Zeolite destruction

Catalytic activity, assessed by cumene cracking on separated fractions and also by analysis of residual coke on catalyst fractions, shows a sharp decline with increasing density (age). This rapid loss of initial activity coincides with zeolite dealumination which is largely completed as a slow rate of zeolite destruction is established. Subsequent loss of crystallinity has little additional effect on activity. The associated loss of microporosity leads to an apparent increase in skeletal density with increasing age. 

The combination of high temperature and steam in the regenerator causes framework dealumination (rapid) and crystalline zeolite destruction (slower). Dealumination both lowers activity and produces important changes in selectivity, while zeolite destruction leads primarily to a loss in activity. 

Measured Ni levels on catalyst are expected to supply an independent age marker, as it has been shown that Ni tends not to migrate following deposition on the catalyst (2). In fact, metals (Ni and V) deposition from the feed onto the catalyst is expected to assist the separation by density/age/activity as older catalyst particles exhibit higher metals levels which contribute to an increase in density. Except for the oldest fractions, which contain the highest metals levels, the older portion of catalyst tends to make less coke as the increase in activity due to increasing metals content is overwhelmed by the loss of activity due to crystalline zeolite destruction (1). 

The estimates in Table IX represent the minimum effect of the loss of micropore volume. Pore-mouth constrictions which lead to partial or complete micropore plugging would serve to increase the contribution of loss of micropore volume to density change. No such pore plugging is expected to occur in the (mesoporous) catalyst matrix or in the mesoporous material that is generated during crystalline zeolite destruction. 

Figure 4), by which time dealumination is largely complete. There follows a steady rate of zeolite destruction which results in an additional 42% loss of crystalline zeolite (relative to fresh catalyst) over the next 80 days (see Figure 4, Fractions B-F). Fraction F, representing the end point of the USY catalyst distribution, retains only 38% of the crystallinity characteristic of fresh catalyst. Fractions A-D, representing the major portion of this equilibrium catalyst, exhibit relative crystallinity retentions ranging from 83 to 66%.

At this point, the 60 to 70% of crystalline zeolite that remains contains a diminishingly small number of acid sites, and further zeolite destruction has little effect on cracking activity. 

The observed zeolite destruction is almost certainly caused by exposure to high-temperature steam in the regenerator since USY zeolite is thermally stable in dry air to temperatures in excess of 1000°C (10). It is unlikely that the steam-induced collapse of crystalline zeolite in this USY catalyst is much affected by the presence of a low concentration of vanadium... 

Vanadate formation (LaV04) occur also in LaY crystals (69). Removal of other charge compensating cations (such as Na+ ions) in the form of vanadates further destabilize the crystal lattice thus promoting and enhancing zeolite destruction. 

Recent work on laboratory catalyst deactivation in the presence of Ni and V by cyclic propylene steaming (CPS) has shown that a number of conditions affect the dehydrogenation activity and zeolite destruction activity of the individual metals. These conditions include find metal oxidation state, overall exposure of the metal to oxidation, the catalyst composition, the total metal concentration and the NiA ratio. Microactivity data, which show dramatic changes in coke and hydrogen production, and surface area results, which show changes in zeolite stability, are presented that illustrate the effect each of these conditions has on the laboratory deactivation of metals. The CPS conditions which are adjustable, namely final metal oxidation state and overall exposure of the metal to oxidation are used as variables which can control the metal deactivation procedure and improve the simulation of commercial catalyst deactivation. In particular, the CPS procedure can be modified to simulate both full combustion and partial combustion regeneration. 

Low Oxygen CPS Deactivation As noted earlier, commercial results (7) indicate that there is a large difference in both activity and selectivity of catalysts from full combustion and partial combustion FCC units. The variation in yields is due to the oxidation states of both Ni and V, as described in the previous section however, the variation in activity is primarily due to the oxidation state of the V, which is very destructive to the zeolite in its oxidized form. Therefore, modifications to laboratory deactivation procedures are required to simulate the deactivation of catalysts from full and partial combustion FCC units. All of the standard deactivation procedures discussed so far have been aimed at simulating full combustion. The modified CPS procedure described in the previous section does change the final metal oxidation state to a reduced state similar to what one would expect from a partial combustion unit however, throughout most of the deactivation, the metals on the catalyst are exposed to full combustion conditions. As such, the zeolite destruction is higher than that expected from a partial combustion unit. 

Rare earths either in zeolite (ZSM-5) or on AI2O3, Mg0-Al203 supports play an important role in SO removal additives by catalyzing the oxidation of S02 into SO3. They also intervene in the processes directed to NO removal. However, the R contained in the zeolite Y of FCC catalysts, reacts with vanadic acid, forming vanadates and leading to the destruction of the zeolite with the corresponding loss of the FCC catalyst activity (Occelli 1991, Yang et al. 1994, Occelli 1996). Vanadium traps based on rare earths can help to reduce zeolite destruction by vanadium (Feron et al. 1992). 

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