(340aj) Developing Tunable Solid Acid Catalysts

Solid acid catalysts are critical components in the production of fine chemicals like fragrances and pharmaceuticals. Compared to the liquid acids traditionally used in these processes (i.e. AlCl­₃, H₂SO_4­), solid acids provide benefits such increased handling safety, reduced corrosiveness, and the ability to be run neat without solvent, which can enhance reaction kinetics and reduce chemical waste generation. Additionally, subsequent separation processes are greatly simplified with a solid catalyst, reducing the overall process’s CAPEX. However, significant effort is required to tune the distribution, strength, and density of acid sites in solid acid materials to optimize their performance for specific applications.

My research focuses on studying the acidity of mixed metal oxide materials, such that they can be better engineered for individual applications. “Overcoated” mixed metal oxide materials were synthesized by stoichiometrically-limited sol-gel deposition of tetraethyl orthosilicate onto Al₂O₃, TiO₂, Nb₂O₅, and ZrO₂ nanopowders and activated by calcination in air. Pyridine diffuse reflectance infrared spectroscopy (DRIFTS) and trimethylphosphine oxide (TMPO) ³¹P MAS NMR experiments were conducted to study the acidic properties of the catalysts as a function of the deposited SiO₂ loading. DRIFTS experiments indicate that SiO₂ deposition generates Brønsted acid sites in all materials, which are likely pseudo-bridging type silanol groups whose acidity is enhanced by the Lewis acidic metal oxide support. TMPO ³¹P MAS NMR experiments rank the composite catalysts’ acidity in the order: SiO₂/Al₂O₃ > SiO₂/Nb₂O₅ > SiO₂/TiO₂ > SiO₂/ZrO₂.

Catalyst performance was evaluated in liquid-phase tetrahydropyranylation, which is a useful reaction for reversibly protecting alcohol, phenol, and acetal functionalities. As shown in Figure 1, the initial activity of the overcoated catalysts rapidly increases with SiO₂ deposition. Since no conversion is observed over the uncoated, Lewis acidic metal oxides, this increase in activity is attributed to the generation of Brønsted sites in the overcoated materials. The differences in activity observed between the series of the materials strongly correlates with their maximum acid strength as determined by ³¹P NMR. Together, these trends indicate the activity of these materials can be optimized through careful selection of the SiO₂ loading and the metal oxide core. In conclusion, we have shown that the deposition of SiO₂ onto Lewis acidic metal oxides generates Brønsted sites at the constructed SiO₂ – MO_x interface. Additionally, the strength and density of Brønsted sites in the composite materials can be independently tuned through synthesis, making these materials promising catalysts for specialty applications.