R. van Grieken

Removal of cyanides in wastewater by supported TiO2-based photocatalysts

Abstract 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 synthesised 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.

Acid hybrid catalysts from poly(styrenesulfonic acid) grafted onto ultra-large-pore SBA-15 silica using atom transfer radical polymerization

Poly(styrenesulfonic acid)-functionalized materials based on poly(styrenesulfonic acid sodium salt) incorporated via aqueous atom transfer radical polymerization (ATRP) initiated from the surface of large-pore mesoporous SBA-15 silica support have been synthesized. The inorganic–organic nature of these hybrid materials makes them particularly desirable for acid-catalyzed reactions that require extended and hydrophobic surface areas with a narrow pore diameter distribution in the mesoporous range. Acidic hybrid materials were prepared by grafting the ATRP-initiator (3-(chlorodimethylsilyl)propyl bromoisobutyrate) on the silica surface, subsequent polymerization of the styrenesulfonic acid sodium salt monomer, and final sodium ion exchange by acid activation. Conventional and ultra-large-pore SBA-15 silica supports with nominal (BJH) pore diameter ranging from 8 to 32 nm were used for the incorporation of different polymer loadings at different polymerization times. The silylation of ATRP-initiator-functionalized SBA-15 supports has allowed a better control of the ATRP within the mesoporous structure. The use of ultra-large-pore SBA-15 supports provides a remarkable increase of the porosity which allowed us to properly allocate the polymer. The hybrid poly(styrenesulfonic acid)-modified materials showed good catalytic activities in the esterification of oleic acid with n-butanol, particularly in terms of intrinsic activity per acid site.

Surface modified amorphous titanosilicate catalysts for liquid phase epoxidation

Abstract Titanium-containing mesoporous silica has been prepared by silylation of mixed oxides obtained via sol–gel method. These samples displayed enhanced activity in the epoxidation of styrene with Bu t OOH in comparison to normal xerogels or other titanium-containing catalytic systems. The influence of the extension of the silylation reaction, as well as the use of different silylating agents, have been investigated. Samples were characterised by FTIR, XRF, 29 Si-MAS NMR, diffuse reflectance UV–Vis and N 2 adsorption–desorption isotherms, X-ray photoelectron spectroscopy (XPS) and FTIR of adsorbed deuteroacetonitrile. The improvement in the catalytic activity achieved has been related to the removal of OH groups by means of calcination or silylation treatments.

Study on the Ti and Al coincorporation into the MFI zeolitic structure

The coincorporation of Ti and Al into the MFI zeolite framework has been investigated using a method based on the wetness impregnation of amorphous Al2O3–TiO2–SiO2 solids with TPAOH solutions and subsequent crystallization under autogenous pressure. The influence of the precursor addition sequence and the conditions during the amorphous cogel formation on the properties of the zeolites obtained have been studied. The Al content in the synthesized zeolite is similar to that of the raw cogel, whereas the Ti content is lower and depends on the degree of Al incorporation in the starting material. The distribution of Al and Ti in the Al-TS-1 samples depends not only on the initial Al content but also on the procedure to prepare the raw cogel. The activity for oxidation process of the Al-TS-1 samples prepared has been tested in n-hexane oxidation. The progressive incorporation of Al into the framework increases the oxidation capacity of Ti atoms compared to TS-1 until a maximum is reached (Al/Ti molar ratio: 0.70) whereas further Al incorporation decreases the catalytic activity to even lower values than in TS-1. This effect has been assigned to changes in the hydrophilic‘vs70,21s\‘vs21,20s\/v\hydrophobic character of the catalyst caused by the interaction between Al and Ti centers, which strongly affects the affinity towards the different compounds in the reaction mixture.

Modification of chiral dimethyl tartrate through transesterification: Immobilization on POSS and enantioselectivity reversal in sharpless asymmetric epoxidation

Modification of dimethyl tartrate has been investigated through transesterification with aminoalcohols to provide reactive functionalities for the covalent bonding of chiral tartrate to polyhedral oligomeric silsesquioxanes. The transesterification of dimethyl tartrate has been widely studied using different catalytic systems and reaction conditions. Through the proper selection of both the catalytic system and the reaction conditions, it is possible to achieve monosubstituted or bis-substituted tartrate derivatives as sole products. All the intermediate chiral tartrate-derived ligands were successfully used in the homogeneous enantioselective epoxidation of allylic alcohols providing moderate enantiomeric excess over the products. Attached amine groups have been used to support the modified tartrate ligands on to a haloaryl-functionalized silsesquioxane moiety. This final chiral tartrate ligand displays reverse enantioselectivity in the asymmetric epoxidation of allylic alcohols with regard to the starting dimethyl tartrate ligand, both molecules having the same chiral sign. However, the POSS-containing ligand can be easily recovered in almost quantitative yield and reused in asymmetric epoxidation reactions. In addition, recovered silsesquioxane-pendant ligand, though displaying decreasing catalytic activity in recycling epoxidation tests, showed very stable enantioselective behavior.

Photoelectrocatalytic study and scaling up of titanium dioxide electrodes for wastewater treatment

Different TiO2 photoelectrodes have been characterized and tested for the photoelectrocatalytic oxidation of methanol. Particulate electrodes (TiO2/Ti and TiO2/ITO) have been shown to notably favour charge-carrier transfer at the electrolyte interface while a thermal electrode (Ti) has been shown to favour charge-carrier separation when applying an electric potential bias according to cyclic voltammetry technique, as a consequence of differences in TiO2 surface between particulate and thermal electrodes. Particulate electrodes lead to a higher photoelectrocatalytic activity for methanol oxidation compared to that of the thermal electrode, probably due to the pure-rutile TiO2 phase composition of the latter and its lower surface area. TiO2/Ti electrode has been shown to be the most effective photoelectrode tested for methanol oxidation since its activity was improved by the combination of the particulate TiO2 layer and the high electrical conductivity of the support. Generally, photocurrent density measured in the photoelectrochemical cell seems to correlate with activity, whereas this correlation is not observed when using a larger photoelectrocatalytic reactor. In contrast, the activity obtained for the scaled-up electrode is found to be similar in terms of surface kinetic constant to that obtained at laboratory scale.

Polystyrene modified hybrid materials based on ordered mesoporous silica

Abstract 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 longrange 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.