Pyridine, adsorption solvent

For adsorption onto alumina, it has been shown (12) that compounds with — NH or —OH groups are adsorbed more strongly from basic solvents than is predicted by Eq. (8-3). This has been referred to as the basic eluent anomaly. For example, such compounds as carbazole, phenol, and aniline give large positive values with solutions of diethylamine or pyridine as solvent. These values tend to increase with increasing acidity of the sample —NH or —OH group, ranging from... 

These methods suffer from the lack of complementarity, and thus the significance of results provided by any of them is limited. A standard practice to detect the Bronsted or Lewis character of surface sites is pyridine adsorption combined with FTIR measurements the number of Lewis or Bronsted sites is more difficult to count, however. Other titration methods use either color indicators and acid or base titrants in nonpolar solvents or the adsorption of gaseous acidic or basic probes. They do not, in general, give consistent quantitative information about the number of acid or base sites even when applied to the same sample. There are several reasons the applicability of titration methods is limited Either the state of the surface is different for different methods or adsorption equilibrium is not always achieved. Another more serious source of discrepancies between titration methods is that probe molecules of different basicities "see" different surface sites. The lack of a uniquely defined thermodynamic scale of acid strength of surface sites makes difficult any correlation between results obtained with different probe molecules. The use of standard catalytic tests for probing the so-called catalytic acidity is not always a better approach, because the mechanistic assumptions involved are neither straightforward nor subject to experimental proof. 

Reference has already been made to the choice of solvent for introducing the mixture to the column. Generally speaking, adsorption takes place most readily from non-polar solvents, such as petroleum ether or benzene, and least from highly polar solvents such as alcohols, esters and pyridine. Frequently the solvent for introducing the mixture to the column and the developer are so chosen that the same solvent serves the dual purpose. 

The results of some of the many aminations of pyridine and its derivatives that have been carried out appear in Table 14. Yields are quoted where possible but these should not be used for quantitative comparisons as reaction and work up conditions vary widely. 2-Alkylpyridines aminate at the vacant a-position, except when the substituent is very large. 2-f-Butylpyridine does not undergo the Chichibabin reaction, probably because the bulky 2-f-butyl group prevents adsorption on to the sodamide surface. In contrast, 2-phenylpyridine undergoes amination in very good yield. Aminations of 2- and 4-methyl-pyridines do not involve attack on the anhydrobases in aprotic solvents, but some ionization does take place in liquid ammonia. 4-Benzylpyridine forms a carbanion (148) which is only aminated with difficulty by a second mole of sodamide (equation 103). 

Before concentration, acid hydrolyzates are neutralized, most commonly with barium carbonate, although such organic bases as methyldioctylamine has been used.81 This step normally causes little loss, except by adsorption on, for example, barium sulfate,82 but the following points are of interest. Neutralization with ammonia has been recommended,83 as the neutral solution may be evaporated directly to dryness without filtration, and the ammonium sulfate formed is insoluble in methyl sulfoxide, a solvent used for trimethyl-silylation. The authors83 also found that, when hydrolyzates are neutralized with ion-exchange resins, the pH of the concentrated solutions may differ by as much as 2 units of pH. D-Fructose has been found to be epimerized by barium carbonate or pyridine, and lead... 

In summary, extraction with carbon dioxide, pyridine and sulfur dioxide can remove the coke from catalyst. The amount of coke removed depends on the extraction temperature, pressure and duration. Consecutive extractions with two solvents appear to remove more coke than the individual solvents do. Adsorption of certain solvents on the catalyst during extraction can poison the catalyst. Therefore, if poisoning solvents are used for decoking, their remains must be removed from the extracted catalyst to restore the catalyst activity. 

Finally, it should be noted that calorimetric measurements can also be used to monitor adsorption phenomena at the solid-liquid interface (in a solvent). This method has been used to measure the adsorption heats evolved upon injection of dilute solutions of pyridine in alkanes ( -hexane, cyclohexane) onto an acidic solid itself in a slurry with -hexane. The amount of free base in solution is measured separately with a UV-Vis spectrometer, leading to an adsorption isotherm that is measured over the range of base addition used in the calorimetric titrations. The combined data from the calorimetric titration and adsorption measurements are analyzed simultaneously to determine equihbrium constants, quantities of sites per gram and acid site strengths for different acid sites on the solid. 

The adsorption of solvent molecules by chelates (126 M) from (123 c) and (124 e) with bivalent metal cations of high coordination number, e.g. Cr was studied. So molecules such as pyridine, CCI4, benzene, CH3OH, n-pentane with a critical diameter of < 7 A are adsorbed. The capacity is higher than using various molecular sieves (except for water). 

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