J.M. Silva

Transformation of an ethylbenzene-o-xylene mixture on HMOR and Pt-HMOR catalysts. Comparison with ZSM-5 catalysts

Abstract The transformation of pure ethylbenzene and of an ethylbenzene (20wt.-%)- o -xylene (80 wt.-%) mixture is carried out on acid and bifunctional HMOR catalysts under the following conditions: fixed bed reactor, temperature 410°C, 0.96 MPa pressure of hydrogen and 0.24 MPa pressure of hydrocarbons, weight of hydrocarbon injected per weight of catalyst and per hour between 10 and 110. On both catalysts the main reaction of pure ethylbenzene is disproportionation, this reaction occurring mainly through diphenylethane intermediates. The main effect of platinum is to allow the bifunctional isomerization of ethylbenzene. In addition to disproportionation reactions, ethylbenzene in mixture with xylene is also transformed through similar mechanisms into dimethylethylbenzene (transethylation) and into ethylmethylbenzene (transmethylation). o -Xylene is rapidly transformed into m -andp-isomers. Large differences exist between MOR and ZSM-5 catalysts. ZSM-5 catalysts are very active for ethylbenzene dealkylation and C 8 naphthenes hydrocracking while MOR catalysts are more active for xylene isomerization and disproportionation, and ethylbenzene hydroisomerization. Disproportionation of ethylbenzene occurs mainly on H-ZSM-5 through a deethylation-ethylation mechanism and on HMOR through diphenylethane intermediates. Steric constraints on the formation of bulky bimolecular intermediates at channel intersections and diffusion limitations are responsible for the particular behaviour of ZSM-5 catalysts.

Influence of platinum on the transformation of an ethylbenzene-o-xylene mixture on H-ZSM-5

Abstract The transformations of pure ethylbenzene and of an ethylbenzene (20wt.-%)- o -xylene (80 wt.-%) mixture were carried out on acid and bifunctional ZSM-5 catalysts under the following operating conditions: fixed bed reactor, temperature 410°C, 0.96 MPa pressure of hydrogen and 0.24 MPa pressure of hydrocarbons, weight of hydrocarbon injected per weight of catalyst and per hour between 10 and 110. On both catalysts the main reactions of pure ethylbenzene are dealkylation and disproportionation, this latter reaction occurring through a dealkylation-alkylation scheme. The main effect of platinum is to increase the rate of dealkylation and to decrease that of disproportionation, which is due to the rapid hydrogenation of ethylene. In addition to dealkylation and disproportionation, ethylbenzene mixed with xylenes is also transformed into dimethylethylbenzene (transethylation), this latter reaction decreasing considerably in the presence of platinum. On the acid and bifunctional catalysts o -xylene is very rapidly transformed into p -andm-xylenes. Xylenes are also transformed through disproportionation and transethylation. Platinum increases slightly the rate of disproportionation at the expense of transethylation.

Influence of the treatment of mordenite by ammonium hexafluorosilicate on physicochemical and catalytic properties

Different samples were prepared by treating a NH 4 MOR sample (Si/Al = 5) with ammonium hexafluorosilicate. The dealumination of the outer surface of mordenite crystallites was more pronounced than that of the bulk. A decrease in the adsorption rate and capacity for organic molecules and in activity and stability for m -xylene transformation was observed. Furthermore, the isomerization selectivity was improved because of inhibition of desorption of bulky products of m -xylene disproportionation. All of these observations can be related to a dealumination of the mordenite limited to the pore mouth with deposit, at the entrance of channels, of aluminum species extracted from the framework.

Dealumination of the outer surface of MFI zeolites by ammonium hexafluorosilicate

The treatment of MFI zeolites with ammonium hexafluorosilicate causes a selective dealumination of their outer surface. This dealumination improves the selectivity to the para isomer of m-xylene isomerization.

n-Hexane hydroisomerisation over bifunctional Pt/MCM-22 catalysts. Influence of the mode of Pt introduction

Three different methods were used to introduce 1.0 wt.% of Pt in bifunctional Pt/MCM-22 zeolite catalysts: ion exchange with Pt(NH 3 ) 4 2+ , incipient wetness impregnation with PtCl 6 H 2 and mechanical mixture with Pt/Al 2 O 3 . The Pt dispersion was estimated by transmission electron microscopy and the hydrogenating activity with toluene hydrogenation at 110°C. From these experiments, it can be concluded that with the ion exchanged sample, platinum was located within the inner micropores and on the outer surface, whereas with the impregnated one, platinum was essentially on the outer surface under the form of large particles. With all the samples there is a fast initial decrease in the activity for n-hexane hydroisomerisation at 250°C. With exchanged and impregnated samples, this decrease is followed by a plateau, the activity value being then higher with impregnated sample. For the sample prepared by mechanical mixture a continuous decrease in activity can be observed. All these differences can be related with the distinct locations of Pt.

30-P-23-Total oxidation of volatile organic compounds-catalytic oxidation of toluene over CuY zeolites

Publisher Summary This chapter discusses the total oxidation of volatile organic compounds, with emphasis on the catalytic oxidation of toluene over CuY zeolites. Transformation of toluene in low concentration in air over CuY zeolites containing different copper contents and silicon/aluminium (Si/Al) ratios is studied at temperatures between 150 and 500°C. Total oxidation is promoted on nondealuminated catalysts and depends on the copper content. The most active catalysts correspond to the catalysts with copper (Cu) contents that are close to the complete exchange of the zeolite. The presence of sodium cocations in CuY zeolites increases their combustion efficiency, by improving the dispersion of ionic copper species and preventing the formation of cupric oxide (CuO) clusters.

Basic Csß — A new support for Pt nanoparticles active in aromatization of parafins

Pt nanoparticles formed in β zeolite when cesium is present as a counter ion are active in the aromatization of n-hexane. The best Pt dispersion and best catalytic behavior are obtained when calcination of Pt exchanged Css is performed at 300°C prior to reduction. This temperature also corresponds to the higher amount of reducible species in calcined Pt/Css. Moreover, the activity can be increased by adapting the conditions of Pt incorporation.

Isomerization of C8 aromatic cut. Improvement of the selectivity of MOR- and MFI-catalysts by treatment with aqueous solutions of (NH4)2SiF6

Summary The treatment of MOR and MFI zeolites with ammonium hexafluorosilicate solutions leads to a preferential dealumination of the crystallite outer surface. The treatment has fully different effects on the activity and selectivity of bifunctional catalysts constituted of Pt/Al 2 O 3 and MOR or H-MFI zeolites for the transformation of an o-xylene-ethylbenzene mixture at 410°C under hydrogen pressure. The activity of treated H-MOR catalyst is lower than that of non-treated catalysts but their isomerization selectivity is increased owing to an inhibition of disproportionation and transalkylation reactions. The decrease in activity can be explained by a partial blockage of the channels which could be caused by a silica deposit at the entrance of mordenite pores during the treatment. The inhibition of disproportionation and transalkylation is due to the decrease in acidity of the outer surface caused by its selective dealumination. On the other hand, the treatment of H-MFI with ammonium hexafluorosilicate causes a significant increase in the rates of ethylbenzene and o-xylene transformation which can be attributed to a decrease in the rate of coke formation on the outer surface of the crystallites. Furthermore, there is a decrease in the selectivity of xylenes disproportionation which involves bulky diphenylmethane intermediates but no change in the selectivity of ethylbenzene disproportionation which occurs on H-MFI through a dealkylation-alkylation mechanism.