Conversion of Methane to Methanol

The direct conversion of methane into methanol, hydrocarbons, or oxygenated fuels has attracted a great deal of interest. At the beginning of the 20th century, one of the first patents was granted in 1905 to Lance and Elworthy [165]. These inventors claimed that oxidizing methane with hydrogen peroxide in the presence of ferrous sulfate could form methanol, formaldehyde, and formic acid. 

Unsupported two-component oxide systems were used by Stroud in 1975 [169]. In their composition, the first component was preferably molybdenum oxide and the second cupric oxide (i.e., Mo03- CuO). The reaction conditions were 20 bar and 485°C, and the yield was 490 g/kg-cat/hr of oxygenated products, including methanol, formaldehyde, ethanol, and acetaldehyde. The work by Stroud used oxygen as the oxidant. Liu et al. [170] used nitrous oxide as the oxidant at 1 bar over the 1.7% Mo/Si02 catalyst. A combined selectivity of 84.6% towards methanol and formaldehyde was obtained with a conversion of 8.1%. They also used a different catalytic system of 1.7% Mo03 supported on Cab-O-SilM-5 silica. Their kinetic study obtained a power law rate expression of the Arrhenius plot for CH4 concentration was 

Khan and Somorjai [171] successfully reproduced the Liu s results, and were able to specify the rate expressions distinctly for formation of methanol and formaldehyde. The rates were measured over 480—590°C. The activation energies for the methanol formation and the formaldehyde formation were 172 17 kJ/mol (below 520°C) and 344+ 17 kJ/mol (below 540°C), respectively. Zhen et al. [172] also showed that silica-supported vanadium pentoxide catalyst resulted in up to 100% selectivity to methanol and formaldehyde at 460°C with the conversion of 0.2%. They also showed that the Y C /SiC system was more active for methanol production than Mo03/Si02 system however, the product selectivity was generally poorer. 

Metal-based catalysts also were used for methane oxidation. Especially over metals such as platinum and palladium, trace amounts of methanol, formaldehyde, and formic acid can be found. Organic halides increased the yield of partial oxidation products and inhibited the complete combustion of methane [173]. Inhibition effects of dichloromethane was observed. Mann and Dosi [174] used a Pd/Al203 catalyst and found that the addition of halogen compounds reduced the conversion of methane in the following order  

They also were able to show from the X-ray powder diffraction of unused and used catalysts that the addition of halogen modifier actually increased the amount of crystalline PdO in the catalyst system. 

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In 1990, Schroder and Schwarz reported that gas-phase FeO" " directly converts methane to methanol under thermal conditions [21]. The reaction is efficient, occuring at 20% of the collision rate, and is quite selective, producing methanol 40% of the time (FeOH+ + CH3 is the other major product). More recent experiments have shown that NiO and PtO also convert methane to methanol with good efficiency and selectivity [134]. Reactions of gas-phase transition metal oxides with methane thus provide a simple model system for the direct conversion of methane to methanol. These systems capture the essential chemistry, but do not have complicating contributions from solvent molecules, ligands, or multiple metal sites that are present in condensed-phase systems. 

The mechanism that has been developed for the conversion of methane to methanol by FeO+ is an excellent example of the synergy between experiment and theory. This mechanism includes two key concepts concerted reaction involving the critical [HO—Fe—CH3] insertion intermediate and two-state reactivity. The reaction proceeds as follows electrostatic interaction between FeO+ and methane produces the [OFe- GHJ entrance channel complex. 

F. Aguirre, Electronic spectroscopy of Au CH2 and intermediates involved in the conversion of methane to methanol by FeO Ph.D. Dissertation, Dept, of Chemistry. University of Massachusetts, 2002. 

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... 

Yoshizawa, K., Shiota, Y., Yamabe, T., 1999, Intrinsic Reaction Coordinate Analysis of the Conversion of Methane to Methanol by an Iron-Oxo Species A Study of Crossing Seams of Potential Energy Surfaces , J. Chem. Phys., Ill, 538. 

Weak M-O ions such as in [PtO]+ may be substituted or the metal reduced to the bare metal ion. The ion [PtCH2]+ was produced by the reaction of Pt+ with methane. The reaction of [PtCH2]+ with oxygen gave [PtO]+, which reacted with methane to give [PtCH2]+ and Pt+. These reactions may form a catalytic cycle for the conversion of methane to methanol (160). 

R. Raja, and R. Ratnasamy, Direct conversion of methane to methanol, Appl. Catal. A158, L7-L15... 

An ozone-sensitized oxidative conversion of methane to methanol has been reported.54 A double-layered Sr on La203 then M0O3 on a silica catalyst bed exhibited significantly higher yields of formaldehyde from a methane-air mixture than did M0O3 on silica alone.55... 

Gas-phase oxidation of methane could be enhanced by the addition of a small amount of NO or N02 in the feed gas.1077 Addition of methanol to the CH4-02-N02 mixture results in a further increase in methane reactivity.1078 Photocatalytic conversion of methane to methanol is accomplished in the presence of water and a semiconductor photocatalyst (doped W03) at 94°C and atmospheric pressure.1079 The yield of methanol significantly increased by the addition of H202 consistent with the postulated mechanism that invokes hydroxyl radical as an intermediate in the reaction. 

Other bidentate N-heterocyclic carbenes were used to form stable chelate complexes. A fine example is the use of palladium NHC complex 24 in the catalytic conversion of methane to methanol (Fig. 10) [111]. In this case the stability of the complexes is a requirement, since the reaction takes place in an acidic medium (trifluoroacetic acid) at elevated temperatures (80 °C) mediated by strong oxidizing agents (potassium peroxodisulfate). 

Fig. 4.45 Approaches for selective conversion of methane to methanol using 02 or H202 as oxidants.

Palladium(II) catalysts, such as PdCl4 in trifluoroacetic acid solvent, have been used for the conversion of methane to methanol (7i). The system uses hydrogen peroxide as oxidant the overall reaction involving the addition of one mole of water is shown in Eq. 26. The function of the... 

Oxidation-reduction reactions Many organic compounds can be converted to other compounds by oxidation and reduction reactions. For example, suppose that you wish to convert methane, the main constituent of natural gas, to methanol, a common industrial solvent and raw material for making formaldehyde and methyl esters. The conversion of methane to methanol may be represented by the following equation, in which [O] represents oxygen from an agent such as copper(II) oxide, potassium dichromate, or sulfuric acid. 

The activation of C-H bonds is one of the elementary steps in chemistry. Intensive research has lead to homogeneous as well as heterogeneous systems which can activate the strong C-H bonds (cf. Section 3.3.6). There are numerous experimental studies which have more recently often been accompanied by theoretical calculations. The two best known examples for the activation of methane are the so-called Shilov system K2PtCl4 [1], which was one of the first systems reported, and the [Pt(bpym)Cl2] system of Periana, which is currently the most active system reported for the direct, low-temperature, oxidative conversion of methane to methanol by platinum salts such as dichloro( /-2-[2,2 -bipyrimidyl])platinum(II) [Pt(bpym)Cl2] with yields of more than 70% and a selectivity of 80% [2]. 

Interestingly, unlike ethane, neither methane nor propane is able to participate in this reaction sequence, the former because the C-H bond of methane is too strong to undergo significant hydrogen-atom abstraction by the Rf. radical and the latter because only primary alkyl radicals are sufficiently reactive to attack (R C0)20. Thus, Sen s observation of Pd(II)-catalyzed conversion of methane to methanol derivative by H202 in trifluoroacetic acid/anhydride mixture was not complicated by the above reaction [26,29]. 

Wang, Y., Otsuka, K. and Ebitani, K. (1995) In situ FTIR study on the active oxygen species for the conversion of methane to methanol. Catal. Lett., 35, 259-263. 

Other applications have been proposed but as yet without success, like the direct coupling of methane to ethylene or the direct conversion of methane to methanol. Recently, some disclosures were made on the reaction of methane with alkanes to yield augmented alkanes. ... 

To promote both the conversion of reactants and the selectivity to partial oxidation products, many kinds of metal compounds are used to create catalytically active sites in different oxidation reaction processes [4]. The most well-known oxidation of lower alkanes is the selective oxidation of n-butane to maleic anhydride, which has been successfully demonstrated using crystalline V-P-O complex oxide catalysts [5] and the process has been commercialized. The selective conversions of methane to methanol, formaldehyde, and higher hydrocarbons (by oxidative coupling of methane [OCM]) are also widely investigated [6-8]. The oxidative dehydrogenation of ethane has also received attention [9,10],... 

This paper reviews research and development In several areas of methanol synthesis technology. Alternatives to the co-precipitated Cu-ZnO-A O and Cu-ZnO-CrgOj catalysts are considered first. Novel processes for syngas conversion are then reviewed, and the paper ends with a discussion of direct conversion of methane to methanol by partial oxidation of natural gas. 

The conversion of methane to methanol and higher alcohols is gaining more and more interest with respect to the production of chemicals and fuels. As natural gas will become the predominant fossil fuel, and crude oil will have passed its maximum market share at the beginning of next century (refs. 1,2), the demand for chemicals and fuels derived from natural gas will increase. Alcohols can be used either as a chemical feedstock or as a liquid secondary energy carrier, depending on the selectivity at which they can be produced. The work at our institute focusses on the production and use of alcohols as synthetic liquid energy carriers (ref. 3), because of their... 

DIRECT CONVERSION OF METHANE TO METHANOL AND HIGHER HYDROCARBONS... 

Direct Partial Oxidation to Methanol. - The direct partial oxidation (DPO) route offers the advantage of direct conversion of methane to methanol. The oxidants include air, oxygen, and nitrous oxide. Recently, Pitchai and Klier, Gesser et al., Foster, Scurrell, and Kuo gave comprehensive reviews on this process. Catalytica Associates also discussed this subject in a client-private report. 

University of Manitoba, Canada, has published [3.27] results of laborattuy tests on direct conversion of methane to methanol. Pressures applied range from 30 to 70 bar, temperatures from 300 to 430° C. At space velocities comparable to those of commercialized processes, the conversion rate is between 4 and 10% of the natural gas feed only, and methanol selectivity is between 70 and 90%, the balance being mainly CO2. As oxidant, pure oxigen has been proposed. 

The seak for cheaper feedstocks has made the researches and industrials to look the possibilities of C1-C4 alkanes. Much work is being done on the conversion of methane to methanol, ethane and ethylene of ethane to vinil chloride, and propane to acrylonitrile. Butane has been already successful oxidized to maleic anhydride. Zeolites can also contribute to upgrade C1-C4 paraffins through the reactions presented in Scheme 1. 

With this equation, we can calculate an equilibrium constant if we know the standard free energy change for the desired reaction. As we learned in Section 10.7, we can often obtain that from tabulated data. Example Problem 12.15 applies Equation 12.7 to the conversion of methane to methanol. 

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