Supported Metallocene Catalysts by Surface Organometallic Chemistry. Synthesis, Characterization, and Reactivity in Ethylene Polymerization of Oxide-Supported Mono- and Biscyclopentadienyl Zirconium Alkyl Complexes:  Establishment of Structure/Reactivity Relationships

Abstract

The reactions of CpZr(CH(3))(3), 1, and Cp(2)Zr(CH(3))(2), 2, with partially dehydroxylated silica, silica-alumina, and alumina surfaces have been carried out with careful identification of the resulting surface organometallic complexes in order to probe the relationship between catalyst structure and polymerization activity. The characterization of the supported complexes has been achieved in most cases by in situ infrared spectroscopy, surface microanalysis, qualitative and quantitative analysis of evolved gases during surface reactions with labeled surface, solid state (1)H and (13)C NMR using (13)C-enriched compounds, and EXAFS. 1 and 2 react with silica(500) and silica-alumina(500) by simple protonolysis of one Zr-Me bond by surface silanols with formation of a single well-defined neutral compound. In the case of silica-alumina, a fraction of the supported complexes exhibits some interactions with electronically unsaturated surface aluminum sites. 1 and 2 also react with the hydroxyl groups of gamma-alumina(500), leading to several surface structures. Correlation between EXAFS and (13)C NMR data suggests, in short, two main surface structures having different environments for the methyl group: [Al](3)-OZrCp(CH(3))(2) and [Al](2)-OZrCp(CH(3))(mu-CH(3))-[Al] for the monoCp series and [Al](2)-OZrCp(2)(CH(3)) and [Al]-OZrCp(2)(mu-CH(3))-[Al] for the bisCp series. Ethylene polymerization has been carried out with all the supported complexes under various reaction conditions. Silica-supported catalysts in the absence of any cocatalyst exhibited no activity whatsoever for ethylene polymerization. When the oxide contained Lewis acidic sites, the resulting surface species were active. The activity, although improved by the presence of additional cocatalysts, remained very low by comparison with that of the homogeneous metallocene systems. This trend has been interpreted on the basis of various possible parameters, including the (p-pi)-(d-pi) back-donation of surface oxygen atoms to the zirconium center.

Surface structural-chemical characterization of a single-site d 0 heterogeneous arene hydrogenation catalyst having 100% active sites

“…However, kinetic poisoning experiments in which the catalytic sites are titrated in situ with H 2 O or t BuCH 2 OH indicate that ≤5% of D-type sites are catalytically significant, likely reflecting, among other factors, the established heterogeneity of alumina surfaces (5,6,9), hence rendering active site structural and chemical descriptions necessarily imprecise. In contrast to these results, chemisorption of such organozirconium precursors on SiO 2 , Al 2 O 3 , and SiO 2 -Al 2 O 3 surfaces having appreciable coverage by weakly acidic OH groups predominantly yields covalently bound, poorly electrophilic Etype species via Zr-CH 3 protonolysis with CH 4 evolution (5,6,10,11). Although the E-type sites may be characterized in some detail by high-resolution solid-state NMR and extended X-ray adsorption fine structure spectroscopy (EXAFS), they display minimal catalytic turnover in the absence of added, complicating activators [e.g., methylalumoxane or B(C 6 F 5 ) 3 ], and the fraction of catalytically significant sites is unknown (12,13).…”

Development of Tungsten‐Based Heterogeneous Alkane Metathesis Catalysts Through a Structure–Activity Relationship

“…[16] The two broad signals at d = 85 and 103 ppm are attributed to different types of methylene carbon atoms (CH 2 tBu), probably associated with the heterogeneity of alumina and possibly with the presence of neutral and cationic W surface complexes. [22,23] The signals at d = 52 and 32 ppm, already observed on silica materials, have been assigned respectively to the quaternary carbon atoms of the neopentylidyne group and to the methyl groups of the various tBu groups. [16] The shoulder at d = 29 ppm, not observed on other supports, may correspond to the presence of small amounts of [(AlÀ CH 2 tBu) À ] fragments.…”

Hybrid materials based on polyethylene and MCM-41 microparticles functionalized with silanes: Catalytic aspects of in situ polymerization, crystalline features and mechanical properties

“…It is generally accepted in these cases that metallocenes are grafted on silica surface by elimination of chloride ligand with hydrogen atoms from silanol groups on the support, generating both monodentate and bidentate surface species. After MAO addition, monodentate µ-oxo surface species may be converted to active cationic sites for the polymerization of α-olefins [29][30][31].…”