Zeolites stability

Fichtner-Schmittler, H., Lohse, U., Engelhardt, G. et al. (1984) Unit-cell constants of zeolites stabilized by dealumination - determination of al-content from lattice-parameters, Cryst. Res. Technol., 19, Kl. 

Catalyst and adsorbent finishing procedures are outlined including preparation of binder-free forms, zeolite stabilization procedures, ion exchange and metal loading procedures, and drying and firing operations. 

Early in our work, we had sought to achieve zeolite stability at high temperatures and in the presence of steam. We sought zeolites that could resist the extreme deactivating conditions of the RCC process. 

Over the past 2 decades zeolites have been exchanged with just about every imaginable cationic species in an attempt to find something that works as well as rare earth does in maintaining zeolite structural stability and catalytic activity. As yet no commercially viable substitute for rare earth has been found which provides the same zeolite stability, activity and product selectivity at a commercial price. 

Probably the only general statement which can be made about the experimental studies on zeolites is that the majority of published data is inapplicable directly to natural minerals. This is due either to the excessively high temperatures under which the experiments are performed, outside of the physical limits of zeolite stability, or to short time spans of observation which do not allow the silicates to come to equilibrium with the fluids of the experiments. Those studies designed to determine zeolite stability indicate that the most silica-poor alkali zeolite, analcite, is not stable above 180°C. More silica-rich species will be found below this temperature. However, the reasons for the crystallization of one or another of the silica-rich alkali zeolites are not yet elucidated. 

Such a local approach is now widely used and works quite well. It provides a natural explanation of the trend toward increasing zeolite stability with Si/Al ratio. It also allows one to follow easily the influence of cations entering a zeolite structure on its stability. Quite a number of such comparative calculations can be found in the literature. We shall confine ourselves to only a few representative examples. 

Components of fluidized cracking catalysts (FCC), such as an aluminosilicate gel and a rare-earth (RE) exchanged zeolite Y, have been contaminated with vanadyl naphthenate and the V thus deposited passivated with organotin complexes. Luminescence, electron paramagnetic resonance (EPR) and Mossbauer spectroscopy have been used to monitor V-support interactions. Luminescence results have indicated that the naphthenate decomposes during calcination in air with generation of (V 0)+i ions. After steam-aging, V Og and REVO- formation occurred. In the presence of Sn, Tormation Of vanadium-tin oxide species enhance the zeolite stability in the presence of V-contaminants. 

In this paper we have examined commercial equilibrium catalysts (Beat) to look for correlations between zeolite and matrix area stability of various catalyst families as a function of sodium. We have performed density separation of Feat samples to measure the sodium distribution as a function of catalyst age. Finally, we have revisited the issue of whether fresh catalyst sodium is equivalent to feedstock sodium in its influence on zeolite stability, catalytic activity and selectivity. 

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. 

Several reasons were proposed to explain the cleaner chemistry. First, the electrostatic fields (0.3 and 0.9 V for Na-Y and Ba-Y, respectively) in the zeolite stabilize the hydrocarbon-02 charge transfer, thereby shifting the transition to visible wavelengths. Second, the use of visible excitation suppresses the secondary photochemistry rampant with UV light. Third, the steric constraints imposed by the zeolite promote product selectivity. This chemistry can be demonstrated by the reaction of 2,3-dimethyl-2-butene (DMB). Figure 26a shows the electronic spectrum... 

Zeolite stability is defrned with respect to a solvent system... 

A recent elegant example of the tailoring the chemical properties of encapsulated metal complexes is the work of Balkus etal. who prepared and studied perfluorinated phthalocyanine complexes of Fe, Co, Cu and Ru (Scheme 25)[230] in NaX. Perfluorinating the complexes enhances the stability and catalytic activity of the catalysts in the oxyfiinctionalisation of light alkanes. The rapid deactivation of the catalysts based on Fe, Co and Cu Fj Pc complexes was overcome by using Ru as the metal center. Similar catalysts, i.e.,Co-phthalocyanine (Co-Pc) encapsulated in zeolite Y, are active catalysts for cyclohexene and 1-hexene epoxidation (Scheme 27)[231]. Comparison of the activity of free and encapsulated Co-Pc has shown that the interaction with the zeolite stabilizes the complex. Co-Pc is still active after 24 hrs reaction whereas the free complex in solution is virtually inactive after 15 minutes. 

An aluminosilicate layer at the crystal surface generated subsequently by an alumination of DAY-S zeolite or directly by the steaming of NaY in order to get DAY-T zeolite stabilizes the high-silica faujasites. The decomposition follows the acid hydrolysis of usual aluminosilicates. Consequently, in this case the kinetic model described in ref [5] is valid at least for the surface layer. 

U.S. 5441661 08/1995 Beaujean et al.l Henkel KgaA surfactant-rich phase can be incorporated into the liquid phase without segregation or separation Nonaqueous liquid detergent containing a hydrated zeolite stabilized by a polar deactivating agent... 

In this paper, oi and Al MASNMP. spectroscopy is used in conjunction with crystallinity, surface area and unit cell size measurements to study individual rare earth exchanged Y zeolites in order to determine the effect of individual rare earths cations on their structure and stability. These methods are used to further probe rare earth induced structural changes that occur during hydrothermal treatment of the zeolites. The studies were extended to also establish the effect of different lanthanum-cerium mixtures on zeolite stability. The data presented and discussed are for lanthanum, cerium, praseodymium and neodymium exchanged Y zeolites, as well as for zeolites exchanged with different lanthanum-cerium mixtures. 

As detailed in the sections above, EXAFS contains directly accessible information on interatomic distances and coordination numbers. Indirectly, such information can in favorable cases be used to speculate about particle sizes and morphologies (e.g., of zeolite-stabilized metal clusters). 

An essential question in zeolite science is the nature of the chemical bond in zeolitic systems. This paper aims to present current understanding and will focus on the covalent as well as electrostatic interactions in these materials. It will appear that structural differences sensitively depend on the balance between the two kinds of interactions. Especially the use of proper quantum-mechanical cluster calculations and the development of new force flelds based on such calculations has resulted in a signiflcant improved understanding of zeolite stability as weU as the acidity of protonic zeolites that are important for catalytic applications. 

R.E. Detterman, N.A. Hamerly, C.A. Lepilleur, A.M. Mazany, D.L. MUe-nius, and A.L. Backman, Halogen containing polymer compounds containing modified zeolite stabilizers, US Patent 6531526, assigned to Noveon IP Holdings Corp., March 11,2003. 

---