Behaviour of Cuznal and Cuznzr Ferrierite-Based Catalysts to Produce DME By CO2 Hydrogenation

The direct hydrogenation of CO2 into dimethyl-ether (DME) has been studied in presence of copper-based methanol (MeOH) selective catalysts and a commercial ferrierite-type zeolite [1 - 5]. These Cu-based systems are Cu-Zn-Al/ferrierite (CZA/FER) and Cu-Zn-Zr/ferrierite (CZZ/FER). Both hybrid catalysts, obtained through gel-oxalate coprecipitation (OX) and wet impregnation (WI), and physical mixed catalysts with 1:2 and 2:1 weight ratio [2] were explored to compare the activity and the selectivity of these different systems. MeOH-selective catalysts were prepared with a 6:3:1 atomic ratio. All the samples were properly characterized with different physic-chemical techniques in order to determine the textural and the morphological nature of the surface of the catalysts. Firstly, physical adsorption measurements of N2 were elaborated to assess the surface area and the porosity of the samples. Secondly, X-rays diffraction (XRD) patterns were analysed to identify the nature of the phases of the catalysts. In addition, FESEM and EDS techniques were carried out to investigate the morphology and the atomic composition of the surfaces. Eventually, temperature programmed reduction (TPR) measurements were performed for observing the surface and bulk reduction of metal oxides. The experimental campaign was carried out in a fixed bed reactor using 250-500 ?m catalyst particles. According to TPR measurements, the samples were activated by means of a reducing process at a temperature of 350 °C, at atmospheric pressure for 3 h. The reducing atmosphere was obtained with a H2/N2 molar ratio equal to 1:9. After the activation process, a degreening test was performed (2.5 MPa, 275 °C and 13.6 Nl/kgcat/h) to stabilize the catalyst before the activity tests. This procedure was helpful in order to put in evidence if a fresh catalyst was affected by a strong and fast deactivation in reacting conditions [3]. This method highlighted that hybrid catalysts (i.e. CZZ/FER OX 1:2) seem to be very active for few hours, then their activity decreases up to 50 % before reaching a stable operating condition. On the contrary, commercial CZA/FER-based catalysts were not affected by fast deactivation. The diminishing of performances may be probably due to a rearrangement of the catalyst structures at high temperatures. For these reasons, both reduced and aged catalysts has also to be characterized in order to identify the causes of the deactivation and improve catalyst performance and stability. The activity tests were carried out operating at 2.5 MPa with a temperature ranging from 200 °C to 300 °C with steps of 25 ° C. A hydrogen (H2), carbon dioxide (CO2) and nitrogen (N2) gas mixture with a H2/CO2/N2 ratio of 3:1:1 was fed to the reactor. Besides, three different weighted hourly space velocity (WHSV) were investigated from 6.6 to 20.0 Nl/kgcat/h. The results of the activity tests highlighted that hybrid catalysts are more selective towards methanol and dimethyl-ether also at higher temperatures, because the reverse water gas shift (WGS) reaction is more inhibited. Indeed, CZA/FER catalysts were less selective and produce non-negligible amounts of by-products (i.e. C2 hydrocarbons) at temperatures higher than 275 °C. At low WHSV (6.6 Nl/kgcat/h) the thermodynamic equilibrium was approached from 250 °C, but the cumulative MeOH/DME yield is lower than 80 g/kgcat/h. Whereas, the cumulative yield increases at higher WHSV (20 Nl/kgcat/h) reaching a cumulative yield of 256 g/kgcat/h. The CO2 conversion always rises as the reaction temperature grows up due to the endothermic reverse WGS reaction, which is favoured at higher temperatures. As illustrated in Fig. 1 (CZA/FER 1:2, 13.6 Nl/kgcat/h), DME selectivity is extremely higher than CO selectivity at low CO2 conver