a background spectrum was taken. The aqueous solutions
0.25 mol Lꢀ1 glucose, 0.01 mol Lꢀ1 catalyst and 100 g water as with catalyst (40 mmol Lꢀ1) and D-glucose substrate (0.75 mmol
the solvent. The system was purged twice with 20 bar oxygen Lꢀ1) were pumped into the reactor, and the liquid inlet and
pressure to remove the residual air out of the reactor. In the outlet valves were closed aer a constant liquid pressure of z 3
following, the reactor was pre-pressurized with about 40 bar bar was reached. Finally, optical absorption spectra between
oxygen pressure, the stirrer was set to 300 rpm and the heating 400–900 nm were recorded.
was switched on. When the desired temperature was reached,
Before performing reoxidation experiments, catalysts were
the oxygen pressure was increased to 60 bar and the stirring pre-reduced (HPA-nred) by glucose in a magnetically stirred two-
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Fig. 2 Schematic illustration of the LCWM setup used in the experiments. FCi, o: fluidic cells in and out, P: pressure indicator, T: temperature
indicator, BPR: backpressure regulator, v: two-way valve.
neck glass reactor (25 mL) equipped with a reux condenser and
temperature control. The reactions took at least 1 hour, with
constant vigorous stirring (700 rpm) in a nitrogen-pre-saturated
solution at 100 ꢁC. The initial concentration ratio between
glucose and the catalysts was 50 : 1.
Logarithmic Arrhenius equation:
EA
ln kðTÞ ¼ ꢀ
RT
þ ln kA
(4)
For reoxidation experiments developed within the LCWM
setup, initial reaction rates were obtained from the initial
slope of the concentration versus time curve (eqn (5)). Due to
the fast gaseous/liquid mass transfer, the oxygen concentra-
tion in the liquid-phase was assumed to be constant. Activa-
tion energy values were calculated by applying eqn (4) for the
reoxidation reaction rate constant (kox) values and
temperatures.
For the reoxidation experiments itself, a constant tempera-
ꢁ
ture was set on the LCWM reactor (80–150 C). The steel outer
tube was set to 2 bar of nitrogen gas. Subsequently, the system
was lled with double-distilled water and a background spec-
trum was taken. Aqueous solutions of HPA-nred catalysts (2–20
mmol Lꢀ1) were pumped into the reactor, and the inlet and
outlet valves were closed to keep a constant liquid pressure of
z6 bar. Continuous absorption spectra measurements between
500 to 800 nm were started. Aer two minutes, rapid exchange
of the gas phase was carried out by discharging N2 into the
fume, ushing with 5 bar of synthetic air, while adsorption
spectra were continuously recorded.
dc0;HPA-red
R2 ¼ ꢀ
¼ ꢀkoxc0;HPA-red
(5)
dt
Analytics
Kinetic description
Characterization of the used HPA-catalysts has been carried
out using different analytical techniques. Elemental analysis
has been done using a PerkinElmer Plasma 400 ICP-OES
device. The determination of hydration water content was
done by performing thermogravimetric analysis (TGA) on
a Setsys 1750 CS Evolution. FTIR spectra were recorded in
a range between 2000 and 400 cmꢀ1 on a Jasco FT/IR-4100
spectrometer equipped with a PIKE GladiATR ATR-accessory
with a resolution of 4 cmꢀ1 at ambient conditions. 31P-NMR
spectra were recorded on a JEOL ECX-400 MHz at room
temperature without resolution enhancement in D2O-solution
showing the same chemical shis according to the litera-
ture.6,28 Powder X-ray diffraction was measured using a STADIP
from Stoe & Cie GmbH. All XRD measurements were carried
out on a at sample holder in transmission geometry (4–60ꢁ/
For the kinetic description we applied the following power law
(eqn (1)) and Arrhenius law (eqn (2)).
Observable reaction rate:
a
robs ¼ kobs glucose
c
(1)
with:
b
kobs ¼ k1ccatalyst,ox
Arrhenius equation:
ꢀ
ꢁ
EA
kðTÞ ¼ kA ꢂ exp ꢀ
(2)
RT
˚
2q, l ¼ 1.54060 A, Ge[111]-monochromator) using a Mythen1K
(Dectris, Baden, Switzerland) detector. Moreover, we measured
Raman-spectroscopy for all applied catalysts with Bruker
Senterra 1 using a NDYAG laser at a wavelength of 532 nm.
Electrochemical measurements (cyclic voltammetry and
square wave voltammetry) were performed in a three-electrode
(3) system connected to a PARSTAT MC Multichannel potentiostat
For determination of the effective substrate oxidation order
and rate constant as well as effective activation energy we used
a linearization of the logarithmic forms (eqn (3) and (4)).
Logarithmic power law:
ln robs ¼ ln kobs + a ln c0,glucose
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with a platinum working electrode, a platinum coil counter
Furthermore, for comparison, the UV/Vis spectra of the
electrode, and an Ag/AgCl reference electrode. All experiments fully reduced, partially reduced and fully oxidized forms of
were carried out at room temperature in a 0.5 M Na2SO4 the catalysts were obtained within the LCWM microreactor
aqueous electrolyte solution. The sample solutions were (see Fig. S7†). For all catalysts, the spectra show a quasi-
purged with nitrogen for 3 min then maintained for 1 min for isosbestic point with the continuous disappearance of
stabilization.
a band (<500 nm) related to Mo peroxo species, and the
Analysis of reaction mixtures has been carried out using appearance of a broad band (>600 nm) typical for the known
liquid and gaseous analysis. Liquid products were analyzed by ‘heteropoly blues’. Nevertheless, as compared in Fig. S8,†
means of HPLC measurements using a High performance HPA-Ta and HPA-Nb partially-reduced spectra present
liquid chromatography from Jasco equipped with a 300 mm ꢂ a slightly deviation from the HPA-Mo behavior. It is evident
8 mm SH1011 Shodex column. As eluent a 5 mmol aqueous a broadening of the heteropoly blue band, reducing visibility
sulfuric acid solution was applied. Gaseous product analysis of the presence of Mo(V) transition systems, and the homo-
was performed using a Varian GC 450 equipped with a 2 m ꢂ intervalence-charge-transfer-peak bands of Mo–O–Mo
0.75 mm ID ShinCarbon ST column.
groups (approx. at 750 and 860 nm, respectively).12 To get
further evidence of this deviation, both catalysts were elec-
trochemically characterized (see Fig. S9 and S10†). Square
wave voltammograms of both catalysts show the appearance
of a dened peak at ꢀ0.4 V in comparison to HPA-Mo. An
additional peak at +0.033 V is only observed for HPA-Ta.
Metal doping therefore also affects the measured electro-
chemical rst reduction potential of the catalysts, which
decreases in the order HPA-Mo > HPA-Nb > HPA-Ta (ꢀ0.433,
ꢀ0.443, ꢀ0.447 V, respectively). Similar behavior has been
previously observed in V-substituted HPA-Mo complexes27
where a decrease in the redox potential was also correlated to
an increase in the oxidation rate constants.
Determination of yields
The yields of the liquid and gaseous products formic acid (FA),
acetic acid (AA), the liquid intermediates glycolaldehyde (GA)
and 5-hydroxymethylfurfural (HMF), CO2 and CO were calcu-
lated by eqn (6).
nproduct;i
Yi ¼
(6)
nC-atoms;glucose
The total carbon yield (TOC) was calculated by eqn (7) and
served as a representative for the substrate conversion as other
products than FA, AA, GA, HMF, CO2 and CO were not detected
using the described analytical methods.
Inuence of Nb- and Ta-doping on product distribution at
P
high glucose conversion
nproduct;i
i
TOC ¼
(7) Motivated by a former work using different Keggin-type
POMs as catalyst,27 we performed long-term batch experi-
ments using glucose as a substrate at 80 ꢁC in order to check
the inuence of Nb- and Ta-doping on the product selectivity
(see Table S1 in the ESI†). However, we could not detect
remarkable product yields for all three catalysts. Conse-
quently, we increased the reaction temperature to 160 ꢁC
nC-atoms;glucose
Results and discussion
Catalyst characterization
The synthesized Keggin-type polyoxometalate catalysts HPA-Mo, (Table 1). Hereby, we could detect high glucose conversion
HPA-Ta and HPA-Nb were characterized by different analytical resulting in total carbon yields between 86 and 96%. As ex-
methods. Elementary analysis using ICP-OES resulted in loadings pected, formic acid (FA) was produced as the major liquid
of 0.23 wt% Nb for HPA-Nb and 0.47 wt% Ta for HPA-Ta. More- product with yields of around 30% followed by acetic acid
over, the elemental composition of the HPA-Mo was determined to (AA) with yields of around 7%. Moreover, glycol aldehyde (GA)
P/Mo ¼ 1/11.94. Thermogravimetric analysis resulted in hydration (approx. 3%) could be detected in all samples. Regarding the
water contents of 8 molecules for HPA-Mo, 9 molecules for HPA-Nb gas phase products, CO2 was the major product with yields
and 14 molecules for HPA-Ta, respectively. Additionally, FTIR, 31P- between 43% (HPA-Nb) and 51% (HPA-Ta). Additionally, 4–
NMR, Raman and XRD measurements were performed to conrm 5% of CO were formed in each experiment.
the Keggin species (see characterization of polyoxometalate cata-
lysts in the ESI, Fig. S1–S6†).
This product distribution is well in line with literature
proposed reaction pathways.7–9 GA is the intermediate product
Table 1 Conversion of glucose using different Keggin-type polyoxometalate catalysts in batch mode at 160 ꢁC. Reaction conditions: 250 mmol
Lꢀ1 glucose, 1 mmol catalyst, 100 g water as solvent, 160 ꢁC, 60 bar oxygen pressure, 1000 rpm stirrer speed, 24 hours reaction time
Entry
Catalyst
FA-yield [%]
AA-yield [%]
GA-yield [%]
CO2-yield [%]
CO-yield [%]
Total carbon yield [%]
1
2
3
HPA-Mo
HPA-Ta
HPA-Nb
28.2
30.0
28.5
6.7
7.0
7.3
3.2
2.3
3.0
46.9
51.3
42.6
4.0
5.1
5.0
89.0
95.7
86.4
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Fig. 3 Reaction scheme of glucose conversion to formic acid and acetic acid using Keggin-type POMs.7–9
of the oxidation of glucose to formic acid and CO2, while AA is of 18% already within the rst hour. Aerwards, its amount
most likely formed by thermal induced conversion of glucose to slightly decreases within the next 3 h. FA yield drastically
HMF and levulinic acid (LA) (see Fig. 3). However, the last two increases from the beginning and is already the major
intermediates could not be detected maybe due to too low species aer 1 h reaction time. Again, the thermal induced
concentrations near the detection limit of our used HPLC. CO reaction pathway with HMF as an intermediate on the way to
could be formed due to the high temperatures by thermal AA plays only a minor role with yields below 5%.
decomposition of the intermediates or FA.
At the highest applied temperature of 160 ꢁC, the
Fig. 3 shows the reaction scheme for the acid catalyzed maximum amount of GA is already reached aer pre-heating
(upper pathway) as well as the HPA-induced (lower pathway) at reaction time zero and then consequently converted to FA.
of glucose oxidation by molecular oxygen in aqueous media. The amount of FA is sharply increasing within the rst hour
and then increases slowly for the next three hours. Here,
signicantly higher AA yields (ca. 5%) could also be detected
Inuence of Nb- and Ta-doping on kinetics of the substrate
over the whole reaction time whereby HMF yield is still close
oxidation step
to the detection limit.
The selectivity and product distribution are mainly inuenced
by the substrate oxidation step (r1 in Fig. 1) as also non-catalytic
side reactions. Therefore, the inuence of the Nb- and Ta-
doping on the product distribution was studied together with
the kinetics of the substrate oxidation step and within a high-
pressure autoclave experiment allowing for product analysis.
In these experiments the inuence of the catalyst reoxidation
step (r2 in Fig. 1), was suppressed by applying oxygen pressures
of 60 bar, where the catalyst reoxidation was shown to be much
faster than the substrate oxidation.21
The other two catalysts HPA-Ta and HPA-Mo show the
same trends with the HPA-catalyzed reaction pathway being
the dominant reaction pathway at all applied temperatures
(see ESI, Fig. S11 and S12†).
The activation energies for the three different catalysts
(Fig. 5a–c) were determined from the temperature variation.
Moreover, the reaction orders of the catalysts were deter-
mined from logarithmic plots of the substrate oxidation reac-
tion rate vs. natural logarithm of the initial concentration of
glucose. The normalized concentration–time proles (Fig. S13–
S15†) as well as the logarithmic plots of reaction rate vs. initial
glucose concentration (Fig. S16 and 17) are shown in the ESI.†
For HPA-Mo, analogous experiments have been performed
To obtain further insight into the kinetics of the oxidation
step r1 samples were taken from the autoclave during the rst
4 h. For the determination of the effective activation energies,
the experiments were performed at different reaction tempera-
showing an effective reaction order of 1.12 with an effective
ꢁ
ꢁ
tures (120 C, 140 ꢁC and 160 C). The liquid product distribu-
tion during the experiments is exemplarily shown for the
catalyst HPA-Nb in Fig. 4.
reaction rate constant of 1.10 mol Lꢀ1 hꢀ1
.
The kinetic
27
parameters obtained are summarized in Table 2.
The determined effective activation energies show
signicant differences between the two doped catalysts and
the conventional HPA-Mo catalyst. The Nb- respectively Ta-
doping has a positive effect on the activation barrier which
could signicantly be reduced from 83 kJ molꢀ1 for HPA-Mo
to around 60 kJ molꢀ1 for the two doped species. Moreover,
the doping seems to have a positive effect on the substrate
oxidation kinetics as it increases the effective rate
constants.
The temperature variation shows that at low temperature
(120 ꢁC, top le), the HPA catalyzed reaction pathway (see Fig. 3)
is dominant. Hereby, the intermediate GA is predominantly
formed to a maximum yield of 20% aer 4 h reaction time. With
increasing reaction time, formic acid yield also increases and
becomes dominant aer 4 h. The thermal induced reaction
pathway leading to AA with HMF as an intermediate plays only
a minor role at this temperature.
At 140 ꢁC (Fig. 4, top right), still the HPA catalyzed reaction
pathway is dominant but GA is formed to its maximum yield
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Fig. 4 HPA-Nb catalyst: time depended yield distribution of the liquid products for different reaction temperatures: (a) 120 ꢁC, (b) 140 ꢁC, (c)
160 ꢁC: reaction conditions: 60 bar O2, 1000 rpm, 1 mmol HPA-Nb, 100 g water, 0.25 mol Lꢀ1 glucose, 4 h fed-batch, and high-pressure
autoclave.
HPA catalysts the concentration change was determined aer
exchanging fast from an inert to an oxygen gas atmosphere.
Fig. 6a shows exemplary the corresponding time dependent
UV/Vis spectra for the reoxidation experiment for HPA-Nb cata-
Inuence of Nb- and Ta-doping on the catalyst reoxidation
kinetics
While the overall selectivity is mainly inuenced by the
substrate oxidation step, the catalyst re-oxidation step (r2 in
Fig. 1) is at envisaged low oxygen pressures rate limiting and
thus critical for the overall reaction rate. As reported by us for
vanadium substituted HPA catalysts the UV/Vis signal of the
HPAs can be employed to deduce within the LCWM reactor the
concentration change from reduced to oxidized HPA catalyst.27
To obtain the inuence of the Nb and Ta substitution on the
reoxidation kinetics a transient response experiment was
carried out within the LCWM reactor, where for fully reduced
ꢁ
lyst at 120 C. Employing the absorption at 750 nm the concen-
tration change of the HPA-Nbred species is given in Fig. 6b for
ꢁ
reoxidation experiments carried out from 80 to 140 C. Similar
results were obtained for the HPA-Tared and HPA-Mored catalyst
and are given in the ESI (see Fig. S18†). Qualitatively the reox-
idation of all three catalysts started to be observable at 100 ꢁC. For
a more quantitative comparison the initial reaction rates were
deduced for all temperatures and activation energies derived (see
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Fig. 5 Arrhenius plots of the temperature variation for the substrate oxidation step for (a) HPA-Nb, (b) HPA-Ta and (c) HPA-Mo. Reaction
conditions: 120–160 ꢁC, 60 bar O2, 1000 rpm, 1 mmol HPA, 100 g water, 0.25 mol Lꢀ1 glucose, 4 h fed-batch, and high-pressure autoclave.
eqn (4) and (5) and values in Table 3). Independent on the metal Mo is similar and approx. 20 kJ molꢀ1. A slight difference can
doping, the reoxidation reactions showed order of magnitude for be observed for HPA-Nb, with a slightly increased activation
the reoxidation rate. The activation energy of HPA-Ta and HPA- energy of 28 kJ molꢀ1. Thus, the Nb substitution can increase the
Table 2 Kinetic data obtained in the high pressure autoclave for substrate oxidation kinetics. Reaction conditions: 160 ꢁC, 60 bar O2, 1000 rpm,
1 mmol HPA-Ta, 100 g water, 0.25–1.00 mol Lꢀ1 glucose, 4 h fed-batch, and high-pressure autoclave
Catalyst
Effective reaction order nobs [—]
Effective rate constant kobs [mol Lꢀ1 hꢀ1
]
Effective activation energy [kJ molꢀ1
]
HPA-Mo27
HPA-Ta
HPA-Nb
1.12
1.20
1.40
1.10
1.16
1.44
83.2
55.1
61.1
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Fig. 6 (a) Example absorption spectra of the reoxidation of pre-reduced HPA-Nb at 120 ꢁC (the blue spectra represents t ¼ 0 min). (b)
Normalized concentration–time profiles at 750 nm of the reoxidation of pre-reduced HPA-Nb catalyst. Reaction conditions: 80–140 ꢁC, 5 bar
synthetic air saturation, 11 mM pre-reduced HPA-Nb initial concentration, LCWM reactor.
reoxidation kinetics at high temperatures slightly. Nevertheless,
A variation of the catalyst concentration from 1.8 to 12 mM at
the benchmarking reoxidation rates of vanadium substituted 120 ꢁC allowed to deduce the reaction orders regarding the
HPAs,27 which can be reoxidized already at 80 ꢁC, cannot be catalyst concentration (Fig. 7a) and rst-order rates resulted for
achieved.
all three HPAs (Table 3).
Table 3 Comparison of kinetic data obtained in LCWM reactor from reoxidation experiments of pre-reduced catalysts
Entry
Catalyst
Reaction order at 120 ꢁC [—]
kreoxidation, at 120 ꢁC [minꢀ1
]
Effective activation energy [kJ molꢀ1
]
1
2
3
HPA-Mo26
HPA-Ta
HPA-Nb
0.99
0.96
0.96
0.219
0.236
0.203
20.43
19.04
28.84
Fig. 7 (a) Initial reaction rates vs. initial concentration of HPA-Nbred, HPA-Tared, and HPA-MOred catalysts at 120 ꢁC and Arrhenius plot for the
reoxidation of HPA-Nbred, HPA-Tared, HPA-MOred catalysts. Reaction conditions: 80–140 ꢁC, 5 bar synthetic air saturation, 1.8–12 mM pre-
reduced HPA-Nb, HPA-Ta, HPA-MO initial concentration, LCWM reactor (see eqn (4) and (5) and Arrhenius plot in (b)).
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acid from water-insoluble and wet waste biomass, Green
Chem., 2015, 17, 5164–5171, DOI: 10.1039/c5gc01474c.
Conclusion
¨
¨
2 J. Albert, R. Wolfel, A. Bosmann and P. Wasserscheid,
Selective oxidation of complex, water-insoluble biomass to
formic acid using additives as reaction accelerators, Energy
Environ. Sci., 2012, 5(7), 7956–7962, DOI: 10.1039/
c2ee21428h.
Selective liquid-phase oxidation of biomass-derived glucose to
formic and acetic acid in aqueous media could be demonstrated
using Nb- and Ta-doped Mo-based Keggin-type poly-
oxometalates. Hereby, we could show that these vanadium-free
catalysts ꢁrequire signicant higher reaction temperatures of
120–160 C to show full glucose conversion in a high-pressure
autoclave setup. The main reaction pathway for the three used
Mo-based POM catalysts leads to formic acid yields of around
3 Z. Zhang and G. W. Huber, Catalytic oxidation of
carbohydrates into organic acids and furan chemicals,
Chem. Soc. Rev., 2018, 47, 1351–1390, DOI: 10.1039/
c7cs00213k.
ꢁ
30% at 160 C whereby the intermediate glycol aldehyde is the
dominant species between 120–140 ꢁC with a maximum yield of
20%. Moreover, only small amounts of acetic acid and its
intermediate HMF could be detected for the whole temperature
range.
4 J. Zhang, M. Sun, X. Liu and Y. Han, Catalytic oxidative
conversion of cellulosic biomass to formic acid and acetic
acid with exceptionally high yields, Catal. Today, 2014, 233,
77–82, DOI: 10.1016/j.cattod.2013.12.010.
5 T. Lu, M. G. Niu, Y. C. Hou, W. Z. Wu, S. H. Ren and F. Yang,
Catalytic oxidation of cellulose to formic acid in
H5PV2Mo10O40 + H2SO4 aqueous solution with molecular
oxygen, Green Chem., 2016, 18, 4725–4732, DOI: 10.1039/
c6gc01271j.
Regarding the kinetics of the glucose oxidation step, the Ta-
and especially the Nb-doping lead to a signicant increase for
the measured effective rate constants as well as the effective
reaction orders compared to the non-doped HPA-Mo catalyst.
Moreover, the effective activation energies could be signicantly
reduced from 80 kJ molꢀ1 for the HPA-Mo catalyst to
61 kJ molꢀ1 for the HPA-Nb and 55 kJ molꢀ1 for the HPA-Ta
catalyst.
¨
¨
6 J. Albert, D. Luders, A. Bosmann, D. M. Guldi and
P. Wasserscheid, Spectroscopic and electrochemical
characterization of heteropoly acids for their optimized
application in selective biomass oxidation to formic acid,
Green Chem., 2014, 16, 226–237, DOI: 10.1039/c3gc41320a.
7 J. Li, D. J. Ding, L. Deng, Q. X. Guo and Y. Fu, Catalytic Air
Oxidation of Biomass-Derived Carbohydrates to Formic
Acid, ChemSusChem, 2012, 5, 1313–1318, DOI: 10.1002/
cssc.201100466.
The inuence of the Nb- and Ta-doping on the reoxidation
kinetics was carried out within a LCWM reactor performing
a transient response experiment, where for fully reduced HPA
catalysts the concentration change was determined aer
exchanging fast from an inert to an oxygen gas atmosphere. The
activation energy for the reoxidation of HPA-Ta and HPA-Mo
was similar and approx. 20 kJ molꢀ1 whereby a slight increase
could be observed for HPA-Nb with 28 kJ molꢀ1. Thus, the Nb
substitution can increase the reoxidation kinetics at high
temperatures slightly. Nevertheless, the benchmarking reox-
idation rates of vanadium substituted HPAs, which could be
8 M. Niu, Y. Hou, S. Ren, W. Wang, Q. Zheng and W. Wu, The
relationship between oxidation and hydrolysis in the
conversion of cellulose in NaVO3 – H2SO4 aqueous solution
with O2, Green Chem., 2015, 17, 335–342, DOI: 10.1039/
c4gc00970c.
9 J. Albert, M. Mendt, M. Mozer and D. Voß, Explaining the
role of vanadium in homogeneous glucose transformation
reactions using NMR and EPR spectroscopy, Appl. Catal., A,
2019, 570, 262–270, DOI: 10.1016/j.apcata.2018.10.030.
10 C. L. Hill and C. M. Prosser-McCartha, Homogeneous
catalysis by transition metal oxygen anion clusters, Coord.
Chem. Rev., 1995, 143, 407–455, DOI: 10.1016/0010-
8545(95)01141-B.
ꢁ
reoxidized already at 80 C, could not be achieved.
To sum up, we could show that Mo-based Keggin-type POMs
could successfully be applied for selective glucose oxidation
especially to glycol aldehyde at temperatures of around 120 ꢁC.
With increasing temperature, the consecutive reaction leading
to formic acid was favored. The Ta- and especially Nb-doping
signicantly increases the selectivity-determining substrate
oxidation kinetics and thus opens new interesting perspectives
for liquid-phase oxidation reactions.
11 A. Proust, B. Matt, R. Villaneau, G. Guillemot, P. Gouzerh
and G. Izzet, Functionalization and post-functionalization:
a step towards polyoxometalate-based materials, Chem.
Soc. Rev., 2012, 41, 7605–7622, DOI: 10.1039/c2cs35119f.
12 I. V. Kozhevnikov, Catalysis by heteropoly acids and
Conflicts of interest
The authors declare no conicts of interest.
multicomponent
polyoxometalates
in
liquid-phase
reactions, Chem. Rev., 1998, 98(1), 171–198, DOI: 10.1021/
cr960400y.
Acknowledgements
13 H. N. Miras, J. Yan, D. L. Long and L. Cronin, Engineering
polyoxometalates with emergent properties, Chem. Soc.
Rev., 2012, 41, 7403–7430, DOI: 10.1039/c2cs35190k.
14 S. Omwoma, C. T. Gore, Y. Ji, C. Hu and Y.-F. Song,
Environmentally benign polyoxometalate materials, Coord.
Chem. Rev., 2015, 286, 17–29, DOI: 10.1016/j.ccr.2014.11.013.
DV, SW and JA thank Julian Mehler supporting the catalyst
synthesis.
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