Noble metal dehydrogenation

The most important process to produce 1-naphthalenol was developed by Union Carbide and subsequently sold to Rhc ne-Poulenc. It is the oxidation of tetralin, l,2,3,4-tetrahydronaphthalene/719-64-2] in the presence of a transition-metal catalyst, presumably to l-tetralol—1-tetralone by way of the 1-hydroperoxide, and dehydrogenation of the intermediate ie, l-tetralol to 1-tetralone and aromatization of 1-tetralone to 1-naphthalenol, using a noble-metal catalyst (58). 1-Naphthol production in the Western world is around 15 x 10 t/yr, with the United States as the largest producer (52). 

UOP Inc. is the key source of technology in this area, having numerous patents and over 70 units operating worldwide (12). The dehydrogenation catalyst is usually a noble metal such as platinum. Eor a typical conversion, the operating temperature is 300—500°C at 100 kPa (1 atm) (13) hydrogen-to-paraffin feed mole ratio is 5 1. 

The initial synthesis of papaverine is due to Pictet, and fittingly enough involved as its key step the name reaction. Acylation of veratrylamine (109) with dimethoxyphenylacetylchlo-ride affords the amide (110). Cyclization by means of phosphorus oxychloride constitutes the same reaction and affords the dihy-droisoquinoline (111). Dehydrogenation by means of a noble metal catalyst affords papaverine (107). ... 

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. 

More recently, noble metal-catalyzed oxidative dehydrogenations have been widely applied to the selective oxidations of alcohols15 and vicinal diols1 1 e.g. 

Copper-catalysts promoted with i) other group VIA or VIIIA metals and ii) alcaline or alcaline earth elements (IA or IIA) are used for selective hydrogenation of various organic compounds (1). Moreover Cu(Co) Zn-Al catalysts were extensively studied for the synthesis of methanol and of light alcohols (2,3). More recently, due to the development of fine chemical processes, detailed studies of copper catalysts were carried out in order to show, like for noble metals, the effect of supports (SMSI), of promoters and of activation-on metal dispersion or reduction, on alloy formation... For example modified copper catalysts are known for their utilization in the dehydrogenation of esters (4-6), in the hydrolysis of nitriles (7), in the selective hydrogenation of nitriles (8), in the amination of alcohols (9)... 

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...

Description The fresh paraffin feedstock is combined with paraffin recycle and internally generated steam. After preheating, the feed is sent to the reaction section. This section consists of an externally fired tubular fixed-bed reactor (Uhde reformer) connected in series with an adiabatic fixed-bed oxyreactor (secondary reformer type). In the reformer, the endothermic dehydrogenation reaction takes place over a proprietary, noble metal catalyst. 

In the adiabatic oxyreactor, part of the hydrogen from the intermediate product leaving the reformer is selectively converted with added oxygen or air, thereby forming steam. This is followed by further dehydrogenation over the same noble-metal catalyst. Exothermic selective H2 conversion in the oxyreactor increases olefin product space-time yield and supplies heat for further endothermic dehydrogenation. The reaction takes place at temperatures between 500°C-600°C and at 4 bar-6 bar. 

Methanol adsorption and decomposition on noble metals have been the subject of many surface-analytical investigations (e.g., References 94,171,320,350,378, 478 94). CH3OH dehydrogenation on palladium catalysts could be a valuable source of synthesis gas or hydrogen, but unfortunately catalyst deactivation by carbon deposits (coking) seriously limits this process (495-498). In this respect, the probability of O H vs. C O bond scission is important, the first path resulting in CO and H2, and the second in carbon or carbonaceous species (CH x = 0-3), CH4, and H2O (see scheme in Fig. 49 details are discussed below). 

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. 

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