Pore formation during coking

Microscopic Investigations of Pore Formation during Coking... 

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. 

Stanislaus, A., Absi-Halabi, M., Khan, Z., Influence of Catalyst Pore Size on Asphaltenes Conversion and Coke-Like Sediments Formation During Catalytic Hydrocracking of Kuwait Vacuum Residues, In Catalysts in Petroleum Refining and Petrochemical Industries. Studies in Surface Science and Catalysis. 1996, Elsevier New York, USA. pp. 189-197. 

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. 

During induction, catalyst activity and selectivities to aromatics and propene increase steadily. Improvement of catalyst performance is due to increase in Ga dispersion and formation of dispersed Ga species (Gao) which are efficient for the heterolytic recombinative release of hydrogen [18,191. The Ga/H-MFI catalyst then reaches its optimal aromatisation performance (stabilisation). Ci to C3 hydrocarbons productions are at their lowest. The gallium dispersion and the chemical distribution of Ga are optimum and balance the acid function of the zeolite. Reversible deactivation during induction and stabilisation of the catalyst is due to site coverage and limited pore blockage by coke deposition. 

INFLUENCE OF CATALYST PORE SIZE ON ASPHALTENES CONVERSION AND COKE-LIKE SEDIMENTS FORMATION DURING CATALYTIC HYDROCRACKING OF KUWAIT VACUUM RESIDUES... 

The deposit of carbonaceous compounds inside the pores or on the outer surface of zeolites (coke) is the main cause of their deactivation during the transformation of organic reactants [1-4]. The cost of this deactivation is very high and great efforts have been and are being made to find methods i) for limiting the formation of coke and its effect on the zeolite activity and ii) for regenerating the activity. The aim of this paper is to illustrate these methods by several examples chosen in the transformation of hydrocarbons. 

Figure 15. Destruction of supported Ni by coke formation during partial oxidation of hydrocarbons, (a) Survivors vs. time at two airJHC ratios (b) effect of average pore diameter upon survival rate (91)...

The aim of the present work was to obtain a better understanding of the coke formation reaction during the process in terms of the kinetics and changes in pore... 

The effect of coke deposition on the MTO reaction is complex. Coke deposition influences either the formation of dimethyl ether (DME) or the DME conversion inside the pores during MTO. However, the effect of coke deposition on the dimethyl ether conversion to light olefins (the DTO process) catalyzed by SAPO-34 is much simpler and can allow us to focus on the effect of intracrystalline coke on the olefin formation from DME. 

Unsaturated residue formed during catalytic reactions that produced paraffins and olefins is the source of alkyl aromatics and nonvolatile residue. When HZSM-5 catalyst is employed, aromatic alkyl chain sizes are restricted to C4 or smaller. The pores of HZSM-5 are large enough to allow formation of small alkyl aromatics by cyclization and dehydrogenation of surface species, but formation of fused unsaturated coke precursors are inhibited. Unlike HZSM-5, larger HY pores facilitate the formation of larger nonvolatile unsaturated coke precursors. 

Figure 7 Formation and growth of coke molecules in the pores of HOFF during n-heptane cracking at 450°C (a) naphthalene, (b) methylphenanthrene and (c) methylchrysene.

---