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Butadiene production from bioethanol and acetaldehyde over tantalum
oxide-supported spherical silica catalysts for circulating fluidized bed
Tae-Wan Kim ⇑, Joo-Wan Kim, Sang-Yun Kim, Ho-Jeong Chae, Jeong-Rang Kim, Soon-Yong Jeong,Chul-Ung Kim ⇑
Research Center for Green Catalysis, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu, Daejeon 305-600, Republic of Korea
h i g h l i g h t s
The effect of size and structural properties of Ta2O5/silica sphere was studied.
The various ETB reaction conditions for Ta2O5/silica sphere were studied.
Ta2O5/Q-6 catalyst showed good mechanical strength and catalytic performance.
Ta2O5/Q-6 catalyst tested for hydrodynamic properties in the cold-bed CFB.
a r t i c l e i n f o
Article history:
Available online 16 October 2014
Keywords:
Ethanol to butadiene
Spherical silica
Tantalum oxide
Circulating fluidized bed
a b s t r a c t
The chemical 1,3-butadiene (BD) is usually produced from ethanol and acetaldehyde (ethanol to BD, ETB)
over Ta2O5/SiO2 catalyst in fixed-bed system. However, the ETB process has a short catalyst-regeneration
cycle due to rapid deactivation of the catalyst by coke. To overcome this problem, a circulating fluidized-
bed (CFB) reactor could be used in the ETB reaction for continuous regeneration of deactivated catalyst.
The catalyst supplied to a CFB reactor must be spherical, show good catalytic performance, and be
mechanically strong. In this study, a series of samples of Ta 2O5 catalyst on silica spheres with different
pore and particle sizes, were prepared. The catalysts were examined for mechanical strength by anattrition test and in ETB reaction in the fixed-bed reactor. The Ta 2O5/Q-6 catalyst, with particles size of
75–150 lm, showed good mechanical strength and the best catalytic performance. This optimum spher-
ical catalyst was tested to examine its hydrodynamic properties in a cold-bed CFB reactor.
2014 Elsevier B.V. All rights reserved.
1. Introduction
The chemical 1,3-butadiene (BD) is an important chemical
intermediate in the petroleum industry because BD is a base mate-
rial used for producing commercially important synthetic rubbers
and polymers [1]. The main application of BD is in the production
of acrylonitrile butadiene styrene and styrene butadiene rubber,
which is used for manufacturing automotive tires. Due to the con-
tinuous growth of the global economy, especially in China and
India, the global automotive tire market is also expected to glow
consistently and to reach about USD 180 billion by 2017. This
potential attracts many investors and researchers to the produc-
tion of BD [2]. At present, the increasing rate of demand for BD
has created a serious deficit in BD production by steam cracking
of naphtha due to the depletion of petroleum, high price of oil,
and environmental issues. These conditions have motivated a push
to develop alternative technologies for BD production from renew-
able, and non-petroleum resources [1]. In addition, long-term
shortages of C4 chemicals are expected to be the outcome of the
availability of huge quantities of natural gas caused by recent
American shale-gas revolution. This has driven a shift to lighter,
gas-based feed-stocks; away from heavier, oil-based feed-stocks,
in the petrochemical industry [3]. This shift to lighter feed-stocks
has resulted in a significant reduction of BD production because
BD is now primarily produced as a byproduct of ethylene cracking
of the oil-based feed-stocks [4]. Ethanol is the most abundant bio-
based, sustainable resource because industrial ethanol is mainly
produced via fermentation of biological-feed-stocks such as sugar
and corn [1,5]. The production of ethanol as a biofuel was greater
than 100 billion liters in 2011 and expected to increase 3–7%
annually in the years from 2012 to 2015 [6]. As the bioethanol
market grows rapidly, the manufacture of bio-ethanol-based
http://dx.doi.org/10.1016/j.cej.2014.09.110
1385-8947/ 2014 Elsevier B.V. All rights reserved.
⇑ Corresponding authors. Tel.: +82 42 860 7257; fax: +82 42 860 7508
(T.-W. Kim). Tel.: +82 42 860 7504; fax: +82 42 860 7508 (C.-U. Kim).
E-mail addresses: [email protected] (T.-W. Kim), [email protected] (C.-U. Kim).
Chemical Engineering Journal 278 (2015) 217–223
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building blocks for the chemical industry has attracted a lot of
attention. Therefore, if technological advances can be achieved in
the near future, BD production using bioethanol would be the most
promising, sustainable and renewable technology among various
on-purpose BD-production technologies [7].
The catalytic conversion of ethanol into BD (ETB) is a well-
known industrial process. The ETB process has been used from
the 1920s, but was scrapped due to lower oil prices enabled byincreasing oil production in early 1960s [1,5,8–14]. However, the
ETB process is now again becoming attractive as a potential alter-
native due to high oil prices and the generous supply of bioethanol
[1,5,7,15,16]. ETB processes are mainly divided into two kinds.
Lebedev’s process has only one step, and Ostromyslensky’s process
has two [1,5,8–15]. The latter was commercialized by the Carbide
and Carbon Chemicals Corporation in the United States [17–22],
and involves two series of reactions (ethanol (EtOH) dehydro-
genation to acetaldehyde (AA), and then EtOH-AA condensation
to BD) in a fixed-bed system. Silica supported tantalum oxide
(Ta2O5/SiO2) is the best-known catalyst for the second step of the
ETB process [17–22].
However, both ETB processes have short catalyst-regeneration
cycles (12 h for the one-step process, and 120 h for the two-step
process) because of the rapid deactivation of the catalyst by coke
formation [17–22]. In order to continuously remove the coke from
the catalyst, a circulating fluidized bed (CFB) reactor could be
applied to the ETB reaction. The CFB catalytic reactors were first
tried to develop fluid-catalytic-cracking (FCC) technology for the
production of gasoline from the oil to replace old bubbling-bed
reactors [23]. Since the 1960s, CFB reactors have been applied to
various chemical processes such as gasification and combustion
of methane, biomass and coal [24,25], FCC, Fischer–Tropsch syn-
thesis, and methanol-to-olefin (MTO). The CFB reactor may easily
control the heat generated by exothermic reactions, enhance mass
transfer in a gas–solid reaction system, and provide continuous
regeneration of the coke-choked catalyst [26]. However, the cata-
lyst used in the CFB reactor need be resistant both to severe heat
and to mechanical stresses due to operating conditions thatinclude high temperature and high velocity flow. A catalyst with
low attrition resistance could cause many problems in the CFB sys-
tem and could eventually shut down the overall process. Therefore,
it is essential that a spherical Ta2O5/SiO2 catalyst with both good
catalytic performance and significant mechanical strength be pro-
vided as the ETB catalyst in CFB reactors.
In this study, we report for the first time spherical-shaped tan-
talum–silica based catalysts in order to apply CFB reactor for the
continuous remove of coke in ETB reaction. The catalytic perfor-mance and mechanical strength of the Ta2O5/SiO2 spherical cata-
lysts were examined using a fixed-bed reactor and an attrition
test, respectively. After selection of the optimum spherical catalyst,
having both high catalytic activity and attrition resistance, we
evaluated the hydrodynamic properties of the optimum catalyst
using a cold-bed, circulating fluidized bed reactor.
2. Materials and methods
2.1. Preparation of Ta 2O5 supported spherical silica catalysts
Commercial silica spheres (CARiACT Q-n, where n stands for
the pore size of the silica) were obtained from Fuji Silysia. All
Fig. 1. CFB reactor apparatus for 1,3-butadiene production from ethanol and
acetaldehyde (1: riser, 2: stripper, 3: regenerator, 4: stripper slide valve, 5:
regenerator slide valve, 6: stripper transfer line, 7: regenerator transfer line, 8: flue
gas control valve, 9: product gas control valve, 10: regenerator air inlet, 11: feed
inlet, 12: riser nitrogen inlet, 13: stripper nitrogen inlet, 14: flue gas outlet, 15:product gas outlet). Fig. 2. Nitrogen sorption isotherms and pore size distributions of the catalysts.
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of the spherical silica-supported 2 wt% tantalum oxide (Ta2O5)
catalysts were prepared by an impregnation method using etha-
nol as a solvent. A measure (100 g) of spherical silica support
was added to 1000 ml of ethanol, which also contained 32 g of
tantalum pentachloride (TaCl5, Aldrich). After stirring for 2 h,
the ethanol was removed using a rotary evaporator and the sam-
ples were dried at 120 C for 10 h. The CARiACT Q-n-supported
Ta2O5 samples (Ta2O5/Q-n) were obtained by calcination at500 C for 5 h.
2.2. Characterization
Nitrogen adsorption isotherms were measured at 196 C on a
Micromeritics Tristar 3000 volumetric adsorption analyzer. Before
the adsorption measurements, all samples were outgassed at
300 C in a degassing station. The Brunauer–Emmett–Teller (BET)
equation was used to calculate the apparent surface area from
the adsorption data obtained at P /P 0 between 0.05 and 0.2. The
total volume of micro- and mesopores was calculated from the
amount of nitrogen adsorbed at P /P 0 = 0.95, assuming that adsorp-
tion on the external surface was negligible compared to adsorption
in the pores. The pore size distributions were calculated by analyz-
ing the desorption branch of the N2 sorption isotherm using the
Barret–Joyner–Halenda (BJH) method. Scanning electron micro-
scope (SEM) images were obtained with a Philips XL-30S fieldemission guns (FEG) SEM operated at 10 kV. The samples were
coated with gold before SEM measurement. The attrition test of
the spherical catalyst was determined using the ASTM D5757-95
method. After 50 g of the catalyst was fluidized by air at 10 l/min
for 5 h, the amount of entrained catalyst collected in the thimble
filter was measured. The attrition rate is defined as the ratio of
the amount of entrained catalyst to the total amount of catalyst
(50 g) used [26]. The bulk density of the catalyst was determined
by measuring the mass and volume of a catalyst sample using a
balance and a graduated cylinder, respectively.
2.3. Catalytic test in a fixed-bed reactor
The production of 1,3-butadiene (BD) from ethanol and acetal-
dehyde was performed in a fixed bed reactor system with a 3/8
inch stainless steel (SUS) tube reactor [7]. The reaction tempera-
ture was controlled by a type-K thermocouple (Omega) and a PID
controller. A mixture of ethanol (99.9 wt%, Samchun) and acetalde-
hyde (85 wt%, Aldrich) with ethanol, to provide an acetaldehyde
molar ratio of 2.5 [17,21], was fed into the catalytic reactor at
0.66 ml/h by a high-performance liquid chromatography (HPLC)
pump. The catalyst (0.25 g) was loaded in the middle of the SUS
tube. Before the reaction, the catalyst was warmed up to thereaction temperature (350 C, heating rate = 5.0 C/min) with a
5 ml/min flow of the carrier gas (N 2). The reaction was then per-
formed with a liquid hourly space velocity (LHSV) of 1.0 h1 at
350 C. The effluent gas products were measured using a gas chro-
matograph (6100GC, Young Lin Instrument Co.) equipped with a
flame ionization detector (FID). Products were detected by the
FID using a capillary HP Plot Q column (0.53 mm id 40 micron
thickness 30 m length). Total conversions and BD selectivities
were calculated using the following equations.
2.4. Catalyst circulation test in the CFB reactor
A schematic diagram of the pilot CFB plant (riser length: 4 m,
riser inner diameter: 14.6 mm) used in the experiments appears
in Fig. 1. Hydrodynamic properties such as the catalyst circulation
rate were examined by means of a cold-bed CFB test (without
heating). First, 2.6 kg of the optimum catalyst was prepared as in
Section 2.1 and loaded into the CFB reactor. The cold-bed test
was performed under 25 psig of N2 gas pressure in a CFB pilot plant
with a mechanical slide valve.
3. Results and discussion
3.1. Properties of the Ta 2O5 /Q-n catalysts
Fig. 2 shows the nitrogen physisorption isotherms and pore size
distributions for the three different Ta2O5/Q-n samples. The capil-
lary condensation step increased and the amount of N2 adsorption
below P /P 0 = 0.2 decreased with an increase in the pore diameter.
This indicates that the number of mesopores in the Ta 2O5/Q-n sam-ple increased, while the number of micropores decreased, with
increasing of pore size. The pore size distribution curves for the
Table 1
Physical properties of catalysts.
Samples Ta (wt%)a S BET (m2 g1)b V t (cm
3 g1)c wavg (nm)d wBJH (nm)
e Dbulk (g/ml)f Attrition test (%)
Ta2O5/Q-3 1.84 688 0.37 2.2 2.0 0.77 0.30
Ta2O5/Q-6 1.90 530 0.58 5.9 7.3 0.53 0.21
Ta2O5/Q-10 1.91 214 1.15 13.3 16.6 0.38 2.93
a Ta, Ta loading measured by ICP-AES.b S BET, apparent BET specific surface area.c V t, total pore volume.d wavg, the average pore size.e
wBJH, the pore size calculated using the BJH method and deduced from the highest point of pore size distribution curve.f Dbulk, bulk density.
Total conversion ¼ ðTotal C moles ðC moleunreacted EtOH þ C moleunreacted AAÞÞ
Total C moles 100
BD selectivity ¼
C moleBD in productsTotal C moles in products except for EtOH and AA 100
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Ta2O5/Q-n samples also show an increase in the mesopore volume
with increasing pore diameter. The detailed structural properties of
the Ta2O5/Q-n samples are summarized in Table 1. The average
pore sizes of the Ta2O5/Q-n catalysts are shown to range from 2.2
to 13.3 nm. The pore sizes of Ta 2O5/Q-3 and Ta2O5/Q-6 calculated
by the desorption branch of the isotherm using the BJH method,
were larger than the average pore sizes due to overestimation by
the BJH method of the range of large mesopores [27]. The specificsurface area gradually decreased with an increase in pore diameter,
while the pore volumes significantly increased from 0.37 to
1.15 cm3/g. This enlargement of pore volume was mostly caused
by the increase in the mesopore volume of the Ta2O5/Q-n catalysts.
The bulk density and mechanical strength of the Ta2O5/Q-n cat-
alysts examined by attrition test are shown in Table 1. The Ta2O5/
Q-6 catalyst exhibited the highest attrition resistance (0.21%),
while Ta2O5/Q-10 showed the lowest attrition (2.93%). With an
increase of pore size, the surface area and bulk density decreased.This indicates that larger pore size might lead to a mechanically
Fig. 3. SEM images of catalysts: (a) Ta2O5/Q-3, (b) Ta2O5/Q-5, and (c) Ta2O5/Q-10.
Fig. 4. EtOH/AA total conversion (open symbol) and BD selectivity (solid symbol) of Ta 2O5/Q-n catalysts (circle: Ta 2O5/Q-3, square: Ta 2O5/Q-6, triangle: Ta 2O5/Q-10).
Table 2
Catalytic performance of Ta2O5 supported Q-n catalysts in a fixed bed reactor at 350 C, LHSV of 1 h1 at 30 h.
Sample EtOH/AA conv. (%) Carbon selectivity (C mol%)
Ethylene Propylene Butene isomers 1,3-BD Ethoxy ethane Ethyl acetate Crotonaldehyde Acetic acid Others*
Ta2O5/Q-3 10.9 4.6 1.1 0.7 47.8 4.9 3.4 18.0 0.6 19.0
Ta2O5/Q-6 31.5 2.7 1.6 1.6 72.0 0.2 0.6 0.3 6.6 14.4
Ta2O5/Q-10 30.6 1.8 1.2 1.6 70.3 2.0 2.0 6.4 3.7 11.0
* Unidentified compounds mainly consisting of heavier compounds than acetic acid in GC chromatography.
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weak catalyst, resulting from a less dense silica framework given
similar-sized silica-support. Thus, the catalyst with the largest
pore size (Ta2O5/Q-10) showed the lowest attrition resistance.
SEM images of the tantalum-oxide-loaded spherical silica cata-
lysts are displayed in Fig. 3. All SEM images of the Ta2O5/Q-n cat-
alysts show a spherical morphology with a particle size around
200–300 lm.
3.2. Catalytic test for ETB in the fixed-bed reactor
To investigate the effects of the mesopore size, and to deter-mine the optimum mesopore size, the catalytic activities of the cat-
alysts (particle size 200–300 lm) with different pore diameters
were examined for ETB reaction in the fixed-bed reactor. Fig. 4
shows the total conversion of ethanol (EtOH) and acetaldehyde
(AA), and the 1,3-butadiene (BD) selectivity of three spherical sil-
ica-supported tantalum oxide catalysts with different pore diame-
ters. The conversions for all samples gradually decreased with
time-on-stream due to the formation of coke in the catalysts, while
the selectivities of BD were almost constant over the entire reac-
tion time. This clearly reveals that the CFB reactor needs to contin-
uously regenerate catalyst deactivated by accumulation of coke.
Among the series of Ta2O5/Q-n catalysts, the total conversion and
the BD selectivity of the Ta2O5/Q-3 catalyst showed the lowest val-
ues among all catalysts due to its smaller pore size. The catalystswith pore size above 6 nm showed good conversion and BD selec-
tivity. The Ta2O5/Q-6 catalyst with the middle mesopore size (aver-
age pore size 5.9 nm) showed the highest catalytic performance
(total conversion = 31.5% and BD selectivity = 72.0%), which was
slightly higher than that of the Ta2O5/Q-10 catalyst (total conver-
sion = 30.6% and BD selectivity = 70.3%). As shown in Table 1, the
mesopore size of the Ta2O5/Q-10 catalyst (13.3 nm) was over two
times larger than that of the Ta2O5/Q-6 catalyst (5.9 nm), whereas
the BET-specific surface area of the Ta2O5/Q-10 catalyst (214 m2/g)
was over two times less than that of the Ta2O5/Q-6 catalyst
(530 m2/g). From structural analyses of the catalysts and results
from the catalytic tests, the Ta2O5/Q-6 catalyst appears to be the
best candidate for ETB reaction because it has both a moderately
large mesopore size and a high surface area, which facilitate goodaccessibility of the reactants and products, as well as good disper-
sion of tantalum oxide within the silica [7]. The detailed product
selectivities are shown in Table 2, including unidentified com-
pounds. These were heavy compounds and would mainly cause
deactivation of the catalyst by creation of coke. The formation of
heavy compounds was gradually reduced with increasing meso-
pore size of the catalysts. This indicates that a catalyst with large
mesopores forms lesser amounts of heavy compounds compared
with catalysts with small mesopores, due to better accessibility
and diffusion of the reactants and the products [7].
The optimum spherical silica-supported tantalum oxide cata-
lyst is Ta2O5/Q-6 because Ta2O5/Q-6 showed both good catalytic
performance and good mechanical strength, which are essentialfor the catalyst in a CFB reactor. To investigate the effect of the par-
ticle size of the Ta2O5/Q-6 catalyst for the ETB reaction, we pre-
pared four Ta2O5/Q-6 samples with different particle sizes
(Fig. 5): 75–150, 75–200, 200–300, and 300–500 lm. As shown
in Fig. 6 and Table 3, the conversions of particles less than
300 lm were 30.1–33.4% at a reaction time of 30 h, and the BD
selectivity increased with decreasing particle size of the catalyst.
Fig. 5. SEM images of Ta2O5/Q-6 catalysts of different particle size: (a) 75–150 lm, (b) 75–200 lm, (c) 200–300 lm, and (d) 300–500 lm.
Fig. 6. EtOH/AA total conversion (open symbol) and BD selectivity (solid symbol) of
Ta 2O5/Q-6 catalysts of different particle size (circle: 75–150 lm, square: 75–200 lm, triangle: 200–300 lm, diamond: 300–500 lm).
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The smallest particle size, 75–150 lm, for the Ta2O5/Q-6 catalyst,
shows the best catalytic performance, while the largest particle
size, 300–500 lm, displayed the lowest catalytic activity. Small
particle sizes should facilitate the diffusion of reactants and prod-
ucts during the catalytic reaction, which is an effect similar to that
from large mesopores as discussed above [7]. In addition, the selec-
tivity of heavy compounds (others in Table 3) in the particle size
range 300–500 lm was 20.0%, which is the largest amount among
the catalysts due to relatively hard diffusion of reactants and prod-
ucts from the catalyst with large particle size, compared with that
of those with small particle size.
3.3. Cold-bed CFB test
As discussed Sections 3.1 and 3.2, the optimum ETB catalyst for
a CFB reactor was Ta2O5/Q-6 with particle size 75–150lm. A 2.6 kg
portion of the optimum catalyst was prepared and loaded into the
circulating fluidized bed (CFB) reactor, as shown in Fig. 1. First, to
evaluate catalyst circulation in the CFB reactor, we performed a
cold-bed test without heating the overall CFB reactor prior to
ETB reaction. The cold-bed test in the CFB reactor is important
because we can determine the rate of catalyst circulation from it,
which is connected with the flow rate of the riser gas and the
degrees of opening of the slide valve. The catalyst circulation rate
(CCR) is controlled by two mechanical slide valves located in the
regenerator bottom (5) and stripper bottom (4), as shown in
Fig. 1. When the degree of opening for the slide valve in the bottom
of the regenerator (5), and the gas velocity in the riser (1) were
fixed, the CCR value could be kept constant by controlling the slide
valve in the bottom of the stripper (4) to keep a constant level of
catalyst in the stripper (2). Therefore, the CCR could be controlled
by the degree of opening of the slide valve in the bottom of the
regenerator (5) alone, which was determined from the measure-
ment of differential pressure between the top and bottom of the
regenerator. A plot of the different pressure of regenerator versus
the catalyst weight loaded into the regenerator is shown in
Fig. 7. The differential pressures increase with the linear increase
in the catalyst weight (up to 1500 g) because the catalyst is filled
from the stand-pipe region of the regenerator, which has a con-
stant pipe diameter. Above 1500 g of catalyst weight, the slope of
the different pressure per the catalyst weight decreased due to
Table 3
Catalytic performance of Ta2O5 supported Q-6 catalysts with different particle size in a fixed bed reactor at 350 C, LHSV of 1 h1 at 30 h.
Particle size (lm) EtOH/AA conv. (%) Carbon selectivity (C mol%)
Ethylene Propylene Butene isomers 1,3-
BD
Ethoxy ethane Ethyl acetate Crotonaldehyde Acetic acid Others*
75–150 33.4 3.2 1.7 1.7 75.2 0.3 0.5 0.3 0.4 16.8
75–200 30.1 4.0 1.7 1.7 74.6 0.2 0.6 0.2 0.3 16.8
200–300 31.5 2.7 1.6 1.6 72.0 0.2 0.6 0.3 2.6 17.4
300–500 24.6 4.0 1.5 1.6 71.0 0.9 0.8 0.2 0.3 20.0
* Unidentified compounds mainly consisting of heavier compounds than acetic acid in GC chromatography.
Fig. 7. Increase of differential pressure between top and bottom of the regenerator
with an increase in the amount of catalyst in the stand pipe region of theregenerator.
Fig. 8. Decrease in the differential pressure of the regenerator with time for degree
of opening of the slide valve in the regenerator at 10 l/min of riser gas velocity.
Fig. 9. Effect of riser gas flow rate and degree of opening of the slide valve on thecatalyst circulation rate (CCR).
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