Solid acids and their use as environmentally

Solid acids and their use as environmentally - Pure Appl....

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Pure Appl. Chem. , Vol. 72, No. 7, pp. 1313–1319, 2000. © 2000 IUPAC 1313 * Pure Appl. Chem. Vol. 72, No. 7, 2000. A special topic issue on green chemistry. Solid acids and their use as environmentally friendly catalysts in organic synthesis* Karen Wilson and James H. Clark Green Chemistry Group, Department of Chemistry, University of York, Heslington, York YO10 5DD, UK Abstract : Tightening environmental legislation is driving the fine and speciality chemicals industries to consider alternative processes that avoid the use of conventional mineral acids. The use of heterogeneous catalysts in these processes would vastly simplify catalyst removal, minimizing the amount of waste formed. However, diffusion limitation of liquids within porous solids dictates that effective solid acids for liquid-phase reactions require the use of mesoporous materials <20–100Å. Recent developments in materials chemistry has led to the discovery of a family of ordered mesoporous silicas which opens up new possibilities for preparing solid-acid catalysts for liquid-phase reactions. This review concentrates on recent developments in the synthesis of new mesoporous solid acids for liquid-phase organic synthesis. INTRODUCTION Catalytic technologies play a key role in the economic development and growth of the chemicals industry and contribute to around 20% of world GNP. A major emerging and challenging area of heterogeneous catalysis is that of environmental pollution control, with tightening legislation on the release of waste and toxic emissions having serious implications for the chemical industry. While heterogeneously catalyzed processes are widely used within the petrochemical industry, many fine and specialty chemicals manufacturers rely on homogeneously catalyzed liquid-phase reactions. Many of these processes were developed at the turn of the century, and focused on product yield, disregarding the environmental impact of inorganic waste and toxic by-products formed during the reaction [1]. A wide range of liquid-phase industrial reactions rely on the use of inorganic or minerals acids. While many of these processes are catalytic, some require (e.g., acylation using AlCl 3 ) stoichiometric amounts of acid. Final isolation of the product necessitates aqueous quenching and neutralization steps to remove the acid, resulting in enormous quantities of hazardous waste, with the cost of disposal of this waste often outweighing the value of the product. Tightening legislation on the emission of hazardous pollutants is driving the industry toward the implementation of innovative “clean technology” including the use of alternative heterogeneously catalyzed processes [2]. Heterogeneous catalysts offer several intrinsic advantages over their homogeneous counterparts: ease of product separation and catalyst re- use; bifunctional phenomena involving reactant activation/spillover between support and active phases; and process advantages through reactor operation in continuous flow versus batch configuration. How-
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Solid acids and their use as environmentally - Pure Appl....

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