Activation processes and polyethylene formation on a phillips model catalyst studied by laser ablation, laser desorption, and static secondary ion mass spectrometry

Frdric Aubriet 0 1 2 Jean-Franois Muller 0 1 2 Pascal G. Di Croce 0 1 2 Paul Grange 0 1 2 0 Unit de catalyse et de chimie des matriaux diviss, Universit catholique de Louvain , Louvain-la-Neuve, Belgium 1 Unit de physico-chimie et de physique des matriaux, Universit catholique de Louvain , Louvain-la-Neuve, Belgium 2 Laboratoire de Spectromtrie de Masse et de Chimie Laser, Universit Paul Verlaine-Metz , Metz, France Claude Poleunis and Patrick Bertrand Since the discovery of the Phillips catalysts, there still is much uncertainty concerning their activation, their molecular structure, the nature of the active chromium sites, and the polymerization mechanisms. Surface techniques are not easy to be used for such study according to the nonconductive behavior of the support. Therefore, model Phillips catalyst is elaborated by spin coating a trivalent chromium precursor on a silicon wafer. The surface characterization of this model catalyst is conducted by laser ablation mass spectrometry (LA-MS), laser desorption/ionization mass spectrometry (LDI-MS), and static secondary ion mass spectrometry (s-SIMS), at different steps of its preparation. To validate our approach, a comparison is also made between the model and the real Philips catalyst. Moreover, the model catalyst efficiency for polyethylene synthesis is evaluated and allows us to discuss the validity of the mechanisms previously proposed to explain the catalytic process. The characterization of Phillips model catalyst by mass spectrometry allows us to better understand the activation processes of such catalyst. Depending on the activation temperature, chromium oxide species are formed and anchored at the support surface. They consist mainly in mono-chromium sites at high temperature. The chromium valence is hexavalent. This model catalyst is active for the polymerization of ethylene. A pseudo-oligomer molecular weight distribution is observed by LA-MS, whereas s-SIMS allows us to elucidate the anchorage of the polymer at activate chromium surface sites. (J Am Soc Mass Spectrom 2006, 17, 406 - 414) 2006 American Society for Mass Spectrometry - Cdustrial catalysts used to perform hydrogenahromium compounds are involved in many intion, dehydrogenation, isomerization, or polymerization of organic compounds. The most important industrial application is the Phillips Cr/SiO2 catalyst for the production of polyethylene [13]. Industrial Phillips catalyst is typically prepared by impregnation of a high surface area material, e.g., silica, with an aqueous solution of a hexavalent Cr(VI)- or a trivalent Cr(III)precursor salt. After activation and eventually a reduction step, heterogeneous systems consisting of small particles (active sites) hidden inside the pores of inorganic supports are obtained. A 0.21.0 Cr wt.% loading of the catalyst is commonly attained. However, only a small portion of anchored chromium is active for polymerization. In spite of its interest, Phillips catalyst is not fully understood to date. A general review dealing with the preparation, the activation, the structure, the ethylene polymerization mechanism, and the characterization of Cr/SiO2 catalyst has been recently published [4]. To improve the catalyst, the authors underline the need of a better understanding of the active site formation and of the catalytic process. Although numerous investigations were performed since the discovery of the Phillips catalysts, much uncertainty still exists concerning their activation [5], their molecular structure [6], the nature of the active chromium sites [7], and the polymerization mechanisms [8] (kinetic control, activity). The growth of polymer at the catalyst surface is poorly understood to date [9]. Since the supports typically used for the anchorage of Phillips catalyst are nonconductive, their analysis by commonly used surface techniques is not easy. To overcome this problem, model Phillips catalysts for ethylene polymerization have been prepared and investigated. Based on ion-scattering experiments, Thne et al.[1012]proposedadescriptionoftheactivation process of a model Phillips catalyst that is active for ethylenepolymerization.Loosetal.[13]andThneet al.[11]determinedbyAFMthemorphologyofthe crystallized polymer layers produced at different polymerization times. A model Phillips catalyst has been elaborated in our group by spin coating a trivalent chromium precursor (chromium acetylacetonate noted here Cr(acac)3) on a silicon wafer. Preliminary results of its characterization by different surface and mass spectrometrytechniqueshavealreadybeenpublished[14, 15].Informationonthecatalystactivationandthe ethylene polymerization efficiency has been obtained. This paper deals with the surface characterization of this model catalyst at the different steps of its preparation. A comparison is also made between the model and the real Philips catalyst. For that purpose, mass spectrometry techniques are used. They consist of laser ablation mass spectrometry (LA-MS), laser desorption/ ionization mass spectrometry (LDI-MS), and static secondary ion mass spectrometry (s-SIMS). Indeed, s-SIMS as well as LDI-MS appear to be well suited to characterize both precursor and chromium oxygenated species present at the surface of model catalyst. Prior works demonstrated the ability of both techniques to characterize in positive detection mode metal acetylacetonate, andespeciallychromiumonebys-SIMS[16,17]or LDI-MS[16,18,19].Generally,associationsbetween chromium atoms and various amounts of acetylacetonate ligands Crx(acac)y are observed. Oxide chromium compounds have been also extensively studied, especially by our group; in that case CrxOy cluster ions are detected[20,21].Moreover,themodelcatalystefficiency for polyethylene synthesis is evaluated. Finally, the characterization of the catalyst surface after polymerization allows us to discuss the validity of the mechanisms previously proposed to explain the catalyticprocess[4]. Sample Preparation The different steps for the model catalyst elaboration consist of the support preparation, its impregnation by the catalyst precursor, and its activation. P-type silicon wafers with (100) surface orientation are used as substratesandarepretreatedasfollows[10].Squarepieces (1.5 1.5 cm2) of wafers are calcinated at 750 C in air for 24 h to present an amorphous SiO2 surface layer. Then, they are cleaned at 70 C with a mixture of H2O2 (30%) and NH4OH (30%) in a ratio 3/2 vol/vol. After several rinses with milli-Q water (HPLC grade produced by a Milli-Q plus system from Millipore, Molsheim, France), the wafers are placed for 30 min in boiling water, to hydroxylize the surface. They are kept in water until the impregnation process to avoid contamination from the atmosphere. Spin coating is performed in air with a CT60 spin coater from Karl Suss Technique (Grove City, PA). The water film is first removed by spinning 3 min at 5000 rpm. Then the wafers are totally covered with three dro (...truncated)