CO₂ hydrogenation to methanol over Pt functionalized Hf-UiO-67 versus Zr-UiO-67

Sustainable methanol formation from CO₂/H₂ is potentially a key process in the post-fossil chemical industry. In this study, Hf- and Zr-based metal-organic framework (MOF) materials with UiO-67 topology, functionalized with Pt nanoparticles, have been tested for CO₂ hydrogenation at 30 bar and 170-240°C. The highest methanol formation rate, 14 mol_methanol mol_Pt^-1 h^-1, was obtained over a Hf-based catalyst, compared with the maximum of 6.2 mol_methanol mol_Pt^-1 h^-1 for the best Zr-based analogue. However, changing the node metal did not significantly affect product distribution or apparent activation energy for methanol formation (44-52 kJ mol^-1), strongly indicating that the higher activity of the Hf-based analogues is associated with a higher number of active sites. Both catalysts showed stable catalytic performance during testing under kinetic conditions, but the addition of 2 vol% water to the feed induced catalyst deactivation, in particular the Hf-MOFs. Interestingly, mainly methanol and methane formation rates decreased, while CO formation rates were less affected by deactivation. No direct correlation was found between catalytic stability and framework stability (crystallinity, specific surface area). Experimental and computational studies suggest that water adsorption strength to the MOF node may affect the relative catalytic stability of Hf-UiO-67-Pt versus Zr-UiO-67-Pt methanol catalysts.This article is part of the discussion meeting issue 'Green carbon for the chemical industry of the future'.