Dimethylether, formation

The same oecurs for CO formation. Such formation seems to take place not only by formaldehyde oxidation, as is commonly accepted, hut also by direct methanol overoxidation and is inhibited by water and even by formaldehyde. Dimethylether formation exhibits a second order kinetics. 

A conmercial catalyst frcm Harshaw was used, a 3 1 mixture of molybdenum trioxide and ferric molybdate, as well as the two separate phases. Kinetic experiments were done previously in a differential reactor with external recycle using these same catalysts as well as several other preparations of molybdenun trioxide, including supported samples. Hie steady state kinetic experiments were done in the temperature range 180-300 C, and besides formaldehyde, the following products were observed, dimethylether, dimethoxymethane, methyl formate, and carbon-monoxide. Usually very little carbon dioxide was obtained, and under certain conditions, hydrogen and methane can be produced. 

Fig (12) Transformation of keto ester (94) to (96) is described. Michael addition leads the formation of the adduct (97) which is subjected to cyclization, aromatization and hydrogenolysis to obtain the phenol (99). This on diazotization, methylation and reduction afforded the amino ether (100). Further diazotization, methanolysis and saponification produce ethyl (+)-camosic acid dimethylether (102). 

A key step is the formation of a stable hydronium ion upon formation of dimethylether. The concept of Bronsted acid-Lewis base catalysis also allows us to understand the formation of ethylene from methanol, as formed in zeolite-catalysed reactions. A possible mechanism is sketched in Fig. 4.68. 

Others acetone, kerosene, dioxane, methylethylketone, toluene, benzene, xylene, ether, ether acetone, dimethylether, diethylether, formaldehyde, acetaldehyde, formic acid, acetic acid and methyl formate... 

The population of surface defects and coordination vacancies drives alkoxide and carboxylate formation and decomposition. When cations have at least two coordination vacancies, bimolecular reactions are possible (e.g., acetone from acetate ions, dimethylether from methoxy groups). 

The fluorenyl salts seem to be particularly susceptible to the formation of solvent separated pairs. Their formation is favoured by the presence of a large charge-delocalized anion plus a small compact cation which can be strongly solvated (or presumably vice-versa). The solvating agent should be highly polar, preferably small in size, or with the ability to offer multiple coordination as with the polyglycol-dimethylethers. With polystyryl salts, the formation of solvent-separated ion-pairs is less extensive. The absorption spectra are not particularly conclusive because the absorption... 

The following questions can in principle be addressed with spectroscopy (1) Zeolite synthesis what are the mechanisms of ZSM-5 synthesis and how do they influence the quality of the catalyst synthesized (2) Catalyst characterization what are the structure and composition of the zeolite, and what is the configuration of the active site for methanol conversion (3) How do methanol and dimethylether interact with the active sites i.e. what species are present in the catalyst in the initial stages of methanol conversion (4) What are the subsequent reaction pathways leading to the final alkane, alkene and aromatic products (5) What causes catalyst deactivation This question concerns both the temporary deactivation associated with coke formation, which can be reversed by oxidative regeneration, and the permanent deactivation which occurs after repeated deactivation-regeneration cycles. 

This methylating function of the second methoxy species, and the correlation between its formation and the first appearance of hydrocarbon products on injection of methanol or dimethylether suggests strongly that it plays a pivotal role in the formation of the first carbon-carbon bonds. Its formation from dimethylether occurs via the protonated ether ... 

This assignment is close to the original suggestion of Ono and Mori (ref. 26), that methyl carbenium ions initiate carbon-carbon bond formation. There seems little support however for their proposed methyl attack on the carbon of methanol. Methylation of the oxygen will form protonated dimethylether... 

The DME catalyst must carry out equilibrium conversion of methanol to dimethylether and water with minimum by-product formation. Less than equilibrium conversion will require more heat to be removed in the ZSM-5 circuit, which will result in higher reactor temperature rise. This will increase catalyst deactivation and decrease yield. The higher temperature rise could be reduced by increasing gas recycle, but this will increase operating costs. Excessive decomposition of methanol (e.g., to CO, C02, H2) will result not only in carbon loss, thereby reducing gasoline yields, but will also affect the composition of recycle gas in the ZSM-5 circuit. For example, one percent methanol decomposition to CO and H2 will increase the ZSM-5 reactor temperature rise by 12%. 

In 1881, Hofmann 6°) reported that the thermal decomposition of tetramethylammonium hydroxide gives trimethylamine and methanol. In 1964, a study by Musker 105> showed that the products resulting from the dry decomposition of tetramethylammonium hydroxide at 135— 140 °C were trimethylamine and dimethylether. Only a trace of methanol was observed. Trace amounts of dimethyl ether was reported as a product of the pyrolysis of cyclohexylmethyl-(3-d-trimethylammonium hydroxide and its formation was explained by a three step mechanism involving a sequence of Su2 reactions 31b An analogous mechanism for the decomposition of tetramethylammonium hydroxide, which would account for the observed products, was proposed by Tanaka, Dunning, and Carter... 

Perhaps the best rationalization for the formation of dimethylether rather than methanol is that the ether is the first product formed which could be evolved from a highly basic polar medium. Both water (hydroxide) or methanol (methoxide) would be expected to be retained in... 

Methanol Conversion. Methanol conversion reactions based on borosilicate catalysts have been studied extensively (10.15,24,28.33.52-54). During the conversion of methanol, the reaction proceeds through a number of steps, to yield dimethylether, then olefins, followed by paraffins and aromatics. The weaker acid sites of borosilicate molecular sieves relative to those of aluminosilicates require higher reaction temperatures to yield aromatics. The use of less forceful process conditions leads to the formation of olefins selectively, instead of a mixture of paraffins, olefins, and aromatics (10.28.53.54). 

Anderson et al. showed that the active sites involved in the conversion of methanol on zeolites are not Lewis acids. Wolthuizen et al., ° however, presented evidence that the presence of Lewis-acid sites enhances the polymerization of ethylene. This is in agreement with the results obtained with HY zeolites,where reaction of ethylene at 80 °C was observed only after dehydroxylation of the Bronsted acid sites into Lewis acid sites. At higher temperatures, ethylene is well-known to react on catalysts with strong Bronsted acid sites.Sayed and Cooney reported the involvement of aluminum Lewis sites in the formation of dimethylether. Haber and Szybalska observed that, when ethanol is converted on boron aluminum phosphates, only dehydration to ethylene takes place on the Bronsted acid sites, whereas, on Lewis acid-base centers, ethanol is mainly dehydrated to diethylether. 

A mechanism that has received a great deal of attention is the oxonium ylide mechanism.Dimethylether is methylated to trimethyloxonium, which is subsequently deprotonated to form surface associated methylene-dimethyloxoniumylide. The next step is either an intramolecular Stevens rearrangement, leading to the formation of methylethyl-ether, or an intermolecular methylation, leading to the formation of ethyl-dimethyloxoniumion. In both cases ethylene is obtained via jS-elimination. 

Kinetic studies on the methanol conversion to hydrocarbons usually consider the methanol/dimethylether mixture as a single species or "lump". This is justified by the observation that the ether formation is much faster than the subsequent reactions, so that the oxygenates are at equilibrium. Based on the autocatalytic nature of the methanol reaction over ZSM-5, Chen and Reagan used the following simple model, assuming that the rate of disappearance of oxygenates (A) is accelerated by their reaction with olefins (B) ... 

Anthony and Singh concluded from a kinetic analysis of the methanol conversion to low molecular weight olefins on chabazite that propylene, methane, and propane are produced by primary reactions and do not participate in any secondary reactions, whereas dimethylether, carbon monoxide, and ethane do. Ethylene and carbon dioxide appear to be produced by secondary reactions. It was also shown that the product selectivities could be correlated to the methanol conversion even though the selectivity and the conversion changed with increasing time on stream due to deactivation by coke formation. 

Compound 17 is the result of a Friedel-Crafts reaction of the carbocation intermediate. Changing the solvent from ethylene glycol dimethylether to alcohol modifies the reactive intermediate from 18a to 18b or 18c. The hydrogenolysis of 18b (or 18c) is more difficult than 18a which leads to a lower stationary concentration in carbocation, therefore minimizing the formation of 17. 

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