Departmental seminar by Ass. Professor James W. Harris, Department of Chemical and Biological Engineering, University of Alabama

Quantifying active sites under reaction conditions is paramount to quantification of per-site turnover frequencies and consequently to direct comparisons of the reactivity of heterogeneous catalysts. In this talk, I will give two examples where we have attempted to separate the functions of, and quantify, multiple types of active sites present on two different classes of heterogeneous catalysts. In the first, I will report on our efforts to prepare supported bimetallic catalysts for oxidative coupling reactions (e.g., coupling of methanol and dimethylamine to form dimethylformamide) across a range of dilute bimetallic (M₁-M₂) nanoparticles of varied M₁ and M₂ identities (M₁= Ag, Ni, Pd, Pt; M₂ = Au, Cu) and M₁:M₂ ratios (0<M₁:M₂<∞).

These studies demonstrate the benefits on reactivity and selectivity of isolating small ensembles of oxyphilic metals within coinage metal nanoparticle hosts, as well as the significant restructuring that occurs when reaction conditions vary between reducing to oxidizing environments. Quantification of active sites via transient wash-out experiments in this chemistry gives unexpected results that lead to improbable quantities of active sites per metal atom and is a topic of our ongoing research. In the second example, I will discuss our contributions in catalysis for the production of sustainable aviation fuel (SAF) from abundant renewable alcohols. This reaction occurs over multifunctional copper-, zinc-, and yttrium-containing dealuminated Beta zeolites, via a series pathway requiring dehydrogenation, aldol condensation, hydrogenation, and dehydrations in a precise sequence.

We have quantified the number of Lewis acidic active sites in these materials via ex situ and in situ chemical titrations, and find that there is evidence for at least two types of yttrium sites that collectively catalyze dehydration, aldol condensation, and Meerwein-Pondorff-Verley reduction. The kinetics of butadiene formation from ethanol-acetaldehyde mixtures over monometallic yttrium- or lanthanum-containing dealuminated Beta zeolite catalysts are disparate, suggesting predictable differences in these metals as primary binding sites for adsorption and conversion of small oxygenates. These findings aid in our ongoing efforts to reduce the cost and carbon footprint of SAF production.