Creation of Al-Enriched Mesoporous ZSM-5 Nanoboxes with High Catalytic Activity: Converting Tetrahedral Extra-Framework Al into Framework Sites by Post Treatment.

Yilai Jiao,Jingfeng Han,Jihong Yu,Xuqing Liu,Yangtao Zhou,Luke Forster,Huanhao Chen,Carmine D'Agostino,Shaojun Xu,Xiaolei Fan,Jinmin Liu,Jinsong Zhang

doi:10.1002/anie.202002416

Abstract

ZSM‐5 zeolite nanoboxes with accessible meso‐micro‐pore architecture and strong acid sites are important in relevant heterogeneous catalysis suffering from mass transfer limitations and weak acidities. Rational design of parent zeolites with concentrated and non‐protective coordination of Al species can facilitate post‐synthetic treatment to produce mesoporous ZSM‐5 nanoboxes. In this work, a simple and effective method was developed to convert parent MFI zeolites with tetrahedral extra‐framework Al into Al‐enriched mesoporous ZSM‐5 nanoboxes with low silicon‐to‐aluminium ratios of ≈16. The parent MFI zeolite was prepared by rapid ageing of the zeolite sol gel synthesis mixture. The accessibility to the meso‐micro‐porous intra‐crystalline network was probed systematically by comparative pulsed field gradient nuclear magnetic resonance diffusion measurements, which, together with the strong acidity of nanoboxes, provided superb catalytic activity and longevity in hydrocarbon cracking for propylene production.

Highlights

Hierarchical zeolites with mesoporous features are a class of inorganic materials for which previous research has confirmed the protective role of the tetrahedral framework Al for Si extraction in alkaline media
We found that a 6 h post-treatment of as-synthesized parent MFI zeolite (AS-MFI) with 0.1m tetrapropylammonium hydroxide (TPAOH) was sufficient to produce ZSM-5 nanoboxes with excellent crystallinity (relative crystallinity (RC) > 98 %, Figure S2b) and mesoporous features (Figure S3 and Table S1)
It was found that, when bulky TIPB was used as the probing species, (i) unlike n-octane and cumene, PFG-nuclear magnetic resonance (NMR) plots of the materials are comparable for all the zeolite samples (Figure 6 c) and (ii) the rate of diffusion within the ZSM-5-P-0.1-6 sample measured across the whole zeolite particle increases, leading to a significant decrease in t values, which becomes comparable with values measured for the parent and conventional zeolites (Table S13)

Summary

Introduction

The use of structural directing agents (SDAs), especially tetrapropylammonium hydroxide (TPAOH), in the modification of MFI zeolites (such as TS-1,[8] silicalite-1[9] and ZSM5[10]) under alkaline conditions can facilitate the recrystallization of leached species from dissolution (i.e., the dissolutionrecrystallization mechanism[8]), and enable the controlled formation of hollow yet mesoporous zeolite crystals (i.e. nanoboxes). Such zeolite nanoboxes are important for catalysis, being able to allow percolation diffusion and providing the opportunity for functionalization (e.g., encapsulation of metal nanoparticles).[10]. The simple and effective strategy solves the challenge of preparing mesoporous ZSM-5 zeolites with low SAR values

Results and Discussion

Conclusion

Conflict of interest