Pyridine sorption

Pyridine sorption studies have shown the presence of both Bronsted and Lewis acid sites in USY zeolites, although to a lesser extent than in the corresponding HY zeolite (51,53). Acidity is maintained even after strong dehydroxylation of USY-B at 820°C. Rehydration of the calcined material did not regenerate significantly Bronsted acid sites, due to irreversible changes in the zeolite framework (51). 

Pyridine sorption studies on EDTA-dealuminated Y zeolites at various temperatures (54,58), as well as measurements of differential heats of adsorption of ammonia on aluminum-deficient Y zeolites (57,59) have led to the conclusion that aluminum-deficient Y zeolites have stronger acid sites than the parent zeolite. 

Kuhl (84) reported a decrease in the number of acid sites in calcined or steamed H-mordenite, due to thermal dealumination of the framework. Barthomeuf et al. (85), using data obtained from ammonia and pyridine sorption, concluded that the total acidity and the number and strength of Lewis sites decreases upon acid-dealumination of H-mordenite to a SiO /Al O ratio of 18, while the Bronsted acidity remained unchanged. 

IR spectra for the pillared bentonites in the OH-stretching region show an intense and broad OH-band centered near 3640 cm this band is shifted to near 3600 cm for the ACH-nontronite sample under study, Fig. 1. After pyridine sorption, only minor changes were observed in these spectra, indicating little reaction of the hydroxyl groups present with pyridine. As the degassing temperature is increased from 200 C to 500 C, OH bands decrease in intensity due to dehydroxylation reactions of the clay lattice. Fig. 1. Dehydroxylation is more facile in the iron-containing ACH-nontronite sample. Fig. IF. 

Pyridine Sorption. After pyridine sorption and degassing in vacuo at (to remove physisorbed pyridine), the intensity of the... 

Fig. 11 FTIR spectra of the ring deformation region after pyridine sorption into H-ZSM-5 (sample No. 3), Li-ZSM-5 (sample No. 5), and Na-ZSM-5 (sample No. 6) monitored through the changes in the absorbance of the band at 1443 cm ...

The assumption that the strong interaction between the strongly acidic adsorbent and the strongly basic adsorbate under our experimental conditions was the reason for the failure to achieve reversibility was confirmed when adsorbents with the same structure but lower (Li-ZSM-5 or Na-ZSM-5) or zero acidity (silicalite-1) were employed [26]. Figure 12 demonstrates the perfect reversibility of pyridine sorption into the slightly acidic Li-ZSM-5 zeolite. 

The thermal stabilities of their corresponding pillar intmlayered montmorillonite clay minerals (PILCs) were characterized through the use of powder X-ray diffraction, differential thermal analyses and surface area measurements. These relative stabilities were also found to increase in the order Gaia-PILC < Alia-PILC < GaAl -PlLC. The relative Brdnsted and Lewis acidities of the montmorillonite itself, and of the clay mineral pillared with the different polyoxocations have been examined through infrared pyridine sorption studies. 

Figure 5-9. IR spectra of pyridine sorbed on expanded montmorillonites that have been (A) air dried or (B) supercritical dried with CO2. Spectra for expanded saponites that have been air dried or supercritical dried with CO2 are given in C and D, respectively. All samples have been degassed in vacuo at (a) 200 C, (b) 300 C, (c) 400 C, and (d)500 C for two hours at each temperature. In (e) is the clay spectra before pyridine sorption.

Green, T.K., and Selby, T.D., Pyridine sorption isotherms of Argonne premium coals Dual-mode sorption and coal microporosity. Energy Fuels, 8(1), 213-218 (1994). 

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