ABSTRACT
Abstract This paper discusses typical properties of powdered precious metal catalysts related to catalytic performance in several organic syntheses. Some new developments and trends in precious metal catalysts, such as new generations of palladium (DeLinkTM), platinum and rhodium heterogeneous catalysts including immobilized homogeneous catalysts (LiganNetTM) for hydrogenation in organic synthesis are discussed. Introduction An important factor in developing fine chemicals and pharmaceuticals, agrochemicals, fragrances and flavors, food additives, and dyes and pigments, is choosing the reaction route to the final product. Catalytic routes have proven to be one of the most effective ways in simplifying the reaction routes to these compounds by increasing product selectivity and reducing waste and hazardous materials handling. Organic synthetic chemists and engineers seek more active, selective and environmentally benign catalysts for their synthesis applications. To meet increasing market demand, catalyst manufacturers are closely working with synthesis chemists and engineers to select and develop better catalysts enabling a wide range of different synthetic transformations. A number of new precious metal catalysts, such as Pd/C, Pt/C and immobilized Rh catalyst, have been successfully commercialized in recent years. Examples of recent developments and trends will be illustrated using several widely practiced reactions, such as debenzylation, nitro group reduction, and chemo-selective and stereo-selective hydrogenations. DeLinkTM – a New Generation of Pd/C Catalysts Activated carbon supported palladium catalysts have been widely used in organic chemical synthesis (1). A comprehensive review surveys the research and development work on preparation of supported palladium catalysts covering
the 1990-2000 period (2). The proven catalysts are: 5-10% Pd on carbon. Sometimes high palladium catalysts, such as Pearlman catalyst (20%Pd(OH)2/C), are preferred for difficult debenzylation reactions. There has not been a significant breakthrough on commercially produced Pd/C catalysts until DeLinkTM catalysts were introduced. In the following sections, we will focus on the effects of catalyst preparation and discuss the properties of DeLinkTM catalysts. Superior ActivityIn the synthesis of fine chemicals, especially in pharmaceutical applications, abundant examples of synthesized molecules containing multiple functional groups are known. Some of the reactive functional groups need to be temporarily protected during synthesis by introducing protecting groups. Using benzyl protection groups is one of the most common methodologies in complex organic synthesis. At the end of synthesis, protected functional groups can be deprotected by hydrogenolysis of the benzyl group. Rylander (3), Freifelder (4) and Seif et al. (5) have reported that an activated carbon supported palladium catalyst is the most commonly used catalyst for debenzylations. Our earlier work (6-8) shows that using debenzylation of 4-benzyloxy phenol (C6H5CH2OC6H4OH, 4-BP) to hydroquinone (C6H4(OH)2) as a model reaction, one can evaluate the relative activity of different catalysts. As shown in Figure 1 and Table 1, the catalytic performance results demonstrate that DeLinkTM 3%Pd/CPS4 (carbon powder support) has activity equal to current commercially supplied 5%Pd/CPS1 and CPS2 catalysts. Using the same technology, DeLinkTM 5% and 10%Pd/CPS4, also are developed in this work. These new catalysts have superior catalytic activity. CPS1 and CPS2 supported catalysts are the regular commercial catalysts and CPS4 supported ones are the new DeLinkTM catalysts. Table 1 summarizes the key properties and reaction rate constants on the catalysts used in this work. This test shows that catalysts with the same palladium metal loading have similar CO chemisorption or metal dispersion, but different activity. The hydrogen uptake curves (Figure 1) show that the reaction completes in less than 5000 seconds for DeLinkTM 5%Pd/CPS4 as compared to about 10,000 seconds for standard catalysts 5%Pd/CPS1 and 5%Pd/CPS2. Figure 1 also shows the reaction is complete in less than 1500 seconds for DeLinkTM 10%Pd/CPS4, but it took up to 4000 seconds to complete the reaction on standard 10%Pd/CPS1 and 10%Pd/CPS2 catalysts. The calculated rate constants of 5%Pd/CPS4 are about twice those of 5%Pd/CPS1 and 5%Pd/CPS2 on both catalyst weight and metal basis (Table 1). DeLinkTM 3%Pd/CPS4 catalyst that is 40% lower than the current commercial catalysts, 5%Pd/CPS2 and 5%Pd/CPS3 has slightly faster hydrogen uptake rate.
