Strongly Lewis Acidic Metal-Organic Frameworks for Continuous Flow Catalysis

Area of Science:

• Materials Science: Synthesis and characterization of novel porous materials.
• Catalysis: Development of heterogeneous Lewis acid catalysts for organic transformations.

Background:

• Highly acidic metal-organic frameworks (MOFs) are of significant research interest for catalytic applications.
• Existing MOFs often require harsh conditions or lack sufficient Lewis acidity for broad applicability.

Purpose of the Study:

• To design and synthesize a strongly Lewis acidic MOF, ZrOTf-BTC, with enhanced catalytic properties.
• To develop a composite material (ZrOTf-BTC@SiO2) for efficient continuous flow catalysis.
• To quantitatively measure the Lewis acidity and compare catalytic performance against homogeneous benchmarks.

Main Methods:

• Two-step transformation of MOF-808 (Zr-BTC) using hydrochloric acid and trimethylsilyl triflate (Me3SiOTf).
• Quantitative Lewis acidity measurement using electron paramagnetic resonance (EPR) and fluorescence spectroscopy.
• Preparation of ZrOTf-BTC@SiO2 composite for continuous flow reactor studies.

Main Results:

• Successfully synthesized ZrOTf-BTC with strong Lewis acidity (ΔE = 0.99 eV), comparable to Sc(OTf)3.
• ZrOTf-BTC exhibited high catalytic activity and stability in various organic reactions (Diels-Alder, epoxide ring-opening, Friedel-Crafts acylation, alkene hydroalkoxylation).
• The ZrOTf-BTC@SiO2 composite demonstrated exceptional turnover numbers (TONs) in continuous flow catalysis.

Conclusions:

• Triflation is an effective strategy to create strongly Lewis acidic sites within MOFs.
• ZrOTf-BTC and its SiO2 composite are highly active and stable solid Lewis acid catalysts.
• The developed MOF@SiO2 composite enables efficient continuous flow catalysis for important organic transformations.