Improvement of catalytic performance of Pd/α-Al₂O₃ in selective acetylene hydrogenation using mixed phases and nanocrystalline α-Al₂O₃

Abstract

The performance in selective acetylene hydrogenation over Pd/α-Al₂O₃ catalysts has been improved using mixed phases and nanocrystalline α-Al₂O₃. The use of mixed phases Al₂O₃ with approximately 64% of α-phase resulted in significant improvement for both acetylene conversion and ethylene selectivity. The presence of small amount of transition-phase in the alumina supports brought about higher BET surface area and Pd dispersion as well as improvement of reduction ability of the Pd/Al₂O₃ catalysts. On the other hand, significant amount of α-Al₂O₃ is necessary for high ethylene selectivity due to the lower amount of ethylene adsorbed. The improvement of ethylene selectivity can also obtained after reduction at 500°C on the solvothermal-derived nanocrystalline α-Al₂O₃ supported Pd. Moreover, the nanocrystalline α-Al₂O₃ powder with average crystallite size 34-68 nm have been synthesized by three different methods, namely, solvothermal, sol-gel, and precipitation. Nanocrystalline porosity of α-Al₂O₃ powder was varied by changing the preparation methods. Although smallest crystallite size of α-Al₂O₃ was obtained via the sol-gel synthesis, the α-Al₂O₃ sol-gel possessed the least amount of specific surface area and pore volume. A narrow pore size distribution with average pore diameter 15 nm can be obtained via solvothermal method while precipitation method yielded α-Al₂O₃ with larger pore size and wider pore size distribution. When employed as a support for Pd catalysts, the α-Al₂O₃ solvothermal provided the highest Pd dispersion and the best catalyst performance for selective hydrogenation of acetylene. The catalytic properties of Pd/α-Al₂O₃ solvothermal were improved in terms of both acetylene conversion and ethylene selectivity. As shown by temperature program studies, the use of solvothermal-derived α-Al₂O₃ facilitated H2 reduction at low temperature and desorption of ethylene and CO