Palladium carbon dioxide activation

Burch, R. and Urbano, F.J. (1995) Methane combustion over palladium catalysts The effect of carbon dioxide and water on the activity, Appl. Catal. A 123, 173. 

Kostic et al. reported the use of various palladium(II) aqua complexes as catalysts for the hydration and alcoholysis of nitriles,435,456 decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.457 Labile aqua or other solvent ligands can be displaced by a substrate. In many cases, the coordinated substrate thus becomes activated toward nucleophilic addition of water or alcohols. 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

Several groups have been successful at the catalytic conversion of carbon dioxide, hydrogen, and alcohols into alkyl formate esters using neutral metal - phosphine complexes in conjunction with a Lewis acid or base (109). Denise and Sneeden (110) have recently investigated various copper and palladium systems for the product of ethyl formate and ethyl formamide. Their results are summarized in Table II. Of the mononuclear palladium complexes, the most active system for ethyl formate production was found to be the Pd(0) complex, Pd(dpm)2, which generated 10/imol HCOOEt per /rniol metal complex per day. It was anticipated that complexes containing more than one metal center might aid in the formation of C2 products however, none of the multinuclear complexes produced substantial quantities of diethyl oxalate. 

Transition metal complexes have proved very useful in both the catalytic and stoichiometric production of cyclic lactones. A series of palladium(O)-phosphine complexes have been shown to be effective for the conversion of three-membered ring systems to cyclic lactones [Eq. (45)] (114). When isopropylidenecyclopropane and [Pd(dba)2]-PPh3 (dba = dibenzylidenea-cetone)(4 1) in benzene were treated with 40 atm carbon dioxide at 126°C for 20 hr, 69% of the lactone (34) was formed. In contrast, when [Pd(diphos)2] was used as the substrate under similar conditions 48% of 35 was produced with only trace amounts of 34. None of the complexes appeared to be active for terminal alkenes such as 36 or 37. 

Platinum in a finely divided form is obtained by the in situ reduction of hydrated platinum dioxide (Adams catalyst) finely divided platinum may also be used supported on an inert carrier such as decolourising carbon. Finely divided palladium prepared by reduction of the chloride is usually referred to as palladium black. More active catalysts are obtained however when the palladium is deposited on decolourising carbon, barium or calcium carbonate, or barium sulphate. Finely divided ruthenium and rhodium, usually supported on decolourising carbon or alumina, may with advantage be used in place of platinum or palladium for some hydrogenation reactions. 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

The reaction achieved considerable attention over the years, and various alterations have been reported. Behr also reported the combination of carbon dioxide, butadiene and ethylene oxide to give the hydroxyester of the acids depicted in Scheme 19. A nickel-catalyzed analogous system using triphenylphosphine or triisopropylphosphite takes a different route as cyclopentanecarboxylic acids are reported as the main product [126]. A palladium catalyst immobilized on a phosphine-decorated polystyrene polymer [127] or on silica also proved to be active [128]. 

Automotive emission control is a major catalyst market segment. These catalysts perform three functions (1) oxidize carbon monoxide to carbon dioxide (2) oxidize hydrocarbons to carbon dioxide and water and (3) reduce nitrogen oxides to nitrogen. The oxidation reactions use platinum and palladium as the active metal. Rhodium is the metal of choice for the reduction reaction. These three-way catalysts meet the current standards of 0.41 g hydrocarbon per mile, 3.4 g carbon monoxide per mile, and 0.4 g nitrogen oxides per mile. 

Carbon monoxide undergoes activated adsorption on the surface of palladium oxide. The maximum for this process, at about 350 mm. pressure, is at about 100°C. The gas taken up during activated adsorption can only be recovered as C02 for the most part (57). In a CO-air stream a slight initial reduction of PdO occurs at 23°C., but in the absence of oxygen, there is no reduction below 76°. This process of reduction decreases in rate with time and does not go to completion below 156°. Carbon dioxide, when present in the gas phase, inhibits the reduction of the palladium at 100°C. because it is adsorbed strongly by the PdO (56). Catalysts have been prepared by the deposition of palladium and platinum on asbestos, on silica gel, and on charcoal. 

The linear dimerisation of butadiene with palladium(II) catalyst precursors has been investigated in [C4Ciim]+ with a variety of different anions.[24] Observed turnover frequencies, which range from 37-49 mol mol h, are affected only slightly by the nature of the ionic liquid or catalyst precursor. Best activities were obtained with four equivalents of triphenylphosphine per palladium at a reaction temperature of 70°C. Contrary to the reaction in THF, no formation of metallic palladium was observed and reuse of the catalyst solution was possible. Pressurising the reaction mixture with 5-10 bar of carbon dioxide led to a decrease in reaction rates, which was explained by decreased substrate solubility in the C02-expanded ionic liquid. 

Palladium(ll)/PliONa catalyst systems are also active in the reaction of butadiene and carbon dioxide, which again forms acids, lactones and esters (187). In the ittechanism of this reaction a hw(ij -allyl) species and a (n -allyl)(tj -atlyl) species are discussed. The insertion of CO2 into the t -allyl bond gives a carboxylate intermediate which liberates the acid.sand the lactones (Scheme 9). 

From a mechanistic point of view the first steps of the catalytic cycle should be similar to the telomerization of butadiene itself (Scheme 2). The catalytic precursor generates the Pd(0) species A that reacts to the bis-(ri -allyl) complex C. The C,C bond formation between two C4 units is followed by insertion of carbon dioxide into a Pd,C bond affording the carboxylate intermediate D. Different pathways have been discussed to describe the multiple product formation (refer to ). Interestingly, a bis-(carboxylato) complex may be prepared directly from the reaction of lactone 1, palladium acetate and P(i-Pr)3. This complex was structurally characterized by Behr and co-workers and shows good activity as catalyst. Reviewing the literature, there are some remarkable facts and open questions of theoretical and technical interest ... 

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