Journal of Molecular Catalysis, 18 (1983) 197 - 202 197

WELL-DISPERSED CATALYSTS OF SUPPORTED CHROMIUM, MOLYBDENUM AND TUNGSTEN CARBONYLS WHICH ARE HIGHLY ACTIVE FOR ALKENE HYDROGENATION

T. J. THOMAS and ALAN BRENNER*

Department of Chemistry, Wayne State University, Detroit, MI 48202 (U.S.A.)

(Received March 28, 1982)

Summary

Traditional catalysts (prepared by the reduction of a salt) of supported Cr, MO, and W compounds have low activities for the hydrogenation of alkenes. This low activity is partly due to the difficulty in fully reducing these catalysts. The use of the zero-valent complexes Cr(C0)6, Mo(CO),, and W(CO), for catalyst synthesis is described. Proper activation can yield well-dispersed and low-valent catalysts primarily containing subcarbonyl complexes which are far more active than traditional catalysts of these elements for the hydrogenation of alkenes.

Introduction

In contrast to Group 8B metals, heterogeneous catalysts derived from salts of Cr, MO, and W have low activities for the hydrogenation of olefins [ 1 - 41. It is likely that much of this difference in activity is due to differ- ences in redox properties. Except at low loadings, supported salts of the Group 8B metals are readily reduced to the zero-valent state (to yield a traditional catalyst). It is well known that both supported metals [5] and metal films [l, 41 of the Group 8B elements are extremely active for the hydrogenation of olefins. Further, metal films of Cr, MO, and W are also very active for the hydrogenation of ethylene [ 1, 41. However, when supported on alumina the trioxides of Cr, MO, and W are reduced to only approximately Cr(II1) [6], Mo(IV) [7], and W(V) [8], respectively, by treatment with H, at 600 “C (even though the bulb oxides of MO and W can be fully reduced). In general, oxides are much poorer catalysts for hydrogenation [3]. However, it has been noted that zero-valent catalyst precursors, such as carbonyl complexes, can be used to prepare well- dispersed and low-valent catalysts of difficult-to-reduce metals [9 - 141 . After activation at 200 “C in flowing He, Mo(CO)JAl,O, was lOOO-fold

*Author to whom correspondence should be addressed.

0304-5102/83/0000-0000/$03.00 0 Elsevier Sequoia/Printed in The Netherlands

198

more active than reduced MoO,/Al,O, for the hydrogenation of propylene [2]. This report extends these results to catalysts derived from Cr(CO), and W(CO)6 and provides additional data on dispersions and the hydro- genation of ethylene for a variety of catalyst activations [ 151.

Experimental

The catalysts were prepared by subliming a carbonyl onto 0.500 g of a high purity y-AlTO, (Conoco Catapal SB, previously calcined in flowing 0, at 500 ‘C, s= 203 m’/g), followed by temperature programmed decom- position (TPDE) in flowing He to the first temperature used for reaction and holding at that temperature for 30 min. Reduction was for 30 min at 600 “C in H2 flowing at 100 cm3/min. Some catalysts were subjected to a redox treatment consisting of oxidation for 15 min at 500 “C followed by reduction at 600 “C for 30 min. Chemisorption of CO was measured at 23 “C! using a highly accurate electronic pressure transducer (accuracy is 0.001 cm3 gas adsorbed). The amount of chemisorption was taken as the difference between two isotherms at approximately 150 torr (a catalyst was evacuated for 5 min after the first isotherm).

Activities were measured using both the dynamic activity spectrum technique [ 21 as well as standard isothermal measurements at 0 “C. The activity spectrum technique is similar to TPDE except the flow of He was replaced with a flow of H, + C,H, and the conversion was measured about every 2 min (the heating rate was 5 K/min). All activity measurements were made at 1 atm, H,/alkene = 4, and a total flow of 30 cm3/min. Activities are expressed in terms of a formal turnover frequency, IV,, which is the number of molecules reacting per unit time per metal atom in the catalyst (regardless of the state of dispersion). Details of the ultra-pure catalyst synthesis, TPDE, chemisorption and activity spectrum techniques, and the automated reaction system have been previously published [ 2, 121.

Results and Discussion

Figures 1 and 2 show the activity spectra of Cr(CO), and W(CO), for the hydrogenation of propylene. After heating to 400 “C both catalysts showed a substantially reduced activity (21 and B-fold, respectively) when cooled to the temperature of the peak in activity, and both catalysts were inactive when cooled to 0 “C. The W(CO), catalyst also showed some meta- thesis activity during heating, but was completely inactive for this reaction when cooled to any temperature below 400 “C!.

Tables 1 and 2 show the activities for the hydrogenation of ethylene of two sets of catalysts activated in various ways. The last column in Table 2 uses data taken from both tables. Table 3 gives the dispersions of a set of fresh catalysts.

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60_

0 100 200 300 400 500

TEMPERATURE OF REACTION AND CATALYST ACTIVATION ('C)

Fig. 1. The activity spectrum of Cr(C0)s/A1203 for the hydrogenation of propylene. The catalyst loading was 0.047% Cr.

60

0 100 200 300 400 500

TEMPEFtATURE OF REACTION AND CATALYST ACTIVATION ('C)

Fig. 2. The activity spectrum of W(C0)6/A120~ for the reaction of propylene. (a) hydro- genation; (0) metathesis. The catalyst loading was 0.32% W.

TABLE 1

Activity* of carbonyl-derived catalysts for the hydrogenation of ethylene

Complex Loading

(%M)

Cr(C0)6 0.09 MO(C0)6 0.17 w(c0)6 0.88

*Activity is expressed as Nf at 0 “C.

Activation

200 OC, He

4 x 10-Z 5 x 10-Z 1.6 x lo-*

600 ‘C, Hz (Hz/He)

6 X 1O-3 0.15 3 x 10-2 0.6 7 x 10-4 0.04

200

TABLE 2

Activity* of carbonyl derived catalysts for the hydrogenation of ethylene

Complex Loading

(%M)

Activation

600 ‘C, He Redox

200 ‘C, He

Redox

Cr(C0)6 0.10 7 x 10-Z 3 x 10-3 13

WC0)6 0.34 2 x 10-J 3 x 10-3 17

w(c0)6 1.34 9 x 10-S 2 x 10-4 80

*Activity is expressed as Nf at 0 “C.

None of the carbonyls showed significant activity until activated above 100 “C. This presumably corresponds to the need to evolve CO to develop coordinative unsaturation (0 region of activation [9] ). It would normally be expected that continued decarbonylation would increase the activity. However, TPDE indicates that above roughly 200 “C for Cr(CO), and 300 “C for Mo(CO), and W(CO), the metals become oxidized by reaction with hydroxyl groups on the alumina [ 161 . Hence, completely decarbonylated catalysts (y region [9] ) are also highly oxidized and are expected to be of low activity. Since these oxides are extremely difficult to reduce when supported, the high activity of the low-valent subcarbonyl form is not regenerated by exposure to H2 at 600 “C.

The effect of oxidation by a-OH is further illustrated by the data in Table 1. Activation at 600 “C in H, leads to catalysts of lower activity than the subcarbonyl form. The opposite response is expected for reducible metals [ 91, and hence it is clear that the H, has not been particularly effec- tive in preventing the oxidation of the catalysts (catalysts of these metals which are both completely decarbonylated and zero-valent are much more active [ 13, 171). However, comparison to direct activation in He at 600 “C (Table 2) indicates that H, can be somewhat beneficial compared to activa- tion in He. The redox form of the catalysts is essentially a reduced oxide, and hence acts as a reference for the activity of a conventionally prepared catalyst. As seen in Table 2, the subcarbonyl catalysts are substantially more active than conventional catalysts.

Data in Table 3 indicate that the subcarbonyl catalysts also have high dispersions compared t,o a reduced oxide. (More detailed studies indicate that a chemisorption stoichiometry of 1 CO per metal atom is probably correct [18] .) Hence, it appears that the enhancement in activity is largely due to the ability to prepare the catalysts in well-dispersed and low-valent states using an organometallic precursor.

It is also of interest to compare these activities with data for other activated carbonyls. Neither photolytic activation at ambient temperature of the hexacarbonyls in solution [ 19, 201 nor thermal activation of solutions near 140 “C [21] yield any activity (Nf < 10e5 and < low8 s-l, respectively) for the hydrogenation of monoolefins. This difference presumably reflects

201

TABLE 3

Percent dispersion*

Complex Loading

(%M)

Activation

200 OC, He 600 “C, He Redox

CrtC0)6 1.3 2.9 0.2 0.0 MO(C0)6 1.1 21 0.2 1.4 w(c0)6 2.2 16 0.0 0.4

*Percent dispersion = 100 (CO/M). Data have been corrected for a background adsorption of 0.02 2 0.01 cm3 CO for the 600 “C and redox catalysts (the background correction is zero for the 200 “C catalysts). Uncertainty in the background correction causes an uncertainty of t 0.5% in the dispersions.

the ability of a support to immobilize the highly reactive coordinatively unsaturated sites formed during activation. A number of carbonyl complexes of the noble metals have also been immobilized by attachment to various supports [ 221. These catalysts have activities of roughly lob3 s-l (extrapo- lated to 0 “C). Thus, the subcarbonyl catalysts of the base metals are seen to be substantially more active than the much more expensive catalysts of immobilized noble metals, even though as metal films the noble metals are roughly loo-fold more active than the Group 6B metals for the hydro- genation of ethylene [ 1, 41. The lower activity of the immobilized catalysts probably reflects the well-defined ligand environment which was maintained which effectively poisons much of the potential activity.

Acknowledgement

Support of this research by DOE is gratefully acknowledged.

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