Pyridine uptake

The adsorption microcalorimetry has been also used to measure the heats of adsorption of ammonia and pyridine at 150°C on zeolites with variable offretite-erionite character [241]. The offretite sample (Si/Al = 3.9) exhibited only one population of sites with adsorption heats of NH3 near 155 kJ/mol. The presence of erionite domains in the crystals provoked the appearance of different acid site strengths and densities, as well as the presence of very strong acid sites attributed to the presence of extra-framework Al. In contrast, when the same adsorption experiments were repeated using pyridine, only crystals free from stacking faults, such as H-offretite, adsorbed this probe molecule. The presence of erionite domains in offretite drastically reduced pyridine adsorption. In crystals with erionite character, pyridine uptake could not be measured. Thus, it appears that chemisorption experiments with pyridine could serve as a diagnostic tool to quickly prove the existence of stacking faults in offretite-type crystals [241]. 

Samples C and D are different from sample A. Their (Si/Al)g and (Si/Al>3 values are equal within 5% suggesting a uniform spatial distribution of A1 atoms. Their BET surface areas are very close to the known values for ZSM-S suggesting absence of pore blockage by extraneous material which is confirmed by their high pyridine uptake. The (N/Al) ratios both in the bulk and on the surface are close to 1 indicating that every A1 atom is associated with one acidic center (able to adsorb one pyridine molecule). Thus these two samples are specially pure and uniform, and they may be used as model solids for further study. 

Also, pyridine was reversibly adsorbed into the essentially nonacidic silicalite-1 (sample No. 4), which has the same MFI structure as H-ZSM-5, but no acid centers. A set of spectra of pyridine uptake into silicalite-1 was scanned in short intervals and is displayed in Fig. 13. 

The results of a very recent study by Attia [3], summarized in Fig. 1, are a good illustration. Carbons of varying surface area were produced by carbonization and activation of date pits in steam, air, or carbon dioxide (at different temperatures and to different extents of burnoff). The correlations of water vapor and pyridine uptakes with the total surface area are seen to be very poor. The author concludes that water vapour adsorption is related to the chemistry of the surface rather than to the extent of the surface area but does not identify, let alone quantify, this chemistry. Regarding pyridine adsorption, the author notes that activation of carbons at low temperatures created acidic sites while treatment at high temperatures led to the generation of basic sites on the surface, but she does not identify these sites. 

The surface acidity of three commercial mordenite (MOR) zeolites with Si/Al ratios of 10, 60, and 80 has been evaluated by adsorption micro calorimetry at 423 K, using pyridine as a probe molecule [210]. As could be expected from the Si/Al ratios, the total pyridine uptakes varied in the order MOR-10 >MOR-60 >MOR-80. The initial differential heats of adsorption were in the range 215-220 kJ mol After a sudden drop, Qdiff changed slightly and stepwise over a relatively wide range of pyridine uptake (at least for MOR-10) and then steeply decreased. The site-energy distributions and the thermokinetic parameters versus coverage were also determined [210]. 

Fig. 3.7 Thermokinetic parameter as a function of pyridine uptake on mordenite samples (from [16])...

White, H. S., Hunt, J., Wolf, H. H., Swinyard, E. A., Falch, E., Krogsgaard-Larsen, R, and Schousboe, A. (1993) Anticonvulsant activity of the gamma-aminobutyric acid uptake inhibitor N-4,4-diphenyl-3-butenyl-4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol. Eur J. 

As stated above, when probes with specific adsorption characteristics are used, additional chemical information can be extracted from adsorption-desorption experiments. Temperature-programmed desorption (TPD) in particular is often employed to obtain information about specific sites in catalysts [55,56], The temperature at which desorption occurs indicates the strength of adsorption, whereas either the amount of gas consumed in the uptake or the amount of desorption upon heating attests to the concentration of the surface sites. The most common molecules used in TPD are NH3 and C02, which probe acidic and basic sites, respectively, but experiments with pyridine, Oz, H2, CO, H20, and other molecules are often performed as well [57-59], As an example, the ammonia... 

In the case of dense, surface-grafted polymer brush thin hlms, cross-links are introduced by the simple addition of solutions containing bis(Pd -pincer) compounds 4a or 4b to grafted PVP brushes (Loveless et al. 2006). Because the association constants for pyridine coordination are similar, the uptake of 4a and 4b from equimolar solutions into the PVP brushes (at constant grafting density and molecular weight) is effectively equivalent, producing samples with comparable structure (number and placement of cross-links). 

Hazell AS, Itzhak Y, Liu HP, Norenberg MD (1997) l-Methyl-4-phenyl-l,2,3,6-tetrahydro-pyridine (MPTP) decreases glutamate uptake in cultured astrocytes. J Nem-ochem 68 2216-2219... 

JV-Nitraminopyridines are reducible both in acid and alkali. In hydrochloric acid the main product from 2-nitraminopyridine was the hydrazino-pyridine, formed in a six-electron reduction, but 2-aminopyridine and 2-chloropyridine were side products, the latter possibly through reaction by an intermediate diazonium compound with chloride. Contrary to nitramines of most primary amines, 2-nitraminopyridine431 is reducible in alkaline solution uptake of the first two electrons forms the 2-pyridyl-N-nitrosamine, which is further reduced to 2-aminopyridine. 

Iron(II) meso-tetrakis(2,4,6-trimethoxyphenyl)porphyrin and the triethoxy analogue have been synthesized by Vaska. These are different to the capped and picket fence porphyrins in that both sides of the porphyrin are sterically hindered. Dioxygen uptake by these iron(II) porphyrins at 25 °C in solution was found to be partially reversible. La Mar and co-workers166 have shown that the porphyrins Fe[T(2,4,6-OMe)3PP], Fe[T(2,4,6-OEt)3PP] and Fe[TpivPP] take up oxygen at low temperatures, in the absence of a base such as Melm or pyridine, to give dioxygen adducts of the... 

Fig. 1.—Effect105 of Temperature on Uptake of Sulfur, Chlorine, and Nitrogen by Cuprammonium Rayon (on Tosylation in Presence of Pyridine during Four Days). (Taken from K. Hess and N. Ljubitsch, Ann., 507, 62 (1933).)...

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