The Dependence of Olefin Hydrogenation and Isomerization Rates on Zirconium Metal-Organic Framework Structure

Zirconium metal-organic frameworks (Zr-MOFs) are a structurally diverse and well-defined class of materials studied in heterogeneous catalysis. Previously, we showed that partial dehydration of the Zr₆O₈node in NU-1000 results in heterolytic H₂cleavage over adjacent Lewis acid and base sites, leading to catalytic conversion of 1-butene. In this work, given the ubiquity of the Zr₆O₈node as a secondary building unit (SBU) in Zr-MOFs, with many different potential MOF topologies and capping ligands surrounding the cluster, we study the influence of thermal pretreatments and MOF topology (MOF-808, NU-1000, UiO-66, and NU-1000-NDC) on the activity of the Zr₆O₈cluster for H₂activation and 1-butene hydrogenation and isomerization. Diffuse reflectance IR in the presence of H₂and pyridine shows that both thermal pretreatment and MOF topology affect the Brønsted acidity of protons generated from H₂activation and their resulting activity for olefin conversion. High isomerization activity of dehydrated NU-1000 is correlated with the formation of μ₃OH species after H₂activation. Additionally, catalytic studies show that the geometry of open coordination sites on individual Zr₆O₈nodes influences butene hydrogenation. For this reason, MOF-808 gives anomalously low hydrogenation activity, despite its relatively high total number of open coordination sites, as calculated either from its crystal structure or from NH₃adsorption. These results reiterate the importance of pretreatment in defining MOF catalytic activity and demonstrate that MOF topology, outside of simply affecting node accessibility, influences reactivity at individual nodes.