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WIREs Energy Environ.
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Glycerol transformation to value added C3 diols: reaction mechanism, kinetic, and engineering aspects

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Glycerol can serve as a starting material for the production of a variety of chemicals currently formed via fossil‐based routes. The selective hydro‐deoxygenation to C3‐diols is one of the most attractive methods for glycerol upgrading. 1,2‐ and 1,3‐Propanediols which are the target products of this reaction are high added value chemicals with a wide range of applications. The reaction mechanisms for both routes (1,2‐ and 1,3‐diols) are presented and analyzed including the strategies followed for mechanistic understanding. 1,2‐Propanediol is proposed to be formed via two mechanisms namely dehydration–hydrogenation and dehydrogenation‐dehydration–hydrogenation. The selective conversion of glycerol to 1,3‐propanediol is suggested to proceed through dehydration–hydrogenation and direct hydrogenolysis mechanisms. The reaction mechanism depends on various factors such as the catalyst formulation, acid and basic character of the system and the H2 origin. A section focusing on engineering issues describes and compares the reaction mode ie batch‐continuous and liquid–gas phase operation. The different reaction configurations have an impact mainly on product distribution and catalyst stability, which is greatly improved when continuous operation is applied. In addition, the impact of reaction temperature, hydrogen pressure, glycerol concentration as well as the reaction kinetics are also discussed. The approach of operating under inert conditions with H2 generated in situ seems to be a very promising concept for process intensification. WIREs Energy Environ 2015, 4:486–520. doi: 10.1002/wene.159 This article is categorized under: Bioenergy > Science and Materials
Pathways of glycerol de‐oxygenation in hydrogen atmosphere.
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Proposed process scheme for the one step 1,2‐propanediol formation from bio‐glycerol stream with in situ H2 production. Reproduced by permission of Elsevier from Ref [].
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Proposed reaction mechanism of glycerol hydrogenolysis with 2‐propanol as a hydrogen donor molecule. M: Metal sites, A: Acid sites, H (blue): hydrogen species from 2‐PO, H (green): hydrogen species from glycerol. Reproduced by permission of Elsevier from Ref [].
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Stability test of the Cu–Al catalyst 20 g Cu–Al catalyst, glycerol:20 wt%, N2 pressure: 20 bar, GHSV: 513 h−1, LHSV:1.53 h−1, temperature: 220 °C. Reproduced by permission of Royal Society of Chemistry from Ref [].
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Glycerol reaction routes over a Cu/SiO2 catalyst, Reproduced by permission of Elsevier from Ref [].
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Possible reaction network over a Ru–Re/C catalyst proposed by Torres et al., Reproduced by permission of American Chemical Society from Ref [].
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Overall reaction routes proposed by Lahr et al. Reproduced by permission of Elsevier from Ref [].
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Reaction pathways included in the kinetic analysis performed by Lahr et al., Reproduced by permission of American Chemical Society from Ref [].
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Effect of temperature on glycerol conversion and product selectivity, catalyst: 5 wt% Ru/SiO2, P = 8 MPa, catalyst/glycerol = 0.006 (wt), 5 h, pure glycerol, Reproduced by permission of American Chemical Society from Ref [].
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Acid and Base assisted dehydration followed by hydrogenation for 1,3‐propanediol formation, Reproduced by permission of John Wiley and Sons from the Ref [].
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Proposed reaction mechanism of glycerol de‐oxygenation over Pt/WO3/ZrO2, Reproduced by permission of Royal Society of Chemistry from Ref .
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Proposed mechanism for glycerol direct hydrogenolysis via alkoxide formation, Reproduced by permission of Elsevier from Ref .
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Proposed glycerol adsorption modes and hydride attack over Ir‐ReOx catalyst, Reproduced by permission of Elsevier from Ref [].
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Proposed mechanism of glycerol conversion into lactic acid and 1,2‐propanediol—dehydrogenation as the first step, Reproduced by permission of John Wiley and Sons from the Ref [].
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Glycerol hydrogenolysis to different observed routes over Ru and Pt catalysts (M = metal) through the dehydrogenation–dehydration–hydrogenation mechanism, Reproduced by permission of Elsevier from Ref .
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(a) Dehydrogenation–dehydration–hydrogenation mechanism for 1,2‐propanediol formation, (b) C–C bond cleavage via retro‐aldol on Ru, and (c) C–C bond cleavage via retro‐Claisen on Cu, Reproduced by permission of Elsevier from Ref .
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Six possible dehydration intermediates for glycerol hydrogenolysis via dehydration–hydrogenation mechanism (Double arrows indicate the possible tautomeric equilibrium between the dehydration intermediates), Reproduced by permission of Royal Society of Chemistry from Ref [].
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Annual number of publications relevant to glycerol conversion to 1,2‐ and 1,3‐propanediols from 1991 to 2013 (Search ‘glycerol’ AND ‘hydrogenolysis’ or ‘hydrodeoxygenation’ in Scopus Database).
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Reaction pathways of glycerol transformation under hydrogen over a Ru/C catalyst with Amberlyst as co‐catalyst, Reproduced by permission of Elsevier from Ref [].
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Acid and Base assisted dehydration followed by hydrogenation for 1,2‐propanediol formation, Reproduced by permission of John Wiley and Sons from Ref [].
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Examples of E1 and E2 mechanism for mono‐alcohol dehydration.
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