Latifa Bergaoui

Silane layers on silicon surfaces: mechanism of interaction, stability, and influence on protein adsorption.

In this work the mechanism of (3-aminopropyl)triethoxysilane (APTES) interaction with silicon surfaces is investigated at the molecular level. We studied the influence of experimental parameters such as time, temperature, and concentration on the quality of the APTES layer in terms of chemical properties, morphology, and stability in aqueous medium. This was achieved using a highly sensitive IR mode recently developed, grazing angle attenuated total reflection (GA-ATR). This technique provides structural information on the formed APTES layer. The topography of this layer was investigated by atomic force microscopy in aqueous medium. The hydrophilicity was also studied using contact angle measurement. Combining these techniques enables discussion of the mechanism of silane grafting. Considerable differences were observed depending on the reaction temperature, room temperature or 90 °C. The data suggest the presence of two adsorption sites with different affinities on the oxidized silicon layer. This also allows the optimal parameters to be established to obtain an ordered and stable silane layer. The adsorption of proteins on the APTES layer was achieved and monitored using in situ quartz crystal microbalance with dissipation monitoring and ex situ GA-ATR analyses.

Al-pillared saponites. Part 3.—Effect of parent clay layer charge on the intercalation–pillaring mechanism and structural properties

A series of synthetic saponites (smectite clays with tetrahedral substitution) have been intercalated by the Al13 polycation followed by pillaring (anchoring of the pillars to the layers on calcination to 500 °C).Even the clays with the highest layer charge have been successfully intercalated and pillared, but the amount of Al intercalated never exceeds one Al13 per six unit cells. This limit appears to be due to steric constraints at the interface between the intercalating solution and the delaminated clay. Furthermore, there is a competition between flocculation and Al13 intercalation: low-layer-charge saponites flocculate quickly and polycation intercalation proceeds only slowly thereafter. The ordering (followed by X-ray diffraction) and surface area of intercalated and pillared samples are well correlated with the amount of Al13 pillars in the interlayers. 29 Si NMR chemical shifts have been correlated with the layer charge: 29Si spectra undergo systematic changes for pillared samples owing to the anchoring of the pillars to the clay layers. 27 Al NMR and IR reveal that the Al13 pillars keep their basic structure on heating to 500 °C, although they lose some terminal water ligands, leaving five-coordinated Als that constitute Lewis acidic centres. Bronsted acidity is also present, owing to the remaining H2O and OH groups on the pillars, and possibly also to Si—OH—Al groups formed in the tetrahedral sheets.These observations allow us to present a schematic picture of the reactions involved in saponite clay pillaring.

Study of Pd(II) adsorption over titanate nanotubes of different diameters

Hydrogenotitanates (HNTs) nanotubes with different diameters were prepared by hydrothermal treatment of TiO(2) (P25) followed by washing with HCl aqueous solution. The prepared samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermal analysis and N(2) adsorption/desorption. In order to determine the palladium uptake ability of different HNT samples, the interaction between HNTs and Pd(II) was subsequently studied in aqueous solution at pH 9 (NH(+)(4)/NH(3) buffer). Transmission electron microscopy showed that the diameter of the nanotubes depends on the preparation conditions. Chemical analysis of residual sodium and thermal studies showed that the chemical formula of the two prepared HNT was H(x)Na(2-x)Ti(2)O(5)H(2)O with x=1.61 or 1.65. The HNTs are mesoporous materials with a multi-walled nanotubular structure and high specific surface area. In order to determine the capacity of palladium retention of different HNTs samples, the interaction between HNTs and Pd(II) was subsequently studied in aqueous solution at pH 9 (NH(+)(4)/NH(3) buffer). The adsorption kinetics of Pd(II) on the HNTs was very fast. The isotherms of Pd(II) on the HNTs showed that the adsorption occurred (1) initially through cationic exchange and (2) when the concentration of Pd(II) is high by precipitation of different Pd salts. The adsorption capacity of Pd(II) is strongly altered by the morphology of the HNTs samples.

Enzyme immobilization on silane-modified surface through short linkers: fate of interfacial phases and impact on catalytic activity.

We investigated the mechanism of enzyme immobilization on silanized surfaces through coupling agents (cross-linkers) in order to understand the role of these molecules on interfacial processes and their effect on catalytic activity. To this end, we used a model multimeric enzyme (G6PDH) and several cross-linking molecules with different chemical properties, including the nature of the end-group (-NCO, -NCS, -CHO), the connecting chain (aliphatic vs aromatic), and geometrical constraints (meta vs para-disubstituted aromatics). There did not seem to be radical differences in the mechanism of enzyme adsorption according to the linker used as judged from QCM-D, except that in the case of DIC (1,4-phenylene diisocyanate) the adsorption occurred more rapidly. In contrast, the nature of the cross-linker exerted a strong influence on the amount of enzyme immobilized as estimated from XPS, and more unexpectedly on the stability of the underlying silane layer. DIC, PDC (1,4-phenylene diisothiocyanate), or GA (glutaraldehyde) allowed successful enzyme immobilization. When the geometry of the linker was changed from 1,4-phenylene diisothiocyanate to 1,3-phenylene diisothiocyanate (MDC), the silane layer was subjected to degradation, upon enzyme adsorption, and the amount of immobilized molecules was significantly lowered. TE (terephtalaldehyde) and direct enzyme deposition without cross-linker were similar to MDC. The organization of immobilized enzymes also depended on the immobilization procedure, as different degrees of aggregation were observed by AFM. A correlation between the size of the aggregates and the catalytic properties of the enzyme was established, suggesting that aggregation may enhance the thermostability of the multimeric enzyme, probably through a compaction of the 3D structure.

Enhancement of biofuels production by means of co-pyrolysis of Posidonia oceanica (L.) and frying oil wastes: Experimental study and process modeling.

Energy recovery from lignocellulosic solid marine wastes, Posidonia oceanica wastes (POW) with slow pyrolysis responds to the growing trend of alternative energies as well as waste management. Physicochemical, thermogravimetric (TG/DTG) and spectroscopic (FTIR) characterizations of POW were performed. POW were first converted by pyrolysis at different temperatures (450°C, 500°C, 550°C and 600°C) using a fixed-bed reactor. The obtained products (bio-oil, syngas and bio char) were analyzed. Since the bio-oil yield obtained from POW pyrolysis is low (2wt.%), waste frying oil (WFO) was added as a co-substrate in order to improve of biofuels production. The co-pyrolysis gave a better yield of liquid organic fraction (37wt.%) as well as syngas (CH4,H2…) with a calorific value around 20MJ/kg. The stoichiometric models of both pyrolysis and co-pyrolysis reactions were performed according to the biomass formula: CαHβOγNδSε. The thermal kinetic decomposition of solids was validated through linearized Arrhenius model.

Acidic properties of a clay prepared from the reaction of zirconyl chloride solution containing sulfate ions with montmorillonite

Sulfated zirconium oxyhydroxide clays were prepared by adding ammonium sulfate to the intercalation solution. The main parameter controlled in this study was the SO4:Zr ratio in solution. The characterization of the catalysts, their acidic properties and catalytic activities were examined by N-2-BET adsorption, chemical analysis, adsorption-desorption of basic molecules and conversion of n-hexane and isopropanol. Two different types of SO4-Zr polycation binding were proposed and related with the acidic properties of these solids. To enhance the acidity of the sulfated zirconium oxyhydroxide clay, the SO4:Zr molar ratio must be higher than 0.125. The higher this ratio, the higher is the activity of these solids. It appears that polymerised entities are more active than individual sulfated polycations. Isopropanol dehydration to propene as well as isomerization of n-hexane seem to be related to the number and strength of Bronsted acid sites

Non-aggressive way using zirconium acetate for preparation of zirconium pillared clay developing high sulfur thermal stability over 830 °C

Pillared zirconium clay modified by sulfate constitutes a new class of materials. Nevertheless, the classical procedure consisting of impregnating these solids with sulfate solutions presents some disadvantages such as low surface area and a poor sulfur thermal stability. In a previous work, we have developed a new in situ sulfation preparation method using zirconyl chloride as zirconium precursor. However, the use of this salt gives an intercalation solution with a very low pH which can affect the clay layers. Furthermore, the addition of sulfate ions to the ZrO2 solution is limited by Hauser salt or polymeric phase precipitation. In this work, zirconium acetate solution was used in order to increase the pH of the pillaring solution and the content of sulfate ions introduced. Different preparation parameters and their effect on the structural and textural properties have been investigated. The resulting materials present a best zirconium-sulfate intercalation with higher sulfate rate and develop very high thermal stability of sulfur even at about 830 ~

Sulfated Zr-pillared saponite: preparation, properties and thermal stability

The intercalation of a saponite with zirconium oligomers containing variable amounts of sulfate has been studied (SO4:Zr molar ratio between 0 and 0.3). Well-ordered intercalated clays with basal spacing between 18 and 20were obtained. For the higher SO4:Zr ratio, a highly polymerized species is intercalated, giving a nanocomposite material with a low surface are (50 m2/g). For the lower ratios, higher surface areas are obtained (160 m2/g). After calcination of those clays, the presence of sulfate induces a loss of cristallinity but surface areas remain important. Thermal evolution of the intercalated compound was followed by mass spectroscopy showing that sulfur is not eliminated from the solids before 450°C (under helium). Above this temperature, two distinct thermal events were observed, suggesting two different modes of linking of sulfates with the polycation, in agreement with Raman data on the SO4−Zr intercalating solution.

A new method for elaborating mesoporous SiO2/montmorillonite composite materials

We report the sol–gel preparation of SiO2/montmorillonite composite materials and the investigation of the effect of the amount of clay and the TEOS concentration on the textural and structural properties of the composites. Pre-swelling of the clay with cetyltrimethyl ammonium results in solids with a larger mesoporous surface area. A decrease in the gel time and an increase in the surface area were observed upon increasing the amount of clay in the reaction medium. These porous solids showed acidic properties, and their acidities were correlated with the amount of the clay mineral. The obtained composites were functionalized by adding manganese, and their catalytic properties were evaluated in the cyclohexene oxidation reaction.Graphical Abstract

Optimization of Hydrothermal and Diluted Acid Pretreatments of Tunisian Luffa cylindrica (L.) Fibers for 2G Bioethanol Production through the Cubic Central Composite Experimental Design CCD: Response Surface Methodology

This paper opens up a new issue dealing with Luffa cylindrica (LC) lignocellulosic biomass recovery in order to produce 2G bioethanol. LC fibers are composed of three principal fractions, namely, α-cellulose (45.80%  ± 1.3), hemicelluloses (20.76%  ± 0.3), and lignins (13.15%  ± 0.6). The optimization of LC fibers hydrothermal and diluted acid pretreatments duration and temperature were achieved through the cubic central composite experimental design CCD. The pretreatments optimization was monitored via the determination of reducing sugars. Then, the 2G bioethanol process feasibility was tested by means of three successive steps, namely, LC fibers hydrothermal pretreatment performed at 96°C during 54 minutes, enzymatic saccharification carried out by means of a commercial enzyme AP2, and the alcoholic fermentation fulfilled with Saccharomyces cerevisiae. LC fibers hydrothermal pretreatment liberated 33.55 g/kg of reducing sugars. Enzymatic hydrolysis allowed achieving 59.4 g/kg of reducing sugars. The conversion yield of reducing sugar to ethanol was 88.66%. After the distillation step, concentration of ethanol was 1.58% with a volumetric yield about 70%.