Selective Synthesis of Lamellar Titania with Carboxylate Precursor and Characterization by Solid-State NMR

摘要

A new means of synthesizing titania mesostructures with ammonium titanyl carboxylates in basic aqueous solution is presented. With a cetyltrimethylammonium bromide (CTAB) ionic template, a lamellar titania was synthesized from an ammonium titanyl oxalate Ti-precursor, and a hexagonally ordered mesoporous titania was synthesized from an ammonium titanyl citrate Ti-precursor. The molecular structure and dynamics of cetyltrimethylammonium (CTA) cations between the lamellar titania layers were studied by solid-state NMR. ~(13)C SP MAS, CP MAS, WISE, and DRSE spectra provided the structural and molecular dynamics information on the CTA cations between the titania layers: (1) bent conformations near the head group, (2) rotation of the methylene carbons in trans conformation about the cetyl chain axes, (3) common rapid methyl group rotation, and (4) interdigitated structures in an antiparallel fashion. The cetyl chains in CTAB are interdigitated as well; however, they are all in trans conformation without any bent. Another difference is that the CTA cations in CTAB have overall lesser mobility than those between the titania layers. The possibility was discussed that the oxalates exist as ligands to the surface Ti of the lamellar titania layers and that the CTA cations interact with the oxalates.