The Methane-Methanol System

The methane-methanol binary is another system where the EoS is also capable of matching the experimental data very well and hence, use of ML estimation to obtain the statistically best estimates of the parameters is justified. Data for this system are available from Hong et al. (1987). Using these data, the binary interaction parameters were estimated and together with their standard deviations are shown in Table 14.1. The values of the parameters not shown in the table (i.e., ka, kb, kc) are zero. 

Parameter Value Standard Deviation Objective Function 

Data for the carbon dioxide-methanol binary are available from Hong and Kobayashi (1988). The parameter values and their standard deviations estimated from the regression of these data are shown in Table 14.2. 

Note Explicit LS (Equation 14.16a) did not converge with a zero value for Marquardt s directional parameter. 

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Table 14.1 Parameter Estimates for the Methane-Methanol System...

Hong, J.H., Malone, P.V., Jett, M.D., and R. Kobayashi, "The measurement and Interpretation of the Fluid Phase Equilibria of a Normal Fluid in a Hydrogen Bonding Solvent The Methane-Methanol System", Fluid Phase Equilibria, 38,83-86(1987). 

Radically different binary phase behavior is found for the methane-TMB and the methane-methanol systems. This suggests that TMB can be extracted from methanol. To verify this conjecture experimental information was obtained on the TMB-methanol-methane system to ascertain whether the weak TMB-methanol complex can be broken by nonpolar methane. Interestingly, carbon dioxide, ethane, and ethylene, all much better supercritical solvents than methane, dissolve both methanol and TMB to such a large extent that they are not selective for either component. But with methane, the interactions between methane and TMB are strong enough to maintain a constant concentration of TMB in the extract phase as TMB is removed from the methanol-rich liquid phase. This means that the distribution coefficient for TMB increases as the concentration in the liquid phase decreases. We know of no other system that exhibits this type of distribution coefficient behavior. 

When the fit is judged to be excellent the statistically best interaction parameters can be efficiently obtained by performing implicit ML estimation. This was found to be the case with the methane-methanol and the nitrogen-ethane systems presented later in this chapter. 

Many investigators have actively studied the electrochemical reduction of C02 using various metal electrodes in organic solvents because these solvents dissolve much more C02 than water. With the exception of methanol, however, no hydrocarbons were obtained. The solubility of C02 in methanol is approximately 5 times that in water at ambient temperature, and 8-15 times that in water at temperatures below 0°C. Thus, studies of electrochemical reduction of C02 in methanol at —30°C have been conducted.148-150 In methanol-based electrolytes using Cs+ salts the main products were methane, ethane, ethylene, formic acid, and CO.151 This system is effective for the formation of C2 compounds, mainly ethylene. In the LiOH-methanol system, the efficiency of hydrogen formation, a competing reaction of C02 reduction, was depressed to below 2% at relatively negative potentials.152 The maximum current efficiency for hydrocarbon (methane and ethylene) formation was of 78%. 

To understand the role of these bacteria in methane cycling, the methane oxidation system must be studied. In methanotrophs, methane is oxidized to methanol by an enzyme called the methane monooxygenase (MMO) (I), which uses methane, molecular oxygen, and reducing equivalents to produce methanol and water. All known methanotrophs contain a membrane-bound MMO, called the particulate methane monooxygenase (pMMO). The presence of this enzyme system is correlated with the complex internal membrane system found in all known methanotrophs. 

Figure 8.20 Critical mixture curves for methanol with methane, ethane, ethylene, xenon, and carbon dioxide (Robinson, Peng, and Chung, 1985 Brunner, 1985 Francesconi, Lentz, and Franck, 1981). The P-T traces for the ethane, ethylene, xenon, and carbon dioxide systems are virtually indistinguishable in the region shown in the graph. But near —50°C the ethane-methanol critical mixture curve turns up abruptly. Not shown in this figure are the three phase lines exhibited by the ethylene-methanol and the ethane-methanol systems at conditions close to their respective critical points.

A topic of current interest is that of methane activation to give ethane or selected oxidation products such as methanol or formaldehyde. Oxide catalysts are used, and there may be mechanistic connections with the Fischer-Tropsch system (see Ref. 285). 

Reaction between carbon monoxide and dihydrogen. The catalysts used were the Pd/Si02 samples described earlier in this paper. The steady-state reaction was first studied at atmospheric pressure in a flow system (Table II). Under the conditions of this work, selectivity was 100% to methane with all catalysts. The site time yield for methanation, STY, is defined as the number of CH molecules produced per second per site where the total number of sites is measured by dihydrogen chemisorption at RT before use, assuming H/Pd = 1. The values of STY increased almost three times as the particle size decreased. The data obtained by Vannice et al. (11,12) are included in Table II and we can see that the methanation reaction on palladium is structure-sensitive. It must also be noted that no increase of STY occurred by adding methanol to the feed stream which indicates that methane did not come from methanol. 

H2PtIV(OH)6 as a stoichiometric oxidant to the catalytic reaction system at 150 °C. In this case, in addition to deuterated methane, methanol is also formed in quantitative yield relative to the added Pt(IV). This suggests that the functionalization step is faster than the oxidation step and leads to the proposal that (above 90 % sulfuric acid) the latter is the rate-limiting step. Below this concentration of acid solvent, studies suggest that the C-H activation step is the rate-limiting step. 

The basis of the high selectivity in this system, confirmed by both theoretical and experimental results, is that the active catalyst [XHg]+ reacts at least 1000 times faster with the C-H bonds of methane than with those of CH3OH, which exists primarily as the protonated, or sulfated forms, [CH3OH2]+ or CH30-S03H, respectively, in sulfuric acid. This greater reactivity of the methane C-H bonds compared with those of methanol can be traced to substantially lower reac-... 

This direct, oxidative condensation of methane to acetic acid in one-pot could be competitive with the current three-step, capital intensive process for the production of acetic acid based on methane reforming to CO, methanol synthesis from CO, and generation of acetic acid by carbonylation of methanol. Key improvements required with the PdS04/H2S04 system, however, will be to develop more stable, faster, and more selective catalysts. Although it is possible sulfuric acid could be utilized industrially as a solvent and oxidant for this reaction, it would be desirable to replace sulfuric acid with a less corrosive material. This chemistry has recently been revisited, verified, and extended by Bell et al., who used Cu(II)/02 as the oxidizing system [22],... 

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