Steam reaction pathways

Scheme 4 Proposed reaction pathway for the steam reforming of ethanol. 

Scheme 6 Proposed reaction pathway for the oxidative steam reforming/ autothermal reforming of ethanol. The dehydration into ethylene intermediate followed by ethylene reforming are not shown. 

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. 

In summary, catalytic C-H transformations in small unfunctionalized alkanes is a technically very important family of reactions and processes leading to small olefins or to aromatic compounds. The prototypical catalysts are chromia on alumina or vanadium oxides on basic oxide supports and platinum on alumina. Reaction conditions are harsh with a typical minimum temperature of 673 K at atmospheric pressure and often the presence of excess steam. A consistent view of the reaction pathway in the literature is the assumption that the first C-H abstraction should be the most difficult reaction step. It is noted that other than intuitive plausibility there is little direct evidence in heterogeneous reactions that this assumption is correct. From the fact that many of these reactions are highly selective toward aromatic compounds or olefins it must be concluded that later events in the sequence of elementary steps are possibly more likely candidates for the rate-determining step that controls the overall selectivity. A detailed description of the individual reactions of C2-C4 alkanes can be found in a comprehensive review [59]. 

In certain bicyclic and polycyclic thiadiazoles the ring can be hydrolytically cleaved to an o-diamine and sulfur dioxide. This process is a reversal of the reaction pathway involved in the synthesis of thiadiazoles from o -diamines and thionyl chloride or iV-sulfinylaniline under anhydrous conditions (see Section 4.26.5.1.1), and is analogous to, but much slower than, the hydrolysis of sulfurdiimides. In contrast to (1), which steam distilled without decomposition, steam distillation of [l,2,5]thiadiazolo[3,4-c][l,2,5]thiadiazole (21) produced only oxamide. Under milder conditions (75 °C, 12 h) a mixture of the diamine (22), oxamide and elemental sulfur was obtained (75JOC2749). Similar hydrolytic instability was observed in thiadiazole rings angularly joined to an anthraquinone (70RCR923) and ben-zofuroxan (78CPB3896). 

Figure 2.22. Proposed reaction pathways for the steam reforming of methanol over Cu-based and Pd/ZnO-based catalysts, (a) Methanol decomposition followed by water-gas shift reaction, (b) Methanol dehydrogenation to formaldehyde/methylformate route.

Figure 2.30. Proposed reaction pathway for the steam reforming of ethanol. Adapted from Velu and Song.167...

Unlike the steam reformer the autothermal one requires no external heat source and no indirect heat exchangers. This makes autothermal reformers simpler and more compact than steam reformers, resulting in lower capital cost. Furthermore, autothermal reformers typically offer higher system efficiency than partial oxidation systems, where the excess heat is not easily recovered. The autothermal reforming reaction is carried out in the presence of a catalyst, which controls the reaction pathways and thereby determines the relative extents of the oxidation and steam reforming reactions. Therefore, in order to achieve the desired conversion and product selectivity an appropriate catalyst is essential. 

Note that H2, CO, and CH4 are shown in Table 2-1 as potentially undergoing direct anodic oxidation. In actuality, direct electrochemical oxidation of the CO and CH4 usually represents only a minor pathway to oxidation of these species. It is common systems analysis practice to assume that H2, the more readily oxidized fuel, is produced by CO and CH4 reacting, at equilibrium, with H2O through the water gas shift and steam reforming reactions, respectively. A simple reaction pathway analysis explains why direct oxidation is rarely the major reaction pathway under most fuel cell operating conditions ... 

Apart from the described radical reaction pathways, there are several important side and consecutive reactions that also proceed in the cracking furnace. The higher the product concentration in the stream (i.e., at high feedstock conversion), the higher is the probability of these side and consecutive reactions. Important side and consecutive reactions include isomerization, cyclization, aromatization, alkylation, and also condensation reactions. The aromatic compounds found in the steam cracker product stream are formed, for example, by cycloaddition reactions of alkenes and dienes followed by dehydrogenation reactions. Moreover, monoaromatic compounds transform into aromatic condensates and polyaromatics (see also Scheme 6.6.2) by the same reactions. Typically, more than 100 different products are found in the product mixture of a commercial steam cracker. 

The partial oxidation (POX) reaction is highly exothennic and provides the heat required for the steam methane reforming reaction that occurs following the eonversion of hydroearbons to earbon monoxide and hydrogen. The partial oxidation reaction is favored by high temperature and high pressure. However, the higher the pressure the more likely that alternative reaction pathways will lead to the formation of earbon. 

Uranium oxide catalysts have been reported to have high activity for oxidation of volatile organics [60]. Activity for oxidation of short chain linear alkanes improved when Cr was added as a modifier. It is reported that the addition of Cr increases the defects density of U3O8 phase. Deep oxidation activity of UsOg was enhanced when 2.6% of water was cofed with VOC [61]. It is proposed that this improvement is due to contributions from the other reaction pathways, such as steam reforming. However, the addition of more water caused the catalytic activity to decrease and similar treatment decreased the VOC oxidation activity of Mn203. 

Figure 12.11 shows the possible reaction pathways in an internally reforming SOFC running on natural gas and steam. In some SOFC applications, especially small-scale and directly reforming systems, a lower steam to carbon ratio is... 

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