Framework asymmetric stretching modes

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. 

Fig. 12. Wavenumbers of various framework vibrations as a function of the mole fraction of aiuminum in tetrahedral (T) sites of the zeolite framework s indicates the slope, i.e., the decrease of the wavenumber (cm ) per 0.1 atom fraction of A1 ion substitution (adopted from [112]). D6R means double six-membered ring, and stand for symmetric and asymmetric stretching modes, respectively (cf. text and list of abbreviations)...

Some vibrational modes of zeolites are sensitive to the amount of aluminum in the framework [93]. The substitution of aluminum for silicon atoms in the zeolite framework changes the T-O-T bond angles (where T is a tetrahedral atom that can be either Si or Al). This is primarily due to the smaller size and different charge density of the aluminum atoms compared to silicon. This results in a shift in frequency for vibrational modes in the zeolite due to external linkages. The T-O-T asymmetric (1100-980 cm ) and symmetric (800-600 cm ) stretching modes as well as structural unit vibrations Mke double four- and double six-rings exhibit a shift to lower frequencies as the aluminum content of the framework is increased. Figure 4.19 shows this relationship for a faujasite-type framework. 

Thus 28 IR active modes are expected to fall in the regions of the vibrations of the orthosilicate anions. Of these, we can expect five modes associated with V3 (asymmetric stretching) and two modes associated with Vi (symmetric stretching), three modes associated with the symmetric deformation (V2) and five with the asymmetric deformation V4, four hindered rotations, four hindered translations, and, finally, five modes associated with Al—O tetrahedra. We actually observe at least 10 components for framework vibrations. Additionally, the low-frequency modes of Na ions are expected to fall in the FIR region [68], where several bands are indeed observed. 

Highly sihceous zeoHtes (with Si/Al ratios 1) are microporous framework alumino-sihcate materials. Discussion of the framework skeletal vibrahons of highly sihceous zeolites is similar to that reported above for silicas. The addition of aluminum in the framework causes shifts in the positions of the sole band. In particular, the asymmetric Si—O—Si stretching modes of framework silicates, usually observed as a complex very strong absorption in the region 1200-1000 cm" , tend to shift down a litde with A1 for Si subshtution. 

As can be realized from Fig. 12, the wavenumbers of, e.g., the asymmetric (v s) and symmetric (Vj) stretching modes were found to be linearly correlated to the mole fraction, n i/lnsi+nAi), of the frameworks of a homologous series of fauja-site-type zeolites. 

The Si02 framework vibrations that occur at about 430, 800, 1070, and 1180cm in the fused silica spectrum (discussed in Section 1.4.2) can be explained by a vibrational calculation on a continuous random network (CRN) [36,37]. The 1070 and 1180 cm bands are assigned to the TO and LO modes of the Si-O asymmetric stretching vibration, respectively. Due to the selection rules, Raman-active modes involve symmetric vibrations, which... 

---