Lutidine probe

Infrared spectroscopy measurements were performed using a Perkin Elmer 580 apparatus. The acid strength distribution of the samples was measured using both calorimetric and volumetric gas-solid titration. Ammonia, pyridine, and branched pyridines (2,6-lutidine and 3,5-lutidine) were the selected probes. They were further dried over activated 3A molecular sieve extrudates and were purified by freeze-thaw techniques. 

In order to more precisely differenciate the acid sites, adsorption of pyridine (pKa=5.25), 3,5-dimethylpyridine (pKa=6.15) and 2,6-dimethylpyridine (pKa=6.72) was carried out at 353 K on the samples. These three basic probes display a lower pKa than ammonia (pKa=9.25) and should titrate less weak acid sites. 2,6-lutidine (2,6-DMP) is supposed to adsorb on Bronsted sites preferently to 3,5-lutidine (3,5-DMP) which should adsorb, as pyridine, on both Lewis and Bronsted sites. This behavior can be explained by the steric hindrance due to the methyl groups, the nitrogen atom being less accessible. For example. Figure 4 shows the differential heats of adsorption of the three probe molecules on the sample with Ti=249 pmol/g pretreated at 773 K. All the curves show a sharp decrease till... 

Similarly, Karge et al. [398] employed pyridine and 2,6-di-tert-butylpyridine to differentiate between internal and external acid sites of zeolite crystallites. In this respect, another possibility is the use of lutidine [139] or quinoline. The latter probe was employed by Corma et al. [693] for the determination of external Bronsted and Lewis acid sites of H, Na-Y and Al, Na-Y zeolites (cf. also [694,695]). For a characterization of the external Bronsted and Lewis acidity of ZSM-5 samples, Keskinen et al. [696] utilized as sufficiently bulky bases trimethylsi-lyldiethylamine and, like Karge et al. [398], 2,6-di-ferf-butylpyridine. For the discrimination of external from internal acid sites of shape-selective H-ZSM-5 catalysts,Take et al. [135] utilized pyridine andabulkytrialkylamine (e.g.,Et3N, n-Pr3N and n-Bu3N) as a pair of probes, with the former indicating the total amount of acid sites. For quantitative evaluation they determined the extinction... 

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). 

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... 

In recent years the tentative to go deeper in the analysis of surfaces (in particular, but not exclusively, of zeolites) suggested the use of sets of molecular probes with similar functionality but different steric hindrance. As for example, different nitriles like those reported in Scheme 9.1 allow to explore the position of the sites in porous materials such as zeolites [82]. Similarly, hindered pyridines, such as 2,6-dimethylpyridine (lutidine, [83]) and 2,6-diterbutyl pyridine can be used [84]. The use of these molecules allow to show that some of the adsorption sites detected by CO and previously supposed to be in the interior of zeolite channels are actually out of the channels [85]. 

Figure 4.24 A schematic of the investigation of HTB AlF2.6(OH)o.4 using adsorption of the probe molecules CO, pyridine, lutidine and Cl]-hydrogen chloride. (Reprinted with permission from D. Dambournet, H. Leclerc, A. Vimont, J.-C. Lavalley, M. Nickkho-Amiry, M. Daturi and J. M. Winfield, Phys. Chem. Chem. Phys. 11, 1369-1379 Copyright (2009) PCCP Owner Societies.)...

The stability of the bijel stracture over time was probed by periodic characterization of the FrrC-Si02/lutidine/water stme-tures. With storage in an incubator at 40 ° C, the structures were maintained, as observed by confocal microscopy, over at least 7 months. These studies revealed the bijel domain morphology to be long-lived, but could not mle out completely the possibility of a very slow aging process and nanopartide reorganization within the lutidine-water interface. ... 

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