Constructing Intrapenetrated Hierarchical Zeolites with Highly Complete Framework via Protozeolite Seeding

Abstract

Creation of intrapenetrated mesopores with open highway from external surface into the interior of zeolite crystals are highly desirable that can significantly improve the molecular transport and active sites accessibility of microporous zeolites to afford enhanced catalytic properties. Here, different from traditional zeolite‐seeded methods that generally produced isolated mesopores in zeolites, nanosized amorphous protozeolites with embryo structure of zeolites were used as seeds for the construction of single‐crystalline hierarchical ZSM‐5 zeolites with intrapenetrated mesopores (mesopore volume of 0.51 cm3g‐1) and highly complete framework. In this strategy, in contrast to the conventional synthesis, only a small amount of organic structure directing agents and a low crystallization temperature were adopted to promise the protozeolites as the dominant growth directing sites to induce crystallization. The protozeolite nanoseeds provided abundant nucleation sites for surrounding precursors to be crystallized, followed by oriented coalescence of crystallites resulting in the formation of intrapenetrated mesopores. The as‐prepared hierarchical ZSM‐5 zeolites exhibited ultra‐long lifetime of 443.9 hours and a high propylene selectivity of 47.92 % at a WHSV of 2 h‐1 in the methanol‐to‐propylene reaction. This work provides a facile protozeolite‐seeded strategy for the synthesis of intrapenetrated hierarchical zeolites that are highly effective for catalytic applications.

The Current Understanding of Mechanistic Pathways in Zeolite Crystallization

“…These are often referred to as protozeolites or embryonic zeolites ,, since they possess elements of the zeolite structure but are very small in size with only short-range order (a few nanometers in length). Studies have demonstrated that seeding growth mixtures with protozeolites can have desirable effects due to their open structures (Figure a), including the promotion of mesoporous features in synthesis products − or the acceleration of crystallization kinetics. , Yu and co-workers demonstrated that ball-milling protozeolites can facilitate the formation of zeolite MFI nanosheets by generating solids with platelike structures where external silanol sites are redistributed in a nonrandom, curvature-dependent fashion with higher probabilities of proximal silanol sites. Local regions with higher densities of silanol sites are more reactive toward crystal growth where a coalescence boundary can develop between the amorphous precursor and the emerging crystalline state (Figure b).…”

Structural Engineering of Hierarchical Zeolite-Based Catalysts

“…The protozeolite-seeded strategy can also be applied to synthesize hierarchical ZSM-5 zeolites with high-quality intrapenetrated mesopores and outstanding hydrothermal stabilities. 33 Benefiting from their high pore connectivity, the hierarchical ZSM-5 zeolites with intrapenetrated mesopores exhibited an ultralong lifetime (443.9 h) and a high propylene selectivity (47.92%) in the MTP reaction.…”

“…In contrast, the mesoporogen-free methods such as kinetic regulation, ,− seed-assisted synthesis, − and steam-assisted synthesis are promising strategies for the efficient and green construction of hierarchical zeolite structures avoiding the usage of mesopore-generating agents. Through kinetic regulation, hierarchical zeolites can be prepared and modulated by controlling both nucleation and growth processes. , For example, our group developed an amino-acid-assisted kinetic regulation method to produce single-crystalline hierarchical ZSM-5 crystals with a two-step crystallization process.…”

“…However, hierarchical zeolites synthesized with crystalline seeds usually exhibit isolated meso-/macropores within the zeolite crystals. Our group developed a protozeolite-seeded approach to realize the construction of hierarchical zeolite crystals with polyhedral faceted mesopores and intrapenetrated mesopores. , Despite the numerous advantages mentioned above, hierarchical zeolites fabricated by mesoporogen-free methods still face the challenge of relatively poor controllability in their pore structures and distributions …”

“…The as-prepared facet-shaped mesoporous ZSM-5 zeolites (ProSeed-Z5-1) with more accessible acid sites and fewer defects exhibited a remarkably improved catalytic performance and a stronger hydrothermal stability in the methanol-to-propylene (MTP) reaction with a higher propylene selectivity of 52.7% and a longer catalytic lifetime of 18.0 h compared to traditional hierarchical ZSM-5 and other counterparts (Figure d). The protozeolite-seeded strategy can also be applied to synthesize hierarchical ZSM-5 zeolites with high-quality intrapenetrated mesopores and outstanding hydrothermal stabilities . Benefiting from their high pore connectivity, the hierarchical ZSM-5 zeolites with intrapenetrated mesopores exhibited an ultralong lifetime (443.9 h) and a high propylene selectivity (47.92%) in the MTP reaction.…”