A comprehensive study on highly active GaZrOx/SAPO-34 catalyst for the methanol-mediated syngas conversion to light olefins

Light olefins (C2=-C4=) are key chemical intermediates mainly produced by oil-based steam cracking and fluid catalytic cracking. As an alternative, a mixture of a hydrogenating metal oxide and an acidic zeotype (OX-ZEO) was found to convert syngas (which can originate from wastes and biomass) to light olefins with a much higher selectivity, but however with relatively weak activity and high CO2 selectivity. Recently, quite high activity was reported for GaZr mixed oxides combined with SAPO-34. Nonetheless, the origin of such activity and the precise reaction mechanism need further investigations.GaZr oxides were synthesized with appropriate amounts of Ga(NO3)3·9H2O and Zr(NO3)4·5H2O by co-precipitation method. SAPO-34 zeotypes were prepared with various Si/Al molar ratios by hydrothermal method. OX-ZEO bifunctional catalysts were tested in a fixed bed reactor from 1 to 25 bar, from 300 to 475 °C under a H2/CO/N2=60/30/10 feed. Products analysis was performed at atmospheric pressure with a microGC equipped with a TCD detector. Structural, textural, redox, and acidic properties were determined before and after reaction by XRD, TEM, SEM, ESR, solid 69Ga NMR, N2 adsorption-desorption isotherms, H2 and CO-TPR, H2 and CO-TPD, ethylene hydrogenation probe reaction and NH3-TPD. In situ and operando DRIFTS and in situ XPS were deployed to characterize oxides and OX-ZEO mixtures.Prepared GaZr oxides with high surface area (>150 m2.g-1) exhibit remarkable performance when physically mixed with SAPO-34 zeotype with CO conversion reaching 40% at P=25 bar and T=400 °C. As shown in Figure 1a, an optimal GaZr5.6O12.7/SAPO-34(Si/Al=1.0) mass ratio of 70/30 enabled such high activity combined with a notable hydrocarbon distribution in light olefins of 87%. Increasing contact time and temperature allowed to reach higher conversions (up to 70%) but was detrimental towards light olefins selectivity thus promoting olefins over-hydrogenation. On the other hand, a high pressure was a prerequisite to achieve good catalytic activity as GaZr5.6O12.7/SAPO-34(Si/Al=1.0) catalyst showed conversion of 21 and only 1% at P=10 and 1 bar, respectively. Unusually, high olefin/parrafin (O/P) ratios were obtained at such high conversion thanks to GaZr5.6O12.7 weak activity towards lower alkenes hydrogenation compared to other common oxides and saturation of the acid sites of SAPO-34 by methanol, coming from highly active GaZr solid solution.The origin of such performance resides in the high H2 dissociation ability of gallium cations as evidenced by the observation of Ga-H hydrides by in situ DRIFT spectroscopy even at T=100 °C. As shown in Figure 1b, formation of formates and their successive transformation to methoxy species is observed under a CO/H2 feed. Presence of hydrides and apparent consumption of OH species is also evidenced. To gain more insights on the nature of active species during CO hydrogenation, a mixture of 30% CO/He was introduced on fresh GaZr5.6O12.7 material with coupled mass spectrometer (MS). An apparent consumption of OH groups associated to the apparition of formates bands proved that hydroxyls participate in the reaction mechanism. Interestingly, H2 was produced during reduction under CO and a band suggesting the presence of Ga-H species was observed. The wavenumber of such hydrides (1991 cm-1) appears slightly different from Ga-H hydrides generated under H2 (1985 cm-1) and we believe that such a difference may come from the different environments generated by both CO and H2 treatments. A possible explanation could be that reductions under CO and H2 lead to different types of oxygen vacancies, as observed on ESR spectra. To gain more information about hydrides, H2-TPD experiments were conducted. A new H2 desorption peak was evidenced at low temperature after reduction suggesting creation of more reactive hydride species. Homolytic H2 dissociation might thus be favored after reduction of the surface as recently demonstrated on sole α-Ga2O3. Since surface hydrides were observed under reaction conditions on GaZr oxides, we suppose that only a fraction of such species react with CO to hydrogenate formates to methoxy species leading to methanol. On the contrary, for OX-ZEO mixture, the Ga-H band disappears suggesting rapid consumption as SAPO-34 introduction boosts CH3OH formation.This work highlights the key role of hydroxyl and hydride species in achieving high performance for the conversion of CO to light olefins. Ongoing in situ XPS, 69NMR and ESR experiments coupled with DRIFT spectroscopy data are expected to provide a clear view of the still-debated redox mechanism happening during CO hydrogenation to methanol.