Competitive oxidation and reduction of aliphatic alcohols over (wo3)3 clusters

The reactions of C1-C4 aliphatic alcohols over (WO ₃) ₃ clusters were studied experimentally and theoretically using temperature-programmed desorption, infrared reflection-absorption spectroscopy, and density functional theory. The results reveal that all C1-C4 aliphatic alcohols readily react with (WO ₃) ₃ clusters by heterolytic cleavage of the RO-H bond to give alkoxy (RO-) bound to W(VI) centers and a proton (H ⁺) attached to the terminal oxygen atom of a tungstyl group (WdO). Two protons adsorbed onto the cluster readily react with the doubly bonded oxygen to from a water molecule that desorbs at 200-300 K and the alkoxy that undergoes decomposition at higher temperatures into the corresponding alkene, aldehyde, and/or ether. Our theory predicts that all three channels proceed over the W(VI) Lewis acid sites with energy barriers of 30-40 kcal/mol, where dehydration is favored over the others. We also present further analysis of the yield and reaction temperature as a function of the alkyl substituents and discuss the origin of the reaction selectivity among the three reaction channels.