N.S. Gnep

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.

Influence of the metal and of the support on the activity and stability of bifunctional catalysts for toluene hydrogenation

Abstract Gas phase hydrogenation of toluene, diluted in inert n -hexane in order to minimise the thermal effects was investigated in a fixed bed reactor at 110°C over series of bifunctional catalysts in which Pt or Pd was deposited on alumina or on an acidic HFAU zeolite. Practically no deactivation was observed when the metals were deposited on alumina, even in mixture with the HFAU zeolite, whereas there was a rapid initial deactivation when Pd and especially Pt were deposited on the acidic zeolite. However, even in this latter case, the activity of the fresh catalysts can be obtained with a good accuracy thanks to the use of a multiple loop value. For given metal and support, a linear correlation was found between the hydrogenating activity and the number of accessible metal atoms. However, Pt was found to be 20–60 times more active than Pd, both metals being more active on HFAU zeolite than on alumina. This acidity effect could be explained by the hydrogenation of aromatic molecules adsorbed on acidic sites by hydrogen spilled over from the metal surface. The initial deactivation of Pt- and Pd-HFAU zeolite catalysts was due to the rapid formation of ‘coke’ molecules inside the zeolite micropores. ‘Coke’ was mainly constituted by C 14 and C 21 products resulting from the acid alkylation of toluene molecules by the olefinic and dienic intermediates of their hydrogenation. While the composition of ‘coke’ as well as its deactivating effect did not depend on the metal (Pt or Pd), the rate of ‘coke’ formation at isoconversion of toluene was found to be lower on Pd than on Pt zeolite catalysts, which explains their slower deactivation.

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.

Reactions involved in the alkylation of isobutane with 2-butene and with propene on a USHY zeolite

Abstract The transformation of an isobutane-2-butene mixture with a 41 molar ratio was carried out in liquid phase at 323 K on a USHY zeolite with a framework Si/Al ratio of 4.5. C5–C14 isoalkanes with about 70 wt.-% of C8 and a small amount of C8 alkenes were obtained. Moreover, about 7% of butene was transformed into undesorbed C10–C30 products containing three unsaturations or cycles. The C5–C14 products resulted from the following reactions: alkylation of isobutane with 2-butene, dimerization of butene, oligomerization-cracking and self-alkylation of isobutane. In order to determine the significance of this latter reaction, the transformation of an isobutane-propene mixture was investigated under similar operating conditions. Trimethylpentanes, and particularly the 2,2,4 isomer, were formed, which confirmed that isobutane self-alkylation was taking place. This reaction was about twice slower than the alkylation of isobutane with propene and twice faster than the dimerization of propene. From the hydrogen balance it was concluded that self-alkylation of isobutane supplied the main part of the hydrogen necessary for the saturation of the products of alkene dimerization and oligomerization-cracking. Hydride transfer would be the limiting step of the formation of isoalkanes. However, isomerization through hydride and methyl shifts of carbenium ion intermediates played also a significant role in the product distribution. This reaction occurred at a rate similar to that of hydride transfer to tertiary carbenium ions but much faster than hydride transfer to secondary carbenium ions.

Conversion of Light Alkanes Into Aromatic Hydrocarbons. 3. Aromatization of Propane and Propene on Mixtures of HZSM5 and of Ga2O3

The activities for propane aromatization of physical mixtures of HZSM5 (25 mg) and Ga2O3 (5 to 200 mg) are much greater than the sum of the activities of the pure components. This synergic effect is characteristic of a reaction in which catalytic sites of the two components (bifunctional catalysis) participate. Experiments with model reactants propane, propene, 1-hexene, 1-heptene, methylcyclohexene and methylcyclohexane confirm that gallium oxide catalyzes the dehydrogenation of alkanes into olefins and of naphthenes into aromatics but is not active for oligomerization and cyclization reactions. The aromatization of propane on the mixture occurs in the following way: on gallium oxide propane is dehydrogenated into propene; propene diffuses from gallium oxide to HZSM5, on which it undergoes oligomerization and then cyclization; naphthenic compounds diffuse from HZSM5 to gallium oxide where they are dehydrogenated into C6-C8 aromatics.

Selective transformation of methanol into light olefins over a mordenite catalyst: reaction scheme and mechanism

Abstract The conversion of a dimethylether–water mixture into hydrocarbons was investigated at 530°C over a commercial mordenite catalyst. A dimethylether–water mixture was chosen instead of methanol in order to limit thermal effects due to the exothermicity of the transformations of methanol into dimethylether and into hydrocarbons. The main reaction products are methanol, C 1 –C 12 hydrocarbons and coke, which is responsible for catalyst deactivation. The distribution of the products was determined on the fresh catalyst over a large range of conversions (up to 100%), which allowed us to propose a complete reaction scheme. The conversion of dimethylether into hydrocarbons is faster than that of methanol, which is formed rapidly by the hydration of dimethylether. Whereas C 3 –C 7 alkenes appear as primary products, ethylene results from secondary cracking. Propene would be formed from a Stevens rearrangement of oxonium ions, and higher alkenes from the rapid methylation of propene. C 6 –C 7 alkenes and probably higher alkenes appearing in traces are transformed into aromatics through cyclization and hydrogen transfer steps, with the simultaneous formation of C 3 –C 4 alkenes. The resulting aromatics are rapidly alkylated by dimethylether or methanol. Methane is formed directly from dimethylether or methanol, but also from the demethylation of polymethylbenzenes or of coke molecules.

Effect of the binder on the properties of a mordenite catalyst for the selective conversion of methanol into light olefins

Abstract The association of an inactive binder with a dealuminated mordenite in a catalyst developed for the selective conversion of methanol into light olefins causes a decrease in the activity of the zeolite and an improvement of its stability. Ammonia thermodesorption and model reactions – n-heptane cracking at 803 K and metaxylene isomerization at 623 K – show that the activity decrease is due to a neutralization of the strongest protonic acid sites by exchange with the alkaline or alkaline-earth cations of the binder during the preparation of the catalyst. This neutralization is also responsible to part of the stability improvement. However this improvement is mainly due to the trapping by the binder of coke precursors with consequently a significant decrease in the amount of coke deposited on the zeolite, as shown by experiments with mechanical mixtures.

Mechanisms of the skeletal isomerization of n-butene over a HFER zeolite. Influence of coke deposits

Abstract On a fresh HFER zeolite (Si/Al=14) at 623 K the skeletal isomerization of n-butene is accompanied by a significant formation of propene and pentenes. A dimerization-cracking mechanism is proposed to explain the simultaneous formation of isobutene, propene and pentenes as primary products. The blockage, caused by carbonaceous deposits (coke), of the access of the reactant to the zeolite pores is responsible for the rapid deactivation observed for the formation of propene and pentenes. Coke is mainly constituted by relatively simple methylaromatics trapped in the zeolite pores. Curiously the rate of butene skeletal isomerization increases first with time-on-stream consequently with formation of coke. This curious positive effect of coke can be related to the development of a monomolecular isomerization mechanism involving, as active sites, benzylic carbocations resulting from adsorption of coke molecules on the acid sites in the pores close to the outer surface of the zeolite crystallites. This monomolecular mechanism does not allow propene and pentenes to be formed, which explains the high selectivity for isobutene of the coked zeolite samples.

o-Xylene Transformation on Acid Catalysts : Influence of Hydrogen Activated on Metallic Sites

Abstract Under hydrogen, nickel mordenites are more stable and more selective for o-xylene isomerization than protonic mordenites. This observation can be attributed to an inhibiting effect of hydrogen activated by nickel on disproportionation and on coke formation. This effect can also be observed by using mixtures of nickel on silica and mordenite as well as mordenite (or silicaalumina) previously treated by hydrogen activated at 450°C on various hydrogenating components. The explanation for this could be that hydrogen activated on metallic sites spilling over the acid catalyst reacts with the carbocations intermediates in disproportionation and coke formation. A decrease of their concentration and consequently a decrease in the rates of these reactions is thus provoked.

Toluene Disproportionation and Coke Formation on Mordenites Effect of Catalyst Modifications and of Operating Conditions

Improving catalytic stability of mordenite has been attempted by using three types of treatment: dealumination, wet air heating, nickel-ion-exchange. The rates of disproportionation and coking on these samples have been compared under different operating conditions: low and high hydrogen or nitrogen pressures. Under nitrogen, mordenite deactivation is very fast. Dealumination increases its activity for disproportionation and coking as well as the deactivation rate, while wet air treatment decreases its coking activity. This decrease may be connected with the elimination of Bronsted acid sites. The catalytic stability of all samples is considerably improved by operating under high hydrogen pressure, the best catalyst being the wet air treated nickel-exchanged mordenite. This stabilizing effect of hydrogen is the result of coking inhibition and of a better distribution of coke in the bulk of the catalyst pellet than under nitrogen. It is suggested that coking inhibition comes from the presence on mordenite of sites capable of activating molecular hydrogen.