Easily separable molecular catalysis

Abstract

Homogeneous metal catalysts offer greater selectivity and controllability because their molecular nature ensures that only one type of active site is present. However, the majority of large-scale, industrial chemical processes employs heterogeneous catalysts because of ease of their separation from products, thermal stability, and amenability to continuous processing. Heterogenization of homogeneous catalysts explores the cross-fertilization of two systems to combine most of their advantages. Some examples are discussed of heterogenized palladium complex catalysts applied for low-temperature CO oxidation, carbonylation for the synthesis of ibuprofen or naproxen, oxidative carbonylation for the synthesis of diphenyl carbonate, and functionalization of methane. There appears no significant change in reaction mechanisms when active centers are transferred from solution to the solid surface. In addition to the easy recovery, heterogenized homogeneous catalysts can have advantages in the productivity over homogeneous catalysts when a suitable support material is selected considering its hydrophilicity and surface reactivity. (C) 2000 Elsevier Science B.V. All rights reserved.