Exploiting polyolefin materials to create a circular economy is possible via catalytic chemical recycling, as they represent a highly refined hydrocarbon source with similarities to crude oil, and this is an important supplement to mechanical recycling of plastics. Whilst some polymers such as poly(ethylene terephthalate) (PET) are amenable to mechanical recycling, polyolefins degrade during reprocessing, and are not suitable for many applications (such as food contact) after the high temperatures and conditions of this classical approach to recycling.
Bifunctional zeolite catalysts have been demonstrated to depolymerise polyolefins via hydrocracking, and this work aims to replace expensive and earth-scarce precious metals such as platinum and palladium with earth abundant transition metal chalcogenides. Nickel sulfide, a common hydrocracking and hydrotreating catalyst for petroleum refining, was synthesized from the single source precursor (SSP) nickel (II) ethyl xanthate. This is a novel application of this synthesis method to supported catalysts, which are traditionally created by wet impregnation of aqueous nickel salts, followed by high temperature sulfidation using H2S, a flammable and highly toxic gas. Avoiding the use of hazardous reagents is another motivating factor in this research.
Polyolefin materials represent a distinct challenge for heterogeneous catalysis due to their high molecular weights; Mass and heat transfer are inefficient due to the high viscosity of most polymer melts, and long carbon chains hinder access to catalytic sites, notably the active site-containing micropores of zeolite catalysts that have proven effective in hydrocarbon transformations in industrial processes.
In order to overcome some of these limitations, we have investigated various aluminosilicate supports, with varying degrees and types of porosity. Zeolite beta, molecular sieve 13X, MCM-41 and SBA-15 were chosen for this initial survey as commercially available support materials with an acidic function.
Dr. Aleksander Tedstone (Manchester) is a post-doctoral researcher in the field of material synthesis and design, currently working to improve catalytic chemical recycling processes for waste plastics. His research interests include porous materials, metal chalcogenides and nanoparticles, and particularly real-world applications in which they can be utilised, as part of the Chemical Engineering and Analytical Science Department at the University of Manchester.