Insight into the effect of promoters (M = Cu, Ag, Zn, Zr) on aldol condensation reaction based on MgO surface in the process of ethanol to 1, 3-butadiene: A comparative DFT study

Based on density functional theory, the performance of aldol condensation of acetaldehyde to 3-hydroxybutanal on MgO surfaces doped with Cu/Ag/Zn/Zr was compared and analyzed, respectively. And the two kinds of coordination structures: terrace surface (Mg-5C-O-5C-T) and the kinks surface (Mg-3C-O-3C-K), were selected as the basis of MgO surface. Among these kinds of promoters, Zn and Ag have a little influence on the surface properties of MgO. While Cu and Zr could modify the acidity-basicity properties of MgO surfaces. Cu as the promoter could reduce the energy barrier of acetaldehyde enolization step on weak basicity site (Mg-5C-O-5C-T), and that of the proton back-transfer step on strong basic site (Mg-3C-O-3C-K) simultaneously. Moreover, Zr could promote both the formation of C-C bond and proton back-transfer step on the strong basicity sites. It is speculated that strong basicity sites on MgO surface could be eliminated by the addition of Zr to avoid the excessive condensation of acetaldehyde, which could result in catalyst deactivation. In conclusion, Cu has a promoting effect on aldol condensation reaction on both two coordination structures of Mg-LC-O-L'C ion pairs. Zr is beneficial to the performance of the strong basic catalyst.