Influence of support friability and concentration of α‐olefins on gas‐phase ethylene polymerization over polymer‐supported metallocene/methylaluminoxane catalysts

Abstract

The effects of support friability (F) and ethylene/comonomer ratios were investigated over supported metallocene/methylaluminoxane catalysts prepared with nine different porous polymeric supports and various comonomer concentrations with a 2-L reactor operated in the semibatch gas-phase mode at 808C and 1.4 MPa. F of the supports was measured with a newly devised method. The performance of the supported catalysts depended on support F as follows. The average homopolymerization activities varied from less than 6 t of polyethylene (PE) (mol of Zr)À1 h À1 for low-F catalysts to 10-20 t of PE (mol of Zr)À1 h À1 for moderate-F catalysts and up to 100 t of PE (mol of Zr) À1 h À1 for the high-F catalysts. The presence of 1-hexene and propylene comonomers increased the activity of the low-F catalysts by up to 20-fold and 50-fold, respectively; that is, there were very marked comonomer effects. Activity enhancement by 1-hexene was less than 3-fold for the moderate-F catalysts, whereas the high-F catalysts showed little activity enhancement. Sometimes, 1-hexene even resulted in activity reductions. Very different particle morphologies were obtained with the catalysts of different F's.

Investigation of silica particle structure containing metallocene immobilized by a sol–gel method

“…This problem could arise from a high resistance of the support or from an insufficient hydraulic force of the growing polymer to breakup as soon as the nascent polymer fills the interior of the support [6,[27][28][29][30]. In fact, the activity of the majority of the metallocenes used in the present study is lower than that found in homogeneous Cp 2 ZrCl 2 and (nBuCp) 2 ZrCl 2 .…”

“…Under these conditions, monomer mass transfer along the particle radius could hinder the polymerization, which subsequently reduces catalytic activity [27,29]. Experimental results addressing this issue have been reported in the literature for supported metallocene catalysts [6,27], silicasupported Ziegler-Natta catalysts [29], and Philipps catalysts [28,29]. Such studies have shown that the fragmentation is closely related to the physical resistance of the support to break into many pieces of 0.1-20 lm in diameter [27][28][29][30].…”

“…According to the literature, the reduction in the catalytic activity has been attributed to the following: (i) deactivation of the metallocene complexes during the immobilization, (ii) inaccessibility of the metallocene complexes to MAO hindering its activation; (iii) generation of active sites with lower propagation rates due to interactions with the support surface, and (iv) restrictions of monomer access to the active sites hindering the chain propagation [6]. Generally, polymers produced with immobilized metallocenes have higher average molecular weight due to a reduction in the rate of termination reactions [6,7]. Conversely, polymers with lower crystalline fractions have been produced with supported metallocene [5].…”

New clay‐supported Ziegler–Natta catalyst for preparation of PE/Clay nanocomposites via in situ polymerization

“…[27][28][29] The morphology of the supports and catalysts was elucidated by SEM ( Figure 7). On the other hand, both yield and texture of the catalyst affect the polymer morphology, and the catalysts are also known to replicate their morphology into the polymer particles.…”

Single-site catalysts in the industrial production of polyethylene

“…The greatest of them is that diffusion limitations reduce the activity of heterogeneous catalysts in the homopolymerization of ethyelene and propylene [29].…”