Zeolites with isolated-framework and oligomeric-extraframework hafnium species characterized with pair distribution...

Area of Science:

• Heterogeneous catalysis
• Solid-state chemistry
• Materials science

Background:

• Zeolites with heteroatoms (Ti, Sn, Hf) function as solid Lewis acids, offering superior catalytic activity compared to bulk oxides.
• Distinguishing framework-incorporated heteroatoms from extraframework species in zeolites is analytically challenging.
• Characterizing the precise structural environment of heteroatoms is crucial for understanding their catalytic mechanisms.

Purpose of the Study:

• To develop and apply advanced analytical techniques for characterizing hafnium (Hf) species within *BEA zeolite frameworks.
• To differentiate between framework-bound and post-synthetically added extraframework Hf species.
• To correlate the structural location of Hf species with their catalytic performance in the aldol condensation reaction.

Main Methods:

• Coupled pair distribution function (PDF) analysis and diffuse reflectance (DR) UV-vis spectroscopy were employed to probe the local atomic structure.
• PDF analysis allowed for the visualization and identification of both framework and extraframework HfOx species.
• Traditional X-ray and neutron diffraction methods were insufficient for detecting these Hf species.

Main Results:

• PDF analysis successfully identified and distinguished framework Hf species from extraframework HfOx species in *BEA zeolites.
• Both framework and extraframework Hf species were undetectable by conventional X-ray and neutron diffraction.
• Catalytic testing of the aldol condensation reaction demonstrated that framework Hf species are the active sites.

Conclusions:

• The study provides direct structural evidence for framework-incorporated Hf species in *BEA zeolites.
• Combined PDF and DR UV-vis spectroscopy offers a powerful approach for characterizing heteroatom incorporation in zeolites.
• Framework Hf species are confirmed as the primary Lewis acid sites responsible for the observed catalytic activity in the aldol condensation.