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 Zr6O8 node in NU-1000 results in heterolytic H-2 cleavage over adjacent Lewis acid and base sites, leading to catalytic conversion of 1-butene. In this work, given the ubiquity of the Zr6O8 node 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 Zr6O8 cluster for H-2 activation and 1-butene hydrogenation and isomerization. Diffuse reflectance IR in the presence of H-2 and pyridine shows that both thermal pretreatment and MOF topology affect the Bronsted acidity of protons generated from H-2 activation and their resulting activity for olefin conversion. High isomerization activity of dehydrated NU-1000 is correlated with the formation of mu 3OH species after H-2 activation. Additionally, catalytic studies show that the geometry of open coordination sites on individual Zr6O8 nodes 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 NH3 adsorption. 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.