Coke Characterization

QUERINI, C.A.

Catalysis

The Royal Society of Chemistry

Lugar: Cambridge; Año: 2004; p. 166 - 209

The formation of coke deposits leading to catalyst deactivation has been a challenge for catalytic technology in many hydrocarbon processes. The development of the fluid catalytic cracking (FCC) process and the continuous catalytic reforming (CCR) are examples of such developments. In these cases, the catalyst, the operating conditions, and the continuous coke removal are integrated and work in a delicate quasi-steady state. In any case, the effective management of catalyst deactivation and catalyst regeneration is the key in many heterogeneously catalyzed processes. The optimization of such complex processes requires the characterization of the coke deposits in order to understand the effect of the operational variables on these deposits and, therefore, minimize its formation and develop effective regeneration strategies. In the last decade, there has been an increase in the research effort in this field. Coke characterization has been included in many papers where deactivation is a major issue. Several characterization techniques have been used to study coke deposits and to obtain information regarding reaction mechanism, deactivation mechanism, and regeneration conditions. One of the most widely used techniques is temperature-programmed oxidation. Because of its simplicity and utility, this technique has been widely accepted and used in the characterization of coke in a large variety of catalytic systems. In this work, the different techniques that have been used for coke characterization on heterogeneus catalysis are presented, using examples of applications reported in the literature. The objective is to present the type of information that each technique provides regarding coke composition, localization, morphology and kinetics of coke gasification, with related references to guide the researcher to previous studies in the field of coke characterization, and more specifically, coke deposits formed on catalytic surfaces. Some of the techniques limited to single crystals or polycrystalline foils, such as low energy electron diffraction (LEED), He scattering, core electron energy loss spectroscopy (CEELS), and metastable deactivation spectroscopy (MDS), are not presented in this work. A. Bell1 reported some examples of application of these techniques. It is not the intention to fully review all the literature regarding coke characterization, but to present selected examples of application of different techniques in some of the most important catalytic systems. The examples for each technique are presented either for the most important reactions on which the technique was applied (e.g. naphtha reforming, dehydrogenation, cracking, etc), or for a given type of catalyst if several studies involving different reactions are discussed (e.g., zeolites, nickel catalysts, etc). Two Tables have been included at the end of the chapter, after the presentation of all the techniques. The first one reports references for several processes where coke characterization is addressed, including the techniques used in each study (Table 5). The second one gives a summary of the information that can be obtained with eahc technique (Table 6).