Effect of WOx on Bifunctional Pd-WOx/Al2O3 Catalysts for the Selective Hydrogenolysis of Glucose to 1,2-Propanediol

摘要

A series of Pd-WOx/Al2O3 catalysts with different contents of WOx were prepared by stepwise incipient wetness impregnations. The influence of WOx on the physicochemical properties of Pd-WOx/Al2O3 catalysts, as well as their catalytic performance for the hydrogenolysis of glucose to 1,2-propanediol (1,2-PDO), was investigated. At low surface W density (0.3-2.1 W nm(-2)), distorted isolated WOx and oligomeric WOx are present on the Pd-WOx/Al2O3 catalysts. Furthermore, isolated WO, are the dominating species on the Pd-WOx(5%)/Al2O3 catalyst. When the W density increased to 3.1 W nm(-2), polymeric WOx species are dominant on the Pd-WOx(30%)/Al2O3 catalyst. The Pd surface area decreased while the acid amount increased with increasing W density. Furthermore, increased Lewis acid sites are provided by isolated WO4 and oligomeric WOx species whereas increased Bronsted acid sites exist on polymeric WOx species. Lewis acid sites promote glucose isomerization to fructose, which is an intermediate in glucose hydrogenolysis to 1,2-PDO. Metal sites catalyze C=O hydrogenation and C-C hydrogenolysis, which avoid the coke formation on catalysts. 1,2-PDO selectivity is dependent on the synergy of Lewis acid and metal sites; however, Bronsted acid sites have no contribution to the 1,2-PDO production. Typically, the Pd-WOx(5%)/Al2O3 catalyst possessing the optimal balance of Lewis acid and the metal site shows a 1,2-PDO selectivity of 60.8% at a glucose conversion of 92.2% and has a lifetime of over 200 h.