Dimethylether, formation zeolites

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 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. 

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. 

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. " ... 

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),... 

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