## 3 Results and Discussion
### Catalytic Tests  
The effect of the GHSV was preliminarily explored at 400 \({}^{\circ}\)C on 20% MgO/AC sample by varying the catalyst load in the reactor while keeping unchanged the total flow rate and the inlet ethanol concentration. Results are reported in Figure 4 in terms of ethanol conversion and butanol yield. As expected, the ethanol conversion increases by reducing the GHSV, as also reported by [PERSON] et al. [15]. Notably, also the process yield to butanol increases for lower values of the GHSV. It should be noticed that the largest amount of catalyst loaded in our experiments (1 g) corresponded to a catalytic bed height of about 3 cm; longer catalyst beds would have exceeded the isothermal zone of the electrical furnace and, therefore, lower values of the GHSV were not explored. On the other hand, a negligible butanol production was obtained performing tests at 350 \({}^{\circ}\)C even using the largest possible dose of catalyst. All tests shown below were performed at GHSV 330 h\({}^{-1}\), that is the value providing the maximum ethanol conversion and butanol yield under isothermal conditions. In Figure 5, the catalytic performance of MgO/AC systems with different loadings of the active phase are compared with results obtained over the bulk MgO catalyst under the same operating conditions. For all samples, in addition to butanol, the main product of the reaction was acetaldehyde. A lot of small and not identified peaks were also detected. Figure 4: Ethanol conversion and butanol yield obtained in catalytic tests on 20% MgO/AC at 400 \({}^{\circ}\)C as a function of the gas hourly space velocity (GHSV). Figure 3: XRD patterns of raw AC and 10, 20, and 30 wt% MgO/AC. Results reported in the figure highlight the key role of MgO dispersion. Indeed, the catalytic activity of bulk MgO is rather low, either in terms of ethanol conversion (<10%) and butanol yield (ca. 2%). However, when dispersed on the high surface area microporous activated carbon, the activity of the same MgO active phase is greatly enhanced, in spite of its lower content in the catalytic bed (only 10%-30% by weight). Best results were obtained with the catalysts containing 20 wt% MgO, which produced 12.5% butanol yield in correspondence of 68% ethanol conversion. For higher MgO contents, both conversion and yield decreased, most likely because of the formation of large MgO aggregates evidenced by reduction of both surface area and pore volume for 30% MgO/AC sample (Table 1). A butanol yield at 350 \({}^{\circ}\)C similar to that obtained by us with 20% MgO/AC was reported by [PERSON] et al. [11] for 3:1 Mg-Al mixed oxide (S.A. = 252 m\({}^{2}\)/g) obtained by thermal decomposition of hydrotalcite. Nevertheless, experimental conditions are not easily comparable. On the other hand, a direct comparison with results obtained with 20 wt% MgO dispersed on \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) pellets [18], tested under the same reaction conditions, suggests that the enhancement in conversion and yield for the AC-supported sample is roughly proportional to the increase of surface area of this catalyst with respect to the alumina-supported one (S.A. = 115 m\({}^{2}\)/g) having an identical loading of MgO. Furthermore, in order to rule out a possible contribution of carbon to the reaction and to confirm the role of MgO dispersion, a mechanical mixture consisting in 20% MgO powder and 80% AC was also tested at 400 \({}^{\circ}\)C under the same space velocity. This sample provided an almost undetectable butanol yield further confirming the superior performance of highly dispersed MgO and excluding, at the same time, that the active carbon support has a direct catalytic activity for the Guerbet reaction. Notably, butanol yields herein reported for all MgO/AC catalysts are significantly higher than those previously obtained on Ni/MgO/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\) pellets tested under the same experimental conditions [18].