Precipitation catalysts

Other Esters. The esterification of acetic acid with various alcohols in the vapor phase has been studied using several catalysts precipitated on pumice (67). 

A particularly interesting system for the epoxidation of propylene to propylene oxide, working under pseudo-heterogeneous conditions, was reported by Zuwei and coworkers [61]. The catalyst, which was based on the Venturello anion combined with long-chained alkylpyridinium cations, showed unique solubility properties. I11 the presence of hydrogen peroxide the catalyst was fully soluble in the solvent, a 4 3 mixture of toluene and tributyl phosphate, but when no more oxidant was left, the tungsten catalyst precipitated and could simply be removed from the... 

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

Displacement of the bound ketone by H2 was directly observed by NMR (Eq. (37)), and an approximate equilibrium constant was determined. The cationic tungsten complex can also be used for catalytic hydrosilylation of ketones. In the case of catalytic hydrosilylation of abphatic substrates using HSiEt3, the catalyst precipitates at the end of the reaction, facilitating recycle and reuse [66],... 

C02 as co-solvent Here, C02 is used to solubilize a catalyst, which is insoluble in the pure organic liquid, in an expanded organic phase. Once the C02 is removed at the end of the reaction, the catalyst precipitates and can be removed by filtration. 

The catalyst system Pd(acac)2/TPPTS (TPPTS = trisulfonated triphenylphos-phine) was used in the experiments in which the telomerization of butadiene with ethylene glycol in TMS systems was investigated. However, the catalyst precipitates from many solvent mixtures as a yellow oil or solid, as soon as a homogenous phase is obtained. For this reason the solubihty of the catalyst was determined in various solvent systems. A solution of the catalyst in the mixture of ethylene glycol and water (si) and toluene (s2) was used in a weight ratio of 1 3. The various mediators s3 were added until a clear solution was formed or the catalyst precipitated. Only with DMF or DMSO can a clear solution be obtained. The addition of the catalyst to the polar phase causes an increase in the amount of s3 required to achieve a homogeneous system in the solvent system si toluene DMF the ratio increases from 1 5 4 to 1 5 4.4. 

Before the reaction, at temperatures below the critical solution point, the catalyst is insoluble in the organic solvent. When heated to temperatures above the critical solution point, the catalyst is soluble in the organic solvent and a homogeneous system is formed in which the catalyzed reaction takes place. After reaction, on cooling to temperatures below the critical solution point, the catalyst precipitates from the organic phase which contains the product. Thus, the catalyst can be easily separated from the product by decantation or filtration and reused. 

Otera has reported that fluorous distannoxanes such as 23, which dissociate to give Lewis acidic species, catalyze transesterifications in or-ganic/fluorous solvent mixtures [8,9]. Although 23 was insoluble in toluene at room temperature, it dissolved at reflux and efficiently promoted the transformation in reaction D of Scheme 4, as well as others. The catalyst precipitated upon cooling, but a fluorous solvent extraction was utilized for recovery (100%). Another thermomorphic fluorous Lewis acid catalyst was developed by Mikami [11]. He found that the ytterbium tris(sulfonamide) 24 could be used for Friedel-Crafts acylations imder homogeneous conditions in CICH2CH2CI at 80 °C, and precipitated upon cooHng to -20 °C (reaction E, Scheme 4). 

In most cases the catalytically active metal complex moiety is attached to a polymer carrying tertiary phosphine units. Such phosphinated polymers can be prepared from well-known water soluble polymers such as poly(ethyleneimine), poly(acryhc acid) [90,91] or polyethers [92] (see also Chapter 2). The solubility of these catalysts is often pH-dependent [90,91,93] so they can be separated from the reaction mixture by proper manipulation of the pH. Some polymers, such as the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers, have inverse temperature dependent solubihty in water and retain this property after functionahzation with PPh2 and subsequent complexation with rhodium(I). The effect of temperature was demonstrated in the hydrogenation of aqueous allyl alcohol, which proceeded rapidly at 0 °C but stopped completely at 40 °C at which temperature the catalyst precipitated hydrogenation resumed by coohng the solution to 0 °C [92]. Such smart catalysts may have special value in regulating the rate of strongly exothermic catalytic reactions. 

Separation of catalysts from high-value products such as fine chemicals or pharmaceuticals is often accomplished by precipitating the catalyst from the product solution. Recycling of these catalysts is feasible, provided that they do not decompose. In industry, catalyst recovery by means of catalyst precipitation is applied only in relatively small batch processes. An example of such a process is the production of (—)-menthol (id) in which an Rh-BINAP isomerization catalyst converts the allylic amine substrate into (R)-citronellal (after hydrolysis of the enamine) in high yield (99%) and with high enantioselectivity (98.5% ee). After distillation of the solvent (THF) and product, the catalyst is recovered from the residue by precipitation with -heptane. 

Following an initial resolution step with 0.5 mol equivalents (R)-mandelic acid in TBME, the crystalline product was filtered and tlie waste isomers in the mother liquors (39% ee) were washed with base and then subjected to racemization with the SCRAM catalyst. Upon completion, the catalyst precipitated and was screened, fresh racemic amine was added, and the whole was resolved a second time. The process was repeated several times, giving the results summarized in Table 13.2. 

Industry employs several techniques for solving these problems [116]. The most common are selective product crystallization, where the catalyst and the excess substrates and reagents are left in the liquid phase, and catalyst precipitation and filtration, where the catalyst is precipitated as a salt from the organic reaction mixture. Other techniques include flash distillation of the product under high vacuum, and liquid/liquid extraction of the catalyst from the reaction mixture. 

In the hydrogenation of tetrahydroxyquinone in presence of the usual palladium catalyst, the yield of m-inositol is small (about 4%), but it can be increased to 20% by the use of a palladium catalyst precipitated on charcoal.21... 

Alternatively, an insoluble fluorous support, such as fluorous silica [43], can be used to adsorb the fluorous catalyst. Recently, an eminently simple and effective method has been reported in which common commercial Teflon tape is used for this purpose [44]. This procedure was demonstrated with a rhodium-catalyzed hydrosilylation of a ketone (Fig. 9.27). A strip of Teflon tape was introduced into the reaction vessel and when the temperature was raised the rhodium complex, containing fluorous ponytails, dissolved. When the reaction was complete the temperature was reduced and the catalyst precipitated onto the Teflon tape which could be removed and recycled to the next batch. 

Disclosed is a stone flour with a catalyst precipitated on or bound thereto as an active substance. The catalyst may be brownstone (manganous oxide), finely divided elementary palladiiun or finely divided elementary ruthenirrm. 

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