Immobilization of the Catalyst

Transition metal catalysis in liquid/liquid biphasic systems principally requires sufficient solubility and immobilization of the catalysts in the IL phase relative to the extraction phase. Solubilization of metal ions in ILs can be separated into processes, involving the dissolution of simple metal salts (often through coordination with anions from the ionic liquid) and the dissolution of metal coordination complexes, in which the metal coordination sphere remains intact. 

After activation by heating, the catalyst was dusted over the surface of a thin polydimethylsiloxane (PDMS) layer, being coated on the PDMS top plate of the micro reactor [19]. Such a modified plate was baked for 1 h at 100 °C. A high surface area and firm immobilization of the catalyst resulted. Then, the micro reactor was assembled from the top and another bottom plate, having at one micro-channel wall the catalyst layer. Stable operation with the PDMS micro reactor up to 175 °C could be confirmed. 

Recyclability can be achieved by heterogenization of the reaction mixture, by binding the catalyst and products to different phases. This can be achieved by (i) immobilization of the catalyst on a solid inorganic or polymeric support (solid-liquid protocols) or (ii) partitioning the catalyst and reagents/products in different liquid phases (liquid-liquid protocols) (see Chapter 9.9 for more details on supported catalysts). 

A list of examples in this section is not exhaustive rather, they have been chosen to illustrate the different approaches used for immobilization of the catalysts for important classes of organic reactions, namely hydrogenation, oxidation, and coupling reactions. Due to the major industrial importance of olefin polymerization (see Chapter 9.1), and although the objectives of immobilization of polymerization catalysts are rather different from the other examples, some references to this will also be given here. 

Besides immobilization of the catalyst, the benefit of the ionic liquids is in this case a reduction of the reaction time from 23 h in toluene to less than 15 h in [C10MIM][BF4] with no loss of selectivity, although the ionic media require slightly higher reaction temperatures (Table 41.16). Furthermore, a stabilization of the ionic catalyst solution against atmospheric oxygen is observed. This stabilization ef-... 

Several strategies were developed to prevent the formation of unreactive dimers [86], with one of the more successful methods being immobilization of the catalyst on solid support. Whereas normally, most immobilized catalysts lose activity in comparison to their soluble analogues, in this case the rate increased, due to the prevention of deactivation by dimerization. Even more convincing, there was a negative correlation between the loading on the resin and the rate of the reaction (Fig. 44.12). 

Figure 2.14 — Continuous configurations coupled on-line to flow-through (bio)chemical sensors involving permanent immobilization of the catalyst (Cat) at the active microzone. Symbol meanings are given in Fig. 2.12. For details, see text.

Figure 5.19 — Flow-through biochemical sensor based on the twofold immobilization of the catalyst (urease) and reagent (an acid-base azo dye) in the sensing microzone for the determination of urea in kidney dialysate. (A) Sensing microzone held in a microcircuit. (B) Valveless flow injection manifold. P pumps T timer S sample W waste. For details, see text. (Reproduced from [57] with permission of Elsevier Science Publishers).

Catalyst instability can be a problem that may be tackled by developing an immobilized catalyst. Organic catalysts do exist that slowly decompose under the conditions necessary for their reaction and release trace amounts of by-products that must be separated from the products. For instance, in photooxigenation reactions catalysed by porphyrin the release of highly coloured materials derived from the catalysts made product purification a real problem. This problem can be solved by immobilization of the catalyst because the decomposed material is also supported and can thus be removed from the reaction medium during the process of catalyst recovery. 

The use of these surface-immobilized electrocatalysts allows for the easy removal of the catalysts from the reaction vessel, and the use of much lower quantities of catalyst which is here highly concentrated in the reaction layer. In many cases the immobilization of the catalyst on the electron source provides its stabilization and allows an marked increase of the turnover frequency compared to the numbers found in related homogeneous systems. Taking... 

Similarly to Bianchini s approach, De Rege [26] also immobilized cationic [((R,R)-Me-duphos (26))Rh-(COD)]OTf complex noncovalently by the hydrogenbonding interaction of triflate counterion with surface silanols ofMCM-41 support. In contrast to the results obtained by Bianchini et al. [25c], the catalytic activity and selectivities of the immobilized 26-Rh complex on MCM-41 were equal to or greater than the homogeneous counterparts (Scheme 2.7). Moreover, the catalysts were recyclable (up to four times, with no loss of activity) and did not leach. Here again, the counteranion was very important for the successful immobilization of the catalyst onto MCM-41. Whereas, the DuPhos-Rh complex with triflate anion was effectively immobilized (6.7 wt% based on Rh), tlie analogous complex with the lipophilic BArp anion [BArp = R(3,l-((. i )2-C J I i was not loaded onto the support. 

Further simplification was attained by immobilization of the catalyst on a solid polymer (49), so that its separation from the product was reduced to mere mechanical filtration the resin bound catalyst could then be used for another run. Ofthe number of polymeric supports investigated, Merrifield (49) and Wang resins were identified as most suitable, exhibiting the same behavior and efficiency [18]. However, owing to the heterogeneous nature of the system, the enantioselectivity of the reaction decreased by 10 15% ee (Table 4.11, entries 6 12). In view ofthe swelling properties, toluene was found to be less suitable than chloroform (compare entries 6 and 7), which makes the method less environment friendly. Furthermore, a conditioning effect was observed for these systems the second run was always found to be more enantioselective than the first one by 10% ee and this level was maintained in the subsequent runs (compare entries 7 with 8 12). The latter effect stems from the... 

Transfer hydrogenation is an unusual case for scale-up, as the problem is how to transfer efficiently the byproducts from solution to the gas phase, compared with hydrogenation, which is the opposite. Unsurprisingly scale-up of the batch process is affected by physical aspects of the reaction such as agitation and surface area of the gas to liquid. One means of overcoming these limitations is continuous operation, and the fast reaction kinetics lends itself to this. With immobilization of the catalyst one can envisage a low cost and efficient continuous flow process. 

Using water as the solvent has not only the advantage of having a mobile support and hence of a de facto immobilization of the catalyst while retaining a homogeneous form of reaction, but also has positive repercussions on the environmental aspects of hydroformylation (cf. Section 5.2 and remarks on p. 338). 

In contrast with the first class of functionalized alkenes, immobilization of the catalyst in aqueous phase results in an enhancement of the catalytic activity [19]. Indeed, it has been observed that the hydroformylation rates of arylic esters having high solubility in water were much higher in biphasic systems than those observed under comparable homogeneous conditions. Except for 2-ethylhexyl acrylate, the initial rate was increased by a factor of 2.4, 12, 2.8, and 14 for methyl, ethyl, butyl, and 2-ethoxyethyl acrylate, respectively (see Figure 1) [20]. One of the most intriguing features is that the hydroformylation rates for ethyl and butyl acrylates in biphasic medium were respectively higher than and comparable with those observed with methyl acrylate. Actually, the water-solubilities of ethyl and butyl acrylates (18.3 and 2.0 g L-1 at 20 °C, respectively) are lower than that of methyl acrylate (59.3 g L-1 at 20°C). 

Additional modifications include the immobilization of the catalyst on an insoluble surface. Silica-anchored ligands have been reported based on the DPP 33 and PYR 34 cores.24 Their use in AD reactions were comparable to the untethered versions of the chiral ligands. Alkenes substituted with alkyl or aryl groups and with internal and terminal double bonds gave diol products with yields ranging 51-93% and optical purities of 61-99% ee. The differentiation between 33 or 34 was the ability to readily recover and recycle the chiral ligands. 

The immobilization of the catalyst xerogel in a porous support was performed with a-Al203 foams with open structure provided by the Mendeleev University of Chemical Technology of Russia, Moscow. The aliunina foam was dipped in the sol-gel solution immediately after introduction of the aqueous ammonia. The impregnated a-alumina foams were removed fi"om the gel after aging and vacuum drying. The excess gel was used to prepare a reference sample. 

Obviously, all applied ionic liquids were stable against the selected oxidizing agents as no hint of any ionic liquid oxidation is found in the publications. For all these published examples catalyst immobilization and potential recyclability serves as motivation to apply the ionic reaction medium. To improve the immobilization of the catalyst, Wu et al. suggested later a bipyridine ligand carrying two 1-methylimidazolium hexafluorophosphate moieties to be a more appropriate ligand system (Fig. 5.3-11) [142]. 

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