On the Influence of Catalyst Pore Structure in the Catalytic Cracking of Polypropylene

Catalytic cracking, one of the core processes of the refining of crude oil, is gaining industrial adoption to convert hard-to-recycle polyolefin plastics back into hydrocarbon feedstocks. While structure-composition-performance relationships for converting shorter hydrocarbons over solid acid catalysts, including zeolite-based materials, have been studied extensively, studies focusing on utilizing polyolefins are only now emerging. In this work, we isolate the effect of the catalyst pore size distribution in the catalytic cracking of polypropylene (PP). This was achieved by preparing a set of amorphous silica-alumina (ASA) materials with close to identical external acidity (~30 µmol/g) from silica supports of pore diameters ranging from 7 to 75 nanometers and particle sizes ranging from 1 to 40 µm. Surprisingly, it was observed that the mesopore pore size had only a minor effect on cracking activity, as measured by thermogravimetric analysis (TGA) at high PP:catalyst ratio. In contrast, at low PP:catalyst ratio, the particle size of the ASA catalyst material determined the minimal cracking temperature required. Smaller ASA catalyst particles (<1 µm) are wetted faster by PP due to the larger particle surface area. We rationalized our results by conducting a simplified simulation of pore intrusion utilizing the Lucas-Washburn equation of capillary flow, which suggested that pore filling could occur before the reaction onset in ramped experiments. Our findings indicate that in optimization of plastic cracking catalysts high external acidity and small particle size will play a significantly larger role compared to mesopore size.