Heterogeneously Catalyzed Hydrogenation Reactions

Reduction by direct addition of hydrogen to a material is a major industrial reaction in numerous synthetic pathways to many common products and pharmaceuticals. This process frequently suffers from mass transfer limitations, and chemists have been forced to circumvent this by applying an overpressure of hydrogen over an already saturated solvent, making the process on a large scale both slow and potentially hazardous. 

A continuous SCF flow reactor, however, is capable of providing a method to hydrogenate organic substrates without long residence times and limitations of mass 

Although this is a simple reaction, it illustrates the principle clearly. The phase behavior has been studied in some detail [64], This case has a particular advantage, as both the transport of reactants and products, towards and away from the active catalytic sites, is not inhibited by phase boundaries. 

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Heterogeneously catalyzed hydrogenation reactions can be run in batch, semibatch, or continous reactors. Our catalytic studies, which were carried out in liquid, near-critical, or supercritical C02 and/or propane mixtures, were run continuously in oil-heated (200 °C, 20.0 MPa) or electrically heated flow reactors (400 °C, 40.0 MPa) using supported precious-metal fixed-bed catalysts. The laboratory-scale apparatus for catalytic reactions in supercritical fluids is shown in Figure 14.2. This laboratory-scale apparatus can perform in situ countercurrent extraction prior to the hydrogenation step in order to purify the raw materials employed in our experiments. Typically, the following reaction conditions were used in our supercritical fluid hydrogenation experiments catalyst volume, 2-30 mL total pressure, 2.5-20.0 MPa reactor temperature, 40-190 °C carbon dioxide flow, 50-200 L/h ... 

Discussion Point DPI While there are many examples of industrial processes that depend on heterogeneously catalyzed hydrogenation reactions, there are very few (apart from the enantioselective processes) which are homogeneously catalyzed. Suggest some reasons for this. 

One synthesis approach towards y-hydroxylated ionic liquids is a Michael-type addition of a protonated ammonium salt to a a, /l-an sal uni led carbonyl compound such as methylvinyl ketone yielding an oxobutyl functionalized cation. Intrinsically unstable due to retro-Michael reaction, this OS could however be transformed by a heterogeneous catalyzed hydrogenation reaction yielding a hydroxyl functionalized TSIL [24] (Fig. 7). 

Another report of hydrogenation under SCF conditions from the patent literature includes claims for a continuous heterogeneously catalyzed hydrogenation reaction process by Subramaniam and Said (182,183). The primary focus of these patents is the in situ mitigation of coke buildup in porous catalysts, but an SCF-mediated hydrogenation process is a cited application (and claim). 

The commonly accepted mechanism of heterogeneously catalyzed hydrogenation involves activation of both the hydrogen and the C—C multiple bond adsorbed on the metal surface. First one hydrogen atom is transferred to the least hindered position of the multiple bond to give a half-hydrogenated adsorbed species. This reaction is fully reversible and ac-... 

Heterogeneously catalyzed hydrogenation of alkenes is generally considered to be a structure-insensitive reaction, as was deduced from numerous studies on more or less complex model catalyst systems [40-54]. However, the following sections will give examples of the opposite case. 

The standard work of Evans [2] as well as a survey of the papers produced in the Journal of Labeled Compounds and Radiopharmaceuticals over the last 20 years shows that the main tritiation routes are as given in Tab. 13.1. One can immediately see that unlike most 14C-labeling routes they consist of one step and frequently involve a catalyst, which can be either homogeneous or heterogeneous. One should therefore be able to exploit the tremendous developments that have been made in catalysis in recent years to benefit tritiation procedures. Chirally catalyzed hydrogenation reactions (Knowles and Noyori were recently awarded the Nobel prize for chemistry for their work in this area, sharing it with Sharpless for his work on the equivalent oxidation reactions) immediately come to mind. Already optically active compounds such as tritiated 1-alanine, 1-tyrosine, 1-dopa, etc. have been prepared in this way. 

Heterogeneously catalyzed hydrogenation is a three-phase gas-liquid-solid reaction. Hydrogen from the gas phase dissolves in the liquid phase and reacts with the substrate on the external and internal surfaces of the solid catalyst Mass transfer can influence the observed reaction rate, depending on the rate of the surface reaction [15]. Three mass transfer resistances may be present in this system (Fig. 42.1) ... 

The hydrogenation reactions occurring on heterogeneous basic catalysts have characteristic features, which distinguish heterogeneous basic catalysts from transition metal catalysts.44 The key features of the base-catalyzed hydrogenation reactions are as follows ... 

In recent years, attempts have been made to make use of the advantages of the supercritical carbon dioxide in chemical reactions. The first technical examples concerning the use of carbon dioxide in a pilot-plant scale chemical reaction were heterogeneous catalyzed hydrogenation and radical polymerization [38-42]. Meanwhile, hydrogenation reactions have been scaled up in a 1000 t/a commercial multipurpose plant. 

In a very recent example (Figure 1.53), INEPT DOSY has been applied to confirm the dinudear nature of the unstable Zirconium intermediate 141 in the reaction of CO with [ZrHCl(Cp)], 140 [344], a model reaction for the heterogeneously catalyzed hydrogenation of CO2 to methanol [345]. As was expected for a binuclear compound, the diffusion coefficient of intermediate 141 is smaller than for the mononuclear 142. 

Catalytic hydrogenation represents a set of reactions that will be extremely important in the production of value-added chemicals from biomass. Already now, they play a significant role in today s industry, and holds great promise for further developments. Here, selected examples of heterogeneously catalyzed hydrogenations of chemicals available from renewables resources are presented. 

All one-step additions to C=C double bonds are mechanistically required to take place cis-selectively (ds > 99 1) (Sections 3.3.1-3.3.3). In addition, the heterogeneously catalyzed hydrogenation of alkenes also usually takes place with very high cw-selectivity, in spite of its being a multistep reaction (Section 3.3.4). 

The addition of hydrogen to olefins is made possible only with noble metal catalysts. They allow for a multistep, low-energy reaction pathway. The noble metal catalyst may be soluble in the reaction mixture in that case we have a homogeneously catalyzed hydrogenation (cf. Section 14.4.7). But the catalyst may also be insoluble then we deal with a heterogeneously catalyzed hydrogenation. 

Reactions on Metal Surfaces Heterogeneously Catalyzed Hydrogenation and Isomerization... 

Step a involves a Friedel-Crafts acylation where HF serves as the catalyst and the solvent for the reaction. Although HF is highly toxic, it can be contained on an industrial scale and completely recycled. The acetic acid by-product can also be recycled. The second step (b) involves heterogeneously-catalyzed hydrogenation of a carbonyl group to give the corresponding alcohol 80 with an atom economy of 100%. Either palladium on charcoal or Raney-nickel can be used as a catalyst. Step c is an excellent example of an alcohol carbonylation that we saw earlier in Section 9-5 this process, like carbonylation of methanol, has an atom economy of 100%. The overall atom economy of the new process, which... 

The structure of substrates may affect reactivity in heterogeneously catalyzed hydrogenations by affecting the rate constant of the reaction, and... 

Balazsik, K., Bartok, M. (2004) Heterogeneous asymmetric reactions-Part 40. New data on repeated use of Pt-alumina catalyst during enantioselective hydrogenation of ethyl pyruvate, J. Mol. Catal. A. Chem. 219,383-389. Bartok, M., Bartok, T., Szollosi, G., Felfoldi, K. (1999) Electrospray ionization mass spectrometry in the heterogeneous catalyzed organic reactions unknown compounds in the pyruvate hydrogenation, Catal. Lett. 61,57-60. 

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