Partial oxidation of ethanol

It has also been proposed that under the acidic conditions found ia whiskeys, ethanol reacts with lignin (qv) to reduce an alcohol-soluble form of lignin (ethanol lignin). This can be converted into coniferyl alcohol, which can be oxidized to coniferaldehyde. The partial oxidation of ethanol lignin can produce siaapic and coniferyl alcohols that can be converted to syfingaldehyde and vanillin, respectively (8). 

Chemistry. Partial oxidation of ethanol (POE) involves reaction between ethanol and oxygen with an appropriate 02/Ethanol molar ratio, up to 1.5 over a suitable metal catalyst for the production of H2 and C02 (eqn (18)). The overall reaction may be considered as a combination of partial oxidation to syngas (eqn (19)) coupled with CO oxidation (eqn (20)). The complete oxidation of ethanol will produce a mixture of H20 and C02 with a huge heat release (eqn (21)). 

Unlike SRE, the POE reaction for H2 production has been reported so far only by a few research groups.101104-108 While Wang et al. os and Mattos et r//.104-106 have studied the partial oxidation of ethanol to H2 and C02 (eqn (18)) at lower temperatures, between 300 and 400 °C using an 02/EtOH molar ratio up to 2, Wanat et al.101 have focused on the production of syngas (eqn (19)) over Rh/Ce02-monolith catalyst in a catalytic wall reactor in millisecond contact time at 800 °C. Depending on the nature of metal catalyst used and the reaction operating conditions employed, undesirable byproducts such as CH4, acetaldehyde, acetic acid, etc. have been observed. References known for the partial oxidation of ethanol in the open literature are summarized in Table 6. 

Table 6 Catalysts for the catalytic partial oxidation of ethanol (CPOX/POE) for hydrogen production...

Fig. 9 Free energy changes in the partial oxidation of ethanol, acetaldehyde and methane. The data of CO oxidation also is included.

Fig. 10 Thermodynamic equilibrium compositions on dry basis for the partial oxidation of ethanol. All species are in gas phase. Initial concentrations of CO, CH4, CH3CHO are taken as zero in the calculation.

Kinetics. During the course of this review, the authors could not find any significant kinetic studies on the partial oxidation of ethanol published in the literature and consequently, the rate and activation energies of the POE reaction are not known. 

Scheme 5 Proposed reaction pathway for the partial oxidation of ethanol. The dehydration into ethylene intermediate is not included. 

While spectroscopic studies attempt to account for the formation of CH4, CO and C02, they do not show the pathway for the formation of large amounts of H2 in the POE reaction even at lower temperature, around 300 °C.108 It is possible that the acetaldehyde formed by the oxidative dehydrogenation could be partially oxidized to H2 and carbon oxides (see eqns (23) and (24) and Fig. 9). Participation of these reactions could account for the formation of large amounts of H2 in the partial oxidation of ethanol. Based on the experimental observation and thermodynamics of the reactions, a reaction pathway for the partial oxidation of ethanol has been proposed and it is shown in Scheme 5. The proposed pathway accounts for the experimental observation of large amount of H2 and C02 in the partial oxidation of ethanol even at lower temperatures, around 300 °C with traces of acetaldehyde and CO as well as small amount of methane.108... 

This review analyzed the chemistry involved, thermodynamics, catalysts used, reaction pathways and mechanisms of various reforming techniques reported for the conversion of ethanol into H2-rich gas. The known reforming processes are broadly classified into three categories, namely steam reforming of ethanol (SRE), partial oxidation of ethanol (POE) and oxidative steam reforming (OSR)/autothermal reforming of ethanol. All these reactions are thermodynamically favorable even at lower temperatures, above 200 °C. 

Ethanol can be converted directly to hydrogen through two main processes, steam reforming of ethanol (SRE) and partial oxidation of ethanol (POE). These two reforming techniques are described by the following equations ... 

The partial oxidation of ethanol was investigated, but with less intensity than in the case of steam reforming. The reason is that the use of the pure partial oxidation process is not advised for bioethanol reforming because bioethanol is an ethanol-water mixture in which removal of all the water entails a significant cost. Therefore, for bioethanol partial oxidation, the process is combined with steam reforming in autothermal schemes with the stoichiometry shown in Equation 6.18. 

H2 and CO, whereas part of the ethoxy species generated on the supports is further oxidized to acetate species, which decomposes to CH4 and/or oxidizes to CO2 via carbonate species [202]. Hence supports with redox properties that help the oxidation of ethoxy species and metals with a high capacity to break C-C bonds and to activate C-H bonds are suitable for use in catalysts applied to the partial oxidation of ethanol. 

Liguras, D.K., Goundani, K., and Verykios, X.E. Production of hydrogen for fuel cells by catalytic partial oxidation of ethanol over structured Ru catalysts. International Journal of Hydrogen Energy, 2004, 29 (4), 419. 

Salge, J.R., Deluga, G.A., and Schmidt, L.D. Catalytic partial oxidation of ethanol over noble metal catalyst. Journal of Catalysis, 2005, 235, 69. 

Derivation Partial oxidation of ethanol, the acetaldehyde first formed condensing with the alcohol. 

Ni/A12Q3 Ceramic Partial oxidation of ethanol (hydrogen prod.) Liguras et al. [212]... 

Acetaldehyde synthesis by dehydrogenation or partial oxidation of ethanol in the vapor phase (Fig. 8.1)... 

The reaction of ethanol with ammonia on zeolite catalysts leads to ethylamine. If, however, the reaction is carried out in the presence of oxygen, then pyridine is formed [53]. MFI type catalysts H-ZSM-5 and B-MFI are particularly suitable for this purpose. Thus, a mixture of ethanol, NH3, H2O and O2 (molar ratio 3 1 6 9) reacts on B-MFI at 330 °C and WHSV 0.17 h 1 to yield pyridine with 48 % selectivity at 24 % conversion. At 360 °C the conversion is 81% but there is increased ethylene formation at the expense of pyridine. Further by-products include diethyl ether, acetaldehyde, ethylamine, picolines, acetonitrile and CO2. When applying H-mordenite, HY or silica-alumina under similar conditions pyridine yields are very low and ethylene is the main product. The one-dimensional zeolite H-Nu-10 (TON) turned out to be another pyridine-forming catalyst 54]. A mechanism starting with partial oxidation of ethanol to acetaldehyde followed by aldolization, reaction with ammonia, cyclization and aromatization can be envisaged. An intriguing question is why pyridine is the main product and not methylpyridines (picolines). It has been suggested in this connection that zeolite radical sites induced Ci-species formation. 

The product of partial oxidation of ethanol mainly consisted of CO2, acetaldehyde and acetic acid. The influence of reaction temperature (Tr), contact time (W/F gcat mole (etoh)h), concentration of rectified spirit in water (weight %) and 02/Ethanol molar ratio on conversion and selectivity to acetaldehyde and acetic acid were studied. Data obtained are listed in tables 1-4 respectively. 

The crystal structure of Sn-Mo catalyst is not clear. The X-ray diffraction pattern of the calcined catalyst suggests that both tin and molybdenum oxide exist separately on the catalyst surface. The reaction mechanism can be explained on the basis, i.e., tin oxide effecting oxidation, while molybdenum oxide effecting reduction to help the partial oxidation of ethanol to acetaldehyde and acetic acid. 

Composite oxide (Sn02, M0O3) is a potential catalyst for partial oxidation of ethanol to acetaldehyde and acetic acid. The operating conditions like temperature, W/F and 02/Ethanol(mole) for selective oxidation of ethanol to acetaldehyde and acetic acid have been optimized. The total selectivity is maximum for low ethanol containing (25 weight% rectified spirit) feed. 

Acetaldehyde can be produced by the partial oxidation of ethanol and the direct oxidation of ethylene. The predominant commercial process, however, is the direct liquid phase oxidation of ethylene. As with many other ethylene-based petrochemicals, acetaldehyde was first produced commercially from acetylene. The acetylene process was developed in Germany more than 70 years ago and was still practiced until the mid-1970s when the high cost and scarcity of acetylene forced it into obsolescence. Another early route to acetaldehyde was based on ethanol. Ethyl alcohol can be either oxidized or alternatively dehydrogenated to acetaldehyde. Site-... 

There were also improvements in acetaldehyde and acetic anhydride manufacture. Ag based catalysts for the partial oxidation of ethanol became available around 1940. When used to oxidatively dehydrogenate ethanol [14], the conversion of ethanol to acetaldehyde was no longer equilibrium limited since the reaction was now very exothermic. Fortunately, the process still displayed excellent selectivity (ca. 93-97%) for acetaldehyde. This technology replaced the older Cu-Cr processes over the period of the 1940-1950 and made ethanol a much more attractive resource for acetaldehyde. When ethylene became available as a feedstock in the 1940 s through 1950 s, ethanol became cheaply available via ethylene hydration (as opposed to traditional fermentation). With ethanol now cheaply available from ethylene, the advent of the Ag catalyzed oxidative dehydration to acetaldehyde rapidly accelerated the shutdown of the last remaining wood distillation units. 

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