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Direct oxidation of isobutane into methacrylic acid over Cs, Ni, and V-substituted H3PMo12O40 heteropoly compounds
l.W. Hightower, W.N. Delgass, E. Iglesia and A.T. Bell (Eds.) 11th International Congress on Catalysis - 40th Anniversary Studies in Surface Science and Catalysis, Vol. 101 © 1996 Elsevier Science B.V. All rights reserved.
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Direct Oxidation of Isobutane into Methacrylic Acid over Cs, Ni, and V-substituted H3PMo 12040 Heteropoly Compounds t Noritaka. Mizuno,*,a Wonchull Han,a Tetsuichi Kudo,a and Masakazu Iwamoto b aInstitute ofIndustrial Science, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan bCatalysis Research Center, Hokkaido University, Sapporo 060, Japan Keggin-type heteropolymolybdates have been found to catalyze the selective oxidation of isobutane with molecular oxygen. The preferable contents of Cs and V are 2.5 and 1, respectively, and the addition of Ni enhanced the catalytic activity for the oxidation of isobutane. The presence of steam did not affected the catalytic' performance very much. CS2.5NiO.OSH1.34PVMoll 040 also catalyzegd the oxidation of propane and ethane. It was suggested by m, XRD, and flp NMR data that the Keggin structure was maintained during the oxidation. 1. INTRODUCTION
The oxidizing properties or strong acidity of heteropoly compounds induce a lot of studies on the heterogeneous and homogeneous catalysis [1]. The additional attractive and important aspects are oxidatively stability and introduction of various components into heteropolyanions and the countercations. Methacrylic acid has been used for the synthesis of methyl methacrylate, an important monomer of resin. Industrial production of methacrylic acid has traditionally been achieved by the reaction of acetone with hydrogen cyanide [2-4]. However, the process uses the dangerous hydrogen cyanide and overproduces solid ammonium bisulfate. Recently, alternative methods, the methylation of propionaldehyde and the oxidation of isobutene, have been developed [2-4]. These processes still have problems to use high-price feedstocks and consist of two-step reactions [2-5]. Therefore, it would be much more desirable to use cheaper feedstock, isobutane and to produce methacrylic acid directly from isobutane and molecular oxygen [2,3]. Oxyfunctionalization of lower paraffins such as methane, ethane, propane, and butanes is industrially important because of the cheapness [2,3,5]. Recently, the active catalysts for the oxidation of lower paraffins have been studied [6-10] and the processes for the oxidation of propane and n-butane have been industrialized [1,11]. Recently the present authors has t We gratefully acknowledge Mr. M. Tateishi for experimental assistance. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan, Yazaki Memorial Foundation for Science and Technology, and Ciba-Geigy Foundation (Japan) for Promotion of Science. * To whom correspondence should be addressed.
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expanded our fundamental knowledge regarding metal-catalyzed 02-based oxidation of cydohexane and other paraffins by using heteropoly compounds [12].
Here we wish to report how catalytic activity of H3PMo12040 for the oxidation ofisobutane is changed by Cs, Ni, and V-substitution.
2. EXPERIMENTAL 2.1. Catalysts H3+xPMo12-xVx040 heteropolyacids were commercially obtained from Nippon Inorganic Colour and Chemical Co., Ltd. The other reagents used were analytical grade and used without further purification. The catalysts were ..prepared as follows: An aqueous solution of nickel nitrate (O.OS mol·dm-D) was added drop~se to an aqueous solution of H3+xPVxMo12-x040 (x = 0 - 4; 0.06 mol·dm- ), followed by. the addition of an aqueous solution of alkaline carbonates (O.OS mol·dm-3 ) at SO ·C. The resulting suspension or solution was evaporated to dryness at SO ·C. The actual composition may be CSqNiO.OSHyPVxMo12-xOz, but in this paper they will be designated as CSqNIO.OSH2.S4+x-qPVxMo12-x040. BET surface areas were measured by means of N2 adsorption using Omnisorp 100MP (Coulter) after samples were evacuated at 300 ·C for 1 h. The Powder X-ray diffraction patterns were recorded on a po}Vder X-ray diffractometer (Materials Analysis and Characterization, MXpiS) by using Cu Ka radiation. The infrared spectra of KBr pellets were recorded on a Paragon 1000PC spectrometer (Perkin Elmer). 2.2. Reaction The reaction was performed in a flow reactor (Pyrex tube, 12 mm internal diameter) at the applied temperature of 260·360 ·C under an atmospheric PNresbsulre. Thelfeed gahs co~sisted 0df 17 v0all%flof isobutane, 33 v01% of 02,.and 1 ow rates were ca. 30 cm 3 ·mm-. 2 a ance un ess ot erwlse state. T ot It was confirmed for CS2.SNiO.OSHO.34PMo12040 that the conversion and selectivity was little changed by the mixing with SiC (l.S g) to prevent an undesirable temperature rise. Prior to the reaction, 1 g of each catalyst was treated in an 02 stream (60 cm iS 'min- 1 ) for 1 h at 300 ·C. The gases at the outlet of the reactor were taken out intermittently with the aid of a sampler directly connected to the system and analyzed by FID and TCD gas chromatograph with FFAP, Porapack Q, and Molecular Sieve SA columns. The conversion and the selectivity were collected after 2 - S h of reaction, when nearly steady-state conversion and selectivity were obtained for each catalyst. Selectivity was calculated on the C4 (isobutane)-basis. The carbon balance was in the range of90 -100%.
3. RESULTS AND DISCUSSION 3.1. Oxidation of Isobutane Figure 1 shows the time course of the oxidation of isobutane catalyzed by CS2.SNiO.OSHl.34PVMoll040 at 320 ·C. The conversion and selectivity reached almost constant after 4 h; e.g., the conversions were 21, IS, 17, IS, and lS% at 1, 2, 3, 4, and S h, respectively. The main products were methacrylic acid (MAA), methacrolein (MAL), acetic acid, and COx (CO + C02). The same products were observed for the other heteropoly catalysts used. The color of CS2.SNiO.OSH1.34PVMoll040 after having been used as
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catalyst was yellow green, suggesting that the oxidation state is rather high under reaction conditions. The preliminary kinetic measurements showed that the pressure dependencies were expressed by -d[i-C4HIOJ/dt = k-Pi-C4HIOl(-O)·P020 at 320°C for CS2.5NiO.08H1.34PVMo11 040, in the range of partial pressure of isobutane, 0.16 - 0.25 atm and oxygen, 0.33 - ~41 atm. Above the partial pressure of isobutane of 0.25 atm, Pi-C4HI0 dependence was reached. Conversion was kept below 16% for the determination of the pressure dependency. Itwas confirmed that conversions linearly increased up to 20% with W (weight of catalyst)! F (flow rate).
80
-~
~ ~ U GO
... ." -;;
• ]=
60 40
~
I:
I: GO
20
~
=
I:
U
0
::i====i===iI
0
1
2
3
:: g
g
4
5
Time!h Figure 1. Time course of the oxidation of isobutane catalyzed by CS2.5NiO.08H1.34PVMoll040 at 320°C. 0, e, _, LJ, and A are conversion ofisobutane and selectivities to MAA, MAL, AcOH, and COx, respectively. The conversion of isobutane monotonously increased with increase in the reaction temperature and reached up to 37% at 360°C. The selectivity to MAL monotonously decreased with increase in the reaction temperature and that to MAA reached the maximum around 285 - 300°C. The decrease of MAA and MAL above 300°C is mainly due to the successive oxidation of MAA and MAL because the amount of COx increased in parallel. The temperature dependency of the yields of MAA and MAL for CS2.5NiO.08H1.34PVMoll040 is shown in Fig. 2. The yield ofMAA reached maximum at 340°C. The value is 9.0% and higher than 6.2%, the maximum value reported at the likely steady-state in patent literatures [13J. The yield of MAL was approximately constant at and above 285°C. Effect of steam on the conversions and selectivities is shown in Table 1. Conversion was changed little by the addition of s 5.2 vol% steam, and the selectivity to MAA slightly increased. The constant conversion is in
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contrast with the great increase of the conversion for the oxidation of methacrolein and acetaldehyde catalyzed by H3PMo12040 [14,15]. The only slight effect of steam in the present system is probably due to (1) the hydrophobicity of Cs salt and/or (2) low partial pressure of steam. 10
:2-~
8
::is
"Cl
=
Cll
6
~
....::is 4
-0
~
Ql
~
2
o "-..._ ..
L...._ _........_ _- - - l
150
450
300
Temp./oe Figure 2. Temperature dependency of yields of MAA and MAL. Catalyst, CS2.5NiO.OSH1.34PVMoll040. • and _, the same as those in Fig. 1. Table 1 Effect of water vapor on oxidation of isobutane over Nio.osCsz.sH1.34PVMo ll 0 40 catalyst at 320 ·C a PHzO Conv. I % I vol% MAA
a
Selectivity / % MAL
AcOH
Sum of yields of COx
MAA+MAL/%
0
15
36
S
6
50
6.6
1.3
15
40
8
12
40
7.2
5.2
15
42
8
6
43
7.5
Isobutane, 17 vol%; 0z, 33 vol%; N z, balance; catalyst, 1.0 g; total flow rate,
ca. 30 cm3 ·min-l .
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3.2. Effect of Cs Salt Formation The catalytic properties of CsxNiO.OSH3.S4-xPVMoll 040 were changed by the Cs contents. The results on CsxNiO.OSH3.S4-xPVMoll040 (x = 0 - 3) catalysts at 340°C are shown in Fig. 3. Note that the results were compared at 340°C because of the lower catalytic activity of Csx NiO.OSH3.S4-xPVMo11040 (x = 0, 1). The conversions were 6, 4, 12, 31, and S% for x = 0, 1, 2, 2.5, and 3, respectively, and the highest conversion was observed at x = 2.5. The yields of MAA on Csx NiO.OsH3.S4-xPVMo11040 were 1.3, 1.S, 5.6, 9.0, and O.S% for x = 0, 1, 2, Thus the substitution of Cs+ for H+ in 2.5, and 3, respectively. NiO.OSH3.S4PVMoll 040 resulted in the great enhancement of the MAA production and the yield reached the maximum around x = 2.5. Similarly the substitution of Cs+ for H+ in H3PM012040 resulted in the great enhancement of the MAA production and the yield reached the maximum around x = 2.5 for CsxH3-xPM012040 catalysts [12]. The surface areas of CsxNiO.OSH4-xPVMoll 040 catalysts were 4.5 and 42.9 m~·g-1 for x = 0 and 2.5, respectively, and increased with the increase in the Cs content. Therefore, the high surface area of CS2.5NiO.OSH1.34PVMoll 040 is one of the factors controlling the high catalytic activity shown in Fig. 3. The catalytic activities per specific surface area decreased from x = 0 to 2.5. The decrease is mainly due to the decrease in the oxidizing power of catalysts with Cs substitution. A similar monotonous decrease of the catalytic activity per surface area was observed for the oxidation of CO, acetaldehyde, and methacrolein and are explained on the basis of the decrease in the oxidizing power of heteropoly catalysts with the Cs+-substitution [15-17].
40..-------------, MAA I2'J AeOH MAL EJ co o CO z
• •
o
1
2
2.S
3
x in CsxNio.osH3.84-xPVMol1040
Figure 3. Effect of Cs+-substitution for H+ in NiO.OSH3.S4PVMoll040 on oxidation of isobutane at 340 ·C.
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3.3. Effect of Ni Addition Figure 4 shows the effect of addition of Ni to CS2HPMo12040 on the catalytic performance. The addition of Ni to CS2HPMo12040 increased the conversion about 1.6 times higher than that on CS2HPMo12040 and the selectivities to MAA and MAL slightly changed. Therefore, the yields of MAA and MAL became higher than those on CS2HPMo12040. Similarly the conversion and yields ofMAA and MAL were increased by the addition of Ni to CS2.5HO.5PMo12040 as shown in Fig. 4 [12b]. The addition of Mn or Fe also increased the yields ofMAA and MAL and Ni most efficiently increased the yields of MAA and MAL [12b). The prefarable content of Ni was O.OS in CS2.5NixHO.5-2xPMo12040: The catalytic activity increased with the increase of x in CS2.5NixHO.5-2xPMo12040, reached maximum at x = O.OS, and decreased above O.S. The decrease in catalytic activity above x = O.OS would be due to the decrease in the protonic acidity.
-MAA -MAL f'JAcOH
-Acetone
ceo
o CO2
Cs2.5llo.sPM0 120 40
CsuNio.osIlo.s.PMo 12040
o
10 20 Conversion and yield / %
Figure 4. Effect of addition of Ni to CS2HPMo12040 CS2.5HO.5PMo12040 on the catalytic performance at 340 ·C.
30
and
3.4. Effect of v5+·Substitution The results on CS2.5NiO.OSHO.34+xPVxMo12-x040 catalysts are shown in Fig. 5. The conversions were 10, 15, 13, 12, and 10% for x = 0, 1, 2, 3, and 4, respectively, and the highest conversion was observed around x = 1. The selectivities to MAA on CS2.5NiO.OSHO.34+xPVxMo12-x040 catalysts were 27,36, 2S, 10, and 4% for x = 0, 1,2,3, and 4, respectively, and the highest selectivity to MAA was also observed at x = 1. It follows that the substitution ofV5 +-for Mo6+ in CS2.5NiO.OSHO.34PMo12040 resulted in the enhancement of the MAA production and the yield reached the maximum (5.4%) at x = 1. It is noted that the yield of MAA on CS2.5NiO.OSH1.34PVMoll040 was about two times higher than that on
}007
CS2.5NiO.OSHO.34PMo12040 (shown in Fig. 4). The maximum yield of MAL was also observed around x = 1 and was 1.4%. Thus the sum of the yields of MAA and MAL upon CS2.5NiO.OSH1.34PVMoll040 reached to 6.S% and was the highest among CS2.5NiO.OSHO.34+xPVxMo12-x040 catalysts. The selectivities to MAA and MAL for the oxidation o~isobutane catalyzed by H3PMo12040 at 340·C were also increased by the V +-substitution: The selectivities to MAA were 4, 30, 34, 14, and 9% for x = 0, 1, 2, 3, and 4, respectively, and those to MAL were IS, 36, 2S, 14, 21%, respectively, although the conversions were in the range of 5 • S% and low. A Similar e~ancement of the catalytic activity of H3PMo12040 by the V +-substitution for the oxidation of n-pentane and n-butane has been reported [10].
20 . . . - - - - - - - - - - - - - - , • •
o
o
1
2
MAA fJ AcOH MAL lEI CO CO 2
o
3
4
x in CS2.sNio.08Ho.34+xPVxM012_x040
Figure 5. Effect ofV5+-substitution for Mo in CS2.5NiO.OSHO.34PMo12040 on oxidation ofisobutane at 320 ·C.
3.5. Oxidation of Propane and Ethane Catalyzed by CS2.5NiO.osHl.34PVMol1040 Figure 6 shows the results of oxidation of propane and ethane as well as isobutane catalyzed by CS2.5NiO.OSH1.34PVMoll040. Propane can be oxidized into acrylic acid (abbreviated by AA), acrolein (ACR), propylene, acetic acid, CO, and C02 at S.5% conversion and 340 ·C. The yield of AA was 1.6% and increased to 4.9% at 400 ·C. In the oxidation of propane, CS2.5FeO.osH1.26PVMoll040 gave the highest yield of acrylic acid, 13% (at 3S0 ·C) [12b], higher than 10.5% reported on highly active V205-P205-Te02 catalyst [IS]. CS2.5NiO.OSH1.34PVMoll 040 also catalyzed the oxidation of ethane: the conversion is 2% and the selectivities to ethylene and C02 were 44 and 56%, respectively, at 350 ·C. Neither corresponding aldehyde nor acid were
1008
observed for the oxidation of ethane and only oxidative dehydrogenation and complete oxidation proceeded. This is probably due to the less reactivity of ethylene formed than propylene and isobutylene [19]. The conversion level decreased with reactants in the order of i-C4HlO > C3RS> C2H6. The order is the same as that of the C-R bond strength. This agreement suggests that the rate-determining step for the oxidation of lower alkanes involves C-H dissociation such as i-C4HI0 ........ i-C4HS and/or i-C4HI0 ........ COx. The first-order dependency of the rate on the partial pressure of isobutane supports the idea.
Isobutane
• •
Propane
MAA MAL
IilAA
ml ACR IZI AeOB
cal\; 8 C 2 84 El co
[II
Ethane
o o
10
20
30
C02
40
Conversion and yield / % Figure 6. Oxidation of propane and ethane as well as isobutane catalyzed by CS2.5NiO.OSH1.34PVMo11040· Catalyst, 1.0 g. Oxidation of isobutane; iJSobutane, 17 vol%; 02, 33 vol%; N2, balance; total flow rate, ca. 30 cm 3 ·min- l ; react. temp., 340 ·C. Oxidation of pr0B,ane; propane, 30 vol%; 02, 40 vol%; N2, balance; total flow rate, ca. 30 cm ·min-I ; react. temp., 340 ·C. Oxidation of ethane; ethane, 33 vol%; 02,33 vol%; N2, balance; total flow rate, ca. 15 cmg·min- I ; react. temp., 350 ·C.
3.6. Stability of CS2.5NiO.osBl.34PVMol1040 Catalyst In order to confirm the structure of CS2.5NiO.OSH1.26PVMo11040 during the reaction, the infrared spectra were measur~d before and after the reaction (Fig. 7). In the range of 700 - 1100 cm- I , the sample showed the intense 1061-cm- 1 , 965-cm- l , S65-cm- 1, and 792-cm- r bands characteristic of Keggin structure [20]. The infrared spectrum little changed after the reaction except for the slight decrease in the intensity of S66-cm- 1 and 797-cm- 1 bands. The slight decrease in the intensity is probably due to the slight reduction of Moo+ to M05+ in Keggin anion [21] and consistent with
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the change of the catalyst color to yellow green (corresponding to slightly reduced state) during the catalytic test. No 1020-cm- 1 band due to free vanadium oxides appeared and the shoulder at ca. 1077 cm- 1 was observed for the spent catalyst, suggesting that vanadium ion is introduced in the Keggin anion [22]. In addition, in the 29 range of5 - 35 ., the XRD patterns of CS2.5NiO.OSH1.26PVMoll 040 showed the signals at 10.5, lS.3, 23.S, 26.1, 30.2, and 32.1 • (cubic system) and little changed after the reaction. No signals due to Mo03.3pd V205 were observed. 1 P NMR spectra of CS2.5NiO.OSH1.26PVMoll 040 little changed during the reaction. Therefore, these data suggest the maintenance of the Keggin structure during b the reaction in the range of 300 - 400 ·C. TG and DTA data support the idea. Such high 1100 900 thermal stability of Keggin 700 anion by the Cs salt formation Wavenumber I em-! has been reported [23]. Further experiments are in Figure 7. Infrared spectra of asprepared and spent progress to investigate the stability in more detail. CS2.5NiO.OSH1.26PVMoll040 (KBr pellet). (a), the as-prepared sample; (b), the 4. CONCLUSION sample after reaction at 340 ·C. To summarize, the above results demonstrate: (1) The example of c.atalytic 0;gdation of lower alkanes by molecular oxygen on Cs+, Ni~+, and V +-substituted heteropolymolybdate species; (2) that the preferred catalyst is CS2.5NiO.OSH1.26PVMol1040; (3) that the yield of MAA reached maximum at 340 ·C and the value is 9.0%, which is higher than the maximum value (6.2%) reported at the likely steady-state in patent literatures.
REFERENCES 1. M. Misono, Catal. Rev. Sci. Eng., 29 (1987) 269; Y. Jeannin and M. Fournier, Pure Appl. Chern., 59 (1987) 1529; M. T. Pope and A. Muller, Angew. Chem. Int. Ed. Engl., 30 (1991) 34; Y. Ono, Perspectives in Catalysis, Blachwell Sci. Publ., London, 1992, p. 431; Y. Izumi, K Urabe and A. Onaka, Zeolite, Clay, and Heteropoly Acid in Organic Reactions, Kodansha-VCH, Tokyo-Weinheim, 1992; N. Mizuno and M. Misono, J.
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Mol. Catal., 86 (1994) 319; T. Okuhara, N. Mizuno, and M. Misono, Adv. Catal., in press. 2. D. Artz, Catalysis Today, 18 (1993) 173. 3. R. A Sheldon, Dioxygen Activation and Homogeneous Catalytic Oxidation, Elsevier Sci. Publ. B.V., Amsterdam, 1991, p.573. 4. M. Misono and N. Nojiri, Appl. Catal., 64 (1990) 1. 5. G. Centi, Catal. Lett., 22 (1993) 53. 6. O. V. Krylov, Catal Today, 18 (1993) 209; R. A Perlana, D. J. Taube, E. R. Evitt, D. G. LOffier, P. R.·Wentrcek, G. Voss and T. Masuda, Nature, 259 (1993) 340; D. A Hickman and L. D. Schmidt, Nature, 259 (1993) 343. 7. S. S. Hong and J. B. Moffat, Appl. Catal. A, 109 (1994) 117 and references 1 - 29 therein. 8. Y.-C. Kim, W. Ueda and Y. Moro-oka, J. Chem. Soc., Chem. Commun., (1989) 652. 9. G. Centi, Ed., Vanadyl Pyrophosphate Catalysts, Catal Today, 16 (1993). 10. M. Ai, Proc. 8th Intern. Congr. Catal., Berlin, 1984, Verlag Chemie, Weinheim, 1985, vol. 5, p. 475.; G. Centi, J. P. Nieto, C. Iapalucci, K. Bruckman and E. M. Serwicka, Appl. Catal., 46 (1989) 197; G. Centi, V. Lena, F. Trifiro, D. Ghoussoub, C. F. Aissi, M. Guelton and J. P. Bonnelle, J. Chem. Soc., Faraday Trans., 86 (1990) 2775. 11. The Chemical Engineer, 13 Sept., (1990) 3. 12. (a)N. Mizuno, T. Tateishi, T. Hirose and M. Iwamoto, Chem. Lett., (1993) 2137; idem., J. Mol. Catal., 88 (1994) L125. (b)N. Mizuno, T. Tateishi, and M. Iwamoto, J. Chem. Soc., Chern. Commun., (1994) 1411; idem., Appl. Catal. A, 118 (1994) L1; idem., ibid., 128 (1995) L165. 13. H. Imai, M. Nagatsukasa, and A. Aoshima, Asahi Chemical Industry Co., Ltd., JP Patent No. 132832 (1987); H. Imai, T. Yamaguchi, and M. Sugiyama, Asahi Chemical Industry Co., JP Patent, No. 145249 (1988); S. Yamamatsu and T. Yamaguchi, Asahi Chemical Industry Co., No. 42033 (1990); K. Nagai, Y. Nagaoka, H. Sato, and M. Ohsu, Sumitomo Chemical Co., Ltd., Eur Patent No. 418657 (1990): The highest yields ofMAA and MAL reported at the likely stationary state were 6.2 and 1.2%, respectively. 14. Y. Konishi, K. Sakata, M. Misono, and Y. Yoneda, J. Catal., 77 (1982) 169. 15. H. Morl, N. Mizuno, and M. Misono, J. Catal., 131 (1991)133. 16. N. Mizuno, T. Watanabe, and M. Misono, J. Phys. Chem., 89 (1985) 80; N. Mizuno, T. Watanabe, and M. Misono, Bull. Chern. Soc. Jpn., 64 (1991) 243. 17. The catalytic activities of CsxH3-xPMo12040 for the oxidation of isobutane per surface area decreased with the increase in x; N. Mizuno, T. Tateishi, and M. Iwamoto, manuscript in preparation. 18. M. Ai, J. Catal., 101 (1986) 389. 19. The formation of isobutene was observed under oxygen poor condition (isobutane 33 vol%; 02, 13 vol%; N2, 54 vol%; total flow rates, ca. 15 cm 3-·min- t ; CS2.5HO.5PMo12040, 1 g; react. temp., 340 ·C). 20. C. Rocchiccioli-Deltcheff, R. Thouvenot, and R. Frank, Spectrochim. Acta, 32A (1976) 587. 21. K. Eguchi, N. Yamazoe, and T. Seiyama, J. Catal., 83 (1983) 32; N. Mizuno, K. Katamura, Y. Yoneda, and M. Misono, J. Catal., 83 (1983) 384; M. Akimoto and E. Echigoya, Chem. Lett., (1981) 1759. 22. D. Casarini, G. Centi, P. Jiru, V. Lena, and Z. Tvaruzkova, J. Catal., 143 (1993) 325. 23. K. Eguchi, N. Yamazoe, and T. Seiyama, Nippon Kagaku Kaishi, (1981) 336.
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Direct Oxidation of Isobutane into Methacrylic Acid over Cs, Ni, and V-substituted H3PMo 12040 Heteropoly Compounds t Noritaka. Mizuno,*,a Wonchull Han,a Tetsuichi Kudo,a and Masakazu Iwamoto b aInstitute ofIndustrial Science, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan bCatalysis Research Center, Hokkaido University, Sapporo 060, Japan Keggin-type heteropolymolybdates have been found to catalyze the selective oxidation of isobutane with molecular oxygen. The preferable contents of Cs and V are 2.5 and 1, respectively, and the addition of Ni enhanced the catalytic activity for the oxidation of isobutane. The presence of steam did not affected the catalytic' performance very much. CS2.5NiO.OSH1.34PVMoll 040 also catalyzegd the oxidation of propane and ethane. It was suggested by m, XRD, and flp NMR data that the Keggin structure was maintained during the oxidation. 1. INTRODUCTION
The oxidizing properties or strong acidity of heteropoly compounds induce a lot of studies on the heterogeneous and homogeneous catalysis [1]. The additional attractive and important aspects are oxidatively stability and introduction of various components into heteropolyanions and the countercations. Methacrylic acid has been used for the synthesis of methyl methacrylate, an important monomer of resin. Industrial production of methacrylic acid has traditionally been achieved by the reaction of acetone with hydrogen cyanide [2-4]. However, the process uses the dangerous hydrogen cyanide and overproduces solid ammonium bisulfate. Recently, alternative methods, the methylation of propionaldehyde and the oxidation of isobutene, have been developed [2-4]. These processes still have problems to use high-price feedstocks and consist of two-step reactions [2-5]. Therefore, it would be much more desirable to use cheaper feedstock, isobutane and to produce methacrylic acid directly from isobutane and molecular oxygen [2,3]. Oxyfunctionalization of lower paraffins such as methane, ethane, propane, and butanes is industrially important because of the cheapness [2,3,5]. Recently, the active catalysts for the oxidation of lower paraffins have been studied [6-10] and the processes for the oxidation of propane and n-butane have been industrialized [1,11]. Recently the present authors has t We gratefully acknowledge Mr. M. Tateishi for experimental assistance. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan, Yazaki Memorial Foundation for Science and Technology, and Ciba-Geigy Foundation (Japan) for Promotion of Science. * To whom correspondence should be addressed.
1002
expanded our fundamental knowledge regarding metal-catalyzed 02-based oxidation of cydohexane and other paraffins by using heteropoly compounds [12].
Here we wish to report how catalytic activity of H3PMo12040 for the oxidation ofisobutane is changed by Cs, Ni, and V-substitution.
2. EXPERIMENTAL 2.1. Catalysts H3+xPMo12-xVx040 heteropolyacids were commercially obtained from Nippon Inorganic Colour and Chemical Co., Ltd. The other reagents used were analytical grade and used without further purification. The catalysts were ..prepared as follows: An aqueous solution of nickel nitrate (O.OS mol·dm-D) was added drop~se to an aqueous solution of H3+xPVxMo12-x040 (x = 0 - 4; 0.06 mol·dm- ), followed by. the addition of an aqueous solution of alkaline carbonates (O.OS mol·dm-3 ) at SO ·C. The resulting suspension or solution was evaporated to dryness at SO ·C. The actual composition may be CSqNiO.OSHyPVxMo12-xOz, but in this paper they will be designated as CSqNIO.OSH2.S4+x-qPVxMo12-x040. BET surface areas were measured by means of N2 adsorption using Omnisorp 100MP (Coulter) after samples were evacuated at 300 ·C for 1 h. The Powder X-ray diffraction patterns were recorded on a po}Vder X-ray diffractometer (Materials Analysis and Characterization, MXpiS) by using Cu Ka radiation. The infrared spectra of KBr pellets were recorded on a Paragon 1000PC spectrometer (Perkin Elmer). 2.2. Reaction The reaction was performed in a flow reactor (Pyrex tube, 12 mm internal diameter) at the applied temperature of 260·360 ·C under an atmospheric PNresbsulre. Thelfeed gahs co~sisted 0df 17 v0all%flof isobutane, 33 v01% of 02,.and 1 ow rates were ca. 30 cm 3 ·mm-. 2 a ance un ess ot erwlse state. T ot It was confirmed for CS2.SNiO.OSHO.34PMo12040 that the conversion and selectivity was little changed by the mixing with SiC (l.S g) to prevent an undesirable temperature rise. Prior to the reaction, 1 g of each catalyst was treated in an 02 stream (60 cm iS 'min- 1 ) for 1 h at 300 ·C. The gases at the outlet of the reactor were taken out intermittently with the aid of a sampler directly connected to the system and analyzed by FID and TCD gas chromatograph with FFAP, Porapack Q, and Molecular Sieve SA columns. The conversion and the selectivity were collected after 2 - S h of reaction, when nearly steady-state conversion and selectivity were obtained for each catalyst. Selectivity was calculated on the C4 (isobutane)-basis. The carbon balance was in the range of90 -100%.
3. RESULTS AND DISCUSSION 3.1. Oxidation of Isobutane Figure 1 shows the time course of the oxidation of isobutane catalyzed by CS2.SNiO.OSHl.34PVMoll040 at 320 ·C. The conversion and selectivity reached almost constant after 4 h; e.g., the conversions were 21, IS, 17, IS, and lS% at 1, 2, 3, 4, and S h, respectively. The main products were methacrylic acid (MAA), methacrolein (MAL), acetic acid, and COx (CO + C02). The same products were observed for the other heteropoly catalysts used. The color of CS2.SNiO.OSH1.34PVMoll040 after having been used as
1003
catalyst was yellow green, suggesting that the oxidation state is rather high under reaction conditions. The preliminary kinetic measurements showed that the pressure dependencies were expressed by -d[i-C4HIOJ/dt = k-Pi-C4HIOl(-O)·P020 at 320°C for CS2.5NiO.08H1.34PVMo11 040, in the range of partial pressure of isobutane, 0.16 - 0.25 atm and oxygen, 0.33 - ~41 atm. Above the partial pressure of isobutane of 0.25 atm, Pi-C4HI0 dependence was reached. Conversion was kept below 16% for the determination of the pressure dependency. Itwas confirmed that conversions linearly increased up to 20% with W (weight of catalyst)! F (flow rate).
80
-~
~ ~ U GO
... ." -;;
• ]=
60 40
~
I:
I: GO
20
~
=
I:
U
0
::i====i===iI
0
1
2
3
:: g
g
4
5
Time!h Figure 1. Time course of the oxidation of isobutane catalyzed by CS2.5NiO.08H1.34PVMoll040 at 320°C. 0, e, _, LJ, and A are conversion ofisobutane and selectivities to MAA, MAL, AcOH, and COx, respectively. The conversion of isobutane monotonously increased with increase in the reaction temperature and reached up to 37% at 360°C. The selectivity to MAL monotonously decreased with increase in the reaction temperature and that to MAA reached the maximum around 285 - 300°C. The decrease of MAA and MAL above 300°C is mainly due to the successive oxidation of MAA and MAL because the amount of COx increased in parallel. The temperature dependency of the yields of MAA and MAL for CS2.5NiO.08H1.34PVMoll040 is shown in Fig. 2. The yield ofMAA reached maximum at 340°C. The value is 9.0% and higher than 6.2%, the maximum value reported at the likely steady-state in patent literatures [13J. The yield of MAL was approximately constant at and above 285°C. Effect of steam on the conversions and selectivities is shown in Table 1. Conversion was changed little by the addition of s 5.2 vol% steam, and the selectivity to MAA slightly increased. The constant conversion is in
1004
contrast with the great increase of the conversion for the oxidation of methacrolein and acetaldehyde catalyzed by H3PMo12040 [14,15]. The only slight effect of steam in the present system is probably due to (1) the hydrophobicity of Cs salt and/or (2) low partial pressure of steam. 10
:2-~
8
::is
"Cl
=
Cll
6
~
....::is 4
-0
~
Ql
~
2
o "-..._ ..
L...._ _........_ _- - - l
150
450
300
Temp./oe Figure 2. Temperature dependency of yields of MAA and MAL. Catalyst, CS2.5NiO.OSH1.34PVMoll040. • and _, the same as those in Fig. 1. Table 1 Effect of water vapor on oxidation of isobutane over Nio.osCsz.sH1.34PVMo ll 0 40 catalyst at 320 ·C a PHzO Conv. I % I vol% MAA
a
Selectivity / % MAL
AcOH
Sum of yields of COx
MAA+MAL/%
0
15
36
S
6
50
6.6
1.3
15
40
8
12
40
7.2
5.2
15
42
8
6
43
7.5
Isobutane, 17 vol%; 0z, 33 vol%; N z, balance; catalyst, 1.0 g; total flow rate,
ca. 30 cm3 ·min-l .
1005
3.2. Effect of Cs Salt Formation The catalytic properties of CsxNiO.OSH3.S4-xPVMoll 040 were changed by the Cs contents. The results on CsxNiO.OSH3.S4-xPVMoll040 (x = 0 - 3) catalysts at 340°C are shown in Fig. 3. Note that the results were compared at 340°C because of the lower catalytic activity of Csx NiO.OSH3.S4-xPVMo11040 (x = 0, 1). The conversions were 6, 4, 12, 31, and S% for x = 0, 1, 2, 2.5, and 3, respectively, and the highest conversion was observed at x = 2.5. The yields of MAA on Csx NiO.OsH3.S4-xPVMo11040 were 1.3, 1.S, 5.6, 9.0, and O.S% for x = 0, 1, 2, Thus the substitution of Cs+ for H+ in 2.5, and 3, respectively. NiO.OSH3.S4PVMoll 040 resulted in the great enhancement of the MAA production and the yield reached the maximum around x = 2.5. Similarly the substitution of Cs+ for H+ in H3PM012040 resulted in the great enhancement of the MAA production and the yield reached the maximum around x = 2.5 for CsxH3-xPM012040 catalysts [12]. The surface areas of CsxNiO.OSH4-xPVMoll 040 catalysts were 4.5 and 42.9 m~·g-1 for x = 0 and 2.5, respectively, and increased with the increase in the Cs content. Therefore, the high surface area of CS2.5NiO.OSH1.34PVMoll 040 is one of the factors controlling the high catalytic activity shown in Fig. 3. The catalytic activities per specific surface area decreased from x = 0 to 2.5. The decrease is mainly due to the decrease in the oxidizing power of catalysts with Cs substitution. A similar monotonous decrease of the catalytic activity per surface area was observed for the oxidation of CO, acetaldehyde, and methacrolein and are explained on the basis of the decrease in the oxidizing power of heteropoly catalysts with the Cs+-substitution [15-17].
40..-------------, MAA I2'J AeOH MAL EJ co o CO z
• •
o
1
2
2.S
3
x in CsxNio.osH3.84-xPVMol1040
Figure 3. Effect of Cs+-substitution for H+ in NiO.OSH3.S4PVMoll040 on oxidation of isobutane at 340 ·C.
1006
3.3. Effect of Ni Addition Figure 4 shows the effect of addition of Ni to CS2HPMo12040 on the catalytic performance. The addition of Ni to CS2HPMo12040 increased the conversion about 1.6 times higher than that on CS2HPMo12040 and the selectivities to MAA and MAL slightly changed. Therefore, the yields of MAA and MAL became higher than those on CS2HPMo12040. Similarly the conversion and yields ofMAA and MAL were increased by the addition of Ni to CS2.5HO.5PMo12040 as shown in Fig. 4 [12b]. The addition of Mn or Fe also increased the yields ofMAA and MAL and Ni most efficiently increased the yields of MAA and MAL [12b). The prefarable content of Ni was O.OS in CS2.5NixHO.5-2xPMo12040: The catalytic activity increased with the increase of x in CS2.5NixHO.5-2xPMo12040, reached maximum at x = O.OS, and decreased above O.S. The decrease in catalytic activity above x = O.OS would be due to the decrease in the protonic acidity.
-MAA -MAL f'JAcOH
-Acetone
ceo
o CO2
Cs2.5llo.sPM0 120 40
CsuNio.osIlo.s.PMo 12040
o
10 20 Conversion and yield / %
Figure 4. Effect of addition of Ni to CS2HPMo12040 CS2.5HO.5PMo12040 on the catalytic performance at 340 ·C.
30
and
3.4. Effect of v5+·Substitution The results on CS2.5NiO.OSHO.34+xPVxMo12-x040 catalysts are shown in Fig. 5. The conversions were 10, 15, 13, 12, and 10% for x = 0, 1, 2, 3, and 4, respectively, and the highest conversion was observed around x = 1. The selectivities to MAA on CS2.5NiO.OSHO.34+xPVxMo12-x040 catalysts were 27,36, 2S, 10, and 4% for x = 0, 1,2,3, and 4, respectively, and the highest selectivity to MAA was also observed at x = 1. It follows that the substitution ofV5 +-for Mo6+ in CS2.5NiO.OSHO.34PMo12040 resulted in the enhancement of the MAA production and the yield reached the maximum (5.4%) at x = 1. It is noted that the yield of MAA on CS2.5NiO.OSH1.34PVMoll040 was about two times higher than that on
}007
CS2.5NiO.OSHO.34PMo12040 (shown in Fig. 4). The maximum yield of MAL was also observed around x = 1 and was 1.4%. Thus the sum of the yields of MAA and MAL upon CS2.5NiO.OSH1.34PVMoll040 reached to 6.S% and was the highest among CS2.5NiO.OSHO.34+xPVxMo12-x040 catalysts. The selectivities to MAA and MAL for the oxidation o~isobutane catalyzed by H3PMo12040 at 340·C were also increased by the V +-substitution: The selectivities to MAA were 4, 30, 34, 14, and 9% for x = 0, 1, 2, 3, and 4, respectively, and those to MAL were IS, 36, 2S, 14, 21%, respectively, although the conversions were in the range of 5 • S% and low. A Similar e~ancement of the catalytic activity of H3PMo12040 by the V +-substitution for the oxidation of n-pentane and n-butane has been reported [10].
20 . . . - - - - - - - - - - - - - - , • •
o
o
1
2
MAA fJ AcOH MAL lEI CO CO 2
o
3
4
x in CS2.sNio.08Ho.34+xPVxM012_x040
Figure 5. Effect ofV5+-substitution for Mo in CS2.5NiO.OSHO.34PMo12040 on oxidation ofisobutane at 320 ·C.
3.5. Oxidation of Propane and Ethane Catalyzed by CS2.5NiO.osHl.34PVMol1040 Figure 6 shows the results of oxidation of propane and ethane as well as isobutane catalyzed by CS2.5NiO.OSH1.34PVMoll040. Propane can be oxidized into acrylic acid (abbreviated by AA), acrolein (ACR), propylene, acetic acid, CO, and C02 at S.5% conversion and 340 ·C. The yield of AA was 1.6% and increased to 4.9% at 400 ·C. In the oxidation of propane, CS2.5FeO.osH1.26PVMoll040 gave the highest yield of acrylic acid, 13% (at 3S0 ·C) [12b], higher than 10.5% reported on highly active V205-P205-Te02 catalyst [IS]. CS2.5NiO.OSH1.34PVMoll 040 also catalyzed the oxidation of ethane: the conversion is 2% and the selectivities to ethylene and C02 were 44 and 56%, respectively, at 350 ·C. Neither corresponding aldehyde nor acid were
1008
observed for the oxidation of ethane and only oxidative dehydrogenation and complete oxidation proceeded. This is probably due to the less reactivity of ethylene formed than propylene and isobutylene [19]. The conversion level decreased with reactants in the order of i-C4HlO > C3RS> C2H6. The order is the same as that of the C-R bond strength. This agreement suggests that the rate-determining step for the oxidation of lower alkanes involves C-H dissociation such as i-C4HI0 ........ i-C4HS and/or i-C4HI0 ........ COx. The first-order dependency of the rate on the partial pressure of isobutane supports the idea.
Isobutane
• •
Propane
MAA MAL
IilAA
ml ACR IZI AeOB
cal\; 8 C 2 84 El co
[II
Ethane
o o
10
20
30
C02
40
Conversion and yield / % Figure 6. Oxidation of propane and ethane as well as isobutane catalyzed by CS2.5NiO.OSH1.34PVMo11040· Catalyst, 1.0 g. Oxidation of isobutane; iJSobutane, 17 vol%; 02, 33 vol%; N2, balance; total flow rate, ca. 30 cm 3 ·min- l ; react. temp., 340 ·C. Oxidation of pr0B,ane; propane, 30 vol%; 02, 40 vol%; N2, balance; total flow rate, ca. 30 cm ·min-I ; react. temp., 340 ·C. Oxidation of ethane; ethane, 33 vol%; 02,33 vol%; N2, balance; total flow rate, ca. 15 cmg·min- I ; react. temp., 350 ·C.
3.6. Stability of CS2.5NiO.osBl.34PVMol1040 Catalyst In order to confirm the structure of CS2.5NiO.OSH1.26PVMo11040 during the reaction, the infrared spectra were measur~d before and after the reaction (Fig. 7). In the range of 700 - 1100 cm- I , the sample showed the intense 1061-cm- 1 , 965-cm- l , S65-cm- 1, and 792-cm- r bands characteristic of Keggin structure [20]. The infrared spectrum little changed after the reaction except for the slight decrease in the intensity of S66-cm- 1 and 797-cm- 1 bands. The slight decrease in the intensity is probably due to the slight reduction of Moo+ to M05+ in Keggin anion [21] and consistent with
1009
the change of the catalyst color to yellow green (corresponding to slightly reduced state) during the catalytic test. No 1020-cm- 1 band due to free vanadium oxides appeared and the shoulder at ca. 1077 cm- 1 was observed for the spent catalyst, suggesting that vanadium ion is introduced in the Keggin anion [22]. In addition, in the 29 range of5 - 35 ., the XRD patterns of CS2.5NiO.OSH1.26PVMoll 040 showed the signals at 10.5, lS.3, 23.S, 26.1, 30.2, and 32.1 • (cubic system) and little changed after the reaction. No signals due to Mo03.3pd V205 were observed. 1 P NMR spectra of CS2.5NiO.OSH1.26PVMoll 040 little changed during the reaction. Therefore, these data suggest the maintenance of the Keggin structure during b the reaction in the range of 300 - 400 ·C. TG and DTA data support the idea. Such high 1100 900 thermal stability of Keggin 700 anion by the Cs salt formation Wavenumber I em-! has been reported [23]. Further experiments are in Figure 7. Infrared spectra of asprepared and spent progress to investigate the stability in more detail. CS2.5NiO.OSH1.26PVMoll040 (KBr pellet). (a), the as-prepared sample; (b), the 4. CONCLUSION sample after reaction at 340 ·C. To summarize, the above results demonstrate: (1) The example of c.atalytic 0;gdation of lower alkanes by molecular oxygen on Cs+, Ni~+, and V +-substituted heteropolymolybdate species; (2) that the preferred catalyst is CS2.5NiO.OSH1.26PVMol1040; (3) that the yield of MAA reached maximum at 340 ·C and the value is 9.0%, which is higher than the maximum value (6.2%) reported at the likely steady-state in patent literatures.
REFERENCES 1. M. Misono, Catal. Rev. Sci. Eng., 29 (1987) 269; Y. Jeannin and M. Fournier, Pure Appl. Chern., 59 (1987) 1529; M. T. Pope and A. Muller, Angew. Chem. Int. Ed. Engl., 30 (1991) 34; Y. Ono, Perspectives in Catalysis, Blachwell Sci. Publ., London, 1992, p. 431; Y. Izumi, K Urabe and A. Onaka, Zeolite, Clay, and Heteropoly Acid in Organic Reactions, Kodansha-VCH, Tokyo-Weinheim, 1992; N. Mizuno and M. Misono, J.
1010
Mol. Catal., 86 (1994) 319; T. Okuhara, N. Mizuno, and M. Misono, Adv. Catal., in press. 2. D. Artz, Catalysis Today, 18 (1993) 173. 3. R. A Sheldon, Dioxygen Activation and Homogeneous Catalytic Oxidation, Elsevier Sci. Publ. B.V., Amsterdam, 1991, p.573. 4. M. Misono and N. Nojiri, Appl. Catal., 64 (1990) 1. 5. G. Centi, Catal. Lett., 22 (1993) 53. 6. O. V. Krylov, Catal Today, 18 (1993) 209; R. A Perlana, D. J. Taube, E. R. Evitt, D. G. LOffier, P. R.·Wentrcek, G. Voss and T. Masuda, Nature, 259 (1993) 340; D. A Hickman and L. D. Schmidt, Nature, 259 (1993) 343. 7. S. S. Hong and J. B. Moffat, Appl. Catal. A, 109 (1994) 117 and references 1 - 29 therein. 8. Y.-C. Kim, W. Ueda and Y. Moro-oka, J. Chem. Soc., Chem. Commun., (1989) 652. 9. G. Centi, Ed., Vanadyl Pyrophosphate Catalysts, Catal Today, 16 (1993). 10. M. Ai, Proc. 8th Intern. Congr. Catal., Berlin, 1984, Verlag Chemie, Weinheim, 1985, vol. 5, p. 475.; G. Centi, J. P. Nieto, C. Iapalucci, K. Bruckman and E. M. Serwicka, Appl. Catal., 46 (1989) 197; G. Centi, V. Lena, F. Trifiro, D. Ghoussoub, C. F. Aissi, M. Guelton and J. P. Bonnelle, J. Chem. Soc., Faraday Trans., 86 (1990) 2775. 11. The Chemical Engineer, 13 Sept., (1990) 3. 12. (a)N. Mizuno, T. Tateishi, T. Hirose and M. Iwamoto, Chem. Lett., (1993) 2137; idem., J. Mol. Catal., 88 (1994) L125. (b)N. Mizuno, T. Tateishi, and M. Iwamoto, J. Chem. Soc., Chern. Commun., (1994) 1411; idem., Appl. Catal. A, 118 (1994) L1; idem., ibid., 128 (1995) L165. 13. H. Imai, M. Nagatsukasa, and A. Aoshima, Asahi Chemical Industry Co., Ltd., JP Patent No. 132832 (1987); H. Imai, T. Yamaguchi, and M. Sugiyama, Asahi Chemical Industry Co., JP Patent, No. 145249 (1988); S. Yamamatsu and T. Yamaguchi, Asahi Chemical Industry Co., No. 42033 (1990); K. Nagai, Y. Nagaoka, H. Sato, and M. Ohsu, Sumitomo Chemical Co., Ltd., Eur Patent No. 418657 (1990): The highest yields ofMAA and MAL reported at the likely stationary state were 6.2 and 1.2%, respectively. 14. Y. Konishi, K. Sakata, M. Misono, and Y. Yoneda, J. Catal., 77 (1982) 169. 15. H. Morl, N. Mizuno, and M. Misono, J. Catal., 131 (1991)133. 16. N. Mizuno, T. Watanabe, and M. Misono, J. Phys. Chem., 89 (1985) 80; N. Mizuno, T. Watanabe, and M. Misono, Bull. Chern. Soc. Jpn., 64 (1991) 243. 17. The catalytic activities of CsxH3-xPMo12040 for the oxidation of isobutane per surface area decreased with the increase in x; N. Mizuno, T. Tateishi, and M. Iwamoto, manuscript in preparation. 18. M. Ai, J. Catal., 101 (1986) 389. 19. The formation of isobutene was observed under oxygen poor condition (isobutane 33 vol%; 02, 13 vol%; N2, 54 vol%; total flow rates, ca. 15 cm 3-·min- t ; CS2.5HO.5PMo12040, 1 g; react. temp., 340 ·C). 20. C. Rocchiccioli-Deltcheff, R. Thouvenot, and R. Frank, Spectrochim. Acta, 32A (1976) 587. 21. K. Eguchi, N. Yamazoe, and T. Seiyama, J. Catal., 83 (1983) 32; N. Mizuno, K. Katamura, Y. Yoneda, and M. Misono, J. Catal., 83 (1983) 384; M. Akimoto and E. Echigoya, Chem. Lett., (1981) 1759. 22. D. Casarini, G. Centi, P. Jiru, V. Lena, and Z. Tvaruzkova, J. Catal., 143 (1993) 325. 23. K. Eguchi, N. Yamazoe, and T. Seiyama, Nippon Kagaku Kaishi, (1981) 336.