Lewis acid sites pyridine adsorbed

Fig.4. Concentration of Bronsted acid sites (A) and Lewis acid sites (B) adsorbing pyridine at 150°C vs t, the number of coke molecules. ( ) after reaction, ( ) after ageing.

Table 7.2 Correspondence between (1) Lewis acid site, (2) adsorbed pyridine 8a mode, and (3) O-H stretch of isolated hydroxyl groups present on the surface of activated H-AI2O3. ...

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

For determination of the number of protonic sites and Lewis acid sites on the surface, the integrated absorbances of the bands at 1450 cm-i (due to pyridine chemisorbed on Lewis acid sites, L-Py) and 1490 cm i (due to both the L-Py and pyridine chemisorbed on protonic acid sites, B-Py) were used with the tangent background for all samples. The values obtained were normalized to the weight of the sample wafer. To obtain the apparent absorption coefficients of the bands, a known amount of pyridine was adsorbed on the sample, and the absorbance of each band was measured. Then, a small quantity of water which is sufficient to convert all Lewis acid sites into protonic acid sites was introduced into the IR-... 

Figure 3.53. IR transmission absorption spectrum of pyridine adsorbed on partly dehydroxylated HY zeolite (Van Bekkum et al, 1991) B = Bronsted acid sites L = Lewis acid sites.

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. 

Infrared spectra of pyridine adsorbed on dehydrated TS-1 and Ti-MCM-41 of comparable Ti content indicated the presence of only Lewis acid sites (Fig. 13). The infrared absorptions at 1595 and 1445 cm-1 are attributed to hydrogen-bonded pyridine (Si/Ti-OH—pyridine) and those at 1580 and 1485 cm-1 to pyridine bonded to weak Lewis acid sites (Fig. 12). Brpnsted sites, if present,... 

Basic molecules such as pyridine and NH3 have been the popular choice as the basic probe molecules since they are stable and one can differentiate and quantify the Bronsted and Lewis sites. Their main drawback is that they are very strong bases and hence adsorb nonspecifically even on the weakest acid sites. Therefore, weaker bases such as CO, NO, and acetonitrile have been used as probe molecules for solid acid catalysts. Adsorption of CO at low temperatures (77 K) is commonly used because CO is a weak base, has a small molecular size, a very intense vc=0 band that is quite sensitive to perturbations, is unreactive at low temperature, and interacts specifically with hydroxyl groups and metal cationic Lewis acid sites.26... 

Lewis acid sites can coordinate with a given indicator molecule to produce an adsorption band identical in position with that produced through proton addition. Even if the indicators used are responsive only to Brpn-sted acids, most basic reagents used to titrate surface acidity (e.g., n-butylamine, pyridine) are strongly adsorbed on surface sites other than Br0nsted acid sites. In this connection, a recent study indicates that adsorption equilibrium is not fully established during titration of silica-alumina with n-butylamine because of the irreversible attachment of amine molecules by adsorption sites at which they first arrive (31). 

Infrared spectral studies of pyridine adsorbed on alkali metal ion-exchanged faujasites have demonstrated the absence of Brpnsted acidity, as reported by Eberly (151), Ignat eva et al. (208), and Ward (156, 209-211). Pyridine is adsorbed weakly by coordination to the alkali metal ions (151, 156). Addition of small amounts of water does not result in formation of Br0nsted acid sites, indicating that the coordinate bound pyridine is not associated with Lewis acid sites in the zeolite framework (210). 

Lefrancois and Malbois (227) determined the types of acidity present on H-mordenite and various cationic forms by obtaining infrared spectra of pyridine adsorbed on the zeolite. H-mordenite activated at 400° contained both Br0nsted and Lewis acid sites. Upon addition of water, the band due to Lewis-bound pyridine disappeared and the Br0nsted site concentration increased. Removal of the added water by evacuation restored some of the Lewis acid sites. 

Infrared spectra of pyridine adsorbed on kaolinite indicated that the dry clay (110°C) contained both Brtfnsted and Lewis acid sites (235). At 1% water content only protonic acid sites were observed. It was not possible to assign the polymerization activity to either type of acid site, since both were present on samples which were catalytically active. 

Infrared has also been used to assess the type of acidity present on a catalyst. The method involves measuring the spectrum of adsorbed pyridine on the catalyst certain characteristic absorption frequencies are assigned to Lewis acid centers (coordinately bound pyridine) and others to Brqnsted sites (pyridine adsorbed as pyridinium ion). 

Infrared Spectra. Figure 2 shows the spectra of pyridine adsorbed on / -alumina. Two types of Lewis acid sites are present strong Lewis acid sites, which still bind pyridine on evacuation at 350°C and characterized by the 1622 and 1454 cm-1 bands and weak Lewis acid sites, characterized by the 1614 and 1450 cm- bands. Br nsted acid sites, which have characteristic bands around... 

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