Production of propionic acid via hydrodeoxygenation of lactic acid over FexOy catalysts

Article abstract of DOI:10.1039/c6ra10096a

The gas-phase hydrodeoxygenation of LA to propionic acid over Fe and its oxides was firstly investigated under various conditions. The catalysts were characterized by nitrogen adsorption-desorption, XRD, FT-IR, H₂-TPR and SEM. Fe₃O₄ as active species was confirmed. Due to that reason, Fe₂O₃ can be efficiently transformed in situ to Fe₃O₄ under an atmosphere containing hydrogen derived from decarboxylation/or decarbonylation reaction of lactic acid, which offers the most excellent catalytic performance. The catalyst is sensitive to reaction temperature. LA conversion and its consumption rate increased with an increase of reaction temperature. Similarly, propionic acid selectivity also increased with reaction temperature in the range of 360-390 °C. But with further enhancement of reaction temperature from 390 to 400 °C, it drastically decreased since the formation rate of propionic acid reduced at 400 °C. The catalyst displayed an excellent adaptability in a wide range of LA LHSV except for 1.3 h^-1. More importantly, at high LA LHSV of 26.3 h^-1, the catalyst offered a satisfactory stability within 100 h on stream. Under the optimal reaction conditions, 96.7% of LA conversion and 46.7% of propionic acid selectivity were achieved.

Full text of DOI:10.1039/c6ra10096a

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Production of propionic acid via
hydrodeoxygenation of lactic acid over FexOy
Cite this: RSC Adv., 2016, 6, 62252
catalysts†
a
a
a
a
a
b
Xinli Li, Zhanjie Zhai, Congming Tang,* Liangwei Sun, Yu Zhang and Wei Bai
The gas-phase hydrodeoxygenation of LA to propionic acid over Fe and its oxides was firstly investigated
under various conditions. The catalysts were characterized by nitrogen adsorption–desorption, XRD, FT-
IR, H
2
-TPR and SEM. Fe
3
O
4
as active species was confirmed. Due to that reason, Fe
2 3
O can be efficiently
transformed in situ to Fe
3
4
O under an atmosphere containing hydrogen derived from decarboxylation/or
decarbonylation reaction of lactic acid, which offers the most excellent catalytic performance. The
catalyst is sensitive to reaction temperature. LA conversion and its consumption rate increased with an
increase of reaction temperature. Similarly, propionic acid selectivity also increased with reaction
ꢀ
temperature in the range of 360–390 C. But with further enhancement of reaction temperature from
ꢀ
ꢀ
3
90 to 400 C, it drastically decreased since the formation rate of propionic acid reduced at 400 C. The
Received 19th April 2016
Accepted 22nd June 2016
ꢁ1
catalyst displayed an excellent adaptability in a wide range of LA LHSV except for 1.3 h . More
ꢁ1
importantly, at high LA LHSV of 26.3 h , the catalyst offered a satisfactory stability within 100 h on
stream. Under the optimal reaction conditions, 96.7% of LA conversion and 46.7% of propionic acid
selectivity were achieved.
DOI: 10.1039/c6ra10096a
www.rsc.org/advances
37
38
39
40
cellulose, wheat straw, sugars, and sorbitol have also been
used to produce LA. Therefore chemicals produced from LA
Introduction
Propionic acid is currently produced in around 4 00 000 tons have displayed a potential perspective.
per year, and is widely used as a food preservative for grain,
The production of propionic acid from LA dates back to 19th
1
–4
4
bread, cake, cheese, meat, etc. Its production mainly relies on century using hydriodic acid as a catalyst. Homogeneous
the oxidization of propanal obtained from ethylene via hydro- precious metal catalysts based on Pt, Pd and Ir, were used for
5–10
41
formylation.
Other processes in propionic acid synthesis this reaction. The reaction mechanism was thought to contain
11,12
include hydrocarboxylation of ethylene,
ethanol, hydrogenation of acrylic acid
carbonylation of two steps: (1) dehydration of LA into acrylic acid; and (2)
and hydration of hydrogenation of acrylic acid into propionic acid. Among these
13
14,15
16–18
acrylonitrile,
and these processes are summarized in precious metal complexes, PtH(Pet
3
)
3
offered the best catalytic
ꢀ
Scheme 1. However theses processes for propionic acid performance (around 50% yield of propionic acid) at 250 C in
4
production are mostly based on raw materials derived from water at pH ¼ 2. Korstanje et al. subsequently reported that
petroleum, which will become scarcer and more expensive with non-noble metal complexes based on molybdenum were also
the rapid development of chemical industries in the long term. used to catalyze the deoxygenation of LA into propionic acid,
The search for new processes for the production of propionic achieving 41% yield of sodium propionate. The major problem
acid based on sustainable feedstock is extremely urgent. Lactic in the homogeneous catalysis is catalyst-product separation.
acid (LA) as sustainable feedstock is widely used to synthesize Thus heterogeneous catalysis which can efficiently overcome
1
9–24
25–27
various chemicals such as acrylic acid,
and polylactic acid.
rich and inexpensive biomass materials such as glycerol,
acetaldehyde,
2,3- above shortcoming is becoming attractive. Among these solid
28,29
30,31
42,43 22,23,28,29
pentanedione
Except for corn starch, catalysts are sulfates,
phosphates,
alkali or alkali
32–36
19,44
45
earth metal-modied zeolites,
mesoporous carbon, metal
25,26,46
47,48
oxides,
and supported platinum.
The reaction
temperature applied in these catalysts is usually selected in the
a
ꢀ
Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, China range of 300–400 C. So far, the best yield of propionic acid
West Normal University, Nanchong, Sichuan, 637002, PR China. E-mail:
ꢀ
(
25%) has been achieved over Pt/Nb O catalyst at 350 C and 50
2 5
tcmtang2001@163.com
b
48
bar H pressure.
2
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu,
To the best of my knowledge, the non-noble metal catalysts
are rarely used to catalyze the gas-phase hydrodeoxygenation
reaction of LA into propionic acid. Especially, iron catalyst has
Sichuan, 610041, PR China
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ra10096a
1
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Scheme 1 Paths for propionic acid production.
never been investigated for this reaction. Here, we presented the the standard FeO sample, JCPDS no. 06-0615. Fresh Fe
3 4
O
use of Fe and its oxides for the selective hydrodeoxygenation of displays six characteristic diffraction peaks centered at 2q ¼
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
LA into propionic acid under an atmospheric pressure without 30.1 , 35.4 , 43.1 , 53.4 , 56.9 and 62.5 , indexed to (220), (311),
additional H
2
.
(400), (422), (511) and (440), respectively, which accords with the
standard Fe O sample, JCPDS no. 65-3107. Fresh Fe O3
3
4
2
displays eight characteristic diffraction peaks centered at 2q ¼
Results and discussion
Characterization of catalysts
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2
4.0 , 33.1 , 35.5 , 40.8 , 49.5 , 54.1 , 62.4 and 63.9 , indexed
to (012), (104), (110), (113), (024), (116), (214) and (300),
respectively, which accords with the standard Fe sample,
2 3
O
BET and SEM. As for a heterogeneous catalyst, its specic
surface area usually has an important inuence in catalytic
activity. For most of heterogeneous catalysts, high specic
surface area has a favorable activity. Table 1 showed the BET
data of Fe and its oxides, and the adsorption isotherms were
JCPDS no. 33-0664. In order to understand the active species for
hydrodeoxygenation reaction of LA to propionic acid, the used
catalysts for 8 h reaction were also determined with XRD char-
acterization, and the results were given in Fig. 2B. It is clearly
observed that all the used catalysts display conformable char-
2
ꢁ1
shown in Fig. S1.† It is clearly seen that Fe (0.18 m g ) and FeO
2
ꢁ1
3 4
acteristic diffraction peaks index to Fe O (JCPDS no. 65-3107).
(
0.02 m g ) have lower specic surface area than Fe
3
O
4
(78.62
(24.05 m g ). Similarly, the pore volume of
samples increases in the following order: FeO < Fe < Fe
Fe . The used catalysts were also measured, and the BET
2
ꢁ1
2
ꢁ1
However, Fe and FeO have only a small part transformed into
Fe and others still remain. Interestingly, as for Fe , it has
completely transformed to Fe , showing nearly 5 times of
intensities of characteristic diffraction peaks stronger than the
used Fe . Subsequently, we further investigate the course of
Fe O transformed into Fe O under the reaction conditions,
2 3
m g ) and Fe O
O
4
2 3
O
2 3
O
3
3 4
O
<
3 4
O
data were shown in Table S1.† All the catalysts increased their
specic surface areas except for Fe O . The morphologic
features of the catalysts were determined by scanning electron
microscope, and the results were shown in Fig. 1. All samples
display an irregular morphologic feature.
3 4
O
3
4
2
3
3 4
and the results were shown in Fig. 3. Indeed, it quickly trans-
formed into Fe . For example, only minor part of Fe exists
in the sample aer 0.5 h of reaction, whereas it has completely
transformed into Fe when the reaction time lengthened to
3
O
4
2 3
O
XRD and FT-IR. Fig. 2A showed the XRD patterns of fresh
samples. Fresh Fe displays two characteristic diffraction peaks
3 4
O
ꢀ
ꢀ
1.0 h. FT-IR techniques were also used to investigate the cata-
lysts, and the results were shown in Fig. 4. Comparison of the
fresh catalysts (before reaction) with the used catalysts (aer
reaction) suggests that characteristic absorption band (around
centered at 2q ¼ 44.7 and 65.0 , indexed to (110) and (200),
respectively, which accords with the standard Fe sample, JCPDS
no. 65-4899. Fresh FeO displays four characteristic diffraction
ꢀ
ꢀ
ꢀ
ꢀ
peaks centered at 2q ¼ 36.0 , 41.9 , 60.6 and 72.7 , indexed to
ꢁ1
5
80 cm ) indexed to Fe O existed in the used catalysts, indi-
(
111), (200), (220), and (311), respectively, which accords with
3 4
cating that Fe and FeO transform in part to Fe
Fe has completely transformed into Fe . This result
accords with that obtained with XRD.
-TPR. Fig. 5A showed the TPR proles of fresh catalysts.
FeO does not display low temperature H -consumption peak,
and only displays high temperature H -consumption peak. Both
Fe O and Fe O display two H -consumption peaks at low
3 4
O , whereas
2
O
3
3 4
O
Table 1 BET data of catalysts
H
2
2
ꢁ1
3
ꢁ1
Catalyst
S
BET (m g
)
Vol (cm g
)
2
ꢁ4
2
Fe
FeO
0.18
0.02
3.66 ꢂ 10
ꢁ4
3
4
2
3
2
1.13 ꢂ 10
0.43
temperature and high temperature, respectively. For that
Fe
Fe
3
O
O
4
3
78.62
24.05
3
+
2
0.21
reason, it is easy to understand that Fe is easily reduced to
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Fig. 2 XRD patterns of fresh catalysts (A) and their corresponding used
catalysts for 8 h reaction (B).
Fig. 3 XRD patterns of fresh catalyst (Fe
experienced after 0.5–1.0 h reaction).
2 3
O ) and its used catalysts
(
3 4 2 3
Fig. 1 SEM images of catalysts (A: Fe; B: FeO; C: Fe O and D: Fe O ).
redox atmosphere. We further investigated the reducibility of
the used Fe O and the results were shown in Fig. 5B. Compared
2
+
2+
2
Fe under the H -containing atmosphere. But, in contrast, Fe
2
3
is difficultly reduced to Fe metal. This result means that it is with fresh Fe O , the H -consumption peak of the used Fe O at
2
3
2
2 3
3
+
2+
easy to construct a recycle between Fe and Fe under the low temperature moved toward lower temperature, close to
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Fig. 5
H
2
-TPR profiles of fresh catalysts (A) and the used Fe
and fresh Fe (B).
2 3
O
catalyst together with its fresh Fe
O
2 3
3 4
O
Fig. 4 FT-IR spectra of fresh catalysts (A) and their corresponding
used catalysts for 8 h reaction (B).
2
ꢁ1
2
ꢁ1
low specic surface areas (ca. 0.18 m g and 0.02 m g
,
2
ꢁ1
respectively, far lower than Fe
3
O
4
(78.62 m
2 3
g ) and Fe O
fresh Fe
ature becomes smaller; and this indicates that small amount of
can reduce the catalyst. In addition, we further calculated
the H -consumption on fresh Fe , Fe and their corre-
3 4 2
O . Notably, its H -consumption peak at low temper-
2
ꢁ1
(
24.05 m g )). As for propionic acid selectivity, it is drastically
inuenced by Fe species. It is well known that propionic acid is
obtained through hydrodeoxygenation of LA, ascribing to
reduction reaction. Ordinarily, the Fe species with low chemical
valence favors to reduction reaction. Accordingly, Fe and FeO
catalysts should display better catalytic performances than
H
2
2
2
O
3
3 4
O
sponding used catalysts, and the results were shown in Table
S2.† Interestingly, the two used catalysts (Fe O and Fe O )
2
3
3 4
almost displayed the coincident results.
Fe
tained. It is obviously observed from the data shown in Table 2
and Fig. 6B that Fe O and Fe O catalysts offer better selectivity
3 4 2 3
O and Fe O . But the opposite results actually were ob-
Evaluation of catalysts
3
4
2 3
of propionic acid than Fe and FeO catalysts. It is noteworthy
Screening of catalysts. Catalytic reactions for gas phase
that the selectivity of propionic acid obtained over the Fe O3
2
hydrodeoxygenation of LA over the Fe and its oxides were per-
3 4
catalyst (46.7%) is slightly higher than Fe O (42.5%). However,
ꢀ
formed at 390 C with LA concentration (20 wt%) and feed ow
the former has stronger oxidation property than the latter. As
mentioned above, the reaction on hydrodeoxygenation of LA to
propionic acid is ascribed to reduction reaction. Evidently,
ꢁ1
rate (1 mL h ), and the results were shown in Fig. 6A and B and
Table 2. To compare the results obtained over the catalysts, the
blank experiment was also performed at the identical condi-
tions. Obviously, the LA conversion (47.5%) is far lower than
those (86.1–96.7%) observed over the catalysts. Furthermore, it
is clearly seen from the data shown in Table 2 that LA conver-
sion gradually increases from 86.1% to 96.7% with change of
2 3
Fe O catalyst can not display better catalytic performance than
Fe O . On the contrary, Fe O catalyst indeed displayed better
3
4
2 3
activity for hydrodeoxygenation of LA to propionic acid than
Fe O and others. Similarly, the higher selectivity of propionic
3
4
4 40
acid over the silica supported heteropolyacid (H PVMo11O )
catalyst in which metals such as V (+5) and Mo (+6) have high
2 3
catalysts from Fe to Fe O . Relatively low conversions of LA
obtained over the Fe and FeO catalysts can be ascribed to their
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3 4
noted that Fe and FeO catalysts in which small Fe O phase
existed displayed worse selectivity of propionic acid than Fe O4
3
and Fe O catalysts. Based on these evidences, we can conclude
2
3
that Fe O is active species for catalytic hydrodeoxygenation of
3
4
LA into propionic acid. Since Fe
we have further investigated the transformation of catalysts by
reaction time course (shown in Fig. 3). Most of Fe has been
transformed into Fe aer 0.5 h of reaction, and it has been
completely transformed into Fe when the reaction length-
ened 0.5 h. This experiment further proved that Fe O can be in
3 4
O is viewed as active species,
2 3
O
3 4
O
3 4
O
2
3
situ rapidly transformed into Fe O under the reaction condi-
3
4
tions. Except for XRD characterization, XPS was also used to
investigate the fresh catalysts and their corresponding used
catalysts, and the results were given in Fig. S2.† High-resolution
XPS proles of Fe 2p for fresh catalysts (Fe
played two obvious peaks at about 724.5 eV and 710.9 eV which
corresponded to the binding energies of Fe 2p1/2 and Fe 2p3/2
2 3 3 4
O and Fe O ) dis-
.
But slight difference existed in the XPS proles of the fresh
catalysts, suggesting that Fe showed different oxidation states
in the fresh catalysts. From high-resolution XPS proles of Fe 2p
in the used catalysts, two obvious peaks occurred at the same
binding energies (724.5 eV and 710.9 eV), indicating that Fe
was completely transformed into Fe during the catalytic
reaction. In addition, H -TPR characterization was also used to
2 3
O
3 4
O
2
investigate the redox properties of catalysts, and the results
were shown in Fig. 5A and B. From the H -TPR proles shown in
2
Fig. 5A, it is clearly seen that both Fe O and Fe O catalysts
3
4
2 3
display a low temperature peak for H consumption in the range
2
ꢀ
ꢀ
ꢀ
of 350–600 C, which are centered at 476 C and 539 C,
Fig. 6 Performance of Fe and its oxides by the time course of LA
a
respectively. However, FeO had no low temperature peak for H
consumption to be replaced by high temperature peak for H
feed flow rate 1 mL h , LA feedstock 20 wt%; reaction temperature, consumption, suggesting that FeO is difficult to reduce into Fe
2
conversion (A) and propionic acid selectivity (B). Catalysts: Fe, 1.24 g,
ꢁ1
3 4 2 3 2
FeO, 0.99 g, Fe O , 0.32 g, Fe O , 0.44 g; carrier gas N , 1 mL min ;
2
ꢁ1
ꢀ
b
3
90 C. LA: lactic acid.
3+
2
in H atmosphere. It is easily concluded that Fe species can be
easily transformed into Fe species under the reductive atmo-
2
+
sphere (ca. H ). It happens to meet with an H atmosphere in
catalytic process since a large quantity of acetaldehyde formed
through decarboxylation/or decarbonylation of LA accompa-
2
2
oxidation state than others was also observed by katryniok and
27
co-workers. Unfortunately, the reasons why the catalysts with
high oxidation state displayed high activity for this reaction
have been short of further discussion so far. In order to search
for the evidences, the catalysts were characterized by XRD and
nying with formation of H
2
together with CO
can be in situ rapidly transformed
3 4
O as active species during the catalytic reaction. Again,
2
as by-
19,23–26
product.
into Fe
from the H
2 3
Thus Fe O
H
2
-TPR techniques, and the results were shown in Fig. 2, 3 and
2
2 3
-TPR graphs of the used Fe O shown in Fig. 5B, low
5. Compared the XRD patterns of fresh catalysts (before reac-
tion) shown in Fig. 2A with those shown in Fig. 2B (aer reac-
tion for 8 h), their diffraction peaks changed evidently except for
a
Table 2 Screening of catalysts
3 4
Fe O catalyst, indicating that Fe species were transformed
under the catalytic reaction conditions. We further analyzed
XRD patterns of the used catalysts using MDI Jade 5.0 soware. Catalystsc
b
Sel. (%)
LA conv. (%)
PA
AD
AA
PD
ACA
Fe O catalyst has been completely transformed into Fe O , and
2
3
3 4
Blank
Fe
FeO
47.5
86.1
91.1
95.2
96.7
7.3
20.8
19.2
42.5
46.7
26.5
17.1
19.8
13.2
12.7
1.6
1.4
1.3
2.0
2.2
0.5
1.1
1.1
1.1
0.9
2.1
2.1
2.0
13.3
14.2
the intensity of its diffraction peaks indexed to Fe
times stronger than the used Fe catalyst. Fe and FeO cata-
lysts have partly been transformed into Fe species, and
3 4
O is about 5
3 4
O
3
O
4
Fe
others still existed in Fe and FeO samples, respectively. Inter- Fe O3
3
O
4
2
estingly, Fe
3
O
4
catalyst has not changed, and still retained
a
Catalyst: 0.38 mL, Fe, 1.24 g, FeO, 0.99 g, Fe
3
O
4
, 0.32 g, Fe
2
O
3
, 0.44 g;
ꢁ
1
ꢁ1
Fe O phase. In four used catalysts, the used Fe O displayed carrier gas N 1 mL min ; feed ow rate 1 mL h , LA feedstock 20 wt%;
3
4
2
3
2
ꢀ
b
reaction temperature, 390 C; TOS, 3–4 h. LA: lactic acid, PA: propionic
acid, AD: acetaldehyde, AA: acrylic acid, PD: 2,3-pentanedione, ACA:
acetic acid. Carbon recovery is above 93% except for Fe and FeO
the strongest intensity of characteristic diffraction peaks
indexed to Fe O phase. Furthermore, the highest selectivity of
propionic acid was also achieved over the Fe O catalyst. It is catalysts (around 85%).
2 3
3
4
c
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2
temperature peak for H consumption more drastically reduced
than fresh Fe O and Fe O , indicating that the used catalyst is
2
3
3 4
easily reduced in the presence of a small quantity of H . Just
2
right, H2 as a reductive atmosphere existed in the reaction
3
+
2+
2+
system can reduce Fe to Fe , and Fe captures an oxygen
atom from LA molecule to form one molecule of propionic acid
3
+
3+
2+
(
2
product), and then it is oxidized to Fe . H reduces Fe to Fe
through drawing on an oxygen atom from iron oxide to form
a water molecule. Besides, another interesting phenomenon
was also observed from the data shown in Table 2. The selec-
tivities of acetic acid over the Fe O and Fe O catalysts are
3
4
2 3
13.3% and 14.2%, respectively, far more than those of Fe and
FeO catalysts (2.1% and 2.0%, respectively). Acetic acid is
possibly formed via catalytic oxidization of acetaldehyde ob-
3
+
45
2+
tained from LA by Fe species, which is reduced to Fe
species. Besides, acetic acid is also possibly formed through
hydration of acetaldehyde accompanying with formation of
3
+
2+
hydrogen, which can also reduce Fe to Fe . As mentioned
2
+
above, Fe species favors to catalyze hydrodeoxygenation of LA
to propionic acid. In order to further learn about the catalytic
ꢀ
performances, lower reaction temperature (360 C) was chosen,
and the results were given in Table S3.† Similar results were
ꢀ
obtained at 360 C. However, there is also a little difference in
which the selectivity of propionic acid over the Fe
higher than Fe . This result further demonstrated that Fe
is active species.
3 4
O is slightly
2
O
3
3 4
O
Reaction temperature. According to the discussion on Fe
and its oxides for catalytic hydrodeoxygenation of LA to pro-
pionic acid, Fe
importantly, Fe
formed into active species of Fe
3
O
4
as active species has been conrmed. More
as precursor can be easily in situ trans-
and offers the best catalytic
performance. Therefore, effect of reaction temperature on
Fig. 7 Effect of reaction temperature on reaction performances: LA
2
O
3
conversion and propionic acid selectivity (A); and area-specific cata-
3
O
4
a
lytic rate (B). Catalyst: 0.38 mL, Fe
2
O
3
, 0.44 g; carrier gas N
2
, 1 mL
ꢁ1
ꢁ1
b
min ; feed flow rate, 1 mL h , LA feedstock 20 wt% ; TOS, 3–4 h. LA:
catalytic performances such as LA conversion, propionic acid lactic acid.
selectivity and area-specic catalytic rate was investigated over
the Fe O as precursor, and the results were shown in Fig. 7A
2
3
ꢀ
and B. In the range of 360–400 C, LA conversion slightly
Liquid hourly space velocity (LHSV). It is known that liquid
increases with an enhancement of reaction temperature. For
hourly space velocity (LHSV) is a vital factor for evaluation of solid
ꢀ
20,42,49,50
example, LA conversion is 90.7% at 360 C, and it rises to 98.4%
catalysts.
The inuence of LA LHSV on reaction perfor-
ꢀ
at 400 C, which increases only 7.7%. As for propionic acid
mance was shown in Fig. 8. The reactions were conducted at 390
ꢀ
ꢁ1
selectivity, it is clearly seen that propionic acid selectivity
C with ow rate of LA changed from 0.5 to 15 mL h (corre-
ꢁ
1
drastically increases from 28.1% to 46.7% with an increase of sponding LHSV ¼ 1.3–39.5 h ). Other operation conditions such
ꢀ
ꢁ1
reaction temperature from 360 to 390 C. However it rapidly
as the ow rate of carrier gas N (1 mL min ) and LA concen-
2
decreases to 19.5% with further increase of reaction tempera-
ture from 390 to 400 C, suggesting that reaction rates of side
reactions increase more quickly than formation rate of pro-
pionic acid. In order to fully understand this view, we correlated
area-specic catalytic rate with reaction temperature, and the
tration (20 wt%) were retained. It is clearly seen from the results
given in Fig. 9 that propionic acid selectivity drastically increases
ꢀ
ꢁ
1
from 33.4% to 46.7% as LA LHSV increases from 1.3 to 2.6 h
Subsequently, propionic acid selectivity remains on around 43–
6% with further increase of LA LHSV. On the other hand, LA
.
4
result was depicted in Fig. 7B. LA consumption rate always conversion slightly decreases from 98.4% to 88.4% as the LA
ꢁ1
increases with an increase of reaction temperature, whereas LHSV increases from 1.3 to 39.5 h , indicating that hydro-
propionic acid formation rate rstly increases with an increase
of reaction temperature, and it subsequently decreases with
further increase of reaction temperature. This result further
supports on the above view, i.e., reaction rate of side reactions
rise more quickly than formation rate of propionic acid at
higher reaction temperature. Thus 390 C is chosen as an
optimal reaction temperature considering LA conversion and
propionic acid selectivity.
deoxygenation reaction of LA to propionic acid is a fast reaction.
The time space yield (YTS) of propionic acid was calculated
according to the following equation.
42,49
m
PA
YTS ¼ m
Cat ꢂ t
ꢀ
where m(PA) is the mass of produced propionic acid (g); m(Cat) is
the mass of catalyst; and t is the reaction time (h). Plotted in
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a
Fig. 8 Effect of LA LHSV on the reaction performance. Catalyst: 0.38
ꢁ
b
1
mL, Fe
3
2
O
3
, 0.44 g; carrier gas N
2
, 1 mL min ; reaction temperature,
ꢀ
90 C; LA feedstock 20 wt%; TOS, 3–4 h. LA: lactic acid.
Fig. 10 Durability of catalyst (A: fresh catalyst, 0.43 g and B: the used
ꢀ
ꢁ
a
2 3
catalyst calcined at 500 C for 5 h, 0.34 g). Catalyst: 0.38 mL, Fe O ;
1
ꢀ
carrier gas N
0 wt%, feed flow rate, 10 mL h
2
, 1 mL min ; reaction temperature, 390 C; LA feedstock
ꢁ
1 b
2
.
LA: lactic acid.
Fig. 9 Time space yield of propionic acid at different LA LHSV.
with an increase of time on stream. For example, LA conversion
reduces with only 26.4% (from 95.7% to 69.3%) within 100 h on
stream. Similarly, propionic acid selectivity also slowly gradu-
ally decreases with time on stream. It is noted that LA feed ow
Fig. 10 is the YTS of propionic acid versus LA liquid hourly space
velocity. From Fig. 9, YTS of propionic acid rapidly increases
with an increase of LA LHSV. But, as the LA LHSV is above 26.3
ꢁ1
ꢁ1
rate of 10 mL h (corresponding LA LHSV ¼ 26.3 h ) is far
more than previous reports on LA conversion into chemicals
ꢁ
1
h
, LA conversion evidently decreases. A case in point is that
2
1,24,26,45,56
such as acrylic acid and acetaldehyde.
Besides, the
ꢁ
1
the residue of LA accounts for 8.5% at LA LHSV of 26.3 h . LA
can not be fully converted at high LA LHSV since LA becomes
difficult to be evaporated due to transfer limitation of heat. It is
known that hydrodeoxygenation reaction of LA into propionic
acid is a typical gas–solid catalytic reaction. Unevaporated LA in
the reaction system does not favor the reaction, resulting in
a low LA conversion. Therefore, it is a key factor for choosing
a suitable LA LHSV to achieve a long-term stability for catalytic
ꢀ
used catalyst was calcined under air atmosphere at 500 C for 5
h, and evaluated the catalytic performance again; and the result
was shown in Fig. 10B. The catalytic performance can be almost
recovered by a simple calcination considering that the used
catalyst reduced from 0.43 g (fresh catalyst) to 0.34 g (the used
catalyst). Therefore the stability experiment suggests that the
catalyst offer excellent durability and recovery.
Mechanistic studies. In order to fully understand the
observed reactivities, we performed several discriminating
experiments on reaction mechanism of propionic acid from LA
over the Fe and its oxides. The possible reaction paths can be
depicted in Scheme 2. Path 1: the acrylic acid is believed as an
intermediate, which is obtained through dehydration of LA, and
ꢀ
hydrodeoxygenation reaction of LA to propionic acid at 390 C.
Durability and recovery. Long-term stability is a very
51–55
important characteristic for a heterogeneous catalyst.
The
catalytic stability of Fe
3
O
4
catalyst in situ transformed from
ꢀ
Fe
h
2
ꢁ
O
3
was investigated at 390 C and LA feed ow rate of 10 mL
1
ꢁ1
(corresponding LA LHSV ¼ 26.3 h ), and the results were
subsequently it is hydrogenated with H to form propionic acid.
2
depicted in Fig. 10A and B. With regard to variation tendency of
LA conversion with time on stream, it slowly gradually decreases
If this path is true, the catalyst should have at least two functions
including catalytic dehydration, and catalytic hydrogenation.
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Scheme 3 The possible paths for hydrogen formation.
Scheme 2 The possible reaction paths for propionic acid synthesis
from LA.
selectivity shown in Table 2. Acetic acid selectivities over the
Fe (14.2%) and Fe (13.3%) are far higher than Fe (2.1%)
2
O
3
3 4
O
and FeO (2.0%). Besides, organic compounds can also partly
react with water to form CO, CO , and H at high reaction
But from the product distribution data (represented using
product selectivity data) shown in Table 2, the acrylic acid
selectivity is low, around 1.3–2.2%, suggesting that the inter-
mediate concentration is low, which is unfavorable to formation
of propionic acid. The other possibility is that hydrogenation
rate of acrylic acid is faster than dehydration rate of LA, resulting
in a low acrylic acid concentration. To discriminate the true or
not on this view, we performed an experiment using only acrylic
2
2
57–63
temperature,
which can provide hydrogen for propionic acid
synthesis from LA.
2
Just right, the in situ formed H as a reductive atmosphere
3+
2+
2+
can reduce Fe to Fe , and Fe captures an oxygen atom from
LA molecule to form one molecule of propionic acid (product),
3+
3+
2
and then it is oxidized to Fe . Then H again reduces Fe to
2+
Fe through drawing on an oxygen atom from iron oxide to
form a water molecule.
ꢁ
1
acid (10 wt%) as a substrate, and H (ow rate, 3 mL min ) as
2
ꢀ
carrier gas at 390 C over the Fe
O
2 3
to synthesize propionic acid.
Acrylic acid conversion is 40%, and propionic acid selectivity is
ꢁ1
3
1.6%, whereas under LA as a substrate and N
2
(1 mL min ) as Experimental section
a carrier gas, LA conversion is 96.7%, and propionic acid selec-
tivity is 46.7%. This result suggests that hydrogenation rate of
acrylic acid is not enough high. In other words, the path 1 is not
major path for propionic acid from LA, and only a minor path.
The path 2: LA is converted to propionic acid via a direct
hydrodeoxygenation (C–OH cleavage). Since path 1 is a minor
path, path 2 should become a major path. In order to further
Materials
Lactic acid (analytic grade, 85–90 wt%) was purchased from
Chengdu Kelong Chemical Reagent Co. and was used for the
synthesis of propionic acid without further purication. Triple-
distilled water was prepared in the laboratory and was used to
dilute lactic acid for desired concentration. Reduced iron
powder (Fe), ferrous oxide (FeO), ferric nitrate (Fe(NO
ferric chloride (FeCl ) and ferrous chloride (FeCl $4H
3
)
3
$9H
2
O),
investigate the effect of external H
experiment using H (ow rate, 3 mL min ) as a carrier gas
substituted for N , and other reaction conditions were retained.
2
, we also performed an
ꢁ
1
3
2
2
O) were
2
purchased from Alfa Aesar. Ammonia water (25–28 wt%), pro-
pionic acid, acetaldehyde, acrylic acid, acetic acid, 2,3-penta-
nedione and n-butanol, together with hydroquinone were
purchased from Sinopharm Chemical Reagent Co., Ltd. Pro-
pionic acid, acrylic acid, acetic acid, 2,3-pentanedione and
acetaldehyde 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.
2
LA conversion is almost unchanged, and propionic acid is
slightly enhanced, suggesting that the catalyst offered a poor
activity for external H (molecular hydrogen), and an excellent
2
activity for in situ hydrogen including an atomic hydrogen (H) or
+
ꢁ
ionized hydrogen (H or H ).
Whether the reaction on propionic acid converted from LA
follows path 1 or 2, hydrogen is necessary. According to previous
27,44,46
investigations,
hydrogen can be formed via decarboxyl-
ation of LA given acetaldehyde, CO and H , or decarbonylation
2
2
Preparation of catalysts
of LA given acetaldehyde, CO and H O, followed with a water-
2
gas shi reaction to produce hydrogen (shown in Scheme 3).
The tail gas was analyzed by GC with a packed column of TDX-
Fe
ammonia water as a precipitant. In a typical experiment, 8.08 g
Fe(NO $9H O was fully dissolved in 100 mL distilled water
2 3
O was prepared with a precipitation method using
2
01 connected to TCD detector, and it was found that CO /CO
3
)
3
2
molar ratio is close to 4 : 1, suggesting that decarboxylation rate
of LA is faster than the decarbonylation rate of LA, or CO is fast
converted to CO through a water-gas shi reaction. These
2
under a stirring state for 1 h at room temperature. Next, the
resultant solution was adjusted to pH ¼ 8–9 to form a reddish-
brown precipitate by dropwise addition of ammonia water (25
wt%). The resulting precipitate was ltered, completely rinsed
processes accompany with producing hydrogen. Another source
is that hydrogen is produced via hydration of acetaldehyde to
ꢀ
with distilled water, dried at 120 C for around 5 h, and calcined
45
produce acetic acid (also shown in Scheme 3). As for the latter
source, it is strongly supported by the data on acetic acid
ꢀ
in muffle furnace at 500 C for 6 h. Fe
3
O
4
was prepared with
$4H
a similar method. In a typical experiment, 4.09 g FeCl
2
2
O
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and 5.00 g FeCl
3
were fully dissolved in 100 mL distilled water
3600 ꢂ 273:15 ꢂ V
cat:
t
C
¼
(1)
under a stirring state for 1 h at room temperature. Next, the
resultant solution was adjusted to pH ¼ 11 to form a precipitate
by dropwise addition of ammonia water (25 wt%). The resulting
precipitate was ltered, completely rinsed with absolute
ethanol. It was dried in the tubular furnace under the argon
22 400 ꢂ ðn þ n
þ n Þ ꢂ T
LA
H2O
C
C
t : contact time (s); Vcat.: catalyst volume (mL); nLA: the moles of
lactic acid passed per hour; n(H2O): the moles of water in lactic
acid aqueous solution feed passed per hour; n : the moles of
carrier gas passed per hour; T: reaction temperature (K).
C
ꢀ
atmosphere at 100 C for around 10 h.
n
0
ꢁ n
1
Conversion=% ¼
ꢂ 100
(2)
n
0
Catalyst characterization
n
p
Selectivity=% ¼ n
ꢂ 100
(3)
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 FT-IR spectra of the catalysts were
recorded in the range of 500–4000 cm on a Nicolet 6700
spectrometer. The morphologic features of the catalysts were
determined by scanning electron microscope (SEM, JSM-6510).
The specic surface areas of catalysts were measured through
nitrogen adsorption at 77 K using Autosorb IQ instrument. Prior
0
ꢁ n
1
where n
0
is the molar quantity of LA fed into reactor, n
molar quantity of LA in the effluent, and n
of lactic acid converted to propionic acid or other byproducts
such as acetaldehyde, acrylic acid, acetic acid and 2,3-
pentanedione.
1
is the
ꢁ1
2
is the molar quantity
ꢀ
to adsorption, the samples were treated at 250 C under vacuum
Conclusions
for 6 h and the specic surface area was calculated according to
the Brunauer–Emmett–Teller (BET) method. X-ray photoelec-
tron spectroscopy (XPS) measurements were performed using
a Thermo ESCALAB 250XI spectrometer. Spectra were obtained
using an aluminium anode (Al Ka ¼ 1486.6 eV) operating at 150
W and a spot size of 500 mm. Redox properties of the samples
The gas-phase hydrodeoxygenation of LA to propionic acid over
Fe and it oxides was rstly investigated at the various condi-
tions. Fe O as active species has been conrmed. Due to that
reason, Fe O can be efficiently in situ transformed to Fe O ,
which offers the most excellent catalytic performance. The
catalyst is sensitive to reaction temperature. LA conversion and
its consumption rate increased with an increase of reaction
temperature. Similarly, propionic acid selectivity also increased
with reaction temperature in the range of 360–390 C. But with
further enhancement of reaction temperature from 390 to 400
3
4
2
3
3 4
2
were estimated by H -TPR on a Quantachrome Instrument. The
sample (ca. 50–60 mg) was purged with dry Ar (50 mL min
ꢁ
1
,
ꢀ
ꢀ
purity > 99.999 v %) at 500 C for 1.0 h, followed by reducing the
t
ꢀ
furnace temperature to 30 C, and switching to a ow of 8.0 v %
t
ꢀ
H
C min
2
/Ar to execute H
2
-TPR in the range of 30–950 C at a rate of 10
ꢁ1
ꢀ
ꢀ
C, it drastically decreased since the formation rate of propionic
acid reduced at 400 C. The catalyst displayed an excellent
adaptability in a wide range of LA LHSV except for 1.3 h . More
importantly, at high LA LHSV of 26.3 h , the catalyst offered
.
ꢀ
ꢁ1
ꢁ
1
Catalyst evaluation
a satisfactory stability within 100 h on stream.
The synthesis of propionic acid from lactic acid over the cata-
lysts was carried out in a xed-bed quartz reactor with a 4 mm
inner diameter operated at an atmospheric pressure without
Acknowledgements
additional H . The catalyst (ca. 440 mg, 20–40 meshes) was
2
placed in the middle of the reactor and quartz wool was placed This work was supported by Scientic Research Fund of
in both ends. Firstly, the catalyst was pretreated at the required Chemical Synthesis and Pollution Control Key Laboratory of
ꢀ
reaction temperature (ca. 390 C) for 1.0 h under N
2
with high Sichuan Province with project number of CSPC-2014-3-1,
ꢁ1
purity (0.1 MPa, 1.0 mL min ). The feedstock (20 wt% solution Scientic Research Fund of China West Normal University
of LA) was then pumped into the reactor (LA aqueous solution with project number of 12B019 and Scientic Research Fund of
ow rate, 1.0 mL h ) and driven through the catalyst bed by Sichuan Provincial Educational Department with project
ꢁ1

nitrogen. The contact time of reactant over the catalyst is number of 14ZA0128, 16ZA0175.
around 0.5 s, and the contact time is estimated according to eqn
42,43,49
(1).
The liquid products were condensed using ice-water
Notes and references
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
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