Synthesis and physicochemical characterization of a novel bismuth silicate templated by tetrapropylammonium bromide and polydiallyldimethylammonium chloride

Abstract

A bismuth silicate (BiSi-1) was hydrothermally synthesized using tetrapropylammonium bromide (TPA·Br) as the organic structure-directing agent and shown by multi-scale characterization to be distinct from known Bi–silicates. Powder X-ray diffraction is indexable with an orthorhombic metric (a = 23.234 Å, b = 17.109 Å, c = 3.897 Å), consistent with a highly anisotropic, possibly layered framework. High-resolution TEM/SAED reveals nanocrystalline, plate-like domains assembled into sub-micrometric aggregates with locally oriented lamellae; lattice fringes (0.27–0.32 nm) match the strongest XRD spacings. Solid-state NMR establishes a silica-rich network with a dominant Q4 population (77%) and minor Q3 (11%) and Q2 (12%) sites; the contact-time dependence of 1H to 29Si cross-polarization is consistent with increasing proximal-proton density from Q4 to Q2. Aging to 24 h sharpens the 29Si lineshape, while calcination progressively removes the OSDA and vicinal hydroxyls; at 750 °C, 29Si spectra indicate framework densification/rearrangement. XANES/EXAFS places bismuth predominantly as Bi3+ in an oxide-like environment with a pronounced Bi–O first shell and no detectable Bi⁰ or Bi–Br contributions. ICP–OES yields a reproducible Bi/Si atomic ratio of 1:3. Thermogravimetry shows stepwise desorption, dehydroxylation, and template removal, with thermal stability maintained to 750 °C. Nitrogen sorption confirms mesoporosity in the as-made and Soxhlet-extracted solids (the latter exhibiting the highest surface area), whereas high-temperature calcination reduces porosity. Collectively, BiSi-1 emerges as a nanocrystalline, anisotropic Bi–silicate whose connectivity, local Bi–O environment, and accessible texture are tunable by aging and post-treatments, positioning it as a promising platform for heterogeneous catalysis and environmental remediation.