Mechanistic Insights into the Meerwein–Ponndorf–Verley Reaction and Relative Side Reactions over MgO in the Process of Ethanol to 1,3-Butadiene: A DFT Study

Abstract

In this work, we performed density functional theory calculations to explore reaction pathways of ethanol to 1,3butadiene on the surfaces of MgO(100) and MgO(110). The overall activation barrier and reaction energy were calculated for the Meerwein−Ponndorf−Verley (MPV) reaction and relative side reactions, cross aldol condensation, and hydrogenation reaction. It is verified that the acidity and basicity sites on MgO(110) are stronger than those on MgO(100), which is favorable for the MPV reaction and cross aldol condensation. On MgO(110), the MPV reaction takes place via the ethoxy and crotonaldehyde route due to dissociated ethanol and directly deoxidizes to produce 1,3butadiene rather than the proton return reaction finally. The ratedetermining step of the MPV reaction is proton transfer reaction on both surfaces. In addition, the cross aldol condensation is less likely to occur on MgO(100) and the hydrogenation side reaction is less likely to occur on MgO(110).

Porous Magnesium Oxide by Twin Polymerization: From Hybrid Materials to Catalysis

“…The para-substituted derivatives 13 b-15 b (Scheme 5) are only detected in very minor amounts. This sensitivity towards the catalyst's basic sites, especially those based on alkaline earth metal oxides, is discussed in detail in references [12,14,89,92]. The recycled catalyst MgO_2-rec, shows, however, a different catalytic behavior when compared to MgO_2.…”

“…[86] The overall mechanism of the MPV reaction is discussed in references. [84,85,[88][89][90][91] Catalysts MgO_1 and MgO_2 can be recycled by centrifugation and consecutive calcination at 400 °C in air for 24 h whereby MgO_1-rec and MgO_2-rec were obtained. Characterization of the material was done by BET and PXRD confirming that the recycled oxides exhibit similar surface areas (230 m 2 g À 1 , MgO_1-rec; 240 m 2 g À 1 , MgO_2-Rec) and comparable crystallite sizes (Table 4).…”

“…A possible explanation for the low overall yields is related to the formation of 1,3-butadiene by direct deoxidization rather than completion of the proton return reaction towards the crotyl alcohol as is discussed in literature. [30,89,91,93,94] The overall yield of the identified products increases from MgO_2 to MgO_2-rec and from MgO_1 to MgO_1-rec indicating that this deoxygenation reaction becomes more hindered in the recycled catalysts compared to the original ones. Due to the non-formation of octatrienal (16) the formation of higher chained side products (> C 10 ) could be excluded for MgO_1 and MgO_2 and their recycled forms.…”

“…In the benzaldehyde reduction, however, side reactions, including the Claisen‐Schmidt condensation typical for benzaldehydes in presence of bases, occur, leading to the reaction of the substrate with acetone, formed in situ from i PrOH, to give benzalacetone ( 9 ) (Scheme 4). [86] The overall mechanism of the MPV reaction is discussed in references [84,85,88–91] …”

A study of the conversion of ethanol to 1,3-butadiene: effects of chemical and structural heterogeneity on the activity of MgO–SiO2 mixed oxide catalysts

“…It is argued that these species are formed either via direct surface reaction between AL and ethanol, 11,90 or by Meerwein-Ponndorf-Verley (MPV) reduction of CAD, which is obtained from AL by aldol addition and condensation. 13,55,91,92 Recently, Yang et al 28 successfully identified the aldol addition product 3-hydroxybutanal (i. e., acetaldol), the intermediate of CAD, but not under commonly used reaction conditions. We never detected acetaldol and never found CAD in the product mixture in concentration that could be considered as strong enough evidence for the role of CAD or CAD-like surface species as an intermediate of the ETB reaction.…”