Quantifying Site Heterogeneity in Microporous Aluminosilicates and Implications for Catalysis.

Area of Science:

• Materials Science
• Catalysis
• Chemistry

Background:

• Microporous materials like zeolites are vital acid catalysts in refining and biomass conversion.
• Hydrothermal conditions create complex catalytic behaviors due to nonframework sites, affecting reproducibility.
• Existing Brønsted and Lewis site classifications are insufficient for understanding steamed aluminosilicates.

Purpose of the Study:

• To develop and validate characterization and titration protocols for quantifying site heterogeneity in steamed aluminosilicates.
• To differentiate between framework, partially hydrolyzed, and extraframework sites.
• To establish a quantitative method for site distribution analysis without catalyst modification.

Main Methods:

• Utilized commercial MFI aluminosilicates (ZSM-5) with varying site heterogeneity and Si/Al ratios.
• Employed temperature-programmed desorption and Fourier-transform infrared spectroscopy (FTIR) protocols.
• Correlated quantified site distribution with propane cracking rate constants.

Main Results:

• Developed a nuanced titration strategy to quantitatively determine aluminosilicate site heterogeneity and Al content.
• Demonstrated the impact of water on Al coordination, even at ambient conditions, especially with higher Al content.
• Quantified site distribution and its effect on propane cracking kinetics.

Conclusions:

• The developed titration strategy provides an accessible method to determine site heterogeneity in dried zeolites.
• This approach aids in evaluating structure-performance relationships for acidic zeolites.
• Findings contribute to more reproducible and reliable catalyst research by addressing site variability.