Dehydrogenation pathway

Isobutene is present in refinery streams. Especially C4 fractions from catalytic cracking are used. Such streams consist mainly of n-butenes, isobutene and butadiene, and generally the butadiene is first removed by extraction. For the purpose of MTBE manufacture the amount of C4 (and C3) olefins in catalytic cracking can be enhanced by adding a few percent of the shape-selective, medium-pore zeolite ZSM-5 to the FCC catalyst (see Fig. 2.23), which is based on zeolite Y (large pore). Two routes lead from n-butane to isobutene (see Fig. 2.24) the isomerization/dehydrogenation pathway (upper route) is industrially practised. Finally, isobutene is also industrially obtained by dehydration of f-butyl alcohol, formed in the Halcon process (isobutane/propene to f-butyl alcohol/ propene oxide). The latter process has been mentioned as an alternative for the SMPO process (see Section 2.7). 

Reduction of peroxidase Compound I back to the resting state can occur by one of four pathways, depending on the reaction catalyzed oxidative dehydrogenation (pathway (1) in Fig. 10.4), oxidative halogenation (2), peroxide disproportionation (3) or oxygen transfer (4) [60]. 

A linear relationship between the stabihty of the metal-oxygen pair (AV ) and the preference for the dehydrogenation pathway is demonstrated. The more stable the surface oxygen atoms (more negative AF ), the less favored is the dehydrogenation... 

Tert-butanol and 1-propanol were chosen because a marked difference in reactivity between the two alcohols was expected a priori. Transition metal oxides may also catalyze the formation of ketones or aldehydes by oxidative dehydrogenation providing they have a significant number of basic/acid site pairs [10], With both HY and WZ, the alcohol dehydration to ether and oxidative dehydrogenation pathways were essentially negligible. 

Figure 18.5 Energy diagrams for a proposed dehydrogenation pathway on a stoichiometric Ti02(l 10) surface. The zero-level energy (E0) as reference is defined as the sum of the energies of the clean surface and of two HCOOH (g).

The electron density at the transition state in the most plausible dehydrogenation pathway (Path 2) is also shown. Note that both the reactant molecules at the transition state interact with the surface. 

On the other hand, the concentrations of the reactants, bridging formate R4 and weakly adsorbed formic add, in Path 2 were sufficient at the surface because no special sites are required for this path and one of the reactants, R4, is the most stable species. Furthermore, high HCOOH coverage by increasing HCOOH pressure increases the coadsorption state in such a manner that a weakly adsorbed HCOO H is located adjacent to a R4 species. Thus Path 2 via the transition state activated in a concerted manner by three Ti4+ ions (Figure 18.5) should be the most plausible dehydrogenation pathway under the reaction conditions [14]. 

The reaction pathways denoted by the intermediates (2) and (4) have been subject to extensive fundamental studies [3,6,31,32] whereas the dehydrogenation pathway was rarely found in such studies. The obvious reason for this problem is that (y) oxygen cannot be generated from the gas phase under conditions accessible in UHV systems [20]. Only after prolonged use of a single crystal the bulk-dissolved... 

The reaction intermediates methoxy and formate seem to occur in two forms each. One is very active and carries the reaction under steady-state conditions. The other is more a spectator species with increased binding energy to the substrate which can, however, undergo complex side reactions and can contribute to the selectivity spectrum of the overall process. The external reaction conditions affect sensitively the co-operation of the dehydrogenation and oxidative dehydrogenation pathways leaving room for the speculation to devise a technical process for undiluted formaldehyde production at reasonable conversions. 

It may also be noted that hydrogen transfer reactions of olefins to aromatics lead to the formation of three moles of paraffins for every mole of aromatics formed (Kaa) over HZSM-5,. The alternative dehydrogenation pathway (Km2) provided by zinc for the conversion of C6-Cg oligomers to aromatics suppresses the hydrogen transfer reactions hence more olefinic molecules are available for the aromatization reaction. Thus, the pathways Kmi and Km2 provided by zinc results in the significant increase in aromatics yield... 

Different formalisms were proposed for the oxidative dehydrogenation mechanism. For reducing sugars in basic solutions it was suggested [25] that aldose anions are adsorbed by the surface and then transfer a hydride (Section 9.3). Because oxidation reactions can also be conducted in neutral or acidic media, a dehydrogenation pathway occurring entirely on the metal surface was proposed... 

The clean Ni surface is a complete dehydrogenator, whereas the surface dosed with half a monolayer of S in an ordered structure results in a surface which is very selective to formaldehyde production, that is, the total dehydrogenation pathway is effectively blocked. 

The direct dehydrogenation pathway favors high turnover efficiencies at low overpotentials through a direct conversion of formic acid to the carbon dioxide (CO2) product [Eq. (3.1)] ... 

Fig. 3.2 Formic acid cyclic voltammetry (a) coupled with differential electrochemical mass spectroscope (b) on Pt in 0.5 M H2SO4. Initially, H COOH was pie-adsorbed at 0.25 V to form a sub-monolayer of Ft- COads- After exchanging the solution with lOmMH COOH, the voltage was scanned at 12.5 mV s, resulting in an initial low-voltage mass signal for C02 (m/e = 44) via the dehydrogenation pathway and an additional signal for C02 due to the dehydration pathway [37]...

Formic acid adsorption onto Pt requires either multiple sites for the dehydration pathway or (Mily one to activate C-H bond for the dehydrogenation pathway [46]. The onset of formic acid electrooxidation has been shown to be effected by both Pt particle size and reactant cmicentration (Fig. 3.5B, C). The dehydration pathway is favored on both the polycrystalline and 8.8 run Pt catalyst surfaces during the forward scan, as is apparent from the low currents and high overpotentials. The higher potentials are required to form the activated hydroxyl complexes required to oxidize the passivating CO moieties to CO2, similar to methanol. The formic acid... 

Demirci investigated the degree of segregation and shifting of d-band centers by metal alloy combinations to improve the direct liquid fuel cell catalyst activity through electronic promotion of the dehydrogenation pathway [57]. He focused on Pt- and Pd-based catalyst for formic acid electrooxidation and looked at the potential impact of surface adatom adsorption of other 3d, 4d, and 5d transition metals. The criteria he imposed for improved catalytic activity on Pt and Pd... 

The type, structure, and electron density clearly determine the reactivity of a catalyst toward formic acid electrooxidation. The catalyst characteristics that promote reactant adsorption in the CH-down orientation exhibit enhanced activity through the dehydrogenation reaction pathway. Pd catalyst initially favors the dehydrogenation pathway but suffers for 30 % activity loss within only 3 h of continuous operation due to accumulation of reaction intermediates on the surface. 

A smaller percentage of Pt surface sites participate in the dehydrogenation pathway, limiting activity to high overpotentials that promote OHads formation to complete the dehydration reaction. 

Abstract Direct formic acid fuel cells offer an alternative power source for portable power devices. They are currently limited by unsustainable anode catalyst activity, due to accumulation of reaction intermediate surface poisons. Advanced electrocatalysts are sought to exclusively promote the direct dehydrogenation pathway. Combination and structure of bimetallic catalysts have been found to enhance the direct pathway by either an electronic or steric mechanism that promotes formic acid adsorption to the catalyst surface in the CH-down orientation. Catalyst supports have been shown to favorably impact activity through either enhanced dispersion, electronic, or atomic structure effects. 

A common method for improving formic acid electrooxidation activity is through the incorporation of foreign adatoms in sub- or monolayer coverages onto metal electrocatalyst surfaces (substrates). Adatoms are usually deposited onto the metal surface either by under potential deposition (UPD) or by irreversible adsorption [17]. The two dominant reaction enhancement mechanisms for the direct dehydrogenation pathway, as described in Sect. 3.3 of the previous chapter for formic acid electrooxidation, are the third-body and electronic effects. The type of enhancement mechanism due to adatom addition is dependent on the substrate/adatom... 

The DiSalvo group at Cornell Uifiversity has intensely studied intermetallics for formic acid electrooxidation and observed significant enhancements in turnover efficiencies [16, 46, 72-79]. Table 4.3 compares the activity of several extended intermetallic surfaces in comparison to a Pt baseline [16]. The onset potential relevant to enhanced reactivity through the direct dehydrogenation pathway was most impacted by the addition of Sb. The introduction of both Sn and Sb into the Pt unit cell negatively impacted the anodic peak current. While Bi increased the peak current, it had an adverse impact on the onset potential. It increased the onset potential by 0.06 V and nearly doubled the peak current. The key challenges related to intermetallics for DFAFCs are surfactant-free synthesis methods and reduced nanoparticle sizes (>10 nm) to improve mass activity of the catalyst [74, 75, 80]. Mastumoto et al. compared the mass activity of PtPb 10-20 nm intermetallic particles to a commercial nanocatalyst [79], During a 9 h hold at 0.197 V vs. RHE, the PtPb intermetallic catalyst demonstrated over a twofold sustained mass activity over that of Pd. 

Figure 4.4. CH3OH stepwise dehydrogenation pathway and corresponding heats of formation on Pt, Ir, Os, Pd, Rh, and Ru [65]. (Reproduced with permission from J Am Chem Soc 1999 121 10928-41. Copyright 1999 American Chemical Society.)...

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