a Only the highest barrier for each reaction is shown.b Includes dehydration of 3-hydroxybutanal.c 31.4 kcal/mol, if the barrier is calculated from Int1_Zn. The barrier height regarding the H2 molecule formation was not considered. In the case of ZnO, the computation showed that ethanol dehydrogenation occurs more easily in comparison MgO where such a result is in line with an experimental study which reports that the introduction of Zn into catalysts such as MgO2/SiO2 or talc enhances the catalytic activity for ethanol dehydrogenation.[48] ZnO appears to be a very good catalyst for aldol condensation and the following dehydration, the barrier of which was about half that of MgO. On the other hand, ZnO’s performance as a catalyst for the MPV reduction is slightly worse than that of MgO. Both oxides show virtually identical barrier height for crotyl alcohol dehydration.Side reactionsOne of the major undesired side reactions of ethanol-to-butadiene conversion is the formation of ethylene via ethanol dehydration.[1] Indeed, metal oxides have been used for ethanol dehydration catalysts.[49,50] One study reported that the Lewis acidity of metal oxides affects the catalytic performance of the dehydration.[50]Dehydration of alcohol occurs via either an E1 or E2-type elimination reaction; according to a previous DFT study on the dehydration reaction over Al2O3, an E2 reaction is favored.[49]PESs for the dehydration of ethanol occurred via the E2 mechanism for ZnO and the E1 mechanism for MgO are shown in Fig. 9. While MgO follows the E1 mechanism, the energetics of the intermediate (INT2_Mg_Dehydration) is virtually identical to that ofTS1_Mg_Dehydration, implying that the reaction mechanism is similar to that of the E2 mechanism. The computed barrier height was 40.5 and 38.9 kcal/mol for ZnO and MgO, respectively. Also, the energetics of TS2_Mg_Dehydration was 41.9 kcal/mol above that of INT1_Mg_Dehydration. This result indicated that the dehydration occurs more easily using ZnO as a catalyst. As the TS cartoon (TS_Zn_Dehydration) in Fig. 9 shows, the dehydration of ethanol via the E2 mechanism included the C-O bond and the C-H bond cleavage, where the former occurs at the Lewis acidic site and the latter at the Lewis basic site. Obviously, the strong Lewis acidity and basicity of ZnO cause the side reaction to occur more easily in comparison with MgO.Figure 9. PESs for ethylene production via dehydration of ethanol catalyzed by MgO or ZnO. (M=Mg, Zn) In addition, the dehydration, e.g., the 3-hydroxybutanal dehydration (Fig. 5) results in the attachment of the OH group on the cluster (seeInt12_Zn or Int12_Mg in Fig. 5) where the OH groups can act as Brøntsed base sites. Thus, we further investigated the dehydration of ethanol on Brøntsed acid/base sites.Figure 10. PESs for ethylene production via dehydration of ethanol on Brøntsed base site. (M=Mg, Zn) Fig. 10 shows that PESs for dehydration of ethanol considering that water was already adsorbed in a dissociative manner as Hhwafffwathe OH group and H on catalyst. INT1_Brønsted is a complex on which the OH group and H are chemically adsorbed on the catalysts; ethanol is also adsorbed on the catalyst. The OH group of ethanol interacts with the metal atom of the metal oxide catalyst. The computed barrier heights of the dehydration were 39.5 and 43.3 kcal/mol for ZnO and MgO, respectively, which are higher than the dehydration at the Lewis acid/base sites. Moreover, as shown in Fig. 10, the energetics of the product (INT2_Brønsted) is higher than that of reactant (INT1_Brønsted), which is in contrast to the dehydration at the Lewis acid/base site. As such, the dehydration that involves the Brøntsed base site is not preferred compared with the dehydration on Lewis acid/base sites. Compared with the ethanol dehydrogenation (see the PESs Fig. 4), ZnO is a better catalyst for dehydrogenation whereas MgO favors dehydration over dehydrogenation because the barrier for the dehydration is lower than that for dehydrogenation.Effects of Lewis acidity and basicityDetails of the PESs of each of the elementary reactions of ethanol-to-butadiene conversion have been discussed up to this point. Now we discuss the barrier heights of the elementary reactions in conjunction with the Lewis acidity/basicity of the metal oxide. One of the most notable features affected by the Lewis acidity/basicity is ethanol dehydrogenation because, as noted, it depends strongly on the metal’s Lewis acidity. As the result of DFT calculations discussed in the previous section indicate, the Mg atom in MgO does not have sufficient Lewis acidity to facilitate the C-H bond cleavage corresponding to the α-­H transfer from ethanol. The C-H bond cleavage is also found in other reactions during ethanol-to-butadiene conversion. For example, TS9-10_M (M=Mg or Zn) shown in Fig. 5 andTS18-19_M (M=Mg or Zn) (Fig. 8) also correspond to the TS of the C-H bond cleavage and the latter occurs on the Lewis basic site. For this C-H bond cleavage, the performance of ZnO as a catalyst is better than that of MgO, which correlates with the stronger Lewis basicity of the O atoms of ZnO. By contrast, the transfer of the OH group, which includes the C-O bond cleavage, appears to occur more easily on MgO. An example isTS19-20_M (M=Mg or Zn). Associated barrier heights are lower for MgO-catalyzed reactions than for ZnO catalyzed reactions of TS19-20_M.Figure 11. Structure of Int19_Mg andInt19_Zn.
To investigate its origin, we present the molecular structure ofInt19_Mg and Int19_Zn , whose details are shown in Fig. 11 along with some structural parameters. As mentioned above, these intermediates are followed by TS19-20_M (M=Mg or Zn), which corresponds to the TS of the C-O bond cleavage leading to butadiene formation. As shown in Fig.11, the distance between the terminal carbon atom and the closest metal (Mg or Zn) atom was computed to be longer forInt19_Mg than for Int19_Zn (2.22 Å for MgO, 2.03Å for ZnO). This result also reflects the strong Lewis acidity of the Zn atom in the ZnO cluster. Also, the distance between the oxygen atom of the OH group and the neighboring metal atom was predicted to be 1.90 Å and 2.18 Å for MgO and ZnO, respectively. As such, the shorter O-Mg distance implies that the transfer of the OH group to the catalyst occurs more easily on MgO than ZnO. Moreover, DFT calculations predicted that the bond distance (4.08 Å) of Zn1-Zn2 is longer than that (3.76 Å) of Mg1-Mg2. Such a result indicates that, structurally, the Mg1-Mg2 distance is similar to the molecular size of crotonaldehyde. Thus, the terminal C atom and the O atom of the OH group of crotonaldehyde can be simultaneously bonded to the catalyst in the case of MgO. In contrast, the Zn1-Zn2 distance is longer than the molecular size of crotonaldehyde, which hinders the simultaneous bonding of the OH group and the terminal carbon of crotonaldehyde to the catalyst. Considering the above results, it could be reasonable to conclude that the lower OH transfer barrier height due to MgO arises from the structural mismatch between the adsorbate and the catalyst.
The ethanol-to-butadiene conversion reaction is very complex, being composed of series of elementary reactions. Depending on the nature of each elementary reaction, the Lewis acidity or basicity of metal oxide catalyst affects the performance of the catalyst in a positive or negative manner, as discussed above. According to our DFT calculations, most energy demanding process in the ethanol-to-butadiene conversion catalyzed by MgO was ethanol dehydrogenation and this step was strongly influenced by the Lewis acidity of the catalyst, of which result is, to some extent, in line with a recent study on ethanol dehydrogenation catalyzed by MgO-SiO2.[52] DFT calculation showed that ZnO is a better catalyst in this step, where such a performance arisen from its stronger Lewis acidity. Moreover, another important aspect of ZnO is that the catalyst favors ethanol dehydrogenation over the dehydration, whose feature was opposite to that of MgO.