Methanol, production thermodynamics

Since other possible transformations, such as, formation of dimethyl ether, higher alcohols, and hydrocarbons, are accompanied with higher negative free-energy change, methanol is thermodynamically a less probable product. Therefore, solely on a thermodynamic basis, these compounds as well as methane should be formed in preference to methanol. To avoid the formation of the former compounds, the synthesis of methanol requires selective catalysts and suitable reaction conditions. Under such conditions, methanol is the predominant product. This indicates that the transformations leading to the formation of the other compounds are kinetically controlled. In the methanol-to-hydrocarbon conversion, dimethyl ether generally is converted similarly to methanol. 

More recently, the use of a pyridinium mediator in an aqueous p-GaP photo-electrochemical system illuminated with 365 nm and 465 nm light has been reported [125], In this case, a near-100% faradaic efficiency was obtained for methanol production at underpotentials of 300-500 mV from the thermodynamic C02/methanol couple. Moreover, quantum efficiencies of up to 44% were obtained. The most important point here, however, was that this was the first report of C02 reduction in a photoelectrochemical system that required no input of external electrical energy, with the reduction of C02 being effected solely by incident fight energy. 

Most of the efficiency loss in methanol production occurs in the reformer (]). This is because high grade fuel must be burned to supply the reforming heat load and combustion is a thermodynamically inefficient process. 

The converged mass and heat balances and the exergy loss profiles produced by the Aspen Plus simulator can help in assessing the thermodynamic performance of distillation columns. The exergy values are estimated from the enthalpy and entropy of the streams generated by the simulator. In the following examples, the assessment studies illustrate the use of exergy in the separation sections of a methanol production plant, a 15-component two-column... 

As shown in Fig. 1.25, to calculate the production of methanol at thermodynamic equilibrium, in accordance with temperature and pressure conditions, use can be made of the expressions of the equilibrium constant as a function of these parameters, le. ... 

Reduction of carbon dioxide can produce a wide variety of possible products. Thermodynamically, the most stable product is methane, but products of intermediate oxidation state such as methanol, methanal, formate, oxalate, carbon monoxide, and elemental carbon are all possibilities (20, 21). 

Methanol Production. Kinetics Surface Science, and Mechanisms Methanol production from CO, CO2, and H2 is an industrial process that yields about 3 X 10 kg per day. The relevant thermodynamic parameters for the two reactions are [127]... 

Copper catalysts are very sensitive to poisonous compounds, especially when they are used in low-temperature processes, because adsorption of poison is thermodynamically favored. The significant poisons for copper catalysts in methanol production are sulfur and chlorine. Sulfur compounds - for example, H2S - form copper sulfides ... 

Methanol synthesis served as the model for the true mechanism. Stoichiometry, thermodynamics, physical properties, and industrial production rates were all taken from the methanol literature. Only the reaction mechanism and the kinetics of methanol synthesis were discarded. For the mechanism a four step scheme was assumed and from this the... 

A portion of the product was heated to reflux with methanolic sodium methoxide to convert it into the thermodynamic mixture of trans- (ca. 65%) and cis- (ca. 35%) isomers. Small amounts of the isomers were collected by preparative gas chromatography using an 8 mm. by 1.7 m. column containing 15% Carbowax 20M on Chromosorb W, and each isomer exhibited the expected spectral and analytical properties. The same thermodynamic mixture of isomers was prepared independently by lithium-ammonia reduction5 of 2-allyl-3-methyl-cyclohex-2-enone [2-Cyclohexen-l-one, 3-methyl-2-(2-propcnyl)-],6 followed by equilibration with methanolic sodium methoxide. 

The usual practice for avoiding the plugging of production facilities by hydrates is to add thermodynamic inhibitors, such as methanol or glycol. A newer concept is the injection of low-dosage additives either kinetic inhibitors, which delay nucleation or prevent the growth of hydrate crystals, or hydrate dispersants, which prevent the agglomeration of hydrate particles and allow them to be transported within the flow [880,1387]. Hydrate control is discussed extensively in Chapter 13. Classes of hydrate control agents are shown in Table 11-9, and additives are shown in Table 11-10. 

The endo isomer was the major product, but the proportion of the endo isomer decreased with temperature, from approximately 90% at 0 °C to 85 % at 60 °C [69]. Ge-dye et al. [71] performed the reaction in methanol solution under MW heating in a closed Teflon container and found that the product contained 79% of the endo isomer at an estimated temperature of 110 °C. A plot of temperature versus percent endo isomer is effectively linear between 0 °C and 60 °C and assuming it remains linear to 110°C, the product should contain 80% endo isomer at this temperature. Thus it was concluded that the change in product composition was due to the change in temperature rather than to some special effect of MW. At higher temperatures there is an increase in the proportion of the exo isomer, which is thermodynamically more stable than the endo isomer. 

A value of kjkp = 17 000 has been determined for partitioning of the acetophenone oxocarbenium ion [12+] in water.15,16 It is not possible to estimate an equilibrium constant for the addition of water to [12+], because of the instability of the hemiketal product of this reaction. However, kinetic and thermodynamic parameters have been determined for the reaction of [12+] with methanol to form protonated acetophenone dimethyl ketal [12]-OMeH+ and for loss of a proton to form a-methoxystyrene [13] in water (Scheme 10).15,16 Substitution of these rate and equilibrium constants into equation (3) gives values of AMeoH = 6.5 kcal mol-1 and Ap = 13.8 kcal mol-1 for the intrinsic... 

Methanol still proceeds through an initial C H bond scission, but reacts with water before the OH bond breaks. Alternatively, formaldehyde formation likely occurs along the same pathway as CO formation. This is true if HCO is an intermediate in the decomposition pathway. Furthermore, the lack of a kinetic isotope effect for CH3OD indicates that formaldehyde is not the product of an initial O-H scission.94 Because formaldehyde and formic acid are not the thermodynamically favored products of methanol oxidation, they must be the result of kinetic limitations preventing the full oxidation to C02, analogous to the production of H202 for the reduction of oxygen (see next section). 

To support the assumption of a kinetic or thermodynamic control, it has been underlined that treatment of a solution of 4-iodo-3-methoxy-l-butene with an etheral solution of HBF4, in acetonitrile, benzene or methanol, affords the corresponding 1,4-iodofunctiona-lized substrates (Nu = NHCOCH3, C6H5, or OCH3) as the major product (equation 61). 

A particularly interesting Michael acceptor is dimethyl 2-hexen-4-ynedioate since it can react at either position of the double or triple bond to form 1,4- or 1,6-addition products. Winterfeldt and Preuss183 treated this substrate with several secondary amines and observed exclusive attack at C-5 with formation of the 1,6-addition products (equation 78). In contrast to this, sodium methanolate added at CM to give the 1,4-adduct as a mixture of E/Z isomers (equation 79) with increasing reaction time, the product distribution was shifted towards the thermodynamically more stable , -product184. Acheson and... 

A catalytic example of C-S bond breakage in benzothiophene has been reported by Bianchini [47], A catalytic desulfurisation was not yet achieved at the time as this is thermodynamically not feasible at such mild temperatures because of the relative stability of metal sulfides formed. Bianchini used a water-soluble catalyst in a two-phase system of heptane-methanol/water mixtures in which the product 2-ethylthiophenol is extracted into the basic aqueous layer containing NaOH. Figure 2.43 gives the reaction scheme and the catalyst. The 16-electron species Na(sulfos)RhH is suggested to be the catalyst. Note that a hydrodesulfurisation has not yet been achieved in this reaction because a thiol is the product. Under more forcing conditions the formation of H2S has been observed for various systems. 

The allylic cation (40), formed in a specific acid-catalysed process, is relatively stable thermodynamically, stable enough towards trapping by nucleophiles that the reaction product obtained is almost invariably the naphthalene elimination product. di-Enediynes (42) are formed regiospecifically when the allylic cation (41) is trapped as shown. The walking of methanol around optically active l-methyl-3-ethylallyl... 

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