On the intimacy of bifunctional catalysts for the conversion of syngas to light olefins

Context and objectivesLight olefins (C2–C4) are key building-block chemicals mostly produced either by steam cracking or fluid catalytic cracking of oil resources, two highly energy-consuming processes. Among alternative processes, the syngas via methanol to olefins (SMTO) and the OX-ZEO single step processes are very promising since very high selectivity to light olefins can be reached. They combine one hydrogenating catalyst to synthesize either methanol or ketene and one acidic zeotype to convert the intermediate into light olefins. In both cases, the intermediate has to diffuse from the first catalyst to the other and their intimacy can significantly influence the catalytic performances.In this work, Cu/ZnO/Al2O3 or MnOx was used as hydrogenating catalyst and SAPO-34 as acidic zeotype The role of intimacy has been investigated by modulating the density of contact between the two functions in different bed arrangements and the distance through preparing MnO@SiO2 core shell structures or diluting the two catalysts in SiO2.Material and methodsCu/ZnO/Al2O3 was commercial catalyst while MnOx and MnOx@SiO2 were prepared from MnCO3 powder or monodispersed suspension by different routes. SAPO-34 powders with different acidities were prepared by hydrothermal method. The catalysts were characterized before and after reaction by various techniques, including ICP/XRF, BET, H2 and CO-TPR, NH3-TPD, XRD, SEM, STEM-HAADF, NMR as well as by in situ DRIFT and Raman spectroscopies. Catalytic performances were compared using a fixed-bed reactor with a feed H2/CO/N2:62/23/15 at 390-450 °C and 25 bar.Main resultsFor both catalytic systems, methanol was shown to be a key intermediate from catalytic data and from the in situ DRIFT observation and evolutions of formates (Fig. 1a) which are formed before methoxy species. The two catalytic systems were active and selective (>50%) to C2-C4 hydrocarbons with a low Olefin/Paraffin (O/P) ratio (