Zeolite signals

Table 5 Experimental results for the chemical shift 3ai of Al NMR zeolites signals observed on...

The first commercial application of truly fluidized catal)4ic cracking was in 1942 at the Standard Oil Co. of New Jersey s Baton Rouge, LA. refinery. The vaporized oil was cracked in a dense, fluidized bed of catalyst ( bed cracking ). Details of that Model I FCC are given by Avidan et al. (1989). Various hardware improvements were made to bed crackers over the ensuing 30 years, but the advent of Davison s synthetic Y zeolites signalled the demise of bed crackers as state-of-art. 

When NOj levels are measured electrochemicaUy, NO and NO2 can lead to opposing signals because NO is oxidized and NO2 tends to be reduced. Moreover, it is preferred to obtain a total NO, measurement instead of only one of the constituents. The latter can be achieved by catalytically equilibrating the feed with oxygen before contact with the sensor by coating an active zeolite layer on top or placing a active catalyst bed in front of the sensor. Both approaches have been demonstrated successfully with a Pt-Y zeohte as active catalyst [74, 75]. The additional advantage of the filter bed is a reduction in the cross-sensitivity with CO due to CO oxidation above 673 K. 

CoSx-MoSx/NaY exhibited doublet bands at 1867 and 1807 cm, accompanying a weak shoulder peak at ca. 1880 cm. These signals are apparently assigned to those of NO molecules adsorbed on Co sulfides. No peaks ascribable to e NO adsorption on Mo sulfide sites were detected at all. What is important in Fig.7 is that in CoSx-MoSx/NaY, coordinative unsaturation sites are present only on the Co sites in spite of the coexistence of the same amount of Mo sulfide species in the zeolite cavities. These results clearly support that the Co sites in CoSx-MoSx/NaY play major roles in the HYD and HDS reactions. 

For infrared spectroscopy, 20-50 mg of the cobalt-exchanged zeolite was pressed into a self-supporting wafer and placed into an infrared cell similar to that described by Joly et al. [21], Spectra were recorded on a Digilab FTS-50 Fourier-transform infrared spectrometer at a resolution of 4 cm-i. Typically, 64 or 256 scans were coadded to obtain a good signal-to-noise ratio. A reference spectrum of Co-ZSM-5 in He taken at the same temperature was subtracted from each spectrum. 

Mn impregnated into MCM-4i, a silicalite containing uniform mesopores of approximately 22 A, catalyzes TBHP epoxidation of alkenes.88 Over Mn-MCM-41, both cis- and trans-stilbene yield trans-stilbene oxide, which the authors conclude signals a radical mechanism.88 In contrast, over Ti—MCM-41, trans-stilbene cannot be oxidized, only cis-stilbene is epoxidized to the cis-stilbene oxide, which suggest a radical-free mechanism.89 Finally, emphasizing the shape selectivity possibilities, only trans-stilbene (not cis-stilbene) can be epoxidized over Mn-ZSM-5, a zeolite with relatively small pores of 5.1 x 5.4 A (Fig. 6.14).88... 

Figure 2 Error signal AFM images of the three different zeolite L preparations where i) and iv) have aspect ratios of 1.5, ii) and v) have aspect ratios of 2.3 and iii) and vi) have aspect ratios of 5.1, and where i)-iii) were taken on the hexagonal (001) face of the crystal and iv)-vi) were taken on a portion of the side walls (100).

The steam treatment does however affect the Al-surroundings in the zeolite crystal. As seen in Fig. 2b, the intensity of both the tetragonally (at 0 ppm) and octahedrally (at 55 ppm) coordinated aluminum species decreases considerably after steam treatments for more than 4 h. Steam treatment for more than 8 h did not lead to a further decrease in the signal intensities. The decreases confirm that aluminum is extracted from the framework during the steaming process, as was also concluded from the 29Si MAS-NMR spectra (Fig. 1 b). This may lead to the formation of additional (micro) porosity, but the aluminum extraction could negatively affect the catalytic activity. 

Similar H NMR spectra were obtained for CHA and LTA zeolites in their hydrated and dehydrated forms. The 29Si spectra show one 29Si(OSi)4 (4Q) signal at -112.7 ppm for... 

The detection of Brpnsted acid sites, SiO(H)Al, is the most recent achievement of 170 NMR of zeolites [119-121]. High magnetic fields and double resonance techniques have allowed the observation of this important species in zeolite HY [120]. Chemical shifts of 21 and 24 ppm have been reported for zeolite HY for the Brpnsted sites in the supercage and sodalite cage, respectively [119]. Quadrupole interaction parameters are Cq = 6.0 and 6.2 MHz and r] = 1.0 and 0.9, respectively. Signal enhancement by 1H-170 cross-polarization has also permitted the detection of the acid sites in zeolite ZSM-5 [119], where they exist with lower abundance than in HY. 

Fig. 7.14 Zeolite thin film FPI chemical sensor, (a) As synthesized outer surface and interference signal, (b) polished outer surface and improved interference signal, and (c) sensor schematic. Reprinted from Ref. 22 with permission. 2008 Molecular Diversity Preservation International...

Lippmaa et al. (38) have also shown that the spectrum of acid-leached USY-A zeolite has a stronger Si(0 Al) signal and a weaker Si(2 Al) signal than the corresponding USY-A spectrum. This was explained by the extraction of some framework Al atoms in addition to non-framework Al. In the vacancies created in the framework by Al extraction, (SiO) Si-OH groups are formed, as was found from the CP spectrum. 

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