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Langmuir 1990, 6, 1774-1783
Chemistry of Organochromium Complexes on Inorganic Oxide Supports. 1. Characterization of Chromocene on Silica Catalysts Shi-Liang Fu and Jack H. Lunsford* Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received April 2, 1990. I n Final Form: June 14, 1990
Chromocene supported on silica catalysts was prepared by sublimation and impregnation techniques and characterizedby infrared spectroscopy. The adsorption of the solvent during impregnation complicates the spectral interpretation. This problem was eliminated by using a sublimation method. Chromocene reacted with hydroxyl groups to form CpCr complexes and surface Cr, on which molecular chromocene was further adsorbed. The infrared results suggest that these complexes were present in an agglomerated state or as clusters regardless of the amount of Cr complexes on the surface. The maximum amount of Cr deposited on the silica decreased as the dehydration temperature was increased. The Cp ligands of surface Cr complexes retained their *-bonded nature and were thermally stable up to ca. 100 "C. Hydroxyl groups that remained after the addition of chromocene also were found to interact with Cr complexes through hydrogen bonding.
Introduction Several types of organochromium complexes can be deposited on an oxide support to form highly active catalysts for olefin polymerization.'-'5 Although such catalysts are not well characterized, the organochromium complexes are thought to bind to the support by reaction with surface hydroxyl^.^^ A variety of Cr species may be present on the surface as a result of concurrent reactions having different stoichiometries. Usually the surface chemistry of the supported organochromium catalysts is very sensitive to the nature of support13J4and the ligands on the For example, chromocene (bis(cyclopentadienyl)chromium,CpnCr) supported on silica forms a very active catalyst, while bis(arene)chromium compounds, such as bis(benzene)chromium and bis(cumene)chromium, require a more acidic support? The attached organic ligands influence the polymerization reaction, as indicated by the unique catalytic behavior of these catalysts compared to other Cr catalysts without organic ligands.*J6 Some catalysts undergo ligandexchange reactions to yield a modified material having catalytic behavior characteristic of the new ligand.H.11 The interesting and important catalytic properties of these
materials have prompted us to systematically study the surface chemistry of supported organochromium complexes. Chromocene on silica, hereafter referred to as the chromocene-derived catalyst, is of special interest because this catalyst is one of the most effective for ethylene polymerization. A unique feature of the chromocene-derived catalyst is the high chain-transfer response to hydrogen during polymerization. Therefore, polymers having a wide range of molecular weights can be tailored by proper control of the hydrogen to ethylene ratio. Fundamental studies have been carried out on catalyst formation and reaction kinetic^.^*^-'^ It is believed that chromocene reacts with hydroxyl groups on the support, with subsequent release of one, or even two, of the ligands as cyclopentadiene. The remaining cyclopentadienyl (Cp) ligand is thought to remain bonded to chromium. Little spectroscopic information is available as to the nature of the organochromium complexes, and the interpretation of the spectra often differs.I7-l9 The present paper describes a study of chromocene-derived catalyts by Fourier transform infrared spectroscopy. Detailed analyses of infrared spectra are given in an effort to characterize further the surface chemistry. The effects of variables such as the solvent, dehydration temperature of the silica, and the chromium content are discussed in detail.
(1) Yermakov, Yu. I.; Zakharov, V. A. Ado. Catal. 1973,24,173. (2)Yermakov, Yu. I. Catal. Reus-Sci. Eng. 1976,13 (l),77. (3)Zakharov, V. A.; Yermakov, Yu. I. Catal. Reo.-Sci. Eng. 1979,19 (I),67. (4)Karol, F. J.; Karapinka, G. L.; Wu, C.; Dow, A. W.; Johnson, R. N.;Carrick, W. L. J . Polym. Sci., Polym. Chem. Ed. 1972,10,2621. (5)Iwasawa, Y.; Chiba, T.; Ito, N.J . Catal. 1986,99,95. (6) Karol, F. J.;Johnson, R. N. J . Polym. Sci., Polym. Chem. Ed. 1975, 13, 1607. (7)Karol, F. J.; Wu., C. J . Polym. Sci., Polym. Chem. Ed. 1974,12, 1549. (8) Freeman, J. W.; Wilson, D. R.; Ernst, R. D.; Smith, R. P.; Klendworth, D. D.; McDaniel, M. P. J . Polym. Sci., Polym. Chem. Ed. 1987, 25,2063. (9)Rebenstorf, B. J . Mol. Catal. 1988,46,109. (10)Karol, F.J.; Brown, G. L.; Davison, J. M. J . Polym. Sci., Polym. Chem. Ed. 1973,11,413. (11)Karol, F. J.; Munn, W. L.; Goeke, G. L.; Wagner, B. E.; Maraschin, N. J. J . Polym. Sci., Polym. Chem. Ed. 1978,16,771. (12)Kozorezov, Y. I.; Seikho, A. Zh. Prikl. Khim. 1983,56,1614. (13)Karol, F. J.; Wu, C.; Reichle, W. T.; Maraschin, N. J. J . Catal. 1979,60,68. (14)McDaniel, M. P.;Leigh, C. H.; Wharry, S. M. J . Catal. 1989,120, 170. (15)Stanovaya, S.S.;Shagilova, A. V.; Grigorev, V. A.; Korobova, N. M.; Karandashova, N. P. Plast. Massy 1981,8,9. (16) McDaniel, M. P. Ado. Catal. 1985,33,47.
0743-7463/90/ 2406-1774$02.50/0
Experimental Section Materials. Davison 952MS silica gel with a surface area of 300 m2/g was used as the catalyst support. Chromocene (Strem Chemicals, sublimed) was purified by sublimation at 40 "C in vacuo (10" Torr). All gases were supplied by Airco and were passed through a molecular sieve or a calcium chloride column prior to use. All solvents (Fischer) were dried over Na/
benzophenone and were distilled under nitrogen. Cyclopentadiene dimer (Aldrich, 95%) was purged with nitrogen for 2 h prior to use. Krytox fluorinated grease (Dupont, LVP grade, L-7), which is inert to the solvents employed, was used on Cajon joints to avoid the contamination of the catalyst. Potassium bromide (Aldrich, 99.999%) was dried in vacuo at 200 "C before pressing with chromocene. (17)McKenna, W. P.;Bandyopadhyay, S.; Eyring, E. M. Appl. Spectrosc. 1984,38,834. (18)Rebenstorf, B.; Larsson, R. J . Mol. Catal. 1981,11, 247. (19)Zecchina, A.; Spoto, G.; Bordiga, S. Faraday Discuss. Chem. SOC. 1989,87,149.
Q
1990 American Chemical Society
Langmuir, Vol. 6, No. 12, 1990 1775
Characterization of Chromocene on S i Catalysts
w
3200
3000
2800
FREQUENCY (cm-') F i g u r e 1. Schematic for infrared cell: (A) magnet, (B)gas reservoir, (C) wafer, (D) furnace, (E) KCl/KRS-5 windows, (F) Cr complex solution, (G) Cr complex, (H) greaseless high vacuum stopcock. C a t a l y s t P r e p a r a t i o n . Chromocene-derived catalysts are highly sensitive to trace impurities such as water and oxygen; therefore, all manipulations were conducted in an inert atmosphere or under vacuum. The quartz cell, as depicted in Figure 1, was used for catalyst preparation, polymerization, and subsequent in situ infrared studies. Silica gel was pressed into a thin wafer of typically 3-5 mg/cm2. Following dehydration of the silica gel wafer a t temperatures between 200 and 900 "C over a period of 10 h, chromocene was deposited on the silica by two techniques, viz., an impregnation method and a sublimation method. In the former method, the silica gel wafer was lowered t o the bottom of t h e cell where chromocene solution of specific concentration was added. The wafer turned black immediately. After mild agitation for 30 min, the remaining solvent was pumped away, and the catalyst was then evacuated overnight to remove residual solvent and excess chromocene. The sublimation method was a vapor deposition process in which chromocene was sublimed a t room temperature onto a silica gel wafer. The intended Cr loading was achieved by varying the sublimation time and the amount of chromocene used for sublimation. Catalysts prepared with silica gel t h a t had been dehydrated a t 200, 450, and 900 "C are designated as Cp2Cr /Si02(200), Cp2Cr/Si02(450), a n d Cp2Cr / SiO2(9O0), respectively. These temperatures were chosen in order to compare chromocene deposition a t three unique stages of surface hydroxylation. A t 200 "C, silica has the highest surface hydroxyl concentration with very little physisorbed water, which is known to oxidize chromocene. Most of the hydroxyl groups are converted t o isolated hydroxyl groups whose concentration reaches a maximum a t 450 "C. The concentration of these isolated hydroxyl groups is reduced to a minimum a t 900 OC without significant sintering of the silica. Analysis of C r Loading. The Cr loading was analyzed by atomic absorption (AA) of an aqueous Cr solution prepared by dissolution of the catalyst. T h e aqueous Cr solution was prepared by first calcining t h e catalyst a t 500 "C in air, which burned off organic species such as polymers and Cp ligands and simultaneously oxidized Cr t o Cr2O3. This was followed by digesting the silica gel in 48";) hydrofluoric acid a t ca. 200 "C. The remaining Cr2O3 was then oxidized to the soluble Cr(V1) ion with 70°;8perchloric acid a t ca. 200 "C, and the resulting solution was diluted to the appropriate volume for measurement.
F i g u r e 2. Difference spectra (background = silica) of impregnated Cp,Cr/Si02(500) catalysts and corresponding solvents adsorbed on silica dehydrated a t 500 "C using the following solutions or solvents: (a) chromocene/hexane, (b)hexane, (c) chromocene/ benzene, (d) benzene, (e) chromocene/toluene, (f) toluene. Atomic absorption analysis was carried out on a Varian AA-30 spectrometer, using the Cr absorption line a t a frequency of 425.4 nm. I n f r a r e d Measurement. Infrared absorption studies were carried out with a Digilab FTS-40 Fourier transform spectrometer operating with a He-Ne laser and a cryogenic MCT detector (-196 "C). Disks of KC1 or KRS-5 (International Crystal Lab.) were used as infrared cell windows. All spectra were recorded a t a n IR beam temperature which was determined to be 5 "C higher than the ambient temperature. Spectral manipulations, such a s subtraction a n d deconvolution, were performed by t h e 3200 D a t a S t a t i o n software. T h e spectral resolution was 2 cm-l, and the frequency was calibrated by using a polystyrene film. Spectra shown in the same figure are offset for a better display. All IR spectra have been normalized to a constant wafer density of 4 mg/cm2.
Results Impregnation vs Sublimation, Impregnated catalysts were prepared by impregnating dehydrated silica with an organic solution of chromocene a t room temperature. Different solvents, including benzene, hexane, and toluene, were used to distinguish spectral features that result from the solvent. These solvents were chosen on the basis of their distinctive types of C-H stretches. The spectra of the C-H stretching region for these catalysts and the corresponding solvents adsorbed on silica are shown in Figure 2. Two convoluted, broad bands centered around 3000 cm-1 were observed for impregnated catalysts. The band in the olefinic stretching region (>3000 cm-l), which has a maximum at 3099 cm-l, is attributed to the attached Cp ligand. Similar bands (3095-3099 cm-I) were previously reported for the same catalyst, and the same assignment was This broad band contained four overlapping peaks a t 3111, 3099, 3087, and 3062 cm-', the relative intensities of which were dependent upon the adsorbed solvent. A second band, which was characteristic of the solvent employed, was observed in the aliphatic C-H
1776 Langmuir, Vol. 6, No. 12, 1990
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Fu and Lunsford
2000
1250
FREQUENCY (cm-') Figure 3. Infrared spectra of (a) sublimed Cp,Cr/Si02(450) catalyst and (b) its silica support dehydrated at 450 "C.
stretching region (27W3OOo cm-l). The close resemblance of this band with that of the correspondingsolvent on silica suggests that it resulted from the adsorbed solvent. This is supported by examining the spectrum of the catalyst prepared by the sublimation method, a process without solvent, which shows substantial suppression of the second band (spectrum a, Figure 3). The weak aliphatic C-H stretching bands in the spectrum of the catalyst impregnated with a chromocene/benzene solution (spectrum d, Figure 2) possibly arose from trace impurities of toluene in the solvent. The frequencies of various bands in the spectra of different impregnated catalysts and their respective solvents in the C-H stretching region are summarized in Table I. The infrared spectra of a sublimed catalyst and its silica support are shown in Figure 3. The most significant difference between spectrum a, Figure 3, and those of impregnated catalysts is the substantial suppression of the aliphatic C-H stretching bands. The intense band of the sublimed catalyst in the olefinic C-H stretching region is very similar to the corresponding band in the spectra of impregnated catalysts. Therefore, this band is analogously attributed to the Cp ligand. The Cp ligand interacted with surface hydroxyl groups, which resulted in a decrease of the dipole moment of C-H bonds and consequently the intensity of C-H stretching bands, as is commonly observed in many adsorbed aromatic systems.*= The involvement of hydroxyl groups in the interaction with Cp ligand is revealed by the spectral changes corresponding to 0-H stretching modes after chromocene deposition. These changes include a large decrease in the intensity of isolated hydroxyl bands at 3746 cm-' and the concurrent appearance of a broad band of interacting hydroxyl groups around 3600 cm-', which is more obvious in the difference spectrum (not shown). The very weak band observed in the aliphatic C-H stretching region may result from trace impurities such as decomposed products or residual solvents trapped in the chromocene crystals. I t also is possible that overtones or combinations of C-C stretching bands at 1425 and 1439 cm-* may have some contributions. T h e vibrational frequencies of a sublimed CpzCr/SiOz(450) catalyst and the literature data of chromocene are compared in Table 11. (20) Wieder, G. M.; Dows, D. A. J . Chem. Phys. 1962,37, 2990. (21) Dows, D. A.; Pratt, A. L. Spectrochim. Acta 1962, 18, 433. (22) Calkiin, G. A,; Kiselev, A. V.; Lygin, V. I. Kinet. Katal. 1964,5, 935.
There are several advantages of the sublimation method over the impregnation method (1)The sublimed catalyst is prepared without the use of solvent, thus eliminating solvent effects (see Discussion). (2) As the sublimation method is carried out under high vacuum, the possibility of inadvertent adsorption of released ligands or decomposed products can be minimized. (3) The sublimation method is much easier than the impregnation method in preparing catalysts for in situ infrared and polymerization studies. Therefore, the sublimation method has been chosen for the remaining studies unless otherwise stated. Infrared Assignment. A large number of cyclopentadienyl complexes have been studied by infrared spectroscopy, and the assignments based on molecular symmetries have been extensively reviewed by Fritz.= The symmetry of Cr complexes on the chromocene-derived catalysts may be considerably lower than that of chromocene (D& however, it is reported that a local symmetry of C5,,is an adequate approximation for the Cp ligand in assigning or predicting vibrational bands of many cyclopentadienyl c o m p l e x e ~ .The ~ ~ local symmetry method assumes that the vibrational modes for the Cp ligand are almost independent of the remaining modes of the complex. On the basis of Cbu local symmetry, nine IR-active fundamental bands (4A1 + 5E1), designated as "characteristic frequencies", are expected for surface cyclopentadienyl chromium complexes. These bands consist of seven Cp ring vibrations and two Cp-Cr vibrations. Although two bands corresponding to the A1 and El modes of C-H stretches are expected for the Cp ligand, four bands a t 3111, 3099, 3087, and 3062 cm-l were observed in the C-H stretching region. Since no first overtones or combination bands are expected in this region to couple with C-H stretches, the observed multiple peaks from band splitting by Fermi resonance can be eliminated.24 Furthermore, the multiple peaks cannot be attributed to the surface heterogeneity, as the same bands were observed for CpzCr/SiOz(450) and CpzCr/SiOz(900) (spectra b and c, Figure 4 0 , which are expected to have a large difference in surface h e t e r ~ g e n e i t y .I~t ~ appears that two factors are responsible for the multiple peaks. These are the splitting of the E1 mode by the lowering of the symmetry from C5,,and the perturbation of C-H stretching modes by their interaction with silanol groups or neighboring Cr species. The latter interaction may account for the high frequency band at 3111 cm-'. The interaction between C-H bands and surface hydroxyl groups has been well established.26127Kieslev and c o - w o r k e r ~have ~ ~ ~studied ~ ~ the adsorption of aromatic compounds on silicas of various hydroxyl concentrations and concluded that different degrees of interaction occurred between the hydroxyl groups and the aromatic rings. The interactions result in a redistribution of the ring electrons, as reflected in a red shift of C-C stretches and a blue shift of C-H stretches. The bands at 1439 and 1425 cm-l (spectrum a, Figure 3) are attributed to the E1 mode of asymmetric C-C stretches of the Cp ring. Analogous to the El mode of C-H stretches, the splitting of the degenerate E1 mode was observed for C-C stretches. Since the electron density of (23) Fritz, H. P. Adu. Organomet. 1964, I , 239. (24) Smith, A. L. Applied Infrared Spectroscopy; Wiley: New York, 1979. (25) Iler, R. K. The Chemistry of Silica; Solubility, Polymerization, Colloid, and Surface Properties and Biochemistry; Wiley New York, 1979. (26) Little, L. H. Infrared Spectra of Adsorbed Species; Academic: New York, 1966. (27) Kieslev, A. V.; Lygin, V. I. Infrared Spectra of Surface Compounds; Wiley: New York, 1975. (28) Galkiin, G . A,; Kiselev, A. V.; Lygin, V. I. Trans. Faraday Soc. 1964,60, 431.
Characterization of Chromocene on Si Catalysts
Langmuir, Vol. 6, No. 12, 1990 1777
Table I. Infrared Bands of Impregnated and Sublimed Chromocene Catalysts a n d Respective Solvents in Diluent o r on Silica in the C H Stretching Repion olefinic stretches, cm-l (3200-3000 cm-l)
aliphatic stretches, cm-1 (3000-2700 cm-1)
samples CpCr/SiOzo (sublimed)
3111
3099
CpCr/SiOz (sol = toluene) toluene/SiOz toluene/CCl.#
3111
3099
CpCr/SiOz (sol = hexane) hexane/SiOz hexane/CCldc
3087
3110
CpCr/SiOz (sol = benzene) benzenelSi02d benzene/CCl4c
3111
3099
3098
3106
b
3062 3087
3121
3062
3069 3062
3032 3030
3063
2953
2928
2874
2859
2959 2950
2928 2921
2874 2874
2858
2980 2987
2944
2901
2999
2941 2959
2904 2929
3084
3066
b
3089
3071 3072
b
3092
2846 2837 2873
2860
Cp/SiOf 3044 2960 2935 2883 2854 a Silica was dehydrated at 450 "C for all samples. Trace amount of impurities or decomposed products was observed (see text). c CC14 was used as a diluent. A similar study was reported in ref 44. e Deposition of dicyclopentadiene vapor on silica. Table 11. Infrared Bands of Chromocene and Chromocene-Derived Catalyst assignments C-H stretches
(UC-H)
overtones or combinations
C-C stretches (UC-C)
chromocene catalysta this work ref 29 3111 sh 3108 w 3099 m 3097 m 3087 sh 3086 m 3062 w
3051 m
2693 w 2488 w 2402 w 2275 w 2235 w 2063 w 1777 br 1683 br
1719 1629 m
chromocene (CpZCr) ref 23 ref 43 3095 3076
1439 m 1425 m 1400 vw 1351 w
1408 m 1340 w
1397 1362 m
1404
1091 8 1038 989 s 960
1089
955 m
1095 s 1055 w 1040 w 992 vs
C-H in-plane deformation or C-C stretches ( ~ c - Hor u c e )
C-H out-of-plane deformation
ref 42 (3356)
(1430)
(TC-H)
882 w 842 w 750 vw 711 m 607 w
840 sh 800 sh 765 vs 620 vw
1091
987
914 890 m 769 ss
829 766
795
663 s
ring-metal vibrations (ucrcP)
435 m 429 408 m 408 A sublimed Cp~Cr/Si0~(450) catalyst. m, medium; s, strong; w, weak; ss, strong & sharp; sh, shoulder; br, broad; vw,very weak.
*
the Cp ring, and thus the C-C bond order, is decreased by interaction with silanol groups,28the 1425-cm-l band is expected to be more intense for catalysts with a high concentration of hydroxyls. Indeed, the intensity of the 1425-cm-' band is higher than that of the 1439-cm-' band for Cp&r/Si02(200), while Cp&r/Si02(450) shows the reverse trend. There is another very weak band at 1350 cm-l which may also result from the C-C stretching modes. By analogy with literature data,lg this band is attributed to a Raman-active mode (E*)which becomes IR visible as
the symmetry is lowered by the support. The C-H outof-plane bending vibration of both A1 and E1 modes is expected between 700 and 900 cm-l, and, unlike other characteristic bands, both the frequencies and the intensities of these vibrations are sensitive to the degree of surface hydroxylation and the surface concentration of Cp ring~.~3*30 Although several bands were observed in this region through the "silica ~indow'',~J1 which is the region where the silica lattice does not absorb strongly, their intensities were greatly affected by the near lattice
(29) Alekesnyan, V. T.; Lokshin, B. V.; Borisov, G.K.; Devyatykh, G. G.;Cmirnov, A. S.; Nazarova, R. V.; Koningstein, J. A.; Gachter, B. F. J. Organomet. Chem. 1977, 124,293.
(30)Nakamoto, K. Infrared and Roman Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1986. (31)Hoffmann, P.; Knozinger, E. Surf. Sci. 1987, 188, 181.
Fu and Lunsford
1778 Langmuir, Vol. 6, No. 12, 1990
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3000
FREQUENCY ( c m 3
Figure 4. Infrared spectra of the 0-H stretching region on (A) pure silica and (B)after chromocene deposition with respect to dehydration temperature: (a) 200 O C , (b)450 O C , (c) 900 "C. The C-H stretching region of B is amplified in C after subtraction of silica background.
absorptions. Therefore, it is difficult to unequivocally assign these bending vibrations. The remaining bands between 2700 and 1500 cm-l are extremely weak and do not appear in the range expected for the fundamental vibrations; therefore, they are assigned to the overtones and combination bands. The absence of the bands for the A1 mode of the C-H in-plane bending vibration and the A1 mode of C-C ring breathing can be accounted for by the presence of a strong lattice absorption in the same region. However, their frequencies can be estimated from the overtones and combination bands to be in the region around 1000 and 1100 cm-', respectively. For comparison, the results of previous infrared studies of chromocene in solid and solution states are also listed in Table 11. However, care must be taken when comparing the infrared studies of chromocene and the chromocenederived catalysts since a t least two factors, which complicate the spectral interpretation, need to be taken into account. First, W i l k i n s ~ nfound ~ ~ that extensive decomposition of chromocene occurred in CS2 and CCld solutions. Decomposition also occurred upon grinding chromocene with Nujol or alkali halides. In this study, several attempts were made to prepare chromocene/ KBr pellets in an Ar atmosphere, and the results showed that various degrees of decomposition occurred, even after prolonged dehydration of KBr in vacuo a t elevated temperature (200 "C). Secondly, although the local symmetry of CsUmay be adequate for interpreting the infrared bands of the Cp ligand in many cyclopentadienyl chromium complexes having unsymmetrical ligands,23 the observed spectrum can be more complicated than the predicted one based on C b symmetry. Thus, when chromocene is coordinated to silica the lower symmetry may split the degenerate mode (El) and activate the originally IR-forbidden mode (A21 or Raman-active mode (Ed, as described earlier. Effects of Dehydration Temperature. Silica was dehydrated a t 200, 450, and 900 "C to achieve different surface concentrations of hydroxyl groups before chromocene deposition. T h e infrared spectra of these dehydrated silicas are shown in Figure 4A. Silica dehydrated at 200 "C exhibited a sharp band at 3746 cm-l and a broad band a t a lower frequency, characteristic of isolated and hydrogen-bonded hydroxyl groups, respect i ~ e l y . ~A ~t higher - ~ ~ dehydration temperatures, the (32) Wilkinson, G. J. Am. Chem. SOC.1954, 76, 209.
hydrogen-bonded hydroxyl groups gradually condensed to release H20. Some hydrogen-bonded hydroxyl groups were thus converted to isolated hydroxyl groups. With further increases in dehydration temperature, the condensation of isolated hydroxyl groups by formation of siloxane (Si0-Si) groups and water became significant; this led to an attenuation of the 3746-cm-' band. Consistent with previous r e s ~ l t s , the ~ ~ high-temperature J~ dehydration (900 "C) did not completely remove the surface hydroxyl groups. The changes in surface hydroxylation correspond well to the spectral changes in the 0-H stretching region, which showed a monotonical decrease of the hydrogen-bonded hydroxyl band, while the intensity of the 3746-cm-' band reached a maximum at ca. 450 "C. The spectra of chromocene-derived catalysts prepared from the above dehydrated silicas, CpzCr/Si02(200), Cp~Cr/Si02(450),and CpzCr/Si02(900), are shown in Figure 4B and 4C. The spectrum of CpzCr/SiOz(900) (spectrum c, Figure 4C) displays the same Cp bands, but at a lower intensity, as that of CpzCr/Si02(450) (spectrum b, Figure 4C). Prolonged sublimation time neither increased the amount of chromocene deposited nor enhanced the intensities of Cp bands in the CpzCr/SiOz. (900) spectrum. This suggests that the amount of chromocene deposited on silica is limited by the number of reactive sites on the surface. These sites are likely the hydroxyl groups, the concentration of which decreases upon increasing the dehydration temperature. Thus, the maximum amount of Cr complexes on CpzCr/Si02(900) is expected to be the lowest of the three catalysts as a result of limited hydroxyls. Impregnation with concentrated chromocene solutions also failed to load excess chromocene on CpzCr/Si02(900). It was found that excess chromocene, seen as red crystals on the wall of the IR cell and as a red tint on the catalyst, was quickly removed after solvent evacuation. It is noteworthy that the intensity of the isolated hydroxyl band a t 3746 cm-' decreased considerably after deposition of chromocene to form Cp&r/Si02(450) and CpzCr/Si02(200). In contrast, little consumption of isolated hydroxyls was observed for CpZCr/ SiOZ(900). This implies t h a t the reactivity and/or accessibility of hydroxyl groups was affected by the dehydration temperature (see Discussion). Despite large differences in the concentrations of isolated and hydrogen-bondedhydroxyl groups for silica dehydrated a t 200 and 450 OC, the integrated absorbance of the C-H stretching bands of Cp ligands for Cp~Cr/Si02(200)and CpzCr/Si02(450) is approximately equal at the same Cr loading level (spectra a and b, Figure 4C). In addition, most of the isolated hydroxyl groups were consumed in both catalysts (spectra a and b, Figure 4B). These results suggest that not only isolated but also hydrogen-bonded hydroxyl groups are reactive toward chromocene. Since the deposition of chromocene takes place on the hydroxyl sites, the maximum amount of Cr complexes which may be deposited on the catalyst is expected to increase with the concentration of hydroxyls on silica or decrease with increasing dehydration temperature of the silica. Indeed, evolution of unreacted chromocene during the chromocene deposition process occurred in the order CpzCr/ Si02(900) > Cr/Si02(450) > Cp!$2r/Si02(200) under the identical sublimation conditions. The order is in agreement with a previous impregnation study, which showed the maximum adsorption of chromocene from a solution of excess chromocene increased with a decrease in silica dehydration temperat~re.'~ There are other spectral changes of Cp bands for catalysts with a high hydroxyl concentration. The
Langmuir, Vol. 6, No. 12, 1990 1779
Characterization of Chromocene on Si Catalysts
X
/I
I
a
8m a
3
0
6
9
Cr LOADING (wt%) 3200
3100
3000
2900
FREQUENCY (cm-l) Figure 5. Difference spectra (background = silica) of the C-H stretchingregion of sublimed Cp&r/Si02(450) catalysts at various Cr loadings: (a) 1.096,(b) 2.5%,(c) 3.4%, (d) L O % , (e) 6.0%, (f)
Figure 6. Absorbance of the 3099-cm-lband of sublimed Cp2Cr/ si02(450) catalysts as a function of Cr loadings.
7.3%.
spectrum of CpzCr/Si02(200) (spectrum a, Figure 4C) shows the enhancement of the 3111-cm-' band a t the expense of lower frequency bands, as compared to that of Cp&r/Si02(450) (spectrum b, Figure 4C). This 3111-cm-l band was previously assigned to the C-H stretching perturbed by the neighboring species. Therefore, the observed spectral changes are attributed to the increased interaction between Cp ligands and hydroxyls, as a consequence of the higher concentration of hydroxyl groups on CpzCr/Si02(200). This type of hydroxyl interaction was less significant for both Cp2Cr/Si02(900) and Cp2Cr/ siOz(450)because of limited hydroxyl groups in the vicinity of cyclopentadienyl Cr complexes. For example, after chromocene deposition only ca. 0.5 hydroxyl group and 1.5 Cr atoms per nm2 surface area were present for a CpzCr/ SiOz(450) catalyst a t a 4 wt % Cr loading. The Cp2Cr/ SiO2(900) catalyst had even smaller concentrations of surface Cr and hydroxyl groups than CpzCr/Si02(450). Chromocene-derived catalysts with silica dehydrated at different temperatures also exhibited different catalytic activities for ethylene polymerization. It was found that CpzCr/Si02(450) had the highest activity, and Cp,Cr/ SiO2(900) was more than 1order of magnitude less active than either Cp&r/Si02(200) or Cp2Cr/Si02(450).33 Effects of Cr Loading. Since Cp2Cr/Si02(450) was the most active catalyst of those prepared from silicas dehydrated between 200 and 900 0C,33this catalyst was used to study the effects of Cr loadings on the formation of surface Cr complexes. The spectrum of silica dehydrated at 450 "C has the highest concentration of isolated hydroxyl groups (3746 cm-l) and very little of the hydrogenbonded hydroxyl band (spectrum b, Figure 4A). The interaction between hydroxyl groups and the Cp ligand, which gives rise to a broad band at ca. 3600 cm-l, can be readily identified because the overlapping bands from hydrogen-bonded hydroxyls are considerably suppressed. The infrared spectra of the C-H stretching region for Cp&r/Si02(450) a t various Cr loadings are shown in Figure 5. It was found that the Cp bands grew proportionally with increasing Cr loading. The intensity of individual bands ,after deconvolution also increased linearly with Cr loading. ~~~
(33) Fu,S. L. Ph.D. dissertation, Texas A&M University, College Station, Texas, ,1990,
3200
3100
3000
3200
3100
3000
FREQUENCY (cm'l)
Figure 7. Difference spectra of (A) sublimed and (B)impregnated Cp~Cr/Si02(450) catalysts upon thermal evacuation at (a) 25 O C , (b) 100 O C , (c) 200 O C , (d) 300 OC, and (e) 500 O C . The linear dependence of the absorbance of the 3099-cm-l band on Cr loading is shown in Figure 6. The linear relationship suggests that Cr and Cp ligands from all surface complexes increase with a constant Cp/Cr ratio regardless of Cr loading. The maximum Cr loading was achieved a t ca. 10 wt % , beyond which chromocene vapor could no longer be deposited on the catalyst. This saturation was followed by the accumulation of red chromocene crystallites in the liquid nitrogen trap. Thermal Stability. An early study by Karol and Wu7 showed that thermal aging of chromocene-derivedcatalysts a t 60 "C did not influence the catalytic activity; however, aging a t 90-350 "C reduced the activity and led t o a modified catalyst having poor chain-transfer response to hydrogen in ethylene polymerization. Thermogravimetric analysis of the modified catalyst indicated that one Cp ligand per Cr site was liberated. In this study, the thermal stability of chromocene-derived catalysts was determined by heating the samples a t elevated temperatures in vacuo. Spectra of the C-H stretching region for both impregnated and sublimed catalysts treated at various temperatures are shown in Figure 7. In agreement with the previous study, thermal treatment a t 100 "C did not alter the spectra significantly, which suggests that the Cp ring remains attached to Cr up to 100 "C. At higher temperatures, the surface Cr complexes began to lose their Cp ligands, as
1780 Langmuir, Vol. 6, No. 12, 1990 indicated by the attenuation of the Cp bands. Furthermore, the intensities of the low-frequencybands (3062 and 3087 cm-l) decreased faster than the high-frequency band (3111cm-l). Since this high-frequency band was earlier assigned to the Cp ring interacting with neighboring species, it is likely that this additional interaction stabilizes the Cp ring on the catalyst. As a result, upon thermal evacuation of the chromocene-derived catalyst, the Cp ligand was released first from the isolated CpCr complexes before the loss of other Cp ligands from interacting Cr complexes. Thermal evacuation a t 500 "C removed most of the Cp ligands from the catalyst, and the remaining Cr metal dispersed on the silica wafer appeared as a shiny black mirror. The thermally modified catalysts showed a reduced activity and a different kinetic profile for ethylene polymeri~ation.3~
Discussion Infrared studies of chromocene-derived catalysts have been previously carried out by various research groups using either impregnation or sublimation methods with slight variations in preparation steps.17-19 In general, the catalysts display two groups of C-H stretching bands characteristic of olefinic and aliphatic C-H stretches, respectively. Despite a general agreement in the assignment of olefinic C-H stretching bands to the Cp ligand, the interpretations of the aliphatic C-H stretching bands are quite different. Eyring and co-workers17 attributed these bands to the adsorbed solvent, while Rebenstorf and LarssonlBsuggested that they resulted from the interaction between cyclopentadienyl hydrogens and silanol groups on the silica. Recently, Zecchina et al.19 assigned these bands to the impurities derived from the decomposition of chromocene. Based on the absence of these bands in the sublimed catalyst, and the fact that they resemble closely those of the corresponding solvent adsorbed on the silica, the present study demonstrates that the aliphatic C-H stretching bands resulted mainly from the adsorbed solvent. The presence of adsorbed solvent not only complicates the spectral interpretation but also influences the deposition of chromocene. For example, some solvents such as benzene have C-H stretching bands which overlap the Cp bands. Furthermore, the adsorbed solvent is not chemically inert on the surface. It is well established that the adsorbed solvent interacts with surface hydroxyl groups.2612' This study showed that the adsorbed solvent also interacted weakly with surface complexes as shown by the differences of the Cp bands between the catalysts prepared by sublimation and those prepared by impregnation with a chromocene/hexane solution (spectrum a, Figure 2). As the adsorbed hexane does not have olefinic C-H stretching bands, the spectral differences must be a result of perturbation by solvent. There are other solvent-induced effects such as stabilization of surface complexes or competition for adsorption sites. Karol and co-workers13 suggested that both physisorption and chemisorption of chromocene occurred on deposition of excess chromocene from decane solution onto the silica, but only chemisorption of chromocene took place when a toluene solution was used. Preadsorbed solvent also precluded the formation of an active catalyst from deposition of bis(benzenelchromium on silica-alumina.33 These problems were overcome by the use of the sublimation method. Although chromocene alone is inactive for ethylene p o l y m e r i ~ a t i o n , ' -it ~ forms an active species when supported on some oxide^.^^^ The inactivity of molecular chromocene may result from the presence of two 7-bonded Cp ligands, which obstructs the coordination of ethyl-
Fu and Lunsford Scheme I. Possible Bonding of Cyclopentadienyl Ligand to Chromium
I
Cr
Cr
I
I
B
C
I / / / / / / / I / / / / / / / I / / / / / / / A
ene. The necessary coordination sites become available when chromocene loses one Cp ligand to form a coordinatively unsaturated CpCr complex on the surface. Although the presence of this CpCr complex on chromocene-derived catalysts has been verified by different t e c h n i q u e ~ , ~ Jthe ~ - ' bonding ~ of the Cp ligand to Cr has not been studied in detail. The importance of the bonding of Cp ligand to Cr should not be neglected, as it may play a role in initiating the catalytic reaction. A t least three types of Cp bonding aze possible for the CpCr complex on the surface, which include the monohapto ($, a-bonded Cp), trihapto (s3, T-allylic Cp), and heptahapto ( 1 5 , r-bonded Cp) configurations, as depicted in A, B, and C of Scheme I, respectively. The a-bonded and 7-bonded Cp ligands can be distinguished by several characteristic spectral differences. First, the spectrum of a a-bonded CpCr complex is more complicated than that of the 7-bonded complex. The r-bonded CpCr complex possesses a local symmetry of Ch; thus, a simple spectrum of nine bands is expected. However, the symmetry of the a-bonded CpCr complex is lowered to a t best C,, resulting in a complex spectrum with all the 27 normal modes of fundamental vibrations being IR-active (15A' + 12A"). The structure of a a-bonded Cp complex is very similar to that of cyclopentadiene, differing only by the replacement of a methylenic proton by a metal; therefore, the spectra of many a-bonded Cp complexes are similar to that of ~yclopentadiene.3~-3~ Second, the a-bonded and r-bonded Cp complexes have different characteristic bands in the C-H stretching region. Only two bands are expected for the C-H stretching vibrations of a r-bonded Cp complex. These correspond to the totally symmetric A1 mode and the doubly degenerate El mode, and both are found above 3000 cm-l due to their olefinic nature. However, all five C-H stretching modes from four olefinic C-H bonds and one aliphatic C-H bond are IR-active for the a-bonded Cp complex. More importantly, the presence of aliphatic C-H stretching bands is unique to the a-bonded Cp complex. Third, the skeletal modes of C-C stretching of the Cp ring are appreciably different for K- and a-bonded Cp complexes. For the 7-bonded Cp complex, all C-C bonds are equivalent with five n-electrons delocalized in the Cp ring. Therefore, two bands are expected for these stretches around 1100 and 1400 cm-l based on a CsU~ymmetry.~3 In the case of the a-bonded Cp complex, a pair of C-C double bonds together with a C-C single bond are present in the Cp ring. The frequencies of C=C stretches are expected at higher wavenumbers around 1500-1600 cm-1,21 some 100 cm-1 red-shifted from the free C=C absorption. These bands are unique to 7-bonded Cp but may have low intensities as a result of conjugation by electron withdrawal to the metal center. Finally, the most noticeable difference between a- and 7-bonded Cp complexes is the presence (34) Davison, A,; Pakita, P. E. Inorg. Chem. 1970,9,289. (35) Cotten, F. A.; Marks, T. J. J. Am. Chem. SOC.1969,92, 728. (36) Gallinella, E.; Fortunato, B.; Mirone, P. J. Mol. Catal. 1967,24,
345.
Langmuir, Vol. 6, No. 12, 1990 1781
Characterization of Chromocene on Si Catalysts
of a strong band at ca. lo00 cm-I unique to the latter species and corresponding to a C-H in-plane bending mode. However, this band is overlapped by the strong silica lattice absorptions, and its presence in this study can only be inferred from weak overtones or combination bands. The present study suggests that the Cp ligand is n-bonded to Cr on the basis of the aforementioned criteria and the infrared results. The infrared spectrum of chromocene-derived catalysts is relatively simple and closely resembles that of other a-bonded CpCr complexes.23 Although the number of the olefinic C-H stretching bands is more than that expected for C5" because of a lowered symmetry and the perturbation by neighboring groups, the absence of aliphatic C-H stretching bands strongly supports the n-bonded nature of Cp ligands. Furthermore, the frequencies of the C-C stretching bands are in agreement with those of n-bonded Cp ligands. No bands corresponding to C=C stretches of a a-bonded Cp ligand were found at a higher frequency. The absence of the C = C stretching bonds also eliminates the possibility of a nallylic Cp structure. Since the Cp ring is relatively fluxional, Le., fast exchange between structures in Scheme I (ring the possibility of changes in the bonding of Cp ligands under polymerization conditions is not ruled out. In contrast to the Phillips catalyst,16a commercial silicasupported inorganic chromium catalyst (Cr/SiOz) which requires a high-temperature (ca. 850 "C) activation step, the chromocene-derived catalyst does not need an activation step. The anchoring of Cr complexes to the support is presumably achieved by the reaction of Cp ligand with hydroxyl groups at room temperature. The decrease in the intensity of the 3745-cm-' band, as shown in Figure 4, and the formation of cyclopentadiene, as demonstrated by Karol et EL^.,^ confirm the participation of hydroxyl groups in the reaction with chromocene. The present study also suggests that the adsorption of the released cyclopentadiene is very limited. Cyclopentadiene has a structure similar to a a-bonded Cp ligand (Scheme IC), which would contribute additional C-H stretching bands if adsorbed on the catalyst.38 In this study, care was taken in acquiring the spectrum of adsorbed cyclopentadiene because of its rapid dimerization. The spectrum was obtained by adding a small amount of cyclopentadiene dimer to a preevacuated IR cell which contained a dehydrated silica wafer. A t such a reduced pressure, the dimer was favorably dissociated into the monomer which was immediately adsorbed on the silica. The resulting spectra (Figure 8) show different bands in the C-H stretching region compared to those of the n-bonded Cp ligand. The absence of these bands in the spectrum of the catalyst indicates that a negligible amount of cyclopentadiene was adsorbed on the surface, in agreement with a previous study which showed that less than 3-4Ob of the released cyclopentadiene was a d ~ o r b e d . ~ It is notable that the isolated hydroxyl band of Cp2Cr/ SiO2(900) and the interacting hydroxyl band of Cp2Cr/ SiOz(450)were observed even at the maximum Cr loading, indicating that not all the hydroxyl were reactive toward chromocene. The presence of unreacted hydroxyls may result from the reduced reactivity and, to a lesser extent, from the inaccessibility of these hydroxyls. The decreased reactivity of interacting hydroxyls of Cp2Cr/Si02(450) is attributed to their interaction with neighboring Cr (37) van Raaij, E.U.; Brintzinger, H. H. J . Organomet. Chem. 1988, 356, 315. (38) Fefer, M.; Small, A. B. In Comprehensioe Organometallic
Chemistry; Wilkinson, G.,Stone, F. G . A,, Abel, E. W., Eds.;Pergamon Press: New York, 1982; Vol. 7, p 417.
2935
w
0 2
3K
sm a
3100
3000
2900
2800
FREQUENCY (cm-') Figure 8. Difference spectra of (a) cyclopentadiene vapor adsorbed on silica dehydrated at 450 O C and (b) after 10h evacuation of the sample used in a.
complexes, which prohibits a further reaction with chromocene. However, the reactivity of isolated hydroxyl groups on Cp&r/SiO2(900) was mainly reduced by the surface and structural changes after high-temperature treatment. Ghiotti et al.39found that a highly distorted surface was formed upon outgassing silica at a temperature higher than 600 "C. The acidity of hydroxyls was also affected by the high-temperature d e h y d r a t i ~ n .These ~~ changes may be responsible for the decreased reactivity of hydroxyl groups. Furthermore, as some hydroxyl groups are located in the interior of small pores, they may not be accessible to chromocene. Davison 952 silica gel has a significant number of small pores, 30-100 A in diameter.l6 Sintering of silica structure at high temperatures can further increase the amount of small pores. In addition, some pores with a small opening, such as ink-bottle shape pores, can be easily clogged by the adsorbates; therefore, the hydroxyls in the interior of the pores will not be accessible to chromocene. Nevertheless, since the molecular dimensions of chromocene are 4.862 X 5.170 A,@ the inaccessibility of hydroxyls should be less important than the reduced reactivity for the presence of unreacted hydroxyls. Zecchina and c o - w o r k e r ~also ~ ~ observed the presence of unreactive hydroxyl groups and attributed them to a kinetic (diffusion-controlled)effect based on the limited penetration depth of chromocene into silica wafer. They proposed a model in which the catalyst wafer was divided into three regions from the outer surface to the inner bulk, each having different concentrations of Cr complexes and hydroxyl groups. The interior region, beyond the reach of chromocene, was white in color and contained only the unreacted hydroxyls. The outer red-colored region was saturated with CpCr complex and molecular chromocene but no hydroxyl groups. In the boundary region between the inner and outer layers, the unreacted hydroxyls and CpCr complex coexisted. However, Zecchina's model is not compatible with the present study, in which a very thin silica wafer was used for chromocene deposition. Chromocene impinging on both sides of wafer easily penetrated to the center. A cross-section cut of a catalyst wafer did not reveal any white region. The catalyst displayed a homogeneous grey/ black color with occasionally a hint of red, depending upon the Cr loading. (39) Ghiotti, G.;Garrone, E. Morterra, C.; Boccuzzi, F. J.Phys. Chem. 1979,83, 2863. (40) Zhuravlev, L. T. Langmuir 1987,3, 316.
1782 Langmuir, Vol. 6, No. 12, 1990
Fu and Lunsford
Scheme 11. Some Proposed Configurations of Agglomerates of Cyclopentadienyl Chromium Complexes on Highly Dehydroxylated Chromocene-Derived Catalysts
Scheme 111. Some Proposed Configurations of Agglomerates of Cyclopentadienyl Chromium Complexes on Slightly Dehydroxylated Chromocene-derived Catalysts
l l / l / l / / / / / I / / / / / / / // / / / / / / / / I / / / / C
B
A
Therefore, the reactivity of hydroxyl groups is temperature dependent rather than kinetic. It is noted that Zecchina's catalyst was prepared from silica dehydrated at 700 "C, which contains mainly isolated hydroxyl groups on the surface (Figure 1,ref 12). The reactivity of these isolated hydroxyls toward chromocene is expected to be small because of the high-temperature treatment, similar to Cp2Cr/Si0~(900)observed in this study. It is important to stress that all the isolated hydroxyl groups reacted on CpzCr/Si02(450) (Figure 4), which would be very unlikely based on the kinetic-effect model. Once the reactive hydroxyl sites are coordinated with CpCr complex, excess chromocene can be molecularly adsorbed on the same sites. Some possible configurations are shown in Scheme 11. The presence of molecular chromocene can be verified by comparing the maximum Cr loading between the experimental and calculated values. Silica dehydrated a t 450 "C has a surface hydroxyl concentration of 2-2.5 OH/nm2.16v41On the basis of this hydroxyl concentration, the deposition of chromocene according to eq 1 would yield a maximum Cr loading of 5.2-6.5 wt Y . This loading, however, is overestimated, as only a fraction of hydroxyl groups is reactive toward chromocene, and some chromocene may react with two hydroxyl groups via eq 2
A
E
C
complexes as depicted in Schemes I1 and 111. As CpzCr/ Sioz(450) had more surface Cr species than CpzCr/SiOz(goo), more molecular chromocene was adsorbed on the former catalyst. This conclusion differs from a previous s t ~ d y which ,~ suggested that chromocene can be adsorbed on the surface by an interaction between the empty d orbital of Cr and the unpaired electrons of siloxane groups. The nature of the bonding (the dotted lines) between molecular chromocene and t h e surface Cr complexes is not clear. It is speculated that the weakly physisorbed chromocene is held by van der Waals forces, while the chemisorbed chromocene involves an interaction between Cr d orbitals and ?r electrons in the Cp ligands. In any event, these types of bonding are by no means stronger than the T bonding (the solid line) of the CpCr complex. The linear relationship between the growth of Cp bands and the Cr loading suggests that the formation of the CpCr complex and the adsorption of molecular chromoceneoccur simultaneously rather than consecutively throughout the sublimation process. As the CpCr complex and adsorbed chromocene (CpzCr) have different numbers of Cp ligands per Cr, the absorbance of the Cp bands per Cr for the former species is expected to be one half of that of the latter. Therefore, if the formation of the CpCr complex is predominant a t the early stage of the sublimation process Cp2Cr + SiOH SiO-CrCp + CpH (1) which is followed by chromocene adsorption, a slope change is expected as the deposition of additional Cr complexes changes from predominantly CpCr species to molecular Cp,Cr 2SiOH (SiO),-Cr + 2CpH (2) chromocene. This is in contrast, however, to the observed constant slope as shown in Figure 6. The simultaneous The present study shows that higher loadings, up to ca. CpCr formation and chromocene adsorption ensure the 10 wt O b , can be actually achieved for CpzCr/SiOz(450) presence of both species regardless of Cr loading. The by extending the sublimation time. This implies the difference between the observed maximum Cr loading (ca. presence of additional Cr complexes a molecular chro10 wt % ) and the calculated values (5.2-6.5 wt 76)implies mocene. Indeed, a small fraction of this excess chrothat each surface CpCr complex may take up one excess mocene was weakly physisorbed and could be removed by chromocene. However, since not all the hydroxyl groups thermal evacuation a t 60 "C. Catalysts prepared by are reacted, a higher ratio of CpzCr/CpCr is possible. impregnation methods were also found to contain excess Karol et a1.4 found that the stoichiometry of the surface chromocene, the amount of which was dependent upon the reaction between chromocene and hydroxyl groups was impregnation temperature and the s01vent.I~The presence dependent upon the dehydration temperature of the silica. of molecular chromocene has been recently verified by Zecchina et al. using UV-vis-NIR reflectance spectros~opy.~~ The authors observed that cyclopentadiene evolution decreased from 86 7; to 43 % of the total Cp ligands when It was found that the amount of molecular chrothe dehydration temperature of silica gel (Davison 952) mocene was greater on CpzCr/Si02(450) than on Cp,Cr/ was increased from 25 to 800 "C. They suggested that SiOz(900). Since the surface area of the silica was only chromocene could either react with two hydroxyl groups slightly reduced by sintering at 900 "C, the smaller amount and lose two Cp ligands to form an exposed Cr species or of the adsorbed chromocene on CpzCr/Si02(900) resulted react with one hydroxyl group with the loss of one Cp from changes in the nature of the surface. Silica dehyligand to form a CpCr complex, as is expressed in eqs 2 drated a t 900 "C had a high concentration of siloxane and 1, respectively. The former pathway (eq 2) was groups and very little chromocene, suggesting that the predominant for silica dehydrated at low temperatures, adsorption of chromocenewas negligible on siloxane groups. while the latter (eq 1) was more important a t higher Instead, chromocene was adsorbed on the surface Cr temperatures. The authors suggested that both Cr species are active for polymerization, with the CpCr complex being (41) Haaland, A. Top. Curr. Chem. 1975,53, 1. (42) Fritz, H.P.Chem. Ber. 1959, 92, 780. more active. One might expect that silica dehydrated at (43) Mar'in, V. P.; Druzhkov, 0. N.; Andrianov, Yu. A.; Arsen'eva; the lower temperature would have less intense Cp bands; Grinval'd. I. I. Zh. Obshch. Khim. 1980.50. 1830. however, the present study shows similar intensities for (44) Galkin, G. A.; Kiselev, A. V.; Lygin; V. I. Russ. J. Phys. Chem. both Cp2Cr/Si02(450) and Cp&r/Si02(200) catalysts at 1962, 36 (8),951.
+
-
Characterization of Chromocene on Si Catalysts
Langmuir, Vol. 6, No. 12, 1990 1783
the same Cr loading. Therefore, the molecular chrobe eliminated by using a sublimation method. Chromocene is also adsorbed on the exposed Cr sites of CpzCr/ mocene reacted with isolated and, to a lesser extent, Si02(200) with the possible configurations shown in Scheme hydrogen-bonded hydroxyl groups to form CpCr complexes 111. The spectral differences between CpzCr/Si02(450)and and exposed Cr species. The released cyclopentadienewas Cp,Cr/Si02(200) in the C-H stretching region (Figure 4C) not significantly adsorbed on the catalyst. However, support a different predominant Cr species and a high molecular chromocene was simultaneously adsorbed on degree of hydroxyl interaction on the latter surface. surface Cr complexes to form polynuclear Cr species Although there are different Cr species on the chroregardless of Cr loading. A small amount of chromocene mocene-derived catalysts, the Cp ligands of these species was weakly adsorbed and can be removed by evacuation remain r-bonded to Cr. I t is suggested that large at 60 "C. Most of the surface Cr complexes were stable agglomeratesof cyclopentadienylchromium complexes are up to 100 "C in vacuo. Not all the hydroxyl groups were predominant on the surface. The presence of polynureactive toward chromocene, and the reactivity of isolated clear Cr species, as suggested in Schemes I1 and 111, is hydroxyl groups decreased with increasing dehydration supported by the predominance of bridging CO bands upon temperature. Consequently, the amount of Cr complexes CO adsorption on chromocene-derivedc a t a l y s t ~ . ~The ~J~*~ that may be formed on the surface, excluding the weakly tendency to agglomeration, which limits the available bound molecular chromocene, decreased with increasing coordination sites on Cr, results in inactive species for dehydration temperature of the silica. The maximum ethylene polymerization. Indeed, less than 1wt % of the amount of Cr deposited on silica dehydrated at 450 "C total Cr is a ~ t i v e . ~Presumably, 3 the active center is an corresponded to ca. 10 wt 7; Cr. Siloxane groups were unisolated monomeric CpCr complex. However, as only a reactive toward chromocene under deposition conditions. small percentage of Cr is active, the infrared spectra will The Cp rings on the catalyst are r-bonded to Cr and not reflect the nature of the active sites. Nevertheless, the interact with neighboring species such as Cr metal or spectra provided valuable information on the formation hydroxyl groups. of chromocene-derived catalysts and the interaction of Cr complexes on the surface. Acknowledgment. This work was partially supported Conclusion by the Robert A. Welch Foundation under Grant A-257. The preparation of chromocene-derivedcatalysts by the A fellowship for S.-L. Fu was provided by the Phillips impregnation method introduced solvent effects which can Petroleum Co.
