M. Guisnet

Coking and deactivation of zeolites: Influence of the Pore Structure

Abstract The modes of coking and of deactivation of zeolites during n-heptane cracking at 723 K were established on the basis of (i) the composition of the carbonaceous compounds responsible for deactivation (coke), (ii) the deactivating effect of the coke molecules and (iii) the reduction by coke of the volume accessible to nitrogen and to n-hexane (kinetic diameter similar to n-heptane). The zeolites [USHY, H Mordenite (HMOR), HZSM5 and H Erionite (HERI)] were chosen to determine the effect of different parameters of the pore structure: (i) pore size, (ii) existence (USHY, HERI) or non-existence of cavities (HMOR, HZSM5), (iii) the possibility for the reactant to diffuse unidirectionally (HMOR) or tridirectionally. The retention of coke molecules is due to trapping in the cavities (or at channel intersections). Their size is intermediate between that of the apertures and that of the cavities (or channel intersections). The coking rate is all the faster when the space available near the acid sites is large and when the coke precursors desorb slowly. On all the zeolites, coke formation occurs through oligomerization of the olefinic cracking products followed by cyclization of the oligomers, transformation through hydrogen transfer into monoaromatics, alkylation of these monoaromatics, then cyclization and hydrogen transfer to give bi-aromatics, tri-aromatics, etc. There is no site poisoning by coke; deactivation occurs through the three following modes: (i) limitation of the access of n-heptane to the active sites, (ii) blockage of the access to the sites of the cavities (or channel intersections) in which the coke molecules are situated and (iii) blockage of the access to the sites of the pores in which there are no coke molecules.

Aromatization of short chain alkanes on zeolite catalysts

Abstract On MFI catalysts, low cost liquefied petroleum gas (LPG ) can be transformed into valuable aromatics (mainly C 6 -C 8 benzenics) and into hydrogen. Unfortunately methane and ethane are also produced in significant amounts. A reduction of the production of these unwanted compounds would render the aromatization process economically more attractive. The reaction pathways of propane aromatization were established on H-ZSM-5 pure or loaded with platinum or with gallium. On H-ZSM-5 the first step is the dehydrogenation and the cracking of the reactant through carbonium ion intermediates. The resulting alkenes (propene and ethylene) undergo rapid successive reactions via carbenium ion intermediates: oligomerization, cyclization, hydrogen transfer. The selectivity to aromatics is limited because of the formation of methane by propane cracking and of alkanes by hydrogen transfer. Platinum increases the rate of propane transformation significantly but a higher production of methane and ethane is found on PtH-ZSM-5 catalysts, owing to the hydrogenolysis of alkanes and of alkylaromatics and to the hydrogenation of ethylene on the platinum sites. Gallium improves both the rate and the selectivity of propane aromatization. The aromatization occurs, like on PtH-ZSM-5, through a bifunctional pathway, gallium catalyzing the dehydrogenation of the alkane reactant to alkenes and of naphthenic intermediates to aromatics, and the acid sites catalyzing the oligomerization of light alkenes and the cyclization of C 6 -C 8 alkenes. The better selectivity to aromatics is obtained for GaMFI catalysts with gallium species well dispersed within the zeolite and being very active for dehydrogenation (e.g. with gallosilicates or galloaluminosilicates steamed under mild conditions). At high temperature, in presence of hydrogen (hence during aromatization) GaMFI catalysts would undergo various modifications: reduction of gallium species, migration within the zeolite, reaction of these species with the protonic sites. The nature of the gallium active species and their role in the various steps are discussed.

Conversion of light alkanes to aromatic hydrocarbons: II. Role of gallium species in propane transformation on GaZSM5 catalysts

Abstract The conversion of propane into benzenic compounds was studied at 530°C on a series of GaHZSM5 catalysts prepared by impregnation of HZSM5 with a solution of gallium nitrate. Gallium species increase the rate of propane conversion and the selectivity for aromatics. These species have dehydrogenating activity, increasing both propane activation and naphthene aromatization. The conversion of propane into benzenic compounds on GaHZSM5 catalysts can be considered, therefore, as a bifunctional process in which dehydrogenation reactions are catalyzed by the gallium species and oligomerization and oligomer cyclization by the acid sites. The active gallium species seem to be gallium oxide dispersed in the zeolite rather than gallium cations in an exchange position.

Infrared spectroscopic study of the acid properties of dealuminated BEA zeolites

Abstract A series of HBEA samples were prepared by dealumination through three different methods (steaming, treatments with hydrochloric acid and ammonium hexafluorosilicate (HFS) solutions) of a parent sample with total and framework Si:Al ratios of 12.5 and 33, respectively, constituted of 20 nm crystallites. The samples were characterized by XRD, nitrogen adsorption and pyridine adsorption–desorption followed by IR spectroscopy. Whereas the three dealumination treatments have practically no effect on crystallinity and nitrogen adsorption properties, they cause large changes in the ranges of the O–H bond stretching modes (3300–3800 cm−1) and of the most intense IR absorption of pyridine (1400–1700 cm−1). Treatments with HCl or HFS cause the most significant changes: disappearance of the 3662 and 3782 cm−1 bands corresponding to extraframework Al species and to tricoordinated Al species partially connected to the framework, respectively, appearance of additional bands at 1603 and 1446 cm−1 ascribed to a new type of pyridine species coordinated to Lewis sites (PyL2) and for desorption temperatures above 350 °C, of a band at 1462 cm−1 generally ascribed to iminium ions, apparently at the expense of the PyL2 band. In agreement with this formation of iminium ions, desorption at high temperatures causes the complete disappearance of pyridinium ions without complete restoration of the acidic bridging hydroxyl band. PyL2 species are proposed to be pyridine molecules coordinated to Lewis sites and interacting through hydrogen bond with protonic sites.

Mechanism of short-chain alkane transformation over protonic zeolites. Alkylation, disproportionation and aromatization

Ethane, propane and butanes can be transformed on large and average pore size protonic zeolites. It is shown that the reaction temperature determines both the mode of alkane activation on the acid sites (i.e. the mode of carbocation formation) and the nature of their transformation. At ambient temperature isobutane (and no other short-chain alkanes) can be activated, but only in the presence of alkenes. This activation occurs by hydride transfer to the carbenium ions resulting from alkene adsorption on the protonic sites of the zeolite. The t-butyl carbenium ions, formed from isobutane, alkylate the alkene molecules through a chain mechanism. At temperatures above 500°C, pure C2–C4 alkanes can be transformed into aromatics, in particular on HMFI. The activation of alkanes occurs, like in superacid solutions, through protolysis of their CH or CC bonds with formation of hydrogen or alkanes and of carbenium ions which desorb as olefins. These olefins are transformed into aromatic products through various reactions: oligomerization-cracking cyclization and hydrogen transfer. On HMFI, the protolytic cleavage of CH and CC bonds is the limiting step of short-chain alkane aromatization. The association of gallium species to HMFI increases significantly the aromatization activity and selectivity of this zeolite and alkane aromatization occurs through a bifunctional scheme. At average temperatures, propane and butanes can be transformed through a dimerization-cracking process (disproportionation). In butane transformation this process is responsible for the formation of propane and pentanes but also for butane isomerization. As is the case at high temperature the activation of alkanes occurs through protolysis. However this protolysis is only the initiation step of the carbenium ion chain mechanism of disproportionation, for protolysis is much slower than the hydride transfer from alkanes to the resulting carbenium ions. The reaction schemes of isobutane alkylation with 2-butene, of butane isomerization and of propane aromatization are described. The influence of the pore structure and of the acidity of the protonic zeolites on their activity and selectivity is discussed.

Conversion of light alkanes into aromatic hydrocarbons. 1-dehydrocyclodimerization of propane on PtHZSM-5 catalysts

Conversion of propane into aromatic compounds (dehydrocyclodimerization) was studied at 530°C on a series of PtHZSM-5 catalysts with different platinum contents. For this reaction, occurring via propene, the product distribution and its change with the conversion rate were compared with those in the conversion of propene on HZSM-5. Large differences were observed; in particular, the selectivity for C6C8 aromatic products was greater on PtHZSM-5 than on HZSM-5 whereas the selectivity for C4 and non-aromatic C5+ was lower. These differences can be explained by the significant increase in the aromatization rate of aliphatic C6+ intermediates, dehydrogenation on the platinum sites being faster than hydrogen transfer on the acid sites. Moreover, a higher production of methane and ethane is observed on PtHZSM-5 catalysts owing to the hydrogenolysis of alkanes and of alkylaromatic products on the platinum sites.

Conversion of light alkanes into aromatic hydrocarbons: VI. Aromatization of C2-C4 alkanes on H-ZSM-5 —reaction mechanisms

Abstract The conversion of ethane, propane, n-butane, isobutane and of the corresponding alkenes was studied at 530°C on a H-ZSM-5 zeolite (Si/Al=40). Butanes transform about 4 times more rapidly than propane and 100 times more than ethane. From propane and butanes the primary products result from dehydrogenation and cracking, cracking being more rapid than dehydrogenation. Ethane undergoes mainly dehydrogenation and leads slowly to methane. It is suggested than these reactions occur through scission of carbonium ions formed by alkane protonation. The more stable the carbonium ion the higher the reaction rate. The olefinic primary products undergo very rapid reactions through carbenium ion intermediates: oligomerization, cyclization of the oligomers followed by aromatic formation through hydrogen transfer from naphthenes to light alkenes. Whatever the reactant, the limiting step of aromatization is the initial formation of alkenes.

Mechanisms of xylene isomerization over acidic solid catalysts

Abstract In this review paper, the gas-phase isomerization of xylene over fresh acidic solid catalysts is shown to occur through the two mechanisms proposed to explain this reaction with Friedel–Crafts catalysts: the well-known intramolecular mechanism which proceeds through methyl shifts in benzenium-ion intermediates and an intermolecular one, involving successively xylene disproportionation followed by transalkylation between the trimethylbenzene and reactant xylene molecules produced. When steric constraints in the vicinity of the acid sites inhibit (medium-pore size zeolites) or limit (large-pore zeolites such as EMT, BEA, etc.) the formation of the bulky diphenylmethane intermediates of transalkylation, xylene isomerization only occurs through the intramolecular mechanism. This is also the case when the catalyst has very strong acid sites because of a rapid transformation of the diphenylmethane intermediates into coke. On the other hand, the intermolecular mechanism becomes predominant when the catalyst has only weak acid sites localized in large cages or in long, non-interconnected channels (e.g., MCM-41). On the example of HFAU catalysts, it is shown that the relative significance of the intra- and intermolecular pathways can be simply determined from the para -/ ortho -xylene ratio obtained in the isomerization of meta -xylene.

Influence of the framework composition of commerical HFAU zeolites on their activity and selectivity in m-xylene transformation

Abstract m -Xylene transformation was carried out at 623 K over a series of commercial HFAU zeolites with framework Si/Al ratios in the 4–100 range. Nitrogen adsorption at 77 K shows that all the samples present, besides the micro-, ultramicro- and mesopores result from the collapse of part of the micropore walls during dearamination. The acidity of the samples was characterized by pyridine adsorption followed by IR spectroscopy. Contrary to what is generally found, the acid strength increases with the density of framework aluminium atoms, hence of protonic sites. The strong acidity of the less dealuminated samples (Si/Al between 4–16) is due to an interaction of framework protonic sites with extraframework aluminium species. The very weak acidity of the more dealuminated samples seems to result from the presence of adjacent OH groups. This large difference in acid strength explains the very large difference in activity of the acid sites of the slightly and strongly dealuminated samples. With all the samples, the classical monomolecular isomerization pathway is accompanied with a bimolecular pathway involving, successively, xylene disproportionation and transalkylation of trimethylbenzenes with m -xylene. The greater the acid site density, the less significant is the proportion of bimolecular isomerization. However, this selectivity change is due to changes in acid strength rather than in site density. On the very strong acid sites of the slightly dealuminated samples the diphenylmethane intermediates of disproportionation would be rapidly transformed into coke precursors and not into the expected toluene and trimethylbenzene products, which would not be the case on the weak acid sites of the strongly dealuminated samples.

Hydroisomerization of n-hexane on platinum zeolites: I. Kinetic study of the reaction on platinum/Y-zeolite catalysts: Influence of the platinum content

Abstract The transformation of n -hexane has been carried out under hydrogen pressure on a series of platinum-stabilized Y-zeolites with platinum contents varying from 0 to 17.7 wt% (platinum area ranging from 0 to 10 m 2 g −1 ). The conventional bifunctional mechanism accounts for the change in the isomerization activity and selectivity with the platinum area and with various operating conditions (temperature, n -hexane and H 2 pressures, H 2 S and NH 3 poisoning). Moreover, the cracking mechanism shifts from a carbonium ion one on small platinum area catalysts to hydrogenolysis on large platinum area catalysts.