Figure 4. PESs for the formation of acetaldehyde from ethanol catalyzed by MgO or ZnO.
Among possible adsorption sites, we first considered a terrace site. A first step of the formation in this case is Int1_Mg followed by the transfer of a H atom of the OH group to the MgO cluster that leads to Int2_Mg whose details are shown in Fig. 4. As a result of TS1-2_Mg , ethanol is now converted to ethoxide (CH3CH2O-). The computed energy barrier for TS1-2_Mg is 1.7 kcal/mol implying that this hydrogen atom transfer is virtually barrierless. OnceInt2_Mg is produced, the reaction can proceed via two different routes: 1) direct interaction of the H atom transferred to the MgO cluster, with one H of the methyl group of ethanol and consequent formation of acetaldehyde and an H2 molecule,Int3_Mg via TS2-3_Mg ; and 2) a stepwise mechanism in which the H atom of the methyl group is first transferred to the cluster, which leads to the formation of acetaldehyde. Both mechanisms have been explored revealing that the former is the only possible one when the MgO catalyst is used, even though a very high energy barrier corresponding to 43.3 kcal/mol is computed. Desorption of H2 molecule and acetaldehyde from Int3_Mgregenerated the catalyst.
When the Mg-apical case is considered, no TS related to the first H transfer has been found, which indicates that ethanol dehydrogenation is not likely to occur at the Mg-apical site of the cluster. Considering the O-apical case, adsorption of ethanol occurs in a dissociative manner. Our attempts to locate a TS that leads to the formation of acetaldehyde at the O-apical site were also unsuccessful.
When the same reaction occurred on the ZnO cluster, the dissociative adsorption of ethanol takes place also on the terrace site, leading to the formation of Int1_Zn , the energy of which is computed to be exothermic by -34.2 kcal/mol. The energetics of Int1_Zn is larger than that of Int2_MgO , whose result correlates with the result of NH3 adsorption on the clusters described above, stressing the strong Lewis acidic nature of Zn in the ZnO cluster. The next step of the dehydrogenation is the α-H atom transfer from ethoxide to the cluster. In the case of ZnO, our attempt to search for a TS that corresponds to TS2-3_Mg , a reaction that the adsorbed H atom on the cluster abstracts the α-hydrogen atom of ethoxide, was unsuccessful. According to our computation, ethanol dehydrogenation catalyzed by ZnO occurs via TS2-3_Zn (Fig. 4). Before the TS occurrence, another intermediate, Int2_Zn is formed, where in case of Int2_Zn , the α-H is pointing to the Zn atom and the O-H bond of ethanol is re-formed. As described in Fig. 4, two H atoms are simultaneously transferred to ZnO atTS2-3_Zn , where one is transferred to Zn and another to O. Such a TS is also found in ethanol dehydrogenation catalyzed by Al2O3.[46] Since the Lewis acidity of Al in Al2O3 is very strong,[47] it is implied that the occurrence of the simultaneous H atom transfer, which is not observed in MgO-catalyzed ethanol dehydrogenation, is associated with the strong Lewis acidity of the metal oxides. The energetics of TS2-3_Zn is 24.5 kcal/mol above Int2_Zn and 31.4 kcal/mol above Int1_Zn . Such values are lower than that of the ethanol dehydrogenation barrier catalyzed by MgO to some extent. As such, our computational results indicate that acetaldehyde production occurs more easily on the ZnO cluster. Int3_Zn corresponds to an intermediate where acetaldehyde, the final product of ethanol dehydrogenation, is adsorbed on the protonic H atom bonded to ZnO. Int3_Zn is followed byInt4_Zn , which is generated upon desorption of acetaldehyde. It is seen in Fig. 4 that the energy barrier (TS3-4_Zn , 19.1 kcal/mol) is required upon H2 molecule formation.Int5_Zn can be described as a hydrogen molecule adsorbed on ZnO, where its desorption re-generates the catalyst. Overall ethanol dehydrogenation is endothermic by 20.1 kcal/mol.