Probe molecules, basic

Without an artificial sink, the membrane retentions are very high, with many basic probe molecules showing R > 80%. With the imposed sink, many of the retentions dropped by as much as 50%. Furthermore, just 0.5% wt/vol cholesterol in dodecane (in addition to the sink) caused increased retention to drop by at least a further 10-30%. It was not possible to form stable cholesterol-containing lipid models under sink conditions with Avanti s egg lecithin acceptor buffer solutions turned significantly turbid in the untenable model 13.1. 

The acidic and adsorptive properties of the samples in gas phase were evaluated in a microcalorimeter of Tian-Calvet type (C80, Setaram) linked to a volumetric line. For the estimation of the acidic properties, NH3 (pKa = 9.24, proton affinity in gas phase = 857.7 kJ.mol-1, kinetic diameter = 0.375 nm) and pyridine (pKa = 5.19, proton affinity in gas phase = 922.2 kJ.mol-1, kinetic diameter = 0.533 nm) were chosen as basic probe molecules. Different VOC s such as propionaldehyde, 2-butanone and acetonitrile were used in gas phase in order to check the adsorption capacities of the samples. 

Another possibility for characterizing zeolite acid sites is the adsorption of basic probe molecules and subsequent spectroscopic investigation of the adsorbed species. Phosphines or phosphine oxides have been quite attractive candidates due to the high chemical shift sensitivity of 31P, when surface interactions take place [218-222]. This allows one to obtain information on the intrinsic accessibility and acidity behavior, as well as the existence of different sites in zeolite catalysts. 

The effect of probe molecules on the 27A1 NMR has attracted some attention recently. In particular, the determination of the quadrupole coupling constant, Cq, is a sensitive means to learn more about the bonding situation at the aluminum in acid sites, and how it reflects the interaction with basic probe molecules. If one of the four oxygen atoms in an AIO4 tetrahedral coordination is protonated, as in a zeolitic acid site, the coordination is somewhat in between a trigonal and a tetrahedral A1 environment [232]. The protonated oxygen decreases its bond order to A1 to approximately half of its size compared to an unprotonated zeolite. 

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... 

Table 4.4 Properties of common basic probe molecules.

While average deprotonation energy is a good measure of the intrinsic Bronsted acid strength of a zeoHte, it is the extrinsic acidity, also impacted by the chemical interaction between the protonated basic probe molecule and the deprotonated zeoHte, that really counts for catalysis. 

The acid sites strength can be determined by measuring the heats of adsorption of basic probe molecules. The basic probes most commonly used are NH3 (pTTa = 9.24, proton affinity in gas-phase = 857.7 kJ/mol) and pyridine (pTTa = 5.19, proton affinity in gas-phase = 922.2 kJ/mol). The center of basicity of these probes is the electron lone pair on the nitrogen. When chemisorbed on a surface possessing acid properties, these probes can interact with acidic protons, electron acceptor sites, and hydrogen from neutral or weakly acidic hydroxyls. 

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. 

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. 

Table 3.11 Position (cm ) of the sensitive IR bands of adsorbed basic probe molecules on different catalyst surfaces. The Lewis acid strength roughtly decreases from top to bottom.

From the Hterature it is apparent that microcalorimetry is very useful in providing informahon on the strength and distribution of acidic and basic sites of catalysts. The technique for determining the acid site distribution is quite well developed, especially if ammonia is used as the basic probe molecule. Moreover, the energehcs of surface reachons, including oxidahon and reduction of metal oxides, oxidahon of adsorbed hydrocarbons or hydrogen and decomposihon reachons can be determined direchy by calorimehy [4]. 

Cardona-Martinez and Dumesic [30] have analyzed the problem of surface mobility of the adsorbates for the particular case of adsorption of basic probe molecules on acid sites of oxides. Without equilibration of adsorbate with surface sites, the measured differential heat would only be an average value of the sites that the molecules adsorb on, and differences among sites would not be detected. Thus, ideally, measurements should be made at as a high a temperature as feasible without desorbing or decomposing the adsorbate. 

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). 

The adsorption on zeolites of a basic probe molecule containing nitrogen can be used to monitor changes in external acidity. The acid sites determined in this way are the first to come into play during contact between catalyst and charge and may have a decisive influence on the activity and selectivity of these solids (Fig. 5.10). 

The weakly basic probe molecules most commonly used are the following sulphur compounds such as H2S, unsaturated hydrocarbons such as ethylene, carbon monoxide and dcu-terated acetonitrile. They are used to detect the strongest acid sites of the solid under study. In these cases protonation does not occur but there is formation of species linked by hydrogen bonding. 

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. 

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.

Proton Affinities, Vertical Ionization Potentials, and Acidic Dissociation Constants of Selected Basic Probe Molecules"... 

A number of methods are used for studying the sorption of basic probe molecules on zeolites to learn more about zeolite acidity. A common disadvantage of all the examinations is that adsorbed basic probe increases the electron density on the solid and, thereby, change the acidic properties of the sites examined. From this aspect it seems advantageous to probe the acid sites with a weak base, e. g., with a hydrocarbon. It was shown that adsorption of alkanes is localized to the strong Brdnsted acid sites of H-zeolites [1, 2]. However, recent results suggest that usually the diffusion in the micropores controls the rate of hydrocarbon transport [3-5]. Obviously, the probe suitable for the batch FR examination of the sites has to be non-reactive and the sorption dynamics must control the rate of mass transport. The present work shows that alkanes can not be used because, due to their weak interaction with the H-zeolites, the diffusion is the slowest step of their transport. In contrast, acetylene was found suitable to probe the zeolitic acid sites. The results are discussed in comparison with those obtained using ammonia as probe. Moreover, it is demonstrated that fundamental information can be obtained about the alkane diffusivity in H-zeolites... 

R spectroscopy is one of the most useful techniques for the characterization of zeolitie materials. It provides information on the nature of OH-groups in the materials and on the framework vibrations. Even more useful is the IR analysis of adsorbed species. Basic probe molecules, such as pyridine, ammonia, or benzene, allow the analysis of acidic sites. CO adsorbed at low temperature also helps to analyze acidic sites, but can also be used for the analysis of noble metal particles in zeolites. 

TPD of basic probe molecules is a method which is often used for the analysis of zeolitic acidity [24], In a parallelized implementation [25] of this technique, the samples to be studied were placed in a parallel channel reactor-body built analogous to the one described by Hoffmann et al. [26], By means of a multiport valve, the effluent from each channel could be switched to a mass spectrometer which was used for the analysis of the desorbed gas. In a typical experiment, after conditioning of the samples and adsorbing ammonia, a temperature ramp was started and in a sequential manner the effluent of each catalyst was fed into the mass spectrometer. Flushing and analysis times per channel were 8 s, so that all 10 samples could be analyzed in somewhat more than one minute. This proved to be sufficient to obtain well resolved TPD curves at a heating rale of 10 K/min. The recorded data corresponded well to curves measured with conventional setups. 

Microcalorimetric NH3 adsorption is one of the powerful techniques for energetic characterization of solid surfaces and provides a direct and accurate method for the quantitative determination of the number of acid sites of different strengths. Microcalorimetry invplves the measurement of differential heats evolved updn adsorption of smeill quantities (micromoles) of basic probe molecules on to the catalysts. Such measurement yields information about the acid strength distribution i.e., the number of sites having the particular heat of adsorption for the basic probe molecule. 

An interesting analysis consists in determining the v OH shift upon adsorption of weaHy basic probe molecules as H2S, CO, C2H4 or C6Hg as shown in table 2, the shift increasing with acid strength. 

After the determination of the dispersive work of adhesion from the Hexane experiment the total work of adhesion can be determined for Dichloromethane. This probe molecule represents a monopolar acid according to the van Oss concept and can be therefore used to determine the base number (ys ) of the surface. For the corresponding determination of the acid number the measurement with a monopolar basic probe molecule is required. Typical candidates for this would be either Toluene or Ethyl acetate. These measurements haven t been completed yet. For this reason the ys -values in the result table are missing. 

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. 

The basic probe molecules of primary interest in this chapter are pyridine, ammonia, i-butylamine, phosphines, and phosphine oxides. All possess several nuclei amenable to solid-state NMR experiments, and, as we shall see, this multinuclear perspective is useful in extracting chemical insights into the nature of the surface. Finally, it is important to point out the early, pioneering contributions of Ian Gay 181] to this area of NMR spectroscopy. It was those initial experiments that demonstrated the potential utility of solid-state NMR methcxis to address important questions in the area of surface chemistry. 

Other useful classes of basic probe molecules used to examine silica, alumina, and silica-alumina surfaces (as well as zeolite systems) include small organic phosphines and phosphine oxides, which rely on the highly convenient P nuclide (/ = 1/2, 100% natural abundance). As Lunsford and coworkers demonstrated for zeolites [89], the P NMR signal of trimethylphosphine is a useful probe for Bronsted acid sites on surfaces. The basis for this approach is the formation of R3P -H B( ) sites at surface Bronsted acid sites, H-B(. ... 

Basic probe molecules 13-P-l 3 Benzaldehyde-ethylcyanoacetate condensation ... 

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