Base-free aerobic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid over Pt/C catalysts synthesized by pulse alternating current technique

Article abstract of DOI:10.1016/j.mencom.2018.07.031

The Pt/C catalysts with various Pt content (5-30 wt%) synthesized via electrochemical pulse alternating current technique have been evaluated for the base-free aerobic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid. The higher Pt content in the catalyst (30 wt%) provides the product yield up to 65% upon performing the process in concentrated (～0.1 M) aqueous solutions of the substrate.

Full text of DOI:10.1016/j.mencom.2018.07.031

Mendeleev
Communications
Mendeleev Commun., 2018, 28, 431–433
Base-free aerobic oxidation of 5-hydroxymethylfurfural
to 2,5-furandicarboxylic acid over Pt/C catalysts
synthesized by pulse alternating current technique
Daria V. Chernysheva,* Victor A. Klushin, Alexander F. Zubenko, Lyudmila S. Pudova,
Oleg A. Kravchenko, Victor M. Chernyshev and Nina V. Smirnova
Department of Technology, M. I. Platov South-Russian State Polytechnic University, 346428 Novocherkassk,
Russian Federation. E-mail: da.leontyva@mail.ru
DOI: 10.1016/j.mencom.2018.07.031
The Pt/C catalysts with various Pt content (5–30 wt%)
synthesized via electrochemical pulse alternating current
technique have been evaluated for the base-free aerobic oxida-
tion of 5-hydroxymethylfurfural to 2,5-furandicarboxylic
acid. The higher Pt content in the catalyst (30 wt%) provides
the product yield up to 65% upon performing the process in
concentrated (~0.1 m) aqueous solutions of the substrate.
Base-free aerobic
oxidation
Pt/C
Pt
Pt
HO
O
CHO HO
2
C
O
2
CO H
O
2
Pt
5
0 nm
C
C
5
-Hydroxymethylfurfural 1 and its derivatives, such as 2,5-furan-
~100% yields.1,9 However, oxidation in alkaline solutions leads
to large quantities of mineral wastes at the product isolation, which
requires acidification to separate free acid, and this increases
the risk of reactor corrosion. Therefore, the aerobic oxidation
of substrate 1 in the absence of base (base-free oxidation) seems
dicarboxylic acid 2 and 2,5-diformylfuran 3 are regarded as
platform chemicals for synthesis of various valuable compounds.
Diacid 2 is one of the most promising bio-based monomers,
which can replace oil-based terephthalic and isophthalic acids
for production of polymers like polyethylene terephthalate in
1
–8
9
environmentally more friendly and suitable for large-scale
1
,8–10
12
future.
Diacid 2 is prepared by the oxidation of substrate 1
industrial applications. The main drawback of base-free oxida-
1
2–15
with the formation of various intermediates and by-products
tion processes
typically consists in necessity for the use of
depending on the reaction conditions (Scheme 1).1
,9,11
Hetero-
diluted (up to 10 m) solutions of compound 1 to minimize by-
product (mainly humines) formation and to prevent catalyst
deactivation. In addition, catalysts with low Pt loading (£5 wt%)
–2
geneously catalyzed oxidations of compound 1 with air or
molecular oxygen are considered to be most promising for
industrial implementation. High activity and selectivity have
been found for precious-metal based catalysts. Platinum-based
1
require prolonged time for oxidation (12–24 h) under base-free
9
12–15
conditions.
Despite high yields of diacid 2, all of these
catalysts on different supports in alkaline aqueous solutions
enables oxidation of hydroxy aldehyde 1 into diacid 2 in up to
methods do not provide high overall process efficiency from
industrial point of view.
In recent years, a new environment-friendly and cheap method
for the preparation of heterogeneous platinum catalysts via
electrochemical dispersion of metal platinum under pulse current
[
O]
O
[O]
O
O
1
6–18
HO
O
OH
electrolysis was developed.
As distinct from many conven-
5
tional ‘chemical’methods, the electrochemical dispersion technique
allows one to tune Pt loadings in a wide range providing narrow
[
O]
[O]
O
1
6–18
Pt nanoparticles size distribution (8–10 nm).
Catalysts Pt/C
O
O
O
obtained via electrochemical dispersion revealed enhanced stability
HO
O
O
OH
1
3
4
combined with good activity for complete oxidation of simple
17,18
[
O]
organic substances in fuel cells.
Nevertheless, potential of these
catalysts for oxidative synthesis of valuable organic compounds
is still unexplored.
O
O
In the current study, we investigated possibilities of base-free
O
O
O
HO HO
OH
oxidation 1 ® 2 with molecular O in concentrated (0.1 m)
2
2
O
aqueous solutions using Pt/C catalysts with enhanced Pt content
(
up to 30 wt%) obtained via electrochemical dispersion of platinum
HO
O
O
metal. To the best of our knowledge, this is the first example of
application of Pt/C catalyst obtained by such a technique in organic
synthesis.
The Pt/C (C is Vulcan XC-72) catalysts were prepared by
electrochemical dispersion of Pt electrodes under pulse alternating
O
O
O
O
O
O
Humins
1
6
current, as was described previously. The platinum loadings
in the catalysts were 5, 15 and 30 wt%. These catalysts are
Scheme 1
©
2018 Mendeleev Communications. Published by ELSEVIER B.V.
–
431 –
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.

Mendeleev Commun., 2018, 28, 431–433
hereafter denoted as Pt/C-5, Pt/C-15 and Pt/C-30, respectively.
(a) 100
1
The average Pt particle size along the [111] direction (D111)
†
80
calculated using the Scherrer equation from XRD data (Figure S1,
Online Supplementary Materials) was 9±1 nm. According to the
TEM images of Pt/C samples (Figure S2), platinum nanoparticles
were uniformly distributed over the surface of carbon support.
Oxidation of substrate 1 was studied at 105°C in a batch-type
autoclave. In catalytic experiments, 0.1 m aqueous solution of 1,
initial molar ratio of 1/Pt = 20:1, oxygen pressure of 0.25 MPa
6
0
40
2
5
2
0
0
3
4
‡
were applied. The reaction mixture was analyzed by high-
performance liquid chromatography (HPLC).§
0
0
0
2
2
2
4
6
8
8
8
10
10
10
Reaction time/h
According to HPLC analyses, the oxidation of hydroxy
aldehyde 1 proceeds sequentially through the partially oxidized
intermediates, 3 and 4 (see Scheme 1), in agreement with the
(b) 100
1
8
6
4
0
0
0
published data for analogous base-free aerobic oxidations over
carbon supported Pt catalysts.1
2,14,15
The product yield curves
are typical of continuous processes (Figures 1 and S5). Higher
Pt contents (15 and 30 wt%) in the studied catalysts provided
higher oxidation rates (see Figure 1). Thus, with Pt/C-5 catalyst
within 6 h nothing of product 2 was detected, whereas Pt/C-15 and
Pt/C-30 catalysts provided the 30 and 63% yields, respectively
3
5
4
20
0
2
(Figures 1 and S5). Prolongation of the reaction time from 6 to
4
6
1
0 h enhanced the product yield substantially (up to 66%) for
Reaction time/h
Pt/C-15 catalyst and only slightly (up to 65%) for Pt/C-30 catalyst.
However, prolongation of the synthesis to 10 h significantly
dropped the selectivity (see Figure 1), apparently owing to
decomposition of intermediate compounds and polymerization
(
c) 100
1
80
6
of substrate 1 into humins. Note that in preparative experiments
4
6
4
2
0
0
0
0
with the prolonged reaction time (10 h) the yields of diacid 2 were
lower because substantial amounts of by-products complicated
its separation and purification.
Remarkably, the Pt/C-5 catalyst with the low Pt content (5 wt%)
furnished very low (~0%) selectivity of the process and promoted
loss of substrate 1 due to formation of unidentified by-products
2
3
5
(
see Figure 1). This may be caused by adverse catalytic activity
4
6
19
of activated carbon support. Thus, it was revealed that activated
carbon (Vulcan XC-72 without Pt) promoted decomposition of
compound 1 (Table 1, entries 4–6). In these experiments, loss
of the latter owing to simple physical adsorption was excluded
by preliminary saturation of activated carbon in large excess of
Reaction time/h
Figure 1 Kinetic curves for catalytic oxidation of compound 1 at 105°C
and 0.25 MPa on (a) Pt/C-5, (b) Pt/C-15, and (c) Pt/C-30. (1) Conversion
of 1, and (2)–(5) yields of 3, 4, 2 and unidentified products, including
humins, respectively.
0
.1 m solution of substrate 1. Since initial 1/Pt molar ratio was
the same in all catalytic experiments, use of the catalyst with low
Pt content (5 wt%) may be regarded as administration of enhanced
Table 1 Effect of carbon content in the Pt/C catalyst on the oxidation of 1.a
Catalyst
components/mg Conversion Yield
UIPb
of 2 (%) (%)
Entry Catalyst
†
The XRD studies and crystallite size determination were performed with
of 1 (%)
Pt
C
an ARL X’TRA powder diffractometer, Thermo Scientific (CuKa target,
l = 1.5418 Å). XRD data were collected in the 2q range of 5–80° using
1
2
3
4
5
6
a
Pt/C-5
5
5
5
0
0
0
95.0
28.0
11.7
95.0
28.0
11.7
74
100
100
18
0
30
63
0
25
15
1
–1
step scan mode with a step size of 0.03° and step scan rate 2.25 deg min .
‡
Pt/C-15
All oxidation experiments were conducted in a batch-type glass-lined
Pt/C-30
stainless-steel autoclave (250 ml) equipped with a thermo-controller, a
pressure gauge and a magnetic stirrer using 0.1 m aqueous solutions of
substrate 1 (5 ml). The initial 1/Pt molar ratio of 20:1 was used in all
experiments. The air was removed from the reactor by flushing oxygen
before catalytic run. Then, the reactor was pressurized with oxygen to
Vulcan XC-72
Vulcan XC-72
Vulcan XC-72
18
12
12
12
0
12
0
Reaction conditions: 0.1 m aqueous solution of 1 (5 ml), 0.25 MPa O ,
0
.25 MPa and heated at 105°C in a thermostat for appropriate time.
For HPLC analysis, the reactor was rapidly cooled to room tempera-
2
b
§
105°C, 6 h. UIP denotes unidentified products, including humins.
ture in water bath, the pressure was reduced to atmospheric, and the
reaction mixture was made alkaline by adding a certain amount of
sodium bicarbonate. Then the catalyst was separated by centrifugation.
The composition of the reaction medium was determined by HPLC using
anAgilent 1260 Infinity LC system equipped with a Discovery Zr-Carbon
column (150 mm × 4.6 mm). The mobile phase was tetrahydrofuran and
aqueous solution of phosphoric acid (0.025 m) with the volume ratio
¶
For isolation of diacid 2 after the catalytic run with the use of Pt/C-30
catalyst at 0.25 MPa within 6 h, the reactor was rapidly cooled to room
temperature in water bath and decompressed, and the mixture was diluted
with water (5 ml). The resulted suspension was heated up to 80–85°C, the
catalyst was separated by hot filtration and washed with a small volume
(~3 ml) of hot water. The filtrate obtained was combined with washings
and evaporated at reduced pressure to a volume of ~1.5 ml, then cooled
to 5°C. The white solid precipitate was collected by centrifugation and
recrystallized once from water to give pure 2,5-furandicarboxylic acid 2
(41 mg, 52% yield). The physical and spectral characteristics of the product
obtained were identical with those described previously.13
–1
of 40:60 at a flow rate of 0.9 ml min , column temperature 60°C, and
detection wavelength of 265 nm. The contents of components 1–5 were
determined using interpolation from calibration curves prepared by the
1
13
using pure authentic substances. H and C NMR spectra of diacid 2
are presented in Figures S3, S4.
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432 –

Mendeleev Commun., 2018, 28, 431–433
3 V. P. Kashparova, V. A. Klushin, D. V. Leontyeva, N. V. Smirnova,
carbon loading into the reaction mixtures causing higher loss
of compound 1 due to decomposition (see Table 1, entry 1).
Mechanism of this decomposition catalyzed by carbon surface is
not clear. Probably, adsorption of substrate 1 on activated carbon
promotes its polymerization to humins since its higher concentra-
tion on carbon surface allows a hydrogen-bonding network to be
formed between sorbate molecules (cf. ref. 6).
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In general, higher Pt content in Pt/C catalysts provides higher
oxidation rate and higher selectivity for base-free oxidation 1 ® 2
in 0.1 m aqueous solutions. According to the obtained results, use
of Pt/C-30 catalyst at 0.25 MPa within 6 h would be preferable.
Under these conditions, diacid 2 was obtained in 52% isolated
2
016, 55, 8338.
7
8
L. V. Romashov and V. P. Ananikov, Chem. Asian J., 2017, 12, 2652.
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¶
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In conclusion, we have demonstrated that Pt/C catalysts with
enhanced Pt content (15–30 wt%) provide selective base-free
aerobic oxidation of concentrated (0.1 m) aqueous solutions of
1
1
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1
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results represent the first example of application of heterogeneous
Pt/C catalyst obtained by pulse alternating current technique in
organic synthesis. More detailed studies are anticipated in the near
future to evaluate various catalyst supports effect and to optimize
oxidation parameters for better yields of product 2.
1
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This work was supported by the Russian Science Founda-
tion (project no. 16-13-10444). The authors are grateful to the
Department of Structural Studies of N. D. Zelinsky Institute of
Organic Chemistry and the Shared Research Center ‘Nanotech-
nologies’ of the M. I. Platov South-Russian State Polytechnic
University for analytical services.
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Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2018.07.031.
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