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-, CH ₃-, 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 (C ₆H₄OH^* 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 69molmolNi Surf-1h ^-1 > diphenyl ether 26molmolNi Surf-1h^-1 > di-p-tolyl ether 1.3molmolNi Surf-1h^-1, 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).