IR and UV-Vis spectroscopie studies of the surface MoCH2 and MoCH-CH3 carbene complexes produced by methylcyclopropane chemisorption over photoreduced silica-molybdena catalysts

Journal

of Molecular

Catalysis,

65 (1991)

393-402

393

IR and UV-Vis spectroscopic studies of the surface Mo=CH, and Mo=CH-CH, carbene complexes produced by methylcyclopropane chemisorption over photoreduced silica-molybdena catalysts K. A. Vikulov,

B. N. Shelimov*

A’. D. Zelinsky Institute of Organic Moscow 117913 (U.S.S.R.)

and V. B. Kazansky

Chemistry,

USSR Academy

of Sciences,

(Received April 12, 1990; revised November 20, 1990)

Abstract As a continuation of our previous study, the interaction of methylcyclopropane (MCP) with surface coordinatively unsaturated Mo4+ ions produced by photoreduction of Moe+/ SiOe catalyst in carbon monoxide was investigated. It was found that MCP chemisorption at room temperature results in the formation of surface carbene species Mo=CH, and Mo=CH-CH,, concentration of the former being higher than that of the latter. Both species are characterized by a charge transfer absorption band at 450-460 nm in the W-Vis spectrum. In the IR spectrum Mo=CH, species exhibit bands at 3080 and 2945 cm-‘, which were earlier attributed to C-H asymmetric and symmetric stretching modes, while for Mo=CH-CHe complexes four bands at 2985, 2910, 2890 and 2850 cm-r were found in the C-H stretching region. Both carbenes are found to be stable in vacua up to 570-620 K and oxygen resistant at 293 K. IR data obtained show fast transformation of Mo=CH-CHe to MO= CHe ln excess gaseous ethylene accompanied by propene evolution into the gas phase; conversely, a fraction of Mo=CH, complexes could be converted to Mo=CH-CH, after propene admission. These findings provide the first direct spectroscopic evidence for the metal-carbene mechanism of heterogeneous propene metathesis over silica-molybdena catalysts. The specific activities of MCP-treated photoreduced Mo/SiOz for propene metathesis were measured and compared to those for cyclopropane-treated catalysts.

Introduction

Earlier it was found that cyclopropane (CP) chemisorption over silica-molybdena catalysts (Mo6+/Si02) photoreduced in CO at room temperature resulted in an enormous increase (by more than one order of magnitude) in their specific activities in the gas phase propene metathesis [ 1, 21 and in the liquid phase 1-hexene metathesis [ 31. Using IR spectroscopy, we have shown [ 1, 41 that this effect could be attributed to the formation of thermally stable carbene complexes MO= CH2, the latter being the product of CP interaction with coordinatively unsaturated Mo4+ ions, produced on the *Author to whom correspondence should be addressed.

0304-5102/91/$3.50

0 Elsevier Sequoia/Printed in The Netherlands

394

catalyst surface by photoreduction in CO of supported Mo6+ ions [5]. Two absorption bands (AR) at 3080 and 2945 cm-’ observed in the IR spectra corresponded to the asymmetric and symmetric stretching vibrations of C-H bonds in Mo=CH2. AR at 450 nm in the UV-Vis spectrum was assigned to a charge transfer in the same complex [ 1, 61. We could also detect in the IR and UV-Vis spectra ABs of a molybdena-cyclobutane intermediate (MOT), which was formed after C2H, chemisorption on carbene complexes Mo=CH,. At present it is generally accepted that carbene and Mo-cyclobutane complexes are the active intermediates for olefin metathesis [ 7 1. For example, propene metathesis proceeds via a chain mechanism through mutual transformations of methylene and ethylidene carbenes: Mo=C+ c=c-c

f

MIEl \

Mo=C-C + C=C-C

d

M

f \

Mo=C-C + C=C (la>

/ t

(lb)

Mo=C + C-C=C-C

Thus, one may expect that on the surface of a real working catalyst there must exist both types of carbene complexes, whose relative concentrations are dependent on their reactivities. Therefore, one of the purposes of this paper was to obtain direct proof of the presence of both Mo=CHa and Mo=CH-CH, species after the propene metathesis reaction over Mo/SiOa catalyst was complete. Catalysts photoreduced in CO were chosen for this purpose as the most promising, since rather high concentrations of Mo=CHa carbenes (up to u 70% of the total MO content) could be obtained by CP chemisorption [ 61. The next task of the present work was to study the MCP interaction with Mo4+ ions in photoreduced catalysts. Taking into account literature data on the interaction of cyclopropane derivatives with CBH5-WCl,-AK& complexes in solution [8] and our own results on CP chemisorption on photoreduced Mo4+/Si02 [ 1, 4, 61, it seemed likely that both methylene and ethylidene carbenes could be formed according to the following scheme:

Y\ //m2 0

n

/My

+

A

c3H6

@a>

m3

0 "MO" /\

CH-CH3

+

'ZH4

(2b)

Thus, MCP chemisorption on Mo4+ ions might reveal how to produce Mo=CH-CH, complexes, to identify them spectroscopically and to study their properties.

395

Experimental

Mo6+/Si02 catalysts (0.1, 1 .O and 2.5 wt.% MO) were prepared by impregnation of industrial KSK-2-5 silica (prewashed in boiling HCl to remove Fe”+ ions) with an aqueous solution of ammonium paramolybdate, followed by drying in air and calcination in oxygen flow at 700-800 K. Before photoreduction in CO, catalyst samples were pretreated in a vacuum system according to the following pattern: calcination in dry oxygen (100 torr, 1 h) at 1073 K, then heating under vacuum (2 10m4 torr, 1 h) at the same temperature. To avoid possible high-temperature reduction of Mo6+ ions, samples were cooled to 600 K in dry O2 (20 torr) and then evacuated to a residual pressure of lop4 torr. Photoreduction of the catalysts (1-2 g) was performed under CO pressure of lo-20 torr in a gas-circulating system at room temperature, using full radiation of a high-pressure mercury lamp (1 kw). The percentage of reduced Mo4+ ions, as determined by measuring the CO2 molecules evolved during irradiation, was equal to 30-35% for 2.5% Mo/SiOz, 45-50% for 1.0% MO/ SiOZ and 70-80% for 0.1% Mo/SiOs. After photoreduction CO was desorbed at 400 K. MCP, CP, ethylene and propene were purified by repeated (usually triple) freeze-pump-thaw cycles. The content of the main component, controlled mass-spectroscopically, was 96-98%. Thermodesorption products and gas-phase compositions during propene metathesis were analyzed with a UTI-100C quadrupole mass-spectrometer. Mixtures of olefins of known composition were used for calibration. Diffuse reflectance spectra in the range of 200-850 nm were recorded using a MgO pellet as a reference with a Hitachi M-340 spectrophotometer. IR spectra in the C-H stretching region were obtained using a Perkin-Elmer 580 B spectrophotometer with a home-made attachment for diffuse reflectance measurements. Catalytic activities in propene metathesis at room temperature were measured in an evacuable gas-circulating system with total reaction volume of 500 cma. Catalyst sample weights were 0.2-0.06 g. Metathesis rates were calculated by applying the Langmuir-Hinshelwood model to the kinetic data as described previously [ 9 1. Results

and discussion

W-Es spectroscopic study of MCP chemisorption LX-I the UV-Vis spectrum of photoreduced Mo/SiOz three ABs with maxima at 340, 645 and 790 run assigned earlier [5] to the d-d transitions in coo&natively unsaturated Mo+~ ions (d2, Td) were observed (Fig. la). After MCP adsorption at 77 K (the number of adsorbed MCP molecules exceeded slightly that of Mo’~ ions in the sample), slow temperature increase to 293 K and subsequent evacuation, a remarkable change in the UV-Vis spectrum occurred (Fig. lb): ABs of the d-d transitions completely disappeared and

396

1

L

300

400

500

600

?LW

600

A ,"fn

spectra of 1% Mo/SiOz sample: (a) photoreduced in CO and outgassed at 320 K; (b) sedately after MCP chem~orption at 293 K on to sampIe (a); (c) after evacuation of sample (b) at 293 K for 1 h; (d) after evacuation of sample (c) at 620 K for 30 min; (e) after oxidation of sample (d) by O2 (P= 1 tow, 30 min, 293 K).

Fig. 1. W-Vii

an intense AB at 480-490 nm emerged. Its maximum shifted gradually (over 1-2 h) to shorter wavelengths (460 nm) and a shoulder appeared near 590-600 nm (Fig. lc). No further change in the UV-Vis spectm was observed while the sample was kept under vacuum at room temperature for a long time (up to several months), but after heat treatment at 600-620 K the ABs of Mo4+ ions were partially restored and the shoulder at 590 nm disappeared (Pig. Id). It should be noted that the transformations of the UV-Vis spectra described above are practically identical to those observed earlier [l] for CP che~so~~on on Mo4+ ions. In accordance with our expectations, this indicates the similar character of the MCP and CP interactions with Mo4+ ions. Taking into account the data reported in [ 1 ], the following explanation for ‘the observed spectra could be proposed. It is likely that the AB at 480490 nm can be assigned to molybdena-cyclobutane complexes formed with Mo4+ ions immediately after MCP chemisorption (see eqns. 2a and 2b). These intermediates gradually decompose to earbene complexes MO-CH,

397

or Mo=CH-CH,, characterized by a closely located AB at 450-460 run (likely a charge transfer band). The shoulder at 590 nm may be attributed to the formation of ethylene and propene r-complexes with bare Mo4+ ions [ 1, 61 and/or to the isomerization of some molybdena-cyclobutanes into butene r-complexes. Olelln r-complexes decompose under vacuum at 500-550 K, and simultaneously the AB at 590 nm disappears while the dd transitions of Mo4+ ions are restored (Fig. Id). The remarkable decrease in intensity of the Mo=CHz and Mo=CH-CH, ABs at 450-460 nm after outgassing the sample at 620 K seems to be attributable to a relatively lower thermal stability of ethylidene carbenes, which can decompose giving rise to ethylene: CH2=CH, Mo=CH-CHa

-

620 K

1

MO

-

620 K

Mo4+ + C 2H4 t

n-complex

Note that Mo=CH2 is stable under vacuum at this temperature [ 11. Equimolar chemisorption of MCP on Mo4+ ions at 293 K followed by thermal treatment at 620 K resulted mainly in ethylene (60%) and propene (30-35%) desorption (the rest were gases of C4Hs composition). Ethylene evolution supports our assumption concerning carbene Mo=CH-CH3 formation after MCP decomposition (eqn. 2a).

IR spectroscopic study of MCP chemisorption No AE%sin the C-H stretching vibration region were observed in the IR spectrum of a photoreduced 2.5% Mo/SiOz sample (Fig. 2a). After MCP chemisorption on this sample as described above (see preceding section) and outgassing at room temperature for 1 h the spectrum exhibits many AE3s(Fig. 2b), among which those at 3080 and 2945 cm-’ are assigned as reported earlier [l, 41 to asymmetric and symmetric C-H stretches of the Mo=CH, carbene, respectively. Upon raising the temperature to 600-650 K and pumping off the sample, the ABs at 3060 and 2960 cm-’ disappeared, while the intensities of all other bands decreased (perhaps this effect is less pronounced for ABs at 3080 and 2945 cm-‘) (Fig. 2c, d, e). These data allow us to assume that four AELsat 2985, 2910, 2890 and 2850 cm-’ might be attributed to the four normal C-H vibrations of carbene Mo=CH -CHa*. This assignment agrees with our assumption on the lower thermal stability of Mo=CH-CHa as compared to that of Mo=CHa. As was shown earlier [ 11, the ABs at 3060 and 2960 cm-’ are likely due to ethylene rr-complexes with Mo4+ ions (cf. the 590 nm AB in the UV-Vis spectrum CFig. lc)). In order to obtain a catalyst containing no active species other than carbene complexes, Mo4+ ions regenerated after thermodesorption were oxidized under mild conditions (1 torr Oa, 293 K, 1 min). This caused the Mo4+ d-d transition in the UV-Vis spectrum to disappear, while the carbene *Note that acetaldehyde, having a similar structure, is characterized the C-H vibration region [lo].

by four IR ABs in

398

d

b

1

'

1

I

2200

2ieo

2000

6 2922

, 2200

3t cm

-I

2. IR spectra of 2.5% Mo/SiOzsample: (a) photoreduced in CO and outgassed at 320 K; (b) after MCP chemisorption on to sample (a) and outgassingat 293 for I h; (c) after evacuation of sample (b) at 600 K for 30 miq (d) after evacuation of sample (c) at 620 K for 30 min; (e) after evacuation of sample (d) at 640 K for 30 min. Fig.

at 450 run remained almost unchanged (Fig. le). No change in the IR spectrum after oxygen treatment was found to occur (Fig. Be). This confirms our earlier tidings for Mo=CH2 [ 1] showing the relatively low activity of such carbenes towards oxygen molecules.

AB

Conversion of carbenes MO =CW, to MO -CH- CH3 and the reverse AS follows from eqn. 1a, Mo=CH2 carbenes should be converted to Mo=CH-CH, under excess ethylene in the gas phase. In order to obtain direct IR spectroscopic evidence for this reaction the following experiments were performed. Catalyst containing only MO= CH2 and MO= CH-CHS species (Fig. 3a) was treated with ethylene (30 torr, 293 K, 10 min) and outgassed at room

399

1

* 3zoo

3wo

33w

2300

2800

3. IR spectra of 1% Mo/SiOz sample photoreduced in CO: (a) after MCP chemisorption and outgassing at 570 K for 30 min followed by Mo4+ oxidation (PO2= 1 torr, 1 min, 293 K); (b) after CSH, admission on to sample (a) and outgassing at 293 K.

Fig.

temperature. In accordance with our expectations this caused the Mo=CH -CHs ABs to disappear (Fig. 3b) and only those of MO- CHz at 3080 and 2945 cm-’ remained in the IR spectrum. The reverse procedure was used for the photoreduced 1% Mo/SiOa treated with CP. After outgassing at 620 K and subsequent oxygen treatment at 293 K, only carbenes Mo=CHa were found by IR (Fig. 4a). After the catalyst was contacted with gaseous propene sufficiently long, eq~~brium concentrations of the metathesis products were attained (18 vol.% ethylene, 18 vol.% butene-2 and 64 vol.% propene); thus, according to eqns. la and lb, steady-state concentrations of carbenes and various molybdena-cyclobutanes should be reached. Outgassing the catalyst at room temperature led to decomposition of molybdena-cyclobutanes, leaving behind only Mo=CH2 (ABs at 3080 and 2945 cm-l) and Mo=CH-CH, carbenes (AlSs at 2985, 2910, 2890 and 2850 cm-‘) (Fig. 4b). Treatment of such a sample in ethylene resulted in disappearance of the MO= CH - CR3 AI3.s(Pig. 4~). Thus, for the fl.rst time mutual carbene transformations in a metathesis reaction over photoreduced Mo/SiOz catalyst have been observed by IR.

Fig. 4. II? spectra of 1% Mo/SiOz sample photoreduced in CO: (a) after CP chemisorption and outgassing at 620 K for 30 min followed by Mo4+ oxidation (PO2= 1 torr, 1 min, 293 K); (b) after treatment of sample (a) in CBH, (30 torr, 30 min) and outgassing at 293 K for 30 min; (c) after treatment of sample (b) in C2H, (30 torr, 30 min) and outgassing at 293 K for 30 min.

Measurements of carbm concentrations Concentrations of Mo=CH, and MO= CH-CHa were measured for a photoreduced 1% MofSiOa sample treated with MCP as described above (see Fig. 4a). First the amount of propene evolved into the gas phase during sample treatment in excess ethylene was determined (eqn. la) and thus the value of [Mo=CH-CH& was obtained (0.54X 10” g-l, Table 1). Then the total number of methylene carbenes [Mo=CH2] = [MO-CH,],+ [MO-CH-CH& (subscript ‘0’ refers to the original sample) was calculated by measuring the amount of COz molecules formed during sample oxidation in oxygen flow at 850-900 K. Thus the concentrations of [Mo=CH,], and [Mo=CH-CH,], were determined (Table 1). The ratio [Mo=CH,],:[Mo=CH-CH,], obtained is in qualitative agreement with that predicted from eqns. 2a and 2b. Indeed, assuming the fo~ation of MOFj and Moq occurs statisti~a~y and the rupture of any C -C bond in the MCP ring has an equal probability, the ratio of these Mo-cyclobutane intermediates should be equal to 2:l. Then, assuming an equal probability for Mo’3, decomposition to Mo=CHs and MO-CH-CH,, one may expect the ratio [MO-CH&:[Mo=CH-CH,], must be close to 2:l. This is in good agreement with the experimental value

401

d .E

6 B

(2.4:1, Table 1). The data in Table 1 indicate that MCP interacts with only 55% of the total Mo4+ ions. This fraction is much smaller than, that found for CP interaction (909/o, Table l), i.e. carbene formation with MCP seems to be more difficult, likely due to some energetic or steric factors. It should also be noted that the fraction of MO*+ ions which could be converted to Mo=CH, by interacting with CP qualitatively agrees with our previous rough estimates [ 61 a isotope exchange

Catalytic

measurements

MCP chemisorption over photoreduced Mo/SiOa catalyst resulted in a sharp increase (almost one order of magnitude) of specific activities (turnover frequencies, TOF) for propene metathesis at 293 K (Table l), which is due to carbene formation on the surface. As follows from Table 1, there is a correlation between specific activities of the catalyst and initial carbene concentrations (c$ experiments 2 and 3). The highest TOFs in our experiments were achieved with photoreduced 0.1% Mo/SiOa treated by CP (TOF= 12.8, Table 1). This is likely explained by a better dispersion of supported Mo6+ ions in such a dilute sample, which resulted in higher conversion of Mo6+ to Mo4+ during photoreduction in CO (70%) as compared to that obtained with 1% Mo/SiOa catalyst (50%) (Table 1, column 3). Correspondingly, the relative yield of Mo=CHa carbenes after CP treatment was higher for the 0.1% Mo/SiOa sample than that for 1% MO/ SiOZ. Acknowledgement

The authors thank Dr. N. M. Abramova for a gift of methylcyclopropane, which was synthesized as described in [ 111. References 1 K. A. Vikulov, I. V. Elev, B. N. Shelimov and V. B. Kazansky, J. Mol. Catal., 55 (1989) 126. 2 I. V. Elev, B. N. Shelimov and V. B. Kazansky, Kin&. Katal., 30 (1989) 895. 3 I. V. Elev, B. N. Shelimov, V. B. Kazansky, M. Ju. Berezin, G. A. Usacheva and P. S. Belov, Kin&. Katal., 30 (1989) 1101. 4 K. A. Vilculov, I. V. Elev, B. N. Shelimov and V. B. Kazansky, Catal. L&t., 2 (1989) 121. 5 B. N. Shelimov, A. N. Pershin and V. B. Kazansky, J. CataZ., 64 (1980) 425. 6 B. N. Shelimov, I. V. Elev and V. B. Kazansky, J. Mol. CataZ., 46 (1988) 187. 7 J. C. Mol and J. A. Moulijn, Catal. Sci. Technol., 8 (1988) 69. 8 P. G. Gassman and T. H. Johnson, J. Am. Chem. Sot., 98 (1976) 6057. 9 B. N. Shelimov, I. V. Elev and V. B. Kazansky, J. Catal., 98 (1986) 70. 10 L. M. Sverdlov, M. A. Kovner and E. P. Krainov, Vibrational Spectra of Poluatomic Molecules, Nauka, Moscow, 1970. 11 J. T. Gradson, K. W. Dreenlee, J. M. Derfer and C. E. Boord, J. Am. Chem. Sot., 75 (1953) 3344.