Vinyl acetate formation in the reaction of acetylene with acetic acid catalyzed by zinc acetate supported on porous carbon spheres

Article abstract of DOI:10.1134/S0036024410050158

A kind of porous carbon spheres (PCS) was prepared by the carbonization of poly(vinylidene chloride) synthesized by suspension polymerization. Structure analyses revealed the existence of bumps and holes on the surface of PCS. The PCS, with the pore size between 0.8-1.2 nm, could be used as the support of zinc acetate because of the regular shape, high specific surface area, and good mechanical strength. Vinyl acetate was produced from acetylene and acetic acid using the PCS-supported zinc acetate (PCS-Zn) under mild conditions. In a single-pass operation performed at 220°C, the conversions of acetic acid and acetylene reached 22.6 and 5.3% respectively while the activity of vinyl acetate formation was above 1000 g mol^-1 h^-1.

Full text of DOI:10.1134/S0036024410050158

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2010, Vol. 84, No. 5, pp. 796–801. © Pleiades Publishing, Ltd., 2010.
CHEMICAL KINETICS
AND CATALYSIS
Vinyl Acetate Formation in the Reaction of Acetylene
with Acetic Acid Catalyzed by Zinc Acetate Supported
1
on Porous Carbon Spheres
FengꢀWen Yan, CunꢀYue Guo, Fang Yan, FengꢀBo Li, QingꢀLi Qian, and GuoꢀQing Yuan
Beijing National Laboratory for Molecular Sciences (BNLMS), Laboratory of New Materials, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, P. R, China
eꢀmail: cyguo@iccas.ac.cn
eꢀmail: yuangq@iccas.ac.cn
Received April 9, 2009
Abstract—A kind of porous carbon spheres (PCS) was prepared by the carbonization of poly(vinylidene
chloride) synthesized by suspension polymerization. Structure analyses revealed the existence of bumps and
holes on the surface of PCS. The PCS, with the pore size between 0.8–1.2 nm, could be used as the support
of zinc acetate because of the regular shape, high specific surface area, and good mechanical strength. Vinyl
acetate was produced from acetylene and acetic acid using the PCSꢀsupported zinc acetate (PCSꢀZn) under
mild conditions. In a singleꢀpass operation performed at 220°C, the conversions of acetic acid and acetylene
–1
–1
reached 22.6 and 5.3% respectively while the activity of vinyl acetate formation was above 1000 g mol
h .
Key words: porous carbon sphere; zinc acetate; vinyl acetate; acetylene.
DOI: 10.1134/S0036024410050158
INTRODUCTION
toxic, inexpensive, and conveniently available [4–6].
In addition to zinc species, other metal compounds
used as additives to improve the catalyst performance
have also been attempted [7, 8].
In the present study, a kind of porous carbon
spheres (PCS) was synthesized and used for the first
time to support zinc acetate (PCSꢀZn). Encouraging
results in the formation of vinyl acetate from acetylene
and acetic acid are expected after employing such a
support that possesses high BET surface area, uniform
pore size distribution, high mechanical strength, and
good stability.
Vinyl acetate (VAc) is one of the 50 chemical feedꢀ
stock produced in large quantity worldwide. VAc, after
a series of syntheses of its derivatives, has found
increasing that wide applications in the production of
polymers such as, polyvinyl acetate and vinyl acetate
copolymers [1]. Currently, VAc is commercially proꢀ
duced using either ethylene and acetic acid (ca. 70%)
or acetylene and acetic acid (ca. 30%) as starting
materials. Although the acetyleneꢀbased method is
economically less profitable compared with the case of
utilizing ethylene, the daily exhausted petroleum
resource and ever increasing oil price make the former
route rather competitive in regions where calcium carꢀ
bide and natural gas are more inexpensive and easily
obtained [2].
EXPERIMENTAL
Preparation of the PCS
The PCS were prepared by the carbonization of
poly(vinylidene chloride) that was synthesized by susꢀ
pension polymerization. Distilled vinylidene chloride
was polymerized in suspending agent (saturated
Since the first synthesis of VAc from acetylene and
acetic acid with zinc acetate adsorbed on charcoal [3],
great efforts have been made in search of more indusꢀ
trially applicable supports and catalytically active
components or their combination [4–6]. Up to now,
considering the properties such as surface area,
mechanical strength, reactant conversion, catalytic
activity, and catalyst lifetime, active carbon has
become almost the only commercial support and zinc
compounds, zinc acetate in particular, have proved to
be the predominant active species because they are less
Na SO4 aqueous solution). The initiator was azobiꢀ
2
sisobutylvaleronitrile (ABVN) [1% (mole ratio) of
monomers]. Bentonite powder was used as the temꢀ
plate of polymeric beads and dispersion phase. Susꢀ
pension polymerization was performed at 40°C for
1
2 h under stirring (350–400 rpm). Poly(vinylidene
chloride) beads of 40–60 mesh were obtained. The
polymeric beads were then washed with hot water to
remove excess bentonite powder and purified in an
1
The article is published in the original.
796

VINYL ACETATE FORMATION IN THE REACTION OF ACETYLENE
, N per sphere
797
Intensity, a.u.
2
F
20
16
a
b
1
0
12
8
400
600
800
1000
Tc, C
°
3
10
30
40
2θ, deg
Fig. 1. XRD patterns of PCS treated at different carbonꢀ
ization temperatures.
Fig. 2. Relationship between the mechanical strength (
F)
and carbonization temperature ( c).
T
acetone stream to remove remaining monomers. The 20 m) was used for separation. The products were also
carbonization was conducted in a quartz tube at 180°C identified by gas chromatographyꢀmass spectroscopy
until volatile molecules, such as HCl, were removed using a Shimadzu model GCMSꢀQP5050 instrument
from the polymer, and a sphere shape was kept. The with a Stabilwax column.
temperature was then gradually elevated to 1000°C
.
All the above processes were conducted in an argon
stream. In this way, PCS were obtained. When the
spheres were cooled, they were washed in a hot soluꢀ
Characterization of PCS
The detection of specific surface area and pore size
tion of acetic acid and water to remove the impurity on distribution of PCS was performed over an STꢀ03
the surface of the material. After washing, the PCS apertometer. Nitrogen sorption analyses were carried
were desiccated for further use. The reagents used in out to characterize the porosity properties. SEM was
the experiment were commercially obtained from performed on a Hitachi S530 scanning electron
Sigma Company.
microscope. AFM was conducted with a Nano Scope
III (Digital Instruments). The samples for AFM were
prepared by pressing the crushed beads to even the surꢀ
face, but the size and structure of micropores were
kept constant. Xꢀray diffraction was performed on a
D/Maxꢀ3B powder diffraction spectroscope.
Preparation of PCSꢀZn
In a typical preparation, 2.0 g of zinc acetate dihyꢀ
drate was dissolved in 10 ml of methanol before 10.0 g
of PCS was impregnated in the solution and stirred for
0
.5 h at room temperature. The mixture was put aside
RESULTS AND DISCUSSION
for 1 h before dried at 80°C in an oven. The Zn loading
of PCSꢀZn was found to be 5.0 wt % as measured by
inductively coupled plasmaꢀatomic emission specꢀ
troscopy (ICPꢀAES POEMS TJA Co.).
Characteristics of the PCS
The material lost about 70% of its weight after carꢀ
bonization and became more carbonaceous, the partiꢀ
cle size of which was around 40–60 mesh. As can be
seen in Fig. 1, the crystallinity of the carbon was higher
after treated at 1000°C than at 400°C. Correspondꢀ
Formation of Vinyl Acetate with PCSꢀZn
For each experiment, approximately 9 g of ingly, PCS had higher thermal stability and mechaniꢀ
PCSꢀZn was packed between quartz wool plugs in a cal strength when treated at higher temperatures. The
quartz Uꢀtube (i.d. = 10 mm) that was placed in an oil relationship between the mechanical strength and the
bath. Acetylene at atmospheric pressure was supplied carbonization temperature is shown in Fig. 2. The
to the Uꢀtube via a flowmeter. Acetic acid was transꢀ influence of the mechanical strength is mainly on the
ported by a proportional pump to an evaporator before stability of the carbon structure. With the elevation of
the Uꢀtube. Products from the reaction of acetylene carbonization temperature, there was a sharp increase
and acetic acid were analyzed by gas chromatography in the specific surface area of the resulting carbon
(
GC) on a Shimadzu model GCꢀ2014 gas chromatoꢀ spheres (Fig. 3). The sorption isotherm of the resultant
graph equipped with a flame ionization detector. PCS was characteristically described as a type isoꢀ
AꢀWAX capillary column (diameter, 0.25 mm; length, therm, and the BET model was used to calculate the
I
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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798
FENGꢀWEN YAN et al.
2
−
1
porosity properties. The specific surface area of the
ssp, m g
000
2
–1
PCS was about 930 m g at the carbonization temꢀ
perature of 1000°C. The pore size distribution of the
PCS is demonstrated in Fig. 4, the ordinate of which
denotes the differential value of pore volume and pore
size. The pores in the PCS are mostly micropores and
the pore size is in the narrow range of 0.8–1.2 nm.
1
8
00
00
Effect of Reaction Temperature
on the Activity of VA Formation
6
Figure 5 shows that the activity of vinyl acetate forꢀ
mation increased monotonously with the increment of
reaction temperatures. The reaction of acetic acid
with acetylene is exothermic:
4
00
600
800
1000
°
T
, C
c
CH COOH + C H
CH COOCH=CH ,
3 2
3
2
2
Fig. 3. Relationship between surface area (ssp) and carbonꢀ
ization temperature (Tc).
(1)
Δ G(298 K) = 65 kJ/mol.
r
Such that it’s a heatꢀreleasing one, the reaction at
60°C produced only about 360 g of vinyl acetate per
1
Δ
V1/Δr1
mole of Zn per hour. Further increase of the temperaꢀ
ture resulted in more formation of vinyl acetate.
Because it’s a common phenomenon that, for a cataꢀ
lytic reaction, more severe diffusionꢀcontrolling influꢀ
ence arises from higher surface area of the catalyst
support, suitable increment of temperature speeds up
the diffusion process, thus expediting the reaction rate
0
.21
0.03
0.02
0.01
[
9]. When the temperature was elevated to 190°C, a
temperature increase of only 30°C led to the value of
activity of 718 g mol
–1
–1
h , which doubles the value
obtained at the reaction temperature of 160°C. Now
that the present case is a singleꢀpass operation, it’s an
encouraging result as expected. To increase the reacꢀ
tion temperature further slowed down the enhanceꢀ
ment tendency of the activities. Additionally, a high
2
0
40
r, Å
–1
–1
activity value of 1040 g mol h was realized at the
reaction temperature of 220°C. For all the experiꢀ
ments performed at different temperatures, byꢀprodꢀ
ucts like acetaldehyde, acetone, and butenal etc.
existed in the reaction mixture were generally below
Fig. 4. Pore size distribution of PCS.
−1
−1
a
1
, g mol h
100
1
.0, 0.01, and 0.015 wt % respectively (data not listed).
This indicates the PCS to be a good laboratoryꢀscale
support in the formation of vinyl acetate with high
selectivity.
900
700
500
300
Effect of Reaction Temperature
on the Conversion of Acetic Acid and Acetylene
Keeping constant of the feed flow rate of acetic
acid and acetylene, the effect of reaction temperature
on the conversion of acetic acid and acetylene was
investigated aiming to obtain some useful hints of the
role of PCS support in the formation of vinyl acetate.
Figure 6 reflects the result. For all the vinyl acetate forꢀ
mation reaction performed in the temperature range
160
180
200
220
°
T, C
160–220°C, the conversion of acetic acid increased
from 7.8 to 22.6% with the rise of temperature. This
increment tendency is in good accord with the change
Fig. 5. Activity dependency on the reaction temperature.
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VINYL ACETATE FORMATION IN THE REACTION OF ACETYLENE
799
−
1
−1
α
, %
α
, %
6
α
, %
a, g mol h
1
24
20
16
12
8
1800
22
18
14
10
6
1
600
400
4
2
1
1200
1
000
160
180
200
220
0.1
0.2
0.3
v
0.4
0.5
0.6
, ml min−
1
T,
°
C
Fig. 6. Effects of reaction temperature on conversions of
acetic acid and acetylene 1)
Fig. 7. Effects of acetic acid flow rate (
of acetic acid ( ) and activity ( ).
v
) on the conversion
(
α
)
(
α
.
α
a
of vinyl acetate formation activities. Because the ratio flow rate of 0.6 ml min–1. Not depicted herein for
of acetylene to acetic acid in the feed for these cases simplicity, as to a constant flow rate of acetylene of
–1
were maintained at 4.7/1 (mol/mol), which is well 200 ml min , the acetylene conversion went up with
above the stoichiometric ratio of 1/1, quite a fraction increasing flow rate of acetic acid. In this case, the
of acetylene was unconverted during the singleꢀpass highest conversion of acetylene reached 9.1% at the
vinyl acetate formation process. The highest value of acetic acid flow rate of 0.6 ml/min for a singleꢀpass
acetylene conversion, 5.3%, was obtained at 220°C
.
operation.
For the catalyst support with much higher surface area
too high reaction temperatures gave rise to marginal
increase in the activities whereas more byꢀproducts
were resulted. Meanwhile, high temperatures imposed
adverse effect to the properties of the catalyst and
more strict requirement for the equipment came forth
accordingly. Considering the fact that there is always
recycling system for unreacted reactants in industrial
processes, both the conversion values of acetic acid
and acetylene are worthy of further investigation into
the potential of PCS as catalyst support for commerꢀ
cial applications.
Because the reaction rate of acetylene and acetic
–4
–4 –1
acid (reaction frequency: 1.3
×
10 –4.2 10 s ) is
×
far below the rates at which acetylene and acetic acid
diffuse to zinc acetate in the pores of PCS, the influꢀ
ence of internal diffusion is negligible. So it’s restricted
to tremendously improve the activity by increasing the
flow rate of acetic acid alone. To curb side reactions
and better the performance of the catalyst at the same
time, increasing of the space velocity is generally an
alternative [10].
As the PCSꢀZn catalyst prepared herein was from
impregnation method, zinc acetate was prone to disꢀ
perse in the micropores as monoꢀlayers. There is modꢀ
erate interaction between the Zn species and the carꢀ
bon support, thus making it easier for the spontaneous
migration and uniform dispersion of zinc acetate as
monoꢀlayers on the surface of the support at certain
temperatures. It’s believed that only the singleꢀlayers
that contacted the carbon support behaved actively in
the formation of vinyl acetate from acetic acid and
acetylene [11].
Effect of Acetic Acid Flow Rate
on Its Conversion and the Activity
As indicated by Fig. 7, increasing the flow rate of
–1
acetic acid from 0.1 to 0.2 ml min led to rapid
increase in the formation of vinyl acetate, jumping
–1
–1
from 1040 to 1430 g mol h . Raising further the aceꢀ
tic acid flow rate resulted in gradually slowing
increase in the activities. A flow rate increase from
–
1
0
1
.5 to 0.6 ml min elevated slightly the activity from
–
1
–1
680 to 1710 g mol h , implying the limited capacꢀ
Effect of Ratio of Acetylene to Acetic Acid
on the Formation of Vinyl Acetate
ity of the PCSꢀZn used in the present research. Also
observed from Fig. 7 is the declining conversion of
acetic acid as its flow rate increased. The conversion
To tap the best performance of PCSꢀZn in the
decreased by about 50% when the flow rate of acetic present study, varying flow rates of acetylene and aceꢀ
–1
acid went up from 0.1 to 0.3 ml min . This tendency tic acid were tested. The result of their effects on the
continued with further increase of the flow rate until conversions of the two reactants and the activities is
a relatively stable value of ca. 9% was reached at the listed in table.
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800
FENGꢀWEN YAN et al.
100
μ
m
50 μm
(а)
(b)
10
μ
m
5 μm
(
c)
(d)
Fig. 8. SEM images of the porous carbon spheres: (a) spherical shape of the spheres, (b) scale like layers on the surface of a sphere,
c) gaps between the layers, and (d) pores between the gaps.
(
Known from the data is that, to obtain the acetic conversions, the activities in vinyl acetate formation
acid conversion above 22%, it’s sufficient to employ also got steady increase with rising ratio of acetylene to
–
1
acetic acid when the flow rate of acetic acid was mainꢀ
the flow rate of acetic acid of 0.1 ml min . Further
increase in acetylene flow rate only resulted in slight
increment of the conversion of acetic acid, 22.6% at
–1
tained at 0.1 ml min . Now that the acetylene introꢀ
duced into the reaction system was in great excess,
continuing increase of the acetylene flow rate ineviꢀ
tably reduced its conversions at fixed acetic acid flow
–1
–1
2
00 ml min , 23.2% at 400 ml min , and 23.8% at
–1
6
00 ml min when the acetic acid flow rate was fixed
–
1
of 0.1 ml min . Simultaneous increase in the flow
at 0.1 ml min–1. Compared to the change of acetic acid
rates of acetylene and acetic acid improved the reacꢀ
–1
–1
tion activity to its highest value of ca. 1850 g mol h
.
However, both the conversions of acetylene and acetic
acid lowered under these conditions. If the activity is
expected to improve in large quantity significant
increase in the total pressure of the reactants will be a
choice. For the sake of operation safety it’s a general
rule to carry out the reaction at atmospheric pressure
of acetylene [12]. It’s postulated that residence time
was not long enough for the reactant molecules, which
were in large excess, to get access to the active Zn sites,
therefore the activity was limited to some extent
accordingly.
Data of vinyl acetate formation of at various ratio of acetyꢀ
lene to HAc
Entry
v
1, ml/min
v
, ml/min
a, g/(mol h)
α, %
α1, %
1
2
3
4
5
6
7
8
200
400
400
600
600
800
800
800
0.1
0.1
0.2
0.1
0.4
0.1
0.4
0.6
1039.2
1067.4
1435.5
1095.0
1586.3
1053.6
1693.2
1849.6
22.6
23.2
15.6
23.8
8.6
5.3
3.5
2.3
2.2
1.2
1.8
1.1
1.0
22.9
9.2
Influence of PCS Structure on the Formation of VAc
6.7
The effective diameter of zinc acetate is 0.77 nm
[13] which lies in the range of the pore diameters of
Note: Reaction conditions: pressure of acetylene 0.1 MPa; temperꢀ
3
ature 220°C; volume of PCSꢀZn in the reactor 12 cm ; v and
1
0.8–1.2 nm of the PCS support. So most of the zinc
v are the flow rate of acetylene and acetic acid respectively,
a is activity, α and α are the conversion of acetic acid and acetate was embedded into the pores of PCS especially
1
acetylene respectively.
at a low zinc loading as in the present research.
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VINYL ACETATE FORMATION IN THE REACTION OF ACETYLENE
801
Compared to planar surface of other carbon supports,
the rugged surface of PCS provided enormous specific
surface area and offered much contact area for the
reactants and zinc acetate. Moreover, the holes also
helped to collect the product and possibly reduced the
extent of side reactions.
CONCLUSION
The porous carbon spheres possessing high
mechanical strength, narrow pore size distribution,
and high surface area proved itself to be an effective
support for zinc acetate. The PCS with bumps and
holes on its surface facilitated both heat transfer and
mass transport during the reaction process. Small in
pore diameter as it is, the PCSꢀZn catalyst still exhibꢀ
ited high activities in the formation of vinyl acetate
from acetic acid and acetylene at atmospheric pressure
and moderate temperatures. The singleꢀpass converꢀ
sions of acetic acid and acetylene were comparable to
others' results and the catalyst was supposed to be used
repeatedly due to the good physical properties of PCS
nm
8
6
4
2
Fig. 9. AFM image of the porous carbon sphere surface.
Scan size: 10.0 nm, Setpoint: 0.0 V, Scan rate: 1.6 Hz,
Number of samples: 256).
(
Because the PCS was synthesized via polymerization support.
method using very pure monomer, it meets the basic
demands on a support: low contents of ash and toxic
2
–1
ACKNOWLEDGMENTS
elements; surface area ca. 1000 m g , and well
developed microporous structure etc. In addition,
The financial support by Jiangsu Sopo Corporation
micropores are advantageous in the adsorption of zinc (Group) Ltd. is acknowledged.
acetate without adverse effect on the pore structure.
As already described, the PCS possessed uniform
size and global morphology. Such a support is suitable
for repeated use due to its excellent mechanical
strength originated from the high crystallinity. Found
in the SEM images (Fig. 8) is that the PCS was of regꢀ
ular spheres (Fig. 8a) with scaleꢀlike layers (Fig. 8b) on
its surface. Such a layer structure is advantageous for
heat transfer and mass transport at large scale. Meanꢀ
while the spherical shape could effectively reduce the
dead volume of the fixedꢀbed reactor. Figure 8c
depicts explicitly the gaps between layers and the uniꢀ
form distance between layers. As noticed in Fig. 8d,
most pores were existed in the limited space between
gaps, such that the reactant molecules easily permeꢀ
ated to the micropores directly via the gaps, thus
greatly reducing the diffusion resistance. In contrast,
the adsorption on common activated carbon had to
proceed step by step, first from the bulk phase into
mesopores and from mesopores into micropores subꢀ
sequently.
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