Structural Stability of Si–C Bonds in Periodic Mesoporous Thiophene-Silicas Prepared under Acidic Conditions

Periodic mesoporous thiophene-silicas with hexagonal (<i>p</i>6<i>mm</i>) symmetry were synthesized using a 2,5-bis­(triethoxysilyl)­thiophene (BTET) precursor in the presence of Pluronic P123 (EO₂₀PO₇₀EO₂₀) and PLGE (EO₁₇(L₂₈G₇)­EO₁₇) triblock copolymers at different acidic conditions. P123-templated mesoporous thiophene-silicas with <i>p</i>6<i>mm</i> ordered structure were prepared in the presence of hydrochloric acid and iron­(III) chloride hexahydrate used as acid catalysts. However, it was found that a relatively large fraction of the Si–C bonds in thiophene-bridging groups were decomposed during the synthesis process. On the other hand, thiophene-silicas synthesized at lower acidic conditions were disordered and nonporous structures. In contrast, PLGE-templated thiophene-silicas with <i>p</i>6<i>mm</i> ordered mesostructure were prepared using copper­(II) perchlorate hexahydrate and boric acid as well as hydrochloric acid. Importantly, up to 97.3% of the Si–C bonds in mesoporous thiophene-silica prepared in the presence of boric acid were retained. Solid state ²⁹Si MAS NMR clearly showed that the structural stability of the Si–C bond is dependent on the acidity and time of the initial self-assembly stage. Also, the thermal stability of the thiophene-bridging groups was shown to be dependent on the acidity of the synthesis gel

Mn-Doped Ordered Mesoporous Ceria–Silica Composites and Their Catalytic Properties toward Biofuel Production

A series of Mn-doped ordered mesoporous ceria–silica composites was synthesized by using hexa­decyl­tri­methyl­ammonium bromide as a soft template under ammonia basic conditions. After template removal, Mn-doped mesoporous ceria–silica composites were thoroughly characterized by small-angle X-ray scattering, which revealed highly ordered 2D-hexagonal (<i>p</i>6<i>m</i>) framework. Other techniques such as wide-angle X-ray diffraction, N₂ adsorption, scanning and transmission electron microscopic analysis with energy-dispersive spectrometry mapping, inductive coupled plasma atomic emission spectrophotometry, ultraviolet–visible spectrometry, and ²⁹Si cross-polarization magic-angle spinning NMR were used to show the effect of Mn incorporation into the ceria–silica composites. The electronic states of the surface Mn and Ce species were also investigated by X-ray photoelectron spectroscopy. These Mn-doped mesoporous ceria–silica composites were successfully reduced under a flowing H₂/N₂ gas mixture at an elevated temperature without noticeable changes in their hexagonally ordered mesostructures. These materials were applied as heterogeneous and reusable catalysts for transesterification of different esters, such as methyl benzoate and ethyl cyanoacetate, in the presence of various alcohols, such as <i>n</i>-octanol and <i>n</i>-butanol, to produce biodiesels under solventless mild conditions

Tailoring Pore Size, Structure, and Morphology of Hierarchical Mesoporous Silica Using Diblock and Pentablock Copolymer Templates

Mesoporous materials of tailored pore size, structure, and morphology are of interests for a wide range of applications. It is important to develop synthetic methods that will allow for easy processing and facile structure modification. Here, we present the preparation of hierarchically structured bimodal mesoporous silicas using water-soluble poly­(lactic acid-<i>co</i>-glycolic acid)-<i>b</i>-poly­(ethylene oxide) (PLGA-<i>b</i>-PEO) diblock copolymer and poly­(lactic acid-<i>co</i>-glycolic acid)-<i>b</i>-poly­(ethylene oxide)-<i>b</i>-poly­(propylene oxide)-<i>b</i>-poly­(ethylene oxide)-<i>b</i>-poly­(lactic acid-<i>co</i>-glycolic acid) (PLGA-<i>b</i>-PEO-<i>b</i>-PPO-<i>b</i>-PEO-<i>b</i>-PLGA) pentablock copolymers as templates. The block copolymers were synthesized through a step-growth polymerization method using a commercial Pluronic F68 macroinitiator. Mesoporous silica samples were obtained by sol–gel chemistry in acidic aqueous solutions. Hexagonally (<i>p</i>6<i>mm</i>) ordered mesoporous silica particles were obtained in the presence of a PLGA-PEO diblock copolymer and exhibited bimodal pore size distributions in the range of 2–9 nm. Core–shell type mesoporous silica particles were obtained in the presence of the PLGA-PEO-PPO-PEO-PLGA pentablock copolymer and exhibited a large pore diameter up to 20 nm with distinct bimodal pore size distributions. The pore size increased when using a longer pentablock copolymer template in strong acid. The physicochemical properties were investigated using small-angle X-ray scattering (SAXS), nitrogen adsorption–desorption, transmission electron microscope (TEM), solid-state ²⁹Si nuclear magnetic resonance (NMR), and scanning electron microscope (SEM), respectively