Probing Surface Acidity

Ammonia (pfCi = 9.24, proton affinity in gas phase = 857.7kj moT ) and pyridine (pfti = 5.19, proton affinity in gas phase = 922.2kJ moT ) are the favored molecules for probing the overall solid acidity, since both Lewis and Bronsted acid sites retain these molecules. However the use of IR spectroscopy or XPS is necessary to distinguish qualitatively and unambiguously between these two types of sites. In addition to NH3 and pyridine, trimethylamine and triethylamine have also been used to probe the acidity of supported oxides. However, it has been mentioned [33] that these two molecules might not be able to equilibrate completely with the surface under typical experimental conditions. The use of substituted pyridines (2,6-dimethylpyridine) has also been considered in order to probe specifically the Bronsted sites [34]. 

Ammonia is among the smallest strongly basic molecules, and its diffusion is hardly affected by a porous structure. In the light of the different possible modes of interaction of this molecule with the oxide surface, it has been found that NH3 mostly coordinates to Lewis sites, but in a few cases the results were interpreted by considering the simultaneous occurrence (to a low extent) of dissociative adsorption leading to NH7 and OH surface species. Dissociative 

Reviews by Gorte and coworkers [35, 36] deal with the adsorption complexes formed by strong and weak bases with acid sites in zeolites. They examine the adsorption enthalpies of a series of strongly basic molecules such as alkylamines, pyridines and imines. These workers also performed studies of the adsorption properties of weak bases, including water, alcohols, thiols, olefins, aldehydes, ketones and nitriles. They report a poor correlation between the differential heats of adsorption on H-MFl zeolites and the enthalpies of protonation in aqueous solutions, but a much better correlation with gas-phase proton affinities [37]. 

Acetonitrile is also an interesting molecule for probing acid sites in catalysts. 

It is a weak base, so no protons are abstracted and actual hydroxyl groups can be observed. It also allows the investigation of both Lewis and Br0nsted acidities [15]. 

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There have been several applications of IGC to the determination of sur ce interactions (15-24). In particular, IGC was applied to several studies of natural polymers. Among them are cellulose (2, wood (26), potato starch as Amylopectin (27) and lignocellosic surfaces (2S). In these studies, die surface diermodynamic characteristic of wood fiber and its relationship to the fiber s water vapor adsorption was determined by IGC (26) Also, the surface ener, surface acid-base flee energy, enthalpy of desorption of acid-base probes, surface acid-base acceptors, and donor parameters were determined by IGC (26). Cellulose was also found by IGC to be sensitive to the presence of adsorbed water which possibly disorders its surface structure. 

Lercher, J.A., Gruendling, C. and Eder-Mirth, G. (1996) Infrared studies of the surface acidity of oxides and zeolites using adsorbed probe molecules, Catal. Today, 27, 353. 

Wakabayashi, F. and Domen, K. (1997) A new method for characterizing solid surface acidity - an infrared spectroscopic method using probe molecules such as N2 and rare gases. Catalysis Surveys from Japan 1 181. 

Chemical composition was determined by elemental analysis, by means of a Varian Liberty 200 ICP spectrometer. X-ray powder diffraction (XRD) patterns were collected on a Philips PW 1820 powder diffractometer, using the Ni-filtered C Ka radiation (A, = 1.5406 A). BET surface area and pore size distribution were determined from N2 adsorption isotherms at 77 K (Thermofinnigan Sorptomatic 1990 apparatus, sample out gassing at 573 K for 24 h). Surface acidity was analysed by microcalorimetry at 353 K, using NH3 as probe molecule. Calorimetric runs were performed in a Tian-Calvet heat flow calorimeter (Setaram). Main physico-chemical properties and the total acidity of the catalysts are reported in Table 1. 

Table III summarizes the parameters that affect Brrfnsted acid-catalyzed surface reactions. The range of reaction conditions investigated varies widely, from extreme dehydration at high temperatures in studies on the use of clay minerals as industrial catalysts, to fully saturated at ambient temperatures. Table IV lists reactions that have been shown or suggested to be promoted by Br nsted acidity of clay mineral surfaces along with representative examples. Studies have been concerned with the hydrolysis of organophosphate pesticides (70-72), triazines (73), or chemicals which specifically probe neutral, acid-, and base-catalyzed hydrolysis (74). Other reactions have been studied in the context of diagenesis or catagenesis of biological markers (22-24) or of chemical synthesis using clays as the catalysts (34, 36). Mechanistic interpretations of such reactions can be found in the comprehensive review by Solomon and Hawthorne (37).

X ray photoelectron spectroscopy (XPS) is powerful in identifying species present at the surface/interface and atoms or functional groups involved in acid-base interactions [116]. Since XPS measures the kinetic energy of photoelectrons emitted from the core levels of surface atoms upon X ray irradiation of the uppermost atomic layers, it can be used to characterize surface acid sites, in combination with base probe molecules adsorption. 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

The purpose of the present work is to incorporate aluminum into the framework of SBA-15 during the synthesis in order to create acid sites on the surface of the material directly and to enhance its activity in acid-catalyzed reactions and to study the stability of SBA and AlSBA molecular sieves under various treatments. The influence of these treatments on the pore size, wall thickness and the environment of Al in these materials are investigated in detail. X-ray diffraction (XRD), Electron Microscopy (TEM) and N2 adsorption were used to characterize the structure, the porosity and the stability of these materials. 27Al MAS NMR was used to ascertain the nature and environment of Al, cumene cracking to test the catalytic activity of parent materials and ammonia chemisorption to probe their surface acidity. 

Figure 13.6 shows a schematic for IGC operation. Inverse, in this instance, refers to the observation that the powder is the unknown material, and the vapor that is injected into the column is known, which is inverse to the conditions that exist in traditional gas chromatography. After the initial injection of the known gas probe, the retention time and volume of the probe are measured as it passes through the packed powder bed. The gas probes range from a series of alkanes, which are nonpolar in nature, to polar probes such as chloroform and water. Using these different probes, the acid-base nature of the compound, specific surface energies of adsorption, and other thermodynamic properties are calculated. The governing equations for these calculations are based upon fundamental thermodynamic principles, and reveal a great deal of information about the surface of powder with a relatively simple experimental setup (Fig. 13.6). This technique has been applied to a number of different applications. IGC has been used to detect the following scenarios ...

For the characterization of the nature of surface sites, probe molecules are needed. In the following, however, we confine ourselves to those probe molecules that can principally be used as poisons under catalytic conditions. Thus, for example, the indicator molecules usually used for the titration of surface acidity and basicity will not be treated. The conditions under which these measurements are carried out differ greatly from those applied during actual catalysis, and the molecular size of the probe molecules is unfortunately usually very large.2 These methods have been reviewed very recently by Tanabe (20) and by Fomi (42). 

The suitability of this adsorption model to characterize quantitative aspects of surface acidic groups gives no indication, however, about the chemical structure of the reactive sites. Only in combination with the chemical probe reactions is it possible to assign the two types of acid sites to carboxylic acid and hydroxy groups, respectively. It is noted that such an approach can also be used to determine ion exchange capacities for metal ion loading required for the generation of dispersed metal-carbon catalyst systems. 

Using large probe molecules43 the activity of the outer surface compared to the microporous activity of Beta materials can be determined. It was shown that for typical TEA-synthesized materials a large fraction of the activity observed could be attributed to the non-shape-selective outer surface acid groups. In order to minimize this non-shape-selective activity various techniques have been employed to passivate the outer surface. 

Desorption of water often converts Bronsted to Lewis acids, and readsorption of water can restore Bronsted acidity. Probe molecules, such as ammonia, pyridine, etc., are used to evaluate Bronsted and Lewis acidity. These compounds may contain water as an impurity, however. Water produced by reduction of metal oxides can also be readsorbed on acid sites. Probe molecules can in some cases react on surface acid sites, giving misleading information on the nature of the original site. Acidity, and accessibility, of hydroxyl groups or adsorbed water on zeolites and acidic oxides can vary widely. Study of adsorbed nitrogen bases is very useful in characterization of surface acid sites, but potential problems in the use of these probes should be kept in mind. 

The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 147 Table 3.8 Molecular probes applied for surface acidity characterization. 

In contrast to acidity characterization with basic probes, the use of acidic molecules to probe surface basicity is far less satisfactory. In fact, all acidic (or electrophilic) molecules (Table 3.12) also contain accessible nucleophilic (basic) atoms. It seems impossible to find a molecule that actually only interacts specifically with basic sites. On the other hand, metal oxides that display significant surface basicity... 

Section 5.3 considered NMR spectroscopic approaches to the bulk characterization of oxides and oxidation catalysts. Gatalytic activity is, however, intrinsically linked with the nature of the catalyst surface and hence a number of techniques have been developed in order to probe this. As discussed in Section 5.1, two of the most significant parameters impacting on catalyst activity are the acid-base characteristics of a surface and the redox properties of the material, and NMR techniques exist to probe both of these characteristics. One of the most common techniques to probe surface structure is GP-MAS NMR, in particular CP from hydrogen to the nucleus of interest-either the metal or the oxygen of the metal oxide. Historically, the source of surface H species has often been those naturally present on the catalyst surface, as chemisorbed hydroxyls or physisorbed water. As such, much of the work in this area involves the study of supports such as Si02. Applications of CP-MAS and other spectroscopic approaches to the study of oxide surfaces are outlined in the following sections. 

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