Dehydration of zeolites

Prior to their use as adsorption agents or after industrial utilization for the adsorption of water, zeolites have to be dehydrated. This is carried out at 450 to 650°C e.g. in a rotary tube furnace or a similar unit. Industrially zeolites charged with water or other compounds are regenerated directly by passing hot dry inert gas through the absorber. 

Removal of water from /.colitcs ( activation ) by healing lo 4. 0 to 

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These findings are in broad agreement with the wide-line measurements by Genser (158) who from the second moment of the 27A1 line in hydrated zeolite Y calculated vQ = 390 kHz, but was not able to observe a signal in dehydrated zeolite. Gabuda et al. (137) observed increased values of vQ after dehydration of zeolites. For the hydrated and dehydrated analcime the values were 270 and 390 kHz, respectively for Na-X, 165 and 285 kHz and for Na-A, 75 and 165 kHz. 

Palancher et al. (2005) Sr-X zeolite Phase deformation + + — Dehydration of zeolite... 

This characteristic was used, for example, to produce diffractometers designed to track the evolution of a sample according to the temperature. Louer and his colleagues studied the stmctural transformations of oxide precursors using a system eqnipped with an oven in which the sample was placed [AUF 90, BEN 93, GUI 95, BEN 96], An example of this type of study is shown in Figure 2.31 where the evolution of a cerium oxide precursor can be observed, showing in particular the presence of an intermediate phase. The same approach was nsed by other authors to study, for example, the dehydration of zeolites [STA 92, STA 94] or also to simultaneously measure, according to the temperature, the evolution of X absorption spectra (EXAFS) and of diffraction patterns [SAN 93, DEN 95]. 

A crystalline phase of beryllosilicate with analcime structure was obtained under hydrothermal conditions from starting material of composition NasBe15Si50ls NaCl. An analcime-like phase was obtained as a nearly pure phase at 200°C. A small amount of sodium chloride coexisted with the above phase. The lattice constant was a = 13.35 it 0.01A. The refractive index was 1.519 it 0.002, higher than that of normal analcime. The broad endothermic peak, indicating the dehydration of zeolitic water, was observed in the temperature range from 150° to 500°C on a DTA curve. The specimen showed little change of structure on heating to 500°C. These results indicate that a beryllosilicate with the analcime structure was prepared by direct synthesis. 

Sodium chloride and chkalovite were thermally stable up to their melting points, which were determined to be about 800°C for sodium chloride and 1050°C for chkalovite by the DTA method. Since a broad peak indicating the dehydration of zeolitic water was not altered by other endothermic peaks resulting from melting of sodium chloride and chkalovite, the existence of zeolitic water could be inferred from the DTA curve. 

One more advantage of Raman spectroscopy is due to the fact that the Raman spectrum of water exhibits only a few signals of low intensity. Thus, careful dehydration of zeolites, which is crucial in many IR experiments, does not play the... 

Dehydration of Zeolites Most zeolites can be dehydrated to some degree without major alteration of their crystal structure they may subsequently be rehydrated, that is, absorb water from the vapour in the liquid phase (Table 6.3). However, many zeolites undergo irreversible structural changes and suffer total structural collapse. 

Activation of zeolites is a dehydration process aceomplished by the application of heat in a high vacuum. Some zeolite crystals show behavior opposite to that of activated carbon in that they selectively adsorb water in the presence of nonpolar solvents. Zeolites can be made to have specifie pore sizes that will increase their seleetive nature due to the size and orientation of the molecules to be adsorbed. Moleeules above a specific size could not enter the pores and therefore would not be adsorbed. 

Molecular sieves are an adsorbent that is produced by the dehydration of naturally occurring or synthetic zeolites (crystalline alkali-metal aluminosilicates). The dehydration leaves inter-crystalline cavities into which normal paraffin molecules are selectively retained and other molecules are excluded. This process is used to remove normal paraffins from gasoline fuels for improved combustion. Molecular sieves are used to manufacture high-purity solvents. 

It is often said that the property of acidity is manifest only in the presence of a base, and NMR studies of probe molecules became common following studies of amines by Ellis [4] and Maciel [5, 6] and phosphines by Lunsford [7] in the early to mid 80s. More recently, the maturation of variable temperature MAS NMR has permitted the study of reactive probe molecules which are revealing not only in themselves but also in the intermediates and products that they form on the solid acid. We carried out detailed studies of aldol reactions in zeolites beginning with the early 1993 report of the synthesis of crotonaldehyde from acetaldehyde in HZSM-5 [8] and continuing through investigations of acetone, cyclopentanone [9] and propanal [10], The formation of mesityl oxide 1, from dimerization and dehydration of... 

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. 

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. 

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). 

The concept of extractive reaction, which was conceived over 40 years ago, has connections with acid hydrolysis of pentosans in an aqueous medium to give furfural, which readily polymerizes in the presence of an acid. The use of a water-immiscible solvent, such as tetralin allows the labile furfural to be extracted and thus prevents polymerization, increases the yield, and improves the recovery procedures. In the recent past an interesting and useful method has been suggested by Rivalier et al. (1995) for acid-catalysed dehydration of hexoses to 5-hydroxy methyl furfural. Here, a new solid-liquid-liquid extractor reactor has been suggested with zeolites in protonic form like H-Y-faujasite, H-mordenite, H-beta, and H-ZSM-5, in suspension in the aqueous phase and with simultaneous extraction of the intermediate product with a solvent, like methyl Aobutyl ketone, circulating countercurrently. 

Here, A is the nearly isotropic nuclear coupling constant, I is the nuclear spin (Iun = I), and m is the particular nuclear spin state. It may be observed that the zero field splitting term D has a second-order effect which must be considered at magnetic fields near 3,000 G (X-band). In addition to this complication nuclear transitions for which Am = 1 and 2 must also be considered. The analysis by Barry and Lay (171) of the Mn2+ spectrum in a CsX zeolite is shown in Fig. 35. From such spectra these authors have proposed that manganese is found in five different sites, depending upon the type of zeolite, the primary cation, and the extent of dehydration. 

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. 

Fontes tt al. [224,225 addressed the acid—base effects of the zeolites on enzymes in nonaqueous media by looking at how these materials affected the catalytic activity of cross-linked subtilisin microcrystals in supercritical fluids (C02, ethane) and in polar and nonpolar organic solvents (acetonitrile, hexane) at controlled water activity (aw). They were interested in how immobilization of subtilisin on zeolite could affected its ionization state and hence their catalytic performances. Transesterification activity of substilisin supported on NaA zeolite is improved up to 10-fold and 100-fold when performed under low aw values in supercritical-C02 and supercritical-ethane respectively. The increase is also observed when increasing the amount of zeolite due not only to a dehydrating effect but also to a cation exchange process between the surface proton of the enzyme and the sodium ions of the zeolite. The resulting basic form of the enzyme enhances the catalytic activity. In organic solvent the activity was even more enhanced than in sc-hexane, 10-fold and 20-fold for acetonitrile and hexane, respectively, probably due to a difference in the solubility of the acid byproduct. 

Zeolite catalysed dehydration of alcohol to linear ether... 

The liquid-phase dehydration of 1-hexanol and 1-pentanol to di-n-hexyl ether (DNHE) and di-n-pentyl ether (DNPE), respectively, has been studied over H-ZSM-5, H-Beta, H-Y, and other zeolites at 160-200°C and 2.1 MPa. Among zeolites with a similar acid sites concentration, large pore H-Beta and H-Y show higher activity and selectivity to ethers than those with medium pores, although activity of H-ZSM-5 (particularly in 1-pentanol) is also noticeable. Increased Si/Al ratio in H-Y zeolites results in lower conversion of pentanol due to reduced acid site number and in enhanced selectivity to ether. Selectivity to DNPE is always higher than to DNHE... 

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