Hydrophobic Polyhedral Oligomeric Silsesquioxane Support Enhanced Methanol Production from CO2 Hydrogenation.

Abstract

The abundance of CO2 from the cement industry, power generation, petroleum production, and combustion of biomass makes it a readily available feedstock to produce chemicals and materials, although it has yet to achieve optimal development. Even though syngas (CO + H2) hydrogenation to methanol is an established industrial process, when the same catalytic system based on Cu/ZnO/Al2O3 is employed with CO2, the water formed as a byproduct reduces the activity, stability, and selectivity of the process. Here, we explored the potential of phenyl polyhedral oligomeric silsesquioxane (POSS) as a hydrophobic support of Cu/ZnO for direct CO2 hydrogenation to methanol. Mild calcination of the copper-zinc-impregnated POSS material affords the formation of CuZn-POSS nanoparticles with Cu and ZnO homogeneously dispersed with an average particle size of 7 and 15 nm supported on O-POSS and D-POSS, respectively. The composite supported on D-POSS was able to reach a 3.8% yield of methanol with a 4.4% of CO2 conversion and with selectivity as high as 87.5% within 18 h. The structural investigation of the catalytic system reveals that CuO/ZnO are electron withdrawers in the presence of the siloxane cage of POSS. The catalytic system metal-POSS is stable and recyclable under H2 reduction and CO2/H2 conditions. We tested the use of microbatch reactors in heterogeneous reactions as a rapid and effective tool for catalyst screening. The increased number of phenyls in the structure of POSS results in an increased hydrophobic character that plays a decisive role in the methanol formation after comparison with CuO/ZnO supported on reduced graphene oxide with 0% selectivity to methanol under the study conditions. The materials were characterized using scanning electron microscopy, transmission electron microscopy, attenuated total reflection Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, powder X-ray diffraction, Fourier transform infrared analysis, Brunauer-Emmett-Teller specific surface area analysis, contact angle, and thermogravimetry. The gaseous products were characterized by gas chromatography coupled with thermal conductivity detectors and flame ionization detectors.