Methane conversion

Ci chemistry can no longer be equated only with syngas chemistry. Nature s own C02 photosynthesis and bacterial methane conversion are also Ci conversion processes. We are far from approaching these processes for practical synthetic use efficiently. Production of methane from carbon dioxide (similarly to carbon monoxide) and hydrogen is a feasible process (methanation).80 Similarly, reduction of carbon dioxide with hydrogen to methyl alcohol81 can be readily carried out, and the method has been industrially developed  

More recently, direct catalytic oxidative condensation of methane to ethane (with metal oxides),57,82-84 as well as to ethylene and acetylene (via high-temperature chlorinative conversion) was explored.76 In all these processes, however, a significant portion of methane is lost by further oxidation and soot formation. The selectivity in obtaining ethane and ethylene (or acetylene), respectively, the first C2 products, is low. There has, however, been much progress in metal-oxide-catalyzed oxidative condensation to ethane. 

Another chemical approach to the chemical conversion of methane involves organometallic reactions.85-89 Interesting work with iridium complexes and other transition metal insertion reactions (rhodium, osmium, rhenium, etc.) were carried out. Even iron organometallics were studied. These reactions take place in the coordination spheres of the metal complexes, but so far the reactions are stoichiometric and noncatalytic.77 In terms of synthetic hydrocarbon chemistry, these conversions are thus not yet practical, but eventually it is expected that catalytic reactions will be achieved. 

The third alternative approach of methane conversion is via electrophilic reactions.77 The electrophilic conversion of methane is based on the feasibility of electrophilic reactions of single bonds and thus saturated hydrocarbons.90 Both C—H and C—C bonds can act as electron donors against strongly electrophilic reagents or superacids. Olah s studies showed that even methane is readily protonated or alkylated under these conditions. Methane with SbF5-containing superacids was found to undergo condensation to C2-C6 hydrocarbons at 50-60°C. 

Combining two methane molecules to ethane and hydrogen is endothermic by some 16 kcal/mol  

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Direct Methane Conversion, Methanol Fuel Cell, and Chemical Recycling of Carbon Dioxide... 

Gross heating value of biomass or methane. Conversion of biomass or methane to another biofuel requires that the process conversion efficiency be used to reduce the potential energy available. These figures do not include additional biomass from dedicated energy plantations. 

The direct methane conversion technology, which has received the most research attention, involves the oxidative coupling of methane to produce higher hydrocarbons (qv) such as ethylene (qv). These olefinic products may be upgraded to Hquid fuels via catalytic oligomerization processes. 

S. Narula, "Economic Prospects for Methane Conversion Technologies", HIChE 1989 SpringMeeting Houston, Tex. 

In addition to these principal commercial uses of molybdenum catalysts, there is great research interest in molybdenum oxides, often supported on siHca, ie, MoO —Si02, as partial oxidation catalysts for such processes as methane-to-methanol or methane-to-formaldehyde (80). Both O2 and N2O have been used as oxidants, and photochemical activation of the MoO catalyst has been reported (81). The research is driven by the increased use of natural gas as a feedstock for Hquid fuels and chemicals (82). Various heteropolymolybdates (83), MoO.-containing ultrastable Y-zeoHtes (84), and certain mixed metal molybdates, eg, MnMoO Ee2(MoO)2, photoactivated CuMoO, and ZnMoO, have also been studied as partial oxidation catalysts for methane conversion to methanol or formaldehyde (80) and for the oxidation of C-4-hydrocarbons to maleic anhydride (85). Heteropolymolybdates have also been shown to effect ethylene (qv) conversion to acetaldehyde (qv) in a possible replacement for the Wacker process. 

At 400—700°C, equihbrium exceeds 99.9% (24). About 5—10% excess sulfur is usually maintained in the reaction mixture to promote high methane conversion and to minimize by-product yield. Carbon disulfide is also formed by the following reaction that is 80% complete at equihbrium at 700°C (47) ... 

M. Baems, "Progress in Methane Conversion Science—Technology—Economics," paper presented at SPUNG Seminar, Norway, Sept. 24—25, 1991. 

Figure 3 illustrates the shift and methanation conversion. The resulting methane is inert and the water is condensed. Thus purified, the hydrogen-nitrogen mixture with the ratio of 3H2 pressed to the pressure selected for ammonia synthesis. 

Figure 8.23. Effect of catalyst potential and work function on the rate of CH4 oxidation to C02 on Pt for a low (1 1) CH4 to 02 feed ratio. Maximum methane conversion is 4%. pCH4= Po2as2 kPa, T, °C, r0, mol O/s.29 Reprinted with permission from Academic Press.

It is known32 reported that the solid electrolyte itself, i.e. Y203-doped-Zr02, is a reasonably selective catalyst for CH4 conversion to C2 hydrocarbons, i.e., ethane and ethylene32 and this should be taken into account in studies employing stabilized Zr02 cells. At the same time it was found54 that the use of Ag catalyst films leads to C2 selectivities above 0.6 for low methane conversions. 

As shown on Fig. 8.49 one can influence dramatically both the total CH4 conversion as well as product selectivity by varying the Ag catalyst potential. Thus under open-circuit conditions (Uwr=U r ) the CH4 conversion is near 0.02 with a C2 selectivity (methane molecules reacting to form C2H4 and C2H6 per total reacting CH4 molecules) near 0.5. Increasing Uwr increases the methane conversion to 0.3 and decreases the selectivity to 0.23, while decreasing Uwr decreases the conversion to 0.01 and increases the... 

The feed gas (CH4) and all gaseous products were analyzed by gas chromatography (GC) and the methane conversion and hydrogen yield were calculated from the area of each peak. The results with respect to methane flow rate are shown in Fig 3. The methane conversion was defined as ... 

Fig. 3 Methane conversion and hydrogen yield as a function of methane flow rates...

The methane conversion and hydrogen yield were investigated as a function of with respect to methane flow rate and both of the two were very high more than 90%. Particle size and sinface area of synthesized carbon were strongly dependent on methane flow rate. Hydrogen produced finm thermal plasma can be applied to fuel cell due to its high purity and carbon black can be applied for the synthesis of rubber industry. 

The effect of catalyst supports on methane conversions and hydrogen yield in the methane decomposition at 998 K and GHSV of2700 h at steady state. 

The effect of metal oxide-Ti02 catalysts on methane conversions and hydrogen y ... 

Volume 36 Methane Conversion. Proceedings of a Symposium on the Production of Fuels and... 

Keller and Bhasin were first to report in 1982 [1] on the catalytic one-step oxidative dimerization or coupling of methane (OCM) to C2 hydrocarbons, ethane and ethylene. Numerous investigations have followed this seminal work and a large number of catalysts have been found which give total selectivity to C2 hydrocarbons higher than 90% at low (<2%) methane conversion [2-6]. 

The reason for the low C2 selectivity values at high methane conversion and thus the reason for the low measured Ycg and YC2H4 yield values of earlier... 

On-line GC analysis (Shimadzu GC 14A) was used to measure product selectivity and methane conversion. Details on the analysis procedure used for batch and continuous-flow operation are given elsewhere [12]. The molecular sieve trap was found to trap practically all ethylene, COj and HjO produced a significant, and controllable via the adsorbent mass, percentage of ethane and practically no methane, oxygen or CO, for temperatures 50-70 C. The trap was heated to -300°C in order to release all trapped products into the recirculating gas phase (in the case of batch operation), or in a slow He stream (in the case of continuous flow operation). 

Figure 2. Effect of methane conversion and applied current on the C2 hydrocarbon (a) and on the ethylene (b) selectivity (filled sjnmbols) and yield (open symbols). (Reprinted with permission from the AAAS, ref. 12).

Interestingly, the ethylene selectivity can increase with increasing methane conversion. This is because of the predominantly consecutive nature of the OCM reaction network ... 

Figure 4. Effect of methane conversion for 1=5 mA on ethylene, ethane and total Cg hydrocarbon selectivity and yield. Lines from kinetic model discussed below. Solid lines CgH j and C2Hg Dashed lines C2...

The ethylene selectivity (Fig. 5) and thus the ethylene yield depend strongly on the adsorbent mass (Fig. 5). For fixed catalyst mass, oxygen supply I/2F and methane conversion there is an optimal amount of adsorbent for maximizing ethylene selectivity and yield (Fig. 5). Excessive amounts of adsorbent cause quantitative trapping of ethane and thus a decrease in ethylene yield according to the above reaction network. This shows the important synergy between the catalytic and adsorbent units which significantly affects the product distribution and yield. 

Figure 5. Effect of adsorbent mass in the molecular sieve trap on the ethylene, ethane and total C2 selectivity at a fixed methane conversion of 15%. Recirculation flowrate 220 cm3 STP/min...

Figure 6 shows typical results obtained with the plug-flow quartz reactor containing 0.5 g of Sr(lwt%)/La203 catalyst operated in the continuous flow recycle mode. The inlet CH partial pressure was 20 kPa (20% CH in He) at inlet flowrates of 7.1 and 14.3 cm STP/min. A 20% O2 in He mixture was supplied directly, at a flowrate Fog, in the recycle loop via a needle valve placed after the reactor (Fig. 1). The methane conversion was controlled by adjusting Fog, which was kept at appropriately low levels so that the oxygen conversion...

Figure 6. Continuous flow steady-state operation (a) Effect of oxygen stream flowrate on C2 selectivity and yield (b) corresponding effect of methane conversion on the selectivity and yield of C2H4 and C2He Catalyst Sr/LaaOa T=750°C recirculation flowrate 200 cm3/min.

Figure 6a shows the effect of F02 on the C2 selectivity and yield. The C2 yield is up to 53%. Figure 6b refers to the same experiments and shows the corresponding elBfect of CH4 conversion on the selectivity and yield of ethylene and ethane. The ethylene yield is up to 50% (65% ethylene selectivity at 76% methane conversion). To the best of our knowledge this is the maximum ethylene yield obtained for the OCM reaction under continuous-flow steady-state conditions. 

Figure 7. Effect of methane conversion on C2 selectivity for some of the best state-of-the-art OCM catalysts (A, based on ref 4), the simulated chromatographic reactor of Aris and coworkers (A, ref. 10) and the present work. ( ) Ag electrocatalyst, single pass (O) Ag electrocatalyst with recycle and trapping (0) Sr/LagOg catalyst, single pass ( ) Sr/La20g catalyst with recycle and trapping. Open symbols, batch operation filled symbols, continuous-flow steady-state operation.

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. 

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