Ethylene, methane conversion

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. 

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. 

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]. 

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 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. 

Model B was especially designed for methane conversion to ethylene [54, 55]. This reaction needs pre-heating to a defined temperature before reaction. This is achieved by ceramic heaters in the housing. In addition, the gases do not enter as a... 

Catalysts were prepared by incipient wetness impregnation of commercial supports using cobalt nitrate as a precursor. Metallic cobalt species were active centers in the ethanol steam reforming. Over 90% EtOH conversion achieved. Nature of support influences the type of byproduct formation. Ethylene, methane and CO are formed over Co supported on A1203, Si02 and MgO, respectively... 

Methane to Ethylene One target is to achieve an ethylene selectively of 90% at a methane conversion level of 50% in a single pass. Additionally, design of novel recycle reactors or membrane systems (to remove the ethylene produced) remain part of the active research. 

Methane Conversion. The results for the conversion of methane on praseodymium oxide are shown in Figure 1 and Table I. The major products were carbon monoxide, carbon dioxide, ethylene, and ethane both in the presence and absence of TCM in the feedstream while small amounts of formaldehyde and C3 compounds were detected. Water and hydrogen were also produced. The catalyst produced low methane conversion (ca. 6%) and selectivity to C2+ compounds (ca. 30%) in the absence of TCM in the feedstream. On addition of TCM the conversion of methane after 0.5 h on-stream was increased by almost two-fold (11.9%) and increased still further to 17.2% after 6 h on-stream. The selectivity to C2+ also increased with time on-stream to 43.3% after 6 h on-stream. It is noteworthy that over the 6 h on-stream with TCM present the ratio increased from 1.0 to 2.1. No methyl chloride was... 

A new catalyst formulation containing alkali metals and W on a silica support gives more promising results.549 Alkali metals are able to lower the phase transition temperature from amorphous silica to a-crystoballite, shown to be critically important for an effective catalyst, while incorporation of W enhances catalytic activity to ensure high methane conversion and excellent ethylene selectivity. An alkali-stabilized tungsten oxo species is thought to be the active site. 

A heterogeneous hydrogen-accumulating system containing porous Ti with 0.4 wt% Ni combined with high-purity Ti chips was tested for methane activation.588 Methane conversion to C1-C4 hydrocarbons reached a value of 20% with 50-55% of ethylene at 450° C and 10 atm. The hydrogen formed was accumulated as TiH2. 

Methane can be catalytically oxidized in the fuel cell mode to simultaneously generate electricity and C2 hydrocarbons by dimerization of methane using a yttria-stabilized zirconia membrane. A catalyst, used as the anode, is deposited on the side of the membrane that is exposed to methane and the cathode is coated on the other side of the membrane. When the catalyst Ag>Bi2C>3 is used as the anode for the reaction at 750> 900X and atmospheric total pressure, the selectivity to ethane and ethylene exceeds 90%. But this high selectivity is at the expense of low power output and low overall methane conversion (less than about 2%). 

Other steps used in the model assume that the heterogeneous conversion of methane is limited to the gas-phase availability of oxygen, O2 adsorption is fast relative to the rate of methane conversion, and heat and mass transports are fast relative to the reaction rates. Calculations for the above model were conducted for a batch reactor using some kinetic parameters available for the oxidative coupling of methane over sodium-promoted CaO. The results of the computer simulation performed for methane dimerization at 800 °C can be found in Figure 7. It is seen that the major products of the reaction are ethane, ethylene, and CO. The formation of methanol and formaldehyde decreases as the contact time increases. 

Ross and co-workers [9,10] have explored the influence of CO2 on the oxidative coupling of methane over the Li/MgO catalyst. They found that carbon dioxide in the gas phase lowers both the methane conversion and the yield of ethane/ethylene products. They also found that carbon dioxide significantly improves the stability of the catalyst against deactivation. Based on the observations of surface species from FTIRS and transient experiments. In addition, most of the observations and experimental results reported to date cover a limited range of methane to oxygen feed ratios. There is a need to study the reaction over a wide range of methane to oxygen ratios and to quantify the effects of carbon dioxide on the reaction rates. 

In all experiments, the major products were ethane and carbon dioxide. Under some conditions, ethylene and carbon monoxide were also observed. In the following, Rj is the C] products (CO2 and CO) formation rate, and R2 is the C2 products (C2H6 and C2H4) formation rate. The methane conversion is defined as (Rj+2R2)/CH4 in feed. The selectivity to C2 products is defined as 2R2/(Ri+2R2), while the C2 yield is defined as the product of conversion and selectivity. Our experimental results indicate that methane does react with carbon dioxide to produce carbon monoxide and either hydrogen or water under reaction conditions, but if oxygen is present, most of the carbon monoxide will be further oxidized to... 

Ethylene was added to the reaction mixture at partial pressures ranging from 0 to 0.01 atm. with different methane to oxygen ratios (i.e. CH4 02 = 4.7, 16.5 and 25) and at T =1023K. Figures 9 shows the methane conversion and C2 selectivity versus C2H4 partial pressure at CH4/O2 = 4.7, respectively. Both methane conversion and C2 selectivity decreased... 

The results in Fig. 2 indicate that the oxygen level in the feed gas is an important variable in the performance of the fixed-bed reactor. This is further demonstrated in Fig. 3 which shows, at a fixed W/F value of 1.5, the effect of oxygen level (between 1.1 and 9.4 vol% 02) on methane conversion, oxygen consumption and selectivity. The increased methane conversion and ethylene/ethane ratio achieved at the higher oxygen levels are obtained at the... 

The production of higher hydrocarbons directly from methane by catalytic oxidative coupling is a novel methane conversion process which warrants further study. When combined with an ethylene oligomerisation step it is a potential alternative to conventional processes, based on synthesis gas, for producing liquid fuels from methane. However, further research is necessary to provide the information required to assess the commercial prospects for this route. 

Figure 4 shows the transient response in the methane conversion with PbO/MgO. As seen in the figure, the oxidation-reduction cycle was repeated three times and the results were substantially the same. Ethane formation was predominant over all periods of the reaction and it lasted for 11 min. Very small amounts of ethylene and carbon dioxide were formed over a period of only 3 min. The initial CH conversion rate to C2 hydrocarbons (4.0 mmol/g h) was... 

We sought to investigate the catalyst longevity, stability and performance at ambient temperature and moderate pressures, and to explore the effects of likely co-feeds from potential upstream methane conversions. In this context, butenes are convenient and appropriate substrates for such a survey. In particular, the use of butenes in catalyst lifetime and stability studies avoids the high costs and potential hazards of large reagent volumes which attend the use of the more reactive lower olefins. Moreover, butenes are likely to be derived either directly or indirectly (e.g., from ethylene) from the conversion of methane. 

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