Beyond Dealumination: Does Fluorine Reshape Zeolitic Acidity?

Area of Science:

• Catalysis
• Materials Science
• Solid-state Chemistry

Background:

• Fluoride post-treatment of ZSM-5 zeolites is common in heterogeneous catalysis.
• Effects are often attributed to dealumination, neglecting fluorine's local chemical influence.
• Zeolitic acidity and catalytic performance are governed by the local chemical environment.

Purpose of the Study:

• Investigate the impact of residual fluorine on ZSM-5 acidity and catalytic behavior.
• Differentiate fluorine's effects from conventional dealumination.
• Elucidate fluorine's influence at the atomic level.

Main Methods:

• Mild fluorination of ZSM-5 via hydrothermal treatment with ammonium fluoride (NH4F).
• Solid-state nuclear magnetic resonance (NMR) spectroscopy: 27Al, 19F, 27Si, and 1H MAS NMR.
• Two-dimensional (2D) 1H double-quantum single-quantum (DQ-SQ) MAS NMR for comparative analysis with silicalite-1.

Main Results:

• Fluorine incorporation perturbs ZSM-5's acidic properties distinctly from dealumination.
• Fluorine induces Brønsted acid site (BAS)-like behavior through local polarization.
• Fluorinated ZSM-5 shows improved catalytic lifetime and aromatic selectivity in methanol-to-hydrocarbon (MTH) reactions.

Conclusions:

• Residual fluorine significantly alters ZSM-5's local chemical environment and acidity.
• Fluorine's electronic influence is key to enhanced catalytic performance, beyond dealumination.
• Fluorinated ZSM-5 offers a promising route for improved heterogeneous catalysis.