Mechanisms of selective cleavage of C-O bonds in di-aryl ethers in aqueous phase

A route for cleaving the C-O aryl ether bonds of p-substituted H-, CH3-, and OH- diphenyl ethers has been explored over Ni/SiO2 catalyst at very mild conditions (393 K, 0.6 MPa). The C-O bond of diphenyl ether is cleaved by parallel hydrogenolysis and hydrolysis (hydrogenolysis combined with HO* addition) on Ni. The rates as a function of Hy pressure from 0 to 10 MPa indicate that the rate-determining step is the C-O bond cleavage on Ni surface. H* atoms compete with the organic reactant for adsorption leading to a maximum in the rate with increasing Hy pressure. In contrast to diphenyl ether, hydrogenolysis is the exclusive route for cleaving a C-O bond of di-p-tolyl ether to form p-cresol and toluene. 4,4'-Dihydroxydiphenyl ether undergoes sequential surface hydrogenolysis, first to phenol and OC6H4OH* (adsorbed), which is then cleaved to phenol (C6H4OH* with added H*) and H2O (O* with two added H*) in a second step. Density function theory supports the operation of this pathway. Notably, addition of H* to OC6H4- OH* is less faliorable than a further hydrogenolytic C-O bond cleavage. The TOFs of three diaryl ethers with Ni/SiO2 in water follow the order 4,4'-dihydroxydiphenyl ether (69 mol mol(Ni surf)(-1) h(-1)) > diphenyl ether (26 mol mol(Ni surf)(-1) h(-1)) > di-p-tolyl ether (1.3 mol mol(Ni surf)(-1) h(-1)), in line with the increasing apparent activation energies, ranging from 4,4'-dihydroxydiphenyl et er (93 kJ mo1-1)< diphenyl ether (98 kJ mo1-1)< di-p-tolyl ether (105 kJ mol(-1)). (C) 2013 Elsevier Inc. All rights reserved.