Zeolites dimethylether from methanol

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. 

The second methoxy species was found to function as a methylating agent. Injection of a pulse of benzene for example at 250°C into ZSM-5 previously exposed to methanol or dimethylether causes a decrease in the concentration of the second methoxy species and the appearance of toluene as a product downstream from the cell (benzene alone does not react over ZSM-5 at this temperature). Similar methylation was observed with ethene (forming propene). With other alkenes the results were less clearcut, since these formed oligomerization products in the absence of methoxy species, but a decrease in the infrared bands of the second methoxy species could still be detected when propene, cyclopentene and 1-hexene were injected into zeolites pretreated with methanol or dimethylether. 

Methanol is first dehydrated to dimethylether (DME). The equilibrium mixture thereof is then converted to light olefins. In the final steps of the reaction path, the Cj-Cg olefins are converted to paraffins, aromatics, naphthenes and higher olefins by polycondensation and alkylation reactions. The importance of light olefins as intermediates in the conversion of methanol to gasoline was soon recognized. As a result, several attempts were made to selectively produce light olefins from methanol on zeolite catalysts, not only on medium-pore zeolites but also on small-pore... 

Several groups have demonstrated that CP/MAS spectra may also be used to characterize the products formed in the zeolite structure from a precursor molecule an approach pioneered by Derouane and co-workers (24), enrichment being used to increase sensitivity (24,25,26). T ical of these studies are the results of Haw and coworkers on the conversion of methanol- to hydrocarbons on zeolite HZSM-5 (25C) shown in Figure 18. As can be seen the spectra show the conversion clearly with the formation of dimethylether (Figure 18b),... 

Influence of Catalyst Preparation. Pebrine reported on the influence of the synthesis conditions of HZSM-5 on the selectivity toward light olefins. Synthesizing ZSM-5 in the presence of a tetra-urea-cobalt(II) complex resulted in an ethylene yield of 24.3 wt% of the hydrocarbon fraction at 43.7% methanol conversion, whereas the conventionally prepared ZSM-5 yielded only 18 wt% ethylene at the same conditions and conversion. Heering et al. mentioned that the conversion of dimethylether on ZSM-5 catalysts crystallized from a sodium-free gel with 1,6-dicunino-hexane as organic base was more selective toward both ethylene and propylene than on a conventionally prepared zeolite in the sodium form from a gel containing tetrapropylammonium. 

It is not always easy to find direct correlations between the heats of adsorption and the catalytic behaviour (activity and/or selectivity). Various attempts at establishing such correlations have been reported in the literature. For instance, a comparison of microcalorimetric measurements with kinetic studies performed over acidic zeolites for methylamine disproportionation reactions, methanol dehydration, and reactions of methanol and dimethylether with methylamines, suggests that acid sites are required in these reactions for the strong adsorption of ammonia and methylamines, while weak adsorption sites are required to facilitate desorption of adsorbed amine species from the acid sites [147]. 

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. " ... 

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