ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

Although technological practice should minimize environmental impact, this is not always economically feasible. During the past decade, for example, there has been increasing global concern over th...

CO2 Reforming of CH4

INCREASING concern about world dependence on petroleum oil has generated interest in the more efficient use of natural gas1–4. The conversion of methane to the common feedstock synthesis gas (carbon monoxide and hydrogen) by steam reforming is already well established5, and we have shown recently that yields of syn-thesis gas in excess of 90% can be obtained at moderate tem-peratures and ambient pressure by partial oxidation, with air or oxygen, over supported transition-metal catalysts6,7. The use of carbon dioxide as an oxidant for conversion of natural gas to synthesis gas is well established in steam reforming5, and is also known in CO2 reforming (for example, the Calcor process8,9), in which the use of excess CO2 yields mainly CO. In the present work, we describe an alternative catalytic strategy for CO2 reform-ing which gives excellent yields (90%) from a stoichiometric (1:1) feed of CO2 and CH4. Carbon deposition ('coking'), which is a hazard of CO2-reforming routes, is suppressed here by the use of catalysts based on platinum-group metals. We show that the exothermic partial oxidation of CH2 and the endothermic CO2-reforming reaction can be carried out simultaneously, thus introducing the possibility of tuning the thermodynamics of the process.

Partial oxidation of methane to synthesis gas using carbon dioxide

Steam reforming reactions will play a key role in new applications of synthesis gas and in a future hydrogen economy. The aim of this review is to provide a coherent description of the catalysis of the reforming reactions. The review is not comprehensive. The first section deals with the applications of synthesis gas and hydrogen and the various synthesis gas technologies. The optimum choice of technology depends on the requirements of the gas composition and the scale of operation. Two examples are included for illustration: synthesis gas for gas-to-liquid plants and hydrogen for fuel cells. The steam reforming process is described in the second section with emphasis on the role of the catalyst and problems related to carbon formation. The third section is a summary of the empirical evidence of the catalysis of the reforming reactions. The methods of characterization are discussed, and data representing sintering, activity trends, and promotion are summarized. The fourth section is a description of the mechanism based on a combination of empirical knowledge with recent data from studies of well-defined surfaces, in-situ high-resolution electron microscopy, and calculations based on density functional theory. The central concept is the role of surface defects as the source of reactivity and the nucleation centers for whisker carbon formation.

Hydrogen and Synthesis gas by Steam- and CO2 reforming

THE diminishing reserves of petroleum oil have focused attention on the possibility of making more efficient use of natural gas, reserves of which are at present considerably under-utilized. Methane is commonly used as a fuel, but it is also the starting material for the production, by steam reforming, of synthesis gas (carbon monoxide and hydrogen), which acts as a feedstock for the synthesis of ammonia and methanol, and can be converted to higher hydrocarbons, alcohols and aldehydes by Fischer–Tropsch catalysis1. The partial oxidation of methane to synthesis gas is also an established industrial process2 but operates at very high temperatures (> 1,200 °C). Here we report that this reaction can be carried out at temperatures of only ∼775 °C by using lanthanide ruthenium oxide catalysts.

Selective oxidation of methane to synthesis gas using transition metal catalysts

The direct conversion of methane to methanol by controlled oxidation

The gas-phase homogeneous partial oxidation of methane with oxygen was studied over a range of operating conditions in a high-pressure tubular reactor. The experimental data indicated that methanol selectivities of 70 - 80% could be achieved at methane conversions of 8 - 10% by operating at suitable reaction conditions. The higher methanol selectivities obtained in this study were probably due to the use of a Pyrex glass liner in the reactor, which may have minimized surface reactions.

Direct conversion of methane to methanol in a flow reactor

Methanol formation at high pressure by the catalyzed oxidation of natural gas and by the sensitized oxidation of methane

Autoxidation and yellowing of methyl linolenate.

A process of directly converting natural gas to methanol employs controlled oxidation. The reaction takes place in an inert reactor, i.e. one having internal surfaces which do not affect the reaction, in the absence of a catalyst. The natural gas is intimately mixed with air or oxygen prior to introduction of the mixed gases into the reactor. Reaction takes place at an elevated temperature of 300° to 500° C. and at an elevated pressure of 10 to 100 atmospheres. The percentage of oxygen in the mixture of reactant gases is kept below 20% by volume and is preferably 2 to 10% by volume. Apparatus for carrying out the method is also provided.

Norman R. Hunter

Papers

The direct conversion of methane to methanol by controlled oxidation

Direct conversion of methane to methanol in a flow reactor

Methanol formation at high pressure by the catalyzed oxidation of natural gas and by the sensitized oxidation of methane

Autoxidation and yellowing of methyl linolenate.

Direct conversion of natural gas to methanol by controlled oxidation