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Theoretical and Experimental Study on Reaction Coupling: Dehydrogenation of Ethylbenzene in the Presence of Carbon Dioxide
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Journal of Natural Gas Chemistry 15(2006)11-20
SClENCE PRESS
Review
Theoretical and Experimental Study on Reaction Coupling: Dehydrogenation of Ethylbenzene in the Presence of Carbon Dioxide Shuwei Chen,
Zhangfeng &in,
Ailing Sun,
Jianguo Wang*
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Shanxi 030001, China [Manuscript received November 21, 2005; revised December 8, 20051
Abstract: Dehydrogenation of ethylbenzene (EB) to styrene (ST) in the presence of COz, in which EB dehydrogenation is coupled with the reverse water-gas shift (RWGS), was investigated extensively through both theoretical analysis and experimental characterization. The reaction coupling proved to be superior to the single dehydrogenation in several respects. Thermodynamic analysis suggests that equilibrium conversion of EB can be improved greatly by reaction coupling due to the simultaneous elimination of the hydrogen produced from dehydrogenation. Catalytic tests proved that iron and vanadium supported on activated carbon or A1203 with certain promoters are potential catalysts for this coupling process. The catalysts of iron and vanadium are different in the reaction mechanism, although ST yield is always associated with COz conversion over various catalysts. The two-step pathway plays an important role in the coupling process over Fe/A12Og, while the one-step pathway dominates the reaction over V/A1203. Coke deposition and deep reduction of active components are the major causes of catalyst deactivation. COZ can alleviate the catalyst deactivation effectively through preserving the active species at high valence in the coupling process, though it can not suppress the coke deposition. Key words: reaction coupling; ethylbenzene dehydrogenation; styrene; carbon dioxide; water-gas shift; reaction mechanism; catalyst deactivation
1. Introduction An ideal process of chemical reaction requests high yield and selectivity of intent products, stable catalysts, mild reaction conditions, and low energy consumption. Moreover, the concepts of green chemistry in the recent years emphasize the atomic economic and environmental benignity for a reaction. However, some reactions are confined thermodynamically and/or kinetically, which may necessitate severe reaction conditions and high energy consump tion and then suffer from low efficiency; while some other reactions are strongly exothermic, which may cause certain troubles t o the reactor design and operation of the heat exchange units. Reaction coupling is an effective approach to improve the behaviors of
reaction confined by the thermodynamic and/or kinetic limits [l].By coupling a dehydrogenation reaction with a hydrogenation reaction, an endothermic reaction with an exothermic reaction, or a reductive reaction with a n oxidative reaction, the equilibrium conversion can be improved greatly and the reaction temperature and the energy consumption can be reduced. Styrene (ST) is commercially produced by the dehydrogenation of ethylbenzene (EB) on the promoted iron oxide catalysts at 600 "C-700 "C, just below the temperature where thermal cracking becomes significant. Due to its highly endothermic and volume-increasing character, a large amount of superheated steam is used to supply heat, lower the partial pressure of the reactant, and avoid the formation of
* Corresponding author. Tel: +86-351-4046092; Fax: +86-351-4041153; E-mail: [email protected].
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Shuwei Chen et al./ Journal of Natural Gas Chernistr.y Vol. 15 No. 1 2006
carbonaceous deposits [2,3]. However, much of the latent heat of steam is lost in the gas-liquid separator. The dehydrogenation of EB to ST in the presence of COz instead of steam, in which EB dehydrogenation is coupled with the reverse water-gas shift (RWGS), is believed to be an energy-saving and environmentally friendly process. EB conversion can be enhanced at a lower temperature (550 "C) and the energy consumption can be reduced significantly. It is estimated that the energy required for producing one ton ST in the new process using C02 is about (6.37.9)x108 J, much lower compared with 6 . 3 ~ 1 0J ~in the current commercial process [4&6]. Since the commercial Fe-Cr-K catalysts do not work effectively in such a coupling system, the catalysts have been screened extensively [4-24]. Fe, V oxides supported on activated carbon (AC) [13-171, V oxide supported on MgO [HI, hydrotalcite-like compounds (Mg-Al-Fe, Mg-A1-V) [19-221, and Fe oxide supported on ZrO2 exhibited high initial catalytic activity for EB dehydrogenation in the presence of C02, but suffered from severe deactivation even for several hours operation. Comparatively, Fe and V oxides supported on A1203 were much more stable together with reasonable activity [4-121. However, the catalysts prepared so far may still not meet the requests for the commercialization, and there exist controversies on the reaction mechanism and causes of catalyst deactivation. To make such a coupling process practicable, we have carried out an extensive investigation on EB dehydrogenation in the presence of C 0 2 through both theoretical analysis and experimental characterization [7-9,13,25-271. With the thermodynamic analysis, the superiority of reaction coupling was displayed and possible reaction pathways were suggested [8,13,25];Potential catalysts for the coupling reaction system were obtained [7,9,13]through extensive catalyst screening; and through catalytic tests and various characterizations, the reaction mechanism and causes of catalyst deactivation were revealed [8,13,27]. In this paper, our efforts on this coupling process were presented to get a better understanding on the benefits of reaction coupling, reaction mechanisms, causes of catalyst deactivation, and roles of CO2 in the coupling system. 2. Thermodynamic analysis
2.1. Advantages of reaction coupling
The dehydrogenation of EB to ST can be expressed by:
When it is coupled with RWGS,
the effects of reaction coupling on the equilibrium EB conversion can be evaluated thermodynamically [25]. The dehydrogenation of EB to ST is an endothermic and volume-increasing reaction; high temperature and low pressure are favored for the conversion of EB. Figure 1 gives the equilibrium conversion of EB in the case of simple dehydrogenation, nitrogen dilution and coupling with RWGS a t different temperatures and different mole ratios of C02 to EB under 0.1 MPa. It can be seen that the equilibrium conversion is quite low in the case of single EB dehydrogenation even a t high temperatures, for example, 69.7% at 690 "C. The conversion can be increased obviously by introducing nitrogen into the reaction system because of the decrease of the partial pressures of all the components in the system, which is one of the reasons for using large amounts of steam in the commercial process. However, this enhancement by dilution is limited, for example, EB conversion increases from 25.2% to 57.8% through dilution by nitrogen (initial mole ratio of Nz/EB=10) at 550 "C.
300
400
500 Temperature ( C )
600
700
Figure 1. Reaction coupling of EB dehydrogenation with RWGS: the effects of the feed composition (in mole ratio) and temperature on the equilibrium conversion of EB at 0.1 MPa (1) EB:C02=1:10, (2) EB:C02=1:5, ( 3 ) EB:C02=1:1, (4) EB:N2=1:10, ( 5 ) EB only
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Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
To enhance the conversion of EB or reduce the operating temperature considerably, it is necessary to eliminate the produced hydrogen in situ by coupling with hydrogen-consuming reactions like RWGS. As shown in Figure 1, the conversion is improved greatly by coupling with RWGS. Moreover, the conversion increases with an increase in the ratio of C02/EB. The conversions are 63.9% and 82.4% at 500 "C and 550 OC, respectively, in the case of the mole ratio of C02/EB being 10. The margin between the lines of dilution and coupling suggests the superiority of the coupling over dilution. It can be then concluded that the reaction coupling can improve the equilibrium conversion of EB at a certain temperature or reduce the operating temperature at a certain ST yield.
two-step pathway is a little higher than that via the one-step pathway at 350 "C-600 "C. In contrast, the COz conversion via the two-step pathway is lower at a temperature higher than 450 "C. Moreover, there will be no hydrogen in the effluent gas when the reaction follows solely the one-step-pathway.
2.2. Different pathways suggested by thermodynamic analysis
There exists always a controversy on the roles of C02 in EB dehydrogenation in the presence of CO2: whether CO2 interacts directly with EB, or just reacts with hydrogen after it is released from EB dehydrogenation. As proposed by Mimura et al. [5], EB dehydrogenation in the presence of CO2 may proceed in two pathways called one-step pathway and two-step pathway. For the one-step pathway, EB is oxidized t o ST with C02 through the direct interaction of CO2 and EB, CsH5-C2H5 f CO2
--+
+
C ~ H ~ - C ~ HCO S
+ HzO
(3) whereas for the two-step pathway, EB is first dehydrogenated to ST with hydrogen formed simultaneously, and then COz reacts with hydrogen via the RWGS,
-
step 1
+
C ~ H S - C ~------+ H ~ C ~ H S - C ~ HH2~ step 2 H2 CO2 CO H20
+
+
300
500
400
600
700
Temperature ( C )
Figure 2. Effects of reaction pathways on the equilibrium conversions of EB (a) and COz (b) under 0.1 MPa: for EB dehydrogenation in the presence of Nz (no coupling ( l ) ) , initial NZ:EB=11; for the coupled EB dehydrogenation in the presence of C02 (in one step (2) or two steps (3)), initial COZ:EB=11
3. Catalyst screening
(4)
Since hydrogen produced in the EB dehydrogenation is eliminated simultaneously by RWGS, the E B conversion can be then improved through the shift of dehydrogenation equilibrium. Through the thermodynamic calculation, the dependence of the equilibrium conversions of EB and C02 for the coupled EB dehydrogenation in the presence of COz through different pathways can be evaluated [8,25].As shown in Figure 2, the equilibrium conversions of EB with the coupling via either pathway are higher than that of EB dehydrogenation in the presence of nitrogen that has only a dilution effect. EB conversion of the coupled dehydrogenation via the
Since the commercial Fe-Cr-K catalysts for EB dehydrogenation with steam do not work effectively in such a coupling system, the catalysts with various supports, active components and promoters were then screened. The catalysts in this work were prepared by impregnation or co-impregnation method [7,8,13]. AC, A1203, Si02, as well as various zeolites like ZSM-5 were used as catalyst supports. The active components Fe, Cu, Pt, V were introduced by the impregnation of supports with aqueous solutions of iron nitrate, copper nitrate, chloroplatinic acid and NH4V03 with oxalic acid, respectively. Alkali metal (Li, K), alkaline earth metal (Ca), transition metal (Mn, Cr, Zr)
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Shuwei Chen et al./ Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
and rare earth metal (La) promoted catalysts were prepared by co-impregnation of a solution containing both the active components and nitrates of the promoters. The catalytic reaction was performed in a stainless steel tube reactor with an inner diameter of 6.0 mm, and about 200 mg catalyst was used per run [7,8,13]. The feed of EB (20.4 mmolEB/(gCat.h) and CO2 were introduced through a micro feeder pump and mass flow controller, respectively. The molar ratio of C02 to EB was fixed at 11. The reaction was generally operated at 550 "C at atmospheric pressure. The effluents, including ST, benzene, toluene, and unreacted EB from the reactor were condensed in a trap by an ice water bath and analyzed with a FID gas chromatograph (Shimadzu GC-7A) equipped with a 3 mmx3 m stainless-steel column of OV-101. The gaseous products (H2, N2, CO, C02, CH4, and C2Hs) were analyzed on-line by a TCD gas chromatograph (Shimadzu GC-9A) equipped with a 3 mmx 3 m stainless-steel column of carbon molecular sieve.
3.1. Supports The catalytic tests over catalysts of various supports revealed that AC and A1203 with high surface area and little acidity are proper catalyst supports for EB dehydrogenation in the presence of C02 at 550 "C [7,8,13]. ZSM-5 supported catalysts give very low selectivity to ST because of the cracking of EB on its acidic sites. AC itself also shows a low activity (ST yield 9.8%) for the coupled reaction. Fe or V supported on AC exhibits high catalytic activity, but suffers from severe deactivation even for several hours' operation. Comparatively, A1203 supported catalysts is much more stable together with reasonable activity. 3.2. Active components
The catalytic behaviors of Fe, V, P t and La supported on AC for EB dehydrogenation in the presence of C02 at 550 "C are listed in Table 1. All these catalysts give high selectivity to ST, but only Fe and V show excellent activity [13]. Similar results were obtained for the catalytic tests over A1203 supported catalysts [7,8]. As shown in Figure 3, for EB dehydrogenation in the presence of COa, Fe and V catalysts show much better activity than P t and Cu catalysts, although all the catalysts give high selectivity to ST. With the time on stream,
the ST yield over both Fe and V catalysts showed a decrease tendency, but the activity of V catalyst decreased much more slowly than that of Fe catalyst. The ST yield over Pt and Cu catalysts increases slowly, but remains at a low level with the time on stream. V supported on A1203 showed the best catalytic behavior among the catalysts investigated, with the ST yield of initial about 45% and still higher than 40% after the reaction lasted for 30 h at 550 "C. Table 1. C a t a l y t i c behaviors of AC-supported c a t a l y s t s w i t h different active components for the EB dehydrogenation i n the presence of C O z Active
EB conversion
ST selectivity
components* Fe(3)/AC
(%I
(%)
(%I
38.0
95.5
36.3
ST yield
V(0.87)/AC
52.0
97.3
50.6
Pt (0.015)/ AC
16.0
97.3
15.6
La(0.75)/AC
21.0
97.8
20.6
Reaction conditions: 550 "C, atmospheric pressure, W/F=4.07 (gcat.h)/mol, COZ/EB=ll; data were acquired after reaction lasted for 1 h. * Numerals in the parentheses indicate loadings of metal species in mmol/g-support. 50
0
5
10
15
20
25
30
Time on stream (h)
F i g u r e 3. C a t a l y t i c behaviors of a l u m i n a s u p p o r t e d c a t a l y s t s w i t h various a c t i v e components for EB dehydrogenation i n the presence of coz (1) Fe(3), (2) V(0.86), (3) Pt(O.Ol), (4) Cu(2.75) Reaction conditions: 550 "C, W/F=4.07 (g,,t.h)/mol, C02/EB = 11, and the numerals in the legend indicate the loadings of metal species in mmol/g-support
3.3. Promoters Various promoters were introduced to improve the catalytic behavior of the supported catalysts for EB dehydrogenation in the presence of CO2. As listed in Table 2 [13], over the catalysts of Fe and V as active components and AC as support, both the conver-
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Journal of Natural Gas Chemistry VoI. 15 No. 1 2006
sion of EB and the selectivity to ST are enhanced significantly with small amounts of alkali, alkaline earth or rare-earth elements as promoters. With the Lipromoted Fe/AC catalyst, EB conversion of 50% and ST selectivity of 98.3% were observed at 550 "C. With La-promoted V/AC catalyst, EB conversion and ST selectivity reached 64% and 97%, respectively. Figure 4 showed the effects of various promoters on the catalytic performances of Fe/A1203 and V/A1203 [7,8]. The catalytic activity of Fe/A1203 can be promoted by the addition of Ca, Mn and Cr, and the highest initial ST yield of about 48% is achieved over Fe(3)Cr(0.3)/A1203. When mixed with vanadium, the activity of Fe/A1203 can be further improved. On the other hand, the addition of Li, Mn and Zr has little influence on the activity of V/Al2O3 catalysts, while the addition of La (0.3 mmol/g) even leads to a decrease in the catalytic ac-
tivity. Among the promoters investigated, only Cr shows an evidently positive effect on the ST yield, which increases from 45% over V(0.57)/A1203 to 50% over V(O.57)Cr(0.3)/A1203. This may be due to that Cr is not only an active component for the dehydrogenation, but also a n effective promoter t o the RWGS. Table 2 . Effect of promoters on the catalytic behaviors of AC-supported Fe and V c a t a l y s t s Active
EB conversion ST selectivity ST yield
components Fe(3)/AC
(760)
(%I
(76)
38.0
95.5
36.3
Fe(3)Li(0.3)/AC
50.0
98.3
49.2
Fe(3)Na( 0.3)/AC V(0.87)/AC V(0.87)Li(O.l5)/AC V(0.87)Mg(0.25)/AC
46.0 52.0 56.0 60.0
98.3 97.3 97.3 97.4
45.2 50.6 54.5 58.4
V(0.87)La(O.O5)/AC
64.0
96.8
62.0
Notes are the same as those in Table 1
50
50
-
h
&
z....
x
45-
2 40 -
35 0
1
2
3
4
5
6
1
8
9
Time on stream (h)
"
0
"
1
'
1
I
2
3
4 5 6 Time on stream (h)
1
I
I
8
I
9
F i g u r e 4. C a t a l y t i c behaviors of Fe(3)/A1203 ( a ) and V(0.57)/A1203 (b) modified w i t h various promoters for EB dehydrogenation i n the presence of C02 (a) (1) None, (2) Ca(0.3), (3) Cr(0.3), (4) Mn(0.3); (b) (1) None, (2) Li(0.3), (3) Cr(0.3), (4) Mn(0.3), (5) Fe(2), (6) Zr(0.3), (7) La(0.3) Reaction conditions: 550 "C, W/F=4.07 (gcat.h)/mol, COZ/EB=11, and the numerals in the legend indicate the loadings of metal species in mmol/g-A1203
4. Reaction mechanism 4.1. Benefits of reaction coupling
In order to elucidate the superiority of reaction coupling, the dehydrogenation of EB was carried out in the presence of CO2 and N2, respectively, over commercial Fe-K catalyst, Fe/AC, FeLi/AC, V/A1203, VCr/A1203 and Fe/A1203 a t 550 "C "7,131. As listed
in Table 3, the commercial catalyst Fe-K shows very low activity for EB dehydrogenation in the presence of N2 or C02 at 550 "C (even with a C02/EB ratio being as high as 41). The selectivity to ST is also the lowest among the catalysts investigated. On other catalysts, the conversion of EB in the presence of C02 is obviously higher than that in the presence of Nz. Just as suggested by thermodynamic analysis [8,25], the conversion of EB can be improved greatly
16
Shuwei Chen et al./ Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
by the reaction coupling with RWGS. Moreover, over FeLi/AC and VCr/A1203 catalysts, the ST yield in the presence of C02 is always higher than that in the presence of nitrogen within 30 h on stream. The
catalyst stability as well as its activity in the presence of C02 is much better than that in the presence of N2, which proved the superiority of the reaction coupling.
Table 3. EB dehydrogenation in the presence of COz or Nz over various catalysts In the presence of C02 EB conversion [%) ST selectivity [%)
Catalysts Fe-K"
1.5
70.0
Fe (3)/AC Fe(3)Li(0.3)/AC
38.0 50.0
95.5 98.3
V(0.57)/A1203 V(0.57)Cr(0.3)/A1203
45.5 50.3
97.8 97.8
35.0
96.3
Fe(3)/A12 0
3
In the presence of N2 EB conversion [%) ST selectivity f%) 1.3 22.0 24.0
72.0 78.8 83.2
37.4
98.1
43.0 34.2
97.0 96.2
Reaction conditions: 550 'C, atmospheric pressure, W/F=4.07 (gcat.h)/mol, C02/EB=11 or N2/EB=11 for the reaction in the presence of CO2 or N2, respectively; data were acquired after reaction lasted for 1 h. * Commercial catalyst for EB dehydrogenation.
4.2. Reaction pathways 50
As mentioned above, there exists two possible reaction pathways for EB dehydrogenation in the presence of COz, namely one-step and two-step pathways. However, it is not easy to distinguish the one-step from the two-step pathways by the catalytic tests. CO (together with the stoichiometric amount of water) and Hz are simultaneously observed in the products for the EB dehydrogenation in the presence of CO2. This implies that the one-step and two-step pathways may coexist in the coupled reaction (Figure 5).
f
h
s
v
9
.E
40
-
35
-
v1
6
I \
5 -
c,H,-CH,-CH,
*2H
(2-1)
-
H,
Figure 5. Role of COz in the coupled El3 dehydrogenation in the presence of COz
43 -
2-
A comparison of ST yields and CO2 conversions for the coupled EB dehydrogenation in the presence of C02 over catalysts of Fe or V supported on A1203 suggests that EB conversion is associated with COz conversion, as shown in Figure 6, the higher the COz conversion is, the higher is the ST yield obtained. The conversion of CO2 also shows the same tendency as the conversion of EB with the time on stream. This means COz participates actively in the coupled reactions and there exists a synergistic effect between the EB dehydrogenation and C02 conversion in the coupling system.
1
L
0
1
2
3
4 5 6 Time on stream (h)
7
8
9
10
Figure 6. ST yield (a) and COz conversion (b) with the time on stream for the coupled EB dehydrogenation in the presence of C O z over the A1203 supported catalysts (1) Fe(3), (2) Fe(3)Ca(0.3), (3) Fe(3)Cr(0.3), (4) V(0.57), (5) V(0.57)Cr(0.3) Reaction conditions: 550 "C, 0.1 MPa, C02/EB=11, and W/F=4.07 (g,,t .h)/mol; the element labels and numerals in parentheses indicate the active components and their loadings in mmol/g-support
Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
However, these catalysts act differently for the single RWGS (with only H2 and COz in the feed), as shown in Figure 7. For the single RWGS, Fe supported on Alz03, especially promoted by Cr, was an effective catalyst; while V supported on Alz03 behaved badly. Contrary to that, for the coupled EB dehydrogenation, V supported on A1203showed much higher C02 conversion, which thereon gave also good activity for the EB dehydrogenation to ST (Figure 6). 8
II
1
\
t t
.,
I I
- - - --
2
0
(1)
1
2
3 4 5 Time on stream (h)
I 6
7
8
Figure 7. COz conversions in the single RWGS over the A1203 supported catalysts (1) V(0.57), (2) V(0.57)Cr(0.3), (3) Fe(3), (4) Fe(3)Cr(0.3) Reaction conditions: 550 "C, 0.1 MPa, C02/H2=11, and W/F=2.04 (gcat.h)/mol; the element labels and numerals in parentheses indicate the active components and their loadings in mmol/g-support
Thus, it was suggested that the catalysts of iron and vanadium are different in the reaction mechanism, although ST yield is always associated with COz conversion over various catalysts [8]. For the EB dehydrogenation in the presence of COz, there perhaps exists two kinds of hydrogen species formed from EB dehydrogenation, namely, active hydrogen species (atomic) and molecular hydrogen. CO2 could either come into contact directly with the active hydrogen species or react with the molecular hydrogen, which corresponded to the one-step and two-step pathways, respectively. The two-step pathway plays an important role in the coupling process over Fe/A1203, while the one-step pathway dominates the reaction over V/A1203. Similar phenomena are observed for the EB dehydrogenation in the presence of COYover the catalysts of Fe or V supported on AC [8]. More characterizations like C02-TPD and H2T P R proved that CO2 can be activated through either the basic sites or the redox sites on the catalyst sur-
17
face [8]. Over Fe supported catalyst with Cr as promoter, the strong basic sites may play an important role; while over V/A1203, the redox cycle is crucial for the COz activation. 5 . Catalyst deactivation
One of the major problems that restrain the process of EB dehydrogenation in presence of COz from being practical is the rapid deactivation of catalyst. The deactivation during reaction may come from the deposition of coke, sintering of active sites and change in the valence state of active vanadium species. In order to explore the causes of catalyst deactivation, a series of catalytic tests for EB dehydrogenation in presence of COz or N2 over V/A1203 catalysts with different vanadium loadings was carried out [27]. 5.1. Effects of Vanadium loading on the catalyst deactivation As shown in Figure 8, the catalytic activity decreases with the time on stream, and the ST yield after 20 h reaction (Yzo)was much lower than the initial ST yield (Y1). The catalytic activity and the extent of deactivation depend on the vanadium loadings and reaction atmospheres [27]. Figure 8 showed that the deactivation of V/A1203 for EB dehydrogenation is distinctly influenced by the vanadium loading. At vanadium loading below 1.50 mmol/g-A1203, the extent of deectivation in 20 h (the difference between Y1 and YzO) increases only slightly with the vanadium loading. However, the catalytic activity is enhanced significantly by the increase in vanadium loading, as indicated by the evident increase in the ST production during the 20 h reaction with the vanadium loading. As the vanadium loading exceeds 1.50 mmol/g-A1203, the extent of deactivation in 20 h increases sharply with the vanadium loading, accompanied with a severe decrease in the catalytic activity and the ST production. These suggested that a t low vanadium loading, the activity of V/A1203 is relatively stable for EB dehydrogenation; the increase in the extent of catalyst deactivation with the vanadium loading is ascribed to the increase in the quantities of ST produced (ST production) or EB consumed (EB consumption) in the same reaction period. However, the catalysts V/A1203 with vanadium loading above 1.50 mmol/g-A1203 suffer from severe deactivation. The presence of CO2 can alleviate the
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Shuwei Chen et al./ Journal of Natural Gas Chemistr.y Vol. 15 No. 1 2006
deactivation of catalysts, especially those with a vanadium loading below 1.50 mmol/g-A1203.
ST production during the same period of reaction. The catalyst V/A1203 with high vanadium loading deactivates severely during the reaction, but the coke deposition was not so severe. These indicate that the coke formation is far from the only cause of catalyst deactivation, although coke deposition may result in decreases in the specific surface area and catalytic activity of V/A1203. There are other factors that cause catalyst deactivation. 350 300 u
h
?
? 250
8 B
v
c
40
1f
I
0.5
8 3
8
I
I
~
I
I
I
I
I
I
I
I
,
,
,
I
I
I
,
,
1.o 1.5 2.0 2.5 Vanadium loading (mmol / g-Al,O,)
I
I
I
,
,
I
200
150
,
3.0
Figure 8. Effects of vanadium loading on the ST yield for EB dehydrogenation over V/A1203 in the presence of COz (a) and Nz (b) (1) Yl, (2) yzo Reaction conditions: 550 "C, 0.1 MPa, COz or Nz/EB=20 (mole ratio), and W/F=8.75 (gcat,h)/mol; Y1 and YZOwere the ST yields acquired after reaction lasted for 1 h and 20 h, respectively
100 I
,
,
,
,
l
,
,
,
,
I
,
,
50 60 70 EB consumption (mmol/g-cat)
40
,
,
80
Figure 9. Relation of coke deposition with EB consumption for EB dehydrogenation in 20 h under different atmospheres over a series of V/A1203 catalysts with various vanadium loadings
5.2. Coke deposition
To search for the causes of catalyst deactivation, the amounts of coke deposition on V/Al2O3 after reaction on stream for 20 h were determined by the thermogravimetric analyses (TGA) under air [27]. As shown in Figure 9, the quantity of coke deposition is actually related to the catalyst activity and EB consumption; the larger the amounts of EB consumed (converted to ST or side products), the more is the coke deposited. There exists a roughly linear relation between the coke content and EB consumption, regardless of either the vanadium loading or the atmosphere for the reactions. At low vanadium loading (<1.50 mmol/g-A1203), C02 can alleviate the catalyst deactivation, despite the fact that it is not able to suppress the coke formation. At high vanadium loading (21.50 mmol/gAl2O3), however, COa can neither suppress the coke deposition nor alleviate the catalyst deactivation. The largest amounts of coke deposit on the catalyst V/Al2O3 with a vanadium loading of 1.50 mmol/gA1203 that exhibits the highest catalytic activity and
100
200
300
400 500 Temperature ('C)
600
700
800
Figure 10. H2-TPR of V/Alz03 (1) Fresh calcined sample, (2) Reduced sample through the first step of H2-TPR, (3) Reduced sample treated with COz at 550 "C for 2 h, (4) Used catalyst for EB dehydrogenation in COz at 550 "C for 20 h, (5) Used catalyst for EB dehydrogenation in Nz at 550 "C for 20 h, with a vanadium loading of 1.50 mmol/g-A1203, (6) Used catalyst with a vanadium loading of 3.00 mmol/g-A1~03for EB dehydrogenation in CO2 at 550 "C for 20 h
Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
5.3. Valence state of active vanadium species The catalyst characterization of XRD, H2-TPR, TGA, UV-Raman and UV-vis spectroscopy further proved the catalytic activity and deactivation extent of V/A1203 also depend on the dispersion and valence state of active vanadium species, which is related to the vanadium loading and reaction atmosphere [27]. With low vanadium loading, the surface vanadium species is still highly dispersed in V/A1203 and most vanadium species remain as oxidative state after EB dehydrogenation lasts for 20 h in the presence of CO2, as shown in Figure 10. At high vanadium loading, the surface vanadium species at the initial high oxidative state was reduced and can not be resumed considerably by C02 during the reaction. Therefore, deep reduction of the surface vanadium species is another cause of catalyst deactivation, which is actually associated with the vanadium loading of V/A1203 and reaction atmosphere. Surface vanadium species at high valence (V") with good reducibility are the active phase for the EB dehydrogenation in the presence of C02. With low vanadium loading (51.50 mmol/g-A1203), the moderate stable activity of V/A1203 for the EB dehydrogenation is attributed to the existence of renewable surface vanadium species at high valence; a major reason for the catalyst deactivation is the coke deposition that is related to the catalytic activity and EB consumption. With high vanadium loading (>1.50 mmol/g-A1203), on the other hand, the deep reduction of the surface vanadium species contributes significantly to the severe deactivation of V/A1203 for the EB dehydrogenation; the coke formation is reduced due to the low catalytic activity and low EB consumption, though it may remain one cause of the catalyst deactivation.
5.4. Action of carbon dioxide
As addressed above, both the deep reduction of the surface vanadium species and coke deposition contribute to the deactivation of V/A1203 for EB dehydrogenation. However, the reactions in the presence of various atmospheres are also different in the causes of catalyst deactivation. Coke deposition is directly related to the EB consumption or ST production and can not be effectively suppressed by CO2; therefore, the portion of catalyst deactivation due to coke deposition can not be alleviated by C02. Deep reduction of the surface vanadium species is another cause of catalyst deactivation, which is actu-
19
ally associated with the vanadium loading of V/A1203 and reaction atmosphere. The catalysts V/A1203 with low vanadium loading (51.50 mmol/g-A1203) own highly dispersed surface vanadium species with proper redox properties. For EB dehydrogenation in the presence of CO2 over V/A1203 with low vanadium loading (11.50 mmol/g-A1203), C02 as a weak oxidant can resume the oxidative state (lattice oxygen) of reduced vanadium species and then alleviate the portion of catalyst deactivation due to the deep reduction of the surface vanadium species. For the reaction over V/A1203 with high vanadium loading (>1.50 mmol/g-A1203), C02 can not resume the oxidative state (lattice oxygen) of reduced vanadium species effectively due to the poor redox properties, which results in the rapid deactivation with the time on stream (large difference between Yl and Y ~ o ) . In the presence of N2, the deactivation of V/A1203 does not exhibit such a sharp variance with the vanadium loading, because an inert atmosphere can not prompt the resumption of the oxidative state of the vanadium species regardless of the vanadium loading. Moreover, EB conversion in the presence of Nz keeps at a relatively low level all the time in comparison with that in the presence of CO2, which results in less coke deposition in despite of severe catalyst deactivation. 6. Closing remarks
Reaction coupling is an effective approach to improve the behavior of reactions confined by the thermodynamic and/or kinetic limits. Through the coupling of EB dehydrogenation with RWGS, the reaction behavior of EB dehydrogenation to ST in the presence of C02 can be improved largely. EB conversion can be enhanced at lower temperature (550 "C) and the energy consumption can be reduced signif icantly. The catalysts of iron and vanadium, supported on AC or A1203, with certain promoters proved to be effective for this coupling process. Although ST yield is always associated with COz conversion over various catalysts, the catalysts of iron and vanadium are different in the reaction mechanism. The twostep pathway plays an important role in the coupling process over Fe supported on AC or Al2O3, while the one-step pathway dominates the reaction over V supported on AC or A1203. Coke deposition and deep reduction of active cat-
20
Shuwei Chen et al./ Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
alyst components are the major causes of catalyst deactivation. Coke deposition is directly related to the EB consumption or ST production and can not be effectively suppressed by CO2, while deep reduction of the surface vanadium species is associated with the vanadium loading and reaction atmosphere. C02 can preserve the active species at high valence, and therefore alleviate the catalyst deactivation in the coupling reactions. The superiority of the reaction coupling in the presence of COz over the single dehydrogenation can be attributed to the fact that, COz, as a weak oxidant, can eliminate hydrogen produced during EB dehydrogenation, resume the oxidative state (lattice oxygen) of reduced vanadium species and alleviate the catalyst deactivation. Acknowledgements The authors are grateful for the financial support of The Sate Key Fundamental Research Project and the Natural Science Foundation of China.
References Towler G, Lynn S. Chem Eng Sci, 1994, 49: 2585 Lee E H. Rev Eng Sci, 1973, 8: 285 Matsui J, Sodesawa T, Nozaki F. Appl Catal A , 1991, 67: 179 Mimura N, Takahara I, SaitoT, Hattori T, Ohkuma K, Ando M. Catal Today, 1998, 45: 61 Mimura N,Saito M. Catal Lett, 1999, 58: 59 Mimura N,Saito M. Catal Today, 2000, 55: 173 Sun A L, &in Z F, Chen S W, Wang J G. Catal Today, 2004, 93-95:273 Sun A L, &in Z F, Chen S W, Wang J G. J Mol Catal A , 2004, 210: 189
[9] Chen S W, Sun A L, &in Z F, Wang J G. Catal Commun, 2003, 4: 441 [lo] Chang J, Vislovskiy V, Park M, Hong D, Yo0 J, Park S. Green Chem, 2003, 5: 587 [ll] Saito M, Kimura H, Mimura N, Wu J, Murata K. Appl Catal A Gen, 2003, 239: 71 [12] Vislovskiy V P, Chang J S, Park M S, Park S E. Catal Commun, 2002, 3: 227 [13] Sun A L, &in Z F, Wang J G. Appl Catal A Gen, 2002, 234: 179 [14] Sugino M, Shimada H, Turuda T, Miura H, Ikenaga N, Suzuki T. Appl Catal A Gen, 1995, 121: 125 [15] Badstube T, Papp H, Kustrowski P, Dziembaj R. Catal Lett, 1998, 55: 169 [16] Sakurai Y, Suzaki T, Ikenaga N, Suzuki T. Appl Catal A Gen, 2000, 192: 281 [17] Badstube T, Papp H, Dziembaj R, Kustrowski P. Appl Catal A Gen, 2000, 204: 153 [18] Sakurai Y, Suzaki T, Nakagawa K, Ikenaga N, Aota H, Suzuki T. J Catal, 2002, 209: 16 [19] Kusrowski P, Rafalska-Lasocha A, Majda D, Tomaszewska D, Dziembaj R. Solid State Ionics, 2001, 141-142:237 (201 Mimura N, Takahara I, Saito M, Sasaki Y, Murata K. Catal Lett, 2002, 78: 125 [21] Carja G, Nakamura R, Aida T, Niiyama H. J Catal, 2003, 218: 104 [22] Ye X, Ma N, Hua W, Yue Y, Mia0 C, Xie Z, Gao Z. J Mol Catal A , 2004, 217: 103 [23] Chang J S, Park S E, Park M S, Chem Lett, 1997: 1123 [24] Ohishi Y, Kawabata T, Shishido T, Takaki K, Zhang Q, Wang Y, Takehira K. J Mol Catal A , 2005, 230: 49 [25] &in Z F, Liu J G, Sun A L, Wang J G. Ind Eng Chem Res, 2003, 42: 1329 [26] Sun A L, &in Z F, Wang J G. Catal Lett, 2002, 79: 33 [27] Chen S W , &in Z F, Xu X F, Wang J G. Appl Catal A Gen, 2006, in press
DIRECT
@
Journal of Natural Gas Chemistry 15(2006)11-20
SClENCE PRESS
Review
Theoretical and Experimental Study on Reaction Coupling: Dehydrogenation of Ethylbenzene in the Presence of Carbon Dioxide Shuwei Chen,
Zhangfeng &in,
Ailing Sun,
Jianguo Wang*
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Shanxi 030001, China [Manuscript received November 21, 2005; revised December 8, 20051
Abstract: Dehydrogenation of ethylbenzene (EB) to styrene (ST) in the presence of COz, in which EB dehydrogenation is coupled with the reverse water-gas shift (RWGS), was investigated extensively through both theoretical analysis and experimental characterization. The reaction coupling proved to be superior to the single dehydrogenation in several respects. Thermodynamic analysis suggests that equilibrium conversion of EB can be improved greatly by reaction coupling due to the simultaneous elimination of the hydrogen produced from dehydrogenation. Catalytic tests proved that iron and vanadium supported on activated carbon or A1203 with certain promoters are potential catalysts for this coupling process. The catalysts of iron and vanadium are different in the reaction mechanism, although ST yield is always associated with COz conversion over various catalysts. The two-step pathway plays an important role in the coupling process over Fe/A12Og, while the one-step pathway dominates the reaction over V/A1203. Coke deposition and deep reduction of active components are the major causes of catalyst deactivation. COZ can alleviate the catalyst deactivation effectively through preserving the active species at high valence in the coupling process, though it can not suppress the coke deposition. Key words: reaction coupling; ethylbenzene dehydrogenation; styrene; carbon dioxide; water-gas shift; reaction mechanism; catalyst deactivation
1. Introduction An ideal process of chemical reaction requests high yield and selectivity of intent products, stable catalysts, mild reaction conditions, and low energy consumption. Moreover, the concepts of green chemistry in the recent years emphasize the atomic economic and environmental benignity for a reaction. However, some reactions are confined thermodynamically and/or kinetically, which may necessitate severe reaction conditions and high energy consump tion and then suffer from low efficiency; while some other reactions are strongly exothermic, which may cause certain troubles t o the reactor design and operation of the heat exchange units. Reaction coupling is an effective approach to improve the behaviors of
reaction confined by the thermodynamic and/or kinetic limits [l].By coupling a dehydrogenation reaction with a hydrogenation reaction, an endothermic reaction with an exothermic reaction, or a reductive reaction with a n oxidative reaction, the equilibrium conversion can be improved greatly and the reaction temperature and the energy consumption can be reduced. Styrene (ST) is commercially produced by the dehydrogenation of ethylbenzene (EB) on the promoted iron oxide catalysts at 600 "C-700 "C, just below the temperature where thermal cracking becomes significant. Due to its highly endothermic and volume-increasing character, a large amount of superheated steam is used to supply heat, lower the partial pressure of the reactant, and avoid the formation of
* Corresponding author. Tel: +86-351-4046092; Fax: +86-351-4041153; E-mail: [email protected].
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Shuwei Chen et al./ Journal of Natural Gas Chernistr.y Vol. 15 No. 1 2006
carbonaceous deposits [2,3]. However, much of the latent heat of steam is lost in the gas-liquid separator. The dehydrogenation of EB to ST in the presence of COz instead of steam, in which EB dehydrogenation is coupled with the reverse water-gas shift (RWGS), is believed to be an energy-saving and environmentally friendly process. EB conversion can be enhanced at a lower temperature (550 "C) and the energy consumption can be reduced significantly. It is estimated that the energy required for producing one ton ST in the new process using C02 is about (6.37.9)x108 J, much lower compared with 6 . 3 ~ 1 0J ~in the current commercial process [4&6]. Since the commercial Fe-Cr-K catalysts do not work effectively in such a coupling system, the catalysts have been screened extensively [4-24]. Fe, V oxides supported on activated carbon (AC) [13-171, V oxide supported on MgO [HI, hydrotalcite-like compounds (Mg-Al-Fe, Mg-A1-V) [19-221, and Fe oxide supported on ZrO2 exhibited high initial catalytic activity for EB dehydrogenation in the presence of C02, but suffered from severe deactivation even for several hours operation. Comparatively, Fe and V oxides supported on A1203 were much more stable together with reasonable activity [4-121. However, the catalysts prepared so far may still not meet the requests for the commercialization, and there exist controversies on the reaction mechanism and causes of catalyst deactivation. To make such a coupling process practicable, we have carried out an extensive investigation on EB dehydrogenation in the presence of C 0 2 through both theoretical analysis and experimental characterization [7-9,13,25-271. With the thermodynamic analysis, the superiority of reaction coupling was displayed and possible reaction pathways were suggested [8,13,25];Potential catalysts for the coupling reaction system were obtained [7,9,13]through extensive catalyst screening; and through catalytic tests and various characterizations, the reaction mechanism and causes of catalyst deactivation were revealed [8,13,27]. In this paper, our efforts on this coupling process were presented to get a better understanding on the benefits of reaction coupling, reaction mechanisms, causes of catalyst deactivation, and roles of CO2 in the coupling system. 2. Thermodynamic analysis
2.1. Advantages of reaction coupling
The dehydrogenation of EB to ST can be expressed by:
When it is coupled with RWGS,
the effects of reaction coupling on the equilibrium EB conversion can be evaluated thermodynamically [25]. The dehydrogenation of EB to ST is an endothermic and volume-increasing reaction; high temperature and low pressure are favored for the conversion of EB. Figure 1 gives the equilibrium conversion of EB in the case of simple dehydrogenation, nitrogen dilution and coupling with RWGS a t different temperatures and different mole ratios of C02 to EB under 0.1 MPa. It can be seen that the equilibrium conversion is quite low in the case of single EB dehydrogenation even a t high temperatures, for example, 69.7% at 690 "C. The conversion can be increased obviously by introducing nitrogen into the reaction system because of the decrease of the partial pressures of all the components in the system, which is one of the reasons for using large amounts of steam in the commercial process. However, this enhancement by dilution is limited, for example, EB conversion increases from 25.2% to 57.8% through dilution by nitrogen (initial mole ratio of Nz/EB=10) at 550 "C.
300
400
500 Temperature ( C )
600
700
Figure 1. Reaction coupling of EB dehydrogenation with RWGS: the effects of the feed composition (in mole ratio) and temperature on the equilibrium conversion of EB at 0.1 MPa (1) EB:C02=1:10, (2) EB:C02=1:5, ( 3 ) EB:C02=1:1, (4) EB:N2=1:10, ( 5 ) EB only
13
Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
To enhance the conversion of EB or reduce the operating temperature considerably, it is necessary to eliminate the produced hydrogen in situ by coupling with hydrogen-consuming reactions like RWGS. As shown in Figure 1, the conversion is improved greatly by coupling with RWGS. Moreover, the conversion increases with an increase in the ratio of C02/EB. The conversions are 63.9% and 82.4% at 500 "C and 550 OC, respectively, in the case of the mole ratio of C02/EB being 10. The margin between the lines of dilution and coupling suggests the superiority of the coupling over dilution. It can be then concluded that the reaction coupling can improve the equilibrium conversion of EB at a certain temperature or reduce the operating temperature at a certain ST yield.
two-step pathway is a little higher than that via the one-step pathway at 350 "C-600 "C. In contrast, the COz conversion via the two-step pathway is lower at a temperature higher than 450 "C. Moreover, there will be no hydrogen in the effluent gas when the reaction follows solely the one-step-pathway.
2.2. Different pathways suggested by thermodynamic analysis
There exists always a controversy on the roles of C02 in EB dehydrogenation in the presence of CO2: whether CO2 interacts directly with EB, or just reacts with hydrogen after it is released from EB dehydrogenation. As proposed by Mimura et al. [5], EB dehydrogenation in the presence of CO2 may proceed in two pathways called one-step pathway and two-step pathway. For the one-step pathway, EB is oxidized t o ST with C02 through the direct interaction of CO2 and EB, CsH5-C2H5 f CO2
--+
+
C ~ H ~ - C ~ HCO S
+ HzO
(3) whereas for the two-step pathway, EB is first dehydrogenated to ST with hydrogen formed simultaneously, and then COz reacts with hydrogen via the RWGS,
-
step 1
+
C ~ H S - C ~------+ H ~ C ~ H S - C ~ HH2~ step 2 H2 CO2 CO H20
+
+
300
500
400
600
700
Temperature ( C )
Figure 2. Effects of reaction pathways on the equilibrium conversions of EB (a) and COz (b) under 0.1 MPa: for EB dehydrogenation in the presence of Nz (no coupling ( l ) ) , initial NZ:EB=11; for the coupled EB dehydrogenation in the presence of C02 (in one step (2) or two steps (3)), initial COZ:EB=11
3. Catalyst screening
(4)
Since hydrogen produced in the EB dehydrogenation is eliminated simultaneously by RWGS, the E B conversion can be then improved through the shift of dehydrogenation equilibrium. Through the thermodynamic calculation, the dependence of the equilibrium conversions of EB and C02 for the coupled EB dehydrogenation in the presence of COz through different pathways can be evaluated [8,25].As shown in Figure 2, the equilibrium conversions of EB with the coupling via either pathway are higher than that of EB dehydrogenation in the presence of nitrogen that has only a dilution effect. EB conversion of the coupled dehydrogenation via the
Since the commercial Fe-Cr-K catalysts for EB dehydrogenation with steam do not work effectively in such a coupling system, the catalysts with various supports, active components and promoters were then screened. The catalysts in this work were prepared by impregnation or co-impregnation method [7,8,13]. AC, A1203, Si02, as well as various zeolites like ZSM-5 were used as catalyst supports. The active components Fe, Cu, Pt, V were introduced by the impregnation of supports with aqueous solutions of iron nitrate, copper nitrate, chloroplatinic acid and NH4V03 with oxalic acid, respectively. Alkali metal (Li, K), alkaline earth metal (Ca), transition metal (Mn, Cr, Zr)
14
Shuwei Chen et al./ Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
and rare earth metal (La) promoted catalysts were prepared by co-impregnation of a solution containing both the active components and nitrates of the promoters. The catalytic reaction was performed in a stainless steel tube reactor with an inner diameter of 6.0 mm, and about 200 mg catalyst was used per run [7,8,13]. The feed of EB (20.4 mmolEB/(gCat.h) and CO2 were introduced through a micro feeder pump and mass flow controller, respectively. The molar ratio of C02 to EB was fixed at 11. The reaction was generally operated at 550 "C at atmospheric pressure. The effluents, including ST, benzene, toluene, and unreacted EB from the reactor were condensed in a trap by an ice water bath and analyzed with a FID gas chromatograph (Shimadzu GC-7A) equipped with a 3 mmx3 m stainless-steel column of OV-101. The gaseous products (H2, N2, CO, C02, CH4, and C2Hs) were analyzed on-line by a TCD gas chromatograph (Shimadzu GC-9A) equipped with a 3 mmx 3 m stainless-steel column of carbon molecular sieve.
3.1. Supports The catalytic tests over catalysts of various supports revealed that AC and A1203 with high surface area and little acidity are proper catalyst supports for EB dehydrogenation in the presence of C02 at 550 "C [7,8,13]. ZSM-5 supported catalysts give very low selectivity to ST because of the cracking of EB on its acidic sites. AC itself also shows a low activity (ST yield 9.8%) for the coupled reaction. Fe or V supported on AC exhibits high catalytic activity, but suffers from severe deactivation even for several hours' operation. Comparatively, A1203 supported catalysts is much more stable together with reasonable activity. 3.2. Active components
The catalytic behaviors of Fe, V, P t and La supported on AC for EB dehydrogenation in the presence of C02 at 550 "C are listed in Table 1. All these catalysts give high selectivity to ST, but only Fe and V show excellent activity [13]. Similar results were obtained for the catalytic tests over A1203 supported catalysts [7,8]. As shown in Figure 3, for EB dehydrogenation in the presence of COa, Fe and V catalysts show much better activity than P t and Cu catalysts, although all the catalysts give high selectivity to ST. With the time on stream,
the ST yield over both Fe and V catalysts showed a decrease tendency, but the activity of V catalyst decreased much more slowly than that of Fe catalyst. The ST yield over Pt and Cu catalysts increases slowly, but remains at a low level with the time on stream. V supported on A1203 showed the best catalytic behavior among the catalysts investigated, with the ST yield of initial about 45% and still higher than 40% after the reaction lasted for 30 h at 550 "C. Table 1. C a t a l y t i c behaviors of AC-supported c a t a l y s t s w i t h different active components for the EB dehydrogenation i n the presence of C O z Active
EB conversion
ST selectivity
components* Fe(3)/AC
(%I
(%)
(%I
38.0
95.5
36.3
ST yield
V(0.87)/AC
52.0
97.3
50.6
Pt (0.015)/ AC
16.0
97.3
15.6
La(0.75)/AC
21.0
97.8
20.6
Reaction conditions: 550 "C, atmospheric pressure, W/F=4.07 (gcat.h)/mol, COZ/EB=ll; data were acquired after reaction lasted for 1 h. * Numerals in the parentheses indicate loadings of metal species in mmol/g-support. 50
0
5
10
15
20
25
30
Time on stream (h)
F i g u r e 3. C a t a l y t i c behaviors of a l u m i n a s u p p o r t e d c a t a l y s t s w i t h various a c t i v e components for EB dehydrogenation i n the presence of coz (1) Fe(3), (2) V(0.86), (3) Pt(O.Ol), (4) Cu(2.75) Reaction conditions: 550 "C, W/F=4.07 (g,,t.h)/mol, C02/EB = 11, and the numerals in the legend indicate the loadings of metal species in mmol/g-support
3.3. Promoters Various promoters were introduced to improve the catalytic behavior of the supported catalysts for EB dehydrogenation in the presence of CO2. As listed in Table 2 [13], over the catalysts of Fe and V as active components and AC as support, both the conver-
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Journal of Natural Gas Chemistry VoI. 15 No. 1 2006
sion of EB and the selectivity to ST are enhanced significantly with small amounts of alkali, alkaline earth or rare-earth elements as promoters. With the Lipromoted Fe/AC catalyst, EB conversion of 50% and ST selectivity of 98.3% were observed at 550 "C. With La-promoted V/AC catalyst, EB conversion and ST selectivity reached 64% and 97%, respectively. Figure 4 showed the effects of various promoters on the catalytic performances of Fe/A1203 and V/A1203 [7,8]. The catalytic activity of Fe/A1203 can be promoted by the addition of Ca, Mn and Cr, and the highest initial ST yield of about 48% is achieved over Fe(3)Cr(0.3)/A1203. When mixed with vanadium, the activity of Fe/A1203 can be further improved. On the other hand, the addition of Li, Mn and Zr has little influence on the activity of V/Al2O3 catalysts, while the addition of La (0.3 mmol/g) even leads to a decrease in the catalytic ac-
tivity. Among the promoters investigated, only Cr shows an evidently positive effect on the ST yield, which increases from 45% over V(0.57)/A1203 to 50% over V(O.57)Cr(0.3)/A1203. This may be due to that Cr is not only an active component for the dehydrogenation, but also a n effective promoter t o the RWGS. Table 2 . Effect of promoters on the catalytic behaviors of AC-supported Fe and V c a t a l y s t s Active
EB conversion ST selectivity ST yield
components Fe(3)/AC
(760)
(%I
(76)
38.0
95.5
36.3
Fe(3)Li(0.3)/AC
50.0
98.3
49.2
Fe(3)Na( 0.3)/AC V(0.87)/AC V(0.87)Li(O.l5)/AC V(0.87)Mg(0.25)/AC
46.0 52.0 56.0 60.0
98.3 97.3 97.3 97.4
45.2 50.6 54.5 58.4
V(0.87)La(O.O5)/AC
64.0
96.8
62.0
Notes are the same as those in Table 1
50
50
-
h
&
z....
x
45-
2 40 -
35 0
1
2
3
4
5
6
1
8
9
Time on stream (h)
"
0
"
1
'
1
I
2
3
4 5 6 Time on stream (h)
1
I
I
8
I
9
F i g u r e 4. C a t a l y t i c behaviors of Fe(3)/A1203 ( a ) and V(0.57)/A1203 (b) modified w i t h various promoters for EB dehydrogenation i n the presence of C02 (a) (1) None, (2) Ca(0.3), (3) Cr(0.3), (4) Mn(0.3); (b) (1) None, (2) Li(0.3), (3) Cr(0.3), (4) Mn(0.3), (5) Fe(2), (6) Zr(0.3), (7) La(0.3) Reaction conditions: 550 "C, W/F=4.07 (gcat.h)/mol, COZ/EB=11, and the numerals in the legend indicate the loadings of metal species in mmol/g-A1203
4. Reaction mechanism 4.1. Benefits of reaction coupling
In order to elucidate the superiority of reaction coupling, the dehydrogenation of EB was carried out in the presence of CO2 and N2, respectively, over commercial Fe-K catalyst, Fe/AC, FeLi/AC, V/A1203, VCr/A1203 and Fe/A1203 a t 550 "C "7,131. As listed
in Table 3, the commercial catalyst Fe-K shows very low activity for EB dehydrogenation in the presence of N2 or C02 at 550 "C (even with a C02/EB ratio being as high as 41). The selectivity to ST is also the lowest among the catalysts investigated. On other catalysts, the conversion of EB in the presence of C02 is obviously higher than that in the presence of Nz. Just as suggested by thermodynamic analysis [8,25], the conversion of EB can be improved greatly
16
Shuwei Chen et al./ Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
by the reaction coupling with RWGS. Moreover, over FeLi/AC and VCr/A1203 catalysts, the ST yield in the presence of C02 is always higher than that in the presence of nitrogen within 30 h on stream. The
catalyst stability as well as its activity in the presence of C02 is much better than that in the presence of N2, which proved the superiority of the reaction coupling.
Table 3. EB dehydrogenation in the presence of COz or Nz over various catalysts In the presence of C02 EB conversion [%) ST selectivity [%)
Catalysts Fe-K"
1.5
70.0
Fe (3)/AC Fe(3)Li(0.3)/AC
38.0 50.0
95.5 98.3
V(0.57)/A1203 V(0.57)Cr(0.3)/A1203
45.5 50.3
97.8 97.8
35.0
96.3
Fe(3)/A12 0
3
In the presence of N2 EB conversion [%) ST selectivity f%) 1.3 22.0 24.0
72.0 78.8 83.2
37.4
98.1
43.0 34.2
97.0 96.2
Reaction conditions: 550 'C, atmospheric pressure, W/F=4.07 (gcat.h)/mol, C02/EB=11 or N2/EB=11 for the reaction in the presence of CO2 or N2, respectively; data were acquired after reaction lasted for 1 h. * Commercial catalyst for EB dehydrogenation.
4.2. Reaction pathways 50
As mentioned above, there exists two possible reaction pathways for EB dehydrogenation in the presence of COz, namely one-step and two-step pathways. However, it is not easy to distinguish the one-step from the two-step pathways by the catalytic tests. CO (together with the stoichiometric amount of water) and Hz are simultaneously observed in the products for the EB dehydrogenation in the presence of CO2. This implies that the one-step and two-step pathways may coexist in the coupled reaction (Figure 5).
f
h
s
v
9
.E
40
-
35
-
v1
6
I \
5 -
c,H,-CH,-CH,
*2H
(2-1)
-
H,
Figure 5. Role of COz in the coupled El3 dehydrogenation in the presence of COz
43 -
2-
A comparison of ST yields and CO2 conversions for the coupled EB dehydrogenation in the presence of C02 over catalysts of Fe or V supported on A1203 suggests that EB conversion is associated with COz conversion, as shown in Figure 6, the higher the COz conversion is, the higher is the ST yield obtained. The conversion of CO2 also shows the same tendency as the conversion of EB with the time on stream. This means COz participates actively in the coupled reactions and there exists a synergistic effect between the EB dehydrogenation and C02 conversion in the coupling system.
1
L
0
1
2
3
4 5 6 Time on stream (h)
7
8
9
10
Figure 6. ST yield (a) and COz conversion (b) with the time on stream for the coupled EB dehydrogenation in the presence of C O z over the A1203 supported catalysts (1) Fe(3), (2) Fe(3)Ca(0.3), (3) Fe(3)Cr(0.3), (4) V(0.57), (5) V(0.57)Cr(0.3) Reaction conditions: 550 "C, 0.1 MPa, C02/EB=11, and W/F=4.07 (g,,t .h)/mol; the element labels and numerals in parentheses indicate the active components and their loadings in mmol/g-support
Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
However, these catalysts act differently for the single RWGS (with only H2 and COz in the feed), as shown in Figure 7. For the single RWGS, Fe supported on Alz03, especially promoted by Cr, was an effective catalyst; while V supported on Alz03 behaved badly. Contrary to that, for the coupled EB dehydrogenation, V supported on A1203showed much higher C02 conversion, which thereon gave also good activity for the EB dehydrogenation to ST (Figure 6). 8
II
1
\
t t
.,
I I
- - - --
2
0
(1)
1
2
3 4 5 Time on stream (h)
I 6
7
8
Figure 7. COz conversions in the single RWGS over the A1203 supported catalysts (1) V(0.57), (2) V(0.57)Cr(0.3), (3) Fe(3), (4) Fe(3)Cr(0.3) Reaction conditions: 550 "C, 0.1 MPa, C02/H2=11, and W/F=2.04 (gcat.h)/mol; the element labels and numerals in parentheses indicate the active components and their loadings in mmol/g-support
Thus, it was suggested that the catalysts of iron and vanadium are different in the reaction mechanism, although ST yield is always associated with COz conversion over various catalysts [8]. For the EB dehydrogenation in the presence of COz, there perhaps exists two kinds of hydrogen species formed from EB dehydrogenation, namely, active hydrogen species (atomic) and molecular hydrogen. CO2 could either come into contact directly with the active hydrogen species or react with the molecular hydrogen, which corresponded to the one-step and two-step pathways, respectively. The two-step pathway plays an important role in the coupling process over Fe/A1203, while the one-step pathway dominates the reaction over V/A1203. Similar phenomena are observed for the EB dehydrogenation in the presence of COYover the catalysts of Fe or V supported on AC [8]. More characterizations like C02-TPD and H2T P R proved that CO2 can be activated through either the basic sites or the redox sites on the catalyst sur-
17
face [8]. Over Fe supported catalyst with Cr as promoter, the strong basic sites may play an important role; while over V/A1203, the redox cycle is crucial for the COz activation. 5 . Catalyst deactivation
One of the major problems that restrain the process of EB dehydrogenation in presence of COz from being practical is the rapid deactivation of catalyst. The deactivation during reaction may come from the deposition of coke, sintering of active sites and change in the valence state of active vanadium species. In order to explore the causes of catalyst deactivation, a series of catalytic tests for EB dehydrogenation in presence of COz or N2 over V/A1203 catalysts with different vanadium loadings was carried out [27]. 5.1. Effects of Vanadium loading on the catalyst deactivation As shown in Figure 8, the catalytic activity decreases with the time on stream, and the ST yield after 20 h reaction (Yzo)was much lower than the initial ST yield (Y1). The catalytic activity and the extent of deactivation depend on the vanadium loadings and reaction atmospheres [27]. Figure 8 showed that the deactivation of V/A1203 for EB dehydrogenation is distinctly influenced by the vanadium loading. At vanadium loading below 1.50 mmol/g-A1203, the extent of deectivation in 20 h (the difference between Y1 and YzO) increases only slightly with the vanadium loading. However, the catalytic activity is enhanced significantly by the increase in vanadium loading, as indicated by the evident increase in the ST production during the 20 h reaction with the vanadium loading. As the vanadium loading exceeds 1.50 mmol/g-A1203, the extent of deactivation in 20 h increases sharply with the vanadium loading, accompanied with a severe decrease in the catalytic activity and the ST production. These suggested that a t low vanadium loading, the activity of V/A1203 is relatively stable for EB dehydrogenation; the increase in the extent of catalyst deactivation with the vanadium loading is ascribed to the increase in the quantities of ST produced (ST production) or EB consumed (EB consumption) in the same reaction period. However, the catalysts V/A1203 with vanadium loading above 1.50 mmol/g-A1203 suffer from severe deactivation. The presence of CO2 can alleviate the
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Shuwei Chen et al./ Journal of Natural Gas Chemistr.y Vol. 15 No. 1 2006
deactivation of catalysts, especially those with a vanadium loading below 1.50 mmol/g-A1203.
ST production during the same period of reaction. The catalyst V/A1203 with high vanadium loading deactivates severely during the reaction, but the coke deposition was not so severe. These indicate that the coke formation is far from the only cause of catalyst deactivation, although coke deposition may result in decreases in the specific surface area and catalytic activity of V/A1203. There are other factors that cause catalyst deactivation. 350 300 u
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? 250
8 B
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40
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8
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,
,
,
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,
,
1.o 1.5 2.0 2.5 Vanadium loading (mmol / g-Al,O,)
I
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I
,
,
I
200
150
,
3.0
Figure 8. Effects of vanadium loading on the ST yield for EB dehydrogenation over V/A1203 in the presence of COz (a) and Nz (b) (1) Yl, (2) yzo Reaction conditions: 550 "C, 0.1 MPa, COz or Nz/EB=20 (mole ratio), and W/F=8.75 (gcat,h)/mol; Y1 and YZOwere the ST yields acquired after reaction lasted for 1 h and 20 h, respectively
100 I
,
,
,
,
l
,
,
,
,
I
,
,
50 60 70 EB consumption (mmol/g-cat)
40
,
,
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Figure 9. Relation of coke deposition with EB consumption for EB dehydrogenation in 20 h under different atmospheres over a series of V/A1203 catalysts with various vanadium loadings
5.2. Coke deposition
To search for the causes of catalyst deactivation, the amounts of coke deposition on V/Al2O3 after reaction on stream for 20 h were determined by the thermogravimetric analyses (TGA) under air [27]. As shown in Figure 9, the quantity of coke deposition is actually related to the catalyst activity and EB consumption; the larger the amounts of EB consumed (converted to ST or side products), the more is the coke deposited. There exists a roughly linear relation between the coke content and EB consumption, regardless of either the vanadium loading or the atmosphere for the reactions. At low vanadium loading (<1.50 mmol/g-A1203), C02 can alleviate the catalyst deactivation, despite the fact that it is not able to suppress the coke formation. At high vanadium loading (21.50 mmol/gAl2O3), however, COa can neither suppress the coke deposition nor alleviate the catalyst deactivation. The largest amounts of coke deposit on the catalyst V/Al2O3 with a vanadium loading of 1.50 mmol/gA1203 that exhibits the highest catalytic activity and
100
200
300
400 500 Temperature ('C)
600
700
800
Figure 10. H2-TPR of V/Alz03 (1) Fresh calcined sample, (2) Reduced sample through the first step of H2-TPR, (3) Reduced sample treated with COz at 550 "C for 2 h, (4) Used catalyst for EB dehydrogenation in COz at 550 "C for 20 h, (5) Used catalyst for EB dehydrogenation in Nz at 550 "C for 20 h, with a vanadium loading of 1.50 mmol/g-A1203, (6) Used catalyst with a vanadium loading of 3.00 mmol/g-A1~03for EB dehydrogenation in CO2 at 550 "C for 20 h
Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
5.3. Valence state of active vanadium species The catalyst characterization of XRD, H2-TPR, TGA, UV-Raman and UV-vis spectroscopy further proved the catalytic activity and deactivation extent of V/A1203 also depend on the dispersion and valence state of active vanadium species, which is related to the vanadium loading and reaction atmosphere [27]. With low vanadium loading, the surface vanadium species is still highly dispersed in V/A1203 and most vanadium species remain as oxidative state after EB dehydrogenation lasts for 20 h in the presence of CO2, as shown in Figure 10. At high vanadium loading, the surface vanadium species at the initial high oxidative state was reduced and can not be resumed considerably by C02 during the reaction. Therefore, deep reduction of the surface vanadium species is another cause of catalyst deactivation, which is actually associated with the vanadium loading of V/A1203 and reaction atmosphere. Surface vanadium species at high valence (V") with good reducibility are the active phase for the EB dehydrogenation in the presence of C02. With low vanadium loading (51.50 mmol/g-A1203), the moderate stable activity of V/A1203 for the EB dehydrogenation is attributed to the existence of renewable surface vanadium species at high valence; a major reason for the catalyst deactivation is the coke deposition that is related to the catalytic activity and EB consumption. With high vanadium loading (>1.50 mmol/g-A1203), on the other hand, the deep reduction of the surface vanadium species contributes significantly to the severe deactivation of V/A1203 for the EB dehydrogenation; the coke formation is reduced due to the low catalytic activity and low EB consumption, though it may remain one cause of the catalyst deactivation.
5.4. Action of carbon dioxide
As addressed above, both the deep reduction of the surface vanadium species and coke deposition contribute to the deactivation of V/A1203 for EB dehydrogenation. However, the reactions in the presence of various atmospheres are also different in the causes of catalyst deactivation. Coke deposition is directly related to the EB consumption or ST production and can not be effectively suppressed by CO2; therefore, the portion of catalyst deactivation due to coke deposition can not be alleviated by C02. Deep reduction of the surface vanadium species is another cause of catalyst deactivation, which is actu-
19
ally associated with the vanadium loading of V/A1203 and reaction atmosphere. The catalysts V/A1203 with low vanadium loading (51.50 mmol/g-A1203) own highly dispersed surface vanadium species with proper redox properties. For EB dehydrogenation in the presence of CO2 over V/A1203 with low vanadium loading (11.50 mmol/g-A1203), C02 as a weak oxidant can resume the oxidative state (lattice oxygen) of reduced vanadium species and then alleviate the portion of catalyst deactivation due to the deep reduction of the surface vanadium species. For the reaction over V/A1203 with high vanadium loading (>1.50 mmol/g-A1203), C02 can not resume the oxidative state (lattice oxygen) of reduced vanadium species effectively due to the poor redox properties, which results in the rapid deactivation with the time on stream (large difference between Yl and Y ~ o ) . In the presence of N2, the deactivation of V/A1203 does not exhibit such a sharp variance with the vanadium loading, because an inert atmosphere can not prompt the resumption of the oxidative state of the vanadium species regardless of the vanadium loading. Moreover, EB conversion in the presence of Nz keeps at a relatively low level all the time in comparison with that in the presence of CO2, which results in less coke deposition in despite of severe catalyst deactivation. 6. Closing remarks
Reaction coupling is an effective approach to improve the behavior of reactions confined by the thermodynamic and/or kinetic limits. Through the coupling of EB dehydrogenation with RWGS, the reaction behavior of EB dehydrogenation to ST in the presence of C02 can be improved largely. EB conversion can be enhanced at lower temperature (550 "C) and the energy consumption can be reduced signif icantly. The catalysts of iron and vanadium, supported on AC or A1203, with certain promoters proved to be effective for this coupling process. Although ST yield is always associated with COz conversion over various catalysts, the catalysts of iron and vanadium are different in the reaction mechanism. The twostep pathway plays an important role in the coupling process over Fe supported on AC or Al2O3, while the one-step pathway dominates the reaction over V supported on AC or A1203. Coke deposition and deep reduction of active cat-
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Shuwei Chen et al./ Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
alyst components are the major causes of catalyst deactivation. Coke deposition is directly related to the EB consumption or ST production and can not be effectively suppressed by CO2, while deep reduction of the surface vanadium species is associated with the vanadium loading and reaction atmosphere. C02 can preserve the active species at high valence, and therefore alleviate the catalyst deactivation in the coupling reactions. The superiority of the reaction coupling in the presence of COz over the single dehydrogenation can be attributed to the fact that, COz, as a weak oxidant, can eliminate hydrogen produced during EB dehydrogenation, resume the oxidative state (lattice oxygen) of reduced vanadium species and alleviate the catalyst deactivation. Acknowledgements The authors are grateful for the financial support of The Sate Key Fundamental Research Project and the Natural Science Foundation of China.
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