Characterization by Diffuse Reflectance IR Spectroscopy

Examples of successful experiments with diffuse reflectance spectroscopy on zeohtes, leading to interesting results and interpretations were reported, for instance, by Kazansky and coworkers [546-554] and Staudte and colleagues [555-560]. Kazansky and his group studied a series of various zeoHtes including sihca-rich ones. Spectra were obtained in the range from 3400 to 7000 cm k As an example. Fig. 26 shows the spectrum of an H-ZSM-5 sample (nsi/nAi=30). 

For the sake of comparison, the diffuse reflectance IR spectrum of amorphous silica was also studied The latter showed bands at 3745,4550 and 7325 cm which were clearly indicative of the fundamental OH stretching vibration of silanol groups, i.e., 9[Si(OH)], its combination with a bending OH mode, i.e.. 

Medin et al. [561] were able to elucidate through DRIFT experiments special features of the Lewis acidity of H-ZSM-5 zeolites, which were loaded with preadsorbed olefins. The authors used DRIFT and as probes CD3CN and NH3 (cf. also Sects. 5.5.2.6.2 and 5.S.2.6.4). On the basis of their experimental results they proposed a mechanism for the formation of strong Lewis sites, which includes breaking of Al-0 bonds in framework fragments like [=OAl-0-SiOs] by basic molecules and a positional inversion of the Al atom. 

Al-OH 3725 cm terminal Si-OH, the oxygen of which was assumed to interact with extra-framework Al-species 3665 cm h Al-OH still coimected to the framework by one or two bonds 3650 cm terminal Si-OH, the proton of which is interacting with extra-framework Al. The study is amended by DRIFT investigations of H2 and C3H8 adsorption. For instance, H2 adsorption on Lewis sites produced a typical band, V (Hj— L),at 4080 cm (cf. also Sects. 5.5.2.1 and 5.5.2.6.3). 

Diffuse reflectance IR spectroscopy (DRIFT) was, furthermore, successfully employed in studying molecular sieves of the new generation [957] such as aluminophosphates (AlPOs) and silicoaluminophosphates (SAPOs). Thus, Zscherpel et al. and Loeffler et al. [563,564] studied by DRIFT the range of fundamental OH stretching, OH deformation and framework vibrations of AIPO4-18 (upon decomposition of the template, tetraethylanunonium hydroxide), the fundamental and combination vibrations of hydroxy groups of SAPO-5 and SAPO-17 as well as the framework region of SAPO-34. In the latter case a comparison was made with the transmission spectrum. 

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