Creating intraparticle mesopores inside ZSM-5 nanocrystals under OSDA-free conditions and achievement of high activity in LDPE degradation

Abstract

This work presents a simple method for creating intraparticle mesopores inside ZSM-5 nanocrystals using a polymer/seed co-assisted approach under OSDA (organic structural directing agent)-free conditions. The effects of anionic polyacrylamide (PAM), and some derived synthesis parameters, including PAM concentration, seed concentration, sodium hydroxide concentration, and crystallization temperature on the textural properties of the final product, and especially on the intraparticle mesopore formation mechanism were systematically investigated by XRD, N2-adsorption, UV-Raman, SEM, TEM, solid state 27Al MAS NMR, and 29Si MAS NMR analyses. The proposed mechanism includes (1) Introducing a seed solution to guarantee sufficient nucleation; (2) PAM acceleration of phase separation by “immobilizing” the generated ultrafine MFI structure building units, and so playing the role of a flocculating agent in the induction period; (3) Creation of intraparticle and interparticle mesopores after removal of PAM species by calcination, which can be pre-designed by simply optimizing the synthesis parameters. The synthesis involves severe conditions and as a consequence the resulting nano-sized ZSM-5 zeolite simultaneously possesses very high crystallinity and considerable intraparticle mesoporosity. Because of the existence of intraparticle mesopores, the produced ZSM-5 nanocrystal agglomerates exhibit excellent catalytic performance in the LDPE degradation reaction. The temperature ascribed to the maximum degradation rate was found to be 22 K lower than that measured for the production of common nano-sized ZSM-5 aggregates that do not exhibit intraparticle mesoporosity.