Layer-by-layer grafting of titanium phosphate onto mesoporous silica SBA-15 surfaces: synthesis, characterization, and applications

Metal phosphates have many applications in catalysis, separation, and proton conduction, but their small surface areas and/or constrained pore structures limit their utilization. Here, we report two new methods for the liquid-phase grafting of titanium phosphate onto mesoporous silica (SBA-15) surfaces: (l) alternate grafting of Ti(OPrⁱ)₄ and then POCl₃ and (2) one-pot grafting of titanium phosphate formed in situ by employing Ti(OPrⁱ)₄ (a base) and POCl₃ (an acid) as an appropriate "acid-base pair". Both the size of mesopores and the content of titanium phosphate can be changed by increasing the number of modification cycles in a stepwise (or layer-by-layer) fashion. The obtained products were characterized by inductively coupled plasma optical emission spectroscopy, X-ray diffraction, N₂ adsorptiondesorption, transmission electron microscopy, ³¹P and ²⁹Si magic-angle spinning NMR, and NH₃ temperatureprogrammed desorption, and their performance in acid catalysis and metal ion adsorption was investigated. This work provides new methodologies for the general synthesis of supported metal phosphates with large surface areas, ordered nanoporous structures, and acid properties.