(560ao) Catalytic Deoxydehydration of Glycerol to Allyl Alcohol with 2-Hexanol As H-Donor – a Detailed Study of the Reaction Mechanism

Allyl alcohol is a very promising starting material for a variety of molecules such as acrylonitrile, acrolein and acrylic acid – all, important intermediates for the chemical industry: The ammoxidation of allyl alcohol to acrylonitrile was reported with a high yield (more than 80%) by Guillon et al.[i],[ii]. The selective oxidation of allyl alcohol to produce acrylic acid yields up to 85 % was submitted at rather low temperatures (300°C) over MoVOx catalyst.[iii] However, the main bottleneck for these applications lies in the sourcing of allyl alcohol. Nowadays, allyl alcohol is produced by the selective catalytic hydrogenation of acrolein, which is obtained by the selective catalytic oxidation of propylene. Recently we reported the synthesis of allyl alcohol with a high yield (>86%) coming from deoxydehydration of glycerol (DODH) over Rhenium based catalyst with 2-hexanol as sacrificial hydrogen-donor and the corresponding reaction mechanism explaining the key role of water.

Experimental

For the DODH of glycerol, a rhenium based catalyst was prepared by an incipient wetness impregnation method using a 75 wt% perrhenic acid aqueous solution (Aldrich). After one hour, the impregnated catalyst was dried at 110 °C for 12 h and then calcined under static air from 110 °C to 500 °C (5 °C / min) for 3 h. The catalytic performance was evaluated using a pressure-resistant glass tube equipped with a magnetic stirring bar, it was loaded with 100mg of catalyst, glycerol (92 mg, 1 mmol), 100 mg, and 2-hexanol (3.3 mL). The reaction temperature inside the tube kept at 148 °C for 2.5h. The conversion and selectivity were determined by GC analysis using benzene(20 mg, 0.25 mmol) as an internal standard.

Results and discussion

The DODH of glycerol over rhenium based catalyst exhibited full conversion after 2.5 h, yielding up to 86% of allyl alcohol. From the reaction curve, it becomes visible that the selectivity in allyl alcohol increased during the 2 first hours of the reaction, suggesting the presence of an intermediate species. With respect to these results, the by-products were deeply analyzed using GC-MS, allowing notably the identification of hexenes, ethers, and acetals. The identification of acetals was most surprising. The latter is formed by condensation of glycerol with acrolein, indicating that acrolein is the intermediate species in the DODH reaction. The latter is formed by dehydration of glycerol and then further hydrogenated to allyl alcohol. The significant formation of acetal at the beginning of the reaction can be explained by the role of water in the equilibrium of this reaction. in other words, the acetalization between acrolein and glycerol is an equilibrium reaction, giving water as a co-product. When the amount of water in the reaction mixture is low, the equilibrium is on the side of the acetals. Here, other identified side reactions come into the game: 2-hexanol was found to be dehydrated to various hexenes and condensated to an ether during the reaction. The as-formed water helps to shift the equilibrium of the acetalization to the reactant side (formation of acrolein), whereby the selectivity of the final product (allyl alcohol) increased with the reaction time. The corresponding mechanism was further validated by using individual reactions with acrolein, water, and 2-hexanol as reactants.

Conclusions

The DODH reaction of glycerol over a heterogeneous rhenium based catalyst using 2-hexanol as H-donor showed increasing selectivity in allyl alcohol over reaction time, suggesting the formation of an intermediate. The deep analysis of the reaction mixture revealed the presence of acetals formed from the reaction between acrolein and glycerol. With respect to the equilibrium of this reaction, water was found to play a key-role.

[i] C. Liebig, S. Paul, B. Katryniok, C. Guillon, J-L. Coururier, J-L. Dubois, F. Dumeignil, W.F. Hoelderich, Appl. Catal. B: Env. 132-133 (2013) 170.

[ii] C. Guillon, C. Liebig, S. Paul, A-S. Mamede, W.F. Hoelderich, F. Dumeignil, B. Katryniok, Green Chem. 11 (2013) 3015.

[iii] T. Murayama, B. Katryniok, S. Heyte, M. Araque, S. Ishakawa, F. Dumeignil, S. Paul, W. Ueda, ChemCatChem 8 (2016) 1