Selective oxidation of 1,2-propanediol to lactic acid over Cu-modified Au/hydrotalcite catalysts

Article abstract of DOI:10.1039/d0nj02526g

Catalytic oxidation of biomass-derived 1,2-propanediol to valuable lactic acid was investigated over various Cu-modified Au supported on Mg-Al hydrotalcite (HT). Compared to those of individual Au-HT, better catalytic performance was achieved over the Cu-modified Au-HT catalysts, mainly due to the suppression of by-product generation. The structural properties of the Cu-Au/HT catalysts were determined by XRD, SEM, HRTEM, and N2 physical adsorption, and a strong interaction was discovered in the Cu-modified Au-HT catalysts. A control experiment verified that the secondary hydroxyl in 1,2-propanediol is unreactive over Au/HT catalysts modified with Cu, which intensively inhibited the generation of the main by-products, formic acid and acetic acid. These findings provided useful guidance for the design of more efficient and stable nano-catalysts for oxidizing glycols to organic acids containing hydroxides.

Full text of DOI:10.1039/d0nj02526g

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Selective oxidation of 1,2-propanediol to lactic acid over Cu
modified Au/hydrotalcite catalysts
a
a
b
a
a
Junying Tian, Hailong Liu, Ping Li, Zhiwei Huang and Jing Chen*
Received 00th January 20xx,
Accepted 00th January 20xx
Catalytic oxidation of biomass-derived 1,2-propanediol to valuable lactic acid were investigated over various Cu modified
Au supported on Mg-Al hydrotalcite (HT). Compared to those of individual Au-HT, the better catalytic performance was
achieved over Cu modified Au-HT catalysts, mainly due to the suppressive generation of byproducts. The structural
DOI: 10.1039/x0xx00000x
properties of Cu-Au/HT catalysts were determined by XRD, SEM, HRTEM, and N physical adsorption, and strong interaction
2
was discovered in the Cu modified Au-HT catalysts. The controlled experience verified that the secondary hydroxyl in 1,2-
propanediol is unreactive over Au/HT catalysts modified with Cu, which intensively inhibited the generation of main by-
products formic acid and acetic acid. These findings provided useful guidance for the design of more efficient and stable
nano-catalysts for oxidizing glycols to organic acids containing hydroxides.
2
and 1.0 MPa air on an AuPd/Mg(OH) catalyst. The oxidation of
Introduction
1,2-propanediol mentioned above provided a promising route
for lactate acid synthesis. Nevertheless, there is still room for
improving the selectivity to lactate acid by suppressing the
Lactic acid is an important fine chemical widely used in the
cosmetic, textile and food industries, and a major monomer for
production of eco-friendly poly-lactic acid (PLA).1-4 Traditional
routes for synthesis of lactic acid in industry generally use HCN,
1
1, 15
generation of byproducts.
Due to two types of hydroxides in 1,2-propanediol, two
competitive pathways probably take place in the its oxidation
acrylonitrile or Cl
2
as raw materials, which are expensive,
1
1-15
reaction (Scheme 1).
Generally, the oxidative
inadequate and environmentally unfriendly.2,
5
Currently,
dehydrogenation of primary hydroxyl into lactaldehyde and
secondary hydroxyl into hydroxyacetone occur concurrently.
Subsequently, the two main intermediates of lactaldehyde and
hydroxyacetone converted to lactate acid or other by-products
via different pathways, respectively. Lactaldehyde could be
directly oxidized to lactic acid quantitatively, while the
conversion of hydroxyacetone into lactic acid also via
arrangement reaction of reactive intermediate of
pyruvaldehyde, which also easily transforming into byproducts
at reation conditions. Therefore, it is a feasible mean to improve
lactic acid selectivity by limiting the pathway 2.
commercial lactic acid is mainly produced by microbial
fermentation, whereas the efficiency is limited by low
productivity and space-time yield and generating large amount
by-product.6 Therefore, chemical synthesis of lactic
, 7
of CaSO
4
acid from renewable biomass with high efficiency has attracted
increasing attention.
The oxidation of glycerol and its derivative of 1,2-propanediol,
obtained from bio-diesel industry, into lactate acid, provides an
alternative and efficient process for production of lactate acid
8-10
because of the structural analogy of C
3
skeleton.
Recent
years, numerous significant works found that supported Pt, Au,
Pd, Ag and heteropolyacid catalysts present excellent catalytic
performance for glycerol and 1,2-propanediol transforming into
In the previous work, the oxidation efficiency of primary
alcohol into aldehyde on Au/SiO catalysts is enhanced by
2
adding copper promoter, indicating that copper is a potential
candidate to change the pathway of 1,2-propanediol
2, 11-17
lactate acid,
and some representative results are shown in
1
8
Table 1. For example, Ma et al. found that 1,2-propanediol
conversion of 97 % and lactic acid selectivity of 84 % were
1
9
oxidation. Herein, various Au catalysts supported on Mg-Al
hydrotalcite (HT) were prepared for selective oxidation of 1,2-
propanediol to lactic acid. The secondary reactions were
obviously suppressed over the Au catalysts modified by Cu
addition, which in favor of increasing lactic acid selectivity.
Additionally, the structure characterization for different
catalysts was conducted for confirming structure-activity
relationship, and the possible reaction mechanism was
proposed.
achieved over an 1.4%Au/Mg(OH)
2
catalyst at the reaction
o
. Yin et al.13 reported that 1,2-
condition of 60 C and 0.3 MPa O
2
propanediol was oxidized to lactate acid with 88 % selectivity at
o
98 % 1,2-propanediol conversion at reaction condition of 60 C
a.
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute
of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000, China.
Email: chenj@licp.cas.cn
b.
College of Science, Gansu Agricultural University, Lanzhou 730070, China.
Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here.] See DOI: 10.1039/x0xx00000x
†
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DOI: 10.1039/D0NJ02526G
Table 1 Some typical reports of catalytic selective oxidation of glycerol and 1,2-propanediol into lactic acid
Lactic acid
Ref.
Substrate
Catalyst
Reaction Conditions
60 °C,0.3 MPa O , NaOH
Conversion (%)
selectivity (%)
1
1
1
1
1
1
,2-propanediol
,2-propanediol
,2-propanediol
,2-propanediol
,2-propanediol
,2-propanediol
Glycerol
1.4%Au/Mg(OH)
2
2
97
98
89
78
90
96
98
99
84
88
92
86
76
87
83
80
18
13
14
20
21
12
22
23
0.75%Au-0.25%Pd/Mg(OH)
2
60 °C, 1.0 MPa O
80 °C, 0.1 MPa O
40 °C , 0.3 MPa O
2
, NaOH
, NaOH
, NaOH
0.5%Au-0.5%Pd/hydroxyapatite
0.5%Au-0.5%Pt/C
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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8
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1
2
3
4
5
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8
9
0
1
2
3
4
5
6
7
8
9
0
98.5%Cu@1.5%Au Nanoparticles
2
100 °C , 1.0 MPa O , NaOH
0.025%Au-1%Pt/TiO
1%Au/CeO
0.4%Au-0.3%Pt/CeO
2
90 °C , 0.5 MPa air, NaOH
90 °C, 0.1MPa air, NaOH
100 °C, 0.5MPa air, NaOH
2
Glycerol
2
into a flask to obtain suspension at room temperature. After
stirring for 1 h, aqueous solution of NaBH solution (NaBH /Au
mol/mol) = 5) was dropped slowly into the suspension. After
4
4
(
aging at room temperature for 6 h, the black precipitates were
filtered and washed several times with deionized water until no
Cl was detected in the spent washing liquid, following by drying
20 °C for 12 h. The nominal content of Au in these catalysts
was at 0.6 wt%. The catalysts were denoted as xCu-Au/HT,
where x represents the nominal copper loading (wt%).
-
1
Catalyst characterization. X-ray powder diffraction (XRD)
Scheme 1 Reaction route of oxidation of 1,2-propanediol into lactate.11-15
patterns were obtained on
diffractometer with nickel filtered Cu K
2
0 mA. N physical adsorption-desorption measurements were
a
Rigaku D/MAX-2400
α
radiation at 40 kV and
3
Experimental
Materials
carried out on a Micromeritics TriStar II instrument. The BET
specific surface area and average pore size were calculated by
the Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-
Halenda (BJH) method, respectively. Inductively coupled
plasma-atomic emission spectroscopy (ICP-AES) measurements
were performed using an Agilent 725-ES. Before tested, the
samples were dissolved by aqua regia and then diluted by
deionized water. Scanning electron microscopy (SEM) was
performed on a scanning electron microscope (Vega3 Tescan)
operated at an acceleration voltage of 20 kV. Transmission
3 2 2
All the reagents used were analytical grade. The Mg(NO ) ·6H O,
Al(NO
Scientific Co., Ltd. Cu(NO
Chemical Reagent Factory. HAuCl
3
)
3
·9H
2
O, NaOH and Na
2
CO
O was purchased from Damao
·3H O and NaBH was bought
3
were produced by Xilong
3
)
2
·3H
2
4
2
4
from Chengdu Chron Chemicals Co., Ltd. and Wuhan Pego
Technology, Ltd., respectively.
Catalyst preparation
Preparation of HT support. The HT support was prepared by co- electron microscopy (TEM) was observed on a transmission
precipitation method reported in previous work. Briefly, a electron microscope (TF20) operated at an acceleration voltage
mixed solution of Mg(NO
solution of NaOH (8.0 M) were dropped concurrently into 100 the TEM micrographs by counting more than 200 particles. High
= 2 : 1) with mechanical stirring angle annular dark-field scanning transmission electron
2 3 3
mL Na CO solution (Al : CO
at 30 °C , while maintaining the pH at approximately 10. After microscopy (HAADF-STEM) and energy-dispersive X-ray
aging at 80 °C for 24 h, the obtained precipitate was filtered and spectroscopy (EDX) analyses were conducted on a Tecnai G2
washed with deionized water until pH~7. And then, the sample F20 transmission electron microscope operated at an
was dried at 120 °C overnight and calcined at 500 °C for 3 h in acceleration voltage of 200 kV.
24
2+
3+
3 2 3 3
) and Al(NO )
(Mg : Al = 3: 1) and of 200 kV, and the particle size was statistically measured from
3+
2-
air.
Catalytic test and product analysis. The 1,2-propanediol
Preparation of Au/HT and Cu-Au/HT catalysts. In the catalysts oxidation reactions were carried out in a stainless steel
preparation, deposition-precipitation method and incipient autoclave (100 ml) at a stirring speed of 800 rpm. In a typical
wetness impregnation method were used to loaded Au and Cu run, 30 g 1,2-propanediol aqueous solution (10 wt%), 3.15 g of
on the support, respectively. Firstly, the Cu modified HT was NaOH (NaOH/1,2-propanediol (mol/mol) = 2) and 1.0 g catalyst
prepared by impregnating over calcined HT support with an were added into the autoclave. Then, the reactor was purged
aqueous solution containing the certain amount of Cu(NO
following by drying at 120 °C for 12 h and reducing at 350 °C in and then heated to the setting temperature, which kept
a H
atmosphere for 3 h. Sequentially, the prescribed amount of constant during the reaction. The products and remained
HAuCl
3
)
2
,
2 2
with O (> 99.99%) three times, pressurized with O to 1.0 MPa
2
solution and prepared Cu/HT or HT samples was added reactant were analyzed by
a
Shimadzu LC-20A high-
4
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performance liquid chromatography (HPLC) with a refractive detected liquid products after acidulation were lactic acid,
index detector (RID) and an RSpak KC-811 column after acidizing formic acid and acetic acid. The conversion and product
with H PO aqueous (1 mol L ). Products were also identified selectivity were calculated as following:
3 4
on an Agilent 7890A/5975C GC-MS with a HP-5MS column. The
View Article Online
DOI: 10.1039/D0NJ02526G
-
1
mol of 1,2-propanediol charged - mol of 1,2-propanediol left
Conversion (%) =
×100
mol of 1,2-propanediol charged
mol of Carbon atoms in product
Selectivity (%) =
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mol of Carbon atoms in 1,2-propanediol charged - mol of Carbon atoms in 1,2-propanediol left
rebuilds from metallic oxides, which could attribute to the
“memory effects” of the hydrotalcite structure.2
5,26
The
Results and discussion
Catalyst characterization
characteristic peak intensity of hydrotalcite decreased gradually
from dried HT to Au/HT and Cu-Au/HT catalysts, indicating the
declining crystallinity of sample with heteroatom Au and Cu
loading. Additionally, no characteristic peaks attributed to Au
(JCPDA 65-2807) or Cu (JCPDA 04-0836) species were observed
in the XRD patterns, indicating that Au and Cu particles were
well dispersed on the HT support.
SEM images of dried HT support, Au/HT and Cu modified
Au/HT catalysts are shown in Fig. 2. A solid lamellar with layered
structure can be observed in the image of dried HT support. And
many stacking holes with different diameters and shapes were
formed by unordered agglomerate of Mg-Al hydrotalcite sheets.
Referring to their parents, no obvious difference present in both
Au/HT and Cu-Au/HT catalysts, indicating that the layered
structure is reserved with addition of Cu and Au during
calcination and reduction treatments, which also verified by
XRD results.
XRD patterns of dried HT support, calcined HT support and
prepared catalysts are shown in Fig. 1. The diffraction peaks at
1
1.7°, 23.6°, 35.0°, 39.7°, 47.1°, 60.9° and 62.4° observed in the
pattern of HT support, Au/HT and Cu-Au/HT catalysts could be
assigned to the (003), (006), (009), (105), (108), (110), and (113)
lattice planes in Mg-Al hydrotalcite, respectively. After
o
calcination at 500 C for 3 h, the characteristic diffraction peaks
of Mg-Al hydrotalcite greatly weakened, and diffraction peaks
of MgO at 42.9° and 62.3° were found. It is worth noting that
the obvious diffraction peaks attributed to Mg-Al hydrotalcite
appear again in the patterns of Au/HT and Cu-Au/HT catalysts
prepared by reductive precipitation method. This finding
indicates that the structure for layered double hydroxide
TEM images of dried HT support, Au/HT and Cu modified
Au/HT catalysts are given in Fig. 3 and particle dispersion results
obtained from the TEM images are listed in Table 2. The porous
2 2
structure, caused by the removal of CO and H O during the
pretreatment process, is observed in all images of samples,
which is responsible for their high surface area. The well-
distributed particles of about 8 nm in diameter are present in
the Au-HT catalyst, while smaller particles of 5.5-7.0 nm are well
dispersed in the entire Cu promoted Au-HT catalyst. These
findings indicate that Cu prevented the aggregation of Au
nanoparticles, which is consistent with the Pt-Cu catalysts
supported on activated carbon and SiO
2
in previous works due
to the strong interactions between Cu and noble metals.2
From the HRTEM images of Au/HT and 0.5Cu-Au/HT (Fig. S1),
the particles observed in both samples show similar lattice
fringes of 0.24 nm, which is close to the {1 1 1} lattice spacing of
fcc metallic gold. These results indicate that the observed
particles in the TEM images of 0.5Cu-Au/HT are mainly Au
particles, instead of the Cu particles or Au-Cu alloy particles.
Furthermore, no obvious change of morphology feature was
found in the sample with different copper loading. It is worth
noting that mean particle size of Au in Au/HT catalyst was 8.0
nm, while diffraction peaks weren’t observed in its XRD pattern.
7, 28
0
The strongest diffraction peak of Au (JCPDA 65-2870) is at 38.2°,
which is near the wide peak of Mg-Al hydrotalcite at 39.7°, so
that this peak of Au may be covered by peak of Mg-Al
hydrotalcite. Moreover, the intensity of other peaks of Au
weren’t strong enough to be observed in the XRD pattern. This
Fig. 1 XRD patterns of HT support, calcined HT support and prepared catalysts
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may be the reason for that diffraction peaks of Au didn’t appear number of Au was much higher than that of Cu in position of the
in the XRD pattern of Au-HT catalyst.
In order to investigated the interaction between Au and Cu, lattice spacing of Au was observed in HRTEM images of 0.5Cu-
the elemental mapping and line-scan tests were conducted on Au/HT.
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metal particles, which may be the reason for that only the
0
.5Cu-Au/HT, and the results are shown in Fig. 4. It can be seen
To further understand the structure of HT support, Au/HT
from the elemental mapping results that the dispersions of Cu and Cu modified Au/HT catalysts, their textural properties are
element and Au element were different. The Cu element existed given in Table 2. The contents of Au and Cu in these catalysts
on the surface of the support with two different statuses, Cu determined by ICP-AES were close to the nominal values. The
particle with high dispersion and aggregated Cu particle. BET specific surface area of Au/HT catalyst was 225 m2 g ,
However, the Au nanoparticle dispersed mainly at the area similar to that of HT support, indicating the pretreatment
consisting of large Cu particles. From the line-scan profiles, it process do not influence textural structure significantly. The
can be found that the atomic ratio of Au/Cu in the metal surface area of Au catalysts modified by copper with 0.05~2.0
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particles was constant. These findings are probably attributed wt% loading increased to 270~290 m g . Meanwhile, the
to strong interaction between Au and Cu. Moreover, the atomic
Fig. 2 SEM images of (a) HT support, (b) Au/HT, (c) 0.05Cu-Au/HT, (d) 0.1Cu-Au/HT, (e) 0.25Cu-Au/HT, (f) 0.5Cu-Au/HT, (g) 1Cu-Au/HT, and (h) 2Cu-Au/HT samples.
Fig. 3 TEM images of (a) HT support, (b) Au/HT, (c) 0.05Cu-Au/HT, (d) 0.1Cu-Au/HT, (e) 0.25Cu-Au/HT, (f) 0.5Cu-Au/HT, (g) 1Cu-Au/HT, and (h) 2Cu-Au/HT sample
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DOI: 10.1039/D0NJ02526G
average pore diameter decreased from ~7.6 nm in HT and propanediol conversion at all Au catalysts tested were above
Au/HT to ~3.6 nm in the Cu modified Au/HT samples. Some 92.8 %, indicating that the activity of these catalysts were high
mesopores of the support might be destroyed and converted to and not influenced by various copper content for 1,2-
smaller mesopores during the extra impregnation, drying and propanediol oxidation at the reaction conditions. Differently,
calcination of the Cu modified catalysts, thus resulted in the the selectivity of lactic acid increased sharply from 75.7 % over
increase of the surface area and decrease of the average pore Au/HT to 89.9 % over Cu-Au/HT catalyst modified with 0.25 wt%
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diameter.
intermediates and products is not hindered in the mesoporous improving copper loading to 2.0 wt%, indicating that moderate
structure. amount of copper was in favor of the production of lactic acid
In addition, the diffusion of substrate, copper, and then decreases gradually to 85.7 % with further
Catalytic performance for 1,2-propanediol oxidation into from 1,2-propandiol oxidation. The acetic acid selectivity
lactate acid. The catalytic performance of 1,2-propanediol to decreased from 16.2 % over Au/HT to the minimum of 6.8 %
lactic acid over Au/HT catalysts with different copper loading over 0.25Cu-Au/HT, and then increased to 9.5 % with further
are shown in Table 3. All Au catalysts with different copper increasing copper loading to 2.0 wt%. Additionally, the formic
loadings are active for 1,2-propanediol oxidation. The 1,2- acid selectivity decreased continuously from 6.8 % to 0.1 % over
Fig. 4 (a) HAADF-STEM image of 0.5Cu-Au/HT catalyst; elemental mapping of (b) Au, (c) Cu and (d) Au & Cu for the area in image (a); (e) line-scan profiles of Au and Cu for the position
marked by yellow line in image (a).
Table 2 The textural properties of HT support and catalysts
Content of Au a
Content of Cu a
S
D
Particle size b
BET
pore
Samples
2
-1
(
wt%)
-
(wt%)
-
-
0.04
0.09
0.23
0.46
0.88
1.80
(m g )
225
225
288
276
276
294
270
279
(nm)
7.8
7.4
3.4
3.3
3.4
3.6
3.6
3.9
(nm)
-
HT
Au/HT
0.62
0.58
0.59
0.60
0.61
0.63
0.64
8.0
5.6
6.1
6.3
6.8
6.7
6.5
0
.05Cu-Au/HT
0
.1Cu-Au/HT
0.25Cu-Au/HT
0.5Cu-Au/HT
1Cu-Au/HT
2
Cu-Au/HT
a
Obtained from ICP measurement. b Measured from the TEM micrographs.
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o
Au/HT catalysts with increasing copper loading to 2.0 wt%. with increasing reaction temperature from 60 to 80 C and kept
These findings indicated that the generation of main side- constant with further increasing temperature to 90 oC. The
products, including acetic acid and formic acid, was suppressed lactic acid selectivity monotonically decreased from 95.2 % to
by adding copper promoter into Au/HT catalysts. Furthermore, 70.1 %, while the acetic acid selectivity significantly increased
the catalytic activity of 0.5Cu/HT was tested, and a conversion from 3.4% to 19.9% with increasing the temperature from 60 to
o
of merely 28.5 % and a selectivity of 78.1 % were obtained. 90 C. The selectivity for another side-product of formic acid
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These results indicated that Au is important for the catalytic maintained below 1.0% in the reaction temperature range we
activity and the addition of Cu is favor of increasing lactate acid. tested, probably due to the different pathways into formic and
Therefore, the higher catalytic performance achieved over Cu- acetic acid. Higher reaction temperature is favorable of the
Au/HT catalysts probably ascribes to the synergistic effect formation of acetic acid in the 1,2-propanediol oxidation, which
between Cu and Au. In addition, total selectivity of detected is responsible for the decreasing lactic acid selectivity.
products was less than 100%, indicating that there were other
undetected products in the reaction. One of them was CO
the alkaline reaction environment, because some gas was from 0.5 to 1.0 MPa, and then kept constant with further
generated in the acidification process of products and the gas increasing pressure to 2.0 MPa. However, the lactic acid
was proved to be CO
the amount of CO was difficult to be obtained.
Effects of the reaction parameters, including the reaction 5.6 % and 0.4 % to 26.5 % and 5.1 % with increasing O
temperature, O pressure and reaction time, were investigated from 0.5 to 2.0 MPa, respectively. These findings indicate that
to obtain high yields of lactic acid over the Cu modified Au/HT the side reactions are promoted by high O pressure.
catalysts. The 0.5Cu-Au/HT, which was one of the most The conversions and selectivity of 0.5Cu-Au/HT in 1,2-
As shown in the Fig. 5b, 1,2-propanediol conversion rapidly
in increased from 83.6 % to 98.5 % when increasing O pressure
2
-
3
2
2
via the reaction with limewater. However, selectivity decreased sharply from 91.6 % to 60.3 %, while the
selectivity to acetic acid and formic acid always increased form
pressure
2
2
2
2
activecatalysts, was selected as a model for the investigations, propanediol oxidation as a function of reaction time at 1.0 MPa
and the results are shown in Fig. 5. The 1,2-propanediol
o
O
2
and 80 C are shown in Fig. 5c. The conversion of 1,2-
conversion continually increased from 62.8 % to approach 100% propanediol increased drastically to 90.3 % in the first 4 h, and
Table 3 The catalytic performance of 1,2-propanediol to lactic acid over Au/HT catalysts with different copper loading.
Selectivity (%)
Catalysts
Conversion (%)
lactic acid
75.7
acetic acid
16.2
10.0
8.1
formic acid
Au/HT
94.2
92.8
98.2
98.2
98.5
99.0
97.1
6.8
4.8
0.3
0
0
.05Cu-Au/HT
85.0
87.8
89.9
88.5
87.6
85.7
0
.1Cu-Au/HT
.25Cu-Au/HT
6.8
7.6
8.3
9.5
0.1
Not detected
0.1
0
.5Cu-Au/HT
1
2
Cu-Au/HT
Cu-Au/HT
0.1
2
Reaction condition: 30 mL 10 wt% 1,2-propanediol aqueous solution, 3.15 g NaOH, 1.0 g Catalyst, 80 °C , 1.0 MPa O , 6.0 h.
Fig. 5 Effect of reaction temperature (a), reaction pressure (b) and reaction time (c) on the catalytic conversion and selectivity over 0.5Cu-Au/HT catalyst. (Reaction condition: 30 g
0 wt% 1,2-propanediol aqueous solution, 3.15 g NaOH, 1.0 g 0.5Cu-Au/HT, (a): 6.0 h, 1.0 MPa O ; (b): 6.0 h, 80 °C ; (c): 1.0 MPa O , 80 °C .
1
2
2
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DOI: 10.1039/D0NJ02526G
then increased gradually to 98.9 % with a further increasing the propanediol oxidation reaction. Afterward, the catalyst was
reaction time to 8 h. The selectivity to lactic acid decreased recharged into the autoclave together with fresh reactant and
slightly from 91.3 to 84.6 % with the increasing reaction time NaOH additive. The conversion and lactic acid selectivity
from 2 to 8 h, which shows the stability of the lactic acid in the decreased slightly from 98.5 % and 88.5 % to 96.5 % and 82.5 %
oxidation reaction. Besides, the selectivity to acetic acid respectively after 5 recycles (Fig. 6a). This result shows an
increased from 5.8 to 10.3 %, whereas the selectivity to formic acceptable stability of the 0.5Cu-Au/HT in the 1,2-propanediol
acid remained almost constant with the increasing reaction oxidation. Furthermore, the reason for the decrease of the
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time from 2 to 8 h.
catalytic performance was studied. It can be discovered from the
The effects of the reaction parameters mentioned above XRD patterns of fresh and used 0.5Cu-Au/HT catalysts (Fig. 6b),
show the possibility for improving the catalytic performance of the peak intensity of HT support change to be a little weaker in
1
,2-propanediol oxidation into lactic acid. For example, at the the hot basic solution of the reaction process. Moreover, no
appropriate reaction conditions (80 °C and 1.0 MPa O ), a obvious change of morphology can be observed in the SEM
complete transformation of 1,2-propanediol and selectivity of image (Fig. 6c) of used catalyst. These results indicated that the
8.5 % to lactic acid were achieved on 0.5Cu-Au/HT catalyst in stability of the HT support was passable in the reaction process.
the presence of sufficient NaOH, which is comparable with In addition, from the TEM image of used catalyst (Fig. 6d), the
those on the other supported Au catalysts. average particle size of active metals was about 8.3 nm, which
2
8
To investigate the recyclability of 0.5Cu-Au/HT catalyst, the was a little bigger than fresh catalyst. To sum up, the growth of
used catalyst was filtrated, washed with deionized water for three metal particles might be the main reason for catalyst deactivation.
times and reduced at 350 C in a H
o
2
flow for 3 h after every 1,2-
Fig. 6 (a) Recycling performance of 0.5Cu-Au/HT catalyst for the catalytic oxidation of 1,2-propanediol (reaction condition: 30 g 10 wt% 1,2-propanediol aqueous solution, 3.15 g
NaOH, 1.0 g 0.5Cu-Au/HT, 80 °C , 1.0 MPa O , 6.0 h); (b) XRD patterns of fresh and used catalysts; (c) SEM image of used catalyst; (d) TEM image of used catalyst
2
Scheme 2 Reaction pathways for catalytic oxidation of 1,2-propanediol 1-6 and functional mechanism of Cu promoter
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Table 4 Oxidation of lactic acid and hydroxyacetone catalyzed by Au/HT and 0.5Cu-Au/HT catalysts
View Article Online
DOI: 10.1039/D0NJ02526G
Selectivity (%)
Formic acid
Substrate
Catalysts
Conversion (%)
Lactic acid
Acetic acid
62.8
Lactic acid
Lactic acid
Hydroxyacetone
Au/HT
0.5Cu-Au/HT
Au/HT
21.8
8.4
95.1
96.2
-
-
1.8
0
10.7
10.2
66.7
16.8
21.3
42.6
37.4
Hydroxyacetone 0.5Cu-Au/HT
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Reaction condition: 30 g 10 wt% aqueous solution of substrate, 3.15 g NaOH, 1.0 g Catalyst, 80 °C , 1.0 MPa O2, 6.0 h
Functional mechanism of Cu promoter. In the oxidation oxidation of secondary hydroxyl of 1,2-propanediol hardly
reaction of 1,2-propanediol into lactic acid, formic acid and occurred over these catalysts. So that, the pyruvaldhyde wasn’t
acetic acid were usually generated as by-products. In the obtained over Cu modified Au/HT catalysts, thus the formation
11-15, 31
previous works
, it was reported that 1,2-propanediol was of formate was inhibited. The reason for this effect may be that
oxidized to lactate through two possible parallel pathways the oxidation activity for primary hydroxyl group of 1,2-
Scheme 2). In the pathway 1, oxidation of the primary hydroxyl propanediol was improved due to synergistic effect between
(
group in 1,2-propanediol occurred firstly to form lactaldehyde. the Au and Cu. It has been reported that there was synergistic
The resultant lactaldehyde can be oxidized to lactic acid rapidly, effect between Au and Cu which arose most probably from the
so that the lactaldehyde wasn’t detected in our experiment. In electronic interaction in the Au-Cu alloy catalyst supported on
2
the pathway 2, oxidation of the secondary hydroxyl group of SiO , and this interaction promoted oxidation of various
,2-propanediol occurred firstly and hydroxyacetone was alcohols to the corresponding aldehydes.7
1
generated. The resultant hydroxyacetone can be oxidized into
lactate acid via another intermediate of pyruvaldehyde.
Moreover, the reason for generation of formic acid and acetic
acid was investigated. Dimitratos et al.11 and Feng et al.
revealed that the pyruvaldehyde, which was an intermediate in
the pathway 2, could converted to formate and acetate in the
alkaline solution (Side reaction 1). Besides, this work discovered
that the lactic acid selectivity decreased and the acetic acid
Conclusion
15 A series of Cu modified Au/HT catalysts were prepared for
catalytic oxidation of 1,2-propanediol to lactic acid. The Cu was
proved as an excellent promoter to increase conversion of 1,2-
propanediol and selectivity of lactic acid. The 1,2-propanediol
conversion and lactic acid selectivity could exceed 98 % and 88 %
respectively over optimized 0.5Cu-Au/HT catalyst at reaction
conditions of 80 oC and 1.0 MPa O2, and the 0.5Cu-Au/HT
selectivity increased, when the reaction time was prolonged (Fig.
2-
4
c). So it was possible that the lactate could convert to CO
3
and
catalyst exhibited good recycling performance. According to the
corresponding results of structure characterization and
reactions, the functions of copper addition for promoting
catalytic performance were proposed in two ways. On the one
hand, oxidation of secondary hydroxyl of 1,2-propanediol didn’t
occurred over Cu modified Au/HT catalysts, thus sharply
decreased the selectivity formic acid and acetic acid generated
from pyruvaldehyde. On the other hand, Cu promoted catalysts
had lower activity for oxidative decarboxylation of lactate than
Au/HT catalyst, thus had lower acetic acid selectivity and higher
lactic acid selectivity. The higher catalytic performance
achieved over Cu-Au/HT catalysts probably ascribes to the
synergistic effect between Cu and Au.
acetate in the alkaline environment by the oxidative
decarboxylation reaction (Side reaction 2).
In order to verify the formation mechanism of formate and
acetate, and investigate the functional mechanism of Cu,
catalytic oxidation reactions of lactic acid and hydroxyacetone
over Au/HT and 0.5Cu-Au/HT were tested, and the results were
shown in Table 4. When the reaction substrate was lactic acid,
its conversion over Au/HT catalyst and 0.5Cu-Au/HT catalyst
were 21.8 % and 8.4 %, respectively. And the main products
were acetic acid over both the catalysts. This results proved that
the lactate could be further oxidized to acetate in the reaction
conditions, and 0.5Cu-Au/HT catalyst had lower activity for this
side reaction than Au/HT catalyst. When the substrate was
hydroxyacetone, its conversion exceeded 95 % and formic acid
selectivity over both catalysts were nearly 10%, indicating that
the formate was formed in pathway 2, which was consistent
Conflicts of interest
There are no conflicts to declare.
1, 2
with the conclusion of previous works . Moreover, the formic
acid selectivity over the two catalysts were similar, which
revealed that the Cu promoter couldn’t reduce the C-C cleavage
of pyruvaldehyde to form the formate. However, it was notable
that over 0.5Cu-Au/HT catalyst, the selectivity of formic acid
was 10.2 % when the substrate was hydroxyacetone, while the
selectivity of formic acid was 0.1 % when the substrate was 1,2-
propanediol (Table 1). These results pointed out that the
oxidation of 1,2-propanediol to lactic acid was almost through
Acknowledgments
This work was supported by CAS "Light of West China" Program
and National Natural Science Foundation of China (21902164,
2
1862003 and 21872155).
the pathway 1 over the Cu modified Au/HT catalysts, and References
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1,2-propanediol was converted to lactic acid over Cu modified Au/hydrotalcite catalysts with high
conversion and selectivity.