ABSTRACT
Metallocene immobilization onto a solid support helps to overcome the drawbacks of homogeneous metallocene complexes in the catalytic olefin polymerization. In this study, valuable insights have been obtained into the effects of pore size, linker composition, and surface groups of metal-organic frameworks (MOFs) on their role as support materials for metallocene-based ethylene polymerization catalysis. Three distinct Zn-based metal-organic frameworks (MOFs), namely, MOF-5, IRMOF-3, and ZIF-8, with different linkers have been activated with methylaluminoxane (MAO) and zirconocene complexes, followed by materials characterization and testing for ethylene polymerization. Characterization has been performed by multiple analytical tools, including X-ray diffraction (XRD), scanning electron microscopy (SEM), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and CO Fourier transform infrared (FT-IR) spectroscopy. It was found that the interactions between MOFs, MAO, and the zirconocene complex not only lead to both catalyst activation and deactivation but also result in the creation of multiple active sites. By alteration of the MOF support, it is possible to obtain polyethylene with different properties. Notably, ultrahigh molecular weight polyethylene (UHMWPE, M W = 5.34 × 106) was obtained using IRMOF-3 as support. This study reveals the potential of MOF materials as tunable porous supports for metallocene catalysts active in ethylene polymerization.
ABSTRACT
Turning waste into valuable products is one of the main challenges of the chemical industry. In this work, chitosan (CS), an abundant, low-cost, and non-toxic biopolymer derived from chitin, was reshaped into beads of ~ 3 mm. Their suitability as a support material for active phase catalyst materials was tested for a zirconium-based Metal-Organic Framework (MOF) with incorporated Pt, namely UiO-67-Pt. Its incorporation was investigated via two procedures: a one-pot synthesis (OPS) and a post-synthetic functionalization (PSF) synthesis method. Scanning electron microscopy (SEM) images show good UiO-67-Pt dispersion throughout the CS beads for the one-pot synthesized material (UiO-67-Pt-OPS@CS). However, this uniform dispersion was not observed for the post-synthetically functionalized material (UiO-67-Pt-PSF@CS). The success of the implementation of UiO-67-Pt was evaluated with ultraviolet-visible and infrared spectroscopy for both composite materials. Thermogravimetric analysis reveals higher thermal stabilities for UiO-67-Pt-OPS@CS composite beads in comparison to pure CS beads, but not for UiO-67-Pt-PSF@CS. The study provides valuable insights into the potential of chitosan as a green, bead-shaped support material for MOFs, offering flexibility in their incorporation through different synthesis routes. It further contributes to the broader goal of the sustainable and eco-friendly design of a new generation of catalysts made from waste materials.
ABSTRACT
Polyolefin catalysts are characterized by their hierarchically complex nature, which complicates studies on the interplay between the catalyst and formed polymer phases. Here, the missing link in the morphology gap between planar model systems and industrially relevant spherical catalyst particles is introduced through the use of a spherical cap Ziegler-type catalyst model system for the polymerization of ethylene. More specifically, a moisture-stable LaOCl framework with enhanced imaging contrast has been designed to support the TiCl4 pre-active site, which could mimic the behaviour of the highly hygroscopic and industrially used MgCl2 framework. As a function of polymerization time, the fragmentation behaviour of the LaOCl framework changed from a mixture of the shrinking core (i.e., peeling off small polyethylene fragments at the surface) and continuous bisection (i.e., internal cleavage of the framework) into dominantly a continuous bisection model, which is linked to the evolution of the estimated polyethylene volume and the fraction of crystalline polyethylene formed. The combination of the spherical cap model system and the used advanced micro-spectroscopy toolbox, opens the route for high-throughput screening of catalyst functions with industrially relevant morphologies on the nano-scale.
