Probes for Basic Sites

The number of probe molecules suitable for studying the basicity of zeolites by application of IR spectroscopy is much more limited than in the case of acidity. The sites of basicity are most likely oxygen atoms of the framework or basic extra-framework species such as CS2O or alkali metal clusters (cf. Volume 3, Chapters 5 and 6 of the present series). 

Frequently proposed probes are pyrrole [371,800-809], carbon dioxide [807, 810], acetic acid [811], boric acid trimethyl ester [812], other weak acids such as 

As an example of pyrrole application. Fig. 50 characterizes the basicity of X- and Y-type zeolites and the effect of the charge-compensating cations on the basicity of the adsorbent As a measure of the base strength, the shift of the NH stretching band of the probe is used (cf.[803]). 

Heidler et al. [806] investigated and discussed by a combination of the energy equalization method and results of the Monte Carlo technique the observed heterogeneity in the spectra of pyrrole adsorbed on basic faujasite-type zeolites (cf. also [804,805]), arriving at the result that there is only one adsorption site for pyrrole but an effect of the nsi/nAi ratio on the orientation of the pyrrole molecule with respect to the six-membered rings. 

As Rymsa et al. [813] have shown, the utilization of deuterochloroform as a probe is another possibility of characterizing the basicity of zeolitic systems by FTIR spectroscopy. The C-D stretching band of CDCI3 was shifted to lower wavenumbers when the probe was adsorbed on basic materials such as M, Na-X, M, Na-Y (M=K, Rb, Cs), Cs-Beta, and the shift increased with increasing size of the cation exchanged into the zeolite. 

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Pyrrole, an amphoteric molecule, is a very effective probe for basic sites in... 

By analogy to the use of small bases (amines, phosphines, phosphine oxides, NiO) as probes of acidic sites on surfaces, one might expect that small acidic molecules could serve as probes of basic sites on surfaces. For this purpose BR compounds show some promise, based on the "B nuclide (/ = 3/2, 80.4% natural abundance) (P. Marchetti and G. E. Maciel, unpublished results). 

Acid-base properties of zeolites are probed by studying their interactions with basic/acidic molecules by appropriate techniques (IR, NMR, calorimetry, TPD). For the external surface region, analogous methods based on XPS detection were developed following the pioneering work of Defosse and Canesson [65]. The probe molecules used are pyridine [55,56,66-72], ammonia [21,43,44,73], and pyrrole and chloroform for basic sites [70,74,75]. Kaliaguine [59] has published a review of the results. 

This study permits to discuss the efficiency of new probe molecules for the characterization of basic sites in zeolites. For MBOH and for methylacetylene, the observed frequencies shift account for the variations in the basic strength of the zeolitic framework oxygen atoms. Interestingly, methylacetylene also informs on the environment of the basic sites and H2S dissociation brings information on the amount of strong basic sites. These results show the high potential and the complementarity of these protic probes to describe the strength, concentration and environment of basic sites. 

Although the equilibrium constant for the formation of ad-nitromethane (27) is very small, strongly basic solutions drive the equilibrium by formation of the ad-anion (28)200. When nitromethane was adsorbed on basic oxides (MgO and CaO) and zeolites (CsX), evidence for the formation of the ad-form was given by means of solid state NMR. The results indicate that nitromethane could be a much more discriminating probe than CO2 for studying basic sites in zeolites201. 

If one wants to understand why such changes occur, one can look at a few of the basic equilibrium properties of such complexes. Figure 1 illustrates the trends which occur when a sample is titrated with copper, monitoring three different parameters. The black dots indicate the relative amount of bound copper as indicated by free copper ions sensed with an ion-selective electrode (Xc of left ordinate). The triangles represent the change of the absorbance of the solution at 465 nm (right ordinate). The curve with the open circles is the relative quenching of the fulvic acid fluorescence (Q of left ordinate). We see that we are able to probe several different types of sites with different types of probes for this multidentate system. 

Adsorption of a specific probe molecule on a catalyst induces changes in the vibrational spectra of surface groups and the adsorbed molecules used to characterize the nature and strength of the basic sites. The analysis of IR spectra of surface species formed by adsorption of probe molecules (e.g., CO, CO2, SO2, pyrrole, chloroform, acetonitrile, alcohols, thiols, boric acid trimethyl ether, acetylenes, ammonia, and pyridine) was reviewed critically by Lavalley (50), who concluded that there is no universally suitable probe molecule for the characterization of basic sites. This limitation results because most of the probe molecules interact with surface sites to form strongly bound complexes, which can cause irreversible changes of the surface. In this section, we review work with some of the probe molecules that are commonly used for characterizing alkaline earth metal oxides. 

Ammonia and pyridine are frequently used as probe molecules for the characterization of acidic surfaces, but they also adsorb on strongly basic sites. Tsyganenko et al. (54) proposed various species resulting from NH3 adsorption on basic solids (Scheme 1). The formation of species I corresponds to hydrogen bonding to a basic surface oxygen, and species II, formed by dissociation to give NH2 and hydroxyl species, involves an acid-base site. Such adsorption requires... 

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 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... 

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. 

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