A model molecule for studying the production of renewable light olefins by cracking biomass derived alkanes

In response to concerns over plastic waste, there has been a shift toward developing green plastics to reduce the environmental impact of petroleum-based materials. Advances in processing technology are key to converting biomass into bio-based monomers for sustainable biopolymers like green polyethylene and polypropylene. Renewable fats and oils, due to their wide availability, biodegradability, cost-effectiveness, and low toxicity, have become important platform chemicals for creating renewable polymers. This study investigates the selective conversion of hydrotreated esters and fatty acids into light olefins using a cracking strategy within a continuous fixed-bed reactor, employing ZSM-5 zeolite as the catalyst. The research focuses on optimizing operational conditions and the physicochemical properties of ZSM-5 to maximize green light olefin production. Key factors like cracking temperature, hydrocarbon partial pressure, and Weight Hourly Space Velocity (WHSV) were optimized to boost conversion rates and light olefin yields while minimizing undesirable reactions, such as hydrogen transfer. The study found that limited diffusion restrictions of linear alkanes occur within the ZSM-5 porous network leading to minimal gains in activity or light olefin selectivity, even when reducing crystal size. Adjusting the silicon-to-aluminum (Si/Al) ratio had little effect on product selectivity, underscoring the zeolite's limited ability to promote bimolecular hydrogen transfer. Moreover, even if positioning acid sites within specific channels allows to limit hydrogen transfer side reactions leading to improved selectivities to light olefins at low cracking temperature of 400 °C. At higher reaction temperature, where cracking mechanisms are promoted and hydrogen transfer reactions are not thermodynamically favoured, no significant enhancement of the confinement effect or promotion of monomolecular reactions over bimolecular ones are observed. Despite these limitations, the study successfully demonstrated the considerable potential for producing light olefins through the cracking of renewable paraffins. Under constant reaction conditions of 600 °C, atmospheric pressure, and an initial hydrocarbon partial pressure of 0.33 bar, high yields of 65 % for light olefins and 35 % for propylene were achieved over ZSM-5 having a Si/Al ratio of 30 at an optimized WHSV of 250 h⁻¹, with near-complete conversion.