Comparative study of nano-ZSM-5 catalysts synthesized in OH- and F- media

This study reports the seeded synthesis of MFI-type (ZSM-5) zeolite in fluoride medium at pH = 8.5. Crystal growth kinetics of the resulting zeolite (ZSM-5-F) as a function of seed content and crystallization temperature is studied. The crystallization time is reduced to 1.5 h and crystals with sizes below 200 nm and a Si/Al ratio of 23.6 are obtained. A zeolite with similar characteristics but synthesized in a hydroxyl medium (ZSM-5-OH) is used to evaluate ZSM-5s synthesized in different crystallization media. Their physicochemical properties are compared and particular attention is paid to the nature, number, and distribution of silanol sites. The two zeolites exhibit similar number of Brφnsted acid sites; however the material synthesized in a hydroxyl medium contains a substantially larger number of surface and internal silanols that impact significantly its catalytic performance in methanol to hydrocarbon transformation. While the two materials exhibit similar selectivity in methanol transformations, the catalyst synthesized in fluoride medium shows superior activity and resistance to deactivation. The results suggest that seeded synthesis in a fluoride medium can be used for the preparation of superior zeolite catalysts

Metal loaded zeolite films with bi-modal porosity for selective detection of carbon monoxide

The assembly of metal containing (Pt and Pd) zeolite Beta nanocrystals in thin films retaining a bi-modal porosity (micropores of 0.73 nm and mesopores of 2.3–2.6 nm) is presented. The high porosity of metal loaded films was examined by adsorption of probe molecules (water and toluene). Besides, the optical properties of the films including surface roughness (10 nm), thickness (200–500 nm), and mechanical stability (Young’s modulus of 0.7–1 GPa) are determined. The zeolite films are further used for detection of carbon monoxide as a single analyte and in a mixture with methanol at different temperatures; it is found that 70% of the Pt and Pd surfaces remain accessible for the CO, but higher partial pressure of the analytes is needed to reach the saturation level

In situ infrared molecular detection using palladium-containing zeolite films

In situ IR detection of carbon monoxide in the presence of hydrocarbons (methanol and pentane) using Pd-containing zeolite thin films is reported. The thin films are prepared by spin coating deposition of nanosized LTL and BEA type zeolites suspensions; the palladium clusters are introduced in the nanosized zeolites by ion exchange followed by γ radiolysis of the coating suspensions. The Pd-containing zeolite films with a thickness of 200 nm are exposed to a single gas (either CO or hydrocarbons) or gas mixtures in the presence of water (100 ppm), and the IR spectra are collected continuously at 25, 75, and 100 °C. The fast recognition of very low concentrations of CO (2-100 ppm) in the presence of highly concentrated vapors of methanol or pentane (400-4000 ppm) with the Pd-containing zeolite films is demonstrated. The detection of CO and hydrocarbons is instant, which is a function of the low thickness of the films, small size of the individual zeolite crystals, and regular size and high stability of the Pd clusters in the zeolite films. The heat of adsorption for all experiments is similar (15 kJ.mol-1), which is explained with weak interactions between the carbon monoxide and palladium clusters in the zeolite films at temperatures below 100 °C. The nanosized zeolites with homogeneously distributed Pd clusters deposited in thin films demonstrate high molecular recognition capacity toward low concentrations of carbon monoxide under real environmental conditions, i.e., in the presence of water and hydrocarbons

Chemical Equilibrium Controlled Etching of MFI-Type Zeolite and Its Influence on Zeolite Structure, Acidity, and Catalytic Activity

International audienceChemical etching with fluoride ions is a new approach for secondary porosity engineering of aluminosilicate zeolite frameworks. We show that diluted HF solutions extract preferentially aluminum from zeolite frameworks. The Brønsted acidity of ZSM-5 treated in such a way decreases, while its structure is unaffected after an HF treatment. With higher HF concentrations, the number of undissociated HF molecules and the concentration of HF2– ions, extracting indiscriminately Al and Si, increase. The addition of NH4F shifts the chemical equilibria to produce more HF2–, avoiding the use of highly concentrated HF solutions; it also suppresses HF dissociation. The etching selectivity of such solutions is concentration-independent and extracts indiscriminately both framework Si and Al. Zeolite dissolution in NH4F-HF solutions starts preferentially at small intergrowth domains and goes deeply in the crystals without a substantial increase of the external surface area. Macropores are produced without altering zeolite acidity. Hierarchical materials obtained by these two approaches are characterized extensively by complementary methods and the catalytic impact illustrated in the m-xylene conversion

Elucidation of Pt clusters in the micropores of zeolite nanoparticles assembled in thin films

Platinum clusters are prepared by γ radiolysis of beta zeolite suspensions (Pt-Beta) in the presence of a templating agent (tetraethylammonium hydroxide, TEAOH). Further, the Pt-Beta suspensions were stabilized with methyl cellulose and deposited in thin films with a thickness of 200 nm on silicon wafers. Elucidation of the size, distribution, and stability of the Pt clusters in the colloidal suspensions and thin films is provided by high-resolution transmission electron microscopy (HRTEM) combined with grazing incident X-ray diffraction (GI-XRD) measurements. The lateral length of the Pt clusters immobilized in the channels of the beta nanocrystals is between 1 and 2 nm. The presence of crystalline fringes with spacings of 0.23 and 1.26 nm corresponding to cubic Pt and zeolite beta are clearly seen in the HRTEM. The homogeneous distribution of the clusters along the film thickness is confirmed by GI-XRD measurements at two penetration depths. Besides, the location of the Pt clusters in the channels of beta nanoparticles is studied by FTIR spectroscopy. The Pt clusters confined in the channels of beta crystals decrease the total micropore volume and also lower the water sorption capacity in comparison with pure beta zeolite

Mitigating coking during methylcyclohexane transformation on HZSM-5 zeolites with additional porosity

International audienc

Corona protein composition and cytotoxicity evaluation of ultra-small zeolites synthesized from template free precursor suspensions

<p>The toxicity of two types of ultra-small zeolites (8-18 nm) with LTL-and EMT-type structures is reported. Both the LTL- and EMT-type zeolites belong to the same group of molecular sieves; they have large pores (7.1-7.5 angstrom) and low silica content (Si/Al = 1.2-2.3). The zeolites are prepared by an environmentally friendly synthetic approach from precursor suspensions without using any organic template. Cellular interactions with the two types of zeolite nanocrystals are evaluated by cell viability, reactive oxygen species and cell life cycle assays. It is found that various concentrations of zeolites have negligible effects on the cell life cycle. Moreover, the LTL- and EMT-types zeolites did not cause extensive oxidative stress on the cells. Although it is seen that the zeolites extensively entered in the cells, there is no sign of toxicity for all employed concentrations of ultra-small EMT and LTL zeolites. Additionally, no abnormality in DNA replication while exposed to the zeolites is observed. Very importantly, the zeolite corona shows a high affinity for fibrinogen, moderate affinity for apoA-II and complement factor 3, and trace affinity for albumin, which is the most abundant protein of human plasma. Thus the zeolite nanoparticles can be considered as very promising material for purification of fibrinogen and lipoproteins. Since fibrinogen is considered as acute phase protein and found to be the most associated biomolecule in the composition of corona at the surface of zeolites, we propose that these nanoparticles can be potentially pro-inflammatory for in vivo applications.</p>