Received: April 13, 2016 | Accepted: May 1, 2016 | Web Released: July 5, 2016
CL-160364
Aqueous Oxidation of Sugars into Sugar Acids Using Hydrotalcite-supported
Gold Nanoparticle Catalyst under Atmospheric Molecular Oxygen
Ravi Tomar,1,2 Jatin Sharma,1,2 Shun Nishimura,1 and Kohki Ebitani*1
1School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292
2M.Tech (CSPT), Department of Chemistry, University of Delhi, Delhi 110007, India
(E-mail: ebitani@jaist.ac.jp)
Hydrotalcite-supported gold nanoparticles show good activ-
ity as a heterogeneous catalyst for the oxidation of mono-
saccharides (xylose, ribose, galactose and mannose) and
disaccharides (lactose and cellobiose) into the corresponding
sugar acids under external base-free conditions in water solvent
using atmospheric pressure of molecular oxygen. The produced
sugar acids were thoroughly identified by 1H-, 13C-, and HMQC-
NMR and ESI-FT-ICR MS spectroscopic techniques.
Recently, we found that an HT-supported Au catalyst could
oxidize 5-hydroxymethylfurfural (HMF) into the corresponding
carboxylic acid,15 and glucosamine-HCl and its derivatives into
α-amino acids, under base-free conditions in water.16 In this
study, we have extended the base-free oxidation system using
an HT-supported Au catalyst to various sugars in water using
atmospheric pressure of molecular oxygen (Scheme 1).
Au/HT was prepared by a deposition-precipitation method
using HAuCl4¢4H2O and HT (Mg/Al = 5.4), supplied by
WAKO and Tomita Pharmaceutical Co., Ltd., respectively (see
Supporting Information (SI)). The oxidation of sugars was
performed in a 30 mL Schlenk flask in 7 mL of water solvent
at 313 K with stirring (500 rpm) under atmospheric O2 flow
(10 mL min¹1). The products were analyzed by using a HPLC
equipped with a Shodex Asahipak NH2P-50 4E column at 308 K
with pure water as the eluent (1.0 mL min¹1). Maleic acid was
used as an internal standard. An isolated product (as white
powder) by evaporation was subjected to 1H-, 13C-, and HMQC-
NMR and ESI-FT-ICR MS spectroscopic techniques.
Keywords: Sugar acid
| Gold nanoparticle catalyst |
Water solvent
Sugar acids have many significant applications in various
industrial fields.1-3 For example, metal complexes with sugar
acids have versatile industrial applications such as water
purification and asymmetric organic syntheses. 1,2,4-Butane-
triol, a precursor of butanetriol trinitrate, has been synthesized
from xylonic acid.4 Lactobionic acid can be applied in the
therapeutic, pharmaceutical, and food grad fields.5
Sugar acids can be obtained from the oxidation of sugars
derived from biomass (cellulose and hemicellulose biomass),
which is considered a cheap, easily available, and renewable
resource in the near future. The present industrial processes that
convert sugars into specialty chemicals are mainly based on
fermentation or enzymatic steps, which require strict reaction
condition control.6 Generally, the chemical oxidation of sugars
has been performed with supported metal catalysts in the
presence of base (pH between 8-9) at 323-343 K using air or
oxygen as an eco-friendly oxidizing agent.7-9 Lactose has been
oxidized into lactobionic acid using supported metal catalysts
with bases such as NaOH.8 Xylonic acid, galactonic acid,
mannonic acid, ribonic acid, and cellobionic acid have also been
produced using supported metal catalysts in alkaline medium.9
In the presence of base, the salts of sugar acids are formed,
which require additional neutralization or acidification.10
Gold nanoparticles and clusters show noticeable catalytic
activity.11 In the case of sugar oxidation, supported Au catalyst
has better activity than Pt.8d Furthermore, Au-based catalysts are
cheaper than Pt-based ones.8b,10 Hydrotalcite (HT) is known to
have a double layered structure with brucite-like layers, which
Table 1 lists the results of xylose oxidation to xylonic acid
in water solvent using different supported Au catalysts under
1 atm O2 with the actual amount of Au loading. The turnover
number (TON) is defined as the ratio of moles of product to
moles of the supported metal. Au/SiO2 was completely inactive
under the present conditions (Entry 5). In the previous study,
Au/Al2O3 was found to be a good catalyst for the oxidation of
O
OH
Au/hydrotalcite, O2 (1 atm)
H2O, 313 K, 150 min
OH
COOH
OH
OH
HO
OH
OH
yield: 95%
OH
xylose
xylonic acid
Scheme 1. Aqueous oxidation of xylose into xylonic acid
using Au/HT catalyst under 1 atm of molecular oxygen.
Table 1. Xylose oxidation into xylonic acid in water using
different supported Au catalysts under 1 atm molecular oxygen
and TONa
Entry Catalyst
wt %b Conv./%c Yield/%c TON
¹
¹
shows a basic nature due to the OH and HCO3 species even
in water. Therefore, HT has been used as heterogeneous base
catalyst for epoxidation, aldol condensation, condensation of
carbonyl group, reduction of unsaturated carbonyl compounds,
and biodiesel synthesis12 and as a metal support.11i Kaneda
et al. reported that Au/HT acted as an efficient catalyst for
the oxidation of various monoalcohols and diols into the
corresponding carbonyl compounds and lactones using molecu-
lar oxygen13 as well as the deoxygenation of epoxides into
olefins.14
1
2
3
4
5
6
Au/HT
Au/CeO2
Au/TiO2
Au/Al2O3
Au/SiO2
®
1.7
1.9
1.8
1.8
1.7
0
100
41.3
37.0
49.7
27.1
0
94.8
9.9
8.1
7.2
0
110
12
9
8
0
0
0
aReaction conditions: catalyst 50 mg, xylose 0.5 mmol, H2O
7 mL, 313 K, 150 min, O2 flow (10 mL min¹1), 500 rpm.
Determined by ICP and HPLC.
b
c
© 2016 The Chemical Society of Japan | 843