Evolution of structure and texture of the Beta zeolite recrystallization-derived materials: Impact of alkaline concentration and exploratory application towards LDPE catalytic pyrolysis

Abstract

The design and preparation of structures involving hierarchical porous systems have attracted interest due to their potential applications in heterogeneous catalysis. In this work, a series of micro/mesoporous composite materials was prepared from Beta zeolite (H-BEA), via a two-step recrystallization method, varying the initial alkaline concentration of the reaction medium (i.e., 0.2, 0.6, 1.2, and 1.8 M aqueous NaOH) generating the MM-02, MM-06, MM-12, and MM-18 samples, respectively. Several techniques, such as FT-IR, XRPD, ²⁷Al MAS NMR, FESEM-EDX, TEM, and N₂ physisorption, were used to study the structure, acidity, and texture of the resulting materials. The alkaline treatment at the lowest concentration led to a material practically preserving the total concentration of acid sites of the H-BEA precursor and resulting in a hierarchical porous structure having intracrystalline mesopores and the intrinsic microporosity of Beta zeolites (MM-02). Meanwhile, a Beta/MCM-41 composite possessing an interesting hierarchical pore system with a non-conventional N₂ isotherm was synthesized using 0.6 M NaOH (MM-06). Besides, the use of stronger alkalinity induced the formation of materials characterized by a poor contribution of microporosity caused by zeolite degradation, but showing a significant mesoporous presence from MCM-41 (MM-12 and MM-18). Finally, exploration assays of Low-Density Polyethylene (LDPE) catalytic pyrolysis were performed on the rich-zeolite hierarchical materials with a sample-to-catalyst mass ratio of 1/7 at 550 °C. The MM-02 catalyst showed the best performance, yielding selectivity values of 51 and 37% to BTEX (Benzene, Toluene, Ethylbenzene, and Xylenes) and alkanes, respectively, with suppression of polycyclic aromatic hydrocarbons (PAH) production.