Carbon as a Catalyst

Carbons are used more widely as supports than as catalysts, despite the fact that they have been found to promote a wide range of reactions. This is owing mainly to the fact that carbons are usually not particularly active catalysts and may weU be destroyed during the course of the reaction. 

In their roles as catalysts, the activity of carbons has been ascribed to several factors, depending on the reaction that is promoted. Hence, for example, Coughlin has attempted to describe most catalytic properties in terms of electronic effectshowever it has been suggested that the decomposition of hydrogen peroxide and nitrous oxide is controlled by the presence of surface complexes. On the other hand, the microporosity of carbons has been suggested to be very important in the catalysed oxidation of hydrogen sulphide.  

in fact, hard to describe carbon as a particular type of catalyst, since it can promote many reactions. A typical cross-section of the relevant literature (Table 2) shows the diversity of some of these reactions. In some cases, the range of reactions is such that the role of impurities as possible catalysts has been investigated. As a result, it is preferable to discuss the catalytic reactions under headings that are fairly individual to carbons, and that reflect one or more aspects considered to be important in each system. 

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The enone 807 is converted into the dienol triflatc 808 and then the conjugated diene 809 by the hydrogenolysis with tributylammonium for-mate[689,690]. Naphthol can be converted into naphthalene by the hydrogenolysis of its triflate 810[691-693] or sulfonates using dppp or dppf as a ligand[694]. Aryl tetrazoyl ether 811 is cleaved with formic acid using Pd on carbon as a catalyst[695]. 

Acyl halides are intermediates of the carbonylations of alkenes and organic-halides. Decarbonylation of acyl halides as a reversible process of the carbo-nylation is possible with Pd catalyst. The decarbonylation of aliphatic acid chlorides proceeds with Pd(0) catalyst, such as Pd on carbon or PdC, at around 200 °C[109,753]. The product is a mixture of isomeric internal alkenes. For example, when decanoyl chloride is heated with PdCF at 200 C in a distillation flask, rapid evolution of CO and HCl stops after I h, during which time a mixture of nonene isomers was distilled off in a high yield. The decarbonylation of phenylpropionyl chloride (883) affords styrene (53%). In addition, l,5-diphenyl-l-penten-3-one (884) is obtained as a byproduct (10%). formed by the insertion of styrene into the acyl chloride. Formation of the latter supports the formation of acylpalladium species as an intermediate of the decarbonylation. Decarbonylation of the benzoyl chloride 885 can be carried out in good yields at 360 with Pd on carbon as a catalyst, yielding the aryl chloride 886[754]. 

Formic acid is a good reducing agent in the presence of Pd on carbon as a catalyst. Aromatic nitro compounds are reduced to aniline with formic acid[100]. Selective reduction of one nitro group in 2,4-dinitrotoluene (112) with triethylammonium formate is possible[101]. o-Nitroacetophenone (113) is first reduced to o-aminoacetophenone, then to o-ethylaniline when an excess of formate is used[102]. Ammonium and potassium formate are also used for the reduction of aliphatic and aromatic nitro compounds. Pd on carbon is a good catalyst[103,104]. NaBH4 is also used for the Pd-catalyzed reduction of nitro compounds 105]. However, the ,/)-unsaturated nitroalkene 114 is partially reduced to the oxime 115 with ammonium formate[106]... 

Silylation of alcohols, amines and carboxylic acids with hydrosilanes is catalyzed by Pd catalysts[l 19], Based on this reaction, silyl protection of alcohols, amines, and carboxylic acids can be carried out with /-butyldimethylsilane using Pd on carbon as a catalyst. This method is simpler and more convenient than the silylation with /-butyldimethylsilyl chloride, which is used commonly for the protection. Protection of P-hydroxymethyl-(3-lactam (125) is an example 120]. 

Liquid-solid-gas where the solid is normally a catalyst such as in the hydrogeneration of amines, using a slurry of platinum on activated carbon as a catalyst. 

Rather unique multicomponent catalysts are those in which one of the components, being volatile, has to be replenished continuously. The oxidation of dilute hydrogen sulfide to elementary sulfur by air with active carbon as a catalyst is such an example. Small amounts of ammonia, if added to the gases entering the reactor, favor and complete this catalytic reaction the ammonia leaves the reactor unchanged (45). 

On the other hand, the linear ester 7 can be prepared as the major product by the carbonylation of a 1-alkene in the presence of a formate ester using Pd(Q) with dppb as a catalyst[12], The linear acid 8 is obtained as the main product by using Pd(OAc)2 or even Pd on carbon as a catalyst and dppb as a ligand in DME in the presence of formic acid or oxalic acid under CO pressure[13]. The linear ester 9 is obtained from a 1-alkene as the main product using PdCI2(Ph3P)2 coordinated by SnCl2[14]. 

Scope of oxidative carbonylation has been studied [83]. The synthesis of acrylic acid or its ester (116) from ethylene has been investigated in AcOH from the standpoint of its commercial production [84]. The carbonylation of styrene is a promising commercial process for cinnamate (117) [80,85,86]. Succinate formation occurs at room temperature and 1 atm of CO using Pd on carbon as a catalyst in the presence of an excess of CuCl2, although the reaction is slow (100% conversion after 9 days) [87],... 

Efficiency of the MHI Process for Treatment of PCBs. The MHI process was developed for the treatment of chlorinated organics varying from neat PCBs to PCB-contaminated liquids and solid material. The MHI processing system provides a total treatment of PCB and PCB-contaminated material. The process involves the removal of PCBs from contaminated containers, recovery of the usable material, and treatment of recovered PCB using an advanced SCWO process. This process operates at 400 °C and 250 bar with sodium carbonate as a catalyst. The MHI system has a capacity of 4 gal/min and could treat a wide range of PCB concentrations (100 ppm-100 wt.%) [125]. The process efficiency is given in Table 7. 

Carbon may also be used as a catalyst without an active component supported on it. This function may overlap with the widespread application of high surface area carbons and sorbents for selective chemisorption processes which are not considered here. The application of carbon as a catalyst in its own right has been reviewed [211-213], All applications are based on the... 

A comprehensive review about particular case studies of carbon as a catalyst [236] provides detailed information and access to the original literature. 

Since the synthesis of Co(NH ))2 is greatly facilitated by the use of carbon as a catalyst [190], Mureinik [196] undertook a careful kinetic study of the reaction... 

The alkylation of pyrroles 131 with epichlorohydrin using potassium carbonate as a catalyst affords l-(2-oxiranyl-methyl)-l/7-pyrroles 132 in modest to high yields (Equation 22) <2002BMC2511>. A similar synthetic route led to indole derivatives 133 (Equation 23). Both compounds 132 and 133 are precursors of isopropanolamines, potential anti-HIV-l-PR agents. 

Three patents have been issued in which two novel methods for forming sulfonic esters are described. A water-soluble derivative, sodium mono-0-(propylsulfonyl)rutin, was prepared by sequential treatment of rutin with sodium in ethanol, followed by propane sul-tone in N,N-dimethylformamide. The second method employed methyl p-toluenesulfonate in N,N-dimethylformamide, with potassium carbonate as a catalyst, for transesterification of the hemiacetal group of aldoses or ketoses. 

Since the x vw-CK-isomer is favored thermodynamically, it can be isolated in high yields by direct oxidation of Co(II) in the presence of the ligands and decolorizing carbon as a catalyst. ... 

Arabinose, xylose, glucose, galactose, and mannose are transformed into the corresponding aldonic acids by air or oxygen passed through their solutions in dilute sodium hydroxide in the presence of 5% platinum on carbon as a catalyst [5, 6] (equation 366). The acids are usually isolated as calcium salts after calcium chloride has been added to the reaction mixture. The classical oxidant for this purpose is bromine water [1165] see equation 368). 

The feasibility of using activated carbon as a catalyst and adsorbent for SO2 removal is demonstrated in processes currently in use in other countries. A series of carbon-based SO2 recovery installations have been operated with continuous recycling for up to two years in West Germany. 

Carbon The uses of carbon as a catalyst support have recently been discussed. Four broad classes of carbon support must be differentiated, activated charcoals, chemically activated charcoals, carbon blacks, and graphites. 

As a consequence of this, it is advantageous to define more closely the scope of this Chapter. The Section on carbon as a catalyst does not include discussion of carbon as a support nor does it include the poisoning of catalysts by carbon. The reactions of carbon are primarily those reactions that are important during gasification and the gasification of cokes and coals has not been considered. 

Most of the above properties are an integral part of the consideration of carbon as a catalyst, and will be discussed in detail later in the text in the context of individual reactions. Two properties are, however, worthy of more discussion. 

Carbon as a Catalyst and Reactions of Carbon Table 2 Reactions catalysed by carbons... 

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