A tunable precious metal-free system for selective oxidative esterification of biobased 5-(hydroxymethyl)furfural

Article abstract of DOI:10.1039/c9gc00840c

Oxidative esterification of biomass-derived 5-(hydroxymethyl) furfural (HMF) and furfural and their derivatives has been performed using a simple MnO₂/NaCN system. The developed method allows the selective one-pot transformation of HMF to dimethyl furan-2,5-dicarboxylate (FDME) in 83% isolated yield without the formation of a free acid. Simplification of FDME production provides the missing link for manufacturing sustainable value-added materials from biomass. Addition of water to the oxidative system allows fine-tuning of reaction selectivity to obtain the previously difficult-to-access pure methyl 5-(hydroxylmethyl)furan-2-carboxylate in one step directly from the unprotected HMF without chromatographic separation.

Full text of DOI:10.1039/c9gc00840c

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A tunable precious metal-free system for
selective oxidative esterification of biobased
Cite this: DOI: 10.1039/c9gc00840c
5-(hydroxymethyl)furfural†
a,b,c
Kirill S. Kozlov,a,b Leonid V. Romashova and Valentine P. Ananikov
*
Oxidative esterification of biomass-derived 5-(hydroxymethyl) furfural (HMF) and furfural and their deriva-
tives has been performed using a simple MnO2/NaCN system. The developed method allows the selective
one-pot transformation of HMF to dimethyl furan-2,5-dicarboxylate (FDME) in 83% isolated yield without
the formation of a free acid. Simplification of FDME production provides the missing link for manufacturing
sustainable value-added materials from biomass. Addition of water to the oxidative system allows fine-
tuning of reaction selectivity to obtain the previously difficult-to-access pure methyl 5-(hydroxylmethyl)
furan-2-carboxylate in one step directly from the unprotected HMF without chromatographic separation.
Received 11th March 2019,
Accepted 9th May 2019
DOI: 10.1039/c9gc00840c
rsc.li/greenchem
equivalent of the most common synthetic polyester, polyethyl-
ene terephthalate (PET, Fig. 1).31–33
Introduction
Biomass processing is one of the most rapidly developing
In the vast majority of cases, PET synthesis starts from
branches of green chemistry, sustainable development and dimethyl 2,5-furandicarboxylate (furan dicarboxylic methyl
engineering.1–7 Due to the huge abundance of renewable ester, FDME) instead of a free acid, since methanol is more
carbohydrates in nature, cellulose and other sugars appear to volatile than water and can be easily separated in the course of
be the most promising sources of organic carbon for the polycondensation. Therefore, direct synthesis of FDME from
chemical industry.8–13 This is the key reason why compounds HMF is a cutting-edge process that is in high demand, since
which can be easily prepared from carbohydrates (HMF, fur- such transformation will shorten the technological pathway
fural, levulinic acid etc.) have been included in the list of plat- and increase its cost-efficiency (Fig. 1). Moreover, the for-
form-chemicals of the chemical industry of the future.14–17 mation of FDCA usually proceeds under strongly basic con-
Nowadays the production of carbohydrate-based chemicals has ditions and acidification of the obtained FDCA salt leads to
reached the industrial level and continues to increase.18 At the the formation of superstoichiometric amounts of inorganic
same time, the search for selective and convenient chemical salt as a byproduct, which is undesirable from the environ-
transformations of biobased substances to value-added com- mental point of view.
pounds remains a challenge of general importance.19–23 The
There are two main types of oxidative processes: catalytic
oxidation of the side-groups in HMF is one of the most inten- oxidation with molecular oxygen and oxidation with stoichio-
sively studied aspects of its chemistry21 and a number of pre-
cious-metal-based catalytic systems have been developed.24–29
The importance of 2,5-furandicarboxylic acid (FDCA)30 is
clearly justified since it is a biomass-based analog of the oil-
based terephtalic acid and therefore a precursor for the green
aZelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
Leninsky prospect 47, 119991 Moscow, Russia. E-mail: val@ioc.ac.ru; http://www.
AnanikovLab.ru
bDepartment of Chemistry, Lomonosov Moscow State University, Leninskiye Gory 1,
Moscow, 119991, Russia
cSaint Petersburg State University, Universitetsky Prospect, 26,
St. Petersburg 198504, Russia
†Electronic supplementary information (ESI) available: Experimental procedures
Fig. 1 Comparison of polyester production by means of oil and
and characterization data. See DOI: 10.1039/c9gc00840c
biomass processing.
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metric oxidants.21 On the one hand, oxidation with molecular
oxygen is attractive since oxygen is a cheap and easily available
oxidant. On the other hand, the requirement of high oxygen
pressure is a critical drawback from the practical point of view,
because it implies the use of specific equipment and extremely
increased safety precautions.34 Utilization of molecular oxygen
carriers is significantly more safe, especially when introducing
the biomass oxidation steps into pre-existing frameworks of
chemical production. In this work we present cyanide-pro-
moted oxidative esterification of HMF and other biomass-
based furanic compounds using MnO2 as a recyclable oxygen
carrier.35–39 The developed oxidation system allows one-pot
oxidative esterification of HMF to FDME and other esters. It
allows the neat preparation of esters of the poorly affordable
5-hydroxymethyl-2-furancarboxylic acid (HMFCA, Fig. 2).
Importantly, in the present study the common commercially
available MnO2 is used as an oxygen carrier instead of
applying the highly expensive precious-metal-based catalytic
systems.
Fig. 3 Oxidative esterification of HMF to MHMFC and FDME.
product with excellent selectivity. Upon increasing the quantity
of MnO2, the product distribution changed, with FDME 2a
becoming a major product. Even at a high oxidant loading (20
equiv.), MHMFC was still produced in a significant amount.
To overcome the formation of MHMFC, we introduced 2,5-
diformylfuran (DFF) in the oxidative esterification process
(Fig. 4), which allowed us to achieve 97% yield of FDME. To
increase the yield of FDME during HMF oxidation, we there-
fore decided to pre-oxidize HMF to DFF. Pre-oxidation can be
easily done using the interaction of HMF with MnO2 in the
absence of cyanide. In this one-pot protocol, an 83% isolated
yield of FDME 2a was achieved. The developed procedure was
tested on the scope of alcohols using both protocols: starting
from HMF with pre-oxidation or starting from DFF in a one-
step manner (Table 1).
The corresponding diesters 2a–f were obtained in good
yields for various alcohols. Amazingly, even the easily oxidiz-
able benzyl (entry 5, Table 1) and allyl (entry 6, Table 1) alco-
hols gave satisfactory results in this reaction. In the case of
allyl alcohols, the reaction was slow enough to allow the iso-
lation of the intermediate monoallyl ester 2f′ (see the ESI† for
details). In the cases of tert-butanol and phenol (entries 7 and
8, Table 1), no formation of esters was detected. This fact can
be explained by the low activity of these alcohols associated
Results and discussion
As stated above, one of the key prospective routes of HMF
application is its transformation to FDME, which is a precur-
sor of various biomass-based polyesters. Among the known
methods of oxidative esterification of aldehydes, the NaCN/
MnO2/MeOH/AcOH oxidative process (known as Corey–
Gilman–Ganem oxidation40) seems to be the most promising
because of the utilization of a cheap oxidant and simple reac-
tion conditions. The classical reaction protocol, which requires
extremely high loadings of MnO2 (20 equiv.) and NaCN (5
equiv.), leads to the formation of FDME in just a moderate
yield of 42%.
In the first part of our study, we optimized reaction con-
ditions and decreased the loading of NaCN and MnO2 by more
than 10 times. The dependence of product distribution on the
quantity of the oxidant is shown in Fig. 3. At a decreased
MnO2 loading (2 equiv.), MHMFC 3a formed as a major
Fig. 2 Different pathways of oxidative esterification of HMF.
Fig. 4 Selective oxidative esterification of DFF to FDME.
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Table 1 Interaction of HMF and DFF with various alcohols under the
CN-promoted oxidative esterification conditionsa
majority of cases, good yields of the ester were achieved.
Remarkably, 5-formyl-2-furancarboxylic acid 1f (FFCA) was
easily transformed into a monoester of FDCA in a moderate
yield by using the described procedure (entry 7, Table 2); this
result is hardly achievable by other methods. p-Anisaldehyde 8
(entry 9, Table 2) was also subjected to oxidative esterification.
The corresponding methyl ester was produced in a nearly
quantitative yield, which indicates that the described pro-
cedure can be successfully applied to non-furanic aromatic
aldehydes as well.
As the next step of our study, we optimized the reaction con-
ditions for selective preparation of MHMFC 3a, a methyl ester,
which is difficult to access directly from HMF. The main
obstacles were the oxidation of HMF to DFF and the overoxida-
tion of MHMFC. To solve these problems, we decided to add
some water (5 vol%) to the reaction medium to regulate the
oxidative activity of MnO2. Fortunately, this approach proved to
be effective and MHMFC 3a was obtained in pure form in a
moderate yield without chromatographic separation; it was iso-
lated by simple extraction (Scheme 1). The method is highly
suitable for lab-scale synthesis of MHMFC and further investi-
gation of its chemistry; the straightforward isolation procedure
is crucial for synthetic applications.41 It is important to note
that in this case we obtain an ester rather than a free acid; the
former is much more stable during storage.
On the basis of the unraveled Corey–Gilman–Ganem reac-
tion mechanism42 we have proposed two possible pathways of
HMF oxidative esterification (Scheme 2). In the first step, HMF
can show two alternative types of reactivity: cyanohydrine for-
mation and oxidation to DFF. According to the obtained
results, we can conclude that the oxidation of the cyano-
hydrine moiety to acylcyanide occurs faster than the oxidation
of the hydroxymethyl group. Addition of water diminishes the
oxidative activity of MnO2, making pathway 2 (Scheme 2) less
favorable and thus switching the selectivity towards the for-
mation of MHMFC. Alternatively, pre-oxidation of HMF to DFF
makes pathway 1 impossible, and the selective formation of
FDME (pathway 2, Scheme 2) is observed.
Yieldb from
HMF, %
Yieldb from
DFF, %
Entry
ROH
Product
1
2
3
4
5
6
7
8
MeOH
EtOH
2a
2b
2c
2d
2e
2f
83
61
54
27
49
33
—
—
97
84
69
38
61
48
—
—
n-PrOH
i-PrOH
BnOHc
AllylOHc
t-BuOH
PhOH
—
—
a Reaction conditions are shown in Table S1. b Isolated yields are
reported. c CH2Cl2 was used as an additional solvent.
with the steric hindrance in tert-butanol and delocalization of
the oxygen electron pair in phenol. As the next step of our
study, we attempted to involve various HMF derivatives and
related compounds in oxidative esterification (Table 2). In the
Table 2 Oxidative esterification of biomass-derived furfuralsa
Entry
1
Substrate
Product
Yieldb, %
97
2
3
89
76
An important part of our study is concerned with the regen-
eration of MnO2. After the reaction, MnO2 was filtered off,
washed with water and methanol, and then dried in an oven at
180 °C in air. The obtained solid was reintroduced in oxidative
esterification and showed activity comparable to the activity of
the starting material. After 7 recycles of MnO2 no considerable
changes in product yield were observed (see Table S2†).
The structure of MnO2 prior to the reaction, after the reac-
tion and after the regeneration was examined by scanning elec-
tron microscopy (Fig. 5). The results indicate that the size of
4
5
6
83
52
61
7
8
9
42
95
98
a Reaction conditions are shown in Table S1. b Isolated yields are
reported.
Scheme 1 Synthesis of MHMFC under modified conditions.
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Sodium cyanide, which is used in the described reaction as
a promoter, is a very cheap and available reagent, especially in
comparison with other reagents known to mediate similar
transformations. Production of NaCN for industrial purposes
exceeds 1 million metric tons annually; efficient procedures of
cyanide utilization and detoxification are well developed and
include thermal hydrolysis, oxidation and microbiological
degradation.43,44 The handling, usage and complete life-cycle
assessment of NaCN-based industrial technologies are a well-
developed area, where toxicity and waste contamination issues
are thoroughly regarded and resolved.45 Environmental factors
for the described oxidative esterification procedure were
roughly estimated (see the ESI†). The obtained values (EF =
0.1–3) correspond to the high performance process suitable
for the production of bulk chemicals.46
Conclusions
Scheme 2 Possible mechanistic pathways of the oxidative esterification
of HMF.
In summary, we have shown that NaCN-promoted oxidative
esterification of biobased furfurals can be readily achieved
with MnO2 as a recoverable oxygen carrier. The suggested
system makes it possible to obtain the highly valuable FDME
directly from HMF without the formation and isolation of
FDCA. Various alcohols and HMF derivatives were successfully
tested in this reaction; good yields of the corresponding esters
confirmed the general applicability of the method. Addition of
water allows fine tuning of the chemical activity of the studied
oxidation system, enabling access to methyl esters of HMFCA
in pure form without chromatographic separation.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was financially supported by the Russian Science
Foundation (RSF Grant #17-13-01176). The authors gratefully
acknowledge Dr Fedor Kucherov for the FFCA sample.
Notes and references
Fig. 5 FE-SEM microimages of MnO2 and the corresponding EDX
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