Lead acetate catalyst deactivator

Several important classes of polar monomers have so far eluded copolymerization by the Pd(II) system. Vinyl chloride insertion, for example, leads to catalyst deactivation following P-halide elimination to form inert chloride species such as 1.32, as shown by Jordan [90], Similarly, attempted vinyl acetate copolymerization results in deactivation by an analogous acetate elimination process, although the ester chelate intermediate that forms after insertion also effectively shuts down the reaction [90], Therefore, -elimination of polar groups represents a significant and unresolved problem for late transition metal polymerization systems unless access of the metal to it is restricted. 

Both objectives have been met by designing special hydrogenation catalysts The most frequently used one is the Lindlar catalyst, a palladium on calcium carbonate combi nation to which lead acetate and quinoline have been added Lead acetate and quinoline partially deactivate ( poison ) the catalyst making it a poor catalyst for alkene hydro genation while retaining its ability to catalyze the addition of H2 to the triple bond... 

Complete reduction to the alkane occurs when palladium on carbon (Pd/C) is used as catalyst, but hydrogenation can be stopped at the alkene if the less active Lindlar catalyst is used. The Lindlar catalyst is a finely divided palladium metal that has been precipitated onto a calcium carbonate support and then deactivated by treatment with lead acetate and quinoline, an aromatic amine. The hydrogenation occurs with syn stereochemistry (Section 7.5), giving a cis alkene product. 

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). 

In acetylenes containing double bonds the triple bond was selectively reduced by controlled treatment with hydrogen over special catalysts such as palladium deactivated with quinoline [565] or lead acetate [56], or with triethylam-monium formate in the presence of palladium [72]. 1-Ethynylcyclohexene was hydrogenated to 1-vinylcyclohexene over a special nickel catalyst (Nic) in 84% isolated yield [49]. 

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]. 

Typical catalyst lifetime is 1-2 years. Preferred operation conditions are temperatures around 150 to 160°C and pressures 8 to 10 bar. Hot spots above 200°C lead to permanent catalyst deactivation. The reactant ratio should ensure an excess of ethylene to acetic acid of about 2 1 to 3 1. Due to an explosion danger the... 

The catalyst plays a crucial role in the technology. A typical modern catalyst consists of 0.15-1.5 wt% Pd, 0.2-1.5 wt% Au, 4-10 wt% KOAc on silica spherical particles of 5 mm [8]. The very fast reaction takes place inside a thin layer (egg-shell catalyst). Preferred conditions are temperatures around 150 to 160 °C and pressures 8 to 10 bar. Hot spots above 200 °C lead to permanent catalyst deactivation. The excess of ethylene to acetic acid is 2 1 to 3 1. Because of explosion danger, the oxygen concentration in the reaction mixture should be kept below 8%. Small amount of water in the initial mixture are necessary for catalyst activation. The dilution of the reaction mixture with inert gas is necessary because of high exothermic effect. Accordingly, the reactor is designed at low values of the per-pass conversions, namely 15 - 35% for the acetic acid and 8-10% for ethylene. The above elements formulate hard constraints both for design and for plantwide control. 

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 ... 

Partial hydrogenation of cumulenes affords m-polyenes selectively in the presence of the Lindlar catalyst (a palladium-calcium catalyst deactivated by lead acetate see p. 19) hydrogenation ceases almost entirely after a rapid absorption of (n — l)/2 moles of hydrogen (n = number of double bonds). According to Kuhn and Fischer,139 tetraphenylbutatriene absorbs one equivalent of hydrogen, yielding l9l9494-tetraphenyl-l93-butadiene ... 

Model oxygen containing compounds, or "oxygenates," having EHI s <1 produce excessive amounts of "coke", which leads to rapid catalyst deactivation (5)(0) using ZSM-5. Thus, as shown in Table I, initially complete acetic acid conversion declines to about 60% after only 3 hours on stream. At that point, the total hydrocarbon yield is less than 10 yrt,%, and the gasoline yield (0 +) is less than 7% (8). 

It is very well documented that the carbon-carbon triple bonds (e.g., alkynes) on catalytic hydrogenation gives the completely reduced product, viz. alkanes. Alkynes can also be reduced partially to give z-alkenes by palladium-calcium carbonate catalyst which has been deactivated (partially poisoned) by the addition of lead acetate (Lindlar catalyst) or Pd-BaSO deactivated by quinoline. The lead treatment poisoned the palladium catalyst, rendering it less active and the reaction is more selective. Some examples are given (Scheme 98). 

Solid acid catalysts such as clays and zeolites are also utilized for phenol acylation however, these processes suffer from catalyst deactivation problems and lack C-selectivity. In the acylation of phenol with acetic anhydride, HZSM-5 zeolite shows a very high ort/io-selectivity (48% o-HAP yield, <1% p-HAP yield), although phenyl acetate is isolated in only approximately 20% yield [115]. The SAR value has a remarkable influence on the selectivity of the process when the reaction is carried out in the presence of HZSM-5(30), HZSM-5(150), and HZSM-5(280) zeolites, the o-HAP yields are 42,40, and 15%, respectively, whereas the O-acylation is noticeably increased. These results mean that C-acylation requires higher Brpnsted acidity and that lower acidity leads to phenyl acetate formation. It must be noted that the reaction performed with an amorphous aluminosilicate acid catalyst gives mostly phenyl acetate without isomer selectivity. These results suggest that the C-acylation of phenol occurs in the channels of zeolites and not on the external surface. 

Ricinoleic acid contains a hydroxyl function at a stereogenic carbon atom. Such additional functional groups may interact with transition-metal catalysts causing directing effects or lead to their deactivation. In the hydroformylation of ethyl ricinoleate, the formed aldehydes are converted immediately into cyclic ethers by acetalization and subsequent dehydration (Scheme 6.87) [36]. 

Lindlar Catalyst = Pd that has been "poisoned" (deactivated) by mixing with CaCO, quinoline and lead acetate... 

The first step of the reaction is likely to be the protonation of ethylene to produce a carbocation that undergoes the direct addition of acetic acid to produce ethyl acetate. The successive addition of ethylene to the carbocation leading to the production of alkene oligomers is a likely side reaction Formation and accumulation of these oligomers could eventually deactivate the catalyst. Detailed studies for a better understanding of the complex reaction mechanism are in progress. 

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