Hydroxyapatite supported lewis acid catalysts for the transformation of trioses in alcohols

Article abstract of DOI:10.1016/S1872-2067(10)60162-3

We prepared hydroxyapatite-supported tin(II) chloride and tin(IV) chloride Lewis acid catalysts. These catalysts showed catalytic activity for the transformation of trioses in alcohols to yield alkyl lactates. Under optimal conditions, n-butyl lactate was obtained in 73.5 yield when dihydroxyacetone and n-butanol were treated with hydroxyapatite-supported tin(II) chloride.

Full text of DOI:10.1016/S1872-2067(10)60162-3

CHINESE JOURNAL OF CATALYSIS
Volume 32, Issue 1, 2011
Online English edition of the Chinese language journal
RESEARCH PAPER
Cite this article as: Chin. J. Catal., 2011, 32: 70–73.
Hydroxyapatite Supported Lewis Acid Catalysts for the
Transformation of Trioses in Alcohols
ZHANG Zehui1,2,*, ZHAO (Kent) Zongbao1,3
1Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
2Graduate University of Chinese Academy of Sciences, Beijing 100049, China
3Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning,
China
Abstract: We prepared hydroxyapatite-supported tin(II) chloride and tin(IV) chloride Lewis acid catalysts. These catalysts showed cata-
lytic activity for the transformation of trioses in alcohols to yield alkyl lactates. Under optimal conditions, n-butyl lactate was obtained in
73.5% yield when dihydroxyacetone and n-butanol were treated with hydroxyapatite-supported tin(II) chloride.
Key words: triose; hydroxyapatite; alkyl lactate; tin(II) chloride; solid catalyst
Lactic acid is widely used as an acidulant, flavorant, and
HO
HO
OH
O
preservative in the food, pharmaceutical, and textile industries.
It is also used to produce biodegradable polylactic acid mate-
rials [1,2]. Microbial fermentation is a state-of-the-art tech-
nology for lactic acid production in industry [3]. However, the
fermentation process presents obvious drawbacks such as the
formation of a stoichiometric amount of salt by pH-regulation
and the consumption of excess energy to recover lactic acid
from the fermentation broth [4]. Lactic acid can also be pro-
duced from trioses when catalyzed by Lewis acids in the
presence of water [5]. When the reaction is complete in an
alcohol solution, dihydroxyacetone (DHA) or glyceraldehyde
(GLV) is transformed into alkyl lactates [6,7]. The use of
Lewis acids as catalysts usually leads to the discharge of highly
toxic byproducts to the environment, contamination of the final
product with metal salts and difficulties in separation. There-
fore, the use of heterogeneous catalysts has attracted much
attention because they are easy to work-up and the quantity of
waste is reduced significantly [8,9]. Recently, commercially
available zeolites have been used to catalyze the transforma-
tion of trioses to produce lactic acid and alkyl lactates in a
good yields [10].
O
OH
DHA
or
SnCl2/HAP
OR
+
H2O
+
ROH
O
OH
Alkyl lactates
GLV
Scheme 1. Transformation of trioses to alkyl lactates.
a promising support material because of its durable acid-base
properties and its high adsorption capacity [11,12]. HAP sup-
ported Lewis acids have been used to catalyze many chemical
reactions [13,14]. In this work, we report the preparation of
the Lewis acids tin(II) chloride and tin(IV) chloride that are
impregnated into hydroxyapatite catalysts (SnCl2/HAP and
SnCl4/HAP) and preliminary results of the transformation of
trioses in alcohols to alkyl lactates promoted by these solid
catalysts (Scheme 1).
1 Experimental
1.1 Preparation of the catalysts
Hydroxyapatite (HAP, Ca10(PO4)6(OH)2) is considered to be
HAP was prepared by the co-precipitation method according
Received 30 June 2010. Accepted 21 September 2010.
*Corresponding author. Tel: +86-411-84379066; Fax: +86-411-84379211; E-mail: zhangzehui@dicp.ac.cn
Foundation item: Supported by the National High Technology Research and Development Program of China (863 Program, 2007AA05Z403) and the Knowledge
Innovation Program of Chinese Academy of Sciences (KGCXZ-YW-336).
Copyright © 2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(10)60162-3

ZHANG Zehui et al. / Chinese Journal of Catalysis, 2011, 32: 70–73
to known procedures [15]. SnCl2/HAP was prepared by the
impregnation method according to known procedures with
some modifications [16]. HAP (2.00 g) was introduced into
100 ml of absolute ethanol containing 4 mmol (0.92 g) of
SnCl2·2H2O. The mixture was stirred for 24 h, filtered, and
washed several times with ethanol. The recovered solid was
dried at 110 °C overnight. Tin(IV) chloride was loaded onto
HAP in a similar way to give the solid catalyst SnCl4/HAP.
SnCl4/HAP
SnCl2/HAP
HAP
1.2 Characterization of the catalysts
A Micromeritics NOVA-4000 automated physisorption in-
strument was used to measure the N2-adsorption isotherms of
the samples at the liquid N2 temperature (–196 °C). The spe-
cific surface area was determined from the linear portion of the
BET plot. The pore-size distribution was calculated from the
desorption branch of the N2-adsorption isotherms using the
Barrett-Joyner-Halenda (BJH) formula. Before the surface area
and pore-size distribution measurements, the samples were
degassed in vacuum for 3 h at 350 °C (for HAP) and 110 °C
(for SnCl2) to remove physically adsorbed components.
X-ray powder diffraction (XRD) patterns of the prepared
samples were measured on a Philips CM-1 powder X-ray dif-
fractometer (Cu KD, Ȝ = 0.1543 nm) operating at 40 kV and 40
mA. Diffraction patterns were collected from 2ș = 5°–90°.
Fourier transform infrared (FT-IR) spectra were recorded on
a Bruker EQUINOX 55 FT-IR spectrometer with a spectral
resolution of 4 cm–1 in the wave number range of 400–4000
cm–1. The samples and KBr were fully dried before the FT-IR
analyses to exclude the influence of water.
20
30
40
2T/(o )
50
60
70
Fig. 1. XRD patterns of HAP, SnCl2/HAP, and SnCl4/HAP.
method was p/p0 = 0.98 (Vp = 0.40 cm3/g). The surface area and
pore volume for SnCl2/HAP were determined to be 43 m2/g and
0.25 cm3/g, respectively. The values for SnCl4/HAP were
almost the same as those for SnCl2/HAP.
XRD patterns of HAP, SnCl2/HAP, and SnCl4/HAP are
shown in Fig. 1. HAP has a typical hexagonal crystal structure
and belongs to the space group P63/m. The lattice parameters of
the HAP sample agreed well with the standard data: a = 0.9412
nm and c = 0.6837 nm. The XRD patterns of SnCl2/HAP and
SnCl4/HAP were similar to that of HAP and their typical dif-
fraction peaks showed no substantial change in intensity. It
should also be noted that no SnCl2 and SnCl4 phases were
detected in the SnClx/HAP sample indicating that SnCl2 and
SnCl4 were highly dispersed. These results indicate that the
modification of HAP by impregnating with SnCl2 and SnCl4
does not change the crystalline structure of the solid material.
The characterization of HAP, SnCl2/HAP, and SnCl4/HAP
was further carried out by FT-IR (Fig. 2). There was no obvious
difference between the support and the catalysts. The bands at
3580 and 631 cm–1 were assigned to the stretching and bending
modes of the OH groups in the hydroxyapatite structure, re-
spectively [17]. The bands at 1098, 1030, and 961 cm–1 were
attributed to the asymmetric and symmetric stretching vibra-
The actual tin loading was determined by inductively cou-
pled plasma/atomic emission spectroscopy (ICP/AES) on an
IRIS Intrepid II XSP instrument (Thermo Electron Corpora-
tion).
1.3 Catalytic test
In a typical run, a mixture containing 100 mg of dihy-
droxyacetone dimer (DHAD) and 8 ml of n-butanol was pre-
heated at 80 °C for 1 h to depolymerize the dimer into mono-
meric DHA. To the mixture was added some catalyst and the
reaction was heated under specified conditions. At different
time intervals, samples were withdrawn and analyzed by gas
chromatography using methyl palmitate as an internal stan-
dard.
SnCl4/HAP
SnCl2/HAP
HAP
2 Results and discussion
2.1 Characterization results of HAP, SnCl2/HAP, and
SnCl4/HAP
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cmꢀ1)
The surface area of the prepared HAP sample was estimated
to be 50 m2/g. The total pore volume (Vp) calculated by the BJH
Fig. 2. FT-IR spectra of HAP, SnCl2/HAP, and SnCl4/HAP.

ZHANG Zehui et al. / Chinese Journal of Catalysis, 2011, 32: 70–73
tion of the phosphate group (PO43–) and the bending modes of
in SnCl2/HAP and SnCl2/HAP was determined to be 2.4 wt%
and 0.8 wt% by ICP-AES, respectively. The fact that
SnCl2/HAP had excellent activity can be attributed to the
stronger ability of tin(II) to exchange with calcium(II) in the
HAP leading to a stable and active structure. Because tin(IV) is
more positive than calcium(II) it is unlikely to remain in the
HAP without breaking the charge balance.
O–P–O were observed at 608, 565, and 475 cm–1 [18]. Fur-
thermore, two bands at 1635 and 3431 cm–1 indicated that free
water was absorbed onto the materials. On the other hand,
impurities were also present in the HAP as shown by the bands
at 1415 and 1450 cm–1, which were attributed to the asymmet-
ric and symmetric stretching vibration of the carbonate group
2–
(CO32–). In addition, the band at 874 cm–1 indicated that HPO4
One of the main advantages of heterogeneous catalysts over
homogeneous catalysts is that the former catalysts can be re-
covered and reused. Thus, SnCl2/HAP was recovered by fil-
tration, washing with n-butanol, and drying overnight at 110
ºC. The recovered catalysts gave a far lower n-butyl lactate
yield even at a reaction time of 10 h (Table 1, entry 7). The
reasons for catalyst deactivation were complex [19]. We
speculate that the water and other byproducts that form in the
reaction are absorbed onto SnCl2/HAP and result in the deac-
tivation of the catalyst.
may also be present in the hydroxyapatite as an impurity.
2.2 Transformation of trioses to n-butyl lactate catalyzed
by different catalysts
In an earlier study, Hayashi et al. [7] screened a number of
Lewis acids for the transformation of trioses in aliphatic al-
cohols to alkyl lactates. They found that tin chlorides have
excellent activities in these reactions. We also carried out a
reaction between DHAD and n-butanol in the presence of tin
chlorides under our reaction conditions (Table 1, entries 1 and
2) and the results were similar to those reported by Hayashi et
al. [7]. However, the use of homogenous Lewis acids has some
drawbacks such as high environmental toxicity and difficulties
in product separation.
2.3 DHAD transformation to lactates catalyzed by
SnCl2/HAP under different conditions
Temperature has a profound effect on the reaction rate and
yield. We investigated the effect of different temperatures (30,
50, 80, and 110 ºC) on the transformation of DHAD to n-butyl
lactate (Table 2, entries 1–4). It was obvious that higher tem-
peratures favored the formation of n-butyl lactate. When the
reaction temperature was 30 ºC, n-butyl lactate was obtained in
6.1% yield even after 24 h and a large amount of DHAD re-
mained (data not shown). With an increase in temperature, the
yields of n-butyl lactate also increased even at shorter reaction
times. The maximum yield of 73.5% was obtained at 110 ºC
over 6 h.
To address the issues associated with a homogenous system,
we immobilized the Lewis acids onto HAP as it is a good solid
material to support Lewis acids. We prepared HAP-supported
tin(II) chloride and tin(IV) chloride catalysts and tested their
activities for the transformation of DHAD to n-butyl lactate. To
our surprise, only SnCl2/HAP gave a comparable yield to that
obtained with SnCl2·2H2O (Table 1, entry 3 vs. entry 1).
SnCl4/HAP gave a significantly lower yield (Table 1, entry 4).
In addition, when GLA was used as an alternative substrate, a
high n-butyl lactate yield was obtained in the presence of
SnCl2/HAP (Table 1, entry 5). Because the catalysts were
composed of the support HAP and the Lewis acids, the reaction
was also carried out using HAP instead of SnCl2/HAP. It was
clear that HAP had negligible catalytic activity toward the
transformation of DHAD (Table 1, entry 6) suggesting that the
catalytic center was the Lewis acid metal ions. The tin content
The effect of catalyst dosage was also investigated (Table 2,
entries 4–6). When the catalyst dosage was increased from 20
mg to 80 mg, the yield of n-butyl lactate increased remarkably
from 14.3% to 73.5%. The higher yield with an increase in
catalyst loading is due to the larger amount of available cata-
lytic sites.
Table 2 Transformation of trioses to alkyl lactates catalyzed by
SnCl2/HAP under different conditions
Table 1 Transformation of trioses to n-butyl lactate catalyzed by Lewis
acids and HAP supported Lewis acids
Alkyl
Catalyst
Time
(h)
24
12
12
6
Yield
(%)
TOF
(h–1)
Entry
t/ºC
alcohol amount (mg)
Time Yield
TOF
(h–1)
1.29
1.47
8.41
7.00
7.19
—
Entry Substrate
Catalyst
1
2
3
4
5
6
7
8
9
n-butanol
n-butanol
n-butanol
n-butanol
n-butanol
n-butanol
methanol
ethanol
80
80
80
80
40
20
80
80
80
30
50
6.1
0.17ꢀ
1.33ꢀ
2.87ꢀ
8.41ꢀ
4.71ꢀ
1.23ꢀ
0.65ꢀ
1.87ꢀ
4.19ꢀ
(h)
6
(%)
77.3
88.4
73.5
20.4
62.8
2.5
23.3
50.1
73.5
54.9
14.3
11.3
21.8
48.8
1
2
3
4
5
6
7
DHAD
DHAD
DHAD
DHAD
GLA
SnCl2·2H2O
SnCl4·5H2O
SnCl2/HAP
80
6
110
110
110
70
6
8
SnCl4/HAP
6
8
SnCl2/HAP
6
12
8
DHAD
DHAD
HAP
12
10
80
SnCl2/HAP (recycled)
35.2
2.42
i-propanol
85
8
Reaction conditions: 100 mg DHAD, 10 mol% Lewis acid or 80 mg solid
catalyst, 8 ml n-butanol, 110 ºC.
Reaction conditions: 100 mg DHAD, 8 ml alcohol. TOF = n(alkyl lac-
tate)/(n(Sn)·t).
TOF = n(butyl lactate)/(n(Sn)·t).

ZHANG Zehui et al. / Chinese Journal of Catalysis, 2011, 32: 70–73
Finally, the reactions between DHAD and other alcohols
terials and chemical products.
were also studied to explore the scope of this method (Table 2,
entries 7–9). Because the reflux temperatures of methanol,
ethanol, and i-propanol are below 110 ºC at ambient pressure
the experiments with these alcohols were carried out at their
reflux temperatures. Unlike the homogeneous systems with tin
chlorides [7] the alcohols had a substantial influence on the
SnCl2/HAP catalyzed transformation of DHAD to alkyl lac-
tates. Higher yields of alkyl lactates were obtained using al-
cohols with more carbons. One important reason was that the
catalytic tin(II) center in the heterogeneous state was less ac-
tive than that in the homogeneous state. On the other hand, a
lower reaction temperature should also contribute to lower
yields for those alcohols with less carbon atoms. Similar
trends were also observed for the influence of temperature on
n-butyl lactate formation (vide ante). Moreover, the steric
effect should also be considered. The larger steric structures
of i-propanol and n-butanol inhibit the formation of the by-
product pyruvaldehyde dibutylacetal by the addition of a sec-
ond alcohol to the reaction intermediate.
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In conclusion, a green catalytic system was developed for
the transformation of trioses to alkyl lactates and the catalyst
was HAP-supported tin(II) chloride. The reaction of DHAD
with n-butanol upon catalysis with SnCl2/HAP afforded a high
product yield of 73.5% over 6 h, which is comparable to that
obtained with SnCl2. This novel catalytic system is easy to
work-up and the quantity of waste is reduced significantly. The
method described herein may be valuable to facilitate the
cost-efficient transformation of biomass into lactic-based ma-
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