Catalytic Hydrodehydration of Cyclohexanone,Hydrogenation of 2-Cyclohexen-1-one,and Dehydrogenation of Cyclohexene over a Mo Chloride Cluster with an Octahedral Metal Framework

A silica gel-supported molybdenum halide cluster, (H₃O)₂(Mo₆Cl₈)Cl₆]·6H₂O (1), developed selective catalytic activity for the condensation of cyclohexanone to cyclohexylbenzene and cyclohex-1-enylbenzene, when it was allowed to react in a stream of helium at 300°C. Halide clusters of Nb, Ta, and W of the same metal framework supported on SiO₂ also catalyzed the condensation at 400°C. However, at 400°C, 1 catalyzed disproportionation, and selectivity increased with increasing temperature, yielding cyclohexene and its dehydrogenation products, 1,3-cyclohexadiene and benzene, and 2-cyclohexen-1-one and its dehydrogenation product, phenol. When the same reaction was performed in a stream of hydrogen above 400°C, hydrodehydration proceeded almost exclusively, producing cyclohexene and its dehydrogenation products. 2-Cyclohexen-1-one was hydrogenated to cyclohexanone under the same reaction conditions utilizing hydrogen molecules, whereas cyclohexene was dehydrogenated via 1,3-cyclohexadiene to benzene. The active site developed on 1 was assumed to be a molybdenum atom, which becomes isoelectronic with the platinum metals by accepting two or more electrons from the ligands.