Direct Uses of Methane

This reaction is of great technological interest in the area of solid oxide fuel cells (SOFC) since it is catalyzed by the Ni surface of the Ni-stabilized Zr02 cermet used as the anode material in power-producing SOFC units.60,61 The ability of SOFC units to reform methane "internally", i.e. in the anode compartment, permits the direct use of methane or natural gas as the fuel, without a separate external reformer, and thus constitutes a significant advantage of SOFC in relation to low temperature fuel cells. 

For the most part we refer to the direct use of methane (100% basis) as a chemical feedstock. The methane... 

Fig. 7.4 Thermodynamically feasible pathways involving direct use of methane.

CH2CI2. A colourless liquid with a chloroform-like odour b.p. 4I°C. Prepared by heating chloroform with zinc, alcohol and hydrochloric acid manufactured by the direct chlorination of methane. Decomposed by water at 200°C to give methanoic and hydrochloric acids. Largely used as a solvent for polar and non-polar substances, particularly for paint removal (30%), dissolving cellulose acetate and degreasing (10%). It is more stable than carbon tetrachloride or chloroform especially towards moisture or alkali. It is somewhat toxic. U.S. production 1981 280000 tonnes. 

Direct conversion of methane [74-82-8] to methanol has been the subject of academic research for over a century. The various catalytic and noncatalytic systems investigated have been summarized (24,25). These methods have yet to demonstrate sufficient advantage over the conventional synthesis gas route to methanol to merit a potential for broad use. 

Direct conversion of methane to ethane and ethylene (C2 hydrocarbons) has a large implication towards the utilization of natural gas in the gas-based petrochemical and liquid fuels industries [ 1 ]. CO2 OCM process provides an alternative route to produce useful chemicals and materials where the process utilizes CO2 as the feedstock in an environmentally-benefiting chemical process. Carbon dioxide rather than oxygen seems to be an alternative oxidant as methyl radicals are induced in the presence of oxygen. Basicity, reducibility, and ability of catalyst to form oxygen vacancies are some of the physico-chemical criteria that are essential in designing a suitable catalyst for the CO2 OCM process [2]. The synergism between catalyst reducibility and basicity was reported to play an important role in the activation of the carbon dioxide and methane reaction [2]. 

Investigation of direct conversion of methane to transportation fiiels has been an ongoing effort at PETC for over 10 years. One of our current areas of research is the conversion of methane to methanol, under mild conditions, using li t, water, and a semiconductor photocatalyst. Research in our laboratory is directed toward ad ting the chemistry developed for photolysis of water to that of methane conversion. The reaction sequence of interest uses visible light, a doped tungsten oxide photocatalyst and an electron transfer molecule to produce a hydroxyl i cal. Hydroxyl t cal can then react with a methane molecule to produce a methyl radical. In the preferred reaction pathway, the methyl radical then reacts with an additional wata- molecule to produce methanol and hydrogen. 

A long-term goal of our research group is to explore and evaluate novel pathways for the direct conversion of methane to liquid fiiels, chemicals, and intermediates. One of our current areas of research is the conversion of methane to methanol, under mild conditions, using li t, water, and a semiconductor photocatalyst. The use of three relatively abundant... 

The large amounts of natural gas (mainly methane) found worldwide have led to extentive research programs in the area of the direct conversion of methane [1-3]. Ihe oxidative transformation of methane (OTM) is an important route for the effective utilization of the abundant natural gas resources. How to increase catalyst activity is a common problem on the activation of methane. The oxidation of methane over transition m al oxides is always high active, but its main product is CO2, namely the product of deep oxidation. It is because transition metal oxides have high oxidative activity. So, they were usually used as the main corrqtonent of catalysts for the conqilete oxidation of alkane[4]. The strong oxidative activity of CH4 over tran on metal oxides such as NiO indicates that the activation of C-H bond over transition metal oxides is much easier than that over alkaline earth metal oxides and rare earth metal oxides. Furthermore, the activation of C-H bond is the key step of OTM reaction. It is the reason that we use transition metal oxides as the mam conq>onent of the OTM catalysts. However, we have to reahze that the selectivity of OTM over transition metal oxides is poor. 

While alkane metathesis is noteworthy, it affords lower homologues and especially methane, which cannot be used easily as a building block for basic chemicals. The reverse reaction, however, which would incorporate methane, would be much more valuable. Nonetheless, the free energy of this reaction is positive, and it is 8.2 kj/mol at 150 °C, which corresponds to an equihbrium conversion of 13%. On the other hand, thermodynamic calculation predicts that the conversion can be increased to 98% for a methane/propane ratio of 1250. The temperature and the contact time are also important parameters (kinetic), and optimal experimental conditions for a reaction carried in a continuous flow tubiflar reactor are as follows 300 mg of [(= SiO)2Ta - H], 1250/1 methane/propane mixture. Flow =1.5 mL/min, P = 50 bars and T = 250 °C [105]. After 1000 min, the steady state is reached, and 1.88 moles of ethane are produced per mole of propane consmned, which corresponds to a selectivity of 96% selectivity in the cross-metathesis reaction (Fig. 4). The overall reaction provides a route to the direct transformation of methane into more valuable hydrocarbon materials. 

Even aliphatic hydrocarbons are susceptible to oxidative carbonylation. From an industrial point of view, the most important process concerns the direct conversion of methane into acetic acid. This transformation has been achieved with Rh(III)-based catalysts using oxygen as the oxidizing agent [149-153], and it is still object of investigations aimed at developing more efficient catalytic systems working under mild conditions. 

First, we will refer to the direct use of hydrocarbon fuels in an SOFC as direct utilization rather than direct oxidation. Second, we recognize that the broadest definition of direct utilization, exclusive from mechanistic considerations, should include rather conventional use of fuel by internal reforming, with steam being cofed to the fuel cell with the hydrocarbon. Indeed, this nomenclature has been used for many years with molten-carbonate fuel cells. However, because internal reforming is essentially limited to methane and because the addition of steam with the fuel adds significant system complexity, we will focus primarily on systems and materials in which the hydrocarbons are fed to the fuel cell directly without significant amounts of water or oxygen. 

In this paper, we summarize results from a small scale methane direct oxidation reactor for residence times between lO and lO seconds. For this work, methane oxidation (using air or oxygen) was studied over Pt-10% Rh gauze catalysts and Pt- and Rh-coated foam and extruded monoliths at atmospheric pressure, and the reactor was operated autothermally rather than at thermostatically controlled catalyst temperatures. By comparing the steady-state performance of these different catalysts at such short contact times, tiie direct oxidation of methane to synthesis gas can be examined independent of the slower reforming reactions. 

Consequently, the direct use of hydrocarbon gases as fuel is usually considered to be impractical, although Whitesides and co-workers18 describe an aqueous fuel cell in which methane reacts with aqueous iron (III) ions over a platinum black catalyst to form CO2 and iron(II) the Fe2+ solution... 

Carbon tetrachloride represents an example of the change to petroleum raw materials in this field. The traditional source of this widely used product has been the chlorination of carbon disulfide, either directly or through the use of sulfur dichloride. Military requirements in World War II caused an increase in demand, and in addition to expansion of the older operations, a new process (28) was introduced in 1943 it involved direct chlorination of methane at 400° to 500° C. and essentially atmospheric pressure. This apparently straight-forward substitution of halogen for hydrogen in the simplest paraffin hydrocarbon was still a difficult technical accomplishment, requiring special reactor construction to avoid explosive conditions. There is also the fact that disposal of by-product hydrochloric acid is necessary here, though this does not enter the carbon disulfide picture. That these problems have been settled successfully is indicated by the report (82) that the chlorination of methane is the predominant process in use in the United States today, and it is estimated that more than 100,000,000 pounds of carbon tetrachloride were so produced last year. 

Dichloromethane was first prepared by Regnault in 1840 by the chlorination of methyl chloride in sunlight. It became an industrial chemical of importance during the Second World War. Two commercial processes are currently used for the production of dichloromethane—hydrochlorination of methanol and direct chlorination of methane (Rossberg etal., 1986 Holbrook, 1993). 

A flow reactor is used for the production of synthesis gas, CO + H2, by direct oxidation of methane and other hydrocarbons in the presence of steam. Preheated streams are mixed and react in a... 

Methylene Chloride tdichtaromethane). CAS 75-09-2. As with the other members of the methyl series of chlorinated hydrocarbons, methylene chloride can he produced hy direct chlorination of methane. The usual procedure involves a modification of the simple methane process. The product from Ihe first chlorination passes through aqueous zinc chloride, contacting methanol at about 100 C. Thus. HCl from chlorination is used to displace the alcohol group, producing additional methyl chloride. This is further chlorinated to methylene chloride. Methylene chloride reacts violently in the presence of alkali or alkaline earth metals and will hydrolyze to formaldehyde in the presence of an aqueous base. Alkvlalion reactions occur at both functions, thus di-suhstiiulioiis result. For example. 

One of the main objective of SOFCs in the future is the use of gaseous mixtures of C0-H2-H20 produced by coal gasification plants or by steam reforming a hydrocarbon fuel, especially methane. Very little data is available about the direct oxidation of methane in SOFCs [96, 97], Steele et al. [97] have recently confirmed the poor electrocatalytic activity of Pt electrodes for the anodic oxidation of methane at 800 °C. Although nickel fulfills major requirements for anode materials when H2 and CO are employed as fuels, its use for the direct oxidation of methane encourages carbon deposition. To overcome this problem, alternative anode materials must be... 

The use of a mixed oxygen ion-electronic conductor membrane for oxygen separation with direct reforming of methane, followed by the use of a mixed protonic-electronic membrane conductor for hydrogen extraction has also been proposed in the literature [34]. The products are thus pure hydrogen and synthesis gas with reduced hydrogen content, the latter suitable, for example, in the Fish-er-Tropsch synthesis of methanol [34]. 

More direct measurements of methane oxidation rates, particularly in wetlands and ocean waters, are needed. The use of stable isotope estimates of methane oxidation, which give an indication of total oxidation, should continue, but direct rate measurements using both " C-CH4 and H-CH4 should be a priority. Pulse-labeling experiments conducted through a growing season are needed to resolve the effect of plant phenology on methane emission. The methane oxidation threshold suggested by a number of open ocean rate measurements should be studied in open ocean samples from areas near and well removed from shelf vent sources. 

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