Inside the reaction mechanism of direct CO2 conversion to DME over zeolite-based hybrid catalysts

This paper aims at shedding more light on the reaction mechanism behind the direct hydrogenation of CO2 streams into DME in presence of hybrid catalysts. Starting from the physico-chemical properties of an optimized CuO-ZnO-ZrO2/HZSM-5 catalytic system, whose synergy among active sites of different nature (i.e., metal/oxide and acid/base) was recently proposed as the key to overcome the typical limitations showed by random mixing of two preformed catalysts, new evidences are herewith reported about catalytic performance, conditions for activation of reactants, limiting steps and product formation. Likewise to other proposed mechanisms at the state of the art, the catalyst efficiency is resulted to be dependent on several catalyst features, related not only to the metal-oxide phase responsible for CO2 activation/hydrogenation, but also to specific characteristics of the zeolite (i.e., porosity, specific surface area, population, location and strength of acid sites, interaction with active metals, ...), influencing the activity-selectivity pattern. The production of DME is conditioned by the rate of methanol formation in proximity of the metal-oxide interface, followed by a rapid transferring of methanol towards the neighboring acid sites of the zeolite. Operando spectroscopic investigations performed under simulated reaction conditions (30 bar, 200-260 degrees C) have shown that the intermediates formation strongly depends upon a concurrence of texture, structure and surface aspects; however, depending on the reaction conditions, methanol formation normally passes through the formation of formate species. From a technological point of view, a reaction pressure of 30 bar appears as the ideal compromise between CO2 conversion and limitation of operative costs, while, due to thermodynamic restrictions, a reaction temperature not higher than 200 degrees C is necessary to attain DME selectivity close to 90 %.