Dehydrated zeolite spectra

Nickel tetracarbonyl and nickel complexes with mixed carbonyl and alkylphosphine ligands in zeolite Y have been studied with EXAFS, NMR, and FTIR spectroscopies (85,240-242). The loading of Ni(CO>4 in dehydrated zeolite X can be as high as 28 wt%, corresponding to 2.75 Ni(CO>4 molecules per supercages. The IR spectrum of Ni(CO>4 was found to be affected by the cations inside the zeolite supercages. The spectrum of... 

Figure 7.9. A. MAS NMR spectum of dehydrated zeolite NaX at a magnetic field of 14.1 T, with the simulated spectrum and the five components used in the simulation. B. Na DOR spectrum acquired at a magnetic field of 9.4 T with an outer rotor speed of 1.5 kHz, with simulated spectrum and the components used in the simulation. C. Na 2D nutation spectrum acquired at a magnetic field of 9.4 T. The asterisk denotes spinning side bands. From Feuerstein et al. (1996), by...

Fig. 27. DR spectrum of fully hydrated zeolite Co(ll)A (1) compared to that of an aqueous Co(II) solution (2). Upon complete dehydration the spectrum drastically changes to (3) assigned to a Co(ll) ion in trigonal planar Dji, coordination by the 6R. Reprinted from [32] with permission of Academic Press, Inc...

The decomposition of iron(ll) acetate ground with NH4-Y, seems to be favoured in presence of water since a less pronounced intensity decrease (60 %) of the doublet typical of this salt was observed when dehydrated zeolite was applied. After heat treatment at 520 K the spectrum is composed of two characteristic doublets (Fig. 3 and Table 3) exhibiting isomer shift values typical of magnetite. Thus, both doublets may be ascribed to a precursor species of magnetite. 

As the intensity of the signals in the NMR spectrum is proportional to the concentration of corresponding species, the H MAS NMR signal intensities can be used to determine the concentration of hydroxyl groups in dehydrated zeolites by comparison of the intensity of different hydroxyl groups with the intensity of some internal reference, for example, certain quantity of adsorbed methane or TMS (tetramethylsilane). 

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. 

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 range of the gzz values is shown clearly by a comparison of the results for the NaY and NaX zeolites. Since the migration of Na+ ions is related to the presence of water (76), it is likely that the type of precursor (Na4)4+ -(H20)x complex formed after a proper degree of dehydration (278) will be strongly dependent on the pretreatment conditions. This will be reflected in the gzz values of the OJ produced during y irradiation by electron transfer from the precursor (278). It is also likely that the OJ can migrate after its formation as shown by Kasai and Bishop (264). These authors (272) have detected a superhyperfine interaction from Na nuclei (I = ) in the EPR spectrum of OJ formed in Na-reduced NaY zeolite and characterized by gzz = 2.113. This value is very close to those observed for alkalisuperoxides trapped in krypton matrices (Ref. 44, Appendix A). 

Figure 4 ESR spectrum (low eld components) of CuNaY zeolites dehydrated at 800° C and taken at room temperature in the X-hand as a function of degree of exchange (numbers on the left)...

Figure 5. ESR spectrum (low field components) of CuNaY zeolite (a = 29%) in the Q-band taken al room temperature as a function of dehydration temperature (numbers on the right are pretreatment temperatures)...

In the spectrum of zeolite KL, at the temperature of evacuation (100°-500°C), stronger water-bound bands (3665, 3685, 3700 cm 1 and 1602, 1630, and 1650-1660 cm 1) are found. Absorbance of these bands decreases above 400°C. A parallelism in the absorbance decrease is shown for the 1602 and 3700 cm 1 bands (preserved to 600°C) at increased dehydration temperature. These bands are not recovered on rehydration, at either low or at high temperatures. Water molecules are, evidently, localized in the secondary system of channels (8). Perhaps water is localized in cancrinite cells. 

Finally, the 29Si CP/MAS-NMR spectrum of a partially dehydrated sepiolite that was subsequently exposed to acetone vapor is presented in Fig. 2c, and is strikingly similar to the spectrum of the original, untreated sepiolite (Fig. 2a). Since zeolitic water molecules are not present in this sample, and in light of the discussion of the partially dehydrated sepiolite sample, it appears that the acetone molecules have penetrated inside the microporous channels and reversed the structural changes that were caused by partial dehydration. Thus Fig. 2c confirms that acetone molecules enter the microporous channels of sepiolite, and are not simply adsorbed on the crystallite exterior surfaces. 

The first spectrum could be recorded 25 s after admission of alcohol to the catalyst. For all the zeolite samples of various crystallite sizes (Table I) at 296 K, the adsorption was complete within 25 s for sec- and isobutyl alcohols. The dehydration process of these alcohols in the zeolitic pores was, however, slower. For a given alcohol (/ -, sec-, or iso-) the kinetics of water elimination were identical for catalysts of different crystallite sizes. This firmly establishes the absence of any diffusion limitation for dehydration for these three alcohols. 

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