Methanol, reaction dissociation

Methanol reactions have also been studied on polycrystalline wafers of UO2 [76]. Two parent desorption states existed for methanol adsorbed at 90 K. Molecularly adsorbed methanol desorbed at 110 K, and methanol generated by surface recombination of methoxides and protons desorbed at 180 K. Carbon (Is) XPS demonstrated that methanol dissociatively adsorbed on the urania surface and that methoxide was the only surface intermediate present above 150 K. Primary reaction products were methane and carbon monoxide at 480 K. Oxygen atoms not removed from the surface as CO were incorporated into the oxide surface isotopically labeled U 02 surfaces did not exchange oxygen with methoxide to produce C 0 [76]. 

Bromination of methane is exothermic but less exothermic than chlorination The value calculated from bond dissociation energies is AH° = -30 kJ Al though bromination of methane is energetically fa vorable economic considerations cause most of the methyl bromide prepared commercially to be made from methanol by reaction with hydrogen bromide... 

This equihbrium favors COS up to ca 500°C. At higher temperatures, COS dissociates increasingly, eg, to 64% at 900°C. The reaction may be mn at 65—200°C to produce carbonyl sulfide if an alkaline catalyst is used (31). A Rhc ne-Poulenc patent describes the manufacture of carbonyl sulfide by the reaction of methanol with sulfur at 500—800°C (32). 

Only recently has a mechanism been proposed for the copper-cataly2ed reaction that is completely satisfactory (58). It had been known for many years that a small amount of carbon dioxide in the feed to the reactor is necessary for optimum yield, but most workers in the field beHeved that the main reaction in the formation of methanol was the hydrogenation of carbon monoxide. Now, convincing evidence has been assembled to indicate that methanol is actually formed with >99% selectivity by the reaction of dissociated, adsorbed hydrogen and carbon dioxide on the metallic copper surface in two steps ... 

The reversibility of aromatic diazotization in methanol may indicate that the intermediate corresponding to the diazohydroxide (3.9 in Scheme 3-36), i. e., the (Z)-or (is)-diazomethyl ether (Ar — N2 — OCH3), may be the cause of the reversibility. In contrast to the diazohydroxide this compound cannot be stabilized by deprotonation. It can be protonated and then dissociates into a diazonium ion and a methanol molecule. This reaction is relatively slow (Masoud and Ishak, 1988) and therefore the reverse reaction of the diazomethyl ether to the amine may be competitive. Similarly the reversibility of heteroaromatic amine diazotizations with a ring nitrogen in the a-position may be due to the stabilization of the intermediate (Z)-diazohydroxide, hydrogen-bonded to that ring nitrogen (Butler, 1975). However, this explanation is not yet supported by experimental data. 

The influence of the presence of sulfur adatoms on the adsorption and decomposition of methanol and other alcohols on metal surfaces is in general twofold. It involves reduction of the adsorption rate and the adsorptive capacity of the surface as well as significant modification of the decomposition reaction path. For example, on Ni(100) methanol is adsorbed dissociatively at temperatures as low as -100K and decomposes to CO and hydrogen at temperatures higher than 300 K. As shown in Fig. 2.38 preadsorption of sulfur on Ni(100) inhibits the complete decomposition of adsorbed methanol and favors the production of HCHO in a narrow range of sulfur coverage (between 0.2 and 0.5). 

When the selectivity of a reaction is controlled by differences in the way molecules are activated on different sites, the probability of the presence of different sites becomes important. An example again can be taken from the activation of CO. For methanation, activation of the CO bond is essential. This will proceed with low barriers at step-edge-type sites. If one is interested in the production of methanol, catalytic surfaces are preferred, which do not allow for easy CO dissociation. This will typically be the case for terrace sites. The selectivity of the reaction to produce methanol will then be given by an expression as in Eq. (1.29a) ... 

The mechanism is thought to involve dissociation of hydrogen, which reacts with molecularly adsorbed CO2 to form formate adsorbed on the surface. The adsorbed formate is then further hydrogenated into adsorbed di-oxo-methylene, methoxy, and finally methanol, which then desorbs. The reaction is carried out under conditions where the surface is predominately empty and the oxygen generated by the process is quickly removed as water. Only the forward rate is considered and the process is assumed to go through the following elementary steps ... 

From the results obtained with in situ reflectance spectroscopy and on-line analytical methods, investigators at Universite de Poitiers proposed a complete mechanism for the electrooxidation of methanol at a platinum electrode. The first step of the electrooxidation reaction is the dissociative adsorption of methanol, leading to several species according to the following equations ... 

Platinum is the only acceptable electrocatalyst for most of the primary intermediate steps in the electrooxidation of methanol. It allows the dissociation of the methanol molecule hy breaking the C-H bonds during the adsorption steps. However, as seen earlier, this dissociation leads spontaneously to the formation of CO, which is due to its strong adsorption on Pt this species is a catalyst poison for the subsequent steps in the overall reaction of electrooxidation of CHjOH. The adsorption properties of the platinum surface must be modified to improve the kinetics of the overall reaction and hence to remove the poisoning species. Two different consequences can be envisaged from this modification prevention of the formation of the strongly adsorbed species, or increasing the kinetics of its oxidation. Such a modification will have an effect on the kinetics of steps (23) and (24) instead of step (21) in the first case and of step (26) in the second case. 

A long series of studies of aromatic nucleophilic substitution included the kinetics of reactions of l-chloro-2,4-bis(trifluoromethylsulfonyl)benzene, 3-nitro-4-chlorophenyl trifluoromethyl sulfone and 2-chlorophenyl trifluoromethyl sulfone with sodium methox-ide or ammonia in methanol . The SO2CF3 group was found to have an enormous accelerating effect, in accord with the value of 1.65, based on the dissociation of anilinium ion. Further examples of the promotion of nucleophilic aromatic substitution by fluoro-substituted sulfonyl groups are given by Yagupol skii and coworkers . 

In reactions [5]-[8] pure electron addition occurs, but in reaction [9] addition and dissociative electron capture giving loss of MeO occur concurrently. Furthermore, CH3 radicals are also formed, together, presumably, with (Me0)2P02 this being an alternative dissociative route. Reaction [10] occurs in methanol, there being no clear sign of the parent anion, P(0Me)3 . This protonation step is also accompanied by dissociative electron capture to give P(0Me)2 radicals. 

As well as an acid-independent pathway, an acid-dependent pathway also occurs for the dissociation of a number of the alkali and alkaline earth metal cryptates (Table 7.1). Similar behaviour to that just discussed for methanol also occurs for reactions in a range of other non-aqueous solvents (Cox, Truong Schneider, 1984). 

Figure 8.4 Hypothetical reaction coordinate diagrams for CO hydrogenation on Pd and Ni the dissociation of CO is more difficult on Pd, making methanol synthesis more favorable than methane formation, which requires C-0 dissociation, and is the preferred pathway on Ni...

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