Zeolite, catalyst deactivation poisoning

Modeling Zeolite Catalyst Deactivation by Coking and Nitrogen Compound Poisoning... 

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 use of solid catalysts and especially zeolites in Fine Chemical synthesis introduces another complication with respect to homogeneous reactions. There is always a progressive decrease of the catalyst activity with increasing reaction time.1191 In some reactions, this deactivation can be due to irreversible chemical transformation of the zeolite catalyst, e.g. reactions with acid reactants causing dealumination and sometimes collapse of the framework. However, in most cases, deactivation results from poisoning of the active sites by the desired reaction... 

In hydrocarbon conversion over zeolite catalysts, the formation and retention of heavy products (carbonaceous compounds often called coke ) is the main cause of catalyst deactivation. 5X 77 XI1 These carbonaceous compounds may poison or block the access of reactant molecules to the active sites. Moreover, their removal, carried out through oxidation treatment at high temperature, often causes a decrease in the number of accessible acid sites due to, e.g., zeolite dealumination or sintering of supported metals. 

The deactivation of a lanthanum exchanged zeolite Y catalyst for isopropyl benzene (cumene) cracking was studied using a thermobalance. The kinetics of the main reaction and the coking reaction were determined. The effects of catalyst coke content and poisoning by nitrogen compounds, quinoline, pyridine, and aniline, were evaluated. The Froment-Bischoff approach to modeling catalyst deactivation was used. 

Sodium on fluid cracking catalyst, FCC, comes from the raw materials used in the catalyst manufacturing process as well as salt contamination in the feedstock. Sodium can deactivate cracking catalysts by poisoning the acid sites on the matrix and zeolite and by promoting sintering of silica-alumina (1). Sodium can act synergistically with vanadium to accelerate the destruction of zeolite (2). 

The cause of catalyst deactivation has been addressed by Iglesia et al. These authors observe by electron microscopy that after catalytic operation the Pt particles that are located outside the L zeolite are covered with a carbonaceous layer, whereas intrachannel Pt particles are apparently free of coke. For these particles the active catalytic life is limited by their agglomeration with concomitant loss of Pt surface area. Sulfur not only poisons active sites on the Pt particles, but also accelerates the rate of agglomeration (334). 

As an example of base-catalyzed reactions over zeolites of different basicity (Cs, Na-Y, Cs, Na-X, CsOH/Cs, Na-Y), the aldol condensation of n-butyraldehyde was explored by Weitkamp et al. [910] using in-situ FTIR to monitor the conversion. During the reaction a band emerged at 1580 cm, which could be ascribed to a COO stretching vibration of a carboxylate anion. These anions were suspected to poison the active basic sites and cause the catalyst deactivation. 

Ammonia is not a poison for metal catalysts (Ni, Cu, Fe), but it may deactivate acidic zeolite catalysts and HDS catalyst. It can be removed over a low-temperature bed with zeolite. 

Chemical poisons from engine lubricant oil, such as Ca, Zn and P, have also been found to interact with Cu/zeolite SCR catalyst and cause catalyst deactivation [19, 49]. Since their concentration is very low in the exhaust stream and there are other catalyst components placed in front of the SCR catalyst which traps a significant portion of the chemicals, only a minor amount of lubricant oil derived chemical poisons will accumulate on the SCR catalysts. In addition, such chemical poisons tend to deposit on the very front section of the SCR catalysts. Therefore, adequate catalyst sizing to take this into account can mitigate the impact of chemical poisons from lubricant oil. 

XPS is the most commonly used and most useful surface analysis method in catalyst characterization. It can be used for both qualitative and quantitative analysis for almost all kinds of catalysts used in heterogeneously catalyzed reactions. XPS studies of oxide, sulfide, fluoride, halide, etc., catalysts,. supported metal catalysts, Raney or gauze metal catalysts and zeolite catalysts are all possible. The samples can be studied in any of the precursor, calcined, reduced, activated, deactivated, aged or poisoned states. Real industrial catalysts can be analyzed as well as fundamental model systems. Quantitative analysis is possible either with the help of empirical sensitivity factors or by standard-free methods. In the latter case appropriate theoretical models are u.sed with photoionization cross-section tables, inelastic mean free path (X.) data and individ-... 

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