Gaseous Acid Adsorption Method

The principle of this method is the same as that of gaseous base adsorption method (2.1.1.B) and all of the latter method can be applied. As sidsorbates, acidic molecules such as carbon dioxide, nitric oxide and phenol vapor have been used. The adsorption of phenol is not necessarily good for the measurement of basic property, because phenol is easily dissociated to adsorb on both acidic and basic sites and hence acidic property affects the adsorption of phenol. Nitric oxide is used for the measurement of unusually strong basic sites. The amount of carbon dioxide irreversibly adsorbed is a good measure of the amount of basic sites on solid surfaces. The TPD profiles of carbon dioxide desorbed from alkaline earth oxides are shown in Fig. 2.9. Since acidic carbon dioxide desorbs at higher temperature from stronger base sites. 

Diphenylamine (pKa = 23) can be used to determine the amount of strong base sites by measuring the amount of diphenylnitroxide radicals by ESR which are formed from diphenylamine in the presence of oxygen by an action of basic sites. 

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For the determination of strength and amount of a solid acid, there are two main methods an amine titration method using indicators and a gaseous base adsorption method. 

Tert-butyl catechol is a weakly acidic molecule with a pKj value of 11.4(6) and hence basic alumina should favour its adsorption by assuming simple acid-base interactions. Hence, we measured the basicity of alumina. The change in surface chemistry due modification of alumina surface with alkali and acid can be measured by two methods, namely (1) Adsorption /Desorption of Gaseous acid molecule (CO2) (2) Benzoic acid titration method... 

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. 

Adsorption measurements of gaseous bases are often used for the determination of surface acidity. The chief advantage of this method over those discussed previously is that adsorption can be measured at or near temperatures at which catalyzed reactions occur. A variety of bases and techniques have been used as described in previous reviews (/ -3). Two representative approaches will be discussed in this critique one consists of using chemisorption measurements of a suitable base at a given set of conditions to count acid sties on a catalyst surface the other is a more comprehensive approach in which adsorption is measured at several temperatures to enable the determination of the thermodynamics of adsorption as a function of surface coverage. 

The advantage of this type of adsorption analysis evidently lies in the stability of the zones theoretically at.least one should be able to use as large columns as desired and therefore to carry out also very difficult separations. The main drawback of the elution method—the tailing— is eliminated. The fact that the different zones are in close contact is a difficulty which,- however, may be eliminated by interposing substances of intermediate adsorption and of a different chemical nature so that they may easily be removed afterwards. Examples of the application of tills procedure may be found in a paper by Hahn and the author (Tiselius and Hahn, 1943), in which various saccharides were separated, and some experiments on amino acids and peptides already mentioned. In case of strong adsorption, the affinity of the adsorbent has to be reduced by pre-treatment with suitable substances as described in the last mentioned paper. Nevertheless, losses are sometimes serious in this method. The experience so far seems to show that with the adsorbents now available the displacement method is applicable chiefly for mixtures of closely related substances, which are able to displace each other completely. However, for fatty acids in organic solvents on carbon the method has failed, but in the separation of gaseous hydrocarbons it seems quite satisfactory (Claesson, 1946). 

Methods utilizing adsorption and desorption of gaseous bases have the advantage that the acid amount for a solid at high temperatures (several hundred degrees centigrade), or under its actual working conditions as a catalyst, can be determined. The... 

Another method is to measure the amount of a gaseous base, such as ammonia, pyridine, or quinoline, adsorbed at elevated temperatures under a specified set of conditions. This has the advantage of allowing the study of a catalyst under conditions more nearly similar to those of reaction. Ammonia has been used extensively. Catalysts may be compared in terms of the amount of ammonia adsorbed as a function of the temperature over a range such as ISO to SOO C. A minimum temperature of about 150 C is necessary to eliminate physical adsorption. By infrared spectra it is possible to distinguish between Bronsted and Lewis acid sites. More details on characterizing the acidity of solid catalysts are described in reviews by Formi (1974), and by Bensei and Winquist (1978). 

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