Efficient and selective conversion of lactic acid into acetaldehyde using a mesoporous aluminum phosphate catalyst

Article abstract of DOI:10.1039/c4gc01779j

Although acetaldehyde is a very important compound and has been utilized as a useful synthon for various important chemicals, it has been synthesized in industry through a petroleum route until now. Herein, we have successfully developed a sustainable route using a heterogeneous catalyst. In the presence of mesoporous aluminum phosphate (MAP3), the decarbonylation reaction of lactic acid proceeded efficiently, with 100% lactic acid conversion and ～92% acetaldehyde selectivity. The catalyst shows high stability for at least 248 h. The unprecedented catalytic performance is due to rich medium acidic sites existing on the catalyst surface. This journal is

Full text of DOI:10.1039/c4gc01779j

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Efficient and selective conversion of lactic acid
into acetaldehyde using a mesoporous aluminum
phosphate catalyst†
Cite this: DOI: 10.1039/c4gc01779j
Congming Tang,*a Jiansheng Peng,a Xinli Li,a Zhanjie Zhai,a Wei Bai,b Ning Jiang,b
Hejun Gaoa and Yunwen Liaoa
Although acetaldehyde is a very important compound and has been utilized as a useful synthon for
various important chemicals, it has been synthesized in industry through a petroleum route until now.
Herein, we have successfully developed a sustainable route using a heterogeneous catalyst. In the pres-
ence of mesoporous aluminum phosphate (MAP3), the decarbonylation reaction of lactic acid proceeded
efficiently, with 100% lactic acid conversion and ∼92% acetaldehyde selectivity. The catalyst shows high
stability for at least 248 h. The unprecedented catalytic performance is due to rich medium acidic sites
existing on the catalyst surface.
Received 14th September 2014,
Accepted 28th October 2014
DOI: 10.1039/c4gc01779j
www.rsc.org/greenchem
rated acids via carbonylation reaction of the corresponding
Introduction
substrates.21–25 Carbon monoxide is also used to synthesize
With the increasing shortage of fossil resources, it is indis- propionaldehyde and other aldehydes via hydroformylation
pensable to develop alternative routes to synthesize bulk reaction of ethylene and other alkenes.26–29 Generally, LA is
chemicals from non-fossil resources.1–3 Production of bulk used as a crucial platform molecule which can be converted to
chemicals from bioresources is generally viewed as a promis- many value added chemicals such as acrylic acid,30–32 acet-
ing route.4–12 Acetaldehyde as an important chemical is widely aldehyde,20 2,3-pentanedione,33,34 propionic acid,35 pyruvic
used for the production of peracetic acid, pentaerythritol, pyri- acid36 and polylactic acid.37 To our delight, the technologies
dine bases, butyleneglycol, and chloral.13,14 At present, acet- on lactic acid synthesis have made much progress and a wide
aldehyde is produced by the Wacker process in which ethylene range of low-cost biomass materials such as cellulose,8
is used as the raw material, and PdCl2–CuCl2 is utilized as the sugars,38 and sorbitol39 have also been used to produce LA.
catalytic system. Due to the increasing depletion of petroleum
Few research studies on decarbonylation of LA to acet-
reserves which are used to produce ethylene via high tempera- aldehyde have been reported so far. Katryniok et al.20 reported
ture pyrolysis or catalytic cracking, this route will be restricted silica supported heteropolyacids for the catalytic decarbonyla-
in the near future. For this reason, significant research is tion of LA to acetaldehyde, achieving 91% conversion of LA as
focused on production of acetaldehyde from biomass. A case well as 81–83% yield of acetaldehyde at 275 °C. More recently,
in point is that acetaldehyde is produced via catalytic dehydro- we have reported metal sulphates as catalysts for the decarbonyl-
genation of ethanol or partial oxidation of ethanol.13,15–19 Acet- ation of LA to acetaldehyde.40 Under the optimal reaction
aldehyde can also be obtained through decarbonylation of bio- conditions, the acetaldehyde yield attains 92.1% at 380 °C.
lactic acid (LA) accompanied by formation of the by-product Although the reported catalysts have a poor durability, the acet-
carbon monoxide.20 However, carbon monoxide is easily separ- aldehyde yield is acceptable. The selective conversion of LA
ated from the mixtures composed of product and unreacted into acetaldehyde catalyzed by medium acidity has been recog-
reagent, and can be further used to synthesize carbonyl com- nized.40 Strong acidity does not favor the decarbonylation of
pounds such as acetic acid, acrylic acid, and other α,β-unsatu- LA to acetaldehyde, but easily leads to the carbon deposition
or coke formation causing deactivation of the catalyst.
In this work, we present an efficient and durable meso-
porous aluminum phosphate catalyst for decarbonylation of
LA into acetaldehyde. 100% LA conversion with ∼92% acet-
aldehyde selectivity has been achieved over the mesoporous
aluminum phosphate at 325 °C. The catalyst shows high stabi-
lity for at least 248 h (Fig. 8).
aChemical Synthesis and Pollution Control Key Laboratory of Sichuan Province,
China West Normal University, Nanchong, Sichuan 637002, PR China.
E-mail: tcmtang2001@163.com
bChengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu,
Sichuan 610041, PR China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/c4gc01779j
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Table 2 NH3-TPD results of catalysts
Results and discussion
Characterization
Acidity amount/mmol g−1
BET and NH3-TPD. It is known that specific surface area is a
pivotal factor for a heterogeneous catalyst since the catalytic
reaction occurs on the surface of the catalyst. Besides, internal
diffusion is also an important factor for the reaction rate.
Therefore the specific surface area and pore structure are
explored and the results are shown in Table 1. The N2 adsorp-
tion–desorption isotherm curves for catalysts and the corres-
ponding pore size distribution curves for catalysts are given in
Fig. S1 and S2.† From the data given in Table 1, we can clearly
see that specific surface areas of catalysts prepared by different
methods vary in a wide range. MAP2 has the lowest specific
surface area, only 40.8 m2 g−1, while MAP3 has the highest
specific surface area, up to 171.1 m2 g−1. Using the Barrett–
Joyner–Halenda (BJH) model, we calculate the pore diameter
based on the desorption branch data of nitrogen. In contrast
to the varying specific surface areas of catalysts, their pore
sizes show a different result, demonstrating that there is no
correlation between the specific surface area and the pore size.
Among all the tested catalysts, the lowest pore diameter is
6.3 nm, far more than the molecular dynamics diameter of
reactants which is less than 1 nm for the majority of reactants.
Thus an internal diffusion does not exist in the decarbonyla-
tion reaction of LA to acetaldehyde. By further analysis, we
have found that the catalytic performance investigated in the
following sections is in disagreement with the specific surface
area of the corresponding catalyst at high reaction tempera-
tures such as 380 °C (shown in Fig. 6). However, at low reaction
temperatures such as 250 °C and 270 °C, the specific surface
area of the catalyst has an evident effect on the catalytic per-
formance due to the relatively low surface specific reaction rate
(shown in Fig. 7).
Weak–medium
(120–400 °C)
Strong
(400–600 °C)
Total acid
Catalysta
amount/mmol g−1
MAP1
MAP2
MAP3
1.08
2.18
3.42
0.28
0.49
0.47
1.36
2.67
3.89
a MAP1, aluminum phosphate prepared by the precipitation method;
MAP2, aluminum phosphate formed using aqueous ammonia as a
precipitant in the presence of citric acid; MAP3, aluminum phosphate
prepared using aqueous ammonia as a precipitant without citric acid.
Fig. 1 NH3-TPD of catalysts.
acidity amount. With the difference between weak–medium
The surface acidity of catalysts is measured by the NH3-TPD acidity and strong acidity amounts, it becomes easy to under-
stand the catalytic performance. As is well known, the medium
acidity favors the decarbonylation of LA to form acetaldehyde
method, and the results are given in Table 2, Table S1† and
Fig. 1. In Fig. 1, only a desorption peak appears in the range of
120–400 °C which is ascribed to the weak–medium acidity. It while strong acidity favors the decomposition of the C–C bond
to form coke or deposit carbon on the surface of the catalyst.
Based on the data of acidity distribution shown in Table 2, it is
is evident that medium acidity accounts for majority while
strong acidity accounts for only a little. For example, from the
data given in Table 2, the strong acidity amount is less than expected that porous aluminum phosphate catalysts should
show an excellent catalytic performance.
one order of magnitude compared to the weak–medium
FT-IR and XRD. FT-IR and XRD are utilized to investigate
the functional groups and structures of catalysts to fully under-
stand the catalysis in decarbonylation reaction of LA to acet-
Table 1 BET data of aluminum phosphate catalysts
aldehyde, and the results are given in Fig.
2 and 3,
SBET
Vol
Pore sizeb
(nm)
respectively. From FT-IR spectra given in Fig. 2, three catalysts
remain constant demonstrating that aluminum phosphate
(AlPO4) is successfully produced by three preparation methods.
But a slight difference exists in peak intensity. For example,
the absorption peak intensity of the MAP1 catalyst is lower
than those of both MAP2 and MAP3. The absorption band at
Catalysta
(m2 g−1
)
(cm3 g−1
)
MAP1
MAP2
MAP3
74.8
40.8
171.1
1.1
0.1
0.9
63.8
6.3
15.5
a MAP1, aluminum phosphate prepared by the precipitation method;
MAP2, aluminum phosphate formed using aqueous ammonia as a 1100 cm−1 can be ascribed to vas(P–O–Al) (asymmetrical stretch-
ing vibration), and that at 498 cm−1 can be ascribed to δs(P–O–Al)
precipitant in the presence of citric acid; MAP3, aluminum phosphate
prepared using aqueous ammonia as a precipitant without citric acid.
(symmetrical deformation vibration),41,42 while the absorp-
b Calculated from desorption branch data on the Barrett–Joyner–
Halenda (BJH) model.
tion band at 3450 cm−1 can be ascribed to v(O–H) of water
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Fig. 2 FT-IR of mesoporous aluminum phosphate catalysts.
Fig. 3 XRD of mesoporous aluminum phosphate catalysts.
Fig. 4 SEM images of catalysts.
adsorbed on the catalysts. In addition, the 1630 cm−1 absorp-
tion band can further demonstrate the existence of water on
the catalyst surface. The acidity of catalysts determined by the
NH3-TPD method comes from Lewis acid sites of Al3+ and
Brønsted acid sites of the water adsorbed on the catalyst.
We subsequently utilize XRD to investigate the structures of
aluminum phosphate catalysts. The XRD patterns are shown
in Fig. 3, and match with the standard AlPO4 (PDF-#52-0211).
All the samples exhibit broad characteristic diffraction peaks
centered at 21.1°, 22.2° and 23°, which can be indexed to
(020), (151) and (240) diffractions, respectively.43 But these
samples have low crystallinity, which can be confirmed from
subsequent SEM images.
MAP2, in comparison with two other catalysts (MAP1 and
MAP3). Furthermore, we also observe the pores of catalysts
(MAP1 and MAP3) formed by the accretion of small catalyst
particles. This can be verified from TEM images (shown in
Fig. S3†). In addition, we find that the catalysts have only three
elements including O, P and Al by Energy dispersive X-ray
Spectroscopy (EDS) analysis (shown in Fig. S4†). Together with
the results from XRD, this result suggests that no impurity
except for the aluminum phosphate component exists in the
three catalysts. In other words, aluminum phosphate as an
efficient catalyst offers an excellent role in synthesis of acet-
aldehyde from lactic acid.
SEM and EDS. SEM images of catalysts are shown in Fig. 4.
The morphological features of the catalysts are influenced by
preparation methods. It is clearly seen that the particle size of
MAP2 is far larger than those of MAP1 and MAP3 catalysts,
which can be used to explain the low specific surface area of
TG of the mesoporous aluminum phosphate catalyst. The
stability of catalysts at high temperatures is very important for
application in chemical industry.1,2,6,16,44,45 A thermal tech-
nique efficient for evaluation of the catalyst stability at high
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Fig. 6 Comparison of different catalysts. (a) Conditions: reaction temp-
erature 380 °C, catalyst: MAP1, 0.1306 g, MAP2, 0.1355 g, MAP3,
0.1300 g, particle size: 20–40 meshes, carrier gas N2: 1 mL min−1, feed
flow rate: 1 mL h−1, LA feedstock: 20 wt% in water. (b) MAP1, aluminum
phosphate prepared by the precipitation method; MAP2, aluminum
Fig. 5 TG and DSC of the mesoporous AlPO4 (MAP3) catalyst under an
air atmosphere.
temperatures. Here, we utilize the TG technique to investigate phosphate formed using aqueous ammonia as a precipitant in the pres-
ence of citric acid; MAP3, aluminum phosphate prepared using aqueous
ammonia as a precipitant without citric acid. (c) Product selectivity: B11,
acetaldehyde; B12, propionic acid; B13, acetic acid; B14, acrylic acid;
B15, 2,3-pentanedione. (d) Blank represented reaction without the
the stability of the catalyst at high temperatures, and the
results are shown in Fig. 5. In Fig. 5, the MAP3 catalyst displays
a weight loss (∼14.9%) in the temperature range of 60–160 °C,
indicating that the lost species is a water molecule determined
using a mass spectrometer. The corresponding DSC curve also
shows a large and broad endothermic peak around 121 °C.
When the temperature is above 160 °C the catalyst remains
stable without weight loss. According to the results from the
TG experiment, we can speculate that the MAP3 catalyst
has a potential stability for catalytic reaction at elevated
temperatures.
catalyst.
surface. However XRD patterns and FT-IR spectra of three cata-
lysts remain constant, demonstrating that aluminum phos-
phate (AlPO4) is successfully synthesized by three preparation
methods. But a slight difference exists in peak intensity. In
FT-IR spectra, the absorption peak intensity of the MAP1 cata-
lyst is lower than those of both MAP2 and MAP3. This result
shows that the density of functional groups in the MAP1 cata-
lyst is also lower than others. Further NH3-TPD data of cata-
Activity
Comparison of aluminum phosphate catalysts. The activity
experiments over the aluminum phosphate catalysts are per- lysts suggest that acidity is also influenced and determined by
the functional group density in catalysts. According to the data
listed in Table 2, the increasing order of the total acid amount
formed at 380 °C with a LA concentration of 20 wt% and the
results are shown in Fig. 6. Prior to evaluation of the catalytic
performance, we carried out a blank experiment under is MAP1 < MAP2 < MAP3. Besides, a substantial proportion
(>79%) of the total acid amount belongs to weak-medium
acids, especially for medium acids. Our recent work40 has
identical conditions. It is clearly seen that the converted LA
accounts for 25% and the selectivity toward acetaldehyde is
62%, while for the major byproduct propionic acid the selecti- demonstrated that acetaldehyde selectivity is related to
medium acidity of the catalyst. Thus it is easy for us to under-
stand the high selectivity of acetaldehyde over the aluminum
vity is 26%, far more than that in the catalytic process. Simi-
larly, other byproducts except for 2,3-pentanedione are also
largely formed. The results using catalysts under identical con- phosphate catalysts. Conversely, the relationship between acet-
aldehyde selectivity and catalyst acidity further shows that
more medium acid over the catalyst is more beneficial to cata-
ditions are better in comparison with the blank experiment.
It is noteworthy that LA is completely converted and the
acetaldehyde selectivity is also drastically enhanced. For lytic formation of acetaldehyde from lactic acid, resulting in
higher selectivity of acetaldehyde. In order to investigate the
mechanism of acetaldehyde formation from lactic acid over
LA conversion, catalysts show no difference at 380 °C, while
for product selectivity, catalysts show different catalytic per-
formances, indicating that the structure of catalysts has an the catalysts, tail gas is analyzed by GC with a TDX-01 packed
column. Carbon monoxide as a major gas by-product is found
in the tail gases. Besides, propionic acid selectivity in catalytic
important influence on the catalytic performance. The increas-
ing order of acetaldehyde selectivity is MAP1 (76.1%) < MAP2
(93.1%) ≈ MAP3 (93.2%). It is known that the product selecti- reactions is much less than that in the blank experiment. It is
generally believed that propionic acid is formed through
vity correlates with catalyst properties determined by catalyst
structures. Thus we utilize XRD and FT-IR to investigate the
hydrogenation of lactic acid or acrylic acid,46,47 while hydrogen
structures of the catalyst and functional groups of the catalyst is produced via decarboxylation of lactic acid. Thus we can
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Fig. 8 Effect of reaction temperature. (a) Conditions: MAP3 used as a
Fig. 7 Effect of reaction temperature over the MAP2 and MAP3 cata-
catalyst, 0.1360 g, particle size: 20–40 meshes, carrier gas N2: 1 mL min−1
,
lysts. Conditions: catalyst: MAP2, 0.1355 g, MAP3, 0.1360 g, particle
feed flow rate: 1 mL h−1, LA feedstock: 20 wt% in water. (b) Product
selectivity: B11, acetaldehyde; B12, propionic acid; B13, acetic acid; B14,
acrylic acid; B15, 2,3-pentanedione.
size: 20–40 meshes, carrier gas N2: 1 mL min−1, feed flow rate: 1 mL h−1
,
LA feedstock: 20 wt% in water.
deduce that acetaldehyde is formed mainly via decarbonyla-
tion of lactic acid accompanied by CO and H2O, rather than
decarboxylation of lactic acid accompanied by CO2 and H2.
Effect of reaction temperature. From the results shown in
Fig. 6, the difference in the performance of MAP2 and MAP3 is
rather small at 380 °C. To fully understand the catalytic per-
formances at low conversions of LA, we carried out the exper-
iments at low temperatures such as 325 °C, 300 °C, 275 °C and
250 °C, respectively, and the results are given in Fig. 7. For
acetaldehyde selectivity at low temperatures, the reaction
temperature showed a slight influence over the MAP2 and
MAP3 catalysts, while for LA conversion the conversion of LA
over the MAP3 is higher than that over MAP2 at less than
300 °C. In order to fully understand this result, we must con-
sider two factors: the surface specific reaction rate (shown in
Table S2†) and the specific surface area (shown in Table 1). At
an identical reaction temperature such as 250 °C, although the
surface specific reaction rate over the MAP2 is higher than that
over the MAP3 due to higher surface acid density on the
former, the total surface reaction rate over the MAP2 is lower
than that over the MAP3 ascribing to a much more specific
surface area of the latter than the former. Thus the conversion
of LA over the MAP3 is higher than that over MAP2 at less than
300 °C. When the reaction temperature increases, the surface
specific reaction rate also increases. As a result, LA can be
completely converted at more than 325 °C over the MAP2 and
MAP3 catalysts.
available silica, Fuji Silysia) supported silicontungstic acid cata-
lyzing the decarbonylation of LA at 275 °C (in terms of acet-
aldehyde yield, the former shows 87% yield while the latter
shows about 81% yield.).20 As for acetaldehyde selectivity, it
slightly fluctuates with the increase of reaction temperature.
For by-products except for acrylic acid, the selectivities display
a trend of slight increase with the increase of reaction tempera-
ture. In terms of the effect of reaction temperature on product
selectivity, the result for the MAP3 catalyst is different from the
results reported in the previous work40 while it is similar to
the results reported by Hong et al.48 According to the results
obtained above, we can conclude that product selectivity is
mainly determined by catalyst acidity. From NH3-TPD patterns
of catalysts shown in Fig. 1, medium acidity accounts for the
majority, while strong acidity accounts for only a little. This is
the main reason for high selectivity of acetaldehyde in decar-
bonylation of LA.
Catalyst stability. Long-term stability is a very important
characteristic for a heterogeneous catalyst.2,6,11,49,50 The cata-
lytic stability of mesoporous aluminum phosphate (MAP3) is
investigated at 325 °C, and the results are shown in Fig. 9. LA
conversion gradually decreases with an increase of time on
stream. For example, LA conversion reduces with only 10%
within 132 h on stream. When the time on stream lengthens
to 248 h, LA conversion still remains above 73%. More impor-
tantly, acetaldehyde selectivity almost remains constant
(>92%) during the whole time on stream. To our great delight,
so far this is the best result concerning LA conversion, acet-
aldehyde selectivity and reaction temperature. Noteworthily,
the reaction temperature (325 °C) used in decarbonylation
reaction over the MAP3 catalyst is lower than other reactions
including decarbonylation and dehydration of LA over the
other catalysts. For the latter, the reactions are generally per-
formed at 340–400 °C.1,40,47,51,52 Although a reaction tempera-
ture of 275 °C was utilized for decarbonylation of LA over the
CARiACT Q-15 supported silicontungstic acid, the stability of
The effect of reaction temperature on the reaction perform-
ance over the MAP3 catalyst is further investigated, and the
results are depicted in Fig. 8. LA conversion drastically
increases with an increase of reaction temperature from 250 to
300 °C. For example, LA conversion is 31% at 250 °C, 92.5% at
275 °C and 100% at 300 °C. It is obviously seen that LA conver-
sion is sensitive to the reaction temperature. It is also noted
that for the MAP3 catalyst at 275 °C the catalytic performance
is more superior to that of the CARiACT Q-15 (a commercially
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of acetaldehyde without further purification. Triple-distilled
water was prepared in the laboratory and was used to dilute
lactic acid for the required concentration. Aluminium nitrate
(Al(NO3)3·9H2O), sodium phosphate (Na(PO4)3·12H2O), phos-
phoric acid, citric acid, acetaldehyde, acrylic acid, propionic
acid, acetic acid, 2,3-pentanedione and n-butanol, together
with hydroquinone, were purchased from Sinopharm Chemi-
cal Reagent Co., Ltd. Acrylic acid, propionic acid, acetic acid,
2,3-pentanedione and acetaldehyde were used as gas chrom-
atography reference materials, and n-butanol was adopted as
an internal standard material. Hydroquinone (0.3 wt%) was
used as a polymerization inhibitor.
Preparation of catalysts
In order to obtain aluminum phosphate catalysts with
Fig. 9 Catalytic stability of the MAP3 catalyst. MAP3 used as a catalyst,
0.1385 g, reaction temperature, 325 °C, particle size: 20–40 meshes,
different structures, three methods were utilized. MAP1 —
carrier gas N2: 1 mL min−1, feed flow rate: 1 mL h−1, LA feedstock: 20 wt% under stirring at room temperature, 7.6 g Na(PO4)3·12H2O and
in water.
7.5 g Al(NO3)3·9H2O were fully dissolved in 50 mL distilled
water, respectively. Subsequently, an aqueous solution of
sodium phosphate was added dropwise to the aqueous solu-
only 5 h on stream has been reported without the experimental
tion of aluminium nitrate to form a white precipitate of alumi-
data for more time on stream. Long-term stability as well as
num phosphate under continuous stirring at room
high selectivity of acetaldehyde is also related to catalyst acidity.
temperature. The resulting precipitate was completely rinsed
As mentioned above, for the MAP3 catalyst predominant
to remove sodium phosphate, sodium nitrate, and aluminium
medium acidity plays an important role in durability and acet-
nitrate using distilled water, and dried at 120 °C for 6 h. Prior
aldehyde selectivity. It is known that strong acidic sites show a
to use, the catalyst was calcined at 550 °C for 6 h. MAP2 —
strong catalysis for decomposition of the C–C bond, resulting in
H3PO4 (85 wt%, 2.3 g) was added to a mixed aqueous solution
deposition of carbon or formation of coke on the catalyst
of Al(NO3)3·9H2O (7.5 g) and citric acid (CA, 4.2 g) under vigor-
surface.11,53,54 Thus the catalyst with more strong acidic sites
ous stirring at ambient temperature, leading to a composition
rapidly deactivates in the process of the catalytic decarbonyl-
in a molar ratio of 1.0 : 1.0 : 1.0 : 86 = Al(NO3)3–CA–H3PO4–H2O.
ation reaction of LA due to covering of the active sites of the
After that, an aqueous ammonia solution (20 wt%) was used to
catalyst surface by the formed carbon or coke.
adjust the pH value of the solution to 5.0 under continuous
stirring at 80 °C. The solid composite was formed after remov-
ing water and all other volatiles by heating the mixed solution
at 90 °C for 10 h. The resulting solid composite was calcined
at 800 °C for 6 h under an air atmosphere to remove the carbo-
Conclusions
The gas phase decarbonylation of lactic acid is performed over
various aluminum phosphate catalysts. The catalytic perform-
hydrate. MAP3 — The preparation of MAP3 is similar to that of
ance is influenced by the preparation methods of catalysts. The
MAP2 except that citric acid was not used while preparing
MAP3 catalyst formed using ammonia as a precipitant, without
MAP3. Besides, MAP3 was calcined at 550 °C, not at 800 °C.
adding citric acid, displays the best catalytic performance. It is
Catalyst characterization
found that the MAP3 catalyst has more dominant medium
acidic sites than others. Furthermore, in comparison with pre-
vious catalysts, less strong acidic sites exist on the catalyst
surface. Due to this, the MAP3 catalyst shows high stability for
at least 248 h. The reaction temperature is also investigated. LA
conversion is drastically influenced with the increase of reaction
temperature. However, the acetaldehyde selectivity slightly fluc-
tuates as the reaction temperature changes. Under the optimal
conditions, 100% LA conversion as well as ∼92% acetaldehyde
selectivity has been achieved at 325 °C.
Powder X-ray diffraction measurement was conducted on a
Dmax/Ultima IV diffractometer operated at 40 kV and 20 mA
with Cu-Kα radiation. The FTIR spectra of the catalysts were
recorded in the range of 500–4000 cm−1 on a Nicolet 6700
spectrometer. The morphological features of the catalysts were
determined using a scanning electron microscope (Philips
XL30 ESEM FEG). TEM measurements were made on a
HITACHI H-8100 electron microscope (Hitachi, Tokyo, Japan)
with an accelerating voltage of 200 kV. The specific surface
areas of catalysts were measured through nitrogen adsorption
analysis at 77 K using the Autosorb IQ instrument. Prior to
adsorption, the samples were treated at 300 °C under vacuum
for 18 h and the specific surface area was calculated according
Experimental section
Materials
Lactic acid (analytic grade) was purchased from Chengdu to the Brunauer–Emmett–Teller (BET) method. The pore size
Kelong Chemical Reagent Co. and was used for the synthesis of catalysts was calculated from desorption branch data on the
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Barrett–Joyner–Halenda (BJH) model. The surface acidity of
the catalyst was estimated by NH3-TPD using the Quanta-
chrome Instrument.
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Catalyst evaluation
The synthesis of acetaldehyde from bio-lactic acid over the
catalysts was carried out in a fixed-bed quartz reactor of 4 mm
inner diameter operated at atmospheric pressure. The catalyst
(0.13–0.14 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 required reaction temperature
(325 °C) for 1.0 h under N2 with high purity (0.1 MPa, 1.0 mL
min−1). The feedstock (20 wt% solution of LA) was then
pumped into the reactor (LA aqueous solution flow rate,
1.0 mL h−1) and driven through the catalyst bed under nitro-
gen. The contact time of the reactant over the catalyst is
about 0.6 s, and the contact time is estimated according to
eqn (1).40,46,55 The liquid products were condensed using an
ice-water bath and analyzed off-line using a SP-6890 gas chromato-
graph with a FFAP capillary column connected to a FID. Quan-
titative analysis of the products was carried out by the internal
standard method using n-butanol as the internal standard.
GC-MS analyses of the samples were performed using the
Agilent 5973N Mass Selective Detector attachment. The reac-
tion tail gas was analyzed using GC with a packed column of
TDX-01 connected to a TCD detector. The conversion of LA
and the selectivity toward acetaldehyde or other by-products
were calculated according to eqn (2) and (3).
3600 ꢀ 273:15 ꢀ Vcat:
tC
¼
ð1Þ
22400 ꢀ ðnLA þ nH O þ nCÞ ꢀ T
2
tC: contact time (s); Vcat.: catalyst volume (mL); nLA: the moles
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of lactic acid passed per hour; n(H O): the moles of water in
2
lactic acid aqueous solution feed passed per hour; nC: the
moles of carrier gas passed per hour; T: reaction temperature (K).
n0 ꢁ n1
Conversion=% ¼
Selectivity=% ¼
ꢀ 100;
ꢀ 100
ð2Þ
ð3Þ
n0
np
n0 ꢁ n1
where n0 is the molar quantity of LA fed into the reactor, n1 is
the molar quantity of LA in the effluent, and np is the molar
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Acknowledgements
This work was supported by the Scientific Research Fund of
Sichuan Provincial Educational Department with the project 23 A. Brennfuhrer, H. Neumann and M. Beller, ChemCatChem,
number of 14ZA0128, the Scientific Research Fund of Chemi- 2009, 1, 28–41.
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Scientific Research Fund of China West Normal University 25 C. M. Tang, X. L. Li and G. Y. Wang, Korean J. Chem. Eng.,
with the project number of 12B019.
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