Selective Hydrogenation for Fine Chemical Synthesis

Pasi Virtanen, Eero Salminen, Paivi Maki-Arvela, andJyri-Pekka Mikkola 

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The three themes of the symposium selective hydrogenation, selective oxidation and acid-base catalysis were introduced by four plenary lectures and two invited communications. A panel concerned with the future of zeolites and other shape-selective materials for fine chemical synthesis was conducted by specialists in the field D. Barthomeuf (University of Paris 6), E. Derouane (University of Namur), L. Forni (University of Milan), M. Gubelmann (Rhone-Poulenc, St Fons), W. Hoelderich (BASF, Ludwigshafen) and G. Perot (University of Poitiers). An exhibition of equipment was held during the symposium on October 3 and 4. Over 20 firms exhibited equipment, chemicals and catalysts which were of interest to researchers involved with the synthesis of functional compounds by heterogeneous catalysis. 

Heterogeneous copper catalysts prepared with the chemisorption-hydrolysis technique are effective systems for hydrogen transfer reactions, namely carbonyl reduction, alcohol dehydrogenation and racemization, and allylic alcohol isomerization. Practical concerns argue for the use of these catalysts for synthetic purposes because of their remarkable performance in terms of selectivity and productivity, which are basic features for the application of heterogeneous catalysts to fine chemicals synthesis. Moreover, in all these reactions the use of these materials allows a simple, safe, and clean protocol. 

Catalytic liquid phase semihydrogenation of acetylenes is an important industrial and laboratory reaction, especially in fine chemical synthesis [1]. The use of supported metal catalysts for this selective hydrogenation readily facilitates the separation of organic products from the catalyst. However, liquid phase reactions with supported catalysts tend towards mass transport limitation [2] and, therefore, the support particles should be between 1 and 10 pm in size this avoids transport limitations and separation problems. With support particles of this size high temperature reduction in a flow of H2 gas is very difficult and to avoid this step it is possible to prepare supported metal particles by decomposing organometallic compounds under mild conditions [3-5]. 

In the following section, methods for the fabrication and deposition of Pd-based and zeolite MMs are discussed, as well as applications in (de) hydrogenation, SR, WGS, partial oxidation (POx) reactions and fine chemical synthesis. The research on Pd-based MMRs for hydrogen separation, purification and production (by dehydrogenation, SR and WGS reactions) has been selected as a case study, as significant research and, therefore, much information can be found in the literature on this field. 

In a previous paper concerning the application of sulfided hydrotreating catalysts to the synthesis of fine chemicals we have already shown that it was possible to obtain selective reactions by taking advantage of the differences in reactivity observed for hydrogenation and hydrogenolysis reactions (ref.l). 

Copper-catalysts promoted with i) other group VIA or VIIIA metals and ii) alcaline or alcaline earth elements (IA or IIA) are used for selective hydrogenation of various organic compounds (1). Moreover Cu(Co) Zn-Al catalysts were extensively studied for the synthesis of methanol and of light alcohols (2,3). More recently, due to the development of fine chemical processes, detailed studies of copper catalysts were carried out in order to show, like for noble metals, the effect of supports (SMSI), of promoters and of activation-on metal dispersion or reduction, on alloy formation... For example modified copper catalysts are known for their utilization in the dehydrogenation of esters (4-6), in the hydrolysis of nitriles (7), in the selective hydrogenation of nitriles (8), in the amination of alcohols (9)... 

Transition metal complexes, zeolites, biomimetic catelysts have been widely used for various oxidation reactions of industrial and environmental importance [1-3]. However, few heterogenized polymeric catalysts have also been applied for such purpose. Mild condition oxidation catalyzed by polymer anchored complexes is attractive because of reusability and selectivity of such catalysts. Earlier we have reported synthesis of cobalt and ruthenium-glycine complex catalysts and their application in olefin hydrogenation [4-5]. In present study, we report synthesis of the palladium-glycine complex on the surface of the styrene-divinylbenzene copolymer by sequential attachment of glycine and metal ions and investigation of oxidation of toluene to benzaldehyde which has been widely used as fine chemicals as well as an intermidiate in dyes and drugs. 

Several methods have been presented for the selective hydrogenation of aromatic rings. As industrial products become more complex, however, further methods are needed for the selective organic transformations required in the production of chemical intermediates and fine chemicals. Of special interest are sulfur-tolerant catalysts and catalysts for the synthesis of chiral complexes. Future research should focus on either of these areas. Much is expected from novel catalysts obtained by immobilizing homogeneous complexes. 

As mentioned earlier, supercritical fluids have a broad potential for application in new processes. In recent years a number of publications reviewed the role of supercritical fluids in technical applications [9-16], and more industrial applications of sc-fluids were established in the synthesis of fine chemicals. In a variety of reactions such as hydrogenations, hydroformylations and Friedel-Crafts reactions, the advantageous use of sc-fluids as solvents has been demonstrated with respect to yield, selectivities, and no work-up procedure [17]. 

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