Controllable Zeolite AST Crystallization: Between Classical and Reversed Crystal Growth

Area of Science:

• Materials Science
• Crystallography
• Chemical Synthesis

Background:

• Crystal growth mechanisms dictate material properties.
• Reversed crystal growth is a nonclassical mechanism, but its origins from amorphous aggregates are not well understood.
• Key factors driving reversed crystallization require further elucidation.

Purpose of the Study:

• To model reversed crystal growth from amorphous aggregates.
• To investigate the role of synthesis conditions and structure-directing agents (SDA) in controlling crystallization pathways.
• To understand the fundamental factors governing the transition between classical and reversed crystal growth.

Main Methods:

• Microscopy techniques were employed to study the inner structure of aggregates.
• Nano X-ray computed tomography was used to analyze crystallinity development.
• Systematic variation of synthesis conditions, specifically the ratio of structure-directing agent to framework elements, was performed.

Main Results:

• A characteristic model for reversed crystal growth was established, detailing inner structure and crystallinity.
• The crucial function of the structure-directing agent in determining the crystallization pathway was identified.
• A clear switch in crystal growth mode was observed: classical growth at low SDA ratios and reversed growth at high SDA ratios.

Conclusions:

• Synthesis conditions, particularly the structure-directing agent concentration, are critical for controlling crystal growth modes.
• The study provides a framework for understanding and manipulating reversed crystal growth.
• Findings offer insights for optimizing synthesis protocols and designing crystalline materials with desired properties.