Rhodium-based catalysts

The switch from the conventional cobalt complex catalyst to a new rhodium-based catalyst represents a technical advance for producing aldehydes by olefin hydroformylation with CO, ie, by the oxo process (qv) (82). A 200 t/yr CSTR pilot plant provided scale-up data for the first industrial,... 

Alternatively, butadiene can be oxidized in the presence of acetic acid to produce butenediol diacetate, a precursor to butanediol. The latter process has been commercialized (102—104). This reaction is performed in the Hquid phase at 80°C with a Pd—Te—C catalyst. A different catalyst system based on PdCl2(NCCgH )2 has been reported (105). Copper- (106) and rhodium- (107) based catalysts have also been studied. 

A new homogeneous process for hydroformylation of olefins using a water-soluble catalyst has been developed (40). The catalyst is based on a rhodium complex and utilizes a water-soluble phosphine such as tri(M-sulfophenyl)phosphine. The use of an aqueous phase simplifies the separation of the catalyst and products (see Oxo process). 

Olefins can be aminomethylated with carbon monoxide [630-08-0] (CO) and amines in the presence of rhodium-based catalysts. Eor example, pipera2ine reacts with cyclohexene [110-83-8] to form W,Af-di-(l-cyclohexylmethyl)-pipera2ine [79952-94-6] (55). 

Earlier catalysts were based on cobalt, iron, and nickel. However, recent catalytic systems involve rhodium compounds promoted by methyl iodide and lithium iodide (48,49). Higher mol wt alkyl esters do not show any particular abiUty to undergo carbonylation to anhydrides. 

The rhodium complexes are excellent catalysts for hydrogenation of NBR. At low temperature and pressure, high catalyst concentrations are used to obtain a better rate of reactions. Due to higher selectivity of the reaction, pressure and temperature can be increased to very high values. Consequently the rhodium concentration can be greatly reduced, which leads to high turnover rates. The only practical drawback of Rh complex is its high cost. This has initiated the development of techniques for catalyst removal and recovery (see Section VU), as well as alternate catalyst systems based on cheaper noble metals, such as ruthenium or palladium (see Sections IV.A and B). 

The addition of allcenes to alkenes can also be accomplished by bases as well as by the use of catalyst systems consisting of nickel complexes and alkylaluminum compounds (known as Ziegler catalysts), rhodium catalysts, and other transition metal catalysts, including iron. These and similar catalysts also catalyze the 1,4 addition of alkenes to conjugated dienes, for example. 

The catalysts used in hydroformylation are typically organometallic complexes. Cobalt-based catalysts dominated hydroformylation until 1970s thereafter rhodium-based catalysts were commerciahzed. Synthesized aldehydes are typical intermediates for chemical industry [5]. A typical hydroformylation catalyst is modified with a ligand, e.g., tiiphenylphoshine. In recent years, a lot of effort has been put on the ligand chemistry in order to find new ligands for tailored processes [7-9]. In the present study, phosphine-based rhodium catalysts were used for hydroformylation of 1-butene. Despite intensive research on hydroformylation in the last 50 years, both the reaction mechanisms and kinetics are not in the most cases clear. Both associative and dissociative mechanisms have been proposed [5-6]. The discrepancies in mechanistic speculations have also led to a variety of rate equations for hydroformylation processes. 

A lot of research has been published on hydroformylation of alkenes, but the vast majority of the effort has been focused on the chemistry of various metal-ligand systems. Quantitative kinetic studies including modeling of rates and selectivities are much more scarce. In this work, we present the approach to modeling of hydroformylation kinetics and gas-solubility. Hydroformylation of 1-butene with a rhodium-based catalyst was selected as a case study. 

In conclnsion, it was shown that the hydrogenolysis of glycerol in the presence of heterogeneous rhodium-based catalysts yielded mainly either 1,2-, or 1,3-propane diol. Many parameters influenced the activity and the selectivity of the catalysts, particnlarly the presence of metal additives and the initial pH value. 1,2-propanediol can be obtained nearly quantitatively at high pH. Further woik is currently under progress in order to optimize this reaction. 

With the advance of three-way catalysis for pollution control, used mainly in automobile catalytic conversion but also for the purification of gas exhausts from stationary sources, a need has arisen to develop a basic understanding of the reactions associated with the reduction of nitrogen oxides on transition metal catalytic surfaces [1,2]. That conversion is typically carried out by using rhodium-based catalysts [3], which makes the process quite expensive. Consequently, extensive effort has been placed on trying to minimize the amount of the metal needed and/or to replace it with an alternatively cheaper and more durable active phase. However, there is still ample room for improvement in this direction. By building a molecular-level picture of theprocesses involved,... 

Different reactions pathways on Rh may explain the intermediate formation of ammonia. NH3 can be obtained via successive reaction steps between adsorbed NHX and dissociated hydrogen species [29]. Alternately, the formation of ammonia may occur via the hydrolysis of isocyanic acid (HNCO) [30]. Isocyanate species are formed by reaction between N and COads on metallic particles. Those species can diffuse onto the support leading to spectator species or alternately react with Hads yielding ultimately HNCO. Previous infrared spectroscopic investigations pointed out that isocyanate species predominantly form over rhodium-based catalysts [31]. 

Ligand (136), an analog of PPh3 with amphiphilic character, was used for making [Rh(CO) (136)(acac)]. The rhodium-based hydroformylation of 1-hexene using catalysts formed in situ... 

Immobilization of homogeneous catalysts for hydrogenation reactions concerns essentially enan-tioselective hydrogenations, important for the synthesis of fine chemicals (see Chapter 9.2). The pioneering work of Pugin et al.131 concerns the synthesis of a rhodium-based catalyst, with a diphosphine-pyrrolidine-based ligand for the hydrogenation of methylacetamide cinnamate (Equation(8)). 

Currently the reaction is carried out using cobalt based catalysts with severe penalties in terms of harsh operating conditions (80 bar CO/H2, 200°C). In addition, substantial loss of substrate (ca. 10%) to hydrogenation makes the overall selectivity to the linear alcohol ca. 80% [15]. Rhodium based systems are capable of giving higher selectivities (>90%) to the desired linear aldeyde product under milder conditions (20 bar, 100°C) [13]... 

Despite the very attractive properties of the rhodium-based system, no commercial plants used it because the low stability of the catalyst meant that the catalyst separation problem prevented commercialisation. Very recently, this situation has changed with the introduction of rhodium-based plant by Sasol in South Africa which uses technology developed by Kvaemer Process Technology (now Davy Process Technology). This batch continuous plant produces medium-long chain aldehydes and the separation is carriedoutbylow pressure distillation [16-18]... 

Eventually, the spent catalyst solution has to leave the oxo loop for work-up. The Ruhrchemie works of Celanese AG in Oberhausen (Germany) operate several rhodium-based oxo processes besides the well-known Ruhrchemie/Rhone-Poulenc process (RCF1/RP, the described low pressure oxo process with TPPTS-modified Rh catalyst), there are the Ruhrchemie process with an unmodified Rh catalyst at high pressure (comparable to the late ICI process [76] this variant is for the benefit of a high iso/n ratio... 

The compound of the distinct three oxo processes, all rhodium-based, enables a highly efficient recovery system to be achieved. Figure 5.17 describes the TPPTS manufacture and its use for the preparation of the rhodium catalyst, using either freshly introduced Rh acetate or recycled Rh 2-ethylhexanoate. The recycle technique of the RCH/RP process and its performance is depicted earlier. Spent Rh-TPPTS solutions are worked-up (see Figures 5.18 and 5.19), the resulting TPPTS returns to the RCH/RP process. The rhodium portion passes also a work-up stage and is reformulated as Rh 2-ethylhexanoate. This Rh salt may serve all various oxo processes of the oxo loop and will compensate for possible Rh losses as mentioned earlier. 

The directed synthesis of biaryls can be achieved using rhodium-based catalysts along with a phosphinite co-ligand (Equation (143)). 

The very first example of the catalytic reductive cyclization of an acetylenic aldehyde involves the use of a late transition metal catalyst. Exposure of alkynal 78a to a catalytic amount of Rh2Co2(CO)12 in the presence of Et3SiH induces highly stereoselective hydrosilylation-cyclization to provide the allylic alcohol 78b.1 8 This rhodium-based catalytic system is applicable to the cyclization of terminal alkynes to form five-membered rings, thus complementing the scope of the titanocene-catalyzed reaction (Scheme 54). 

A single example of the reductive cyclization of allenic carbonyl compounds is reported, which employs a rhodium-based catalyst in conjunction with Et3SiH as terminal reductant.113 This protocol promotes hydrosilylation-cyclization to form both five- and six-membered rings with exceptional levels of yy -diastereocontrol. As revealed... 

The use of the diphosphine PHANEPHOS (see Scheme 1.24) permitted Bar-gon, Brown and colleagues to detect and characterize a dihydrido intermediate in the hydrogenation of the enamide MAC by a rhodium-based catalyst The PH IP NMR technique was employed, and showed one of the hydrogen atoms to be agostic between the rhodium center and the /1-carbon of the substrate [85]. By using the same diphosphine and technique it was also possible to detect two diastereomers of the dihydride depicted in Scheme 1.25, which may also be detected using conventional NMR measurements [86]. 

It has been established that usually two monodentate ligands (phosphorami-dites, phosphites or phosphonites) are present in their rhodium-based hydrogenation catalysts. This would allow the possibility of testing catalysts based on two different monodentate ligands. Initially, this does not seem very appealing, as the suspected outcome would be the formation of a mixture of the heterocatalyst and the two homocatalysts (Scheme 36.13). 

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