(36b) Copper-Exchanged Zeolitic Imidazolate Framework-8 Enable Selective CO2 Hydrogenation to Methanol

In view of mitigating CO₂ emissions and cut the dependency of our reliance on fossil feedstocks, number of approaches were proposed to capture and convert CO₂ to useful fuels or chemicals. While methanol (MeOH) synthesis via CO₂ hydrogenation might become practically relevant in the future, many researchers working on to develop an active, selective, and stable catalyst with low-cost. Currently, a copper-based catalyst is industrially used to produce methanol from a mixture of CO/CO₂/H₂ at 200–300 ºC and elevated pressures (50 – 100 bar) since 1960s. Despite high methanol productivity over the benchmark Cu/ZnO/Al₂O₃ catalyst, this catalytic system suffers with faster deactivation and selectivity issues (reverse water gas-shift reaction, and methanation). To address the issues with selectivity and stability of the catalyst, a number of multimetallic composite systems have been proposed, that is, CuO/ZnO/ZrO₂, supported (e.g. ZnO, SiO₂, ZrO₂) Pd, and In-Pd, Au/CeO_x/TiO₂, Ni-Sn/In-ZrO₂, Ni/In₂O₃, Ni-Ga/SiO₂, In₂O₃/Co₃O₄, and etc. However, despite they achieve high selectivity towards methanol with better stability, these mix-component systems are able to convert CO₂ to methanol at low conversions per pass and/or the high cost of some of the materials used in making the catalysts did not see further scope for scale-up and commercialization. In this reference, we explore the synergistic interaction of copper stabilized Zn-MOF (ZIF-8) for CO₂-based methanol synthesis through a comprehensive experimental and theoretical program. In situ IR spectroscopy and surface adsorption techniques along with thermal, volumetric, and microscopy analyses uncovered the nanometric exchange and construction of the selective copper species in ZIF-8 framework. Density functional theory used to understand structural rearrangements of copper exchanged ZIF-8 and the reactivity of complementary promoted surfaces.

Materials and methods

The ion-exchange and impregnation procedure for ZIF-8 sample was carried out with excess metal nitrate (in methanol or water) solution at 50-80 ºC. After evaporation-exchange procedure, the sample was dried at 100 ºC for 24 h prior to any pre-treatment, characterization, and catalytic testing of the material. Textural properties were studied by means of N₂ adsorption-desorption (Micromeritics ASAP 2040) at -196 ºC. Crystallographic analyses were performed via powder X-ray diffraction in a Bruker D8 advanced diffractometer equipped with a Bragg-Brentano geometry fitted with a copper tube operating at 40 kV and 40 mA. Diffractograms were acquired over a 2θ range of 10-80º. Metal-support interactions were studied by means of temperature-programmed H₂ reduction (Altamira AMI-200ip). Chemical composition of the powdered samples was analyzed by ICP-OES (Varian, Inc./Agilent Model 7200-ES) and high resolution X-ray photoelectron spectroscopy in a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic Al Kα x-ray source (hν = 1486.6 eV). High-pressure In situ Diffuse Reflectance FT-IR-Spectroscopy (DRIFTS) measurements were performed on a Nicolet 6700 FTIR spectrometer with a liquid-nitrogen cooled MCT detector using a high-temperature reaction cell (Harrick) equipped with a temperature programmer and connected to a gas-dosing device. Scanning electron microscopy imaging was performed with secondary electrons in an FEI TENEO VS microscope using 2 kV acceleration voltage and 5 mm working distance. Catalytic tests were performed using a Flowrence^® Avantium parallel reactor consisting of 16 tubular fixed-bed quartz reactors. In each reactor, ~ 0.05 g of sieved catalyst particles (150 - 300 mm) were loaded onto a 9.5 cm long coarse SiC bed that ensures the catalyst bed lies on the isothermal zone of the reactor. The reactors were pressurized with a mixed feed containing 20 vol % of CO₂ and 80 vol % of H₂ to 50 bar using a membrane-based pressure controller in the temperature range of 225 – 300 ºC and the effluent stream was analyzed using a gas chromatograph equipped with TCD and FID detectors.

Results and discussions

The prepared catalysts were extensively characterized by various adsorption and spectroscopic techniques to understand the surface, bulk, and morphological properties. The elemental composition of all the samples were investigated by ICP-OES, STEM-EDX, and XPS analyses and percentages of each element determined from ICP-OES analysis are in line with the nominal loadings. A higher BET surface area observed for ZIF-8 sample (~1160 m²/g) decreased significantly after Cu exchange or impregnation. However, Cu-exchanged ZIF-8 sample showed much higher surface area (~870 m²/g) compared to the Cu impregnated ZIF-8 (~25 m²/g) and benchmark Cu-ZnO-Al₂O₃ (~78 m²/g) catalysts. The thermogravimetric analysis of Cu ion-exchanged catalyst showed relatively better thermal stability in comparison to Cu impregnated sample. The structural stability of ZIF-8 MOF after Cu-exchange is also confirmed with in situ diffuse reflectance infrared spectroscopy at reaction temperature (250 °C) under high pressure (25 bar). In reference to the testing of these materials in CO₂ reduction methanol, a higher space-time-yield was observed for Cu exchanged MOF in comparison to the impregnated and benchmark catalysts. The higher productivity observed for Cu-exchanged catalyst was attributed to the presence of higher CO₂ sorption capacity (CO₂-TPD), uniform distribution of Cu nanoparticles (SEM and HR-TEM), improved thermal stability (TGA and XRD) caused after Cu exchange. The higher thermal stability of the Cu-exchanged ZIF-8 catalyst was attributed to the increase in the ZnO crystallite size determined from XRD analysis of the catalyst. The high-pressure in situ diffuse reflectance infrared spectroscopic studies of the catalysts revealed the possible reaction intermediates in order to understand the reaction mechanism. Density functional theory is also used to understand the gas (CO₂+H₂) adsorption properties and reaction mechanism of CO₂ hydrogenation to methanol. Finally, in addition to the high activity and selectivity of the prepared catalytic system, the catalyst also showed good stability for more than 36 h of continuous operation in CO₂ to methanol reaction conditions. The used MOF catalyst was also characterized by XRD, XPS and HR-TEM analyses that showed insignificant differences in comparison to the fresh catalyst indicating its robust nature under the reaction conditions used in this study.

Conclusions

A detailed kinetic, mechanistic and theoretical investigations were carried out for a newly synthesized MOF material in the CO₂ hydrogenation to methanol. The investigations revealed superior catalytic performance for the designed catalyst in comparison to the benchmark catalyst. Highly uniform distribution of active sites in the molecular organic framework in addition to the presence of required CO₂ and H₂ sorption sites were found to be the key reasons for its high catalytic activity, selectivity and stability of the reported system in this study.

Acknowledgements

The authors gratefully acknowledge the financial support, resources and facilities provided by the King Abdullah University of Science and Technology (KAUST).