Oxidative dehydrogenation of lactic acid to pyruvic acid over Pb-Pt bimetallic supported on carbon materials

Article abstract of DOI:10.1016/j.apcata.2017.01.010

Various carbon materials, including activated carbon, flake graphite, carbon black and multiwall carbon nanotubes, were adopted as supports for lead-promoted platinum catalysts in the investigation of aqueous-phase oxidative dehydrogenation of lactic acid to produce pyruvic acid. Among them, the nanoscale 3Pb-1Pt/carbon black catalyst showed the highest TOF (769?h^?1) and yielded 60% pyruvic acid within 20?min under mild conditions. N₂adsorption, Raman spectroscopy, thermogravimetry, SEM, TEM and XPS were performed to investigate their different structures, graphitization degrees, metal particle sizes and surface oxidation states. Under the current conditions, smaller support particle size is essential for the enhanced lactic acid transformation rate by favoring the diffusion of reactants; less graphitic structure of catalyst is also beneficial by improving metal particle dispersion and facilitating the exposure of metals.

Full text of DOI:10.1016/j.apcata.2017.01.010

Accepted Manuscript
Title: Oxidative dehydrogenation of lactic acid to pyruvic acid
over Pb-Pt bimetallic supported on carbon materials
Authors: Chen Zhang, Tao Wang, Yunjie Ding
PII:
S0926-860X(17)30018-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.01.010
APCATA 16121
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Applied Catalysis A: General
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Accepted date:
9-12-2016
9-1-2017
12-1-2017
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Highlights
3Pb-1Pt/carbon black catalyst yielded 60% pyruvic acid within 20 min.
Various carbon materials were studied in pyruvic acid production from lactic acid.
Support particle size and metal particle size affected lactic acid transformation.
The graphitic degree of catalyst influenced the metal particle size.
1 / 19

Oxidative dehydrogenation of lactic acid to pyruvic acid over
Pb-Pt bimetallic supported on carbon materials
Chen Zhang a, c, Tao Wang a, *, Yunjie Ding a, b,
*
a
Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of
Sciences, Dalian 116023, PR China
b State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian
116023, PR China
c University of Chinese Academy of Sciences, Beijing 100049, PR China
* Corresponding author. Tel.: +86 411 84379143; fax: +84379143.
E-mail address: dyj@dicp.ac.cn (Y. Ding) and wangtao@dicp.ac.cn (T. Wang)
2 / 19

Abstract
Various carbon materials, including activated carbon, flake graphite, carbon black and
multiwall carbon nanotubes, were adopted as supports for lead-promoted platinum
catalysts in the investigation of aqueous-phase oxidative dehydrogenation of lactic
acid to produce pyruvic acid. Among them, the nanoscale 3Pb-1Pt/carbon black
catalyst showed the highest TOF (769 h-1) and yielded 60% pyruvic acid within 20
min under mild conditions. N2 adsorption, Raman spectroscopy, thermogravimetry,
SEM, TEM and XPS were performed to investigate their different structures,
graphitization degrees, metal particle sizes and surface oxidation states. Under the
current conditions, smaller support particle size is essential for the enhanced lactic
acid transformation rate by favoring the diffusion of reactants; less graphitic structure
of catalyst is also beneficial by improving metal particle dispersion and facilitating the
exposure of metals.
Keywords: Oxidative dehydrogenation; Pyruvic acid; Lactic acid; Carbon materials
3 / 19

1. Introduction
Facing the increasing concerns in fossil feedstock exhaustion as well as
greenhouse effects, scientists are paying much attention on deriving biomass-based
platform molecules to produce fuels and commodity chemicals. Lactic acid (LA), i.e.,
2-hydroxy-propanoic acid, is one of the molecules with great appeal. It can be
generated by fermentation of various renewable resources derived from agricultural
crops (e.g., sugars and starch) and waste biomass streams (e.g., wood molasses) [1]. It
can also be produced by chemical conversion of biomass-based resources including
cellulose and glycerol [2, 3]. As it contains both hydroxyl group and carboxylic acid
group, LA can undergo versatile reactions, such as polymerization, dehydration, and
esterification, to generate biodegradable poly-lactic-acid plastics, acrylic acid and
ethyl lactate [4, 5].
Recently, researchers have also investigated the oxidative dehydrogenation of LA
to yield pyruvic acid (PA) [6, 7]. PA is widely adopted in food, cosmetic and
pharmaceutical industries [8]. In spite of its expanding applications, the major
technology for PA production is still based on the dehydrative decarboxylation of
tartaric acid in the presence of stoichiometric amounts of KHSO4, developed by
Erlenmeyer in 1881, which has high consumption of energy [6, 9]. Therefore, an
alternative way to produce PA from LA with high atom efficiency at mild conditions
would be highly desirable.
The oxidative dehydrogenation of LA to PA has been conducted in both gas and
liquid phases. In vapor phase, several transitional metal catalysts were studied,
including vanadium-based mixed oxides, MoO3-based oxides, Ni-Nb oxides and iron
phosphate [7, 10-13]. However, the gas phase reactions generally consume more
energy because of high reaction temperature (typically over 200 °C). The high
temperature favors successive side reactions, leading to PA yield generally lower than
50%.
In contrast, the liquid phase reaction can suppress the above problems. Catalysts
used in liquid phase reactions commonly consist of noble metals as active sites (e.g.,
Pt or Pd), heavy metals as additives (e.g., Pb, Te and Bi), and carbon as support
4 / 19

[14-16]. For example, Sugiyama et al. reported a 0.95Te-5Pd/C catalyst giving a PA
yield of 68.2% under pressurized oxygen, which is by far the highest [15]. Yet it took
5 h to obtain its highest PA yield. Considering the use of noble metals, the current
transformation efficiency of catalyst for PA production is still less satisfactory.
Meanwhile, various carbon materials, including activated carbon, graphite and
multiwall carbon nanotubes could serve as supports, but the application of them in LA
oxidative dehydrogenation to PA is quite limited. Thus, the exploration of various
carbon materials as supports might provide new solutions for the improvement of the
catalytic efficiency for PA production from LA.
Herein, four types of carbon materials, including activated carbon, flake graphite,
carbon black and multiwall carbon nanotubes, were adopted as supports for Pb-Pt
bimetallic catalysts in the oxidative dehydrogenation of LA to produce PA.
Characterizations including N2 adsorption, Raman spectroscopy, thermogravimetry,
SEM, TEM and XPS were performed to uncover the structure-performance
relationship of the catalysts.
2. Experimental
2.1. Catalyst preparation
Activated carbon (AC, almond shell, 100 - 200 mesh, 1241 m2/g; Tangshan
United Carbon, China), flake graphite (Graphite, 3500 mesh, 38 m2/g; Xianfeng
Nano, China), carbon black (CB, 846 m2/g; Ketjenblack 300JD, AkzoNobel,
Netherlands), and multiwall carbon nanotubes (MWCNTs, 324 m2/g; Timesnano,
China) were used as catalyst supports. In a typical preparation experiment of
Pb-promoted Pt/carbon catalyst, certain amount of Pb(NO3)2 was dissolved in the
mixed solution of deionized water (3 mL) and ethanol (1 mL), impregnated onto 1.0
g of carbon support and dried at 120 °C for 12 h. Then, it was reduced by H2 (400 °C,
4 h). Sequentially, the Pb/carbon sample was dispersed in 200 mL of deionized water
under vigorous stirring, and an H2PtCl6 (Shanghai Jiuyue, China) solution was
reduced and deposited on the as-prepared Pb/carbon sample by
deposition-precipitation method at 60 °C [3]. The catalyst was denoted according to
its metal loadings, e.g., 3Pb-1Pt/AC indicates that the weight percentages of Pb and Pt
5 / 19

are 3.0 wt% and 1.0 wt%, respectively.
2.2. Catalyst characterization
The pore structures of the catalysts were studied by N2 physical adsorption on a
Quantachrome Autosorb-1 instrument, and the samples were degassed at 250 °C for
12 h prior to analysis. The graphitization degrees of the catalysts were measured by
Raman spectroscopy on a Bruker Optics Senterra Raman confocal microscope
spectrometer at ambient conditions with an excitation wavelength at 532 nm. The
thermal stability of carbon support was tested by thermogravimetry (TG)
measurement on a Setaram Setsys 16/18 analyzer from room temperature to 900 °C at
a heating rate of 10 °C/min and an air flow rate of 40 mL/min. The catalyst
morphologies were investigated on a FEI Quanta 200F field emission scanning
electron microscope (SEM) operating at 20 kV and a JEM-2100 transmission electron
microscope (TEM) operating at 200 kV. The X-ray photoelectron spectroscopy (XPS)
was conducted on a Thermo ESCALAB 250Xi X-ray spectroscopy spectrometer. The
spectra were excited by the monochromatic Al Kα source (1486.6 eV) at 15 kV. The
charging effects were corrected using the C ls peak at 284.6 eV.
2.3. Lactic acid oxidative dehydrogenation reaction
The oxidative dehydrogenation of LA was performed in a 100 mL three-neck
flask at ambient pressure in an oil bath. LA solution (25.0 g, 0.2 mol/L), LiOH•H2O
(purity at 90%, 0.21 g) and catalyst (0.25 g) were mixed together and heated to 90 °C
in the reactor under stirring at a rate of 800 r/min and oxygen bubbling of 100 mL/min.
The starting pH of the solution was around 12, while the ending pH was around 8.
The pH was not adjusted during the reaction. The products were analyzed by the same
procedure as we reported previously [3]. For simplicity, the reactants were denoted as
their acid forms.
3. Results and discussion
3.1. Catalyst characterization
3.1.1. N2 physisorption of 3Pb-1Pt/carbon catalysts
6 / 19

The pore structures of the 3Pb-1Pt/carbon catalysts were shown in Fig. S1 and
Table 1. Samples a (3Pb-1Pt/Graphite) and d (3Pb-1Pt/MWCNTs) exhibited the type
IV isotherms, indicating the existence of mesopores probably caused by the
aggregation of nanoparticles or the inter-nanotube cavities [17]. Samples b
(3Pb-1Pt/AC) and c (3Pb-1Pt/CB) showed the combination of type I and type IV
isotherms, characteristic of materials containing both mesopores and micropores. The
shape of hysteresis loops for samples a, b, and c are corresponding to the H3 type,
confirming the presence of disordered mesoporous structures with slit-shaped
micropores or plate-like particles [18]. Meanwhile, sample d showed an H1 type
hysteresis loop, which is related with cylindrical shape pores with open ends and
uniform distributions [19].
Table 1
Texture properties of 3Pb-1Pt/carbon catalysts.
a
b
c
d
SBET
Smeso
Vpore
Vmeso
Entry
Catalyst
(m2/g)
(m2/g) (mL/g)
(mL/g)
0.16
1
2
3
4
3Pb-1Pt/Graphite
3Pb-1Pt/AC
38
38
0.16
0.83
1.71
2.44
1122
756
386
614
297
0.53
3Pb-1Pt/CB
1.61
3Pb-1Pt/MWCNTs
297
2.44
a Calculated by the Brunauer-Emmett-Teller method.
b, d Calculated by the t-plot method.
c Determined from the adsorbed nitrogen at a relative pressure (p/p0) of 0.99.
Their BET surface areas were also calculated, with 38 m2/g for sample a, 1122
m2/g for b, 756 m2/g for c, and 297 m2/g for d (Table 1). In line with previous
discussion, the mesopore surface areas of samples a and d were equal to their
respective total surface areas, confirming their mesopore-structures. For sample b, its
mesopore area only counts for a small part in the total area, indicating that its large
surface area mainly belonged to micropores, which are generally difficult to access
during the reaction. However, for sample c, it showed the largest mesopore surface
area of 614 m2/g. Apparent differences were also observed in their total pore volumes.
The total volume sequence is as follows: sample d (2.44 mL/g) > c (1.71 mL/g) > b
7 / 19

(0.83 mL/g) > a (0.16 mL/g), and the mesopore volumes are consistent with the orders
of the total volume. This might be related with the support density as well as their
pore shapes, because cylindrical nanotubes (sample d) tends to occupy more free
space than plates or slits [20].
3.1.2. Raman spectroscopy of 3Pb-1Pt/carbon catalysts
The Raman spectra of 3Pb-1Pt/carbon catalysts are displayed in Fig. 2.
Characteristic G (1595 cm-1) and D (1350 cm-1) bands were observed in all the four
samples. The G band is ascribed to the in-phase vibrations of the graphite lattice, and
the D band is attributed to the defective graphitic structures. The 2D band (or known
as G’ band) at 2700 cm-1, which was originated from the stacking order of nanosheet
structures, was only observed in samples a and d [21]. The relative intensity ratio
ID/IG of samples, commonly correlated with the structural purity of graphitic materials,
was calculated from the Raman spectra [22, 23]. The low ID/IG ratio of sample a
indicates a highly graphitic structure, whereas the higher ratio for sample b suggests
more disorder or amorphous forms. The graphitization of samples d and c were
between that of samples a and b. The higher disorder degree would also lead to a
broader G band, as shown in Fig. 2. The TG profiles (Fig. S2) of these carbon
supports further confirmed the highly graphitized nature of sample a, of which onset
decomposition temperature was the highest, whereas that for sample b was the lowest,
indicating its more amorphous framework [17, 24].
G
D
ID / IG = 0.4
a
ID / IG = 2.3
ID / IG = 2.0
ID / IG = 1.2
b
c
d
500
1000
1500
2000
2500
3000
3500
4000
wave number (cm-1)
Fig. 1. Raman spectroscopy of a) 3Pb-1Pt/Graphite, b) 3Pb-1Pt/AC, c) 3Pb-1Pt/CB, and d)
3Pb-1Pt/MWCNTs catalysts.
8 / 19

3.1.3. SEM and TEM analyses of 3Pb-1Pt/carbon catalysts
Morphologies of the 3Pb-1Pt/carbon catalysts were characterized by SEM (Fig. 3,
left row). The support particle sizes of samples a and b are generally much larger than
samples c and d. The former ones are the sheet-like or flake-like micrometer-scale
particles, but the latter ones are the well-defined nanospheres or nanotubes. This
agrees well with the above discussion (Section 3.1.1) on their pore shapes.
The TEM images (Fig.3, middle row) further showed the catalyst morphology.
Sample a displays a flat plate-like layer with limited edges or defects. Compared with
a, samples b and c present more amorphous carbon and defects on the surface. For
sample d, hollow structures with the walls (about 10 layers) formed by graphene
sheets were detected. This observation matches with their graphitization degree
determined by Raman spectroscopy.
100
90
a3
80
d = 2.1 0.8 nm
70
60
50
40
30
20
10
0
0-0.9 0.9-1.8 1.8-2.7 2.7-3.6 3.6-4.5 4.5-5.4
Particle size (nm)
100
90
b3
80
d = 1.4 0.4 nm
70
60
50
40
30
20
10
0
0-0.9 0.9-1.8 1.8-2.7 2.7-3.6 3.6-4.5 4.5-5.4
Particle size (nm)
100
90
80
70
60
50
40
30
20
10
0
c3
d = 1.3 0.4 nm
0-0.9 0.9-1.8 1.8-2.7 2.7-3.6 3.6-4.5 4.5-5.4
Particle size (nm)
9 / 19

100
90
80
70
60
50
40
30
20
10
0
d3
d = 1.7 0.5 nm
0-0.9 0.9-1.8 1.8-2.7 2.7-3.6 3.6-4.5 4.5-5.4
Particle size (nm)
Fig. 2. SEM, TEM images and particle size distributions of a) 3Pb-1Pt/Graphite, b) 3Pb-1Pt/AC, c)
3Pb-1Pt/CB, and d) 3Pb-1Pt/MWCNTs catalysts.
The particle size distribution (Fig.3, right row) was statistically measured from
the TEM micrograph by counting more than 200 particles. Since the dark spots of the
samples could be Pt, Pb or their alloy, we collectively termed them as the Pt-Pb
cluster. For sample a, the average cluster size was around 2.1 nm. While for samples
b, c and d, the average cluster sizes were obviously smaller, being 1.4 nm, 1.3 nm and
1.7 nm, respectively. In other words, the dispersion of metals over the catalysts were
in the order of c > b > d > a. The cluster sizes of above catalysts showed no obvious
relationship with their respective surface areas. Instead, such variation of metal cluster
size might be closely related with the graphitization of the carbon support [25].
Considering that the metals tend to be situated on the edges or corners of the support,
lower graphitization would expose more such defects. Thus, cluster sizes were quite
small on samples b and c, moderate on sample d, and slightly aggregated on more
graphitic sample a.
3.1.4. XPS analyses of 3Pb-1Pt/carbon catalysts
The X-ray photoelectron spectroscopy was performed to investigate the metal
oxidation states and the Pt/Pb molar ratio on the surface of the 3Pb-1Pt/carbon
catalysts. The Pt 4f spectra of 3Pb-1Pt/carbon catalysts were shown in the left row,
with the two doublet peaks originated from the spin-orbital splitting of the Pt 4f7/2 and
Pt 4f5/2 states, and they were deconvoluted into two groups. For samples a, b and d,
the deconvoluted peaks at 71.3 - 71.6 eV were ascribed to the metallic form of Pt, and
the peaks at 72.6 - 73.5 eV were ascribed to the Pt2+ species, e.g., the Pt-O-Pb state or
the Pt(OH)2 state [26-29]. For sample c, the deconvoluted peak at 72.5 eV also
10 / 19

corresponded to the Pt2+ species, but almost no metallic Pt was detected on its surface.
Instead, the binding energy (BE) peak at 74.6 eV indicated the presence of Pt4+
species (e.g., PtO2) [28] or other forms of Pt2+ species (e.g., PtO) [29]. The Pb 4f
spectra of 3Pb-1Pt/carbon catalysts were shown in the right row, with the two doublet
peaks generated by the spin-orbital splitting of the Pb 4f7/2 and Pb 4f5/2 states. The Pb
4f spectra of these four samples were of similar shape and binding energy. The Pb
4f7/2 peaks at 138.3 - 138.6 eV corresponded to the Pb2+ species, e.g., PbCO3,
Pb(OH)2 and PbO species [30-32].
According to Barrett et al., the Pt 4f
BE peak of bulk Pt foil was at 71.1 eV
7/2
[33]. In our case, the metallic Pt generally showed higher BE peaks than 71.1 eV,
which may be caused by the smaller particle size of Pt [34], as shown by TEM images
(Fig. 2). In addition, compared with monometallic 1.0wt%Pt/AC catalyst (Pt 4f
7/2
peak at 71.7 eV) [3], the introduction of Pb lead to the formation of both Pt0 species at
lower state (71.3 eV) and Pt2+ species on 3Pb-1Pt/AC catalyst. On the one hand, the
presence of metallic Pb would donate electrons to Pt species and lead to the shifting
of Pt BE peaks to lower state [35]. On the other hand, the promoter metal Pb is easily
oxidized by oxygen, and then the Pt species would be surrounded by oxidized Pb
species [36]. That may explain the formation of Pt species at higher oxidation state.
This proposition was also corroborated by the O 1s spectra (Fig.S3, right row), which
showed shoulder peaks at higher BE. Moreover, the higher oxidation state of Pt on
sample c corresponded well with its C 1s and O 1s spectra (Fig. S3, sample c), which
all displayed shoulder peaks at higher BE.
11 / 19

Pt 4f 5/2
Pb 4f 7/2
73.5 eV
Pb 4f 5/2
a2
a1
b1
c1
74.8 eV
76.8 eV
Pt 4f 7/2
143.3 eV
138.3 eV
71.5 eV
nPt/nPb = 0.79
75.9 eV
74.6eV
b2
72.6 eV
143.6 eV
138.6 eV
138.4 eV
71.3 eV
nPt/nPb = 0.17
75.8 eV
77.9 eV
74.6 eV
72.5 eV
c2
143.4 eV
143.5 eV
nPt/nPb = 0.26
74.9 eV
d1
76.5 eV
73.2 eV
71.6 eV
d2
138.5 eV
nPt/nPb = 0.19
86 84 82 80 78 76 74 72 70 68 66
152 150 148 146 144 142 140 138 136 134
B.E.(eV)
B.E.(eV)
Fig. 3. XPS spectra: Pt 4f (left row) and Pb 4f (right row) of catalysts. Samples: a)
3Pb-1Pt/Graphite, b) 3Pb-1Pt/AC, c) 3Pb-1Pt/CB, and d) 3Pb-1Pt/MWCNTs.
The surface Pt/Pb molar ratios determined by XPS data are reported in Fig. 3,
right row. The surface Pt/Pb molar ratios show evident distinction among different
carbon supported catalysts, even though their initial loadings and preparation method
were the same. Theoretically, the 3.0wt% Pb and 1.0wt%Pt would result in a Pt/Pb
molar ratio of 0.35. In fact, Pt was greatly enriched on the surface of
3Pb-1Pt/Graphite, whereas Pb was enriched on the surface of the other three catalysts.
3.2 Results of lactic acid oxidative dehydrogenation
Table 2 shows the results of LA oxidative dehydrogenation over monometallic
1Pt/AC, 3Pb/AC catalysts and bimetallic 3Pb-1Pt/carbon catalysts. Similar to
previous studies which were performed under atmospheric pressure [14, 16], the
monometallic 1Pt/AC catalyst could hardly convert LA, with a LA conversion of
1.5% after 60 min. Tsujino et al. attributed such limited activity of Pd/C in LA
oxidative dehydrogenation reaction to the inability of Pd/C in oxygen absorption
under the reaction conditions [14]. When Pb was introduced to Pt/C catalysts, a
12 / 19

dramatic increase in catalytic activity was observed (Table 2), which is also in line
with literatures [14, 16]. It’s proposed that post-transition metals or semi-metals, such
as Pb, Bi and Te, have some covalent nature, which allows them to modify the Pt
electronic structures (as shown in Fig.3, XPS spectra) and to facilitate the adsorption
of oxidizing species O or OH radical [36]. Then the catalyst activity in LA oxidative
dehydrogenation to PA was greatly enhanced. It’s also worth noting that under
pressurized oxygen atmosphere, monometallic 5Pd/C catalyst would become active
[15]. This might be because that Pt/C or Pd/C catalyst has the potential in catalyzing
the oxidative dehydrogenation of α-position of the hydroxyl group when proper
amount of oxygen species were adsorbed, and high-pressure conditions favored the
adsorption of oxygen species over 5Pd/C catalyst.
Table 2 Catalytic performance of 3Pb-1Pt/carbon catalysts in lactic acid oxidative
dehydrogenation. a
TOF
Time
(min)
60
Conversion
(%)
Product selectivity (%)
Catalyst
(h-1) b
PA
AA c
5.5
-
OAd
5.6
-
Others
34.6
-
1Pt/AC
3Pb/AC
-
-
1.5
54.4
-
60
0
20
39.7
40.5
41.2
31.3
56.9
59.5
65.7
70.7
72.4
54.4
74.5
74.9
97.6
96.5
92.1
98.5
91.1
79.1
91.3
80.6
67.7
98.9
82.3
81.1
1.6
2.4
3.0
0.9
1.5
2.1
1.6
1.5
1.5
1.0
1.6
2.1
0.8
1.0
1.2
0.2
1.0
1.6
0.0
0.9
1.4
0.1
1.2
1.8
0
3Pb-1Pt/Graphite
3Pb-1Pt/AC
465
366
769
637
40
0.1
3.7
0.4
6.4
17.2
7.1
17
60
20
40
60
20
3Pb-1Pt/CB
40
60
29.4
0
20
3Pb-1Pt/MWCNTs
40
14.9
15
60
a
Reaction conditions: t = 90 °C, p = 0.1 MPa, catalyst = 0.25 g, LA (0.2 mol/L) = 25 g, LiOH•
H2O (90%) = 0.21 g, FO2 = 100 mL/min, stirring rate = 800 r/min. b TOF was calculated by the 20
min data. c AA = acetic acid. d OA = oxalic acid.
13 / 19

Since monometallic Pb/AC catalyst was inactive under our conditions, Pb mainly
served as an additive for the catalysts. Hence, we calculated the substrate/metal ratio
based on the loading of Pt. The LA/Pt molar ratio in our case is 390, which is at least
1.8 times of previous reports (< 213) [14-16]. The 3Pb-1Pt/CB catalyst converted
65.7% of LA within 20 min with a PA selectivity of 91.3%. The 3Pb-1Pt/MWCNTs
catalyst showed a LA conversion of 54.4% with PA selectivity up to 98.9% in 20 min.
Even for the less active 3Pb-1Pt/AC and 3Pb-1Pt/Graphite catalysts, over 30% of LA
conversion and over 95% of PA selectivity were achieved in such a short reaction
period. In general, the 3Pb-1Pt/carbon catalysts exhibited obviously elevated catalytic
efficiency than literatures.
The comparison of catalytic results among the 3Pb-1Pt/AC catalyst (entry 1) in
these work and previous typical catalysts (entry 2-4) were shown in Table S1.
Considering that one of the major difference between current work (Table 2, LA at 0.2
mol/L) and previous work (LA at 0.5 mol/L) is the concentration of LA [14-16], we
tested the 3Pb-1Pt/AC catalyst in a 0.5 mol/L LA substrate. The 1 h yield of PA over
3Pb-1Pt/AC was slightly decreased from 47% to 40% when LA concentration
increased from 0.2 mol/L to 0.5 mol/L, confirming the influence of LA concentration.
Lower LA concentration means longer contact time, with other conditions remaining
the same. To better understand the relation between catalyst activity and contact time,
we conducted two series of reactions with different catalyst amount (0.13 - 0.5 g) over
both 3Pb-1Pt/CB and 3Pb-1Pt/AC catalysts (Fig. S4). For nanoscale 3Pb-1Pt/CB
catalyst, the increase in catalyst amount did not result in increase in LA conversion,
which implies that the reaction over 0.25 g 3Pb-1Pt/CB was not in the kinetic regime.
However, for micrometer-scale 3Pb-1Pt/AC catalyst, there is a linear relation between
catalyst amount and LA conversion, suggesting the limitation of kinetic factors.
Meanwhile, the lower LA concentration (or the contact time) was not the only
factor for the overall better catalytic efficiency of 3Pb-1Pt/carbon catalysts. With 0.5
mol/L LA as substrate, comparison between Table S1, entries 1, 2 and 4 again
14 / 19

illustrated the enhanced catalytic efficiency of 3Pb-1Pt/AC. It yielded 40% of PA
within 1 h, with noble metal loading at 1.0 wt% and catalyst amount of 0.25 g,
whereas 2 g of 3Pb-5Pt/C required 3 h to yield 60% of PA. Even though the catalyst
amount of 0.95Te-5Pd/C was the same as 3Pb-1Pt/AC, it required 5 h and 1.0 MPa of
pressurized oxygen to obtain its highest yield.
Another possible explanation for our better catalytic performance is the metal
particle size. Although TEM images of the previous catalysts were not shown, we
could easily tell from the XRD patterns that their metal particles were generally large
(> detection limit of 4.5 nm) [14, 15]. However, the metal particles in our work are
relatively small. Smaller metal particle means better metal dispersion, which would
provide more exposure of active sites to facilitate the reaction. Thus, even at same
substrate concentration (LA, 0.5 mol/L), the 3Pb-1Pt/AC catalyst still showed higher
reaction efficiency than the reported 3Pb-5Pt/C catalyst [14].
When focused on the activity of the four catalysts in this work, the initial reaction
rate of the catalysts represented by TOF indicates that the nanoscale catalysts are
superior to the micrometer-scale catalysts. Specifically, the nanosphere catalyst
(3Pb-1Pt/CB) had a higher TOF than the nanotube catalyst (3Pb-1Pt/MWCNTs),
which may be related with the shorter length of the former support (shown in Fig.2).
Similarly, the 3500-meshed 3Pb-1Pt/Graphite catalyst converted LA at a faster rate
than the 200-meshed 3Pb-1Pt/AC catalyst at the initial stage. Then, we tested the
reaction on both bigger (40-60 mesh) and smaller (ball milled for 1 h) AC supported
3Pb-1Pt catalysts (Table S2). The catalyst with bigger AC particle size displayed
reduced catalytic activity, whereas the ball-milled AC supported 3Pb-1Pt/AC catalyst
showed a similar TOF (615 h-1) and PA selectivity with the nanoscale catalysts (e.g.,
3Pb-1Pt/MWCNTs) in Table 2. Therefore, the support particle size is an important
factor for obtaining high transformation efficiency.
When reaction time was prolonged to 40 min or longer, the catalyst performance
in LA conversion showed some changes. The two nanoscale catalysts still maintained
at high LA conversions, but the gap between the activities of 3Pb-1Pt/MWCNTs and
3Pb-1Pt/CB was narrowed. The activity of 3Pb-1Pt/AC even surpassed the
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3Pb-1Pt/Graphite catalyst. This suggests that there are other factors affecting the
catalyst activity. After the diffusion of substrate onto the surface of the catalyst, it
would be activated through the interaction with the metal clusters. Therefore, a better
metal dispersion would favor such interactions. Then, the reaction rate of the
3Pb-1Pt/Graphite might fall behind because of its lower metal dispersion.
In addition, the graphitization degree and the shape of the support might impact
the catalyst activity by holding Pt-Pb cluster of different size. According to the TEM
images of fresh catalysts (Fig.2) and spent catalysts (reacted for 6 h, Fig. S5), the
Pt-Pb cluster on Graphite support increased from around 2.1 nm (fresh) to 2.5 nm
(spent). Whereas the aggregation phenomenon was limited on AC support (fresh: ca.
1.4 nm vs. spent: ca. 1.5 nm). This is due to the high graphitization degree of Graphite
and low graphitization degree of AC support [25]. Thus, the 3Pb-1Pt/AC catalyst
would provide more active sites and favor LA transformation. As for the two spent
nanoscale catalysts, the average metal particle size on CB support (less graphitic) was
smaller than that on MWCNTs support (more graphitic). But the clusters increased
slightly from around 1.3 nm to 1.5 nm on CB support, while maintaining a stable size
on MWCNTs support. Possibly, this is related with the shape of the support, i.e. the
nanotubes confinement effect [37]. Therefore, the 3Pb-1Pt/MWCNTs catalyst showed
better catalytic activity when time was prolonged.
Moreover, the surface molar ratio of Pt/Pb as well as the oxidation states of Pt
and Pb species might also affect the catalyst performance. Under current conditions,
the proper ratio of Pt/Pb should be around 0.26 (corresponding to 3Pb-1Pt/CB
catalyst); the surface oxidation states of metals might influence the activation of
reactant, but more investigation is still necessary to obtain detailed information.
Table 3
Comparison of PA selectivity over 3Pb-1Pt/carbon catalysts at similar LA conversion. a
Time
(min)
15
Conversion
(%)
Product selectivity (%)
Catalyst
PA
AA b
OA c
Others
0
3Pb-1Pt/Graphite
3Pb-1Pt/AC d
27.0
95.4
98.5
2.0
2.6
20
31.3
0.9
0.2
0.4
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3Pb-1Pt/CB
5
28.1
24.2
95.4
96.0
1.1
0.7
0.5
0.6
3.0
2.7
3Pb-1Pt/MWCNTs
11
a
Reaction conditions: t = 90 °C, p = 0.1 MPa, catalyst = 0.25 g, LA (0.2 mol/L) = 25 g, LiOH•
H2O (90%) = 0.21 g, FO2 = 100 mL/min, stirring rate = 800 r/min. b AA = acetic acid. OA =
oxalic acid. d Data reproduced from Table 2.
c
Table 3 displays the comparison of PA selectivity over 3Pb-1Pt/carbon catalysts
at similar LA conversion (around 27%). The difference in PA selectivity among the
3Pb-1Pt/carbon catalysts was quite limited. That might be because that the activity to
afford PA originates from the Pb-Pt interactions (Pb-Pd interactions in Tsujino’s work),
and the effect of support is minor [14]. Then, all the 3Pb-1Pt/carbon catalysts
displayed a descending PA selectivity with time on stream (Table 2), in accordance
with literatures [14]. In contrast, the selectivity for further oxidation product (AA and
OA) increased significantly [36]. Since Pt was an excellent catalyst in oxidation
reactions [38], we supposed that the decline in PA together with the increase in AA
and OA were related with the oxidative decomposition of PA. The oxidation reaction
of PA over 3Pb-1Pt/AC catalyst confirmed the proposition (Fig. S6). As Fig. S6 shows,
the conversion of PA resulted in the obvious increase in both AA and OA selectivity.
Therefore, we proposed a possible reaction pathway for the oxidative
dehydrogenation of LA to PA and its major byproducts in Scheme 1.
Scheme 1. Proposed reaction pathway for oxidative dehydrogenation of LA to PA and major
byproducts.
4. Conclusions
The Pb promoted Pt catalysts supported on various carbon materials were
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investigated to be effective for oxidative dehydrogenation of LA to produce PA under
mild reaction conditions. The 3Pb-1Pt/carbon catalysts showed much improved
catalytic efficiency than those previously reported, related with both substrate
concentration and metal particle sizes. For the 3Pb-1Pt/carbon catalysts, smaller
support particle size as well as smaller metal particle size are important for the
enhanced LA transformation efficiency, and the metal particle size is mainly affected
by the graphitization degree of the carbon support. In addition, the oxidation state and
molar ratio of Pt and Pb were different over various carbon materials, and this would
also affect their respective activity, which still requires further research.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(21176236).
Appendix A. Supplementary data
Supplementary data to this article can be found online.
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