Spectra of pyridine

Fig. XVIII-20. Spectra of pyridine adsorbed on a water-containing molybdenum oxide (IV)-Al203 catalyst L and B indicate features attributed to pyridine adsorbed on Lewis and Brpnsted acid sites, respectively. (Reprinted with permission from Ref. 191. Copyright 1976 American Chemical Society.)...

Albrecht M G and Greighton J A 1977 Anomalously intense Raman spectra of pyridine at a silver electrode J. Am. Chem. Soc. 99 5215-17... 

Fleischmann M, Hendra P J and McQuillan A J 1974 Raman-spectra of pyridine adsorbed at a silver electrode Chem. Phys. Lett. 26 163-6... 

There are, at present, two overriding reasons an experimentalist would choose to employ laser Raman spectroscopy as a means of studying adsorbed molecules on oxide surfaces. Firstly, the weakness of the typical oxide spectrum permits the adsorbate spectrum to be obtained over the complete fundamental vibrational region (200 to 4000 cm-1). Secondly, the technique of laser Raman spectroscopy is an inherently sensitive method for studying the vibrations of symmetrical molecules. In the following sections, we will discuss spectra of pyridine on silica and other surfaces to illustrate an application of the first type and spectra of various symmetrical adsorbate molecules to illustrate the second. 

Figure 6P.2 contains the Raman spectra of pyridine adsorbed on silica gel at three temperatures as reported by Schrader and Hill [Rev. Sci. Instrum, 46 (1335), 1975]. Four bands are apparent at 991, 1006, 1032, and 1069 cm F If the band intensities are proportional to surface concentrations and if each band is associated with one vibrational degree of freedom of an adsorbed species, what is your interpretation of these data ... 

Infrared spectra of pyridine adsorbed on dehydrated TS-1 and Ti-MCM-41 of comparable Ti content indicated the presence of only Lewis acid sites (Fig. 13). The infrared absorptions at 1595 and 1445 cm-1 are attributed to hydrogen-bonded pyridine (Si/Ti-OH—pyridine) and those at 1580 and 1485 cm-1 to pyridine bonded to weak Lewis acid sites (Fig. 12). Brpnsted sites, if present,... 

Fig. 13. FTIR spectra of pyridine adsorbed on dehydrated TS-1 and Ti-MCM-41 [from Srinivas etal (152)].

Figure 6. EELS spectra of pyridine carboxylic acids adsorbed at Pt(lll). Experimental conditions (A and B) adsorption from 1 mM NA in 10 mM KF at pH 3, followed by rinsing with 2 mM HF (pH 3) (C) adsorption from 10 mM KF (pH 3), followed by rinsing with 0.1 mM KOH (pH 10) other conditions as in Figure 4. A. Adsorption at -0.2 V vs. Ag/AgCl (pH 3). Continued on next page.

Figure 5. Reflection infrared spectra of pyridine films (a) pyridine adsorbed on Ni(111) at 180 K (b) pyridine adsorbed on Ni(111) at 320 K (c) electrochemicaily grown poly(pyridine) on Pt (d) 3,5-lutidine adsorbed on Ni(111) at 180 K (e) 3,5-lutidine polymer on Ni(111) at 750 K (f) 2,6-lutidine adsorbed on Ni(111) at 180 K...

The SERS spectra of pyridines contain many other bands but those mentioned above have proven to be particularly useful in characterizing the surface species. 

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. 

Unfortunately, it cannot be applied to pyridine securely because of the indeterminate effect of charge delocalization on the para chemical shift. Nonetheless the sign and magnitude of the effects in phenyl-lithium would go a long way towards removing the discrepancies in the pnmr spectra of pyridine and pyridinium ion. 

It is important to understand the catalyst characteristics in detail, which in turn helps to understand the catalyst better and correlate the structure and composition of the catalysts with its performance, so that further improvement of the catalyst is possible. Acidity is an important property which influences the overall activity of the alkylation catalysts and the same was studied for Cui.xZnxFc204 by IR and TPD methods. The changes in acidity with respect to catalyst composition and temperature were studied through pyridine adsorption followed by IR measurements. In situ FTIR spectra of pyridine adsorbed on Cui xZnxFe204 between 100 and 400°C (Figme 23) indicated Lewis acidity is the predominant active centers available on the surface [14]. 

Figure 23. Temperature dependent DRIFT spectra of pyridine adsorbed on three selected compositions from Cul-xZnxFe204 (x = 0.05, 0.5 and 1) between 100 and 400oC.

Comparative studies of the effects of (1 Ln = La Gd, Dy -> Yb R = R = Bu ) on the n.m.r. spectra of pyridine revealed that the Dy complex was the best high-field shift reagent whereas the Yb complex was the most effective low-field shift reagent. contact shifts were also observed in the spectra of [Bu4N]3[LnClg] (Ln = La Sm, Er, Tm, or Yb). The dpm complexes were also effective in causing shifts of the spectrum of 2,4,6-tri-fluoroaniline. °... 

Figure 2 Infra-red spectra of pyridine after evacuation at increasing temperatures. Sample G5-Ce calcined at 680°C and pyridine adsorption at room temperature.

Figure 3 Infra-red spectra of pyridine on sample G5-Ce, steamed at 550°C. Adsorption of pyridine at room temperature, then evacuation at different temperatures.

Figure 4 Infra-red spectra of pyridine adsorbed on sample G5-Ce steamed at 550°C (spectrum a) and 650°C (b) and on a FCC commercial catalyst steamed at 775°C (c). Pyridine adsorption at 25°C and desorption at different temperatures.

Fig. 2. IR spectra of pyridine sorbed on A) Wyoming ACH-bentonite B) Texas ACH-bentonite C) Fe-bentonite D) ACH-(Fe-bentonite) E) (ACH,Fe)-bentonite and F) ACH-nontronite. 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.

We next give Tables 6, 7 and 8 to show representative values for various chemical shifts, couplings and solvent effects in and 13C NMR spectra of pyridine and its iV-oxide and protonated derivatives, in addition to those in the general section (Chapter 2.01), to show the general character of the results obtained and their susceptibility to variation of structure and media. 

Table III summarizes experimental and theoretical data on the first three absorption bands (Lb, La, Bb) in the spectra of pyridine, quinoline, isoquinoline, acridine, and their benzo and dibenzo derivatives.

Ellis and co-workers published a 13C MAS study of ethylamine on solid acids in 1981 (157). Maciel and Haw (158, 159) published NMR studies of pyridine as a probe molecule on solid acids in 1983. We have recently begun to reexamine the 15N spectrum of pyridine on zeolites and other solid acids (160). At low temperatures pyridine is remarkably sensitive to the kinds of acid sites present. Figure 28 shows 15N spectra of pyridine adsorbed on HY samples before and after dealumination. Dealumination in this case seems to make four kinds of Lewis sites distinguishable by NMR of adsorbed pyridine, suggesting pyridine as a good candidate for... 

Fig. 28. 36-MHz l5N CP/MAS spectra of pyridine-15N on zeolite HY. The experimental conditions were all the same for (a) and (b), except that sample b was extensively dealuminated by increasing the activation temperature to SSITC (400°C for sample a). Both spectra were acquired at 77 K to prevent chemical exchanges on the NMR time scale, (a) The single resonance at —176 ppm as well as its associated sidebands indicates protonation of pyridine by the Brpnsted sites, (b) In addition to the protonated pyridine, four additional resonances at -68, -88, —116, and -140 ppm are also seen, indicating complexation of pyridine with different extraframework Lewis sites.

Figure 3. FTIR spectra of pyridine absorption on PSM and AMM materials after degassing at (a) room temperature and (b) 200°C.

Figure 3. Infrared spectra of pyridine adsorbed on AgFSM-16 before and after sulfiding, a) AgFSM-16 evacuated at 500°C for 2 hours, b) AgFSM-16 sulfided at 200°C...

Figure 71 shows the 13C spectra of pyridine at different loading levels and after a pretreatment with HC1. It is seen that the resonance of the y carbon of pyridine (middle peak) is very sensitive to its chemical state, and Maciel et al. developed a model of pyridine exchange between Bronsted, Lewis, and hydrogen-bonding acid sites as well as the physisorbed state. The same authors used the 15N signal of a fixed quantity of adsorbed pyridine as a kind of urface indicator. Pyridine is displaced by n-butylamine according to the sequence... 

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-... 

Lefrancois and Malbois (227) determined the types of acidity present on H-mordenite and various cationic forms by obtaining infrared spectra of pyridine adsorbed on the zeolite. H-mordenite activated at 400° contained both Br0nsted and Lewis acid sites. Upon addition of water, the band due to Lewis-bound pyridine disappeared and the Br0nsted site concentration increased. Removal of the added water by evacuation restored some of the Lewis acid sites. 

Infrared spectra of pyridine adsorbed on kaolinite indicated that the dry clay (110°C) contained both Brtfnsted and Lewis acid sites (235). At 1% water content only protonic acid sites were observed. It was not possible to assign the polymerization activity to either type of acid site, since both were present on samples which were catalytically active. 

Infrared Spectra. Figure 2 shows the spectra of pyridine adsorbed on / -alumina. Two types of Lewis acid sites are present strong Lewis acid sites, which still bind pyridine on evacuation at 350°C and characterized by the 1622 and 1454 cm-1 bands and weak Lewis acid sites, characterized by the 1614 and 1450 cm- bands. Br nsted acid sites, which have characteristic bands around... 

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