Coke deposition procedure

Regeneration of noble metal catalysts to remove coke deposits can successfully restore the activity, selectivity, and stabiUty performance of the original fresh catalyst (6—17). The basic steps of regeneration are carbon bum, oxidation, and reduction. Controlling each step of the regeneration procedure is important if permanent catalyst damage is to be avoided. 

As an example, Figure 3.1.10 illustrates the use of this procedure for elucidating the location of coke deposits on zeolite catalysts [62]. Samples of zeolites H-ZSM-5... 

A problem with monofunctional reactions, e.g., cracking, alkylation, etc. is that they have a tendency to quickly deactivate because of coke deposition. This problem is usually not of concern with bifunctional reactions, e.g., those that employ a metal function in addition to the acid sites. However, we avoided the use of metal function because of the possible unknown modifications that could be introduced to a given sample by the metal deposition procedure. This is especially important when dealing with samples like VPI-5. Thus, to minimize the rate of deactivation, the alkylation experiments were conducted at 463 K. This low temperature introduces another problem, namely, the adsorption of reactants and products. At the experimental conditions employed here, the catalyst bed becomes saturated at time of 10 minutes or less (depending on sample). From this point onward, deactivation is clearly observable via the decrease in conversion with time. The data reported here were obtained at 11-13 minutes on-line. Since meta-diisopropylbenzene proceeds through several reaction pathways that lead to a number of products, it is most appropriate to compare the catalytic data at the constant level of conversion. Here we report selectivities at approximately 25 % conversion. For each catalyst, the results near 25 % conversion were repeated three times to ensure reproducibility. 

The catalytic performance and the effect of coke deposition on the activity and selectivity, were found to be strongly dependent on the preparation procedure, on the reduction temperature, and on the type of support. At low reduction temperatures, the deposition of coke on Ni-Al, (Ni-Al-Ti)sg, and Ni-Ti samples decreased the selectivity to ethylene whereas on (Ni-Al-Ti)imp, deposition of coke increased selectivity. At high reduction temperatures, with only the exception of the Ni-Ti catalyst, coke deposition increased the selectivity to the desired product. The pattern of deactivation by coke was also different for the different samples. In Ni-Ti and (Ni-Al-Ti)sg samples, coke formation strongly diminished their activity and simultaneously increased methane production. With Ni-Al and (Ni-Al-Ti)imp samples, coke did not cause an significant deactivation or an increase in methane yield. Finally, catalysts... 

In recent years, attention has tended to be focused on coke deposition in zeolites (6, 7) in order to characterise the coke formed. In one specific study Groten et al (8) carried out a study of coke formation using zeolite USHY with n-hexane as reactant, but in this case, as in others (6, 7) it was necessary to deposit excessive amounts of coke (> 5%) to enable characterisation of the coke deposits to be achieved. However, if demineralisation of the catalyst is used to concentrate the coke as in the present work the inherently quantitative single pulse excitation (SPE) NMR procedure may be used to characterise coke deposits on FCC catalysts at realistic levels of ca 1% by weight. 

Before studying the reactivity of the carbonaceous deposit, the reactant mixture above was passed over the catalyst, a procedure known to form hydrocarbon or coke deposits [20, 16]. The catalyst was then heated to 773 - 823 K at 10 K min in a mixture of NO (2000 ppm), O2 (2%) balance helium. The extent of NOx conversion was monitored continuously by the chemiluminescent analyser. 

Table 1 presents the n-hexane conversion, selectivity to isomers and coke deposited after reaction for catalysts prepared by using two different platinum precursors tetraammine platinum nitrate and hexachloroplatinic acid. Both materials were calcined at different temperatures after platinum addition. For both platinum precursors, run under standard operational conditions, the optimum calcination temperature for catalytic activity was 500 °C. The amount of coke is small and the TPO profiles of the coked samples (not shown) are similar for all catalysts. Coke is completely burnt off at temperatures below that at which the catalyst was calcined after the metal addition. This is an important feature, because regeneration procedures would not affect the metal function. 

Subsequent investigations, including IINS, were carried out to characterize the various resistances of such cokes to controlled after-treatments, such as oxidation or hydrogasification processes, as a basis for determining the feasibility of catalyst reactivation. The presence of metallic contaminants (iron, cobalt, and nickel) was of relevance, not only to the deposition of cokes and the catalytic transformation of the carbon structure, but also to the dynamic processes in the controlled decomposition of the material in catalyst regeneration procedures 50). 

As described by Gerritsen et al [23] in a Cyclic Deactivation procedure the catalyst is deactivated by means of several reaction and regeneration (coke burning) cycles. This is essential for the realistic deposition and aging of the metals. 

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