Zeolites dehydrating catalysts

The important groups of dehydration catalysts are oxides, aluminosilicates (both amorphous and zeolitic), metal salts and cation exchange resins. Most work on mechanisms has been done with alumina. 

Catalytic hydrogenation of carbon dioxide was studied for the simultaneous synthesis of methanol and dimethyl ether (oxygenates). Various combinations of methanol synthesis catalysts and methanol dehydration catalysts have been examined for the hydrogenation. The hybrid catalyst of Cu/ZnO/CraOs and CuNaY zeolite was found to be very efficient for the production of oxygenates. 

Zeolites represent an interesting class of alcohol dehydration catalysts because their properties can be modified systematically by ion-exchange or chemically by replacement of silica, thus changing their acidity and therefore activity and selectivity. 

Numerous studies, including mechanistic and kinetic investigations mostly with simple alcohols, have been performed with molecular sieves as dehydration catalysts [8,32-34]. Although highly active these are rarely used for converting alcohols with complicated structures to alkenes. The reason is that these catalysts are not selective-a prevalent El mechanism, i. e. the involvement of carbocationic intermediates, and parallel inter- and intramolecular processes result in the formation of isomeric alkenes and ethers. Alcohols with specific structure, however, can be selectively transformed to alkenes. For example, 1-phenyl-1-ethanol is transformed to styrene in 95 % yield over HZSM-5 zeolite at 493 K [34]. Ether formation, however, was shown to be significant when a-(p-tolyl)ethanol was reacted over zeolite HY [35]. A low concentration of the reactant alcohol inside the zeolite is required to prevent such dimerization-type reaction a suitable competing solvent should be selected. 

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. " ... 

Catalysis by Metal Oxides and Zeolites. Metal oxides are common catalyst supports and catalysts. Some metal oxides alone are industrial catalysts an example is the y-Al202 used for ethanol dehydration to give ethylene. But these simple oxides are the exception mixed metal oxides are more... 

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. 

These experiments demonstrate that the surface-catalyzed 2 reaction is far more effident than either the or C pathway for the dehydrative coupling of alcohols over the solid add catalysts tested. High selectivity to configurationally inverted chiral ethers ensues, espedally in the case of the HZSM-5 catalyst, in which the minor C or C paths were further suppressed by "bottling" of 3-ethoxypentane by the narrow zeolite chaimels. 

The water-insoluble salts such as Cs2,5Ho., iPWi204o efficiently catalyse dehydration of 2-propanol in the gas phase and alkylation of m-xylene and trimethyl benzene with cyclohexene this catalyst is much more active than Nafion-H, HY-zeolite, H-ZSM-5, and sulphated zirconia (Okuhara et al., 1992). 

Catalytic Reactions. The copper catalysts were pre-reduced at 270°C with H2 before the catalytic test. Citral (0.1 g) was dissolved in toluene or heptane dehydrated over zeolites (8 ml) or not and the solution transferred under N2 into a glass reaction... 

A very attractive method for the preparation of nitroalkenes, which is based on the reaction with NO, has been reported. Treatment of alkenes at ambient pressure of nitrogen monoxide (NO) at room temperature gives the corresponding nitroalkenes in fairly good yields along with P-nitroalcohols in a ratio of about 8 to 2. The nitroalcohol by-products are converted into the desired nitroalkenes by dehydration with acidic alumina in high total yield. This simple and convenient nitration procedure is applied successfully to the preparation of nitroalkenes derived from various terminal alkenes or styrenes (Eq. 2.27).53 This process is modified by the use of HY-zeolites instead of alumina. The lack of corrosiveness and the ability to regenerate and reuse the catalyst make this an attractive system (Eq. 2.28).54... 

Heterogeneous catalysts, particularly zeolites, have been found suitable for performing transformations of biomass carbohydrates for the production of fine and specialty chemicals.123 From these catalytic routes, the hydrolysis of abundant biomass saccharides, such as cellulose or sucrose, is of particular interest. The latter disaccharide constitutes one of the main renewable raw materials employed for the production of biobased products, notably food additives and pharmaceuticals.124 Hydrolysis of sucrose leads to a 1 1 mixture of glucose and fructose, termed invert sugar and, depending on the reaction conditions, the subsequent formation of 5-hydroxymethylfurfural (HMF) as a by-product resulting from dehydration of fructose. HMF is a versatile intermediate used in industry, and can be derivatized to yield a number of polymerizable furanoid monomers. In particular, HMF has been used in the manufacture of special phenolic resins.125... 

A delaminated zeolite with an Si/Al ratio of 29, derived from the layered zeolite Nu-6(1), was employed as catalyst for dehydration of xylose at 170 °C, using a water-toluene biphasic reactor-system.140 This material, designated del-Nu-6(l), proved to be efficient for this transformation, giving 47% selectivity to furfural at 90% xylose conversion. 

Durable changes of the catalytic properties of supported platinum induced by microwave irradiation have been also recorded [29]. A drastic reduction of the time of activation (from 9 h to 10 min) was observed in the activation of NaY zeolite catalyst by microwave dehydration in comparison with conventional thermal activation [30]. The very efficient activation and regeneration of zeolites by microwave heating can be explained by the direct desorption of water molecules from zeolite by the electromagnetic field this process is independent of the temperature of the solid [31]. Interaction between the adsorbed molecules and the microwave field does not result simply in heating of the system. Desorption is much faster than in the conventional thermal process, because transport of water molecules from the inside of the zeolite pores is much faster than the usual diffusion process. 

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