Revisiting glucose isomerization and epimerization catalyzed by Cr-MIL-101 in aqueous phase: Kinetics and deactivation mechanism

Understanding the kinetics and mechanism of deactivation process favors the design of more effective strategies to improve catalyst function. We herein investigate the deactivation mechanism of Cr-MIL-101 catalyst during aqueous-phase glucose isomerization and epimerization. The study on effects of synthesis protocols and post-activation excludes the possibility of mass-transfer limitation in confined nanocage as the origin of limited catalytic activity. Contrary to the previous observation, the isotopic-tracer and NMR spectroscopy demonstrates that the formation of mannose proceeds by two mechanisms, i.e., the intramolecular 1, 2-carbon shift and two-step isomerization. Thermodynamic profiling reveals that Cr-MIL-101-catalyzed aqueous-phase glucose transformation exhibits a low intrinsic activity and deactivation phenomenon. Theoretical calculations confirm the non-selective nature of trinuclear chromium (III) clusters within the framework architecture, which demonstrate comparable adsorption affinities toward hexoses (glucose, fructose, and mannose). Additionally, the residual terephthalic acids, solvent water, and certain by-products can bind to the Cr sites, indicating that catalyst deactivation is primarily due to Cr-site-blocking. Based on these findings, a set of generalized phenomenological kinetic and deactivation kinetics models capturing the most relevant features of deactivation were developed and validated by experimental dataset. These models quantitatively describe the kinetic and deactivation behaviors, explicitly accounting for the underlying mechanism of competitive binding. Eventually, kinetic parameters derived from model regression analysis are discussed in relation to the underlying reaction pathways. The potential strategies to enhance intrinsic catalytic activity and mitigate catalyst deactivation are proposed.