Pyridine as probe molecule

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. 

Acidity of the PILC. Acidic properties were studied by infrared spectrometry adsorbing pyridine as probe molecule on self supported wafers prepared by pressing the PILC into thin films (15... 

Microcalorimetry experiments with NH3 and pyridine as probe molecules indicated that insertion of Ga into the offretite aluminosilicate structure increased the overall acid sites strength of the crystals while decreasing its acid sites density [255], The observed heterogeneity of acid site strength distribution of H,Ga,Al-offretites was attributed to some extra-framework Al(Vl) and Ga(Vl) species generated during the ion exchange and calcination procedures used to prepare H-offretite crystals. 

Sur ce acidity was determined by FT-IR analysis of catalysts after preadsorption of pyridine as probe molecule, according to a procedure already described [12]. 

One of the most tedious experiments in zeolite analysis is the assessment of acidity using pyridine as probe molecule. Activation, pumping and equilibration times are very time consuming, and thus such experiments typically take up to a day per sample, especially, if... 

There are several probe molecules for which infrared spectroscopy can differentiate between adsorption on Bronsted and Lewis acid sites and even estimate the amounts adsorbed. Pyridine is the most widely used because it gives well-resolved bands when protonated by Bronsted acid sites (e.g., 1540 and 1640 cm ) or when coordinated to Lewis acid sites (1450 and 1620 cm ). The values of extinction coefficients are available in the literature [121] for these bands, which makes possible semiquantitative measurements, separately, of Lewis and Bronsted sites. Ammonia, with a smaller kinetic diameter that enables it to reach more easily the acid sites in smaller pores, can also be used to distinguish betwen Bronsted and Lewis acid sites however, the use of ammonia is less reliable, mainly because the resulting IR bands overlap each other [122]. Another base that can distinguish between Bronsted and Lewis acid sites is quinoline because its size is greater than that of pyridine quinoline can also be used to differentiate between acid sites at the external surface and those in pores smaller than its kinetic diameter (6 A). Bronsted sites can be selectively measured with IR methods by using substituted pyridines as probe molecules [123]. 

The total acidity has been determined gravimetrically using pyridine as probe molecule adsorbed on the surface. The total irreversibly adsorbed pyridine amounts was measured at 100 C. Results was expressed as mmol of adsorbed pyridine per surface area of support. 

Micro calorimetric experiments with ammonia and pyridine as probe molecules have been used to investigate the effects of framework Ga on the acidic properties of several zeolites [235-241]. 

Fig. 4.17 TPD curve for /-alumina, using pyridine as probe molecule [38]...

Auroux et al. [38] have studied acid properties of silica, alumina and multilayers of silica on alumina (SA) and alumina on silica (AS), obtained by grafting. The surface acidity of the pure oxides and samples obtained by grafting, SA and AS, of both Lewis and Bronsted type, has been investigated by TPD and microcalorimetry, using pyridine as probe molecule. 

Microcalorimetric experiments with ammonia and pyridine as probe molecules have been used to investigate the effects of framework Ga on the acidic properties of several zeolites [121-127]. Experiments of NH3 adsorption microcalorimetry, together with FTIR results from pyridine thermodesorption, have shown that the isomorphous substitution of A1 by Ga in various zeolite frameworks (offretite, faujasite, P) leads to reduced acid site strength, density, and distribution [122-125]. To a lower extent, a similar behaviour has also been observed in the case of a MFI framework [126, 127]. A drastic reduction in the acid site density of H,Ga-offretites has been reported, while the initial acid site strength remained high [122, 124]. 

The acidity measurements were carried out in a Cahn 1000 electrobalance using pyridine as probe molecule adsorbed on the catalyst surface. The irreversibly adsorbed pyridine amounts were determined at 273 K. 473 K and 573 K. Results are expressed as mmol of pyridine irreversibly adsorbed per surface area of catalyst. 

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. 

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 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... 

Br0nsted acidity of zeolite protons is essential for catalytic reactions such as isomerization and cracking and has been studied extensively 15,264). Several characterization methods for acid sites in zeolites have been developed this subject has been covered in recent reviews (265,266). Pyridine and other basic molecules are often used in IR work as probe molecules for Brpnsted and Lewis acid sites (267). Trimethylphosphine has also been used as a probe for the determination of zeolite acidity by IR or NMR (96,268). 

Finally, calorimetric measurements can also be used to monitor adsorption phenomena on the surface of solid catalysts in contact with a liquid phase (in a solvent). For example, the so-called cal-ad method [30-33] has been used to measure the adsorption heats evolved upon addition of dilute solutions of pyridine in -hexane to a solid acid catalyst (TS, H-ZSM-5) in a slurry wifli -hexane. The amount of free base in solution is measured separately using a UV-Vis spectrophotometer [30,31]. A similar technique has been used to determine the acidic character of niobium oxide and niobium phosphate catalysts in different solvents [34,35], using aniline and 2-phenyl-ethylamine as probe molecules. 

Isomerization of w-butane and adsorpion of pyridine as probe reactions CO, CD3CN, and C5H5N as probe molecules, problems of selection of suitable probe molecule NH3 and CDCI3 as probe molecules, basic center described as VO-+H4N, correlation between catalytic activity, NFLj band intensity, and AvCD of CDCI3 measured by DRIFTS... 

The sur ce FT-IR stey of a nanosize aluminum nitride powder definitely brought evidence of the specific chemical composition of its first atomic layer. The unavoidable contamination, mainly by atmospheric water, implies the presence of oxygen and hydrogen in this first layer. As a result, a comparison with the alumina sur ce appeared quite reasonable. Indeed, methanol and pyridine used as probe molecules showed the same behavior on both material sur ces. The acidity of the AIN sur ce was proven by the presence of two types of Al Lewis sites. However for AIN, a specific dissociative adsorption mechanism of acidic methanol could be possibly explained by the presence of weakly basic AI3N sites. Besides, the isotopic exchange... 

The accessibility and acidity of HZSM-35 zeolite were studied by H and P solid-state MAS NMR spectroscopy after adsorption of deuterated pyridine or trimethylphosphine as probe molecules. ... 

One of the widely used methods for analyzing the strength of acid sites is temperature-programmed desorption spectroscopy (TDS), which usually applies GC or MS for the analysis. It seems more appropriate to perform TDS in an IR cell or to combine IR and MS during thermal desorption, in order to monitor the types of sites released from the probe molecules as has been successfully applied to faujasite and mordenite samples using NH3 and pyridine as probes [42-44]. 

When ammonia or pyridine were used as probe molecules to study the acidity of zeoUtes at a given temperature [14, 16], the time needed to establish equilibrium after the addition of doses at first increases with increasing coverage (Fig. 3.7). This is due to the fast adsorption rate because the molecules are bonded irreversibly to the... 

Acidic properties were analyzed by several complementary methods in order to obtain informations on the nature, number and strength of the acid sites. The adsorption of pyridine, used as probe molecule, was studied by IR spectroscopy on self-supported wafers obtained by pressing the PILCs into thin films. Lewis and Bronsted sites give characteristics peaks having well established positions around 1450 and 1550 cm respectively. Evacuations at increasing temperatures were useful to investigate the acid strength. ... 

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. 

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