The Multifaceted Role of Methylaluminoxane in Metallocene-Based Olefin Polymerization Catalysis

Abstract

In single-site olefin polymerization catalysis, a large excess of cocatalyst is often required for the generation of highly active catalysts, but the reason for this is unclear. In this work, fundamental insight into the multifaceted role of cocatalyst methylaluminoxane (MAO) in the activation, deactivation, and stabilization of group 4 metallocenes in the immobilized single-site olefin polymerization catalyst was gained. Employing probe molecule FT-IR spectroscopy, it was found that weak Lewis acid sites, inherent to the silica-supported MAO cocatalyst, are the main responsible species for the genesis of active metallocenes for olefin polymerization. These weak Lewis acid sites are the origin of AlMe2+ groups. Deactivation of metallocenes is caused by the presence of silanol groups on the silica support. Interaction of the catalyst precursor with these silanol groups leads to the irreversible formation of inactive metallocenes. Importantly, a high concentration of MAO (14 wt% Al) on the silica support is necessary to keep the metallocenes immobilized, hence preventing metallocene leaching and consequent reactor fouling. Increasing the loading of the MAO cocatalyst leads to larger amounts of AlMe2+, fewer silanol groups, and less metallocene leaching, which all result in higher olefin polymerization activity.

Intra‐ and Interparticle Heterogeneities in Solid Activators for Single‐Site Olefin Polymerization Catalysis as Revealed by Micro‐Spectroscopy

“…The catalytic activity for the corresponding catalysts, expressed in kg polyethylene produced per mmol zirconium per hour,i si ndicated in Ta ble 1b. [11] Ar eference catalyst, containing only the parent silica impregnated with the catalyst precursor and denoted as Zr/Si-0Al, was not active in the olefin co-polymerization reaction, indicating the necessity of the MAO activator. In what follows, we presentt he results obtained with FT-IR micro-spectroscopy in combination with pyridine as probe molecule for acid sites.…”

“…Previously,w eh ave demonstrated the MAO/SiO 2 solid activators to have ac oncentration of weak Lewis acid sites that linearly correlates with MAO loading. [11] This weak Lewis acid site is observed in pyridine FT-IR spectroscopy via bands at 1615 and1 450cm À1 .O ther bands in this region characterize pyridine when adsorbed on non-acidic silanol groups. [21,22] Upon temperature treatment at 200 8C, only the bands ascribed to pyridine adsorbed on acid sites are still present.I ns ummary,F igure 1s hows the pyridine FT-IR spectra of as olid activator particlec ontaining al ow amounto fs ilanol groups and as ignificant amounto fw eak Lewis acid sites (found in sample Si-17Al) and as olid activator particle containing ah igh amount of silanol groups with an egligible amount of acid sites (found in sample Si-6Al).…”

“…[6][7][8][9][10] Recently,w eh ave reported the influence of MAO loading on metallocenea ctivity in the silica-supported single-site olefin polymerization catalyst. [11] With increasingM AO loading, the amount of weak Lewis acid sites in the solid activator( MAO/ SiO 2 )i ncreases, showing al inear correlation with olefin polymerization activity.T hese weak Lewis acid sites in supported MAO materials are therefore assigned to the active species in metallocenea ctivation. In accordance with literature, the weak Lewis acid sites are consideredt he origin of mobile AlMe 2 + groups,w hichh ave often been ascribed to the active species for metallocene activation.…”

“…In our previousw ork, we have reported that an increase in MAO loading leads to the creationo fm ore weak Lewis acid sites in the supported activator. [11] These weak Lewis acid sites (W-LAS) are the main responsible speciesf or metallocene activation for olefin polymerization catalysis.T he presence of silanol groups on the silica support, on the other hand, was reportedt oc auset he deactivation of metallocenes. They are, therefore, undesirable in the solid activators.…”

“…[2] Am inimum MAO loading is, therefore, required to scavenge all silanol groups on the support surface. [11] In this work, we have extended our characterization studies towardss ingle activatorp article analysis. To that end, we have studied the influence of the MAO loading on the creationo f active sites in the MAO (weak Lewis acid sites) for metallocene activation and on the level of silanol scavenging within single solid activator particles, employing infrared micro-spectroscopy in combination with pyridine as ap robe molecule for acid sites.…”