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 H2 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 H2 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 OC6H4OH* is less favorable 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 image > diphenyl ether image > di-p-tolyl ether image, in line with the increasing apparent activation energies, ranging from 4,4′-dihydroxydiphenyl ether (93 kJ mol−1) < diphenyl ether (98 kJ mol−1) < di-p-tolyl ether (105 kJ mol−1).