Recent Catalyst Improvements

Phenylboronic and 4-chloroacetophenone combine efficiently according to eq. (2) at certain palladacycle catalysts (Structure 3), which are easily available from Pd(OAc)2 and appropriate phosphines PR3 (e. g., R = o-tolyl). Turnover numbers (TONs) of 75 000 are achieved with only 0.001 mol% 3 [3]. Several reports on the catalysts of type 3 have substantiated the discovery of 1995 [3a]. As in the Heck coupling, no aryl scrambling is observed with the palladacycle catalysts [4]. 

After the first report in 1998 [5], Pd complexes of A -heterocyclic complexes demonstrated their high catalyst efficiency in the Suzuki coupling [6-8]. Both isolated Pd /Pd -complexes such as Structures 4-6 [5-7] and in situ systems like 7 and 8 [9, 10] have been used. A -Heterocyclic carbenes as ligands in transition-metal catalysis were also described [11]. 

The least reactive aryl chlorides can thus be activated. For example, the coupling following eq. (3) yields up to 90 % of the desired products [6]. The catalyst loading of 0.02-0.05 mol % Pd is the lowest known as yet for the Suzuki coupling 

The catalyst system 11, with an optimized P/Pd ratio of 1.0-1.5, does not require electron-withdrawing substituents. Instead, p- and o-substituted aryl chlorides having R = CH3, OCH3, NH2 give typical yields between 82 and 92% [13]. The electron richness and the steric bulk of tris(r-butyl)phosphane seem to be the origin of the good catalytic performance. Obviously, only a single phosphine is attached to the zerovalent Pd in the active state of the catalyst. 

Bisarylphosphines combined with Pd(OAc)2 or Pd2(dba)3 - for example. Structure 12 - have proven successful in the Suzuki coupling of non-activated aryl chlorides, too [14—17]. Related catalysts work with 0.5-2.0 mol% Pd at 80-130 °C [18]. Or /jo-substitution on one or both of the coupling partners is possible, and both electron-donating and electron-withdrawing functional groups are tolerated. It seems that P,0-chelation occurs in these particular catalysts [19, 20]. 

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In a more recent and improved approach to cyclopropa-radicicol (228) [ 110], also outlined in Scheme 48, the synthesis was achieved via ynolide 231 which was transformed to the stable cobalt complex 232. RCM of 232 mediated by catalyst C led to cyclization product 233 as a 2 1 mixture of isomers in 57% yield. Oxidative removal of cobalt from this mixture followed by cycloaddition of the resulting cycloalkyne 234 with the cyclic diene 235 led to the benzofused macrolactone 236, which was converted to cyclopropa-radicicol (228). 

The DKR of amine is more challenging compared to that of secondary alcohol since no metal catalysts have been known for the efficient racemizahon of amine. Reetz et al. reported for the first time the DKR of amine, in which 1-phenylethylamine was resolved by the combination of lipase and palladium (Scheme 4). In this procedure, CALB and Pd/C were employed as the combo catalysts. However, the DKR required a very long reaction time (8 days) at 50-55°C and provided a poor isolated yield (60%). Recently, an improved procedure using Pd on alkaline earth salts as the racemizahon catalyst was reported by Jacobs et al. " The DKR reachons were performed at 70°C for 24-72 h and 75-88% yields were obtained with 99% or greater enanhomeric excess. 

As mentioned above one of the fundamental attributes ascribed to homogeneous catalysts is superior activity at low temperature. However, even within classes of such catalysts, improvements in catalyst activity can be made allowing operation at lower temperatures, thus reducing or avoiding completely this mode of catalyst decay. One such example can found in recent advances in palladium catalysed ethene carbonylation (Equation 1.1). 

In Chapter 8 we will discuss the hydroformylation of propene using rhodium catalysts. Rhodium is most suited for the hydroformylation of terminal alkenes, as we shall discuss later. In older plants cobalt is still used for the hydroformylation of propene, but the most economic route for propene hydroformylation is the Ruhrchemie/Rhone-Poulenc process using two-phase catalysis with rhodium catalysts. For higher alkenes, cobalt is still the preferred catalyst, although recently major improvements on rhodium (see Chapter 8) and palladium catalysts have been reported [3],... 

The ACH process has recently been improved, as stated by Mitsubishi Gas. Acetone-cyanohydrin is first hydrolized to 2-hydroxyisobutylamide with an Mn02 catalyst the amide is then reacted with methylformiate to produce the methyl ester of 2-hydroxyisobutyric acid, with coproduction of formamide (this reaction is catalyzed by Na methoxide). The ester is finally dehydrated with an Na-Y zeolite to methylmethacrylate. Formamide is converted to cyanhydric acid, which is used to produce acetone-cyanohydrin by reaction with acetone. The process is very elegant, since it avoids the coproduction of ammonium bisulphate, and there is no net income of HCN. Problems may derive from the many synthetic steps involved, and from the high energy consumption. 

The Sohio technology is based on a catalyst of bismuth an4 molybdenum oxides. Subsequent catalyst improvements came from the use of bismuth phosphomolybdate on a silica gel, and more recently, antimony-uranium oxides. Each change in catalyst was motivated Jby a higher conversion rate per pass to acrylonitrile. 

Upon the addition of Lewis (i.e. All ) and protonic non-compSexing. acids (i.e. HPF5) promoters to [Ru(00)3X3] catalysts, improvements in selectivity to valuable products (acetic acid + ethyl acetate) and in reaction rate are observed. This is believed to be due to an acceleration by acids of the alkyl migration-carbonyl insertion step of the process, as discussed in detail in a recent paper (10). 

Earlier work in this laboratory showed that chromium oxide supported on alumina is a good catalyst for the conversion of olefins (ref. 1) as well as paraffins (ref. 2) to nitriles with high selectivities, by reaction of NO with the hydrocarbons (nitroxidation). Recent work (ref. 3) reported preliminary results of the nitroxidation of paraxylene as an extension of the use of C Oj-Al Oj to the catalytic synthesis of aromatic nitriles. It should be mentioned that only few data are available in the literature related to the nitroxidation of aromatic hydrocarbons. Teichner et al (ref. 4 ) reported interesting results of selective synthesis of benzonitrile by nitroxidation of toluene on NiO-AlgO catalysts. Improvements of the catalytic activity and selectivity in this reaction were reached by use of C Og-Al. which also exhibits striking properties in the synthesis of paratolunitrile by contact of NO with paraxylene (ref. 3). 

Fretbuerger, M A.. Buss. W. C. Bridge, A. G. Recent catalyst and process improvements in oommerdal rbenifonning , SPRA Annual Meeting. New Orleans, Louisiana (22/25 March 19( U). 

Recently, a more stable Rh catalyst for methanol carbonylation based on the crosslinked polyvinylpyridine system has been disclosed in which the degree of crosslinking of the resin support is as high as 60 % [115c-e]. This catalyst improvement is the basis for the potential development of a commercial methanol carbonylation acetic acid process named Acetica . This process is being offered for license by Chiyoda and UOR Even with this announcement, there are still considerable doubts whether heterogenized carbonylation catalyst systems can compete with the low-water homogeneous Rh- and Ir-catalyzed processes (cf. Sections 2.1.1 and 3.1.1.3). 

All licensors agree on the necessity of hydrotreating the feed to lower the level of poisons for the platinum-based reforming catalyst. Temporary poisons are sulfur and nitrogen, while As, Pb, and other metals are permanent poisons. Proper conditions of hydrogen, pressure, temperature, and space velocities are able to reduce these poisons to the acceptably low levels of modern catalysts. Numerous process design modifications and catalyst improvements have been made in recent years. 

The total capacity of the semiregenerative units exceeded 5.0 million b/d while CCR units reached 3.8 million b/d. The simultaneous use of CCR technology and bimetallic catalysts has given UOP a unique position in the field of catalytic reformer process licensing. Recent catalyst formulations have improved both aromatic and reformate yields. UOP has improved the performance of the conventional platforming process by incorporating a CCR system. The process uses stacked radial-flow reactors and a CCR section to... 

A large number of papers have been published on the process modeling and optimization of the etherification process. More details could be found in a handbook. The most important aspect of process improvement is catalyst improvement because the Amberlyst ion-exchange resin used in the MTBE synthesis has an upper thermal stability limit of less than 100°C and there is a need to develop other acidic catalysts with higher thermal stability. Some of the recent papers have described the use of zeolites. 

Reviewing the work on tire Pt-Ru electrocatalysts is beyond the scope of this article. We will briefly comment on some key advances in this area. Although early discovery by Petrii, and Bockris and Wrob Io wa established the catalytic activity of Pt-Ru alloys for methanol oxidation, despite of active investigation that followed, even the optimum composition of Pt-Ru is yet to be firmly settled. An early explanation for the mechanism by which bimetallic catalysts improve upon the performance of pure Pt, that is, the bifunctional mechanism proposed by Watanabe and Motoo, was recently challenged. 

A process has been introduced (88) based specifically on improved selection and control of operating variables. Potentially the most important new developments have been in the area of reforming catalysts. Up to a few years ago, work in this area centered on metal distribution studies (95) and general optimization (51). This work has been covered in reviews (14, 94, 135). Very little has been published on more recent catalyst development although some papers have been presented (53, 81, 111, 128, 144). Available information on the use of such new catalysts points to greatly improved stability, particularly at severe conditions, and also to increased yields by reduction of hydrocracking. There is no indication that the use of such catalysts involves any basic changes in... 

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