The poisoning resistance to sulfided and oxygenated compounds of some VIII Group PYGAS selective hydrogenation catalysts based on metals was assessed. Low content alumina supported Rh, Pd, Ru and Pt catalysts (0.35 wt%) were prepared from chlorided precursors. In the case of the palladium catalysts a nitrogenated precursor was also used. The catalysts were mainly assessed in the catalytic test of selective styrene hydrogenation in the presence or absence of known poisons. Model feedstocks spiked with thiophene, thiophane and tetrahydrofuran were used. The catalysts were further characterized by means of chemical analysis, XPS, TPR and chemisorption. The results indicate that chlorided precursors yield more sulfur resistant catalysts. The effect was attributed in part to the formation of oxychlorinated species, refractory to reduction, that leave the metal in an electron deficient state, thus inhibiting the formation of strong poison-metal bonds, the chloride species could also be a steric factor that can contribute to the sulfur resistance of the catalyst. Pd based catalyts had the highest activity and resistance to poisons of all the metals tested. This superior performance was attributed in part to the total occupancy of the 4d electronic levels of the Pd metal that was supposed to promote the rupture of the H 2 bond during the hydrogenation reaction. Keywords Low metal loading catalysts Á Selective hydrogenation Á Sulfided and oxygenated poisons List of Symbols PF Poison free condition TE Thiophene TA Thiophane THF Tetrahydrofuran D Metal dispersion S Metal specific surface (m 2 g met -1 ) d Average metal particle size (Å ) V HAds (CNPT) Volume of hydrogen chemisorbed at normal conditions (cm 3 ) PA Atomic weight of the metal (g mol -1 ) w Metal content per unit gram of catalyst M Mass of sample (g) m Stoichiometry of chemisorption of the gas on the surface metal atoms V m CNPT Volume of the chemisorbed monolayer of the adsorbate at normal conditions (cm 3 g -1 ) N Avogadro's number (6.023 9 10 23 ) V N Molar volume of the adsorbate at normal conditions r Number of metal atoms per square meter (atoms m -2 ) xAdsorption stoichiometry (number of hydrogen atoms adsorbed per unit of metal atom) qDensity of the metal (g m -3 ) M Red /M Tot Reduced metal amount and total metal amount ratio wt M% Nobel metal content (weight percentage) T Red Reduction temperature (K) r°E S Initial styrene hydrogenation rate in poison free condition (mol g cat -1 min -1 ) DESRelative deactivation per concentration of poison (ppm -1 )
Recebido em 27/4/09; aceito em 30/10/09; publicado na web em 12/3/10 Semi-hydrogenation of alkynes has industrial and academic relevance on a large scale. To increase the activity, selectivity and lifetime of monometallic catalysts, the development of bimetallic catalysts has been investigated. 1-Heptyne hydrogenation over low-loaded Pd and Ni monometallic and PdNi bimetallic catalysts was studied in liquid phase at mild conditions. XPS results suggest that nickel addition to Pd modifies the electronic state of palladium as nickel loading is increased. Low-loaded Pd catalysts showed the highest selectivities (> 95%). The most active prepared catalyst, PdNi (1%) , was more selective than the Lindlar catalyst.
Palladium, platinum, and ruthenium supported on activated carbon were used as catalysts for the selective hydrogenation of 1-heptyne, a terminal alkyne. All catalysts were characterized by temperature programmed reduction, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. TPR and XPS suggest that the metal in all catalysts is reduced after the pretreatment with H2 at 673 K. The TPR trace of the PdNRX catalyst shows that the support surface groups are greatly modified as a consequence of the use of HNO3 during the catalyst preparation. During the hydrogenation of 1-heptyne, both palladium catalysts were more active and selective than the platinum and ruthenium catalysts. The activity order of the catalysts is as follows: PdClRX > PdNRX > PtClRX ≫ RuClRX. This superior performance of PdClRX was attributed in part to the total occupancy of the d electronic levels of the Pd metal that is supposed to promote the rupture of the H2 bond during the hydrogenation reaction. The activity differences between PdClRX and PdNRX catalysts could be attributed to a better accessibility of the substrate to the active sites, as a consequence of steric and electronic effects of the superficial support groups. The order for the selectivity to 1-heptene is as follows: PdClRX = PdNRX > RuClRX > PtClRX, and it can be mainly attributed to thermodynamic effects.
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