Catalyst recyclability

In the latter twentieth century, spent automotive catalysts have emerged as a significant potential source of secondary Pt, Pd, and Rh. In North America, it has been estimated that 15.5 metric tons per year of PGM from automotive catalysts are available for recycling (22). However, the low PGM loading on such catalysts and the nature of the ceramic monoliths used have required the development of specialized recovery techniques as well as the estabhshment of an infrastmcture of collection centers. These factors have slowed the development of an automotive catalyst recycling iadustry. 

Rhodium was about three times the price of gold through 1988—1989 until skyrocketing to 74/g ( 2300/troy oz) in early 1990. Thus precious metal catalyst costs requite an absolute minimum level of use and maximum number of catalyst recycle uses when batch processing is employed. Starting material contaminants may effect catalyst poisoning, though process routes to overcome this by feed stream pretreatment may be devised (37,60). 

A number of StHle coupling reactions have been reported by Handy et al. [95]. With PdCl2(PhCN)2/Ph3As/Cul in [BMlM][Bp4], good yields and good catalyst recyclability (up to five times) were reported for the reaction between a-iodenones and vinyl and aryl stannanes (Scheme 5.2-19). However, the reported reaction rates were significantly lower than those obtained in NMP. 

Easy product separation and catalyst recycling Lower cost of chemical processes... 

Slightly later, and independently of Cole-Hamilton s pioneering work, the author s group demonstrated in collaboration with Leitner et al. that the combination of a suitable ionic liquid with compressed CO2 can offer much more potential for homogeneous transition metal catalysis than only being a new procedure for easy product isolation and catalyst recycling. In the Ni-catalyzed hydrovinylation of... 

Jacobsen subsequently reported a practical and efficient method for promoting the highly enantioselective addition of TMSN3 to meso-epoxides (Scheme 7.3) [4]. The chiral (salen)Cl-Cl catalyst 2 is available commercially and is bench-stable. Other practical advantages of the system include the mild reaction conditions, tolerance of some Lewis basic functional groups, catalyst recyclability (up to 10 times at 1 mol% with no loss in activity or enantioselectivity), and amenability to use under solvent-free conditions. Song later demonstrated that the reaction could be performed in room temperature ionic liquids, such as l-butyl-3-methylimidazo-lium salts. Extraction of the product mixture with hexane allowed catalyst recycling and product isolation without recourse to distillation (Scheme 7.4) [5]. 

Supported copper catalysts have also been described/29 340 The main impetus for the development of supported ATRP catalysts has been to facilitate catalyst removal and, in some cases, to allow for catalyst recycling. 

Catalyst to substrate ratio (% w/w) Catalyst recycle Time (h) 6(a) HOjC COjH 2(a)... 

A copper-free Sonogashira coupling reaction in ionic liquids and its application to a microflow system for efficient catalyst recycling, Org. Lett. 4, 10 (2002) 1691-1694. 

Dijkstra, H.P, van Klink, G.P.M, van Koten, G. (2002) The Use of Ultra- and Nanofiltration Techniques in Homogeneous Catalyst Recycling. Acceleration Chemistry Research, 35, 798-810. 

Gruttadauria, M., Riela, S., Lo Meo, P., D Anna, F., Noto, R. (2004) Supported Ionic Liquid Asymmetric Catalysis A New Method for Chiral Catalysts Recycling, the Case of ProUne-Catalysed Aldol Reaction. Tetrahedon Letters, 45(32), 6113-6116. 

An interesting example of the use of a second liquid phase is to facilitate catalyst recycle when the reactor is operated as a slurry reactor. 

Gas-expanded liquids (GXLs) are emerging solvents for environmentally benign reactive separation (Eckert et al., op. cit.). GXLs, obtained by mixing supercritical CO2 with normal liquids, show intermediate properties between normal liquids and SCFs both in solvation power and in transport properties and these properties are highly tunable by simple pressure variations. Applications include chemical reactions with improved transport, catalyst recycling, and product separation. 

Catalyst recycling. The solid Pd-doped perovskite catalysts are easily filtered from the reaction mixtnre for reuse. The activity of the recycled BaCco 95Pdo.o502 95 catalyst was investigated in the coupling of 4-bromoanisole with 4-phenylboronic acid. The results in Table 27.3 show that high activity was retained even after seven cycles of catalyst use. 

Thermomorphic solvent mixtures have been tested for hydroformylation of 1-octene and 1-dodecene to determine the ease of product recovery and catalyst recycling. Using both batch and continuous reactors, we demonstrated the efficacy of a biphasic, thermomorphic, system that had the following advantages ... 

Little to no catalyst leaching and high catalyst recyclability. 

Economics. Comparison of the material and energy balance for our process and the cobalt-based BASF higher olefin process (8), we foimd that our process reduced the capital investment required by over 50% due to the fact that we require far fewer unit operations, and because the operating pressure is much lower. In sutmnary, the thermomorphic solution developed by TDA allows easy catalyst recycle, which, when coupled with the lower pressure operation possible with Rh catalysts (compared to the cobalt-based process) lowers both capital and operating costs for current oxidation (oxo) plants of similar capacity. 

Figure 1.7 Catalyst recycling in the hydrosilylation reaction under inert (N2) conditions with several platinum precursors, [K2PtCI4] (black), [PtCI2(C6Hno)]2 (dark grey), [PtCI2(PPh3)2] (white), and [Pt(PPh3)4]...

The interest in catalyst recyclability has led to the development of biphasic catalysts for hydro-boration.22 Derivitization of Wilkinson s catalyst with fluorocarbon ponytails affords [Rh(P (CH2)2(CF2)5CF3 3)3Cl] which catalyzes FIBcat addition to norbornene in a mixture of C6FnCF3 and tetrahydrofuran (TF1F) or toluene (alternatively a nonsolvent system can be used with just the fluorocarbon and norbornene) to give exo-norborneol in 76% yield with a turnover number up to 8,500 (Scheme 4). Mono-, di- and trisubstituted alkenes can all be reacted under these conditions. The catalyst can be readily recycled over three runs with no loss of activity.23... 

Some attempts to obtain a polymer-supported catalyst for chiral asymmetric allylic alkylation have been reported with some success in respect of catalyst recycling on activity, but with unsatisfactory results concerning enantioselectivity.188,189... 

Scheme 6.93) [192]. Using either of the two solvent systems, all studied cycloaddition reactions were completed in less than 1 min upon microwave irradiation at 50 °C employing 3 mol% of the catalyst. An additional advantage of using the ionic liquid l-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) as solvent is that it facilitates catalyst recycling. 

Feed to tails ratio may be defined as the ratio between the liquid fed to column (4) and the liquid in the catalyst recycle. Higher feed/tails ratios contribute to higher conversion since with only catalyst and heavy solvent being recycled more of the reactor volume is available for product. 

In hydroformylating with a polar ligand modified rhodium catalyst to give a relatively non-polar aldehyde product, after the flash column, the catalyst solution is extracted with a non-polar solvent. Polar catalyst recycles from the extractor to the reactor. The non-polar solvent is removed and recycled to the extractor (see Figure 2.6). 

Figure 5.3. Basic flow-sheets of a) a conventional, homogeneously catalyzed process and b) an aqueous-biphasically, homogeneous catalytic process. 1, Reactor 2 Separators) 3, Catalyst separator 4, Make-up 5, Further purification and processing, Gas recycles) 7, Catalyst recycle 8, Reactant feed 9, Withdrawal of high...

The membrane process, incorporated into the catalyst recycle either in the main stream or in the side stream, may also separate the reaction products from the remaining catalyst (Figure 5.8). 

CHAPTER 7 CATALYST RECYCLING USING IONIC LIQUIDS... 

Since the focus of this contribution is clearly on catalysis and catalyst recycle using the ionic liquid methodology it is not possible to go into more detail on the materials science aspects of ionic liquids. However, it should be clearly stated that at least some understanding of the ionic liquid material is a prerequisite for its successful use as a liquid catalyst support in catalysis. Therefore, the interested reader is strongly encouraged to explore the more specialized literature [28],... 

In contrast, we intend to demonstrate the principle aspects of catalyst recycling and regeneration using the ionic liquid methodology. These aspects will be explored in more detail for the example of Rh-catalysed hydroformylation (see Section 7.2). First, however, we will briefly introduce important general facts concerning transition metal catalysis in ionic liquids (see Section 7.1.2). This will be followed by a consideration of liquid-liquid biphasic reactions in these media from an engineering point of view (see Section 7.1.3). 

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