Anion-Controlled Cation-Exchange Process: Intercalating α-Titanium Phosphate through Direct Ion Exchange with Alkylammonium Salts.

Abstract

Several alkylammonium salts were used in the study of α-titanium phosphate (α-TiP) intercalation chemistry. The characterization results demonstrated that the expected intercalation by direct ion exchange could be successfully achieved without any addition of an extra amine substance. Our findings are different from the current opinion that by the ion-exchange method, without the assistance of bases, large cations are difficult to exchange into the narrow interlayer space of α-tetravalent metal phosphate directly because of the small interlayer distance. Studies found that alkylammonium cations, for example, n-butylammonium cation, could be directly exchanged into the interlayer space merely by choosing salts with appropriate anions such as phosphate, phosphite, sulfite, citrate, and malate ions. In the case of phosphates, besides n-butylammonium, the exchange of n-hexylammonium, cyclohexylammonium, and pyridinium with interlayer protons was investigated and successfully accomplished as well. The uptake values for these four cations were 0.420, 0.595, 0.571, and 0.335 g/g, respectively. A mechanism study revealed that although the relevant exchange reaction seemed only to involve the proton of α-TiP and the alkylammonium cation of the salt, the strength of the conjugate acid of the anion from the salt-the counterion-was proven to be the key factor in this process.