Swelling and delamination behaviors of birnessite-type manganese oxide by intercalation of tetraalkylammonium ions

Intercalations of tetramethylammonium (TMA), tetraethylammonium (TEA), tetrapropylammonium (TPA), and tetrabutylammonium (TBA) ions into layered birnessite-type manganese oxide, BirMO(H), were studied in aqueous tetraalkylammonium hydroxide solutions with different concentrations. Expansions of the interlayer were not observed clearly in the TEA, TPA, or TEA systems. Stable colloidal suspensions were obtained in the TMA system at TA(1)/H-s (molar ratio of tetraalkylammonium ions over exchangeable protons in BirMO(H) larger than 1. XRD analyses for these colloidal sediments in the wet state showed a short-range swelling by the TMA(+) intercalation, with an increase in the basal spacing (d(bs)) from 0.73 to 1.56 nm. The basal spacing decreased stepwise from 1.56 through 1.27 to 0.96 nm with a decrease of the relative humidity during drying. The chemical analysis results showed that the cation intercalation progressed by an ion exchange mechanism. The intrinsic selectivity coefficient for intercalation had a tendency to decrease with an increase in the ionic radius of guest cations, probably owing to the larger steric hindrance for larger ions. The intercalation of TBA(+) ions could be achieved by a two-step reaction involving treatment with (TMA)OH followed by (TBA)OH. The basal spacing increased from 1.56 to 2.19 nm due to the TBA(+) intercalation. No long-range swelling was clearly observed. However, delamination took place and the layered structure collapsed when TMA(+)-intercalated samples obtained at TA(1)/H-s = 25 were washed with distilled water. Delamination was also observed in the washing of the TBA(+)-intercalated sample. Freeze-drying of the washed slurry gave a flakelike manganese oxide, which supports the finding of delamination in wet conditions. Upon air-drying, the reassembling of the manganese oxide sheet progressed and the amorphous phase reverted to the layered structure with basal spacing of 0.96 nm. The origins of short-range swelling and the delamination observed in the BirMO(H)-TMA(+) system are discussed in terms of attractive and repulsive forces by electrostatic interaction, hydration of interlayer cations, and interlayer hydrogen bonding.