Tailoring Heterogeneous Catalysts at the Atomic Level: In Memoriam, Prof. Chia-Kuang (Frank) Tsung

Professor Chia-Kuang (Frank) Tsung made his scientific impact primarily through the atomic-level design of nanoscale materials for application in heterogeneous catalysis. He approached this challenge from two directions: above and below the material surface. Below the surface, Prof. Tsung synthesized finely controlled nanoparticles, primarily of noble metals and metal oxides, tailoring their composition and surface structure for efficient catalysis. Above the surface, he was among the first to leverage the tunability and stability of metal-organic frameworks (MOFs) to improve heterogeneous, molecular, and biocatalysts. This article, written by his former students, seeks first to commemorate Prof. Tsung's scientific accomplishments in three parts: (1) rationally designing nanocrystal surfaces to promote catalytic activity; (2) encapsulating nanocrystals in MOFs to improve catalyst selectivity; and (3) tuning the host-guest interaction between MOFs and guest molecules to inhibit catalyst degradation. The subsequent discussion focuses on building on the foundation laid by Prof. Tsung and on his considerable influence on his former group members and collaborators, both inside and outside of the lab.

Shielding against Unfolding by Embedding Enzymes in Metal-Organic Frameworks via a de Novo Approach

We show that an enzyme maintains its biological function under a wider range of conditions after being embedded in metal-organic framework (MOF) microcrystals via a de novo approach. This enhanced stability arises from confinement of the enzyme molecules in the mesoporous cavities in the MOFs, which reduces the structural mobility of enzyme molecules. We embedded catalase (CAT) into zeolitic imidazolate frameworks (ZIF-90 and ZIF-8), and then exposed both embedded CAT and free CAT to a denature reagent (i.e., urea) and high temperatures (i.e., 80 °C). The embedded CAT maintains its biological function in the decomposition of hydrogen peroxide even when exposed to 6 M urea and 80 °C, with apparent rate constants k_obs (s^-1) of 1.30 × 10^-3 and 1.05 × 10^-3, respectively, while free CAT shows undetectable activity. A fluorescence spectroscopy study shows that the structural conformation of the embedded CAT changes less under these denaturing conditions than free CAT.

Formation of hollow and mesoporous structures in single-crystalline microcrystals of metal-organic frameworks via double-solvent mediated overgrowth

The creation of hierarchical porosity in metal-organic frameworks (MOFs) could benefit various applications of MOFs such as gas storage and separation. Having single-crystalline microcrystals instead of poly-crystalline composites is critical for these potential applications of MOFs with hierarchical porosity. We developed a room temperature synthetic method to generate uniform hollow and mesoporous zeolitic imidazolate framework-8 (ZIF-8) microcrystals with a single-crystalline structure via overgrowing a ZIF-8 shell in methanol solution on a ZIF-8 core with water adsorbed in the pores. The cavities formed as a result of the different solvent micro-environment. This double-solvent mediated overgrowth method could be applied to prepare other MOFs with hierarchical porosity.