Pyridine probing Bronsted acid sites

The aluminum is incorporated in a tetrahedral way into the mesoporous structure, given place to Bronsted acidic sites which are corroborated by FTIR using pyridine as probe molecule. The presence of aluminum reduces the quantity of amorphous carbon produced in the synthesis of carbon nanotubes which does not happen for mesoporous silica impregnated only with iron. It was observed a decrease in thermal stability of MWCNTs due to the presence of more metal particles which help to their earlier oxidation process. 

Two types of probe molecules have been used for the detection of Lewis and Bronsted acid sites. The first involves the adsorption of relatively strong basic molecules such as pyridine, ammonia, quinoline, and diazines. The second kind involves the adsorption of weak base molecules such as CO, NO, acetone, acetonitrile, and olefins. The pioneering works of Parry27 and Hughes and... 

Ammonia (pfCi = 9.24, proton affinity in gas phase = 857.7kj moT ) and pyridine (pfti = 5.19, proton affinity in gas phase = 922.2kJ moT ) are the favored molecules for probing the overall solid acidity, since both Lewis and Bronsted acid sites retain these molecules. However the use of IR spectroscopy or XPS is necessary to distinguish qualitatively and unambiguously between these two types of sites. In addition to NH3 and pyridine, trimethylamine and triethylamine have also been used to probe the acidity of supported oxides. However, it has been mentioned [33] that these two molecules might not be able to equilibrate completely with the surface under typical experimental conditions. The use of substituted pyridines (2,6-dimethylpyridine) has also been considered in order to probe specifically the Bronsted sites [34]. 

This has been attributed to the use of ammonia as a probe molecule, since this cannot distinguish between Lewis and Bronsted acidity. Cornaglia and coworkers [79] measured the acid sites using pyridine and acetonitrile. However, the pyridine results showed no correlahon between the activity and selectivity to maleic anhydride and either the Lewis-to-Bronsted acid sites ratio (L/B acidity ratio) or the Lewis acid site concentration. 

The Bronsted acid sites of HY zeolites dealuminated either by conventional treatment (steaming + acid leaching) or isomorphous substitution (fluorosilicate) have been characterized at each step of the preparation procedures through IR spectroscopy of probe molecules with various basic strengths (pyridine, C2H4,... 

Most acidity studies have been made using basic molecules such as ammonia, pyridine, and piperidine as probes. These molecules have the property that their interaction with Bronsted acid sites, Lewis acid sites, and cations and their hydrogen-bonding interactions give rise to different species detectable by infrared spectroscopy. Thus, adsorption on Bronsted acid sites gives rise to ammonium, pyridinium, and piperidinium ions with characteristic absorption frequencies of 1475, 1545, and 1610 cm"1, respectively. Adsorption on Lewis acid sites—tricoordinated aluminum... 

Table 2.20 summarizes the spectral parameters of differently bound DMP. Compared to pyridine, lutidine seems to have some advantages as a probe. The most important aspect is that the spectra of protonated lutidine seem to contain information on the strength of the Bronsted acid sites. It was demonstrated (256), with a series of FAU zeolites, that the positions of the Vga and v(NH) bands (2975-2494 cm ) of DMPH depend on the acid... 

The adsorption of strong bases such as ammonia or pyridine on Bronsted sites results in proton donation. The protonated bases remain within the solid, themselves showing hydrogen bonding to the framework. The infrared spectrum then resembles that of the ammonium (or pyridinium) cation. The pyridinium ion has a characteristic infrared spectrum with absorbances at ca. 1540 and 1490 cm Strong bases of this kind have very high enthalpies of interaction, and titrate any acid sites that are present. As such, they are not sensitive probes of acid site strength. 

There are several probe molecules for which infrared spectroscopy can differentiate between adsorption on Bronsted and Lewis acid sites and even estimate the amounts adsorbed. Pyridine is the most widely used because it gives well-resolved bands when protonated by Bronsted acid sites (e.g., 1540 and 1640 cm ) or when coordinated to Lewis acid sites (1450 and 1620 cm ). The values of extinction coefficients are available in the literature [121] for these bands, which makes possible semiquantitative measurements, separately, of Lewis and Bronsted sites. Ammonia, with a smaller kinetic diameter that enables it to reach more easily the acid sites in smaller pores, can also be used to distinguish betwen Bronsted and Lewis acid sites however, the use of ammonia is less reliable, mainly because the resulting IR bands overlap each other [122]. Another base that can distinguish between Bronsted and Lewis acid sites is quinoline because its size is greater than that of pyridine quinoline can also be used to differentiate between acid sites at the external surface and those in pores smaller than its kinetic diameter (6 A). Bronsted sites can be selectively measured with IR methods by using substituted pyridines as probe molecules [123]. 

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