Functionalized Spray-dried Chitosan Microspheres as Support for Sustainable Metallocene Olefin Polymerization Catalysis

Abstract

With the ever-increasing multi-billion plastic market, polyolefins are still used as the primary building block for packaging and other consumer products. The discovery of supported metallocene catalysts gives the possibility to tailor the polymer structure in a way that has not been reached before by either Phillips or Ziegler-Natta catalysts. Metallocenes require the co-catalyst methylaluminoxane (MAO) for activation and are generally supported on porous silica materials. Great effort has been continuously made to synthesize these catalysts sustainably and biodegradable and optimize productivity. Furthermore, silica supports are unsuitable for multi-phase rubber incorporation to produce high-impact resistance polypropylene (hiPP). In contrast to rubber incorporation, a different polymer phase can be incorporated into the polymer matrix using spray-dried chitosan microspheres as a novel support for a metallocene olefin polymerization catalyst. A set of catalysts with different co-catalyst loading (CTS-(23-46)Al) have been synthesized to find the optimal MAO loading with pyridine and CO probe molecule Fourier Transform Infrared (FT-IR) spectroscopy. The total and weak Lewis acid site (LAS) concentration increased with higher alumina content. This higher weak LAS concentration provides more AlMe2+ species to activate the metallocene catalyst, which increases the polymerization activity. Therefore, an MAO loading of 46 wt% should be achieved to maximize the polymerization activity. In addition to this nonporous catalyst, a porous catalyst is synthesized by template-assisted spray drying using polystyrene nanospheres. The early-stage polymerization kinetics of both supported catalysts is characterized by in-situ Diffusive Reflectance Infrared Fourier Transform Spectroscopy (DRIFT) spectroscopy, which displayed a fast initial activity followed by a slower steady-state activity. Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) is utilized to visualize the fragmentation behavior by analyzing the Al co-catalyst distribution in the PE polymer particle after gas-phase and slurry polymerization. Fragmentation of the catalyst was only visible after gas-phase polymerization for both nonporous and porous catalysts. The FIB-SEM analysis reveals distinct fragmentation patterns, with smaller particles exhibiting bisectional fragmentation and larger particles undergoing layer-by-layer fragmentation. Under slurry conditions, the porous catalyst exhibits a unique porous PE structure, showing potential for hiPP production.