Ionic reactions and pyrolysis of glycerol as competing reaction pathways in near- and supercritical water

Bühler, W.; Dinjus, E.; Ederer, H. J.; Kruse, Andrea; Mas, C.

doi:10.1016/s0896-8446(01)00105-x

Cited by 425 publications

(274 citation statements)

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“…Trace amounts of acrylic acid and ethylene were also detected. This wide variety of products reflects the complexity of the reaction mechanisms involved in the hydrothermal decomposition of glycerol, which can be summarized into the coexistence of competing ionic and free radical pathways [10]. The ionic path dominates at high-water density -low temperature and high pressure -a situation in which the ion product of water is high enough to allow the existence of free protons and hydroxyl ions that may catalyze ionic reactions, and the static dielectric constant of water is high enough to stabilize ionic species and reaction intermediates.…”

Section: Methodsmentioning

confidence: 99%

“…At 510 °C and 350 bar the ion product of water is six orders of magnitude lower than for water at room temperature and the dielectric constant is close to that of steam, which indicates that the ionic route will play a minor role in our case and that the free-radical pathway should dominate. The latter is summarized in Figure 6, where a simplified schematic of a more rigorous mechanism [10] is given. Hydrogen transfer reactions form unstable •CH 2 CHOHCH 2 OH (Int1) and •COHCHOHCH 2 OH (Int2) radicals.…”

Section: Methodsmentioning

confidence: 99%

“…Hydrothermal chemistry in sub-or supercritical water (SCW), either non-catalytic or with the addition of homogeneous and heterogeneous catalysts, has received considerable attention in recent years. Non-catalytic hydrothermal processing yields a variety of aldehydes, alcohols and gases [10], but adequate catalysts and control of reaction conditions maximize the yield of specific products like acrolein [11,12].…”

Section: Introductionmentioning

confidence: 99%

“…Hydrogen, methane and syngas have been produced by hydrothermal gasification of biomass substrates and model compounds such as glucose, cellulose and lignin in supercritical water [13][14][15][16][17][18], and glycerol gasification has been investigated as well [10,19]. Supercritical water has particular properties that provide a highly reactive and homogeneous medium for the conversion of organic molecules.…”

Section: Introductionmentioning

confidence: 99%

“…Non-catalytic glycerol decomposition in SCW proceeds through a complex reaction mechanism that is summarized in two competing pathways, ionic and free-radical, whose predominance depends on water density and acidity [10,22]. At low temperature and high pressure (i.e., high water density) a set of ionic reactions forms acetaldehyde, formaldehyde and acrolein as main products.…”

Section: Introductionmentioning

confidence: 99%

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Catalytic gasification of glycerol in supercritical water

May

Salvadó

Torras

et al. 2010

Chemical Engineering Journal

100

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The conversion of glycerol in supercritical water (SCW) was studied at 510 -550°C and a pressure of 350 bars using both a bed of inert and non-porous ZrO 2 particles (hydrothermal experiments), and a bed of 1 % Ru/ZrO 2 catalyst particles. Experiments were conducted with a glycerol concentration of 5 wt% in a continuous isothermal fixed-bed reactor at a residence time between 2 and 10 s. Hydrothermolysis of glycerol formed water-soluble products such as acetaldehyde, acetic acid, hydroxyacetone and acrolein, and also gases like H 2 , CO and CO 2 .The catalyst enhanced the formation of acetic acid, inhibited the formation of acrolein, and promoted the gasification of the glycerol decomposition products. Hydrogen and carbon oxides were the main gases produced in the catalytic experiments, with only minor amounts of methane and ethylene. Complete glycerol conversion was achieved at a residence time of 8.5 s at 510 °C, and at around 5 s at 550 °C with a 1 wt% Ru/ZrO 2 catalyst. The catalyst was not active enough to achieve complete gasification, since high yields of primary products like acetic acid and acetaldehyde were still present. Carbon balances were between 80 and 60 % in the catalytic experiments, decreasing continuously as the residence time was increased. This was attributed partially to the formation of methanol and acetaldehyde, which were not recovered and analyzed efficiently in our set-up, but also to the formation of carbon deposits. Carbon deposition was not observed on the catalyst particles but on the surface of the inert zirconia particles, especially at high residence time. This was related to the higher concentration of acetic acid and other acidic species in the catalytic experiments, which may polymerize to form tar-like carbon precursors. Because of carbon deposition, hydrogen yields were significantly lower than expected; for instance at 550 ºC the hydrogen yield potential was only 50 % of the stoichiometric value.

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%