Pyridinium ion bands

The IR spectra of pyridine adsorbed on RE—Y zeolite calcined at 480°C indicate that its acidity is predominantly of the Br0nsted type (2/2). Only a weak absorption at 1450 cm 1 due to Lewis-bound pyridine is observed, whereas the pyridinium ion band at 1550 cm-1 is quite intense. Such measurements show that calcination at higher temperatures de-... 

For the samples studied, no pyridine adsorbed on Lewis acid sites was detected, indicating that the zeolite had not been partially dehy-droxylated to form such sites. The band reflecting pyridine-cation interaction was detected only after about 16 alkaline earth ions had been introduced into the unit cell. It grew steadily in intensity as more divalent ions were introduced into the structure. In Figure 1, the intensity of the pyridinium ion band, expressed as absorbance/sample mass, is plotted as a function of the per cent exchange by divalent ion. The concentration of acid sites is a function of the degree of exchange. 

Solid-state ion exchange with CaCl2/H-MOR and MgC /H-MOR was also monitored by IR. Figure 7 demonstrates, as an example, the removal of acidic OH groups (spectrum 2a) in H-MOR due to replacement of the protons by Ca +. Subsequent pyridine adsorption gave rise to a sharp band at 1446 cm typical of pyridine coordinatively bonded to calcium cations and a smaller band at 1455 cm which is due to pyridine attached to true Lewis sites (spectrum 2b). Most likely, both types of pyridine coordination contributed to the small signal at 1610 cm. Note, however, that only a very weak pyridinium ion band at 1540 cm was observed. Another sample, which had been prepared via solid-state ion exchange (spectrum 2a), was contacted with small amounts... 

Adsorption of pyridine subsequent to spectrum c gave rise to a band at 1452 cm typical of pyridine coordinatively bonded to lanthanum cations only a tiny pyridinium ion band at 1542 cm was observed. When, however, generation of spectrum d (i.e. after HgO contact of the sample) was followed by pyridine adsorption, the band of acidic OH groups at 3630 cm was completely removed and a strong band at 1542 cm (pyridium ions) appeared. 

Fig. 2. Sample Si-0.13. Recovery of the 3640 cm- (a) and 3575 cm- (b) OH bands upon evacuation of pyridine at increasing temperatures. Decrease in the 1540 cm-i pyridinium ion band (c).

Fig. 4. Derivatives of the curves c (pyridinium ion bands) in figure 2 and 3. a SAPO-37, b HYD.

Dzwigaj et al. [449] re-interpreted the HF OH-band generally appearing in the mid IR spectra of Y-type zeohtes. They offered evidence for their suggestion that the HF-OH band of, e.g., H,Na-Y or H,Mg-Y is composed of two components with wavenumbers of 3648 and 3660 cm originating from two different kinds of OH groups. This result was supported by pyridine adsorption experiments, which showed the appearance of two pairs of pyridinium ion bands, viz. at 1543/1627 and 1552/1636 cm (cf. Sect. 5.5.2.6.2). 

Consumption of acid OH groups upon introduction of Mn cations led to a decrease in Brpnsted and a concomitant increase in Lewis acidity, as indicated by IR using pyridine as a probe. The decrease in Bronsted acidity was indicated by a decrease in the intensity of the pyridinium ion band at 1540 cm" observed upon pyridine adsorption on Mn-ZSM-5 samples that had been prepared by SSIE. A corresponding increase in the Lewis acidity effected an enhancement of the absorbance around 1450-1454 cm" which is indicative of pyridine coordinated to electron pair acceptors or Lewis sites such as Mn +. This was compared with similar IR measurements on the parent H-ZSM-5. Figure 51 shows a linear correlation between the density of Bronsted and Lewis acid sites of the parent H-ZSM-5 as well as of Mn,H-ZSM-5 samples measured by this technique. 

Z values are obtained from Eq. (8-76) for solvents having Z in the approximate range 63-86. In more polar solvents the CT band is obscured by the pyridinium ion ring absorption, and in nonpolar solvents l-ethyl-4-carbomethoxy-pyridinium iodide is insoluble. By using the more soluble pyridine-1-oxide as a secondary standard and obtaining an empirical equation between Z and the transition energy for pyridine-1-oxide, it is possible to measure the Z values of nonpolar solvents. The value for water must be estimated indirectly from correlations with other quantities. Table 8-15 gives Z values for numerous solvents. 

Spectroscopy. In the methods discussed so far, the information obtained is essentially limited to the analysis of mass balances. In that re.spect they are blind methods, since they only yield macroscopic averaged information. It is also possible to study the spectrum of a suitable probe molecule adsorbed on a catalyst surface and to derive information on the type and nature of the surface sites from it. A good illustration is that of pyridine adsorbed on a zeolite containing both Lewis (L) and Brbnsted (B) acid sites. Figure 3.53 shows a typical IR ab.sorption spectrum of adsorbed pyridine. The spectrum exhibits four bands that can be assigned to adsorbed pyridine and pyridinium ions. Pyridine adsorbed on a Bronsted site forms a (protonated) pyridium ion whereas adsorption on a Lewis site only leads to the formation of a co-ordination complex. 

Parry (344) determined the infrared spectrum of pyridine adsorbed on rj-alumina dehydrated at 450°. Characteristic differences in the 1400-1700 cm region exist in the spectra of pyridine adsorbed via hydrogen bonds, pyridinium ions, and pyridine coordinately bonded to electrophilic sites. Pyridinium ions are characterized by a strong band at 1540 cm and a very strong band at 1485-1500 cm" coordinately bonded pj ridine has a strong absorption at 1447-1460 cm". No evidence was found for the existence of Bronsted sites on the alumina surface. 

It is clear from the above observations that pyridine molecule interacts on the catalyst surface in the following three modes (1) interaction of the N lone pair electron and the H atom of the OH group, (2) transfer of a proton from surface OH group to the pyridine forming a pyridinium ion (Bronsted acidity), and (3) pyridine coordination to an electron deficient metal atom (Lewis acidity). Predominant IR bands, vga and vigb, confirms that the major contribution of acidity is due to Lewis acid sites from all compositions. Between the above two modes of vibrations, Vsa is very sensitive with respect to the oxidation state, coordination symmetry and cationic environment [100]. A broad feature for v a band on Cu containing... 

In acetone, only one displaced band is observed, and clearly no complete hydrogen transfer is involved. In the more basic pyridine it might well be expected to occur markedly with the better proton-donors. Fig. 3 shows that for pyridine, and all the more acidic materials but pyrrole, new bands appear at lower frequencies, which might be attributed to new species, such as the pyridinium ion. (With pyrrole, although there is a particularly broad bonded IS —H band, no other maxima were apparent.)... 

We have accordingly made a number of measurements on pyridinium salts. In agreement with Lord and Merhifteld [8] we find the broad structureless band at about 2460 cm 1 in the spectrum of pyridine hydrochloride in chloroform solution, which they attributed to Nh— Ho., Cl" bonding. In addition we find this raised to about 2570 cm 3 in ethanol solution. The solid perchlorate, sulphate and chloride (in KBr discs) have practically identical spectra (to 4 tu.) in which the broad band now occurs centred on 2740 cm 1. Pyridiniain perchlorate dissolved in pyridine shows a similar band at 2530 cm 3. This is presumably due to solvation of the pyridinium ion through —H N... 

Jacobs and Uytterhoeven (199, 200) observed a band in the 3700 to 3675 cm-1 region in addition to the bands reported by Ward. The intensities of the acidic bands at 3650 and 3550 cm-1 were greater than those observed by Ward, which probably resulted from a lesser degree of aluminum removal. The new bands at 3700 and 3600 cm-1 arose from hydroxyls that were nonacidic to ammonia (199, 200) and pyridine (198, 199), although bands from pyridinium ions were observed in the IR spectrum. The latter bands were attributed to interaction of pyridine with the 3650 cm-1 hydroxyls (200). Jacobs and Uytterhoeven (199) and Scherzer and Bass (198) attributed the 3700 and 3600 cm-1 bands to structural hydroxyl groups associated with removal of aluminum from the zeolite framework. The 3600 cm-1 band arose from weakly acidic hydroxyls (200) since the band was removed by treatment with 0.1 W NaOH solution. The 3700 cm-1 band was unaffected by a similar treatment. 

Eberly (151) found that exposure of dehydroxylated alkaline earth samples to water vapor resulted in reappearance of the acidic hydroxyl bands at 3650 and 3550 cm 1. Subsequent exposure to pyridine resulted in interaction with the 3650 cm 1 groups and the formation of pyridinium ions. A band at 3585 cm-1 has also been reported (156), which appears simultaneously with the reformation of the acidic hydroxyl groups upon exposure to water. This band does not react with pyridine and is thought to arise from hydroxyl groups associated with the cations that result from dissociation of the added water. 

Determined from peak height/sample mass of the 1545-cm1 band that arises from pyridinium ion. 

The acidic sites of solid acids may be of either the Brpnsted (proton donor, often OH group) or Lewis type (electron acceptor). Both types have been identified by IR studies of solid surfaces using the pyridine adsorption method. The absorption band at 1460 cm 1 is assigned to pyridine coordinated with the Lewis acid site, and another absorption at 1540 cm 1 is attributed to the pyridinium ion resulting from the protonation of pyridine by the Brpnsted acid sites. Various solids displaying acidic properties, whose acidities can be enhanced to the superacidity range, are listed in Table 2.6. 

Infrared absorption can be used to estimate the relative amounts of Lewis and Bronsted acid sites on cracking catalysts. Bases complex with Lewis acid sites while proton transfer to the base occurs at Bronsted acid sites. Each has distinct, well-resolved infrared bands. For example, pyridine forms a complex with the Lewis acid site and produces an infrared absorption band at approximately 1450 cm-1. Pyridinium ions form at Bronsted sites and produce an absorption band at approximately 1540 cm-1. The relative intensities of these two bands can be used to estimate the relative amounts of Lewis vs. Bronsted acid sites. 

The IR spectra of pyridine adsorbed on SO4/ZrO2-l(650°C) are shown in Fig. 12, where both the pyridinium ion (at 1540 cm 1) and coordinately bonded pyridine (at 1440 cm-1) are observed, the decrease in the former band being seen together with the increase in the latter band after evacuation at high temperature (spectrum C) (150, 151). Spectrum D shows the changes that occur upon the addition of water to the sample the increase in the 1540-cm 1 band indicates that a considerable amount of Bronsted acid has been formed, and the decrease in the 1440-cm 1 band shows a... 

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