Why Does Industry Not Use Immobilized Transition Metal Complexes as Catalysts?

Abstract

Much effort has gone into the immobilization of homogeneous catalysts based on the idea that in this way the catalysts could be not only separated more easily from the product but also reused several times, thus reducing the cost of the catalyst use. So far none of these immobilized catalysts have been used by industry. In this article we critically review the use of immobilized homogeneous catalysts from the point of view of process development for the pharmaceutical and fine chemical industry. The first and foremost question that needs to be answered is: will immobilizing a homogeneous catalyst really lead to lower costs? The answer is thus far always no. This is caused mostly by the fact that homogeneous catalysts are not stable and thus there is little point in immobilizing them. The second reason is the extra added cost that is incurred in immobilizing the catalysts. Other problems are lower rates, sometimes lower selectivities and metal leaching. Three different areas are discussed. The research on immobilized metathesis catalysts is analyzed in detail; in general the immobilized catalysts do not achieve sufficient turnovers to be interesting for industrial use. Very many publications have appeared on immobilized palladium catalysts that were used for CC bond‐forming reactions, such as Suzuki, Heck or Sonogashira reactions. These catalysts are invariably converted to nanoparticles after the first run. Although these catalysts can be reused, there is no reason to use an expensive support based on immobilized ligands. This also does not protect the product from palladium contamination. Even more effort has gone into the immobilization of homogeneous hydrogenation catalysts. Most of these catalysts suffer from the same problems as the other immobilized catalysts: catalyst deactivation, low turnover numbers, and leaching of the metal. In addition, the heterogenization adds complexity to the system, increasing risk and prolonging process development.magnified image

The Multifaceted Reactivity of Single‐Atom Heterogeneous Catalysts

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Catalytic Enantioselective Flow Processes with Solid‐Supported Chiral Catalysts

“…The conceptualization of organocatalysis that followed the ground‐breaking works of List, Lerner and Barbas on enamine catalysis and MacMillan on iminium ion catalysis paved the way for the extension of these simple (yet hitherto elusive) concepts to hundreds of related reactions . This fever also arrived to the main players in the field of solid‐supported catalysts, allured by the fact that these species would not suffer the problem of metal leaching –…”

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

“…[61] However,these approaches are costly, time-consuming,and rarely provide materials with improved catalytic performances. [62] Similarly,e ncapsulation strategies have not yielded highly active systems,m ainly as ar esult of the incompatibility of the molecular precursors with the conditions employed for the synthesis of the inorganic matrix. [63] Catalysts immobilized by adsorption and electrostatic interactions,with the exception of few particular cases, often lack stability because of the weak interaction between the molecular complex and the support leading to leaching under the reaction conditions.…”