Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 19-27
ings, although statistically significant, were small. These observations can be explained in terms of the above reaction steps. The coal-derived solvent used for these experiments, SRC-I1 heavy distillate, was produced by a noncatalytic liquefaction process (Nowacki, 1979) and therefore contained a low concentration of hydroaromatic donor solvent components. Without a catalyst, a significant amount of gas-phase hydrogen cannot be transferred to dehydrogenated polynuclear hydroaromatics in order to replenish those consumed. Thus, for the uncatalyzed experiments, lack of hydrogen donors in the hydrogen transfer step severely limits the conversion of coal to low mw product. However, for the catalyzed experiments, hydrogen donors may be replenished in situ by reaction 3. With an adequate concentration of hydrogen donors, conversion of coal to low mw product in the presence of a catalyst is increased because the hydrogen transfer step is not rate limiting. The small differences in conversions between experiments with different catalysts and active metal loadings may be due to the consequences of either kinetic or thermodynamic constraints. The bond fragmentation step, not the catalytic hydrogenation step, may be rate limiting, or alternately, all of the catalysts may rapidly hydrogenate the donor solvent components to their thermodynamic concentration limits. Both effects would diminish the differences in the apparent activities of the catalysts. For example, although these experiments indicate that Pd hydrous titanate increases coal conversion only a few percent more than that of the Ni hydrous titanate, the rate of hydrogenation of solvent components to their thermodynamic concentration limits may be much greater with the Pd catalyst. Kinetic studies of catalytic hydrogenation of solvent alone or with much simpler model chemical systems (e.g., phenanthrene or pyrene) must be performed to quantify the differences in hydrogenation activity of the catalysts. These results indicate that catalysts prepared using hydrous metal oxide ion exchangers show promise for
19
applications to coal liquefaction and other hydrogenation processes. The hydrous titanate catalysts, even at low active metal loadings of 1%,are equally effective for conversion of coal to low molecular weight product as a commercial Ni-Mo/alumina catalyst containing 15% by weight active metals. Considering the versatility of these inorganic ion-exchange compounds for adjustment of substrate acidity and basicity, and addition of promoter elements, it is possible that these materials can be used to produce improved multifunctional catalysts for not only coal liquefaction but a variety of processes. Registry No. Ti(O-i-C3H7)4, 546-68-9; NaTizQ5H,60704-88-3; Ni(Tiz05H),,94090-50-3;Mo(Tiz05H)2,94090-51-4;Pd(TizQsH)z, 94090-52-5;Ni, 7440-02-0;Mo, 7439-98-7;Pd, 7440-05-3.
Literature Cited Dosch, R. G. Sandia National Laboratories Report SAND 78-0710, Albuquerque, NM, June 1978. Dosch, R. G. Sandia National Laboratories Report SAND 80-1212, Aibuquerque, NM, Jan 1981. Gorin, E. "Fundamentals of Coal Liquefaction", Chapter 27 in "Chemistry of Coal Utilization", 2nd Suppl. Vol., Elliot, M. A,, Ed.; Wiley: New York, 1981. Hankey, D. L.; Hammetter, W. F.; Dosch, R. G. 34th Pacific Coast Regional Meeting, American Ceramic Society, Oct 1981. Johnston, K. Fuel, 1984, 63(4), 463. Kotfenstette, R. J. Sandla National Laboratories Report SAND 82-2495, Albuquerque, NM, March 1983. Nowacki, P. "Coal Liquefaction Processes"; Noyes Data Corporation, Park Rage, NJ, 1979. Rao, A. K.; Pillai, R. S.; Lee, J. M.; Johnson, T. W. "Recent Advances in Two-Stage Coal Liquefaction at Wilsonville"; Eighth Annual EPRI Contractors' Conference on Coal Liquefaction, Palo Alto, CA, May 1983. Schindler, H. D.; Chen, J. M.; Potfs, J. D. "Integrated Two-Stage Liquefaction. Topical Technical Progress Report, Steady State Illinois No. 6 Program Period April 1, 1982 - July 6, 1982"; The Lummus Co., Report 1480449, Dist. Category UC-Sod, April 1983. Stephens, H. P.; Chapman, R. N. Am. Chem. Soc ., Div. Fuel Chem. Pfepr. 1983, 28(5),161. Tauster, S. J.; Fung, S.C.; Baker, T. K.; Horseiey, J. A. Science 1981, 211, 4487. Whitehurst, D. D.; Mitchell, T. 0.; Farcasiu, M. "Coal Liquefaction"; Academic Press: New York, 1980.
Received for review May 7, 1984 Revised manuscript received October 5, 1984 Accepted October 11, 1984
Characterization and Activity of Some Mixed Metal Oxide Catalysts Tetsuro Selyama, Noboru Yamazoe, and Kolchl Eguchl Department of Materiels Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasugakoen, Kasuga-shi, Fukuoka 8 16, Japan
One of the most important factors which affect the catalytic activities of metal oxide catalysts in the oxidation process is the activation of oxygen. The properties of oxygen can be modified in mixed oxide catalysts, leading to promoter actions. This paper describe&three examples of catalysis for which lattice oxygen, adsorbed oxygen, or absorbed oxygen is responsible. Oxidation catalysis of heteropoly compounds is operated by the redox cycle of Keggin anions where the bridging oxygen participates in the reaction. The redox process in methacrolein oxidation is strongly affected by additives such as arsenic and copper. Adsorbed oxygen plays a major role in biacetyi formation from methyl ethyl ketone on spinel type oxides under the influence of acid-base properties of catalysts. Spinel type oxides, e.g., CuCo,O, and Co2Ni04,show excellent activity and selectivity for biacetyl formation. A large amount of absorbed oxygen is incorporated into the lattice defects of perovskite-type oxides. Partial substitution of Sr for La greatly affects the absorption behavior of oxygen as well as oxidation activity of LaCoO,.
Introduction Among many factors which affect the catalytic oxidation process, one of the most important ones is the activation of oxygen. Usually, two types of activated forms of oxygen 0198-4321/85/1224-0019$01.50/0
may be discerned, namely, lattice oxygen and adsorbed oxygen. However, we wish to add one more type, absorbed oxygen, which plays major roles in some oxide catalysts with defect structures. The three types of activated oxygen 0 1985 American Chemical Society
20
Ind. Eng. Chem. Prod.
Res. Dev., Vol. 24,No. 1, 1985
have widely different properties in catalytic oxidation. Lattice oxygen has been shown to be responsible for many useful partial oxidation reactions. The catalytic activities of lattice oxygen are often determined by thermodynamic properties of oxide because redox mechanisms are usually operative. Oxygen atoms in a crystal lattice may be classified into several equivalent groups according to their structural environments. In such a case, only the specific lattice oxygen often participates in oxidation reactions, because the reactivity of oxygen is strongly dependent on the kind of neighboring metal cations as well as M-0 bonding distance. Adsorbed oxygen is known to be formed on cation sites of the surface of oxides. The reactivity of adsorbed oxygen differs extremely from that of the lattice oxygen because of their charge and bonding strength. The adsorbed oxygen is generally regarded as unselective species in partial oxidation. However, there are some characteristic partial oxidation reactions for which the adsorbed oxygen is responsible. The reactions proceed particularly at low temperatures, being affected by the acid-base properties of oxide surfaces. Some oxide lattices contain large extents of oxygen vacancies which can accommodate oxygen from the gas phase. Such sorbed oxygen tends to be more reactive than normal lattice oxygen mainly due to weak bonding strength. The vacancy formation can be controlled by doping foreign cations. The kind of doping metal and its amount affect the amount as well as the reactivity of sorbed oxygen. To know the real nature of catalytic oxidation reactions, it should be necessary to make clear differences in reactivity of lattice, sorbed, and adsorbed oxygen species because in complicated systems more than single oxygen species react concurrently with reactant molecules. However, few papers so far have compared the reactivities of the three oxygen species. In addition, it will also be of great importance to reveal how the reactivity of each oxygen species is affected by the kind of component metal cations. In this paper, three typical examples of catalytic oxidation reactions are picked out from our recent studies in each one of which only a specific type of oxygen participates. The first is the catalysis by heteropoly compounds in the oxidation of methacrolein to methacrylic acid, where lattice oxygen at a particular site of Keggin unit is responsible for the oxidation. The second is the biacetyl formation from methyl ethyl ketone on spinel type and other mixed oxides where adsorbed oxygen seems to play a major role. The third is the catalysis by perovskite type oxides for the catalytic combustion of hydrocarbons where essential roles are played by absorbed oxygen. Redox Mechanism of Heteropoly Compounds The catalytic oxidation of methacrolein to methacrylic acid is being increasingly employed to produce methyl methacrylate (Ohara, 1977; Eguchi et al., 1979). This process was recently industrialized by a Japanese company by using 12-molybdophosphates as active catalysts. The structure of 12-molybdophosphates is characterized by its large cluster anion called Keggin anion (Tsigdinos, 1978) as shown in the coordination polyhedral model (Figure 1). It consists of 12 octahedra of MOO, surrounding a central tetrahedron PO4. There are several combinations in Keggin anion components, but the P-Mo system is the most effective as an oxidation catalyst. A Keggin anion, in its fully oxidized form, contains 1 2 hexavalent molybdenum and 40 oxygen atoms. Since the catalytic oxidation seems to proceed by the redox cycle of the polyanions, the bonding types of lattice oxygen are of great importance.
Obl atom :
: P5+
(As5+, Si4', et
Ob2
Figure 1.
Ge4+)
Coordination Dolvhedral model of Keggin unit,
NO-0 NO-Cb-MO
c
( e ) x= 6.31
1200
1000
800
600
Wave Number / cm-I
Figure 2. Infrared spectra of potassium dodecamolybdophosphate reduced with HZ.
The Keggin anion possesses 3 bonding types of oxygen, i.e., 24 bridging oxygen atoms, o b , 12 terminal oxygen atoms, 0,,and 4 oxygen atoms coordinated to heteroatom, 0,. By a contact with reactant molecules, oxygen ions are consumed, accompanied by reduction of molybdenum ions into a pentavalent state. The difference in reactivity of oxygen species has long been discussed in many other redox systems of oxides. The situation in the Keggin anion is rather simple because the site and amount of oxygen species are already known. We carried out spectroscopic measurements of reduced molybdophosphates to know which and how many oxygen ions are first consumed by the reduction of the Keggin anion. First, the reduction process of polyanions was investigated by quantitative IR measurements. Figure 2 shows the change of IR spectra of the potassium salt with H2 reduction. Hereafter, the degree of reduction, x , is defined as the number of introduced electrons per Keggin anion. The unreduced sample showed IR bands characteristic of the 3 bonding types of oxygen in the anion (Lyhamn et al., 1976), i.e., P-0, (1065 cm-I), Mo-0, (960 cm-I), and Mo-Ob-Mo (800 and 870 cm-l). These bands are called O,, O,, and Ob bands, respectively. As the reduction progressed, 0, and O b bands were significantly decreased, while the 0, band decreased only gradually. Each IR absorbance is plotted vs. the degree of reduction with H2 in Figure 3. It is noted that the absorbances of 0, and
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985 Lowest GupledNO
0.2
21
A -
0,l 0
0.3 0,2
0.1 0
e
0
870
cm-1
0
0.6 U
0.2
0,4
8
0.2
8
e
Figure 5. Electron density distribution of HOMO and LUMO. 0.8
0 0
2
4
6
I'
8
Degree of Reduction, x / e l a n i o n
Figure 3. Changes in IR intensities of K 3 P M o 1 2 0 ~ zwith / 2 the degree of reduction with H2: (- - -) expected line. C6H12
H2
C4H60
evac. a t 2 0 0 " ~
K
2
0.2
evac, U
0
0.2
0.4
.
100°C 0.6
0.8
Degree o f Reduction w i t h H i /e/anion
Figure 6. Changes in the spin concentration with reduction degree of 12-molybdophosphoric acid evacuated at various temperatures: (-) obsd, recorded at -100 OC, corrected the Mo6+ concentration formed by evacuation; (- - -) calcd.
Figure 4. Reduction processes of 12-molybdophosphateion.
Obdecrease linearly with x and almost disappear at x equal to 4. As we reported previously (Eguchi et al., 1983) these bands are characteristic of unreduced anion and disappear when the anion symmetry is deteriorated with reduction. The slopes of these bands indicate that 4 electrons are introduced to each polyanion in the initial stage of reduction. On the other hand, intensity of the Ot band should reflect the concentration of Ot because this band is insensitive to the anion symmetry. The integrated absorbance of Ot is unchanged at the initial stage of reduction. This means that not the terminal oxygen but the bridging oxygen is consumed at this stage. At more deeply reduced region, the integrated absorbance of Ot gradually decreased. The slope of its decrease agreed well with that expected from the consumption of 0,. While the reduction with Hzis initiated with the formation of 4e-reduced intermediates as mentioned, 2 electron reduction dominates in the reduction with methacrolein and cyclohexane (Figure 4). This difference may be associated with the molecular size of reactants. Large molecules, such as cyclohexane and methacrolein, react only with the polyanion at the surface, while hydrogen molecules are small enough to enter the pore and can reduce the bulk polyanion by a direct contact up to the 4-electron reduced state. In every reduction, the bridging oxygen showed exclusive reactivity at x less than 4. The high reactivity of the bridging oxygen shown from IR results was supported from a molecular orbital calculation. The electronic structure calculation of whole polyanion cluster was carried out by the model-potential
x,
method (Katsuki and Inokuchi, 1982) by cooperation of Professor Taketa et al. Among the valence orbitals, the highest occupied and the lowest unoccupied MO directly influence the chemical reactivity of the cluster. The contributions of atomic orbitals to these molecular orbitals are shown in Figure 5. The highest occupied level is found to consist of almost pure 2p component of the bridging oxygen. This appears to indicate that the bridging oxygen serves as the adsorption site of a reactant molecule. The LUMO consists of Mo 4d orbitals together with a partial contribution from 0 2p of the bridging oxygen. This level is supposed to accept the electrons transferred from a reactant molecule. Since this level has an antibonding character with respect to the Mo-Ob bond, the electron transfer should result in weakening of this bond. Thus the MO calculation well supports the experimental results that the bridging oxygen is first consumed with reduction. The reduced states of molybdenum were further investigated by ESR. Figure 6 shows the effect of evacuation on spin concentration of Mo5+. The increase in spin concentration with evacuation temperature appears to be caused by suppression of the spin-lattice relaxation (Hall and LoJacono, 1977). It is noted that after evacuation at 300 "C the spin concentration almost coincided with the values expected from the reduction degree. This means that Mo5+ions scarcely interact with each other after the evacuation treatment. This eliminates a possibility that two Mo5+are bonded to a single lattice oxygen ion, since the ESR signals are generally unobservable in such a configuration owing to the strong coupling of spins. Taking into account the 4e-reduced state based on the IR result, 4 Mo5+ions are accommodated in these Mo3OI3units, one by each. This suggests that the bridging oxygen atoms o b 1
22
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985
Table I. Degree of Reduction of the Catalyst after Methacrolein Oxidation deg of redn, e/anion ~~~
sur-
bulk
reaction temp, OC 280 300 320 350 375
(IR) 0.25 0.51 0.79 0.95 1.25
face (ESR) 0.18 0.64 0.56 0.99 1.26
(XPS) 2.5
Table 111. Reduced States of the Catalysts after Oxidation of Methacrolein (T= 320 "C;W / F = 2.0 g s/cmS) deg of redn, e/anion cat (W (XPS) >2O 4.2 As5+ (As/P = 1.5) + As5' Cu2+ (As/P = Cu/P = 0.5) 1.3 4.1 H,PMO~~O +,Cu2+ ~ (Cu/P = 0.5) 0.5 0.8
+
+
IR bands were too broad to estimate exact values.
2.6 5.3
T
= 350
"C. :00
Table 11. Effect of Additives on Catalytic Oxidation of Methacrolein on 12-Molybdophosphoric Acid" C4H60
C4H602
conv, 70 selec, % 35.2 37.6 H3PM012040 33.9 43.1 + As5+ (As/P = 0.1lb As5+ (As/P = 0.5)b 17.0 57.5 13.6 61.2 + As5' (As/P = 1.5)b 46.6 20.1 + Cuz+ (Cu/P = 0.5)b 36.0 35.9 + Co2+ (Co/P = 0.5) + As5++ Cu2+ (As/P = Cu/P = 0.5)* 39.8 53.3 I 1/zP205+ 12Mo03 + '/,Asz05 2.6 0 ' IC 20 30 25.8 49.9 H3PMoiz040 + '/4Asz% Electrc~egativity of r e t a i Ion, Xi (NH4)3AsMoiz040 21.6 66.5 Figure 7. Correlation between selectivity and electronegativity of H3PM012040 + H3A~M012040 22.2 48.5 component metal ion of catalyst: W / F = 2.7 s g of cat./cm3, Po2/ PMEK= 19.3, PMEK= 0.03 atm. a T = 320 "C; W / F = 2.0 g 5/cm3. *Prepared from homogeneous solution of H3PMo12040and H3As04and/or C U ( N O ~ ) ~ . cat.
+
which are shared by neighboring Mo3OI3units are consumed. Based on this information on the reduction process, we investigated the redox behavior of 12-molybdophosphoric acid in the catalytic oxidation of methacrolein. The degree of reduction shown in Table I was estimated from the IR absorbances, which agreed fairly well with the Mo5+spin concentration. It is obvious that the degree of reduction tends to increase with a rise in reaction temperature. The integrated absorbance for the Ot band was again unchanged, indicating that the active species in the catalytic oxidation are also the bridging oxygens. As observed by XPS, the degree of reduction at the surface was greater than that of the bulk catalyst. This confirms that the reaction takes place preferentially at the surface of the catalyst . A number of combinations of oxides are reported as active catalysts for production of methacrylic acid in Japanese patents (Ohara, 1977). Since most of the catalysts contain Mo and P, molybdophosphates are expected to be the active species of this reaction. However, some other elements seem to be necessary to enhance the catalyst performances. The promoting effects of arsenic and copper were investigated (Table 11). As the amount of As in the catalyst increased, the selectivity to methacrylic acid was clearly enhanced, while the oxidation activity was gradually lowered. In contrast, an addition of Cu promoted the oxidation activity of the catalyst, though the catalysis became unselective. When both additives were contained, the selectivity as well as the activity was noticeably increased. A similar effect of As on the selectivity is also observed with molybdoarsenate and with a mixture of molybdophosphoric and molybdoarsenic acids. This indicates that arsenic is present as a heteroatom in the Ascontaining catalysts. Considering that Cu cations are easily exchanged with the acidic protons, copper ions seem to be situated as countercations. After the use for the catalytic oxidation of methacrolein, the As and f or Cu added catalysts were subjected to the measurements of the reduced
states. In Table 111, the catalysts were listed in increasing order of the degree of reduction. Since the As-added catalyst was strongly reduced, As seems to enhance the reducibility of the catalyst. The decrease in oxidation activity is probably due to suppression of the reoxidation by As. In contrast, the addition of Cu is obviously effective in restoring the oxidation state in either a P-Mo or an As-P-Mo system. Copper appears to be the oxygen carrier to the polyanion. The As-containing catalysts have deeply reduced surface layers even when Cu is present. Such a situation leads to suppression of the multistep oxidation of methacrolein to CO and COz.
Oxidation of Methyl Ethyl Ketone to Biacetyl There have been few examples of useful partial oxidation using 02-or 0-,probably except for the well-known case of ethylene oxidation on silver catalyst. Recently, however, we have found that methyl ethyl ketone (MEK) is selectively oxidized into either acetic acid + acetaldehyde or biacetyl (BA) as follows (Seiyama et al., 1977; Takita et al., 1977, 1979; Yamazoe et al., 1983). The best performCH$OCH2CH3
+
02
/ 1 '
CH300H
+
CH3COCOCH3
CH3CH0
+
H2O
(V2O5-MoO,)
(C030,)
ance data show 80% selectivity. There has been evidence indicating that the oxidation proceeds by the participation of Oz-adsorbed species, and both reactions appear to go via a common peroxide type intermediate. For these reactions, the lattice oxygen seems not to be reactive or unselective. The catalysts become selective a t low temperature (lower than ca. 200 "C) where only the adsorbed oxygen participates in the reaction. The selectivity for biacetyl as well as that for acetaldehyde and acetic acid are correlated with the electronegativity (xi)of the component metal ions of oxide catalysts in Figure 7. There is a trend that acidic oxides favor the scission reaction while weakly basic oxides favor the biacetyl formation. Figure 8 shows a proposed reaction
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985 23 Table IV. Oxidation of cat. ConMnOA CoAlz04 Mgc0204
-
ZnCoz04 CoCr20, FezCo04 (33304 u 0204
cc
Co2Ni04
MEK over the Cobalt-Spinels (at 200 conv, % 40.1 21.9 11.5 2.3 26.5 8.2 29.5 7.1 26.3
r
' r : Specific rate of MEK conversion,
I
?
02 ~
?I
33.1 2.6 1.3
BA 10.1 18.1 23.5
AcH 41.1 51.1 66.6
1.6 4.3 1.4 7.8 46.4 18.8
39.0 46.9 49.4 56.1 61.2 72.0
35.0 21.5 13.9 0.3 19.7 4.2
selectivity, % AcOH 41.9 17.4 4.4 0 3.2 26.3 6.0 0 6.0
COZ 6.2 13.4 5.5
EtCHO 0
26.0 28.4 10.4 31.3 19.1 17.2
0 0 0.1 0.3 0 0.6
0 0
mol s-l m-2. BA: biacetyl; AcH: acetaldehyde; AcOH: acetic acid; EtOH: propionaldehyde.
- F:C-CHj
H3C-CH -0-Me-0-Me-
OC)O
?;?
?
? EMK -0-Me-0-Me- -----, -0-Me-0-Me-
/ / / / / / / I / / / / //////I/////
/////////////
0 11
basic catalysts
C03O4
, ,
I
+
-0-Me-0-Me-
CH3-C-C-CH3 + H 2 0 11
11
0 0
/ / / / / / / I//// / 0
H3C-CH acidic catalysts
5'2"
8,
$-CH,
,
H oI -0-Me-0-Me-
+
CHjCHO
+ CH3COOH
////////I I/// Figure 8. Reaction scheme of MEK oxidation.
0'
[ I
'
160
v
180
200
220
I
T / "C
mechanism. The reaction order of oxygen, approaching unity, is consistent with the assumed participation of adsorbed molecular oxygen. The formation of 02-ions was confirmed on the surface of catalysts such as Ag/Si02 and V205/Si02 (Lunsford, 1973), while it was strongly suggested on NiO and a-Fe203(Iwamoto et al., 1976, 1978). Moreover, the 02-ions on V205/Si02were shown to react with MEK to produce the scission products. On the basis of these facts we assume that 02-ions are the reactive species taking part in BA formation as well as in the scission reaction. On acid catalysts such as V205 the peroxide type intermediate is subjected to nucleophilic attack of 02-,while on basic catalysts hydrogen abstraction (probably as a proton) occurs to give respective products. Of the catalysts tested,Co304and Co2Ni04were active and selective for BA formation. We therefore, investigated the catalytic activities for MEK oxidation using the spinel type oxides containing cobalt (Yamazoe et al., 1983). As an example, the result of the oxidation over Fe2CoO4is shown in Figure 9. The formation of biacetyl was observed more or less on all the oxides in a low-temperature range (150 to 300 "C). The selectivity to BA decreased with an increase in temperature, BA formation being replaced by complete oxidation. The results of oxidation at 200 "C are listed in Table IV. Based on the product distribution at 200 "C, the oxides were classified into two groups; one group which is selective for BA and the other which favors oxidative scission of MEK. In the first group, Co2Ni04had the highest selectivity for BA, while CuCo204showed the highest specific rate for MEK conversion. In the latter group, Co,Mn04 gave equimolar amounts of acetic acid and acetaldehyde, but acetaldehydewas obtained predominantly on MgCo204 and CoA1204. It was further found that the catalytic activities of the cobalt-spinels for biacetyl formation or partial MEK oxidation were dependent on the types of spinel, i.e., (A)
Figure 9. Oxidation of MEK over FezCoO4: P O J P M E K = 12; WIF = 2.1 s g of cat./cm3.
0'"
PME,
= 0.03 atm,
2+
2.1
6.3
4.2 - A H ~ O
J x
IO-^
Figure 10. The specific rate of MEK oxidation over cobalt spinels as correlated with the heat of formation of the component oxides, MO,: (0) oxide of MCo204type; ( 0 )oxides of CoM2O4type.
M2+Co3+204, (B) (C) from the correlation between the catalytic activities and the electronegativity of component metal ions of cobalt-spinels (Yamazoe et al., 1983). Among the spinels of the same type, the activities are well correlated not only with the electronegativities of metal ion, but with the heat of formation of component oxides, MO,, as shown in Figure 10. I t is supposed that the oxides with a smaller heat of formation have better oxygen adsorptive property and show higher catalytic activity. The deviation of the activities of CozNi04and Fe2Co04,taking into consideration that these belong to inverse spinel, suggests that the cobalt ions at the A sites (tetrahedral) are more active than those at the B sites (octahedral). Apart from these correlations, temperature-programmed desorption measurements were also investigated. Oxygen was preadsorbed by cooling the samples in O2 (100 torr)
24
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985 sreeO3-ceo2 0
650 -
SrTiO3-TiO2
0 CaTi03-Ti02
18w03-~03
Y \ (1
550 -
2 c
I
100
300
200
T
400
I
500
/ "C
450-
Figure 11. TPD chromatograms of oxygen desorbed from the cobalt spinels.
0 LaCo03-Co3Oq
La?nO3-O-Mn2o3
450
550 TBO" 1 K
-7.0
Figure 13. Comparison of catalytic activities for propylene oxidaand component B oxide (BO,); tion of perovskite-type oxide (ABO,) T, temperature at which the rate of propylene oxidation reaches 10-8.6 mol m-2 5-l.
I
1
-6.5 104
-6,0 mount o f desorbed 02
I
-5.5
.mol / m 2 - c a t ,
Figure 12. Correlation between amount of desorbed O2 and catalytic activity of Co-spinel oxides; O2 desorption, R.T.-500 OC.
from 500 "C to room temperature. Figure 11shows TPD chromatograms of oxygen from the cobalt spinels. The desorption of oxygen is observed in every sample. Espec i d y CuCo20, exhibits a large desorption peak from a low temperature. It is seen in Figure 12 that the catalytic activities of the oxides ran parallel with the amount of adsorbed oxygen on the oxides. This fact confirms the participation of the adsorbed oxygen also for spinel type oxide catalysts.
Characterization of Perovskite-Type Oxide Catalysts Perovskite-type oxides containing transition metals are attracting great attention as catalysts for complete oxidation of hydrocarbons as well as electrochemical reduction of oxygen (Voorhoeve, 1977). As far as we know, the use of perovskite-type oxides as catalysts was first reported by Meadowcroft in 1970 for the electrochemical reduction of oxygen. Soon after that, Voorhoeve et al. (1972) reported the high catalytic activity of perovskite oxides for heterogeneous oxidation. These studies triggered many studies thereafter which are related to exhaust control catalysts and electrode catalysts. In perovskite-type oxide, represented by AB03, the B site cation is surrounded octahedrally by oxygen, and the A site cation is located in the cavity made by these octahedra. In this oxide system, the replacement of A and/or B site cations by other metal cations often brings about the formation of lattice defects. Although such defects have been correlated with high catalytic activity, little is known of this relationship. Perovskite-type oxides with A and/or B sites partly substituted desorb or absorb a
Temperature / "C
Figure 14. TPD chromatograms of oxygen from Lal-,Sr,Co03-,; oxygen preadsorption 800 OC-R.T., 100 torr.
large amount of oxygen. The nature and reactivity of absorbed oxygen may be greatly different from other oxygen which forms a rigid crystal lattice, because the absorbed oxygen is more weakly bonded t o metal cations than the normal lattice oxygen. Such weakly bonded oxygen is considered to be effective to complete combustion. Both the oxygen-sorptiveproperties of the compounds in relation with the defect structure and the role of absorbed oxygen and surface state in the catalytic activity were investigated. First of all, we have investigated a general aspect of unsubstituted perovskite oxide catalysts (ABO,). The activity of several perovskite oxides for catalytic oxidation of propylene with that of the component B oxides (BO,,) is compared in Figure 13. The activity is expressed in terms of temperature (9 at which the total oxidation of propylene takes place at a given rate. The plots below the straight line show that activity of B oxides is enhanced by forming perowkite structure, while the negative effects are indicated by the plots above the straight line. As shown in the figure, some oxides such as LaCoO, and LaF'eO, show positive effects. Roughly speaking, however, the plots are distributed near the straight line. This shows that the activity of unsubstituted perovskite oxides is determined mainly by component B oxides, and that the most active catalysts are those which contain Co and Mn. More interesting effects come out when the perovskite oxide is partially substituted for A or B sites. The effects of partial substitution are seen most obviously on the oxygen-sorptiveproperties. TPD chromatograms of oxygen from Sr-substituted LaCoO, are shown I
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985 25 Table V. The Amounts of Oxygen Desorbed from La,,Sr$00~~ amt of O2 desorbed: pmol O2 g-' surface area, cat. m2 g-1 a(e,) m,) 29.4 (3.4) LaCoOs 2.2 2.7 (0.3) 72.0 (3.5) L%&O.ZCoO3-6 5.2 81.6 (3.9) 5.7 207.3 (9.1) 85.5 (3.8) L%.6sr0.4c003-d 172.0 (3.2) SrCO02.5,b 13.4 348.0 (6.5)
'-1
z 2.0
:7
I
I
1. m
i
E
"8, and 8, denote surface coverages (in unit of surface monolay0
er).
200
600
400
800
TmDerature / "C
in Figure 14. Oxygen was preadsorbed by cooling the samples from 800 "C to room temperature in an oxygen atmosphere. All chromatograms are characterized by the appearance of two desorption peaks, a and 0. a is a broad plateau-like peak appearing below ca. 800 "C, while @ is a very sharp one centered around 820 "C. Curve 1 for unsubstituted sample shows only a small @ peak, while curves 2 to 4 for Sr-substituted samples additionally show a peaks in the lower temperature region. The amounts of oxygen desorbed at a and @ peaks are presented in Table V. The amounts of both a and @ oxygen increase with an increase in Sr substitution. The surface coverages of these oxygens which are presented in parentheses far exceed unity except for a case of a-oxygen from Sr-unsubstituted oxide. This indicates that both a and /3 oxygen are not only adsorbed on the surface but they are also absorbed in the bulk. The situation is quite similar when La ions are substituted by other alkali or alkali earth metals. We have confirmed that in every case A site substitution increases the a and @ desorption of oxygen. The effect of A site substitution is considered by substituting Sr for La as an example. The substitution of divalent Sr for trivalent La requires charge compensation, which is achieved either by the formation of tetravalent Co or positive holes as shown in formula 1, or by the formation of oxygen vacancies as shown in formula 2. Formula 3 is a mixture of the above two.
Figure 15. TPD chromatograms of oxygen from LaM03 (M = Cr, Mn, Ni, Fe, and Co); oxygen preadsorption 800 "C-R.T., 100 torr.
La3+1~xSr2+xCo3+03~,~2Vox~~ (2)
La3+1~~Sr2+xCo3+1~x+26C04+x~zs03~6V06 (3) Thus we have to know how the two types of oxygen desorption, a and @, are associated with such defect formation. The @ desorption is observed for the substituted as well as unsubstituted samples and similar in shape for all samples. This suggests that the @ desorption is more specific to B cation, though it is also affected by A site substitution. This suggestion was investigated by using various B cations. Figure 15 shows TPD chromatograms of La-based oxides where B sites are 3d transition metals from Cr to Ni. The compounds with Co, Mn, and Ni show large desorption peaks in the elevated temperature range, while those with Cr and Fe do not show any oxygen desorption in the temperature range examined. The onset temperatures of these large desorption peaks for Mn-, Ni-, and Co-containing samples were found to be related with the thermal decomposition temperatures of respective component oxides. Furthermore, the decomposition temperatures of Cr and Fe oxides are far above the temperature range examined, and no desorption peaks were observed for corresponding perovskite samples. These results indicate that the desorption of @ oxygen is ascribable to partial reduction of B site cations to lower valencies. In order to elucidate the effect of A site substitution, Sr-substituted LaCo03, LaFe03, and LaMn03+6are sub-
L
? 0
z
p:
La,-,Sr,FeO,-,
a
x =0.2 _------x = o --
_-- _ _ _ _ _ _ _---
La1-xSrxMn03t,
,p x = 0.2
- - x-= 0-. ----< 4
26
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985
300
400
500 60C 700 Temperature / "C
800 -
Figure 17. 'TPD chromatograms of oxygen from La&ro,4Co03-6 after hydrogen pulse a t 300 "C.
5
2.1
2,2
2.3
103.~-l K-'
Figure 19. Arrhenius plots of the rate of C,Hlo oxidation over Lal_,Sr,Co03_b;W / F = 1.2 s g of cat./cm3.
I
- 1
40.2
200
600
100 -emerature /
80C
"C
F i g u r e 20. TPD chromatograms of oxygen from La, ?Sr0sFeyCo,,03_6; oxygen preadsorption 800 "C-R.T., 100 torr. Y ir
LO~.~S~~SCI~,_~
Figure 18. Rate of oxygen reduction (a), specific reactivity of aoxygen (b), and amount of a-oxygen (c) as correlated with n.
the TPD chromatograms of oxygen after various amounts of hydrogen were pulsed over the oxygen preabsorbed L%.6Sro.4Co03-6at 300 "C. With increasing amounts of pulsed hydrogen, CY oxygen is successively eliminated from the lower temperature region to higher temperature region, and finally &oxygen is also eliminated. This indicates that both a- and 0-oxygen are reactive to hydrogen, but CY oxygen is more active than 0. Experiments have been done to see how hydrogen uptake increases as hydrogen is repeatedly pulsed on various Sr-substituted samples. The initial slopes of hydrogen uptake were taken as a measure of reactivity and plotted as a function of Sr concentration x in Figure 18. Curve a is the rate of oxygen reduction vs. x thus obtained. It is estimated that a maximum reactivity is attained around the x value of 0.4. Curve c represents the amount of oxygen vs. x , which increases with increasing x . If we assume that total reactivity of each sample, that is curve a, is given by the product of the amount of a-oxygen and the specific reactivity of a-oxygen, the specific reactivity of a-oxygen is estimated to decrease with x as shown by curve b. In other words, total reactivity seems to result from these two factors, one increasing with x the other decreasing with X.
The total reactivity just discussed seems to relate with the catalytic activity under oxidation conditions. The rate of n-butane oxidation over the samples with x = 0,0.2, and 0.4 is shown in Figure 19. The highest catalytic activity is obtained for the sample of x = 0.4 followed by 0.2 and 0. Thus we consider that under catalytic oxidation condition, the amount and reactivity of oxygen determine the catalytic activity. How to control them seems to be important for the catalytic point of view.
-
i
Figure 21. Catalytic activity for Hz02decomposition (80 "C); HzOz 1/202 + H,O.
So far, we have been concerned with oxides which contain a single kind of B site cation. We refer to a possibility of modifying the properties of perovskite oxides with a combination of adequate B cations. For example, Figure 20 shows TPD chromatograms of oxygen from complex oxides where the B site is occupied by Fe and Co ions at various compositions. By the combination of Fe and Co, the amounts of oxygen desorbed are increased, especially in the lower temperature region. In coincidence with this change, the samples with mixed B site cations have been found to have catalytic activities for HzOzdecomposition in the liquid phase which are superior to the compounds with a single kind of B cation, as shown in Figure 21. In the methane oxidation reaction, the catalytic activity of the compound with mixed B site cations is also higher than those of the compounds with mono B cation (Figure 22). It has been shown by many workers that perovskite-type oxide which has only Co at B site is one of the most active oxidation catalysts, while one which has only
27
Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 27-32
Registry No. 02,7782-44-7;methacrolein, 78-85-3;methyl ethyl ketone, 78-93-3. Literature Cited -6
1,2
1.4 1.6 103.~-lK - ~
1,8
Figure 22. Arrhenius plots of the rate of methane oxidation over
L~.*Sro.zFeyCol-y03-6. Fe is less active. Accordingly, the results obtained here suggest a possibility of finding more excellent catalysts by selecting a proper combination of B site cations. Finally, we would briefly refer to the surface structure of perovskite-type oxides. According to our XPS measurements, the surface of Sr-substituted LaCoO3 is subject to compositional changes during high-temperature calcination. These changes can lead to a deactivation of catalytic properties of the compounds.
Eguchi, K.; Aso, 1.; Yamazoe, N.; Seiyama, T. Chem. Lett. 1878, 1345. Eguchi, K.; Toyozawa, Y.; Yamazoe, N.; Seiyama, T. J . Catal. 1883, 83, 32. Hall, W. K.; Lalacono, M. "Proceedings, 6th International Congress on Catalysis", London, 1977; p 246. Iwamoto, M.; Yoda, Y.; Egashira, M.; Seiyama, T. J . phys. Chem. 1876, 80, 1989. Iwamoto, M.; Yoda, Y.; Yamazoe, N.; Seiyama, T. Bull. Chem. SOC.Jpn. 1878, 5 1 , 2765. Katsuki, S.;Inokuchi, M. J . Phys. SOC.Jpn. 1882, 5 1 , 3652. Lunsford, J. H. Catal. Rev. 1873. 8 , 135. Lyhamn, L.; Cyvin, S. J.; Cyvin, B. N.; Brunvoil, J. Z . Naturforsch. 1876, A31, 1589. Meadowcroft, D. B. Nature (London) 1870, 226, 847. Ohara, T. Shokubai(Catalyst) 1877, 19, 157. Seiyama, T.; Nita, K.; Maehara, T.; Yamazoe, N.; Takita, Y. J . Catal. 1877, 49, 164. Takita, Y.; Nita, K.; Maehara, T.; Yamazoe, N.; Seiyama, T. J . Catal. 1877, 50, 364. Takita, Y.; Nita, K.; Yamazoe, N.; Seiyama, T. Engineering Sciences Report, Kyushu University 1979; Vol. I , p 1. Tsigdinos, 0. A. Top. Curr. Chem. 1878, 76, 1. Voorhoeve, R. J. H.; Remeika, J. P.; Freeland, P. E.; Matthias, B. T. Science 1872, 177, 353. Voorhoeve, R. J. H. "Advanced Materials in Catalysis", Academic Press: New York, 1977; p 129. Yamazoe, N.; Hidaka, S.;Arai, H.; Seiyama, T. Oxld. Commun. 1883, 4 , 287. Yamazoe, N.; Noguchl, M.; Seiyama, T. Nippon Kagaku Kalshi 1883, 1983, 470.
Receioed for review April 26, 1984 Accepted September 4, 1984
Nature of Active Sites and Coking Reactions in a Pillared Clay Mineral Marlo L. Occelll' and Joseph E. Lester Gulf Research d Development Company, Pittsburgh, Pennsylvania 15230
FTIR studies of chemisorbed pyridine confirm that (Na or Ca) bentonites pillared with alumina clusters contain both Lewis and Brernsted acid sites. At 400 OC, acidity is mainly of the Lewis type and acid site strength is comparable to that observed in HY. Formation of catalytic coke from 1-hexene and toluene on these materials is significantly slower than that measured for zeolites. However, coke make from gas oil conversion at microactivity test (MAT) conditions is at least a factor of 2 greater than on HY zeolite. The pillared clay high coke make during gas oil cracking is attributed to its open, two-dimensional microporous structure and high Lewis acidity, which promotes polycondensation reactions and coke formation.
Introduction
Montmorillonitespillared with N+(CH3)*and N+(CzH6)4 cations were first reported by Barrer and MacLeod (1955). While the molecular sieving properties of these materials were known (Barrer, 1978), their catalytic potential was ignored, probably because of their limited thermal stability. In the late 1970's, synthesis of heat-stable (500 to 600 "C), high-surface-area bentonites pillared with large inorganic proppants was reported by several workers (Brindley and Sempels, 1977; Lahav et al., 1978; Yamanaka and Brindley, 1979; Vaughan et al., 1979; Shabtai et al., 1980). Literature recognition of the catalytic properties of clays pillared kith alumina began appearing in 1979-1980 (Vaughan et al., 1979; Vaughan and Lussier, 1980). Their high cracking activity is believed to be associated with the'nature of the 0196-432118511224-0027$01.50/0
acidity introduced by the aluminum oligomers used to expand the clay structure. Similar catalysts were synthesized by Shabtai (1980) and used in studying the rate of cumene and isopropyl naphthalene dealkylation. Of particular interest to the petroleum industry is their selectivity to light cycle gas oil (LCGO) production when cracking gas oil fractions (Lussier et al., 1980; Occelli, 1983). In addition to cracking, pillared clays are active catalysts for methanol conversion, for alkylating toluene with ethylene (Occelli et al., 1984), and may represent a new class of versatile heterogeneous catalysts. Thermal stability of pillared clays in 95% steam-5% nitrogen mixtures is limited to temperatures below 600 "C. They also deactivate readily because of coke formation in most hydrocarbon conversion reactions. The high coke 0 1985 American Chemical Society
