Selective Dehydration of Alkanediols into Unsaturated Alcohols over Rare Earth Oxide Catalysts

Syntheses of unsaturated alcohols in the vapor-phase catalytic dehydration of alkanediols over rare earth oxides are reviewed. CeO 2 effectively catalyzes the dehydration of 1,3-butanediol to produce 3-buten-2-ol and trans -2-buten-1-ol. Heavy rare earth oxides such as Er 2 O 3, Yb 2 O 3, and Lu 2 O 3 selectively catalyze the dehydration of 1,4-butanediol to produce 3-buten-1-ol. In the dehydration of 1,5-pentanediol, Yb 2 O 3, Lu 2 O 3, and Sc 0.5 Yb 1.5 O 3 catalysts efficiently work to produce 4-penten-1-ol. The active and selective oxides are composed of large particles with well-crystallized fluorite or bixbyite structure. Small oxide particles with poor crystallinity decrease the selectivity to unsaturated alcohols because of their dehydrogenation ability. In the reactions of different alkanediols, the reactivity of alkanediol depends on the length between the OH groups as well as on the geometry of the catalyst surface, which is affected by the distance between rare earth cations. For example, over CeO 2, the reactivity order of the alkanediols is 1,3-butanediol > 1,4-butanediol > 1,5-pentanediol > 1,6-hexanediol. Quantum calculations support a probable reaction mechanism: OH groups and the H of the position-2 methylene group of 1,3-butanediol are interacted with the surface Ce 4+ to form a tridentate coordination, and the abstraction of the position-2 H by Ce 4+ is the initial step of 1,3-butanediol dehydration in the formation of unsaturated alcohols.