Infrared band spectroscopy zeolites

Infrared Spectroscopy. Infrared (IR) spectroscopy is probably the one instrumental technique that has been applied most often to the study of catalysts in general and to acidity in particular. IR spectroscopy can be used to detect Brpnsted acidity in zeolites directly by measurement of the OH stretching bands and to ascertain structural information based on the abundance of various OH bands and in the metal framework region. 

From the infrared spectroscopy of adsorbed CO it appears that aging treatments, as low as 923 K, lead to a migration of the active isolated copper ions to inaccessible sites. In these conditions an agglomeration is not detected but, after aging at 1173 K, an agglomeration is evidenced both by XRD and by the infrared bands of CO adsorbed on partially reduced bulk CuO oxide. These accessible copper oxide crystallites are probably located at the external surface of the zeolite and are inactive. In fact, the activity remains correlated to the number of Cu VCu isolated ions deduced from the infrared spectra of adsorbed CO and located in the zeolite structure. This correlation holds whatever the treatment and whatever the Si/Al ratio (Table 4). 

The isomorphous substitution of tetrahedral A1 in zeolite structure with elements such as gallium, boron or iron has been described in a large number of papers. In the case of Fe for instance, the presence of Fe h jp the framework gives rise to new infrared bands assigned to Si-O-Fe bonds (8). Mossbauer spectroscopy, which is a very valuable tool in this case,... 

D correlation analysis is a powerful tool applicable to the examination of data obtained from infrared spectroscopy. The correlation intensities, displayed in the form of 2D maps, allow us to correlate the shift induced by CO adsorption and acidity of sites in dealuminated zeolites. Results are in accordance with previous results, obtained using only IR measurements, proving the validity of this technique. New correlations allowed the assignment of very complex groups of bands, and 2D correlation revealed itself as a great help for understanding acidity in dealuminated zeolites. 2D correlation has allowed us to validate the model obtained by NMR. 

Infrared spectroscopy has proven to be a very informative and powerful technique for the characterization of zeolitic materials. Most infrared spectrometers measure the absorption of radiation in the mid-infrared region of the electromagnetic spectrum (4000-400 cm or 2.5-25 xm). In this region of the spectrum, absorption is due to various vibrational modes in the sample. Analysis of these vibrational absorption bands provides information about the chemical species present. This includes information about the structure of the zeolite as well as other functional... 

Infrared spectroscopy can be used to obtain a great deal of information about zeolitic materials. As mentioned earlier, analysis of the resulting absorbance bands can be used to get information about the structure of the zeolite and other functional groups present due to the synthesis and subsequent treatments. In addition, infrared spectroscopy can be combined with adsorption of weak acid and base probe molecules to obtain information about the acidity and basicity of the material. Other probe molecules such as carbon monoxide and nitric oxide can be used to get information about the oxidation state, dispersion and location of metals on metal-loaded zeolites. 

Infrared spectroscopy has been used to help solve or determine the structure of zeolites. The technique is particularly useful for identifying the presence of double four- and six-rings as well as five-membered pentasil rings. In the structural characterization of beta zeolite, Newsam and coworkers used a variety of techniques including IR, electron microscopy (TEM), X-ray diffraction (XRD) and sorption data to solve the stacked, faulted structure [57]. The presence of IR absorption bands at 1232 and 560cm indicated that the structure contained five-member pentasil building units. 

Nitrogen dioxide, N02, is a fairly small molecule with an unpaired electron and may be expected to be a selective molecule for electron-deficient or Lewis acid sites. Nevertheless, only very little spectroscopic information on the nature of surface species formed on adsorption of N02 is available. Naccache and Ben Taarit (242) have shown by infrared spectroscopy and ESR that N02 forms Cr+N02+ and Ni+N02+ complexes on chromium and nickel zeolites. Thus, N02 behaves as an electron donor and reducing agent in these zeolites. Boehm (243) has studied the adsorption of N02 on anatase and on tj-A1203, which were pretreated at very low temperatures of only 100°-150°C. At 1380 cm-1, a band which is to be attributed to a free nitrate ion, was observed. Boehm (243) has explained the formation of the nitrate ion by the disproportionation by basic OH ions ... 

Infrared spectroscopy has been used for many years to probe acid sites in zeolites. Typically, strong bases such as ammonia or pyridine are adsorbed, and the relative or absolute intensities of bands due to Lewis acid adducts or protonated Bronsted acid adducts are measured. The basicity of ammonia or pyridine is however much stronger than that of most hydrocarbon reactants in zeolite catalysed reactions. Such probe molecules therefore detect all of the acid sites in a zeolite, including those weaker acid sites which do not participate in the catalytic reaction. Interest has recently grown in using much more weakly basic probe molecules which will be more sensitive to variations in acid strength. It is also important in studying smaller pore zeolites to use probe molecules which can easily access all of the available pore volume. 

The low intensity of the Raman bands intrinsic to zeolite structures is an advantage when attempting to observe adsorbed molecules (in contrast to the situation in infrared spectroscopy, where large regions of... 

As can be seen in Fig. 5, N conversion using H-ZSM-11 zeolite seems to be correlated with the number of Bronsted sites on the external surface (deduced from measurements of methylene blue adsorption capacity) and not with the total niunber of Bronsted sites (determined by the total pyridine adsorbed on Bronsted sites and desorbed at 150°C by FT-IR spectroscopy), using the literature data on the integrated molar extinction coefficients [17], (for infrared absorption bands of pyridine adsorbed on solids acid catalyst [17], providing no dependence of the integrated coefficients on the catalyst or strength of the sites). 

Zeolites. The weak Raman signals arising from the aluminosilicate zeolite framework allow for the detection of vibrational bands of adsorbates, especially below 1200 cm which are not readily accessible to infrared absorption techniques. Raman spectroscopy is an extremely effective characterization method when two or more colored species coexist on the surface, since the spectrum of one of the species may be enhanced selectively by a careful choice of the exciting line. A wide range of adsorbate/zeolite systems have been examined by Raman spectroscopy and include SO2, NO2, acety-lene/polyacetylene, dimethylacetylene, benzene, pyridine, pyrazine, cyclopropane, and halogens. Extensive discussions of these absorbate/zeolite studies are found in a review article by Bartlett and Cooney. ... 

The powder X-ray diffraction patterns were measured in a D-500 SIEMENS diffractometer with a graphite seeondary beam monochromator and CuKoj contribution was eliminated by the DIFFRAC/AT software to obtain a monochromatic CuKa,. The Unit Cell Size (UCS) was measured following the ASTM D-3942-90 procedure. The Surface areas were measured by nitrogen adsorption at 75 K on a Micromeritics Accusorb 2100 E equipment using the ASTM method D-3663-78. Temperature Programmed Desorption (TPD) of ammonia and pyridine adsorption by Infrared Spectroscopy (IR) were used to characterize the acidity of the zeolites. For IR-Pyridine the spectra were recorded each 100°C and the characteristic bands of Lewis and Bronsted acid sites (1444 cm" and 1540 cm, respectively) were integrated in order to obtain the total acid sites. 

Infrared spectroscopy can be used to monitor the crystallinity of ZSM-5 preparations through observation of bands due to vibrations of the zeolite lattice. 

Infrared spectroscopy. The IR spectra of Ga-ZSM 5 (Fig. 2) also provide evidence for the incorporation of gallium into the zeolite framework during crystal growth. The asymmetric T-O-T stretching vibration band is shifted to lower wave numbers by... 

Infrared spectroscopic studies have shown that adsorbed carbon monoxide interacts with Brensted acid Si(OH)Al groups of the zeolite H-ZSM-5 forming hydrogen-bonded H-CO and H—OC species, which are characterized by C-0 stretching IR absorption bands at 2175 and 2112 cm", respectively. By means of variable-temperature FTIR spectroscopy, these C-bonded and O-bonded adducts were found to be in a temperature dependent equilibrium which can be described as ZH CO = ZH- OC, where Z stands for the zeolite framework. The corresponding enthalpy change was found to be AH° = 4.2 kJ mol", as derived from a van t Hoff analysis of the intensity of the corresponding IR absorption bands as a function of temperature. 

As can be seen, two bands at 102 and 73 cm" arise. However, a reliable interpretation of this spectrum is quite difficult. The key problem in interpreting far-infrared spectra of silicon-rich zeolites such as ZSM-5 is connected with the fact that, due to the low cation concentration, structural information about cation sites are so far rather scarce. Under the mentioned conditions it would certainly be a substantial progress if the vibrational assignment in the far-infrared region could be assisted by other suitable cation-sensitive techniques which provide additional information. One way, as chosen in Ref. [363], is to start from X-ray absorption spectroscopy (XAS) giving access to the local environment of the cations and their coordination spheres. For the dehydrated Ba-ZSM-5 sample a six-fold oxygen coordination at a distance of 2.75 A was obtained for Ba " ions by EXAFS analysis of the XAS spectrum. In a second step, positions in the unit cell of ZSM-5, which fulfill these criteria, were searched by computer simulation... 

Examples of building up large molecules inside a zeolite structure via a reaction were frequently reported under the term ship-in-the-bottle synthesis . For instance, very early Romanowski et al. [844] and, somewhat later, Schulz-Ekloff et al. [845] produced phthalocyanines in faujasite-type zeolites and investigated the pro ducts, inter alia, by IR spectroscopy. Cobalt-phthalocyanine encapsulated in zeolite EMT was prepared and, inter alia, characterized via IR spectroscopy by Ernst et al. (cf. [846] and references to related work therein). A number of typical bands of CoPc-EMT in the mid infrared (1600-1200 cm ) were observed and interpreted. 

Time-resolved in-situ micro-infrared spectroscopy was applied for the first time by Nowotny et al. [862] on a process occurring in a zeolite single crystal. These authors monitored the thermal decomposition of the template (tripropylamine, TPA) in single crystals of MFI materials with nsi/n i ratios below 1000 (silicalite-1), 122, and 31 (ZSM-5). Bands of the CH stretching modes at 2981, 2943,2882 and 2746 cm and of the CH bending vibrations at 1473 and 1458 cm" were observed. From the presence of the latter two bands the authors concluded that at ambient temperature TPA is rather immobile in the channels of the as-synthesized ZSM-5 crystals. From their measurements they arrived at the conclusions that... 

XRD is useful to determine cation location in the faujasite structure. For some light cations such as Li or for structures with many possible sites, far infrared spectroscopy may give valuable information. It has been applied mainly to faujasites (figure 13) (80-82) and quite less to A or ZSM-5 (82). In each structure the cations in the various sites give rise to bands at a given wavenumber (80) which shift upon adsorption of H2O (81,82) or other adsorbates such as H2S, C6H6... (81). These results clearly indicate that the interaction cation - adsorbate described above implies also a change in the interaction of the cation with the zeolite framework. The deconvolution of the spectra may lead to the evaluation of the cation population in each site. 

The study of the dynamics of N isotope transfer under adsorption-desorption equilibrium (NO -1- O2 + He) revealed two types of NOx complexes, and their concentrations and formation rates (depending on NO and O2 concentrations) were estimated. According to in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) data, these complexes are assigned to nitrite-nitrate (1520 cm" ) and N02 species (2130 cm" ). Note that nitrite-nitrates and N02 differ clearly in the rates of their formation. Under the reaction conditions, the concentrations of both active species drop considerably. Therefore, two parallel reaction pathways were proposed that involve both active complexes. The rates of NOx complexes interaction with methane were also calculated, and the reaction with participation of N02 species was shown to proceed about 2.5 times faster than that of nitrite-nitrate. The N02 species was determined to form at the interface between CoO clusters and acid OH groups in zeolite (or at the paired Co -OH sites). This finding agrees well with in situ DRIFTS data that indicates that the N02 formation correlates with a drop in the acid OH group band intensity. 

---