Improvement of the Activity and Selectivity of Low-loaded Pt Catalysts

C. BETTI; J. BADANO; M. JULIANA MACCARRONE; C. VERA; M. E. QUIROGA

Beijin

Otro; World Resources Forum 2012 (WRF 2012); 2012

Institute of Process Engineering, Chinese Academy of Sciences, China

In the petrochemical industry gasoline and BTX (Benzene, toluene and xylenes) streams coming from the cracking of petroleum cuts must be purified in order to minimize the concentration of olefins and diolefins. The widespread method of purification is the selective hydrogenation of vinyllic compounds, keeping the aromatic rings unaltered. The employed catalysts must have a great resistance to different sulfur compounds. A particular type of supported catalysts is that of the highly dispersed bimetallic catalysts, that have properties clearly different from those of the monometallic catalysts. Many studies on bimetallic catalysts for selective hydrogenation have been performed using both surface characterization techniques and catalytic activity/selectivity tests. The greater part of these studies has involved bimetallic catalysts in reactions of selective hydrogenation of hydrocarbons of low molecular mass. In recent works our group has studied the effect of the precursor salt and the temperature of reduction on the activity and poison resistance of monometallic catalysts for the selective hydrogenation of styrene. The objective of this work is to extend this research to the reaction system of the bimetallic Pt-Ni/gamma-alumina catalyst and the model feedstock of styrene contaminated with thiophene. The influence of the preparation of bimetallic Pt¨CNi catalysts on their activity and sulfur resistance during styrene semi-hydrogenation was studied. The preparation variables assessed were the sequence of impregnation and the kind of nickel precursor used. The catalysts were characterized by ICP, TPR, XPS and pyridine TPD. All catalysts showed high selectivities to ethylbenzene (>98%). The XPS results point to the presence of different Pt0, Pt¦Ä−and Pt¦Ä+OxCly species. The presence of Pt¦Ä−species would indicate the formation of a metallic bond or a Pt¨CNi alloy. The bimetallic catalysts were more active than monometallic Pt and the following activity order was found: Pt¡Ö NiClPt ¡Ü NiNPt < PtNiCl < PtNiN. After poisoning with 300 ppm of thiophene the following order of sulfur resistance was found: PtNiCl < Pt << PtNiN ¡Ö NiClPt¡Ö NiNPt. Species of Pt¦Ä−with high availability of electrons were found on the bimetallic catalysts suggesting that an electronic effect would be partly responsible for their higher conversion of styrene, either in the presence or the absence of thiophene. The bimetallic catalysts prepared by successive impregnation of Ni and Pt had a high surface content of chlorine, a low amount of Ni0 and a high total acidity. These catalysts were the most sulfur resistant because of steric and electronic effects. The great poisoning of the PtNiCl bimetallic catalyst would be due to an electronic effect: the easy adsorption of the thiophene sulfur atoms over the relatively abundant Ni0 and Pt¦Ä−species with high availability of electrons.-alumina catalyst and the model feedstock of styrene contaminated with thiophene. The influence of the preparation of bimetallic Pt¨CNi catalysts on their activity and sulfur resistance during styrene semi-hydrogenation was studied. The preparation variables assessed were the sequence of impregnation and the kind of nickel precursor used. The catalysts were characterized by ICP, TPR, XPS and pyridine TPD. All catalysts showed high selectivities to ethylbenzene (>98%). The XPS results point to the presence of different Pt0, Pt¦Ä−and Pt¦Ä+OxCly species. The presence of Pt¦Ä−species would indicate the formation of a metallic bond or a Pt¨CNi alloy. The bimetallic catalysts were more active than monometallic Pt and the following activity order was found: Pt¡Ö NiClPt ¡Ü NiNPt < PtNiCl < PtNiN. After poisoning with 300 ppm of thiophene the following order of sulfur resistance was found: PtNiCl < Pt << PtNiN ¡Ö NiClPt¡Ö NiNPt. Species of Pt¦Ä−with high availability of electrons were found on the bimetallic catalysts suggesting that an electronic effect would be partly responsible for their higher conversion of styrene, either in the presence or the absence of thiophene. The bimetallic catalysts prepared by successive impregnation of Ni and Pt had a high surface content of chlorine, a low amount of Ni0 and a high total acidity. These catalysts were the most sulfur resistant because of steric and electronic effects. The great poisoning of the PtNiCl bimetallic catalyst would be due to an electronic effect: the easy adsorption of the thiophene sulfur atoms over the relatively abundant Ni0 and Pt¦Ä−species with high availability of electrons.