Direct Methanol Formation

Direct oxidation of methane to methanol is an obvious dream reaction  

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In complexes where the carbomethoxy or acyl group is adjacent to the metal-complexed acyl group (e.g., 145.a), photolysis affords a 1 1 mixture of isoprenic diene complexes (E-152 and Z-152) directly. The formation of an intermediate allylketene complex (153) may be demonstrated by methanol trapping, followed by aerial oxidation to quantitatively yield the diester 154. 

To achieve, then, high acetic acid selectivity directly from synthesis gas (eq. 1) it is necessary to balance the rates of the two consecutive steps of this preparation - ruthenium-carbonyl catalyzed methanol formation (10) (Figures 2 and 5) and cobalt-carbonyl catalyzed carbonylation to acetic acid (Figure 6) - such that the instantaneous concentration of methanol does not build to the level where competing secondary reactions, particularly methanol homologation (7, H), ester homologation (12, 13), and acid esterification (1 ), become important. 

Because of the pure performance of traditional Cu catalysts in the hydrogenation of C02, efforts have been made to find new, more effective catalysts for direct C02 hydrogenation. The problem is to improve selectivity, specifically, to find catalysts that display high selectivity toward methanol formation and, at the same time, show low selectivity in the reverse water-gas shift reaction, that is, in the formation of CO. It appears that copper is the metal of choice for methanol synthesis from C02 provided suitable promoters may be added. Special synthesis methods have also been described for the preparation of traditional three-component Cu catalysts (Cu-ZnO-A1203 and Cu-Zn0-Cr203) to improve catalytic performance for C02 reduction. 

Thus, a predominantly new means of formaldehyde production by direct methanol oxidation with hydrogen peroxide under homogeneous conditions without methanol formation stage was suggested. 

Some indications of what may be happening are gained from analysis of the acetic acid terminated products at various reaction times (Fig. 18). With butyllithium initiation a large amount of methanol (based on initiator) can be isolated at very short reaction times, whereas in the diphenylhexyllithium initiated process only a slower and smaller methanol formation is observed. Now this must correspond to lithium methoxide formed either directly in step (20), or in the termination of product (20), or from the cyclization reaction, or from termination of active chains [177] in reaction (21),... 

These authors consider the increase of the methanol formation rate to result from the direct contribution of electrons and positive holes produced by gamma irradiation in the solid. The free carriers are able to modify the adsorption equilibria of Ha and CO, because these reactants, according to the authors, are adsorbed as ions on the surface. They consider that the observed unit G may be explained by admitting 20% of the electrons produced by radiation in the solid to be effective for catalytic reaction, 20 e.v. being necessary for the production of one electron. 

When palladium salts are used for methanol oxy-carbonylation to DMC, reaction conditions are milder than using copper only however, methanol and CO selectivities are lower owing to the formation of DM0 as a by-product and to the higher ratio between CO2 and DMC production rates. Despite the large amount of work on the catalytic systems, no process based on gas-phase direct methanol oxy-carbonylation to DMC has been established. 

It is interesting to note that some of the earliest claims to processes capable of yielding oxygenated organic compounds from water gas mixtures included the formation of acids and esters. Indeed, it is probable that the subsequent success attained by the leaders in the field in directing the reactions exclusively to methanol formation served as a stimulus to those who had hopes of forming acids and esters directly. 

As mentioned earlier in section 2.2, a two-step mechanism via intermediate formation of methanol has been proposed by Adebajo et al, [21-23, 26, 36] for the oxidative mcthylation of benzene with methane over acidic zeolites in a high-pressure batch reactor. In view of this mechanism, a preliminary investigation has been carried out by these workers [24] on the reaction of toluene with methane over acidic ZSM-5 catalysts in a batch reactor containing residual air to determine the actual contribution of direct mcthylation (via intermediate methanol formation) to the observed reaction products. The reactions were carried out at 400 C and 6.9 MPa pressure. The major reaction products obtained by these wwkers were benzene and xylenes. Smaller amounts of ethylbenzene, trimethylbenzene and other higher aromatics were also produced. Over acidic catalysis, the conversion of toluene can, in principle occur through two different reaction pathways mcthylation by methane via methanol (as in the case of benzene mcthylation) and disproportionation, as shown in equations (4) and (S) below ... 

Min et al. [35] experimented on high-catalyst loading with 60% carbon and 40% Teflon backing claimed to be the most efficient electrode for direct methanol/proton exchange membrane fuel cell (PEMFC). The catalysts used were platinum and ruthenium which formed an alloy at an atomic ratio 1 1. The formation of the alloy was seen in XRD as there were no pure metal peaks found. The alloy formation of Pt and Ru promotes oxidation of methanol at lower temperatures. The 60% carbon backing makes it evident that the lower the percentage of carbon increases the efficiency. 

Dimethylether. Several strategies for the production of dimethyl ether (DME) are described, e.g. direct synthesis from syngas according to equation (8.5) or via dehydration of methanol according to equation (8.6). From a mechanistic point of view direct synthesis proceeds also via methanol formation and subsequent release of water but without procedural isolation of methanol. The process can also be designed to yield both methanol and DME. Established methanol catalysts are employed for methanol formation and typical dehydration catalysts are solid-acid catalysts, e.g. alumina, silica-, phosphorus- or boron-modified alumina, zeolite, (sili-co)aluminophosphates, tungsten-zirconia or sulfated-zirconia. " ... 

Whereas uniform distribution of water within the membrane is desired, the permeability of the material to reactants (i.e., hydrogen or methanol and oxygen) has to be low to prevent direct chemical reaction between fuel and oxidant, which may lead to hotspots and, eventually, pinhole formation. Methanol permeability is a major challenge in the direct methanol fuel cell (DMFC), largely because methanol transport is strongly correlated with water transport, leading to significant penalties in fuel efficiency and poor cathode performance [189]. 

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