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Langmuir 1991, 7, 1172-1178
Chemistry of Organochromium Complexes on Inorganic Oxide Supports. 3. Interactions of Nitrogen Oxides with Chromocene on Silica Catalysts Shi-Liang Fu and Jack H. Lunsford' Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received July 26, 1990. In Final Form: November 19, 1990
The interactions of nitrogen oxides with chromocene on silica catalysta were studied by using Fourier transformed infrared spectroscopy. The adsorptionof NO on the catalysts yielded intense N-0 stretching bands at 1804and 1696 cm-1 and weaker bands at 1778and 1680cm-1 that are characteristic of dinitrosyl complexes. In addition, a weak band at 1533 cm-1 is attributed to a bridging nitrosyl. The weak band of a mononitrosyl complex at 1660cm-l also was observed. The formation of these species was dependent upon the NO pressure, with mononitrosyls being predominant at a low pressure (0.02Torr) and dinitrosyls at higher pressures. These nitrosyl complexes were very stable on the surface. The NO ligands could not be removed by room temperature evacuation, nor could they be oxidized or displaced by 0 2 or CO. Nitric oxide displaced the preadsorbed CO or coadsorbed with CO to form CpCr(C0)2(NO),CpCr(NO), and CpCr(NO)z complexes. The formation of various coordination complexes verified the coordinatively unsaturated nature of the catalysts. On the basis of the N-0 stretching frequencies of various nitrosyl complexes on the catalysts, the valence of Cr is approximatelyCr6+. The nitrosyl complexes did not react with 0 2 but exposure to gas phase NO2 resulted in the formation of N,O species. Correspondingly, Cr and the Cp ligands were oxidized. The interaction of nitrous oxide with the catalysts was very weak, and thus no surface complexes were formed.
Introduction Chromocene (bis(cyclopentadienyl)chromium,Cp2Cr) deposited on a dehydrated silica forms an active catalyst (hereafter referred to as chromocene-derivedcatalysts) for ethylene polymerization.'-3 The importance of chromocene-derived catalysts for the commercial production of polyethylene has prompted a few fundamental ~tudies;~-lO however, the surface chemistry of these catalysts remains largely unexplored. Of particular interest is the coordinatively unsaturated nature of the catalysts, which is responsible for the catalytic activity. The coordinative unsaturation of surface Cr allows chromocenederived catalysts to interact with simple probe molecules, such as CO and NO. The resultant complexes can be readily characterized by spectroscopic techniques, which provide valuable information as to the nature of the catalysts.11112 Nitric oxide is one of the most common ligands in transition-metal c~mplexes.'~Nitric oxide is also readily adsorbed on many supported metals or metal oxides to form various surface species, such as nitrosyl and nitro complexes.12 The formation of these species and the (1) Karol, F. J.; Karapinka, G. L.; Wu, C.; Dow, A. W.; Johnson, R. N.; Carrick, W. L. J. Polym. Sci., Part A-1 1972, 10, 2621. (2) Karol, F. J.;Wu, C.; Reichle, W. T.; Maraschin, N. J. J. Catal. 1979,
60, 68. (3) Kozorezov, Yu. I.; Aleksandrova, A. L. Plast. Massy 1983,11,51. (4) Karol, F. J.; Brown, G. L.; Davison, J. M. J. Polym. Sci., Polym. Chem. Ed. 1973, 11,413. (5) Karol, F. J.; Wu, C. J. Polym. Sci., Polym. Chem. Ed. 1974, 12, 1549. (6) Freeman, J. W.; Wilson, D. R.; Ernst, R. D.; Smith, P. D.; Klendworth, D. D.; McDaniel, M. P. J.Polym. Sci.,Part A Polym. Chem. 1987, 25,2063. (7) Zecchina, A.; Spoto, G.; Bordiga, S.J.Chem Soc., Faraday Discuss. 1989, 87, 149. (8) Armstrong, D. R.; Fortune, R.; Perkin, P. G. J. Catal. 1976,42,435. (9) McKenna, W. P.; Bandyopadhyay, S.;Eyring, E. M. Appl. Spectrosc. 1984,38, 834. (10) Rebenstorf, B.; Larsson, R. J. Mol. Catal. 1981,11, 247. (11) Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Marcel Dekker: New York, 1967. (12) Kung, M. C.; Kung, H. H. Catal. Reu.-Sci. Eng. 1985,27, 425. (13) Lukehart, C. M. Fundamental Transmission Metal Organometallic Chemistry; Wadsworth: Belmont, CA, 1985.
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frequencies of N-0 stretching are highly dependent upon the nature of the adsorbent. Therefore, NO has been extensively used as a probe molecule to characterize the supported metal catalysts.12 Nitric oxide adsorption is also a useful complement to CO adsorption, as the N-0 stretching frequency is more sensitive to the changes in the nature of the metal center than the CO frequency.12 The surface chemistry of the interactions between chromocene-derived catalysts and carbon oxides has been previously investigated in this series of studies by using Fourier transformed infrared spectro~copy.'~In the present study, chromocene-derived catalysts were further characterized by their interactions with various nitrogen oxides. The effects of NO pressure and silica dehydration temperature on NO adsorption were studied, as were the thermal stability of the resulting complexesand the effect of oxygen addition. In addition, the reaction between NO2 and chromocene-derivedcatalysts, the coadsorption of NO with 02,CO, and NzO, and the adsorption of NO on catalysts with preadsorbed CO were also studied. Isotopic mixtures were used to aid in the interpretation of the infrared results. The spectral information was integrated and compared to various homogeneous analogues to elucidate the nature of these catalysts.
Experimental Section Materials. Davison 952MS silica gel having a surface area of 300m2/g was used as the catalyst support. Chromocene (Strem Chemicals, sublimed) was purified by sublimation at 40 "C in vacuo ( 4 0 4Torr). Carbon monoxide (Matheson, UHP grade, 99.8%)was passed through a Cru/Si02 column to remove trace amount of oxygen prior to use. The NO2 and N204 impurities were removed from NO (Matheson,CP grade, 99%) by vacuum distillation using both a n-pentane slush bath (-131"C) and a liquid nitrogen bath (-196"C). This was followed by several cycles of freeze-pump-thaw at liquid nitrogen temperature to remove the more volatile impurities. Nitrogen dioxide was prepared by oxidizing the purified NO with 02,which was followed by the same purification technique as was used for NO. A small fraction of NO2 was dimerized to form N204 with characteristic (14) Fu, S. L.; Lunsford, J. H. Langmuir 1990, 6, 1774 and 1784.
0 1991 American Chemical Society
Interactions of Nitrogen Oxides with Chromocene bands centered at 1750 and 1260 cm-l.l5 The 1:l N160/NW mixture was prepared from NO via an oxidation-reduction cycle using 1 8 0 2 as an oxidant. In this cycle, N W was first oxidized by a stoichiometrically excess amount of l802. After the unreacted '802 was removed,the isotopicmixtures of NO2 were reduced by chromocene-derivedcatalysts (seeResults and Discussion)to form a 1:l mixture of N180/N180, which was spectroscopically verified by the intensity of the respective Q bands at 1876 and 3 nitrous oxide, N15N0 (MSDIsotopes, 1827~ m - ~ . l 516N-labeled 99%), 0 2 (UHP grade, 99.99%),and l*Oz (Miles, 98.92% 180, 0.088% 170)were used without further purification. Experimental Techniques. Chromocene-derivedcatalysts were prepared by deposition of chromocene onto a thin silica gel wafer (ca. 4 mg/cm2) using a sublimation technique that has been described previ0us1y.l~Unless otherwise stated, silica gel wafers were dehydrated at 450 O C over a period of 10 h prior to chromocene deposition, and the resulting catalysts had a Cr loading of ca. 4 w t % . Each adsorption process or reaction was carried out at the IR beam temperature (ca. 30 "C). Normally a pressure of 4.5 Torr was used for reaction/chemisorption, and 0.02 Torr was used in the low-pressure experiments. Infrared Measurement. A special quartz cell was used for catalyst preparation and subsequentin situ infrared spectroscopic studies.14Infrared spectra were recorded by usinga Digilab FTS40 FTIR spectrometer equipped with a He-Ne laser (at 632.8 nm) and an MCT detector (at -196 "C). All spectra are reported as difference spectra, attained by subtraction of the catalyst spectrum from the spectrum following chemisorption/reaction. Spectra shown in the same figure are offset to various degrees for a better display and comparison. The frequencies of overlapping bands were determined by deconvolutionor by taking the second derivativeof the differencespectrum. Both processes were performed by the Data Station 3200 software, which manipulates IR data in the Fourier-transformeddomain.l'J8The spectral resolution was taken at 2 cm-l, and the frequency was calibrated against a polystyrenefilm. Allspectrawere normalized to a similar wafer density of ca. 4 mg/cm*.
Results and Discussion
NO Adsorption. Spectral Assignment. Nitric oxide did not adsorb on dehydrated silica (at 450 "C) under the experimental conditions; therefore, the adsorption of NO was limited to surface Cr. Previous studies117J4have shown that at least three types of Cr complexes are present on the chromocene-derived catalysts, correspondingto chromocene losing two, one, and none of its Cp ligands. These complexes are not isolated from one another; instead, they agglomerate to form polynuclear complexes on the surf a ~ e . ~The J ~infrared spectra in the N-0 stretching region of NO adsorbed on these complexes are shown in Figure 1 (spectra a and b), which displays two intense bands at 1804 and 1696 cm-l, corresponding to the N-0 stretches of nitrosyl c o m p l e x e ~ . ~ ~ JUnlike 3 J ~ the adsorbed CO which rearranged slowly,7J4 these nitrosyls were very stable on the surface. The initial adsorption rate of NO was also greater than that of CO; ca. 90% of the full intensity of the nitrosyl bands was attained within 1min. Deconvolution of these nitrosyl bands showed that each band was a composite comprised of an intense band at a higher frequency and a weak band at a lower frequency (spectra c and d, Figure 1). The resulting four bands are classified into an intense band pair a t 1804/1698 cm-l and two weak bands a t 1778 and 1680 cm-l. The proportional growth of the intense band pair at 1804/1698 cm-' and the tendency of NO to adsorb in pairs suggest that they belong (15) Nightingale,R. E.; Downie,A. R.; Rotenberg, D. L. J.Phys.Chem. 1954,58, 1047. (16) Ohlsen, J. R.; Laane, J. Prog. Inorg. Chem. 1980,27, 465. (17) Cameron, D. G.;Moffatt, D. J. J. Test. Eual. 1984, 12, 78. (18) Kauppinen, J. K.; Moffatt, D. J.; Mantach, H. H. Appl. Spectrosc. 1981, 35,271. (19) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley-Interscience: New York, 1986.
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Figure 1. Difference spectra of NO adsorbed on CpzCr/SiO2(450) catalyst after (a) 0.5 min and (b) 100 min. (c) Second derivative of spectrum b. (d) Deconvolution of spectrum b.
to a strongly coupled dinitrosyl complex.12fl The bonding between NO and Cr is similar to that of the adsorbed CO, which involves a u bonding (from a 5u orbital of NO to vacant d orbitals of Cr) and 7r backbonding (from filled d orbitals of Cr to 27r orbitals of NO).12J3 However, NO is the only diatomic molecule that has an unpaired electron in ita 277 antibonding orbitals. Thus, NO has a greater tendency than CO to be adsorbed in pairs. Zecchina et al.20pointedout that greater stability was achieved through the mutual interaction of the unpaired electron on each NO. The formation of a dinitrosyl complex is further supported by comparing the observed frequencies with those of homogeneous model compounds, namely the cyclopentadienylchromium dinitrosyl complexes (CpCr(N0)zL in Table I). It is apparent from Table I that the N-O stretching frequencies are dependent upon the nature of the ligand L. The complexes with a neutral ligand L, Cp(Cr(NO)ZL,have lower N-0 stretching frequencies than those with an anionic ligand L-, CpCr(NO)Z+L-,as a result of less 7r backbonding for the latter complexes which have the positive charge localized in the Cr metal. By analogy with dinitrosyls on transition metals,l2the high-frequency band is assigned to the symmetric N-0 stretching and the lower frequency band to the asymmetric stretching. Since Cr-N-0 with a bent structure generally has a lower N-0 stretching frequency (
