J. Phys. Chem. C 2008, 112, 6439-6449
6439
Nature of ≡SiOCrO2Cl and (≡SiO)2CrO2 Sites Prepared by Grafting CrO2Cl2 onto Silica Cori A. Demmelmaier,† Rosemary E. White,† Jeroen A. van Bokhoven,§ and Susannah L. Scott*,†,‡ Departments of Chemical Engineering and Chemistry, UniVersity of California, Santa Barbara, California 93106-5080, and Institute for Chemical and Bioengineering, ETH Zurich, 8093 Zurich, Switzerland ReceiVed: December 19, 2007; In Final Form: February 11, 2008
The room-temperature reaction between chromyl chloride and Sylopol 952 silicas pretreated at 200, 450, and 800 °C was investigated using IR, XANES, and EXAFS spectroscopy, as well as by DFT modeling. On the silicas pretreated at 200 and 450 °C, the structurally uniform sites formed by the reaction with one surface hydroxyl group are described as ≡SiOCrO2Cl. Unreacted silanols persist on these silicas even in the presence of excess CrO2Cl2, and on the silica pretreated at 200 °C some participate in hydrogen bonding with the grafted monochlorochromate sites. On the silica pretreated at 800 °C, both ≡SiOCrO2Cl and (≡SiO)2CrO2 sites are formed. The latter are produced despite the absence of hydrogen-bonded hydroxyl pairs on the support. The origin of the chromate sites is proposed to be the reaction between CrO2Cl2 and hydroxylsubstituted siloxane 2-rings. These rings are likely formed at 800 °C by condensation between a pair of vicinal silanols in which one of the silanols is also a member of a geminal pair.
Introduction The reactions of volatile metal halides such as chromyl chloride (CrO2Cl2) with surfaces are of interest in both materials chemistry and heterogeneous catalysis. For example, the deposition of CrO2Cl2 onto oxide substrates yields chromium oxide overlayers (CrO2 or Cr2O3), which have applications in optical elements, sensors, and magnetic materials.1-7 High temperatures and laser or electron beam irradiation are often used in these processes to produce the thin films, sometimes in the presence of other gases such as O2. The molecular layering method, which requires milder reaction conditions, involves alternating exposures of the surface to CrO2Cl2, water vapor, and H2.8,9 Treatment of γ-Al2O3 with only CrO2Cl2 and water at 150 °C was reported to produce a CrO3 covering on the support.10 A study of CrO2Cl2 deposition onto the (110) surface of TiO2 revealed molecular adsorption via the oxo ligands at -145 °C, followed by desorption of CrCl2 upon heating to 430 °C.11 A mixture of CrO2Cl2 and n-hexane was shown to give a layer of Cr2O3 on silica upon heating.5 Upon heating above 300 °C under vacuum, autoreduction of CrO2Cl2 grafted onto amorphous silica occurs, accompanied by the formation of Cl2.12 Silica is a particularly important substrate for chromyl chloride deposition. Silica-supported chromium catalysts are usedindustriallytomakeabout40%oftheworld’spolyethylene13-15 and are also active for the selective oxidation of hydrocarbons.16-18 Catalysts prepared via grafting of CrO2Cl2 may possess more uniform silylchromate sites, (≡SiO)2CrO2, and consequently be more active, than those made by the conventional wet impregnation of chromate.13,19 Even at low loadings, the latter method results in incomplete dispersion of the metal, with the formation * Author to whom correspondence should be addressed. Fax: 1-805893-4731. E-mail:
[email protected]. † Department of Chemical Engineering, University of California, Santa Barbara. ‡ Department of Chemistry, University of California, Santa Barbara. § Institute for Chemical and Bioengineering.
of small Cr2O3 particles,20 while the dispersed sites present both dioxo- and monooxochromium(VI) structures.21 The desire to control active site structures in heterogeneous olefin polymerization catalysts is motivated by the low fraction of Cr sites that become activated upon exposure to the olefin substrate (typically, ca. 1%).13 This complicates studies of their reaction mechanisms and increases the amount of catalyst residue in the polyethylene product. In chromate catalyst preparation, the calcination temperature has a tremendous influence on the ethylene polymerization activity.13 The changing nature of the interaction between silica surfaces pretreated at various temperatures and CrO2Cl2 vapor has been the subject of several studies. Volkova et al. deposited CrO2Cl2 onto a large-pore silica gel from a stream of flowing N2 at 180 °C.22 The silica itself was pretreated at 180 °C to remove adsorbed water. On the basis of the evolution of HCl and the finding that no chloride was retained on the surface, the grafting reaction was formulated as in eq 1:
2 ≡SiOH + CrO2Cl2 f (≡SiO)2CrO2 + 2 HCl
(1)
where ≡SiOH represents a Q3 site on the silica surface. Plyuto et al. reported a different reaction stoichiometry for CrO2Cl2 reacting at 100 °C with silicas pretreated between 500 and 800 °C, eq 2:23
≡SiOH + CrO2Cl2 f ≡SiOCrO2Cl+ HCl
(2)
Using IR spectroscopy, Nishimura and Thomas deduced that CrO2Cl2 merely adsorbs on isolated hydroxyl groups via a hydrogen-bonding interaction at 25 °C; upon heating to 450 °C, this weakly bound CrO2Cl2 was thought to desorb.24 Grafting was proposed to occur only on vicinal silanol pairs, giving rise directly to the chromate sites (≡SiO)2CrO2. Mehandjiev et al. reported the formation of a mixture of sites ≡SiOCrO2Cl and (≡SiO)2CrO2 upon reaction of CrO2Cl2 at 170 °C with a silica gel pretreated at temperatures ranging from 200 to 700 °C, based
10.1021/jp7119153 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/01/2008
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on measured Cl/Cr ratios from 0.5 (200-500 °C) to 0.7 (600700 °C).25 In addition, an EPR signal of Cr(V) was detected; however, this may reflect oxidation of adventitious hydrocarbons by Cr(VI) because a new IR vibration at 1750 cm-1 was also reported. In the most comprehensive study of CrO2Cl2 grafting to date, McDaniel reported the deposition of CrO2Cl2 in flowing nitrogen at 200 °C onto the surface of a wide-pore silica pretreated at temperatures ranging from 200 to 900 °C.26 On the basis of the amount of Cr and Cl retained on the silica, a mixture of ≡SiOCrO2Cl and (≡SiO)2CrO2 sites was inferred, except in the case of silicas pretreated at 800 °C and above. For those materials, Cl/Cr ratios of 1.0 and greater were found. The exclusive formation of ≡SiOCrO2Cl sites was assumed and, in addition to the reaction of CrO2Cl2 with isolated surface hydroxyls (eq 2), grafting onto strained siloxane bonds was proposed, eq 3:
≡SiOSi≡ + CrO2Cl2 f ≡SiOCrO2Cl+ ≡SiCl
(3)
For all silica pretreatment temperatures, a decrease in the Cl/ Cr ratio observed upon heating the Cr-modified silicas under either Ar or O2 was attributed to the transformation of ≡SiOCrO2Cl sites to (≡SiO)2CrO2. To date, no spectroscopic tools, other than IR analysis of silanol vibrations, have been used to elucidate the structures of grafted CrO2Cl2 on silica. Furthermore, grafting has been studied only at elevated temperatures, when transformations subsequent to the initial reaction of CrO2Cl2 with the surface may cause changes in the structures of the grafted sites. Finally, the use of chloride content to infer the grafting stoichiometry and the structures of the anchored chromium sites in these experiments may be problematic because the HCl formed as a reaction product in eqs 1 and 2 is known to chlorinate the silica surface at reaction temperatures above 175 °C,27-29 eq 4.
≡SiOH + HCl f ≡SiCl+ H2O
(4)
In this contribution, we probe the nature of the first reaction between CrO2Cl2 and silica, using a combination of IR, XANES, and EXAFS spectroscopies, as well as DFT computational modeling of the grafted sites. The uniformity of these sites as a consequence of the mild reaction conditions is expected to facilitate the investigation of their relationship to the active sites of the Phillips catalyst. Experimental and Computational Methods Sample Preparation. The silica used in this study is Sylopol 952, a silica gel from Grace-Davison with a BET surface area of (258 ( 1) m2/g, a pore volume of 1.61 mL/g, and an average particle size of 112 µm. This silica is used in commercial formulations of the Phillips (Cr/SiO2) ethylene polymerization catalyst. It was pretreated by heating to the desired temperature (200, 450, or 800 °C) under dynamic vacuum (
