Direct synthesis and the morphological control of highly ordered two-dimensional p6mm mesoporous niobium silicates with high niobium content

Here we demonstrate the preparation of highly ordered two-dimensional P6mm mesoporous niobium silicates with unprecedented loadings of niobium in the silica framework (up to n_Si/n_Nb ratio of 6.1) and highly winding morphologies maintaining regularly aligned arrays of nanochannels through a simple adjustment of the molar water to hydrochloric acid ratio (n_H2O/n_HCl) in the synthesis gel using nonionic surfactant as a template in a highly acidic medium. X-ray diffraction (XRD), N₂ adsorption, high-resolution scanning electron microscopy (HRSEM), energy-dispersive spectrometry (EDS), and high-resolution transmission electron microscopy (HRTEM) results provided strong evidence of the presence of highly ordered two-dimensional structure in the NbSBA-15 materials with a very high content of niobium and winding morphologies, whereas UV-visible diffuse reflectance spectroscopy (UV-vis DRS) confirmed the presence of tetrahedrally coordinated niobium in the silica framework. Moreover, the loading of Nb in the silica framework can be controlled by a simple adjustment of the n _H2O/n_HCl and the amount of Nb source in the synthesis gel. Thus, a hitherto unachieved loading of niobium in the silica framework with a controlled manner, considered by many researchers to be impossible to achieve for the highly ordered mesoporous SBA-15 materials because of the highly acidic synthesis condition and the framework connectivity constraints, has been reported. We also found that the Nb incorporation could be a way to control the morphlogy of the NbSBA-15 materials. The morphology of the materials changed from rod to semispherical to "winding road" shape with decreasing the n_Si/n_Nb from 22.5 to 6.1. Moreover, the detailed mechanism on the morphological control as a function of Nb content has also been proposed. This simple one-step technique to control the morphology, which would be of general interest for the application requiring the materials with different morphology, and Nb loading in a highly acidic medium synthesis have not been reported so far.