## 3 Results and Discussion
### Effect of Temperature  
The experiments focused on studying the effect of temperature on the reaction indices have been conducted feeding ternary mixtures of H\({}_{2}\) + CO + CO\({}_{2}\) with CO\({}_{2}\)/CO molar ratios between 1/3 and 1 under the following conditions: pressure, 30 bar; space time, 10 g\({}_{\rm cat}\) h (mol\({}_{\rm C}\))\({}^{-1}\); H\({}_{2}\)/CO\({}_{\rm x}\) molar ratio, 3. Figure 2 shows the results of the effect of temperature over CO\({}_{\rm x}\) conversion (Figure 2a), product yield (Figure 2b) and CO\({}_{2}\) conversion (Figure 2c). These results are shown as an example and correspond to a CO\({}_{2}\)/CO molar ratio of 1/3 in the feed. It is observed that the maximum CO\({}_{x}\) conversion and DME yield (Figure 2a,b, respectively) are reached at 300 \({}^{\circ}\)C. These results differ from those obtained with the same catalyst feeding syngas (without CO\({}_{2}\)), where the maximum is attained within the 275-300 \({}^{\circ}\)C temperature range, and CO\({}_{x}\) conversion and DME yield are higher, while methanol yield is lower than that shown in Figure 2b [35]. Moreover, it is also remarkable that methanol yield (Figure 2b) remains almost constant within the whole temperature range studied. This result is characteristic of a CO\({}_{2}\)/CO ratio of 1/3, as it has been proven (results not shown) that for higher values of this ratio, the yield of methanol shows a growing trend with temperature. On the other hand, when comparing these results with those obtained using syngas feeds [35], it is observed that paraffin yield diminishes slightly co-feeding CO\({}_{2}\), being the decrease more evident for feeds richer in CO\({}_{2}\).  
When CO\({}_{2}\) utilization is analyzed in Figure 2c, it is necessary to distinguish two regions: (i) the area where the net CO\({}_{2}\) conversion is negative, that is, where there is formation of CO\({}_{2}\), due to the predominant role of the reverse WGS reaction (reverse Equation (3)). In this case, the value will correspond (in absolute terms) to CO\({}_{2}\) yield; and (ii) the area of positive CO\({}_{2}\) conversion. In general, the effect of temperature over CO\({}_{2}\) conversion shows the opposite trend to that of DME yield (Figure 2b). The results plotted in Figure 2c show a positive value of CO\({}_{2}\) conversion at 250 \({}^{\circ}\)C (7.5%), while above this temperature there is a net formation of CO\({}_{2}\), with a maximum at 325 \({}^{\circ}\)C. Regarding the study of the capability of the process for the utilization of CO\({}_{2}\), it should be noted that temperatures below 250 \({}^{\circ}\)C have not been studied, as under these reaction conditions both CO\({}_{x}\) conversion and DME yield are very low. On the other hand, it was observed that for CO\({}_{2}\)/CO ratios above 1/3 (results not shown) the effect of temperature on CO\({}_{2}\) conversion is attenuated: (i) at 250 \({}^{\circ}\)C, the conversion of CO\({}_{2}\) is similar to that obtained with a CO\({}_{2}\)/CO molar ratio of 1/3; (ii) at 275 \({}^{\circ}\)C, the CO\({}_{2}\) conversion value is low; (iii) at 300 \({}^{\circ}\)C, the net CO\({}_{2}\) formation is lower than that of CO\({}_{2}\)/CO = 1/3. It must also be noted, that the results correspond to per-pass conversion values. This conversion can therefore be increased operating on a larger scale, by recycling the (CO + CO\({}_{2}\) + H\({}_{2}\)) gas stream, which are easily separated from the products (DME, methanol and water) by condensation. Note that this limitation of the per-pass conversion by thermodynamics is greater in the reaction of CO\({}_{2}\) hydrogenation to methanol, which therefore, also requires the recycling of the unconverted reactant gases to improve the economics of the process [37].  
Figure 2: _Cont._