Small-pore zeolites dehydration

In substrate selectivity, access to the catalytically active site is restricted to one or more substrates present in a mixture, e.g. dehydration of a mixture of n-buta-nol and isobutanol over the small pore zeolite, CaA, results in dehydration of only the n-butanol [38] while the bulkier isobutanol remains unreacted. Product... 

Dehydration of the small-pore zeolite analcime (NaAlSi20e) has also been studied by high-temperature Na NMR measurements up to 500°C (Kim and Kirkpatrick 1998) in which changes in the Na peak positions and widths with temperature were interpreted in terms of exchange between the Na sites and the effect of motional averaging of the intensity of the ( 1/2, 3/2) satellite transitions. 

Alcohol Dehydration using New Shape Selective Zeolites. - The shape selectivity of zeolites has been referred to and exploited in numerous ways. Nevertheless, within the last ten years a new class of zeolite (Type ZSM-5) has been reported, which possesses pore openings, intermediate in size between the large and small pore zeolites its sieving properties in respect of alkyl aromatics and various aliphatic hydrocarbons have been reported. 

In 1960, Weisz, Frilette, and co-workers first reported molecular-shape selective cracking, alcohol dehydration, and hydration with small pore zeolites (6,7), and a comparison of sodium and calcium X zeolites in cracking of paraffins, olefins, and alkylaromatics (8). In 1961, Rabo and associates (9) presented data on the hydroisomerization of paraffins over various zeolites loaded with small amounts of noble metals. Since then, the field of zeolite catalysis has rapidly expanded,... 

Titanosilicalites such as ETS-4 represent a new class of crystalline microporous molecular sieves, similar to zeolites in their general structure but significantly different in their composition. Like the small pore zeolites ETS-4 has a three dimensional channel structure controlled by 8-membered oxygen rings but the dimensions of the unit cell and hence both the size and shape of the 8-ring windows change dramatically with the dehydration temperature [48]. Provided... 

Strong acids are able to donate protons to a reactant and to take them back. Into this class fall the common acids, aluminum hahdes, and boron trifluoride. Also acid in nature are silica, alumina, alumi-nosihcates, metal sulfates and phosphates, and sulfonated ion exchange resins. They can transfer protons to hydrocarbons acting as weak bases. Zeolites are dehydrated aluminosilicates with small pores of narrow size distribution, to which is due their highly selective action since only molecules small enough to enter the pores can reacl . 

Shape selective catalysis as typically demonstrated by zeolites is of great interest from scientific as well as industrial viewpoint [17], However, the application of zeolites to organic reactions in a liquid-solid system is very limited, because of insufficient acid strength and slow diffusion of reactant molecules in small pores. We reported preliminarily that the microporous Cs salts of H3PW12O40 exhibit shape selectivity in a liquid-solid system [18]. Here we studied in more detail the acidity, micropore structure and catal3rtic activity of the Cs salts and wish to report that the acidic Cs salts exhibit efficient shape selective catalysis toward decomposition of esters, dehydration of alcohol, and alkylation of aromatic compound in liquid-solid system. The results were discussed in relation to the shape selective adsorption and the acidic properties. 

One of the most promising techniques for studying transition metal ions involves the use of zeolite single crystals. Such crystals offer a unique opportunity to carry out single crystal measurements on a large surface area material. Suitable crystals of the natural large pore zeolites are available, and fairly small crystals of the synthetic zeolites can be obtained. The spectra in the faujasite-type crystals will not be simple because of the magnetically inequivalent sites however, the lines should be sharp and symmetric. Work on Mn2+ in hydrated chabazite has indicated that there is only one symmetry axis in that material 173), and a current study in the author s laboratory on Cu2+ in partially dehydrated chabazite tends to confirm this observation. 

Information published during thepast few years about the faujasite class of zeolites indicated that they present a possibly unique system in which the necessary conditions might be met. Sherry (4, 5) reported that rare earths, as compared with alkali or alkaline earth metals, are readily exchanged into Linde X from dilute aqueous solutions, and that they strongly favor the zeolite phase. When such an exchanged zeolite is dehydrated by heating to 350-700° C, the lanthanide ions move into the small pore system (6>, 7) after which they are not readily exchanged back out of the crystal. Smith (8) has reviewed the structure of lanthanide X and Y zeolites. 

Zeolite Catalysts. - Crystalline silica-aluminates or zeolitic supports differ in three important respects from the alumina and silica supports discussed above. First, they are very strong cation exchangers and tend to stabilize low valent cations, making them more difficult to reduce completely. Secondly, they in general have a pore structure commensurate with atomic dimensions. This microporous structure is such as to restrict the adsorption or exclude altogether the more bulky ions. Together with the highly polar surface this small pore size also makes them very difficult to dehydrate at low temperatures. Finally, the catalyst structure itself tends to be thermally and hydrolytically unstable, particularly at a low pH. 

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... 

The most straightforward cause of shape selectivity is the discrimination between molecules on the basis of their diffusion rates through the channels or cage windows. Microporous solids act as true molecular sieves, because the well-defined pores are able to select molecules on the basis of differences in dimensions of 0.1 A or less. Examples of strong molecular sieving effects include the selection of normal alkanes over branched ones by small-pore solids and the selection of para-substituted over ortho- and meta-substituted aromatics over medium-pore zeolites. This type of selectivity according to molecular diffusion rate may act on both reactant and product molecules. The much faster dehydration of n-butanol compared to isobutanol over Ca-A demonstrated by Frilette and Weisz is the classic example of reactant diffusion... 

If the water content is driven off (usually by heating to 350 °C in a vacuum), the dehydrated zeolite becomes an avid absorber of small molecules, especially water. The size of the molecules that can be absorbed is limited by the zeolite pore diameter, which is different for different zeolites (Table 7.1) a given zeolite (e.g., zeolite 3A) can be a highly selective absorber of, say, small amounts of water from dimethyl sulfoxide (DMSO) solvent. For this reason, dehydrated zeolites are often called molecular sieves. 

To achieve a significant adsorptive capacity an adsorbent must have a high specific area, which implies a highly porous structure with very small micropores. Such microporous solids can be produced in several different ways. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles, followed by dehydration. Carbon adsorbents are prepared by controlled burn-out of carbonaceous materials such as coal, lignite, and coconut shells. The crystalline adsorbents (zeolite and zeolite analogues are different in that the dimensions of the micropores are determined by the crystal structure and there is therefore virtually no distribution of micropore size. Although structurally very different from the crystalline adsorbents, carbon molecular sieves also have a very narrow distribution of pore size. The adsorptive properties depend on the pore size and the pore size distribution as well as on the nature of the solid surface. 

Most surface area determinations are based on measurements of the low temperature adsorption of nitrogen or krypton on the solid and use of the BET theory. This procedure may not give reliable results because the products are chilled well below reaction temperature, possibly resulting in the sealing of internal pores. Volumes of gases adsorbed are sometimes small, as observed for dehydrated alums [37] and decomposed ammonium perchlorate [48], where the areas are consistent with product crystallites of linear dimensions between 1 and 3 pm. The results indicate, however, that little, if any, zeolitic material is formed [36]. The surface area of a solid may also be estimated from electron micrographs. Density measurements may be used to complement area measurements. 

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