Lewis Acidic Aluminosilicates: Synthesis, ²⁷Al MQ/MAS NMR, and DFT-Calculated ²⁷Al NMR Parameters

Area of Science:

• Materials Science
• Catalysis
• Solid-state Chemistry

Background:

• Porous aluminosilicates are vital Lewis acid catalysts in synthesis.
• The specific aluminum species involved in their catalytic activity are not well understood.
• Understanding these species is crucial for designing more efficient catalysts.

Purpose of the Study:

• To synthesize model aluminosilicate networks with defined aluminum centers.
• To characterize the structure and properties of these aluminum species.
• To correlate the structure of aluminum centers with their catalytic potential.

Main Methods:

• Nonhydrolytic sol-gel condensation using a spherosilicate building block and aluminum precursors.
• Comprehensive characterization via multinuclear MAS NMR (1H, 13C, 27Al, 29Si, 31P), FTIR, ICP-OES, gravimetry, and N2 adsorption.
• Advanced 27Al TQ/MAS NMR and DFT calculations for precise aluminum center assignment and correlation analysis.

Main Results:

• Successfully prepared aluminosilicate networks containing [L-AlO3] and [AlO4]- centers.
• Established a direct correlation between the symmetry of aluminum centers and their quadrupole coupling constant (CQ).
• Achieved single-site materials with exclusively [TEPO-AlO3] or [AlO4]- centers and proposed a mechanism for unexpected [AlO4]- formation.

Conclusions:

• The study provides a detailed understanding of aluminum speciation in porous aluminosilicates.
• The findings enable the rational design of tailored aluminosilicate catalysts with specific Lewis acid sites.
• This work advances the field of heterogeneous catalysis by elucidating structure-activity relationships at the atomic level.