Catalytic Hydrolysis of 4-Nitrophenyl Phosphate by Lanthanum(III)-Hectorite

Abstract

The hydrolysis of 4-nitrophenyl phosphate (NPP), to produce 4-nitrophenol and inorganic phosphate, is catalyzed by an aqueous suspension of La3+ ion-bound hectorite clay (La3+-hectorite) at 50 °C. Sodium exchanged hectorite (Na+-hectorite) does not promote the hydrolysis of NPP under the same reaction conditions suggesting that La3+ ions are actively involved in the catalysis. Initial rates of the reaction are dependent upon the amount of clay present but independent of NPP concentrations in the range of 0.5−2.5 mM, indicating that these conditions produce a steady state. Reactions run over an extended period of time demonstrate that the rate drops off dramatically after a period of about 5 h, suggesting inhibition of the reaction by phosphate that is produced during hydrolysis. This inhibition is confirmed by experiments in which phosphate ion is present at the start of the reaction. Extended reaction times also indicate the catalytic nature of the reaction whereby 1.5 turnovers of substrate (relative to the theoretical number of moles of La3+ ions present) are accomplished after 9 days. A bell-shaped plot of pH versus initial rate demonstrates that the reaction is pH dependent. Initial rates rise from a pH of 5.5 to a maximum at pH 7.5 and then drop off again at pH values greater than 8.0. This plot fits to a theoretical model in which two La3+-bound water molecules are sequentially deprotonated, the first leading to the active catalyst, and the second producing an inactive species. A catalytic mechanism for the reaction is presented (for pH 7.5), on the basis of experimental results presented herein, in which hectorite-bound La3+ ions play an active role.