Microwave-assisted base-free oxidation of glucose on gold nanoparticle catalysts

Article abstract of DOI:10.1016/j.catcom.2015.11.014

Base-free oxidation of glucose to gluconic acid with supported gold catalysts was studied using microwave heating. High conversion and selectivity were obtained in a remarkably shortened reaction time; in 10 min gluconic acid was obtained with up to 76% yields with 0.09 mol% Au/Al₂O₃ and hydrogen peroxide as oxidant. Very high turn-over frequencies over 10,000 h^-1 were measured for the microwave assisted oxidation. Moreover, the catalyst activity remained constant in four consecutive runs, even though minor particle growth was observed.

Full text of DOI:10.1016/j.catcom.2015.11.014

Catalysis Communications 74 (2016) 115–118
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Microwave-assisted base-free oxidation of glucose on gold
nanoparticle catalysts
Sari Rautiainen a, Petra Lehtinen a, Marko Vehkamäki a, Klaus Niemelä b, Marianna Kemell a,
Mikko Heikkilä a, Timo Repo a,
⁎
a
Laboratory of Inorganic Chemistry, Department of Chemistry, A.I. Virtasen aukio 1, P.O. Box 55, 00014, University of Helsinki, Finland
VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, 02044, VTT, Finland
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Base-free oxidation of glucose to gluconic acid with supported gold catalysts was studied using microwave
heating. High conversion and selectivity were obtained in a remarkably shortened reaction time; in 10 min
gluconic acid was obtained with up to 76% yields with 0.09 mol% Au/Al2O3 and hydrogen peroxide as oxidant.
Very high turn-over frequencies over 10,000 h−1 were measured for the microwave assisted oxidation. More-
over, the catalyst activity remained constant in four consecutive runs, even though minor particle growth was
observed.
Received 17 September 2015
Received in revised form 16 November 2015
Accepted 17 November 2015
Available online 18 November 2015
Keywords:
Oxidation
© 2015 Published by Elsevier B.V.
Carbohydrates
Heterogeneous catalysis
Gold
Microwave chemistry
1. Introduction
from cellulose through acid hydrolysis followed by the catalytic oxida-
tion [12]. Consequently, there is great interest in developing catalysts
Catalytic transformations of carbohydrates are essential for the sus-
tainable production of chemicals [1–3]. Gluconic acid, a product of selec-
tive oxidation of glucose, is widely used e.g. as a food additive, in
pharmaceuticals and as a chelating agent in detergents [4]. Currently,
gluconic acid is manufactured by enzymatic glucose oxidation with an-
nual production estimated at 60,000 tons [5]. In this process, low pH
causes deactivation of the enzymes and therefore sacrificial Brönsted
base is required to neutralize the formed carboxylic acid [6]. In search
for more efficient methods, the aerobic oxidation of glucose catalyzed
by Pd group metals has been extensively studied [7]. In recent years,
gold nanoparticle (Au NP) catalysts have gained increasing attention
due to their exceptional selectivity, activity and stability compared
to catalysts based on palladium or platinum [6,8,9]. With metal
oxide supported Au NPs turn-over frequencies (TOF) of up to
160,000 molGlc molsurface A−u1 h−1 have been reported in oxidation of
glucose to gluconic acid [10].
that are highly active in acidic conditions [13].
Early studies showed that Au NPs can oxidize glucose also without
the additional base [14]. However, the catalytic activity was significantly
lower at acidic conditions than at alkaline pH and leaching of Au from
the carbon support led to catalyst deactivation. Therefore, the research
on Au catalyzed carbohydrate oxidation has mainly focused on alkaline
conditions [6], and base-free glucose oxidation has gained more atten-
tion only recently [15–17]. In comparison to alkaline oxidation, which
is carried out e.g. with air or oxygen bubbling at temperatures below
60 °C, the reported base-free oxidations require pressurized oxygen
and/or significantly longer reaction times to reach reasonable conver-
sions (Table 1). Also, acidic conditions are clearly demanding for the cat-
alysts; deactivation due to Au leaching, sintering, adsorption of organic
species or support instability were reported. Apparently, significant de-
velopment is required for base-free oxidation to be a viable alternative
to alkaline oxidation.
Noble metal catalysts require alkaline conditions (pH 9–10) to main-
tain high activity and consequently, the corresponding gluconate salt is
produced [11]. However, oxidation without pH control eliminates both
the need for an added base during the oxidation and the use of a strong
acid to isolate gluconic acid from the generated salt. As such, base-free
oxidations would enable direct, one-pot production of gluconic acid
We initiated studies on base-free glucose oxidation using Au NPs
supported on different metal oxides under pressurized oxygen. Simi-
larly to the earlier reports, we observed low conversions and catalyst
deactivation leading to decreased activity upon recycling. To promote
the oxidation, we introduced microwave assisted heating. Employing
such systems have led to dramatically improved yields and decreased
reaction times in organic synthesis [18] as well as in heterogeneous ca-
talysis [19]. To our knowledge however, the use of microwave irradia-
tion in Au NP catalysis has been studied only in oxidation of benzylic
⁎
Corresponding author.
E-mail address: timo.repo@helsinki.fi (T. Repo).
http://dx.doi.org/10.1016/j.catcom.2015.11.014
1566-7367/© 2015 Published by Elsevier B.V.

116
S. Rautiainen et al. / Catalysis Communications 74 (2016) 115–118
Table 1
Previous reports on base-free glucose oxidation with supported Au NPs.
Entry
Catalyst
Glc:Au
T
a (°C)
Time (h)
Oxidant
Conv. (%)
Select. (%)
Ref.
1
2
3
4
1% Au/C
1000
140
1600
1000
100
65
60
6
2
24
2
3 bar O2
2.3 bar O2
airb
100
76
57
100
96
100
88
[14]
[15]
[16]
[17]
0.5% Au/CeO2
0.5% Au/MgO
1% Au/CMK-3c
110
3 bar O2
92
a
Conventional heating.
Atmospheric air.
Au supported on mesoporous carbon.
b
c
and aliphatic alcohols by gold on mesoporous silica [20]. Herein, we re-
port very high catalytic activity for Au NP catalyzed base-free oxidation
of glucose under microwave irradiation (Scheme 1).
3. Results & discussion
Microwave-assisted oxidation of glucose was carried out using hy-
drogen peroxide as oxidant, since the use of pressurized oxygen in
closed microwave vials was unfeasible with the current system (Biotage
Initiator). The low solubility of oxygen in water can be a limiting factor
on the oxidation rate, whereas these mass transfer limitations are
avoided with H2O2 [26,27]. Hydrogen peroxide decomposes on gold cat-
alysts producing dioxygen via intermediate peroxo and superoxo spe-
cies [28]. Using H2O2 and Au/Al2O3, Saliger et al. reported very high
selectivity and activity in alkaline oxidation of glucose [29]. Instead of
hydrogen peroxide however, the effective oxidizing agent in the reac-
tion was a species formed by decomposition of H2O2, presumably
dioxygen.
The catalyst support affects the H2O2 decomposition rate [13] and
therefore, we introduced Au NP catalysts with three different support
materials including MgO, MgAl2O4 and Al2O3. Immediate O2 formation
occurred with all catalysts upon addition of H2O2 to the reaction mix-
ture. The oxidant-to-substrate ratio was optimized to 2.2 equiv. of
H2O2 and applied to all catalytic studies (Table S1).
2. Experimental
2.1. Catalyst preparation and characterization
Au/MgO was prepared by deposition-precipitation with urea ac-
cording to reference [21]. Au/Al2O3 was prepared by direct ion-
exchange method; preparation and characterization were reported in
ref. [22]. The MgAl2O4 spinel support was prepared by coprecipitation
and Au was deposited on the support by deposition precipitation as de-
scribed previously [23]. Au loading was determined by AAS (Atomic Ab-
sorption Spectrophotometer, PerkinElmer 3030) after dissolving the
catalyst into aqua regia. Au particle size distribution was determined
by transmission electron microscopy (TEM) based on 100 particles,
and gold dispersion calculated from the particle size distribution ac-
cording to ref. [24]. See Supplementary data for detailed experimental
procedures and catalyst characterization.
Magnesia has been previously used as support in base-free oxidation
of glucose and glycerol [16,30], which inspired to test Au/MgO in our
studies. To our surprise however, microwave-assisted oxidation of glu-
cose with Au/MgO resulted in complete dissolution of the support. The
previous studies reported only minor instability of the magnesia sup-
port; ppm-levels of Mg2+ leached into the acidic solutions [16,30]. In
search of a more stable catalyst support, Au NPs were supported on
MgAl2O4 spinel, which is known for high hydrothermal stability [23,
31]. Particle size analysis of the 2.3 wt.% Au/MgAl2O4 catalyst revealed
a bimodal distribution; TEM analysis showed small Au particles with
mean diameter of 3.8 nm, whereas very large particles with sizes over
100 nm were detected with FESEM (see SI). Presumably, the small
NPs are responsible for the activity of the catalyst, and therefore disper-
sion and TOF values were calculated according to the particle size de-
tected by TEM (Table 2, entry 1). Moderate conversion (23%) and high
selectivity (94%) to gluconic acid were obtained with the Au/MgAl2O4
catalyst at 100 °C. Increasing temperature to 120 °C improved the con-
version to 54% while selectivity remained the same.
2.2. Microwave oxidation and product analysis
The oxidation of glucose (Glc) was conducted in sealed glass vials
using a Biotage Initiator microwave reactor with a 2.45 GHz magnetron.
The instrument measures temperature of the reaction mixture using IR
and adjusts the heating power accordingly (Fig. S4). A 20 ml glass vial
was charged with catalyst (6 mg), D-glucose (110 mg, 0.61 mmol),
water (5 ml) and H2O2 (1 equiv.). The reaction mixture was heated
with microwave irradiation for 5 min at the chosen reaction tempera-
ture after which the second H2O2 portion (1 equiv.) was added with a
syringe through the septum. The mixture was heated at the same tem-
perature for another 5 min, giving a total reaction time of 10 min. The
reaction mixture was stirred with magnetic stirring at 600 rpm. The
products were identified using 1H-NMR and GC/MS, and yields were de-
termined by HPLC. Reproducibility of the results was confirmed in re-
peated experiments; 1–2% deviation in conversion and max. 5%
deviation in selectivity was detected in at least two repeated experi-
ments. After a typical experiment, the pH value of the reaction solution
was between 2.4 and 3. In these conditions, gluconic acid forms intra-
molecular esters, gluconolactones. While these lactones have often
been reported as intermediates or byproducts, their formation from
gluconic acid depends on the pH of the solution according to the chem-
ical equilibrium [25]. These can easily be hydrolyzed to gluconic acid
and therefore we included the lactones in the yield.
Alumina supported gold catalysts are among the most active Au cat-
alysts for glucose oxidation in alkaline conditions [10] and furthermore,
their long-term stability has been demonstrated [32]. Recently, we
showed that a 1.8 wt.% Au/Al2O3 catalyst oxidizes both aldoses and
uronic acids at alkaline conditions with high activity [22]. Herein, the
same catalyst was very active in base-free glucose oxidation; 83% con-
version and 87% selectivity were obtained at 120 °C giving an extremely
−1
high TOF value of 12,900 molGlc mol
h−1 (Table 2, entry 2). Con-
surface Au
trol experiment without any catalyst using 2.2 equiv. H2O2 at 120 °C
gave 12% conversion and 51% selectivity. Respectively, 19% conversion
and 41% selectivity were obtained using the Al2O3 support alone.
The effect of temperature on the oxidation was studied further with
Au/Al2O3. Conversion doubled when temperature increased from 80 °C
to 140 °C, and simultaneously, the selectivity decreased from 97% to 85%
due to secondary reactions and product degradation (Fig. 1). High
gluconic acid yields were obtained both at 100 °C and 120 °C; 62% and
72%, respectively. At 140 °C, only a small increase in yield (76%) was
Scheme 1. Base-free oxidation of glucose with Au NP catalysts.

S. Rautiainen et al. / Catalysis Communications 74 (2016) 115–118
117
Table 2
Microwave-assisted base-free oxidation of glucose to gluconic acid with Au catalysts.a
Entry
Catalyst
Au loading (wt.%)
d (nm)
Dispersion
Glc:Au
Conv. (%)
Select. (%)
TOFb (h−1
)
1
2
Au/MgAl2O4
Au/Al2O3
2.3 0.12
1.8 0.15
3.8 1.0c
2.4 0.6
0.27
0.43
870
1110
54
83
93
87
10,400
12,900
a
Reaction conditions: 6 mg catalyst, 110 mg glucose, 120 °C, 10 min, 2.2 equiv. H2O2.
TOF value calculated from conversion: TOF = nGlc ∗ conv. / (nAu ∗ DAu ∗ t).
Also large (N100 nm) particle detected with SEM (see SI).
b
c
observed, so the subsequent experiments were carried out at lower
temperatures to ensure high selectivity. Based on GC/MS analyses, the
main byproducts were keto-gluconic acids and 4–5 carbon atom aldonic
and aldaric acids with small amounts of glucuronic acid. HPLC analyses
showed traces of formic acid. Even though carbohydrate reactions are
usually carried out at low temperatures (20–60 °C) to prevent side reac-
tions and isomerization, the selectivities obtained herein at high tem-
peratures are similar to those previously reported under acidic
conditions at considerably lower temperatures [15]. The radically short-
ened reaction time likely prevents further side reactions.
The role of the heating method was investigated by comparing mi-
crowave heating with conventional oil bath under otherwise similar
conditions. Conversions obtained by microwave heating were substan-
tially higher than on oil bath (Fig. S5); at 120 °C only 62% conversion
was obtained on oil bath in contrast to 83% with MW. Selectivity was
similar with both methods. Although the reasons for the enhanced reac-
tions with MW are still unclear, aqueous glucose solution is a strong mi-
crowave absorber [33] and consequently, the reaction mixture is heated
rapidly and uniformly under MW irradiation. On the other hand, alu-
mina has low microwave absorbing capacity [19], which prevents
superheating of the catalyst ensuring high selectivity.
Recycling of Au/MgAl2O4 resulted in considerable loss of activity and
selectivity compared to the fresh catalyst; second run at 120 °C pro-
duced gluconic acid with 86% selectivity at 36% conversion (cf. Table 2,
entry 1). Remarkably, the Au/Al2O3 catalyst maintained both activity
and selectivity upon recycling; 86% conversion and 84% selectivity
were obtained in second run at 120 °C (cf. Table 2, entry 2). Further-
more, no deactivation of Au/Al2O3 was detected in four consecutive
runs at 100 °C (Fig. 2).
Possible causes for catalyst deactivation are particle sintering, metal
leaching and poisoning or adsorption of organic species on the catalyst
surface. In case of Au/MgAl2O4, the mean particle size increased to
4.4 nm after one run. In addition, aggregation of Au NPs was detected
with TEM (Fig. S11), indicating that the deactivation of Au/MgAl2O4 is
caused mainly by Au particle sintering. TEM analysis of the recycled
Au/Al2O3 showed an increase in mean particle size from 2.4 nm to
3.6 nm after two runs (Fig. S10) and this is in accordance with previous
observation on Au NPs sintering under acidic conditions [15]. Despite
the observed particle growth the oxidation activity of the catalyst
remained unchanged. Within experimental error of AAS, gold content
of both recycled catalysts was similar to the fresh ones' and no gold
was detected in the reaction solutions, indicating that leaching does
not occur. Organic species such as reactants or products can adsorb on
the catalyst surface and decrease catalytic activity. Weakly adsorbed
species could be removed by washing [17], but more strongly adsorbed
molecules require calcination of the catalyst [15]. However, no indica-
tion of adsorbed species was detected in thermogravimetric analysis
of our recycled catalysts. As demonstrated here, the high activity and re-
usability make Au/Al2O3 a very attractive catalyst for base-free glucose
oxidation.
Finally, we studied decomposition of hydrogen peroxide to dioxygen
with the catalysts and the supports, as we hypothesized that the oxida-
tion activity would be linked to the rate of H2O2 decomposition. Both
supports produced oxygen at similar, low rates (Table S2). Comparison
of the supports with the corresponding catalysts showed that gold is
highly active in decomposing H2O2; initial rate of oxygen generation
with both catalysts was multiple times higher compared to the sup-
ports. Of the two catalysts, Au/Al2O3 was drastically more active and
maintained the very high rate throughout the measurement, whereas
the initial rate with Au/MgAl2O4 decreased significantly during the ex-
periment (Fig. S9). These results correspond well with the oxidation ac-
tivity and stability of the studied catalysts.
4. Conclusions
In summary, we have shown that microwave heating is very effi-
cient in promoting base-free oxidation of glucose to gluconic acid with
gold nanoparticle catalysts. Using hydrogen peroxide as oxidant,
gluconic acid yield reached 76% in only 10 min reaction time. While
temperature had a significant effect on both conversion and selectivity,
over 87% selectivities were obtained even at 120 °C. Comparison with
conventional heating under similar conditions confirmed the efficiency
of microwave heating. Moreover, the oxidation activity of the studied
catalysts was clearly associated with their ability to decompose hydro-
gen peroxide to dioxygen. Recycling experiments and catalyst charac-
terization were carried out to study the stability of the catalysts. The
performance of Au/Al2O3 remained constant during four consecutive
runs, even though small increase in Au particle size was observed.
Fig. 1. Effect of temperature in microwave-assisted oxidation of glucose with 2.2 equiv. of
Fig. 2. Catalyst recycling in microwave-assisted oxidation of glucose with 2.2 equiv. of
H2O2 and 0.09 mol% Au/Al2O3 in 10 min.
H2O2 and 0.09 mol% Au/Al2O3 in 10 min at 100 °C.

118
S. Rautiainen et al. / Catalysis Communications 74 (2016) 115–118
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This work was funded by Tekes — the Finnish Funding Agency for
Technology and Innovation (project no. 40224/11) and University of
Helsinki Doctoral Programme in Chemistry and Molecular Sciences
(CHEMS). The authors would like to thank Electron Microscopy Unit
of the Institute of Biotechnology, University of Helsinki for providing
laboratory facilities.
Appendix A. Supplementary data
Detailed experimental procedures, catalyst characterization and
product analysis are available in the Supporting information. Supple-
mentary data associated with this article can be found in the online ver-
sion, at http://dx.doi.org/10.1016/j.catcom.2015.11.014.
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