Protozeolite‐Seeded Synthesis of Single‐Crystalline Hierarchical Zeolites with Facet‐Shaped Mesopores and Their Catalytic Application in Methanol‐to‐Propylene Conversion

Abstract

Hierarchical zeolites integrating intrinsic micropores with secondary meso-and/or macropores afford superior catalytic properties for enhanced mass transportation and more accessible active sites. The mesopores generated by using mesoporogens or framework etching are usually irregular with abundant defective sites and low hydrothermal stability. Here, using protozeolite nanoparticles as seeds, we succeeded in the synthesis of single-crystalline hierarchical ZSM-5 zeolites with hexagonal mesopores faceted by complete microporous frameworks. The protozeolites played a key role in the formation of faceted mesopores achieved via intraparticle ripening process. Thanks to the unique texture properties and more accessible acid sites at the open location, the as-prepared zeolites exhibited remarkedly long lifetime (18 h) and high propylene selectivity (52.7 %) with propylene/ethylene ratio of 3.64 in the methanol-to-propylene reaction (WHSV = 12.5 h À 1 ) compared to the counterparts.

Tetrapropylammonium Hydroxide Treatment of Aged Dry Gel to Make Hierarchical TS-1 Zeolites for Catalysis

“…In detail, the diffuse diffraction rings and TEM images of ST­(90 °C) show that it is amorphous with the irregular mesoporous structure (Figure a). After the 2-h TPAOH treatment, the SAED pattern of HTS-0.1-0.1-2H shows a broad diffraction ring, suggesting that the material has a medium-range order . In addition, the morphology of HTS-0.1-0.1-2H changed to aggregates of nanoparticles (Figure b).…”

“…XRD patterns (Figure 3e) of the pristine dry gel ST(90 °C) show the very weak characteristic diffraction peaks around 23°, which shows the amorphous feature of the aged dry gel with partial MFI structure. 21,34,35 After a 2-h TPAOH treatment, the intensity of the characteristic peaks was enhanced in HTS-0.1-1.0-2H, indicating the increase in the crystallinity in the solid phase. 21 By increasing the treatment time to 3 h, crystallization to the MFI structure was achieved with HTS-0.1-1.0-3H having the RC value of 72%.…”

“…21,34,35 After a 2-h TPAOH treatment, the intensity of the characteristic peaks was enhanced in HTS-0.1-1.0-2H, indicating the increase in the crystallinity in the solid phase. 21 By increasing the treatment time to 3 h, crystallization to the MFI structure was achieved with HTS-0.1-1.0-3H having the RC value of 72%. UV−vis analysis shows that ST(90 °C) contains ∼67% extraframework Ti species and ∼33% framework Ti species (Figure 3f).…”

“…After the 2-h TPAOH treatment, the SAED pattern of HTS-0.1-0.1-2H shows a broad diffraction ring, suggesting that the material has a medium-range order. 21 In addition, the morphology of HTS-0.1-0.1-2H changed to aggregates of nanoparticles (Figure 4b). For HTS-0.1-0.1-3H, its SAED pattern shows the diffraction spots which are arranged irregularly, indicating the formation of the crystalline phase after treating the dry gel for 3 h, and the aggregates are composed of disordered accumulation of nanocrystals (Figure 4c).…”

“…Serrano et al prepared an aged zeolite precursor which was used to prepare TS-1 and ZSM-5 nanocrystalline aggregates by silanization. , Recently, Yu et al employed the aged precursor as seeds for the synthesis of single-crystal hierarchical ZSM-5 zeolites with hexagonal mesopores faceted by microporous domain. The aged precursor played a key role in the formation of faceted mesopores via intraparticle ripening process . Hence the aged precursors show promise to synthesize novel hierarchical zeolites.…”