Oxo catalysts

Several strategies to immobilize the p-oxo catalysts on an electrode surface or in a membrane have been employed. However, no available data about their efficiency as modified electrodes for water oxidation have been given.482-486 It should be noted that [ (bpy)2RuIII(OH2) 2(M C))]4+ is also an excellent electrocatalyst for oxidation of chloride to chlorine (better than for the oxidation of H20 into 02) at 1.20 V vs. SCE in 0.05 M HC1 solution,487 or at a modified electrode prepared by incorporation of the complex by ion-exchange into polystyrene sulfonate or Nafion films.482,4 8... 

Only the biphasic method, specially of aqueous-biphasic catalysis, has provided a fundamental remedy to the problem of stress-free and economical recovery and recycle of homogeneous oxo catalysts [12]. The fact that the catalyst, which still acts homogeneously, is dissolved in water, thus in a polar solvent, and remains dissolved, enables it to be separated from the nonpolar products without problems and with minimal effort after reaction. 

With the RCH/RP process, it is possible to hydroformylate propene up to pentenes with satisfying space time yields. On the other hand, heavier aldehydes such as Cio (iso-decanal) or higher from the hydroformylation of nonene(s), decenes, etc. can not be separated from the oxo catalysts by conventional means such as distillation due to thermal instability at the required temperatures and thus especially needs the careful aqueous-biphasic separation technique. There are numerous attempts to overcome the problem of low reactivity of higher alkenes which is due to low miscibility of the alkenes in water [26,27b, 50a,58d]. These proposals can briefly be summarized as ... 

The classic oxo catalyst is octacarbonyldicobalt at 130-175°C and 250 atm. This reacts with hydrogen to give hydridotetraearbonyl eobait, the active eatalyst in the oxo process. 

An important development in the past 15 years in hydroformylation technology was the introduction of biphasic homogeneous catalysis. Kuntz (62) expressed the basic idea of a new generation of water-soluble oxo catalysts with triphenylphosphane trisulfonate (tppts as the sodium salt) as a ligand for a rhodium-complex-catalyzed hydroformylation process. Ruhrchemie AG adapted the idea on the basis of research done at Rhone-Poulenc and developed it into an industrially viable process, which was... 

Future Trends. In addition to the commercialization of newer extrac-tion/decantation productfcatalyst separations technology, there have been advances in the development of high reactivity oxo catalysts for the conversion of low reactivity feedstocks such as internal and -alkyl substituted cr-olefins. These catalysts contain (as ligands) ortho-t-butyl or similarly substituted arylphosphites. which combine high reactivity, vastly improved hydrolytic stability, and resistance to degradation by product aldehyde, which were deficiencies of earlier, unsubstituted phosphites. 

In an effort to develop new chiral BINOL-Ti complexes, chemical modifications of the chiral complex (f )-BINOL-Ti(OPr )2 (R-2) that can easily be prepared by simply mixing ( PrO)4Ti and (/ )-BINOL in the absence ofMS4A have been studied [37c-e]. A dimeric form has been reported for the single-crystal X-ray structure of complex R-2 [38], (I )-BINOL-Ti-p3-oxo complex, prepared via hydrolysis of complex R-2 has been shown to serve as an efficient and moisture-tolerable asymmetric catalyst [37d,e]. It is noteworthy that the (/ )-BINOL-Ti-)i3-oxo catalyst [37e] shows a remarkable level of (+)-NLE (asymmetric amplification), thereby attaining the maximum enantioselectivity for this system by using (/ )-BINOL with only 55-60% ee as the chiral source (consult Scheme 8C. 14). 

Considerable effort over the years has been devoted to a search for new oxo catalysts. This has been motivated by a desire to minimize the less valuable isobutyraldehyde/alcohol and also to lower oxo reaction temperatures and the high pressures (3-4000 psi) associated with the conventional cobalt process for reduced capital investment and increased energy savings. 

The oxo catalyst may be modified to function as a hydrogenation catalyst as well and, using a 2 1 ratio of hydrogen to carbon monoxide, alcohols are produced directly. 

As another example of innovation in the oxygen carrier system, a simple self-organizing phenanthroline iron(III) p-oxo catalyst 32 has been developed and applied to the rapid and efficient epoxidation of a variety of alkenes, including the often recalcitrant terminal olefins. Thus, vinylcyclohexane 33 provides the corresponding epoxide 34 within 5 min at 0°C in 90%... 

Example 6.2. Hydroformylation with "oxo" catalyst. Hydroformylation of olefins... 

Example 6.5. Olefin hydroformylation with phosphine-substituted cobalt hydrocarbonyl catalyst [30], The pathway 6.9 of olefin hydroformylation with the "oxo" catalyst, HCo(CO)4, has been shown in Example 6.2 in Section 6.3. For phosphine-substituted catalysts, HCo(CO)3Ph (Ph = organic phosphine), the pathway olefin — aldehyde is essentially the same. However, these catalysts also promote hydrogenation of aldehyde to alcohol (Examples 7.3 and 7.4) and of olefin to paraffin (Example 7.5). Moreover, straight-chain primary aldehydes under the conditions of the reaction undergo to some extent condensation to aldol, which is subsequently dehydrated and hydrogenated to yield an alcohol of twice the carbon number (e.g., 2-ethyl hexanol from n-butanal see Section 11.2). The entire reaction system is... 

Example 8.3. Phosphine-substituted cobalt hydrocarbonyls as hydroformylation catalysts. Extensively studied catalyst systems with complex equilibria include phosphine-substituted hydrocarbonyls of cobalt, HCo(CO)3Ph, where Ph stands for a tertiary organic phosphine. They are modifications of the original oxo catalyst, HCo(CO)4. Like the latter, they catalyze the oxo or hydroformylation reaction of olefins to aldehydes one carbon number higher ... 

It is hence not surprising that Otto Roelen s initial investigations into homogeneous coordination catalysts in oxo synthesis proved a source of much frustration (reviewed in [3]). It was only the work of Adkins and Krsek [6], Storch et al. [7], Berty and Markd [8] and Natta [9] that confirmed oxo catalysts to be homogeneous in nature. The intense activity associated with hydroformylation and oxo... 

A plethora of metal complexes have been stated to catalyze the hydroformylation reaction. Oxo catalysts typically consist of a transition metal atom (M) which enables the formation of a metal carbonyl hydride species. Optionally, these complexes may be modified by ligands (L). A general composition is represented by Structure 1. 

Co, Rh, Pt, and Ru belong to the group of six transition metals forming the most active oxo catalysts. Today s hydroformylation plants operate exclusively with catalysts based on rhodium or cobalt, namely HCo(CO)4, HCo(CO)3PBu3 and HRh(CO)(PR3)3 [9] (see Section 2.1.1.4). 

Industrial hydroformylation is currently performed in two basic variants the homogeneous processes, where the catalyst and substrate are in the same liquid phase (Shell, UCC, BASF, etc.), and the two-phase process with a water-soluble catalyst (RCH/RP). These processes will be discussed in detail in Section 2.1.1.4. Gas-phase hydroformylation with heterogeneous catalysts plays no role today. The immobilization of homogeneous catalysts will be discussed in Section 3.1.1. Special applications such as SLPC (supported /iquid-phase catalysts) [43] and SAPC (supported aqueous-/7hase catalysts) [44] are not considered further here. Heterogeneous oxo catalysts are not within the scope of this book they are discussed further elsewhere [267]. 

The n/i selectivity of modified oxo catalysts increases with lower partial pressure of carbon monoxide and with high concentration of ligand. The effect of temperature is less pronounced. Under such conditions the predominant catalyst species is coordinated by more than one phosphine ligand. The metal center presents a more sterically hindered environment to the olefin and the formation of linear alkyl and acyl species is favored. Table 1 summarizes experimental evidence for these effects [8]. 

In contrast to its organic-soluble derivative, HRh(CO)(TPPTS)3 does not take up a second molecule CO to form HRh(CO)2(TPPTS)2, even at syngas pressures of 20 MPa. As has been shown, the latter compound generates by dissociation of either carbon monoxide or TPPTS the unsaturated species HRh(CO)(TPPTS)2 or HRh(CO)2(TPPTS), which is responsible for the formation of linear and branched aldehydes (Scheme 4). As HRh(CO)(TPPTS)2 is formed by TPPTS dissociation from the starting compound HRh(CO)(TPPTS)3, and HRh(CO)2(TPPTS) is only obtained by an equilibrium reaction from HRh(CO)2(TPPTS)2, the observed increased n/i selectivity for water-soluble rhodium oxo catalysts becomes comprehensible. 

Figure 1. The different methods of separation and recycling of oxo catalysts for the reaction S + A-B —> P [73] (a) aqueous diphasic operation membrane technique ...

Figure 2. Different approaches of the variation of the application phase of oxo catalysts. FBS = fluorous biphase [multiphase] system PEG = polyethylene glycol NAIL = non-aqueous ionic liquid.

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