Transport limitations in polyolefin cracking at the single catalyst particle level

Area of Science:

• Chemical Engineering
• Materials Science
• Catalysis

Background:

• Catalytic cracking chemically recycles polyolefins into valuable hydrocarbons like naphtha and aromatics.
• Traditional cracking catalysts are optimized for liquid or gaseous feedstocks, not large polymer molecules.
• Polyolefins exhibit high melt viscosity, posing mass transport challenges for catalyst performance.

Purpose of the Study:

• To investigate mass transport limitations in the catalytic cracking of polypropylene (PP).
• To understand the impact of polymer molecular weight (M_w) on cracking efficiency.
• To evaluate the role of catalyst accessibility in polyolefin conversion.

Main Methods:

• Thermogravimetric experiments with PP of varying molecular weights and catalysts of different porosities.
• In situ optical microscopy and electron microscopy to analyze catalyst-polymer composites at micro- and nano-scales.
• Correlation of polymer melt viscosity with catalyst accessibility and cracking performance.

Main Results:

• Low M_w PP cracked efficiently below 275 °C, while high M_w PP required temperatures 150 °C higher.
• High melt viscosity of high M_w PP severely limits catalyst-polymer contact and access to internal catalyst pores.
• Inner catalyst active sites are underutilized with high M_w polymers due to poor diffusion.
• Catalyst activity can be overestimated when using low M_w model polymers.

Conclusions:

• Mass transport limitations, primarily due to high melt viscosity, significantly impact polyolefin catalytic cracking.
• Polymer molecular weight is a critical factor determining cracking efficiency and catalyst utilization.
• Pre-treatment to reduce polyolefin melt viscosity may be necessary for efficient chemical recycling via catalytic cracking.