Hydrogenation of organic substances

Solubihty of H2 in most organic solvents is very low. As hydrogenation is conducted in traditional solvents, diffusion of H2 from bulk gas phase to the H2/liquid interface, absorption at the H 2/liquid interface, and diffusion into Hquid bulk phase are necessary, and this influences the progress of the reaction. H2 is completely miscible with SCFs, and its concentration at the catalyst surface can be greatly increased, so high reaction rates can be achieved [21]. 

Excellent synergy effect of Pd/C with solid Lewis acids exists in the hydrogenation of phenol to cyclohexanone with almost 100% selectivity under mild conditions. CO2 could enhance the apparent reaction rate, and the phase behavior influenced 

COj is an abundant, nontoxic, nonflammable, easily available, and renewable carbon source. Using CO2 as a carbon source to produce useful chemicals benefits the sustainable development of our society. In addition, sc conditions can enhance the reactivity of CO2 [39]. In recent years, SCCO2 and SCCO2-IL systems have gained a growing interest since CO2 can play a dual role as both the reactant and solvent. Many valuable substances has been obtained, such as dimethyl carbonate, cyclic carbonate, polycarbonates, N,N -disubstituted ureas, urethanes, formic acid, other organic acids, and others [40]. 

The reaction has also been carried out in the scC02/[bmim][PF5] system with Wilkinson s catalyst RhCl(PPh3)3 [47]. In this biphasic reaction system, the homogeneous transition-metal catalyst was immobilized in the IL phase, and the products could be isolated from the SCCO2 phase. As a result, high selectivity, catalyst recycling, and product recovery could be achieved for the hydrogenation of COj in the presence of dialkylamines. 

Hydrogenation of CO2 in methanol can give methyl formate. A Pd/Cu/ZnO/ alumina nanocatalyst prepared by a simple coprecipitation method shows good activity on the reaction with high yields ( 20%) and excellent selectivity ( 96%) at 150 °C [51]. The activity could be influenced greatly by doping the catalyst with other metals such as Ru, Ni, Au, and Pd, and the last one was found to be the best doper with an activity of 38.15 h .  

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Examination of the catalog shelf-list section for titles in the field of hydrogenation reveals a number of texts in this field—e.g., Hydrogenation of Organic Substances by C. Ellis, and The Fischer-Tropsch and Related Syntheses by H. A. Storch et al. Ellis book. Chapter 55, Reduction of Carbon Oxides, is specific to the problem in question. All the other texts listed in this section should, of course, be examined also, well as the Kirk-Othmer, Ullmann, and Thorpe encyclopedias, and the books on catalysis. 

Rhenium sulfides are effective catalysts for hydrogenation of organic substances and they have the advantage over heterogeneous platinum metal catalysts in that they are not poisoned by sulfur compounds.811 An inorganic reduction that they catalyze is that of NO to N20 at 100°.8b... 

Why is there such a deep interest in this field We may point out some of the probable reasons, such as (a) a hope to elucidate the HER mechanism as a prototype for heterogeneous reaction kinetics, (b) to obtain a systematic view on the catalytic properties of various metals, (c) the existence of many related important fields such as corrosion of metals, electrolytic hydrogenation of organic substances, etc., and (d) some more up-to-date reasons, such as efficient hydrogen production or use of hydrogen in fuel cells, and seeking a prospect toward the so-called hydrogen economy. 

C. Ellis, Treatise on Hydrogenation of Organic Substances, 3"" Edition, D. Van Nostrand, New York, 1930. 

Reactions of organic substances at the dropping electrode usually involve hydrogen ions a typical reaction can be represented by the equation... 

Fig. 8. Relationship between relative reactivities S and the ratios of the initial reaction rates rj°/VA° of alkylphenols to phenol in the hydrogenation on Ni-catalyst containing 8.4% (wt.) AUOa at 160°C and initial molar ratio of hydrogen to organic substances G = 19. Alkyl substituents in phenols Me—methyl, Et—ethyl, Pr—n-propyl, i-Pr— isopropyl, s-Bu—sec-butyl, t-Bu—terc-butyl.

For the cathodic reduction of organic substances, electrodes of two types are used the platinum and the mercury type. Those of the first type (platinum metals, and in alkaline solutions nickel) exfiibit low polarization in hydrogen evolution their potential can be pushed in the negative direction no further than to -0.3 V (RHE). Hydrogen readily adsorbs on these electrodes, which is favorable for reduction... 

Vincenzini M, Materassi R., Tredici M.R., Florenzano G. (1982) Hydrogen production by immobilized cells. I. Light dependent assimilation of organic substances by Rhodopseudomonaspalustris. Int J. Hydr. Energy., 7 231-236... 

Hydrogen sulfide also reacts with many types of organic substances. Reaction with methanol at high temperatures in the presence of an acidic catalyst yields methanethiol and dimethylsulfide ... 

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