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Catalytic activity of low-valent chromium supported on Y-zeolites
Journal Elsevier
of Molecular Catalysis, 78 (1993) 57-66 Science Publishers B.V., Amsterdam
57
M3059
Catalytic activity of 1owLvalent chromium supported on Y-zeolites Takayuki Komatsu, Reiko Miyoshi, Seitaro Namba* and Tatsuaki Yashima** Department (Beceived
of Chemistry,
Tokyo Institute
of Technology,
Meguro-ku,
Tokyo
152 (Japan)
February 25, 1992; revised June 10, 1992)
Abstract The catalytic activity and selectivity of low-valent chromium species supported on Yzeolites were studied for the dehydrogenation of cyclohexane and for the hydrogenation of 1,3-butadiene. Chromium species having average oxidation numbers (AON) of 0.02-0.37 were prepared by decomposition of Cr(CO)e adsorbed on HNa-Y zeolites with various values of proton content. The activity of the low-valent chromium species in the dehydrogenation of cyclohexane into benzene was high compared with Cr(II)- and Cr(III)exchanged Y-zeolites and was also higher than that of the conventional chromia/siIica and chromia/ahunina catalysts. The activity decreases with increasing AON of chromium. The Cr(0) species was assigned as the active site. A sharp drop in the activity was observed when the AON of chromium was increased above 0.2. For the hydrogenation of 1,3-butadiene, the activity also decreased with increasing AON. Cr(0) was again thought to be the active site. The selectivity for the formation of butene isomers changed when the AON exceeded 0.2, that is, 1,2-addition of hydrogen was dominant at AON below 0.2 and 1,4-addition began to increase above 0.2. These results indicate that the changes in the electron density and in the morphology of Cr(0) species occur within a small range of the AON around 0.2. Simple models for the low-valent chromium species are proposed.
Introduction Supported chromia catalysts are known to catalyze the dehydrogenation of saturated hydrocarbons. Many researchers have tried to clarify the active site in relation to the oxidation state of the supported chromium. Bridges et al. [ 1 ] have studied the dehydrogenation of cyclohexane over Cr,O&l,O, catalysts. They found that formation of methylcyclopentane occurred on the alumina surface, along with dehydrogenation on the chromia surface. The addition of potassium poisoned the acid sites of alumina, resulting in an increase in the dehydrogenation activity. Reijen et al. [2] have also studied the dehydrogenation of cyclohexane over the potassium-promoted Cr203/ *Present address: Department of Materials, The Nishi-Tokyo Kitatsuru-gun, Yamanashi 409-01, Japan. **Author to whom correspondence should be addressed.
0304-5102/93/$06.00
0 1993 - Elsevier
University,
Uenohara-cho,
Science Publishers B.V. AI1 rights reserved
58
A1203. The addition of HZ0 caused the oxidation of Cr(II) to Cr(III), which resulted in a decrease in activity. They concluded that Cr(I1) was the active site. In the case of the dehydrogenation of ethane over Cr20,/Al,0, catalysts, Cr(II), which was formed from Cr(II1) by reduction in dry hydrogen, was reported to be active [3]. Cr(II) was also more active than Cr(III) for the dehydrogenation of isobutane [ 4 1. These results indicated that the lower the oxidation state, the higher the activity for the dehydrogenation. Therefore, chromium species having oxidation states lower than Cr(I1) may have higher activity. However, for the conventional supported chromia catalysts prepared by an impregnation method, it is hard to reduce chromium to oxidation states lower than + 2. Cr(CO& has been reported to be a possible chromium source for a new type of chromium catalysts [5-lo]. Chromium was fixed on a support through the decomposition of adsorbed Cr(CO)G at high temperatures. By using a highly dehydroxylated alumina as a support, low-valent chromium species were obtained with the average oxidation number (AON) of chromium below + 1 [5, 61. We have prepared chromium-supported Y-zeolites using Cr(CO), and HNa-Y zeolites [ 7, 81. The oxidation states of chromium were controlled by changing the preparation conditions and the proton concentration in HNa-Y. Higher dehydration temperatures of HNa-Y and lower decomposition temperatures of adsorbed Cr(CO)G gave chromium species with low AON. Lower proton concentrations also helped to get the lower-valent chromium species. Thus we could adjust the AON of chromium from 0.02 to 1 .ll. In the present work, we prepared chromium-supported zeolites which consisted of chromium species in very low oxidation states using Cr(CO), and HNa-Y zeolites, and carried out the dehydrogenation of cyclohexane over these catalysts. The purpose of this study is to clarify the catalytic properties of the low-valent chromium species. We have already studied [B] the relation between activity and oxidation state for the chromium catalysts supported on Y-zeolites. Cr(0) was suggested to be the active site for the hydrogenation of ethylene, and Cr(I) the active site for the polymerization of ethylene. In these reactions, the activities changed by more than one order of magnitude when the AON of chromium was varied from 0.02 to 0.26. Therefore, not only the valence state but also other functions of the active chromium species, Cr(0) and Cr(I), might be changed within the small range of AON values. In order to clarify what happened to the chromium species in low oxidation state, the hydrogenation of 1,3-butadiene was also carried out over the same series of catalysts. The hydrogenation of 1,3-butadiene has been extensively studied by Wells and co-workers [ 11-141. They proposed a reaction scheme consisting of two reaction paths, 1,2-addition and 1,4addition, for alumina-supported iron, cobalt, nickel and copper catalysts [ 111 and for alumina-supported rhodium, palladium and platinum catalysts [ 121. Contamination with sulfur and other nonmetals varied the selectivity for the two reaction paths [ 13, 141. Okamoto et al. [ 15, 16 ] have studied the same reaction over various nickel catalysts. From the results of the catalytic
59
reactions and XPS spectra of nickel, they concluded that small changes in the electron density of nickel metal caused the change in selectivity. In the case of our chromium catalysts, the low-valent chromium species should mainly contain Cr(O), which suggests that the catalysts behave like metal catalysts. Therefore, small changes in the electron density may be monitored by the selectivity in the hydrogenation of 1,3-butadiene.
Experimental Catalysts Two kinds of chromium supported on Y-zeolites were prepared from Na-Y (Toyo Soda Ind. Co., Lot Y-30; Si/Al= 2.88). Details of the catalyst preparation have been described previously [7]. The first, Cr/HNa-Y, was prepared by gas-phase adsorption of Cr(CO), (Aldrich) on H(x)Na-Y having various degrees of proton exchange, x(0-65%). The proton exchange was carried out in a conventional manner with NH4C1 aqueous solution followed by calcination at 743 K in air. A specific amount of Cr(CO)G powder was added to the dehydrated HNa-Y under argon atmosphere. After flash evacuation at 295 K, Cr(CO& vapor was adsorbed at 333 K. The Cr(CO),/H(x)Na-Y was heated in vacua at 723 K to release its CO ligands quantitatively, resulting in the formation of Cr/HNa-Y. The concentration of supported chromium was adjusted to two chromium atoms per supercage of Y-zeolite. The second, Cr-Y, was prepared by an ion-exchange technique using the same Na-Y and an aqueous solution of CrC13. The degree of ion exchange was 47%, corresponding to an average stoichiometry of one chromium atom per supercage. Chromia-alumina (Cr,O,: 10 wt.%) was prepared by impregnation [ 171 with chromic anhydride (Wake Pure Chemical Ind. Ltd.) on -y-alumina (Nishio Ind. Co., type 30-60). Chromia-silica (Crz03: 5 wt.%) was prepared by a similar impregnation technique [ 181 using chromic anhydride and fumed silica (Cab-o-Sil@, M-5). The amount of supported chromium in Cr/HNa-Y and the degree of proton exchange in H(x)Na-Y were determined by atomic absorption spectroscopy and flame photometry, respectively. Apparatus and procedure The average oxidation number (AON) of chromium in Cr/I-INa-Y was determined from the amounts of Ha, CO, CO2 and CH., evolved during the temperature-programmed decomposition (TPDE) of Cr(CO)G adsorbed on HNa-Y [ 71. These gases were analyzed with a quadrupole mass spectrometer (ULVAC, MSQ-15OA). The dehydrogenation of cyclohexane was carried out using a glass circulation system with a dead volume of 300 ml. In the case of Cr/HNa-Y, HNa-Y (0.20 g) was put in a quartz tube attached to the system and Cr/ HNa-Y was prepared in situ using a side arm containing Cr(CO)G. The
60
reaction was carried out after the decomposition of Cr(CO)G/HNa-Y in vacua at 773 K. Other catalysts were placed in the quartz tube and heated in vczczw at 773 K before the reaction. Cyclohexane was circulated at an initial pressure of 50 torr and at a reaction temperature between 573 K and 773 K. The gas-phase mixture was analyzed by gas chromatography. The hydrogenation of 1,3-butadiene was carried out in a similar manner. Cr/HNa-Y was prepared in situ by the decomposition of Cr(CO),/HNa-Y at 473 K. The initial pressures of 1,3-butadiene and hydrogen were 100 and 150 torr, respectively. These gases were mixed and the mixture was circulated through the catalyst at temperatures of 273-473 K. 1,3-butadiene were purified by repeating a Cyclohexane and freeze-pump-thaw cycle. Hydrogen was passed through a manganese/silica trap to remove possible residual oxygen and water. Results
and discussion
In our previous work [7, 81 we reported that the oxidation state of chromium in Cr/HNa-Y prepared by the gas-phase adsorption technique could be controlled by changing preparation conditions. Lower dehydration temperatures and higher degrees of proton exchange of the parent HNa-Y resulted in higher oxidation states, because protons in HNa-Y oxidized the zero-valent chromium of Cr(C0)6 during the decomposition of Cr(C0)6 adsorbed on HNa-Y. Decomposition at higher temperatures also accelerated the oxidation of chromium. Thus we could control the average oxidation number (AON) of chromium in the range 0.02-l .l 1. It was also reported [S] that the chromium species in these catalysts consist of Cr(0) and a small amount of Cr(1) and Cr(II). Figure 1 shows a plot of the AON of chromium against the degree of proton exchange. The dehydration temperature of HNa-Y was 823 K and
0
20
40
60
Desree Of Droton exchanse /9o
0
0.1
0,2
0.3
0.4
AON
Fig. 1. Effect of degree of proton exchange in Y-zeolites on average chromium in Cr/HNa-Y. HNa-Y was dehydrated at 823 K.
oxidation
number of
Fig. 2. Effect of average oxidation number of chromium on the activity for dehydrogenation of cyclohexane. The conversion of cyclohexane was obtained after 1 h of the reaction at 723 K.
61
the decomposition temperature of CI-(CO)~ was 723 K. As expected, the AON increased with the proton concentration, and the AON could be adjusted from 0.13 to 0.37. In order to investigate the catalytic properties of the low-valent chromium species in these catalysts, the dehydrogenation of cyclohexane was carried out. The conversion over the parent HNa-Y was negligible compared with that over CrMNa-Y catalysts. The reaction products over the Cr/HNa-Y catalysts were benzene and a trace of cyclohexene. Figure 2 shows the percentage conversion of cyclohexane into benzene after a reaction time of 1 h over various Cr/HNa-Y having AON of 0.13-0.37 (see Fig. 1). The catalysts of lower AONs gave high conversions. In the preparation of the catalysts, Cr(CO),/HNa-Y was decomposed at a high temperature (773 K) to keep the Cr/HNa-Y stable at the reaction temperature, 723 K. As mentioned above, the AON is higher when Cr(CO)JHNa-Y is decomposed at a higher temperature. Accordingly, 0.13 was the lowest AON obtained through the decomposition at 773 K. The activity decreased drastically when the AON was increased from 0.19 to 0.25. It is suggested that a chromium species with very low oxidation state, probably Cr(O), is responsible for the high dehydrogenation activity. In the case of the hydrogenation of ethylene, we have already reported [8] that the Cr(0) species were the active site of Cr/HNa-Y catalysts. However, it should be considered that in Fig. 2, Cr/HNa-Y with AON of 0.25 or 0.37, which gave very low activities, still have a significant amount of Cr(0). A slight change in the electron density and/or in the morphology of Cr(0) species might affect the activity. We have investigated the adsorption of NO on Cr/HNa-Y [8]. The amount of adsorbed NO (about 1.2 NO/Cr) did not change significantly when the degree of proton exchange was varied from 0 to 75%. This indicates that the dispersion of chromium did not change greatly in CrLHNa-Y used for the dehydrogenation of cyclohexane. Therefore, the drastic change in the dehydrogenation activity shown in Fig. 2 could not result from a change in the dispersion of chromium. A migration of chromium into specific sites inaccessible to cyclohexane molecules should also be considered. Cr(0) atoms are weakly bound to the surface, while Cr(I) cations are fixed to the surface by Cr-0 bonds. Therefore, Cr(0) might migrate into such specific sites. If so, the activity would increase with increasing number of chromium cations, which could anchor the Cr(0) atoms to the sites accessible to the reactant. The activity, then, would increase with increasing AON, which does not coincide with the result shown in Fig. 2. These considerations result in the conclusion that slight changes in the electron density are the most probable cause of the activity change. The activities of other chromium catalysts were examined to compare the low-valent chromium species with Cr(II) and Cr(III) cations. Figure 3 shows conversion versus time plots for various chromium catalysts. The amounts of chromium were equal for all the catalysts. It is clear that Cr/ Na-Y (a) is the most active catalyst, followed by Crz03/Al,03 (b), Cr203/ SiOa (c) and Cr,O, (d). Therefore, the activity of the low-valent chromium, Cr(O), may be higher than that of Cr(II1). The activities of Cr(III)-Y (e) and
62
1.0
(b) y
CC)
c
Cdl
0
-1.0
\ F
11 "0
30
60
Reaction time /mln
I
1.3
1.4
1.5
1.6
103/1
Fig. 3. Change in conversion of cyclohexane with reaction time for the dehydrogenation at 723 K over (a) Cr/Na-Y, (b) Cr,03/Al,03, (c) Cr,O,/SiO,, (d) Cr,O,, (e) Cr(III)-Y and (f) Cr(II)-Y. Fig. 4. Arrhenius plot for the dehydrogenation H(35%)Na-Y and (A) Crz03/A120,.
of cyclohexane
over (0)
Cr/Na-Y, (0)
Cr/
Cr(II)-Y (f) prepared by the reduction of Cr(III)-Y with hydrogen at 773 K for 2 h were both very low, even when compared with unsupported Cr,O,. This suggests that the isolated chromium cations are much less active for the dehydrogenation of cyclohexane. Figure 4 shows Arrhenius plots for the dehydrogenation over Cr/Na-Y, Cr/H(35%)Na-Y and Cr,0&41,03 catalysts. The rate constant k was obtained from the initial rate, assuming a first-order dependence on the pressure of cyclohexane. The apparent activation energies for these catalysts were 42, 41 and 142 kl mol-‘, respectively. As shown in Pigs. 2 and 3, Cr/H(35%)Na-Y had AON = 0.20 and was less active than CrLNa-Y (AON = 0.13). Nevertheless, the activation energies for these catalysts were almost the same, suggesting that the active sites of these catalysts were the same. On the other hand, the activation energy for Cr203/A1203, where Cra’ was the dominant species, was much higher than that for Cr/Na-Y or Cr/H(35%)Na-Y. These results support the idea that the active site for the dehydrogenation of cyclohexane over Cr/HNa-Y with various degrees of proton exchange is the same Cr(0) species. Phillipson et al. [ 111 have studied the hydrogenation of 1,3-butadiene over iron, cobalt, nickel and copper catalysts. They concluded that the reaction occurred through 1,2- and 1,4-addition of hydrogen to the diene and that the dominant route depended on the degree of sintering or on the completeness of reduction during preparation. We carried out the hydrogenation of 1,3butadiene to get information about the morphological and electrical changes in the low-valent chromium species of Cr/HNa-Y within a low AON region. Various CriHNa-Y catalysts with chromium AON between 0.02 and 0.29 were examined for the expected change in activity and selectivity at around AON = 0.2, which was the critical AON for the dehydrogenation of cyclohexane, as indicated above (see Fig. 2). At the temperatures tested (273-473 K),
63
reaction products were 1-butene, trcms- and cti-2-butene and a small amount of n-butane. Figure 5 shows the effect of the AON of chromium in Cr/HNa-Y on the conversion and selectivity for the hydrogenation of 1,3-butadiene. The conversion decreased when the AON increased from 0.2 to 0.3. Although this decrease in activity is not so drastic as that for the dehydrogenation of cyclohexane (Fig. 2), the low-valent chromium species seems again to be the active site. It should be noted that the selectivity also changed with AON, that is, the selectivity for 1-butene decreased and that for cti-2-butene increased as the AON increased from 0.2. Okamoto et al. [19] reported that the hydrogenation of 1,3-butadiene over Cr(CO& encaged in Na-X zeolites gave cti-2-butene with high selectivities (> 98.5%) at 330-350 K. They have proposed a reaction mechanism in which cti-2-butene is formed through (q4-C4HG)Cr(CO), and 1-butene through ($-C4HG)Cr(CO), and (q2C4H,),Cr(C0)a. Their catalyst contained some CO ligands on each chromium under the reaction conditions. The chromium species in our catalysts, Cd HNa-Y, do not have any CO ligands. Therefore, the selectivity of Cr/HNa-Y was different. Figure 6 shows the changes in the selectivities to the butenes and butane with reaction time on Cr/Na-Y (AON= 0.02). During the initial 20 min, the selectivity to 1-butene slightly increased, while that to cti-2-butene decreased. If the isomerization of butenes occurs concomitantly, 1-butene should be converted mainly into Pans-2-butene to approach thermodynamic equilibrium (at 373 K, I-butene: 7%, trans-2-butene: 65%, cti-2-butene: 28%). In the case of metallic catalysts, the rate of isomerization of 1-butene was reported to depend on the hydrogen pressure [ 201. To determine the influence of the isomerization, the hydrogenation of 1,3-butadiene was carried out under various initial pressures of hydrogen (loo-250 torr) with the pressure of 1,3-butadiene kept constant (100 torr). As shown in Fig. 7, the selectivity to each butene did not change with the hydrogen pressure after 1 h of reaction over Cr/Na-Y (AON = 0.02). The isomer distribution of butenes was far from the equilibrium composition. Therefore, the isomerization of butenes
GON
Fig. 5. Effect of average oxidation number of chromium on (0) the conversion of 1,3-butadiene, and on the selectivities to (0) 1-butene, (A) cis-2-butene, (I) truns-2-butene and (V) butane for the hydrogenation of 1,3-butadiene over Cr/Na-Y at 373 K for 1 h.
.
O-TiFF=s 0 Reaction
tlme/mln
Lx
0
100
Pressure
200
of
300
Hz /Tort
Fig. 6. Changes in selectivities to (0) 1-butene, (A) cti-2-butene, (I) tmns-2-butene and (V) butane with reaction time for the hydrogenation of 1,3-butadiene over Cr/Na-Y at 373 K. Fig. 7. Effect of the partial pressure of hydrogen on the selectivities to (0) I-butene, (A) cti-2-butene and (m) trcms-2-butene for the hydrogenation of 1,3-butadiene. The reaction was carried out over Cr/Na-Y at 373 K for 1 h.
can be neglected in the discussion about butene composition in the products of the hydrogenation of 1,3-butadiene. Phillipson et al. [ 111 have proposed a reaction mechanism for the hydrogenation of 1,3-butadiene over some metal catalysts: 1-butene is produced only by 1,2-addition via n-bonded species, while bans- and c&+2butene are produced by 1,4-addition via v-allylic species as well as by 1,2addition via the r-bonded species. According to their mechanism, on the catalysts with high electron density, the r-bonded species are stable and so 1-butene is the main product. As the electron density decreases, the 7-r-allylic species become stable and the selectivity to 2-butenes increases. In the case of CrAINa-Y shown in Fig. 5, 1-butene was the main product at lower AON, which was in accordance with the simple idea that the lower the AON, the higher the electron density of chromium species. The selectivity began to change when the AON increased above 0.2, where the selectivity for 1-butene decreased. This result could be explained by the change in electron density of active chromium species, though the change in the AON was relatively small. The drastic change in the activity for the dehydrogenation of cyclohexane shown in Fig. 2 could also be explained by the change in electron density. We propose possible models of the active chromium species (Fig. 8) based on the relation between AON and activity. When the AON of chromium is 0.0 (a), all the chromium atoms are Cr(0) and they do not have strong electrical interactions with the surface of Y-zeolites. Therefore, the chromium atoms migrate through the supercages of Y-zeolite to form aggregates. The number of chromium atoms constituting one aggregate is not necessarily five, as indicated in Fig. 8(a). However, there is not enough space inside Y-zeolite for the chromium atoms to form a large aggregate. The largest cavity, supercage, has a diameter of only about 1.3 run. Moreover, to keep a certain dispersion of chromium, the diameter of the aggregate must be smaller than 1.3 nm. When the AON is 0.2 (b), a portion of the chromium
65
Cr(0)
Cr(l)
(a) AON =O.O
Cc) AON=0,3
n
Fig. 8. Models of active chromium species
of Cr/HNa-Y.
atoms should be oxidized to higher oxidation states by surface hydroxyl groups of HNa-Y to form Cr-0 bonds, which anchor the chromium cations to specific sites of HNa-Y. As a result, unoxidized chromium atoms migrate to the chromium cations to form aggregates. ESR studies on Cr/HNa-Y [ 71 indicated that the samples with AON between 0.15 and 1.I1 did not give a signal due to Cr(II1) and that only a weak signal due to Cr(II) was detected. For example, 2% of the total number of chromium atoms were Cr(I1) for the sample with an AON of 0.26. Therefore, the chromium cation anchored to the surface is almost exclusively Cr(I), which results in the statistical stoichiometry shown in Fig. S(b), four Cr(0) attached to one Cr(I). When the AON is 0.3, the number of Cr(0) attached to one Cr(1) cation should decrease to about two, on average, as shown in Fig. 8(c). The interaction between Cr(0) and Cr(I) in model (c) may lower the electron density on Cr(0) to some extent. As mentioned above, Cr(0) is thought to be the active site for the dehydrogenation of cyclohexane and for the hydrogenation of 1,3-butadiene. Therefore, the decrease in electron density may cause the decrease in catalytic activities, though the electron density of Cr(0) in model (b) could be slightly lower than that in model (a). As can be seen in Fig. 8, the Cr(1) cation in model (b) is covered by Cr(0) atoms and is thus shielded from gas-phase molecules to some extent. On the other hand, Cr(1) in model (c) has more space to interact directly with gas-phase molecules. If direct interaction between the Cr(1) cation and the reactant molecules hinders the reactions, the change in the extent of exposure of Cr(1) cations is another possible factor for the drastic change in catalytic activities at AONs around 0.2. A similar effect of Cr(1) might cause the change in selectivity as well as that in activity. The significant change in
66
butene selectivity indicated in Fig. 5 could result from this effect, in addition to the effect of the decrease in electron density.
References 1 J. M. Bridges, G. T. Rymer and D. S. MacIver, J. Phys. Chem., 66 (1962) 871. 2 L. L. Van Reijen, W. M. H. Sachtler, P. Cossee and D. M. Brouwer, Proc. 3rd Int. Congr. Catal., Amsterdam, 1964, p. 829. 3 F. M. Ashmawy, J. Churn. Sot., Faraday Trans., I, 76 (1980) 2096. 4 Ch. MarciIly and B. Delmon, J. Catal., 24 (1972) 336. 5 A. Brenner, D. A. Hucul and S. J. Hardwich, Inorg. Chem., 18 (1979) 1478. 6 T. J. Thomas, D. A. Hucul and A. Brenner, ACS Symp. Ser., 192 (1982) 267. 7 J. K. Wang, T. Komatsu, S. Namba, T. Yashima and T. Uematsu, J. Mol. Catal., 37 (1986) 327. 8 J. K. Wang, S. Namba and T. Yashima, J. Mol. Catal., 53 (1989) 155. 9 A. Kazusaka and R. F. Howe, J. Catal., 63 (1980) 447. 10 Y. Ben Taarit, G. Wicker and C. Naccache, Magnetic Resonance in Colloid and Intwfme Science, Reidel, Dordrecht, 1980, p. 497. 11 J. J. Phillipson, P. B. Wells and G. R. Wilson, J. Chern. Sot. A, (1969) 1351. 12 J. Bates, 2. K. Leszczynsky, J. J. Phillipson, P. B. Wells and G. R. Wilson, J. Chmn. Sot. A, (1970) 2435. 13 J. Grant, R. B. Moyes and P. B. Wells, J. Catal., 51 (1978) 355. 14 M. George, R. B. Moyes, D. Ramanarao and P. B. Wells, J. Catal., 52 (1978) 486. 15 Y. Okamoto, Y. Nitta, T. Imanaka and S. Teranishi, J. Catal., 64 (1980) 397. 16 Y. Okamoto, K. Fukino, T. Imanaka and S. Teranishi, J. Catal., 74 (1982) 173. 17 R. C. Archibaid and R. S. Greensfelder, In& Eng. Chxm., 37 (1945) 356. 18 B. Fubini, G. Ghiotti, L. Stradella, E. Garrone and C. Morterra, J. Catal., 66 (1980) 200. 19 Y. Okamoto, H. Onimatsu, M. HOI?, Y. Inui and T. Imanaka, Catal. I&t., 12 (1992) 239. 20 G. C. Bond, G. Webb, P. B. Wells and J. M. Winterbottom, J. Chem. Sot., (1965) 3218.
of Molecular Catalysis, 78 (1993) 57-66 Science Publishers B.V., Amsterdam
57
M3059
Catalytic activity of 1owLvalent chromium supported on Y-zeolites Takayuki Komatsu, Reiko Miyoshi, Seitaro Namba* and Tatsuaki Yashima** Department (Beceived
of Chemistry,
Tokyo Institute
of Technology,
Meguro-ku,
Tokyo
152 (Japan)
February 25, 1992; revised June 10, 1992)
Abstract The catalytic activity and selectivity of low-valent chromium species supported on Yzeolites were studied for the dehydrogenation of cyclohexane and for the hydrogenation of 1,3-butadiene. Chromium species having average oxidation numbers (AON) of 0.02-0.37 were prepared by decomposition of Cr(CO)e adsorbed on HNa-Y zeolites with various values of proton content. The activity of the low-valent chromium species in the dehydrogenation of cyclohexane into benzene was high compared with Cr(II)- and Cr(III)exchanged Y-zeolites and was also higher than that of the conventional chromia/siIica and chromia/ahunina catalysts. The activity decreases with increasing AON of chromium. The Cr(0) species was assigned as the active site. A sharp drop in the activity was observed when the AON of chromium was increased above 0.2. For the hydrogenation of 1,3-butadiene, the activity also decreased with increasing AON. Cr(0) was again thought to be the active site. The selectivity for the formation of butene isomers changed when the AON exceeded 0.2, that is, 1,2-addition of hydrogen was dominant at AON below 0.2 and 1,4-addition began to increase above 0.2. These results indicate that the changes in the electron density and in the morphology of Cr(0) species occur within a small range of the AON around 0.2. Simple models for the low-valent chromium species are proposed.
Introduction Supported chromia catalysts are known to catalyze the dehydrogenation of saturated hydrocarbons. Many researchers have tried to clarify the active site in relation to the oxidation state of the supported chromium. Bridges et al. [ 1 ] have studied the dehydrogenation of cyclohexane over Cr,O&l,O, catalysts. They found that formation of methylcyclopentane occurred on the alumina surface, along with dehydrogenation on the chromia surface. The addition of potassium poisoned the acid sites of alumina, resulting in an increase in the dehydrogenation activity. Reijen et al. [2] have also studied the dehydrogenation of cyclohexane over the potassium-promoted Cr203/ *Present address: Department of Materials, The Nishi-Tokyo Kitatsuru-gun, Yamanashi 409-01, Japan. **Author to whom correspondence should be addressed.
0304-5102/93/$06.00
0 1993 - Elsevier
University,
Uenohara-cho,
Science Publishers B.V. AI1 rights reserved
58
A1203. The addition of HZ0 caused the oxidation of Cr(II) to Cr(III), which resulted in a decrease in activity. They concluded that Cr(I1) was the active site. In the case of the dehydrogenation of ethane over Cr20,/Al,0, catalysts, Cr(II), which was formed from Cr(II1) by reduction in dry hydrogen, was reported to be active [3]. Cr(II) was also more active than Cr(III) for the dehydrogenation of isobutane [ 4 1. These results indicated that the lower the oxidation state, the higher the activity for the dehydrogenation. Therefore, chromium species having oxidation states lower than Cr(I1) may have higher activity. However, for the conventional supported chromia catalysts prepared by an impregnation method, it is hard to reduce chromium to oxidation states lower than + 2. Cr(CO& has been reported to be a possible chromium source for a new type of chromium catalysts [5-lo]. Chromium was fixed on a support through the decomposition of adsorbed Cr(CO)G at high temperatures. By using a highly dehydroxylated alumina as a support, low-valent chromium species were obtained with the average oxidation number (AON) of chromium below + 1 [5, 61. We have prepared chromium-supported Y-zeolites using Cr(CO), and HNa-Y zeolites [ 7, 81. The oxidation states of chromium were controlled by changing the preparation conditions and the proton concentration in HNa-Y. Higher dehydration temperatures of HNa-Y and lower decomposition temperatures of adsorbed Cr(CO)G gave chromium species with low AON. Lower proton concentrations also helped to get the lower-valent chromium species. Thus we could adjust the AON of chromium from 0.02 to 1 .ll. In the present work, we prepared chromium-supported zeolites which consisted of chromium species in very low oxidation states using Cr(CO), and HNa-Y zeolites, and carried out the dehydrogenation of cyclohexane over these catalysts. The purpose of this study is to clarify the catalytic properties of the low-valent chromium species. We have already studied [B] the relation between activity and oxidation state for the chromium catalysts supported on Y-zeolites. Cr(0) was suggested to be the active site for the hydrogenation of ethylene, and Cr(I) the active site for the polymerization of ethylene. In these reactions, the activities changed by more than one order of magnitude when the AON of chromium was varied from 0.02 to 0.26. Therefore, not only the valence state but also other functions of the active chromium species, Cr(0) and Cr(I), might be changed within the small range of AON values. In order to clarify what happened to the chromium species in low oxidation state, the hydrogenation of 1,3-butadiene was also carried out over the same series of catalysts. The hydrogenation of 1,3-butadiene has been extensively studied by Wells and co-workers [ 11-141. They proposed a reaction scheme consisting of two reaction paths, 1,2-addition and 1,4addition, for alumina-supported iron, cobalt, nickel and copper catalysts [ 111 and for alumina-supported rhodium, palladium and platinum catalysts [ 121. Contamination with sulfur and other nonmetals varied the selectivity for the two reaction paths [ 13, 141. Okamoto et al. [ 15, 16 ] have studied the same reaction over various nickel catalysts. From the results of the catalytic
59
reactions and XPS spectra of nickel, they concluded that small changes in the electron density of nickel metal caused the change in selectivity. In the case of our chromium catalysts, the low-valent chromium species should mainly contain Cr(O), which suggests that the catalysts behave like metal catalysts. Therefore, small changes in the electron density may be monitored by the selectivity in the hydrogenation of 1,3-butadiene.
Experimental Catalysts Two kinds of chromium supported on Y-zeolites were prepared from Na-Y (Toyo Soda Ind. Co., Lot Y-30; Si/Al= 2.88). Details of the catalyst preparation have been described previously [7]. The first, Cr/HNa-Y, was prepared by gas-phase adsorption of Cr(CO), (Aldrich) on H(x)Na-Y having various degrees of proton exchange, x(0-65%). The proton exchange was carried out in a conventional manner with NH4C1 aqueous solution followed by calcination at 743 K in air. A specific amount of Cr(CO)G powder was added to the dehydrated HNa-Y under argon atmosphere. After flash evacuation at 295 K, Cr(CO& vapor was adsorbed at 333 K. The Cr(CO),/H(x)Na-Y was heated in vacua at 723 K to release its CO ligands quantitatively, resulting in the formation of Cr/HNa-Y. The concentration of supported chromium was adjusted to two chromium atoms per supercage of Y-zeolite. The second, Cr-Y, was prepared by an ion-exchange technique using the same Na-Y and an aqueous solution of CrC13. The degree of ion exchange was 47%, corresponding to an average stoichiometry of one chromium atom per supercage. Chromia-alumina (Cr,O,: 10 wt.%) was prepared by impregnation [ 171 with chromic anhydride (Wake Pure Chemical Ind. Ltd.) on -y-alumina (Nishio Ind. Co., type 30-60). Chromia-silica (Crz03: 5 wt.%) was prepared by a similar impregnation technique [ 181 using chromic anhydride and fumed silica (Cab-o-Sil@, M-5). The amount of supported chromium in Cr/HNa-Y and the degree of proton exchange in H(x)Na-Y were determined by atomic absorption spectroscopy and flame photometry, respectively. Apparatus and procedure The average oxidation number (AON) of chromium in Cr/I-INa-Y was determined from the amounts of Ha, CO, CO2 and CH., evolved during the temperature-programmed decomposition (TPDE) of Cr(CO)G adsorbed on HNa-Y [ 71. These gases were analyzed with a quadrupole mass spectrometer (ULVAC, MSQ-15OA). The dehydrogenation of cyclohexane was carried out using a glass circulation system with a dead volume of 300 ml. In the case of Cr/HNa-Y, HNa-Y (0.20 g) was put in a quartz tube attached to the system and Cr/ HNa-Y was prepared in situ using a side arm containing Cr(CO)G. The
60
reaction was carried out after the decomposition of Cr(CO)G/HNa-Y in vacua at 773 K. Other catalysts were placed in the quartz tube and heated in vczczw at 773 K before the reaction. Cyclohexane was circulated at an initial pressure of 50 torr and at a reaction temperature between 573 K and 773 K. The gas-phase mixture was analyzed by gas chromatography. The hydrogenation of 1,3-butadiene was carried out in a similar manner. Cr/HNa-Y was prepared in situ by the decomposition of Cr(CO),/HNa-Y at 473 K. The initial pressures of 1,3-butadiene and hydrogen were 100 and 150 torr, respectively. These gases were mixed and the mixture was circulated through the catalyst at temperatures of 273-473 K. 1,3-butadiene were purified by repeating a Cyclohexane and freeze-pump-thaw cycle. Hydrogen was passed through a manganese/silica trap to remove possible residual oxygen and water. Results
and discussion
In our previous work [7, 81 we reported that the oxidation state of chromium in Cr/HNa-Y prepared by the gas-phase adsorption technique could be controlled by changing preparation conditions. Lower dehydration temperatures and higher degrees of proton exchange of the parent HNa-Y resulted in higher oxidation states, because protons in HNa-Y oxidized the zero-valent chromium of Cr(C0)6 during the decomposition of Cr(C0)6 adsorbed on HNa-Y. Decomposition at higher temperatures also accelerated the oxidation of chromium. Thus we could control the average oxidation number (AON) of chromium in the range 0.02-l .l 1. It was also reported [S] that the chromium species in these catalysts consist of Cr(0) and a small amount of Cr(1) and Cr(II). Figure 1 shows a plot of the AON of chromium against the degree of proton exchange. The dehydration temperature of HNa-Y was 823 K and
0
20
40
60
Desree Of Droton exchanse /9o
0
0.1
0,2
0.3
0.4
AON
Fig. 1. Effect of degree of proton exchange in Y-zeolites on average chromium in Cr/HNa-Y. HNa-Y was dehydrated at 823 K.
oxidation
number of
Fig. 2. Effect of average oxidation number of chromium on the activity for dehydrogenation of cyclohexane. The conversion of cyclohexane was obtained after 1 h of the reaction at 723 K.
61
the decomposition temperature of CI-(CO)~ was 723 K. As expected, the AON increased with the proton concentration, and the AON could be adjusted from 0.13 to 0.37. In order to investigate the catalytic properties of the low-valent chromium species in these catalysts, the dehydrogenation of cyclohexane was carried out. The conversion over the parent HNa-Y was negligible compared with that over CrMNa-Y catalysts. The reaction products over the Cr/HNa-Y catalysts were benzene and a trace of cyclohexene. Figure 2 shows the percentage conversion of cyclohexane into benzene after a reaction time of 1 h over various Cr/HNa-Y having AON of 0.13-0.37 (see Fig. 1). The catalysts of lower AONs gave high conversions. In the preparation of the catalysts, Cr(CO),/HNa-Y was decomposed at a high temperature (773 K) to keep the Cr/HNa-Y stable at the reaction temperature, 723 K. As mentioned above, the AON is higher when Cr(CO)JHNa-Y is decomposed at a higher temperature. Accordingly, 0.13 was the lowest AON obtained through the decomposition at 773 K. The activity decreased drastically when the AON was increased from 0.19 to 0.25. It is suggested that a chromium species with very low oxidation state, probably Cr(O), is responsible for the high dehydrogenation activity. In the case of the hydrogenation of ethylene, we have already reported [8] that the Cr(0) species were the active site of Cr/HNa-Y catalysts. However, it should be considered that in Fig. 2, Cr/HNa-Y with AON of 0.25 or 0.37, which gave very low activities, still have a significant amount of Cr(0). A slight change in the electron density and/or in the morphology of Cr(0) species might affect the activity. We have investigated the adsorption of NO on Cr/HNa-Y [8]. The amount of adsorbed NO (about 1.2 NO/Cr) did not change significantly when the degree of proton exchange was varied from 0 to 75%. This indicates that the dispersion of chromium did not change greatly in CrLHNa-Y used for the dehydrogenation of cyclohexane. Therefore, the drastic change in the dehydrogenation activity shown in Fig. 2 could not result from a change in the dispersion of chromium. A migration of chromium into specific sites inaccessible to cyclohexane molecules should also be considered. Cr(0) atoms are weakly bound to the surface, while Cr(I) cations are fixed to the surface by Cr-0 bonds. Therefore, Cr(0) might migrate into such specific sites. If so, the activity would increase with increasing number of chromium cations, which could anchor the Cr(0) atoms to the sites accessible to the reactant. The activity, then, would increase with increasing AON, which does not coincide with the result shown in Fig. 2. These considerations result in the conclusion that slight changes in the electron density are the most probable cause of the activity change. The activities of other chromium catalysts were examined to compare the low-valent chromium species with Cr(II) and Cr(III) cations. Figure 3 shows conversion versus time plots for various chromium catalysts. The amounts of chromium were equal for all the catalysts. It is clear that Cr/ Na-Y (a) is the most active catalyst, followed by Crz03/Al,03 (b), Cr203/ SiOa (c) and Cr,O, (d). Therefore, the activity of the low-valent chromium, Cr(O), may be higher than that of Cr(II1). The activities of Cr(III)-Y (e) and
62
1.0
(b) y
CC)
c
Cdl
0
-1.0
\ F
11 "0
30
60
Reaction time /mln
I
1.3
1.4
1.5
1.6
103/1
Fig. 3. Change in conversion of cyclohexane with reaction time for the dehydrogenation at 723 K over (a) Cr/Na-Y, (b) Cr,03/Al,03, (c) Cr,O,/SiO,, (d) Cr,O,, (e) Cr(III)-Y and (f) Cr(II)-Y. Fig. 4. Arrhenius plot for the dehydrogenation H(35%)Na-Y and (A) Crz03/A120,.
of cyclohexane
over (0)
Cr/Na-Y, (0)
Cr/
Cr(II)-Y (f) prepared by the reduction of Cr(III)-Y with hydrogen at 773 K for 2 h were both very low, even when compared with unsupported Cr,O,. This suggests that the isolated chromium cations are much less active for the dehydrogenation of cyclohexane. Figure 4 shows Arrhenius plots for the dehydrogenation over Cr/Na-Y, Cr/H(35%)Na-Y and Cr,0&41,03 catalysts. The rate constant k was obtained from the initial rate, assuming a first-order dependence on the pressure of cyclohexane. The apparent activation energies for these catalysts were 42, 41 and 142 kl mol-‘, respectively. As shown in Pigs. 2 and 3, Cr/H(35%)Na-Y had AON = 0.20 and was less active than CrLNa-Y (AON = 0.13). Nevertheless, the activation energies for these catalysts were almost the same, suggesting that the active sites of these catalysts were the same. On the other hand, the activation energy for Cr203/A1203, where Cra’ was the dominant species, was much higher than that for Cr/Na-Y or Cr/H(35%)Na-Y. These results support the idea that the active site for the dehydrogenation of cyclohexane over Cr/HNa-Y with various degrees of proton exchange is the same Cr(0) species. Phillipson et al. [ 111 have studied the hydrogenation of 1,3-butadiene over iron, cobalt, nickel and copper catalysts. They concluded that the reaction occurred through 1,2- and 1,4-addition of hydrogen to the diene and that the dominant route depended on the degree of sintering or on the completeness of reduction during preparation. We carried out the hydrogenation of 1,3butadiene to get information about the morphological and electrical changes in the low-valent chromium species of Cr/HNa-Y within a low AON region. Various CriHNa-Y catalysts with chromium AON between 0.02 and 0.29 were examined for the expected change in activity and selectivity at around AON = 0.2, which was the critical AON for the dehydrogenation of cyclohexane, as indicated above (see Fig. 2). At the temperatures tested (273-473 K),
63
reaction products were 1-butene, trcms- and cti-2-butene and a small amount of n-butane. Figure 5 shows the effect of the AON of chromium in Cr/HNa-Y on the conversion and selectivity for the hydrogenation of 1,3-butadiene. The conversion decreased when the AON increased from 0.2 to 0.3. Although this decrease in activity is not so drastic as that for the dehydrogenation of cyclohexane (Fig. 2), the low-valent chromium species seems again to be the active site. It should be noted that the selectivity also changed with AON, that is, the selectivity for 1-butene decreased and that for cti-2-butene increased as the AON increased from 0.2. Okamoto et al. [19] reported that the hydrogenation of 1,3-butadiene over Cr(CO& encaged in Na-X zeolites gave cti-2-butene with high selectivities (> 98.5%) at 330-350 K. They have proposed a reaction mechanism in which cti-2-butene is formed through (q4-C4HG)Cr(CO), and 1-butene through ($-C4HG)Cr(CO), and (q2C4H,),Cr(C0)a. Their catalyst contained some CO ligands on each chromium under the reaction conditions. The chromium species in our catalysts, Cd HNa-Y, do not have any CO ligands. Therefore, the selectivity of Cr/HNa-Y was different. Figure 6 shows the changes in the selectivities to the butenes and butane with reaction time on Cr/Na-Y (AON= 0.02). During the initial 20 min, the selectivity to 1-butene slightly increased, while that to cti-2-butene decreased. If the isomerization of butenes occurs concomitantly, 1-butene should be converted mainly into Pans-2-butene to approach thermodynamic equilibrium (at 373 K, I-butene: 7%, trans-2-butene: 65%, cti-2-butene: 28%). In the case of metallic catalysts, the rate of isomerization of 1-butene was reported to depend on the hydrogen pressure [ 201. To determine the influence of the isomerization, the hydrogenation of 1,3-butadiene was carried out under various initial pressures of hydrogen (loo-250 torr) with the pressure of 1,3-butadiene kept constant (100 torr). As shown in Fig. 7, the selectivity to each butene did not change with the hydrogen pressure after 1 h of reaction over Cr/Na-Y (AON = 0.02). The isomer distribution of butenes was far from the equilibrium composition. Therefore, the isomerization of butenes
GON
Fig. 5. Effect of average oxidation number of chromium on (0) the conversion of 1,3-butadiene, and on the selectivities to (0) 1-butene, (A) cis-2-butene, (I) truns-2-butene and (V) butane for the hydrogenation of 1,3-butadiene over Cr/Na-Y at 373 K for 1 h.
.
O-TiFF=s 0 Reaction
tlme/mln
Lx
0
100
Pressure
200
of
300
Hz /Tort
Fig. 6. Changes in selectivities to (0) 1-butene, (A) cti-2-butene, (I) tmns-2-butene and (V) butane with reaction time for the hydrogenation of 1,3-butadiene over Cr/Na-Y at 373 K. Fig. 7. Effect of the partial pressure of hydrogen on the selectivities to (0) I-butene, (A) cti-2-butene and (m) trcms-2-butene for the hydrogenation of 1,3-butadiene. The reaction was carried out over Cr/Na-Y at 373 K for 1 h.
can be neglected in the discussion about butene composition in the products of the hydrogenation of 1,3-butadiene. Phillipson et al. [ 111 have proposed a reaction mechanism for the hydrogenation of 1,3-butadiene over some metal catalysts: 1-butene is produced only by 1,2-addition via n-bonded species, while bans- and c&+2butene are produced by 1,4-addition via v-allylic species as well as by 1,2addition via the r-bonded species. According to their mechanism, on the catalysts with high electron density, the r-bonded species are stable and so 1-butene is the main product. As the electron density decreases, the 7-r-allylic species become stable and the selectivity to 2-butenes increases. In the case of CrAINa-Y shown in Fig. 5, 1-butene was the main product at lower AON, which was in accordance with the simple idea that the lower the AON, the higher the electron density of chromium species. The selectivity began to change when the AON increased above 0.2, where the selectivity for 1-butene decreased. This result could be explained by the change in electron density of active chromium species, though the change in the AON was relatively small. The drastic change in the activity for the dehydrogenation of cyclohexane shown in Fig. 2 could also be explained by the change in electron density. We propose possible models of the active chromium species (Fig. 8) based on the relation between AON and activity. When the AON of chromium is 0.0 (a), all the chromium atoms are Cr(0) and they do not have strong electrical interactions with the surface of Y-zeolites. Therefore, the chromium atoms migrate through the supercages of Y-zeolite to form aggregates. The number of chromium atoms constituting one aggregate is not necessarily five, as indicated in Fig. 8(a). However, there is not enough space inside Y-zeolite for the chromium atoms to form a large aggregate. The largest cavity, supercage, has a diameter of only about 1.3 run. Moreover, to keep a certain dispersion of chromium, the diameter of the aggregate must be smaller than 1.3 nm. When the AON is 0.2 (b), a portion of the chromium
65
Cr(0)
Cr(l)
(a) AON =O.O
Cc) AON=0,3
n
Fig. 8. Models of active chromium species
of Cr/HNa-Y.
atoms should be oxidized to higher oxidation states by surface hydroxyl groups of HNa-Y to form Cr-0 bonds, which anchor the chromium cations to specific sites of HNa-Y. As a result, unoxidized chromium atoms migrate to the chromium cations to form aggregates. ESR studies on Cr/HNa-Y [ 71 indicated that the samples with AON between 0.15 and 1.I1 did not give a signal due to Cr(II1) and that only a weak signal due to Cr(II) was detected. For example, 2% of the total number of chromium atoms were Cr(I1) for the sample with an AON of 0.26. Therefore, the chromium cation anchored to the surface is almost exclusively Cr(I), which results in the statistical stoichiometry shown in Fig. S(b), four Cr(0) attached to one Cr(I). When the AON is 0.3, the number of Cr(0) attached to one Cr(1) cation should decrease to about two, on average, as shown in Fig. 8(c). The interaction between Cr(0) and Cr(I) in model (c) may lower the electron density on Cr(0) to some extent. As mentioned above, Cr(0) is thought to be the active site for the dehydrogenation of cyclohexane and for the hydrogenation of 1,3-butadiene. Therefore, the decrease in electron density may cause the decrease in catalytic activities, though the electron density of Cr(0) in model (b) could be slightly lower than that in model (a). As can be seen in Fig. 8, the Cr(1) cation in model (b) is covered by Cr(0) atoms and is thus shielded from gas-phase molecules to some extent. On the other hand, Cr(1) in model (c) has more space to interact directly with gas-phase molecules. If direct interaction between the Cr(1) cation and the reactant molecules hinders the reactions, the change in the extent of exposure of Cr(1) cations is another possible factor for the drastic change in catalytic activities at AONs around 0.2. A similar effect of Cr(1) might cause the change in selectivity as well as that in activity. The significant change in
66
butene selectivity indicated in Fig. 5 could result from this effect, in addition to the effect of the decrease in electron density.
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