Pd—Pb catalyst

Alcohols or phenols protected by benzyloxycarbonyl groups are very smoothly reduced over Pd/C using phosphinic acid or sodium phosphinate as hydrogen donor. Effective cleavage of an allylic r-amine has been demonstrated (equation 16) utilizing ammonium formate as donor. The lack of tegiospeci-ficity could possibly be overcome by selective use of catalysts. For example, double bond migration can be influenced by Pd/Hg or Pd/Pb catalysts. ... 

New Pd-Pb catalysts comprising of the intermetallic compound PdsPb has been reported with improved properties over commercially available Lindlar s catalysts for the selective reduction of 2-butyne and phenylacety-lene, in which overreduction and isomerisation are diminished. While commercially sourced Lindlar catalyst (Pd-Pb-CaCOs) was found to be comprised of palladium with highly dispersed lead covering its ciystallites, the intermetallic compound PdsPb catalyst appears to contain much more homogeneously dispersed alloy crystallites, although with lead enriched in the surface layer. ... 

In two-metal co-catalyst Pd-Pb-bromide catalytic packages, Pd TON also increases with lead concentration, but decreases with concentration of a second co-catalyst (Figure 2,b). However, we found that addition of a small amount of titanium or cerium is justified because it improves reaction rate and eliminates the induction period characteristic of the lead only package. As a result of optimization, the best performing catalytic packages contain about 100 molar equivalents (vs. Pd) lead, 2-4 molar equivalents of a second co-catalyst (Ti or Ce), and 600 - 800 equivalents of a quaternary bromide. 

The Pd(0)-catalyzed electroreductive coupling of aryl halides (303) is a currently relevant topic. In the electroreduction of aryl halides (307) the replacement of the halogen atom by hydrogen predominantly takes place giving (306). Difficulties are encountered, however, when aryl-aryl coupling products (305) via (304) are wanted (Scheme 116). An efficient electroreductive coupling of aryl bromides (307) (X = Br) and iodides (307) (X = I) into biaryls (310) has been shown to occur in a DMF/Et4NOTs/(Pb cathode) system in the presence of Pd(0) and/or Pd(II) catalysts (Scheme 117) [440]. 

Significant improvement of the activity of selectivity of Pd on Si02 could be achieved in the hydrogenation of acetylene by adding Ti, Nd, or Ce oxides to the catalyst.395 The metal oxides modify both geometrically and electronically the Pd surface. They retard the sintering of the dispersed Pd particles, suppress the formation of multiply bound ethylene, and facilitate the desorption of ethylene. The beneficial effect of lead in the hydrogenation of 1,3-butadiene over a Pd-Pb-on-... 

Requires a transition metal catalyst, Pt, Pd, Ni, etc. Addition is supra-facial from least hindered side. Rearrangements can occur. Alkynes are reduced to c/ s-alkenes over Lindlar catalyst, Pd-Pb (Section 11-2). 

A new Pd-Pb alloy catalyst has been reported to be more selective than commercial Lindlar catalyst in the hydrogenation of triple bonds, that is, in the hydrogenation of 2-butyne to (Z)-2-butene and phenylacetylene to styrene.34 The high selectivity of the alloy catalyst was confirmed in the syntheses of (Z)-l 1-hexade-cenyl acetate and (Z)-l 1-tetradecenyl acetate where particularly high stereoselectivity was required. 

A CaC03 supported Pd-Pb alloy catalyst was found to be more selective in alkyne hydrogenation than the Lindlar catalyst.23 Styrene was produced in over 95% selectivity by the hydrogenation of phenyl acetylene over this catalyst (Eqn. 16.12). Further hydrogenation to ethyl benzene was significantly less than that observed using Lindlar s catalyst. The Z (cis) alkene was formed in >99% selectivity at 100% conversion in the hydrogenation of 11-hexadecynyl acetate Eqn. 16,13).23... 

Two procedures for metal introduction in chitosan base were used impregnation and coprecipitation. According to the first procedure the metal deposition on chitosan micro beads was carried out from aqueous and alcohol solutions of NazPdCU, HzPdCU, RhCb, Rh2(CH3COO)4, ZnS04 and Pb(CH3COO)2. Pd and Pb/Zn in bimetallic catalysts was deposited by subsequent precipitation. Pd-Pb (Zn) atomic ratios were 1/1. Metal contents in the resulting samples were 0.5 - 4%. 

In a previous work [13], we reported on the preparation of carbon-supported bimetallic Bi-Pd catalysts by the thermal degradation of Bi and Pd acetate-type precursors under nitrogen at 773 K and described their catalytic properties in glucose oxidation. The formation of various BixPdy alloys (BiPd, BiPds, Bi2Pds) or, at least, associations on the surface of these catalysts during the activation step was heavily suspected. Alloy formation in supported bimetallic Pd-based catalysts has been mentioned several times in the literature in die presence of other promoting elements, like Pb or Te [14-16] and is sometimes assumed as responsible for the deactivation of the catalysts. 

The use of palladium in the three-way catalysts as a substitute for more expensive metals such as Pt or Rh is an important both economic and strategic objective. Indeed, in the commonly used Pt-Rh based catalysts, the Pt/Rh ratio (= 0.2) is much lower than that of the Pt mine, leading to a deficit in the Rh supplies. Actually, the main reason for the preferential use of Pt in these depoliution catalysts is a lower sensitivity than Pd to poisoning by Pb (1-4), Nowadays, the residual Pb concentration level in unleaded gasoline is negligible and many efforts have been devoted in the last years to the development of Pd based catalysts. 

However, FTP results from the same catalysts aged with lead-containing fuel indicate a severe deterioration in NOx performance, as shown in Figure 2. Although the aging with a leaded fuel deactivates the Pd-only catalyst activity in all sections, it is clear that Pb impedes NOx performance the most, confirming Brisley et al. findings. 

Since the converter used in the FTP consisted of both A and B catalysts, a separate study was conducted to differentiate which catalyst is more resistant to Pb-poisoning. Both A and B catalysts were separately aged under RAT-820°C for 50 hours. To enhance the Pb impact, a 12mgPb/gallon fuel was used for aging additionally, the converter volume was kept relatively small at 26.3 ifr. These two converters were evaluated using a 2.2L L4 vehicle. To further track and differentiate the Pb impact between these two Pd-only catalysts, only Phase 2 of FTP was conducted. Results are shown in Figure 7 ... 

It is apparent that both Pd-only TWC catalysts suffered a marked loss in NOx reduction. Nonetheless, with a Pd loading of 200 g/ft for the A catalyst, twice as much as the B catalyst, the A catalyst shows more deterioration in NOx performance than does the B catalyst. To ascertain why the Pd-only catalyst is more susceptible to Pb poisoning, and why the. e is a marked difference in Pb resistance among the Pd-only catalysts, we conducted analytical studies on these samples using XPS, XRD, XRF, EMP and TEM elemental mapping. 

Ultimately, a Pb resistant catalyst should include other precious metals. A Rh-containing catalyst, such as the Pd/Rh catalyst, for example, offers high Pb resistance, especially in NOx performance, as shown in Figure 11. 

Utilization of lead as a promoter has been developed further by the formulation of true Pd-Pb alloy catalysts these have even higher seleetivities than the Lindlar analogs, in the hydrogenation of 11-hexadecynyl acetate and 12-tetrahydropyrany-loxy-3-tetradecyne (insect sex pheromones) [2] ... 

LindUr Catalyst. Pd -Pb-CaC03. Prepn Ljndlar, Heir. Chim. Acta 35, 446 (1952) Lindlar, Dobuis cited by Fieser, Fieser. Reagents for Organic Synthesis (New York, 1967) p 566. 

The best catalysts for the liquid phase oxidation of alcohols and aldehydes with molecular oxygen contain Pd and/or Pt and are developed industrially. In general, elements like Bi or Pb substantially increase the overall catalytic performances (activity, sometimes selectivity) and/or the lifetime of the Pd/C and Pt/C catalysts. In the selective oxidation of glucose to gluconic add, the presence of Bi as promoter of Pd/C catalysts is known to increase significantly the catalytic activity and to cancel the deactivation occurring during the first minutes... 

---