Recoverable immobilized catalysts

In this chapter, we will present some contemporary results from our laboratory aimed toward blending the desirable properties of homogeneous and heterogeneous catalysts to create highly active, selective, and recoverable immobilized catalysts. The chapter covers organometaUic catalytic systems for polymerization reactions as well as small molecule reactions. Formation of single-site catalysts, recyclability, and stability/leaching issues will be addressed, as will studies on the effects of the immobilization procedure and the nature of the support structure on catalytic performance. Finally, a brief discussion on projected future directions for immobilized catalysts is presented. 

Hu, A.G., Yee, G.T. and Lin, W.B. (2005) Magnetically recoverable chiral catalysts immobilized on magnetite nanopartides for asymmetric hydrogenation of aromatic ketones. Journal of the American Chemical Society, 127 (36), 12486-12487. 

Marquardt and Liining 28) immobilized concave pyridines on two types of dendrimers (9 and 10) to obtain a recoverable acylation catalyst. For comparison... 

Three key conditions must be met to design a uniformly reactive, recoverable, and recyclable polymerization catalyst (1) the synthetic protocol used to make the immobilized catalyst must lead to only one type of active site on the surface, (2) the support material must be able to allow sufficient transport of reactants to and polymer from the active site, and (3) at the end of the reaction, the active site must not be irreversibly changed or decomposed [23]. Research in our lab has thus far sought to investigate these points using the atom transfer radical polymerization (ATRP) of methyl methacrylate as a model reaction. 

A latent biphasic system is a miscible solvent mixture that will become biphasic by the addition of a small amount of an additive. For example, a mixture of 10 mL of heptane, 9.2 mL of ethanol, and 0.8 mL of water would be miscible near room temperature. However, addition of a small amount (200 ]iL) of water or the addition of some salt would make this mixture biphasic. Such solvent mixtures that are at the cusp of immiscibility are useful as homogeneous media for catalysis and, after perturbation, as biphasic systems for separation. If a soluble polymer-immobilized catalyst is present that is by design phase-selectively soluble in one or the other phases of the biphasic mixture, it is possible to design recoverable reusable homogeneous catalysts with such latent biphasic systems. 

The same research group designed a soluble catalyst 102 with the main aim of avoiding the problems associated with the heterogeneous systems, and related to the common supported catalysts [207]. The main advantage of this system is the inverted solubility pattern that this catalyst exhibits, since it is soluble in non-polar solvents and insoluble in polar media (Fig. 9). This feature simplified the recovery (up to 99 %) and re-use of the catalyst at least five times without loss of activity, improving the results obtained with catalyst 100 (for the preparation of other immobilized catalysts easily recoverable, see [208]). 

In 2008, Chinese researchers attempted to increase the performance and recoverability of IL-supported catalysts by attaching the imidazolium tag they contained to a polymeric matrix, for example, polystyrene or silica gel, or to the 1,2,3-triazolyl-derived spacer group (Figure 22.7). Immobilized catalysts 96 [105], 97 [106], 98a [107], and 98b [108] of these types were highly efficient in asymmetric reactions of cyclic ketones with (i-nitrostyrene derivatives under neat conditions to give the corresponding Michael adducts with a diastereomeric excess and enantiomeric purity not inferior to those attained under the action of catalysts 84a-d. Catalysts 96-98 were readily separated from the products and efficiently catalyzed Michael reactions up to eight times without a decrease in diastereo- and enantioselectivity. 

Chiral phase-transfer catalysis (PTC) is a very interesting methodology that typically requires simple experimental operations, a mild reaction conditions and inexpensive and/or environmentally benign reagents, and which is amenable to large-scale preparations [15]. The possibihty of developing recoverable and recyclable chiral catalysts has attracted the interest of many groups. Indeed, the immobilization of chiral phase-transfer catalysts has provided the first demonstrations of the feasibility of this approach. 

Immobilization of such catalytically highly efficient HPA into an insoluble, readily recoverable solid acid is, therefore, an interesting and significant research target because environmentally benign solid-acid catalysts should replace problematic sulfuric acid and aluminum chloride. Several efforts have been made to immobilize HPA. Active carbon tightly entraps HPA inside its pores to form an insoluble solid acid that catalyzes liquid-phase organic reactions in polar media... 

Karimi, B. and Enders, D. (2006) New IV-heterocyclic carbene palladium complex/ionic liquid matrix immobilized on silica application as recoverable catalyst for the Heck reaction. Org. Lett., 8, 1237-40. 

While considerable progress has been made in the field of immobilized metathesis catalysts, a supported catalyst system capable of meeting the economic requirements of catalytic activity and lifetime necessary for a viable, industrially-relevant process has yet to be developed. However, the foundation provided by these research programs aimed at sustainability will help guide future efforts in the field, where emphasis can be placed on achieving an ideal recoverable catalyst, or better yet, an ideal catalyst for olefin metathesis processes. 

---