Ethanol partial oxidation

Wanat et al. performed ethanol partial oxidation over different rhodium containing catalysts on ceramic monoliths [195], Similar to partial oxidation of methanol (see Section 4.2.1), the rhodium/ceria sample performed best, 95% ethanol conversion was achieved at reaction temperatures exceeding 700 ° C and 0/C ratios between 0.66 and 1.0. Similar to methanol, carbon monoxide selectivity was high, exceeding 85%, while methane and ethylene were observed as by-products. 

Acetaldehyde, first used extensively during World War I as a starting material for making acetone [67-64-1] from acetic acid [64-19-7] is currendy an important intermediate in the production of acetic acid, acetic anhydride [108-24-7] ethyl acetate [141-78-6] peracetic acid [79-21 -0] pentaerythritol [115-77-5] chloral [302-17-0], glyoxal [107-22-2], aLkylamines, and pyridines. Commercial processes for acetaldehyde production include the oxidation or dehydrogenation of ethanol, the addition of water to acetylene, the partial oxidation of hydrocarbons, and the direct oxidation of ethylene [74-85-1]. In 1989, it was estimated that 28 companies having more than 98% of the wodd s 2.5 megaton per year plant capacity used the Wacker-Hoechst processes for the direct oxidation of ethylene. 

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). 

It is well established that the main products of ethanol electro-oxidation on Pt in acidic media are acetaldehyde and acetic acid, partial oxidation products that do not require C—C bond breaking, with their relative yields depending on the experimental conditions [Iwasita and Pastor, 1994]. Apart from the loss of efficiency associated with the partial oxidation, acetic acid is also unwanted, as it constitutes a catalyst poison. 

Cu-CuO% nanoparticles (with a content of about 10 wt.%) on titania are effective for the production of hydrogen under sacrificial conditions [176-178], A fairly low concentration of Cu (2.5 wt.%) was sufficient to allow promising H2 production from ethanol-water and glycerol-water mixtures in the case of CuO% nanoparticles encapsulated into porous titania [179]. A key limitation of this system is photocorrosion under oxidizing conditions (oxygen and carboxylic adds as by-products of partial oxidation of the sacrificial agent). However, in the presence of UV irradiation, Cu photodeposition can occur, preventing loss of Cu [179]. 

Partial oxidation is also mentioned as a process to convert ethanol to hydrogen.124 Another novel technology for ethanol to hydrogen has been described by Toci and Modica.125 It is based on cracking ethanol vapors by "cold-plasma-chemical processing" in the presence of a Ni-based catalyst. 

H2 production from ethanol (as well as methanol) employs these methodologies either as such or after slight modifications, especially in the ATR process, wherein a separate combustion zone is usually not present (Scheme 3). A mixture of ethanol, steam and 02 with an appropriate ethanol steam 02 ratio directly enters on the catalyst bed to produce syngas at higher temperature, around 700 °C.18,22 The authors of this review believe that under the experimental conditions employed, both steam reforming and partial oxidation could occur on the same catalyst surface exchanging heats between them to produce H2 and carbon oxides. The amount of 02 may be different from what is required to achieve the thermally neutral operation. Consequently the reaction has been referred to as an oxidative steam reforming... 

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. 

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