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Characterization of bifunctional Pt/H[Ga]ZSM5 and Pt/H[Al]ZSM5 catalysts: II. Evidences of a Pt-Ga interaction

Abstract:

The purpose of this work is to elucidate how platinum supported on a gallosilicate of the MFI type interacts with extra-framework and/or framework gallium, thus affecting the properties of bifunctional catalysts of the Pt/H[Ga]MFI type. To this end, a gallosilicate and an aluminosilicate of the MFI type, with a Si/M atomic ratio ≅ 15 (M = Ga, Al) were synthesized and characterized by X-ray diffraction (XRD), nitrogen physical adsorption at -196°C, and chemical analysis by ICP-AES. These zeolites were used as supports in the preparation of a series of bifunctional catalysts with varying Pt content: 0.10 wt.% Pt/H[Ga]ZSM5, 0.50 wt.% Pt/H[Ga]ZSM5, 1.00 wt.% Pt/H[Ga]ZSM5 and 1.00 wt.% Pt/H[Al]ZSM5, which were assessed by transmission electronic microscopy (TEM); dispersion of the supported phase was found to range within 50 and 80%. The supports (gallosilicate and aluminosilicate) as well as the bifunctional catalysts were characterized by X-ray photoelectron spectroscopy (XPS) to determine the chemical species on their surfaces. The presence of extra-structural gallium was observed in Pt/H[Ga]ZSM5, which could be found most probably as Ga 2O 3; the presence of Pt 0, Pt-O ads, and PtO in Pt/H[Ga]ZSM5 and Pt/H[Al]ZSM5 was also observed. It was also evidenced an increase in surface gallium concentration as the content of platinum increases in the Pt/H[Ga]ZSM5 solids. The bifunctional catalysts were catalytically tested under standard conditions by the acetone transformation reaction. The results of the catalytic test confirms that the 1.00 wt.% Pt/H[Al]MFI catalyst shows an initial global activity four times higher than the 1.00 wt.% Pt/H[Ga]MFI one, and additionally a selectivity to the desired product (methyl isobutyl ketone, MIBK), three times higher. The activity and selectivity results observed for the Pt/H[Ga]MFI solids are remarkably similar to those found for the pure H[Ga]ZSM5 support. These results clearly show that the platinum metallic centers on the Pt/H[Ga]ZSM5 catalysts are not active for the hydrogenation reactions of the olefinic and/or carbonylic double bonds, probably due to passivation caused by the gallium species on the surface of the bifunctional catalysts. © 2004 Elsevier B.V. All rights reserved.