Pyridine probing Lewis acid cation sites

Similarly, IR investigation of CO adsorption on molecular sieves was used to characterize Lewis acidity of cations (C-sites) and true Lewis acidity (L-sites) [ 740]. The interaction of CO with cations (acid C-sites) was dealt with already in Sect. 5.5.2.2. In particular, Angell and Schaffer [595] have carried out a detailed study of CO adsorption on a series of X- and Y-type zeoHtes containing monovalent and divalent cations of alkali, alkaline earth and transition metals. A linear relationship was found between the position of the IR stretching band of adsorbed CO and the Coulomb field, q/r, of the respective cationic adsorption center. This is similar to the observation made by Ward in the case of pyridine attached to cations (vide supra). It should be noted, however, that CO, like pyridine, is not capable of entering the sodalite cages and the hexagonal prisms of the faujasite structure, so that the cations located there are not detected by these probes. 

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... 

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... 

Although they are not as widely used as pyridine, substituted pyridines have found their own place in the characterization of hydroxyl groups. It was proposed (470) that, for steric reasons, 2,6-dimethylpyridine (DMP, lutidine) does not interact with Lewis acid sites and is thus a proton-specific probe. Later, it was demonstrated that DMP (247,256,471-473) (as well as 2,4,6-trimethyl pyridine or collidine (474)) stiU forms coordination bonds with coordinatively unsaturated surface cations. This bond is only weakened by the steric interference of the methyl groups with the surface but not prevented. In any case, the steric hindrance leads to preferential interaction ofDMP with hydroxyl groups (471-473). DFT calculations suggest that the proton transfer is promoted by the stabilization of the lutidinium ion on the deprotonated site, rather than by the intrinsic acidity of the acid site itself (475). 

Consumption of acid OH groups upon introduction of Mn cations led to a decrease in Brpnsted and a concomitant increase in Lewis acidity, as indicated by IR using pyridine as a probe. The decrease in Bronsted acidity was indicated by a decrease in the intensity of the pyridinium ion band at 1540 cm" observed upon pyridine adsorption on Mn-ZSM-5 samples that had been prepared by SSIE. A corresponding increase in the Lewis acidity effected an enhancement of the absorbance around 1450-1454 cm" which is indicative of pyridine coordinated to electron pair acceptors or Lewis sites such as Mn +. This was compared with similar IR measurements on the parent H-ZSM-5. Figure 51 shows a linear correlation between the density of Bronsted and Lewis acid sites of the parent H-ZSM-5 as well as of Mn,H-ZSM-5 samples measured by this technique. 

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