Molybdenum-Catalyzed Conversion of Diols and Biomass-Derived Polyols to Alkenes Using Isopropyl Alcohol as Reductant and Solvent

Abstract

Chemical processes capable of reducing the high oxygen content of biomass-derived polyols are in demand in order to produce renewable substitutes for chemicals of fossil origin. Deoxydehydration (DODH) is an attractive reaction that in a single step transforms a vicinal diol into an alkene, but the reaction requires a homogeneous catalyst, a reductant, and a solvent, which are typically expensive, unsustainable, or inefficient. Herein, we present the use of molybdenum(VI)-based compounds, in particular the cheap and commercially available (NH 4 ) 6 Mo 7 O 24 •4H 2 O, as catalysts for the DODH of vicinal diols in isopropyl alcohol ( i PrOH), which serves as both the solvent and reductant. The reaction proceeds at 240−250 °C in a pressurized autoclave, and the alkene yield from simple aliphatic diols can be as high as 77%. The major byproducts are carbonyl compoundsformed by dehydration of the dioland the alcohols formed by transfer hydrogenation of the carbonyl compounds; the total yield of reduced species (i.e., alkene and alcohols) can be as high as 92%. The DODH of glycerol yields allyl alcohol, which undergoes subsequent Mo-catalyzed deoxygenation to propylene driven by the oxidation of i PrOH; a major byproduct is the homocoupled product 1,5-hexadiene. Further insight in this Mo-catalyzed deoxygenation is gained by an investigation of model compounds: The allylic alcohol 1-hexen-3-ol is deoxygenated to hexene isomers in a yield of 65%, while benzyl alcohol is deoxygenated to toluene in a yield of 93%. The DODH of erythritol yields 39% 2,5-dihydrofuran, while the DODH of the proposed intermediate 1,4-anhydroerythritol yields 75%. The mechanism of the DODH of 1,4-anhydroerythritol was investigated by means of density functional theory (DFT), and the rate-determining step (24.1 kcal/mol) was found to be reduction of a molybdenum(VI) diolate to a molybdenum(IV) diolate.

Catalytic Transfer Hydrogenolysis as an Effective Tool for the Reductive Upgrading of Cellulose, Hemicellulose, Lignin, and Their Derived Molecules

“…Following this approach, the total yield of reduced species (such as alkene and alcohols) can be as high as 92% at 240-250 °C. The DODH of erythritol can reach a yield of 39% to 2,5-dihydrofuran [60]. Schüth and coworkers investigated the conversion of glycerol into allyl alcohol, through an initial dehydration to acrolein, by using iron oxide as a catalyst [57].…”

“…The same procedure was applied to erythritol that has been converted into 2,5-dihydrofuran at 210-220 • C [59]. During the dehydration step, formic acid acts as acid catalyst and, in the reductive step, as hydrogen donor [60].…”

“…In this work the deoxydehydration (DODH) of glycerol and erythritol is performed with the commercially available (NH 4 ) 6 Mo 7 O 24 ·4H 2 O catalyst, in the presence of 2-propanol, acting both as solvent and reducent agent. Following this approach, the total yield of reduced species (such as alkene and alcohols) can be as high as 92% at 240-250 • C. The DODH of erythritol can reach a yield of 39% to 2,5-dihydrofuran [60].…”

Vanadium‐Catalyzed Deoxydehydration of Glycerol Without an External Reductant

“…Yet under continuous‐flow conditions, triethyl orthoformate rivals formic acid as a reagent for the DODH of glycerol . Comparably, secondary alcohols have literature precedence as reductants in Mo‐ and Re‐catalysed DODH . Yet their use on an industrial scale would require the recycling of the secondary alcohols by the hydrogenation of the corresponding ketones.…”

A Radical Mechanism for the Vanadium-Catalyzed Deoxydehydration of Glycols

“…2 Homogeneous reactions using rhenium catalysts, for which detailed mechanistic information (computational and experimental) is available, [3][4][5][6][7][8][9][10][11][12][13][14][15] are showing great potential in processes ranging from the deoxydehydration of glycols to the deoxygenation of epoxides, alcohols and carbonyl compounds, as does the derivatization of biomass polyols catalyzed by rhodium or iridium complexes. 16 The discovery of the activity of Mo(VI) and V(V) complexes, based on cheaper and more abundant metals, in the DODH of glycols [17][18][19][20][21][22][23][24] led us to focus on the study of the mechanism of these transformations. These complexes are versatile in terms of manipulation and derivatization, hence the goal is designing more efficient catalytic systems.…”