Aldol condensation of furfural and acetone on layered double hydroxides

Article abstract of DOI:10.1134/S0023158410010131

The Aldol condensation of furfural (Fur) with acetone (Ac) to 4-(2-Furyl)-3-buten-2-one (FAc) is one of the most important processes in the aqueous-reforming of oxygen-containing biomass derivatives and has been carried out in the presence of solid-base catalysts, calcined-rehydrated Layered Double Hydroxides (LDH). The Mg-Al Layered Double Hydroxides has been prepared by the coprecipitation, calcination and regeneration from mixed oxides by rehydration. The catalyst prepared with different Mg/Al molar ratios showed different catalytic performance and the best catalyst was with the Mg/Al molar ratio of 2.5. Phenol adsorption showed that the best catalyst had the largest numbers of accessible basic sites. The appropriate rehydration temperature and time for mixed oxides obtained by calcination were also investigated. The Mg-Al LDH catalysts can be regenerated by calcination at 773 K and rehydration in decarbonate water, but the regeneration is complex and incomplete. In addition, the catalyst calcined at high temperature also had activity, which was attributed to the formation of the Mg-Al spinels.

Full text of DOI:10.1134/S0023158410010131

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2010, Vol. 51, No. 1, pp. 75–80. © Pleiades Publishing, Ltd., 2010.
Published in Russian in Kinetika i Kataliz, 2010, Vol. 51, No. 1, pp. 81–87.
Aldol Condensation of Furfural and Acetone
1
on Layered Double Hydroxides
Huihui Liu, Wenjie Xu, Xiaohui Liu, Yun Guo, Yanglong Guo, Guanzhong Lu, and Yanqin Wang
Lab for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology,
Shanghai 200237, People’s Republic of China
eꢀmail: wangyanqin@ecust.edu.cn, xhliu@ecust.edu.cn
Received December 18, 2008
Abstract—The Aldol condensation of furfural (Fur) with acetone (Ac) to 4ꢀ(2ꢀFuryl)ꢀ3ꢀbutenꢀ2ꢀone (FAc)
is one of the most important processes in the aqueousꢀreforming of oxygenꢀcontaining bioꢀmass derivatives
and has been carried out in the presence of solidꢀbase catalysts, calcinedꢀrehydrated Layered Double
Hydroxides (LDH). The Mg–Al Layered Double Hydroxides has been prepared by the coꢀprecipitation, calꢀ
cination and regeneration from mixed oxides by rehydration. The catalyst prepared with different Mg/Al
molar ratios showed different catalytic performance and the best catalyst was with the Mg/Al molar ratio of
2.5. Phenol adsorption showed that the best catalyst had the largest numbers of accessible basic sites. The
appropriate rehydration temperature and time for mixed oxides obtained by calcination were also investiꢀ
gated. The Mg–Al LDH catalysts can be regenerated by calcination at 773 K and rehydration in deꢀcarbonate
water, but the regeneration is complex and incomplete. In addition, the catalyst calcined at high temperature
also had activity, which was attributed to the formation of the Mg–Al spinels.
DOI: 10.1134/S0023158410010131
1
1
. INTRODUCTION
Double Hydroxides (LDHs) have been found to be the
promising candidates of aldolization catalysts [2–5].
Layered Double Hydroxides (LDHs) known as
anionic clays, are natural or synthetic materials conꢀ
sisting of the positively charged bruciteꢀlike sheets
with the general formula of
People are now suffering from the pressures of both
economic growth and environmental protection. It’s
important to protect, exploit and use energy sources.
Renewable resources has been exploited and used all
over the world considering the advantages of environꢀ
mental protection, increasing employment and energy
sources safety. Among these renewable resources, bioꢀ
mass energy is an important energy and it is the fourth
energy besides coal, petroleum and natural gas and has
a high position in energy system. Furfural, which is
available from the renewable farm and forest plant in
industry, can be used as a starting material for the C8
alkane by the Aldol condensation of furfural and aceꢀ
tone, and further by the hydrogenation and dehydraꢀ
tion; and for the C13 alkane by the Aldol condensation
of C8 and furfural with the similar processes [1]. The
two alkanes can be used as renewable resources.
2
+
3+
x
n–
[
M M (OH) ][A ]
2
x/
⋅ yH O,
n 2
1−x
where M2+ and M3+ represent divalent and trivalent
cations in the octahedral sites within the hydroxyl layꢀ
3+
2+
3+
ers, x is equal to the ratio of M /(M + M ) with a
value varying in the range of 0.17–0.50, and A is an
exchangeable interlayer anion [6–8], Carbonates are
the interlayer anions in naturally occurring hydrotalꢀ
cites. However, the number of counterbalancing ions
is essentially unlimited, and LDHs intercalated by varꢀ
ious simple inorganic, polyoxometalate, and complex
as well as organic anions have been synthesized.
Aldol condensation plays an important role in the
The Mg–Al LDHs can be used as catalyst both in
aforementioned process. Normally, this kind of reacꢀ the form of Mg(Al)O mixed oxides obtained by the
tion is practiced industrially by using homogenous calcination of an Mg/Al hydrotalcite precursor and
base catalysts, such as sodium and calcium hydroxide. the rehydrated form of these calcined materials [9].
However, these processes would generate significant The rehydrated hydrotalcites possess Brønsted basic
water streams that must be neutralized and leads to an sites and in most studies these catalysts are more active
additional disposal cost. Developing a new active and and selective to the acrossꢀcondensation products
stable solid base catalyst can simplify the process, proꢀ than the mixed oxides [10–15]. Roelofs et al. [13, 16]
mote clean manufacturing, and decrease the producꢀ demonstrated that less than 5% of the total number of
–
tion cost. Among the solid base catalysts, activated OH in the rehydrated samples was available for the
hydrotalciteꢀlike compounds (HTLCs) or Layered condensation reactions and the activity was attributed
–
to the localized OH at the edge of the reconstructed
platelets of the layered structures. While, Climent et al.
1
The article is published in the original.
75

76
HUIHUI LIU et al.
2.2. Catalyst Activation
The MgAlꢀmixed oxides (CꢀLDO) were obtained
by calcination of CꢀLDH at 673 K for 4 h in the air
–1
with a heating rate of 5 K min . The mixed oxides
were rehydrated in deꢀcarbonated water at a given
temperature for several hours. Then the activated catꢀ
alysts were obtained by drying under vacuum at 373 K
–
and denominated as CꢀRLDH, but with OH ions as
the counterions in the interlayer.
а
b
c
2
.3. Characterization
XRD powder patterns were collected on a Rigaku
D/Max 2550VB/PC instrument using monochromaꢀ
K radiation (
00 mA) and IR spectra were recorded with a Nicolet
tized Cuꢀ
1
λ
= 0.154056 nm, 40 kV, and
10
20
30
40
θ, deg
50
60
70
α
1
2
Nexus 670 spectrophotometer.
Fig. 1. XRD patterns of
a
, CꢀLDH;
b
, CꢀLDO; and
The adsorption of phenol was used to characterize
the available basic sites of the solid base. The experiꢀ
ment was carried out in cyclohexane at 298 K and the
amount of phenol adsorbed by the solids was measured
c
, CꢀRLDH (Mg/Al = 2.5).
–
[
11] suggested that the OH sites in the rehydrated
samples are of medium basic strength and thus well
adapted to the Aldol condensation.
by UVꢀVis spectroscopy (
λ
= 271.6 nm) [17, 18].
max
Nitrogen adsorption and desorption isotherms
were measured at 77 K with a Micromeritics
ASAP2020M sorption analyzer. Before the measureꢀ
ments, the samples were outgassed at 373 K for 8 h.
In this paper, Layered Double Hydroxides (LDHs)
were prepared by coꢀprecipitation and characterized
by XRD, FTꢀIR and BET. The condensation of furꢀ
fural and acetone was carried out in the presence of
activated MgAl–LDHs and the influence of the prepꢀ
aration method; Mg/Al molar ratios; rehydration
temperature and time on the catalysis properties were
investigated.
2.4. Catalytic Activity Measurement
The catalytic tests were performed in a sealed botꢀ
tle. 0.5 g catalyst was added to a mixture of 14.5 g aceꢀ
tone and 2.4 g furfural and kept at 373 K for 10 h. After
reaction, the liquid was taken from the reaction mixꢀ
ture and analyzed by a PerkinElmer Clarus 500 gas
chromatography with a SEꢀ54 column. A flame ionꢀ
ization detector (FID) was used to analyze the final
products, and an internal standard quantitative analyꢀ
sis method was used.
2. EXPERIMENTAL
2.1. Preparation of the Catalyst Precursor
In the condensation of furfural and acetone, FAc
was the major and aim reaction, product. Because of
the symmetry of acetone molecule, a further condenꢀ
sation with furfural will happen, which led to the forꢀ
mation of 1,4ꢀpentadienꢀ3ꢀone, 1,5ꢀdiꢀ2ꢀfuranyl
The LDH samples were prepared by coꢀprecipitaꢀ
tion from an aqueous solution of magnesium and aluꢀ
minum nitrate (solution A) and a highly basic carbonꢀ
ate solution (solution B). Solution A, containing
Mg(NO3)2
(
⋅
6H O and Al(NO3)3 9H O was 0.6 M
⋅
2
2
(
F2Ac), as a minor product. The Aldol condensation
Al + Mg) with an Mg/Al molar ratio equal to 2, 2.5,
of acetone itself can occur, but the corresponding
product was not found in our analysis by GC because
of the thermodynamic limitation.
or 3. Solution B was prepared by dissolving appropriꢀ
ate amounts of Na CO and NaOH in distilled water
2
3
2
−
in order to obtain 0.1 M [CO ] and 1.2 M [OH–].
3
In the synthesis procedure, solution B was placed
in a 500 ml PTPE reactor and solution A was slowly
dropped into it with vigorous stirring at a constant
temperature of 313 K. After that, the gel was stirred at
3. RESULTS AND DISCUSSION
3.1. Characterization of the Catalysts
The catalyst precursor was synthesized by coꢀpreꢀ
cipitation with an Mg/Al molar ratio of 2.5. After calꢀ
cination and rehydration, the activated catalyst was
obtained. The XRD patterns of the asꢀprepared preꢀ
cursor, CꢀLDH (a); calcined mixed oxide, CꢀLDO (b)
and rehydrated, active catalyst, CꢀRLDH (c) were
presented in Fig. 1. The XRD pattern of the asꢀpreꢀ
363 K for 24 h. Then the solid was obtained after filꢀ
tered, washed with distilled water and dried at 373 K
for 24 h. The asꢀsynthesized Layered Double Hydroxꢀ
ides by this way was denominated as CꢀLDH, with
2
−
CO3 as the counterions in the interlayer.
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ALDOL CONDENSATION OF FURFURAL AND ACETONE
77
Vads, cm g−
3
1
5
4
3
2
1
00
00
00
00
00
0
b
а
b
c
c
a
2000
1600
1200
Wavenumbers, cm−
800
400
1
0
0.2
0.4
0.6
0.8
1.0
P/P0
Fig. 2. FIꢀIR spectra of (
a
) CꢀLDH, (
b) CꢀLDO, and
Fig. 3.
N
2
adsorptionꢀdesorption isotherms of the different
) CꢀLDH, ( ) CꢀLDO, and ( ) CꢀRLDH.
(c
) CꢀRLDH (Mg/Al = 2.5).
samples: (
a
b
c
pared LDH (Fig. 1
symmetric peaks for (003), (006), (110) and (113) tern of the rehydrated sample (Fig. 2
planes, as well as broad symmetric peaks for the (009), reappeared, which showed that the layered structure
a
) showed wellꢀdefined sharp and to the collapse of the layered structure. But in the patꢀ
), these peaks
c
2
−
(
015), and (016) planes. These peaks were characterisꢀ
tic reflections of clay minerals with layered structure
19]. The sharp peaks showed that the wellꢀcrystallized
hydrotalcites were formed. After calcination, all these
characteristic peaks of the layered structure were disꢀ
appeared in the XRD pattern of the MgAlꢀmixed
–
was reconstructed with OH mostly replaced CO3 in
the interlayer. These results are accorded with the preꢀ
vious reports [20–22].
[
Figure 3 showed the N adsorption–desorption
2
isotherms of the three samples. Upon calcination, the
porosity is increased in CꢀLDO, in accordance with
oxides (Fig. 1
ture was collapsed. The peaks in Fig. 1
teristic reflections of MgO. In the XRD pattern of the
rehydrated sample (Fig. 1 ), these characteristic peaks
b
), which meant that the layered strucꢀ
2
⎯1
the increase in BET surface area from 35 to 123 m g
Table 1). Such behavior has been reported for LDH
b
were characꢀ
(
containing carbonates and assigned to the formation
of craters through the layers due to evolution of CO2
c
of the layered structure were reappeared, which
showed that the layered structure was returned. Due to
the memory effect of hydrotalcites, the rehydration of
MgꢀAl mixed oxides led to the layered structure’s
reconstruction.
and H O [23]. Rehydration of CꢀLDO in the liquid
2
phase by stirring produced an increase in surface area
due to the rupture of particles and a marked exfoliaꢀ
tion of the crystals.
The FTꢀIR spectra of CꢀLDH (
and CꢀRLDH ( ) were presented in Fig. 2. In Fig. 2
the peak at 1360 cm was attributed to the vibration of
a), CꢀLDO (b),
c
a,
3.2. Aldol Condensation of Furfural and Acetone
–1
3
.2.1. Influence of the Mg/Al molar ratio. Due to
, the the particularity of MgAlꢀLDH, the ratio of Mg/Al
peak at 1360 cm was changed and shifted to determined the structure of LDH. For lower Mg/Al
2
−
CO3 existed in the interlayer of LDH. In Fig. 2
b
–1
–
1
1
388 cm with much weaker strength. This meant ratio, the increase in the number of neighboring Alꢀ
2
−
containing octahedral led to the formation of
that CO3 was almost eliminated by calcination, but
Al(OH) , whereas high Mg/Al ratio led to the segregaꢀ
3
2
−
not completely. In Fig. 2c, the vibration peak of CO3
reappeared at 1360 cm , which meant that a little taining octahedral in the Bruciteꢀlike sheets [24]. Pure
tion of Mg(OH) due to the high density of Mgꢀconꢀ
2
–1
2
−
amount of CO3 still existed during the process of calꢀ
cination and rehydration. In low spectrum band, the
peak around 555 cm–1 was attributed to the shifting
vibration of Al–OH or Mg–OH. The peak at 687 cm
was attributed to the bending vibration of Al–OH or
Mg–OH, and the peak at 787 cm was attributed to
the stretching vibration of Al–OH or Mg–OH. All
these peaks were disappeared in the MgAlꢀmixed
Table 1. Structural properties of the samples
–1
–1 Pore volume,
2
Samples
Phase
SBET, m g
3
–1
cm g
–1
CꢀLDH
CꢀLDO
Periclase
Meixnerite
35
123
133
0.37
0.62
0.53
CꢀRLDH Meixnerite
oxides (Fig. 2b), which showed that the calcination led
KINETICS AND CATALYSIS Vol. 51
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78
HUIHUI LIU et al.
Table 2. Reaction results of the catalysts with different Mg/Al
Figure 5 showed that all the three catalysts could
absorb phenol, which meant that they all owned surꢀ
face basic sites, and CꢀRLDH with Mg/Al = 2.5 has
the most basic sites, which accordance with its best
catalytic activity. In addition, we noted that the selecꢀ
tivity to FAc increased as the amount of base site on
the surface of material increased. The reason for rehyꢀ
drated catalyst with Mg/Al = 2.5 owned the most basic
sites could be explained as follows: on the one hand,
the Al content decreased with increasing the Mg/Al
molar ratio from 2 to 2.5, which led to the numbers of
the positive charge on layer decreased. Accordingly,
molar ratio
Conversion
of Fur, %
Selectivity
to FAc, %
Yield
of FAc, %
Mg/Al
2
: 1
.5 : 1
: 1
62.4
78.6
66.4
67.3
72.3
55.2
41.3
56.9
36.6
2
3
Table 3. Reaction results of the catalysts at different rehydraꢀ
tion temperature (Mg/Al = 2.5)
–
the number of OH formed from aqueous solution in
interlayer counterpoised the positive charge on layer
was decreased, which brought the decrease of the
accessible basic sites. On the other hand, as Mg/Al was
close to 3, steady layered structure for LDH was easily
formed, because the Mg/Al of natural hydrotalcite was
Temperature, Conversion
Selectivity
to FAc, %
Yield
of FAc, %
K
of Fur, %
3
23
33
43
63
74.6
73.2
78.6
74.9
50.8
59.2
72.3
75.4
37.9
43.3
56.9
56.5
3
3
3
–
equal to 3. And OH could be arranged in order on
steady structure, which led to high density of basic
sites. The two factors were contradicted, which led to
the catalyst with Mg/Al = 2.5 had the most basic sites
relative to 2 and 3.
hydrotalcites with high crystallinity could be prepared
at Mg/Al ratios between 2 and 4 [25, 26]. So, three
samples with different Mg/Al ratio were prepared to
investigate the influence of Mg/Al ratio on the cataꢀ
lytic activity (Table 2).
3.2.2. Influence of the rehydration temperature and
time. Catalysts rehydrated at different temperatures
were prepared and their catalytic performance was
listed in Table 3.
All the rehydration time were fixed at 2 h. Table 3
It can be seen from Table 2 that the catalyst with showed that the selectivity and yield to FAc increased
Mg/Al ratio equal to 2.5 showed the best catalytic with the rehydration temperature from 323 to 343 K,
activity with furfural conversion reaching to 78.6% but the further increase of the rehydration temperature
and the selectivity to FAc being 72.3%. From XRD from 343 to 363 K didn’t change the selectivity and
analysis, the three catalysts all have the layered strucꢀ yield greatly.
ture (Fig. 4), so the difference of the activity and selecꢀ
When fixed the rehydration temperature (343 K)
tivity to FAc was not come from the crystal structure, and prolonged the rehydration time, the catalytic
but may be attributed to the numbers of surface basic activity also changed, but not too much. The results
sites and strengths. The numbers of the accessible were presented in Table 4.
basic sites of these catalysts were measured by phenol
absorption and presented in Fig. 5.
From Table 4, the catalyst with longer rehydration
time didn’t showed better catalytic performance. So
Amount absorbed, mmol/g
0
0
0
0
0
0
.24
.20
.16
.12
.08
.04
0
b
, 2.5 : 1
а
, 2 : 1
a
, 2 : 1
c
, 3 : 1
b
, 2.5 : 1
c
, 3 : 1
70
10
20
30
40
θ, deg
50
60
0
0.5
1.0
1.5
2.0
2
Equilibrium concentration, mmol/L
Fig. 4. XRD patterns of the three different samples:
) CꢀLDH, ( ) CꢀLDO, and ( ) CꢀRLDH.
Fig. 5. The adsorption of phenol from cyclohexane on differꢀ
ent catalysts: ( ) CꢀLDH, ( ) CꢀLDO, and ( ) CꢀRLDH.
(a
b
c
a
b
c
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ALDOL CONDENSATION OF FURFURAL AND ACETONE
79
we believed the appropriate rehydration temperature Table 4. Reaction results of catalysts of different rehydration
time (Mg/Al = 2.5)
and time for Mg–Al mixed oxides were 343 K and 2 h,
respectively.
Conversion
Selectivity
to FAc, %
Yield
of FAc, %
τ
, h
3.2.3. Reutilization and regeneration. For solid base
of Fur, %
catalyst, its reusability and regeneration was imporꢀ
tant. The reusability was conducted over a used cataꢀ
lyst from a typical experiment after cooling down the
reactor to room temperature, recovery of the solid by
washing with acetone, and drying under vacuum at
2
3
4
78.6
84.1
80.2
72.3
66.3
68.9
56.9
55.8
55.3
3
73 K for 1 h. Showed that the furfural conversion over
the catalyst after one run was much lower than that on
the fresh catalyst (45% vs. 73%). The deactivation of
the catalyst may be due to the adsorption of the reacꢀ
tion products (polyunsaturated molecules such as
FAc) or products derived from furfural oligomerizaꢀ
tion during the reaction. Because the catalyst after one
run was deeply brown, while the fresh was white.
These deposits maybe prevent the accessibility of the
Table 5. Catalytic performance of the catalyst with high calciꢀ
nation temperatures (Mg/Al = 2.5)
Temperature, Conversion
Selectivity
to FAc, %
Yield
of FAc, %
K
of Fur, %
8
73
18.6
61.4
80.3
84.6
30.4
63.3
64.9
60.4
5.7
38.9
52.1
51.1
973
–
active OH centers, and than lead to lower conversaꢀ
1073
173
tion. Importantly, the selectivity to FAc was remained
constant (77%), which indicated that these deposits
1
–
did not modify the nature of the active OH centers.
The regenerated LDH was obtained by calcined the increase the calcination temperature, the conversion
used catalyst at 773 K for 4 h in air in order to remove
the carbonaceous deposits, followed by rehydration in
decarbonated water at 343 K. The conversion of furꢀ
fural was slightly recovered, but still lower than that of
the fresh one (55% vs. 73%), but the selectivity to FAc
was largely increased to 97%. Furthermore, the regenꢀ
erated catalyst was slightly yellow, which showed that
the calcination temperature was not sufficient to comꢀ
pletely eliminate the carbonaceous deposits. But
of furfural was increased slowly, but the selectivity to
FAc reached maximum at 1073 K, than decreased a
little bit at 1173 K. To explain this, their structures
were analysized by XRD measurement (Fig. 6). After
calcination at 873 K, MgO was formed as shown in
Fig. 6
but a Lewis alkali, different from MgAl–OH ꢀLDHs,
which is a Br nsted alkali. According to the previous
a and MgO had strong basicity as it was known,
–
ø
work [9], Lewis alkali was not an effective catalyst for
the crossꢀcondensation, so it is explained why the
mixed oxides calcined at 873 K had weak catalytic perꢀ
formance. When the calcination temperature reached
7
73 K was the allowed highest temperature, because
the layered structures can not be reconstructed when
the calcination temperature was above 773 K. Thereꢀ
fore, the regeneration of the LDH was not complete,
which explained the lower furfural conversion. It can 973 K, the structure was a bit different from that calꢀ
be proposed that the regeneration process reduce the cined at 873 K, because the new peaks appeared
number of active sites in the sample, rendering lower around 32
catalytic activity, and affect the quality of the regenerꢀ
ated centers, as the FAc selectivity is increased; the
latter is still under consideration.
°
and 59
°
in pattern
b
, which is correꢀ
3
.2.4. Calcination with high temperature. MgAl–
2
−
CO3 LDHs treated by calcination below 773 K and
rehydration could turn into MgAl–OH ꢀLDHs with
–
the same structure, which could catalyze the condenꢀ
sation of furfural with acetone, but the recovery of the
catalyst is complex and incomplete. When the calcinaꢀ
tion temperature was higher than 773 K, it would conꢀ
vert to mixed oxides with basicity. So we calcined
а
b
c
2
−
MgAl–CO3 ꢀLDHs at different temperatures higher
than 873 K, and used the mixed oxides as catalysts
directly.
d
As shown in Table 5, for mixed oxides calcined at
73 K, the conversion of furfural and the selectivity to
10
20
30
40
2θ, deg
50
60
70
8
FAc were very low. When the calcination temperature
was increased to 973 K, the conversion of furfural and
the selectivity to FAc were enhanced greatly. Further
Fig. 6. XRD patterns of the samples calcined at (
) 973, ( ) 1073, and ( ) 1173 K.
a) 873,
(
b
c
d
KINETICS AND CATALYSIS Vol. 51
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80
HUIHUI LIU et al.
sponded to the spinelꢀtype structure. When LDH was
calcined at 1073 K, the peaks indexed to spinel phase
appeared, and the catalytic activity got to the maxiꢀ
mum. Further increase of the calcination temperature
led to the sharpening of the XRD pattern, indicating
the well crystallization of the spinel phase, while the
conversion and selectivity changed a little bit. All the
above analysis presented that MgAlꢀspinel could be
used as catalyst for the condensation of furfural with
acetone.
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1
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is being under study because Mg–Al LDHs are hard to
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ACKNOWLEDGMENTS
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This work was supported financially by the 973
Program of China (no. 2004CB719500), National
Natural Science Foundation of China (no. 20673037),
New Century Excellent Talents in University, China
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KINETICS AND CATALYSIS Vol. 51
No. 1
2010