Adsorption of pyridine into

Measurements of adsorption of pyridine into acid zeolites were severely impeded by the strong interaction of the adsorbate molecules with the adsorption sites, that is, centers of Bronsted and/or Lewis acid types, i.e., acidic OH groups and only threefold coordinated framework Al or Al-containing extra-framework species, respectively ( true Lewis sites see Vol. 4, Chap. 1,... 

IR spectra of the samples with and without pyridine were recorded on an FT-IR spectrometer (PE 430) with a resolution of 1 cm. In order to measure the pyridine adsorption, the samples were pressed into thin pellets, and placed into a quartz cell with CaF2 windows. The sample pellets were evacuated at 400°C for 2 h (< 10 Torr). After cooling down to room temperature, the pellets were exposed to pyridine environment (10 Torr) at 25°C. IR spectra were recorded after adsorption of pyridine for 1 h and evacuation at 150, 250, 350, and 450°C for 1 h. 

On a pc-Au electrode in 1 M NaF vertically oriented pyridine molecules have been observed at 0.7 V (versus Ag/AgCl), applying in situ IR. In contrast, they have not been detected at this potential in electrochemical method [240]. Considering the fact that adsorption of pyridine on gold electrodes is a replacement reaction and taking into account the results obtained from quartz crystal microbalance experiments, the conclusion has been made that adsorption of one pyridine molecule is accompanied by the removal of 10-12 water molecules [241]. 

Catalysts were characterized by means of X-ray diffraction (Phillips diffractometer PW3710, with CuKa as radiation source), UV-Vis-DR spectroscopy (Perkin-Elmer Lambda 19) and chemical analysis. Measurements of surface acidity were carried out by recording transmission FT-IR spectra of samples pressed into self-supported disks, after adsorption of pyridine at room temperature, followed by stepwise desorption under dynamic vacuum at increasing temperature (Perkin-Elmer mod 1700 instrument). The procedure for chemical analysis is described in detail in ref. (13). 

The nature of the acidity was investigated in order to explain the catalytic activity of the calcined Co-substituted P-zeolite and the role played by the aluminic sites of this solid. A pyridine adsorption followed by IR spectroscopy measurements was performed on the calcined catalyst. It has been shown that adsorption of pyridine emphasized two distinct bands at 1548 cm and 1451 cm corresponding respectively to the adsorption on Brbnsted and Lewis sites [22], In the case of the calcined Co-substituted zeolite, only a weak band at 1548 cm appeared in the IR spectrum. Thus, we deduced that very few Bronsted sites were present in the catalyst. This could explain that the oxidation of cyclohexane into adipic acid in the presence of calcined Co-substituted aluminic P-zeolite was not inhibited. 

To investigate the acid properties of these materials, adsorption/desorption experiments with pyridine as the probe molecule were carried out. FT-IR spectra of samples were recorded after adsorption of pyridine and desorption at increasing temperatures. The acid strength is subdivided into i) very weak, i.e., acid sites able to retain pyridine only after evacuation at 21°C, ii) weak, i.e., acid sites able to retain pyridine after evacuation at 100°C, and iii) medium, i.e., acid sites able to retain pyridine after evacuation up to 200°C. The results are reported in Table 3. 

When the strong acidity of the MFI zeolite (H-ZSM-5) was removed via ion exchange, as in the case of Li-ZSM-5 or Na-ZSM-5, or was intrinsically absent, as in the case of silicalite-1, adsorption of pyridine was entirely reversible, and reasonable ( corrected ) diffusion coefficients were obtained. As an example. Fig. 12 illustrates the reversible pyridine adsorption into and desorption out of Li-ZSM-5. The diffusion coefficient of pyridine transport in... 

Bases like pyridine are more strongly bound to such Lewis acid centers than to Bronsted acid centers, as can be shown by IR spectroscopy and temperature-controlled desorption. Figure 7-6 shows the transformation of Bronsted into Lewis acid centers on calcination of an HY zeolite, monitored by IR spectroscopic measurements on the adsorption of pyridine. 

The organic adsorbate in a small vessel was put into a phosphorous oxide desiccator and its vapor was allowed to adsorb on the clay at room temperature. When the adsorbate was solid, a clay sample was Immersed into the cyclohexane solution and dried in the desiccator. In the adsorption of pyridine, barium oxide was used as a drying agent. The formation rate of colored complexes was dependent on the adsorbate molecules and the dryness of montmorillonite. In the case of anisole or phenol, the color began to appear at once when the adsorbate was brought into contact with the dried montmorillonite, but it took about two weeks to finish the reaction in pyridine adsorption. 

Acidic properties were analyzed by several complementary methods in order to obtain informations on the nature, number and strength of the acid sites. The adsorption of pyridine, used as probe molecule, was studied by IR spectroscopy on self-supported wafers obtained by pressing the PILCs into thin films. Lewis and Bronsted sites give characteristics peaks having well established positions around 1450 and 1550 cm respectively. Evacuations at increasing temperatures were useful to investigate the acid strength. ... 

Zhang W, Shi L, An Y, Wu K, Gao L, Liu Z, Ma R, Meng Q, Zhao C, He B. Adsorption of poly(4-vinyl pyridine) unimers into polystyrene-Wocfe-poly(acrylic acid) micelles in ethanol due to hydrogen bonding. Macromolecules 2004 37 2924-2929. 

Infrared spectroscopic measurement was performed by Jasco FT-IR 230S using an in-situ cell. Hydrogen sulfide adsorption was carried out by introducing 40 Torr of hydrogen sulfide into the cell at 200°C, followed by evacuation at the same temperature for 0.5 hour. Pyridine adsorption was performed by introducing 10 Torr of pyridine vapour into the cell at 150°C, followed by evacuation at the same temperature for 0.5 hour. 

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