Cleaving of C-O bonds of substituted phenols in aqueous phase has been explored with mono- and dual- functional catalysts containing Ni and HZSM-5. Conventionally the aromatic ring of phenol is hydrogenated in the first step and the C-O bond of the resulting cyclohexanol is subsequently dehydrated. The overall rate of phenol hydrodeoxygenation increases with the acid concentration irrespective of the concentration of accessible Ni atoms. This suggests at first sight that hydrogenation of the aromatic ring is quasi-equilibrated leaving the acid site catalyzed dehydration as the kinetically relevant step. The situation is, however, more complex, as Ni also appears to participate in the C-O bond cleavage. For catechol, the higher activity  induced by the two hydroxy groups causes hydrogenation of the aromatic ring and the hydrogenolysis of the phenolic �OH group to take place in parallel with a ratio of 4:1. The saturated cyclohexane-1,2-diol can be further dehydrated over HZSM-5 or hydrogenolysed on Ni to realize full hydrodeoxygenation. The presence of proximal Br�nsted acid and metal sites increases the activity of Ni in the presence of H2, and the effect is stronger for hydrogenation than for hydrogenolysis. However, not only the metal sites are influenced by the zeolite, but also the acid catalyzed reactions are enhanced by the presence of the metal. Both pathways of 2-methoxycyclohexanol from guaiacol reacting either via hydrogenolysis by Ni to cyclohexanol or via acid catalyzed demethoxylation and dehydration with subsequent hydrogenation of the intermediately formed olefin are enhanced by the synergy. Thus, in general, hydrogenation and hydrogenolysis on Ni as well as hydrolysis and dehydration on acid sites of HZSM-5 are synergistically enhanced by the proximity of both sites.

Cooperative action of metal and acid sites for C-O bonds cleavage

247th National Meeting (2014)

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