Mechanism of Hydrodeoxygenation of Acrolein on a Cluster Model of MoO₃

Abstract

We have explored the potential energy surface for reactions of acrolein on a Mo 3 O 9 cluster model of the MoO 3 surface to investigate the thermodynamics and kinetics of hydrogenation and selective hydrodeoxygenation. In the presence of hydrogen, conversions of acrolein to allyl alcohol, 1-propanol, and propene are all thermodynamically favorable, but the selectivity is controlled kinetically to form the least favorable product, allyl alcohol. We propose a mechanism in which coordinatively unsaturated Mo sites (i.e., oxygen vacancies) selectively chemisorb acrolein. On the basis of experimental and theoretical evidence, the active phase of the catalyst is a reduced hydrogen bronze, H x MoO 3-y , and surface hydroxyl sites are occupied when x is in the range 1.1-1.2. Surface hydroxyls are important for both oxygen vacancy formation and as Brønsted acids. The reaction rate is essentially controlled by protonation of the C-1 carbon of chemisorbed acrolein. Additional reaction barriers for proton donation to the C-2 or C-3 sites are similar in magnitude (104-134 kJ/mol), limiting the rates of formation of propene and 1-propanol, respectively. In contrast, the selectivity toward allyl alcohol is determined by a smaller O-H bond formation barrier (33 kJ/mol). The estimated reaction rate is comparable to the rate of oxygen vacancy creation, so that operation in a continuous flow process appears to be feasible. The calculated reaction barrier for C-O scission is 104 kJ/mol; we discuss the advantages of other oxides, particularly WO 3 , that have stronger metal oxygen bonds and stronger Brønsted acidity of surface hydroxyls.

One-step propylene formation from bio-glycerol over molybdena-based catalysts

“…It has been reported that Brønsted acidity directs the reaction towards acrolein-propene formation while Lewis acidity towards acetol production. 50 Taking into account the experimental results of this study, as well as relative reports, 30,43,51,52 it could be proposed that molybdenum oxide, at a reduced state, and surface hydroxyls formed due to water presence and reduction procedure, 30,53,54 acting as Brønsted acids, provide the active sites for the reaction to occur. It is of great importance that selectivity to propene increases with glycerol conversion while the overall yield to propene reaches the value of 67.6%, at 6 h of reaction time.…”

Pt/Al2O3 Catalyzed 1,3‐Propanediol Formation from Glycerol using Tungsten Additives

“…Possibly, a glycerol tungstic acid ester is formed as an intermediate, which, in combination with tungsten's favorable acidic properties, leads to 13PD selectivity. [23][24][25] Interestingly, 20 to 40 % 13PD formation is observed if a tungsten containing acidic additive is used (Figure 2 c-e) in combination with a Pt based catalyst. Moreover, only very small amounts of 12PD were observed.…”

Hydrodeoxygenation of model compounds and catalytic systems for pyrolysis bio-oils upgrading

“…The intermediate species undergoes C-O cleavage and forms a final product as shown in Figure 10 (b). In the reactions of HDO, if the metaloxygen bond is too weak, it would be difficult for the catalysts to abstract oxygen from oxy-compounds, but if metal-oxygen bond is too strong, it is difficult to create surface vacancies to adsorb oxy-compound [117]. It follows the Sabatier rule that oxides with intermediate metal-oxygen bond strength are the best better supports [118].…”

“…Pretreatments of WO 3 and MoO 3 in H 2 produce active phases as H y WO 3-z (x is in the range of 0.9-1.3) [145] and H x MoO 3-y (x is in the range of 1.1-1.2) [117]. This new class of heterogeneous catalysts for HDO of allyl alcohol and acrolein was reported by It is interesting to investigate the activity between sulfided catalysts and corresponding reduced catalysts for HDO process.…”