A DFT study on the aldol condensation reaction on MgO in the process of ethanol to 1,3-butadiene: understanding the structure–activity relationship

Abstract

Using periodic density functional theory calculations, the aldol condensation of acetaldehyde to 3-hydroxybutanal over dehydroxylated MgO surfaces with and without structure defects was investigated. Compared with the C-C coupling step, the enolization step via proton transfer of the α-hydrogen of acetaldehyde to the MgO surface or the proton back-transfer step to form the desired 3-hydroxybutanal has a higher energy barrier, indicating that the proton transfer process is the key step for the aldol condensation on MgO. To highlight the effect of water, we also calculated the proton transfer steps in the presence of water and studied the reaction pathways over the partially hydroxylated MgO surface. The results show that water can participate in the proton back-transfer step by donating a proton to the alkoxide anion to form the 3-hydroxybutanal, thus reducing the activation energy; the surface OH groups induce a lowering of the activation energy barriers for the overall reaction. The results of the electronic structure analysis indicate that a strong Lewis acid-weak/medium base pair may have the best performance for aldol condensation, such as Mg-O-D produced by divacancy defects and Mg-OH produced by the dissociative adsorption of water. A strong Lewis acid generated by low-coordinated Mg can adsorb and stabilize the acetaldehyde molecule near the catalyst surface which is beneficial for the abstraction of an α-proton from an acetaldehyde molecule, and a medium or weak Brønsted base is favorable for the proton back-transfer step.

Element Effects on High-Entropy Alloy Vacancy and Heterogeneous Lattice Distortion Subjected to Quasi-equilibrium Heating

“…Divacancy is a non-equilibrium defect in a crystal lattice where two adjacent atoms are missing 27,54,55 . In principle, divacancy formation should decrease entropy.…”

Identifying the roles of acid–base sites in formation pathways of tolualdehydes from acetaldehyde over MgO-based catalysts

“…The enolate then executes a nucleophilic attack onto the carbonyl-C of the second acetaldehyde, forming the C 4 aldol which dehydrates to 2-butenal. [63][64][65] Previous studies of the vapor phase aldol condensation of acetaldehyde have reported the active 2-butenal-forming site to be a medium-strength, M-O-type site. 19,23,65,66 Tolualdehydes (ortho-and para-) can form through 2 main routes.…”

Mechanistic Investigation on Ethanol to Butadiene Conversion Reaction over Metal Oxide Clusters: A Computational Study

“…A number of computational studies on the reaction mechanism of ethanol-to-butadiene conversion have been reported. [7,[25][26][27][28] For examples, Chieregatoet al performed a computational modeling coupled with an experimental study on a MgO cluster to investigate the reactivity of ethanol. [24] As a model for MgO nanocrystals, (MgO) 10 or (MgO) 16 clusters were used.…”

“…Zhang et al carried out a periodic DFT study on the aldol condensation reaction on MgO. [25] According to their results, the barrier height of the C-C coupling step in the aldol condensation reaction was computed to be lower than the proton transfer step. Baltrusaitiset al also used periodic DFT methods to explore the ethanol-to-butadiene reaction mechanism.…”

“…[3,29] The introduction of ZnO has been reported to enhance the catalytic performance of MgO/SiO 2 [4] , which is of interest for investigating details of the performance at the molecular level. There have been several computational mechanistic studies on some metal oxides such as MgO, [25,26] ZrO 2 , [27] Zr-SBA-15 [28] , but to our knowledge, no study has dealt with a detailed computational investigation on full mechanistic aspects described in Scheme 1. Moreover, the atomistic mechanism on the enhanced catalytic performance by ZnO as a catalyst for the ethanol-to-butadiene conversion has still been elusive.…”