Lewis acid sites adsorbing basic probes

Basic molecules such as pyridine and NH3 have been the popular choice as the basic probe molecules since they are stable and one can differentiate and quantify the Bronsted and Lewis sites. Their main drawback is that they are very strong bases and hence adsorb nonspecifically even on the weakest acid sites. Therefore, weaker bases such as CO, NO, and acetonitrile have been used as probe molecules for solid acid catalysts. Adsorption of CO at low temperatures (77 K) is commonly used because CO is a weak base, has a small molecular size, a very intense vc=0 band that is quite sensitive to perturbations, is unreactive at low temperature, and interacts specifically with hydroxyl groups and metal cationic Lewis acid sites.26... 

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. 

It was proven that microcalorimetry technique is quite well developed and very useful in providing information on the strength and distribution of acidic and basic sites of catalysts. When interpreting calorimetric data, caution needs to be exercised. In general, one must be careful to determine if the experiments are conducted under such conditions that equilibration between the probe molecules and the adsorption sites can be attained. By itself, calorimetry only provides heats of interaction. It does not provide any information about the molecular nature of the species involved. Therefore, other complementary techniques should be used to help interpreting the calorimetric data. For example, IR spectroscopy needs to be used to determine whether a basic probe molecule adsorbs on a Brpnsted or Lewis acid site. 

In a broad sense, an acid site can be defined as a site on which a base is chemically adsorbed. Conversely, a basic site is a site on which an acid is chemically adsorbed. Specifically, a Bronsted acid site has a propensity to give a proton, and a Bronsted base has the tendency to receive a proton. Additionally, a Lewis acid site is capable of taking an electron pair and a Lewis basic site is capable of providing an electron pair. These processes can be studied by following the color modifications of indicators, and by using infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies, and calorimetry of adsorption of the probe molecules (see Chapter 4). 

Apparently, CH4 molecules cannot discriminate between Lewis basic and Lewis acid sites of MgO, at variance with adsorbed CO which is a well-established probe for acid centers. However, due to the exclusive adsorption affinity of methane for low-coordinated sites, these molecules can be employed to probe the relative amoimt of morphological defects of MgO samples obtained under different experimental conditions. 

Since the catalytic role of oxide surfaces is often related to the presence of Bronsted and Lewis acidic sites on the surface, it is desirable to characterize the acidic surface properties and the respective concentrations of these species. Information about the Bronsted sites can be obtained from H MAS chemical shifts, which are strongly correlated with acidities 14,29]. Complementary information is available from multinuclear MAS-NMR studies of basic probe molecules adsorbed to the surface. Interaction of the probe molecules with acidic sites is expected to cause characteristic shift effects compared to the resonances of the same molecules in either unperturbed or physisorbed states, and hence quantitative information about site populations should be available. Although a number of such investigations have been carried out, it is often difficult to compare results obtained in different laboratories on the same system because experimental details such as sample history, surface coverage, and impurities (frequently water) have large effects on the spectra. 

Investigations of the surface Lewis acidity of aluminas have mainly been performed by adsorbing basic probes after previous dehydroxylation of the samples by outgassing. Based on spectroscopic results, most authors agree that at least three different types of Lewis acid sites (with weak, medium, and high acid strength) exist on transition aluminas (293). 

Acidity and basicity are relative properties. Many compounds are amphoteric and behave as acids or as bases according to a partner. Metal oxides are classified as acidic, amphoteric or basic. Experimentally, this classification corresponds to the adsorption of probe molecules[7, 8]. NH3 is a base probe molecule that reacts with the electron deficient metal atoms (Lewis acid) or the protons adsorbed on the hydrated surface, CO2 is usually considered as acidic and thus it is expected to adsorb more strongly on basic sites. According to this classification, Ti02 belongs to an amphoteric species and MgO to a basic species. A general difficulty for such classifications is that the order can vary with the choice of the probe. The Hard and Soft Bases and Acids theory[9, 10] responds to the necessity to refine the model with a second scale it is better to couple... 

Infrared spectroscopy has been used for many years to probe acid sites in zeolites. Typically, strong bases such as ammonia or pyridine are adsorbed, and the relative or absolute intensities of bands due to Lewis acid adducts or protonated Bronsted acid adducts are measured. The basicity of ammonia or pyridine is however much stronger than that of most hydrocarbon reactants in zeolite catalysed reactions. Such probe molecules therefore detect all of the acid sites in a zeolite, including those weaker acid sites which do not participate in the catalytic reaction. Interest has recently grown in using much more weakly basic probe molecules which will be more sensitive to variations in acid strength. It is also important in studying smaller pore zeolites to use probe molecules which can easily access all of the available pore volume. 

In order to more precisely differenciate the acid sites, adsorption of pyridine (pKa=5.25), 3,5-dimethylpyridine (pKa=6.15) and 2,6-dimethylpyridine (pKa=6.72) was carried out at 353 K on the samples. These three basic probes display a lower pKa than ammonia (pKa=9.25) and should titrate less weak acid sites. 2,6-lutidine (2,6-DMP) is supposed to adsorb on Bronsted sites preferently to 3,5-lutidine (3,5-DMP) which should adsorb, as pyridine, on both Lewis and Bronsted sites. This behavior can be explained by the steric hindrance due to the methyl groups, the nitrogen atom being less accessible. For example. Figure 4 shows the differential heats of adsorption of the three probe molecules on the sample with Ti=249 pmol/g pretreated at 773 K. All the curves show a sharp decrease till... 

Adsorption complexes of methane at MgO are interesting because they relate to the conversion of methane to ethylene and methanol. In particular, oxidative coupling of methane on metal-oxide catalysts attracted great attention [119]. Usage of methane as a probe to identify and characterize adsorption sites of different acid strength on oxide catalysts is another important aspect. Because CH4 is not easily captured by surfaces of metal oxides, the nature of the interaction of methane with surface sites was little understood until recently. A FTIR spectroscopy investigation of methane on MgO at 173 K revealed adsorbed molecular species preferentially bound at Lewis basic sites CH4 adsorption on a Lewis acid-base pair has also been proposed [120]. 

It is often difficult to determine the nature of the adsorbed species, or even to distinguish between the different kinds of adsorbed species from the calorimetric data. In many cases this technique fails to distinguish between cations md protonic sites due to the insufficient selectivity of the adsorption. For example, the differential heats of NH3 adsorption on strong Lewis centres and strong Brdnsted sites are relatively close to each other. This can make it difficult in some cases to discriminate Lewis and Bronsted sites solely by adsorption microcalorimetry of basic probe molecules if no complementary techniques are used. Because no exact information can be obtained regarding the nature of the acid centres from the calorimetric measurements, suitable IR, MAS NMR, and/or XPS [36] investigations are necessary to identify these sites. However,... 

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 molecular probe technique in combination with XPS has seldom been used since the early 1970s and has mainly been applied to zeoKtes [162-164]. These studies were aimed at identifying and quantifying Lewis acidic and basic sites at catalyst surfaces by monitoring the BE shifts of Nls from adsorbed pyridine [162,163] and pyrrole [164], respectively. We shall discuss the application of this approach to molecular and polymeric species. 

Lewis acidic and basic sites, respectively, correspond to electron acceptor and electron donor sites whereas Brensted acidic and basic sites correspond to proton donor and proton acceptor sites. For a con lete characterization, different molecules should be used to prote these sites, but it must be kept in mind that a molecule can probe different types of sites at the same time. A striking exaraple is the CH3CN probe-molecule which can form at least four different species by adsorbing on different sites (hydrogen bonding, coordination on metallic ion, coordination on OH" ion, reaction with ion). Moreover, acidic and basic characteristics are interdependent, since their formation depends on the stoichiometry, the crystalline phase, the synthesis conditions, and the inq)urities or the contaminants. Therefore, several experiments have to be successively run with different probe molecules to get a good knowledge of the surfece reactivity. 

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