Stepwise tuning of metal-oxide and acid sites of CuZnZr-MFI hybrid catalysts for the direct DME synthesis by CO2 hydrogenation

Multifunctional CuZnZr-zeolite catalysts were investigated for the direct synthesis of dimethyl ether (DME) from carbon dioxide (CO2) hydrogenation. Physical mixtures prepared by pre-pelletized CuZnZr methanol catalysts and home-made MFI-type zeolites as well as hybrid systems prepared by coprecipi- tation of metal-oxide precursors in solutions containing the zeolites were tested in a plug-flow reactor at 240◦C, pressure of 3.0–5.0MPa and space velocity comprised between 2500 and 10,000NL/kgcat/h. Preliminary experiments in methanol (MeOH) synthesis reaction from CO2 –H2 mixtures allowed to indi- viduate a suitable CuZnZr composition ensuring good activity in CO2 conversion, high MeOH selectivity and limited CO formation. Parallel tests in MeOH-to-DME reaction were also carried out to investigate the functionality of the zeolites prepared. The results disclosed that an optimal Si/Al ratio is necessary to obtain dehydration catalysts characterized by suitable acidity and good resistance to deactivation in pres- ence of water. The performance of the multifunctional CuZnZr-MFI catalysts in the direct CO2-to-DME hydrogenation reaction showed that, by controlling the coprecipitation of CuZnZr methanol synthesis precursors, the availability of the active sites is maximized over the zeolite surface. That allowed to reach both high CO2 conversion levels (up to 24%) and high rate of MeOH dehydration to DME.