Homogeneous Dehydrogenation

Hougen and Watson in an analysis of Kassel s data for the homogeneous, vapor-phase dehydrogenation of benzene in a tubular-flow reaetor eonsidered two reaetions ... 

A convenient synthesis of A -3-ketones in the 5 5 series uses DDQ in one step. This introduction has to be done indirectly because of the unfavorable direction of enolization. In this scheme, advantage is taken of the equilibrated formylation at C-2 of 5i5-3-ketones. Dehydrogenation of the 2-formyl derivative (72) proceeds rapidly with DDQ and deformylation is achieved in the presence of a homogeneous catalyst. A related approach involves preparation of the 2i -bromo-5i5-3-ketone by bromination of the 2-formyl compound (72). ... 

There are many ways to produce acetaldehyde. Historically, it was produced either hy the silver-catalyzed oxidation or hy the chromium activated copper-catalyzed dehydrogenation of ethanol. Currently, acetaldehyde is obtained from ethylene hy using a homogeneous catalyst (Wacker catalyst). The catalyst allows the reaction to occur at much lower temperatures (typically 130°) than those used for the oxidation or the dehydrogenation of ethanol (approximately 500°C for the oxidation and 250°C for the dehydrogenation). 

Another recent patent (22) and related patent application (31) cover incorporation and use of many active metals into Si-TUD-1. Some active materials were incorporated simultaneously (e.g., NiW, NiMo, and Ga/Zn/Sn). The various catalysts have been used for many organic reactions [TUD-1 variants are shown in brackets] Alkylation of naphthalene with 1-hexadecene [Al-Si] Friedel-Crafts benzylation of benzene [Fe-Si, Ga-Si, Sn-Si and Ti-Si, see apphcation 2 above] oligomerization of 1-decene [Al-Si] selective oxidation of ethylbenzene to acetophenone [Cr-Si, Mo-Si] and selective oxidation of cyclohexanol to cyclohexanone [Mo-Si], A dehydrogenation process (32) has been described using an immobilized pincer catalyst on a TUD-1 substrate. Previously these catalysts were homogeneous, which often caused problems in separation and recycle. Several other reactions were described, including acylation, hydrogenation, and ammoxidation. 

Saito and coworkers134 reported on the homogeneous reverse water-gas shift reaction catalyzed by Ru3(CO)i2. Conditions employed were 20 ml of N-methyl-2-pyrrolidone solution 0.2 mmol Ru3(CO)i2 1 mmol bis(triphenylphosphine)immi-nium chloride and C02-H2 1 3 under 80 kg/cm2 at 160 °C. The major products were CO (15.1 mmol), H20 (21.6 mmol), and methanol (0.8 mmol). As no formic acid was detected, and because the authors only detected Ru cluster anion species H3Ru4(CO)i2, H2Ru4(CO)i22, and HRu3(CO)n, they concluded that the mechanism did not involve formic acid as an intermediate. Rather, they proposed that the mechanism proceeds by dehydrogenation of a metal hydride, C02 addition, and electrophilic attack from the proton to yield H20, as outlined in Scheme 48. 

The high activity of iridium PCP pincer complexes in transfer dehydrogenation has been applied in a very elegant approach to devise the first homogeneous alkane metathesis process (Equation 12.5) [3]. 

Alkane dehydrogenation has been demonstrated as a suitable method for the functionalization of polyolefins such as atactic poly(l-hexene) under homogeneous conditions (Equation 12.6) [23]. 

Figure 4. Comparison of the behavior of VSil545 in propane oxidative dehydrogenation using N2O or O2 as oxidizing agents. Exp. conditions as in Fig. 1. The dotted lines represent the propane conversion and propylene selectivity observed in the absence of the catalyst (homogeneous gas phase). The activity of the catalyst in the absence of O2 or N2O is similar to that observed in the homogeneous gas phase, but the selectivity to propylene (around 50-60%) is lower.

As seen from the discussions, there are numerous heterogeneous catalysts suitable for the dehydrogenation of alkanes. In sharp contrast, homogeneous catalysts are rare. Exceptions are the Ir pincer complexes (7a and 7b), which are extraordinarily active and robust catalysts.333... 

A heterogeneous route, however, cannot be ruled out, either. Similarly, ethane can dehydrogenate to ethylene either in a homogeneous or in a surface reaction. 

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