Chemisorption pyridine

As expected for silica-alumina as a mixed oxide (see also Section IV.B.5), the PyH+ and PyL species are observed simultaneously (160, 205,206,221-223). Two distinct types of Lewis acid sites could be detected (19b mode at 1456 and 1462 cm-1, respectively) on a specially prepared aluminum-on-silica catalyst (160). On water addition, the Lewis sites can be converted into Br nsted sites (160, 205, 221), The effect of Na+ ions on the acidity of silica-aluminas has been studied by Parry (205) and by Bourne et al. (160). It can be concluded from Parry s results that Na+ ions affect both types of acid sites, so that alkali poisoning does not seem to eliminate the Br nsted sites selectively. For quantitative determination of the surface density of Lewis and Br nsted acid sites by pyridine chemisorption, one requires the knowledge of at least the ratio of the extinction coefficients for characteristic infrared absorption bands of the PyH+ and PyL species. Attempts have been made to evaluate this ratio for the 19b mode, which occurs near 1450 cm-1 for the PyL species and near 1545 cm-1 for the PyH+ species (160,198,206,221,224,225). The most reliable value as calculated from the data given by Hughes and White (198) seems to be... 

It is interesting to note that the cis-trans ratios obtained by Ghorbel and coworkers (312) on 1-butene isomerization were mostly unaffected by the chemisorption of the different poisons and corresponded to the thermodynamically determined ratio. The same result was obtained by Knozinger and Aounallah (377), when 1-butene was isomerized in a recirculating reactor at 120°C on tj-A1203 (activated at 500°C) that was partially poisoned by pyridine. In micro-catalytic pulse experiments, on the other hand, one observes kinetically determined primary product distributions with cis-trans ratios of about 2 (370). Different types of active sites appear, therefore, to be involved in the doublebond shift and in the cis-trans isomerization, the latter remaining unaffected by pyridine chemisorption. 

In Fig. 41, distinct differences in the amounts of chemisorbed pyridine can be seen for HZSM-5 samples coked by n-hexane and mesitylene cracking. Whereas coke from mesitylene only slightly inhibits the pyridine chemisorption, coke from n-hexane leads to a much stronger inhibiting effect. This result supports the model of controlled coke deposition derived... 

Fig. 41. Pyridine chemisorption as a function of the coke content a, n-hexane coke v, mesitylene coke (131).

It should be noted that whenever equation (27) applies and dehydroxylation happens solely through the Uytterhoven step, assuming the stoechiometry of pyridine chemisorption is one molecule adsorbed per either Brbnsted or Lewis acid site, then the N/Al ratio must be equal to one. 

The total acidity (i.e. both intra- and intercrystalline) of both the zeolite catalysts, obtained by the pyridine chemisorption, is consistent with that measured by the toluene disproportionation reaction. The acid sites measured by the chemisorption of pyridine (at 400°C) are essentially strong zeolitic acid sites. No chemisorption of pyridine (at 400 C) on... 

Ward et al. (65, 71) reported on the NaHY and MgHY systems. The 2 systems behave as a mixture of the 2 components. Sodium hydrogen Y was studied as a function of the sodium content from 100 to 0% exchanged (71). For the calcination conditions used (480°C), the series of samples were all in the Bronsted acid form. The concentration of acid sites detected by pyridine chemisorption increased linearly as the sodium was removed until only about 16 sodium ions were left. This corresponds to the number of easily removed sodium ions which can be exchanged with ammonium ions and subsequently form hydroxyl groups of the 3650 cm 1 type. Further exchange yields hydroxyl groups in the small-pore structure which are not accessible and/or not sufficiently acidic to react with pyridine. 

Pyridine chemisorption on hydrogen mordenite has been studied (11). As the calcination temperature was increased from 200° to 600°C, the population of Bronsted acid sites decreased and of Lewis acid sites increased. Above 300°C, 2 absorption bands (1462 and 1455 cm-1) were observed, suggesting the presence of 2 types of Lewis acid sites. Cannings suggested the following structures ... 

Table 5-4. Pyridine chemisorption data for some expanded clays and an HY-type zeolite (Linde s LZY-82) with BET surface area of 740 mVg. Bronsted (B) and Le (L) acid site density has been obtained by dividing the integrated absorbance (in the 1557-1525 cm region and in the 1472-1425 cm region) by the wafer thickness (mg/cm )...

The probe molecule pyridine has often been used more recently in the study of clay surface acidity by IR methods. Figure 7 shows data of pyridine chemisorption on a synthetic mica-montmorillonite catalyst as an example (62,63) of the types of bands of interest. The spectra are interpreted as showing chemisorption of pyridine at both protic and aprotic sites. The clay was first heated for 15 h at 650°C under vacuum and then cooled and spectrum A taken. Note that there are no bands in the 1400-1700 cm region, the residual hydroxyl near 3450 cm S and edge silanol hydroxyl at 3747 cm . Pyridine vapor was then chemisorbed and spectrum B taken. The bands at 1456 cm and 1547 cm are assigned to Lewis and Bronsted sites, respectively. Since the 3747 cm edge silanol band decreases, it is assumed that these protons are involved in the mechanism. Lewis sites predominated under these particular conditions. 

FIGURE 7 IR spectra of pyridine chemisorption on synthetic mica-montmorUlonite showing the transformation of pyridine to pyridinium ion by interaction with water. L, B = Lewis and Bronsted acid sites (63). (Reproduced with permission from the Mineralogi-cal Society of Great Britain and Ireland.)... 

Figure 7.11 Schematic representations of (a) the proposed acid sites of activated T -alumina and (b) the complexes formed on pyridine chemisorption, (i) The weak Lewis acid site, (ii) the medium-weak Lewis acid site, (iii) the medium-strong Lewis acid site, and (iv) the strong Lewis acid site. The asterisks in (a) indicate coordinative unsaturation. Reproduced with permission from Ref. [60].

X-ray diffraction (XRD), pyridine chemisorption, and microactivity test (MAT) results have been used to characterize a sample of natural rectorite pillared with alumina clusters. After reaction with chlorhydrol, a pillared product was obtained that after drying at 100°C/10h had d(OOl) spacing of 28.7 A. The pillared rectorite retained its structure even after calcining in air at 800 C/5h or after steam aging at 760°C/5h with steam at 1 atm. Thus, pillared rectorites have thermal and hydrothermal stability comparable to that of zeolites with the Faujasite structure. 

Natural rectorites pillared with alumina clusters have thermal as well as hydrothermal stability far superior to that of similarly prepared montmorilIonite catalysts. Their stability is comparable to that of zeolites with the Faujasite structure. Pyridine chemisorption experiments have indicated that these materials contain both B and L acid sites and that at cracking conditions acidity is essentially of the L-type. Steam-aged (760°C/5h) pillared rectorites at MAT conditions have cracking activity comparable to that of similarly steam-aged commercial FCC containing an estimated 35% CREY. Coke selectivity (as well as particle density) will have to be improved for ACH-rectorites to compete with zeolite-containing FCC. 

T. R. Kantner, and K. H. Rhee, 1964. The nature of the acidic sites on a silica-alumina. Characterization by infrared spectroscopic studies of trimethylamine and pyridine chemisorption. J. Phys. Chem. 

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