Ammonia adsorption, Lewis acid sites

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... 

Prior to solving the structure for SSZ-31, the catalytic conversion of hydrocarbons provided information about the pore structure such as the constraint index that was determined to be between 0.9 and 1.0 (45, 46). Additionally, the conversion of m-xylene over SSZ-31 resulted in a para/ortho selectivity of <1 consistent with a ID channel-type zeolite (47). The acidic NCL-1 has also been found to catalyze the Fries rearrangement of phenyl acetate (48). The nature of the acid sites has recently been evaluated using pyridine and ammonia adsorption (49). Both Br0nsted and Lewis acid sites are observed where Fourier transform-infrared (FT IR) spectra show the hydroxyl groups associated with the Brpnsted acid sites are at 3628 and 3598 cm-1. The SSZ-31 structure has also been modified with platinum metal and found to be a good reforming catalyst. 

Ammonia TPD is very simple and versatile. The use of propylamine as a probe molecule is starting to gain some popularity since it decomposes at the acid site to form ammonia and propene directly. This eliminates issues with surface adsorption observed with ammonia. The conversion of the TPD data into acid strength distribution can be influenced by the heating rate and can be subjective based on the selection of desorption temperatures for categorizing acid strength. Since basic molecules can adsorb on both Bronsted and Lewis acid sites, the TPD data may not necessarily be relevant for the specific catalytic reaction of interest because of the inability to distinguish between Bronsted and Lewis acid sites. 

The acidic/basic properties of zeolites can be changed by introdnction of B, In, Ga elements into the crystal framework. For example, a coincorporation of alnminnm and boron in the zeolite lattice has revealed weak acidity for boron-associated sites [246] in boron-snbstitnted ZSM5 and ZSMll zeolites. Ammonia adsorption microcalorimetry gave initial heats of adsorption of abont 65 kJ/mol for H-B-ZSMll and showed that B-substituted pentasils have only very weak acidity [247]. Calcination at 800°C increased the heats of NH3 adsorption to about 170 kJ/mol by creation of strong Lewis acid sites as it can be seen in Figure 13.13. The lack of strong Brpnsted acid sites in H-B-ZSMll was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation with methanol. 

Magnesium oxide is considered to exhibit basic properties (20). It is thus not unexpected that neither Br nsted nor Lewis acid sites could be detected by ammonia adsorption (180,181). Hydrogen-bonding is the only type of interaction that ammonia probably undergoes with surface oxide ions on dehydroxylated surfaces (180) and with surface OH groups on hydroxylated surfaces (181). 

Infrared spectroscopic studies regarding the adsorption of pyridine on both anatase and rutile have been reported (136, 176, 194, 216,217). Hydrogen-bonded pyridine is readily desorbed on pumping at room temperature, whereas pyridine held by coordinatively unsaturated Ti4+ ions is thermally stable up to approximately 400°C. As ammonia, pyridine forms two distinct coordinately held species (176, 217) indicating the existence of two types of Lewis acid sites, which should correspond to Ti4+ ions in different stereochemical environments. According to Primet et al. (176), the more stable species is chemisorbed on type... 

Ai (140) measured the acid site concentration by the adsorption of ammonia. No correlation was found between the P/V ratio, the acidity, and the catalytic activity. This result has been attributed to the use of ammonia as a probe molecule that cannot distinguish between Lewis and Br0nsted acidity. Comaglia et al. (137) measured the acid sites using pyridine and acetonitrile as probes. However, the pyridine results showed no correlation between the Lewis to Br0nsted acid site ratio or the Lewis acid site concentration and the activity and selectivity of the catalyst for MA formation. 

Figure 9.4 Regions in a typical curve of differential heats of adsorption versus adsorbed amount. All regions (a, b, c, d) can be observed for zeolite samples presenting both Lewis and Bronsted acid sites, as probed by ammonia adsorption. For oxides presenting only Lewis acid sites, the regions a, c and d are observed.

For decationated mordenites, an increase in the ammonia adsorption temperature caused a significant increase in the differential heat of adsorption at low coverages, the largest increase of the initial heat being from 130 to 170 kJ mol" , corresponding to Lewis acid sites on a 90% decationated mordenite that was dehydroxylated at 923 K (88-90). The effect of increasing the... 

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. 

The greater acid strength of Bronsted- and Lewis-acid sites associated with framework and extra-framework gallium species, respectively, is also reflected by the greater heats of ammonia adsorption. These effects are not surprising since, as generally known from chemistry textbooks, gallium oxide is more acidic than aluminum oxide. 

Microcalorimetric studies of the adsorption of ammonia on TS-1 with various Ti contents have shown that these solids possessed relatively strong Lewis acid sites that are not present in silicalite-1. The number and strength of these sites do not increase linearly with the Ti content. 

Ammonia adsorption on Lewis sites is stronger than that on Bronsted sites [97]. In situ infrared spectroscopy has been used to monitor surface coverages by various species under reaction conditions. Temperature programmed desorption shows that no NO decomposition occurs in the temperature range 100-600 K. By means of in situ FTIR spectroscopy it was observed that the fractional surface coverages by ammonia on the Bronsted and Lewis acid sites were 0.26 and 0.39, respectively, at 573 K. No adsorption of NO was found. Moreover, it was stated that water does not block the sites for ammonia adsorption. 

Most acidity studies have been made using basic molecules such as ammonia, pyridine, and piperidine as probes. These molecules have the property that their interaction with Bronsted acid sites, Lewis acid sites, and cations and their hydrogen-bonding interactions give rise to different species detectable by infrared spectroscopy. Thus, adsorption on Bronsted acid sites gives rise to ammonium, pyridinium, and piperidinium ions with characteristic absorption frequencies of 1475, 1545, and 1610 cm"1, respectively. Adsorption on Lewis acid sites—tricoordinated aluminum... 

Early applications of IR spectroscopy in zeolite research go back to studies by Szymanski et al. [201],Bertsch and Habgood [202], Tsitsishvili [203], Watanabe andHabgood [204] and especially the pioneering works of Uytterhoeven,Christ-ner and Hall [205] and Cant and Hall [206] who, in particular, investigated the formation of OH groups on the external and internal surface of Y-type zeolites as, somewhat later. Ward [207] also did. Hall s group also studied the adsorption of small molecules such as ammonia [205] and ethylene [208-210] and employed pyridine as a probe to discriminate Bronsted and Lewis acid sites (cf. 

NHs adsorption microcalorimetry has been used to characterize the acid sites of a H-USY zeolite and another USY sample in which the strong Lewis acid sites were poisoned with ammonia. Poisoning of the Lewis acid sites did not affect the rate of deactivation, the cracking activity, or the distribution of cracked products during 2-methylpentane cracking. Thus, strong Lewis acid sites do not seem to play any important role in cracking reactions [148]. 

There are several probe molecules for which infrared spectroscopy can differentiate between adsorption on Bronsted and Lewis acid sites and even estimate the amounts adsorbed. Pyridine is the most widely used because it gives well-resolved bands when protonated by Bronsted acid sites (e.g., 1540 and 1640 cm ) or when coordinated to Lewis acid sites (1450 and 1620 cm ). The values of extinction coefficients are available in the literature [121] for these bands, which makes possible semiquantitative measurements, separately, of Lewis and Bronsted sites. Ammonia, with a smaller kinetic diameter that enables it to reach more easily the acid sites in smaller pores, can also be used to distinguish betwen Bronsted and Lewis acid sites however, the use of ammonia is less reliable, mainly because the resulting IR bands overlap each other [122]. Another base that can distinguish between Bronsted and Lewis acid sites is quinoline because its size is greater than that of pyridine quinoline can also be used to differentiate between acid sites at the external surface and those in pores smaller than its kinetic diameter (6 A). Bronsted sites can be selectively measured with IR methods by using substituted pyridines as probe molecules [123]. 

At least two families of acid sites for ammonia adsorption are present on zeolites surfaces, namely (i) Brpnsted acid sites, where ammonia is strongly bonded, and (ii) Lewis acid sites, where ammonia is weakly adsorbed, especially in the presence of water [25, 26]. Furthermore, the presence of physisorbed ammonia cannot be ruled out, especially at low temperatures. 

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