Ms. Morey et al., Hydrothermal and postsynthesis surface modification of cubic, MCM-48, and ultralarge pore SBA-15 mesoporous silica with titanium, CHEM MATER, 12(4), 2000, pp. 898-911
We describe the introduction of titanium centers to cubic MCM-48 and SBA-15
mesoporous silica by hydrothermal and postsynthetic grafting techniques. M
CM-48 was hydrothermally prepared with a gemini surfactant that favors the
cubic phase and leads to a high degree of long-range pore ordering. This ph
ase was chosen due to its high surface area (1100-1300 m(2)/g) and its thre
e-dimensional, bicontinuous pore array, SBA-15, synthesized with a block co
polymer template under acidic conditions, has a surface area from 600 to 90
0 m(2)/g and an average pore diameter of 69 Angstrom, compared to 24-27 Ang
strom for MCM-48, Alkoxide precursors of titanium were used to prepare samp
les of Ti-MCM-48 and Ti-SBA-15, We have detailed the bulk and molecular str
ucture of both the silica framework and the local bonding environment of th
e titanium ions within each matrix. X-ray powder diffraction and nitrogen a
dsorption shows the pore structure is maintained despite some shrinkage of
the pore diameter at high Ti loadings by grafting methods, UV-visible and R
aman spectroscopy indicate that grafting produces the least amount of Ti-O-
Ti bonds and instead favors isolated tetrahedral and octahedral titanium ce
nters. High-resolution photoacoustic FTIR spectra demonstrated the presence
of intermediate range order within the silicate walls of MCM-48, establish
ed the consumption of surface silanols to form Si-O-Ti bonds by grafting, a
nd resolved the characteristic IR absorbance at 960 cm(-1), occurring in ti
tanium silicates, into two components. All three spectroscopic techniques,
including in situ Raman, reveal the reactive intermediates formed when the
materials are contacted with hydrogen peroxide.