The use of solid catalysts has a number of advantages compared to catalysis in solution. The most important advantage is their discrete state, usually stationary, which enables easy separation of the product from the catalyst. Although catalysis by solids in organic technology was largely restricted till about the mid-1970s to bulk chemicals produced by continuous processes, it has since been extended to organic intermediates and fine chemicals (which are usually medium to small-volume production in batch processes). We devote this chapter to a brief review of the major types of solid catalysts used in the production of intermediates and fine chemicals. Though these reactions can be carried out in both the vapor and liquid phases, the substrates used in organic synthesis are often relatively complex liquid molecules which tend to decompose under harsh conditions. Hence it is usually desirable to operate under softer conditions, thus preserving the liquid state of the substrate and preventing any likely decomposition to unwanted products. Because catalysis by solids will almost certainly play a major role in organic syntheses of the future, surface science studies involving complex organic molecules are being increasingly undertaken (see, e.g., Rylander, 1979, 1985; Molnar, 1985; Kim and Barteau, 1989; Joyner, 1990; Idriss et al., 1992; Schulz and Cox, 1992, 1993; Pierce and Barteau, 1995; and the recent review by Smith, 1996). However, this book will not be concerned with such mechanistic considerations. There are a few classes of catalysts that have acquired a degree of prominence during the last decade in the synthesis of organic intermediates and fine chemicals that marks them as uniquely relevant in the context of industry’s irreversible shift to green technology. In addition to the homogeneous catalysts considered subsequently in Chapter 9, they include a wide variety of solid catalysts. These catalysts can be classified in two ways: (1) as distinct classes of catalysts that cut across different types of reactions, including dissolved catalysts supported on solids; and (2) as catalysts specific to different types of reactions. It is also possible to control catalytic action by using appropriate solvents/additives.
Heterogeneous Strategies For Selective Conversion of Lignocellulosic Polysaccharides
