Basicity probe

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. 

Differential heats of NH adsorption were measured for the samples outgassed at different temperatures ranging from 400 to 800°C. Ammonia was chosen as a basic probe because its size is small, which may limitate diffusion effects in small pore zeolite materials. The variations of the differential heats of adsorption are plotted in fig. 3 as a function of the successive pulses of... 

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... 

The acido-basic properties of water molecules are greatly affected in restricted media such as the active sites of enzymes, reverse micelles, etc. The ability of water to accept or yield a proton is indeed related to its H-bonded structure which is, in a confined environment, different from that of bulk water. Water acidity is then best described by the concept of proton-transfer efficiency -characterized by the rate constants of deprotonation and reprotonation of solutes - instead of the classical concept of pH. Such rate constants can be determined by means of fluorescent acidic or basic probes. 

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. 

Figure 4.2 Schematic of basic probe designs transmission cell, immersion probe (retroreflecting and transmission), attenuated total internal reflection.

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. 

The subtraction curve in Fig. 3A (solid) is compared with that obtained by ramping the same column load twice with no probe, and then a third time after injecting the basic probe (dotted curve, Fig. 3B). The near congruence of the curves resulting from the first and third ramps demonstrates that acidic sites are part of the glass structure (silanols and silicates) and are not susceptible to desorption in this temperature range. 

The basic probe system has been refined to avoid the effects of finite input resistance as much as possible and to increase accuracy, sensitivity and spatial resolution in field and charge investigations. Three versions are briefly described below. 

Internal versus External and Extraframework Sites in Zeolite Acid Catalysis the Use of Hindered Basic Probes Catalytically active sites also exist on the external surface and at the pore mouth of zeolite crystals. These sites are considered to be responsible for unwanted non-selective catalysis. On the other hand, H-zeoUtes also catalyze reactions of molecules that do not enter the cavities because of their larger size. So, the external surface of zeolites is certainly active in acid catalysis. Additionally, the bulk and surface Si/Al compositions of a zeolite could be different and different preparation procedures can be chosen to modify this ratio. 

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.

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... 

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. 

Another way to characterize acidity is to study the differential heat of adsorption of a basic probe compound, such as ammonia or pyridine, by microcalorimetry as a function of uptake. This technique yields the distribution of acid strength relative to coverage, but unfortunately does not differentiate between Br0nsted and Lewis... 

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). 

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