Effect of temperature and extraframework cation type on CHA framework flexibility

Area of Science:

• Materials Science
• Chemistry
• Crystallography

Background:

• Zeolite sorption properties depend on framework composition (Si/Al ratio), extraframework (EF) cations, and temperature.
• The CHA (CHA) framework's flexibility under varying conditions is crucial for its applications.
• Contrasting literature results exist regarding the high-temperature (HT) behavior of Na-CHA.

Purpose of the Study:

• Investigate the flexibility of the CHA framework as a function of EF cation content and temperature (20-350 °C).
• Clarify the high-temperature (HT) behavior of Na-CHA.
• Determine the role of temperature and EF cation type in structural response to heating.

Main Methods:

• Analysis of two CHA forms (Na-CHA and Cu-CHA) with a Si/Al ratio of 2.
• Investigation of structural changes using temperature-dependent analysis.
• Evaluation of framework flexibility and cation influence on structural transformations.

Main Results:

• Na-CHA exhibits a significant unit-cell volume contraction (-12%) and symmetry lowering (R-3m to I2/m) at 75 °C.
• This Na-CHA transformation is reversible upon exposure to ambient conditions after dehydration.
• Cu-CHA shows a different dehydration path with minor framework changes and positive thermal expansion post-dehydration.

Conclusions:

• The type of extraframework cation (Na+ vs. Cu2+) dictates the structural response of the CHA framework to temperature changes.
• Understanding these temperature-induced framework transformations is essential for optimizing zeolite performance in gas separation processes.