Aurélie Vicente

Hierarchical ZSM-5 zeolites in shape-selective xylene isomerization: role of mesoporosity and acid site speciation.

The isomerization of o-xylene, a prototypical example of shape-selective catalysis by zeolites, was investigated on hierarchical porous ZSM-5. Extensive intracrystalline mesoporosity in ZSM-5 was introduced by controlled silicon leaching with NaOH. In addition to the development of secondary porosity, the treatment also induced substantial aluminum redistribution, increasing the density of Lewis acid sites located at the external surface of the crystals. However, the strength of the remaining Brønsted sites was not changed. The mesoporous zeolite displayed a higher o-xylene conversion than its parent, owing to the reduced diffusion limitations. However, the selectivity to p-xylene decreased, and fast deactivation due to coking occurred. This is mainly due to the deleterious effect of acidity at the substantially increased external surface and near the pore mouths. A consecutive mild HCl washing of the hierarchical zeolite proved effective to increase the p-xylene selectivity and reduce the deactivation rate. The HCl-washed hierarchical ZSM-5 displayed an approximately twofold increase in p-xylene yield compared to the purely microporous zeolite. The reaction was followed by operando infrared spectroscopy to simultaneously monitor the catalytic performance and the buildup of carbonaceous deposits on the surface. Our results show that the interplay between activity, selectivity, and stability in modified zeolites can be optimized by relatively simple post-synthesis treatments, such as base leaching (introduction of mesoporosity) and acid washing (surface acidity modification).

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.

Ultrastructure of cellulose crystallites in flax textile fibres

Seven varieties of flax (Linum usitatissimum) fibres were analyzed in order to gain a deeper insight into the morphological features of the crystalline assembly. Different spectroscopic techniques and a chemical bleaching process were used to provide an accurate description of the lateral arrangement of the polysaccharide chains within the fibre cell wall. The flax fibres were analyzed in their natural state and after an extraction treatment of the non-crystalline components such as hemicelluloses, pectins and phenolics. The chemical bleaching process consisted of a Soxhlet extraction in toluene, a sodium chlorite treatment and an alkaline extraction of the residual hemicelluloses. Solid-state 13C nuclear magnetic resonance (NMR) confirmed the sequential removal of the non-cellulosic components from the flax cell wall. Both wide-angle X-ray diffraction (WAXD) and solid-state 13C NMR provided measures of the crystallite thicknesses and overall crystallinities before and after treatment. The existence of non-cellulosic highly ordered paracrystalline domains was also evidenced by proton spin relaxation time calculation. Whereas the overall crystallinity determined by WAXD decreased after treatment, the cellulose crystallinity calculated with the help of the solid-state 13C NMR slightly increased. This is explained by the difference in chemical selectivity between these two techniques and by the paracrystalline state of both hemicelluloses and pectins. Strong adhesion between cellulose crystallites, hemicelluloses and pectins in the fibres was evidenced by low spin–spin relaxation times and by an increase in crystallite thickness after bleaching. A simple model is proposed that describes the rearrangement of the macromolecules during the bleaching process.

Ultra-fast framework stabilization of Ge-rich zeolites by low-temperature plasma treatment

A novel one-step post-synthesis method for structural stabilization of germanium (Ge) silicate molecular sieves is presented. The modification includes low-temperature SiCl4 and TiCl4 plasma treatment of as-synthesized Ge-silicates. During plasma reaction template removal, partial extraction of Ge from the framework and incorporation of Si and Ti in micropore space takes place. The modified material is further subjected to high-temperature calcination, which completes the incorporation of replacing cations in the zeolite framework. The feasibility of this approach for stabilization and modification of Ge-silicate molecular sieves is exemplified by employing Ge-rich BEC-type zeolite as model material. In situ IR spectroscopy is used to monitor the interactions of SiCl4 and TiCl4 precursors with zeolite material and to follow the structural changes provoked by Ge substitution. Ultimate products and their intermediates were subjected to comprehensive characterization employing complementary methods (XRD, SEM/TEM, TG/DSC, NMR, N2 sorption and chemical analysis). The set of experimental data unambiguously proves successful stabilization of the zeolite framework. Stabilized Ti-containing BEC-type materials were used for methanol photooxidation in order to verify catalytic activity of zeolites obtained by this innovative approach.

One-pot synthesis of silanol-free nanosized MFI zeolite

The synthesis of nanostructured zeolites enables modification of catalytically relevant properties such as effective surface area and diffusion path length. Nanostructured zeolites may be synthesized either in alkaline media, and so contain significant numbers of hydrophilic silanol groups, or in expensive and harmful fluoride-containing media. Here, we report and characterize, using a combination of experimental and theoretical techniques, the one-pot synthesis of silanol-free nanosized MFI-type zeolites by introducing atomically dispersed tungsten; this prevents silanol group occurrence by forming flexible W-O-Si bridges. These W-O-Si bonds are more stable than Si-O-Si in the all-silica MFI zeolite. Tungsten incorporation in nanosized MFI crystals also modifies other properties such as structural features, hydrophobicity and Lewis acidity. The effect of these is illustrated on the catalytic epoxidation of styrene and separation of CO2 and NO2. Silanol-free nanosized W-MFI zeolites open new perspectives for catalytic and separation applications.

Ionothermal synthesis of FeAPO-5 in the presence of phosphorous acid

The ionothermal crystallization of FeAPO-5 molecular sieves in the presence of phosphorous acid (H3PO3) has been investigated. The use of H3PO3 enabled the formation of a metastable intermediate phase (FeNKX-2) that transforms into a more open-framework crystalline phase (FeAPO-5). The initial raw materials dissolved rapidly in the presence of the [bdmim]Cl polar ionic liquid, and the addition of the Fe3+ salt resulted in the crystallization of the FeNKX-2 intermediate. At this stage, the [bdmim]+ cation did not play the role of a pore filler for the FeNKX-2 crystalline structure. Consecutive phase transformation from the FeNKX-2 to the FeAPO-5 phase occurred under prolonged ionothermal treatment, and during this stage, the tetrahedral Fe3+ species was found to not only participate in the construction of the FeAPO-5 framework but also act as an intermediary electron-transfer medium. The fast crystallization of FeAPO-5 was explained by the presence of Fe3+ as an intermediate electron-transfer medium promoting the fast release of phosphorus nutrients (P5+) from the phosphite (P3+) reservoir that were further required for the crystallization of the FeAPO-5 molecular sieves. The use of ionic liquids as dual solvents and templates in combination with H3PO3 as an alternative phosphorus source thus opens the possibility to synthesize other microporous materials via a phase transformation approach.

Modeling short-range substitution order and disorder in crystals: Application to the Ga/Si distribution in a natrolite zeolite

Atomic substitutions are a central feature of the physicochemical properties of an increasing number of solid-state materials. The complexity that this chemical disorder locally generates in otherwise crystalline solids poses a major challenge to the understanding of the relationships between the structure and properties of materials at the atomic and molecular level. Strategies designed to efficiently explore the ensemble of local chemical environments present in disordered crystals and predict their signatures in local spectroscopies such as solid-state nuclear magnetic resonance (NMR) are therefore essential. Focusing on the Ga/Si disorder in the framework of rubidium-exchanged gallosilicate natrolite zeolite (Rb-PST-1) with a high Ga content (SiGa=1.28), we show how the structure-generation approach implemented in the new program supercell (Okhotnikov et al. [26]) provides an excellent basis for the understanding of complex experimental spectroscopic data. Furthermore, we describe how exhaustive explorations of atomic configurations can be performed to seek local structural ordering and/or disordering factors. In the case of Rb-PST-1, we more specifically explore the possibility to form and to detect the presence of thermodynamically unfavorable Ga-O-Ga connectivities. While particularly adapted to the description of dense materials, we demonstrate that this approach may successfully be used to reproduce and interpret the distributions of local structural distortions (i.e., the geometrical disorder) resulting from the chemical disorder in systems as complex as microporous zeolites.

High-Visible-Light Photoactivity of Plasma-Promoted Vanadium Clusters on Nanozeolites for Partial Photooxidation of Methanol

Cold VCl3-plasma is employed for the preparation of highly dispersed vanadium oxide clusters on nanosized zeolite. Different types of zeolites, such as EMT, FAU (z.X), and Beta, are used. The activity of the prepared catalysts is studied in the selective photooxidation of methanol under polychromatic visible and UV irradiations. The physicochemical properties and catalytic performance of plasma-treated zeolite Beta (P-V2O5@Beta) catalyst is compared with zeolite Beta (V2O5@Beta) and amorphous silica (V2O5@SiO2) impregnated vanadium oxide catalysts. Pure V2O5 is used as a reference material. The set of catalytic data shows that plasma-prepared zeolite Beta based catalyst displays the highest activity. Complementary characterization techniques including XRD, N2-sorption, FTIR, ionic exchange, pyridine adsorption, Raman, NMR, TPR, and EDX-TEM are used to study the impact of the preparation approach on the physicochemical properties and catalytic performance of photocatalysts.

Formation mechanism of three-membered ring containing microporous zincosilicate RUB-17

The crystallization process of RUB-17 (RSN-type), a zeolite-type zincosilicate, was studied in order to shed light on the zeolite crystallization mechanism. The sequence of crystallization events from the formation of the initial gel to the complete transformation into a zeolite-type material was investigated. Complementary methods, including XRD, TG/dTG, Raman, 29Si MAS NMR, and SEM, that allowed studying both short and long-range order in the solids were used. The RSN-type structure contains 3-, 4-, 5- and 6-membered rings (MRs) that allowed elucidation of the formation of different building units in the course of zeolite formation. The set of experimental data revealed that the three-membered ring (3MR) was the unit preferentially formed during the induction period. At this stage of gel evolution, the presence of larger rings was not detected. The latter were observed only after the appearance of long-range order in the solid, proved by X-ray diffraction analysis. Hence, the formation of the RSN-type structure was related to the large 3MR population during the induction stage.

Nanosized Na-EMT and Li-EMT zeolites: selective sorption of water and methanol studied by a combined IR and TG approach

Nanosized EMT-type zeolite crystals in sodium (Na-EMT) and ion-exchanged lithium (Li-EMT) forms were prepared. The sorption behavior of Li(Na)-EMT samples towards water, methanol and a mixture of both (50 : 50) was studied by combined thermogravimetric and infrared spectroscopic methods. The stability of the samples prior to and after the sorption measurements in two subsequent cycles was confirmed by X-ray diffraction, N2 sorption and NMR spectroscopy. The high sorption capacity of the Li-EMT sample towards water was demonstrated. It was found that the methanol is replaced by water faster in the Li-EMT sample in comparison to the Na-EMT sample. At low temperature, the methanol shows weak adsorption on each cationic site and no side products during desorption for both samples were obtained.