Pyridine, adsorption on silica

Fig. 8. Diflierential heat of adsorption for pyridine adsorbed on silica-supported oxides that showed only Lewis acidity. (Adapted from Ref. 104.)...

Dumesic and co-workers studied the activity of isopropanol dehydration (247) on a series of silica-supported oxide catalysts as well as the acidic properties of these materials using IR spectroscopy and TGA of adsorbed pyridine (59) and adsorption microcalorimetry of pyridine at 473 K (18,104). Samples that showed only Lewis acidity were at least one to two orders of magnitude less active than the samples that displayed Brpnsted acidity. The activity of the latter samples increased in the order Sc < Ga < Al + This is the same order found for differential heats of pyridine adsorption on the Brpnsted acid sites, and a good correlation between the heats and the activity was found. No correlation was found with the initial heats or for the samples that had only Lewis acidity. 

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. 

Ismail, H.M. Fouad, N.E., and Zaki. M.I.. Nitrogen and pyridine adsorption on chromia-coated silica and alumina catalysts Probing the chromia dispersity. Adsorpt. Sci. Technol.. 8( 1). 34-43 (1992). 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

When a more acidic oxide is needed, amorphous silica-alumina as weU as meso-porous molecular sieves (MCM-41) are the most common choices. According to quantum chemical calculations, the Bronsted acid sites of binary sihca-alumina are bridged hydroxyl groups (=Si-OH-Al) and water molecules coordinated on a trigonal aluminum atom [63]. Si MAS NMR, TPD-NH3 and pyridine adsorption studies indicate that the surface chemistry of MCM-41 strongly resembles that of an amorphous sihca-alumina however, MCM-41 has a very regular structure [64, 65],... 

The Bronsted or proton acidity of the surface hydroxyl groups on silica and alumina is weak (51, 52, 53). Evidence for this comes from IR studies of pyridine adsorption, no surface pyridinium species (py H+)... 

Maciel et al. (370,371,374) combined l3C and 15N CP/MAS NMR to study the adsorption of pyridine on silica-alumina. Hydrogen bonding was found to be the dominant interaction at high loading levels (0.5 to 1 monolayer). At lower coverages, a Lewis acid-base complex dominates and the pyridine is significantly less mobile. Bronsted complexes are found when the surface has been pretreated with HC1 gas. 

Tomida et al. (73) investigated the temperature-programmed desorption of n-butylamine from silica-alumina and alumina. The desorbed amine products were different in the two cases. n-Butylamine and n-butene were obtained from silica-alumina dibutylamine and n-butene were obtained from alumina. In a subsequent paper by Takahashi et al. (73a), the authors conclude that two types of adsorption sites on silica-alumina account for the desorption behavior of n-butylamine. One type chemisorbs the amine and the other catalyzes the decomposition of the amine to lower olefins at temperatures above 300°C. On the other hand, amine decomposition was not observed when pyridine was desorbed from silica-alumina. The effects of sodium poisoning on desorption behavior of n-butylamine and pyridine were also examined. 

The question of the acidity of silica, alumina and silica-alumina surfaces has always been of great interest to catalytic scientists. Previously, transmision infrared spectroscopy, particularly of pyridine adsorption, has been used to distinguish the presence of Lewis and Bronsted acid sites on oxide surfaces (24). The frequency shift of the surface OH group during adsorption now... 

Tables XIII I76-I79), XIV (I80-I83), and XV present a survey of micro-calorimetric studies performed for silica, alumina, and silica-alumina, respectively. Silica displays relatively low heats of adsorption for both basic probe molecules (e.g., ammonia, triethylamine, n-butylamine, pyridine, and trimethylamine) and acidic probe molecules (e.g., hexafluoroisopropanol), indicating that the surface sites on silica are both weakly acidic and basic. Most of the adsorption over silica is considered mainly to be due to hydrogen bonding and van der Waals interaction. Infrared and gravimetric adsorption measurements of pyridine adsorbed on SiO at 423 K have shown that more than 98% of the pyridine adsorbed was hydrogen bonded (62). The differential heats of ammonia 18, 74, 85, 105, 140, 147) and triethylamine (18, 71, 94. 105, 176) on silica show a considerable decrease as the adsorption temperature is increased.

When these bases are compared in terms of their respective proton affinities, the order of basic strength is ammonia < n-butylamine < pyridine < trimethylamine < piperidine < triethylamine, which is the same order observed with microcalorimetric measurements. In fact, plots of the initial differential heat of adsorption of ammonia, pyridine, trimethylamine, and triethylamine on silica-alumina and on silica as a function of the proton affinity give linear correlations, as can be seen in Fig. 7 (18, 105). 

The adsorption of ammonia, pyridine, trimethylamine, and triethylamine on silica and silica-alumina was studied microcalorimetrically by Cardona-Martinez and Dumesic (18, 105). The calorimetric results of this study were correlated successfully in terms of Drago parameters for each catalyst. These parameters describe well the acidic properties of silica and the strongest sites (Lewis acid sites) on silica-alumina and may allow the prediction of heats of adsorption for a wide range of basic molecules with known Drago parameters on these sites. Parameters to describe the strength of the Brpnsted sites could not be determined because the contribution from these sites could not be studied independently. 

Bulk Metal Oxides. Extensive Raman chemisorption studies on high surface area alumina and silica supports have been performed because of the industrial importance of these oxides and their weak background Raman vibrations. The most informative studies resulted from the adsorption of pyridine since this probe molecule is very sensitive to the type of acid sites (Brpnsted and Lewis) present on the silica and alumina surfaces. On the alumia support, Lewis pyridine was predominately observed and on the silica support both Lewis and Brpnsted pyridine were observed. " The surface concentrations of pyridine on the silica surface were very small in comparison to the pyridine coverages on the alumina surface. The signal intensities were dramatically enhanced by the... 

Fig. 5 Binary surface excess isotherms for adsorption of liquid mixtures (A) benzene (1) + cyclohexane (2) on silica gel and (B) pyridine (1) + -heptane (2) on silica gel and alumina.

In contrast to gas phase adsorption, pyridine, adsorbed from a cyclohexane solution on silica gel, immersed as a mull into Nujol, displays remarkably sharp bands of the tt-tt transition with only a small red shift (Fig. 4). This structured spectrum is nearly identical with that of pyridine in hydroxylated solvents, and in the solid state at low temperature. 

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