IR framework spectra

For pure Si-MCM-41. this band has been assigned to the Si-O stretching vibrations and the presence of this band in the pure siliceous is due to the great amount of silanol groups present. A characteristic absorption band at about 970 cm 1 has been observed in all the framework IR spectra of titanium-silicalites. It was also reported that the intensity of 970 cm 1 band increased as a function of titanium in the lattice[17] and this absorption band is attributed to an asymmetric stretching mode of tetrahetral Si-O-Ti linkages [18] in the zeolitic framework. The increase in intensity of this peak with the Ti content has been taken as a proof of incorporation of titanium into the framework. 

The framework IR spectra were recorded in a Nicolet (60 SXB model) instrument using KBr pellet technique. 1 l"Sn MAS NMR spectra were obtained at 111.82 MHz on a Broker MSL-300 NMR instrument. Typically around 3000 transients were signal averaged before Fourier transformation. The chemical shifts were referenced externally to tetramethyltin. 

Figure 2. Framework IR spectra of Sn- silicalites of MFI(a), MEL(b) and MTW(c) structures compared with Sn-impregnated silicalite-1 (d) and pure Sn02 (e).

The framework IR spectra of metallosilicates reveal a band at -960-970 cm, the band being attributed to Si-O vibrations of defect (internal) silanol groups associated with the metal locations in the framework [18]. As a consequence, the presence of the band is generally used as an evidence for the presence of the metal in lattice positions associated with defect sites [6,7]. The framework IR spectra of the calcined samples A and B are presented in Fig. 3. The spectrum of B has a band at 967 cm" and not that of A suggesting the absence of defect silanols and, presumably, the absence of vanadium in the framework positions in the latter sample. 

The IR spectra of silicon oxides, in the framework region mode, is dominated by a strong absorption around 1000 cm due the anti-symmetric stretching of the Si - 0 - Si unit (Raman inactive mode) and by a less intense absorption around 800cm due the symmetric stretching of the Si-O-Si unit (Raman active mode). In the transparency window between these two modes, the IR spectra of TS-1 shows an additional absorption band located at 960 cm ... 

Fig. 4 a IR spectra, in the OH stretching region, of from top to bottom, TS-1 samples (full line spectra) with increasing Ti content, from 0 (silicalite-1, dashed spectrum) to 2.64 atoms per imit cell. All samples have been activated at 120 °C. Adapted from [24] with permission. Copyright (2001) by the ACS. b Schematic representation of the preferential location of Ti atoms and Si vacancies in the MFI framework (upper part) and their interplay (lower part). Yellow and red sticks represents Si and O of the regular MFI lattice blue balls refer to Ti, and red and white balls to O and H of defective internal OH groups... 

For example, clusters identified by IR spectra and extraction as Ir4(CO)i2 on y-Al203 were found by EXAFS spectroscopy to have an Ir-Ir coordination number of nearly 3, consistent with the tetrahedral structure of the metal frame EXAFS spectroscopy produces the equivalent result for sohd Ir4(CO)i2 [27]. EXAFS spectroscopy is the most appropriate method for determination of framework structures of supported clusters, but it is limited by the errors to clusters with at most about six metal atoms. Thus, it has been used to determine frameworks that are triangular (EXAFS first-shell metal-metal coordination number of 2), tetrahedral (EXAFS first-shell metal-metal coordination number of 3), and octahedral (EXAFS first-shell metal-metal... 

The interaction of CO and acetonitrile with extra-framework metal-cation sites in zeolites was investigated at the periodic DFT level and using IR spectroscopy. The stability and IR spectra of adsorption complexes formed in M+-zcolitcs can be understood in detail only when both, (i) the interaction of the adsorbed molecule with the metal cation and (ii) the interaction of the opposite end of the molecule (the hydrocarbon part of acetonitrile or the oxygen atom of CO) with the zeolite are considered. These effects, which can be classified as the effect from the bottom and the effect from the top, respectively, are critically analyzed and discussed. 

The IR spectra of CO adsorbed on CuK-FER with the same Cu/Al ratio (Cu/Al = 0.09) but different Si/Al ratio (nominal value of Si/Al ratio are 8.6 and 27.5) were taken at the same experimental conditions (Figure IB). Since dual cation sites capable of bridging CO should be more abundant in the FER matrix with a lower Si/Al ratio (i.e., with a higher concentration of Al in the framework and, thus, higher concentration of extraframework cations in the zeolite), the band at 2138 cm"1 should be more prominent for the sample with lower Si/Al ratio. Indeed, it is clearly seen that the band at 2138 cm"1 is more pronounced in the case of the CuK-FER sample with lower Si/Al ratio (Figure IB). 

In addition, there are some weaker broad bands that are typically observed between 1900 and 1500 cm" that have been assigned to overtones of framework vibrations. The IR spectra in Figure 4.22 are truncated at 1300 cm" because the absorbance of the sample is too high to measure at lower frequencies (<1200 cm" ). This is due to the very strong T-O-T stretching vibrations of the zeolite as mentioned in the previous section on framework IR measurements. 

Calcined and steamed FAU samples also have complex hydroxyl IR spectra. Figure 4.25 shows the difference between an ammonium ion-exchanged FAU before and after steaming and calcination. The very simple, easily interpretable hydroxyl spectrum of the ammonium exchanged FAU sample is transformed into a complex series of overlapping hydroxyl bands due to contributions from framework and non-framework aluminum atoms in the zeolite resulting from the hydrothermal treatment conditions [101]. 

Dihydrogen bonding energies can be determined in solution by variable-temperature IR spectra in the framework of van t Hoffs method or from the frequency shifts, Av (HX) [or integral intensity, AA(XH)], determined in the IR spectra of proton-donor components, which correlate with the bonding energy. 

Methoxymurrayanine (3) (3-formyl-l,6-dimethoxycarbazole) (15) was isolated from the roots of C. lansium (23). The roots of C. lansium were used to treat bronchitis and malaria. The UV (/Imax 227, 239, 251, 294, 335, and 349 nm) and IR spectra showed the presence of a 3-formylcarbazole framework. The H-NMR data were similar to murrayanine (9), with an additional methoxy group for one of the aromatic hydrogens at the C-ring of the carbazole. Since the signal for H-5 (5 7.90) is not ortho-coupled, an additional methoxy could be located at C-6. This structural conclusion was also supported by the C-NMR spectrum. 

Bulk amounts of elements were determined by atomic absorption spectrophotometry. The amount of framework A1 was determined by Al MAS NMR. The acidic properties of the metallosilicates were determined by IR and NH3-TPD measurements. Before the IR measurements, the sample wafer was evacuated at 773 K for 1.5 h. In the observation of pyridine adsorbed on metallosilicates, the sample wafer was exposed to pyridine vapor (1.3 kPa) at 423 K for 1 h, then was evacuated at the same temperature for 1 h. All IR spectra were recorded at room temperature. NH3-TPD experiments were performed using a quadrupole mass spectrometer as a detector for ammonia desorbed. The sample zeolite dehydrated at 773 K for 1 h was brought into contact with a 21 kPa of NH3 gas at 423 K for 0.5 h, then evacuated at the same temperature for 1 h. The samples were cooled to room temperature, and the spectra obtained at a heating rate of 10 K min from 314 to 848 K. 

III.B. In order to analyze the wavefunction in a chemically more intuitive way, it is useful to localize it. In the framework of AIMD this is, for example, done by calculating the maximally localized Wannier functions (MLWF) and the corresponding expectation values of the position operator for a MLWF basis the so-called maximally localized Wannier centers (MLWCs), see Fig. 1 (67-72). With the help of the MLWC it is possible to compute molecular dipole moments (72-82). Furthermore, it is possible with the MLWC to obtain molecular properties, e.g., IR spectra (75,76,82-85). 

For catalytic application it is necessary to incorporate hetero-atoms into the silica framework. Several samples have been synthesised using different aluminia precursors. The metal content was determined by X-ray fluorescence analysis, UV-VIS spectra, IR spectra and solid state NMR spectroscopy, respectively. X-ray fluorescence analysis provides information about the metal content of the samples. By variation of the metallic precursor concentration the metal content of the product could be enhanced up to 10 % w/w. 

Figure 2. IR spectra of framework vibrations of MCM-41 and amorphous Si02-...

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