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Characteristics of a V2O5 coated membrane reactor for the selective oxidation of 1-butene
~
A PT PA LE IY DSS CA L I A: GENERAL
ELSEVIER
Applied Catalysis A: General 156 (1997) 239-252
Characteristics of a V 2 0 5 coated membrane reactor for the selective oxidation of 1-butene S.I. Hong a, J.H. J u n g b'* aDepartment of Chemical Engineering, Korea University, Seoul 136-701, South Korea bDepartment of Chemical Engineering, Kyonggi University, Suwon-si, Kyonggi-do 442-760, South Korea Received 5 August 1996; received in revised form 16 January 1997; accepted 22 January 1997
Abstract The selective oxidation of butene to maleic anhydride has been carried out in a V205 coated membrane reactor. From the V205 sol made by fusing and melting, the V205 coated membrane with a uniform surface without crack is prepared on support. The mean pore diameter and specific surface area of the prepared membrane is 30 A and 1.1 m2/g, respectively. When the V205 coated membrane was heated to 450°C, the V205 [010] plane was grown selectively, and at around 500 nm the charge was changed from 0 2 - to V 5+ in the selective oxidation reaction that produced maleic anhydride from butene. The maximum selectivity of 95% was obtained. This high selectivity shows that the V205 coated membrane reactor is very efficient for the selective oxidation.
Keywords: Membrane reactor; Selective oxidation; Vanadium pentoxide; Sol-gel
1. Introduction Selective oxidation of butane or butene to maleic anhydride is the most industrially important reaction among the reactions using V205 catalyst [1,2]. When maleic anhydride is produced from Ca hydrocarbon, the selectivity of maleic anhydride is low, because selective oxidation and total oxidation always proceed competitively. Though fixed bed reactors or fluidized bed reactors have been mainly used for selective oxidation for the past 30 years, the selectivity of maleic anhydride has not been higher than 69% [3]. Recently developed membrane reactors have been introduced because they offer some advantages. A membrane reactor plays the role of both catalyst and * Corresponding author. 0926-860X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 8 6 0 X ( 9 7 ) 0 0 0 5 1 - 3
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membrane, and comes in two forms [4]. First, one of the products is separated by membrane, and chemical equilibrium is shifted, as a result the conversion is increased. This type of reactor can be used for water gas shift reaction, dehydrogenation and so on [5,6]. Second, the formation of byproducts is suppressed due to presence of membrane between the two reactants, so that selectivity is improved. This second type can be applied for the selective oxidation and hydrogenation reaction that produce a lot of byproducts during the reaction, but has not been adequately studied compared to the first type of membrane reactor. Therefore it is very important to prepare the second type membrane reactor using V205 catalyst for selective oxidation. V205 coated film has been mostly studied to characterize semiconductor, not membrane [7,8]. The V205 coated film can be prepared by RF sputtering [8], vacuum evaporation [9], CVD [10] and sol-gel [7,11]. Among these methods, the sol-gel process is the most appropriate method for preparation of membrane reactors, because it is readily applicable to tubular supports. V205 sol is prepared mostly using vanadium alkoxide (VO(OC3H7)3) [12], and can also be made by passing vanadic acid through ion exchange resin and polymerizing it [13]. Another method for the preparation of V205 sol is to pour the melted V205 into distilled water, which uses the property that amorphous V205 dissolves easily in water [7]. The objective of this study was to prepare the V205 coated membrane reactor using sol-gel method, and to investigate reaction characteristics of the prepared membrane reactor through the selective oxidation reaction of butene to maleic anhydride. The instrumental analysis of the V205 membrane has been also performed to find the optimum conditions in the preparation of membrane reactors.
2. Experimental 2.1. Preparation of a V205 coated membrane reactor Sol-gel method was selected for preparing V205 membrane, because we obtain relatively thick and dense V205 films. V205 sol was prepared by dissolving the melted V205 into distilled water [7]. As a support of the membrane reactor a Vycor® tube (250 mm long and 7 mm wide) with a quartz tube connected to either end was used. In order to coat the support interior with V205 sol, one end of the support was connected to syringe and the other end of the tube was put into V205 sol. After the sol was sucked in, it was allowed to stay within the support for certain time, and then pushed the remaining solution out. The thickness of the coated membrane was controlled by the length of time the V205 sol remaining in the support, and by the viscosity of sol. The coated tube was dried at room temperature for 24 h, and in order to crystallize the V205
S.I. Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252
241
membrane, the furnace temperature was raised by 1°C per minute and kept at 450°C for 1 h. The V205 membrane was also prepared by dip coating the V205 sol on silicon wafer to characterize the membrane surface. 2.2. Apparatus of the membrane reactor
The V205 coated Vycor connected to the quartz tube was fixed within the 1/2 inch wide quartz tube by using CAJON® ultra-tort fitting. Both nitrogen, the transport gas, and butene were passed through the Vycor tube under the application of high-pressurized air to the outside of the Vycor tube. The oxygen and the butene were met and interacted on the surface of the V205 coated membrane. At this time the concentration of the mixture of butene and nitrogen was sustained at 1%, and the surface velocities (volumetric flow rate/surface area of reactor) of butene and nitrogen were set to 500 cm/h and 200 cm/h, respectively. The reaction was performed at temperatures between 200°C and 450°C. Maleic anhydride and unreacted compounds were analyzed by on-line gas chromatography. 2.3. Characterization
Both the specific surface area and the pore size distribution were measured by adsorbing nitrogen while the existence of macro pores was confirmed by mercury porosimetry. The crystal structure of V205 as a function of the crystallization temperature and time was investigated by X-ray diffractometry. The weight loss and the crystallization temperature were determined using TG and DSC. In order to observe the magnitude of charge transition of the V205 coated membrane during heat treatment, UV-Diffuse Reflectance Spectroscopy (DRS) was used and MgO was chosen as a reference. The morphology of the V205 sol and the pore size and crystallinity of the V205 coated membrane were observed by TEM, while the thickness of the V205 coated membrane was measured by SEM. To analyze the unreacted butene and maleic anhydride in the membrane reactor, gas chromatograph equipped with FID and Tenax-GC® column was used. Also to analyze the CO and CO2, gas chromatograph equipped with TCD and molecular sieve 5A and Porapak Q column was used.
3. Results and discussion 3.1. Surface area and pore diameter
The specific surface area of V205 coated membrane was 1.1 ma/g, and the average pore diameter of the V205 coated membrane measured by nitrogen adsorption was approximately 30 A. Macro pores identified by mercury intrusion did not exist, and the porosity was approximately 9%.
S.I. Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252
242
(v2o5}
PeaK- 320.58
Peak from: 2 9 1 . 9 3 to: 349.35 Onset- 301.70 J/g - 55.28
7.5
3.75
Temperature (Cl
Fig. 1. DSC pattern of V205 film with heating rate 10°C/min.
From results of isotherm adsortion-desorption curve, V205 membrane has slitshaped pore structure. Hence pore mostly exist between the layers of V205, and there are nearly non-porous on the surface of V205 layer.
3.2. Thermal analysis The analysis of DSC crystallization in Fig. 1 started at 291°C and was completed at 349°C with the exothermic energy of 55.28 J/g. According to TG analysis in Fig. 2, the total weight loss was moderate from room temperature to 400°C while sudden weight loss occurred between 350°C and 370°C. The total weight loss occurred when the moisture within the membrane was eliminated and the weight loss ratio was approximately 16%. The weight loss occurring at 350°C seems to be lattice oxygen within the membrane. Therefore, it is clear that the heat treatment temperature of V205 coated membrane must be raised the temperature higher than 400°C when crystallization is completed.
3.3. X-ray diffraction analysis The X-ray diffraction results as a function of the heat treatment temperature of the V205 coated membrane are shown in Fig. 3. The V205 coated membrane prior
S.L Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252 100 ~
243
~0.4
0.8
95
o~ dWt dT
0
*el
go
0.8
Temperature
(°C)
Fig. 2. TG pattern of V205 film with heating rate 10°C/min.
to calcination was an amorphous structure without any characteristic peak. When the membrane was heated to 200°C the crystallization was slightly developed, and the crystallization continued to 400°C. But at temperatures over 400°C the crystal never grow. Therefore, the heat treatment temperature should be higher than 400°C. The characteristic peak of V205 occurred at 20=15.4 °, 20.3 °, 22 °, 25.6 °, 26.2 °, and 31 °. Each peak shows [200], [010], [110], [210], [101], [400] plane. In the case of the membrane heated up to 200°C, [200] plane, [010] plane, [110] plane and [400] plane started to occur but the rate of crystal growth is low. The membrane that was heated up to 300°C showed only the great growth of [010] plane while the [210] plane and [010] plane grew slightly. The membrane which was heated at 450°C showed the great growth in [200] plane, [010] plane, [110] plane and [400] plane, but the crystallization rates on other planes were negligibly small. Of the four crystalline structure, the active sites of V205 for oxidation reaction are [010] plane at 20.3 ° and [101] plane at 26.2 °, where [010] plane is the main active site for selective oxidation [14]. Therefore, the higher the rates of intensity of [010] plane and [101] plane are, the more favorably the selective oxidation occurs. In the case of the V205 coated membrane the morphological factor which is defined by 1[0101/11101]is ten times higher than those of the ordinary V2Os-based catalysts.
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[ 010 ] [2~0]~ [f01]~[400] 10
"
20
=
30
40
I0
20
(a)
30
40
(b) [ 010 ]
[ 010 ] 30]
[4OO]
[ 2OO
T [101]i400]
10
20
T
30
40
10
(c)
'
T
20
30
40 = 2 0
(d)
Fig. 3. XRD pattern of V205 films with calcination condition: (a) before calcination; (b) 20-200°C; (c) 20300°C; (d) 20-400°C.
3.4. UV diffuse reflectance spectroscopy The results from the UV-DRS analysis of the V 2 0 5 coated membrane is plotted in Fig. 4. The V205 coated membrane prior to heat treatment and the V205 coated membrane heated up to 200°C did not show absorption peaks that were caused by the charge transition from 0 2 - to V 5+. However in the V205 coated membrane heated up to 300°C, some absorption peaks appeared between 500 and 600 nm, and the strongest absorption peak occurred in the membrane that was heated up to 450°C. This is the typical characteristic of V205 catalyst and it is known that the stronger the charge transition, is the better the selective oxidation occurs [ 15,16].
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8
¢.)
0 300
I 400
I 500
I 600
I 700
800
Wave length (nm) Fig. 4. UV-DRS pattern of vanadium films with calcination condition: (a) 20-300°C; (b) 20-350°C; (c) 20400°C; (d) 20-450°C; (e) 20-200°C; (f) before calcination.
Therefore, we were able to see that the V 2 0 5 coated membrane prepared by the sol-gel process has catalytic activities which is suitable for the selective oxidation.
3.5. Electron microscopy analysis An electron microscope picture of V 2 0 5 sol prepared by melting is shown in Fig. 5. V205 sol is a typical polymeric sol and has a needle-like structure. The needle-like structure was reported to appear at V205 sol from vanadium alkoxide [13], so the V205 sol used in this study and the V205 sol synthesized from alkoxide are the same. The cross sectional image of V205 coated membrane on support is shown in Fig. 6. The V205 layer has the uniform thickness of approximately 0.8 gtm. The surface image of heated V205 coated membrane is shown in Fig. 7. There was no crack on the membrane surface, and clean surface was formed.
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Fig. 5. The picture of prepared V205 sol (× 100 k).
The electron diffraction pattern of selected area was analyzed in order to identify the structure of the Vz05 coated membrane both before and after heat treatment. The V205 coated membrane before heat treatment had a typical amorphous structure as shown in Fig. 8(a) while the V205 coated membrane followed by heat treatment showed a regular crystalline structure as shown in Fig. 8(b). It is believed that this crystalline structure is the orthorhombic structure of V205 [010] plane.
3.6. Partial oxidation reaction The conversion of 1-butene in the membrane reactor was comparatively low as shown in Fig. 9. The maximum conversion was only 22% and 33% respectively when the surface velocity was 500 cm/h and 200 cm/h. On the other hand, the selectivity of maleic anhydride in the membrane reactor was very high as shown in Fig. 10. When the surface velocity was 200 cm/h and 500 cm/h, the maximum selectivity was 80% and 95% at 350°C, respectively. The total oxidation occurs when butene reacts with the gas phase oxygen, where the selective oxidation occurs when butene reacts with the lattice oxygen. In the
S.L Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252
Fig. 6. The cross sectional picture of V205 coated membrane on the support ( x 5 k).
Fig. 7. The picture of V205 film after calcination at 450°C (x250 k).
247
248
S.L Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252
(a)
(b)
Fig. 8. The electron diffraction patterns of V2Os films: (a) before calcination; (b) after calcination at 450°C. 50
40
//11
I
0-100
I 200
300
i
Temperature (°C)
I 400
Fig. 9. Conversion of 1-butene over V205 coated membrane reactor at 15 psi g of 0 2 pressure. (m): surface velocity 500 cm/h; (A): surface velocity 200 cm/h.
249
S.L Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252 100
I
80
60 *P'I
20
01~
100
I
I
I
I
I
200 300 Temperature (°12)
I
400
Fig. 10. Selectivity of maleic anhydride over V205 coated membrane reactor at 15 psi g of surface velocity 500 cm/h, (A): surface velocity 200 cm/h.
0 2
pressure. (11):
membrane reactor, butene and gas phase oxygen are separated by the membrane, and butene and oxygen in the gas phase cannot react directly, and therefore the total oxidation does not occur. On the other hand butene adsorbed on the V205 membrane reacted only with the lattice oxygen in the V205 membrane, and the desired lattice oxygen was supplied from the other side of V205 membrane. Therefore the selective oxidation occurred only at the membrane reactor and the selectivity of maleic anhydride was increased. The conversion in the membrane reactor was very low compared to that in the fixed bed reactor, because the amount of V205 catalyst coated on the membrane reactor was only 0.01 cc, which restricts the reaction area very small. The conversion at the surface velocity of 500 cm/h was lower than that at 200 cm/h, because the amount of oxygen through the V205 membrane wall does not change with surface velocity of butene. However the selectivity of the former case was higher reversely because the contact time between butene and catalyst is reduced. When contact time is long, a portion of maleic anhydride reacts with the catalyst again, and is tend to be converted into CO2 and so on, and therefore lowers the selectivity of maleic anhydride. Because butadiene and acetic acid are produced under 250°C, and a portion of lattice oxygen is used for total oxidation above 400°C, the selectivities of maleic anhydride under 250°C and above 400°C
S.L Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252
250
60
o 40 *~-4
o 20
O 100
~ 200
300
Temperature (*C)
400
Fig. 11. Conversion of 1-butene with 02 pressure over V2Os coated membrane reactor (surface velocity: 200 cm/h). (e): 02 pressure 15 psi g; (11): 02 pressure 30 psi g; (A): 02 pressure 45 psi g.
are relative low. In order to increase the conversion in the membrane reactor, the membrane reactor should be made in hollow fibers, or the unreacted butene should be recycled. Butene can be condensed easily because it has a high liquefaction temperature. If the membrane process through recycling is developed, both the selectivity and the yield could be increased. When the oxygen partial pressure applied from the outside was changed to 15, 30 and 45 psi g, the effect of oxygen pressure was observed. As shown in Figs. 11 and 12, the conversion increases and the selectivity decreases as the oxygen partial pressure increases. Especially when the pressure increases from 15 to 30 psi g the decrease in the selectivity was slight, but when pressure was raised to 45 psi g the selectivity showed a big drop. This is due to the fact that, the permeation rate of oxygen through membrane was too low as the oxygen pressure was low. The oxygen was supplied only by the adsorption of oxygen on the V2Os membrane. When the pressure was above a certain value, the oxygen was permeated through the membrane and reacted directly with butene, which causes the total oxidation. Not only the amount of total transferred oxygen molecule but also the amount of transferred lattice oxygen make the difference of selectivity between 30 and 45 psi g. The total amount of transferred oxygen molecule is large at 45 psi g, but
S.I. Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252
251
tO0
8°I 7/ 20
100
200
300
Temperature (*C)
400
Fig. 12. Selectivity of maleic anhydride in a V205 coated membrane reactor at 02 pressure (surface velocity: 200 cm/h). (o): 02 pressure 15 psi g; (ll): 02 pressure 30 psi g; (A): 02 pressure 45 psi g.
the amount of transferred lattice oxygen that is used to selective oxidation is about the same at 30 psi g and 45 psi g. The reactions of the membrane reactor and the fixed bed reactor are not directly comparable, because the catalyst structure and the reaction mechanism of the membrane reactor is basically different from those of the fixed bed reactor. However, comparing to previous other studies [1,2], the membrane reactor produces higher selectivity as the same conversion than the fixed bed does. Also, the decrease in conversion due to the increase in the selectivity is relatively small.
4. Conclusions
From the V205 sol made by fusing and melting, the V205 coated membrane with a uniform surface is obtained without cracking on the support. The mean pore diameter and the specific surface area of the prepared membrane is 30 ,~ and 1.1 mZ/g respectively. The calcination temperature of the V205 coated membrane must be set to the temperature higher than 400°C when the crystallization is completed. When the V205 coated membrane is heated to 450°C, a VeO5 [010]
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plane is grown selectively, and at around 500 nm the charge transition from 0 2 - to V 5+, which is a typical characteristic of V~O5 catalysts, is observed. As the V205 coated membrane reactor is applied to the selective oxidation reaction which produces maleic anhydride from butene, the maximum selectivity is 95%. This result shows that the V205 coated membrane reactor can control the total oxidation reaction effectively and increase the efficiency of the oxidation reaction.
Acknowledgements The authors gratefully acknowledge the Korea Science and Engineering Foundation (Project No. 911-1002-028-2) for support.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
G. Centi and E Trifiro, Chem. Rev., 88(1) (1988) 55. E Cavani, Ind. Eng. Chem. Process Res. Dev., 22 (1983) 565. B.K. Hodnett, Catal. Rev., 27 (1985) 373. V.T. Zaspalis, Adv. Ceram. Chem. Proc. Eng., (1988) 103. A.M. Champagnie, T.T. Tsotsis, R.G. Minet and I.A. Webster, Chem. Eng. Sci., 45(8) (1990) 2423. N. Itoh, AIChE J., 33 (1987) 1576. J. Bullot, E Cordier, O. Gallais, M. Gauthier and J. Livage, J. Non-Cryst. Solids, 68 (1984) 123. T.E. Philips, R.A. Murphy and T.O. Poebler, Mat. Res. Bull., 22 (1987) 1113. EC. Case, Appl. Optics, 26(8) (1987) 1550. K. Inumaru, T. Okuhara and M. Misono, Chem. Lett., (1990) 1207. L. Hou and S. Sakka, J. Non-Cryst. Solids, 112 (1989) 424. R.C. Mehrotra, Z. Anorg. Allg. Chem., 327 (1964) 311. J.J. Legengre and J. Livage, J. Colloid Inter. Sci., 94(1) (1983) 75. G. Marian and M. Todeusz, J. Catal., 83 (1983) 472. M. Inomata, K. Mori, A. Miyamoyo, U. Toshiaki and Y. Murakami, J. Phys. Chem., 87 (1983) 754. G. Lischke, W. Hanke, H.G. Jersehkewith and G. Ohlmann, J. Catal., 91 (1985) 54.
A PT PA LE IY DSS CA L I A: GENERAL
ELSEVIER
Applied Catalysis A: General 156 (1997) 239-252
Characteristics of a V 2 0 5 coated membrane reactor for the selective oxidation of 1-butene S.I. Hong a, J.H. J u n g b'* aDepartment of Chemical Engineering, Korea University, Seoul 136-701, South Korea bDepartment of Chemical Engineering, Kyonggi University, Suwon-si, Kyonggi-do 442-760, South Korea Received 5 August 1996; received in revised form 16 January 1997; accepted 22 January 1997
Abstract The selective oxidation of butene to maleic anhydride has been carried out in a V205 coated membrane reactor. From the V205 sol made by fusing and melting, the V205 coated membrane with a uniform surface without crack is prepared on support. The mean pore diameter and specific surface area of the prepared membrane is 30 A and 1.1 m2/g, respectively. When the V205 coated membrane was heated to 450°C, the V205 [010] plane was grown selectively, and at around 500 nm the charge was changed from 0 2 - to V 5+ in the selective oxidation reaction that produced maleic anhydride from butene. The maximum selectivity of 95% was obtained. This high selectivity shows that the V205 coated membrane reactor is very efficient for the selective oxidation.
Keywords: Membrane reactor; Selective oxidation; Vanadium pentoxide; Sol-gel
1. Introduction Selective oxidation of butane or butene to maleic anhydride is the most industrially important reaction among the reactions using V205 catalyst [1,2]. When maleic anhydride is produced from Ca hydrocarbon, the selectivity of maleic anhydride is low, because selective oxidation and total oxidation always proceed competitively. Though fixed bed reactors or fluidized bed reactors have been mainly used for selective oxidation for the past 30 years, the selectivity of maleic anhydride has not been higher than 69% [3]. Recently developed membrane reactors have been introduced because they offer some advantages. A membrane reactor plays the role of both catalyst and * Corresponding author. 0926-860X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 8 6 0 X ( 9 7 ) 0 0 0 5 1 - 3
240
S.I. Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252
membrane, and comes in two forms [4]. First, one of the products is separated by membrane, and chemical equilibrium is shifted, as a result the conversion is increased. This type of reactor can be used for water gas shift reaction, dehydrogenation and so on [5,6]. Second, the formation of byproducts is suppressed due to presence of membrane between the two reactants, so that selectivity is improved. This second type can be applied for the selective oxidation and hydrogenation reaction that produce a lot of byproducts during the reaction, but has not been adequately studied compared to the first type of membrane reactor. Therefore it is very important to prepare the second type membrane reactor using V205 catalyst for selective oxidation. V205 coated film has been mostly studied to characterize semiconductor, not membrane [7,8]. The V205 coated film can be prepared by RF sputtering [8], vacuum evaporation [9], CVD [10] and sol-gel [7,11]. Among these methods, the sol-gel process is the most appropriate method for preparation of membrane reactors, because it is readily applicable to tubular supports. V205 sol is prepared mostly using vanadium alkoxide (VO(OC3H7)3) [12], and can also be made by passing vanadic acid through ion exchange resin and polymerizing it [13]. Another method for the preparation of V205 sol is to pour the melted V205 into distilled water, which uses the property that amorphous V205 dissolves easily in water [7]. The objective of this study was to prepare the V205 coated membrane reactor using sol-gel method, and to investigate reaction characteristics of the prepared membrane reactor through the selective oxidation reaction of butene to maleic anhydride. The instrumental analysis of the V205 membrane has been also performed to find the optimum conditions in the preparation of membrane reactors.
2. Experimental 2.1. Preparation of a V205 coated membrane reactor Sol-gel method was selected for preparing V205 membrane, because we obtain relatively thick and dense V205 films. V205 sol was prepared by dissolving the melted V205 into distilled water [7]. As a support of the membrane reactor a Vycor® tube (250 mm long and 7 mm wide) with a quartz tube connected to either end was used. In order to coat the support interior with V205 sol, one end of the support was connected to syringe and the other end of the tube was put into V205 sol. After the sol was sucked in, it was allowed to stay within the support for certain time, and then pushed the remaining solution out. The thickness of the coated membrane was controlled by the length of time the V205 sol remaining in the support, and by the viscosity of sol. The coated tube was dried at room temperature for 24 h, and in order to crystallize the V205
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241
membrane, the furnace temperature was raised by 1°C per minute and kept at 450°C for 1 h. The V205 membrane was also prepared by dip coating the V205 sol on silicon wafer to characterize the membrane surface. 2.2. Apparatus of the membrane reactor
The V205 coated Vycor connected to the quartz tube was fixed within the 1/2 inch wide quartz tube by using CAJON® ultra-tort fitting. Both nitrogen, the transport gas, and butene were passed through the Vycor tube under the application of high-pressurized air to the outside of the Vycor tube. The oxygen and the butene were met and interacted on the surface of the V205 coated membrane. At this time the concentration of the mixture of butene and nitrogen was sustained at 1%, and the surface velocities (volumetric flow rate/surface area of reactor) of butene and nitrogen were set to 500 cm/h and 200 cm/h, respectively. The reaction was performed at temperatures between 200°C and 450°C. Maleic anhydride and unreacted compounds were analyzed by on-line gas chromatography. 2.3. Characterization
Both the specific surface area and the pore size distribution were measured by adsorbing nitrogen while the existence of macro pores was confirmed by mercury porosimetry. The crystal structure of V205 as a function of the crystallization temperature and time was investigated by X-ray diffractometry. The weight loss and the crystallization temperature were determined using TG and DSC. In order to observe the magnitude of charge transition of the V205 coated membrane during heat treatment, UV-Diffuse Reflectance Spectroscopy (DRS) was used and MgO was chosen as a reference. The morphology of the V205 sol and the pore size and crystallinity of the V205 coated membrane were observed by TEM, while the thickness of the V205 coated membrane was measured by SEM. To analyze the unreacted butene and maleic anhydride in the membrane reactor, gas chromatograph equipped with FID and Tenax-GC® column was used. Also to analyze the CO and CO2, gas chromatograph equipped with TCD and molecular sieve 5A and Porapak Q column was used.
3. Results and discussion 3.1. Surface area and pore diameter
The specific surface area of V205 coated membrane was 1.1 ma/g, and the average pore diameter of the V205 coated membrane measured by nitrogen adsorption was approximately 30 A. Macro pores identified by mercury intrusion did not exist, and the porosity was approximately 9%.
S.I. Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252
242
(v2o5}
PeaK- 320.58
Peak from: 2 9 1 . 9 3 to: 349.35 Onset- 301.70 J/g - 55.28
7.5
3.75
Temperature (Cl
Fig. 1. DSC pattern of V205 film with heating rate 10°C/min.
From results of isotherm adsortion-desorption curve, V205 membrane has slitshaped pore structure. Hence pore mostly exist between the layers of V205, and there are nearly non-porous on the surface of V205 layer.
3.2. Thermal analysis The analysis of DSC crystallization in Fig. 1 started at 291°C and was completed at 349°C with the exothermic energy of 55.28 J/g. According to TG analysis in Fig. 2, the total weight loss was moderate from room temperature to 400°C while sudden weight loss occurred between 350°C and 370°C. The total weight loss occurred when the moisture within the membrane was eliminated and the weight loss ratio was approximately 16%. The weight loss occurring at 350°C seems to be lattice oxygen within the membrane. Therefore, it is clear that the heat treatment temperature of V205 coated membrane must be raised the temperature higher than 400°C when crystallization is completed.
3.3. X-ray diffraction analysis The X-ray diffraction results as a function of the heat treatment temperature of the V205 coated membrane are shown in Fig. 3. The V205 coated membrane prior
S.L Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252 100 ~
243
~0.4
0.8
95
o~ dWt dT
0
*el
go
0.8
Temperature
(°C)
Fig. 2. TG pattern of V205 film with heating rate 10°C/min.
to calcination was an amorphous structure without any characteristic peak. When the membrane was heated to 200°C the crystallization was slightly developed, and the crystallization continued to 400°C. But at temperatures over 400°C the crystal never grow. Therefore, the heat treatment temperature should be higher than 400°C. The characteristic peak of V205 occurred at 20=15.4 °, 20.3 °, 22 °, 25.6 °, 26.2 °, and 31 °. Each peak shows [200], [010], [110], [210], [101], [400] plane. In the case of the membrane heated up to 200°C, [200] plane, [010] plane, [110] plane and [400] plane started to occur but the rate of crystal growth is low. The membrane that was heated up to 300°C showed only the great growth of [010] plane while the [210] plane and [010] plane grew slightly. The membrane which was heated at 450°C showed the great growth in [200] plane, [010] plane, [110] plane and [400] plane, but the crystallization rates on other planes were negligibly small. Of the four crystalline structure, the active sites of V205 for oxidation reaction are [010] plane at 20.3 ° and [101] plane at 26.2 °, where [010] plane is the main active site for selective oxidation [14]. Therefore, the higher the rates of intensity of [010] plane and [101] plane are, the more favorably the selective oxidation occurs. In the case of the V205 coated membrane the morphological factor which is defined by 1[0101/11101]is ten times higher than those of the ordinary V2Os-based catalysts.
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[ 010 ] [2~0]~ [f01]~[400] 10
"
20
=
30
40
I0
20
(a)
30
40
(b) [ 010 ]
[ 010 ] 30]
[4OO]
[ 2OO
T [101]i400]
10
20
T
30
40
10
(c)
'
T
20
30
40 = 2 0
(d)
Fig. 3. XRD pattern of V205 films with calcination condition: (a) before calcination; (b) 20-200°C; (c) 20300°C; (d) 20-400°C.
3.4. UV diffuse reflectance spectroscopy The results from the UV-DRS analysis of the V 2 0 5 coated membrane is plotted in Fig. 4. The V205 coated membrane prior to heat treatment and the V205 coated membrane heated up to 200°C did not show absorption peaks that were caused by the charge transition from 0 2 - to V 5+. However in the V205 coated membrane heated up to 300°C, some absorption peaks appeared between 500 and 600 nm, and the strongest absorption peak occurred in the membrane that was heated up to 450°C. This is the typical characteristic of V205 catalyst and it is known that the stronger the charge transition, is the better the selective oxidation occurs [ 15,16].
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8
¢.)
0 300
I 400
I 500
I 600
I 700
800
Wave length (nm) Fig. 4. UV-DRS pattern of vanadium films with calcination condition: (a) 20-300°C; (b) 20-350°C; (c) 20400°C; (d) 20-450°C; (e) 20-200°C; (f) before calcination.
Therefore, we were able to see that the V 2 0 5 coated membrane prepared by the sol-gel process has catalytic activities which is suitable for the selective oxidation.
3.5. Electron microscopy analysis An electron microscope picture of V 2 0 5 sol prepared by melting is shown in Fig. 5. V205 sol is a typical polymeric sol and has a needle-like structure. The needle-like structure was reported to appear at V205 sol from vanadium alkoxide [13], so the V205 sol used in this study and the V205 sol synthesized from alkoxide are the same. The cross sectional image of V205 coated membrane on support is shown in Fig. 6. The V205 layer has the uniform thickness of approximately 0.8 gtm. The surface image of heated V205 coated membrane is shown in Fig. 7. There was no crack on the membrane surface, and clean surface was formed.
246
S.L Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252
Fig. 5. The picture of prepared V205 sol (× 100 k).
The electron diffraction pattern of selected area was analyzed in order to identify the structure of the Vz05 coated membrane both before and after heat treatment. The V205 coated membrane before heat treatment had a typical amorphous structure as shown in Fig. 8(a) while the V205 coated membrane followed by heat treatment showed a regular crystalline structure as shown in Fig. 8(b). It is believed that this crystalline structure is the orthorhombic structure of V205 [010] plane.
3.6. Partial oxidation reaction The conversion of 1-butene in the membrane reactor was comparatively low as shown in Fig. 9. The maximum conversion was only 22% and 33% respectively when the surface velocity was 500 cm/h and 200 cm/h. On the other hand, the selectivity of maleic anhydride in the membrane reactor was very high as shown in Fig. 10. When the surface velocity was 200 cm/h and 500 cm/h, the maximum selectivity was 80% and 95% at 350°C, respectively. The total oxidation occurs when butene reacts with the gas phase oxygen, where the selective oxidation occurs when butene reacts with the lattice oxygen. In the
S.L Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252
Fig. 6. The cross sectional picture of V205 coated membrane on the support ( x 5 k).
Fig. 7. The picture of V205 film after calcination at 450°C (x250 k).
247
248
S.L Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252
(a)
(b)
Fig. 8. The electron diffraction patterns of V2Os films: (a) before calcination; (b) after calcination at 450°C. 50
40
//11
I
0-100
I 200
300
i
Temperature (°C)
I 400
Fig. 9. Conversion of 1-butene over V205 coated membrane reactor at 15 psi g of 0 2 pressure. (m): surface velocity 500 cm/h; (A): surface velocity 200 cm/h.
249
S.L Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252 100
I
80
60 *P'I
20
01~
100
I
I
I
I
I
200 300 Temperature (°12)
I
400
Fig. 10. Selectivity of maleic anhydride over V205 coated membrane reactor at 15 psi g of surface velocity 500 cm/h, (A): surface velocity 200 cm/h.
0 2
pressure. (11):
membrane reactor, butene and gas phase oxygen are separated by the membrane, and butene and oxygen in the gas phase cannot react directly, and therefore the total oxidation does not occur. On the other hand butene adsorbed on the V205 membrane reacted only with the lattice oxygen in the V205 membrane, and the desired lattice oxygen was supplied from the other side of V205 membrane. Therefore the selective oxidation occurred only at the membrane reactor and the selectivity of maleic anhydride was increased. The conversion in the membrane reactor was very low compared to that in the fixed bed reactor, because the amount of V205 catalyst coated on the membrane reactor was only 0.01 cc, which restricts the reaction area very small. The conversion at the surface velocity of 500 cm/h was lower than that at 200 cm/h, because the amount of oxygen through the V205 membrane wall does not change with surface velocity of butene. However the selectivity of the former case was higher reversely because the contact time between butene and catalyst is reduced. When contact time is long, a portion of maleic anhydride reacts with the catalyst again, and is tend to be converted into CO2 and so on, and therefore lowers the selectivity of maleic anhydride. Because butadiene and acetic acid are produced under 250°C, and a portion of lattice oxygen is used for total oxidation above 400°C, the selectivities of maleic anhydride under 250°C and above 400°C
S.L Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252
250
60
o 40 *~-4
o 20
O 100
~ 200
300
Temperature (*C)
400
Fig. 11. Conversion of 1-butene with 02 pressure over V2Os coated membrane reactor (surface velocity: 200 cm/h). (e): 02 pressure 15 psi g; (11): 02 pressure 30 psi g; (A): 02 pressure 45 psi g.
are relative low. In order to increase the conversion in the membrane reactor, the membrane reactor should be made in hollow fibers, or the unreacted butene should be recycled. Butene can be condensed easily because it has a high liquefaction temperature. If the membrane process through recycling is developed, both the selectivity and the yield could be increased. When the oxygen partial pressure applied from the outside was changed to 15, 30 and 45 psi g, the effect of oxygen pressure was observed. As shown in Figs. 11 and 12, the conversion increases and the selectivity decreases as the oxygen partial pressure increases. Especially when the pressure increases from 15 to 30 psi g the decrease in the selectivity was slight, but when pressure was raised to 45 psi g the selectivity showed a big drop. This is due to the fact that, the permeation rate of oxygen through membrane was too low as the oxygen pressure was low. The oxygen was supplied only by the adsorption of oxygen on the V2Os membrane. When the pressure was above a certain value, the oxygen was permeated through the membrane and reacted directly with butene, which causes the total oxidation. Not only the amount of total transferred oxygen molecule but also the amount of transferred lattice oxygen make the difference of selectivity between 30 and 45 psi g. The total amount of transferred oxygen molecule is large at 45 psi g, but
S.I. Hong, J.H. Jung/Applied Catalysis A: General 156 (1997) 239-252
251
tO0
8°I 7/ 20
100
200
300
Temperature (*C)
400
Fig. 12. Selectivity of maleic anhydride in a V205 coated membrane reactor at 02 pressure (surface velocity: 200 cm/h). (o): 02 pressure 15 psi g; (ll): 02 pressure 30 psi g; (A): 02 pressure 45 psi g.
the amount of transferred lattice oxygen that is used to selective oxidation is about the same at 30 psi g and 45 psi g. The reactions of the membrane reactor and the fixed bed reactor are not directly comparable, because the catalyst structure and the reaction mechanism of the membrane reactor is basically different from those of the fixed bed reactor. However, comparing to previous other studies [1,2], the membrane reactor produces higher selectivity as the same conversion than the fixed bed does. Also, the decrease in conversion due to the increase in the selectivity is relatively small.
4. Conclusions
From the V205 sol made by fusing and melting, the V205 coated membrane with a uniform surface is obtained without cracking on the support. The mean pore diameter and the specific surface area of the prepared membrane is 30 ,~ and 1.1 mZ/g respectively. The calcination temperature of the V205 coated membrane must be set to the temperature higher than 400°C when the crystallization is completed. When the V205 coated membrane is heated to 450°C, a VeO5 [010]
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252
plane is grown selectively, and at around 500 nm the charge transition from 0 2 - to V 5+, which is a typical characteristic of V~O5 catalysts, is observed. As the V205 coated membrane reactor is applied to the selective oxidation reaction which produces maleic anhydride from butene, the maximum selectivity is 95%. This result shows that the V205 coated membrane reactor can control the total oxidation reaction effectively and increase the efficiency of the oxidation reaction.
Acknowledgements The authors gratefully acknowledge the Korea Science and Engineering Foundation (Project No. 911-1002-028-2) for support.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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