## 3 Results and Discussion
### Catalytic Tests
In that case, the butanol yield was limited to 6%, in spite of the addition of the metal which was shown to promote the first step of the reaction mechanism. All tests presented up to now were carried out under dry conditions, but water is generally present in the ethanol feed or it can be produced as a by-product, and it may cause irreversible or reversible catalyst deactivation. Therefore, in order to investigate those effects, some catalytic tests were repeated on the most-performing sample (20% MgO/AC) starting from dry feed conditions, then adding 2 vol % of H\({}_{2}\)O for a total of 9 h on stream, and eventually switching back to a dry feed. The results of this sequence are reported in Figure 6.  
Figure 5: Ethanol conversion and butanol yield obtained in catalytic tests on pure MgO and AC supported MgO catalysts at T = 400 \({}^{\circ}\)C and GHSV = 330 h\({}^{-1}\). Notably, the addition of water induced a significant reduction of ethanol conversion from 68% down to ca. 40%, but the yield to butanol was not affected and remained around 12.5%. This implies that water is capable to enhance the process selectivity to butanol, suppressing some of the undesired side reactions consuming ethanol to form unidentified products, excluding acetaldehyde. The effluent stream from the catalytic reactor operated under wet feed conditions was monitored for a total of 9 h measuring a rather stable butanol production. Results in Figure 6 obtained after the removal of water from the feed indicated that its effects were not completely reversible: the ethanol conversion increased to ca. 61%, slightly below the initial level, but, most important, the yield to butanol increased up to 14.5%. This could be associated to an increase of the surface area that occurs when water weakly gasifies the activated carbon. Indeed, the surface area of the catalyst after the exposure to wet reaction conditions at 400 \({}^{\circ}\)C increased up to 750 m\({}^{2}\)/g due to an evident increase of both kinds of micropores, as shown in Figure 7. This is in agreement with results reported by [PERSON] et al. (2016) who regulated the pore size distribution of their activated carbon through steam activation of coal containing different amount of potassium hydroxide. They found an optimal KOH fraction determining the maximum increase of micropores in the range 1-2 nm. The presence of water during the reaction with the consequent formation of basic Mg(OH)\({}_{2}\) could promote the large increase of micropores observed in the catalyst used under wet condition. Furthermore, the much higher solubility of magnesium hydroxide can promote some re-dispersion of the active phase thus leading to a catalyst with further enhanced properties compared with those attainable by the incipient wetness impregnation of AC. On the other hand, the total pore volume of the catalyst decreased down to 0.27 cm\({}^{3}\)/g, mostly due to a marked loss of mesopores (Figure 7), which might be related to the deposition of some by-products of reaction. Figure 6: Effect of 2 vol. % H\({}_{2}\)O addition to the feed (for a total of 9 h) and removal on the ethanol conversion and butanol yield obtained with a 20%MgO/AC sample tested at T = 400 \({}^{\circ}\)C and GHSV = 330 h\({}^{-1}\). In order to highlight the key role of the activated carbon support in providing high water tolerance to supported MgO catalysts, the same sequence of experiments above described was performed on a Ru/MgO/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\) system reported in Reference [18] that achieved the best performance among a series of Me/MgO/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\) catalytic pellets. The experiments were carried out at 350 \({}^{\circ}\)C representing the best operating temperature for this material [18]. Results are shown in Figure 8 and clearly demonstrate that water addition induced a significant loss of activity for this catalyst and severely inhibited butanol production. In fact, both ethanol conversion and butanol yield dropped to less than 50% of their original value under dry feed conditions.