Dehydration of methanol

Dimethyl Ether. Synthesis gas conversion to methanol is limited by equiUbrium. One way to increase conversion of synthesis gas is to remove product methanol from the equiUbrium as it is formed. Air Products and others have developed a process that accomplishes this objective by dehydration of methanol to dimethyl ether [115-10-6]. Testing by Air Products at the pilot faciUty in LaPorte has demonstrated a 40% improvement in conversion. The reaction is similar to the Hquid-phase methanol process except that a soHd acid dehydration catalyst is added to the copper-based methanol catalyst slurried in an inert hydrocarbon Hquid (26). 

Mobil MTG and MTO Process. Methanol from any source can be converted to gasoline range hydrocarbons using the Mobil MTG process. This process takes advantage of the shape selective activity of ZSM-5 zeoHte catalyst to limit the size of hydrocarbons in the product. The pore size and cavity dimensions favor the production of C-5—C-10 hydrocarbons. The first step in the conversion is the acid-catalyzed dehydration of methanol to form dimethyl ether. The ether subsequendy is converted to light olefins, then heavier olefins, paraffins, and aromatics. In practice the ether formation and hydrocarbon formation reactions may be performed in separate stages to faciHtate heat removal. 

Water is also formed in the acid cataly2ed dehydration of methanol to give dimethyl ether. The solution is acidic because of the presence of the HI. 

The results in Table 3 show that H-mordenite has a high selectivity and activity for dehydration of methanol to dimethylether. At 150°C, 1.66 mol/kg catal/hr or 95% of the methanol had been converted to dimethylether. This rate is consistent with that foimd by Bandiera and Naccache [10] for dehydration of methanol only over H-mordenite, 1.4 mol/kg catal/hr, when extrt lat to 150°C. At the same time, only 0.076 mol/kg catal/hr or 4% of the isobutanol present has been converted. In contrast, over the HZSM-5 zeolite, both methanol and isobutanol are converted. In fact, at 175 X, isobutanol conversion was higher than methanol conversion over HZSM-5. This presents a seemingly paradoxical case of shape selectivity. H-Mordenite, the zeolite with the larger channels, selectively dehydrates the smaller alcohol in the 1/1 methanol/ isobutanol mixture. HZSM-5, with smaller diameter pores, shows no such selectivity. In the absence of methanol, under the same conditions at 15(fC, isobutanol reacted over H-mordenite at the rate of 0.13 mol/kg catal/hr, higher than in the presence of methanol, but still far less than over H M-5 or other catalysts in this study. 

In a recent publication, Chang and Silvestri have discussed this reaction in detail (109). They reported that under conditions of low (ca. 10%) conversion substantial amounts of dimethyl ether, formed by the reversible dehydration of methanol, are present and 78% of the primary hydrocarbon product consists of C2-C4 olefins. Also, if dimethyl ether, in the absence of water, is used instead of methanol, essentially the same hydrocarbon product distribution is obtained. On the basis of these observations, Chang and Silvestri suggest the reaction path shown below ... 

H(hydrogen)-mordenite catalyst The crystallites were approximate parallelepipeds, the long dimension of which was assumed to be the pore length. Their analysis was based on straight, parallel pores in an isothermal crystallite (2 faces permeable). They measured (initial) rates of dehydration of methanol (A) to dimethyl ether in a differential reactor at 101 kPa using catalyst fractions of different sizes. Results (for two sizes) are given in the table below, together with... 

As early as 1835, attempts to prepare the parent carbene (CH2) by dehydration of methanol had been reported. It is interesting to note that at that time the... 

Clinoptilolite Isomerization of n-butene, the dehydration of methanol to dimethyl ether, and the hydration of acetylene to acetaldehyde... 

The main reactions of the MTG/MTO process can be summarized as follows the first is the dehydration of methanol to DME on acidic zeolite catalysts. The equilibrium mixture of methanol, DME, and water is then converted to light alkenes, which react further to form higher alkenes, n- and Ao-alkanes, aromatics, and naphthenes by hydrogen transfer, alkylation, polycondensation, isomerization, and other secondary reactions. 

Until now, the detailed mechanism involved in the MTG/MTO process has been a matter of debate. Two key aspects considered in mechanistic investigations are the following the first is the mechanism of the dehydration of methanol to DME. It has been a matter of discussion whether surface methoxy species formed from methanol at acidic bridging OH groups act as reactive intermediates in this conversion. The second is the initial C—C bond formation from the Ci reactants. More than 20 possible mechanistic proposals have been reported for the first C-C bond formation in the MTO process. Some of these are based on roles of surface-bound alkoxy species, oxonium ylides, carbenes, carbocations, or free radicals as intermediates (210). 

At reaction temperatures of r< 523 K, the conversion of methanol on acidic zeolites is dominated by a dehydration of methanol to DME 210). Two mechanisms have been proposed for the formation of DME. In the indirect pathway (Eqs. (27a, b)), methanol molecules adsorbed on bridging OH groups react first to give methoxy species (ZOCH3), which subsequently react with another methanol molecule to give DME 280,281) ... 

Fig. 13. Dehydration of methanol on a Brdnsted acid site (a) shows the side-on complex, (b) the transition state, and (c) the dissociative complex of surface methoxy and water. Reprinted with permission from Ref. 221. Copyright 1995 American Chemical Society.

After discussing the dehydration of methanol and formation of DME, we are able to illustrate a number of key theoretical concepts. The first is that carbocation fragments are found in transition states, rather than as stable intermediates. Furthermore, the nature of these species is different from what is predicted from gas-phase studies, experimental or theoretical. The cluster, i.e., the zeolite, controls the stabilization of this carbocationic fragment. Second, we see that each different reaction requires a different transition state, rather than formation of a transition state that can be converted in a number of possible reactions. (This latter view received some support as a result of different processes possessing very similar activation barriers.)... 

Commercially, formaldehyde is sold as u. IKt aquruus so lulion Good reactivity, low price, and important end-use markets have combined lo give formaldehyde its growth mi pcliiv Formaldehyde is produced by vnpor-pha.se oAidulnm or by dehydration of methanol Its mam muricis are in production ot phenolic and urea resins The price of formaldehyde has historically been coupled closely to the price of methanol... 

However, 2% Ti02 dispersed on Si02 is also a catalyst for the oxidation of methanol, and therefore this reaction does not allow one to discriminate between framework titanium and Ti02. It has been found that in the presence of TS-1 containing Al3 +, the only reaction product is dimethyl ether formed by the dehydration of methanol ... 

Acid-catalyzed dehydration of methanol would give dimethyl ether (follow eq. 8.8) ... 

Roozeboom et al106 in an investigation of both unsupported V2 Os and a number of supported catalysts observed that at low temperatures dehydration of methanol to dimethyl ether is a side-reaction on some catalysts and at higher temperatures consecutive oxidation of dimethyl ether and/or formaldehyde to CO. Selectivity to formaldehyde increased with decreasing reducibility of the catalyst, which itself was a function of the catalyst-support interaction. 

Dimethyl ether (DME) can be produced by dehydration of methanol using alumina as solid catalyst. The main reaction is given by... 

Figure 17.9 Catalytic activities ofW03/Sn02 with various concentrations of W for the dehydration of methanol to dimethyl ether at 210°C. The concentration ofW 80% (A), 40% (A), 20% ( ), 10% (O), 5% ( ), 2% ( ).

The catalytic activity of heteroion-exchanged TSM, Ti Zn -TSM, is different from the activities of Ti - and Zn -TSMs. The results of the methanol conversion over the catalysts 35, 36) are summarized in Table V, which includes the data for Ti -TSM from Table IV. The reaction conditions are the same as given in Table IV. The heteroion-exchange reaction was conducted using a mixed solution of Ti(IV) and Zn(II) chlorides (mole ratio = 9 1). The resultant precipitate was washed with distilled water repeatedly and quickly to obtain Ti" Zn -TSM and Ti Zn -TSM/Cl, respectively. Ti -TSM catalyzes the dehydration of methanol to give dimethyl ether and a small amount of hydrocarbons, mainly methane, as described in the preceding section. The catalytic activity of Ti Zn -TSM is less than one-sixth as low as that of Ti -TSM, although only one-tenth of the Ti" in Ti -TSM has been replaced with Zn, inactive for the... 

The catalytic dehydration of methanol (ME) to form dimethyl ether (DME) and water was earried out over an ion exehange eatalyst [K. Klusacek, Collection Czech. Chem. Commun.,49, 170(1984)]. The packed bed was initially filled with nitrogen and at r = 0 a feed of pure methanol vapor entered the reactor at 413 K, 100 kPa, and 0.2 cmVs. The following partial pressures were recorded at the exit to the differentialreaetor containing 1.0 g of catalyst in 4.5 cm of reactor volume. 

Dimethyl ether is prepared by the reaction of bituminous or lignite coals with steam in the presence of a finely divided nickel catalyst. This reaction produces formaldehyde, which is then reduced to methanol and dimethyl ether. Dimethyl ether may also be prepared by the dehydration of methanol. 

It can be considered that these pillared products will be intercalated by accompanying with proton to produce a solid acid catalyst, because they exhibited acidity as shown in Table 1. To examine the acidic property of the catalysts dehydrations of methanol and 1-butanol were attempted by a flow reactor. The dehydration products of methanol were dimethyl ether and water, and those of 1-butanol were 1-, cis-2-, and trans-2-butenes and water. At relatively low temperature (250°C to 300°C) in hydration of 1-butanol a... 

Dimethyl ether (CH3OCH3) is a good substitute for environmentally harmful propellants in aerosol spray cans. It is produced by the dehydration of methanol ... 

Methanol.—Strong homogenous acids, e.g. phosphoric acid, " catalyse the dehydration of methanol to a mixture of hydrocarbons. Nominal Lewis acids. 

The production of diisopropyl ether by the dehydration of isopropyl alcohol and the simultaneous removal of the product diisopropyl ether and water at the reaction zone was reported to increase the conversion of this equilibrium limited reaction. The dehydration of methanol to produce dimethyl ether was also reported. ... 

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