The effect of lanthanum promoter on the selectivity of Pd/zeolite-X in methanol synthesis

Abstract

The promoting effect of various amounts of lanthanum on Pd/zeolite-X in enhancing selectivity in CO hydrogenation toward methanol/oxygenates has been investigated. The catalysts were characterized by temperature-programmed reduction, O₂-H₂ titration, and temperature-programmed desorption. The dispersion and H₂ chemisorption of Pd/zeolite-X catalysts were almost unchanged on La promotion. The virtually unchanged chemisorption capacity of the Pd/zeolite-X catalysts on La promotion is attributed to the cationic exchange method adopted to introduce La after Pd introduction into the zeolites. The La₂O₃ formed during calcination was partially reduced during reduction with hydrogen due to its contact with palladium. PdLaO_x entities formed thereby trigger the activation of coordinated CO/H₂ and facilitate the hydrogenation of CO to methanol/oxygenates at low temperature. Thus, the Pd sites responsible for methanol synthesis are stabilized by La at low and high pressures. Dimethyl ether, dominant for low La content Pd/zeolite-X catalysts, may be ascribed to the acid sites developed, but other mechanisms cannot be ruled out. The effect of the zeolite matrix lies in the stabilization of PdLaO_x entities, but otherwise it behaves in a similar fashion to other supports. The zeolite lattice, however, ensures the maintenance of small PdLaO_x particles. The turnover frequency values obtained with the optimum La promotion on Pd/zeolite-X are consistent with the reported values observed on Pd/La₂O₃ catalysts. The zeolites are therefore potential alternative supports in the preparation of suitable bimetallic systems.