Cracking catalysts active sites

Although this function is presented to lie on the well established time on stream theory, it is very empirical, because cracking catalyst active sites do not disappear because of long time on stream, but rather because of coke foulirtg. Moreover q may change with time [4]. 

Zeolites as cracking catalysts are characterized hy higher activity and better selectivity toward middle distillates than amorphous silica-alumina catalysts. This is attrihuted to a greater acid sites density and a higher adsorption power for the reactants on the catalyst surface. 

The deposition of carbon on the E-cat during cracking will temporarily block some of the catalytic sites. The carbon, or more accurately the coke, on the regenerated catalyst (CRC) will lower the catalyst activity and, therefore, the conversion of feed to valuable products (Figure 3-15). 

A good example for reactant shape selectivity includes the use of catalysts with ERI framework type for selective cracking of linear alkanes, while excluding branched alkanes with relatively large kinetic diameters from the active sites within the narrow 8-MR zeolite channels [61, 62]. Here molecular sieving occurs both because of the low Henry coefficient for branched alkanes and because of the intracrystalline diffusion limitations that develop from slow diffusivities for branched alkane feed molecules. 

The effect of steam treatment of ZSM-5 on its cracking activity and selectivity was measured with experiments using n-hexadecane feed. With the thermally treated ZSM-5 catalyst, concentration of the unconverted n-hexadecane in the product was not measurable while 507e of the feed was unconverted with the steam treated ZSM-5 catalyst (Table II). The lower limit of conversion with the thermally treated catalyst corresponding to detection limit of n-hexadecane is 99.99%. This lower limit suggests at least an order of magnitude reduction in apparent first order rate constant of the ZSM-5 catalyst upon steam treatment. The small reduction in crystallinity upon steaming cannot fully explain the dramatic activity loss. Loss of active sites due to dealumination of ZSM-5 can be postulated to explain the reduction in activity. 

Another heteroatom in the feed which affects performance and product quality is nitrogen. Basic nitrogen in the feed tends to lower cracking activity by adsorbing onto the acid sites of the surface of the cracking catalyst. The work of Wu and Schaffer (5), showed that not all nitrogen compounds were equally harmful. They... 

Figures 13 and 14 also show that hydrotreating the catalytic cracker feedstock increases the zeolite cracking. C3, and C5+ compounds are possible products of primary zeolite cracking. These figures show that hydrotreating of the feedstock results in larger yields of these primary cracking products and hence more valuable products. This improvement is most likely due to the heteroatom removal and the saturation of aromatic compounds during hydrotreating which tend to block active sites and reduce the activity of the catalyst.

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. 

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. 

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