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doi.org/10.1002/cctc.202100632
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In a previous paper, we could explain the effect of different
Each catalytic oxidation reaction was conducted at 90 C and
vanadium species occurring in aqueous solution with nuclear
magnetic resonance (NMR) and electron paramagnetic reso-
nance (EPR) spectroscopy. In detail, we could show that under
aerobic conditions (20 bar oxygen atmosphere) substituted V5+
species are the predominantly active species in the oxidation of
glucose to formic acid. Moreover, under anaerobic conditions
(20 bar N2 atmosphere), paramagnetic acid-bound vanadyl
species catalyse the transformation of glucose to lactic acid.[24]
Very recently, some of us found that using different alcoholic
solvents like methanol and ethanol can manipulate the reaction
mechanism of HPA-5 in liquid phase biomass oxidation. The
outstanding result of a nearly perfect yield of methyl formate
(>99%) from glucose using methanol as a solvent instead of
water lead to the question which specific influence the solvent
exerts on the catalyst. We could unambiguously prove that the
reaction mechanism in methanol differs from the common one
in aqueous media. Especially the total oxidation to CO2 can be
completely supressed in methanol.[26] The impressive solvent
effects were also recognized by Deng et. al.[27] and Lu et. al.[14]
for similar reaction systems but not further investigated.
20 bar oxygen for 24 h. The conversion of glucose was around
100% in every tested solvent. However, depending on the
solvent the product composition varied. Using water as a
solvent lead to formic acid as the main product (Y=56%) and
CO2 (43%) as the only by-product at full glucose conversion. In
pure methanol there is only one product, which is methyl
formate, the product of the esterification of formic acid and the
solvent methanol. Formic acid, however, undergoes further
esterification in all alcoholic solvents. Using ethanol as a
solvent, ethyl formate (ester of formic acid in ethanol) occurs as
the main product with a yield of 49%. A significant amount
(Y=18%) of ethyl acetate (ester of acetic acid in ethanol) was
formed as a by-product. In the gas phase, CO2 as well as CO are
found in a combined yield of 33%. In n-propanol as well as in
n-butanol the same product compositions like using ethanol
were detected. The combined yield of CO2 and CO in both
solvents are higher (45% and 43%) than in ethanol and nearly
as high as in water.[26]
Depending on the solvent, the different product composi-
tions and the reaction mechanisms strongly differ. In water
(Scheme 1) the main product is formic acid with the side
product CO2.[24,28]
The aim of this study is to explain the solvent effect on the
oxidation of glucose with vanadium containing heteropolyacids
in more detail. For this purpose, we investigated different
alcohol-water mixtures via NMR and EPR spectroscopy and
identified the predominant species being responsible for
catalysing different glucose transformation pathways.
Heteropolyacids (HPAs) are well-known redox catalysts
based on the fast and reversible multielectron transfer.[21,29] The
reduction of the vanadium-oxygen (VÀ O) species catalyses the
oxidative CÀ C bond cleavage under water elimination
(Scheme 1). After every CÀ C bond cleavage the vanadium-
oxygen species are oxidized by oxygen from the aerobic
atmosphere to complete the catalytic cycle or activate an
oxidant to form an intermediate that oxidizes the reactants.[30]
To get deeper insight into the catalytic active vanadium-oxygen
species, a former study of some of our group analysed the HPA-
5 catalyst in water before and after the reaction with 51V-NMR
and EPR spectroscopy.[24] The 51V-NMR-spectra show an isomer-
ization reaction of the HPA-5 catalyst. It is well known in
literature that aqueous HPA-5 solution contains a complex
mixture of different species and isomers due to different pH-
dependent equilibrium reactions of VÀ MoÀ P-substituted HPAs
in aqueous solution (for further description see[31]). Therefore,
the vanadium-NMR spectra of HPA-5 (H8PV5Mo7O40) in aqueous
solution shows lower and higher vanadium-substituted Keggin
species such as: H9PV6Mo6O40 (HPA-6), H7PV4Mo8O40 (HPA-4),
Results and Discussion
Catalytic Glucose Oxidation using Various Solvents
In a previous study, some of our group investigated the effect
of different solvents on the HPA-5 catalysed reaction of glucose
to formic acid.[26] Interestingly, significant differences in product
selectivity were observed (see Table 1). Hereby, besides water
as a standard solvent for glucose oxidation also alcoholic
solvents like methanol, ethanol, n-propanol and n-butanol were
tested. The experiments were performed in a tenfold screening
plant with a batch mode reactor setup consisting of ten 20 mL
autoclaves. Each reactor was filled with 1 mmol glucose as
substrate, 0.1 mmol HPA-5 catalyst dissolved in 10 g solvent.
Table 1. Yield and conversion of the oxidation of glucose using different solvents.[26]
Entry
Solvent
Conversion
[%]
Yield
Yield
Yield
CO2
[%]
Yield
CO
[%]
formic acid/corresponding ester[a]
[%]
acetic acid/corresponding ester[b]
[%]
1
2
3
4
5
Water
Methanol
Ethanol
n-propanol
n-butanol
100
100
100
100
100
56
100
49
47
37
0
0
18
9
43
0
21
24
27
1
0
12
21
16
20
[a] Corresponding ester of formic acid in the different solvents: methanol: methyl formate, ethanol: ethyl formate, n-propanol: propyl formate, n-butanol:
butyl formate.[b] Corresponding esters of acetic acid in the different solvents: methanol: methyl acetate, ethanol: ethyl acetate, n-propanol: propyl acetate, n-
°
butanol: butyl acetate. Reaction conditions: 1 mmol glucose, 0.1 mmol catalyst dissolved in 10 g solvent, 20 bar O2, 90 C, 24 h, 1000 rpm; Conversion and
product yields determined as described in the corresponding section of the experimental part.
ChemCatChem 2021, 13, 1–10
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