Catalytic Aerial Oxidation of Biomass-Derived Furans to Furan Carboxylic Acids in Water over Bimetallic Nickel–Palladium Alloy Nanoparticles

Article abstract of DOI:10.1002/cctc.201600942

Bimetallic Ni_1?xPd_x (0.10≤x≤0.75) alloy nanoparticle catalysts were synthesised and successfully employed for the catalytic aerial oxidation of biomass-derived furans, such as 2-furfuraldehyde (furfural), 2-furfuryl alcohol (furfuryl alcohol), 5-hydroxymethyl-2-furfural (5-HMF), 5-methyl-2-furfural (MF) and 5-methyl-2-furfuryl alcohol (MFA), to selectively afford the corresponding furan carboxylic acids (2-furoic acid, furan-2,5-dicarboxylic acid (FDCA) and 5-methyl-2-furoic acid (MFCA)) in water at 80 °C. Among the studied Ni_1?xPd_x nanoparticle catalysts, Ni_0.90Pd_0.10 nanoparticle catalyst outperformed the others, achieving high yields of the corresponding furan carboxylic acid products. The presence of Ni in the Ni_1?xPd_x nanoparticle catalysts was advantageous, because it not only enhanced the catalytic activity for the facile oxidation of biomass-derived furans using aerial oxygen to achieve high catalytic turnover, but also provided excellent stability to the Ni_0.90Pd_0.10 nanoparticle catalyst towards air and water and thus significantly enhanced its recyclability (up to 10 catalytic runs). The experiments revealed that the catalytic oxidation of 5-HMF proceeded by the initial oxidation of the formyl group to carboxylic acid, and, subsequently, the conversion of alcohol to carboxylic acid via the formyl group to form FDCA. Moreover, the one-pot direct transformation of fructose to furan carboxylic acid products (such as FDCA) was also achieved by using the Ni_0.90Pd_0.10 nanoparticle catalyst.

Full text of DOI:10.1002/cctc.201600942

Accepted Article
Title: Catalytic aerial oxidation of biomass-derived furans to furan
carboxylic acids in water over bimetallic Ni-Pd alloy nanoparticles
Authors: Kavita Gupta, Rohit Kumar Rai, Ambikesh Dhar Dwivedi, and
Sanjay Kumar Singh
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Catalytic aerial oxidation of biomass-derived furans to furan
carboxylic acids in water over bimetallic Ni-Pd alloy nanoparticle
Kavita Gupta,[a] Rohit K. Rai,[a] Ambikesh D. Dwivedi,[a] and Sanjay K. Singh*[a,b]
[a] Discipline of Chemistry, School of Basic Sciences, Indian Institute of Technology Indore, Indore 453 552,
Madhya Pradesh, India. E-mail: sksingh@iiti.ac.in
[b] Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology Indore, Indore
453 552, Madhya Pradesh, India
Abstract
Bimetallic Ni1-xPdx (0.10 ≤ x ≤ 0.75) alloy nanoparticle catalysts were synthesized and successfully
employed for the catalytic aerial oxidation of biomass-derived furans, such as 2-furfuraldehyde
(furfural), 2-furfuryl alcohol (furfuryl alcohol), 5-hydroxymethyl-2-furfural (5-HMF), 5-methyl-2-
furfural (MF) and 5-methyl-2-furfuryl alcohol (MFA), to selectively afford corresponding furan
carboxylic acids (2-furoic acid, furan-2,5-dicarboxylic acid (FDCA), 5-methyl-2-furoic acid (MFCA))
in water at 80 °C. Among the studied Ni1-xPdx nanoparticle catalysts, Ni0.90Pd0.10 nanoparticle catalyst
outperformed to achieve high yields for corresponding furan carboxylic acid products. Presence of Ni
in Ni1-xPdx nanoparticle catalysts was found to be advantageous, as it not only enhanced the catalytic
activity for facile oxidation of biomass-derived furans using aerial oxygen to achieve high catalytic
turnover, but also provided excellent stability to the Ni0.90Pd0.10 nanoparticle catalyst towards air and
water, and thus significantly enhanced recyclability (up to 10 catalytic runs) was achieved.
Experimental evidences revealed that the catalytic oxidation of 5-HMF proceeds via the initial
oxidation of formyl group to carboxylic acid, and subsequently the conversion of alcohol to
carboxylic acid via formyl group to form FDCA. Moreover, one-pot direct transformation of fructose
to furan carboxylic acid products (such as FDCA) was also achieved using Ni0.90Pd0.10 nanoparticle
catalyst.
Keywords: Biomass-derived furans; aerial oxidation; alloy bimetallic nanoparticles; nickel;
palladium
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Introduction
Conversion of biomass into fuel and chemicals has been developed in last few decades as an
efficient methodology to substitute, partial or full, the fossil fuel and petroleum products.[1-12]
Huge efforts are being devoted towards the development of new and efficient methodology to
extract maximum from biomass or biomass-based chemicals.[7-12] In this context, biomass-
based chemicals such as furans, 2-furfuraldehyde (furfural), 2-furfuryl alcohol (furfuryl
alcohol), 5-hydroxymethyl-2-furfural (5-HMF), 5-methyl-2-furfural (MF) and 5-methyl-2-
furfuryl alcohol (MFA) have received potential attention, particularly 5-HMF, as the most
promising key platform substrate for the production of value-added fine chemicals and fuel
components using several catalytic processes.[13-30] One of the important transformations is
including selective oxidation of furfural and its derivatives to important chemicals such as 2-
furoic acid, 2,5-furandicarboxylic acid (FDCA) and 2,5-diformylfuran (DFF) [equation 1],
for diverse applications in food, pharmaceuticals and polymer industries.[31-37] For example,
2-furoic acid is being widely used as preservative and flavouring agent.[32-34] FDCA, obtained
from the selective oxidation of 5-HMF, has been identified as the top-12 value added
chemicals from biomass.[35] FDCA has shown potential to serve as a bio-based polymer
building block by replacing petroleum-derived terephthalic acid, for the production of crucial
polymers like polyethylene terephthalate (PET) and polybutylene terephthalate (PBT).[36]
Au nanoparticles based catalyst has been widely used in the oxidation reaction
because of its high affinity towards oxygen.[38,39] Therefore, several groups have explored
monometallic Au and bimetallic Au-Cu and Au-Pd catalysts for the selective oxidation of 5-
HMF to FDCA.[40-47] Beside that Au catalyst showed some deactivation in the presence of
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carboxylate products, most of these reactions were performed under the high pressure (20
bar) of O2 gas and in several cases with strong base, NaOH, to achieve high selectivity to
FDCA.[39-47] Other than Au catalysts, Pt, Ru and Pd based catalysts were also reported for the
oxidation of 5-HMF. In this direction, Pt-based catalysts have exhibited high efficiency for
the complete oxidation of 5-HMF to FDCA, however , in the presence of strong base and
high pressure of O2 or air.[39,48-56] For example, recently Han et al. explored oxidation of 5-
HMF to FDCA over Pt/C-O-Mg catalyst at 110 °C and 1.0 MPa O2 gas.[56] On the other side,
Ru catalyst showed only partial oxidation of 5-HMF to DFF at 110 °C with high pressure of
O2, 2.0 MPa, using organic solvents.[57,58] Like Au, Pd catalyst also showed high activity
towards oxidation of alcohols and carbonyl compounds.[59-61] Several Pd nanoparticle
catalysts were reported for the catalytic aerobic oxidation of 5-HMF to FDCA.[41-44,62-66] In
particular, Pd/PVP (PVP = polyvinylpyrrolidone) catalyst (1.8 nm) showed higher yield for
FDCA at 90 °C in the presence of 1.25 equiv. NaOH.[62] Interestingly, for Pd nanoparticle
catalysts the content of oxygen was reported to be very crucial, as at low O2 flow by-products
may block the surface active sites, whereas under higher O2 content atomic oxygen may block
the surface.[62] Moreover, alkaline solution decreases the stability of Pd nanoparticles and
therefore it is difficult to recycle Pd nanoparticles.[63] Alloying Pd with Au in Au8Pd2/AC
(AC = activated carbon) catalyst, higher activity for 5-HMF oxidation to FDCA, in
quantitative yield, was obtained but in the presence of 2 equiv. of NaOH under 3 MPa O2 at
60 °C.[43] Au-Pd/CNT (CNT = carbon nanotube), though, catalysed 5-HMF to FDCA under a
base-free oxidation condition, but only at 100 °C under 0.5 MPa O2 or 1.0 MPa air after 12
h.[44] Notably, presence of Pd in Au-Pd catalyst enhanced the oxidation of hydroxyl group of
5-HMF, and therefore higher selectivity was achieved for FDCA, in comparison to Au
catalyst which preferentially catalysed 5-HMF to 5-hydroxymethyl-2-furancarboxylic acid
(HFCA).[44] Despite that Au or Pd catalyst showed higher catalytic activity, they also suffered
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from several drawbacks such as requirement of strong base, high reaction temperature and
high air or O2 pressure for the oxidation reaction. Hence, there is a thrust to search for other
highly active catalysts which can activate aerial oxygen event at atmospheric pressure, cost-
effective and could be highly stable under the reaction condition. In this context, we have
recently developed a highly active low-cost Ni-Pd nanoparticle catalyst for Suzuki-Miyaura
reaction.[67-69] This catalyst showed high synergistic interaction, brings by the electronic
charge transfer between Ni and Pd, to achieve high catalytic activity with low Pd leaching
and high recyclability. We anticipated that such strong synergistic interaction of Ni and Pd in
Ni-Pd alloy catalyst could be beneficial for the catalytic oxidation reaction.
In our continuous effort towards the upgradation of biomass-derived furans,[13,14]
herein, we report bimetallic Ni1-xPdx (0.10 ≤ x ≤ 0.75) alloy nanoparticles for the aerial
oxidation of biomass-derived furans (furfural and its derivatives e.g. 5-HMF). High catalytic
conversion of these furans selectively into corresponding furan carboxylic acids was achieved
using Ni0.90Pd0.10 nanoparticle catalyst at lower temperature, 80 °C. Studied catalyst,
Ni0.90Pd0.10, showed high stability in water under aerial oxygen atmosphere and thus
1
displayed high recyclability. Using time-scaled H NMR and UV-visible spectroscopic
studies, attempts were made to gain more insights about the reaction pathway.
Experimental Section
Materials and Instrumentation. All metal salts and chemicals used in this study were of
high-purity. Chemicals were purchased from Sigma Aldrich, Merck and Alfa Aesar.
Transmission Electron Microscopic (TEM) imaging was carried out using PHILIPS CM 200
with operating voltages 20-200kv and resolution 2.4 Å. Elemental mapping and energy
dispersive X-ray spectroscopy (EDS) point analysis were performed on FEI Tecnai G2, F30
TEM with operating voltage of 200 kV. Scanning electron microscopic (SEM) images and
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elemental mapping data were collected with a Supra55 Zeiss (operating voltage 5 kV)
equipped with an Oxford Instrument EDS X-ray spectrometer. Inductively coupled plasma
atomic emission spectroscopy (ICP-AES) analysis was achieved with ARCOS, Simultaneous
ICP Spectrometer of SPECTRO Analytical Instruments. Powder X-ray diffraction (XRD)
measurements were performed over the dried particles using a Rigaku SmartLab, Automated
Multipurpose X-ray Diffractometer at 40 kV and 30 mA using CuK radiation (λ = 1.5418
1
13
Å). H NMR (400 MHz) and C NMR (100 MHz) were recorded in CDCl3 or DMSO-d6
solvents on a Bruker Avance 400 spectrometer at 298 K using tetramethylsilane (TMS) as an
external standard. Chemical shifts are reported in ppm relative to the centre of the singlet at
7.26 ppm for CDCl3 and 2.49 ppm for DMSO-d6 in 1H NMR and centre of triplet at 77.0 ppm
13
for CDCl3 and centre of septet at 39.5 ppm for DMSO-d6 in C NMR. Coupling constants
are reported in Hertz (Hz) and the splitting patterns of NMR peaks are assigned as: s, singlet;
d, doublet; t, triplet; m, multiplet. High-resolution mass spectra (HRMS) spectra were
recorded on a micrOTF-Q II mass spectrometer. GC-MS analysis was performed on
Shimadzu GCMS-QP2010 Ultra with EI source in MS using Rtx-5MS column and Helium as
a carrier gas. For the GC-MS analysis injection temperature was 280 °C with injection in spilt
mode, column oven temperature held at 40 °C for 5 min and then increased upto 280 °C at
the rate of 20 °C/min and kept hold for 13 min at 280 °C.
General procedure for preparation of Ni1-xPdx (0.10 ≤ x ≤ 0.75) nanoparticle
catalysts: Bimetallic Ni0.90Pd0.10 nanoparticles were prepared following our previously
reported aqueous-phase co-reduction process of Ni2+ and Pd2+ salts using
polyvinylpyrrolidone (PVP), as a stabilizer.[67-69] Typically, an aqueous solution of NaBH4
(0.0125 g in 5 mL water) was added dropwise to the solution of potassium
tetrachloropalladate(II) (0.0016 g, 0.005 mmol), nickel(II) chloride hexahydrate (0.0107 g,
0.045 mmol) and PVP (0.0250 g) in water (5 mL) at room temperature. The content of the
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flask was sonicated for 10 min that resulted in a black suspension of nanoparticles.
Analogously, other bimetallic Ni1-xPdx alloy nanoparticles with different molar ratios of Ni
and Pd, as well as monometallic Ni and Pd nanoparticles were prepared using their
corresponding metal salts. A physical mixture of Ni + Pd nanoparticles was prepared by
mixing separately prepared monometallic Ni and Pd nanoparticles in 9:1 molar ratio.
General procedure for the catalytic aerial oxidation of furan derivatives: In a two neck
round bottom reaction flask, having freshly prepared 10 mL aqueous suspension of 5 mol%
nanoparticles (based on metal salts), 1 mmol of furan derivative was added. The reaction
mixture was stirred at 80 °C with a continuous flow of air in an oil bath for a desired reaction
time. The progress of the reaction was monitored by thin layered chromatography (TLC).
After the completion of the reaction, catalyst was recovered from the reaction mixture by
centrifugation at 6000 rpm x 2 for 10 min. To the reaction mixture, 5 mL of brine solution
and 3 mL of 1.2 M HCl was added and then extracted using diethyl ether (5 x 10 mL). The
organic layer was dried over anhydrous Na2SO4, filtered off and then solvent was evaporated
under reduced pressure. The oxidised products of 5-HMF were obtained from the reaction
mixture using 5 mL brine and 10 mL of 1.2 M HCl followed by extraction with diethyl ether
(10 x 10 mL) and evaporation under reduced pressure. Resulted products were identified by
13
1H NMR, C NMR, HRMS and GC-MS. Conversion and selectivities of the products were
obtained by 1H NMR. Purification of FDCA, obtained by the oxidation of 5-HMF, was done
using column chromatography with a mixture of dichloromethane and methanol, from
99.9:0.1 to 99:1 as an elute. Recyclability of Ni0.90Pd0.10 nanocatalyst was performed under
the optimized reaction conditions, where catalyst was recovered from the reaction mixture by
centrifugation (6000 rpm x 2 for 10 min), washed with ethanol (2 mL x 1) and distilled water
(2 mL x 2), and then dispersed in 10 mL distilled water for next catalytic run.
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Result and Discussion
Synthesis and characterization of bimetallic Ni-Pd alloy nanoparticles.
Bimetallic Ni0.90Pd0.10 alloy nanoparticles were prepared by aqueous-phase co-reduction of an
aqueous solution of potassium tetrachloropalladate(II) and nickel(II) chloride hexahydrate
salts, using NaBH4 as a reducing agent in the presence of polyvinylpyrrolidone.[67-69] Ni1-xPdx
(x = 0.250.75) nanoparticles with varying Ni to Pd ratios were also synthesised accordingly.
Nanoparticles were collected by centrifugation, dried under vacuum at room temperature and
used for the characterization. TEM images of bimetallic Ni0.90Pd0.10 nanoparticles revealed
the particle size in the range of 24 nm (Figure 1). Moreover, TEM-EDS elemental mapping
and point analysis showed the presence of both Ni and Pd metals in alloy Ni0.90Pd0.10 and
suggesting a uniform distribution of both metals in the bimetallic Ni-Pd nanoparticles (Figure
1 and Figure S1 in the supporting information). Furthermore, The TEM-EDS and ICP
analysis of Ni0.90Pd0.10 nanoparticles showed the ratio of Ni to Pd approximately 9:1. Powder
XRD diffraction pattern of Ni0.90Pd0.10 nanoparticles showed a major peak at 2ϴ = 40°, which
can be indeed to (111) lattice plane. A shifting towards little higher 2ϴ value in comparison
to Pd(111) and lowers 2ϴ value with respect to Ni(111) diffraction plan suggesting an alloy
composition of Ni0.90Pd0.10 nanoparticles (Figure S2 in the supporting information).
Consistent with XRD, selected area electron diffraction (SAED) pattern also revealed the
presence of fcc (111) lattice plane (Figure 1).
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Figure 1. a-b) TEM image of a) fresh and b) used Ni0.90Pd0.10 nanoparticles. c)
SAED pattern, d) EDS point analysis, e) FEG-TEM images and f-g) their
corresponding EDS elemental mapping showing Pd (red) and Ni (green) for fresh
Ni0.90Pd0.10 nanoparticles.
Catalytic aerial oxidation of biomass-derived furans. Catalytic aerial oxidation of
biomass-derived furans was carried out over Ni1-xPdx (x = 0.10  0.75) nanoparticles under
moderate reaction conditions in aqueous medium using furfuryl alcohol as a model substrate.
Furfuryl alcohol was used for initial optimization of reaction, as direct conversion of primary
alcohol to carboxylic acid is often difficult in comparison to formyl group (as in furfural). All
the catalytic reactions were performed using freshly prepared Ni-Pd nanoparticles, obtained
from the reduction of respective metal salts (in specified Ni to Pd molar ratio) by NaBH4,
without further purification. To our delight, complete conversion of furfuryl alcohol to 2-
furoic acid was achieved in 4 h with >99% selectivity in the presence of 5 mol% Ni0.90Pd0.10
nanoparticles in water under continuous air bubbling at 80 °C (Table 1, entry 1).
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Table 1. Reaction optimization for the catalytic oxidation of furfuryl alcohol over Ni1-
xPdx nanoparticle catalysts in water.[a]
Entry
Catalyst
T (°C)/Time (h) Conv. (%)
Sel. (%)
(2-furoic acid) (furfural)
1
Ni0.90Pd0.10
Ni0.90Pd0.10
Ni0.90Pd0.10
Ni0.90Pd0.10
Ni0.90Pd0.10
Ni0.90Pd0.10
Ni0.90Pd0.10
80/4
60/4
60/8
100/3
80/3
80/3
80/3
>99
67
>99
>99
93
--
2
--
3
>99
>99
>99
40
7
4
>99
98.5
33
--
5
1.5
67
>99
--
6[b]
7[c]
8
1.3
--
--
without catalyst 80/3
--
9
Ni
80/3
80/3
80/3
80/3
80/3
80/3
80/3
--
--
--
10[d]
11
12
13
14
15
Pd
24
>99
97.5
98
--
Pd
>99
19
2.5
2
Ni0.90+Pd0.10
Ni0.75Pd0.25
Ni0.50Pd0.50
Ni0.25Pd0.75
>99
>99
>99
99
1
97
3
96
4
[a] Reaction conditions: furfuryl alcohol (1.0 mmol), catalyst 5 mol%, air bubbling, water
(10 mL), [b] under open atmosphere (without air bubbling), [c] under Ar atmosphere, [d] 0.5
mol% catalyst.
Decreasing the reaction temperature to 60 °C or increasing to 100 °C led to the
complete conversion of furfuryl alcohol to 2-furoic acid in 8 h and 3 h, respectively (Table 1
entries 2-4). Complete conversion of furfuryl alcohol with ~99% selectivity for 2-furoic acid
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was also achieved at 80 °C in 3h (Table 1 entry 5). Reaction performed without air bubbling
through external source, but in open atmospheric conditions, was found to be very slow and
only 40% conversion was achieved at 80 °C in 3 h, suggesting the decisive role of aerial
oxygen in the catalytic oxidation reaction (Table 1 entry 6). Notably, very poor conversion
(~1%) was observed under Ar atmosphere (Table 1, entry 7). Noteworthy, reaction performed
in the absence of catalyst could not proceed (Table 1, entry 8). In contrary to high catalytic
activity of bimetallic Ni0.90Pd0.10 nanoparticles catalyst, monometallic Ni nanoparticles
catalyst was not active at all (Table 1, entry 9). Though, complete conversion of furfuryl
alcohol was achieved with 5 mol% Pd, but reaction performed with 0.5 mol% Pd
nanoparticles (amount of Pd equivalent to those in 5 mol% Ni0.90Pd0.10 nanoparticles) only
24% conversion was achieved (Table 1, entries 10-11). Moreover, only ~20% conversion of
furfuryl alcohol was observed for the reaction performed using a physical mixture of Ni and
Pd nanoparticles (Table1, entry 12). Literature reports revealed that furfural and furfuryl
alcohol get decompose or polymerize in the presence of acid and water, however, we
achieved a considerable good yield of 2-fuoic acid in aqueous medium using Ni0.90Pd0.10
nanoparticle catalyst.[70-73]
Interestingly, reaction performed using Ni1-xPdx nanoparticles with varying Ni to Pd
ratio also resulted in complete conversion of furfuryl alcohol, but not with complete
selectivity for 2-furoic acid (Table 1, entries 13-15). Among all the studied Ni1-xPdx
nanoparticles screened, Ni0.90Pd0.10 displayed highest TOF (65.3 h-1) with complete selectivity
for 2-furoic acid, which is 10 times higher than the TOF of Pd nanoparticles (6.4 h-1) (Figure
2). Hence, further catalytic reactions were performed using the high performing Ni0.90Pd0.10
nanoparticle catalyst.
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Figure 2. Catalytic activity of various nanoparticles catalysts (5 mol%) for the
oxidation of furfuryl alcohol to 2-furoic acid under air bubbling at 80 °C for 3 h in
water. [a] 0.5 mol% Pd nanoparticles, [b] physical mixture of Ni and Pd nanoparticles
in 9:1 molar ratio.
1
Kinetics of the catalytic reaction was studied, using time scaled UV-visible and H
NMR spectroscopy, to gain more insight into the reaction progress and the reaction pathway
for the catalytic aerial oxidation of furfuryl alcohol to 2-furoic acid in the presence of
Ni0.90Pd0.10 nanoparticle catalyst. The UV-visible spectrum (Figure S3 in the supporting
information) was recorded for the Ni0.90Pd0.10 catalyzed aerial oxidation of furfuryl alcohol to
2-furoic acid under the optimized reaction conditions, for a period of 04 h. The initial highly
intense absorption band at 215 nm, corresponding to furfuryl alcohol, showed continuous fall
in intensity with the progress of the catalytic reaction, consequently a new band at 277 nm,
corresponding to furfural, appeared over a period of 045 min. Further, with the progress of
the reaction, the intensity of the absorption band at 277 nm decreased continuously with the
appearance of a new absorption band at 246 nm, corresponding to 2-furoic acid.
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Concurrently, after 4 h only one prominent band at 246 nm was left, suggesting the complete
conversion of furfuryl alcohol to 2-furoic acid and that the oxidation reaction proceeded via
furfural intermediate.
1
Analysing the catalytic reaction by H NMR at different interval of time showed a
continuous decrease in the intensity of the peaks comes due to furfuryl alcohol with the
appearance of peaks corresponds to furfural and 2-furoic acid (Figure 3). Peak identification
1
of furfural during H NMR studies further reinforced that the catalytic oxidation reaction of
furfuryl alcohol to 2-furoic acid was proceeding through the formation of furfural.
Chloroform-d
c
d
e
4h
3h
b
b
b
a
2h
1h
0h
9
8
7
6
5
Chemical Shift (ppm)
1
Figure 3. Time-dependent H NMR study for the catalytic aerial oxidation of furfuryl
alcohol to 2-furoic acid in the presence of Ni0.90Pd0.10 nanoparticle catalyst.
Notably, using NaBH4 to in situ synthesize Ni-Pd nanoparticles was found to be
advantageous, as during this reduction process NaBH4 generated sodium metaborate
(NaBO2), which further facilitated (as a base) the catalytic oxidation of furfuryl alcohol to 2-
furoic acid. Effect of the base was found to be crucial, as for the reaction in the presence of
washed Ni0.90Pd0.10 nanoparticles, free from NaBO2, only 5% of conversion of furfuryl
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alcohol to 2-furoic acid was achieved under optimized reaction conditions. Furthermore,
using other bases (0.25 equiv.) such as sodium carbonate, sodium tert-butoxide and sodium
hydroxide, complete conversion of furfuryl alcohol to 2-furoic acid over Ni0.90Pd0.10
nanoparticle catalyst was also achieved, but with higher yields of 2-furoic acid in comparison
to in situ generated NaBO2 (Table S1in the supporting information). Increasing the amount of
base to 0.5 equiv could not enhance the yield, therefore, we further explored catalytic
oxidation reaction using Na2CO3 (0.25 equiv). Furthermore, earlier reports revealed the effect
of solvent polarity on the oxidation of 5-HMF and showed a high conversion to FDCA in
water as compare to organic solvents.[64,65] Similarly, we also observed higher activity of
Ni0.90Pd0.10 nanoparticle catalyst for the oxidation of furfuryl alcohol in water as compare
organic solvent like toluene, ethanol and methanol (Table S2 in the supporting information).
Catalytic activity of Ni0.90Pd0.10 nanoparticles was further explored for the aerial
oxidation of several other furan derivatives, such as furfural, MF, MFA and 5-HMF, under
the optimized reaction condition: 5 mol% Ni0.90Pd0.10, 0.25 equiv. Na2CO3 at 80 °C in water.
Earlier reports revealed that the presence of strong base e.g. NaOH, decompose or degrade
furans (such as 5-HMF), whereas with mild base degradation of furans can be avoided and
even higher yields of the oxidized product (such as FDCA from 5-HMF) can be achieved.[63]
Moreover, strong base may also adversely affect the activity of stabilizing effect of PVP,
which led to the agglomeration and decreased catalytic activity.[62] Interestingly, complete
conversion of furfural to 2-furoic acid was achieved in 1 h, suggesting that the oxidation of
formyl group to carboxylic acid is a facile process (Table 2, entry 2). Analogous to furfural
and furfuryl alcohol, aerial oxidation of 5-methyl-furfural (MF) and 5-methyl-furfuryl
alcohol (MFA) selectively to 5-methyl-2-furoic acid (MFCA) was also accomplished in 1.5 h
and 4 h, respectively over Ni0.90Pd0.10 nanoparticle catalyst (Table 2, entries 3, 4).
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Table 2. Catalytic aerial oxidation of furan derivatives over Ni0.90Pd0.10
nanoparticle catalyst.[a]
Entry Furans
1
Time (h) Products
4
Conv./Sel.(%)
>99/>99 (46)[b]
2
3
1
>99/>99 (42)[b]
1.5
>99/>99 (47)[b]
>99/>99 (54)[b]
4
4
4
5[c]
>99/86:11:3
(FDCA:HFCA:FFCA)
{70 (FDCA)}[b]
[a] Reaction conditions: furans (1.0 mmol), air bubbling, Na2CO3 (0.25 equiv.) water
(10 mL), 80 °C. [b] isolated yield. [c] reaction with 1 equiv. of Na2CO3.
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Figure 4. Effect of Na2CO3 amount on the catalytic aerial oxidation of 5-HMF.
High reactivity towards the catalytic oxidation of 5-HMF was also observed with
Ni0.90Pd0.10 nanoparticles catalyst, where due to the availability of an alcoholic and a formyl
group, several intermediate oxidized product were observed (Figure 4, and Table 2, entry 5).
The Ni0.90Pd0.10 nanoparticle catalyzed aerial oxidation of 5-HMF with 0.25 equiv. of Na2CO3
was found to be slow and only 58% conversion was achieved with 5-formyl-furan-2-
carboxylic acid (FFCA) as the major product (59% selectivity). The slow conversion of 5-
HMF was presumably due to a low amount of base which led to competition between 5-HMF
and its products, HFCA and FFCA, for their further oxidation to FDCA. Further, increase in
Na2CO3 content to 0.50 and 0.75 equiv., significantly increased the conversion of 5-HMF to
85% and >99%, respectively, but the selectivities towards FDCA were only 14 and 67%
respectively. Notably, using 1 equiv. of Na2CO3 resulting in a complete conversion of 5-HMF
with 86% selectivity for FDCA (70% isolated yield) (Table 2, entry 5). Using >1 equiv. of
Na2CO3 or preforming the catalytic reaction at 100 °C could not increase the selectivity for
FDCA significantly (Table S3 in the supporting information). Observation of several
intermediate oxidized products during the catalytic oxidation of 5-HMF, suggesting a
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stepwise oxidation of 5HMF; (step a) oxidization of formyl group to carboxylic acid and
(step 2) alcohol to carboxylic acid via formyl group (Scheme 1).
Scheme 1. Schematic illustration of the plausible reaction pathway for the catalytic
aerial oxidation of 5-HMF to FDCA.
Stability, recovery and recyclability of Ni0.90Pd0.10 nanoparticle catalyst.
Impressively, the highly active Ni0.90Pd0.10 nanoparticle catalyst also showed high stability in
water and air. Therefore, recyclability test of Ni0.90Pd0.10 nanoparticle catalyst was attempted
for the catalytic oxidation of furfuryl alcohol to 2-furoic acid in the presence on 0.25 equiv.
of Na2CO3 under optimized reaction conditions. Noteworthy, Ni0.90Pd0.10 nanoparticle catalyst
displayed significantly high activity and can be recycled as high as 10 consecutive catalytic
runs without any significant loss in conversion and selectivity (Figure 5). ICP-AES analysis
showed no significant leaching of Ni or Pd for the Ni0.90Pd0.10 nanoparticle catalyst during the
catalytic oxidation reaction. Furthermore, SEM-EDS analysis of the spent Ni0.90Pd0.10
nanoparticle catalyst revealed the presence of both Ni and Pd in approx. 9:1 ratio (Figure S1
in the supporting information). Ni0.90Pd0.10 nanoparticle catalyst was also recycled for 6
catalytic runs for the catalytic oxidation of 5-HMF to FDCA (Figure S4 in the supporting
information).
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10.1002/cctc.201600942
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Figure 5. Recyclability of the Ni0.90Pd0.10 nanoparticle catalyst for the oxidation of
furfuryl alcohol to 2-furoic acid.
We also attempted the direct catalytic transformation of fructose to FDCA (and other
furan carboxylic acid products) in the presence of Ni0.90Pd0.10 nanoparticle catalyst (Scheme
2). In a one-pot two-step catalytic reaction, initially fructose was treated with 0.5 equiv. of
NH4Cl in 2-propanol at 80 °C for 12 h.[73] Thereafter, reaction mixture was added 8 mL
water, 1.5 equiv. of Na2CO3 and stirred again for another 8 h at 80 °C with air bubbling in the
1
presence of 5 mol% Ni0.90Pd0.10 nanoparticle catalyst. H NMR analysis of the reaction
mixture inferred the formation of HFCA and FDCA as the major products with 74 and 22%
selectivities, respectively, whereas FFCA (2.5%) and 5-HMF (1.5%) were also appeared as
minor products. Important to mention here, that the formation of reaction product FDCA,
analogous to that obtained from 5-HMF, suggesting the direct transformation of fructose to
furan carboxylic acids (FDCA, HFCA or FFCA) via 5-HMF can be achieved using
Ni0.90Pd0.10 nanoparticle catalyst under suitably tuned one-pot reaction conditions.
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10.1002/cctc.201600942
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Scheme 2. One-pot transformation of fructose to furan carboxylic acids.
The observed high catalytic activity and recyclability displayed by the studied
bimetallic Ni0.90Pd0.10 nanoparticle catalyst towards aerial oxidation of biomass-derived
furans in water is of significant importance, as most of the earlier reports catalysts showed
oxidation of furans only at high temperature and/or high O2 gas or air pressure. Moreover,
earlier studies also showed, that Pd catalysts displayed high catalytic activity, mostly, in the
presence of O2 than air.[64] For instance, Pd/PVP catalyst were active for the oxidation of 5-
HMF to FDCA only at 90 °C with 1.25 equiv. of NaOH.[62] It is worth mentioning here, that
the studied Ni0.90Pd0.10 nanoparticles catalyst displayed higher TOF (35 h-1) (with respect to
Pd content in Ni0.90Pd0.10 nanoparticles catalyst) in comparison to earlier reported several
monometallic and bimetallic Pd nanoparticles catalyst (~5 h-1) for the oxidation of 5-HMF to
FDCA (Table S4 in the supporting information).[44,62-66] Literature revealed that, as
adsorption energy of O2 on the surface of Pd nanoparticles is low, adsorption of O2 takes
place strongly that resulted with the elongation of O-O bond distance as compare to free
O2.[74,75] Elongation of bond followed with the decrement of O2 bond order that results filling
of π* antibonding orbital of O2 by the transfer of electrons from the metal at the time of
adsorption.[74,75] Therefore, we anticipated that a highly negatively charged Pd centre could be
a favourable site for the facile activation of O2 (even in the presence of very low content of
O2, e.g. as in air). In this context, our earlier report showed a highly activated Ni-Pd
nanoparticle catalyst for the facile oxidative addition of aryl halide, where via DFT
calculations we showed that Ni, which has less ionization energy as compare to Pd, could
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10.1002/cctc.201600942
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transfer electronic charge to the Pd centre in the bimetallic Ni0.9Pd0.10 nanoparticles.[67-69]
Transfer of electron from Ni to Pd makes Pd more electron rich and facilitates for the
adsorption of more oxygen on the surface of Pd centre which resulted in the enhanced
catalytic activity of Ni0.90Pd0.10 for the oxidation of furan derivatives as compare to their
monometallic nanoparticles.
Conclusion
In summary, we successfully explored various bimetallic Ni1-xPdx (0.10 ≤ x ≤ 0.75) alloy
nanoparticle catalyst for the aerial oxidation of biomass-derived furan, furfural, furfuryl
alcohol, 5-HMF, 5-methyl furfural (MF) and 5-methyl furfuryl alcohol (MFA), to their
corresponding carboxylic acid products, 2-furoic acid, furan-2,5-dicarboxylic acid (FDCA),
5-methyl-2-furoic acid (MFCA) at 80 °C in water in the presence of a mild base. Among
various bimetallic Ni1-xPdx (0.10 ≤ x ≤ 0.75) nanoparticles, along with monometallic Ni and
Pd nanoparticles, and a physical mixture of Ni and Pd monometallic nanoparticles, bimetallic
Ni0.90Pd0.10 nanoparticle catalyst outperformed to achieve high catalytic turnover for the
corresponding furan carboxylic acids. Moreover, the studied Ni0.90Pd0.10 nanoparticle catalyst
also displayed excellent stability towards air and water, and therefore recycled up to ten
catalytic runs for the aerial oxidation of furfuryl alcohol without any significant loss in
catalytic activity. Experimental evidences revealed that the catalytic oxidation of 5-HMF
proceeds via the initial oxidation of formyl group to carboxylic acid, and subsequently the
conversion of alcohol to carboxylic acid via formyl group to form FDCA. Furthermore, high
catalytic activity of Ni0.90Pd0.10 nanoparticle catalyst was also explored for one-pot direct
transformation of fructose to furan carboxylic acid products (such as FDCA). Through our
study, we demonstrated a significant synergy between Ni and Pd to achieve high catalytic
activity (TOF) for the aerial oxidation of biomass-derived furans to corresponding furan
carboxylic acids. We believe, the present study on low-cost high-performing bimetallic Ni-Pd
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10.1002/cctc.201600942
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nanoparticle catalyst for the selective aerial oxidation of biomass-derived furans, could
provide perceptions towards the development to highly active catalysts for several other
important catalytic transformations including biomass transformation to value added
chemicals and others.
Acknowledgements
Authors thank IIT Indore, CSIR New Delhi and SERB-DST New Delhi for financial support.
SIC, IIT Indore and SAIF, IIT Bombay are acknowledged for instrumental facilities. K.G.
and R. K. R. thank CSIR, New Delhi for their SRF grants. A. D. D. thanks CSIR New Delhi
(01(2722)/13/EMR-II) for financial support for his fellowship.
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Graphical Abstract
Highly effective catalytic aerial oxidation of biomass-derived furans was achieved using a
high-performing low-cost and recyclable bimetallic Ni0.90Pd0.10 alloy nanoparticles catalyst
under moderate reaction conditions in water.
26
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