Cracking catalysts chemisorption measurements

The type of study outlined above yields meaningful constants, G and k3B0, which can be used to distinguish two possible sources of differences between various cracking catalysts. The constant G can be considered to measure the chemisorption capability of the catalyst for the reactant, and k-Jh measures its capability to crack the chemisorbed reactant. 

Strong acid sites of the zeolite with and without silica binder were measured by the chemisorption of pyridine at 400 C. The acid sites were also measured in terms of the activity of the zeolite catalysts in acid catalyzed model reaction, disproportionation of toluene at 500 C. Acid sites on the external surface of zeolite crystals or intercrystalline acid sites of the zeolite catalysts were measured in terms of the iso-octane (which cannot enter in ZSM-5 zeolite channels even at 400°C [18, 19]) cracking activity at 400 C [11]. The results showing the influence of silica binder on both the intracrystalline and intercrystalline acidity of the zeolite catalyst are presented in Tables 1 and 2. 

All these difficulties arising from the interaction of cracking catalysts with an aqueous medium can be avoided by using the adsorption of gaseous substances of basic properties for the investigation of these catalysts. It is the authors belief that the only really valid demonstration and measurement of the acidity of cracking catalysts has to be based on the observation of chemisorption of basic substances from a... 

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. 

P. H. Emmett Johns Hopkins University)-. The adsorption measurements to which I referred in my paper included normal butane, normal heptane, and normal octane at a presssure of 4 mm. and at temperatures up to about 350°. In addition, measurements were made at 1 atm. pressure by a different technique on normal butane at temperatures as high as 531°. These measurements, therefore, extended into the region in which butane begins to crack quite readily. At no point was any appreciable chemisorption of any of these hydrocarbons detected, though a deposition of carbon caused a gradual weight increase in the catalyst when the latter was held at 530° in butane. 

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