Ultrafast surfactant-templating of *BEA zeolite: An efficient catalyst for the cracking of polyethylene pyrolysis vapours

Abstract

Introducing intracrystalline mesoporosity to zeolites is an effective way to reduce their diffusion limitations and enhancing their catalytic performance in reactions where large molecules are involved. The emergence of the so-called surfactant-templating technique has enriched the toolbox of methods to prepare mesoporous zeolites, in which amphiphilic molecules are used to assist in and direct the generation of mesopores in a pre-existing zeolite structure under alkaline conditions. We herein present ultrafast surfactant-templating of *BEA zeolite, which is achieved in just a few minutes with a tubular reactor that features fast heating. Two *BEA zeolites of the same Si/Al ratio, but differing in crystal size, are employed to demonstrate the concept. Tuning the interplay between the surfactant concentration and the alkalinity leads to the ultrafast generation of mesoporosity in the *BEA zeolite with the larger crystal size (BEA (L)) in 5 min; while it fails to generate uniform mesopores to the *BEA zeolite with the smaller crystal size (BEA (S)) under otherwise identical conditions. The ultrafast surfactant-templating of *BEA (S) is achieved only when it is partially dealuminated through a pre-treatment with acid. Our results provide new insights on the synergetic effect between the surfactant and the base that governs the surfactant-templating process and on how to produce intracrystalline mesoporosity with narrow pore size distribution under ultrafast conditions (2 min). The resultant mesoporous *BEA zeolites have been tested in the catalytic cracking of the vapours produced by pyrolyzing low-density polyethylene, a promising reaction for converting waste plastics into fuels. Improved selectivity towards aromatic hydrocarbons and light olefins is observed as a result of enhanced accessibility and availability of active sites, validating the superiority of the surfactant-templated *BEA zeolites.