Palladium catalyst deactivator

The presence of other functional groups ia an acetylenic molecule frequendy does not affect partial hydrogenation because many groups such as olefins are less strongly adsorbed on the catalytic site. Supported palladium catalysts deactivated with lead (such as the Liadlar catalyst), sulfur, or quinoline have been used for hydrogenation of acetylenic compound to (predominantiy) cis-olefins. 

Dehydrolinalool, obtained by ethynylation of 6-methyl-5-hepten-2-one, can be converted into dehydrolinalyl acetate with acetic anhydride in the presence of an acidic esterification catalyst. Partial hydrogenation of the triple bond to linalyl acetate can be carried out with, for example, palladium catalysts deactivated with lead [73]. 

For special purposes, e.g., for partial reduction of triple to double bonds138 or of cumulenes to polyenes,139 the catalyst may be deactivated by partial poisoning. Double bonds are not hydrogenated in presence of these catalysts. Lindlar138 gives the following directions for preparation of a palladium catalyst deactivated by lead acetate and supported on calcium carbonate ... 

Although the process is of significance, it has not well studied. Since the initial development of the CTA hydropurification process in 1960s , only a few papers have been published, mainly regarding catalyst deactivation [2]. Recently, Samsung Corporation, in collaboration with Russian scientists, developed a novel carbon material-CCM supported palladium-ruthenium catalyst and its application to this process [3]. However, pathways and kinetics of CTA hydrogenation, which are crucial to industrialization, are not reported hitherto. 

GL 18] ]R 1] ]P 19a] For a sputtered palladium catalyst, low conversion and substantial deactivation of the catalyst were foimd initially (0.04 mol 1 60 °C 4 bar 0.2 ml min ) [60, 62]. Selectivity was also low, side products being formed after several hours of operation (Figure 5.25). After an oxidation/reduction cycle, a slightly better performance was obtained. After steep initial deactivation, the catalyst activity stabilized at 2-4% conversion and about 60% selectivity. After reactivation, the selectivity approached initially 100%. As side products, all intermediates except phenylhydroxylamine were identified. 

Several palladium catalysts for formation of aryl sulfides from aryl halides have been investigated more recently. A combination of Pd2(dba)3 and DPEphos catalyzed the formation of a broad range of diaryl sulfides in the presence of 1 mol.% palladium and NaO-t-Bu base in toluene solvent.12,rThe highest yields of alkyl aryl sulfides were obtained from aryl triflates and n-butyl thiol catalyzed by a combination of palladium acetate and BINAP. However, these reactions contained 10 mol.% catalyst, were long, and required deactivated aryl triflates. A combination of Pd2(dba)3 and DPPF catalyzed the coupling of thiols with resin-bound aryl halides.121... 

In their experiments, an unsubstituted dppf-complex was compared with the analogous dendritic complex (Figure 4.23). After 35 exchanged reactor volumes the dendritic catalyst still showed a conversion of 77% (maximum 85% after 10 reactor volumes) while the unsubstituted catalyst deactivated from 70% to 15% at the end of the catalytic run. The drop in activity for both systems can be completely explained by their retention (97% and 99.8% for the unsubstituted and the dendritic complex, respectively). This means, no deactivation of the catalyst occurred during catalysis, which is often the case for palladium-catalyzed continuous catalysis. 

In line with the above mechanism, catalyst deactivation by formation of palladium black can be retarded by increasing the [P]/[Pd] ratio, however, only on the expense of the reaction rate. Bidentate phosphines form stronger chelate complexes than TPPMS which may allow at working with lower phosphine to palladium ratios. Indeed, the palladium complex of sulfonated XANTPHOS (51) proved to be an effective and selective catalyst for hydroxycarbonylation of propene, although at [51]/[Pd] < 2 formation of palladium black was still observed. The catalyst was selective towards the formation of butyric acid, with 1/b = 65/35 [41]. 

Alkynes may also be hydrogenated, initially to alkenes, and then further to alkanes. By suitable modification of the catalyst, it has proved possible to stop the reaction at the intermediate alkene. Typically, platinum or palladium catalysts partially deactivated (poisoned) with lead salts are fonnd to be suitable for reduction of alkynes to alkenes. Again, syn addition is observed. 

Palladium catalysts are more often modified for special selectivities than platinum catalysts. Palladium prepared by reduction of palladium chloride with sodium borohydride Procedure 4, p. 205) is suitable for the reduction of unsaturated aldehydes to saturated aldehydes [i7]. Palladimn on barium sulfate deactivated with sulfur compounds, most frequently the so-called quinoline-5 obtained by boiling quinoline with sulfur [34], is suitable for the Rosenmund reduction [i5] (p. 144). Palladium on calcium carbonate deactivated by lead acetate Lindlar s catalyst) is used for partial hydrogenation of acetylenes to cw-alkenes [36] (p. 44). 

Before 1970 there was very little unleaded gasoline on the market, but by 1974 all gas stations were offering it. In 1974, unleaded fuel had become a necessity for most new cars because of their catalytic converters placed in the exhaust system. These contain platinum or palladium compounds that act as a surface catalyst to bum the hydrocarbons more completely. But lead coats the platinum and palladium and deactivates the converters, so unleaded gas must be used. Up to 4 g/gal of lead could be used in the 1970s, but this was decreased to 0.1 g by 1986. Since 1995 no leaded gas could be used in the U.S. Fig. 7.6 shows the dramatic shift from leaded to unleaded gas between 1975 and 1992. 

The transformation of alcohols to the corresponding carbonyl compounds or carboxylic acids is one of the few examples in which a heterogeneous (solid) catalyst is used in a selective, liquid phase oxidation (7,2). The process, which is usually carried out in an aqueous slurry, with supported platinum or palladium catalysts and with dioxygen as oxidant, has limited industrial application due to deactivation problems. 

Substituted benzyl chlorides were carbonylated using a Pd/tppts catalyst in aqueous/organic two phase systems under basic reaction conditions to afford the sodium salts of the corresponding phenylacetic acids. After acidification the phenylacetic acid dissolved in the organic phase and could be readily separated from the Pd/tppts catalyst contained in the aqueous phase (Figure 12) 466-468 TOFs up to 21 h 1 (turnover number, TON=165) and phenylacetic acid yields up to 94% were obtained at 70°C, 1 bar CO, tppts/Pd=10, NaOH/substrate=3/2 in an aqueous/toluene (1/1) two phase system in a batchwise procedure.466 The TOFs were improved to a maximum of 135 h 1 (TON=1560) in a continuous operation mode. Palladium catalysts modified with binas (Table 2 25) exhibited low catalytic activity (TONs up to 140) in the carbonylation of benzyl chloride 466 In strongly acidic media (pH=l) the Pd/25 catalyst was active and remained stable during the reaction which contrasts with Pd/tppts where palladium black was observed. However, the catalyst was completely deactivated after three cycles.466... 

Such a stabilization of the palladium catalyst can also be achieved in homogeneous liquid phase by the use of appropriate ligands. Thus, it has recently been shown that palladium(II) hydroxamates are effective catalysts for the acetoxylation of ethylene with high selectivity and a high turnover (>200) (equation (162), whereas Pd(OAc)2 rapidly becomes deactivated and precipitates in the form of metallic palladium.419 It is probable that the bidentate hydroxamate ligand stabilizes the hydride Pd—H species and prevents palladium from precipitating. 

With most hydrogenation catalysts of the platinum and nickel groups (Sections 4.2.61, p. 459 and 4.2.50, p. 450), a mixture of products is obtained even if an attempt is made to stop the reaction at the half-way stage. An alkene may only be obtained in good yield if particular attention is paid to the selection of a deactivated hydrogenation catalyst. An early, highly effective formulation of a deactivated palladium catalyst is Lindlar s catalyst (Section 4.2.54, p. 453), but palladium-on-barium sulphate in the presence of quinoline, or palladium-on-calcium carbonate, is also recommended. In these cases it is advisable to... 

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