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

Abstract

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)

Cerium Oxyhydroxide Clusters: Formation, Structure, and Reactivity

“…[99][100][101] In the present LA experiments, large cluster anions are formed by ejection of Ce x O y species into the gas phase 73,80 that are stabilized by collisions with the pulse gas. The Ce x O y species may be ionic or neutral, and may exist in their ground state or in an excited state.…”

“…The formation of cluster ions containing a large number of hydroxyl groups was unexpected, since prior laser ablation experiments of metal oxides showed that one or at most two OH groups were present in anion and cation clusters. [99][100][101] In the present LA experiments, large cluster anions are formed by ejection of Ce x O y species into the gas phase 73,80 that are stabilized by collisions with the pulse gas. The Ce x O y species may be ionic or neutral, and may exist in their ground state or in an excited state.…”