Coking of zeolite catalysts

Studies of catalysts deactivation by coke are abundant in the literature most of them are usually conducted at high temperatures (around 500°C) using metal catalysts supported on oxides with low surface area such as silica, aluminas or silica-alumina [2 and references therein]. The deactivation by coke of zeolite catalysts has also been studied and such studies have mostly been done for high temperature reactions such as the conversion of n-hexane or the isomerization of xylenes [2,4]. However, low temperature coke formation (20-25°C) combining the effect of high acidity and size specificity for a high coking component such as nickel, has not yet been considered from the point of view of the presence of compounded effects of crystalline structure and location of metal particles. 

Moreover, the macro - and micro -FTIR techniques enable us to obtain spectra in situ from a working catalyst, since the cells used (see Sect. 2.1.1) may be operated as flow-through reactors. Thus, coking of zeolite catalysts upon reaction of ethane or ethylbenzene was investigated in situ, and the decrease of diffusivities (e.g., of benzene) in the coking samples was measured as a function of the amount of coke deposited [15]. Similarly, the sorption of para-, meta-, and ortho-diethylbenzene from the gas phase into H-ZSM-5... 

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. 

The presence of zeolite catalysts increases the amount of gaseous hydrocarbons produced during pyrolysis but decreases the amount of pyrolysis oil. Further, significant quantities of coke were formed on the surface of the catalysts in the course of pyrolysis. The catalysts reduced the yield of e.g., as styrene and cumene, in favor of naphthalene. The zeolite catalysts, especially Y-Zeolite, were found to be very effective in removing volatile organo bromine compounds. However, they were less effective in removing antimony bromide from the highly volatile products of pyrolysis (133). 

The fundamental description of coking, deactivation and regeneration of zeolite catalysts will be mainly based on results obtained on acid zeolites, coke being formed during gas phase transformation of hydrocarbons. 

To obtain the coking mechanism of zeolite catalyst for SCFP alkylation of benzene, two kinds of the zeolite used in LP and SCFP alkylation processes were analyzed by using the conventional catalyst analysis methods. Fresh zeolite is also analyzed for comparison. 

Now zeolite catalysts have been employed by most FCCUs. Although zeolite catalysts have a much higher initial activity as compared to amorphous catalysts, coke deposit on the catalyst particles rapidly lowers their activity. As the carbon content of zeolite catalysts increases by 0.1 wt%, the activity decreases by 2-3 units. Generally the carbon content of regenerated zeolite catalysts should not be allowed to exceed 0.2 wt. %, or preferably less than 0.1 wt. % in the case of ultrastable Y zeolite (USY). Therefore, how to decrease CRC efficiently for zeolite catalysts in FCCUs has become a significant problem. 

Table 31 provides some details of zeolite catalyst usage. The best kuowu is that of zeohte Y as a fluidized bed catalyst (FCC) to crack crude oil for gasoliue production. The zeohte is used as a promoter aud comprises up to 50% of a small composite with a clay or silica binder. As such it can remain stable throughout many cycles in the catalyst riser (at 480-520 °C), where it meets the downstream of crude oil followed by a steam blast to release the cracking products, and then a stream of air in a regenerator (590-730 °C). This regeneration process cleans the catalyst of coke and the residual oil products. 

MASUDA HASHIMOTO Deactivation of Zeolite Catalysts by Coke... 

NMR Techniques for Studying the Coking of Zeolite-Based Catalysts... 

Bonardet, J.L., Barrage, M.C. and Fraissard, J. (1995) NMR techniques for studying the coking of zeolite-based catalysts, in Deactivation and Testing of Hydrocarbon-Processing Catalysts (eds... 

Metal oxides on zeolites have also found use as redox catalysts. High-temperature (700-750 °C) dehydroaromatization of methane under nonoxidizing conditions has been explored with a number of zeolitic catalysts modified with transition metal ions. Although coke formation at these high temperatures is a problem, calcined molybdate-impregnated ZSM-5 shows unparalleled activity of up to 8 % methane conversion with 100 % selectivity towards aromatics. Surface studies of these Mo HZSM-5 catalysts indicate that M0O3 crystals are on the external zeolite surface [123]. 

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