Sol-gel synthesis of ZrO2-SiO2 catalysts for the transformation of bioethanol and acetaldehyde into 1,3-butadiene

Article abstract of DOI:10.1039/c5ra22623f

A series of ZrO₂-SiO₂ catalysts synthesized by sol-gel method were investigated for the 1,3-butadiene formation from bioethanol and acetaldehyde. The influence of ZrO₂ content and reaction conditions on catalytic performance were studied. The catalysts were characterized by N₂ adsorption-desorption analysis, temperature-programmed desorption of NH₃ (NH₃-TPD), IR spectroscopy of adsorbed pyridine (Py-IR), Fourier Transform Infrared Resonance (FTIR), X-ray diffraction (XRD) and Transmission Electron Microscope (TEM). These characterization results indicated that suitable intensity of acid sites, especially Lewis acid, are very crucial to obtain an active catalyst. The catalysts showed significantly higher selectivity for C4 chemicals, and 2 wt% ZrO₂ content reached the highest selectivity to BD (69.7%), at space velocity (WHSV) of 1.8 h^-1.

Full text of DOI:10.1039/c5ra22623f

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Sol-gel synthesis of ZrO -SiO catalysts for the transformation of
2
2
bioethanol and acetaldehyde into 1,3-butadiene
a,b
a,b
a,b
a,b,
a,b
Zheng Han , Xiang Li , Minhua Zhang *, Zongzhang Liu , Meixiang Gao
a
Key Laboratory for Green Chemical Technology of Ministry of Education,
R&D Center for Petrochemical Technology, Tianjin University,
Tianjin 300072, P R China,
b
Collaborative Innovation Center of Chemical Science and Engineering,
Tianjin 300072, China
*
Corresponding author: Tel: +86ꢀ22ꢀ27401826; fax: +86ꢀ22ꢀ27401826
Eꢀmail address: mhzhang@tju.edu.cn (Minhua Zhang)
ABSTRACT: A series of ZrO ꢀSiO catalysts synthesized by solꢀgel method
2
2
were investigated for the 1,3ꢀbutadiene formation from bioethanol and acetaldehyde.
The influence of ZrO content and reaction conditions on catalytic performance were
studied. The catalysts were characterized by N adsorption–desorption analysis,
2
2
Temperatureꢀprogrammed desorption of NH3 (NH ꢀTPD), IR spectroscopy of
3
adsorbed pyridine (PyꢀIR), Fourier Transform Infrared Resonance (FTIR), Xꢀray
diffraction (XRD) and Transmission Electron Microscope (TEM). These
characterization results indicated that suitable intensity of acid sites, especially Lewis
acid, are very crucial to obtain an active catalyst. The catalysts showed significantly
higher selectivity for C4 chemicals, and 2wt% ZrO content reached the highest
2
ꢀ1
selectivity to BD (69.7%), at space velocity (WHSV) of 1.8h .
2 2
Keywords: 1,3ꢀButadiene, Ethanol, Acetaldehyde, ZrO ꢀSiO , Solꢀgel
1

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1
. INTRODUCTION
1
,3ꢀButadiene (BD) is an important chemical product in petrochemical industry. As
one of the most important basic organic chemical materials, it is widely used in
producing synthetic rubber such as acrylonitrile butadiene styrene (ABS), acrylonitrile
1
butadiene (NBR) and styreneꢀbutadiene (SBR). Recently, BD has received more
2
attention owing to the rapid economic growth of developing countries.
Nowadays, approximately 95% of BD is predominantly produced by isolation from
3, 4
naphtha steam cracker fractions generated during ethylene production.
However,
the recent trends in lightening of the feedstock for steam cracking, as a result of the
recent shale gas boom, threaten BD production and are expected to significantly affect
5
its price. Moreover, aware of the gradually decreasing resources of nonꢀrenewable oil
and the deterioration of the environment, it is crucial to develop alternative
technologies for BD production from renewable, nonꢀpetroleum resources such as
2
bioethanol. This is not a new process, as the catalytic conversion of ethanol into BD
was ever used industrially during the world war two when oil extraction was
extraordinarily expensive. With the advancement of bioethanol industry and the
increasingly large volumes of bioethanol production, bioethanol again, gradually
6
becomes a promising feedstock in the formation of BD.
Since the early 1990s, two processes have been reported for the synthesis of BD
7
from ethanol, that is, oneꢀstep process and twoꢀstep process. The direct conversion of
ethanol to BD was called oneꢀstep process and first discovered by Sergey Lebedev
8
ꢀ12
using a variety of mixed metal oxide catalysts, typically MgO/SiO2
or
2

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1
3ꢀ15 16
ZnO/Al
2
O
3
.
While much earlier in the USA, a twoꢀstep process has been
1
7
implemented by Ostromyslensky in which partial ethanol (EtOH) dehydrogenation
1
8, 19
to acetaldehyde (AA)
as the first step, followed by the transformation of the
2
0
mixture of ethanol and acetaldehyde into BD. The mechanism of BD formation
from ethanol or the mixture of ethanol and acetaldehyde is very complicated and is
still a subject of debate. However, the following five steps are applicative to both
2, 12, 20ꢀ22
oneꢀstep and twoꢀstep processes and commonly accepted by most researchers
:
(1) acetaldehyde formation from ethanol; (2) aldol condensation of acetaldehyde to
acetaldol; (3) dehydration of acetaldol to crotonaldehyde; (4) Meerwein–Ponndorf–
Verley reaction between crotonaldehyde and ethanol to obtain crotyl alcohol and
acetaldehyde; and (5) dehydration of crotyl alcohol to BD. The ideal catalyst for this
process thus should be active for both dehydration and dehydrogenation reactions.
Although it remains undecided whether the twoꢀstep process should be preferred
over the oneꢀstep process, some available bibliographic data showed that for some
1
6
catalysts the added acetaldehyde in the feed improved the selectivity of BD.
However, after several papers published in the 1940s, few catalyst studies of twoꢀstep
2
process has been reported. As early as 1947, tantalum, zirconium or niobium oxide
2
3
promoted silica were used to study the twoꢀstep process of BD formation. At
o
ꢀ1
3
25ꢀ350 C and a space velocity of 0.4ꢀ0.6 h , 2wt% Ta O content reached the
2 5
2 2
highest selectivity to BD (67%); 1.6wt% ZrO on the SiO showed a BD selectivity of
2
4
5
9% under comparable reaction conditions. Later on, Corson et al screened many
metallic and nonꢀmetallic oxides and concluded that catalysts like tantalaꢀsilica and
3

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zirconiaꢀsilica exhibited excellent catalytic activity in the twoꢀstep process. Jones et
25
al reported the utility of a variety of silica impregnated biꢀ and trimetallic catalysts
for the conversion of ethanol into BD; Zr : Zn system impregnated on silica was
supposed as the most promising catalyst and the highest BD selectivity to be 67.4%
was found. Commonly, the structure of the catalysts may have an influence on the
2
6, 27
2
catalytic performance.
Chae et al focused their study on the structure of ordered
mesoporous silica supports. Along with research thorough, zirconia was later reported
28
1, 5,
as a successful replacement for tantalum oxide. Although many recent researchers
2
9ꢀ33
focused on the oneꢀstep process by using MgO/SiO as their catalysts, the
2
twoꢀstep process with ZrO
study.
2
In this study, ZrO ꢀSiO catalysts prepared via solꢀgel method were characterized
2 2
ꢀSiO catalysts is still a fascinating subject worthy of
2
and their catalytic performances for the formation of BD from bioethanol and
acetaldehyde were investigated. The effects of pore structures and surface acidity on
2
the catalytic activity were studied. The ZrO contents and experimental conditions,
such as the reaction temperature, the molar ratio of ethanol to acetaldehyde and the
WHSV were optimized. Different characterization means were performed to
determine the influences of acid sites and surface structure on the catalytic process.
2
. EXPERIMENTS
2
.1. Catalyst preparation
A series of ZrO
2
ꢀSiO
2
2
with different ZrO contents up to 8.4wt% were prepared by
4

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the solꢀgel method. Briefly, zirconium oxynitrate (Alladin Industrial Corporation, AR),
and tetraethyl orthosilicate (TEOS, Tianjinꢀguangfu Corporation, AR) were used as
the precursors and ethanol as the coꢀsolvent. The appropriate amounts of zirconium
oxynitrate were dissolved in the solvent, deionized water. The solution was heated to
o
3
0 C under stirring to thoroughly mix the components, then ethanol (40mL), nitric
acid (80mL, 2M) and tetraethyl orthosilicate were added in the solution in sequence.
o
After 24 hours’ standing, the solution was then dried for 6 hours at 110 C and
o
34
subsequently calcined in a muffle furnance for 6 hours at 650 C (ramping rate,
o
5
C/min).
2
.2. Catalyst testing
The catalytic conversion of ethanol (99wt%) and acetaldehyde (99wt%, 97wt%)
to BD was carried out in a fixed bed quartz reactor. The reaction was performed at
o
3
20ꢀ410 C and atmospheric pressure after pretreating the catalysts in nitrogen (50
ml/min) at reaction temperature for 1 hour. The mixed solution of ethanol and
o
acetaldehyde with a molar ratio of 0.5ꢀ4.5 was introduced into a vaporizer (120 C) by
a metering pump. The dry gas was analyzed online by Agilent 7890A equipped with a
CarbonꢀPLOT column (30 m, 0.535 mm, 3 ꢁm) and Thermal Conductivity Detector
(TCD), while liquid product was collected by a cold trap and also analyzed by Agilent
7
890A equipped with a HPꢀINNowax (30 m, 0.32 mm) and Thermal Conductivity
Detector (TCD).
In this study, total conversions and BD selectivities were calculated by the
5

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2
following equations, which were confirmed by the previous literature .
(
Total C moles − (C moleunreacted EtOH +C moleunreacted AA ))
Total conversion=
×100
Total C moles
C moleBD in products
BD selectivity=
×100
Total C moles in products except for EtOH and AA
EtOH, ethanol; AA, acetaldehyde; BD, 1,3ꢀbutadiene.
2
.3. Characterization
o
Nitrogen adsorptionꢀdesorption isotherms were obtained at ꢀ198 C using a
Micromeritics Tristar 3000 instrument. Before the measurement, all samples were
o
pretreated at 200 C for 12 h. The BrunauerꢀEmmettꢀTeller (BET) method was used to
calculate the surface areas.
The strength of acid sites was determined by NH
3
ꢀTPD using a Micromeritic
o
Autochem II 2920. Sample was first preꢀtreated at 200 C for two hours under a
helium (99.999%) flow rate of 50 mL/min. Subsequently, by exposing the sample
under the flow of ammonia (Argon 99% Ammonia 1%) at rate of 20 mL/min for one
o
and a half hours at 70 C, the absorption process was completed. After the baseline
o
o
o
was stable at 70 C, the temperature was then ramped to 800 C (15 C/min) in helium
at rate of 50mL/min. All the results were recorded by Thermal Conductivity Detector
(
TCD).
Thermo Scientific Nicolet 560 FTIR spectrometer with 64 scans at 4cm resolution
was used to determine the type of acid of the samples. The samples were preꢀtreated
ꢀ1
o
ꢀ3
at 200 C under the vacuum of 10 Pa for two hours. When the samples were cooled to
room temperature, IR background spectra were recorded. Before flushing with
6

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nitrogen to remove physisorbed pyridine, the sample was exposed to pyridine vapor at
o
room temperature for 30 min. Pyridine absorbed spectras were recorded at 200 C.
High purity nitrogen flowed constantly during the experiment.
The infrared spectra was obtained using a Nicolet 560 spectrometer with a scanning
ꢀ1
ꢀ1
range between 4000 ~ 400 cm and at a resolution of 4 cm . The samples were
prepared with the addition of KBr at the weight proportion of 99%.
Xꢀray diffraction analysis was obtained using a Rigaku D/Max 2500 type Xꢀray
powder diffractometer with Cu Kα radiation operating at 40 kV and 150 mA. The
θ
scan range was 2 = 5 to 90° with a speed of 5°/s.
Transmission electron microscopy figures were obtained using a Tecnai G2 F20
operated at 200 kV. The samples were prepared on the porous copper grids by
dispersing in ethanol.
3
. RESULTS AND DISCUSSION
3
.1. Catalyst characterization
As shown in Figure 1, all the N
2
adsorptionꢀdesorption isotherms for different ZrO
2
3
5
contents exhibited type IV isotherms with H1 hysteresis loop as defined by IUPAC.
This indicates that all the catalysts have mesoporous structures. In addition, the total
adsorption volume decrease gradually with the increasing of ZrO loading. Mostly, it
is because that more ZrO loading led to the smaller pore volumes. BET surface areas
measured by N sorption for different ZrO contents (1.4wt%, 2.0wt%, 2.8wt%,
.6wt%, 8.4wt%) are 743m /g, 606m /g, 589m /g, 568m /g, 528m /g, respectively.
2
2
2
2
2
2
2
2
2
5
7

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The solꢀgel synthesized catalysts have relatively high surface areas. XRD patterns of
these catalysts only showed the characteristic peaks of SiO (2θ=23º) (Figure 2). No
ZrO diffraction was detected, suggesting the highly dispersed nature of ZrO
2
2
2
.
1.4
2.0
2.8
5.6
8.4
300
250
200
150
100
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
Fig.1. N adsorptionꢀdesorption isotherms of ZrO ꢀSiO catalysts with different ZrO
2
2
2
2
contents
f
e
d
c
b
a
10
20
30
40
50
60
70
80
90
2θ/°
Fig.2. XRD patterns of ZrO
2
2 2
ꢀSiO catalysts with different ZrO contents (a) 0%；
（
b）1.4%；（c）2.0% ；（ d）2.8%；（e）5.6% ；（ f）8.4%
NH ꢀTPD was performed to further investigate the acidic properties of the
3
catalysts (Figure 3). In low temperature range, all samples showed a broad peak from
8

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o
1
00 to 170 C conforming the existence of weak acid sites. When the temperature
o
exceeds 170 C, no obvious peaks were observed, indicating the mere existence of
weak acid sites on the surface. Figure 3 also shows that with the increasing of ZrO
2
loading, the strength of weak acid increased obviously to the high temperature,
contributing to the enhancement of total number of acid sites in Table 1. The
enhanced strength of these samples indicates that the addition of ZrO
2
contributed to
3
6
forming the new weak sites on the surface of the samples . PyꢀIR was also used to
ꢀ1
compare the change of the type of acidity. Both Brønsted acid sites (1545cm ) and
ꢀ1
ꢀ1
Lewis acid sites (1445cm , 1595cm ) were observed (Figure 4). The peak located at
ꢀ1
1
445 cm belongs to pyridine adsorbed on Lewis acid sites which has a slight shift
ꢀ1
ꢀ1
comparing to the 1450 cm . According to former research the peak at 1450 cm
could suffer some shift when the size or charge of the cation constituting the acid site
3
7
ꢀ1
changes. 1595 cm also attributes to pyridine adsorbed on Lewis acid sites but
corresponds to a different vibration mode. When the ZrO content increased from 1.4
2
to 8.4wt%, the surface Brønsted acid sites almost had no change, but the Lewis acid
sites increased significantly. Combined with the NH3ꢀTPD results, it indicated that
the addition of ZrO
2
can only increase the intensity of Lewis acid. As shown in Figure
ꢀ1
ꢀ1
4
, at 1445 cm and 1595 cm , the characteristic peak areas of lower ZrO
2
contents
(
1.4wt% and 2.0wt%) were much smaller than 5.6wt% and 8.4wt%, whereas Table 1
clearly showed that as the ZrO content increases, the growth of the acid sites number
was steady. This probably because the molecular volume of pyridine is bigger than
2
NH
3
2 2
, and pyridine can’t enter into the internal surface and holes of ZrO ꢀSiO
9

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catalysts sufficiently. So the result of PyꢀIR mostly reflected the external surface type
of acidity. With the ZrO content from 2.0 to 5.6wt%, the adsorption of ZrO on the
, which
2
2
internal surface was saturated, while the external surface appeared more ZrO
2
can be detected by PyꢀIR easily.
f
e
d
c
b
a
5
0
100
150
200
250
300
350
400
T/°C
Fig.3. NH ꢀTPD profiles of ZrO ꢀSiO catalysts with different ZrO contents (a) 0%;
3
2
2
2
(b）1.4%; (c) 2.0%; (d) 2.8%; (e) 5.6%; (f) 8.4%
L
L
B
8.4%
6.5%
2.0%
1.4%
1400
1450
1500
1550
1600
ꢀ1
Wavenumber (cm )
Fig.4. PyꢀIR spectra of ZrO
2 2 2
ꢀSiO catalysts with different ZrO contents
(a) 1.4%; (b) 2.0%; (c) 5.6%; (d) 8.4%
Table 1 The amounts of weak acidic sites of ZrO -SiO catalysts with different
2
2
1
0

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2
ZrO contents
2
ZrO content
Number of Acid Sites
mmol/g)
(wt %)
(
0
0.10
0.11
0.13
0.12
0.15
0.17
1.4
2.0
2.8
5.6
8.4
FTIR spectra method was used to explore the existence of typical group between
ZrO
2
and SiO
2
2 2 2
. Figure 5 exhibited the infrared spectrum of ZrO ꢀSiO and pure SiO .
ꢀ1
Both of ZrO
2
ꢀSiO and pure SiO showed a strong band noticed at 1080 cm
2
2
ꢀ1
accompanied with a shoulder at 1220 cm . These belong to the typical asymmetric
3
8,39
ꢀ1
ꢀ1
stretching vibrations of SiꢀOꢀSi.
Two other bands at 800 cm and 460 cm are due
39, 40
to symmetric stretching and bending vibrations of SiꢀOꢀSi bonds, respectively.
ꢀ1
The band at 1630 cm indicates the existence of the physisorbed molecular water on
41
ꢀ1
the surface of the catalyst. The frequency at around 960 cm appears different in
two samples which contributes to the stretching vibration of SiꢀOꢀZr groups if the
ꢀ1
bands observed at higher frequencies. As to the pure SiO
2
, 960 cm is due to the
42
SiꢀOꢀ groups. These results indicated that those ZrO
2
ꢀSiO samples didn’t mingle
2
mechanically, whereas had some interactions and finished the ethanol and
acetaldehyde to BD process cooperatively.
11

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b
a
1
800
1600
1400
1200
1000
800
600
400
ꢀ
1
Wavenumber(cm )
Fig. 5. Infrared spectrum of the samples (a) 2% ZrO
2
2 2
ꢀSiO ; (b) SiO
As demonstrated in Figure 6, the leafy shape of SiO
2
is shown in (a). The
agglomeration of the particles can also be seen in the same image, but in a small
amount. However in (b), clots are obvious as the black cloudy sites in the image
shown and there is no trace of crystal structure of ZrO which elucidates that ZrO is
2
2
at its amorphous state or the crystal particles are so small that can’t be seen. That is to
say, the ZrO is well dispersed on the SiO support, which is in good agreement with
the results of XRD.
2
2
1
2

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2
Fig. 6. Transmission electron microscopy (TEM) pictures of (a) SiO , (b) 2%
3
4
2 2 2 2 2
ZrO ꢀSiO (100nm), (c) ZrO , (d) 2% ZrO ꢀSiO (5nm)
3
.2. Catalyst testing
4
3
As Zr has different acidic and basic properities , varying the ZrO
2
content was
hypothesized to have an effect on the delicate acidꢀbase balance and at the same time
determined the optimal content for BD yield. The influence of different ZrO
2
contents
contents:
on catalytic performance was studied using solꢀgel method with five ZrO
2
1
.4wt%, 2.0wt%, 2.8wt%, 5.6wt% and 8.4wt%. These samples may not show distinct
differences in morphology, but they emerged high variability in catalytic activity. For
these catalysts, activity and selectivity are reported in Figure 7, providing the first
2
data of catalyst performance. As the ZrO content increased, dehydration activity (i.e.,
ethylene and diethyl ether selectivity) increased and the increasing dehydration
activity matched well with the increased weak acid sites. This was evidenced by the
aforementioned NH
3
ꢀTPD and IRꢀPy results, and presumably the increased
dehydration activity was only affected by Lewis acid sites. As the secondary product
28
of acetaldehyde , BD mostly reflected the dehydrogenation activity. Actually, the
1
3

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44
dehydrogenation is a competitive process compared to its dehydration. Thus
dehydration of ethanol to ethylene and diethyl ether should be avoided to attain high
16
BD yields. However, the catalysts for BD formation should be active for both
7
dehydration and dehydrogenation. Then, the dehydrogenation and dehydration
components should be present in an optimal ratio. In this article, as the ZrO
increased, the selectivity of BD was increased at first and then decreased remarkably.
wt% ZrO content reached the highest selectivity to BD. Presumably, this ZrO
2
content
2
2
2
content balanced the dehydrogenation and dehydration processes. What’s more, the
work by Jones et al. has confirmed that a degree of acidity in the support is critical as
several steps in the mechanism are acid catalyzed and Zr(IV) is Lewis acidic, which is
25
believed to enhance the activity. Ordomaky et al. also suggested that Lewis acid
45
sites play important role in the aldol condensation of acetaldehyde. It suggested that
the acidity of catalysts could be beneficial to the transforming of acetaldehyde into
BD, but high selectivity of BD required a suitable acid sites rather than as much as
possible.
1
4

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55
50
45
40
35
30
25
20
15
10
5
Diethyl ether
Ethylene
Butadiene
Butene
1
2
3
4
5
6
7
8
9
ZrO Content (wt%)
2
Fig.7. Effect of ZrO
2
content on 1,3ꢀbutadiene and other main products selectivity over
ZrO ꢀSiO catalysts
2
2
(
Conditions: 0.5g catalyst, The ethanolꢀtoꢀacetaldehyde feed ratio: 2.5:1, WHSV=1.8g EtOH/g
ꢀ1
o
cat h , T=350
C
. Note: Other minor products included propylene, ethyl acetate, 1ꢀbutanol and
+. Selectivity to all minor products were<10% ).
C
6
Interestingly, despite the varied ZrO content, the sum of BD selectivity and
2
butene selectivity was always very high. This result indicated that ZrO
2 2
ꢀSiO catalysts
were beneficial to the generation of C s. A balance of acidic and basic sites were
4
present on ZrO
2 2
ꢀSiO catalysts which, upon further optimization of reaction
2
8
conditions, could be used for the BD formation.
The effects of the different parameters, such as the reaction temperature, the molar
ratio of ethanol to acetaldehyde and the WHSV on the selectivity of BD and butene
were studied (See Figure 8).
In order to determine the effect of temperature, the reaction was conducted at
o
temperature from 290 to 410 C. The optimum ZrO
2
content obtained in the previous
section were used here. As Figure 8 (a) shows, the total conversion of ethanol and
1
5

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acetaldehyde appeared to increase apparently with an increase in the temperature. At
o
3
20 C, the selectivity of BD reached a top value and butene was low enough.
o
Therefore, in our experiment, 320 C was chosen as the optimum temperature for this
system.
Generally speaking, the change of WHSV mostly led to a great difference of the
conversion. This law was consistent with our experiment. As showed in Figure 8 (b),
despite the increase of WHSV, the selectivity of BD and butene had little change,
while the total conversion had a decreasing trend. The maximum BD selectivity was
achieved at 1.8, which was then chosen as the optimum WHSV.
The effect of ethanolꢀtoꢀacetaldehyde molar ratio on the selectivity of BD and
butene was determined using ratios of 0.5 to 4.5. All other conditions were those
obtained in the previous sections. As Figure 8 (c) shows, the molar ratio made the
greatest influence on the selectivity. With the increase of ethanolꢀtoꢀacetaldehyde
molar ratio, the selectivity of BD appeared to increase too. The highest BD selectivity
was obtained at 3.5, then dropped slightly at 4.5. But, the change of butene selectivity
exhibited a completely opposite situation. Therefore, the feed ratio was very
important to the selectivity of BD. Too much or too little didn’t do benefit to the
conversion to BD. The molar ratio of 3.5:1 was the optimum condition.
1
6

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Conversion
Butadiene selectivity
Butene selectivity
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
(a)
2
80
300
320
340
360
380
400
420
Temperature (°C)
Conversion
Butadiene selectivity
Butene selectivity
(b)
90
80
70
60
50
40
30
20
10
0
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
ꢀ
1
WHSV (h )
Conversion
Butadiene selectivity
Butene selectivity
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
(
c)
0
1
2
3
4
5
Ethanol/Acetaldehyde (mol/mol)
Fig.8. Effect of temperature (a); WHSV (b) and ethanol/acetaldehyde molar ratio (c)
on the total conversion and the selectivity of butadiene and butene over 2wt% ZrO2
content catalysts
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Table 2 shows the total conversion of ethanol and acetaldehyde, the selectivity of
main products and unidentified compounds on ZrO ꢀSiO catalysts at appropriate
2
2
conditions. Through the optimization of reaction conditions, these catalysts indeed
demonstrated high activity to BD. The relatively high conversion and BD selectivity
2
were closely associated with the high surface area (about 606m /g), which offers a
good dispersion of active sites (zirconium oxide). What’s more, as discussed in
Section 3.1, it is supposed that the acidꢀbase properties and the synergistic effects of
ZrO and SiO played a more important role in the process of ethanol and
2
2
acetaldehyde to BD.
2 2
Table 2 Catalytic performance of 2wt% ZrO supported SiO catalysts in 1,3-BD
o
-1
production at 320 C, WHSV=1.8 h at 3h.
a
Catalyst
E:A
E/A
Carbon selectivity (C mol%)
b
con.(%)
Ethylene Propylene Butene 1,3ꢀBD Ether AA Ethyl
acetate
Butanol Others*
c
d
d
ZrO
ZrO
2
2
2
ꢀSiO
ꢀSiO
ꢀSiO
2
2
2
3.5:1 42.2
3.5:1 45.4
4.5:0 30.2
14.2
16.0
85.8
0.9
2.0
0
1.0
0.9
0
65.8
69.7
0.7
9.2
8.2
8.4
—
—
5.0
5.2
0.8
0
0.9
0.5
0
3.6
2.2
0
ZrO
a
b
c
The ethanolꢀtoꢀacetaldehyde feed ratio. The total conversion of ethanol and acetaldehyde. The concentration of
d
acetaldehyde was 99%. The concentration of acetaldehyde was 97%.
*
Unidentified compounds mainly consisting of heavier compounds in GC chromatography.
In an attempt to further improve the catalytic data, the influence of the
concentration of acetaldehyde were studied. Both 99% and 97% purities of
acetaldehyde were used as feed after mixing with appropriate amount of ethanol. 97%
purity showed a higher activity both in the total conversion of ethanol and
acetaldehyde and in the selectivity of BD. In addition, the amounts of the unidentified
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8

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compounds and some side products, such as ethyl and butanol got a certain degree
reduction too. However, due to the unidentified impurities of 97% purity acetaldehyde,
this phenomenon can’t be easily explained up to now. Presumably, the impurities
contained some intermediate products in the ethanol to BD process, which had the
same active effect with the addition of acetaldehyde in the feed.
Strangely enough, when no acetaldehyde was added to the feed there was a
dramatic decrease in the selectivity towards BD compared to the mixed feed of
ethanol and acetaldehyde, see entry 2 and entry 3 of Table 2. Without the addition, the
catalyst mainly played the role of dehydration. This is implying that with the added
25
acetaldehyde, ethanol is being preferentially reacted in the aldol condensation. It is
supposed that the ZrO ꢀSiO catalysts have weak ability in dehydrogenation, but
2
2
benefit aldol condensation with rather high handling capacity. The addition of
acetaldehyde is very necessary, otherwise we should enhance the dehydrogenation
ability of ethanol with other methods.
4
.
CONCLUSION
A series of ZrO ꢀSiO catalysts were prepared by solꢀgel method in the process of
2
2
1
,3ꢀbutadiene formation from ethanol and acetaldehyde. The catalysts exhibited a
high dispersion of ZrO
2
and obtained high activity. The highest 1,3ꢀbutadiene
o
selectivity of 69.7% was obtained under the optimal conditions: 320 C
,
2wt% ZrO
2
ꢀ1
content, 3.5:1 molar ratio of ethanolꢀtoꢀacetaldehyde and a WHSV of 1.8h .
Furthermore, the influence of the acidity and surface structure of the catalysts was
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studied in order to illustrate the reason of high selectivity for 1,3ꢀbutadiene. The
solꢀgel synthesized catalysts showed suitable weak acid sites. Moreover, concluded
from PyꢀIR, Lewis acid is a crucial element in the formation of BD. Both XRD and
TEM confirmed that ZrO was highly dispersed in the amorphous silica.
2
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38x190mm (96 x 96 DPI)