DIRECT CO2 CONVERSION TO DME: CATALYTIC FEATURES CONTROLLING DEACTIVATION OF HYBRYD SYSTEMS

The production of dimethyl ether (DME) by catalytic hydrogenation of CO2 is an issue receiving a particular attention [1] because of the enormous environmental and economic interest related to the reduction of CO2 emissions into the atmosphere. Conventionally, by CO2 hydrogenation on metal and oxide sites MeOH is first generated, while DME is formed by MeOH dehydration on acid sites. Recently, we focused the attention on the development of novel hybrid CuZn-Zr-zeolite catalytic systems to obtain high DME yields via CO2 hydrogenation in one step [2-4]. Results obtained confirmed the achievement of higher DME productivity with respect to the conventional mechanical mixtures [5] usually proposed. In view to verify the real potentiality of different hybrid systems prepared, the attention was focused on probing their stability on time which is fundamental for industrial applications. Therefore, several endurance tests, under typical operation conditions (260 °C; 3.0 MPa; 8,800 NL/kgcat/h; CO2/H2/N2 3/9/1) have been carried out by using as carrier different home-made zeolites (MFI, BEA, FER, MOR, Y) characterized by defined framework topology and acidity. Despite a similar initial activity-selectivity pattern, a markedly different deactivation trend of different system investigated was observed. What clear resulted was that the catalytic stability is mainly depending upon the nature and strength of the specific interaction among metal-oxide(s) active species and zeolite surface, whose extent leads to a rearrangement of oxide clusters induced by water formation that determines a full control both on particles sintering and the loss of acidity induced by H+/Cu2+ ion exchange [6]. The best results were obtained using the MOR- and MFI-based hybrid catalysts which, in respect of the other systems investigated allowed to better stabilize on surface the active phase.