Efficient Conversion of Bio-Lactic Acid to 2,3-Pentanedione on Cesium-Doped Hydroxyapatite Catalysts with Balanced Acid–Base Sites

Article abstract of DOI:10.1002/cctc.201701332

We report the design and synthesis of cesium-doped hydroxyapatite for direct and high-yield conversion of biobased lactic acid to 2,3-pentanedione (72.3 %). Cs species derived from CsNO₃ at high temperature of calcination is introduced into the hydroxyapatite structure to regulate its acid–base properties. It is found that a balance of acid–base chemistry favors the condensation of lactic acid to 2,3-pentanedione. As a result, the undesired reactions such as lactic acid dehydration, decarbonylation, and coking are suppressed. Instead, a concerted catalysis between surface basic site and acidic site for lactic acid condensation to 2,3-pentanedione dominates on the cesium-doped hydroxyapatite catalyst, leading to a highly selective process for direct conversion of bio-lactic acid to 2,3-pentanedione.

Full text of DOI:10.1002/cctc.201701332

Accepted Article
Title: Efficient Conversion of Bio-lactic Acid to 2,3-Pentanedione on
Cesium Doped Hydroxyapatite Catalysts with Balanced Acid-
Base Sites
Authors: Xinli Li, Liangwei Sun, Weixin Zou, Ping Cao, Zhi Chen,
Congming Tang, and Lin Dong
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ChemCatChem
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Efficient Conversion of Bio-lactic Acid to 2,3-Pentanedione on
Cesium Doped Hydroxyapatite Catalysts with Balanced Acid-
Base Sites
Xinli Li , Liangwei Sun , Weixin Zou , Ping Cao , Zhi Chen , Congming Tang*[a,b], and Lin Dong[c]
[
a]
[b]
[c]
[a]
[a]
Abstract: We report the design and synthesis of cesium doped
have displayed an unsatisfied performance, ca., low 2,3-
hydroxyapatite for direct and high-yield conversion of bio-lactic acid
pentanedione selectivity and high acrylic acid selectivity. For
o
to 2,3-pentanedione (72.3%). Cs species derived from CsNO
3
at
example, over the 9%NaNO
3
/SBA-15 catalyst at 340 C, the
high temperature of calcination is introduced into the hydroxyapatite
structure to regulate its acid-base properties. It is found that a
balance of acid-base chemistry favors the condensation of lactic acid
to 2,3-pentanedione. As a result, the undesired reactions such as
lactic acid dehydration, decarbonylation and coking are suppressed.
Instead, a concerted catalysis between surface basic site and acidic
site for lactic acid condensation to 2,3-pentanedione dominates on
the cesium doped hydroxyapatite catalyst, leading to a highly
selective process for direct conversion of bio-lactic acid to 2,3-
pentanedione.
selectivities of 2,3-pentanedione and acrylic acid are 62.1% and
18.4%, respectively, and LA conversion is low, only 58.8%.[
Furthermore, most of all these catalysts are short of evidence of
catalyst stability. Thus these have prompted us to further
investigate on the relation between the property derived from
catalyst structure as well as its components and activity to
achieve an efficient catalyst for condensation of LA into 2,3-
pentanedione. Hydroxyapatite is used as biomedical materials
due to an excellent biocompatibility. Recently, it is also extended
to efficient catalysts for dehydration of LA to acrylic acid.[3d, 8] In
comparison with its bare form, the modified hydroxyapatite
displayed better properties in application.[ For example, in zinc
incorporated hydroxyapatite, the preference of Zn for the Ca2
site may facilitate uptake and release of Zn by biological
hydroxyapatite.[10]
6b]
9]
Introduction
Chemicals and fuels are obtained from abundant, renewable
biomass and its derivatives via catalytic conversion, which
provides a viable route to alleviate our strong dependence on
increasingly depleting fossil fuels.[1] For that reason, lactic acid
In this work, we developed a novel approach for preparation of
cesium doped hydroxyapatites, and investigated their catalytic
performance for condensation of LA to 2,3-pentanedione. In
experiments performed to date, 2,3-pentanedione yields as high
as 72.3% have been achieved, with acetaldehyde and acrylic
acid, both being still valuable, as byproducts. More importantly,
the catalyst is very stable for at least 40 h on stream (Figure 1).
The unprecedented catalytic performance relates to synergy
between basic sites and acidic sites located on the surface of
cesium doped hydroxyapatite catalyst.
(
LA) is particularly attractive due to its facile synthesis by
biomass fermentation together with chemical conversion of
glycerol, and expected increased availability and reduced cost.[2]
Recent years, conversion of LA into chemicals such as acrylic
acid,[3] acetaldehyde,[4] poly-lactic acid,[5] and 2,3-pentanedione[6]
has become a heated topic for the chemical industry to decrease
the use of petrochemical reserves. Currently, synthesis of 2,3-
pentanedione is mainly via oxidation of methyl propyl ketone in
the presence of hydroxylamine hydrochloride catalyzed by
NaNO
2
and HCl.[7] Besides, it can also be obtained by
condensation of acetol with hexanal catalyzed by acid. Here, a
promising opportunity arises to replace petroleum-based
feedstock by renewable lactic acid for greener and more
sustainable 2,3-pentanedione production.
In the past decades, the investigation on condensation of LA
into 2,3-pentanedione has focused on carbon/silicon supported
on alkali metal nitrates or phosphates as catalysts.[6,
Although the reaction conditions are optimized, these catalysts
7b-d]
[
[
[
a]
b]
c]
Xinli Li, Ping Cao, Zhi Chen and Prof. Congming Tang
School of Chemistry and Chemical Engineering
Chongqing University of Technology
Chongqing 40054, PR China
E-mail: tcmtang2001@163.com (C. M. Tang)
Liangwei Sun
Chemical Synthesis and Pollution Control Key Laboratory of
Sichuan Province
China West Normal University
Figure 1. LA conversion and 2,3-pentanedione selectivity vs time on stream
using cesium doped hydroxyapatite. (a: Catalyst, 0.38 mL, catalyst with Ca :
o
P : Cs = 1.622 : 0.958 : 1.667, calcination temperature: 700 C, carrier gas N
2
:
1
mL/min, feed flow rate: 1 mL/h, LA feedstock: 20wt% in water, reaction
o
temperature, 290 C. b: LA: lactic acid, 2,3-PD: 2,3-pentanedione.)
Nanchong, Sichuan, 637002, PR China
Weixin Zou and Prof. Lin Dong
Jiangsu Key Laboratory of Vehicle Emissions Control, Center of
Modern Analysis,
Nanjing University, Nanjing 210093, PR China
Supporting information for this article is given via a link at the end of
the document.
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ChemCatChem
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FULL PAPER
Results and Discussion
could be observed (Figure S3), in agreement with the XRD
results.
Catalyst characterization
Nitrogen physisorption results show in Table S1 and 2. The
specific surface area reduces from 38.3 to 2.0 m /g with
2
To incorporate the multiple functionality needed for a catalytic
conversion of process that directly converts bio-lactic acid to 2,3-
increase of calcination temperature while the average pore size
for all samples remains 3.8 nm (Table S1). From the results
showed in Table S2, the specific surface area for samples with
pentanedione,
a number of cesium doped hydroxyapatite
materials were prepared by the means of inducing cesium
component and high–temperature calcination, which favors
cesium component dispersion in the structure of hydroxyapatite
to regulate acid-base properties. The physical properties of
these catalyst materials will be firstly described.
2
varying Cs contents slightly fluctuates between 7.2 and 9.1 m /g.
Similarly, the average pore size also ranges from 3.4 to 3.8 nm.
Hydroxyapatite and cesium doped hydroxyapatite materials
with varying Ca/Cs ratios, unless otherwise noted, were
prepared with one-pot synthesis, and characterized by XRD, FT-
IR, SEM-EDX, surface area / pore size measurements. Figure 2
showed the wide-angle X-ray diffraction (XRD) patterns of
catalyst materials with Ca : P : Cs = 1.622 : 0.958 : 1 at different
calcination temperatures. Hydroxyapatite phase is confirmed
o
among all the samples except for the sample calcined at 800 C.
CsNO
3
phase is also evidently determined in the samples which
o
were calcined at lower 600 C, and it disappears with further
increase of calcination temperatures, ca. 700 C. Although
CsNO
o
o
3
disappears at higher than 700 C, Cs is doped
homogeneously into the structure of hydroxyapatite, which is
confirmed by EDX analysis (Figure S1, A) together with its
corresponding elemental mapping (Figure S1, B). These similar
phenomena were also observed in the samples with different Cs
Figure 2. XRD patterns of catalysts at different calcination
temperatures (Ca : P : Cs = 1.622 : 0.958 : 1)
o
doped amounts at calcination temperature of 700 C (Figure 3).
3
However, the CsNO phase occurred obviously when the Cs
doped amount increased largely, ca., ratio of Ca : P : Cs being
up to 1.622 : 0.958 : 2.333. Samples (a-g) can hardly observe
the characteristic diffraction peaks for CsNO
3
phase since the
o
calcination temperature (700 C) is higher than its discomposed
o
temperature (600 C). Furthermore, the diffraction peaks indexed
to hydroxyapatite also reduce in peak intensity, suggesting that
Cs is well doped into structure of hydroxyapatite. To obtain
further insight into Cs-hydroxyapatite support interactions, we
studied the oxidation state of Cs in catalyst before and after
o
calcination at 700 C by X-ray photoelectron spectroscopy (XPS,
Figure 4). The binding energy (BE) was corrected for surface
charging by taking the C 1s peak of contaminant carbon as a
reference at 284.6 eV. The Cs 3d5/2 BE in the catalyst calcined
o
at 700 C is about 723.36 eV, lower than that before calcination
(
723.76 eV). This phenomenon can be explained that before
o
-, while Cs changes
calcination at 700 C, Cs combines with NO
to combine with PO 3-
3
after calcination because NO -
4
3
Figure 3. XRD of catalyst with different loading CsNO
at calcination temperature of 700 C
a : Ca : P = 1.622 : 0.958, b : Ca : P : Cs = 1.622 : 0.958 : 0.334, c : Ca :
P : Cs = 1.622 : 0.958 : 0.667, d : Ca : P : Cs = 1.622 : 0.958 : 1, e : Ca :
P : Cs = 1.622 : 0.958 : 1.333, f : Ca : P : Cs = 1.622 : 0.958 : 1.667, g :
Ca : P : Cs = 1.622 : 0.958 : 2, h : Ca : P : Cs = 1.622 : 0.958 : 2.333
3
amount
decomposes at high temperature. Electronegativity for
element is stronger than P element. This also confirmed that Cs
component is well doped into the structure of hydroxyapatite. In
N
o
addition, FT-IR spectra is also used to investigate the structure
of catalyst (Figure S2 and 3). Absorption band at ~1396 cm-1
-
ascribed to NO
3
disappears when the calcination temperature
o
-1
increases to 700 C while the new absorption band at ~ 910 cm
occurs, which confirms the strong interaction between Cs and
PO 3
~
-
(Figure S2). At higher Cs content, the absorption band at
phase
4
1385 cm-1 indexed to aggregation of a separate CsNO
3
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ChemCatChem
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catalytic activity increases with an increase of Cs content.
However, catalytic activity remains constant (~90% of LA
conversion and 70% of 2,3-pentanedione selectivity) when Cs
content is above 1 : 1.622 for Cs/Ca molar ratio. But excessive
Cs content such as Cs/Ca molar ratio = 2.33 : 1.622 does not
favor the catalytic performance (LA conversion = 70% and 2,3-
pentanedione selectivity = 60.9%). To the best of our knowledge,
direct conversion of LA to 2,3-pentanedione with both a high
selectivity (~70%) and a high LA conversion (~90%) has never
been reported. These observations indicate that calcination and
the Cs-doped amount enable improvement of acid-base
properties of the hydroxyapatite for direct conversion of LA to
2,3-pentanedione.
Table 1. Catalytic Activity Results for Hydroxyapatite Doped with Cs at
Different Calcination Temperatures[a]
Area-specific catalytic rate
Sel. [%][b]
LA
(μmol·h-1·m -2
)
Calcined
conv.
[%]
Figure 4. XPS spectra of catalyst (Ca : P : Cs=1.622 : 0.958 : 1)
o
temp./ C
2,3-
PD
LA
PD
o
before and after calcination at 700 C (sample 1, uncalcination;
AD
PA
AA
ACA
consumption
103
formation
22
o
sample 2, calcined at 700 C)
-
72.5
73.6
76.9
88.2
89.9
51.4
~ 10
42.1
53.7
59.8
64.6
66.9
65.5
-
16.0
11.7
9.5
7.7
7.2
7.2
6.4
6.2
8.0
15.4
17.7
17.8
15.6
15.1
14.0
6.2
6.1
1.5
1.7
1.4
1.8
Catalytic performance
400
325
88
5
6
7
8
9
00
00
00
00
00
428
128
182
278
243
-
Table 1 shows the performance of hydroxyapatite doped with Cs
at different calcination temperatures for the condensation
reaction of LA to 2,3-pentanedione. Initially, LA conversion
lineally increases with an increase of calcination temperature, up
10.9
10.1
7.4
564
831
744
617
o
to 700 C, and then it decreases drastically with further increase
a: Catalyst, 0.38 mL, Ca/P/Cs = 1.622:0.958:1, particle size: 20~40 meshes,
of calcination temperature. In contrast with effect of calcination
temperature on LA conversion, the selectivity to 2,3-
pentanedione is relatively sensitive to calcination temperature.
carrier gas N
2
: 1 mL/min, feed flow rate: 1 mL/h, LA feedstock: 20wt% in water,
o
reaction temperature, 300 C, TOS: 4~5 h. b: LA: lactic acid, 2,3-PD: 2,3-
pentanedione, AD: acetaldehyde, PA: propionic acid, AA: acrylic acid, ACA:
acetic acid.
o
When the calcination temperature increases from 120 C (drying
temperature) to 700 oC, the selectivity to 2,3-pentanedione
increases from 42.1% to 66.9%. But more than 700 oC, the
selectivity to 2,3-pentanedione also drastically decreases. For
Table 2. Effect of Cs Doped Amount on Catalytic Performance of the
Condensation Reaction of LA[a]
Area-specific catalytic rate (μmol·h-
o
example, at calcination temperature of 900 C, 2,3-pentanedione
Ca/Cs
molar
ratio
LA
Sel. [%][b]
1·m -2
)
is hardly detected, and LA conversion is very low, only ~10%.
Again, correlating the catalyst performance with its calcination
temperature discloses that volcano-type dependence appears
between 2,3-pentanedione production rate and calcination
temperature, which reveals an important factor of choosing
calcination temperature for selective production of 2,3-
conv.
[%]
2
,3-
AC
A
LA
AD
PA
AA
PD formation
PD
consumption
98
1
.622
:
:
:
:
:
:
:
:
~
10
-
-
-
1.622
10.
5
50.5
66.7
89.9
90.5
91.4
86.3
70
54.3
68.8
66.9
69.2
69.9
72.7
60.9
2.8
9.1
7.9
6.2
6.6
5.4
5.9
8.7
3.1
2.4
1.4
1.6
1.2
1.4
2.8
457
646
831
866
906
789
823
124
222
278
300
317
287
251
o
0.334
pentanedione, 700 C being the optimal calcination temperature
1
.622
.667
11.
1
in our experiment conditions performed for condensation
reaction of LA into 2,3-pentanedione. Under the fixed calcination
temperature (700 oC), we further investigate the effect of Cs
doped amount on the catalytic performance (shown in Table 2).
On the bare hydroxyapatite catalyst (without Cs component), the
catalytic activity is very low, LA conversion being around 10%,
and 2,3-pentanedione is hardly detected. It is noted that
hydroxyapatite is an excellent catalyst for dehydration of LA to
8.1
0
1.622
1
15.
1
10.1
9.2
1
1
1
1
1
2
1
2
.622
.333
.622
.667
.622
13.
2
13.
9
9.3
11.
1
acrylic acid, and low 2,3-pentanedione selectivity (around 1%) is
8.6
o
reported at 360 C of reaction temperature.[3d, 8a] When a small
.622
.33
12.
7
amount of Cs component is doped into hydroxyapatite structure
14.6
(
Ca/Cs molar ratio = 1.622 : 0.334), LA conversion of 50.5% as
a: Catalyst, 0.38 mL, fixed Ca / P molar ratio = 1.622 : 0.958, calcination
well as 2,3-pentanedione selectivity of 54.3% is achieved,
indicating that additional catalytic chemistry is taking place. In
the range of low content of Cs (Ca/Cs molar ratio > 1.622 : 1),
o
temperature: 700 C, particle size: 20~40 meshes, carrier gas N
2
: 1 mL/min,
feed flow rate: 1 mL/h, LA feedstock: 20wt% in water, reaction temperature,
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o
3
00 C, TOS: 4~5 h. b: LA: lactic acid, 2,3-PD: 2,3-pentanedione, AD:
were obtained under the optimal reaction conditions tested in
these studies (Figure 5A).
acetaldehyde, PA: propionic acid, AA: acrylic acid, ACA: acetic acid.
The reaction mechanism on condensation of LA to 2,3-
pentanedione has been investigated over the sodium salts/silica
or silica-alumina catalysts.[6b, 7a, 7d] In those studies, the sodium
lactate in-situ formed via reaction of initial sodium salts with
lactic acid is believed as an important stable intermediate for
conversion of LA to 2,3-pentanedione, and is confirmed by
experimental evidence. In the process for formation of 2,3-
pentanedione from LA, the two key steps including
decarboxylation of LA and ketonization accompanying with loss
of water are involved. The catalyzed decarboxylation reaction
requires basic sites while the catalyzed ketonization with loss of
water requires acidic sites. In other words, developing a
bifunctional catalyst which possesses both acidic sites and basic
sites on its surface has a potential chance for catalyzing the
condensation of LA to 2,3-pentanedione efficiently. According to
previous studies, hydroxyapatite has been found to have an
excellent activity for dehydration of LA due to the presence of
the appropriate acidic sites on its surface.[
3d, 8a, 11]
For that
reason, hydroxyapatite has been chosen as a catalyst support
due to an existence of appropriate acidic sites for dehydration
reaction of LA and its derivatives. But, possessing inherent acid-
base properties on its surface can not well catalyze the
condensation of LA to 2,3-pentanedione. Thus under the high
3
temperature calcination, active precursor (CsNO ) has been
doped into its structure to regulate acid-base properties via in-
situ synthesis by means of one-pot method, and the results has
been shown in Figure 6, S4 and S5. Acidity density
Figure 5. performance of catalyst with Ca : P : Cs = 1.622 : 0.958 : 1.667 for
the condensation of LA as
a function of residence time and reaction
characterized with NH
temperature is enhanced, but a volcano-type appears between
basicity density characterized with CO -TPD and calcination
3
-TPD increases linearly when calcination
temperature (A), and effect of LA concentration at 290 oC (B). a: 0.23g of
o
catalyst, calcination temperature: 700 C, LA concentration: 20 wt%, TOS: 4~5
2
h. b: LA: lactic acid, 2,3-PD: 2,3-pentanedione, AD: acetaldehyde, PA:
propionic acid, AA: acrylic acid, ACA: acetic acid.
temperature. Due to this, basicity/acidity ratio functions as
calcination temperature, obtaining a similar volcano-type curve.
Furthermore, we correlated area-specific catalytic rates including
LA consumption rate and 2,3-pentanedione formation rate with
basicity/acidity ratio, and the results were shown in Figure 7.
The area-specific catalytic rates increase slowly with
basicity/acidity ratio in the range from 4.0 to 7.5, but decrease
drastically as the basicity/acidity ratio is lower than 4.0. From the
data shown in Figure 7, it seems that catalytic activity for
conversion of LA to 2,3-pentanedione continuously increases
with enhancement of basicity/acidity ratio under the ratio of
basicity/acidity > 4. But this is not true from the data shown in
Multiple factors including reaction temperature, residence time,
and LA concentration were found to affect LA conversion and
2,3-pentanedione selectivity on the hydroxyapatite doped with
Cs (Ca : P : Cs = 1.622 : 0.958 : 1.667). LA conversion
drastically increases while 2,3-pentanedione selectivity has only
a slight fluctuation with residence time (Figure 5A), suggesting
that condensation reaction of LA to 2,3-pentanedione dominates.
In addition, we also found that with an increase of reaction
temperature, the selectivity to 2,3-pentanedione decreases while
the selectivities to by-products such as acetaldehyde and acrylic
Table
basicity/acidity ratio is more than 19 on bare hydroxyapatite
without Cs doped amount), while the activity for condensation
2
together with Figure 6B. For example, the
acid increase. Under a relatively low reaction temperature
o
(
290 C), the effect of LA concentration on reaction performance
(
was investigated (Figure 5B). 2,3-Pentanedione selectivity
increases with an increase of LA concentration while LA
conversion decreases. According to previous kinetic studies,
formation of 2,3-pentanedione from LA via condensation
reaction is second order in LA concentration while other
reaction of LA to 2,3-pentanedione is very low, LA conversion
being ~10%. Interestingly, the Cs component is doped into the
structure of hydroxyapatite, resulting in a rapid decrease of the
basicity/acidity ratio, and it remains to 7~8 although the Cs
doped amount changes. The results were supported by the
evidence obtained with the diffuse reflectance infrared Fourier
transform spectra (DRIFTS) of adsorbed pyridine on catalysts
because Brönsted acidic sites and Lewis sites increased in
contrast with bare hydroxyapatite (Figure S6). More importantly,
all these catalysts with doped Cs offer an acceptable selectivity
accompanied conversion reactions of LA to acrylic acid and
acetaldehyde is first order in LA concentration.[7c,
7d]
Thus
elevated LA concentration favors the formation of 2,3-
pentanedione, to improve 2,3-pentanedione selectivity.
Importantly, note that 2,3-pentanedione yields as high as 72.3%
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ChemCatChem
10.1002/cctc.201701332
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to 2,3-pentanedione (54.3~72.7%). From the above
observations, we can conclude that both calcination temperature
and Cs doped amount can provide a balance of surface acid-
base chemistry in the Cs doped hydroxyapatite, which favors the
activity for condensation of LA to 2,3-pentanedione.
form (I). After deprotonation, the process undergoes a Claisen
condensation, followed by decarboxylation to produce form (II).
In this step, a large number of CO
trace of CO was detected in the on-line reaction tail gas analysis
shown in Figure S7). Step 2, acidic sites attack β-OH of form (II)
2
was detected while only
(
to remove a molecular water, and form another enolate (III).
Form (III) rapidly isomerizes into product of 2,3-pentanedione.
Scheme 1. Proposed mechanism for the condensation of LA to
2,3-pentanedione. Reaction process highlighted in green is
preferred pathway, while the pink highlighted steps are
undesirable ones. Detailed reaction pathways see Scheme S1.
Conclusions
We report efficient conversion of bio-lactic acid to 2,3-
pentanedione, a highly value-added fine chemical useful for
flavoring chemicals, using
a multifunctional cesium doped
hydroxyapatite catalyst. The unique modification of cesium
doped hydroxyapatite provides a balance of the surface acid-
base chemistry in the catalyst. Consequently, undesirable
reactions such as dehydration and decarbonylation of lactic acid
which are mainly believed to catalyze by acidic sites, are largely
suppressed, while the surface basic site-acidic site synergically
catalyzed lactic acid condensation reaction to 2,3-pentanedione
dominates on the cesium doped hydroxyapatite. In this way, a
highly selective (as high as 72.3% yield) process for direct
conversion of bio-lactic acid to 2,3-pentanedione on the cesium
doped hydroxyapatite has been achieved.
Figure 6. The acid-base properties of catalysts versus calcination temperature
(
A, (Ca : P : Cs=1.622 : 0.958 : 1)) and the Cs doped amount (B, calcination
o
temperature, 700 C)
Experimental Section
Materials
Lactic acid (85~90 wt%) was purchased from Chengdu Kelong Chemical
Reagent Co. and was used for the synthesis of 2,3-pentanedione without
further purification. Triple-distilled water was prepared in the laboratory
and was used to dilute lactic acid for demanded concentration. Cesium
Figure 7. Catalytic rate versus base/acid ratio
Based on these experimental evidences, the possible reaction
mechanism was proposed (shown in Scheme 1 and S1). In the
mechanism, two steps are crucial for the formation of 2,3-
pentanedione from lactic acid. Step 1, the basic sites on the
surface of catalyst attack H at α-carbon to produce an enolate
2 3 4
nitrate, Ca(OH) , H PO , 2,3-pentanedione, propionic acid, acetaldehyde,
acrylic acid, acetic acid and n-butanol, together with hydroquinone were
purchased from Sinopharm Chemical Reagent Co., Ltd. 2,3-
Pentanedione, propionic acid, acetaldehyde, acrylic acid and acetic acid
were used for gas chromatograph reference materials, and n-butanol
was utilized as internal standard material. Hydroquinone (0.3 wt %) was
used as a polymerization inhibitor.
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Preparation of catalysts
charging by taking the C 1s peak of contaminant carbon as a reference
at 284.6 eV.
The Cs doped hydroxyapatite catalysts were prepared by an
evaporation-driven oriented assembly (EDOA) approach in
Ca(OH) /H PO /H O/CsNO mixed solution. Typically, for Cs doped
hydroxyapatite with Ca : P : Cs = 1.622 : 0.958 : 1, 1.85 g (25 mmol) of
Ca(OH) was dissolved in 100 mL H O, and stirred vigorously for 30 min
to form clear and transparent solution. Sequentially, 1.0 mL
14.76mmol) of H PO (85 wt%) was added dropwise under vigorous
stirring for 60 min to form white murky solution. Next, 3.0
15.41mmol) of CsNO was added to the resultant solution under
a
basic
Catalyst evaluation
2
3
4
2
3
The condensation of lactic acid to 2,3-pentanedione over the catalysts
was carried out in a fixed-bed quartz tubular reactor with a 4 mm inner
diameter operated at atmospheric pressure. The catalyst (ca. 0.23~0.25
g, 20~40 meshes) was placed in the middle of the reactor and quartz
wool was placed in both ends. Firstly, the catalyst was pretreated at the
2
2
a
(
3
4
a
g
(
3
2
required reaction temperature (ca. 300 °C) for 1.5 h under N with high
vigorous stirring for 300 min. After that, the obtained solution was
transferred into a volumetric flask, and left it in a drying oven to
purity (0.1 MPa, 1.0 mL/min). The feedstock (20 wt% solution of LA) was
then pumped into the reactor (LA aqueous solution flow rate, 1.0 mL/h)
and driven through the catalyst bed by nitrogen. Residence time is
calculated according to equation 1. The liquid products were condensed
evaporate solvent H
00 oC for another 10 h to completely remove the solvent. The obtained
O at 70 oC for 8 h, then second-step evaporate at
2
1
o
white powder was calcined at 700 C for 600 min to use as a catalyst for
LA condensation to 2,3-pentanedione. The bare hydroxyapatite (without
Cs component) was prepared using a similar method.
using ice-water bath and analyzed off-line using
a SP-6890 gas
chromatograph with a FFAP capillary column connected to a FID.
Quantitative analysis of the products was carried out by the internal
standard method using n-butanol as the internal standard material. GC-
MS analyses of the samples were performed using Agilent 5973N Mass
Selective Detector attachment. The reaction tail gas was analyzed using
GC with a packed column of TDX-01 connected to TCD detector. The
conversion of LA and the selectivity toward 2,3-pentanedione or other by-
products were calculated according to equations 2, 3 and 4.
Catalyst characterization
Powder X-ray diffraction measurement was conducted on a Dmax/Ultima
IV diffractometer operated at 40 kV and 20 mA with Cu-Ka radiation. The
FTIR spectra of the catalysts were recorded in the range of 500~4000
cm–1 on a Nicolet 6700 spectrometer. The morphologic features of the
catalysts were determined by scanning electron microscope (SEM, JSM-
w
-
----- (1)
tR 
6510). The specific surface areas of catalysts were measured through
F
nitrogen adsorption at 77 K using Autosorb IQ instrument. Prior to
adsorption, the samples were treated at 250 oC under vacuum for 6 h
and the specific surface area was calculated according to the Brunauer-
Emmett-Teller (BET) method. Surface acid and base properties of the
−1
t
R
.
: Residence time (hgmL ); w : catalyst mass (g); F: feed (LA
aqueous solution) flow rate (mL/h).
3 2
samples are estimated by NH -TPD and CO -TPD, respectively, on a
n0  n
1
----- (2),
Conversion/ % 
Selectivity/% 
100
100
Quantachrome Instrument. The sample (ca. 50~60 mg) is purged with
n0
o
dry Ar (50 mL/min, purity> 99.999vt%) at 400 C for 1.0 h, followed by
reducing the furnace temperature to room temperature, and switching to
np
np
------ (3),
----- (4)
a flow of 8v
t
3
% NH /Ar or 10 v
t
% CO
2
/Ar for 1 h to execute NH
3
or CO
2
Yield / %  n0 100
n0  n
1
adsorption. Then, NH
3
or CO
2
adsorbed on the sample is desorbed in the
range of 80~600 C at a rate of 10 oC/min. FTIR of adsorbed pyridine
experiments were performed on a Bruker Tensor 27 FTIR spectrometer
equipped with Praying Mantis™ accessory and a high-pressure reaction
cell (Harrick Scientific Products Inc.) for in situ Diffuse Reflectance
Infrared Fourier Transform spectroscopy (DRIFTS) measurements.
Briefly, ~50 mg of finely ground sample was loaded in the reaction cell.
The sample was then ramped to 400 ⁰C (10 ⁰C/min) for in-situ purging
under flowing pure Ar (flow rate 15 mL (STP)/min). After holding at 400
o
Where n0 is the molar quantity of LA fed into reactor, n1 is the
molar quantity of LA in the effluent, and np is the molar quantity
of lactic acid converted to 2,3-pentanedione or other byproducts
such as acetaldehyde, propionic acid, acrylic acid, and acetic acid.
⁰C for 2 hours, the sample was cooled to ambient temperature in pure Ar
(15 mL (STP)/min), a background spectrum was then taken. Pyridine was
introduced into the reaction cell by flowing He (5 mL (STP)/min) through
a bubble generator at ambient temperature until saturation (judged by the
appearance of the 1596 cm-1 feature assigned to physisorbed pyridine).
Acknowledgements
This work was supported by Sichuan province science and
Physisorbed pyridine was removed with pure Ar purging (15 mL
o
(
STP)/min) at 100 C. The sample was then cooled to ambient
technology
department
application
foundation
project
temperature and IR spectra were acquired. X-ray photoelectron
spectroscopy (XPS) measurements were performed using a Thermo
Scientific K-Alpha spectrometer, equipped with a monochromatic small-
spot X-ray source and a 180o double focusing hemispherical analyzer
with a 128-channel detector. Spectra were obtained using an aluminum
anode (Al Kα = 1486.6 eV) operating at 72 W and a spot size of 400 μm.
Survey scans were measured at a constant pass energy of 200 eV and
region scans at 50 eV. The background pressure was 2 × 10-9 mbar and
during measurement 3 × 10-7 mbar Argon because of the charge
compensation dual beam source. Data analysis was performed using
CasaXPS software. The binding energy was corrected for surface
(
2017JY0188 ） ， the opening foundation of Jiangsu Key
Laboratory of Vehicle Emissions Control (NO. OVEC 032), and
Scientific Research Fund of Sichuan Provincial Educational
Department (16ZA0175).
Keywords: cesium doped hydroxyapatite • 2,3-pentanedione •
condensation • lactic acid • green synthesis
[1] a) P. Liu, E. J. M. Hensen, J. Am. Chem. Soc. 2013, 135, 14032-
14035; b) J. M. Sun, K. K. Zhu, F. Gao, C. M. Wang, J. Liu, C. H. F.
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201701332
FULL PAPER
Peden, Y. Wang, J. Am. Chem. Soc. 2011, 133, 11096-11099; c) L.
S. Sharninghausen, J. Campos, M. G. Manas, R. H. Crabtree, Nat.
Commun. 2014, 5, 1-7; d) D. M. Alonso, J. Q. Bond, J. A. Dumesic,
Green Chem. 2010, 12, 1493-1513.
[5] A. J. Amass, K. L. R. N'Goala, B. J. Tighe, F. Schue, Polymer 1999,
40, 5073-5078.
[6] a) L. W. Sun, X. L. Li, C. M. Tang, Acta Phys. -Chim. Sin. 2016, 32,
2327-2336; b) J. F. Zhang, X. Z. Feng, Y. L. Zhao, W. J. Ji, C. T. Au,
J. Ind. Eng. Chem. 2014, 20, 1353-1358.
[
2] a) M. Dusselier, P. Van Wouwe, A. Dewaele, E. Makshina, B. F. Sels,
Energy Environ. Sci. 2013, 6, 1415-1442; b) J. L. Xu, H. Y. Zhang, Y.
F. Zhao, B. Yu, S. Chen, Y. B. Li, L. D. Hao, Z. M. Liu, Green Chem.
[7] a) M. S. Tam, G. C. Gunter, R. Craciun, D. J. Miller, J. E. Jackson,
Ind. Eng. Chem. Res. 1997, 36, 3505-3512; b) D. C. Wadley, M. S.
Tam, P. B. Kokitkar, J. E. Jackson, D. J. Miller, J. Catal. 1997, 165,
162-171; c) G. C. Gunter, R. H. Langford, J. E. Jackson, D. J. Miller,
Ind. Eng. Chem. Res. 1995, 34, 974-980; d) G. C. Gunter, D. J.
Miller, J. E. Jackson, J. Catal. 1994, 148, 252-260; e) M. S. Tam, R.
Craciun, D. J. Miller, J. E. Jackson, Ind. Eng. Chem. Res. 1998, 37,
2360-2366.
2013, 15, 1520-1525; c) N. Murakami, M. Oba, M. Iwamoto, Y.
Tashiro, T. Noguchi, K. Bonkohara, M. A. Abdel-Rahman, T. Zendo,
M. Shimoda, K. Sakai, K. Sonomoto, J. Biosci. Bioeng. 2016, 121,
89-95; d) Y. L. Zhang, Z. Shen, X. F. Zhou, M. Zhang, F. M. Jin,
Green Chem. 2012, 14, 3285-3288; e) Y. Li, M. Nielsen, B. Li, P. H.
Dixneuf, H. Junge, M. Beller, Green Chem. 2015, 17, 193-198.
[
3] a) X. H. Zhang, L. Lin, T. Zhang, H. O. Liu, X. F. Zhang, Chem. Eng.
J. 2016, 284, 934-941; b) G. M. Lari, B. Puertolas, M. S. Frei, C.
Mondelli, J. Perez-Ramirez, ChemCatChem 2016, 8, 1507-1514; c)
Z. Guo, D. S. Theng, K. Y. Tang, L. L. Zhang, L. Huang, A. Borgna,
C. Wang, Phys. Chem. Chem. Phys. 2016, 18, 23746-23754; d) B.
Yan, L. Z. Tao, Y. Liang, B. Q. Xu, ACS Catal. 2014, 4, 1931-1943;
e) J. S. Peng, X. L. Li, C. M. Tang, W. Bai, Green Chem. 2014, 16,
[8] a) V. C. Ghantani, S. T. Lomate, M. K. Dongare, S. B. Umbarkar,
Green Chem. 2013, 15, 1211-1217; b) A. Fihri, C. Len, R. S. Varma,
A. Solhy, Coordination Chemistry Reviews 2017, 347, 48-76.
[9] K. Kaneda, T. Mizugaki, ACS Catal. 2017, 7, 920-935.
[10] Y. Z. Tang, H. F. Chappell, M. T. Dove, R. J. Reeder, Y. J. Lee,
Biomaterials 2009, 30, 2864-2872.
[11] Y. Matsuura, A. Onda, S. Ogo, K. Yanagisawa, Catal. Today 2014,
226, 192-197.
108-111.
[
4] a) C. M. Tang, Z. J. Zhai, X. L. Li, L. W. Sun, W. Bai, J. Catal. 2015,
329, 206-217; b) B. Katryniok, S. Paul, F. Dumeignil, Green Chem.
2010, 12, 1910-1913.
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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1
.
Cesium doped hydroxyapatite
Xinli Li, Liangwei Sun, Weixin Zou, Ping
Cao, Zhi Chen, Congming Tang*, and
Lin Dong
catalysts offered an efficient activity
for condensation of lactic acid to 2,3-
pentanedione.
Page No. – Page No.
2.
The unprecedented catalytic
performance relates to synergy
between basic sites and acidic sites
located on the surface of cesium
doped hydroxyapatite catalyst.
Efficient Conversion of Bio-lactic
Acid to 2,3-Pentanedione on Cesium
Doped Hydroxyapatite Catalysts with
Balanced Acid-Base Sites
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