Cracking catalysts acidity measurements

To measure the site energy distribution or other surface properties of powders by measuring heats of immersion as a function of the amount of preadsorbed wetting liquid. Heats of immersion of the partly covered surfaces reveal the site energy distributions. For acid sites on cracking catalysts, for example, adsorbates of different basicity can be used to develop a topographical map of the surface activity. 

Fig. 26. Comparison of acidity of NiP/Al, MoP/Al, and NiMoP/Al catalysts as measured by cyclopropane (CP) cracking as a function of P content [reprinted with permission from Iwamoto (67)].

P. E. Eberly, Jr. Yes. This seems plausible. The band appears at 1438 cm" and does not change with the nature of the rare earth ion. The reversible Bronsted acidity does not correlate well with catalyst activity. Pretreatments were identical for the cracking studies in Figure 6 and the acidity measurements in Figure 7. 

Addition of flourine to H-mordenite enhanced considerably the acid strength of this catalyst but decreased the ratio of Brpnsted to Lewis acidity (167). Using IR spectroscopy of adsorbed pyridine, X-Ray diffraction, catalytic activity tests for cumene cracking, and microcalorimetric measurements of ammonia adsorption, it was shown that some of the acidic hydroxyl groups were substituted with fluorine and that the inductive effect of fluorine increased the acid strength of the remaining hydroxyl groups. 

In a different study, Masuda et al. (140) found two different linear relationships between the activity for cumene cracking at 623 K and the total acidity as determined from calorimetric measurement of NH3 adsorption at room temperature. One correlation was for amorphous silica-alumina with different loadings and treatments, and the other correlation was for zeolitic silica-alumina with similar loadings and treatments. Other catalysts did not follow these correlations. The apparent lack of correlation between the different sets of catalysts is probably due to the fact that the total acidity measured includes both active and nonactive sites. It is likely that for a set of... 

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. 

Reference has already been made to the fact that cracking catalysts have definite and measurable acidic properties. Present concepts of the... 

For platinum catalysts no interference with the measured reaction rate has been observed with acidic catalyst present in the reactor. For example, mixing equal parts of 46 A.I. (CAT-A evaluation) silica-alumina cracking catalyst with platinum catalyst did not alter the rate measurements obtained from the platinum catalyst alone. 

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... 

Niu and Hofmann [ 164] related the activity of USY, H-ZSM-5, and H-MOR zeolite catalysts in ethylbenzene conversion to the acidity measured by IR spectroscopy using pyridine adsorption and TPD of NH3. Catalytic measurements at 300 °C did not show deactivation over all three catalysts, while increasing the reaction temperature to 400 °C led to a strong deactivation for H-MOR and some deactivation of USY. The conversion over H-ZSM-5 was not affected by deactivation. At 300 °C only disproportionation reactions were observed, while at 400 °C large contributions of cracking were found (i.e., benzene to diethylbenzene ratios above 1) over all three catalysts. 

As has been mentioned previously, one is most likely to find analogies to catalytic reactions on solids with acidic and/or basic sites in noncatalytic homogeneous reactions, and therefore the application of established LFERs is safest in this field. Also the interpretation of slopes is without great difficulty and more fruitful than with other types of catalysts. The structure effects on rate have been measured most frequently on elimination reactions, that is, on dehydration of alcohols, dehydrohalogenation of alkyl halides, deamination of amines, cracking of the C—C bond, etc. Less attention has been paid to substitution, addition, and other reactions. 

Several different test reactions have been suggested to evaluate the catalytic activity of an acid catalyst as a measure of the number and strength of the active sites. The ideal test reaction is experimentally easy, fast, reproducible, requires only a small amount of catalyst, has simple kinetics, and should show little deactivation. It should also not be diffusion limited and affected by the particle or crystal size. While no one reaction fits all these criteria perfectly, we and apparently others - find that hexane cracking comes closer to the ideal than most other reactions. 

As expected from the TPD results, Al-sapo was more active for the cracking of cumene on a per weight of catalyst basis than Al-mont. In order to compare the catalytic activity on a basis of active sites, we evaluated the number of active sites on these catalysts. TPD spectra were measured with varying the temperature of ammonia adsorption. Typical results on Al-mont are shown in Fig. 2. By integrating these spectra, the concentration of acid sites corresponding to different strength of acidity can be determined. 

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