Catalytic transformations of cyclohexanol on Group VIII metal catalysts

Abstract

Transformations of cyclohexanol and cyclohexanone have been studied over various Group VIII metals as catalysts. For each metal the predominant reaction of cyclohexanol was dehydrogenation to cyclohexanone. Two main groups of metals can be distinguished. Selectively dehydrogenating metals are those where dehydrogenation stops at the stage of cyclohexanone (Os, Co, Fe, Re, Ru). Aromatizing metals catalyze also the further dehydrogenation of cyclohexanone to aromatics (Pd, Pt, Ni). Rh and Ir occupy an intermediate position: they dehydrogenate in nitrogen and aromatize in hydrogen. Radiotracer methods show that cyclohexanone is the intermediate of phenol formation, except for Pt and Pd where there is a “direct” route of phenol formation from cyclohexanol. Benzene is the product of the hydrogenolytic splitting of the phenolic OH group. Dehydration of cyclohexanol to cyclohexene is not important, although it occurs over some dehydrogenating metals. Ru is the only metal where there is considerable additional formation of benzene via cyclohexene. Hydrogenolysis of the alcoholic OH group was not observed. Hydrogenolysis of the CC bond of the ring is favoured by hydrogen carrier gas; it is considerable over dehydrogenating metals as well as Rh and Ir. The enhanced reactivity of cyclohexanol as compared with cyclohexane is due to the presence of the OH group facilitating the interaction of the molecule with the surface. Knor's model of localized/free-electron interplay as well as the number of unpaired d electrons could be used to interpret the different activity of various metals.