Heterogeneous catalyst platinum-catalyzed reactions

In heterogeneous catalysis, solids catalyze reactions of molecules in gas or solution. As solids - unless they are porous - are commonly impenetrable, catalytic reactions occur at the surface. To use the often expensive materials (e.g. platinum) in an economical way, catalysts are usually nanometer-sized particles, supported on an inert, porous structure (see Fig. 1.4). Heterogeneous catalysts are the workhorses of the chemical and petrochemical industry and we will discuss many applications of heterogeneous catalysis throughout this book. 

Both platinum and an enzyme can be considered heterogeneous catalysts in the sense that the catalytic activity occurs on their surfaces or near them. Even so, some would classify an enzyme as a homogeneous catalyst. The most important difference, however, is one of specificity. Platinum is a rather nonspecific catalyst, catalyzing many different reactions. An enzyme, however, is quite specific, usually catalyzing only one reaction rather than all reactions of a given class. 

Transition metal catalyzed reactions are becoming commonplace in synthetic chemistry. Heterogeneous or homogeneous catalysts containing valuable metals such as palladium, platinum, rhodium and ruthenium are frequently used in the manufacture of active pharmaceutical ingredients (API).[1] The use of such catalysts can lead to metal contamination of the product. The amount of precious... 

Scheme 1 a The [2 + 2] cycloaddition product of prochiral trans 2-butene with Si dimers of the Si(100) surface leads to chiral adsorbate complexes, b Hydrogenation of prochiral a-keto esters over platinum is a heterogeneously catalyzed reaction leading to chiral alcohols. Using cinchonidin as chiral modifier makes this surface reaction enantioselective. In a similar fashion, TA-modified nickel is a highly enantioselective catalyst for /3-keto ester hydrogenation... 

However, it is kinetically inert and requires a catalyst for this and many other of its reactions. Generally, heterogeneous catalysts such as CuO and Rh203 are used, though homogeneous catalysis has been reported for some reactions of nitric oxide. For example, a number of platinum metal complexes, as well as metals and metal oxides, can catalyze the oxygen transfer reaction ... 

Manners first proposed that the transition-metal-catalyzed ROP occurred via a homogenous mechanism.157 However, a heterogenous catalytic cycle has been reported.158 The proposed mechanism for the Pt(l,5-cod)2 (cod-cyclooctadiene) catalyzed reaction is shown in Scheme 2.24. The Pt(l,5-cod)2 forms a [2]platinasilaferrocenophane through oxidative addition to the zero-valent Pt complex via elimination of a 1,5-cod ligand. Platinum colloids are then formed by the elimination of the second 1,5-cod ligand these platinum colloids are proposed to be the active catalysts. The polymers are then formed by subsequent oxidative addition and reductive eliminations at the colloid surface. 

This salt, which is soluble in acetic acid, is recommended as a homogeneous catalyst for exchange of hydrogen in aromatic hydrocarbons for deuterium.1 The substrate, acetic acid, heavy water, and hydrochloric acid are allowed to react in an evacuated, sealed tube at 25-120°. Aliphatics exchange only slowly by this technique. No dimerization (e.g., benzene — diphenyl) is observed. This reaction is observed with heterogeneous platinum-catalyzed exchange with heavy water. 

As for heterogeneous catalysts, the addition of hydrogen is catalyzed by a large variety of materials, but synthetically useful procedures generally employ nickel or the platinum metals. In the latter case, the best results are obtained if the metal is finely divided over the surface of an inert support. Many materials can be used as catalyst supports, however, carbon or alumina are suitable for the majority of reactions. Calcium and barium carbonate or sulfate are also frequently used if less reactive catalysts are desired. The influence of the support is generally small compared to the effect of the metal3. The choice of metal is especially important when stereoselectivity is desired because different metals can catalyze the formation of different diastereomers upon hydrogenation. 

Hydride complexes of platinum have received considerable study since the preparation of PtHCl(PEt3)2- Spectroscopic studies by NMR techniques have been widely used because of the structural information which can be obtained from coupling constant data to Pt and other nuclei. Platinum is widely used as a heterogeneous catalyst, and vibrational studies on platinum hydride complexes have been useful for comparison of a hydrogen atom bonded to a single platinum with that bonded to a surface. Complexes of platinum have been used to catalyze hydrogenation, hydrosilylation and isomerization reactions with alkenes and alkynes, as well as H/D exchange reactions on alkanes. Hydride complexes are frequently proposed as intermediates in these reactions, and the pathways related to the known chemistry of hydride complexes. 

The oxidation of carbon monoxide takes place in the car exhaust cleaning by means of catalysts based on the platinum metals and represents the simplest heterogeneously catalyzed reaction [54,55]. It involves chemisorption of CO and dissociative chemisorption of oxygen, and COad -f Oad react with each other to CO2 via the Langmuir-Hinshelwood mechanism. As an example. Fig. 6.15 shows the ordered structures formed by these adsorbates on a Rh(l 11) surface [56]. The CO molecules are in this case bonded to the surface "on top" and always exhibit the tendency to form densely packed adlayers if the coverage becomes high enough (Fig 6.15a). The O atoms, on the other hand, occupy... 

Postsynthetically modified frameworks were also used as heterogeneous catalyst for the same reaction. Proch et al. [104] loaded MOF-177 with platinum nanoparticles via gas-phase infiltration of volatile [MejPtCp ] precursor under reduced pressure followed by reduction under hydrogen pressure. While this material efficiently catalyzes the oxidation of benzylic and allylic alcohols under solvent and base-free conditions at room temperature, it could not be recycled because of stability issues. 

Liljenroth (1918) was the first to describe a nonlinear phenomenon in heterogeneous catalysis. He studied the platinum-catalyzed oxidation of ammonia and analyzed the stabdify of the steady state of this process. Apparently, Davies (1934) was the first to systematically observe nonlinear phenomena and to point out the chemical nature of the rate hysteresis. In the 1950s, Boreskov and colleagues (Boreskov et al., 1953 Kharkovskaya et al., 1959) studied the oxidation of hydrogen over metal catalysts and found that in a certain range of reaction parameters, very different values of the steady-state reaction rate corresponded with the same gas composition. These data were obtained at isothermal conditions. For more historical information on multiplicity of steady states, see Yablonskii et al. (1991) and Marin and Yablonsky (2011). 

Selective ammoxidation is practiced commercially using heterogeneous catalysts, that is, gaseous reactants at temperatures in excess of300°C are contacted with a solid catalyst. Except for methane ammoxidation to HCN, the reactions are catalyzed by solid metal oxides. In the case of methane ammoxidation, the catalyst is generally platinum metal. 

The first example of a heterogeneously catalyzed hydroamination of an alkene appeared in a 1929 patent in which it is claimed that NHj reacts with ethylene (450°C, 20 bar) over a reduced ammonium molybdate to give EtNH2 [24]. An intriguing reaction was also reported by Bersworth, who reacted oleic acid with NH3 in the presence of catalysts like palladium or platinum black or copper chromite to give the hydroamination product in quantitative yields [25]. However, this result could not be reproduced [26]. 

This and similar instruments (3,4) that allow one to study reaction rates and product distributions on small-area crystal and catalyst surfaces have been used in our studies of the mechanism of heterogeneous catalysis and the nature of active sites. These studies, which concentrated primarily on hydrocarbon reaction as catalyzed by platinum crystal surfaces, will be reviewed in the next section. 

In the majority of catalytic reactions discussed in this chapter it has been possible to rationalize the reaction mechanism on the basis of the spectroscopic or structural identification of reaction intermediates, kinetic studies, and model reactions. Most of the reactions involve steps already discussed in Chapter 21, such as oxidative addition, reductive elimination, and insertion reactions. One may note, however, that it is sometimes difficult to be sure that a reaction is indeed homogeneous and not catalyzed heterogeneously by a decomposition product, such as a metal colloid, or by the surface of the reaction vessel. Some tests have been devised, for example the addition of mercury would poison any catalysis by metallic platinum particles but would not affect platinum complexes in solution, and unsaturated polymers are hydrogenated only by homogeneous catalysts. 

Several metals have been used to catalyze redox reactions the most commonly studied are platinum and gold. There is debate concerning the exact catalytic nature - homogeneous or heterogeneous - of these catalysts. We summarize the results on the dependence of the reaction rate and TOP for redox reactions catalyzed by colloidal nanoparticles on (1) particle size, (2) local particle environment, (3) particle concentration, and (4) particle shape. 

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