Synthesis of Self-Pillared Zeolite Nanosheets by Repetitive Branching

Abstract

Hierarchical zeolites are a class of microporous catalysts and adsorbents that also contain mesopores, which allow for fast transport of bulky molecules and thereby enable improved performance in petrochemical and biomass processing. We used repetitive branching during one-step hydrothermal crystal growth to synthesize a new hierarchical zeolite made of orthogonally connected microporous nanosheets. The nanosheets are 2 nanometers thick and contain a network of 0.5-nanometer micropores. The house-of-cards arrangement of the nanosheets creates a permanent network of 2- to 7-nanometer mesopores, which, along with the high external surface area and reduced micropore diffusion length, account for higher reaction rates for bulky molecules relative to those of other mesoporous and conventional MFI zeolites.

Hierarchy concepts: classification and preparation strategies for zeolite containing materials with hierarchical porosity

“…increase in conversion and/or selectivity as well as better resistance to coke formation. 37,84,202 Based on the very promising results, attempts to commercialise hierarchically porous zeolites have been initiated and are still in progress. 66,203,204 Although the results of such catalytic studies indicate the potential for technical applications for hierarchically porous systems, from the scientific point of view their interpretation in terms of mass transport/reaction relationships is still a challenging task and matter of debate.…”

“…Their hierarchical porosity consists of the zeolitic micropores and additional intercrystalline porosity created by the three-dimensional nanosheet assembly. 84 The repetitive branching of the MFI-type zeolite nanosheets was found to be based on a twinning mechanism due to the formation of MEL-type zeolite domains within the MFI structure. MEL and MFI are isostructural (structural twins).…”

“…Non-templating methods are either based on the aggregation of nanocrystals to form intercrystalline mesopores 65,82,83 or on the controlled crystallisation of amorphous gels into zeolite crystals with intracrystalline mesopores 48 or on the selective modification of the zeolitic crystal growth direction (twinning). 84 The aggregation or self-assembly of nanocrystals is the widespread method to create additional porosity without utilising any mesoporogen. 65,82,83 Although promising, the self-assembly of nanocrystals results in unstable intercrystalline mesopores or macropores, which can be easily lost due to the (sometimes) very tight agglomeration of the nanocrystals, especially due to treatment stresses during catalytic applications.…”

“…9), but also as 'morphology modifiers' by inducing twin formation. 84 However, such organic molecules are often difficult to prepare and require a post-synthetic treatment for their removal.…”

Exploring Zeolite Chemistry with the Tools of Surface Science: Challenges, Opportunities, and Limitations

“…40,41 At variance, the model system described here is not a truncated framework that would result in surfaces with dangling bonds and/or silanol groups. Instead, the H-2dH zeolite film is a standalone structure with all bonds covalently saturated within the framework, and only weakly interacting with the metal support.…”