María José Torralvo

Effect of the template and functional monomer on the textural properties of molecularly imprinted polymers.

Molecularly imprinted polymers (MIPs) for zearalenone analysis have been synthesized using the template mimics cyclododecyl 2,4-dihydroxybenzoate (CDHB), resorcinol and resorcylic acid. The MIPs are photochemically prepared from 2-(diethylamino)ethyl methacrylate (2-DAEM), 4-vinylpyridine (VIPY), 2-hydroxyethyl methacrylate (HEMA) or 1-allylpiperazine (1-ALPP) as the functional monomers, trimethylolpropane trimethacrylate (TRIM) as cross-linker, azobis(isobutyronitrile) as initiator and acetonitrile as porogen. Non-imprinted polymers have been also synthesized for reference purposes. The textural properties of the novel polymers (BET areas, pore volumes and pore size distributions) have been determined from nitrogen adsorption-desorption isotherms. These parameters have shown to be strongly dependent on the presence of the template and the monomer nature. Scanning electron microscopy and solvent uptake experiments support these findings. Microporosity contributes less than 7% to the total pore volume for all the polymers prepared. Interestingly, a 3.5 nm pore opening is observed for all the polymers and additional pore apertures in the 20-40 nm region for VIPY-, HEMA- and 2-DAEM-based MIPs whereas a much wider opening size distribution has been measured for the 1-ALPP-based MIP. Molecular modeling and, particularly, (1)H NMR experiments demonstrate the strong (2:1) complex formed between 1-ALPP and the diphenolic CDHB (K(11)=4.7 x 10(4)M(-1) and K(12) = 2.6 x 10(2)M(-1) in acetonitrile) that make the corresponding MIP the most suitable for zearalenone recognition in real samples.

Macroporous silica and titania obtained using poly[styrene-co-(2-hydroxyethyl methacrylate)] as template

Ordered macroporous silica and titania have been prepared using poly[styrene-co-(2-hydroxyethyl methacrylate)] (PS-HEMA) latex arrays as templates. Polystyrene (PS) particles have also been used to template macroporous titania. Arrays of spherical latex particles, with average diameters of 260 nm (PS-HEMA) and 390 nm (PS), have been obtained by either evaporation of the solvent at room temperature or by deposition on filtration membranes. In both cases, 3D extended regions with cubic close packing of microspheres are observed, but in the samples obtained after evaporation of the solvent the self-assembled particles show a higher degree of order. The oxides were synthesized by hydrolysis of sodium silicate or titanium isopropoxide (diluted in n-propanol or hexane) which were infused into the cavities of the latex arrays. Two different methods have been used to fill the cavities with the precursor: filtration with suction and capillary forces. When solidification of the ceramic matrix took place, the template was eliminated by heating at 500 °C. As replica of the latex assemblies, periodic porous silica and titania were obtained. Sheets of amorphous silica, as thin as 4–7 nm, form the wall of the interconnected pores whose diameters range from 200 to 220 nm. In macroporous titania obtained from titanium isopropoxide diluted in n-propanol and imprinted on PS-HEMA particles, the pore diameters range from 160–180 nm. The porous network is shaped by walls that are 8–12 nm thick and are formed from anatase nanocrystals with a major dimension of 8–20 nm. Pore diameters, crystallite sizes and thicknesses are similar when hexane is used as the solvent of the inorganic precursor. However, in macroporous titania imprinted on PS particles, nanocrystals with a major dimension of 6–10 nm form the pore wall with a thickness of 6 to 8 nm. Interactions between the titania precursor and the latex particles seem to affect the microstructural parameters. The porous skeleton of titania is preserved after treatment at 600 °C for 16 hours but the pore wall thickness slightly increases with crystallite growth and in some particles disordered areas appear.

Micro/nano-structural properties of imprinted macroporous titania and zirconia

Some factors affecting micro-/nano-structural parameters of imprinted titania and zirconia have been analysed. Latex particles with different compositions were selected to be used as templates in order to investigate the effect of template–inorganic precursor interaction on the microstructural properties of the oxides. Titanium isopropoxide or zirconium n-propoxide diluted in n-propanol or hexane were used as precursors of titania and zirconia respectively. Samples have been prepared with different alkoxide/solvent ratios and in some cases hydrochloric acid was employed as catalyst. The chemical composition of the template can affect the wall structure, wall thickness and structural contraction. A large crystal wall structure is observed in titania imprinted on poly[styrene-co-(2-hydroxyethylmethacrylate)] and poly[styrene-co-acrylic acid], but when imprinted on polystyrene, a fibre-like wall structure, thinner pore wall and lower structural contraction, are obtained. Chemical interaction between the latex particles and the metal alkoxide seem to be less important for zirconium n-propoxide than for titanium isopropoxide. Therefore, zirconia imprinted on polystyrene, poly[styrene-co-(2-hydroxyethylmethacrylate)] and poly[styrene-co-acrylic acid] shows a similar fibre-like wall structure. Pore walls made of small and homogeneous crystallites have been observed in zirconia obtained using hexane as solvent, probably due to the stability of alkoxy bridges that leads to a low hydrolysis rate. Small crystals building a thin and continuous pore wall are also formed when hydrochloric acid is added to the precursor solution and the hydrolysis takes place in ambient conditions. These microstructural features can be understood as due to both the easy protonation and the hindered condensation process under the experimental conditions used.

Supercritical fluid deposition of Ru nanoparticles into SiO2 SBA-15 as a sustainable method to prepare selective hydrogenation catalysts

Ru nanoparticles were successfully deposited onto mesoporous SiO2 SBA-15 using supercritical CO2 (scCO2). The use of scCO2 favoured the metal dispersion and Ru nanoparticles uniformly distributed throughout the support were obtained. Different precursors and methodologies were employed: impregnation with Ru(tmhd)2(COD) in scCO2 at 80 °C and 13.5 and 19.3 MPa and further reduction in H2/N2 at 400 °C at low pressure, reactive deposition of Ru(tmhd)2(COD) with H2 in scCO2 at 150 °C and reactive deposition of RuCl3·xH2O with ethanol in scCO2 at 150 and 200 °C. The size of the particles was limited in one dimension by the pore size of the support. The metal loading varied with the methodology and experimental conditions from 0.9 to 7.4% Ru mol. These materials exhibited remarkable catalytic activity. The Ru/SiO2 SBA-15 materials prepared by reactive deposition with H2 in scCO2 were selective catalysts for the hydrogenation reactions of benzene and limonene, allowing the production of partly hydrogenated hydrocarbons that may serve as building blocks for more complex chemicals. scCO2 is shown to be a green solvent that allows the preparation of efficient heterogeneous catalysts to design sustainable processes. Furthermore, in the hydrogenation of limonene, scCO2 was also used as the solvent.

Textural evolution of zirconia and yttria-doped zirconia on thermal treatment under different conditions

Abstract Pure zirconia and yttria-doped zirconia samples have been prepared from the corresponding microporous gels under different thermal treatment conditions. Doped materials show a delay in the high symmetry (cubic or tetragonal) to low symmetry (monoclinic) phase transformation, compared to pure ZrO 2 . However, as in pure zirconia, thermal treatment leads to the textural degradation of doped samples. Ageing of gels at 723 K in air yields mesoporous powders. On increasing the yttria content in the sample, adsorption capacity and mesopore surface area decrease and the most probable pore radii are slightly smaller. Differences observed in the porous texture of pure and doped zirconia could be justified in terms of the different packing of the primary particles in the gels. In both pure and doped gels, the evolution of the porous structure is highly affected by the conditions at which thermal dehydration is carried out. Under vacuum, the development of a mesoporous texture is hindered. Condensation and elimination of water produce the most important reduction of the micropore volume.

Textural characterization of zirconia and yttria-doped zirconia powders

Abstract Pure zirconia and yttria-doped zirconia powders were obtained by precipitation from zirconium and yttrium salt solutions and then treated at 723 K in air. The evolution of the texture and structure has been studied as a function of the treatment time of the powders. After 2 h of treatment, doped samples correspond to the cubic form and pure zirconia to the tetragonal form. With increase in the time of treatment, the transformation to phases with lower symmetry (cubic→tetragonal→monoclinic) occurs but it is delayed as the yttria content in the sample is increased. However, a degradation of the porous texture promoted by thermal treatment is observed in both pure and doped samples, and the textural stability is not improved by addition of yttria. The elimination of volatile species from the starting gels must be responsible for the differences observed in the textural evolution.