Hydrothermal and Postsynthesis Surface Modification of Cubic, MCM-48, and Ultralarge Pore SBA-15 Mesoporous Silica with Titanium

Abstract

We describe the introduction of titanium centers to cubic MCM-48 and SBA-15 mesoporous silica by hydrothermal and postsynthetic grafting techniques. MCM-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 phase was chosen due to its high surface area (1100-1300 m 2 /g) and its three-dimensional, bicontinuous pore array. SBA-15, synthesized with a block copolymer template under acidic conditions, has a surface area from 600 to 900 m 2 /g and an average pore diameter of 69 Å, compared to 24-27 Å for MCM-48. Alkoxide precursors of titanium were used to prepare samples of Ti-MCM-48 and Ti-SBA-15. We have detailed the bulk and molecular structure of both the silica framework and the local bonding environment of the titanium ions within each matrix. X-ray powder diffraction and nitrogen adsorption shows the pore structure is maintained despite some shrinkage of the pore diameter at high Ti loadings by grafting methods. UV-visible and Raman spectroscopy indicate that grafting produces the least amount of Ti-O-Ti bonds and instead favors isolated tetrahedral and octahedral titanium centers. High-resolution photoacoustic FTIR spectra demonstrated the presence of intermediate range order within the silicate walls of MCM-48, established the consumption of surface silanols to form Si-O-Ti bonds by grafting, and resolved the characteristic IR absorbance at 960 cm -1 , occurring in titanium 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.

Visible light absorption of binuclear TiOCoII charge-transfer unit assembled in mesoporous silica

“…We anticipate preferential reaction of the precursor with TiOH groups because of the higher acidity of titanol compared to silanol groups as indicated by a red shift of the OH stretch mode. 24,25 This may result in a higher proportion of bimetallic sites relative to isolated Co II centers than predicted by the relative abundance of TiOH and SiOH groups. Infrared spectra of the as- In order to investigate the possibility of a hydroxo rather than an oxo bridge between the anchored Ti and Co centers, a TiCo II -MCM-41 material calcined at 350 o C was exposed to 3 Torr pyridine gas.…”

Post-synthesis of TiO2 Dispersed Inside the Pore Channels of SBA-15 and its Photocatalytic Activity for the Degradation of Methylene Blue

“…Recently, direct synthesis of Ti-substituted SBA-15 with structural integrity under hydrothermal conditions in the absence of acid has also been studied using titanium tetrachloride and tetraethyl orthosilicate as the titanium and silica sources, respectively [9]. In addition, several strategies for the post-synthesis have been used to prepare Ti-containing mesoporous silicas, such as molecular dispersion [10,11], grafting [12][13][14] and impregnation [15][16][17]. These post-synthesis methods, in general, incorporate titanium species both in the pore channel and on the external surface of mesoporous silicas.…”

A zinc oxide-coated nanoporous carbon adsorbent for lead removal from water: Optimization, equilibrium modeling, and kinetics studies

“…In other words, q m is the maximum adsorption capacity and b is the adsorption affinity onto the adsorption sites and it is related to energy of adsorption (Morey et al 2000). Also, the Freundlich model is given by Eq.…”