Pyridine, adsorption effects

Figure 4 Effect of the Mg/Fe atomic ratio in Mg/Fe/O catalysts on the number of acid sites, as determined by pyridine adsorption (o), on the Q C-methylation ratio (v), on the ortho/para-C-methylation ratio (a), and on the 2,5-DMP/2,3-DMP selectivity ratio (/ ), in the liquid-phase methylation of m-cresol [4],...

Figure 4.28 Effect of steaming and calcination on Bronsted and Lewis acid site strength distributions of a FAU-type zeolite as determined by pyridine adsorption/desorption IR.

Figure 3. Effect of pyridine adsorption at 260°C on low-frequency OH band...

P. E. Eberly, Jr. 1 share your concern about structural changes with temperature. Consequently, we have consistently given consideration to what conditions would be most desirable for measuring acidity. We have effected a compromise. At reaction temperatures much above 260 °C, pyridine can undergo some decomposition, and this clouds the experimental results. We selected 260°C for pyridine adsorption. This is higher than that used by other investigators and hence should yield acidities which more nearly represent those at the cracking temperature of 274°C used in this study. 

As to the effect of dealumination on acidity it is well demonstrated that the acid strength increases as A1 is removed from the framework. Pyridine adsorption showed that not only... 

In this equation, we find the different source and the selectivity between the two solutes, which can differ by the difference in energy of adsorption (Si° - 82°), molecular size (As2 - Asi), and secondary adsorption effect (Aeas)i - (Aeas)2- These considerations are valuable for a large number of compounds, but in the case of some isomers, the second terms of Eq. 13 can be considered practically equal to zero and only difference sources for selectivity remain in the difference in energy of adsorption and the secondary adsorption effect. The secondary adsorption effect plays a major role in the separation of the isomers. For instance, in adsorption on silica gel with benzene-pyridine (90 10, vol/vol) as eluent, the following Aeas values - 0.90 0.17 and + 0.05 0.11 for m-hydroxy-benzaldehyde and (9-hydroxybenzaldehyde, respectively. 

Parrillo et al. [242] have used micro calorimetric measurements of ammonia and pyridine adsorption to compare the acid sites in H-[Fe]ZSM-5, H-[Ga]ZSM-5, and H-[Al]ZSM-5. On each of the molecular sieves, the differential heats of adsorption for both ammonia and pyridine were constant up to a coverage of one molecule per Brbnsted site. The differential heats at coverages below 1 1 were identical on each of the materials, with values for ammonia of 145 5, 150 5, and 145 5 kjmol- on H-[Fe]ZSM-5, H-[Ga]ZSM-5, and H-[Al]ZSM-5, respectively, and for pyridine of 195 5, 200 5, and 200 5kjmol i on H-[Fe]ZSM-5, H-[Ga]ZSM-5, and H-[Al]ZSM-5, respectively. The authors [242] concluded that the microcalorimetric heats of adsorption for ammonia and pyridine at Brbnsted acid sites formed by framework Fe(III) and Ga(III) were very similar to heats of adsorption at Al(OH)Si sites, and that the three samples were effectively equivalent proton donors. In contrast, they foimd very different reactivity measurements for n-hexane cracking and propene oligomerization on the same materials. The authors claimed that heats of adsorption for strong bases do not reflect differences in inherent acid strength and may not be related to catalytic activity in any simple manner. 

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