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Selective oxidation of hydrocarbons catalyzed by heteropoly compounds
3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.
Selective oxidation compounds
of
hydrocarbons
35
catalyzed
by
heteropoly
Makoto Misono, Noritaka Mizuno, Kei Inumaru, Gaku Koyano, and Xin-Hong Lu Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Selected characteristic features of heteropoly catalysts for the selective oxidation of hydrocarbons are described based on recent studies from our laboratory as well as from other groups. 1. C O R R E L A T I O N PERFORMANCE
BETWEEN
REDOX
PROPERTIES
AND
CATALYTIC
Fundamental correlations between redox properties and catalytic activity have successfully been established for the hydrogen form and alkali salts of 12molybdophosphoric acid [1]. Provided that the contributions of surface- and bulk-type catalysis are properly taken into account, good monotonic relationships are obtained between the catalytic activity for oxidation and the reducibility (or the oxidizing power) of the catalyst. The rate of oxidation of aldehydes, a surface-type reaction, correlates linearly with the surface reducibility of the catalyst, and the rate of oxidative dehydrogenation of cyclohexene, a bulk-type reaction, with the bulk reducibility [2].
~ a
1
~
2
~ ~0
m
t'~
3 0
N
~m ~o
1 2
L
0
.
|
2O
I
I
I
I
!
4O 6O 80 N MAA Yield/% Figure 1. Effect of V and Cs contents on the yield of methacrylic acid (MAA) at 350~ over H3+xPMo1~.-xVxO40and Cs2.75H0.2~+xPMo12_xVxO40catalysts.
36 The quantitative agreement between the rates of catalytic oxidation observed experimentally and those predicted from the reduction and oxidation rates of the catalysts measured independently demonstrated that the catalytic oxidation proceeded by redox cycles of the catalysts, that is, the redox mechanism or Marsvan Krevelen mechanism [3]. However, attempts to find similar relationships for mixed-metal heteropoly compounds such as molybdovanadophosphates have not been successful. This has been due to the low thermal stability of these compounds. For example, PMollVO4o and PMoloV204o decomposed to PM012040 and VOx above 200~ [4]. We attempted to stabilize the heteropolyanions by forming their cesium salts. Although the possibility of slight decomposition could not be excluded, high yields were obtained for the conversion of isobutyric acid to methacrylic acid (MAA) as shown in Fig. 1 [5]. A crystalline vanadium phosphorous oxide may be regarded in a broad sense to be a heteropoly compound. By applying e x s i t u and in s i t u spectroscopies (Raman spectroscopy, infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD), electron diffraction (ED)), we found that the surface of (VO)2P207 was reversibly oxidized to the X1 (5) phase of VOPO4 under reaction conditions in the oxidation of n-butane to maleic anhydride [6]. For example, the in s i t u Raman spectra measured at steady state flow reaction conditions showed that the surface changed reversibly between (VO)2P207 and X1 phase depending on the butane/oxygen ratio in the feed [7]. Correspondingly, the catalytic selectivity varied reversibly (Table 1). Table 1 Changes in the selectivity to maleic anhydride over (VO)2P207 with the partial pressure of n-butane Partial pressure Conversion a/% Selectivity/% Phase b of C4Hlo/% 1.5 c 53.8 52.4 P 0.75 c 52.0 48.8 P 0.25 c 56.8 23.4 P (+ X) 0.25 d 56.0 22.8 P +X 0.75 d 51.3 44.1 P 1.5 d 45.5 61.7 P The flow rate of the feed was adjusted at each step to obtain approximately the same conversion, b Determined by in situ Raman. P: (VO)2P207 and X: XI. r Partial pressure of butane was decreased stepwise from 1.5% (17%, partial pressure of 02 in parentheses) to 0.75 (18.5)% and then 0.25 (19.5)% after N2 treatment of the catalyst at 500~ The balance was N2. ~ Partial pressure of butane (02 content) was increased stepwise from 0.25% (19.5%, partial pressure of 02) to 0.75 (18.5)% and 1.5 (17)% after treatment in air at 460~ ,
,
,
,
37 2. S E L E C T I V E OXIDATION OF ALKANES There have been several a t t e m p t s to obtain oxygenated products from lower alkanes (C2 - C5) by using heteropoly catalysts. It has been reported t h a t the hydrogen form of H3PMo12040 catalyzes the oxidation of lower alkanes to aldehydes and carboxylic acids [8] and t h a t the substitution of V 5§ for Mo 6+ modified the catalytic activity and selectivity [1, 9 - 12]. By optimizing the q u a n t i t y and type of constituent elements of heteropolyanions and counter cations, fairly good yields were obtained for the oxidation of isobutane [13 - 17]. Recently, it was found t h a t acidic cesium salts of Keggin-type heteropolymolybdates can efficiently catalyze the oxidation of isobutane to methacrylic acid with molecular oxygen. The optimal contents of Cs and V were 2.5 and 1, respectively, and the addition of Ni enhanced the catalytic activity even further will be discussed below [13- 16]. The results for CsxH3-xPMo1204o catalysts are shown in Table 2 [13]. The highest conversion was observed around x = 2.5 - 2.85. The main products were methacrylic acid (MAA), methacrolein (MAL) and acetic acid (AcOH). The substitution of Cs for H in H3PMo12040 resulted in a great e n h a n c e m e n t of the MAA production and the yield reached a m a x i m u m at x = 2.5. The sum of the yields of MAA and MAL on Cs2.sHo.sPMo1204o reached 5.1%. The catalytic properties of Cs2.sHo.~PMo~2040 changed by the addition of transition metal ions [16]. The addition of Ni, Mn, or Fe increased the yields of MAA and MAL. In the case of Ni, the yields of MAA and MAL reached 6.5 and 1.5%, respectively. In contrast, Co, Cu, Hg, Pt, and Pd decreased the yields. The results for Cs2.sNioosHo.34+xPMo12-xVxO4o catalysts are shown in Table 3 [16]. The conversions were 10 - 15%. The highest selectivity to MAA was also observed at x = 1. It follows t h a t the substitution of V 5+ for Mo 6+ in Cs2.sNio.0sHo.34PMo~204o resulted in the e n h a n c e m e n t of MAA production and the yield reached a m a x i m u m at x = 1. Table 2 Oxidation of isobutane over CsxH3-xPMo12040 at 340~ a x Surface Conv. Rate Selectivity 5/% area /% /10 .5 mol /m 2 g-1 min -1 m 2 MAA MAL AcOH CO
Sum of
yields of MAA+ C02 MAL/% 0 1.1 7 1.34 4 18 8 44 26 1.5 1 2.1 6 0.60 23 17 10 32 18 2.4 2 5.9 11 0.39 34 10 7 29 21 4.8 2.5 9.5 16 0.36 24 7 7 41 21 5.1 2.85 46.0 17 0.08 5 10 5 44 37 2.4 3c 46.0 8 0.04 0 10 6 32 35 0.8 a Isobutane, 17 vol%; 02, 33 vol%; N2, balance; catalyst, 1.0 g; total flow rate, ca. 30 cm 3 min-1, b Calculated on C4 (isobutane)-basis. c The selectivity to acetone was 17%.
38 Table 3 Oxidation of isobutane catalyzed by Cs2.sNio.osHo.sa+~PMol2-xV~O4o at 320~ a X Conv./% Selectivity 5/% MAA MAL AcOH CO CO2 0 10 27 12 5 30 26 1 15 36 9 6 25 24 2 13 28 8 6 25 33 3 12 10 8 9 35 38 a,b Experimental conditions. See Table 2. Thus, the Keggin-type heteropolymolybdates such as Cse.5Ni0.08Hl.s4PMollVO40 fairly selectively catalyze the oxidation of isobutane into methacrolein and methacrylic acid with molecular oxygen. At 340~ the yield of methacrylic acid reached 9.0%. The 9.0% yield of A A was greater than the highest value of 6.2% reported in the patent literature at similar steady-state conditions [10]. Figure 2 shows a good correlation between the rates of oxidation of isobutane and non-catalytic reduction of catalysts by CO. The correlation noted in Fig. 2 indicates that the catalytic activity is controlled by the oxidizing ability of catalysts. It has been suggested for the case of CsxHs-~PMo1204o catalysts that the factors controlling the catalytic activity are the o 2 oxidizing ability and the O protonic acidity of catalysts [16]. r 9
C s2.5Mn+o.08H 1.5-0.08nPMo I 1VO 40
(M = Ni z+, Fe 3+) also catalyzed the oxidation of propane and ethane [18- 20]. Here, the rate and role of vanadium is of concern [21]. It is interesting that the reduced heteropoly compounds showed higher selectivity to methacrylic acid for the oxidation of isobutane [10, 13, 15, 22, 23]. Ueda et al. applied reduced 12-molybdophosphoric acid to the oxidation of propane and obtained 50% selectivity to acrylic acid and acrolein at 12% conversion [22, 23].
O
O
H
~
Cs2.85~
o~ r
0
S~Cs2 / ~ - - - Cs3
1 2 3 Rate of reduction by CO /10 .6 mol min-1 m-2 Figure 2 Correlation between the rates of catalytic oxidation of isobutane and those of non-catalytic reduction of catalysts by CO. Csx and H represent CsxHs-~PMoy204o and HsPMo12040, respectively.
39 3. SELECTIVE HYDROXYLATION OF BENZENE Various oxidants such as dioxygen, hydrogen peroxide, and alkyl hydroperoxides have been applied for the oxidation of hydrocarbons in the homogeneous liquid phase catalyzed by heteropoly catalysts [1, 24]. Below are presented results on the selective hydroxylation of benzene to phenol with H202 catalyzed by Keggin-type heteropolyanions. It is known t h a t Fenton or related reagents also catalyze this reaction [25]. A research group that includes one of the present authors has previously reported that the reaction proceeded selectively by using H202 and vanadium-substituted heteropolymolybdates or tungstates [26]. The present study is an extension of this earlier study. Recently the same reaction was attempted by using wellcharacterized K salts of vanadium-substituted heteropolytungstates [27]. The selectivity based on benzene was high, but the yield based on H202 was not given. Heteropoly compounds obtained commercially, H3+~PMo12-~V~O40 (x = 0 - 4), were purified by extraction with ether and subsequent recrystrallization. They are abbreviated as PMol2-xVx hereafter. Their IR spectra agreed with those reported in the literature [27]. 51V-NMR spectrum of PMoloV2 in aqueous solution showed that PMoloV2 was a mixture of three to four positional isomers of PMol0V2 and contained PMol~V at less t h a n 30% level. For comparison, NaVOa, V203, V204, and V205 were used. In the case of V2Ox, sulfuric acid was added in order to dissolve the catalyst. This addition resulted in improved yields. The reaction was usually carried out at 20 - 70~ in a four neck flask, by adding dropwise 10 ml of 0.08 M H202 aqueous solution (0.8 mmol) into a mixture of benzene 10 ml and water 15 ml. Catalyst (0.05 - 0.3 mmol) was dissolved in the water phase before the reaction. Hydroxylation of benzene took place in the water phase and a majority of phenol formed was transferred to the benzene phase. The concentration of water and benzene phases were analyzed periodically by liquid chromatography with o-cresol as a standard. The evolution of oxygen gas by the decomposition of H202 was measured volumetrically. The yield of phenol on the basis of H202 consumed tended to increase in parallel with the catalytic activity of each catalyst for the H202 decomposition in the absence of benzene. The sudden introduction of H202 into the reaction system caused the evolution of a significant amount of oxygen. After a short induction period the yield of phenol increased rapidly with time. When the concentration of H202 in the reaction system was kept low by adding dropwise very slowly a diluted H202 solution, the yield of phenol increased remarkably, the unproductive decomposition of H202 being suppressed. The selectivity to phenol was almost 100 % on the basis of benzene and reached above 90 % on the basis of H202. Typical results thus obtained at 65~ are summarized in Table 4. The yields are in the order of PMol0V2 > PMo9V3 > PMosV4 > PMo~V >> PMol2. NaVO3 and V2Ox showed modest performance. The turnover based on the catalyst was about 3 in the case of PMoloV2. It was confirmed that, upon the addition of H202
40 Table 4 Yields of phenol from the oxidation of benzene by hydrogen peroxide and vanadium compounds Yield of Selectivity of Catalyst Selectivity of Catalyst Yield of phenol/% phenol/% phenol/% phenol/% (H202 (benzene (benzene (H202 basis) basis) basis) basis) 65~ 65~ 45~ VO(C5H702)2 46.6 PMo1204o 0 0 92.1 NaVO3 23.8 PMo11VO4o 9.0 0 100 56.0 V205 (I-12804) 52.5 PMoloV204o 92.6 69.2 100 81.6 V204 (H2SO4) 21.8 PMooV304o 90.1 90.7 100 V203 (a2so4) 13.5 PMosV404o 68.1 73.6 100 Catalyst, 0.3 mmol; H20, 15 ml; C6H6, 10 ml. 10 ml of 0.08 M H202 was added dropwise very slowly. Reaction time: 1.5 h. after the reaction, the oxidation proceeded again at a similar rate. Therefore, there was little deactivation of catalyst. The optimum pH range for the phenol yield was 2 to 3 for PMoloV2 and PMo9V3, and 1 to 2 for PMollV and PMosV4. According to the UV-vis spectra, PMoIoV2 and PMogV3 were stable in the pH ranges of 2 - 3 and 1.5 - 3.5, respectively. The temperature dependencies are shown in Fig. 3. The optimum reaction temperature and time appear to depend on the polyanion species. 100 The IR spectra of the reaction solutions showed that 80the structures of the Keggin polyanions remained unchanged 60 r during the reaction. When the concentration of H202 was 40 O increased, however, new b a n d s CD .t-.4 appeared in the 500 - 600 cm -1 20 region possibly due to peroxo I I species. It seems that active 0 "AI " 10 20 30 40 50 60 70 80 peroxo species are formed by the Temperature/~ reaction of vanadiumFigure 3. Temperature dependencies of the substituted polyanions and activity for the oxidation of benzene with H2Oe or that other active oxygen hydrogen peroxide for 1.5h reaction time. species are derived from the Catalyst, 0.3 retool; H20, 15 ml; C6H6, 10 ml. peroxo species. Either or both 10 ml of 0.08 M H202 was added dropwise species are probably active for very slowly. the hydroxylation of benzene. A; H4PMollVO4o, A; H5PMoloV2040, The species tend to deactivate O; H6PMogV304o, 0; H:PMosV404o, by reaction between them (e. g., ["l; H3PM012040, X; V205 (H2804) dimerization), as indicated by
41 the fact that the unproductive decomposition of H202 to oxygen and water became dominant when their concentration was high. The induction period observed when H202 was added suddenly probably corresponds to the period for the formation of the active species. Thus, by choosing appropriate vanadium-substituted heteropolymolybdates and keeping the concentration of H202 low, efficient hydroxylation of benzene to phenol was achieved. The highest yield based on H202 was 93%, where the selectivity with respect to benzene consumed was 100%. 4. CONCLUSION Characteristic features of vanadium containing heteropoly catalysts for the selective oxidation of hydrocarbons have been described. MAA yield from isobutyric acid was successfully enhanced by the stabilization of the vanadiumsubstituted heteropolyanions by forming cesium salts. As for lower alkane oxidation by using vanadium containing heteropoly catalysts, it was found that the surface of (VO)2P207 was reversibly oxidized to the X1 (5) phase under the reaction conditions of n-butane oxidation. The catalytic properties of cesium salts of 12-heteropolyacids were controlled by the substitution with vanadium, the Cs salt formation, and the addition of transition metal ions. By this way, the yield of MAA from isobutane reached 9.0%. Furthermore, vanadium-substituted 12molybdates in solution showed 93% conversion on H202 basis in hydroxylation of benzene to phenol with 100% selectivity on benzene basis. REFERENCES
1. 2.
T. Okuhara, N. Mizuno, and M. Misono, Advan. Catal., 41 (1996) 113. M. Misono, N. Mizuno, H. Mori, K. Y. Lee, and T. Okuhara, Stud. Surf. Sci. Catal., 67 (1991)87. 3. N. Mizuno, T. Watanabe, and M. Misono, J. Phys. Chem., 94 (1990) 890. 4. E. Cadot, C. Marchal, M. Fournier, A. Teze, and G. Herve, in Polyoxometalates: From Platonic Solids to Anti-retroviral Activity (Eds. M. T. Pope and A. Muller), Kluwer, Dordrecht, 1994, p 315. 5. K.Y. Lee, S. Oishi, H. Igarashi, and M. Misono, Catal. Today, 33 (1997) 183. 6. G. Koyano, F. Yamaguchi, T. Okuhara, and M. Misono, Catal. Lett., 41 (1996) 149. 7. G. Koyano, T. Saito, and M. Misono, Chem. Lett., (1997) in press. 8. H. Krieger and L. S. Kirch, Rhom and Haas Co., Eur. Patent No. 0010902 (1979). 9. G. Centi, M. Burttini, and F. Trifiro, Appl. Catal., 32 (1987) 353; J. B. Moffat, ibid., 146 (1996) 65. 10. H. Imai, T. Yamaguchi, and M. Sugiyama, Asahi Chemical Industry Co., JP 145249 (1988). 11. K. Nagai, Y. Nagaoka, H. Sato, and M. Osu, Sumitomo Chemical Co., JP 106839 (1991).
42 12. G. Centi, J. L. Nieto, C. Iapalucci, K. Bruckman, and E. M. Serwicka, Appl. Catal., 46 (1989) 197; F. Cavani, E. Etienne, M. Favaro, A. Galli, F. Trifiro, and G. Hecquet, Catal. Lett., 32 (1995) 215. 13. N. Mizuno, M. Tateishi, and M. Iwamoto, J. Chem. Soc. Chem. Commun., (1994) 1411. 14. N. Mizuno, M. Tateishi, and M. Iwamoto, Appl. Catal. A: General, 118 (1994) L1. 15. N. Mizuno, W. Han, T. Kudo, and M. Iwamoto, Stud. Surf. Sci. Catal., 101 (1996) 1001. 16. N. Mizuno, M. Tateishi, and M. Iwamoto, J. Catal., 163 (1996) 87. 17. F. Cavani, E. Etinne, M. Favaro, A. Gall, F. Trifiro, and G. Hecquet, Catal. Lett., 32 (1995) 215. 18. N. Mizuno, M. Tateishi, and M. Iwamoto, Appl. Catal. A: General, 128 (1995) L165. 19. N. Mizuno, D. J. Suh, W. Han, T. Kudo, and M. Iwamoto, J. Mol. Catal. A: Chemical, in press. 20. N. Mizuno, W. Han, and T. Kudo, Chem. Lett., (1996) 1121. 21. R. Bayer, C. Marchal, F. X. Liu, A. Teze, and G. Herve, J. Mol. Catal. A: Chemical, 110 (1996) 65. 22. W. Ueda, Y. Suzuki, W. Lee, and S. Imaoka, Stud Surf. Sci. Catal., 101 (1996) 1065. 23. W. Ueda and Y. Suzuki, Chem. Lett., (1995) 541. 24. C. L. Hill and C. M. P-M. Charta, Coord. Chem. Rev., 143 (1995) 407. 25. E. g., S. Tamagaki, M. Sasaki, and W. Takagi, Bull. Chem. Soc. Jpn., 62 (1989) 153; S. Ito, A. Mitarai, K. Hikino, M. Hirama, and K. Sasaki, J. Org. Chem., 57 (1992)6937. 26. R. D. Huang, X. H. Lu, B. J. Zhang, and E. B. Wang, Chinese Chem. Lett., 4 (1993) 319. 27. K. Nomiya, H. Yanagibayashi, C. Nozaki, K. Kondoh, E. Hiramatsu, and Y. Shimizu, J. Mol. Catal., 114 (1996) 181.
Selective oxidation compounds
of
hydrocarbons
35
catalyzed
by
heteropoly
Makoto Misono, Noritaka Mizuno, Kei Inumaru, Gaku Koyano, and Xin-Hong Lu Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Selected characteristic features of heteropoly catalysts for the selective oxidation of hydrocarbons are described based on recent studies from our laboratory as well as from other groups. 1. C O R R E L A T I O N PERFORMANCE
BETWEEN
REDOX
PROPERTIES
AND
CATALYTIC
Fundamental correlations between redox properties and catalytic activity have successfully been established for the hydrogen form and alkali salts of 12molybdophosphoric acid [1]. Provided that the contributions of surface- and bulk-type catalysis are properly taken into account, good monotonic relationships are obtained between the catalytic activity for oxidation and the reducibility (or the oxidizing power) of the catalyst. The rate of oxidation of aldehydes, a surface-type reaction, correlates linearly with the surface reducibility of the catalyst, and the rate of oxidative dehydrogenation of cyclohexene, a bulk-type reaction, with the bulk reducibility [2].
~ a
1
~
2
~ ~0
m
t'~
3 0
N
~m ~o
1 2
L
0
.
|
2O
I
I
I
I
!
4O 6O 80 N MAA Yield/% Figure 1. Effect of V and Cs contents on the yield of methacrylic acid (MAA) at 350~ over H3+xPMo1~.-xVxO40and Cs2.75H0.2~+xPMo12_xVxO40catalysts.
36 The quantitative agreement between the rates of catalytic oxidation observed experimentally and those predicted from the reduction and oxidation rates of the catalysts measured independently demonstrated that the catalytic oxidation proceeded by redox cycles of the catalysts, that is, the redox mechanism or Marsvan Krevelen mechanism [3]. However, attempts to find similar relationships for mixed-metal heteropoly compounds such as molybdovanadophosphates have not been successful. This has been due to the low thermal stability of these compounds. For example, PMollVO4o and PMoloV204o decomposed to PM012040 and VOx above 200~ [4]. We attempted to stabilize the heteropolyanions by forming their cesium salts. Although the possibility of slight decomposition could not be excluded, high yields were obtained for the conversion of isobutyric acid to methacrylic acid (MAA) as shown in Fig. 1 [5]. A crystalline vanadium phosphorous oxide may be regarded in a broad sense to be a heteropoly compound. By applying e x s i t u and in s i t u spectroscopies (Raman spectroscopy, infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD), electron diffraction (ED)), we found that the surface of (VO)2P207 was reversibly oxidized to the X1 (5) phase of VOPO4 under reaction conditions in the oxidation of n-butane to maleic anhydride [6]. For example, the in s i t u Raman spectra measured at steady state flow reaction conditions showed that the surface changed reversibly between (VO)2P207 and X1 phase depending on the butane/oxygen ratio in the feed [7]. Correspondingly, the catalytic selectivity varied reversibly (Table 1). Table 1 Changes in the selectivity to maleic anhydride over (VO)2P207 with the partial pressure of n-butane Partial pressure Conversion a/% Selectivity/% Phase b of C4Hlo/% 1.5 c 53.8 52.4 P 0.75 c 52.0 48.8 P 0.25 c 56.8 23.4 P (+ X) 0.25 d 56.0 22.8 P +X 0.75 d 51.3 44.1 P 1.5 d 45.5 61.7 P The flow rate of the feed was adjusted at each step to obtain approximately the same conversion, b Determined by in situ Raman. P: (VO)2P207 and X: XI. r Partial pressure of butane was decreased stepwise from 1.5% (17%, partial pressure of 02 in parentheses) to 0.75 (18.5)% and then 0.25 (19.5)% after N2 treatment of the catalyst at 500~ The balance was N2. ~ Partial pressure of butane (02 content) was increased stepwise from 0.25% (19.5%, partial pressure of 02) to 0.75 (18.5)% and 1.5 (17)% after treatment in air at 460~ ,
,
,
,
37 2. S E L E C T I V E OXIDATION OF ALKANES There have been several a t t e m p t s to obtain oxygenated products from lower alkanes (C2 - C5) by using heteropoly catalysts. It has been reported t h a t the hydrogen form of H3PMo12040 catalyzes the oxidation of lower alkanes to aldehydes and carboxylic acids [8] and t h a t the substitution of V 5§ for Mo 6+ modified the catalytic activity and selectivity [1, 9 - 12]. By optimizing the q u a n t i t y and type of constituent elements of heteropolyanions and counter cations, fairly good yields were obtained for the oxidation of isobutane [13 - 17]. Recently, it was found t h a t acidic cesium salts of Keggin-type heteropolymolybdates can efficiently catalyze the oxidation of isobutane to methacrylic acid with molecular oxygen. The optimal contents of Cs and V were 2.5 and 1, respectively, and the addition of Ni enhanced the catalytic activity even further will be discussed below [13- 16]. The results for CsxH3-xPMo1204o catalysts are shown in Table 2 [13]. The highest conversion was observed around x = 2.5 - 2.85. The main products were methacrylic acid (MAA), methacrolein (MAL) and acetic acid (AcOH). The substitution of Cs for H in H3PMo12040 resulted in a great e n h a n c e m e n t of the MAA production and the yield reached a m a x i m u m at x = 2.5. The sum of the yields of MAA and MAL on Cs2.sHo.sPMo1204o reached 5.1%. The catalytic properties of Cs2.sHo.~PMo~2040 changed by the addition of transition metal ions [16]. The addition of Ni, Mn, or Fe increased the yields of MAA and MAL. In the case of Ni, the yields of MAA and MAL reached 6.5 and 1.5%, respectively. In contrast, Co, Cu, Hg, Pt, and Pd decreased the yields. The results for Cs2.sNioosHo.34+xPMo12-xVxO4o catalysts are shown in Table 3 [16]. The conversions were 10 - 15%. The highest selectivity to MAA was also observed at x = 1. It follows t h a t the substitution of V 5+ for Mo 6+ in Cs2.sNio.0sHo.34PMo~204o resulted in the e n h a n c e m e n t of MAA production and the yield reached a m a x i m u m at x = 1. Table 2 Oxidation of isobutane over CsxH3-xPMo12040 at 340~ a x Surface Conv. Rate Selectivity 5/% area /% /10 .5 mol /m 2 g-1 min -1 m 2 MAA MAL AcOH CO
Sum of
yields of MAA+ C02 MAL/% 0 1.1 7 1.34 4 18 8 44 26 1.5 1 2.1 6 0.60 23 17 10 32 18 2.4 2 5.9 11 0.39 34 10 7 29 21 4.8 2.5 9.5 16 0.36 24 7 7 41 21 5.1 2.85 46.0 17 0.08 5 10 5 44 37 2.4 3c 46.0 8 0.04 0 10 6 32 35 0.8 a Isobutane, 17 vol%; 02, 33 vol%; N2, balance; catalyst, 1.0 g; total flow rate, ca. 30 cm 3 min-1, b Calculated on C4 (isobutane)-basis. c The selectivity to acetone was 17%.
38 Table 3 Oxidation of isobutane catalyzed by Cs2.sNio.osHo.sa+~PMol2-xV~O4o at 320~ a X Conv./% Selectivity 5/% MAA MAL AcOH CO CO2 0 10 27 12 5 30 26 1 15 36 9 6 25 24 2 13 28 8 6 25 33 3 12 10 8 9 35 38 a,b Experimental conditions. See Table 2. Thus, the Keggin-type heteropolymolybdates such as Cse.5Ni0.08Hl.s4PMollVO40 fairly selectively catalyze the oxidation of isobutane into methacrolein and methacrylic acid with molecular oxygen. At 340~ the yield of methacrylic acid reached 9.0%. The 9.0% yield of A A was greater than the highest value of 6.2% reported in the patent literature at similar steady-state conditions [10]. Figure 2 shows a good correlation between the rates of oxidation of isobutane and non-catalytic reduction of catalysts by CO. The correlation noted in Fig. 2 indicates that the catalytic activity is controlled by the oxidizing ability of catalysts. It has been suggested for the case of CsxHs-~PMo1204o catalysts that the factors controlling the catalytic activity are the o 2 oxidizing ability and the O protonic acidity of catalysts [16]. r 9
C s2.5Mn+o.08H 1.5-0.08nPMo I 1VO 40
(M = Ni z+, Fe 3+) also catalyzed the oxidation of propane and ethane [18- 20]. Here, the rate and role of vanadium is of concern [21]. It is interesting that the reduced heteropoly compounds showed higher selectivity to methacrylic acid for the oxidation of isobutane [10, 13, 15, 22, 23]. Ueda et al. applied reduced 12-molybdophosphoric acid to the oxidation of propane and obtained 50% selectivity to acrylic acid and acrolein at 12% conversion [22, 23].
O
O
H
~
Cs2.85~
o~ r
0
S~Cs2 / ~ - - - Cs3
1 2 3 Rate of reduction by CO /10 .6 mol min-1 m-2 Figure 2 Correlation between the rates of catalytic oxidation of isobutane and those of non-catalytic reduction of catalysts by CO. Csx and H represent CsxHs-~PMoy204o and HsPMo12040, respectively.
39 3. SELECTIVE HYDROXYLATION OF BENZENE Various oxidants such as dioxygen, hydrogen peroxide, and alkyl hydroperoxides have been applied for the oxidation of hydrocarbons in the homogeneous liquid phase catalyzed by heteropoly catalysts [1, 24]. Below are presented results on the selective hydroxylation of benzene to phenol with H202 catalyzed by Keggin-type heteropolyanions. It is known t h a t Fenton or related reagents also catalyze this reaction [25]. A research group that includes one of the present authors has previously reported that the reaction proceeded selectively by using H202 and vanadium-substituted heteropolymolybdates or tungstates [26]. The present study is an extension of this earlier study. Recently the same reaction was attempted by using wellcharacterized K salts of vanadium-substituted heteropolytungstates [27]. The selectivity based on benzene was high, but the yield based on H202 was not given. Heteropoly compounds obtained commercially, H3+~PMo12-~V~O40 (x = 0 - 4), were purified by extraction with ether and subsequent recrystrallization. They are abbreviated as PMol2-xVx hereafter. Their IR spectra agreed with those reported in the literature [27]. 51V-NMR spectrum of PMoloV2 in aqueous solution showed that PMoloV2 was a mixture of three to four positional isomers of PMol0V2 and contained PMol~V at less t h a n 30% level. For comparison, NaVOa, V203, V204, and V205 were used. In the case of V2Ox, sulfuric acid was added in order to dissolve the catalyst. This addition resulted in improved yields. The reaction was usually carried out at 20 - 70~ in a four neck flask, by adding dropwise 10 ml of 0.08 M H202 aqueous solution (0.8 mmol) into a mixture of benzene 10 ml and water 15 ml. Catalyst (0.05 - 0.3 mmol) was dissolved in the water phase before the reaction. Hydroxylation of benzene took place in the water phase and a majority of phenol formed was transferred to the benzene phase. The concentration of water and benzene phases were analyzed periodically by liquid chromatography with o-cresol as a standard. The evolution of oxygen gas by the decomposition of H202 was measured volumetrically. The yield of phenol on the basis of H202 consumed tended to increase in parallel with the catalytic activity of each catalyst for the H202 decomposition in the absence of benzene. The sudden introduction of H202 into the reaction system caused the evolution of a significant amount of oxygen. After a short induction period the yield of phenol increased rapidly with time. When the concentration of H202 in the reaction system was kept low by adding dropwise very slowly a diluted H202 solution, the yield of phenol increased remarkably, the unproductive decomposition of H202 being suppressed. The selectivity to phenol was almost 100 % on the basis of benzene and reached above 90 % on the basis of H202. Typical results thus obtained at 65~ are summarized in Table 4. The yields are in the order of PMol0V2 > PMo9V3 > PMosV4 > PMo~V >> PMol2. NaVO3 and V2Ox showed modest performance. The turnover based on the catalyst was about 3 in the case of PMoloV2. It was confirmed that, upon the addition of H202
40 Table 4 Yields of phenol from the oxidation of benzene by hydrogen peroxide and vanadium compounds Yield of Selectivity of Catalyst Selectivity of Catalyst Yield of phenol/% phenol/% phenol/% phenol/% (H202 (benzene (benzene (H202 basis) basis) basis) basis) 65~ 65~ 45~ VO(C5H702)2 46.6 PMo1204o 0 0 92.1 NaVO3 23.8 PMo11VO4o 9.0 0 100 56.0 V205 (I-12804) 52.5 PMoloV204o 92.6 69.2 100 81.6 V204 (H2SO4) 21.8 PMooV304o 90.1 90.7 100 V203 (a2so4) 13.5 PMosV404o 68.1 73.6 100 Catalyst, 0.3 mmol; H20, 15 ml; C6H6, 10 ml. 10 ml of 0.08 M H202 was added dropwise very slowly. Reaction time: 1.5 h. after the reaction, the oxidation proceeded again at a similar rate. Therefore, there was little deactivation of catalyst. The optimum pH range for the phenol yield was 2 to 3 for PMoloV2 and PMo9V3, and 1 to 2 for PMollV and PMosV4. According to the UV-vis spectra, PMoIoV2 and PMogV3 were stable in the pH ranges of 2 - 3 and 1.5 - 3.5, respectively. The temperature dependencies are shown in Fig. 3. The optimum reaction temperature and time appear to depend on the polyanion species. 100 The IR spectra of the reaction solutions showed that 80the structures of the Keggin polyanions remained unchanged 60 r during the reaction. When the concentration of H202 was 40 O increased, however, new b a n d s CD .t-.4 appeared in the 500 - 600 cm -1 20 region possibly due to peroxo I I species. It seems that active 0 "AI " 10 20 30 40 50 60 70 80 peroxo species are formed by the Temperature/~ reaction of vanadiumFigure 3. Temperature dependencies of the substituted polyanions and activity for the oxidation of benzene with H2Oe or that other active oxygen hydrogen peroxide for 1.5h reaction time. species are derived from the Catalyst, 0.3 retool; H20, 15 ml; C6H6, 10 ml. peroxo species. Either or both 10 ml of 0.08 M H202 was added dropwise species are probably active for very slowly. the hydroxylation of benzene. A; H4PMollVO4o, A; H5PMoloV2040, The species tend to deactivate O; H6PMogV304o, 0; H:PMosV404o, by reaction between them (e. g., ["l; H3PM012040, X; V205 (H2804) dimerization), as indicated by
41 the fact that the unproductive decomposition of H202 to oxygen and water became dominant when their concentration was high. The induction period observed when H202 was added suddenly probably corresponds to the period for the formation of the active species. Thus, by choosing appropriate vanadium-substituted heteropolymolybdates and keeping the concentration of H202 low, efficient hydroxylation of benzene to phenol was achieved. The highest yield based on H202 was 93%, where the selectivity with respect to benzene consumed was 100%. 4. CONCLUSION Characteristic features of vanadium containing heteropoly catalysts for the selective oxidation of hydrocarbons have been described. MAA yield from isobutyric acid was successfully enhanced by the stabilization of the vanadiumsubstituted heteropolyanions by forming cesium salts. As for lower alkane oxidation by using vanadium containing heteropoly catalysts, it was found that the surface of (VO)2P207 was reversibly oxidized to the X1 (5) phase under the reaction conditions of n-butane oxidation. The catalytic properties of cesium salts of 12-heteropolyacids were controlled by the substitution with vanadium, the Cs salt formation, and the addition of transition metal ions. By this way, the yield of MAA from isobutane reached 9.0%. Furthermore, vanadium-substituted 12molybdates in solution showed 93% conversion on H202 basis in hydroxylation of benzene to phenol with 100% selectivity on benzene basis. REFERENCES
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