Polytype distributions in low-defect zeolite beta crystals synthesized without an organic structure-directing agent

• Zeolite beta crystals were synthesized without an organic structure-directing agent.
• Direct visualization with HRTEM confirms the crystals are virtually defect free.
• Proportions of BEA and BEB polytypes were measured directly from HRTEM micrographs.
• The distribution of the two polytypes can be explained by a simple stochastic growth mechanism.
• Crystallization of multimembered rings on {001} surfaces leads to epitaxial growth.

Design of efficient and environmentally benign zeolitic materials at the nanoscale is becoming increasingly important for a range of industrial applications. Here we report a high resolution transmission electron microscopy investigation of the atomic level structure of zeolite beta synthesized without an organic structure-directing agent (OSDA). Direct visualization of zeolite beta crystals confirmed them to be essentially defect free, with domain boundaries the only disruption to the pore channel network. Direct visualization enabled the distribution of BEA and BEB polytype subdomains to be quantified, and the overall abundance ratio was calculated to be BEA:BEB = 9:11. By assuming this ratio corresponds to the probability of forming a new BEA or BEB layer on a (001) surface at the tip of a crystal, the experimentally observed polytype distributions can be described by a simple probability function, suggesting that epitaxial crystal growth occurs stochastically in a state close to chemical equilibrium.

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