Desorption of pyridine

Figure 4 Temperature programmed desorption of pyridine pyridine adsorbed at 47SK, heating rate 20K/mtn...

Figure 6. Fl lK spectra after desorption of pyridine at RT of a) H-AIMCM-41-16 and b) H-NbMCM-41-16 [3]...

This would indicate that (3 has stronger acid sites than dealuminated HYD samples, something which agrees with IR data of both solids, where the wavenumber of the accesible acidic hydroxy groups is 3620-3630 cm-1 on dealuminated HY, and 3612 cm- in HJ3 zeolites (ref. 8), A similar conclusion was reached by measuring the desorption of pyridine on these two types of large pore zeolites. 

Fie. 15. Thermal desorption of pyridine from heteropolyacids (from Ref. 125). H3PW12 refers to H3PWl20 o. 

In the last decade, a number of publications has been devoted to this subject. These studies are either based on spectroscopic techniques (IR, NMR) or on desorption techniques (temperature programmed desorption of pyridine and water). In all of these models the distinction between free and bridged silanols, trapped water and intraglobular hydroxyls is the key problem. 

Enthalpy changes on adsorption and desorption of probe molecules on catalyst surfaces may also be followed by differential thermal analysis (DTA) (67) although this method has been used only sporadically in the past. The experimental techniques have been described by Landau and Molyneux (67) very recently. As an example, Bremer and Steinberg (68) observed three endothermic peaks during the desorption of pyridine from a MgO-Si02 catalyst these peaks were assigned as three different chemisorption states of pyridine. 

The other enhancement is termed the chemical enhancement and can result from a charge-transfer (C—T) or bond formation of the metal and adsorbate, which can increase a, the molecular polarizability (55). For the pyridine/Ag system, the C—T band appeared on adsorption of pyridine on Ag and disappeared reversibly on desorption of pyridine (56). It was observed that the stronger the C—T band, the stronger the SERS spectrum. 

Figure 3. Thermal desorption of pyridine (a) HjPWuO, (b) H3PMoI2O40, (c) NaH2PWl20 o, (d) Na3PWI2O40, (e) CS3PW11O40, (f) Si02-Al203. Pyridine ab(ad)sorbed at 298 K was desorbed at each temperature for 1 h by evacuation.

The surface acid-base properties of polycrystalline MgO surfaces have been assessed by means of thermogravimetry and DSC of desorption of pyridine and CO2 in the room temperature to 400 °C temperature range [44]. The endotherms and corresponding AH of desorption were discussed in relation with results determined previously using differential adsorption calorimetry and taking into account the structure, surface area and defects of the studied surfaces. 

Several authors have discussed the existence of two kinds of BrOnsted acid sites in high silica zeolites. For example Datka and Tuznik [48] measured the desorption of pyridine fi-om H-ZSM-S at various temperatures while monitoring the changes in intensity of the 3610 cm and the 1545 and 1455 cm bands. They found that a certain amount of pyridinium ion was decomposed after the disappearance of the 1455 cm band and before the reappearance of the 3610 cm. They ascribed this amount to pyridine chemisorbed on weak BrOnsted acid sites. 

Pyridine adsorption on a self-supported wafer or on the solid deposited on a silicon slide, are compared. Results are similar and Fig. 4 summarized the spectra obtained by stepwise desorption of pyridine from cloverite deposited on silicon. At room temperature, pyridine interacts with hydro ls at 3673 and 3700 cm L The 944 cm band is also perturbed confirming that it is a vibrational band due to hydroxyl groups. The pyridine species formed are characterized as pyridinium species (1545 cm band) and coordinated species (1450 and 1610 cm bands). 

IR spectroscopy was applied for the measurements of adsorption and desorption of pyridine. Prior to the pyridine adsorption, all samples were activated in vacuum (lO Pa) at 675K except when they were previously evacuated at 975K. Pyridine adsorbed on Bronsted acid sites, true Lewis acid sites and sodium cations was characterised by IR bands at -1550, -1455 and -1442 cm", respectively [12]. Table 2 shows the ratio of the absorbance at -1550 cm 1 (PyB) to that at -1455 cm 1 (] L) for various samples. These results indicate that all samples, even when evacuated at 975K possessed Bronsted acid sites. However, the Lewis acid sites dominated except with the samples treated at lower temperatures. One should stress that evacuation at 975K led to a higher reduction of Bronsted acid sites than heating at the same temperamre in air. The lower PyB/PyL ratio observed on niobium-modified NH4NaY... 

Table 2 - Acidity as measured by TPD of NH, and IR with adsorption and desorption of pyridine at different temperatures.

Liang used n.m.r. to characterize amines adsorbed on hydrated silica-alumina they found evidence for protonated species on sites which were not sterically hindered. Further work on the thermal desorption of pyridine and n-butylamine from silica-aluminas (13 and 25 wt % AI2O3) showed that the stronger acid sites, where pyridine was adsorbed, were of varied acid strength, and both number and strength of the sites were affected by alkali poisoning. An i.r. study of pyridine adsorbed on sodium-poisoned silica-alumina also showed both Lewis and Bronsted sites to be affected. 

FT/IR measurements of adsorbed NH3 on NiSMM catalysts have shown that the number of sites, most probably Bronsted sites, increases due to Ni reduction ( ). This suggests that the newly formed acidic sites are responsible for the increased activity of NiSMM compared with e.g., beidellite, in which no Ni is substituted for Al in the octahedral sites. The newly formed sites should then be highly acidic. In order to verify this statement we have studied the thermal desorption of pyridine from Pd-NiSMM before and after reduction, by means of FT/IR spectroscopy. For comparison, the same study was made with Pd-exchanged synthetic beidellite. 

Population of surface acid sites determined by temperature-programmed desorption of pyridine and cyclohexylamine ... 

We indeed found a correlation between the temperature of desorption of pyridine and the selectivities to (I) and (II), even for NaY and RbY, as rather basic zeolites. The correlation between pyridine desorption temperature and selectivity to (I) was almost linear (Fig. 2), whereas a volcano shaped correlation with a maximum at 220 °C was found for the selectivity to (II) (Fig. 3). To our surprise, there was no reasonable correlation between the temperature of ammonia desorption and the selectivity to either (I) or (II) (Fig. 4). 

Figure 6. Reciprocal integrated absorption at 1445 cm-1 versus time for the desorption of pyridine from silica gel at 80 °C.

The observed second-order desorption of pyridine is believed to be evidence that the majority of surface silanols are paired on the surface in either a vicinal or geminal configuration. Further evidence for the pairing of the majority of silanols on the silica surface was presented elsewhere... 

Temperature programmed desorption (t.p.d.) of pyridine The variations of the rates of desorption of pyridine from SAP05-1 and SAP05-3 during... 

Temperature of the maximum of the rate of desorption of pyridine from strong acid sites... 

Fig. 1 Rates of desorption of pyridine during t.p.d. from SAP05-1 (a) and SAP05-3 (b)...

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