Dehydration Y-zeolites

The distribution of cations in a hydrated zeolite is mainly controlled by their sizes and can be described by a statistical model. In the dehydrated state, most of the cations are located on the intraframework sites their occupancies are governed by mutual repulsions and cation—framework interactions [1]. By which, the environments of the framework silicon atoms and their corresponding ssi NMR spectra are affected [2,3]. The chemical shift and lineshape of Si NMR have been found to depend on the nature and the distribution of cations in the small sodalite and double hexagonal prism (D6R) cavities of the dehydrated Y zeolites [3] The irreversible migration of La3 ions from the supercages to the small sodalite and/or D6R cavities by... 

For the hydrated Y zeolites, sample spinning rates of 3 to 5 KHz were used in most cases. A typical spectrum was acquired using 1000 scans, a recycle time of 0.5 sec. and an rf excitation pulse width of 2.0 microsec, which is less than 1/4 of a 90° pulse width of sodium in solution. Under these conditions, the integrated intensity of the different sodium NMR lines in the spectrum closely approximates the concentration of different sodium species giving rise to the NMR lines. For the dehydrated Y zeolites, samples, spinning rates of 6 to 9 KHz were used. A typical spectrum was acquired using 5000 scans, a 2 sec. recycle time and a pulse width less than that of a solids 45° pulse (about 2.5 microsec). 

In this study we report that the sodium-23 MASNMR spectra of Na cations in dehydrated Na-Y and mixed cation Y zeolites can be observed at 96 MHz. Using MAS rates in excess of 5 KHz reveals structure to the spectrum which can be associated with lines arising from cations sites in the different Y zeolites cages. However, it is estimated that only about 70 to 80% of the Na atoms present in the zeolite may be observable by NMR under these conditions. As an example of the sodium-23 MASNMR spectra obtainable from the dehydrated Y zeolites, the spectra of a Na-Y and the three mixed cation Y zeolites are presented in Figure 3. For the (NH, Na), (Ca,Na) and (La,Na)-Y zeolites, the degrees of cation exchange were 56, 58, and 65%, respectively. In contrast to the NMR spectra of the hydrated Y zeolites presented in Figure... 

Sodium-23 MASNMR measurements have been used to examine the extent to which this method can be used to determine the cation distribution in hydrated and dehydrated Y-zeolites. Results have been obtained on Na-Y and series of partially exchanged (NH, Na)-Y, (Ca,Na)-Y and (La,Na)-Y zeolites which demonstrate that the sodium cations in the supercages can be distinguished from those in the smaller sodalite cages and hexagonal prisms. For the hydrated Y zeolites, spectral simulation with symmetric lines allows the cation distribution to be determined quantitatively. 

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... 

Figure 4.33 IR spectrum of a dehydrated H,Na-Y zeolite in OH stretching region. (Reprinted from Introduction to Zeolite Science and Practice, Studies in Surface Science and Catalysis, Vol. 58, J.H.C. van Hooff, J.W. Roelofsen, Techniques of Zeolite Characterization, pp. 241-283. Copyright 1991. With permission from Elsevier.)...

The ACH process has recently been improved, as stated by Mitsubishi Gas. Acetone-cyanohydrin is first hydrolized to 2-hydroxyisobutylamide with an Mn02 catalyst the amide is then reacted with methylformiate to produce the methyl ester of 2-hydroxyisobutyric acid, with coproduction of formamide (this reaction is catalyzed by Na methoxide). The ester is finally dehydrated with an Na-Y zeolite to methylmethacrylate. Formamide is converted to cyanhydric acid, which is used to produce acetone-cyanohydrin by reaction with acetone. The process is very elegant, since it avoids the coproduction of ammonium bisulphate, and there is no net income of HCN. Problems may derive from the many synthetic steps involved, and from the high energy consumption. 

The crystal structure analysis of palladium-exchanged zeolite allows the determination of initial cation positions in the dehydrated porous framework. Similar studies after reduction by hydrogen at various temperatures should permit the observation of palladium removal from the cation sites and thus the estimation of the reduction level. Moreover, the presence of metal on the external surface is easily detected. Hence, x-ray diffraction techniques should give a good picture of hydrogen reduction of palladium in Y zeolites. 

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. 

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. 

Alkaline earth-exchanged samples examined by Ward (211) were more resistant to thermal dehydroxylation hydroxyl bands were present in the spectra after dehydration at 500°C. The concentration of OH groups was, however, much smaller than found in H—Y zeolite, and was dependent on the cation type. An almost linear inverse relationship was found between the alkaline earth cation radius and the concentration of acidic hydroxyl groups (210). 

Hopkins (161) found that a steady decrease in n-heptane cracking activity occurred over La- and Ca-exchanged Y zeolites as the catalyst calcination temperature was increased from 350° to 650°C. The lanthanum form was about twice as active as the calcium form. Reduction in activity with increasing activation temperature was attributed to removal of acidic framework hydroxyl sites as dehydration becomes more extensive. The greater activity of La—Y with respect to the calcium form was thought to result from the greater hydrolysis tendency of lanthanum ion, which would require more extensive dehydration to result in the same concentration of acidic OH groups as found on Ca—Y. 

Sanple Preparation. Copper Y zeolite was prepared by ion exchange of sodium Y (Linde SK-40) zeolite with aqueous Cu(N03)2 solution. CuigNa24Y(Cu Y) (11) was obtained by stirring a slurry of 50g of NaY in 1 dm of 0.1 M Cu Oj at 25° for 4 h. The copper content was determined by spectrophotometry of Cu + after dissolution of exchanged sieve. The Cu Y zeolite was washed, air dried, and 2g samples were dehydrated in an apparatus (Fig. la) under vacuum first at room temperature and then at 100°C, 200°C, 300°C, and 400°C, being held at each temperature for one hour. 

Cu Y. The absorption spectra of hydrated and dehydrated CuIJY zeolites are shown in Figs. 5 and 6, respectively. The dehydrated Cu Y zeolite also displayed a weak photoluminescence at 540 nm, in qualitative accord with the reports of partial autoreduction of Cu to Cu upon dehydration, which amounts to approximately 20% of Cu converted to Cu at the dehydration temperature of 400°C (3). The sharp peaks at 5200 and 7000 cm- in Fig. 5 are the (v+6) and (2v) vibrational bands of water (14). Their absence in Fig. 6 demonstrates that the dehydration of Cu Y is complete. Also, absence of the silanol (2v) band at 7300 cm- (I5) shows that hydroxyl groups are absent in the dehydrated Cu Y as well as in all subsequently treated copper zeolites. The broader bands between 9000 and 16000 cm and above 30000 cm- are electronic absorption spectra of the copper species in the hydrated and dehydrated Cu Y, as follows from their comparison with the spectra of NaY and CuxY. 

The piiotoluminescence of Cu Y with adsorbed oxygen-evidenoe for and quantitative analysis of resonance energy transfer. The Cu- -Y zeolite used in this study was prepared by reduction of the dehydrated CuIXNa Y zeolite in hydrogen. The reduction reaction may be written as... 

The decay time of the green (540 the dehydrated Cu Naj Y zeolite was from 112 to 28 and 6 ps upon exposure... 

Butyrolactone reacts rapidly and reversibly with ammonia or an amine forming 4-hydroxybutyramides, which dissociate to the starting materials when heated. At high temperatures and pressures, the hydroxybutyramides slowly and irreversibly dehydrate to pyrrolidinones this dehydration is accelerated by use of a copper-exchanged Y-zeolite or magnesium silicate. 

All Ru exchanged Y zeolites were dehydrated in a glass vacuum line at 623 K and reduced under static hydrogen at the same temperature using a procedure similar to that of Uytterhoeven et al..(7,<5) Dispersions for all samples exceeded 0.9. For coadsorption studies of ethylene and hydrogen, ethylene was preadsorbed at 77 K and hydrogen was introduced immediately afterwards. 

Unidentate carbonate species are formed on heating bivalent cation-exchanged Y zeolites in C02 (286). The Ca2+ ions are involved in the formation of this surface carbonate (281) in CaY zeolites. Jacobs et at. (281) have shown that lattice oxygen must be incorporated in C02 to form the carbonate on a dehydrated zeolite. Provided there are some residual water molecules retained in the zeolite, carbonate formation is explained by the following reactions ... 

The initial exchange site of Ag into X and Y zeolites is most probably SI in the hexagonal prisms or possibly SI displaced into the 8-cage (15,16) from the hexagonal prisms. When Ag° is formed by electron reduction of Ag+ at 77 K the Ag° does not seem to remain in the SI or SI sites. Instead it appears to be able to move to site SU. Partial dehydration studies suggest that Ag° moves toward SI as dehydration occurs. 

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