Probe pyridine

On co-adsorbing phenol and methanol, the protonation of methanol occurs on the active acid sites as the labile protons released from the phenol reacted with methanol. Thus protonated methanol became electrophilic methyl species, which undergo electrophilic substitution. The ortho position of phenol, which is close to the catalyst surface, has eventually become the substitution reaction center to form the ortho methylated products (Figure 3). This mechanism was also supported by the competitive adsorption of reactants with acidity probe pyridine [79]. A sequential adsorption of phenol and pyridine has shown the formation of phenolate anion and pyridinium ion that indicated the protonation of pyridine. 

The spectroscopic probe pyridine-N-oxide was used to characterize polar microdomains in reverse micelles in supercritical ethane from 50 to 300 bar. For both anionic and nonionic surfactants, the polarities of these microdomains were adjusted continuously over a wide range using modest pressure changes. The solubilization of water in the micelles increases significantly with the addition of the cosolvent octane or the co-surfactant octanol. Quantitative solubilities are reported for the first time for hydrophiles in reverse micelles in supercritical fluids. The amino acid tryptophan has been solubilized in ethane at the 0.1 wt.% level with the use of an anionic surfactant, sodium di-2-ethylhexyl sulfosuccinate (AOT). The existence of polar microdomains in aggregates in supercritical fluids at relatively low pressures, along with the adjustability of these domains with pressure, presents new possibilities for separation and reaction processes involving hydrophilic substances. 

Decomposition of organic cations in offretite and ZSM-34 crystals was followed via FTIR spectroscopy by Occelli et al. [873] in that the decrease of the bands of the organic species and the concomitant appearance of the new bands in the OH stretching region was monitored. The protonic nature of the newly formed hydroxy groups was confirmed by their interaction with the probe pyridine and generation of the bands of pyridinium ions (cf. Sect. 5.5.2.6.2). 

Figure 1 shows the aggregation behavior of AOT in liquid cyclohexane and supercritical fluid ethane. The systems are one-phase without added water. Surfactant aggregation is indicated by the solvatochromic probe pyridine A -oxide. Pyridine A -oxide was used because of its small size and large dipole moment (/x = 4.3 D), which allow it to partition to the center of reverse micelles instead of being trapped at the surfactant interface. This molecule is a blue shift indicator in that its U V absorption maximum shifts to lower... 

In two-phase systems, however, where surfactant and water can partition between a fluid and a liquid phase, significant pressure effects occur. These effects were studied for AOT in ethane and propane by means of the absorption probe pyridine N-oxide and a fluorescence probe, ANS (8-anilino-l-naphthalenesulfonic acid) [20]. The UV absorbance of pyridine A-oxide is related to the interior polarity of reverse micelles, whereas the fluorescence behavior of ANS is an indicator of the freedom of motion of water molecules within reverse micelle water pools. In contrast to the blue-shift behavior of pyridine N-oxide, the emission maximum of ANS increases ( red shift ) as polarity and water motion around the molecule increase. At low pressures the interior polarity, degree of water motion, and absorbance intensity are all low for AOT reverse micelles in the fluid phase because only small amounts of surfactant and water are in solution. As pressure increases, polarity, intensity, and water motion all increase rapidly as large amounts of surfactant and water partition to the fluid phase. The data indicate that the surfactant partitions ahead of the water thus there is a constant increase in size and fluidity of the reverse micelle water pools up to the one-phase point. An example of such behavior is shown in Fig. 4 for AOT in propane with a total fVo of 40. The change in the ANS emission maximum suggests a continuous increase in water mobility, which is due to increasing fVo in the propane phase, up to the one-phase point at 200 bar. 

Fig. 78. Description of the kinetics of solid-state ion exchange in the system CuCl/Na-Y through a diffusion model the symbols represent experimental data derived from the measured integrated absorbances of the probe (pyridine), the broken lines represent results of the fitting to the diffusion model (for details, see text after [289], with permission)...

When two equivalents of pyridine were added to the nmr sample and the probe heated to 80° C, the enol formate 61 decreased and phenyl cyclopropyl ketone 58 appeared at a rate approximately ten times faster than in the previous buffered system. The observation of intermediate 61 and the kinetic results, together with the observed induction periods, are consistent with the idea that some and perhaps all of the rearranged product ketone in the solvolysis of this system arises via double-bond participation in 61 rather than triple-bond participation and a vinyl cation (80). 

Another SBU with open metal sites is the tri-p-oxo carboxylate cluster (see Section 4.2.2 and Figure 4.2). The tri-p-oxo Fe " clusters in MIL-100 are able to catalyze Friedel-Crafts benzylation reactions [44]. The tri-p-oxo Cr " clusters of MIL-101 are active for the cyanosilylation of benzaldehyde. This reaction is a popular test reaction in the MOF Hterature as a probe for catalytic activity an example has already been given above for [Cu3(BTC)2] [15]. In fact, the very first demonstration of the catalytic potential of MOFs had aheady been given in 1994 for a two-dimensional Cd bipyridine lattice that catalyzes the cyanosilylation of aldehydes [56]. A continuation of this work in 2004 for reactions with imines showed that the hydrophobic surroundings of the framework enhance the reaction in comparison with homogeneous Cd(pyridine) complexes [57]. The activity of MIL-lOl(Cr) is much higher than that of the Cd lattices, but in subsequent reaction rans the activity decreases [58]. A MOF with two different types of open Mn sites with pores of 7 and 10 A catalyzes the cyanosilylation of aromatic aldehydes and ketones with a remarkable reactant shape selectivity. This MOF also catalyzes the more demanding Mukaiyama-aldol reaction [59]. 

Time-resolved luminescence quenching measurements using the probe Tb(pyridine-2,6-dicarboxylic acid)i and the quencher bromophenol blue show the existence of micellar clusters in AOT-based w/o microemulsions. The fast exchange appearing over several microseconds was attributed to intracluster quenching, whereas the slow exchange on the millisecond time scale was attributed to intercluster exchange [243]. 

Adsorbed CO and NO were used as probes to Investigate the effects of Co concentration and sulfide on the nature and numbers of exposed metal sites on reduced catalysts containing 1 to 6 wt% Co and 8 wt%. Mo on three alumina supports. Exposure of Mo Ions decreased with Increased Co concentration. Exposure of Co Ions typically reached a maximum at 2-4% Co. Sulfide decreased exposure of all metal Ion sites and Increased exposure of reduced metals. Effects of preadsorbed pyridine and 2,6-lutldlne, known poisons, on the exposure of metal sites, plus other evidence. 

For probing the nature of the acid sites by X-ray photoelectron spectroscopy (XFS), the samples were evacuated before gaseous pyridine was adsorbed. Excess pyridine was desorbed at 1S0°C, and then samples were pressed onto a sample stub imder Nj and loaded into the SCIENTA ESCA-300 instrument without exposure to air. Sample charging was minimized by using a Qood gun while acquiring the experimental data. 

While our discussion will mainly focus on sifica, other oxide materials can also be used, and they need to be characterized with the same rigorous approach. For example, in the case of meso- and microporous materials such as zeolites, SBA-15, or MCM materials, the pore size, pore distribution, surface composition, and the inner and outer surface areas need to be measured since they can affect the grafting step (and the chemistry thereafter) [5-7]. Some oxides such as alumina or silica-alumina contain Lewis acid centres/sites, which can also participate in the reactivity of the support and the grafted species. These sites need to be characterized and quantified this is typically carried out by using molecular probes (Lewis bases) such as pyridine [8,9],... 

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. 

One of the first studies to predict log P by using potential energy fields calculated using the GRID and CoMFA approaches was done by Kim [60]. The author investigated H, CH3 and H2O probes, and calculated the best models using the hydro-phobic probe H2O for relatively small series (20 or less compounds each) of furans, carbamates, pyridines and pyrazines. A similar study was performed by Waller [61] who predicted a small series of 24 polyhalogenated compounds. Recently, Caron and Ermondi [62] used a new version of Cruciani s software, VolSurf [63], to predict the octanol-water and alkane-water partition coefficients for 152 compounds with r = 0.77, q = 0.72, SDEP = 0.60 for octanol-water and r = 0.76, q = 0.71, SDEP = 0.85 for alkane-water. 

To probe interactions between active silanol sites and the isothiazolin-based biocides a number of model probes were investigated 12. The adsorbates (1-methylpyrro lidin-2-one, pyridine, pyrrolidine, pyrrole, 2-methylthiophene, 2-octyl-4-isothiazolin-3-one, 4,5-dichloro-2-octyl-4-isothiazolin-3-one and 2-cyclopenen-l -one,) varied in basicity, polarity and 7i-character. The amounts of the adsorbates retained by... 

Furthermore, the membrane retentions of the lipophilic probe molecules are dramatically decreased in the presence of the sink condition in the acceptor wells, as shown in Fig. 7.27. All molecules show R < 35%, with progesterone and phenazo-pyridine showing the highest values, 34% and 26%, respectively. 

Heterocyclic nitrogen donors and their adducts with zinc chloride have been studied.623,624 A large number of other ligand systems have also been characterized, for example, zinc halide adducts of 2,2-dimethylpropane-1,3-diamine and hexamethylphosphoramide have been studied.625,626 The formation of mixed ligand complexes with chloride and substituted pyridines has been studied.627 The zinc tris(pyridyl) chloride anion has also been structurally characterized.628 Manganese(II) ions have been used to probe the stereochemistry in reactions of zinc halides with pyrazine.629... 

The specific surface area of the fresh and used catalysts was measured by nitrogen adsorption method (Sorptometer 1900, Carlo Erba Instruments). The catalysts were outgassed at 473 K prior to the measurements and the Dubinin equation was used to calculate the specific surface area. The acidity of investigated samples was measured by infrared spectroscopy (ATI Mattson FTIR) by using pyridine (>99.5%, a.r.) as a probe molecule for qualitative and quantitative determination of both Bronstcd and Lewis acid sites (further denoted as BAS and LAS). The amounts of BAS and LAS were calculated from the intensities of corresponding spectral bands by using the molar extinction coefficients reported by Emeis (23). Full details of the acidity measurements are provided elsewhere (22). 

The Nitrogen Availability Assay [376] consists of growth tests under defined conditions using mineral salts medium and organonitrogen compounds as sources of carbon and/or nitrogen. Probe molecules include quinoline, pyridine, carbazole, and porphyrin. Growth tests are performed using six conditions ... 

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. 

The number of sites titrated by NH3 and pyridine are similar except for sample Al-SBA-15(15) which means a good accessibility of pyridine in the solid pores without any steric hindrance. On the contrary, the integral heats of adsorption are higher when using pyridine due to its higher protonic affinity in gas phase compared to NH3 and the way in which probe molecules bind on the solid surface [6, 7]. 

The aluminum is incorporated in a tetrahedral way into the mesoporous structure, given place to Bronsted acidic sites which are corroborated by FTIR using pyridine as probe molecule. The presence of aluminum reduces the quantity of amorphous carbon produced in the synthesis of carbon nanotubes which does not happen for mesoporous silica impregnated only with iron. It was observed a decrease in thermal stability of MWCNTs due to the presence of more metal particles which help to their earlier oxidation process. 

The samples are named as Fe/Al-MCM41 (nSi/nAl). Pyridine as a molecule probe was adsorbed on dried samples for the acidic sites characterization by FTIR. MWCNTs were synthesized using the CCVD of ethylene as described elsewhere [7]. 

Acidity of both zeolites was investigated by adsorption of ammonia, pyridine, d3-acetonitrile and pivalonitrile ((CH3)3CCN) used as probe molecules followed by FTIR spectroscopy. All samples were activated in a form of self-supporting wafers at 450 °C or 550 °C under vacuum for 1 h prior to the adsorption of probe molecules. 

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