Neighboring Zn-Zr Sites in a Metal-Organic Framework for CO2 Hydrogenation

ZrZnOx is active in catalyzing carbon dioxide (CO2) hydrogenation to methanol (MeOH) via a synergy between ZnOx and ZrOx. Here we report the construction of Zn2+-O-Zr4+ sites in a metal-organic framework (MOF) to reveal insights into the structural requirement for MeOH production. The Zn2+-O-Zr4+ sites are obtained by postsynthetic treatment of Zr-6(mu(3)-O)(4)(mu(3)-OH)(4) nodes of MOF-808 by ZnEt2 and a mild thermal treatment to remove capping ligands and afford exposed metal sites for catalysis. The resultant MOF-808-Zn catalyst exhibits >99% MeOH selectivity in CO2 hydrogenation at 250 degrees C and a high space-time yield of up to 190.7 mgMeOH gZn(-1) h(-1). The catalytic activity is stable for at least 100 h. X-ray absorption spectroscopy (XAS) analyses indicate the presence of Zn2+-O-Zr4+ centers instead of ZnmOn clusters. Temperature-programmed desorption (TPD) of hydrogen and H/D exchange tests show the activation of H-2 by Zn2+ centers. Open Zr4+ sites are also critical, as Zn2+ centers supported on Zr-based nodes of other MOFs without open Zr4+ sites fail to produce MeOH. TPD of CO2 reveals the importance of bicarbonate decomposition under reaction conditions in generating open Zr4+ sites for CO2 activation. The well-defined local structures of metal-oxo nodes in MOFs provide a unique opportunity to elucidate structural details of bifunctional catalytic centers.

Automated summary

In this study, ZrZnOx was shown to be active in catalyzing the hydrogenation of CO2 to methanol through a synergy between ZnOx and ZrOx. The construction of Zn2+-O-Zr4+ sites in a metal-organic framework was achieved to reveal insights into the structural requirements for methanol production. The catalytic activity of the resulting MOF-808-Zn catalyst was stable and exhibited high selectivity and yield, with XAS analyses and TPD experiments providing important insights into the catalytic mechanism.