Aging Coke Formation

Catalyst Aging Coke Formation.—In most, possibly all, organic reactions catalysed by zeolites, high molecular weight products accumulate within the zeolite pores and deactivate the catalyst, a phenomenon distinct from deactivation due to zeolite structure modification or collapse. Under some conditions the extent of dehydrogenation of the deposit is sufficient for it to be... 

Even if 5A zeolite is widely used in iso-paraffin separation from an n/iso paraffin mixture, the adsorbent is affected by a slow deactivation mainly due to coke formation inside the molecular sieve porosity. Its aging phenomenon decreases its sorption properties. According to previous studies, 5A zeolite deactivation results essentially from heavy carbonaceous compound formation in a-cages blocking the 5A zeolite microporosity [1-2]. 

As the catalyst ages, the rate constants decrease since the number of active sites is reduced by coke formation ... 

A large buildup of iron and titanium was found in a narrow band of the catalyst exterior of a spent catalyst. Improved catalyst aging is likely to occur by the use of an ebullated bed reactor, primarily by decreased interparticle coke formation as well as by mild abrasion of metal contaminants. 

At this point, it is necessary to distinguish between experiments in which mixtures are introduced as feed for the purpose of accelerated aging and those for other purposes. The early papers of Blue and coworkers [4,5], for example, introduced tetralin and decal in with various butenes, but this was done primarily to find a detailed mechanism of coke formation using isotopic techniques. This is not of interest in this work. In this work, we report on the effect of accelerated coking, brought about by cokegenerating additives of various types and at various levels. 

Coke deposition and aging are basic catalyst constraints, constraints lifted in ceitain applications by the discovery of ZSM-5 and related zeolite compositions and pore Structures. It was recognized a number of years ago that coke formation within tiie pores of a zeoUte can be a shape selective reaction (ref. 1). This area has now been excellently reviewed by M. Guisnet and P. Magnoux (ref, 2). 

Rollman recently described some interesting work on coke formation in several zeolites, which suggests that coking and aging rates are intrinsic properties of the zeolite pore structure. Adsorption measurements and the catalytic conversion of five hydrocarbons were used to compare zeolite performance. 

The crystalline aluminosilicate-catalyzed aldol condensation of acetophenone to form dypnone has been reported (27). As shown in Table XXIII, hydrogen zeolites were the most effective catalysts for this conversion. Operation at low temperatures in the liquid phase is critical for this reaction, to avoid both coke formation and condensation with aromatic solvents. Catalyst aging was rapid, however. Only transient conversions of acetone to mesitylene were obtained over REX or H-mordenite at 315° owing to rapid intracrystalline self-condensation and coke formation. 

With the introduction of faujasite zeolite into petroleum cracking, interest in vapor phase alkylation was renewed. There were several reported studies on the use of faujasite or mordenite to ethylate benzene.They were very active, but associated with rapid aging attributed to coke formation. Therefore, a feasible commercial alkylation using faujasite as a catalyst never evolved. 

Lower dilution levels did not allow sufficient depolymerlzatlon and higher dilution caused excessive depolymerlzatlon In the aged solutions. Pillared clays prepared from aged dilute solutions had an enhanced microstructure which showed an Increased activity for selectively cracking large molecules to the light cycle oil range. This microstructure Is lost In the presence of steam which also reduces the formation of catalytic coke. Addition of rare earth zeolite to pillared clay can partially overcome the effects of this loss of microstructure. 

These deposits responsible for fouling can block out the reactants and prevent them from reaching the active sites, or even block the internal pores of the catalyst. Hydrocarbons and aromatics are usually the cause of coking. The chemical nature of the carbonaceous deposits relies on many parameters temperature, pressure, feed composition, nature of products, and catalyst age share the responsibility of the residue formation on catalysts. 

The term aging generally describes a loss in the activity—or selectivity—observed in a catalytic process after a certain period of reaction time. Aging may result from some change in the nature or number of catalyst sites, or in the accessibility of the sites to reactant molecules. Thus, such factors as formation of hydrogen-deficient organic residues ( coke ), selective adsorption of impurities from the charge ad-... 

Coking, Aging and Regeneration of Zeolites Xtl- Composition of Coke Formed on USHY during Phenol Alkytation with Methanol-Mode of Formation M. GuisneL I. Neves, F. R. Ribeiro, C. Canaff, P. Magnoux and G, Perot... 

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