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

CAROLINA P. BETTI; JUAN M. BADANO; M. JULIANA MACCARRONE; CARLOS R. VERA; MÓNICA E. QUIROGA

Beijing

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

In the petrochemical industry gasoline and BTX (Benzene, toluene and xylenes) streamscoming from the cracking of petroleum cuts must be purified in order to minimize the concentrationof olefins and diolefins. The widespread method of purification is the selective hydrogenation ofvinyllic compounds, keeping the aromatic rings unaltered. The employed catalysts must have agreat resistance to different sulfur compounds. A particular type of supported catalysts is that of thehighly dispersed bimetallic catalysts, that have properties clearly different from those of themonometallic catalysts. Many studies on bimetallic catalysts for selective hydrogenation have beenperformed using both surface characterization techniques and catalytic activity/selectivity tests.The greater part of these studies has involved bimetallic catalysts in reactions of selectivehydrogenation of hydrocarbons of low molecular mass.In recent works our group has studied the effect of the precursor salt and the temperature ofreduction on the activity and poison resistance of monometallic catalysts for the selectivehydrogenation of styrene. The objective of this work is to extend this research to the reactionsystem of the bimetallic Pt-Ni/γ-alumina catalyst and the model feedstock of styrene contaminatedwith thiophene. The influence of the preparation of bimetallic Pt Ni catalysts on their activity andsulfur resistance during styrene semi-hydrogenation was studied. The preparation variables assessedwere the sequence of impregnation and the kind of nickel precursor used. The catalysts werecharacterized by ICP, TPR, XPS and pyridine TPD.All catalysts showed high selectivities to ethylbenzene (>98%). The XPS results point to thepresence of different Pt0, Ptd- and Ptd+OxCly species. The presence of Ptd- species would indicate theformation of a metallic bond or a PtNi alloy. The bimetallic catalysts were more active thanmonometallic Pt and the following activity order was found: Pt = NiClPt <  NiNPt < PtNiCl <PtNiN. After poisoning with 300 ppm of thiophene the following order of sulfur resistance wasfound: PtNiCl < Pt << PtNiN = NiClPt = NiNPt. Species of Ptd- with high availability of electronswere found on the bimetallic catalysts suggesting that an electronic effect would be partlyresponsible 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 surfacecontent of chlorine, a low amount of Ni0 and a high total acidity. These catalysts were the mostsulfur resistant because of steric and electronic effects. The great poisoning of the PtNiCl bimetalliccatalyst would be due to an electronic effect: the easy adsorption of the thiophene sulfur atoms overthe relatively abundant Ni0 and Ptd- species with high availability of electrons.