MORE ACTIVE AND SULFUR RESISTANT BIMETALLIC Pd-Ni CATALYSTS

Quim. Nova, Vol. 41, No. 2, 151-156, 2018 http://dx.doi.org/10.21577/0100-4042.20170152 Carolina Bettia, Nicolás Carraraa, Juan Badanoa, Cecilia Lederhosa, Carlos Veraa,b and Mónica Quirogaa,b,* a Instituto de Investigaciones en Catálisis y Petroquímica, INCAPE (FIQ-UNL, CONICET), Colectora Ruta Nac. N° 168 Km 0, Paraje El Pozo, 3000 Santa Fe, Argentina b Departamento de Química, Facultad de Ingeniería Química, Universidad Nacional del Litoral, Santiago del Estero 2829, S3000AOJ Santa Fe, Argentina Artigo MORE ACTIVE AND SULFUR RESISTANT BIMETALLIC Pd-Ni CATALYSTS Recebido em 07/07/2017; aceito em 02/10/2017; publicado na web em 13/11/2017 The influence of the kind of metal precursor and the sequence of impregnation on the properties of Pd-Ni catalysts was evaluated during the test reaction of selective hydrogenation of styrene to ethylbenzene by means of physicochemical characterization. The focus was put on the final hydrogenating activity and the resistance to deactivation by sulfided compounds (thiophene). The used techniques of characterization were ICP, XPS, XDR, TPR, CO chemisorption and TEM. XPS results indicated the presence of different Pd species: Pdδ-, Pd0 and Pdδ+. In the case of the Ni containing catalysts, Ni0 and NiO species were also detected. These palladium and nickel species would be responsables of the variation of activity and sulfurresistance of the catalysts. NiClPd catalysts had a higher resistance to deactivation by sulfur poisoning. This was associated to a higher concentration of Pdη+ClxOy species that would prevent the adsorption of thiophene by both steric and electronic effects. It could also be due to the lower concentration of Pd0 and Ni0 on these catalysts, as compared to those shown by the PdNiCl catalysts. Both the Pd0 and Ni0 species are more prone to poisoning because of their higher electronic availability. Keywords: selective hydrogenation; bimetallic catalysts; sulfur resistance; palladium; nickel. INTRODUCTION Many petrochemical processes and applications in biotechnology, pharmacy and agrochemicals are based on the catalytic heterogeneous hydrogenation of hydrocarbons. The main objectives of these processes are to get the highest possible yields in conditions of reasonably mild pressure and temperature, and at the lowest cost. Petroleum hydrocarbon mixtures contain unsaturated compounds and many aromatic compounds. For this reason it is important that the reaction may be selective to the desired products. Particularly the heterogeneous reactions of selective hydrogenation of vinylic compounds that keep the aromatic nucleus intact are of great interest and usefulness in the petrochemical industry. A clear example are the pyrolysis gasolines that are a low quality subproduct of cracking processes for olefin production.1,2 Selective hydrogenation permits obtaining processed gasoline that meets the quality and octane conditions for its addition to the gasoline pool. Besides aromatic compounds like Benzene, Toluene and Xylene (BTX) can be extracted and used for other petrochemical uses. The hydrogenation of styrene to ethylbenzene is considered as a model for the study of the purification of pyrolysis gasoline because styrene is the most refractory compound for hydrogenation. Both Pd and Ni catalysts are used for styrene hydrogenation.3,4 The main problem of the PyGas streams is that they contain high amount of sulfided compounds, usually between 300 and 2000 ppm1,3,5 that poison the catalysts. Thiophene is one of the main sulfided compounds causing deactivation of the catalysts by poisoning. In a previous work with monomethalic catalysts the effect of chlorine for the sulfurresistance was studied. In Badano et al.,6 it was found that chloride complex species (Pdxδ+OyClz and the Ptxδ+OyClz, with δ < 2) present in the surface could hinder the adsorption of poisons via a steric factor (big size of chlorine ligands) or an electronic one (high electronegativity). Besides, bimetallic catalysts are potentially *e-mail: thioresistant systems. In a previous work our group studied Pt-Ni and Pt-W catalysts, and reported electronic and steric effects that improved the activity or the sulfur resistance.7,8 The objectives of this work were three: (i) the synthesis of bimetallic Pd-Ni catalysts varying the order of impregnation of the metals and the kind of precursor; chlorided and nitrogenated precursors were used; (ii) the catalytic evaluation of these catalysts; (iii) the assessment of the resistance of the catalysts to deactivation by sulfur poisoning. Four bimetallic catalysts were synthesized: PdNiN, NiNPd, PdNiCl and NiClPd. Results of activity and thioresistance were compared to the properties of a monometallic Pd catalyst. EXPERIMENTAL Catalysts preparation Bimetallic catalysts were prepared by successive impregnation of the metal precursors in solution. The used support was γ-Al2O3 Ketjen CK 300. This was previously calcined in order to stabilize its surface area (SBET: 224 m2 g-1). For the incorporation of the metals to the support the technique of incipient wetness impregnation was used. Impregnating solutions with dissolved PdCl 2 , NiCl 2 or Ni(NO3)2.6H2O were used to obtain monometallic catalysts of Pd, NiCl and NiN. Monometallic catalysts were dried with 24 h in a stove at 373 K, then they were calcined in dried air for 3 h at 823 K and finally they were reduced for 1 h at 673 K in a hydrogen flow of 110 mL min-1. During the impregnation of the second metal, an acidified solution of PdCl2 was impregnated over the monometallic catalysts of NiN and NiCl, for obtaining the bimetallic catalysts NiNPd and NiClPd respectively. Similarly, acidified solutions of NiCl2 and Ni(NO3)2.6H2O were impregnated over Pd monometallic catalysts to obtain the PdNiCl and PdNiN catalysts, respectively. The resulting bimetallic catalysts were dried for 24 h in a stove at 373 K, calcined for 3 h at 823 K and reduce for 1 h at 673 in a hydrogen stream of 110 mL min-1 before the reaction test. Betti et al. 152 Catalysts characterization Quim. Nova performed in batch mode using a PTFE coated, stainless steel, stirred tank reactor. The reaction of styrene hydrogenation was performed at 333 K, 20 bar hydrogen pressure, 1200 rpm stirring rate, 0.3 g catalyst mass and 200 mL solution of 5% (v/v) of styrene in toluene, n-decane was used as an internal standard for chromatography analysis. Some tests were performed with the feed of the standard test additivated with 600 ppm of thiophene, in order to assess the catalysts resistance to sulfur poisoning. Reactants and products were analyzed in a Shimadzu gas chromatograph equipped with a flame ionization detector and a 30 m, J&W InnoWax capillary column (cat. number 19091N-213). The mass concentration of Pd and Ni in the final catalysts was determined by plasma induction atomic emission spectroscopy (ICPOES). The equipment used was a Perkin Elmer 2100 and the samples had to be previously digested in (...truncated)