Zeolite extinction coefficient

IR extinction coefficients as a criterion for chemical activation upon adsorption propene interaction with cationic forms of y zeolite... 

Keywords IR spectroscopy, integral molar extinction coefficients, propene, adsorption, zeolites. 

The density of Bronstcd and Lewis acid sites was determined by IR spectroscopy (Nicolet 710) of adsorbed pyridine, after desorption at 250°C, using the molar extinction coefficients previously obtained by Emeis [11]. The acid strength distribution of selected zeolites was studied by NH3-TPD in an Autochem 2910 Equipment (Micromeritics) coupled to a quadrupole mass spectrometer. First, NH3 was adsorbed at 175°C until saturation and then desorbed by increasing the temperature up to 800°C at a heating rate of 10°C/min. 

Table 4.5 Selected experimentally determined IR extinction coefficients for surface species and adsorbates on zeolites. (Adapted from Karge and Geidel [87]).

Species Zeolite type Extinction coefficient (cm mmol Remarks ) Band position (cm- )... 

T-O-T stretch measured by framework IR, as discussed in Section 4.5.3.2. The comparison of areas as described above does provide quantitative information about the relative changes in acidity between the samples since the area is direction proportional to the concentration (Beer-Lambert law, discussed in Section 4.5.2.) It most cases, this relative, but quantitative comparison between samples is sufficient to provide information about how various treatments or modifications have altered acid site distributions. Since extinction coefficients can change with zeolite type (Table 4.5), these comparisons are best for samples of the same zeolite type. Therefore, caution should be used when comparing data from samples with different zeolite structures. 

The ratio of extinction coefficients El/Eb was given the value 1.5 as suggested by Rhee et aL [47] for high silica zeolites. Then (B/L) ratios were calculated from the relative intensity ratios of the three component Njj peaks, under two different hypotheses. First (B/L)3 was calculated assuming that peak 2 corresponded to a Lewis acid site and then... 

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). 

It has been shown that the relatively broad band around 1910 cm, shown in literature (10) for NO adsorbed on Cu zeolites of various structures, consists from several individual bands as indicated by the spectra analysis, described above and depicted in Fig. 3. When NO adsorption is employed for the Cu ion siting characterization, it is necessary to prevent partial reduction of the readily reducible Cu sites. The reduced Cu ions are easily detected in the spectrum as Cu" nitrosyl complexes (Fig. 4B, bands at 1835, 1811, and 1775 cm for Cu-ZSM-5). If no Cu nitrosyl complexes are detected, the spectra correspond to Cu -NO mononitrosyl complexes of all the accessible Cu sites. The integrated intensity of the individual IR bands of Cu -NO can be used for the semiquantitative evaluation of the individual Cu sites, when no bands originate from the mutual interaction of adsorbed NO species, and under assumption that the extinction coefficients of all the adsorbed NO are similar. However, there is a problem of the hidden Cu sites, which cannot be solved by the specific NO adsorption. 

Brpnsted acidity (mmol pyridine (g catalyst) ) of the different zeolites was measured at 623 K and calculated by use of the extinction coefficients given in Ref. 64. 

Figure 4. Spectra of the complex ion Co(OH)u2 from (2) (km, the molar extinction coefficient, curve 1), of molecular sieve Co(II)A with adsorbed water (2), and of dehydrated Co2+-exchanged Type X and Y zeolites (3,4)...

The catalysts were characterized by X-ray diflfractometry (XRD) and infi-ared (IR) spectroscopy. Acidity of the catalysts was tested by pyridine adsorption monitored by IR spectroscopy. Self-supported wafers pressed fi-om zeolite powder (thickness 15 mg cm ) were placed in the sample holder and outgassed at 770 K in vacuum (final vacuimi was better than 10 Pa) for 2 horns followed by cooling to room temperature where the spectrum of the activated zeolites were registered. 1.33 kPa pyridine was adsorbed at 473 K for 1 hour followed by evacuation at the same temperature for 1 h. For calculating the concentration of acid sites extinction coefficients available in the literature were used [15]. 

Vb and Vg stand for the wavenumbers of the beginning and the end of the band, respectively, T (v) and T"(v) for the transmittance along the base line and the band contour, respectively. Modern instrumentation usually allows routine base Hne determination and band integration to evaluate Aj, . In IR and Raman spectroscopy of zeoHtes and adsorbate/zeohte samples, the extinction coefficient, or e,(c), is usually unknown. Thus, if knowledge of the absolute concentrations is required, e, has to be determined in separate experiments (cf., e.g., [ 129-135,]). In such experiments, the absorbance has to be measured of zeolite samples covered with a known number of functional surface groups or loaded with well-defined amounts of adsorbate in order to obtain caUbration cmves, Ajn, vs. c. An example is shown in Fig. 7. 

Fig. 7. Calibration curve for the extinction coefficient of benzene sorbed into zeolite H-ZSM-5 integrated absorbance of the typical benzene band at 1478 cm" vs. the barometrically measured amoimt adsorbed [132]...

Table 2. Experimentally derived IR extinction coefficients of surface species of zeolites and adsorbates on zeolites... 

Species Zeolite, adsorbent Extinction coefficients (cm mmol ) or (cm mmol" ) Remarks Band (cm ) References... 

The concentration of A1 and Fe isomorphously substituting silicon in the frameworks of mordenite, ferrierite, ZSM-5, and ZSM-22, as well as in other zeolites were determined in NH4 and dehydrated/deammoniated forms by IR and Al NMR [01B3]. It has been shown that the intensity of N-H vibration at 1445 cm" of fully exchanged NH4-zeolites with the determined extinction coefficient represented a quantitative measure of the concentration of NH4 ions and, accordingly, the concentration of A1 and Fe in the fiamework positions of the hydrated zeolites [01B3]. 

The concentration of Bronsted and Lewis acidic sites (Table 10.3) was calculated based on the intensity of the respective bands and their corresponding molar extinction coefficients (e) by applying Lambert-Beer law. The quantification of sites showed that the total number of acidic sites of the zeolite did not change after addition of Cu and that a modification in the nature of the acidic sites occurs, wherein Bronsted sites are transformed into Lewis sites. An evaluation of the methanol oxidation catalysts, used as a reaction model, showed that activity and selectivity are influenced by the distribution of acidic sites. 

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