What Is Measured When Measuring Acidity in Zeolites with Probe Molecules?

Area of Science:

• Materials Science
• Physical Chemistry
• Catalysis

Background:

• Zeolites are crucial porous materials with tunable acidity.
• Brønsted acid sites in zeolites are key catalytic centers.
• Understanding acidity is vital for catalyst design and performance.

Purpose of the Study:

• To investigate how confinement effects within MOR and MFI zeolites influence acidity measurements.
• To analyze the interaction of different probe molecules (CO, NH3, pyridine) with Brønsted acid sites.
• To elucidate the factors contributing to the heterogeneity of acidity in zeolites.

Main Methods:

• Theoretical calculations were employed to model probe molecule adsorption.
• Adsorption at various sites within MOR and MFI zeolite structures was simulated.
• Interactions between probe molecules and Brønsted acid sites were analyzed.

Main Results:

• Weak bases like CO form adducts whose interactions are modulated by pore dimensions and dispersion forces.
• Strong bases like NH3 and pyridine undergo protonation, forming ion pairs.
• Adsorption enthalpies reflect both proton transfer and ion pair stabilization, revealing acidity heterogeneity.

Conclusions:

• Confinement effects significantly alter probe molecule interactions and acidity measurements in zeolites.
• The nature of the probe molecule (weak vs. strong base) dictates the adsorption mechanism and observed acidity.
• Zeolite acidity can be heterogeneous due to complex interactions within the confined pore structure.