Acid probe molecules

For basicity measurements, the number of acidic probes able to cover a wide range of strength is rather small [166]. The most common acidic probe molecules used are CO2 (p/fa = 6.37) and SO2 (p/fa = 1.89). Carboxylic acids such as acetic acid can also be used but dimmers can be formed, particularly at high coverage. Pyrrole may also be used, particularly at low adsorption temperature, but has sometimes shown some amphoteric character [103]. Hexafluoroisopropanol has also been used to characterize the surface basicity of some solids [145]. 

The surface of alumina presents strong acid and basic sites, as demonstrated by the differential heats of adsorption of basic probe molecules such as ammonia [169- 171] and pyridine [169,172] or of acidic probe molecules such as SO2 [169,171] and CO2 [173,174]. Table 13.2 presents a survey of microcalorimetric studies performed for AI2O3. 

The snrface acid-base properties of supported oxides can be conveniently investigated by studying the adsorption of suitably chosen basic-acidic probe molecules on the solid. As shown, acidic and basic sites are often present simultaneously on solid surfaces. The knowledge of the detailed amphoteric character of supported metal oxides is of extreme interest due to the possibility of using them as catalysts in different reactions in which acidity governs the reaction mechanism. 

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 technique has been fruitfully used to characterize acid and basic sites in many catalysts, in particular for zeoHtes and metal oxides [143]. It has also been applied for POMs [144]. It consists of measuring the differential heats of adsorption when adsorbing successive increments of a basic probe molecule such as ammonia or pyridine for acidity characterization or of an acid probe molecule such as GO2 or SO2 to characterize basicity. The technique produces a histogram of the acid-base strength as a function of coverage, in particular when heterogeneity in strength exists. The data should then be compared with ammonia or pyridine desorption data from IR and thermal desorption experiments (see above). 

In most recent calorimetric studies of the acid-base properties of metal oxides or mixed metal oxides, ammonia and n-butylamine have been used as the basic molecule to characterize the surface acidity, with a few studies using pyridine, triethylamine, or another basic molecule as the probe molecule. In some studies, an acidic probe molecule like CO2 or hexafluoroisopropanol have been used to characterize the surface basicity of metal oxides. A summary of these results on different metal oxides will be presented throughout this article. Heats of adsorption of the basic gases have been frequently measured near room temperature (e.g., 35, 73-75, 77, 78,81,139-145). As demonstrated in Section 111, A the measurement of heats of adsorption of these bases at room temperature might not give accurate quantitative results owing to nonspecific adsorption. 

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.

For basicity measurements, the number of acidic probes able to cover a wide range of strength is rather small [41]. Moreover, a difficulty stems from the fact that some acidic probe molecules may interact simultaneously wifri cations (such as Na ). The ideal probe molecule should be specific to basic sites and should not be amphoteric. It should not interact with unwanted types of basic sites or give rise to chemical reactions [41]. For instance CO2 (pKa = 6.37) is a suitable probe to determine and characterize, simultaneously, the surface basicity as well as the Lewis acidity of acidic metal systems. It can form caibonate-like species on the former sites, whereas it can be molecularly coordinated in a linear form at the latter sites [42]. Moreover, the energetic features of the adsorption of CO2 on various molecular sieves, over a large domain of temperature and pressure, can provide interesting information on the nature of the adsorbate-adsorbent interactions [43]. Similar problems may arise when using SO2 as an acidic probe, despite the fact that SO2 (pKa = 1.89) is more acidic than CO2 and, thus, more likely to probe the total basicity of the surface. 

The number of acidic probe molecules that are able to cover wide range of strength is rather small. Moreover the difficulty in evaluating surface basic properties stems from the fact that these molecules may interact simultaneously with cations (such as Na+). The ideal probe molecule should be specific to basic sites, it should distinguish interaction with oxide ion and hydroxyls and it should not give rise to chemical reactions. 

A wide variety of NMR methods are being applied to understand solid acids including zeolites and metal halides. Proton NMR is useful for characterizing Brpnsted sites in zeolites. Many nuclei are suitable for the study of probe molecules adsorbed directly or formed in situ as either intermediates or products. Adsorbates on metal halide powders display a rich carbenium ion chemistry. The interpretation of NMR experiments on solid acids has been greatly improved by Ae integration of theoretical chemistry and experiment. 

It is often said that the property of acidity is manifest only in the presence of a base, and NMR studies of probe molecules became common following studies of amines by Ellis [4] and Maciel [5, 6] and phosphines by Lunsford [7] in the early to mid 80s. More recently, the maturation of variable temperature MAS NMR has permitted the study of reactive probe molecules which are revealing not only in themselves but also in the intermediates and products that they form on the solid acid. We carried out detailed studies of aldol reactions in zeolites beginning with the early 1993 report of the synthesis of crotonaldehyde from acetaldehyde in HZSM-5 [8] and continuing through investigations of acetone, cyclopentanone [9] and propanal [10], The formation of mesityl oxide 1, from dimerization and dehydration of... 

The probe molecules of greatest historical interest in catalysis are the Hammett indicators [13]. The difficulty of making reliable visual or spectrophotometric observations of the state of protonation of these species on solids is well known. We have recently carried out the first NMR studies of Hanunett indicators on solid acids [ 14]. This was also the occasion of the first detailed collaboration between the authors of this article, and theoretical methods proved to strongly compliment the NMR experiments. The Hanunett story is told after first reviewing the application of theoretical chemistry to such problems. Central to the application of any physical method in chemistry is the process of modeling the relationship between the observables and molecular structure. However often one does this, it is rarely an exact process. One can rationalize almost any trend in isotropic chemical shift as a function of some variation in molecular structure - after the fact, but the quantitative prediction of such trends in advance defies intuition in most nontrivial cases. Even though the NMR spectrum is a function... 

Spectroscopy. In the methods discussed so far, the information obtained is essentially limited to the analysis of mass balances. In that re.spect they are blind methods, since they only yield macroscopic averaged information. It is also possible to study the spectrum of a suitable probe molecule adsorbed on a catalyst surface and to derive information on the type and nature of the surface sites from it. A good illustration is that of pyridine adsorbed on a zeolite containing both Lewis (L) and Brbnsted (B) acid sites. Figure 3.53 shows a typical IR ab.sorption spectrum of adsorbed pyridine. The spectrum exhibits four bands that can be assigned to adsorbed pyridine and pyridinium ions. Pyridine adsorbed on a Bronsted site forms a (protonated) pyridium ion whereas adsorption on a Lewis site only leads to the formation of a co-ordination complex. 

Table 7.4 summarizes the key pharmacokinetic (PK) and physicochemical properties of the selected probe molecules, consisting of bases, acids, and neutral species. 

Table 4.1. Conceptional criteria for the selection of probe molecule to characterize solid acids (from ref. [31])...

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. 

Romero Sarria, F., Blasin-Aube, V., Saussey, J. et al. (2006) Trimethylamine as a Probe Molecule To Differentiate Acid Sites in Y-FAU Zeolite FTIR Study, J. Phys. Chem. B, 110, 13130. 

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