Aerobic oxidation of C4-C6 α,ω-diols to the diacids in base-free medium over zirconia-supported (bi)metallic catalysts

Article abstract of DOI:10.1039/c9nj01695c

Oxidation of aliphatic α,ω-diols is a potentially interesting route to the production of valuable α,ω-diacids or ω-hydroxy acids for various polymer synthesis. 1,4-Butanediol (BDO), 1,5-pentanediol (PDO) and 1,6-hexanediol (HDO) are particularly attractive since they may be obtained from lignocellulosic biomass. The aqueous aerobic oxidation of these diols to the corresponding diacids was investigated in water over a set of Au, Pt, Au-Pt and Au-Pd catalysts supported on zirconia at 70 °C or 90 °C under 40 bar air. The nature of the metallic catalyst influenced the distribution of products as oxidation proceeded. The longer the carbon chain linking the terminal alcohol groups, the higher the yield of the diacid. The best yields of succinic acid, glutaric acid and adipic acid reached 83, 84 and 96% from BDO, PDO and HDO, respectively, over Au-Pt/ZrO₂. There was some evidence of decarbonylation of the α,ω-hydroxyaldehyde at the early stage of the reaction. The presence of the hydroxyl substituent in 1,2,6-hexanetriol significantly slowed the oxidation rates compared with HDO. Besides, oxidation of PDO or HDO was highly selective to the ω-hydroxycarboxylate in moderate alkaline medium (NaOH/diol = 2) over Au/ZrO₂ (90-93%).

Full text of DOI:10.1039/c9nj01695c

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Diallo, A. Said, D. Da Silva Perez, C. Nikitine, L. Puchot, Y. Habibi, C. Pinel, N. Perret and M. Besson, New J.
Chem., 2019, DOI: 10.1039/C9NJ01695C.
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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

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Aerobic oxidation of C4-C6 α,ω-diols to the diacids in base-free medium over
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zirconia-supported (bi)metallic catalysts.
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Modibo Mounguengui-Diallo,a Achraf Said,a Denilson Da Silva Perez,b Clémence Nikitine,c
Laura Puchot,d Youssef Habibi,d Catherine Pinel,a Noémie Perret,a and Michèle Besson*a
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Univ Lyon, Univ Claude Bernard, CNRS, IRCELYON, UM5256, 2 Avenue Albert Einstein 69626
Villeurbanne (France)
LGPC, CNRS/CPE Lyon/UCBL-Université de Lyon, 43 Boulevard du 11 Novembre 1918, 69616
Villeurbanne (France)
Institut FCBA, InTechFibres, 38044 Grenoble (France)
Luxembourg Institute of Science and Technology (LIST), 5 Avenue des Hauts-Fourneaux, L-4362
Esch-sur-Alzette, (Luxembourg)
*E-mail : michele.besson@ircelyon.univ-lyon1.fr
Abstract
Oxidation of aliphatic α,ω-diols is a potentially interesting route to the production of valuable
α,ω-diacids or ω-hydroxy acids for various polymer synthesis. 1,4-Butanediol (BDO), 1,5-
pentanediol (PDO) and 1,6-hexanediol (HDO) are particularly attractive since they may be
obtained from lignocellulosic biomass. The aqueous aerobic oxidation of these diols to the
corresponding diacids was investigated in water over a set of Au, Pt, Au-Pt and Au-Pd catalysts
supported on zirconia at 70°C or 90°C under 40 bar air. The nature of the metallic catalyst
influenced the distribution of products as oxidation proceeded. The longer the carbon chain
linking the terminal alcohol groups, the higher the yield of diacid. The best yields of succinic
acid, glutaric acid and adipic acid reached 83, 84 and 96% from BDO, PDO and HDO,
respectively, over Au-Pt/ZrO2. There were some evidences of decarbonylation of the α,ω-
hydroxyaldehyde at the early stage of the reaction. The presence of hydroxyl substituent in
1,2,6-hexanetriol significantly slowered the oxidation rates compared with HDO. Besides,
oxidation of PDO or HDO was highly selective to the ω-hydroxycarboxylate in moderate
alkaline medium (NaOH/diol = 2) over Au/ZrO2 (90-93%).
Keywords: aerobic oxidation; α,ω-diols; diacid; Au, Pt, Pd-based catalysts
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1.
Introduction
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Terminal diols including 1,6 hexanediol (HDO), 1,5 pentanediol (PDO) and 1,4-
butanediol (BDO) are useful chemical intermediates that find applications in the polymer
industry for a variety of polyesters, elastic fibers, and polyurethanes.1,2 Oxidation of
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these α,ω−diols yields the corresponding ω-hydroxycarboxylic acids and/or diacids that
are also important monomers for polymer synthesis.3-5 For instance, adipic acid is used
in the production of nylon 66, and 6-hydroxyhexanoic acid is a precursor to caprolactam.
The α,ω-diols are currently produced by hydrogenation of the petroleum-derived
corresponding diacids or their esters.1,6 In the last decades, policy decisions have been
oriented toward sustainable strategies aiming to reduce fossil feedstock dependency.7-10
More specifically, in the recent years the production of these α,ω−diols has been
proposed from lignocellulose derivatives to provide a competitive interesting alternative
to the fossil routes (Scheme S1). Thus, BDO is obtained from sugars via fermentation
of glucose on a commercial scale,11 or by hydrogenolysis of erythritol produced through
the fermentation of glucose by some yeasts.12-14 Up to 90% BDO yield could be obtained
from 1,4-anhydroerythritol using a physical mixture of ReOx-Au/CeO2 and ReOx/C in
dioxane.15 Recent research on the production of C5 and C6 diols reported the synthesis
of PDO from furfural, and HDO from 5-hydroxymethylfurfural (HMF) or
levoglucosenone (LGO).1,16-29 For instance, a multistep catalytic conversion of 2-
hydroxytetrahydropyran (2-HO-THP) via sequential dehydration, hydration, ring-
opening tautomer rearrangement, and hydrogenation to PDO in 84% overall yield was
described;26-28 the same route was applied from tetrahydropyran-2-methanol (THP2M)
to HDO with a 34% overall yield (Scheme S1).29
The selective catalytic oxidation of these diols in aqueous solution using
molecular oxygen as the oxidant is then an important transformation for the manufacture
of biomass-derived ω-hydroxy or di-carboxylic acids. Despite the fact that mono- and
bi-metallic catalysts based on Pt, Pd or Au have received broad attention for oxidation
of monoalcohols in the presence or absence of a base, oxidation of diols has only
received limited attention.30-33 The product selectivity to the
ω-hydroxycarboxylic acid
or the diacid depends on the reaction conditions, the length of the carbon chain of the
diol, and the catalyst.34-44
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Thus, the
ω
-hydroxycarboxylate product could be obtained with selectivity oVfie9w0Ar-ticle Online
DOI: 10.1039/C9NJ01695C
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100% at high conversion over supported Au catalysts in the presence of a base in the
oxidation of 1,3-propanediol,34 or ethyleneglycol.35 In the presence of Au-Pt/H-
mordenite,36 or Au-Pd /hydrotalcite (HT),37 the selectivity to 3-hydroxypropionic acid
(3-HPA) was 85% at low conversion (< 40%) in water. Comparison of various α,ω-diols
over Pt/C showed that oxidation of ethyleneglycol was 100% selective to glycolic acid
at ca. 90% conversion under acidic conditions at 70°C under 10 bar O2. However, with
increasing carbon number in the substrate, increasing amounts of diacids were produced
in addition to the hydroxycarboxylic acids.38 Thus, under the same reaction conditions,
the selectivity to the diacid increased from 4% malonic acid, to ca. 30% succinic and
glutaric acids, and to ca. 62% adipic acid. Another method consisting in oxidation of
HDO using a N,N-dimethyldodecylamine N-oxide (DDAO) capped Au40Pd60 /HT and
30% aq. H2O2 provided a 81% yield of 6-hydroxyhexanoate in aqueous basic media at
80°C.5,39
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As concerns transformation to the diacids, oxidation of HDO at 70°C under 10
bar O2 yielded 97% of adipate in 1M NaOH aqueous solution over Au/C,40 while the
reaction over Pt/C in 0.35 M acetic acid produced 85% adipic acid.41,42 In base-free
media, Pt/C showed better activity in comparison to Au and Pd catalysts for BDO
oxidation.43 Au-Pt was still more performant, nevertheless, the highest yield of succinic
acid was 16%.
Our previous study focused on the oxidation of HDO (0.1M) in water over
different mono- and bimetallic catalysts under 40 bar of air (HDO/metal=100) reported
higher yields using ZrO2 support rather than activated carbon.44 Yield of adipic acid of
80% and 96% was obtained over Pt/ZrO2 and Au-Pt/ZrO2, respectively, at 70°C; up to
96% was also observed over Au-Pd/ZrO2 at higher temperature of 90°C; Au/Pd and
Au/Pt with a molar ratio close to 1 gave the highest yields. These high catalytic
efficiencies make these catalysts attractive materials to study further this type of
oxidation reaction.
In the present work, the oxidation reaction of HDO, PDO, and BDO was
compared over a set of ZrO2-supported Pt, Au-Pt, and Au-Pd catalysts.
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2. Experimental
2.1. Catalyst preparation and characterization
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The catalysts were prepared by wet impregnation (or co-wet impregnation in the case of
bimetallic catalysts) of ZrO2 support (MEL Chemicals, XZO 632/18, 139 m2 g-1,
mesoporous, average pore diameter 9 nm) followed by NaBH4 reduction as described
previously.44,45
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The actual metal loadings were obtained by Inductively Coupled Plama Optical
Emission Spectroscopy (ICP-OES) on an Activa Jobin-Yvon instrument. Prior to the
analysis, the materials were mineralized in a H2SO4-HNO3 mixture at 250-300°C, and
then in aqua regia at 150-200°C. The specific surface areas (SBET) were measured by N2
adsorption at -196°C using BET method on a Micromeritics ASAP 2020 apparatus, after
outgassing of the samples under vacuum at 250 °C for 3 h. The X-ray diffraction (XRD)
patterns were recorded on a D8 Advance A25 diffractometer (Bruker) using CuKα
source (λ = 1.54184 Å). The crystalline phases were identified by reference to the JCPDS
data files and the average crystallite size of the metallic nanoparticles was calculated
from line broadening according to Scherrer equation using EVA software. Transmission
Electron Microscopy (TEM) measurements were performed on a JEOL 2010 instrument
operated at an accelerating voltage of 200 kV. The samples were prepared by ultrasonic
dispersion in ethanol. A drop of dispersed catalyst was deposited onto a carbon-coated
copper grid and the solvent was evaporated in air at room temperature.
2.2. Catalytic reaction tests
The catalytic oxidation reactions were performed in a batch reactor (Hastelloy, 300 mL)
equipped with a magnetically driven gas inducing stirrer and heating jacket. Substrate
(0.1M; 1,6-hexanediol, Alfa Aesar, 97%; 1,5-pentanediol, Aldrich, 96%; 1,4-butanediol,
Aldrich, 99%; 1,2,6-hexanetriol, Alfa Aesar, 96%) in 150 mL of ultra-pure water,
eventually sodium hydroxide, and the appropriate amount of catalyst to get a
substrate/total metal molar ratio of 100 were introduced into the reactor. After sealing
and purging the reactor with Ar, the suspension was heated to the reaction temperature
of 70°C (or 90°C). Afterwards, the reactor was pressurized to 40 bar with air and stirred
at 1200 rpm, which initiated the reaction. Liquid samples were periodically collected
through a sample dip-tube connected to a valve, and the catalyst was filtered before
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analysis. After reaction, the autoclave was cooled down and the catalyst waDsOrI:e1c0.o10v3e9/rCe9dNJ01695C
by filtration.
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2.3. Product analysis
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After filtration through a 0.45 µm PVDF membrane and dilution by a factor of 20, liquid
samples were analyzed using a HPLC Shimadzu-10A equipped with Photodiode Array
(PDA) and differential refraction (RI) detectors in series. Product separation was carried
out with a Phenomenex Rezex ROA Acid H+ column thermostated at 65°C with 10 mM
H2SO4 in water flowing at 0.6 mL min-1. The retention times and the calibration curves
for 1,6-hexanediol, 6-hydroxyhexanoic acid, 1,5-pentanediol, glutaric acid, 1,4-
butanediol, succinic acid, 1,2,6-hexanetriol, 2-hydroxyadipic acid were determined by
injecting known concentrations of standards. Lactol (2-hydroxytetrahydrofuran, 2-
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hydroxytetrahydropyran,
butyrolactone, δ−valerolactone,
6890N on a VF WAX column. Other intermediate products (such as the C5 and C6
hydroxyaldehydes and -oxoacids, or 5-hydroxypentanoic acid) that are not
2-oxepanol)
and
lactone
products
(γ-
ε
-caprolactone) were analyzed using a GC Agilent
ω-
ω
commercially available were assigned according to (i) the order of appearance during
the reaction, and (ii) the retention order expected for the molecules on the ion-exclusion
column with strong cation exchange stationary phase, i.e. carboxylic acids with higher
pKa values show longer retention; they were assigned an arbitrary response factor.
The total organic carbon (TOC) in aqueous solution was analyzed using a TOC
analyzer equipped with an ASI-autosampler (Shimadzu TOC-VCHS). The measured TOC
was compared to the carbon balance calculated from HPLC to check the accuracy of the
HPLC analysis. These values were also compared to the initial TOC value in order to
check the possible adsorption on the catalytic materials or the degradation into gaseous
products.
3. Results and discussion
3.1. Characterization of catalysts
Table 1 reports the actual composition of the catalysts, the metal average crystallite sizes
calculated from the Scherrer equation, and the specific surface areas. The total metal
loading was between 3.6 and 6.4 wt%. The monometallic Pt/ZrO2 and Au/ZrO2 showed
the characteristic peaks of face-centered structure of Pt (JCPDS 04-001-0112) and Au
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(JCPDS 00-004-0784), respectively, with crystallite size of 7 nm and 11 nm,
respectively. The Au/Pd (0.77) and Au/Pt (0.84) molar ratio of the bimetallic catalysts
used in this study was slightly lower than unity. The XRD patterns of Au-Pt/ZrO2 and
Au-Pd/ZrO2 showed obvious diffraction peaks between those corresponding to the (111)
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planes of the (fcc) gold (2θ = 38.2°) and platinum (2θ = 39.8°) or palladium (2θ = 40.2°)
lattices, that were assigned to Au-Pt and Au-Pd alloy.43 Only single alloy structures were
observed and no monometallic phase of Au, Pd, or Pt was discernible. The composition
of the alloy determined by XRD according to the Vegard law matched relatively well the
actual bulk composition determined by ICP. The crystallite size of Au-Pt/ZrO2 (5 nm)
was smaller than the one of Au/ZrO2 (11 nm), as previously observed with this
preparation procedure.44,45 The SBET did not change significantly upon addition of the
metal (Table 1).
Table 1. Chemical composition and structural parameters of the ZrO2-supported Au, Pt,
Au-Pt and Au-Pd catalysts.
b
Catalyst
Au a
Pd a
Pt a
Total
metal
loading a
Bulk molar
ratio Au:Pd or
Au:Pt
dXRD
SBET
(wt%) (wt%) (wt%)
(nm) (m2.g-1)
(wt%)
ZrO2
-
-
-
-
-
139
Au/ZrO2
Pt/ZrO2
3.6
3.7
6.4
4.1
11
7
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129
128
128
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-
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3.6
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Au-Pt/ZrO2
Au-Pd/ZrO2
2.8
2.5
0.77
0.84
5 c
9 c
1.6
a Metal content measured by ICP-OES, b crystallite size from the Scherrer equation, c alloy.
Representative TEM images of the monometallic catalysts are shown in Figure 1
.
The nanoparticles of Pt or Au prepared with the same procedure as previously for
bimetallic Au-Pt and Au-Pd catalysts supported on TiO2 or ZrO2, i.e. NaBH4 reduction,
showed comparable morphology, i.e. individual and agglomerated wormlike
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nanoparticles with sizes between 5 to 15 nm, consistent with the values obtained by
XRD.44,45
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a
b
Fig. 1. Representative TEM images of (a) Au/ZrO2, (b) Pt/ZrO2.
3.2. Catalytic oxidation to the diacids in water
Base-free conditions that allow the direct formation of the carboxylic acids are preferable
to avoid the disadvantages arising from the production of salts. Reasonable performances
in the oxidation of alcohols have been reported for Pt catalysts in the absence of base in
water.36,46,47 Further, the combination of gold with Pt or Pd may also avoid the use of a
base, while high conversion and selectivity to the acid are obtained depending on the
nature of the support used.43,45,48-54 The resistance to overoxidation by oxygen was also
improved. On the contrary, monometallic Au catalysts are only significantly active at
this low temperature in strong alkaline medium.55,56 It has been suggested that the base
promotes H-abstraction of the alcohol to form an alkoxide, i.e. the first step of oxidative
dehydrogenation.30,57,58 Therefore, the oxidation reaction of the α,ω-diols (HDO, PDO,
BDO) was investigated in water, i.e. under unadjusted pH, in the presence of Pt/ZrO2,
Au-Pt/ZrO2 and Au-Pd/ZrO2 at 70°C under 40 bar air.
3.2.1. Concentration–time profiles over the different catalysts
The reactions were carried out using a diol/total metal molar ratio of 100 during 48 h.
Figure 2 shows the evolution of diol conversion, yields of products, carbon balance, and
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TOC measurement as a function of time for BDO, PDO, and HDO oxidatiDoOnI: 1o0v.10e3r9/tCh9NeJ01695C
Pt, Au-Pt and Au-Pd catalysts during the first period of 8 h.
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The TOC measurements and the carbon balance values (calculated from
chromatography measurements) were fairly close, though some of the intermediate
products were not available for calibration, demonstrating a relatively accurate analysis
of the different products in the aqueous phase. Comparison with the initial introduced
TOC value evidenced a more or less significant carbon loss in the liquid phase that was
rapidly observed from the beginning of the reaction. This may be attributed to adsorbed
compounds, C-C bond cleavage products formed by decarbonylation of the aldehyde,
decarboxylation of the product acids or diacids, or overoxidation.38,44 The fact that the
loss of carbon in the liquid phase occurs from the beginning rather suggests a possible
decarbonylation of the aldehyde intermediate products formed (vide infra). Indeed,
decarbonylation of aldehyde was demonstrated to occur over Pt catalysts during
oxidation of HDO and during isotope-enriched 13C-BDO, using surface-enhanced
Raman and solid-state 13C NMR spectroscopies.59 CO formed was subsequently further
oxidized to CO2 transferred to the gas phase. However, we could not detect possible
olefinic species produced by decarbonylation in the liquid phase by HPLC.
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0
70
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20
10
0
70
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10
0
70
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20
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0
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2
3
4
5
6
0
1
2
3
4
5
6
7
8
Time (h)
Time (h)
Time (h)
Fig. 2. Reaction profiles as a function of time for: (a-c) BDO, (a’-c’) PDO, and (a”-c”) HDO
oxidation reactions in water under base-free conditions over: (a-a”) Pt/ZrO2, (b-b”) Au-Pt/ZrO2,
and (c-c”) Au-Pd/ZrO2. Reaction conditions: 0.1 M diol, 70°C, 40 bar air, diol/metal molar ratio
= 100. () BDO, PDO, or HDO conversion, () ALD, ω-hydroxyaldehyde, () GBL, (▲)
HA, ω-hydroxyacid, () AA, ω-oxoacid, and () DA, α,ω-diacid, () carbon balance, and
() TOC measurement. Figures 3a”-c” are adapted from ref 43.
From the reaction profiles in Figures 2a-c, Figures 2a’-c’, and Figures 2a”-c” one
may observe that under the same reaction conditions BDO was globally converted faster
than PDO, and PDO faster than HDO. More precisely, over Pt and Au-Pt, conversion of
BDO was complete within 1 h (Figure 2a and Figure 2b), conversion of PDO was total after
2 h (Figure 2a’ and Figure 2b’), whereas significantly higher reaction times of 3 h and 6
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h, respectively, were required for HDO total conversion (Figure 2a” and Figure 2b”).
Similarly, over Au-Pd, conversions of BDO and PDO were of ca. 94% after 8 h (Figure 2c
and Figure 2c’), whereas conversion of HDO attained only 55% (Figure 2c”). The results
highlight that the structure of the linear diol, i.e. the length of the carbon chain linking
the two primary alcohols, is an important factor. On the other hand however, the present
results are the opposite of those reported in a previous study by Ide et al for the order of
reactivity over Pt/C in water; they measured similar TOF of 0.18-0.19 s-1 for PDO and
HDO oxidation, whereas the TOF decreased for BDO (0.15 s-1), 1,3-propanediol (0.047
s-1), and ethylene glycol (0.032 s-1) at 70°C under 10 bar O2.38 Conflicting results may
be obtained using the same metal but different supports and reaction conditions. Finally,
combining Au and Pt in a Au-Pt bimetallic catalyst did not enhance the conversion rates
of BDO, PDO, and HDO oxidation relative to the monometallic Pt/ZrO2 catalyst, while
Au-Pd/ZrO2 exhibited much lower activity. As previously mentioned, the morphology and
particle sizes of the Pt mono and Au-Pt and Au-Pd bimetallic materials were quite
similar. The difference in activity is then in agreement with previous studies focused on
glycerol oxidation under base-free conditions: a significantly higher activity was
obtained over Au-Pt than Au-Pd of same composition synthesized by immobilization of
colloidal nanoparticles on a MgO support.60
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The reaction of BDO, PDO and HDO proceeded through first the oxidation of
one of both alcohol functions to yield the
ω-hydroxyacid (HA) via the ω-
hydroxyaldehyde (ALD), and then oxidation of the second alcohol group to provide the
diacid (DA) via AA (Scheme 1).
O
O
n
OH
OH
OH OH
HO
OH
O
OH
OH
O
n
O
n
O
O
O
n
n
n
ALD
HA
AA
DA
n =
n =
n =
2 BDO
3 PDO
4 HDO
O
O
n
n
O
OH
lactone
lactol
Scheme 1. Reaction pathway associated with BDO, PDO, and HDO oxidation. ALD =
4-hydroxybutanal, 5-hydroxypentanal, or 6-hydroxyhexanal, HA = 4-hydroxybutanoic
acid, 5-hydroxypentanoic, or 6-hydroxyhexanoic acid, AA = 4-oxobutanoic acid, 5-
oxopentanoic, or 6-oxohexanoic acid, DA = succinic acid, glutaric, or adipic acid, lactol
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= 2-hydroxytetrahydrofuran, 2-hydroxytetrahydropyran, or 2-oxepanol, lactone = γ-
5
6
butyrolactone, δ−valerolactone, or ε-caprolactone
.
7
8
9
The profiles for product yields in oxidation of BDO, PDO and HDO as a function
of time over the different catalysts are consistent (i) with the fact that oxidation of
alcohols in aqueous solution proceeds through first the oxidation to an aldehyde, and
second to a carboxylic acid via the corresponding hydrate,30-32 (ii) with the character of
a sequential reaction already previously described for oxidation of α,ω−diols,38,44 and
(iii) the decrease of pH as a function of the formation of carboxylic acid functions. The
intermediates in the reactions appear in the order of the successive reactions. The
amplitude of their concentrations depended on the rates of production and consumption.
In addition, an equilibrium between the hydroxyaldehyde or hydroxyacid with the
corresponding lactol or lactone by intramolecular cyclisation, respectively, as well as
oxidation of the lactol to the corresponding lactone, are likely to take place. First, the
commercially available lactol form of 4-hydroxybutanal (2-hydroxytetrahydrofuran) could not
be observed during oxidation of BDO. Nor could a peak assignable to the C5 or C6 lactol be
identified by neither HPLC nor GC during reaction, while the cyclic hemiacetal is usually
reported to be the major species in the equilibrium, especially if a 5- or 6-membered ring
is formed.61,62 By contrast, although the lactone was not detected during oxidation of
PDO or HDO, cyclisation of 4-hydroxybutanoic acid was clearly observed during oxidation
of BDO as γ-butyrolactone (GBL) was identified among the products formed. Depending on
the number of carbon atoms, the equilibrium in acidic aqueous medium encountered in the
present study changed from the open chain form (n = 3 or 4) to the lactone form (n = 2).
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For instance, δ-valerolactone is not stable in water and is easy to be hydrolysed to the
hydroxyacid.63 High yields in lactonization of α,ω-diols with O2 was only reported in
toluene solvent over Au using hydrotalcite as a basic support.64 It is worth mentioning
that the lactone products of PDO and HDO were neither observed by Ide et al during
oxidation of these diols in aqueous phase.38 On the other hand, GBL is particularly stable
among the family of lactones and exists in equilibrium with the free acid in aqueous solution.65
3.2.2. Yield-oxidation level curves over the different catalysts
For a better comparison of the results independently of the reaction time, the dependence
of the yield of the different products as a function of the extent of conversion were
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compared for the different substrates over the Pt, Au-Pt and Au-Pd catalysts during the
period of 48 h (Figure 3). The level of conversion was defined as ([ALD] + 2∙[HA]+ 2
[lactone] + 3∙[AA] + 4∙[DA])/[diol]0. It takes into account the different oxidation steps
(from alcohol to aldehyde and then carboxylic acid functional group). The maximum
value of 4 means that the total amount of diols initially introduced into the reactor has
been converted up to the diacids. Figure 3 highlights that the product distribution as a
function of conversion level clearly depended on the diol and catalyst used.
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(a) BDO, Pt
(a') PDO, Pt
(a'') HDO, Pt
100
80
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20
0
100
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20
0
100
80
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40
20
0
0
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0
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4
0
0
0
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4
4
4
0
0
0
1
2
3
4
BDO conversion level
HDO conversion level
PDO conversion level
(b) BDO, Au-Pt
(b') PDO, Au-Pt
(b'') HDO, Au-Pt
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
1
2
3
1
2
3
1
2
3
4
BDO conversion level
PDO conversion level
HDO conversion level
(c) BDO, Au-Pd
(c') PDO, Au-Pd
(c'') HDO, Au-Pd
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
1
2
3
1
2
3
1
2
3
4
BDO conversion level
PDO conversion level
HDO conversion level
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Fig. 3. Yield of products as a function of conversion level. Reaction conditions: 0.1 M diol (a-
c: BDO; a’-c’: PDO; a”-c”: HDO), 70°C, 40 bar air, diol/metal molar ratio = 100. () ALD,
(▲) HA, () GBL, () AA and () DA.
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8
9
During conversion of BDO over Pt/ZrO2 (Figure 3a) a high maximum yield of 45% of
4-hydroxybutanal ALD was detected. Then, oxidation of this linear form of ALD formed 4-
hydroxybutanoic acid HA, which was in equilibrium with the lactone GBL. The proportion of
GBL detected in this reaction (maximum yield of 40%) was high compared to that of HA
(maximum yield 11%). The highest yield of (HA + GBL) was observed at the time of total ALD
disappearance. Under similar reaction conditions, Ide and Davis did not observe formation of
the linear hydroxyacid in the oxidation of BDO, but only the lactone.38 Later, HA and GBL
were further oxidized to 4-oxobutanoic acid (AA) and succinic acid (DA). The former was
formed in very low amounts, which suggests a rapid transformation of this intermediate to
succinic acid. After 48 h reaction time, the yield of succinic was 61% in addition to AA and
GBL accounting for 11%. The use of Au-Pt catalyst (Figure 3b) had a similar effect on the
evolution of products as a function of conversion level, and the final yield of succinic acid was
66% at 48 h. To our knowledge, the highest yield in succinic acid after oxidation of BDO
reported in the literature up to now is of 23%.38,43 In the presence of Au-Pd/ZrO2 catalyst (Figure
3c) neither HA nor AA were detected during the reaction, suggesting that once formed they
were very efficiently transformed to succinic acid, which was formed in significant yields from
the beginning. In contrast, we further noted that GBL was poorly reactive. The possible reason
is the stability of GBL under these conditions and the very slow hydrolysis to hydroxybutanoic
acid. Indeed, an additional experiment of oxidation of an aqueous solution of GBL at 70°C over
the Au-Pt catalyst confirmed a low GBL conversion of only 30% after 24 h. Since oxidation of
GBL was substantially slow, the final yield in DA was only 42% after 48 h along with 40% of
remaining GBL.
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The profiles as reaction progressed for PDO and HDO oxidation over Pt/ZrO2
were slightly different (Figure 3a’ and Figure 3a”), showing that the length of the carbon
chain has also importance with regards to the relative reaction rates of formation and
transformation of the different intermediates. It is clear that ALD (oxidation of one of
the primary alcohol functions of PDO or HDO) was the initial product formed with high
initial selectivity; the yield of ALD reached a maximum around 40% for both substrates.
Then, comparatively, the maximum yield of HA intermediate formed was much higher
during PDO reaction than during HDO reaction. This difference may be due either to a
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higher rate of oxidation of the aldehyde function to the carboxylic function (ALD to HA)
for the C5 compound, or because the second alcohol function of 5-hydroxypentanoic
acid was oxidized to the corresponding AA at a lower rate than the alcohol of 6-
hydroxyhexanoic acid. This would suggest again that the subsequent oxidation of the
second alcohol function will also be performed at different rates depending on the chain
length between the carboxylic acid and the alcohol. The maximum yield of HA was 40%
and 17% from PDO and HDO, respectively. After 48 h reaction time, the oxidation of
all intermediates was complete and final yields of the diacid of ca. 67% and 81% were
obtained, respectively.
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In the presence of the bimetallic Au-Pt/ZrO2 catalyst (Figure 3b’ and Figure 3b”)
the sequences of PDO and HDO oxidation reactions were the same as over Pt/ZrO2.
However, oxidation of the aldehyde function in ALD intermediate to yield HA was very
significantly accelerated during PDO reaction. Indeed, 5-hydroxypentanal was oxidized
to the hydroxyacid within less than 1 h and the maximum yield of HA was as high as
70%. In contrast, at similar conversion level of ca. 1.8 during HDO reaction, the yield
of 6-hydroxyhexanal was still of 22% and the yield of HA was of 22%. Then, the rate of
oxidation of the second alcohol group in HA was significantly slower, which is
consistent with some results of Ide et al.38 In the comparison of oxidation rates of α,ω
diols and -hydroxyacids, they reported that oxidation of 1,3-propanediol was
-
ω
significantly more rapid than the oxidation of 3-hydroxypropionic acid, and oxidation of
HDO was about 4-5 times faster than oxidation of 6-hydroxyhexanoic acid HA over Pt/C
under acidic conditions.38 These two experiments confirm the very different behaviour
of the C5 and C6 diols observed over Au-Pt, and the high sensitivity to the chain length
linking the terminal functions. On this point, it should be mentioned that oxidation of
PDO or HDO over Au-Pt under different reaction conditions in terms of temperature
(60°C or 90°C) or diol/metal ratio (150 and 200 instead of 100) systematically generated
a higher maximum yield of HA in the case of PDO reaction (not shown). To understand
the difference in reactivity of PDO and HDO over Pt-based catalysts, further studies on
the geometric structures of the AuPt alloy have to be considered in relation with the
substrate structure. Finally, all intermediates were totally oxidized within reaction time
of less or around 48 h to yield ca. 84% and ca. 96% DA, respectively.
The Au-Pd/ZrO2 catalyst also showed very different profiles compared with those
obtained with the Pt-based catalysts, due to the notable difference in reactivity to oxidation of
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an alcohol and an aldehyde function on the systems examined. As observed oveDrOtI:h1e0.1A039u/C-P9NtJ01695C
catalyst, ALD was rapidly converted to HA in the presence of the Au-Pd catalyst (Figure
3c’ and Figure 3c”). Thus, once ALD was formed over Au-Pd the oxidation of the aldehyde
function to HA was very rapid, so that the maximum yield of ALD was less than 5% in both
reactions, whereas the yield of HA was high. Similarly, the second alcohol function in HA was
slowly oxidized to the aldehyde, the latter being converted to the carboxylic acid at a high rate.
The maximum yield of AA formed was less than 2%. Finally, glutaric acid was obtained with
70% yield after 48 h; the yield of adipic acid was 80% in addition to 9% yield of HA remaining
in the solution.
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60
Table 2 summarizes the influence of the carbon chain length between terminal alcohol
groups on the formation of the diacid DA. In summary, the final yield of DA increases with the
carbon chain length of the α,ω-diol whatever the catalyst. Regardless the substrate, the highest
carbon balance was obtained in the the presence of Au-Pt catalyst. Very high yields of succinic
acid (66%), glutaric acid (84%) and adipic acid (96%) were obtained. Those are the highest
reported in the literature for oxidation of C4-C6 diols to the corresponding diacids.
Table 2. Oxidation of diols at 70°C under 40 bar air at S/M=100 in base-free medium.
Catalyst
t100
(h) a
1
Yield b (%)
GBL AA
(substrate)
ALD
HA
3
DA
61
66
42
67
84
70
81
96
80
Pt (BDO)
0
0
0
0
0
0
0
0
0
8
17
41
-
0
0
0
0
0
0
2
0
0
Au-Pt (BDO)
Au-Pd (BDO)
Pt (PDO)
1
2
24
2
0
0
Au-Pt (PDO)
Au-Pd (PDO)
Pt (HDO)
2
0
-
24
3
0
-
0
-
Au-Pt (HDO)
Au-Pd (HDO)
6
0
-
44
9
-
a Time to achieve total conversion of diol. b At 48 h of reaction.
Furthermore, examination of the reaction profiles shows that the reaction rates of
oxidation of the diol were initially high, however, they decreased as reaction progressed,
whatever the active metal. Deactivation phenomena are well known in the selective oxidation
of alcohols over noble metal catalysts. Different causes have been suggested and explored.66
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The chemical stability of the solid catalysts was verified by analysis of the reaction media at
the end of the reactions were. The amounts of Au, Pt, or Pd were below the detection
limit (< 0.3 mg L-1) of ICP-OES analysis. So, leaching was disgarded as the possible cause
of inhibition. Overoxidation of surface active metallic sites of Pd or Pt catalysts by molecular
oxygen has been mentioned in the literature,67-69 however, it is worthnoting that for bimetallic
systems, Au acts usually as a promoter to enhance resistance to oxygen poisoning.70 Therefore,
the bimetallic catalysts are more resistant under oxygen rich conditions and we excluded
poisoning of the catalyst due to the use of oxygen. Another possibility is the irreversible
adsorption of carboxylic acids which are known to block the access to the active sites in the
absence of a base.59 On this basis, and looking at the shapes of the reactions profiles we
proposed that the inhibitory effect resulted from the production of the intermediates or
products such as hydroxyacid HA, aldehyde-acid AA and diacid DA, and their strong
binding to the metallic surface. The extent of inhibition probably depends on the
conversion level and amount of acid formed as reaction progresses. The results may be
correlated with the inhibitory effect of organic acids reported previously in the literature.
For instance, the addition of 0.35 M acetic acid or propionic acid to a 0.1 M HDO
aqueous solution resulted in a decrease of the TOF from 0.27 s-1 to 0.19 s-1 over a Pt/C
at 70°C with 10 bar O2.38
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3.2.3. Effect of reaction temperature over Au-Pt/ZrO2
The Au-Pt/ZrO2 catalyst was selected to examine a higher temperature in order to
accelerate the reactions, in particular oxidation rate of GBL. Figure 4 shows the reaction profile
for BDO oxidation at 90°C. Noteworthy, the transformation of GBL is significantly increased
at this temperature compared to 70°C (Figure 2 b). In both cases very small amounts of HA
were analysed. At higher temperature, the equilibrium towards the open chain form and
subsequent oxidation to succinic acid is increased. The yield of succinic acid was considerably
improved to 81%.
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Fig. 4. Oxidation of BDO over Au-Pt/ZrO2 at 90°C under 40 bar of air. () BDO conversion,
() 4-hydroxybutanal, ALD, () γ-butyrolactone, GBL, (▲) 4-hydroxybutanoic acid, HA, ()
4-oxobutanoic acid, AA, and () succinic acid, DA yields, () carbon balance, and () TOC
measurement.
Additional oxidation experiments of PDO and HDO were subsequently performed at
this reaction temperature of 90°C (Figure S1). As expected, all reactions were accelerated. The
yield of glutaric and adipic acid was 78% and 88% after 8 h, respectively, i.e. slightly lower
than at 70°C (84% and 96%, respectively). In both cases a good carbon balance was achieved.
3.2.4. Effect of the presence of an hydroxy substituent on the diol
Finally, the presence of an additional hydroxy group substituent on the diol chain was
analysed. Thus, we compared oxidation of 1,6-hexanediol (HDO) and 1,2,6-hexanetriol (HTO)
at 70°C under 40 bar of air in water using Pt/ZrO2, Au-Pt/ZrO2 and Au-Pd/ZrO2. The results are
summarized in Table 3 and are illustrated in Figure 2b” and Figure 5 over Au-Pt/ZrO2.
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Table 3. Comparison of diol (HDO) and triol (HTO) oxidation over Pt, Au-Pt and Au-Pd
catalysts at 70 °C under 40 bar air in water, Substrate/metal ratio = 100.
5
6
7
8
Catalyst
t100 a (h)
Yield b (%)
9
(substrate)
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ALD
HA
AA
2
DA
81
21
96
49
80
9
Pt (HDO)
3
>48
6
0
0
0
38
0
Pt (HTO)
17
0
Au-Pt (HDO)
Au-Pt (HTO)
Au-Pd (HDO)
Au-Pd (HTO)
0
24
0
14
9
17
0
44
0
>48
12
37
9
aTime to achieve total conversion, bat 48 h of reaction
Fig. 5. HTO oxidation in water under 40 bar of air at 70°C over Au-Pt/ZrO2. () conversion,
yields of () ALD, (▲) HA, () AA, and () DA, () carbon balance, and () TOC
measurement.
The reaction products observed during HTO oxidation are shown in Scheme 2: 1,2-
dihydroxyhexanal (ALD), 1,2-dihydroxyhexanoic acid (HA), 1-oxo-2-hydroxyhexanoic acid
(AA) and 2-hydroxyadipic acid (DA) were formed. Oxidation of the secondary alcohol was not
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expected, in accordance with known reactivity of alcohols in the literature, i.e. PDt,OAI: 1u0.-1P03t9/aCn9NdJ01695C
Au-Pd promote the oxidation of a primary alcohol rather than that of a secondary
alcohol.34,59,71,72
8
9
OH
O
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OH
H
O
O
OH
OH
O
HO
HO
OH
(HTO)
O
OH
acid
1,2-dihydroxyhexanal (ALD)
1,2-dihydroxyhexanoic
(HA)
(AA)
1,2,6-hexanetriol
O
OH
OH
O
HO
O
OH
OH
acid
O
acid
2-hydroxyadipic
(DA)
1-oxo-2-hydroxyhexanoic
Scheme 2. Reaction pathway for HTO oxidation over Au-Pd, Pt and Au-Pt catalysts supported
on ZrO2 under 40 bar air at 70°C.
The first oxidation reaction will concern either the alcohol of the 1,2-diol part of the
molecule, or the non-substituted alcohol function in 6-position. The two primary alcohol
functions in HTO are differentiated, one now possesses an hydroxy group in alpha position.
This substituent induces a steric hindrance. The nature of the substituent may be an other
influencing factor that will slow down oxidation of HTO. Indeed, it was reported in the
literature that the rate of oxidation of a primary alcohol over Pt/C was dependent on the presence
and proximity of an additional alcoholic group in the molecule. Thus, for butanol and
butanediols the following order of initial TOF was established: BuOH > 1,4-BDO > 1,3-BDO
> 1,2-BDO.38 Likewise, over Pt/C the turnover frequency for 1,4-BDO oxidation was ca. 7
times larger than for 1,3-BDO, the one for 1,6–HDO oxidation was five times larger than for
1,2-HDO.38 Furthermore, intramolecular hydrogen bonding could not be excluded, which may
alter the reactivity. The rate of oxidation of HTO is then expected to be slower and the 6-
hydroxy group should react predominantly.
Actually, whatever the catalyst, HTO was notably less reactive than HDO, and the time
to achieve total conversion was drastically increased. In the presence of the most efficient
catalyst for diacid formation (Au-Pt/ZrO2), HTO conversion was complete only after 24 h vs.
6 h for HDO oxidation (Table 3). Reaction times even superior to 48 h were required for
complete conversion over Pt and Au-Pd.
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Once formed, ALD was rapidly oxidized during HTO oxidation over Au-Pt (Figure 5),
following the same trend as during HDO oxidation (Figure 2b”). The rapid conversion of ALD
to the corresponding hydroxyacid would suggest that the non-substituted terminal primary
alcohol of HTO was oxidized first.
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In contrast, subsequent oxidation of the second primary alcohol function in HA was very
slow during HTO reaction, so that the yield of HA attained 78% at its maximum. The 1-oxo-2-
hydroxyhexanoic acid compound was formed and reached 17% after 48 h, while 2-
hydroxyadipic acid yield was only of 49% at the same time.
3.2.5. Exploratory kinetic modeling
In order to further compare the behavior of the catalysts in PDO and HDO oxidation, we
did some exploratory kinetic modeling. We considered a stirred tank reactor in batch
mode with a constant volume of liquid over the reaction, and the liquid phase consists
mainly of water. In addition, all experiments were performed with a constant mass of
metallic active phase (substrate/metal = 100). Oxygen concentration in liquid phase
could be assumed constant due to the constant pressure during reaction and sufficient
mixing. We considered the simplified network of PDO and HDO oxidation without
cyclisation to the lactol or the lactone (Scheme 1).
A first simple model considering just pseudo-first order reaction for all steps was
considered. However this model poorly fit the experimental data. The concentration data
deviated rapidly and extensively from the theoretical trend obtained from this kinetic
model.
Therefore, we applied a formal Langmuir-Hinshelwood-Hougen-Watson
(LHHW) mechanism including terms for the adsorption of reactant and products in the
absence of both external and internal diffusion limitations. However, this model did not
take into account the possible decarbonylation reaction of ALD. The resulting four rate
equations based on the LHHW model can be given by:
ꢂ
ꢇ
ꢃꢄꢅꢆ ꢃꢄꢅꢆ
ꢀ1 = ꢁ1 1+ꢂ
(1)
(2)
ꢇ
+ꢂ
+ꢂ
ꢇ
+ꢂ
ꢇ
+ꢂ
+ꢂ
ꢇ
+ꢂ
ꢇ
ꢊꢈ ꢊꢈ
ꢈꢉꢊ ꢈꢉꢊ
ꢋꢈ ꢋꢈ
ꢈꢈ ꢈꢈ
ꢃꢄꢅꢆ ꢃꢄꢅꢆ
ꢂ
ꢇ
ꢈꢉꢊ ꢈꢉꢊ
ꢀ2 = ꢁ2 1+ꢂ
ꢇ
ꢇ
+ꢂ
ꢇ
ꢇ
+ꢂ
ꢇ
ꢊꢈ ꢊꢈ
ꢈꢉꢊ ꢈꢉꢊ
ꢋꢈ ꢋꢈ
ꢈꢈ ꢈꢈ
ꢃꢄꢅꢆ ꢃꢄꢅꢆ
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ꢋꢈ ꢋꢈ
ꢀ3 = ꢁ3 1+ꢂ
(3)
(4)
ꢇ
+ꢂ
+ꢂ
ꢇ
+ꢂ
ꢇ
+ꢂ
+ꢂ
ꢇ
+ꢂ
+ꢂ
ꢇ
ꢊꢈ ꢊꢈ
ꢈꢉꢊ ꢈꢉꢊ
ꢋꢈ ꢋꢈ
ꢈꢈ ꢈꢈ
ꢃꢄꢅꢆ ꢃꢄꢅꢆ
ꢂ
ꢇ
ꢈꢈ ꢈꢈ
ꢀ = ꢁ4
4
1+ꢂ
ꢇ
ꢇ
+ꢂ
ꢇ
ꢇ
ꢇ
ꢊꢈ ꢊꢈ
ꢈꢉꢊ ꢈꢉꢊ
ꢋꢈ ꢋꢈ
ꢈꢈ ꢈꢈ
ꢃꢄꢅꢆ ꢃꢄꢅꢆ
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where diol represents PDO or HDO, ri and ki denote the reaction rate and the rate
constant of step i, respectively, Cj the concentration of reaction compound j in the liquid
phase and Kj the corresponding adsorption coefficient. The sum in denominator takes
into account all components of the reaction mixture. The material balances of each
component in a batch reactor are coupled and simultaneously resolved:
ꢑꢒ
ꢄ
ꢌꢍꢀꢎ
=
(5)
ꢏꢍꢐ
ꢑꢓ
with νi is the stoichiometric coefficient of component i, r is the reaction rate
(equations (1-4)) and ni number of mol.
Equations (5) were tested with the experimental data by using non linear
regression analysis with Matlab 9.0 software. The aim of the data fitting procedure is to
minimize the mean squared difference between the calculated values of each
concentration (5 compounds) with the values obtained from the experimental data as
shown below:
ꢒ
2
min ꢔ = ��ꢕꢍꢖꢗꢑ − ꢕꢍꢘꢙꢚ
�
ꢍ=1
where n is the number of experimental points recorded during each run.
As examples, the comparison of experimental data with the simulated ones for
concentration-time curves for HDO in the presence of Au-Pt and Au-Pd are provided in
Figure 6 over the period of 10 h. Experimental data are given as points, while the solid
lines correspond to the predicted values. The other kinetic modelling curves are depicted
in the Supplementary material as Figure S2
.
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100
80
60
40
20
0
5
6
7
8
HDO mod
(a)
ALD mod
HA mod
AA mod
DA mod
HDO
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ALD
HA
AA
DA
0
2
4
6
8
t(h)
100
(b)
HDO mod
80
60
40
20
0
ALD mod
HA mod
AA mod
DA mod
HDO
ALD
HA
0
2
4
6
8
AA
t (h)
Fig. 6 Comparison of experimental (points) and predicted (lines) concentration-time
profiles for HDO oxidation over (a) Au-Pt and (b) Au-Pd catalysts. Reaction conditions:
0.1 M diol, 70°C, 40 bar air, diol/metal molar ratio = 100.
The model seems relatively consistent with the experimental results at least at the
early reaction time of the reactions (Fig. 6, Fig. S2) and the fitting between the
experimental and predicted data of the different reaction systems for the diol and ALD
is quite satisfying at the beginning of the reactions. The adequacy of the model then
deviates more or less, as ALD transformation progresses.
The calculated values of kinetic and adsorption parameters under the reaction
conditions resullting from the present optimization are listed in Table 4.
Table 4. Estimated values of kinetic parameters determined from kinetic model (1)-(4).
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Substrate/
Catalyst
PDO/
Pt
PDO/
Au-Pt
1653
363
1027
21
PDO/
Au-Pd
1029
1713
290
4
HDO/
Pt
HDO/
Au-Pt
442
422
3710
13
DHOI:D10O.10/39/C9NJ01695C
Au-Pd
620
1102
2402
7
Kinetic
k1
1032
1273
1308
54
866
976
3123
21
constant
(mmol L-1 h-1)
k2
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k3
k4
Adsorption
constant
(L mmol-1)
Kdiol
KALD
KHA
KAA
KDA
24
30
7
22
35
13
15
734
9
65
17
39
102
2
10
40
9
5
322
2618
1181
461
4783
1860
623
2654
1867
1038
3193
807
As expected, the adsorption coefficients of diols (Kdiol) are low, and the k1 values
confirm the higher conversion rates of PDO than those of HDO, i.e. the influence of
carbon chain length. Then, some discrepencies and short comings between model and
experiment can be identified. During PDO reaction, the experimental conversion of ALD
(the aldehyde-alcohol) to HA (the acid-alcohol) was significantly faster over Au-Pt than
over Pt. But the kinetic constant k2 for ALD conversion with Au-Pt was calculated as
being lower than with Pt catalyst (363 vs. 1273 mmol L-1 h-1), while adsorption was of
the same magnitude. Besides, comparison of the rates of oxidation of the second alcohol
group (k3) of HA to those of the first alcohol group (k1) of the diol indicates kinetic
constants k3 that are the same or slightly lower than k1 for PDO reaction over the
catalysts; in contrast, the k3 constants are a few times higher for HDO reactions. This
fact is coherent with the low maximum of HA concentrations observed for HDO.
The discrepancies may be strongly related to the carbon balance decrease in the
liquid phase. Indeed, as noted supra, decarbonylation of the aldehyde intermediates was
highly suspected, generating olefinic species during oxidation of diols. In addition to the
competitive adsorption of oxidation products, these strongly adsorbed products may
deactivate the catalyst.58 The formation of these products was not detected by
chromatography and could not be taken into account in the present model, which
evidently must be improved. A kinetic model should be designed which integrates
degradation pathway of ALD by decarbonylation.
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The modeling over the whole range of the reactions (Figure S3) also highlights
that the model then deviates more during the transformation of PDO than that of HDO.
However, the reasons why the olefinic species should have a larger detrimental effect
for PDO than for HDO reaction is not clear and should be studied further.The parity
diagram shown in Figure 7 for the reaction of HDO over Au-Pt illustrates that yields are
overestimated by the model that does not consider decarbonylation.
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1
0.8
0.6
conversion
ALD
0.4
HA
AA
0.2
DA
y=x
0
0
0.2
0.4
0.6
0.8
1
Predicted value
Fig. 7. Parity plot of experimental and predicted (from the reaction model) values of
reactant conversion and product yields for HDO reaction over Au-Pt catalyst. The dotted
lines show the +/-10% deviation.
3.3 Oxidation of PDO and HDO to the corresponding ω-hydroxycarboxylate in the presence
of Au/ZrO2 and a base
During all reactions performed in pure water, ω-hydroxyacid HA was an intermediate to the
diacid and the yield never exceeded 63% in base-free medium. Nonetheless, as mentioned in
the introduction, literature has highlighted that supported monometallic gold catalysts were
highly selective for the oxidation of ethyleneglycol and 1,3-propanediol to their
corresponding ω-hydroxycarboxylate in aqueous solution under basic conditions.57,72
Moreover, no production of the dicarboxylate was observed, even when 3-hydroxypropionic
acid was used as the substrate. Thus, the Au/ZrO2 was examined for selective production of the
ω-hydroxycarboxylate from PDO and HDO under alkaline conditions, using NaOH as the base.
Figure S4 shows the catalytic oxidation of PDO at 70°C under 40 bar air and diol/Au=100 in
the presence of increasing amounts of NaOH.
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With the addition of 1 eq of NaOH relative to the diol, PDO was very rapidDlyOIc: 1o0n.10v3e9r/Cte9dNJ01695C
within the first hour to attain 75% conversion; then, the reaction stopped (Figure S4a). The
amount of base introduced was not sufficient to neutralize all hydroxyacid formed, which was
strongly adsorbed on the metal surface and inhibited the reaction. The main product was 5-
hydroxypentanoate with a maximum yield of 68%. When the amount of NaOH was doubled (2
eq), PDO conversion was complete after 0.5 h. The reaction was very selective to 5-
hydroxypentanoate and a sharp maximum of 92% HA yield was observed (Figure S4b). Then,
the yield decreased significantly by conversion to glutarate and strong inhibition of the
transformation of 5-hydroxypentanoate was noticed with formation of the dicarboxylate.
Neither ALD nor AA were detected in significant amount, showing that the aldehyde function
was rapidly converted to the carboxylate under alkaline conditions. With further increase of the
amount of NaOH (4 eq.), the maximum yield of 5-hydroxypentanoate measured was 82% after
1 h; it was fully converted after 6 h to yield ca. 94% glutarate. Thus, under basic conditions
Au/ZrO2 produced only HA and DA, the proportion of which depended on the amount of base
introduced. Using 2 eq. of NaOH in presence of this catalyst gave a good compromise to achieve
a good yield of the ω-hydroxycarboxylate, in a short time.
4
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Figure S5 shows the results for oxidation of HDO over Au/ZrO2 in the presence of 2 eq
NaOH. As observed under neutral conditions, HDO is less reactive than PDO. A wide
maximum for 6-hydroxyhexanoate yield at 90-93% was obtained under these conditions.
To conclude, the use of Au/ZrO2 under moderately alkaline conditions is a valuable
system for the selective production of C5 and C6 hydroxyalkanoates, useful precursors of
polyesters. Over 90% yield was obtained, higher than the yield of 6-hydroxyhexanoate obtained
over Au-Pd/HT using hydrogen peroxide in basic solution.5,39 This is consistent with results of
Prati et al who oxidized ethyleneglycol to glycolate and 1,2-propanediol to lactate over Au/C
with very high selectivity in the monocarboxylate at high diol conversion.56
4. Conclusions
The aqueous aerobic oxidation of α,ω-diols has been investigated over a series of Au, Pt, Au-
Pt and Au-Pd catalysts supported over ZrO2, synthesized using wet (co)impregnation and
NaBH4 reduction. The nature of the catalyst and the operating conditions clearly affect the
reaction rate and the distribution of products as oxidation proceeds. We have reported the highly
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chemoselective oxidation of the diols to the diacids in water over Pt/ZrO2, Au-Pd/ZrO2 and Au-
Pt/ZrO2.
Diacid yield increases with carbon chain length of diol whatever the catalyst used. So,
5
6
7
8
Au-Pt/ZrO2 was the most efficient catalyst and yielded 83% of succinic acid from BDO at 90°C
under 40 bar air, and 84% of glutaric acid from PDO and 96% adipic acid from HDO at 70°C
under the same air pressure. The vicinal diol in 1,2,6-HTO is selectively oxidized at the primary
alcohol after an initial oxidation of the unsubstituted primary alcohol function to the acid; the
presence of the hydroxyl substituent affects the activity of the catalyst and reaction rates were
significantly lower than for HDO oxidation. A preliminary LHHW model taking into account
adsorption of reactant and oxidation reaction products was able to adequately reproduce the
experimental observations in terms of diol conversion and product yields at the beginning of
the reaction. However in addition to the competitive adsorption of oxidation products,
decarbonylation of the intermediate aldehydes and the formation of strongly adsorbed olefin
products should be investigated further.
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Interestingly also, catalytic oxidation in the presence of Au/ZrO2 and 2 eq of NaOH
resulted in the formation of ω-hydroxycarboxylate with high yields (90-93%).
Acknowledgements
This work was financially supported by the Agence Nationale de la Recherche (ANR) within
the French-Luxembourg program under Award Number ANR-15-CE07-0017-02.
Conflict of interest
The authors declare no conflict of interest.
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