Silica Zirconia Mixed Oxides Research Articles

Sol-gel immobilization of lipase enzyme on FTO modified with silica-zirconia and gold nanoparticles films for design of triglyceride impedimetric biosensor

A novel electrochemical sensing platform was created using the formation of a sequence of films, over a conductive fluorine-doped tin oxide glass (FTO), obtained using the sol-gel method and the dip coating technique. Firstly, thin films of silica-zirconia mixed oxides were deposited, in sequence an ionic silsesquioxane film stabilizing and controlling the size of gold nanoparticles (6.5 ± 2.4 nm) was settled, and after the lipase enzyme, obtained from Candida rugosa, was entrapped into a silica film maintaining its operational stability. The films were characterized by scanning electron microscopy (SEM) with EDX, UV–Vis spectroscopy and X-ray photoelectron spectroscopy (XPS). The thickness of each film on the FTO glass was evaluated by optical profilometry. The presence of the enzyme on the platform was confirmed by cyclic voltammetry with the ρ-nitrophenyl palmitate method. This platform was applied successfully as a biosensor for tributyrin (TB) determination by electrochemical impedance spectroscopy EIS, showing low limit of detection (LD) of 1.86 μmol L−1 and high sensitivity of 5.37 μΩ μmol−1 L. The biosensor presented a low KMapp value of 22.69 μmol L− 1 and the Vmaxapp value of 85.57 μmol L−1 min−1 suggesting that the enzyme, immobilized with this method retained activity promoting a fast enzymatic reaction with the TB.

Gelation of amino acid-based amphiphiles in water-based mixed solvent systems: reusable catalytic templates for nanostructured silica and silica–zirconia photocatalyst

We describe a gelation study of a series of different fatty acid–amino acid conjugated amphiphiles in various mixed solvent systems with water. Field emission scanning electron microscopy (FESEM), polarized optical microscopy (POM) and X-ray diffraction (XRD) study reveals that the self-organization of the amphiphile molecules in the mixed solvent leads to the formation of crystalline fibers, which form the opaque white gel. This amino acid amphiphile-based gel acts as a catalyst as well as a template for the hydrolysis/condensation of tetraethoxysilane (TEOS) to silica and forms a composite gel. The methanol extraction of the opaque white as-prepared composite gel results in the formation of a transparent nanostructured silica gel. The recovered amphiphilic gelator, after a methanol wash, can be reused for preparing a gel, which can subsequently be used as a catalyst to prepare a nanostructured silica gel again. The FESEM study confirms that the formed nanostructured silica gel is made up of spherical silica nanoparticles. We also extend this gel-based catalysis strategy to prepare silica–zirconia mixed oxide nanostructures. TEM examination reveals the formation of spherical silica–zirconia nanoparticle of high surface area as confirmed through BET surface area measurement. Finally, the photocatalytic activity of silica–zirconia mixed oxide is investigated towards methylene blue degradation. The mixed oxide shows higher photocatalytic efficiency than neat zirconia nanostructures.

MoO3/SiO2-ZrO2 Catalyst: Eeffect of Calcination Temperature on Physico-chemical Properties and Activities in Nitration of Toluene

12 wt % molybdena was deposited over 1:1 silica zirconia mixed oxide support and the resultant catalyst was calcined between the 500 to 700 oC range of temperature. The samples were characterized by XRD, FT-IR, BET, SEM, NH3-TPD and pyridine adsorbed FT-IR techniques. Nitration of toluene was studied as a model reaction over the prepared catalysts and parameters like effect of reaction temperature, effect of various solvents, catalyst reusability are studied. It was found that conversion of toluene varies with the presence of Brönsted acid sites over the catalyst surface and para-nitrotoulene selectivity is associated with pore size of the catalyst. Over the same catalysts, nitration was extended for some other aromatics. Avoid of sulfuric acid in the present process is an interesting concern in view of green chemistry. © 2012 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0)

Aminolysis of Styrene Oxide Over Heterogeneous Acidic Catalysts

Silica alumina and silica zirconia mixed oxides are shown to be effective and regioselective catalysts for the aminolysis of styrene oxide under very mild experimental conditions, giving the corresponding primary β-amino-alcohol in good to excellent yield.

Preparation of Cu +/SiO 2-ZrO 2 catalysts for the oxidative carbonylation of methanol to dimethyl carbonate

Silica-zirconia mixed oxide (SiO 2-ZrO 2) was synthesized by sol-gel method; with SiO 2-ZrO 2 as the support, CuCl 2/SiO 2-ZrO 2 catalyst was then prepared by ion-exchange and used in the liquid oxidative carbonylation of methanol to dimethyl carbonate (DMC). The results showed that the introduction of zirconium species into the amorphous silica leads to the formation of Si-O-Zr bonds and Brønsted acid sites. When the CuCl 2/SiO 2-ZrO 2 catalyst is calcined at 300–550°C in an inert atmosphere, CuCl 2 is dispersed and bounded on the surface of SiO 2-ZrO 2 support; CuCl 2 can be auto-reduced to CuCl, which is then ion-exchanged with the surface Brönsted acid sites of the SiO 2-ZrO 2 support to form Cu + species. As a result, most of copper remains to be highly dispersed on the surface of SiO 2-ZrO 2 and only a small amount of chlorine is left over. The CuCl 2/SiO 2-ZrO 2 catalyst calcined at 450°C exhibits excellent activity in the oxidative carbonylation of methanol; under the conditions of 140°C, 2.4 MPa and a CO/O 2 ratio of 2, the conversion of methanol and the selectivity to DMC reach 10.0% and 79.4%, respectively.

Alkyl chain grafting on silica–zirconia mixed oxides: preparation and characterization

Silica–zirconia mixed oxide substrates were prepared and grafted with C18 alkyl chains using n-octadecyltrichlorosilane. Si : Zr molar ratios in the mixed oxide samples before and after alkyl chain modification were determined by X-ray photoelectron spectroscopy (XPS). Nitrogen sorption measurements provided the porous features of these materials. By means of solid-state 29Si nuclear magnetic resonance (NMR) the amounts of the silanol groups present in the as-prepared, humidified and grafted mixed oxide systems as well as the degree of cross-linking of the silanes were obtained. 13C NMR spectroscopy was used to examine the conformational order of the n-octadecyl chains grafted on the silica–zirconia mixed oxide substrates. Similar information was available from variable temperature Fourier transform infrared (FTIR) investigations. Scanning electron microscopy (SEM) measurements and energy dispersive X-ray (EDX) analyses yielded the morphological features of the solid substrates and information about the distribution of the alkyl chains on the oxide surfaces. The annealing temperature for the preparation of the mixed oxides and the elemental composition of the samples, i.e. relative amount of the oxide components, were found to determine the surface properties and in turn the alkyl chain assembly on the mixed oxide surfaces. FTIR and solid-state 13C NMR data gave evidence for an unusual high conformational order of the grafted C18 alkyl chains in all samples, irrespective of the low surface coverage, which exceeds comparable C18 and C30 grafted silica and metal oxide systems. This finding was attributed to the formation of island structures with aggregated octadecyl chains of high conformational order due to strong intermolecular interactions. Differences in the conformational order of the various samples most likely arise from vertical polymerization which varies with the actual sample.

Combined use of XAFS, XRD and TEM to unravel the microstructural evolution of nanostructured ZrO2–SiO2 binary oxides: from nanometres down to the molecular domain

In this paper, the detailed study of the microstructural evolution under annealing of zirconium-based inorganic–organic hybrid materials to give silica–zirconia mixed oxides was addressed by X-ray absorption fine structure (XAFS) spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The silica materials embedding different amounts of ZrO2 nanoparticles were prepared by copolymerisation of the organically modified oxozirconium cluster (Zr4O2(OMc)12 (OMc = methacrylate)) with methacryloxypropyltrimethoxysilane (MAPTMS). By the free radical copolymerisation of the oxoclusters bearing 12 methacrylate groups with the methacrylate-functionalised siloxanes, a stable anchoring of the clusters to the silica network was achieved. The thermal treatment of these hybrids at high (≥500 °C) temperatures yielded the SiO2–ZrO2 mixed oxides. The microstructural evolution upon heating was studied at increasing temperatures, namely 500, 600, 700, 900, 1000 and 1300 °C. Furthermore, different samples characterised by different Zr : Si atomic ratios and annealed at 1000 °C were comparatively analysed to study the effect of the composition on the evolution of the hybrids to give the mixed oxides. In a third experiment, samples characterised by the same composition were annealed at the same temperature by using either a conventional muffle or a microwave oven in order to evidence whether the different processing could also affect the microstructural features of the final oxide materials. Through XRD and XAFS it was demonstrated that at temperatures above 800 °C, crystallisation of tetragonal zirconia occurs in the samples of high zirconium concentration treated in muffle, whereas amorphous oxide materials form upon annealing in microwave oven. The presence of zirconia nanoclusters having an average size of 5–10 nm was evidenced by TEM.

Advanced Drug Delivery Vectors with Tailored Surface Properties Made of Mesoporous Binary Oxides Submicronic Spheres

The synthesis of mesoporous silica-zirconia mixed oxides that can be used as efficient drug delivery carriers for bisphosphonates based drugs has been performed via spray-drying process. These silica-zirconia mixed oxides used for the first time as drug delivery systems exhibit mesostructures with tunable acidity. The two selected drugs were alendronate and zoledronate, which carry similar phosphonate heads but different amine tails in terms of acidity and hydrophobicity. The incorporation of different zirconium amounts into the mesostructured silica network creates Lewis and Bronsted acid surface sites allowing to control the surface properties of the mesoporous matrix and therefore the amount of phosphonate adsorbed into the mesopores. This effect occurs even at low ZrO2 contents and produces a noticeable modification on the release behavior of bisphosphonates. The partial retention of the drug through phosphonates complexation by zirconium sites leads to a more sustained release of the drug. These bina...

Silica–zirconia mixed oxide samples by an hybrid materials based innovative preparation procedure: Influence of preparation procedure and composition on active sites

Abstract In this work, the interaction of amorphous silica–zirconia mixed oxide samples obtained from inorganic–organic silica-based hybrid materials with pyridine and CO2 was studied to investigate their acid/base character. Several silica–zirconia mixed oxide powders characterized by different [Zr/Si] atomic ratios were prepared and treated at increasing temperatures both in a conventional muffle and with microwave technology. The powder samples were characterized with Diffuse Reflectance Infrared Fourier Transform (DRIFT) and X-ray Photoelectron (XP) spectroscopies. The surface acidic and basic active sites were investigated (with DRIFT spectroscopy) by chemisorbing probe molecules (pyridine, carbon dioxide). The obtained results revealed the presence of both Lewis and Bronsted acidic sites on the amorphous silica–zirconia mixed oxide powder surfaces. Several acidic sites characterized by different strength were observed; the acidic sites distribution is markedly influenced by the sample composition and by the heat treatment: more numerous acidic sites form on the surface of the samples treated with microwaves with respect to the muffle treated ones; the increment of the temperature and the decrease of the zirconium content cause a significant decrement of the acidic sites. No basic sites were revealed.

Kinetics of reduction of supported nanoparticles of iron oxide

A series of catalysts prepared by dispersing iron oxide on supports of different nature and acidity has been studied. Silica (S) and silica-zirconia mixed oxides (SZ) with different ZrO2 content (from 5 to 45 mass%) were used as supports for the iron oxide phase which was deposited over them by an equilibrium-adsorption method. The red-ox properties of the Fe-catalysts were studied by temperature programmed reduction (TPR) technique. The two well defined and narrow TPR peaks observed could be associated with the reduction steps: I) Fe2O3→FeO and Fe2O3→Fe(0) (at ca. 400°C) and II) FeO→Fe(0) (at ca. 800–900°C). The temperature of the second-step-peak increased with the zirconia content in the support, likely because of the stronger interaction of the iron oxide phase with the support. Activation parameters for the two step-reduction processes were obtained by a simple computation procedure applied to the TPR profiles.

Synthesis of amino-functionalized mesoporous silica–zirconia mixed oxide using sodium silicate and zirconium carbonate complex

Amino-functionalized mesostructured silica–zirconia mixed oxide has been synthesized through a very convenient one step synthesis route using water soluble sodium silicate, zirconium(IV) carbonate complex and 3-aminopropyltriethoxysilane in the presence of anionic surfactant sodium dodecyl sulphate under basic condition, exhibiting excellent adsorption properties towards arsenate ions. Moderately high surface area has been achieved using Si/Zr ratio 2.06 and 10 mol% 3-aminopropyltriethoxysilane with respect to the amount of silica. The presence of mixed oxide framework is clearly visible from the peak shift of Si–O–Si stretching frequency in FTIR spectra. The S BET and pore volume of the composite was found to be 420 m 2 g −1 and 0.4396 ml g −1 respectively with uniform and narrow pore size distribution centered at 42.8 Å. The mesoporous composite showed good absorption properties towards arsenate anion and anionic dye molecules.

FTIR and reaction studies of the acylation of anisole with acetic anhydride over supported HPA catalysts

The behaviours of two heteropoly acid (HPA) catalysts, one supported on a commercial silica and the other on a silica–zirconia mixed oxide, were compared in the acylation of anisole with acetic anhydride. Both supported catalysts and the latter support, but not the silica, were active for the reaction, although all underwent a deactivation process before achieving complete conversion of anisole to p-methoxyacetophenone. The greatest yield to p-methoxyacetophenone was obtained for HPA/SiO 2 while the extent of deactivation was in the order HPA/SiO 2–ZrO 2 > SiO 2–ZrO 2 > HPA/SiO 2. Although acetate species accumulated on the SiO 2–ZrO 2 and HPA/SiO 2–ZrO 2 catalysts during the reaction, these species are not thought to play a role in the deactivation process. The relative extent of deactivation was consistent with the calculated ratios of adsorption coefficients which determine the strength of adsorption of product to that of anisole and indicate that product inhibition is greatest in the case of HPA/SiO 2–ZrO 2. Calculated absorption coefficients for pyridine adsorbed on the samples failed to distinguish between the nature of the HPA species dispersed on the two different supports.

Addition of zirconia in Ni/SiO 2 catalyst for improvement of steam resistance

Nickel catalysts supported on silica–zirconia mixed oxides were prepared by homogeneous precipitation in sol–gel-derived wet silica gel. Their structural properties and catalytic performance in steam-reforming of methane were investigated from the viewpoint of steam resistance. Ni/SiO 2 without zirconia readily loses its catalytic activity during the reaction because the coarsening of silica that occurs in the presence of high-temperature steam hinders the active surface of Ni. The addition of zirconia drastically increases the steam resistance of silica. Ni/SiO 2-ZrO 2 catalyst shows steady activity in steam-reforming of methane without any changes in pore structure.

Modification of the Acid Properties of Silica–Zirconia Aerogels by in Situ and ex Situ Sulfation

Sulfated silica–zirconia mixed oxides containing 33 mol% Zr were prepared by sol-gel routes with the sulfate/zirconia molar ratio varying between 0.2 : 1 and 0.3 : 1. The influence of the added sulfate introduced by both in situ and ex situ methods on the bulk and surface properties was examined and in particular the role of sulfate in modifying the acid site density. Both silica and sulfate ( in situ) were well distributed throughout the solid, thus delaying the onset of crystallization of zirconia phases. Ex situ sulfuric acid treatment resulted in extraction of a proportion of the zirconia to the surface to form small amorphous zirconia clusters, thus increasing Lewis acidity at the expense of Brønsted acid sites. As the sulfate concentration was increased beyond an estimated 2.82 SO 4 2− per square nanometer of exposed zirconia, this Lewis acidity was replaced by Brønsted acid sites. The maximum number of Brønsted acid sites which could be created by sulfate treatment was only marginally greater than the site density on a nonsulfated sample, although the site strength was much greater for the former. The distribution of components and in particular the role played by segregated, amorphous zirconia at the surface is discussed in terms of the modification of both Lewis and Brønsted acid sites.

On Determination of Acid Site Densities on Sulfated Oxides

Combined infrared and gravimetric studies have been performed of pyridine adsorption on sulfated and non-sulfated silica-zirconia mixed oxides in order to quantify the site densities of both Bronsted and Lewis acid sites. The values determined for the integrated molar absorption coefficients for the non-sulfated silica-zirconia mixed oxides were 1.24 cm μmol-1 at 1540 cm-1 and 1.56 cm μmol-1 at 1450 cm-1 and were independent of the Si:Zr ratio and other pre-treatment and preparation variables. On the other hand, values ranging from 0.64 to 1.66 cm μmol-1 at 1540 cm-1 for the pyridinium ion and from 1.00 to 2.08 cm μmol-1 for the Lewis-bound pyridine were determined for the sulfated samples. It is proposed that the reduction in absorption coefficient for the pyridinium ion with increasing sulfate density corresponds with the transformation of monomeric to dimeric or polymeric sulfate species which bring two adjacent adsorbed pyridine molecules into close proximity. The findings of this study raise considerable concern over the validity of acid site densities determined for sulfated materials where calculations have been performed based on literature values for the absorption coefficients.

Influence of Si/Zr ratio on the formation of surface acidity in silica-zirconia aerogels

A series of silica-zirconia mixed oxides were prepared by sol-gel routes wherethe zirconia content was varied from 9 to 75 mol%. The bulk and surface properties were analysed and compared with those of a sample wherezirconia was deposited as an external layer on a silica surface thereby avoiding internal mixing of the component oxides. Spin-lattice relaxation measurements made by 29 Si MAS NMR provide a useful estimation of the relative size of domains containing silica only and thus the degree of mixing. High levels of silica distribution throughout the mixed oxide prevented crystallisation of zirconia phases even at high Zr:Si ratios. The extent of mixing at the surface and throughout the bulk is discussed in relation to the generation of both Lewis and Bronsted acid sites. Increased levels of mixing and heteroatom linkages throughout the bulk did not necessarily result in an increase in the density of surface Bronsted acid sitesgenerated.

Surface and bulk properties of silica–zirconia mixed-oxides: acid vs base catalysed condensation

The influence of acid and alkali catalysed condensation on the nature of silica–zirconia mixed oxides derived from sol–gel preparations has been compared. The bulk and surface properties have been characterised and where appropriate compared with results for a sample of Aerosil 200 where an equivalent zirconia loading was deposited by precipitation. Results indicate that the sol–gel preparations generate solids with similar surface areas, but only alkali catalysed condensation leads to solids with large mesopore volumes. Both solids give similar acid site densities although solids prepared by alkali catalysed condensation contained a higher proportion of strong Brønsted acid sites. An increased acid site density was not created by increasing the degree of mixing of the two components.

Synthesis, structure, and properties of poly(dimethylsiloxane) networks reinforced by in situ‐precipitated silica–titania, silica–zirconia, and silica–alumina mixed oxides

AbstractSeveral novel fillers, specifically silica–titania, silica–zirconia, and silica–alumina mixed oxides, have been successfully precipitated into poly(dimethylsiloxane) (PDMS) networks using a sol‐gel approach. The resulting filled networks were found to have very good mechanical properties. In comparison with networks filled only with silica, these materials had good extensibilities as well as high strengths. Filler particle diameters were generally several hundred angstroms, but were found to decrease with increase in crosslink density of the networks. The distributions of particle size were relatively narrow, and there was very little particle aggregation. The presence of in situ silica–titania and silica–zirconia mixed oxides can also increase the onset temperature for degradation of the PDMS. © 1995 John Wiley & Sons, Inc.

Inorganic‐organic composites containing mixed‐oxide phases

AbstractSeveral novel fillers, specifically silica‐titania, silica‐zirconia and silica‐alumina mixed oxides, have been successfully precipitated into poly(dimethylsiloxane) (PDMS) networks using a sol‐gel approach. The resulting filled networks were found to have very good mechanical properties. In comparison with networks filled only with silica, these materials had good extensibilities as well as high strengths. Filler particle diameters were generally several hundred Angstroms, but were found to decrease with increase in crosslink density of the networks. The distributions of particle size were relatively narrow, and there was very little particle aggregation. The presence of in situ silica‐titania and silicazirconia mixed oxides can also increase the onset temperature for degradation of the PDMS. Swelling equilibrium measurements indicated good bonding between these mixed‐oxide fillers and the PDMS. The presence of the PDMS was found to shorten gelation times in these systems, and to improve the impact strengths of the composites.