Catalytic dehydrogenation noble metals

Synthesis from Citronellol. Citronellol is hydrated to 3,7-dimethyloctan-l,7-diol, for example, by reaction with 60% sulfuric acid. The diol is dehydrogenated catalytically in the vapor phase at low pressure to highly pure hydroxydihydrocitronellal in excellent yield. The process is carried out in the presence of, for example, a copper-zinc catalyst [68] at atmospheric pressure noble metal catalysts can also be used [69]. 

Catalytic reforming has become the most important process for the preparation of aromatics. The two major transformations that lead to aromatics are dehydrogenation of cyclohexanes and dehydrocyclization of alkanes. Additionally, isomerization of other cycloalkanes followed by dehydrogenation (dehydroisomerization) also contributes to aromatic formation. The catalysts that are able to perform these reactions are metal oxides (molybdena, chromia, alumina), noble metals, and zeolites. 

A major problem in noble metal catalyzed liquid phase alcohol oxidations -which is principally an oxidative dehydrogenation- is poisoning of the catalyst by oxygen. The catalytic oxidation requires a proper mutual tuning of oxidation of the substrate, oxygen chemisorption and water formation and desorption. When the overall rate of dehydrogenation of the substrate is lower than the rate of oxidation of adsorbed hydrogen, noble metal surface oxidation and catalyst deactivation occurs. 

Some other catalytic events prompted by rhodium or ruthenium porphyrins are the following 1. Activation and catalytic aldol condensation of ketones with Rh(OEP)C104 under neutral and mild conditions [372], 2. Anti-Markovnikov hydration of olefins with NaBH4 and 02 in THF, a catalytic modification of hydroboration-oxidation of olefins, as exemplified by the one-pot conversion of 1-methylcyclohexene to ( )-2-methylcycIohexanol with 100% regioselectivity and up to 90% stereoselectivity [373]. 3. Photocatalytic liquid-phase dehydrogenation of cyclohexanol in the presence of RhCl(TPP) [374]. 4. Catalysis of the water gas shift reaction in water at 100 °C and 1 atm CO by [RuCO(TPPS4)H20]4 [375]. 5. Oxygen reduction catalyzed by carbon supported iridium chelates [376]. - Certainly these notes can only be hints of what can be expected from new noble metal porphyrin catalysts in the near future. 

Figure 13.20 Methylcyclohexane conversion to toluene as a function of reactor temperature in a membrane and a nonmembrane reactor [45]. Reprinted with permission from J.K. Ali and D.W.T. Rippin, Comparing Mono and Bimetallic Noble Metal Catalysts in a Catalytic Membrane Reactor for Methyl-cyclohexane Dehydrogenation, Ind. Eng. Chem. Res. 34, 722. Copyright 1995, American Chemical Society and American Pharmaceutical Association...

Numerous metals have been evaluated as bifunctional catalysts. Those used are noble metals and non-noble or transition metals. Platinum and palladium have the highest catalytic activity. The noble metal content is usually 1% or less, whereas that of non-noble metals is larger 1-30%. The concentration of dispersed metal on supports plays an important role in the activity of the catalyst, e.g. the activity of hydrogenation/dehydrogenation increases then decreases with the concentration of metals. There are some typical reactions of bifunctional catalysts. 

In spite of the commercial successes of these two millisecond reactors, few processes other than NH3 oxidation (a superoxidation) and HCN synthesis (oxidative dehydrogenation or ammoxidation) have been carried out on a large scale. In the automotive catalytic converter, contact times over noble metals on wash coated extended ceramics are used with contact times of -0.1 sec with temperatures of 400°C. 

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

Alternative cocatalysts, such as [Fe(pc)]20 (pc = phthalocyanine) and AIBN, have also been reported for catalytic aerobic DDQ oxidation reactions, though these have seen less widespread application than NO [28]. Aerobic quinone-catalyzed dehydrogenation reactions using less oxidizing quinones, such as chloranil, have been reported using polymer-incarcerated noble metal catalysts reported by Kobayashi and colleagues [29, 30]. 

The utility of carbide and nitride catalysts has prompted numerous studies of their reactivity that use carbide and nitride overlayers as the catalyst rather than bulk carbides or nitrides. This approach permits careful manipulation of the surface metal/nonmetal stoichiometry, which is crucial to probing reactivity. These studies consistently reveal the catalytic activity of carbide and nitride overlayers and, in several cases, the similarities between their behavior and that of noble metal catalysts. For example, the same benzene yield and reaction pathway for the dehydrogenation of cyclohexane was observed for both p(4x4)-C/Mo(110) and Pt(l 11) surfaces. Furthermore, carbon-modified tungsten may be a more desirable catalyst for direct methanol fuel cells than Pt or Ru surfaces because the transition metal carbide exhibits higher activity toward methanol and water dissociation and is more CO-tolerant. ... 

Several non-noble metal-catalyzed iV-alkylation reactions have also been reported in recent years. In 2010, Ramon, Yus and co-workers reported Cu(OAc)2-catalyzed A-alkylation of poor nucleophilic amine derivatives and alcohols (Eq. 30) [125]. Control experiments indicated that a base was indispensable in the reaction to force the alcohol dehydrogenation step, which was later confirmed by DFT calculations reported by Liu, Huang and co-workers [63]. In 2011, Ramon and co-workers reported the results of their own mechanistic studies and proposed two possible catalytic cycles [126]. The main aldehyde-free cycle, depicted with plain arrows, requires the presence of a base. The minor cycle, depicted in dashed arrows, may proceed when an aldehyde exists in the reaction media (Scheme 23). In the same year, Li and co-workers also disclosed a CuCl-catalyzed A-alkylation of heteroarylamines [127]. 

Noble metal salts, for example, of Pd(II) or Pt(II), undergo reduction by primary and secondary alcohols inhomogeneous solution. Indeed, the ability of alcohols to reduce Pd(II) was described as early as 1828 by Berzelius, who showed that K2PdCl4 was reduced to palladium metal in an aqueous ethanolic solution [25]. The reaction involves a [3-hydride elimination from an alkoxymetal intermediate and is a commonly used method for the preparation of noble metal hydrides (Eq. (5.1)). In the presence of dioxygen this leads to catalytic oxidative dehydrogenation of the alcohol, for example, with palladium salts [26-30]. 

Very high olefin yields were reported by Schmidt and co-workers,who first proposed the oxidative dehydrogenation of hght alkanes over insulated noble metal coated monoliths at contact times of a few milhseconds. This new concept of catalytic reactor had been previously applied by the same group to methane partial oxidation and was extended to test the reactivity of C2-C6 alkane/air fuel-rich feeds. Ceramic foam monoliths (with 45 and 80 ppi) were mostly studied as supports of noble metals and bimetallic catalysts. 

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