Examinando por Autor "van Grieken, R."

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Direct synthesis and post-oxidation of SBA-15 and MCM-41 functionalized with butenyl groups.
(ELSEVIER, 2005) Garcia, R.A.; van Grieken, R.; Iglesias, J.; Morales, V.; Martin, J.
Olefins and derivatives are interesting compounds to anchor onto the surface of silica supports since their reactivity allow to form a large variety of organic functionalities. In this work, the synthesis of mesostructured silicas (MCM 41 and SBA 15 type materials) functionalized with butenyl groups is studied through the use of post synthetic grafting or direct synthesis procedures. Results confirm the higher functionalization degree achieved in the final mesostructured materials when using direct synthesis procedures. Unlike MCM 41 materials, which become microporous when high organic content are attached on, SBA 15 type butenyl functionalized silicas keep the features of a mesoporous solid. Post synthetic and in-situ oxidations of butenyl groups during the preparation of the mesostructured silicas have been carried out using H2O2 as oxidant, leading to materials functionalized with glycol species attached to inorganic mesoporous materials, which are attractive to be used as supports for anchoring different catalytic species.
Direct synthesis of mesoporous M-SBA-15 (M = Al, Fe, B, Cr) and application to 1-hexene oligomerization
(ELSEVIER, 2009) van Grieken, R.; Escola, J. M.; Moreno, J.; Rodríguez, R.
M-SBA-15 materials (M = Al, B, Cr, Fe) were prepared by different direct synthesis methods, characterized and tested catalytically in the 1-hexene oligomerization reaction at 125ºC. Al-SBA-15 were synthesized within the range Si/Al=12-86 using aluminium isopropoxide as aluminium source. 27Al MAS NMR spectra point out that the three Al-SBA-15 samples contain more tetrahedral aluminium than octahedral and the ratio tetrahedral / octahedral diminished with increasing aluminium contents. B-SBA-15 was prepared with a Si/B=51-106 using two direct synthesis methods (sol-gel and hydrothermal) and three different boron sources (solid boric acid, aqueous boric acid and boron isopropoxide). The best results in terms of boron incorporation (Si/B = 51) were achieved with solid boric acid and the sol-gel method, although a lower degree of mesoscopic ordering was obtained compared to the hydrothermal procedure. 11B MAS NMR showed that trigonal boron is the principal coordination state obtained after calcination. Cr-SBA-15 was also achieved by direct synthesis method at different pH (1.5, 3, 5) but the incorporation degree was low, at best Si/Cr = 240 at pH = 5. UV-vis spectroscopy indicated that all Cr species were Cr6+ formed during the calcination step. Oligomerization of 1-hexene at 125ºC showed that the highest conversion (~ 30%) was attained over Al-SBA-15 (Si/Al=30) although Cr-SBA-15 material exhibited close conversion (21%) despite its low heteroatom content (Si/Cr = 240). Dimers were the major products over Al-SBA-15, Cr-SBA-15 and Fe-SBA-15 catalysts (selectivity ¿ 40%) while strikingly, sol gel B-SBA-15 lead mostly to heavy oligomers (> 60%), with less than 10 % of dimers.
Perfluorinated nafion-modified SBA-15 materials for catalytic acylation of anisole
(ELSEVIER, 2008) Martínez, F.; Morales, G.; Martín, A.; van Grieken, R.
Mesoporous SBA-15 silica materials have been functionalized with perfluorosulfonic acidic Nafion resin using a post-synthetic impregnation method. The remarkable physicochemical properties of hexagonally ordered SBA-15 silicas (high surface area, large and narrow pore size distribution and thermal stability) make them particularly attractive for the immobilization of strongly acidic perfluorosulfonic sites. The loading of Nafion resin was varied between 10 and 20 wt%. The synthesized Nafion/SBA-15 hybrid materials were evaluated in the Friedel Crafts acylation of anisole. The optimum catalytic performance of Nafion/SBA-15 hybrid material with a resin loading around 13 wt% was compared with other perfluorosulfonic acid-based catalysts either commercially available or prepared following recipes reported in the literature. The influence of several variables on the acylation of anisole, such as the reaction temperature and the anisole/acylating agent ratio, has also been assessed by means of a factorial design of experiments. The catalytic activity of Nafion-modified SBA-15 materials is strongly affected by the reaction temperature, whereas a lesser effect was evidenced for the reaction mixture composition in the range under study. The optimal operation conditions in terms of anisole conversion were achieved at 150ºC and equimolar anisole/acetic anhydride mixture. The deposition of poly-acetylated by-products on the catalytic acid sites is mainly responsible for the catalyst deactivation, whereas the leaching of sulfonic groups and their contribution in the anisole acylation via homogeneous catalysis has been ruled out. Finally, the recovery of the catalytic activity by regeneration of the spent catalyst by refluxing in nitric acid solution was demonstrated. The characterization of the regenerated catalyst indicates that the regained activity is likely related to the displacement of the deactivating by-products out of the perflurorosulfonic acid sites during the nitric acid regeneration.
Polystyrene modified hybrid materials based on ordered mesoporous silica
(ELSEVIER, 2008) van Grieken, R.; Morales, G.; Martin, A.; Martinez, F.
Organically-modified mesoporous silica materials have been prepared by direct co-condensation of styrylethyltrimethoxysilane (STETMOS) and tetraethylorthosilicate (TEOS) in one-pot synthesis. The polimerizable nature of the styryl-containing precursor induces the formation of anchored polystyrene blocks over the silica surface, which are amenable to be functionalized with acid groups via sulfonation. The resultant organosulfonic-modified mesostructured silica materials have exhibited hexagonal long-range mesoscopic arrangement with extended surface areas and narrow mean pore size distributions. The acid capacity of these materials was assessed on the acid-catalyzed reaction test of anisole acylation with acetic anhydride.
Preparation of titanium molecular species supported on mesostructured silica by different grafting methods
(ELSEVIER, 2002) Calleja, G.; van Grieken, R.; García, R.; Melero, J.A.; Iglesias, J.
Titanium supported on SBA-15 mesoporous silica has been synthesised containing different titanium loadings prepared by chemical grafting using titanocene dichloride as precursor and over different treated silica surfaces. A comparative study using MCM-41 mesoporous silica as support is also reported. The type of silica support and its surface properties as well as the initial concentration of Ti precursor in the organic solution influences clearly the incorporation of Ti species. The materials after grafting treatment were characterized by different conventional techniques including XRD, FT-IR DR UV-Vis and nitrogen adsorption. The titanium containing SBA-15 silica shows hexagonal mesoscopic order and pore sizes up to 70 Å with surface areas up to 600 m2/g and titanium content ranging from 1 to 3 wt. %. DR UV-Vis of Ti containing SBA-15 silica after removal of the organic ligand shows the presence of Ti isolated species tetrahedrally coordinated and the absence of bulky TiO2 phases. Likewise, these materials upon calcination were catalytically active for the epoxidation of styrene with TBHP exhibiting a significant selectivity toward the epoxide.
Removal of cyanides in wastewater by supported TiO2-based photocatalysts
(ELSEVIER, 2002) Aguado, José; van Grieken, R.; López Muñoz, M.J.; Marugán, J.
Titania supported samples on different types of silica have been prepared through a sol-gel method followed by hydrothermal processing. The photocatalytic activity of the samples was tested for free cyanides photo-oxidation. As compared to commercial TiO2 all the synthesized materials showed not only similar photocatalytic efficiencies but improved recovery properties. The degradation of iron (III) cyanocomplexes was also studied in the absence or presence of titania catalyst. In all cases, a photoinduced CN¿ released from the complex takes place through a homogeneous process. The further oxidation of those cyanides ions to cyanate species is significantly enhanced in the presence of the catalyst in which mesostructured SBA-15 silica is used as support.
Synthesis and characterization of SBA-15 materials functionalized with olefinic groups and subsequent modification through oxidation procedures
(ELSEVIER, 2010) van Grieken, R.; Iglesias, J.; Morales, V.; García, R.A.
The synthesis of alkenyl-SBA-15 materials has been carried out through different methodologies. The direct-synthesis method used for the functionalization of SBA-15 involving the co-condensation of the alkenyl-silicon precursor together with silicon alkoxides is clearly superior than any other post-synthetic pathway, even if acid catalysts are used to drive the post-synthetic grafting of olefin groups. The direct-synthesis methodology has been deeply studied within this work, involving the influence of the silica precursor prehydrolysis time, and the loading of different olefin-silanes on the quality of the mesostructure. Small olefinic functionalities, like vinyl groups, can be incorporated in a very efficient manner up to achieve organic loadings as high as 50% mole of the olefin precursor to TEOS. The modification of these alkenyl groups through oxidation can be accomplished through post-synthetic oxidation of the olefinic group or even through an in-situ methodology where the oxidant ¿ hydrogen peroxide ¿ is added together with the rest of the precursor into the synthesis gel to lead complete oxidation of the double bonds towards glycolic functionalities. The resultant materials show potential as hydrophilic adsorbents and catalytic supports.
Synthesis of size-controlled silica-supported TiO2 photocatalysts
(ELSEVIER, 2002) van Grieken, R.; Aguado, José; Lopez-Muñoz, M.J.; Marugan, J.
Supported nanocrystalline titanium dioxide has been prepared by a sol-gel method through the use of mesoporous silica materials. The work has been addressed to study the effect of the different type of silica support on the mean titania particle size and their effect in the photocatalytic activity. The prepared samples have been characterised by powder X-ray diffraction, nitrogen adsorption, UV-vis spectroscopy, EDS, and TEM and their photocatalytic activity has been checked for cyanide oxidation in liquid phase. It is observed a correlation between the textural properties of the chosen silica support and the diameter of the obtained titania particles. The use of mesostructured SBA-15 silica support with a pore diameter around 7 nm has shown to be an effective method to obtain photocatalysts with controlled TiO2 particle size even at high semiconductor contents (up to 60% wt).