Maguy Jaber

Framework Stabilization of Ge-Rich Zeolites via Postsynthesis Alumination

The use of organic structure directing agents in zeolite syntheses has dramatically extended the number of zeolite structure types during the past decades. However, for about 20% of all known zeolite structure types, the necessary postsynthesis elimination of organic templates by high-temperature combustion leads to structure collapse, where the particularly strongly affected are Ge-rich zeolites. Here, we present a treatment approach that leads to zeolite structure stabilization by postsynthetic isomorphous substitution of Al for Ge. An important advantage of this new method is that no preliminary elimination of the organic structure directing agent from zeolite pores is required; thus it can be applied to microporous materials that cannot withstand the high temperature combustion of organic templates. The experimental data unambiguously show that besides framework stabilization the postsynthesis treatment facilitates incorporation of active sites in the zeolite framework. The feasibility of this new approach is corroborated by alumination of a BEC-type material. The presented method is expected to broaden the practical utilization of many microporous materials by improving their thermal stability.

Structure, Orientation and Stability of Lysozyme Confined in layered Materials

The confinement of lysozyme in 3 layered materials based on montmorillonite and lamellar double hydroxides exhibiting different surface charges was studied. The protein structure and orientation in these materials were determined by X-ray diffraction, time resolved fluorescence and fluorescence anisotropy. For montmorillonite exchanged with sodium and modified with a non-ionic surfactant (tri-ethylene glycol mono n-decyl ether), the lysozyme was found to be located in the interlayer space with the “end-on” and “side-on” orientations, respectively. Conversely, no lysozyme intercalation was observed with a lamellar double hydroxide modified with an anionic surfactant (sodium octylsulfate), since the protein was adsorbed on the surface of the particles. Fourier transformed infrared spectroscopy analysis shows that lysozyme confinement in the interlayer space preserves its structure after dehydration, whereas some structural changes were observed for lysozyme adsorbed on the particle surface.

A comparative study of the catalysis of peptide bond formation by oxide surfaces

It is well-known that amino acids deposited on some inorganic oxides undergo peptidic condensation. It is seldom realised however that a large diversity of behaviours can be observed in such systems. Here we use the apparently simple case of glycine-non-porous silica as a reference system, in which glycine (Gly) dimerisation to diketopiperazine (DKP) is easy to evidence, especially when using TG in combination with NMR. We then proceed to compare it with other AA deposited on the same support on the one hand, with Gly deposited on other mineral surfaces on the other hand. In a final section, we provide more detailed mechanistic information on the glycine condensation process on silica, including kinetic data and a (13)C solid-state NMR follow up of the species at various stages of thermal condensation. The best mechanism to rationalise these data involves a crucial step of isomerisation from zwitterion to neutral glycine, and the participation of several distinct types of surface sites probably consisting of silanol ensembles.

New range of Al–Mg organoclays with tailored hydrophobicity: incorporation of fluoride as a local probe to study the octahedral character

Abstract A series of inorganic–organic hybrids with a phyllosilicate-like structure were prepared using a single-step templating sol–gel procedure. The synthesis involves reaction of magnesium nitrate and aluminum acetylacetonate with octyltriethoxysilane, to give products where saponite is the inorganic parent. These hybrids were characterized by X-ray diffraction, solid-state 29 Si, 13 C and 27 Al nuclear magnetic resonance, infrared spectroscopy and thermogravimetry. The results showed that the hybrid materials exhibit lamellar structures, similar to those found in natural inorganic silicate saponite. The influence of the substitution of magnesium and silicon by aluminum was evident from 29 Si and 13 C NMR studies. A high degree of condensation was obtained. The hydrophobic character was evaluated by wettability measurements. These materials hold promise as sorbents for solid-phase extraction of organic pollutant molecules, due to their highly hydrophobic character.

Effect of Nontronite Smectite Clay on the Chemical Evolution of Several Organic Molecules under Simulated Martian Surface Ultraviolet Radiation Conditions

Most of the phyllosilicates detected at the surface of Mars today are probably remnants of ancient environments that sustained long-term bodies of liquid water at the surface or subsurface and were possibly favorable for the emergence of life. Consequently, phyllosilicates have become the main mineral target in the search for organics on Mars. But are phyllosilicates efficient at preserving organic molecules under current environmental conditions at the surface of Mars? We monitored the qualitative and quantitative evolutions of glycine, urea, and adenine in interaction with the Fe(3+)-smectite clay nontronite, one of the most abundant phyllosilicates present at the surface of Mars, under simulated martian surface ultraviolet light (190-400 nm), mean temperature (218 ± 2 K), and pressure (6 ± 1 mbar) in a laboratory simulation setup. We tested organic-rich samples that were representative of the evaporation of a small, warm pond of liquid water containing a high concentration of organics. For each molecule, we observed how the nontronite influences its quantum efficiency of photodecomposition and the nature of its solid evolution products. The results reveal a pronounced photoprotective effect of nontronite on the evolution of glycine and adenine; their efficiencies of photodecomposition were reduced by a factor of 5 when mixed at a concentration of 2.6 × 10(-2) mol of molecules per gram of nontronite. Moreover, when the amount of nontronite in the sample of glycine was increased by a factor of 2, the gain of photoprotection was multiplied by a factor of 5. This indicates that the photoprotection provided by the nontronite is not a purely mechanical shielding effect but is also due to stabilizing interactions. No new evolution product was firmly identified, but the results obtained with urea suggest a particular reactivity in the presence of nontronite, leading to an increase of its dissociation rate.

Laponite and hybrid surfactant/laponite particles processed as spheres by spray-drying

Laponite elementary disc crystals dispersed in water in the presence or absence of non-ionic Brij58 surfactant have been used as nanobuilding blocks to form laponite and laponite/surfactant spherical particles by spray-drying. The size distribution of the spherical spray-dried particles ranges from 70 to 1000 nm (average size about 220 nm). Although raw laponite (RL) was almost totally exfoliated in water, the spray-dried spherical particles were found partially delaminated and stable when spread into water. Based on a full characterization by several techniques such as dynamic light scattering, scanning and transmission electron microscopies, X-ray diffraction, thermal analyses, nitrogen volumetric sorption and solid-state multi-nuclear NMR studies, a mechanism has been proposed to explain the formation of the spherical micron size particles: a face-to-face and edge-to-edge stacking of the elementary disc crystals leads to the formation of few-layer laponite platelets that aggregate within the restricted volume of water droplets during the drying stage. In the presence of Brij58, elementary disc crystals are covered by surfactant molecules adsorbed on their surface. As a consequence, surfactant molecules fill the interlayer space to form spherical Brij58/laponite nanocomposite particles. These spray-dried laponite and Brij58/laponite spherical particles present a high potential for sorption applications.

Clays and Clay Minerals as Layered Nanofillers for (Bio)Polymers

This introductory chapter presents the most relevant structural, physical, and chemical properties of clay minerals for the formation of nanocomposites with polymers. The general principles of silicates classification are outlined in order to better understand the structures of the various types of clay minerals as phyllosilicates. Cation exchange capacity (CEC), surface area, porosity, and rheological properties of clay minerals are briefly discussed. The physico-chemical properties of clay mineral layers, including the reactivity at the edges surfaces, are introduced together with their consequences for the various mechanisms of clay-polymer interactions. The chapter closes on a brief presentation of synthetic clay minerals and a general introduction to clay polymer nanocomposites.

Formation of Activated Biomolecules by Condensation on Mineral Surfaces – A Comparison of Peptide Bond Formation and Phosphate Condensation

Many studies have reported condensation reactions of prebiotic molecules, such as the formation of peptide bonds between amino acids, to occur to some degree on mineral surfaces. We have studied several such reactions on the same divided silica. When drying steps are applied, the equilibria of peptide formation from glycine, and polyphosphate formation from monophosphate, are displaced to the right because these reactions are dehydrating condensations, accompanied by the emission of water. In contrast, the equilibrium of AMP dismutation is not significantly favored by drying. The silica surface plays little role (if any) in the thermochemistry of the condensation reactions, but is does play a significant kinetic role by acting as a catalyst, lowering the condensation temperatures with respect to bulk solids. Of course, the surface also catalyzes the inverse hydrolysis reactions.

Stabilization of ribofuranose by a mineral surface.

The existence of the ribose moiety in biomolecules poses two problems for prebiotic chemistry. First, the exclusive presence of the furanose isomer in RNA has to be accounted for since furanose is a minor form in solution and does not exist in crystals. Second, all D-ribose polymorphs are unstable in aqueous medium so that a stabilization mechanism has to be invoked. We observed that the adsorption on mineral surfaces as amorphous silica protects the sugar from degradation processes. Moreover, this silica surface, used as realistic chert model, is able to increase significantly the proportion of ribofuranose compared to ribopyranose forms. The interaction between surface and sugar was analyzed by 13C NMR. Our results show a very significant chemical and thermal stabilization of the adsorbed sugar by a silica surface and an almost twofold increase of ribofuranose compared to ribose in solution.

Structural studies of adsorbed protein (betalactoglobulin) on natural clay (montmorillonite)

In this work, the adsorption of a small globular protein (betalactoglobulin, BLG), on a natural montmorillonite clay (Mt) was investigated in acidic buffer (pH = 3). The combination of different characterization techniques such as zetametry, X-ray diffraction, transmission electronic microscopy, fluorescence and solid state nuclear magnetic resonance spectroscopies shed light on the interaction mechanism between the clay mineral and the proteins. For low BLG concentration, a slight increase of the interlayer spacing of the clay mineral was noticed as well as structural changes of the protein. In contrast, as the concentration of BLG increased, the adsorption led to a partial exfoliation of the clay mineral, accompanied with significant secondary structural changes of the protein characterized by a loss of β-sheet organization. Altogether, our results revealed an unexpected adsorption scheme where the increase of the BLG/Mt weight ratio of the hybrid material leads to a partial exfoliation of the Mt, but at the expense of the protein native structure.