Furuduy Discuss. Chem. SOC., 1989, 87, 149-160 Interaction of Chromocene with the Silica Surface, and Structure of the Active Species for Ethene Polymerization Adriano Zecchina,” Giuseppe Spoto and Silvia Bordiga Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali dell’ Universita di Torino, V. P. Giuria 7, 10125 Torino, Italy The anchoring process of Cr(Cp)? on silica hydroxyl groups occurs by elimination of CSH, and formation of SSi-0-CrCp mononuclear species. These anchored species are then able to adsorb incoming Cr(Cp), to give catalytically inactive dimeric species. These reactions are heavily diffusion- controlled. Interaction of CO and NO with mononuclear anchored species gives well defined dicarbonylic and dinitrosylic compounds, while the reac- tion of CO with dinuclear (catalytically inactive) species gives much more complex polycarbonylic compounds.The SSi-0-CrCp mononuclear surface species are the active sites for ethene polymerization. The chain- initiation mechanism probably consists of the formation of a metallocyclic structure. Many mechanistic and structural problems associated with ethene polymerization on supported chromium catalysts [Phillips Cr/ Silica and Union Carbide Cr( Cp)’/ Si02 catalysts]’-’ are far from being completely understood. In particular the structure of the active centres and of the mechanism of chain initiation and propagation on the Cr/Silica (Phillips) catalyst have been debated for over 30 years.‘” Less attention has been paid to the Cr(Cp),/SiO, (Union Carbide) catalyst, as only a few contributions can be found in the more recent literature.’ Our group has extensively studied the surface chemistry of the Cr/SiOz system in the hope of elucidating the structure of the catalytic centres and understanding the major features of the polymerization mechanism.’ However, even on simplified versions of the Phillips catalyst, many problems remain unresolved and require further experi- mental efforts.In view of the similarity of the Phillips and Union Carbide catalysts (same support, same metal in identical, or similar, oxidation state) we thought that, in addition to continuing work on the Cr/ Silica system, some general information concern- ing common (and hence general) features could be achieved from a comparison of the two catalysts.In this paper we report our first spectroscopic results on the structure of the active sites in the Union Carbide catalyst and on the polymerization of ethene occurring on them at ca. 320 K. Experimental The Cr(Cp),/Si02 catalyst was prepared directly in a suitably designed i.r. cell by room-temperature vacuum sublimation and subsequent adsorption from the gas phase of Cr( Cp), onto an SiO, pellet (Aereosil, surface area 380 m’ g-I) previously outgassed at 973 K under high vacuum (lo-’ Torrt). t 1 Torr = 101 325/760 Pa. 149150 Chromocene on Silica and Ethene Polymerization 1.0 e, C f sl % 0.1 4( 10 3500 3000 2500 2000 1500 1000 500 wavenumber/cm-' Fig. 1. ( a ) 1.r. spectrum of a silica sample outgassed at 973 K. ( h ) 1.r. spectrum of the same silica sample contacted with Cr( Cp), vapour.The i.r. spectra of the adsorbent and of the adsorbed species have been recorded in transmission mode with a Bruker IFS 113V F.t.i.r. spectrometer (4 cm-' resolution). The u.v.-visible-n.i.r. spectrum has been recorded in the diffuse reflectance configur- ation on a Varian 2390 spectrophotometer equipped with a diffuse reflectance attachment. Results and Discussion The Anchoring Process In fig. 1 the spectrum of a silica sample activated in L'acuo at 973 K is shown together with that of the same sample after interaction with Cr(Cp), (from the gas phase). The following can be seen: (i) the narrow peak at 3748 cm-', which is due to the stretching mode of isolated surface hydroxyl groups of silica, is greatly but not completely eroded; (ii) a low intensity and broader (AV,,,,== 100 cm- ' ) band centred at 3590 cm-' is formed. Owing to its position and half-width, this band can safely be attributed to perturbed (hydrogen-bonded) OH groups; ( i i i ) new peaks appear at 3099 cm-' (weak and complex) and 1424 cm-' (very weak).On the basis of the literature datax-'" they are assigned to the v ~ . ~ and vCc- modes, respectively, of a Cp ring. In some experiments, additional weaker peaks were observed in the 3000-2800 and 2100-1950 cm-' regions. Because they were not present when fresh chromocene was used, we conclude that they are associated with small variable amounts of impurities derived from the decomposition of Cr(Cp),. As the reactivity of the SiO,/Cr(Cp), system towards CO, NO and C2H4 was the same in either presence or their absence, they will not be further discussed.In a few experiments a large excess of chromocene was dosed by increasing the time of exposure of the sample to the Cr(Cp), vapour; however, we never succeed in consuming all the OH groups of the surface.A. Zecchina, G. Spoto and S. Bordiga 151 The disappearance of the silanols and the appearance of the Cp- vibrations can undoubtedly be explained by the reaction: SSi-OH +Cr(C,H,), - ZSi-OCrC5H5+C5H6 ( 1 ) whi -h has already been proposed for the interaction of surface silanols with chromocene in organic s ~ l u t i o n s . ' ~ ~ The incomplete consumption of the OH groups, even under an excess of reactant, is not a consequence of incomplete availability of the hydroxyls towards the interaction with gaseous rnolecules (for instance because of their location in narrow pores).In fact it has been known for many years"." that the free OH groups of Aereosil can interact with adsorbed species. In the following, we shall develop considerations which demonstrate that this is a kinetic (diff usion-controlled) effect. In fact the heavy Cr(Cp), vapour impinging the pellet from both sides (see scheme 1, where a section of the pellet is shown) is immediatelj adsorbed (both chemically and physically) onto the surface of the microparticles, and can thus form a penetration front which divides the pellet into two parts. The first part is fully saturated, or even super-saturated, by the Cr(Cp), vapour and is characterized by the presence of both anchored [following reaction( 1 ) ] and weakly and/or physically adsorbed chromocene.In this saturation region all the OH groups have been consumed. The second part is characterized by absence of penetration of the vapour: the surface of the silica microparti- cles is completely free from adsorbed chromocene and the surface silanols are unreacted. Between these two regions a narrow interface can exist where the Cr concentration changes abruptly from supersaturation values to zero. On this basis it is evident that the intensity of the unreacted OH groups gives information about the extension of the Cr( Cp),-free region of the pellet. In the boundary layer, dividing the super-saturated and clean regions, unreacted OH and anchored chromocene coexist: only in this narrow region does the possibility exist of finding unreacted silanols interacting with adjacent anchored Cr(Cp), via hydrogen bonding. If the boundary layer is very thin, the hydrogen bonding will involve only a very small fraction of hydroxyl groups (as observed i n the experiment). As a matter of fact, if after Cr(Cp), adsorption the silica pellet is cut perpendicular to the main faces, the presence of an inner white part and of external red-coloured layers is immediately noted.Moreover, the boundary between the regions is sharp. On this basis we consider the previously advanced hypothesis as the correct one, as it explains simultaneously: ( i ) the incomplete OH consumption; (ii) the small hydrogen-bonding effects; ( i i i ) the existence of incomplete diffusion and the appearance of regions which are red and white. The presence in the external layers of both reacted and unreacted chromocene can be confirmed by u.v.-visible-n.i.r.reflectance spectroscopy (a typical spectrum is shown in fig. 2). In fact, by comparison with the known spectrum of molecular Cr(Cp), in homogeneous conditions,'3 the peaks at ca. 29 000 (shoulder) and 24 000 cm-' are assigned to the L- M (charge transfer) and d-d transitions of unreacted or weakly perturbed chromocene. The remaining bands at 36000 (strong) and ca. 18 000cm ' (weak and broad) are assigned to analogous L - M and d-d transitions of the anchored clean pellet Scheme 1 front152 Chromocene on Silica and Ethene Polymerization I I 1 5 4 3 2 1 wavenumber/ lop4 cm-' Fig.2. U.v.-visible-n.i.r. reflectance spectrum of Cr(Cp), adsorbed on Si02 outgassed at 973 K. species. Owing to the lower strength of the total ligand field in the anchored species (vide infra), the d-d transition occurs at lower frequency. The existence of weakly adsorbed chromocene in the external layers of the pellet is also confirmed by a simple final outgassing experiment. In fact, if after adsorption at room temperature, the sample is outgassed under mild conditions (323 K), a fraction of weakly adsorbed chromocene is desorbed and is found unmodified as a dark-red (the colour characteristic of the chromocene microcrystals) condensation ring on the internal wall of the liquid-nitrogen trap of the vacuum line. At the same time the intensity of the i.r. modes of the Cp- ring decreases and the band due to weakly perturbed OH groups disappears. On the basis of all these considerations, scheme 2 is proposed for the external regions of the silica surface after interaction with Cr(Cp), where the weakly adsorbed Cr(Cp), is supposed to interact weakly with the anchored, highly coordinatively unsaturated, CrCp moiety to form more fully coordinated species. The weakness of this interaction explains the observed downward shift of the d-d transition of the anchored species with respect to Cr(Cp),.In the boundary regions (scheme 3) several species coexist, including hydrogen- bonded species and coordinatively unsaturated Si -0-CrCp groups. Cr(Cp), does not penetrate the inner regions (scheme 4) because of incomplete diffusion. Schemes 2 and '4 predominate.Scheme 2A. Zecchina, G. Spoto and S. Bordiga 153 Scheme 3 /H 0 I /H 0 I /H 0 I Scheme 4 Desorption of Cr(Cp), under mild conditions increases the fraction of coordinatively unsaturated species with respect to fully coordinated species. As will be shown later, this process is far from being complete at 323 K. Moreover, the process cannot be forced to completion by using higher outgassing temperatures because decomposition processes begin to occur and these complicate the situation instead of simplifying it (results not reported for the sake of brevity). The Structure of the Adsorbed Species (as probed by CO and NO) Adsorption of CO on a sample previously contacted with Cr(Cp), at room temperature gives the spectrum illustrated in fig.3( a ) , while the adsorption of CO on a sample which was first contacted with Cr(Cp)2 and then evacuated at 323 K, in order to remove part of weakly adsorbed chromocene, is shown in fig. 3(6). On the basis of the considerations developed above, the two samples differ in the relative amounts of the species (a) and ( b ) in scheme 5 whose concentrations are C, >> Cb for the first case and C,, 3 C, for the second case. On this basis, it is expected that the spectrum of the first sample should be dominated by the bands associated with the carbonylic complexes mainly derived from ( a ) by reaction with CO, while the spectrum of the second sample should be dominated by the bands of the carbonylic complexes derived from species (6) and/or from ( a ) by expulsion of Cr(Cp), [a process which could be favoured by lower surface concentration of weakly bound Cr(Cp),] (vide infra).A band pair at 2054 and 1985 cm-' predominates in fig. 3(6), while a sextet at 1920, 1895, 1832, 1775, 1628 and 1579 cm-' is more abundant in fig. 3(a). As the intensity of the components of the doublet at 2054 and 1985 cm-' decreases or increases in a strictly parallel way by decreasing or increasing the CO pressure in the 40-0 Torr interval, it is inferred that they are associated with a single simple reversible complex, having cis- dicarbonylic structure, derived from ( a ) and/or (6) species as shown below: tCO -CO SSi-0-CrCp SSi-O-CrCp(CO)2 SSi-O-CrCp..-CpCrCp SSi-O-CrCp(CO),+ CpCrCp. Although it is probably less important, the second reaction must be considered also, because in a CO atmosphere the equilibrium of the ligand-substitution reaction could be shifted to the right simply by mass-action effects when the surface concentration of weakly adsorbed Cr(Cp), is not too large.(2) tC'O -CO154 0.5 e, E: -2 2 D 0.0 Chromocene on Silica and Ethene Polymerization I li 1 A 2200 2000 1800 1600 1400 wavenumber/ cm- I 0.5 0.0 2200 2000 1800 1600 1400 wavenumber/cm-' Fig. 3. ( a ) 1.r. spectrum obtained after dosing 40 Torr CO onto a freshly prepared Cr(Cp),/SiO, sample. ( b ) Spectrum obtained after dosing 40 Torr CO onto a Cr(Cp),/Si02 sample previously outgassed for 1 h at 323 K. I n both spectra the peak at 1424cm-' is due to the ucc mode of a Cp- ring. On the basis of the intensity ratio of the two peaks, the angle formed by the two oscillators is ca.85 '.I4 The sextet of bands at 1920, 1895, 1832, 1775, 1628 and 1579 cm-' derives from the interaction of CO with the ( a ) dimers in an excess of Cr(Cp),. As the ( a ) dimers are not active in ethene polymerization (vide infra), we shall not discuss into detail the structure of the carbonylic complexes derived from them. We mention only that: ( i ) the sextet is the sum of two correlated triplets (1920, 1832, 1628 cm-* and 1895, 1775, 1579 cm-I), presumably belonging to two different (polycarbonylic) species; ( i i ) the ratio between the two triplets changes slightly from one sample to the other and with time, Unlike the doublet at 2054 and 1985 cm-', the two triplets at lower frequencies do not change in intensity on decreasing the equilibrium pressure of CO.It is thus inferredA. Zecchina, G. Spoto and S. Bordiga 155 1 .o 0, C -e 0.5 0, 2 0.0 n ' * c 0 I I3c 0 1 3 ~ ~ ~ / 2200 2000 1800 1600 wavenumber/cm-' Fig. 4. 1.r. spectra recorded after contacting Cr(Cp),/SiO, samples previously outgassed at 323 K with: ( a ) 40 Torr "CO; ( b ) 40 Torr I3CO; ( c ) 40 Torr of a '2CO/'3C0 1 : 1 mixture. that the corresponding carbonylic species are more stable. It is also worth mentioning that the rate of formation of the dicarbonylic species is larger than that of the polycar- bonylic species. Further insight into the structure of the carbonylic species is obtained from the isotopic '2CO/"C0 substitution experiments (fig. 4). It can be seen that when the 1:l mixture is used, the doublet at 2054 and 1985 cm-' splits into three doublets (two bands of the '2CO-'2C0 complex, two bands of the mixed '2CO-'3C0 species and two bands of the '3CO-'3C0 species) with 1 : 2 : 1 approximate intensity ratios.These results definitely confirm the cis-dicarbonylic nature of the species hypothesized in reaction (2). The isotopic substitution pattern of the two triplets derived from the dinuclear species ( a ) is more complex and it will not be discussed in detail. It is sufficient to mention here that the spectrum of the 1 : 1 mixture is not the sum of the spectra of the pure "CO and "CO species. This indicates considerable coupling between the CO oscillators within these species. In particular, the splitting of the two higher-frequency peaks at 1920 and 1832 cm-' suggest that two preferentially coupled CO groups are present in the polycarbonylic polynuclear complex.In a separate study we shall advance a more detailed hypothesis on their structure. We can anticipate here that we are probably156 Chromocene on Silica and Ethene Polymerization 1 .o 0 c .f! s % 0.0 1 , 2000 1900 1800 1700 1600 1500 wavenumber/cm-' Fig. 5. 1.r. spectrum obtained after contacting a Cr(Cp),/SiO, sample with 0.5 Torr NO. dealing with a salt-like dimeric species [ 2Si-O-Cr(CO),Cp]- [ Cr( Cp)J+ and/or [ Cr( CO),]-[ Cr( Cp)J+ similar to that described in ref. ( 15) and ( 16). The interaction of NO with adsorbed Cr(Cp), (fig. 5) is simpler than that of CO, as only two main peaks (with constant intensity ratio) are always seen at 18 13 and 1707 cm-' (with a shoulder at 1793 cm-') on both concentrated and diluted samples.This result can be explained by assuming that the two peaks belong to a cis-dinitrosylic complex derived from (a) and (6) as follows: +NO SSi-0-CrCp - ~Si-O-CrCp(NO)2 %3-O-CrCp-.CpCrCp - fSi-0-CrCp( NO),+ CpCrCp. Unlike CO, NO (being a stronger ligand) displaces all the weakly bound Cr(Cp), (even in the presence of an excess of it) without giving further dinuclear polynitrosylic products. It is worth noting that the frequency and intensity ratio of this doublet are very similar to those observed for cis-dinitrosylic complexes formed on Cr"/~ilica.'~ The previous assignment is consequently strongly reinforced. +NO The Ethene Polymerization Centres As is well ethene polymerizes on SiOz samples functionalized with Cr(Cp),, and the reaction can be easily investigated by i.r.spectroscopy following the evolution (in the presence of ethene) of the i.r. spectrum of the growig polymeric chain.A. Zecchina, G. Spoto and S. Bordiga 157 4000 3500 3000 2500 2000 1500 wavenumber/cm- ' Fig. 6. 1.r. spectra obtained at increasing CzH4 contact time (last spectrum recorded after 20 min) illustrating the polymerization reaction on a Cr(Cp)2/ SiO, sample previously outgassed at 323 K. The spectrum of the SO2 pellet (activated at 973 K) before anchoring chromocene is also reported for comparison. A typical spectrum is shown in fig. 6 for a sample previously contacted with Cr(Cp), and then outgassed at 323 K to remove a fraction of the weakly adsorbed chromocene.It is remarkable that the rate of the reaction (not very large under this ethene pressure) is much lower on samples which did not undergo the outgassing procedure at 323 K. This observation indicates that Cr(Cp), bound weakly to the anchored (6) structures acts as a poison. This is not unexpected because the Cp- ligand of chromocene can fill the coordination vacancies that are necessary for the polymerization. Moreover, in the conditions investigated here (it is conceivable that under pressures of the order of several atmospheres the situation could be different) ethene, unlike CO and NO, is not strong enough a ligand to displace the Cr(Cp), from the ( a ) species. It is noticeable that the polymerization is also totally poisoned by CO and NO.Following these considerations, we conclude that the polymerization centres are the mononuclear coordinatively unsaturated ( b ) species. For the reasons discussed above, the ( b ) centres are very scarce on freshly prepared samples (because of diffusion problems) and are present only in the boundary layers between supersaturated and clean regions of the pellet. This explains the low polymeriz- ation activity (at least at the low ethene pressures used in this investigation) of the freshly prepared samples. Outgassing at 323 K partially removes the weakly adsorbed poisoning chromocene: this explains the increase of the polymerization activity observed after the thermal treatment. Unfortunately, as mentioned before, removal of all the weakly adsorbed Cr(Cp), is not possible without causing undesired chemical transforma- tions.This makes the problem of building a system with maximum catalytic activity apparently impossible. A way to overcome this problem is illustrated in fig. 7, where the effect of anchoring Cr(Cp), in an ethene atmosphere is reported. The experimentChromocene on Silica and Ethene Polymerization 4000 3500 3000 2500 2000 1500 wavenumberjcm- ' Fig. 7. 1.r. spectra recorded at increasing contact time (last spectrum after 20 min) illustrating the effect on the ethene polymerization rate of anchoring chromocene on silica in presence of 70 Torr C,H, following the experimental procedure described in the text. The spectrum of the virgin Si02 sample (outgassed at 973 K) is also reported for comparison.As in fig. 6, the peaks in the 2100-1950 cm-' region are due to adsorbed impurities (see text). Notice that the absorbance smle is the same as in fig. 6. was performed as follows: a silica pellet outgassed in vacuo at 973 K in the usual way was brought in the immediate vicinity of a Cr(Cp), film sublimed on the internal wall of the i.r. cell and kept a t low temperature (ca. 263 K) to avoid vaporization (and hence interaction through the gas phase with the OH groups of the silica pellet). Then C2H, was dosed by filling the cell with 60Torr of gas. The temperature of the film was then allowed to increase rapidly up to 323 K in order to cause fast sublimation of the metallocene on the adjacent silica pellet in the presence of ethene. After this operation the i.r.spectra were recorded at fixed time intervals. From the results reported in fig. 7, the following interesting facts emerge: ( i ) the surface OH groups are nearly completely consumed ensuring maximization of the anchoring reaction and hence formation of the maximum number of potentially active centres); (ii) the polymerization activity is much increased with respect to the previous experiments. The explanation of these phenomena is as follows. The anchoring process occurs in the presence of ethene in the usual way by elimination of C,H, and formation of the ( b ) structure. However, when ethene is present, the freshly formed ( b ) structures begin to act immediately as catalytic centres with subsequent fast growth of the polymeric chain attached to the Cr atom (scheme 6).The incoming Cr(Cp), finds the coordination vacancies already saturated by the growing polymeric chain. Consequently, it cannot be adsorbed on the pre-existing anchored chromocene sites and cannot poison the catalytic centres. Moreover, in absence of these sites, the excess chromocene can now more freely migrate inside the pellet and react with almost all the OH groups of the microparticles. In conclusion, the higher consumption of OH groups and the absence of any negative effect of unreacted chromocene is simultaneously explained on the basis of the same concept.A. Zecchina, G. Spoto and S. Bordiga 0 1 ,7i\ 0 C2H4, I Cr-Polymeric chain / + Scheme 6 @ A Cr 159 J Scheme 7 The Polymerization Mechanism As shown in fig. 6 and 7, the main features of the growing polymeric chain are represented by peaks at 2918 and 2851 cm-' (asymmetric and symmetric stretching frequencies of CH2 groups) and peaks at 1472 and 1465 cm-' (bending modes of the same groups).This spectrum is very similar to that observed on Cr"/Si02 (Phillips catalyst). However, unlike this system, no broad bands are observed at ca. 2750 cm-', which were assigned to an agostic interaction.* It is remarkable that, even during the first polymerization stages, no definite signs of the presence of groups other than CH2 were found. These observations suggest that the chain initiation consists of the formation of a metallocyclic structure following the mechanism in scheme 7 and that the propagation corresponds to an insertion of the C2H4 molecules into the ring following the mechanism in scheme 8 with the formation of long, doubly anchored, chains without end groups.Following this idea chain termina- tion (not observed in our low-pressure, low-temperature experiments) can be represented by scheme 9. The major difficulty with this reaction scheme is the following: how can ethene be inserted into the metallocycle when the Cr atom is apparently fully saturated (in fact, as the C5HS usually behaves an an 77' ligand occupying three coordination vacancies, the Cr is sixfold coordinated). A plausible answer lies in the well established fluxionality of the Cp- ligand (especially with Group VI metal derivatives)I6 which can change from 7' to q3 configur- ation (ring slippage). In fact this transformation could allow the ethene to be coordinated Scheme 8160 Chromocene on Silica and Ethene Polymerization Scheme 9 by the metallic centre before insertion into the metallocycle.Moreover, we have to consider also the fact that the Group VI metals give a rich variety of seven-coordinated compounds. '*,I9 It is quite conceivable that both these factors operate simultaneously here, thus giving a highly versatile catalytic character to the metallic centre of structure ( 6 ) . We thank Prof G. 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