Effect of alkali additives over nanocrystalline Co-Cu-based perovskites as catalysts for higher-alcohol synthesis

A series of LaCo0.7Cu0.3O3-delta perovskites prepared by mechano-synthesis using different alkali additives was characterized by several techniques (AAS, BET, XRD, H-2-TPR) and tested under typical higher-alcohol synthesis conditions. Using group I elements as additives allowed substantial enhancement of the specific surface area of nanocrystalline LaCo(1-x)Su(x)O(3-delta) perovskites. The catalyst surface area varied with the cationic radius of the alkali metal used. The perovskite precursors were prereduced in situ at 500 degrees C under flowing H-2/Ar, providing copper/cobalt metals dispersed over an oxide support, La2O3. Reaction tests were carried out at 275 degrees C, 1000 psi, VVH = 5000 h(-1), and H-2/CO/He = 8/4/3. The products included paraffins, olefins, normal alcohols, and carbon dioxide. The alkali ions remaining on the surface of the reduced LaCo0.7Cu0.3O3-delta perovskite had a strong influence on the CO conversion, WGS reaction, selectivity, and alcohol and hydrocarbon productivity. An optimum content of alkali promoter in this study was in the range of 0.1-0.3 wt%. The promotional effect of such alkali metals on CO hydrogenation was to improve the propagation of the hydrocarbon chain of both hydrocarbons and alcohol products. The carbon number distributions of normal alcohols and hydrocarbon products were consistent with the ASF plot. The chain growth probability factors of products were correlated with the alkali electronegativity. Alcohol selectivity was in the range of 25.7-48.9%. Alcohol productivity was strongly dependent on the presence and content of residual alkali additives and on reaction temperature. The presence of an alkali promoter was necessary for higher-alcohol synthesis from syngas. (c) 2006 Elsevier Inc. All rights reserved.