Zeolites poisoning

Anotlier important modification metliod is tire passivation of tire external crystallite surface, which may improve perfonnance in shape selective catalysis (see C2.12.7). Treatment of zeolites witli alkoxysilanes, SiCl or silane, and subsequent hydrolysis or poisoning witli bulky bases, organophosphoms compounds and arylsilanes have been used for tliis purjDose [39]. In some cases, tire improved perfonnance was, however, not related to tire masking of unselective active sites on tire outer surface but ratlier to a narrowing of tire pore diameters due to silica deposits. 

Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. In general, there are two types of catalyst deactivation that occur in a FCC system, reversible and irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms zeolite dealu-mination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium. 

Vanadium also promotes dehydrogenation reactions, but less than nickel. Vanadium s contribution to hydrogen yield is 20% to 50% of nickel s contribution, but vanadium is a more severe poison. Unlike nickel, vanadium does not stay on the surface of the catalyst. Instead, it migrates to the inner (zeolite) part of the catalyst and destroys the zeolite crystal structure. Catalyst surface area and activity are permanently lost. 

There are several theories about the chemistry of vanadium poisoning. The most prominent involves conversion of VjOj to vanadic acid (H-iVO ) under regenerator conditions. Vanadic acid, through hydrolysis, extracts the tetrahedral alumina in the zeolite crystal structure, causing it to collapse. 

The alumina content, the amount of rare-earth, and the type and amount of zeolite affect catalyst tolerance to vanadium poisoning. 

An active matrix can also serve as a trap to catch some of the vanadium and basic nitrogen. The high boiling fraction of the FCr feed usually contains metals and basic nitrogen that poison the zeolite. One of the advantages of an active matrix is that it guards the zeolite from becoming deactivated prematurely by these impurities. 

Vanadium in the feed poisons the FCC catalyst when it is deposited on the catalyst as coke by vanadyl porphydrine in the feed. During regeneration, this coke is burned off and vanadium is oxidized to a oxidation state. The vanadium oxide (V O ) reacts with water vapor in the regenerator to vanadic acid, HjVO. Vanadic acid is mobile and it destroys zeolite crystal through acid-catalyzed hydrolysis. Vanadic acid formation is related to the steam and oxygen concentration in the regenerator. 

As with many zeolite-based processes, the zeolite is not used alone since it is highly active, resulting in rapid coke deactivation and poisoning... 

Analytical electron microscopy permits structural and chemical analyses of catalyst areas nearly 1000 times smaller than those studied by conventional bulk analysis techniques. Quantitative x-ray analyses of bismuth molybdates are shown from lOnm diameter regions to better than 5% relative accuracy for the elements 61 and Mo. Digital x-ray images show qualitative 2-dimensional distributions of elements with a lateral spatial resolution of lOnm in supported Pd catalysts and ZSM-5 zeolites. Fine structure in CuLj 2 edges from electron energy loss spectroscopy indicate d>ether the copper is in the form of Cu metal or Cu oxide. These techniques should prove to be of great utility for the analysis of active phases, promoters, and poisons. 

Several previous studies have demonstrated the power of AEH in various catalyst systems (1-11). Often AEM can provide reasons for variations in activity and selectivity during catalyst aging by providing information about the location of the elements involved in the active catalyst, promoter, or poison. In some cases, direct quantitative correlations of AEM analysis and catalyst performance can be made. This paper first reviews some of the techniques for AEM analysis of catalysts and then provides some descriptions of applications to bismuth molybdates, Pd on carbon, zeolites, and Cu/ZnO catalysts. 

Mechanism of 2,3,7,8-TCDD was not established so far means of specific therapy as to this compound poisoning are not available. Experiments with animals have shown that activated carbon, zeolite (subject to introduction of sorbents immediately after poison), unithiol, Liv-52, carsil, festal, guaranteed survival of 20-50% laboratory rats [6],... 

Fejes, P. Kiricsi, I. Hannus, I. Tihanyi, T. Kiss, A., Poisoning of acidic centres in zeolites with sodium azide, Imelik, B. Naccache, C. Taarit, Y. Ben Vedrine, J.C. Coudrier, G Praliaud, H., Eds., Catalysis by Zeolites, Studies in Surface Science and Catalysis 5, Elsevier Sci. Publ. Co. Amsterdam, 1980, pp. 135-140. 

The zeolite catalysts require high operating temperatures. Although this reaction condition favors the production of an isomerate with a RON of about 78, these catalysts possess a tolerance to feedstock poisons such as sulfur or water. 

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