Selective formation of formic acid from biomass-derived glycolaldehyde with supported ruthenium hydroxide catalysts

Article abstract of DOI:10.1039/c9cy00271e

Ceria-supported ruthenium hydroxide catalysts, Ru(OH)_x/CeO₂, comprising micro- A nd nanoparticle supports were prepared, characterized and examined for the catalytic aerobic oxidation of glycolaldehyde (GAD) to formic acid (FA) in water under mild and base-free conditions. Under optimized reaction conditions, both of the examined catalyst types gave a high FA yield of around 80% at full GAD conversion with low CO₂ formation (>95% carbon balance), while intermediate oxidation products were obtained under milder conditions and humins dominated under harsher conditions. The nanoparticle catalysts were more active than analogous catalysts based on microparticle ceria, which had a lower BET surface area and lower content of surface Ru(OH)_x species relative to other Ru species. Notably, the superior nanoparticle-based catalyst also proved to be reusable in three consecutive reaction runs with essentially unchanged catalytic performance. This study demonstrates a benign and green protocol to convert GAD-a versatile platform molecule obtainable from biomass-derived carbohydrates-into the large-volume commodity chemical FA which has multiple industrial applications.

Full text of DOI:10.1039/c9cy00271e

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Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
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Page 1 of 13
Catalysis Science & Technology
Selective Formation of Formic Acid from Biomass-derived
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DOI: 10.1039/C9CY00271E
Glycolaldehyde with Supported Ruthenium Hydroxide Catalysts
A. Modvig and A. Riisager*
Centre for Catalysis and Sustainable Chemistry, Department of Chemistry, Technical University
of Denmark, DK-2800, Kgs. Lyngby, Denmark
Corresponding author: ar@kemi.dtu.dk, +45 45252233
Abstract
Ceria-supported ruthenium hydroxide catalysts, Ru(OH)x/CeO2, comprising micro- and
nanoparticle supports were prepared, characterized and examined for the catalytic aerobic
oxidation of glycolaldehyde (GAD) to formic acid (FA) in water under mild and base-free
conditions. Under optimized reaction conditions, both of the examined catalyst types gave
high FA yield around 80% at full GAD conversion with low CO2 formation (>95% carbon
balance), while intermediate oxidation products were obtained at milder conditions and
humins dominated at more harsh conditions. The nanoparticle catalysts were more active than
analogous catalysts based on microparticle ceria, which had lower BET surface area and lower
content of surface Ru(OH)x species relatively to other Ru-species. Notably, the superior
nanoparticle-based catalyst also proved reusable in three consecutive reaction runs with
essentially unchanged catalytic performance. The study demonstrate a benign and green
protocol to convert GAD - a versatile platform molecule obtainable from biomass-derived
carbohydrates – into the large-volume commodity chemical FA which has multiple industrial
applications.
Keywords: Formic acid; glycolaldehyde; oxidation; ruthenium hydroxide; heterogeneous
catalysis
1

Catalysis Science & Technology
Page 2 of 13
1. Introduction
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Increasing environmental concerns and more strict legislation require development of
DOI: 10.1039/C9CY00271E
sustainable chemical processes relying on alternatives to fossil feedstock. In this context, the
high carbon-content of lignocellulosic biomass comprised in polymeric carbohydrates and
lignin (44-53%) render forestry, agro- and agricultural biomass promising renewable sources
for production of carbon-containing bulk and fine chemicals.1,2
Thermal degradation of biomass-derived carbohydrates result in formation of the smallest -
hydroxy sugar, glycolaldehyde (GAD), as a by-product. Alternatively, GAD can be produced in
higher yields from glucose in critical water and from bio-oils.3-5 GAD is reactive and can be
transformed into several other chemicals with important applications, including diamines or
ethanolamines by direct amination (polymers and pharmaceuticals),6 C6 and high-value C4
sugars by aldol condensation (food and pharmaceuticals),7,8 lactic acid and ethylene glycol by
hydrothermal treatment and hydrogenation (polymer building blocks)9-11 and glyoxal/glycolic
acid/glyoxylic acid by oxidation.12-14 This large reaction diversity makes GAD a versatile
biomass-derived platform molecule with potential to be explored also as starting material for
other attractive commodity chemicals like formic acid (FA).
FA is globally produced on an annual scale of 0.6 Mt and used in several industries and
products, including leather, agricultural, pharmaceutical, chemical, textile, rubber and
catalysis.15-17 More recently, FA has also been found feasible for hydrogen storage due to its
fast and selective release of H2 and CO2 under mild conditions.18,19 Currently, FA is
predominantly produced in industry by a two-step gas-phase process involving formation of
methyl formate from methanol and CO followed by hydrolysis of methyl formate, affording FA
and methanol which is recycled.16 However, a liquid-phase oxidation process using water as
solvent where carbohydrates and other forms of biomass affords FA in high yields has recently
been commercialized using a vanadium-substituted heteropolyacid catalyst.20,21 The catalyst
system is homogeneous, but easy catalyst separation and reuse would be obtained by use of a
heterogeneous catalyst.
Heterogeneous ruthenium-based catalysts have been widely applied for oxidations,22-30
hydrogenations31-34 amination,31 C-C bond formation,35,36 etc. Additionally, catalytic upgrading
of biomass and derived compounds have been reported over various supported ruthenium
oxide catalysts.16,36-42 The many applications of ruthenium in catalysis has rendered it a very
well-studied metal in both homogeneous and heterogeneous systems.43-54 More recently,
Mizuno and Yamaguchi reported high activity of supported ruthenium hydroxide catalysts
(Ru(OH)x/support) in liquid-phase oxidations of alcohols, aldehydes and amines.55-59 The
catalysts were easily prepared by deposition-precipitation and dried at low temperature
without further activation by calcination or reduction, affording highly dispersed ruthenium
hydroxide species on the support surface.59 The ruthenium hydroxide species provide a Lewis
acid site on the ruthenium center and a Brønsted basic site of the hydroxide group within the
same catalyst. Oxidation of ethanol to acetic acid has previously been reported by our group
over a microparticulate ceria-supported ruthenium hydroxide catalyst, where the ceria support
was selected due to its oxygen transfer properties.40
In this work, catalysts based on ruthenium hydroxide supported on nano- and microparticulate
ceria (i.e. Ru(OH)x/CeO2) were studied for the oxidative conversion of GAD into FA under mild
conditions with oxygen as oxidant in a base-free aqueous media (Scheme 1). Water was
selected as solvent since biomass-derived GAD typically is produced by aqueous-phase
reforming, and because aerobic oxidation is safe to perform in aqueous media, which is not
the case for many organic solvents. The formation of FA with supported Ru(OH)x is proposed to
2

Page 3 of 13
Catalysis Science & Technology
proceed via a mechanism involving oxidative dehydrogenation of the hydrated aldehyde
View Article Online
(geminal diol) of GAD, which is dominant in aqueous solution, with the ceria support providing
stabilization for intermediate surface Ru-species. The introduced catalyticDOI: 10.1039/C9CY00271E
process
demonstrate a benign and green route to produce the versatile commodity chemical FA from a
feedstock derived from biomass.
Ru(OH)x/CeO2
O
Ru1
Ru2
: CeO2 nanoparticles
: CeO2 microparticles
HO
O
H
OH
O2, H2O
GAD
FA
Scheme 1: Aerobic oxidation of glycolaldehyde (GAD) to formic acid (FA) with ruthenium hydroxide supported on
ceria nano- or microparticles.
2. Experimental
2.1 Materials
Ruthenium(III) chloride (purum, ̴41% Ru, Sigma-Aldrich), cerium(IV) oxide (<50 nm “nano”,
≥99.9%, Sigma-Aldrich), cerium(IV) oxide (<5 μm “micro”, ≥99.9%, Sigma-Aldrich), sodium
hydroxide (≥98%, Sigma-Aldrich), glycolaldehyde dimer (GAD, Sigma-Aldrich), formic acid (FA,
≥98%, Sigma-Aldrich), oxygen (99.5%, Air Liquide Denmark), pressurized air (Air Liquide
Denmark). All reagents were used without further purification.
2.2 Catalyst Preparation
Supported ruthenium hydroxide catalysts based on ceria nanoparticles (Ru1) and
microparticles (Ru2) were prepared by deposition-precipitation according to published
procedures.40,54,56,58,62,63 RuCl3 (246.8 mg, 1.2 mmol) was dissolved in MilliQ water (143 mL)
affording a solution with a ruthenium concentration 8.3 mM. Nano- or microparticular CeO2
support (9.76 g) was added to the solution which was left stirring for 15 min before the pH was
adjusted to 13.5 by addition of 1M NaOH ( ̴28 mL) and the mixture left stirring for additionally
18 h. Then, the catalyst was removed by filtration, washed thoroughly with MilliQ water until
neutral pH of the filtrate was obtained and finally oven dried at 140 oC for 40 h.
2.3 Catalyst Characterization
Surface areas were determined from nitrogen physisorption at liquid nitrogen temperature on
a Micrometrics ASAP 2020 apparatus. Samples were outgassed in vacuum at 150 °C for 6 h
prior to measurements. The total surface areas were calculated according to the
Brunauer−Emmett−Teller (BET) method. Transmission electron microscopy (TEM) images were
recorded on a FEI Tecnai Transmission Electron Microscope operated at 200 kV. Samples were
dispersed on a lacy amorphous carbon support film prior to the imaging. Energy dispersive X-
ray spectroscopy (EDX) analysis were performed on an adjacent Oxford INCA system, coupled
with a scanning electron microscope (SEM) from Quanta 200 ESEM FEG operated at 10 kV and
spot size of 2.0. The ruthenium content was determined from the average of 4-7
measurements on selected areas of the analyzed samples. X-ray powder diffraction (XRPD)
patterns were recorded using a Huber G670 powder diffractometer (Cu−Kα radiation, λ =
1.54056 nm) in the 2θ interval 3-100°. X-ray photoelectron spectroscopy (XPS) spectra were
obtained using a ThermoScientific system at room temperature with a monochromatic Al Kα X-
ray source (1484.6 eV). Survey spectra were acquired by accumulating four sweeps in the
range 0-1400 eV at a pass energy of 200 eV. Ru3p scan spectra were acquired by accumulating
40 sweeps in the range 450-510 eV at a pass energy of 50 eV. Spectral processing was carried
out using Avantage 4.87 software using a Powell fitting algorithm and a Smart background
3

Catalysis Science & Technology
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subtraction for deconvolutions. Sample charging effects were eliminated by correcting the
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DOI: 10.1039/C9CY00271E
observed spectra with the Ce3d3/2 binding energy value of 900.00 eV.
2.4 Catalytic Oxidation of Glycolaldehyde
All oxidation reactions were performed in customized mini high-pressure reactors (V = 18.4
mL). The reactors were charged with GAD (5.9 mmol), Ru(OH)x/CeO2 (114.8 mg, corresponding
to 0.23 mol% Ru) and MilliQ water (4.62 g). The reactors were then flushed and pressurized
o
with oxygen gas (1-30 bar), heated to the desired temperature (23-150 C) while constantly
stirring at 500 rpm for 1-70 h. After the reaction, the high-pressure vessels were rapidly cooled
to room temperature (23 oC) in an ice-bath, and the gas-phase collected for CO2 analysis by IR
(Quantek NDIR detector equipped with a Hammond Drierite drying tube) and the liquid-phase
for product analysis by HPLC (Agilent Technologies 1200 equipped with an Aminex HPX-87H
column from Bio-Rad, 300 mm x 7.8 mm x 9 μm, solvent: 5 mM H2SO4, 24 oC with a flow rate of
0.3 mL/min).
3. Results and Discussion
3.1 Catalyst Characterization
The BET surface areas of the prepared Ru1 and Ru2 catalysts varied by approximately a ten-
fold as a result of the different support particles sizes, and were both comparable to the
pristine nano- and microparticulate ceria materials, respectively (Table 1). The metal content in
the catalysts was determined from XRF and EDX analyses (Supporting Information, Figures S1-
Entry Catalyst
CeO2
support
Nano
BET Surface Area (m2/g)
Ru Content (wt%)a
CeO2
42
Catalyst
XRF
EDX
1
2
Ru1
Ru2
39
3
1.4 (0.6)
1.0 (0.1)
1.3 (0.4)
1.1 (0.3)
Micro
3
S2 and Table S1), affording ruthenium contents between 1.0-1.4 wt% in very good agreement
with the expected value (1.2 wt%) from the catalyst syntheses.
Table 1: BET surface areas and ruthenium content of the prepared catalysts.
a Standard deviation in parentheses.
Ruthenium metal or ruthenium oxide nanoparticles were not observed on recorded TEM
images of the catalysts (Figure 1), which is in line with previous observations for
Ru(OH)x/support catalysts.60 However, small particles surrounding the ceria support was
observed, forming a veil-like constitution around the ceria particles (Figure 1a and 1c). The
veil-like substance was most eminent in the images of Ru1, based on ceria nanoparticles.
The XRD diffraction pattern of Ru2 was found to be identical to the pattern recorded for the
microparticulate ceria (Supporting Information, Figures S3-S4), revealing no crystallinity of the
Ru(OH)x phase in accordance with literature.40 In contrast, difference in the diffraction patterns
for Ru1 and the nanoparticulate ceria was observed as additional peaks appeared in the 2θ
region 5-45° for Ru1. Notably, the additional peaks were apparently not associated to
crystalline ruthenium or ceria species, and the lower 2θ region indicated a (sub)crystalline
substance with a larger unit cell compared to that of ceria. Moreover, the peak width was
comparable with that of the nanoparticulate ceria suggesting similar particle size of the
unidentified crystalline specie (possibly an impurity). The identity of the crystalline specie
remains presently unknown, but it could possibly be associated to the veil-like substance
observed in TEM.
4

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Catalysis Science & Technology
View Article Online
DOI: 10.1039/C9CY00271E
Figure 1: TEM images of Ru1 (a and b) and Ru2 (c and d) catalysts.
Spectrum
RuO3
Ru1 (fresh)
Ru3p3/2
Ru1 (calcined)
Ru3p3/2
Ru(OH)x
RuO2
Ru0
Background
Sum
472
470
468
466
464
462
460
458
472
470
468
466
464
462
460
458
Binding Energy (eV)
Binding Energy (eV)
Ru2 (fresh)
Ru2 (calcined)
Ru3p3/2
Ru3p3/2
472
470
468
466
464
462
460
458
472
470
468
466
464
462
460
458
Binding Energy (eV)
Binding Energy (eV)
Figure 2: XPS Ru3p scan of Ru1 (top) and Ru2 (bottom) with freshly prepared catalysts (left) and calcined catalysts
(350 oC) (right).
5

Catalysis Science & Technology
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From XPS analysis, cerium, oxygen and ruthenium were detected in the survey spectra of Ru1
View Article Online
and Ru2 (Supporting Information, Figure S5). Deconvolution of the Ru3p3/2 area afforded four
different Ru-containing species typically assigned64 to RuVIO3, hydrous RuIVO2 (i.e. DROuI:(1O0.H10)3x9),/C9CY00271E
RuIVO2 and Ru0, which also previously have been identified in supported Ru(OH)x catalysts.65
However, more recent and detailed analysis suggests that the peaks corresponding to the +6
oxidation state may be a satellite peak of hydrated/anhydrous RuIVO2 and Ru(OH)X may be a
Ru(OH)3 phase with trivalent ruthenium.66 The contribution from the Ru-species was compared
based on the calculated area under the curves and presented in Figure 3. For the freshly
prepared Ru1 and Ru2 catalysts, the predominant Ru-component was found to be Ru(OH)x (71
and 58%) which decreased to 35 and 22% after calcination, respectively. Instead, the area of
RuO2 increased indicating formation of RuO2 on the surface induced by calcination, as
expected.
100
Ru1 (fresh)
Ru1 (calcined)
90
Ru2 (fresh)
Ru2 (calcined)
80
70
60
50
40
30
20
10
0
Ru0
RuO3
Ru(OH)x
RuO2
Ru-specie
Figure 3: XPS convoluted area distributions of the Ru3p3/2 area (455-473 eV) of freshly prepared and calcined Ru1
and Ru2 catalysts.
3.2 Oxidation of Glycolaldehyde
Effect of temperature, pressure and reaction time
Aerobic oxidation of GAD was performed with the prepared Ru(OH)x/CeO2 catalysts Ru1 and
Ru2 in base-free, aqueous medium at mild conditions. All reactions were analyzed for GAD, FA,
CO2, acetic acid (AA), glycolic acid (GA), glyoxylic acid, glyoxal (GLO), oxalic acid (OA),
formaldehyde, lactic acid, various tetroses and hexoses, however, only the major products FA,
GA, GLO, AA and CO2 (>2% yield) were quantified (Figure 4).
O
O
O
O
O
HO
C
O
OH
OH
H
OH
O
Glyoxal
GLO
Glycolic Acid
GA
Acetic Acid
AA
Formic Acid
FA (Ru1)
CO2
CO2
FA (Ru2)
Figure 4: Major products of the oxidation of GAD to FA.
Initially, the effect of reaction temperature was studied (Figure 5). The GAD conversion and FA
product selectivity increased with increasing temperature until reaching full conversion and
o
maximum selectivity at 80 and 90 C using the Ru1 (88.8% selectivity) Ru2 catalyst (81.7%
selectivity), respectively. At these optimal temperatures (and below), an excellent carbon
balance >95% was obtained for all reactions, whereas higher temperatures led to gradually
decreased FA selectivity due to formation of humins and CO2 resulting in lower carbon
balance.
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Catalysis Science & Technology
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DOI: 10.1039/C9CY00271E
Figure 5: Effect of reaction temperature on the oxidation of GAD with Ru1 (left) and Ru2 (right) catalysts. Reaction
conditions: GAD (356.1 mg, 5.9 mmol), Ru(OH)x/CeO2 (114.8 mg, 0.23 mol% Ru from 1.2 wt% Ru(OH)x/CeO2), H2O
(4.6 mL), O2 (30 bar), 16 h, 500 rpm.
o
Next, the GAD oxidation was examined at various pressures of oxygen at 90 C (Figure 6).
Several condensation products and low FA selectivity was observed using 1 bar of O2 for both
of the Ru catalysts. Likewise, applying 10 bar of air (i.e. PO2  2 bar) also resulted in low FA
selectivity and yield, possibly due to limited availability of oxygen in the aqueous phase under
such moderate oxygen pressure due to low gas solubility. In line with this, significantly higher
FA selectivity was obtained at higher partial pressure of oxygen reaching a maximum at 10 bar
for both catalysts.
Figure 6: Effect of oxygen pressure on the oxidation of GAD with Ru1 (left) and Ru2 (right) catalysts. Reaction
conditions: GAD (356.1 mg, 5.9 mmol), Ru(OH)x/CeO2 (114.8 mg, 0.23 mol% Ru from 1.2 wt% Ru(OH)x/CeO2), H2O
(4.6 mL), 90 oC, 16 h, 500 rpm.
Study of the reaction time showed that essentially full conversion (99.7%) was obtained after 8
h of reaction with the Ru1 catalyst, affording 80.5% yield of FA (Figure 7). Increasing the
reaction time did not affect the FA yield (16-70 h), revealing either high product stability over
time at the applied reaction conditions or gradual FA formation from the by-products GLO and
GA which were almost fully converted over time. Changing the catalyst to the
microparticulate-based Ru2 afforded close to full conversion only after 16 h (99.5%) and 81.9%
FA selectivity, showing that the Ru2 catalyst was less active than Ru1 in agreement with the
trend observed in the temperature study above. Moreover, with Ru2 the FA yield slightly
decreased over time implying decreased product stability as the GLO and GA by-products
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Catalysis Science & Technology
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remained largely unconverted. Hence, possibly also in the reaction with Ru1 was FA not
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DOI: 10.1039/C9CY00271E
entirely stable at prolonged reaction time.
Figure 7: Effect of reaction time on the oxidation of GAD with Ru1 (left) and Ru2 (right) catalysts. Reaction
conditions: GAD (356.1 mg, 5.9 mmol), Ru(OH)x/CeO2 (114.8 mg, 0.23 mol% Ru from 1.2 wt% Ru(OH)x/CeO2), H2O
(4.6 mL), O2 (10 bar), 90 oC, 500 rpm.
Effect of substrate concentration and catalyst loading
Since the Ru1 catalyst was found to be the most active of the prepared catalysts its catalytic
performance was further studied at varied GAD concentrations and catalyst loadings (Figure
8). Relatively low GAD concentrations affected the catalytic activity negatively resulting in
decreased GAD conversions and formation of trace amounts of various unidentified
condensation products (Figure 8, left). Likewise, formation of humins was prominent at
relatively high GAD concentrations, indicating that 1.3 M was the optimal GAD concentration
under the applied conditions (corresponding to the GAD concentration applied in the above-
described results). When the Ru1 catalyst amount was increased an increased catalytic activity
and FA yield was obtained (as expected), until a maximum was reached in Ru/GAD ratio
corresponding to 0.23 mol% Ru (Figure 8, right). At higher catalyst loadings, the FA yield
decreased due to humins formation.
Figure 8: Effect of the concentration of GAD (left) and Ru/GAD ratio (right) on the oxidation of GAD with Ru1
catalyst. Reaction conditions: GAD (356.1 mg, 5.9 mmol), Ru(OH)x/CeO2 (114.8 mg, 0.23 mol% Ru from 1.2 wt%
Ru(OH)x/CeO2), H2O (4.6 mL), O2 (10 bar), 90 oC, 8 h, 500 rpm.
8

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Catalysis Science & Technology
Varying the Ru loading to either 0.6 or 2.4 wt% in the nanoparticle-based ceria catalysts
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DOI: 10.1039/C9CY00271E
yielded the catalysts Ru3 and Ru4, respectively (see Supporting Information, Figure S1 and
Table S1 for XRF and EDX analysis). Application of these catalysts resulted in lower selectivity
to FA compared to the Ru1 catalyst with 1.2 wt% Ru and formation of humins was pronounced
with the low metal loading (Table 2 entries 3-5). Notably, a decreased catalytic activity was
observed with the high Ru loading of 2.4 wt%, indicating formation of less active Ru species on
the ceria surface. Absence of catalyst or use of ceria nanoparticles alone afforded only low
GAD conversions (3.6-7.2%) and corresponding low FA yields (entries 1 and 2), as expected.
Moreover, pre-calcination of the Ru1 catalyst led to a significant decrease in the FA selectivity
to 56.8% (entry 6), mostly likely due to transformation of active Ru(OH)x species on the surface
to less active or inactive RuO2 as also observed by the XPS analysis (Figure 3).
Entry Ru loading GAD Conversion FA Selectivity
Product Yield (%)
(wt%)
(%)
3.6
7.2
98.9
99.7
85.0
99.2
98.0
95.6
(%)
37.6
9.7
49.3
80.8
59.2
56.8
75.6
81.6
GLO
0.5
1.5
12.6
6.2
10.9
12.5
5.2
GA
0.0
0.2
7.1
4.2
3.1
4.3
3.9
5.2
AA
0.0
0.1
FA
1.4
0.7
CO2
0.0
0.0
b
1
2
3
4
5
6
7
8
-
-
c
0.6
1.2
0.3 48.8 2.7
0.3 80.5 2.6
0.2 50.3 1.4
0.1 56.3 1.6
0.3 74.1 1.7
0.2 78.0 2.0
2.4
1.2d
1.2e
1.2f
10.0
Table 2. Effect of ruthenium loading and reuse of nanoparticle-based ceria catalysts for the oxidation of
GADa
a
Reaction conditions: GAD (356.1 mg, 5.9 mmol), Ru(OH)x/CeO2 (0.23 mol% Ru), H2O (4.6 mL), O2 (10
bar), 90 oC, 8 h, 500 rpm. b Without added catalyst. c Nanoparticle ceria alone.d Pre-calcined Ru1 catalyst
(350 °C, 6 h, static air). e First recycling of Ru1 catalyst. f Second recycling of Ru1 catalyst.
Catalyst reuse
Reusability of the Ru1 catalyst was assessed by performing consecutive reactions runs, where
the catalyst between each run was isolated by filtration, thoroughly washed with water
o
(MilliQ) until neutral pH of the filtrate followed by drying (140 C, 40 h). The catalyst could
successfully be recycled twice without any significant decrease in FA yield, which remained
around 80% (Table 2 entries 4, 7 and 8). However, a minor decrease in GAD conversion
occurred during the reuse suggesting a gradual decrease in catalyst activity, indicating that
trace amounts of Ru species leached during recycling due to insufficient immobilization to the
support. In line with this, elemental analysis by ICP on the reaction solution after the third
catalyst recycle revealed presence of Ru, but reliable quantification was not possible due to
concentration measurements close to the detection limit.
Reaction mechanism for GAD oxidation
Aerobic oxidation with supported Ru(OH)x catalysts in water has been described to occur by a
reaction mechanism involving oxidative dehydrogenation.60 An analogous mechanism can also
be elucidated for the formation of GLO, GA and OA via the hydrated form of GAD (i.e. germinal
diol) which dominates in aqueous solution,61 whereas formation of FA (and AA) involving C-C
cleavage is not directly deduced by this mechanism. Instead, a plausible mechanism for the
oxidation of GAD to FA could involve condensation of the germinal diol of GAD with surface
Ru(OH)x species (I) affording the bridged Ru specie (II), which by oxidative cleavage releases
two molecules of FA (Figure 9). The latter reaction step leaves two partially charged Ru species
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(III), possibly stabilized by the ceria surface. Finally, hydration of the partially charged Ru
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species regenerates Ru(OH)x and a Ru-hydride specie (IV), which is re-oxidized by the oxygen
to regenerate the Ru(OH)x species (I) via a peroxide intermediate, as also previouslyDpOrIo: 1p0o.1s0e39d/C9CY00271E
with supported Ru(OH)x catalysts.63
HO
O
OH
O
2 H2O
(II)
Run+
Run+
O
OH
OH
H2O
2x
HO
HO
GAD
H
OH
O
OH
½ O2
FA
Run+
Run+
Run+
HO
HO
H
(I)
O
CeO2
H
Run+
(III)
½ O2
Run+
Run+
Run+
Run+
HOO
OH
HO
H
(V)
(IV)
O2
Figure 9: Proposed reaction mechanism for the aerobic oxidation of GAD to FA in water with Ru(OH)x/CeO2.
Hydrothermal treatment of GAD under basic conditions can form FA and AA by reaction
pathways comprising condensation intermediates such as, e.g. glyceraldehyde,
dihydroxyacetone, formaldehyde, lactic acid, threose, lerythrulose, erythrose and acrylic acid
(for details see supporting information Figure S1).9 In this study, the GAD oxidation reactions
were performed without added base and none of the mentioned products were detected by
analysis of the liquid reaction mixtures, indicating that such condensations were not
dominating. However, since the Ru(OH)x/CeO2 catalyst contains both Lewis acidic- and
Brønsted basic sites (as previously mentioned) some FA formation by aldol condensations and
oxidation of formaldehyde via the Cannizzaro reaction cannot be excluded.
4. Conclusions
Catalysts based on ruthenium hydroxide supported on ceria were applied for selective aerobic
oxidation of biomass-based GAD into FA in water under base-free conditions. Optimization of
catalyst and reaction parameters, i.e. reaction time, temperature, O2 pressure, GAD
concentration, support and catalyst metal loading, resulted in formation of up to 80.8% FA
yield at full GAD conversion at relative benign conditions of 10 bar O2 and 80 °C using a catalyst
based on nanoparticle ceria support. This catalyst could also be recycled in three consecutive
reaction runs with only minor change in catalytic performance. In comparison, lower activity
was generally obtained for catalysts based on ceria microparticles having lower surface area
and less surface Ru(OH)x species relatively to other Ru-species. Interestingly, all reactions
performed at 80-90 °C resulted only in minor formation of CO2 as over-oxidation product
making Ru(OH)x/ceria an useful catalyst system for valorizing biomass-based oxygenates by
aerobic oxidation.
Conflicts of interest
There are no conflicts to declare.
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Catalysis Science & Technology
Acknowledgements
View Article Online
This work was supported by the Bio-Value platform (biovalue.dk) under the SPIR initiative by
DOI: 10.1039/C9CY00271E
The Danish Council for Strategic Research and The Danish Council for Technology and
Innovation (case number 0603-00522B).
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