Catalyst palladium clusters

The catalytic applications of Moiseev s giant cationic palladium clusters have extensively been reviewed by Finke et al. [167], In a recent review chapter we have outlined the potential of surfactant-stabilized nanocolloids in the different fields of catalysis [53]. Our three-step precursor concept for the manufacture of heterogeneous egg-shell - nanocatalysts catalysts based on surfactant-stabilized organosols or hydrosols was developed in the 1990s [173-177] and has been fully elaborated in recent time as a standard procedure for the manufacture of egg-shell - nanometal catalysts, namely for the preparation of high-performance fuel cell catalysts. For details consult the following Refs. [53,181,387]. 

In contrast to the usual Wacker-conditions, optimum rates and catalyst stability in the Pd/batophenanthroHne-catalyzed olefin oxidations was observed in the presence of NaOAc (pH s 11.5). Under such conditions, the catalyst-containing aqueous phase could be recycled with about 2-3 % loss of activity in each cycle. In the absence of NaOAc precipitation ofPd-black was observed after the second and third cycles. Nevertheless, kinetic data refer to the role of a hidroxo-bridged dimer (Scheme 8.1) rather than the so-called giant palladium clusters which could easily aggregate to metallic palladium. 

Okamoto K, Akiyama R, Yoshida H, Yoshida T, Kobayashi S. Formation of nanoarchitectures including subnanometer palladium clusters and their use as highly active catalysts. J Am Chem Soc 2005 127 2125-2135. 

Major trends can be discerned for Pd-catalysts, aimed at increasing the stability and activity. First is the use of palladium-carbene complexes [178]. Although activities are still modest, much can be expected in this area. Second is the synthesis and use of palladium nanoparticles. For example, the giant palladium cluster, Pd561phen6o(OAc)i8o [179], was shown to catalyze the aerobic oxidation of primary allylic alcohols to the corresponding a,/funsaturated aldehydes (Fig. 4.66) [180]. 

A. Yu. Vasifkov, A. Yu. Olenin, V.A. Seigecv, A.N. Karavanov, E.G. Olenina, and V.M. Gryaznov, Membrane catalysts for hydrogenation with cryochemically synthesized palladium clusters, J. Cluster ScL 2 117 (1991). 

The interaction of chlorobenzene with the catalyst PdY-4.0 has also been studied in situ at the palladium K-edge. The emergence of a chlorine shell at 1.9A after one hour reaction time can be seen in Figure 6. Part of the chlorine could be removed by hydrogen treatment at approximately 50-80 °C under development of hydrogen chloride, and further chlorine was removed by an oxidative treatment at 450°C in air. The hydrogen treatment leads to palladium clusters (shell at 2.25 A) with a coordination number being about of that of bulk palladium, i.e., of a cluster size somewhat like 13 A. By subsequent reoxidation the clusters could only partly be reoxidized. 

Reetz MT, Lohmer G (1996) Propylene carbonate stabilized nanostructured palladium clusters as catalysts in Heck reactions. Chem Commun (Camb) 16 1921... 

Choi, K.-M., Mizugaki, T., Ebitani, K., Kaneda, K. Nanoscale palladium cluster immobilized on a Ti02 surface as an efficient catalyst for liquid-phase Wacker oxidation of higher terminal olefins. Chem. Lett. 2003, 32, 180-181. 

Reaction Mechanism. To understand the size-dependent reactivity of palladium clusters on MgO surfaces in more detail, combined Fourier transform infrared (FTIR) and thermal desorption (TDS) studies were performed. The cluster model catalysts were first exposed to 1 Langmuir of CO at 90 K and subsequently to the same amount of NO. Upon linearly heating the model catalysts, the product molecules C02 and were detected by mass spectrometry as a function of the cluster size for Pd with n < 30. While for Pd4, the formation of C02 is negligible, Pdg and Pdso form C02 at 305 K or 145K and 300K, respectively (Fig. 1.97). 

Moiseev and coworkers showed [10,13] that giant palladium clusters with an idealized formula Pd56iL5o(OAc)igo (L = phenanthroline or bipyridine) are highly active catalysts for allylic oxidation of olefins. The catalytically active solution was prepared by reduction of Pd(OAc)2, e. g. with H2, in the presence of the ligand, L, followed by oxidation with O2. The giant palladium cluster catalyzed the oxidation of propylene to allyl acetate under mild conditions. Even in 10% aqueous acetic acid, allyl acetate selectivity was 95-98 % [10]. Oxidation catalyzed by Pd-561 in water afforded a mixture of allylic alcohol (14%), acrolein (2%), and acrylic acid (60%), and only 5% acetone [10]. 

Semmelhack demonstrated that the addition of alkali metal chloride salts can sometimes markedly increase the yields of coupling products in heterogeneous Pd-catalyzed reactions, especially when the olefin component contains an amide function [15]. It has been claimed that palladium-grafted mesoporous material (MCM-41), designated Pd-TMSll, is one of the most active heterogeneous catalysts for the Heck reaction and enables C-C formation with activated and non-activated aryl substrates [16a,b]. Nanoscale particles of palladium clusters prepared by the ultrasonic reduction of Pd(OAc)2 and NR4X in THF or methanol, were also active for C-C couplings [17]. 

It is noteworthy that besides highly dispersed palladium clusters, ionic Pd was also detected by ESR in Pd/Li-AbOa catalysts prepared by anchoring PdCb onto alumina modified with butyl lithium. These catalysts were highly active and selective in the hydrodechlorination of substituted aromatic compounds. The coexistence of Pd and Pd° in the Pd/Li-AbOa catalysts suggests the possible formation of mixed metal ion - metal nanocluster ensemble sites. 

Beller et al. have shown for the first time that palladium colloids are effective catalysts for the olefination of aryl bromides (Heck reaction). Reetz et al. have studied Suzuki and Heck reactions catalyzed by preformed palladium clusters and palladium/nickel bimetallic clusters and further progress was achieved by Reetz and Lohmert using propylene carbonate stabilized nanostructured palladium clusters as catalysts in Heck reactions. In addition, the use of nanostructured titanium clusters in McMurry-type coupling reactions has been demonstrated by Reetz et... 

PdCl2-catalyzed oxypalladation of 4-pentynoic acid generates the y-lactonic alkenylpalladium 266, and its protonolysis affords the y-methylene-y-lactone 267 with regeneration of Pd(II) [113]. A palladium cluster is a very active catalyst of the cyclization, and the S-methylene-S-lactone 268 was obtained from 5-hexynoic acid in high yield at 40°C in a few minutes [114]. 

---