Activity rhodium catalysts

The most common oxidation states, corresponding electronic configurations, and coordination geometries of iridium are +1 (t5 ) usually square plane although some five-coordinate complexes are known, and +3 (t7 ) and +4 (t5 ), both octahedral. Compounds ia every oxidation state between —1 and +6 (<5 ) are known. Iridium compounds are used primarily to model more active rhodium catalysts. 

Figure 5.2-1 Stabilization of the active rhodium catalyst by addition of the ionic liquid...

Capture of Active Catalyst Using Solid Acidic Support with H2 Elution. The limit on the practical life of a catalyst solution may be determined by several factors including the presence of poisons or inhibitors, the buildup of less soluble materials such as the oxidation products of organophosphorus ligands, or an increase in the concentration of heavy aldehyde condensation products in the catalyst solution. In the latter case, there may be substantial amounts of active catalyst, but it is in a solvent that is unsuitable. Alternately, active rhodium catalyst may have been carried over with product. Technology has been disclosed [39] that permits the isolation of an active metal hydride catalyst. Steps include ... 

All Group VIII, IX and X transition metals show some catalytic activity for hydroformylation, although cobalt and rhodium are the most active, rhodium catalysts being 104 times more reactive. More recently, platinum catalysts containing the trichlorostannate ligand have been shown to be selective catalysts that effect hydroformylation under mild conditions.6... 

Industrially the straight chain isomer is generally the most desired product and hence the normal/iso product ratio obtained for a given catalyst is of importance. Further, the hydrogenation activities of catalysts vary considerably such that alcohols can in some cases be obtained in a single step (222). The first catalysts developed for this reaction were based on cobalt carbonyl and later cobalt carbonyl phosphine complexes. However, more recently attention has been focused on the intrinsically much more active rhodium catalysts (222, 223). A simplified mechanism for (223) cobalt- and rhodium-catalyzed hydroformylation has been proposed which involves the following steps ... 

Figure 5.2-1 Stabilization of the active rhodium catalyst by addition of the ionic liquid [BMIM][PF6] as co-solvent during distillative product isolation - apparatus for distillative product isolation from the ionic catalyst layer.

Besides the "immobilized" CF3SO3H, another homogeneous catalyst is anionic [Rh(00)212]. This was the first active rhodium catalyst for the carbonylation of methanol to acetic acid. Recently, Chiyoda and UOP introduced the Acetica process, a novel technology based on an "immobilized" [Rh(CO)2l2] on a polyvinyl pyridine resin. Compared with the existing homogeneous process, immobilization increases catalyst concentration in the reaction mixture. 

The second-generation process corresponds to the use of the more active rhodium catalysts (Celanese Corporation, Union Carbide, BASF, Mitsubishi). These processes are performed under mild reaction conditions and generate many fewer byproducts. The selectivity for linear aldehydes is also increased. Thanks to the use of phosphine ligands, the thermal stability of the catalyst is increased and the recycling can be performed by distillation with moderate rhodium losses. 

Another example of successful SILP gas-phase reaction is the rhodium-catalyzed carbonylation of methanol [37]. The technical importance of this reaction is indicated by the Monsanto process, the dominant industrial process for the production of acetic acid (and methyl acetate), carried out on a large scale as a homogeneous liquid-phase reaction [38]. Using [Rh(CO)2l2] anions as the catalyticaUy active species, Riisager and coworkers have developed a new silica SILP Monsanto-type catalyst system [39] 21, in which the active rhodium catalyst complex is part of the IL itself. The SILP system was prepared by a one-step impregnation of the silica support using a methanoUc solution of the IL [BMIM]I and the dimeric precursor species [Rh(CO)2l]2, as depicted in Scheme 15.5. 

Sometimes, also polynuclear clusters such as Rh4(CO)j2 or Rh6(CO)26 were submitted to the formation of rhodium catalysts [18]. Metallic rhodium embedded in inorganic materials (carbon, AI2O3) was tested for mini-plant manufacturing. In this context, the frequently phosphorus ligands [PPhj, P(OPh)3] were added with the intention to detach rhodium from the heterogeneous layer (activated rhodium catalyst = ARC) [19, 20] More recently, ligand (Xantphos, PPhj, BIPHEPHOS)-modified or unmodified rhodium(O) nanoparticles were used as catalyst precursors for solventless hydroformylation [21]. It is assumed that under the reaction conditions these metal nanoparticles decompose and merge into soluble mononuclear Rh species, which in turn catalyze the hydroformylation. 

Under these conditions, the bisdiazaphospholane ligand with electron-withdrawing groups induced the highest conversion, which corresponds to the well-accepted experience in hydroformylation that electron-poor phosphines produce particularly active rhodium catalysts. Interestingly, the relevant catalyst is even more active than those derived from the diphosphites Chiraphite and Kelliphite. 

An activated rhodium catalyst (ARC) [42], that is, rhodium layered on 5% alumina with PPhj as a modifier, was utilized to propose a hypothetical plant with 1000 mt scale production [43,44]. The hydroformylation was conducted in a mini-plant operating as a batch process (Figure 6.13). The Rh catalyst was recycled in a rotary oven at high temperatures. 

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