Catalytic Selective Oxidation - American Chemical Society

Chapter 12

Active Crystal Face of Vanadyl Pyrophosphate for Selective Oxidation of n-Butane

Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch012

T. Okuhara, K. Inumaru, and M. Misono Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan

Catalytic properties of the crystal faces of (VO) P O for selective oxidation of butane were examined by using large plate-like crystallites with average size of 5 μm and thickness of 40 nm. The (VO) P O produced maleic anhydride with 60%-selectivity in the butane oxidation at713K. When SiO was deposited on the surface of (VO) P O (the atomic ratio of Si to V at the surface was 6) by the reaction of Si(CH3)4 + O at 773 K, the surface lost the catalytic activity. TEM-EDX observation revealed that SiO was located both on the basal (100) face, in which V (=O)-O-V sites are present, and on the side faces such as (001). The SiO deposited (VO) P O wasfracturedinto small crystallites by applying pressure on it to create the side faces predominantly. It was found that the fractured sample was active mainly for the formation of CO and CO from butane. Therefore, it is concluded that the face which is selective for the formation of maleic anhydride is (100), while the side faces such as (001) are active for non-selective oxidation. When the parent (VO) P O (without SiO -deposition) was fractured by the same method, the activity increased a little and the selectivity to maleic anhydride decreased from 60% to 56%. This fact supports the above conclusion. 2

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V-P oxides are known to be efficient for the production of maleic anhydride (abbreviated as MA) from n-butane (1-3). A single-phase (VO) P O has been inferred to be the active catalyst phase (4-6). In addition, the catalytic properties of (VO) P O varied depending on the microstructure of (VO) P O particles (7,8). Some claimed that the 2

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0097-6156/93/0523-0156$06.00/0 © 1993 American Chemical Society In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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active phase is a combination of (VO)2P2Û7 and VOPO4 (9-11). Volta et al. have proposed that M A is formed on (VO)2P207 surface with the participation of (11). On the other hand, P-rich surface phases have been reported to be active on the basis of XPS analysis (12,13). The active site of (VO>2P207 for the butane oxidation is thought to be V +(=0)-0-V +, which is located on the (100) face of (VO) P2C>7 U), where XRD lines were indexed following Gorbunova and Linde (14). Busca et al. have proposed that these sites are responsible for the first step, i.e., the dehydrogenation of butane to butene adsorbed on the surface (15). Some researchers inferred that the defects in the (100) face play an important role in the reaction (1,5). We have previously demonstrated by means of XPS that the surface compositions of various (V0)2P207 prepared with different methods were close to those of the bulk (P/V = 1) (16). This supports that the V pair sites are active. However, no direct evidence for the active sites and active faces has been presented yet. The difficulty in determining them is due to the fact that most of efficient (V0)2P2Û7 consist of non-uniform fine crystallites. In the present study, it has been attempted to differentiate the reactivities of the crystal faces of (V0)2P207 and assign the effective face by using large plate-like crystallites of similar size. 4


Active Crystal Face of Vanadyl Pyrophosphate

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2

4

+

Experimental Catalysts. To obtain large plate-like crystallites of (V0)2P207> the following method was adopted after several trials and errors. (VO)2P207 was prepared from a well-defined precursor, VOHPO4O.5H2O, which was obtained by reduction of VOPO42H2O with 2-butanol by the same method as described in the literature (17,18). In our previous papers, this was called C-4 (7,8). Powder of V2O5 was added to an aqueous solution of H3PO4 (40%) (P/V atomic ratio = 7). Then the solution was stirred for several hours at room temperature. The precipitate formed (probably VOP04-nH20, n> 2) was refluxed at 393 Κ for 2 h. In order to dehydrate the precipitate to VOPO42H2O, it was suspended into an aqueous solution of H3PO4 (85%) and the solution was again refluxed at 393 Κ for 2 h. The obtained crystallites were confirmed to be VOP04- 2H2O by XRD (10,19). By the reduction with 2-butanol at 358 Κ for 2 h in 2-butanol solution, VOHPO4O.5H2O was formed (see below). It was necessary to stir the solution slowly to avoid the destruction of the crystallites during the above procedure. The obtained VOHPO4O.5H2O was treated in a flow of N2 at 823 Κ for 5 h to form ( ν θ ) 2 Ρ 2 θ γ . In order to cover the surface of (VO)2P2C>7 with the S1O2 overlayer, ( ν θ ) 2 Ρ 2 θ γ was treated with a mixture of Si(CH3)4 and O2 as follows.

In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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CATALYTIC SELECTIVE OXIDATION

After (VO)2P207 was evacuated at 773 Κ for 1 h in a closed circulation system (250 cm ), the mixture of Si(CH3)4 (200 Torr) and O2 (70 Torr) was introduced into the system at the same temperature. Si(CH3)4, CO, and C O 2 as well as O2 were analyzed with an on-line gas chromatograph. The resulting sample is denoted by Si02/(VO)2P207» f ° which the amount of S1O2 was evaluated from the pressure decrease of Si(CH3)4. The crystallites of ÇVC»2l?2 7 which was prepared by the same method as the present study, to be (100). The same electron diffraction pattern was obtained for the basal plane of (VO)2P207 in the present study. The S E M micrographs of the fractured (VC»2P207 and S1O2/ (VO)2P207 are given in Figure 3. Crystallites of (VO)2P2C>7 changed by the fracture into smaller ones with average size of 3 μπι (Figure 3a). In this way, the side faces like (001) were newly created. When the crystallites were fractured 5 times, they became much smaller (about 1 μπι) as shown in Figure 3b. Figures 3c and 3d clearly showed that Si02/(VO)2P2Û7 had the same crystallite size and shape as the parent ( V O 2 P 2 O 7 (Figure 3c) and the change of the crystallites upon the fracture was also the same as that of (VC»2P207In Table 1, the size of crystallite, surface area, and density of Si on ( V O 2 P 2 O 7 are summarized. From the surface area (17 m ^ g " for (νθ)2Ρ2θγ), the density (3.2 g e m ) and the size (5 μπι in length), the thickness was calculated to be 40 nm, which is comparable with that from the line-width of XRD (20 nm). The surface areas of (VO)2P207 and Si02/(VO)2P207 changed little upon the fracture, showing that the surface areas of the side faces, which were newly created, were small. This is reasonable because the fraction of the side faces is estimated to 1.6 and 7.4% of the total surface area for the crystallites with the sizes of 5 and 1 μπι, respectively, assuming the thickness of 40 nm. If the fracture brought about cleavage along (100) plane of the crystallites to change thickness to half (20 nm), the surface area would become about twice. Thus little change of the surface area indicates that the cleavage along (100) plane upon the fracture is little. For Si02/(VO)2P207, the density of Si is expressed as the ratio of Si to V atoms present at the surface, S i / V f , where the density of V f was evaluated from the crystallographic data (14). The value of S i / V f was 6 (Table 1). In order to determine the location of S1O2 on the surface of (VO)2P207» TEM-EDX was applied. As shown in Figure 4, the electron beam (size; 50 nm χ 10 nm) was focused on two positions; position I is the surface at the center of the crystal face of (100), position II the edge. By E D X analysis, the Si peak was detected on both positions, while the ratio of Si/V peak intensity (Si; 1.739 keV, V; 5.426 keV) for position II was about 10 times as large as that for position I. This difference does not necessarily correspond to the difference i n the density of Si. The amount of V atoms, which are analyzed by EDX, is proportional to the sample volume that the electron beam passed, since the sample volume at position II is about half of that at position I, i.e., the amount of V detected 1

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Figure 2. S E M micrographs of (a) V O P 0 - 2 H 0 , (b) V O H P O 0 . 5 H O , (c) ( ν θ ) Ρ Ο γ . Magnification: 2000 4

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Figure 3. S E M micrographs of (a) ( ν θ ) Ρ Ο γ fractured once, (b) ( ν θ ) Ρ Ο γ fractured 5 times, (c) Si0 /(VO) P C>7, and (d) S i 0 / ( V O ) P 0 7 fractured once. Magnification: 2000 2

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Table 1. Crystallite Size, Surface Area, and Si/Vsurf Atomic Ratio of Catalysts 0

Size (thickness^) Surface area

Catalysts

a

(VO2P2O7 Si02/(VO) P20 2

7

SilVsurjd

5 μπι (40 nm) 5 μπι (40 nm)

17 10

6.0

3 μπι (40 nm)

11

6.0

-

Si02/(VO) P207 2


fractured (VO2P2O7

17 fractured once 3 μπι (40 nm) 1 μπι (40 nm) 5 times 18 Average size measured by SEM, ^Estimated from surface area, Measured by BET method, dAtomic ratio of Si to V f , where V f is the amount of surface vanadium atoms. a

c

s u r

s u r

by E D X at position II must be half of that at position I. On the other hand, the surface area analyzed at position II is estimated to about 2.5 times greater than that at position I, considering the thickness (40 nm) of the crystallites and the beam size (Figure 4). Thus, i f the density of S1O2 is equal on both (100) and the side face, the ratio of Si/V peak intensity for position II becomes 5 times that for position I. This value is not far from the experimental value, 10. Therefore, it is likely that the density of S1O2 is similar on both faces or somewhat higher at the side faces. In any event, it is concluded that S1O2 was deposited to a similar extent on the basal face and on the side faces. Catalytic Oxidation of Butane. The dependences of the conversion of

butane on W/F for (VO)2P2Û7 and Si02/(VO)2P207 are given in Figure 5, where W is the catalyst weight and F is the flow rate of butane. The conversion increased with the increase in W/F. The catalytic activity and the selectivity of oxidation of butane are summarized in Table 2. The parent (VO)2P2Û7 produced MA with the selectivity of about 60% at the conversion of 46%, which is in agreement with the previous data (7,8). Si02/(VO)2P207 showed a very low activity, indicating that the active sites on the surface of (VO)2P20 were effectively poisoned by S1O2. When the S i / V f ratio was less than 6, the conversion increased gradually with time and M A was formed at the later stage of the reaction. In this case, S1O2 overlayers probably peeled off from the surface of (S0)^2^1 because of the incomplete formation of the S1O2 overlayers. 7

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Figure 4. T E M micrograph of and illustration of E D X analysis, (a) T E M micrographs of SiO2/(VO)2P2O7(xl60000). E D X analysis was performed at positions I and II, (b) Illustration of E D X analysis (beam size; 50 nm χ 10 nm) 80

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W/F / 10 g mol" h Figure 5. W/F dependencies of the conversion for the butane oxidation over ( ν θ ) 2 Ρ 2 θ γ . (Ο) ( ν θ ) 2 Ρ 2 θ γ , (•) ( ν θ ) 2 Ρ 2 θ γ fractured 5 times, ( · ) Si02/(VO)2P207, (•) Si02/(VC»2P207 fractured once.



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When the inactive Si02/(VO)2P20 was fractured, it exhibited a certain activity and produced mainly CO and CO2, with the formation of a small amount of crotonaldehyde (10%). This result clearly indicates that the side faces such as (001) and (210) are non-selective for the formation of MA. Since the parent (VO)2P207 S M A with 60%selectivity, it is deduced that the (100) face, i n which V +(=0)-0-V + pair sites are located, is selective for the formation of MA. No change i n the crystallite size and shape was detected by S E M after the reaction. Table 1 shows that the surface area of Si02/(V0)2P207 increased by 1 2 . g - l by the fracture, which is consistent with the estimated value from the S E M measurement. The change in the conversion with time was small over Si02/(V0)2P207 fractured once. If the increase i n the reaction rate (from 0.05 χ 10~5 to 0.68 χ 10"5 mol-g^-min" ) by the fracture is attributed to the surface (side faces) created, the activity per unit surface area is 0.63 χ 10-5 mol •m"2.min~l. A possible reason of the apparent high activity of the side faces created by fracture is that the fractured faces consisted of not only side faces but also (100), etc., because of the roughness of the fractured surface or peeling of S1O2 overlayers by the fracture. The typical time courses of the butane oxidation over (VO)2P20 d the fractured (V0)2P207 (Si0 -free) are shown in Figure 6. The fractured (VO)2P2Û7 showed high initial activities as compared with the parent (VO)2P207- The initial activity increased as the fracture was 7

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Table 2. Catalytic Activities and Selectivities for Oxidation of Butane

Catalysts

a

(VO) P20 Si02/(VO)2P2Û7 2

Catalytic activity Selectivity I10~5 molg'l-min'l to MAI % (Conversion / %) (46) 1.80 (0.11) 60 0.05 (0.005) 0 (2)

7

Si02/(VO) P207 fractured 2

0.68 (0.062)

1.2

(20)b

(VO) P 0 fractured once 56 (47) 1.90 (0.11) 5 times 2.26 (0.13) 56 (52) Reaction rate: Butane; 1.5%, O2; 17% at 713 K. The figures in the parentheses are the rates divided by the total surface area; 10"5 mol-m" min-1, ^Crotonaldehyde was formed with the selectivity of 10%. 2

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repeated (from 1 to 5). Over (VO)2P207 fractured 5 times, the %conversion decreased appreciably and the selectivity increased with time at the initial stages of the reaction. This result shows that the side faces


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Figure 6. Time courses of butane oxidation over (VO)2P207 and (VO)2P207 fractured at 713 K. (O); parent, (•); fractured once, (Δ); fractured 5 times. The feed gas consisted of butane 1.5%, O2 17%, and N2 (balance). created freshly tend to be deactivated under the reaction conditions. As shown i n Table 2, the stationary catalytic activities of the fractured ( V O 2 P 2 O 7 were slightly higher than that of the parent (VO)2P2C>7- O the other hand, the selectivities of the fractured ( V O 2 P 2 O 7 (without S1O2 deposition) were slightly lower than that of the parent (VC»2P207- This result is consistent with the conclusion that the side faces are non selective. We reported previously that the addition of H 3 P O 4 (probably present as PO2.5 on the surface after the calcination) to (VO)2P2U7» which was prepared by an organic solvent method (5), enhanced the selectivity to M A at high conversion levels, while the activity decreased (20). Considering the low selectivity of the side faces, it is possible that the H 3 P O 4 added preferentially deactivated the side faces to suppress the formation of CO and CO2 and/or the secondary oxidation of the product M A there. In conclusion, the crystal faces of ( V O 2 P 2 O 7 had different reactivities for butane oxidation; the (100) face was selective for the formation M A and the side faces such as (001) are active for non-selective oxidation as illustrated in Figure 7. n


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C4H10 + O2

COx Figure 7. Selective and non-selective faces of (VO)2P207 f ° oxidation of butane.


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Acknowledgments We would like to thank to J E O L for the T E M - E D X measurement. This work was i n part supported by Tokuyama Science Foundation and a Grant-in-Aid for Scientific Research (No. 04805081) from the Ministry of Education, Science and Culture of Japan. Literature Cited (1) Centi, G.;Trifiro, F.; Ebner, J. B.; Franchetti, V. M. Chem. Rev., 1988, 88, 55. (2) Hodnett, Β. K. Catal. Rev. Sci. Eng., 1985, 27, 373. (3) Contractor, R. M.; Bergna, H. E.; Horowitz, H. S.; Blackstone, C. M.; Malone, B.; Torardi, C. C.; Griffiths, B.; Chawdhry, U.; Sleight, A. W. Catal. Today, 1987, 1, 40. (4) Shimoda, T.; Okuhara, T.; Misono, M. Bull. Chem. Soc. Jpn., 1985, 58, 2163. (5) Busca, G.; Cavani, F.; Centi, G.; Trifiro, F. J. Catal., 1986, 99, 400. (6) Moser, T. P.; Schrader, G.; J. Catal., 1985, 92, 216. (7) Misono, M.; Miyamoto, K.; Tsuji, K.; Goto, T.; Mizuno, N.; Okuhara, T. Stud. Surf. Sci. Catal., 1990, 55, 605. (8) Okuhara, T.; Tsuji, K.; Igarashi, H.; Misono, M. Catalytic Science and Technology, 1990, Vol. 1, Kodansha, Tokyo-VCH, Weinheim, p. 443. (9) Hodnett, B. K.; Delmon, B. Appl. Catal., 1985, 15, 141. (10) Bordes, E.; Courtine, P. J. Catal., 1977, 57, 236. (11) Harrouch Batis, N.; Batis, H.; Ghorbel, Α.; Vedrine, J. C.; Volta, J. C. J. Catal., 1991, 128, 248; Abdelouahab, F. B.; Olier, R.; Guilhaume, N.; Lefebvre, F.; Volta, J. C. J. Catal., 1992, 134, 151. (12) Yamazoe, N.; Morishige, H.; Teraoka, Y. Stud. Surf. Sci. Catal., 1989, 44, 15. (13) Cornaglia, L. M.; Caspani, C.; Lombardo, E. Appl. Catal., 1991, 74, 15. (14) Gorbunova,Y. E.; Linde, S. A. Dokl. Akad. Nauk SSSR, 1979, 245, 584.


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(15) Busca, G.; Centi, G.; Trifiro, F.; Lorenzelli, V. J. Phys. Chem., 1986, 90, 1337. (16) Okuhara, T.; Nakama, T.; Misono, M. Chem. Lett., 1990, 1941. (17) Bordes, E.; Courtine, P. J. Solid State Chem., 1984, 55, 270. (18) Johnson, J. W.; Johnston, D. C.; Jacobson, A. J.; Brody, J. F. J. Am. Chem. Soc., 1984, 106, 8123. (19) R'kha, C.; Vandenbarre, M. T.; Livage, J. J. Solid State Chem., 1986, 63, 202. (20) Okuhara, T.; Misono, M. Catal. Today, in press. 1992


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Catalytic Selective Oxidation - American Chemical Society

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