Intercalation compounds as precursors for oriented catalysts

Synthetic Materials, 4 (1982) 319 - 330

319

INTERCALATION COMPOUNDS AS PRECURSORS FOR ORIENTED CATALYSTS

JEAN-CLAUDE VOLTA Institut de Recherches sur la Catalyse, C.N.R.S., 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex (France)

(Received January 14, 1981)

Summary The oxyhydrolysis of MoC15 Ceylon graphite intercalation compounds, in the 400 - 500 °C temperature range, gives MoO3-graphite oriented catalysts. (010) MoO3 crystalline faces are parallel to the (001) graphite sheets. Catalytic properties in propylene oxidation are governed by the distribution of the MoO3 faces and controlled by the temperature of preparation. From the example of the MoOa-graphite system, it appears that intercalation compounds can be precursors for oriented catalysts with a strong interaction of the graphite support with the active phase, which improves the catalytic properties.

1. Introduction During the last twenty years, many studies have been devoted to the use of intercalation compounds in catalysis. The most important survey on this subject was published in 1974 by Boersma [1]. It is concerned with the catalytic properties of alkali metal-graphite intercalation compounds. In this review, Boersma discusses the specific reactivity of these compounds for the ortho-para hydrogen conversion, the hydrogenation of unsatured hydrocarbons, the Fischer-Tropsch synthesis, the double bond olefin isomerisation, the dimerization of propylene and isobutene, and the synthesis of ammonia. The author refers also to the work of H~rold [2] concerning the stability of these compounds in the presence of gaseous reactants. X-ray data show a progressive destruction of the C24 K intercalation compound by reaction with 02, NzO, NO, NOz, SO2, CO and CO2 at temperatures from 100 °C up to 500 °C. Most catalytic reactions involving these gases occur in this temperature range. Referring to these compounds as catalysts is questionable, however; it would be better to regard intercalation compounds as specific reagents for these reactions. Indeed, the total chemical nature of the intercalation c o m p o u n d is not preserved during the catalytic process. 0379-6779/82/0000-0000/$02.75

© Elsevier Sequoia/Printed in The Netherlands

320

Many results have been published over the years concerning the use of other intercalation compounds as catalysts. Some work has been patented, but few authors have paid attention to the exact nature of the intercalation compound after catalysis. Hooley studied the interactions of H2 and CO with FeC13-graphite intercalation compounds [3, 4]. He found that activated CsFeC13 (CI) catalyzes the production of hydrocarbons, but at a slower rate than an activated mixture of graphite and hydrated ferric oxychloride (CM) [4]. The interesting point was that the selectivity of the reaction was very different for CI and CM. This difference was not due to intercalated iron, but it was postulated that there were different distributions of crystal faces and edges of Fe on the two catalysts, which would produce different selectivities. We were interested in studying this particular point in the field of catalysis on oxides, in order to obtain catalysts with better selectivity for the oxidation of propylene to acrolein. Indeed, some patents have claimed good results for this reaction on MoO3-graphite catalysts prepared via intercalation compounds [5, 6]. The structure of molybdenum pentachloride-graphite has been well established by Syme-Johnson in the case of a 4th stage compound [7]. In this publication we show that an MoCl~-graphite intercalation compound can be a precursor for an oriented MoO3-graphite catalyst, which is the effective catalyst in the propylene oxidation reaction to acrolein. The increase of selectivity compared with conventional catalysts is explained by a specific distribution of the MoO3 crystal faces. Some aspects of this study have already been published [8, 9].

2. Experimental G-MoCI5 intercalation compounds were prepared with different graphites in a vacuum system by the two temperature tube method [10] in the 280 - 330 °C temperature range. Parameters controlling intercalation structure have been studied, and our results agree with former publications [ 11, 12]. A 3rd stage intercalation compound (44.3% intercalated MoCI~ ) was obtained with Ceylon graphite under the conditions: T6 = 280 °C, A T = 10 °C, t = 14 days (see X-ray diagram (Fig. 1)). rrr(oosi

ITI(o04)

2 Iv(0

lctoo:/ :co~ 3,0

2,s

m~o.i 2,0

1,s

rrrloo21 lp

rrr~o,j 5

~,,,

~"

Fig. 1. X-ray diagram of a 3rd stage MoCl5-Ceylon graphite intercalation compound.

321 Molybdenum trioxide-graphite catalysts were prepared by oxyhydrolysis (OXH) of an intercalated (34.9% MoC15) (INT.) and impregnated (34.9% MoC15) (IMP.) MoCl~-Ceylon graphite compound. Preparation runs were performed using a Sartorius microbalance. Gas composition and flow rate were controlled during the heating procedure which consisted of a linear increase of temperature (A T = 0.1 - 16 °C/min) from 25 to 500 °C, followed by an isothermal treatment (t = 6 - 61 h) at the final oxyhydrolysis temperature, ToxH. Resulting solids were examined by IR spectroscopy, X-ray diffraction, electron transmission and microdiffraction microscopy and scanning electron microscopy. The IR spectra of the samples diluted in KBr (2%) were recorded on a Perkin-Elmer 580 spectrophotometer in the 1300 - 250 cm-z range with ordinate expansion and compared with that of pure MOO3. Examination by X-ray diffraction of the same solids was carried out on a Siemens goniometer equipped with a quartz front monochromator with Cu Ks radiation. Electron microscopy studies of the MoO3-graphite catalysts were undertaken using a JEOL 100C electron microscope. For scanning electron microscopy examinations, samples were coated with a thin gold layer by a plasma discharge technique and examined at the accelerating potentials of 40 kV. The high quality image was given by secondary electrons. Solids were simultaneously examined by transmission electron microscopy and microdiffraction on a JEM 100 CX electron microscope. Catalytic propylene oxidation on MoO3-supported catalysts was carried out in a differential microreactor at a low conversion (< 1%) and with different contact times in order to compare the initial selectivities of the catalysts. Experimental conditions were: Po2 = PC3H+ = 100 Tort, P s 2 = 560 Tort, T = 375 °C. Acetaldehyde, propanal, acrolein, acetone and CO2 were analyzed chromatographically. Molybdenum trioxide-silica catalysts were prepared by the same procedure and compared with the MoO3-graphite catalysts in the same reaction in order to study the influence of support orientation on the catalytic activity.

3. Results and discussion

3.1. Study of the oxyhydrolysis of the MoCls-graphite intercalation compound The transformation of the MoC15-graphite intercalation compound during oxyhydrolysis has been studied by IR spectroscopy, radiocrystallographic analysis and continuous analysis of gases evolved from the thermobalance during the oxyhydrolysis treatment. The spectroscopic study of samples prepared at different TOXH is presented in Fig. 2. With increasing temperature, a decrease of the band at 351 cm -1, attributed to the Mo-C1 vibration [13], is observed. A band at

322 f requency(cm -1) 1000

800

600

400

OXH 0.57.I 2 5 0 - ~

Fig. 2. Study of the G - M o O 3 (INT.) catalysts by IR spectroscopy. TABLE 1 IR spectra Frequencies, p (era - 1 )

Samples MoO 3 OXH 280 OXH 343 OXH 400 OXH 470 OXH 485

Optical density D v related to 10 mg MoO a 989

873

605

3.71 0.03 0.16 0.26 0.33 0.35

4.93 0.05 0.24 0.39 0.50 0.53

5.14 0.25 0.41 0.59 0.63

MoCI 5

Mo02CI 2

351

980

0.80 0.53 0.42 0.15 0.05

0.02 0.09 0.05 ---

MoO 3 (%)

100 0.9 4.5 7.6 10.0 10.6

980 cm -1, due to the Mo-O vibration in MoO2C12 [14], increases with increasing temperature up to 343 °C and then decreases (Table 1 ). Beyond 343 °C, the progressive appearance of the four major bands due to vibrations of MoO3 (989,873,605 and 375 cm -1) conforms with the final transformation of the intermediary oxychloride to molybdic oxide. This is seen by the continuous analysis of Cl~ and HC1 for a linear increase of the oxyhydrolysis temperature. Two maxima for HC1 evolution are observed at 140 °C and 380 °C and a maximum for C12 at 210 °C (Fig. 3). The variation of the optical density of the characteristic bands of MoC15, MOO2C12 and MoO3

323

k evolved gases 15( - (arbit. units)

100

cI 2 t

5O

i

i

0

100

200

300

400

TIOc)

Fig. 3. Continuous analysis of HCI and CI 2 evolved during oxyhydrolysis of the G-MoCI 5 LCG {temperature increase: 3 °C/min).

MoCI5 MoO2CI2

I(~/°~M003

i

•

~,

Mo02CI2

MOLl 5

~y--'-

~

/

1.0

0.1

~

/'

2,, 5

//

/

',, " t

~M

',

o0 3

"• ,

v ~\

0

200

300

4()0

'e h .

500

T{OC}

Fig. 4. IR results: % MoO~ calculated from the optical density, Du, of the MoO 3 bands; of the 351 cm -~ band (Mo-C1 vibration in MoC15); D~/IO mg of the 980 cm - 1 band ( M o -O vibration in MOO2C12).

Dv/lO mg

shows (Fig. 4) the gradual transformation of the intercalated MoC15-graphite c o m p o u n d to the MoO3 supported graphite catalyst via the transient formation of M002C12. The study of the MoC15-MoO2 C12-M003 system conducted by Eliseev [15, 16] shows that the oxyhydrolysis of MoC15 to MoO3 by increasing temperature can be summarized according to the scheme: MoC15 + H20

-+ MoOC13 + 2HC1

(1)

MoOC13 + ~-O2

-~ M002C12 + ~C12

(2)

M002C12 + H20

-+ MoO3 + 2HCI.

(3)

1

324 C_~oo2)

M°O3 X rays

1 (o4o) JR 1

,0. _:lOlL

,0=1/1i .

OXH

A

315- ......

310- ~-

2}~ .... 2)........... :.).... L

315

3'0

215

470 400 343

4'0

2'0

115

~|'l

280 200 MoCI 5 LCG

3=0 215 2~0 Fig. 5. X-ray diagramsof the G-MoO3 (INT.) catalysts. Such a scheme is in agreement with the evolution of HC1 and C12 shown in Fig. 3. Measurement of the chloride content in these gases and the variation of the weight of the sample in the thermobalance confirm these successive reactions. From Fig. 3, it appears that the complete transformation of molybdenum 0xychloride is not attained before 400 °C. The radiocrystallographic study of the samples confirms these results. The structure of the intercalation compound is progressively destroyed and characteristic MoO8 X-ray lines appear with restoration of the graphitic structure (Fig. 5). Moreover, the shape of the nucleated MoOa crystallites is strongly anisotropic. The higher the oxyhydrolysis temperature, the more pronounced the anisotropy in the [0k0] direction. It is observed that the relative intensities of the (0k0) and (021) lines are reversed compared with those of the ASTM standard. It can be concluded that the (0k0) MoOa planes are parallel to the (001) graphite planes. Consequently, growth of MoOa crystallites is preferentially observed perpendicularly to graphitic planes.

3.2. Catalytic study of the oriented MoOa-graphite catalysts --Influence of preparation conditions on catalytic results No results have yet been published on the influence of the orientation of crystal oxides on catalytic properties. When we arrived at the important

325

S(~o) 6(3

5C 4C 3£ 2C ,....... ~ ....... v ............. m ....... ~ 3

10

~oo

,~o

d)o

*" TOXH (°c)

Fig. 6. Catalytic results of the mild oxidation of p r o p y l e n e o n MoO 3 catalysts: S(%) : initial acrolein selectivity; TOXH: oxyhydrolysis temperature; 1: MoO 3 (INT.)-graphite catalysts; 2: MoO 3 (IMP.)-graphite catalysts; 3: MoO 3 (IMP.)-silica catalysts.

conclusion that molybdenum oxide prepared via MoC15-intercalated graphite (LCG) was oriented on the graphite sheets, we changed the preparation conditions in order to modify the catalytic results in the mild oxidation of propylene by redistributing the MoO3 crystal faces. Indeed, MoOa is known as a selective catalyst for propylene oxidation to acrolein. Several MoOa (INT. }-graphite catalysts were prepared in the 400 - 500 °C temperature range where transformation of the intercalation compound is achieved. The specific influence of the orientation of MoOa on graphite on the catalytic properties is illustrated in Fig. 6 for the MoOagraphite catalysts compared with the MoOa-silica catalysts. It appears that the MoO3 (INT. )-graphite catalysts present a strong increase in selectivity at 470 °C compared with the MoOa (IMP.)-graphite catalysts, a result which is not observed on the MoO3-silica catalysts. The variation in selectivity for acrolein cannot be explained by different support acidities. The physicochemical study of the catalysts suggests a relation between the selectivity for acrolein and the development of specific faces of MoO3 on graphite. The role of graphite is essential in this scheme as can be seen in Fig. 6: a silica support without an ordered structure cannot induce preferential orientation. Electron diffraction microscopy and scanning electron microscopy confirmed that the MoO3 crystal growth is epitaxial on gral~hite. The orientation of this support determines the process of nucleation and growth of the M o O 3 grains owing to a strong isomorphism between the two structures. The shape of the MoO3 crystals is dependent on the final treatment temperature, as seen in Fig. 7, and on the heating time at this temperature, as observed in Fig. 8. The electronic diffraction study carried out on MoOa crystals at the edge of the graphite sheets confirmed, in all cases, the orientation of the (010) MoO3 planes on the (001) cleavage of graphite. This result was mentioned previously by Oberlin [17]. Additionally, two preferential orientations were observed: [001] MoOa tl [100] graphite and

326

4

420.6h

10~

(a)

471.6h

10~

(b)

496.6h (c)

10#

Fig. 7. Scanning electron microscopy results: influence of the oxyhydrolysis temperature.

[100] MoO3 II [100] graphite as seen in Fig. 9. This is consistent with the SEM examination, Fig. 9. It is possible to identify the edge planes of the MoO3 crystals on the scanning electron microscope photographs. From a histogram giving the crystal dimensions in different planes and directions which was drawn for each sample examined, the mean size was calculated and is given in Table 2. The MoO3 crystal size is maximum in all dimensions at 471 °C. Beyond this temperature, a volatilization of MoO3 occurs, which is facilitated by water vapor from the preparation process, as first reported b y Millner [ 1 8 ] . The increase in the heating time of the final isothermal treatment involves an increase of

327

420.6h

, lOu 4

(a)

496.6h

(c)

420.61h

I

lOu

(b)

~ lOp. I

496.61h

I 10p't

(d)

Fig. 8. Scanning electron microscopy results: influence of the heating time.

the MoOa crystal size at 420 °C and a decrease at 496 °C. Assuming all faces are accessible to the reaction gases, a comparison, Table 3, of the percentage area of the terminal faces with selectivity for acrolein and CO2 formation, shows that, whatever the preparation conditions, there is a good correlation between the selectivity for acrolein and the proportion of the (100) MoO3 face on the one hand, and between the selectivity for CO2 and the proportion of the (010) MoO3 face on the other. This study confirms the specificity of the MoO3 crystalline faces for mild oxidation o f propylene, as has already been proposed b y radiocrystallographic study [ 8].

328

TI,00 • 101

l

01.00

_O 100

_O 101

0.0 0

•

•

00F

•

000

o_

_O 101

1 o.o

To.o

0

0

001

•

_o

101

101

o

o

o

0 1.0

1 1.0

0 1.0

[001 ] MoOJ/[ 100] GRAPH ITE •

/

01.00

• 001

• 101

•

•

10.0 _O

1 O0

•

100

101

~1.00

000

100

•

o_

001

101 o

1 1.0

[100] MOO3//[100] GRAPHITE

MoO3 DIFFRACTION SPOTS

O GRAPHITE DIFFRACTION SPOTS

Fig. 9. Comparison of the diffraction and scanning electron microscopy studies.

4. Conclusions Oriented MoO3-graphite catalysts can be prepared by oxyhydrolysis of MoCls-graphite intercalation compounds. By changing the preparation parameters, it is possible to obtain different distributions of MoO3 crystal faces and thereby to change the catalytic properties in the mild oxidation of propylene. It has been possible to correlate the selectivity for acrolein and CO2 formation to the development of the (100) and (010) MoO3 faces, respectively. These results seem general, since oxyhydrolysis of SbC15-graphite intercalation compounds gives oriented-Sb204-graphite catalysts [19]. It appears that to obtain oriented oxides is conditional on a structural isomorphism with graphite.

O

329 TABLE 2 Molybdenum trioxide crystal mean length (from the SEM study) Samples Preparation conditions TOXH (°C)

Heating progr, (°C/min)

Heating time (h)

420 420 471 496 496

0.5 0.5 0.5 0.5 0.5

6 61 6 6 61

,

Yb

MoO 3 crystals mean length in corresponding planes and directions (pro)

(100)

(010)

(101)

(£01)

[001]

[100]

[101]

[101]

18 40 20 9 5

6 13 12 6 3

16 40 28 12 0

~ ~ O l

[010]

(001) [100]

1 4 6 2.5 1.5

0 0 0 0 3

[,0,]

(oo,),[,oo]4..J ;LV'Vj V

TABLE 3 Comparison of SEM and catalytic results Samples

420, 420, 471, 496, 496,

6h 61 h 6h 6h 61 h

Area of terminal crystals faces (%)

Selectivity (%)

Sacro Sacro SCO2

(100)

(010)

(£01) (101)

(001)

Acrolein C O 2

(%) (%) (%) (100) (101) (010) (101)

10 16 18 18 28

85 76 61 64 55

5 8 21 18 0

0 0 0 0 17

24.4 32.8 42.0 41.6 52.0

2.4 2.1 2.3 2.3 1.9

62.9 55.7" 44.0 44.2 36.0

4.9 4.1 2.0 2.3 --

0.7 0.7 0.7 0.7 0.7

I n t e r c a l a t i o n c o m p o u n d s t h u s o f f e r an o p p o r t u n i t y t o s t u d y t h e react i o n specificity in catalysis o f t h e o x i d e crystalline faces. We c o n s i d e r t h a t s o m e results p u b l i s h e d in this field m a y be e x p l a i n e d b y t h e fact t h a t t h e effective catalysts are o r i e n t e d m e t a l o r o x i d e crystals w i t h specific distribut i o n s o f crystalline faces, as p r e v i o u s l y r e p o r t e d b y H o o l e y [ 4 ] .

330

References 1 M. A. M. Boersma, Catal. Rev. Sci. Eng., 10 (2) (1974) 243 - 280. 2 D. Saehr and A. H~rold, Bull. Soc. Chim. Ft., (1965) 3130. M. Colin and A. H~rold, C. R. Acad. Sci., Set. C, 269 (1965) 1302. N. Daumas and A. H~rold, Bull. Soc. Chim. Fr., (1971) 1598. 3 S. Parkash, S. K. Chakrabartty and J. G. Hooley, Carbon, 15 (1977) 307 - 310. 4 S. Parkash, S. K. Charkrabartty and J. G. Hooley, Carbon, 16 (1978) 231 - 234. 5 Sagami Chem0 Res. Center, Jap. Pat. 061867 (1974). 6 Sagami Chem. Res. Center, Jap. Pat. 130281 (1974). 7 A. W. Syme Johnson, Acta Cryst., 23 (1967) 770 - 779. 8 J. C. Volta, B° Moraweck, W. Desquesnes and G. Coudurier, React. Kinet. Catal. Lett. 12 (3) (1979) 241 - 246. 9 J. C. Volta and B. Moraweck, J. Chem. Soc., Chem. Commun., (1980) 338 - 339. 10 A. H~rold, Bull. Soc. Chim. Ft., 5 (1955) 999. 11 J. G. Hooley, Carbon, 10 (1972) 155 - 163. 12 J. G. Hooley, Carbon, 11 (1973) 225 - 236. 13 J. R. Ferraro, Low Frequency Vibrations in Inorganic and Coordination Compounds, Plenum Press, New York, 1971, p. 111. 14 F. A. Schroeder, B. Krebs and R. Mattes, Z. Naturforsch. B, 27 (1972) 22. 15 I. A. Glukhov, S. S. Eliseev and M. S. Pulatov, Russ. J. Inorg. Chem., 15 (5) (1970) 730 - 731. 16 S. S. Eliseev and V. A. Zhilenko, Dokl. Akad. Nauk Tadzh., S.S.R., 11 (1) (1968) 41 - 45. 17 M. Oberlin, A° Siat and R. Hocart, C. R. Acad. Sci., 234 (1952) 2377. 18 T. Millner and J. Neugebauer, Nature (London), 163 (1949) 601. 19 J. C. Volta, to be published.