Quinoline chemisorption

Fig. 3. Quinoline chemisorption at 315° as a function of activity for cracking light East Texas gas-oil (32). O, Si02-Al203 Houdry type S) , SiOj-1% Al203 , clay catalyst (fil-trol) O, SiOj-MgO V, Si02-Zr02. (Reprinted with permission of the American Chemical Society.)...

For all cases studied, the Y zeolites are considerably more acidic than the X zeolites. This may be unexpected since there are more ion exchangeable cations in X zeolites and consequently more potential acid sites. It is possible that the weaker electrostatic fields in X zeolites do not result in the production of the maximum number of hydroxyl groups. In fact, for Ca and Mg, the Bronsted acidity increased linearly with decreasing aluminum content of the zeolite (69). Quinoline chemisorption also has been used. However, the results indicate that only Lewis acid sites are detected (10). 

Fio. 10. Quinoline chemisorption at 316°C. as a function of activity for cracking light... 

Physical properties, notably the specific surface areas, have been proposed by some authors as a measure for the activity of catalysts. This correlation is successful only when applied to catalysts which resemble one another in their composition and in their method of preparation. That surface area cannot be considered to be of exclusive importance to catalytic activity is demonstrated by the rather extreme examples given in Table VII. On the other hand, the fact that the capacity for quinoline chemisorption is quantitatively related to the activity of cracking catalysts is shown by Fig. 8 obtained with catalysts of various compositions, methods of preparation, and activities. The amount of quinoline chemisorbed thus measures a general property of this entire class of catalysts which is fundamentally related to their ability to act as catalysts. 

The adsorption of cumene and inhibitors on active cracking sites follows a Langmuir type of isotherm. This means that there is little or no interaction among the chemisorbed molecules on the surface. This might be expected to be the case as studies of the chemisorption of the inhibitor quinoline by similar catalyst (14) show that the surface is sparsely covered with active sites (<5% of internal surface area covered with chemisorbed quinoline at 315°C.). In addition, the active sites are homogeneous with respect to adsorption energies. 

In our earlier studies [8-10] the Monte Carlo method has been applied to simulate the adsorption of the modifier-substrate into the Pt(lll) surface. The results of these simulations indicated that the above complex, as a supramolecule, maintains its entity after chemisorption (see Figure 2). Slight differences were observed in the orientation of the quinoline ring towards the Pt(l 11) surface, i.e., orientations either parallel or slightly bent to the Pt(lll) surface were obtained (8-10). However, the shielding effect provided by the quinoline ring was maintained in each case. 

The studies of Mills, Boedeker, and Oblad (2) on the chemisorption of the inhibition quinoline on similar catalysts can be used to place an upper limit on the value of Bo. Their data show that 1.27 X quinoline mole-cules/sq. m. are required to reduce the cumene cracking activity to essentially zero. 

The second major aspect of the surface chemistry of chromia-alumina that has to be considered is the acidic nature of its surface. The exact chemical nature of the acid sites of solid oxide catalysts such as alumina or silica-alumina has been a subject of considerable research and speculation for a number of years, yet despite these efforts a fully satisfactory chemical description of catalyst acidity has not been obtained. Nevertheless, in the case of chromia-alumina, there is good evidence for the existence of acid sites of one kind or another on the surface. Voltz and Weller (29), for example, studied the chemisorption of quinoline on chromia-alumina, with and without potassium promotion, and at the same time measured their titrable acidities in aqueous suspensions. Both methods indicated that chromia-alumina was acidic, and that the addition of potassium decreased the acidity. This observation was supported by the fact that the double bond isomerization of 1-pentene, normally an acid-catalyzed reaction, proceeded quite readily over pure chromia-alumina, but less readily over a chromia-alumina treated with potassium. 

At constant temperature the amount of physically adsorbed quinoline varies with its partial pressure in the nitrogen stream, whereas the amount of chemisorbed quinoline remains practically independent of the partial pressure of quinoline. This is illustrated in Fig. 5 for a silica-alumina catalyst of medium activity. Such an effect is to be expected since physical adsorption increases, as a rule, with increasing pressure of the adsorbate in the gas phase, whereas chemisorption, in many cases, can reach saturation at even very low concentrations of the adsorbate in the gas phase. 

The effect of temperature on the chemisorption of quinoline on a catalyst was investigated for a temperature range of 260-480°. The results are given in Table I. It should be pointed out that the division made here between chemisorbed and physically adsorbed quinoline is somewhat arbitrary. Undoubtedly, there is some overlapping of the range of the chemical forces and van der Waals forces involved in the adsorption of the basic nitrogenous compounds. As a catalyst sample becomes saturated with the nitrogenous compound, the final amounts... 

Other methods for determining the surface acidity of a catalyst are also available. For example the sum of Bronsted and Lewis centers can be determined by chemisorption of basic substances such as ammonia, quinoline, and pyridine. 

From the practical point of view, the selective chemisorption has been widely applied in the study of acidity of solid catalysts, whose surface acidity of the solid catalyst is usually measured by the adsorption of basic nitric compounds. Ammonia, p3rridine, quinoline, trimethylamine, etc. are used for the measurement of solid surface acidity. Due to the adsorption isotherms relating to the distribution of adsorption heat on the uneven surface, the number of adsorption sites distributed according to the intensity of acid can be roughly estimated by the changes of adsorption isotherms of basic materials. 

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