P. Magnoux

Organic chemistry of coke formation

Abstract The modes of formation of carbonaceous deposits (“coke”) during the transformation of organic compounds over acid and over bifunctional noble metal-acid catalysts are described. At low reaction temperatures, ( 350°C), the coke components are polyaromatic. Their formation involves hydrogen transfer (acid catalysts) and dehydrogenation (bifunctional catalysts) steps in addition to condensation and rearrangement steps. On microporous catalysts, the retention of coke molecules is due to their steric blockage within the micropores.

Deactivation by coking of zeolite catalysts. Prevention of deactivation. Optimal conditions for regeneration

Abstract The deactivation of acid zeolite catalysts used in hydrocarbon transformations is mainly due to the deposit inside the pores of heavy secondary products generally known as coke. It is shown how the rate of coking and the deactivating effect of the coke molecules are affected by the pore structure and the acidity of the zeolites as well as by the operating conditions. Directives for minimizing the deactivation by coking are proposed: (1) choice of tridimensional zeolites without trap cavities (large cavities with small apertures); (2) adjustment of the density and strength of the acid sites to the lowest values necessary for the selective formation of the desired products; and (3) choice of operating conditions in order to avoid the formation of coke-maker molecules (alkenes, polyaromatics). The regeneration of zeolites is generally carried out through coke combustion under air or oxygen flow. The detrimental effect that water, produced by coke oxidation, has on the zeolite activity can be limited by using a two-stage generation process, the hydrogen atoms of the coke molecules being oxidized at the low temperature of the first stage.

Coking, aging, and regeneration of zeolites. III: Comparison of the deactivation modes of H-mordenite, HZSM-5, and HY during n-heptane cracking

The rates of deactivation by carbonaceous residues (coke) of HY, H-mordenite, and HZSM-5, which have similar initial activities in n-heptane cracking are quite different: HZSM-5 forms very little coke and is very stable, while HM deactivates much faster than HY. The deactivation mechanisms of these zeolites could be specified by comparing as a function of the coke content: (i) the cracking activity of these zeolites and their capacity for adsorption of n-hexane, (ii) the volume apparently and really occupied by coke, and (iii) the number of coke molecules and of sites on which NH3 can no longer be adsorbed. On HY, polyaromatic molecules are very rapidly formed on the strongest acid sites, these molecules obstructing partially or completely the access of the reactant to the acid sites. On HZSM-5 deactivation is initially due to the “coverage” of the acid sites, located at channel intersections, by alkylaromatics with 1 or 2 rings; later on (above 3% coke content) polyaromatic molecules, formed on the external surface, block access to a part of the pore volume. On HM deactivation is due to pore blockage: even at very low coke contents (1% coke), coke can block the access of n-heptane to a pore volume 10 times greater than the volume really occupied by the coke.

New Technique for the Characterization of Carbonaceous Compounds Responsible for Zeolite Deactivation

Abstract A new technique has been developed for characterizing the carbonaceous compounds deposited in zeolites, responsible for their deactivation (“coke”). The technique consists in treating the coked samples at room temperature by a solution of hydrofluoric acid at 40% in order to dissolve the zeolite and to liberate the internal “coke”. This treatment does not cause any transformation of the carbonaceous compounds as shown by the tests carried out with two reactive hydrocarbons : 1-tetradecene and 9-methylphenanthrene impregnated on an inert solid. The soluble components of “coke” extracted by an organic solvent (CH2Cl2 are analyzed by classical techniques : G. C., H. P. L. C., H-N. M. R., M. S… Two examples are given here to show the interest of this technique. The first concerns the effect of the reaction temperature (120-450°C) on the composition of the “coke” formed during propene transformation on a USHY zeolite; the reactional steps involved in “coke” formation were able to be defined. In the second example, the mode of deactivation of three protonic zeolites : USHY, H mordenite and HZSM5, during dimethylether conversion into hydrocarbons, was specified by using results obtained from adsorption measurements on the “coked” zeolites and from analysis of the carbonaceous compounds.

Fundamental description of deactivation and regeneration of acid zeolites

The deactivation of acid zeolite catalysts is mainly due to the deposit within the pores or on the outer surface of the crystallites of heavy secondary products generally known as coke. Coke formation depends mainly on the zeolite pore structure and on the reaction temperature both of which determine the nature of the reactions involved and the retention of coke molecules (through condensation or trapping). The formation of coke molecules begins inside the micropores; however the growth of coke molecules trapped in cavities close to the outer surface of the crystallites leads to highly polyaromatic molecules which overflow onto this outer surface. With monodimensional zeolites (e.g. mordenite) and zeolites having large cavities with narrow apertures (e.g. erionite) deactivation occurs mainly through pore blockage. With tridimensional zeolites such as Y and MFI deactivation is mainly due to a competition for adsorption between reactant and coke molecules. The oxidation of coke molecules begins by their hydrogen atoms with formation of oxygenated compounds which can undergo various reactions: decarbonylation, decarboxylation, condensation. Radical cations formed through reaction cf molecular oxygen on coke molecules adsorbed on protonic sites would be intermediates in coke oxidation.

Coking, aging, and regeneration of zeolites: II. Deactivation of HY zeolite during n-heptane cracking

Heavy carbonaceous compounds (coke) are deposited very rapidly during n-heptane cracking on a stabilized HY zeolite at 450/sup 0/C, decreasing its activity, modifying its selectivity and its adsorption properties. Deactivation of the zeolite can be explained mainly by partial or total blocking of access to the active centers, even for low coke content. Indeed, while the decrease of the pore volume accessible to n-hexane (hence to the reactant n-heptane) is about 3 times less than the decrease of the activity, calorimetric study of NH/sub 3/ adsorption shows that the sites which no longer adsorb NH/sub 3/ are the strongest. Moreover, the diffusivity of n-hexane in the pore volume which remains accessible is definitely smaller than in the coke-free zeolite. Finally, the pore volume which has become inaccessible to n-hexane is, even at low coke content, about three times smaller than the volume really occupied by coke (estimated from its density). This deactivation through pore blockage is due to the fact that, even at low coke content, the coke molecules are sufficiently large to prevent access of the reactant to the supercages of the zeolite.

Coking, aging, and regeneration of zeolites: VII. Electron microscopy and EELS studies of external coke deposits on USHY, H-OFF, and H-ZSM-5 zeolites

External coke deposits produced by n-heptane cracking on USHY, H-OFF, and H-ZSM-5 zeolites have been located by a combination of transmission electron microscopic observations and electron energy loss spectroscopic (EELS) measurements performed with a field-emission gun scanning transmission electron microscope equipped with an electron spectrometer. The local structure of coke is determined from the fine structure at the high-energy side of the CK EELS peak, in comparison with reference compounds such as graphite, coronene, and pentacene. In H-ZSM-5 and H-OFF the coke forms an external envelope around the zeolite crystal and stands as an empty mold after zeolite extraction. Its structure is similar to that of coronene (polyaromatic-pregraphitic). In USHY part of the coke is in the form of 1-nm-large carbon filaments protruding from the zeolite mesopores and micropores. Its structure is more like that of pentacene (linear polyaromatic).

Roles of acidity and pore structure in the deactivation of zeolites by carbonaceous deposits

Deactivation of zeolite acid catalysts during hydrocarbon transformations is mainly due to the formation and the retention inside the pores of heavy secondary products. The effects of pore structure of zeolites and of their acidity on the rate of formation of these carbonaceous compounds, on their composition and on their deactivating effect are examined. The roles of strength and density of active acid sites (often protonic sites) are generally limited in comparison to the roles played by size and shape of cavities (or channel intersections) and by the size of their apertures. The formation of carbonaceous compounds (coke) and the deactivation that they cause are clearly shape selective processes. Their formation occurs through a nucleation-growth pathway. In the range of temperatures of refining and petrochemical processes nucleation is due to trapping in the cavities of coke precursors. Condensation and hydrogen transfer reactions are involved both in the formation of coke precursors and in their growth. Four modes of deactivation can be distinguished: (1) limitation or (2) blockage of the access of the reactant to the active sites of a cavity in which one coke molecule is located, due to steric reasons or to a competition for adsorption between reactant or coke molecules and (3) steric limitation or (4) blockage of the access of the reactant to the active sites of cavities or of channels in which no coke molecule is located. The deactivation of monodimensional zeolites and of zeolites with trap cavities (large cavities with small apertures) only occurs through Modes 3 and 4, hence is very rapid. With the other types of zeolites, deactivation passes successively from Mode 1 to Mode 4 as the coke content increases; Modes 3 and 4 are due to the coke molecules which overflow onto the outer surface of zeolite crystallites.

Coke formation and coke profiles during the transformation of various reactants at 450°C over a USHY zeolite

Abstract Coking of a USHY was studied in a fixed-bed reactor at 450°C. Different model compounds were used: an alkane ( n -heptane), an alkene (propene), a naphthenic (methylcyclohexane) and aromatics ( m -xylene, 1-methyl-naphthalene). Those reactants present very different coking rates in the order: 1-methyl-naphthalene>propene⪢ m -xylene> n -heptane>methylcyclohexane. The coke profile along the reactor is a function of the coke precursors. By using a triple-bed reactor, it is shown that the coking process proceeds primarily with olefins and aromatic reactants and secondarily with alkanes and naphthenes. In the first case, coke is preferentially deposited on the first part of the bed, whereas in the second case, the coke deposit is rather concentrated on the last part of the bed (after formation of coke maker molecules). GC-MS analysis has shown that whatever the reactant, coke is mainly constituted by small polyaromatic compounds with four to six aromatic rings located in the zeolite pores. Also, highly polyaromatic compounds located either inside the pores or on the external surface of zeolite crystallite were observed.

Zeolite-catalyzed rearrangement of phenyl acetate

Etude de la transposition de Fries en presence de trois catalyseurs acides solides. Pourcentage de compose ortho par rapport au compose para suivant le catalyseur utilise