A novel route for cleaving C-O bond of three p-substituted diphenyl ethers (H-, CH3-, and OH-) has been developed over Ni/SiO2 catalysts in the aqueous phase at mild conditions (393 K, 6 bar H2). The C-O bond of diphenyl ether is cleaved by the parallel hydrogenolysis and hydrolysis routes on the Ni site. In contrast with diphenyl ether, the C-O bond in di-p-tolyl ether is exclusively cleaved by hydrogenolysis to p-cresol and toluene. 4,4'-Dihydroxydiphenyl ether is catalytically cleaved via a two-step sequential hydrogenolysis on Ni surface, that is, it is firstly cleaved to phenol (C6H5O� added with H�) and HOC6H4O� (adsorbed), and in turn HOC6H4O� (adsorbed) is further cleaved to phenol (C6H5O� added with H�) and water (O� added with 2H�) in the second step. The critical HOC6H4O� fragment that is neutralized to hydroquinone in the batch kinetics study causes the discrepancy from true in situ surface reaction to the observation of static state kinetics. In the aqueous phase TOF of three aryl ethers with Ni/SiO2 follows the order: 4,4'-dihydroxydiphenyl ether (69 h-1) > diphenyl ether (26 h-1) > di-p-tolyl ether (1.3 h-1), which agrees well with the sequence of apparent activation energy, that is, 4,4'-dihydroxydiphenyl ether (93 kJ�mol-1) < diphenyl ether (98 kJ�mol-1) < di-p-tolyl ether (105 kJ�mol-1).

Mechanisms of selective cleavage of aryl ether bond in the lignin-derived dimers in the aqueous phase

246th National Meeting (2013)

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