Palladium thermal cycling

Pure palladium becomes brittle in the presence of hydrogen during thermal cycling due to dimensional changes caused by a transformation between two phases (a and ft) of palladium hydride around 300°C. To avoid metal embrittlement and resulting membrane cracking or distortion, pure palladium membrane should not be exposed to hydrogen at temperatures below 300°C. To increase resistance to embrittlement, Pd is alloyed with... 

Not only the permeability, permselectivity and mechanical properties, but also the catalytic properties are affected by the two hydride forms that can exist in palladium. The a phase corresponds to solid solutions with a H/Pd ratio of about 0.1 and the P phase with a H/Pd ratio of about 0.6. The phase change is associated with a large change in lattice constant that often leads to microcracks and distortion in the palladium membrane. As a result, the mechanical properties are reduced. The transformation depends on the operating conditions such as temperature and hydrogen partial pressure. Repeated thermal cycles, for example, between 100 and 250 C under 1 atm of hydrogen pressure can make a 0.1 mm thick Pd foil expand to become 30 times thicker [Armor, 1992]. 

Palladium membranes must possess sufficient strength to operate under large pressure differentials at elevated temperatures for extended periods of time. Additional desirable characteristics include resistance to common contaminants such as sulfur and the ability to thermally cycle under hydrogen. The use of palladium alloys can help satisfy physical and chemical stability requirements. 

Figure 5.4 Comparison of effects on a palladium-copper alloy membrane by thermal cycling (A) a Monel membrane support, (B) a 304 stainless steel membrane support.

To ensure high permeabilities, it is important to work with low membrane thickness without compromising membrane integrity. For this purpose, several techniques for the production of composite membranes, in which thin palladium alloy layers are deposited onto porous supports have been developed (Fig. 9.8) and are summarized by Drioli et al. [11]. The main problems related to composite membranes concern the achievement of defect-free deposited layers which maintain performance both with time, and also with thermal cycling. Usually, the dif-... 

Thermal cycling of Pd—H-alloys in the duplex phase causes brittleness due to stresses generated by changes of the lattice dimensions for different quantities of dissolved hydrogen. Palladium-silver alloys with 20-25 wt% silver dissolve higher amounts of hydrogen than pure palladium (Fig. 3.1-259). 

There are a number of other surface finishes used in the industry, such as Electroless Nickel Electroless Palladium Immersion Gold (NiPdAu), Immersion Silver, Immersion Gold, Immersion Tin, OSP and Electrolytic Nickel Gold. There are reliability and process trade offs with each surface finish. That is why it is recommended that strain/strain rate characterization and thermal cycling be performed for each set of surface finish before it is selected for the specific end-use conditions in which it will be used. The industry test methods used to evaluate different surface finishes are outlined in detail in the next chapter. 

The challenge, for these membranes, is related to stabihty under thermal cycling, due to a mismatch in thermal expansion coefficients between the ceramic and the palladium, particularly when hydrogenated (Ayturk et al., 2006). At high temperature, total stress can be considered as the sum of thermal stress and H2 stress due to H2 dissolution within the Pd lattice. As shown in Table 3.5, the thermal expansion coefficient of stainless steel is about three times higher than that of alumina. It is also interesting to note that Hastelloy C and zirconia oxide exhibit very close values. Pall... 

Najera et al. [92] have reported that for the coupling of aryl halides with organoboronic adds, complexes 23-26 are adequate catalysts, giving TONs between 102 and 10s. These pallada-cycles exhibit greater aerial and thermal stability than palladium]0) complexes. 

The carbonylation of aryl halides with alcohols and amines catalysed by palladium complexes with triphenylphosphine ligand is the convergent and direct route to the synthesis of aromatic esters as well as aromatic amides. Even though these palladium complexes are widely employed as the best catalytic system, those catalysts are difficult to separate and reuse for the reaction without further processing. The major drawbacks are oxidation of triphenylphosphine to phosphine oxide, reduction of palladium complex to metal and termination of the catalytic cycle. The phosphine-free, thermally stable and air resistant catalyst (1) containing a carbon-palladium covalent bond (Figure 12.3) has been found to be a highly selective and efficient catalyst for the carbonylation of aryl iodides.[1]... 

Hemnann et have indicated that the standard palladacycle trans-di(fju-acetato)-bis[o-(di-o-tolylphosphino)benzyl]-dipaUadium(II) (A) might be a catalyst precursor to active palladium(O) complexes (Scheme 41). In other words, the palladacycle may act as a thermally stable reservoir for the real catalytic species, which is released by heterolytic Pd—C bond cleavage and is activated by subsequent reduction. If this is the acmal case a tfaditional catalytic cycle via Pd(0)/Pd(II) has to be postulated also with palladacycles. In addition, for cross-coupling and amination reactions there is strong evidence for the reduction mechanism of phosphapaUadacycle A into a Pd(0) species.f ... 

Mechanistic studies performed with Freeh s pincer catalyst in the Heck reaction excluded catalytic cycles with the involvement of homogeneous palladium(O) species, as indicated by the results obtained from the (recently developed) dibenzyl-test, which is directly applicable under the reactions conditions applied [24aj. Dibenzyl formation was - in contrast to Heck reactions catalyzed by palladium(O) complexes of type [Pd(PR3)2, where Pd /Pd" cycles are operative - not detectable by gas chromatography-mass spectrometry (GC/MS) when reaction mixtures of aryl bromide, olefin, benzyl chloride ( 10 mol% relative to aryl bromide), catalyst, and base were thermally treated. On the other hand, experimental observations, such as quantitative poisoning experiments with metallic mercury and CS2, which were shown to eflfidently inhibit catalysis, as well as analysis of the reaction profiles showed sigmoidal-shaped kinetics with induction periods and hence indicated that palladium nanoparticles are the catalytically active form... 

A year later, Blacque and Freeh [24c] performed comprehensive DFT studies on the thermal feasibility of Pd /Pd cycles in pincer-catalyzed Heck reactions and convincingly showed that pincer-type Pd intermediates are indeed thermally accessible with aryl bromides at elevated temperatures and hence are generally to be considered as reactive intermediates in pincer-catalyzed reactions with aryl halides at elevated reaction temperatures (for details, see below). Shortly thereafter, Vicente and coworkers [41] published the first oxidative addition of an aryl iodide on the metal center of a paUadium(II) pincer complex 2-iodobenzoic acid was found to smoothly undergo oxidative addition on the paLladium(II) center of [(ONC)Pd(OAc)]... 

Computational Investigations on the Thermal Feasibility of Pd"/Pd Cycles of Palladium Pincer-Catalyzed Heck Reactions... 

W,4 Thermal Feasibility of Pd fPd Cycles of Palladium Pincer-Cataiyzed Heck Reactions 273... 

These computational investigations showed for the first time that catalytic cycles with the involvement of Pd intermediates are indeed thermally accessible for palladium pincer complexes under Heck reaction conditions and hence are a true alternative to palladium nanoparticle-catalyzed versions of the Heck reaction. This, however, does not imply that Pd /Pd mechanisms are operative in any case for palladium pincer complexes in the Heck reaction. In contrast, palladium nanoparticles have been often shown to be the catalyticaUy active form of pincer-type Heck catalysts, as it is, for example, the case for the aminophosphine-based palladium pincer Heck catalyst [2,6-CgH3(NHP(piperidinyl)2)2Pd(Cl)] (10) (under the reaction conditions applied) - the pincer complex with the highest electron density on the metal center and thus where the lowest energy path was calculated. Therefore, it is reasonable to anticipate that palladium pincer Heck catalysts exist that operate via Pd /Pd mechanisms whereas others serve as sources of palladium nanoparticles. This hypothesis got strong experimental support from... 

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