Coke on Zeolites

2 Coke on Zeolites. - A ZSM-5 catalyst used in n-hexadecane cracking, was treated with 40 wt% HF after two successive extraction steps with CCI4 and CH2CI2. The released coke was extracted with CH2CI2 to separate soluble and insoluble coke residues l The insoluble fraction was studied with NMR (see section 9). 

Coke deposited on HY zeolite during n-heptane reaction at 450°C, was completely dissolved by CI2CH2 after dissolution of the support with HF . This is not usual, since in most studies there is a fraction of insoluble coke. The coke formed on PtUSHY and PtHMOR catalysts during benzene hydrogenation at 80°C was also completely solubilized in CI2CH2 after support dissolution with 40% HF °. In this case, since the coke was deposited at low temperature, such a high solubility could be expected. 

The coke deposited during the isobutane alkylation with C4 olefins on zeolites was extracted with a mixture of methanol/toluene, and with CI2CH2. None of these solvents were effective to remove the coke. Only a very small fraction was removed, which was attributed to the dissolution of external coke. The coke molecules formed inside the channels have a large size and therefore cannot diffuse out of the pores . 

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Figure 14. Abundance of various types of coke on zeolite H-Y during reaction with propene. The soluble fractions (A-C, black symbols) represent different aromatic structures which form during the first polymerization steps. The insoluble coke is the final product of polymerization which is affected by the solid state acidity of the zeolite, represented here as the Si A1 ratio.

The stability of catalyst is one of the most important criteria to evaluate its quality. The influence of time on stream on the conversion of n-heptane at SSO C is shown in Fig. 5. The conversion of n-heptane decreases faster on HYl than on FIYs with time, so the question is Could the formation of coke on the catalyst inhibit diffusion of reactant into the caves and pores of zeolite and decrease the conversion According to Hollander [8], coke was mainly formed at the beginning of the reaction, and the reaction time did not affect the yield of coke. Hence, this decrease might be caused by some impurities introduced during the catalyst synthesis. These impurities could be sintered and cover active sites to make the conversion of n-heptane on HYl decrease faster. 

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... 

In a series of investigations of the cracking of alkanes and alkenes on Y zeolites (74,75), the effect of coke formation on the conversion was examined. The coke that formed was found to exhibit considerable hydride transfer activity. For some time, this activity can compensate for the deactivating effect of the coke. On the basis of dimerization and cracking experiments with labeled 1-butene on zeolite Y (76), it is known that substantial amounts of alkanes are formed, which are saturated by hydride transfer from surface polymers. In both liquid and solid acid catalysts, hydride transfer from isoalkanes larger than... 

The zeolite to matrix surface area ratio can be used for optimization of catalysts for catalytic cracking of atmospheric residues. For North Sea long residues this ratio should be as large as possible, but the ratio should not exceed an upper limit. For the main catalyst type (A) used in this investigation the upper limit of the ZSA/ MSA ratio was around 3.5. There is also a lower limit for the matrix surface area. If the matrix surface area is lower than this limit, the catalyst will not be able to crack all the heavy components in the residue feed, and the coke on the matrix will increase dramatically. This will prevent the catalyst from working properly. Different type of catalysts must be optimized individually, as well as different type of long residues. 

It is possible to conclude from the TPO spectra that the signal with the peak of highest intensity located between 610°C and 617°C, for the case of light feedstock and low CCR, can be attributed to catalytic coke. This type of coke can reach values up to 89% of the total coke on the catalyst. In addition, it can be also concluded that catalytic coke yields are a direct function of the number framework aluminums in the zeolite structure. 

On zeolites, coke is built by a mechanism initiated on acid sites, and the extent of this deposit is often correlated with the A1 content of the solid. In odCB conversion the stability of the catalyst, expressed as the kd constant, does not appear directly correlated with the aluminium content of the solid. Actually, for that reaction, tarry materials are formed both by... 

Catalytic activity, assessed by cumene cracking on separated fractions and also by analysis of residual coke on catalyst fractions, shows a sharp decline with increasing density (age). This rapid loss of initial activity coincides with zeolite dealumination which is largely completed as a slow rate of zeolite destruction is established. Subsequent loss of crystallinity has little additional effect on activity. The associated loss of microporosity leads to an apparent increase in skeletal density with increasing age. 

Differences between various zeolites and matrix components of the catalyst have been accounted for by fitting coke deactivation rates (exponent n), coking and cracking activities (A and k), and adsorption coefficients (kA and k A). Similar ideas on deactivation of a composite cracking catalyst have been presented by Dean and Dadyburjor (13). Coke on catalyst then becomes the sum of the coke on the matrix and the coke on the zeolite ... 

Figures 1A and 1B show the adsorption isotherms of xenon on the Na, H-ZSM-5 and H-ZSM-5 zeolites, respectively. From the comparison, one sees that xenon uptake decreases slightly (about 10%) with coke content in the Na, H-ZSM-5 with a low (1%) coke content, on zeolite H-ZSM-5, and decreases only slightly more with heavy coking (12%).

The formation of coke on acid zeolite catalysts depends on i) the characteristics of the acid sites and of the pore structure of the zeolite and ii) the nature of the feed and the operating conditions (T,P). 

Three examples have been chosen for describing coking of zeolites. The first concerns the formation of coke during the transformation of propene, of toluene and of a mixture propene toluene at 120°C and 450°C on a HZSM5 zeolite. All the coke components are soluble in methylene chloride. Most of them are located inside the pores indeed they are not dissolved by direct soxhlet extraction of the coked zeolite samples. 

Deactivation of zeolites, like that of the other porous catalysts occurs in two ways, the first one in which at the maximum one active site per coke molecule is deactivated, the second in which several active sites are deactivated. The two modes of zeolite deactivation are shown in Figure 7. The effect of coke on the activity and on the pore volume accessible to the reactant is also indicated. 

From a series of experiments in this reactor, the deactivation effect of coke on a complex reaction mechanism may be obtained. This is illustrated for the catalytic cracking of n-hcxane on a US-Y zeolite catalyst. On a faujasite, the coke formation deactivates the main reactions, but not the coking reaction. Moreover, the coke formation induces selectivity changes, which can be explained by the distribution of acid site strength in Y-zeolites and the acid strength requirements of the various reactions. 

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