Real‐time Analysis of a Working Triethylaluminium‐Modified Cr/Ti/SiO₂ Ethylene Polymerization Catalyst with In Situ Infrared Spectroscopy

Abstract

A diffuse reflectance infrared Fourier‐transform (DRIFT) study has been conducted at 373 K and 1 bar on an industrial Cr/Ti/SiO2 Phillips‐type catalyst modified with, and without, triethylaluminium (TEAl) as co‐catalyst. The reaction rate of the polymerization of ethylene, as monitored by the increase in the methylene stretching band of the growing polyethylene (PE), has been investigated as a function of the titanium content. After an initial period of mixed kinetics, with the reaction rate significantly higher for the TEAl‐modified catalysts compared with the non‐modified catalysts, the polymerization proceeded as a pseudo‐zero‐order reaction with a reaction rate that increased as a function of titanium loading. Furthermore, it was found that the higher Ti loading caused the appearance of more acidic hydroxyl groups and modified the Cr sites by making them more Lewis acidic, ultimately shortening the induction time and increasing the initial polymerization rate.

Genesis of MgCl2‐based Ziegler‐Natta Catalysts as Probed with Operando Spectroscopy

“…[27] The final amount of titanium that is grafted on MgCl 2 with this procedure is close to 1.0 wt%, as determined by ICP analysis. Then the TiCl 4 vapors reach with the activated MgCl 2 at 90 8C.…”

“…[12][13][14][15] The high dilution and the intrinsic heterogeneity of the catalytic sites, the co-presence of spectator species and the high air sensitivity of the catalysts and their activators are among the main reasons for the lack of Ziegler-Natta operando spectroscopic studies. [24,25] Recently, operando spectroscopies were successfully applied to disclose the genesis of the Cr active sites in the Cr VI /SiO 2 Phillips catalyst [26] and the molecular-level properties of Phillips-type catalysts modified by aluminum-alkyls, [26][27][28] demonstrating that these methods have progressed enough to meet the technical challenges posed by olefin polymerization catalysts. [24,25] Recently, operando spectroscopies were successfully applied to disclose the genesis of the Cr active sites in the Cr VI /SiO 2 Phillips catalyst [26] and the molecular-level properties of Phillips-type catalysts modified by aluminum-alkyls, [26][27][28] demonstrating that these methods have progressed enough to meet the technical challenges posed by olefin polymerization catalysts.…”

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

“…[227,228] Thea ssociated process produces al arge proportion of the worldsh ighdensity polyethylene (HDPE). Even if the Phillips catalyst does not require ac o-catalyst, AlEt 3 or BEt 3 enhance the activity, shorten the induction period, [231][232][233][234] tend to broaden the molecular weight distribution, and increase the amount of branching in the polymer. Thec alcined material is active in the polymerization of ethylene in the absence of aco-catalyst or activator, although an induction period is observed under the reaction conditions (Scheme 15).…”

“…Even if the Phillips catalyst does not require ac o-catalyst, AlEt 3 or BEt 3 enhance the activity, shorten the induction period, [231][232][233][234] tend to broaden the molecular weight distribution, and increase the amount of branching in the polymer. [234,[236][237][238][239] CrO 3 /Al 2 O 3 is less active than CrO 3 /SiO 2 in the polymerization of ethylene,b ut CrO 3 /Al 2 O 3 can produce polymer with an ultrahigh molecular weight. [8,229] CrO 3 on silica-titania (CrO 3 /SiTiO x )has ashorter induction period and higher polymerization rate than CrO 3 /SiO 2 .The polymer has abroader molecular weight distribution and higher longchain branch levels than the polymer produced with CrO 3 / SiO 2 .…”

Eine Brücke zwischen industriellen und wohldefinierten Trägerkatalysatoren

“…Das kalzinierte Material ist ohne Cokatalysator oder Aktivator für die Ethylenpolymerisation aktiv, wenn auch unter den Reaktionsbedingungen eine Induktionsperiode beobachtet wird (Schema ). Obwohl der Phillips‐Katalysator keinen Cokatalysator benötigt, erhöhen AlEt 3 oder BEt 3 die Aktivität, verkürzen die Induktionsperiode, verbreitern tendenziell die Molekulargewichtsverteilung und erhöhen das Ausmaß an Verzweigung im Polymer …”