Metal-organic frameworks (MOFs) show great prospect as catalysts and catalyst support materials. Yet, studies that address their dynamic, kinetic and mechanistic role in target reactions are scarce. In this study, exceptionally stable MOF catalysts consisting of Pt nanoparticles (NPs) embedded in Zr-based UiO-67 MOFs, UiO-67-Pt, was subject to steady-state and transient kinetic studies involving H/D and 13C /12C exchange, coupled with operando infrared spectroscopy and density functional theory (DFT) modeling, targeting methanol formation from CO2/H2 feeds at 170-240 °C and 1-30 bar pressure. The UiO-67 MOFs consisted of Zr6O4(OH)4 nodes connected by 90 % biphenyl-4,4’-dicarboxylic acid linkers (BPDC) and 10 % bipyridine-4,4’-dicarboxylic acid linkers (BPY), forming 3-dimensional, porous frameworks with fcc structure. The BPY linkers served as anchoring sites to PtCl2, which was subsequently in-situ reduced to Pt NPs with appx. 3.5 nm diameter during pretreatment in 10% H2/He flow at 350 °C. Comparison between UiO-67-Pt and a dense Pt/ZrO2 catalyst yielded 42% methanol selectivity over UiO-67-Pt, but only 6 % methanol selectivity over Pt/ZrO2, at similar CO2 conversion at 170 °C, 30 bar and CO2/H2/inert = 1/6/2. Transient studies of the UiO-67-Pt catalysts revealed that CO is formed at the Pt NPs, while methanol and methane are formed at the interface between the Pt NPs and defect Zr nodes. Methanol, but not methane, is formed via formate species attached to the Zr nodes. Water co-feed experiments did not change the methanol steady state reaction rate, but had a substantial inhibiting effect on CH4 formation. The results obtained in this study yield enhanced insight in methanol and byproduct formation mechanisms over UiO-67-Pt MOFs, and provide important leads to further catalyst and process design.

Mechanistic studies of CO2 hydrogenation to methanol, methane and CO over Pt-containing Zr-MOFs

ACS Spring Meeting 2021

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