Philippe Ayrault

Infrared spectroscopic study of the acid properties of dealuminated BEA zeolites

Abstract A series of HBEA samples were prepared by dealumination through three different methods (steaming, treatments with hydrochloric acid and ammonium hexafluorosilicate (HFS) solutions) of a parent sample with total and framework Si:Al ratios of 12.5 and 33, respectively, constituted of 20 nm crystallites. The samples were characterized by XRD, nitrogen adsorption and pyridine adsorption–desorption followed by IR spectroscopy. Whereas the three dealumination treatments have practically no effect on crystallinity and nitrogen adsorption properties, they cause large changes in the ranges of the O–H bond stretching modes (3300–3800 cm−1) and of the most intense IR absorption of pyridine (1400–1700 cm−1). Treatments with HCl or HFS cause the most significant changes: disappearance of the 3662 and 3782 cm−1 bands corresponding to extraframework Al species and to tricoordinated Al species partially connected to the framework, respectively, appearance of additional bands at 1603 and 1446 cm−1 ascribed to a new type of pyridine species coordinated to Lewis sites (PyL2) and for desorption temperatures above 350 °C, of a band at 1462 cm−1 generally ascribed to iminium ions, apparently at the expense of the PyL2 band. In agreement with this formation of iminium ions, desorption at high temperatures causes the complete disappearance of pyridinium ions without complete restoration of the acidic bridging hydroxyl band. PyL2 species are proposed to be pyridine molecules coordinated to Lewis sites and interacting through hydrogen bond with protonic sites.

Acid properties of dealuminated beta zeolites studied by IRspectroscopy

The concentration of acid sites in non-dealuminated (Si/Al=10), and dealuminated (Si/Al=20–90) H-beta zeolites has been studied by quantitative IR measurements of pyridine sorption. The number of Bronsted sites was found to be less than that of Al atoms. Most probably this was due to the presence of Al in extraframework positions (Al–OH groups and Lewis acid sites were found). Some positively charged extraframework Al species may neutralize the charge of AlO 4 - and lower the concentration of protonic sites. Dealumination decreased the amount of extraframework Al species and, hence, the difference between the amounts of Al and Bronsted sites. Dealumination also decreased the number of Lewis acid sites. IR studies of pyridine desorption evidenced that, as well as strong Bronsted sites (Si–OH–Al), there were also weak Bronsted sites, not Si–OH–Al; their nature is not clear. The contribution of such weak Bronsted sites decreased with the extent of dealumination. IR studies of pyridine desorption and benzene sorption, evidenced that Si–OH–Al groups were heterogeneous in H-beta zeolites. Dealumination removed the less acidic hydroxy groups first.

Influence of coke on the acid properties of a USHY zeolite

The composition of coke and its effect on the acid sites were determined with samples of a USHY zeolite (with total and framework Si/Al ratios of 3.1 and 5.4, respectively) used for m-xylene transformation at 520 and 720 K. At low coke contents, coke is mainly constituted by methyl substituted polyaromatic compounds with three (520 K) or four (720 K) aromatic rings trapped in the zeolite pores. Coke causes a decrease in the intensity of the IR hydroxyl band. The most acidic hydroxyls, i.e., the bridging hydroxyls in interaction with extra-framework aluminum species are most affected by coke, whereas no interaction is observed between the non-acidic hydroxyls and the coke molecules. Pyridine adsorption shows that, while the number of protonic sites able to retain pyridine adsorbed decreases by coking, this was not the case for the Lewis sites. Pyridine adsorption facilitates the desorption under vacuum of coke molecules from the coked zeolites, the effect being more significant for coking at 520 K. This indicates that the retention of coke molecules is not only due to their low volatility (at 520 K) or to their steric blockage (720 K), but also to their adsorption on the acid sites.

Surface modifications of illite in concentrated lime solutions investigated by pyridine adsorption

The surface properties and functionality of an illite-containing clay mineral in alkaline solutions containing increasing quantities of calcium hydroxide were investigated using FTIR monitored pyridine adsorption at various temperatures. Results showed that the binding behavior of electron donor pyridine molecules to illite platelets is mostly governed by electron acceptor sites located at the edges of the clay particles. The binding of bulky hydrated calcium ions at the surface of the illite platelets decreases the surface area of illite. Moderate thermal treatments up to 450°C do not affect the structure of the clay mineral but strongly decrease the number of Lewis and Brönsted sites available at the edges of the clay platelets.

Acid properties of mazzite zeolites studied by IR spectroscopy

Abstract The adsorption of ammonia, pyridine and benzene on non-dealuminated (Si/Al = 4.4) and dealuminated (Si/Al = 10 and 30) mazzite samples was followed by FT-IR spectroscopy. The dealumination of mazzites (by steaming and acid treatment) made all their narrow channels accessible to pyridine. Pyridine and ammonia adsorption and desorption showed that the broad OH band at approximately 3600 cm −1 was composed of several submaxima corresponding to hydroxyl groups of various localizations and various acid strengths. With all the mazzite samples, the concentrations of Bronsted acid sites determined by ammonia adsorption were comparable with the values calculated from the chemical composition of zeolites (amounts of Al minus amount of Na) whereas those determined from pyridine adsorption were lower than these values. The greatest difference was found for the non-dealuminated MAZ-4.4, in which pyridine could not reach the sites of the narrow channels. The values of the extinction coefficients corresponding to the OH groups located in the large channels were higher than those corresponding to the OH groups of the narrow channels, thus showing the stronger acidity of the former OH groups. The acid strength of OH groups increased with the dealumination, as seen from the increase in the corresponding extinction coefficient and frequency shift Δν due to the hydrogen bonding of hydroxyl groups with benzene. Furthermore, the protonic acidity of the dealuminated mazzite of Si/Al = 30 was found to be stronger than that of mordenite and beta zeolite with comparable Si/Al.

Dealumination of zeolites Part VIIIAcidity andcatalytic properties of HEMT zeolites dealuminated bysteaming

IR spectroscopy has been used to characterize the hydroxy groups of a series of HEMT samples dealuminated by steaming (framework Si/Al ratios between 4.5 and 52) and their interaction with pyridine. Three OH bands are observed in the spectrum of the non-dealuminated sample (HEMT 4.5): at 3631 cm -1 [assigned to (HF)OH located in the large cages]; at 3551 cm -1 [(LF)OH located in the sodalite cages]; and at 3740 cm -1 (terminal defect SiOH groups). A large number of new OH bands appear in dealuminated samples; two of them, present in mildly dealuminated samples and located at 3599 and 3525 cm -1 , correspond to protonic sites stronger than those corresponding to the (HF) and (LF)OH bands. These bands were assigned to (HF) and (LF)OH groups interacting with cationic extraframework aluminium species located in sodalite cages. The other bands which correspond to extraframework species or to SiOH groups are generally slightly acidic or non-acidic. However, a band appearing at 3610 cm -1 in the more dealuminated samples is due to very acidic OH groups of silica alumina debris. With all the samples, the number of Bronsted sites was found to be lower than the number of framework aluminium atoms, the difference being particularly pronounced for the less dealuminated samples. This can be explained by the inaccessibility to pyridine of some of the (LF)OH groups, by the partial exchange of protonic sites by cationic extraframework aluminium species and, with HEMT 4.5, by a partial dehydroxylation during pretreatment. The maximum activity per protonic site for m-xylene transformation, found with the mildly dealuminated samples, was attributed to the presence of very strong acid sites resulting from the interaction of the OH groups with extraframework species. Transition state shape selectivity in the hypocages is proposed to explain the low value of the disproportionation/isomerization ratio and the preferential formation of 1,2,4-trimethylbenzene found with all the HEMT samples (compared to that with HFAU zeolites).

Influence of the ageing of zeolite coke on composition and deactivating effect

ABSTRACT The ageing of coke formed during m-xylene transformation at 250°C over a USHY zeolite was investigated under nitrogen flow at the reaction temperature. This ageing treatment causes a decrease in the zeolite coke content and an increase in the size and aromaticity of coke molecules. Pyridine adsorption followed by IR spectroscopy, carried out on the coked samples before and after ageing, shows that ageing causes a significant decrease in the concentrations of protonic and Lewis acid sites able to retain pyridine adsorbed at 150°C. The deactivating effect of coke is also more pronounced after than before ageing. A change in the mode of deactivation from site poisoning to pore blockage is proposed to explain these observations.

Effect of low frequency ultrasound on the surface properties of natural aluminosilicates

Structural and surface properties of different natural aluminosilicates (layered, chain and framework structural types) exposed of 20 kHz ultrasound irradiation (0-120 min) in aqueous and 35 wt%. aqueous H2O2 dispersions were studied by X-ray diffraction (XRD), dynamic light scattering (DLS), nitrogen adsorption-desorption, thermal analysis, and Fourier transform infrared spectroscopy (FTIR) techniques. It was confirmed that sonication caused slight changes in the structure of investigated minerals whereas their textural properties were significantly affected. The aqueous dispersions of montmorillonite (Mt), clinoptilolite (Zlt), glauconite (Glt) and palygorskite (Pal) were represented by several particles size fractions according to DLS-study. Ultrasound irradiation produced a decrease of the average particle diameter by 4-6 times in water and by 1.3-5 times in H2O2 dispersions except for Pal, which underwent strong agglomeration. A significant increase of total pore volume and pore diameter was observed for Glt sonicated in H2O2 dispersions whereas for Pal mainly micropore volume sharply increased in both aqueous and H2O2 dispersions.

Non Thermal Plasma Functionalized 2D Carbon–Carbon Composites as Supports for Co Nanoparticles

AbstractNovel two-dimensional carbon–carbon composites made of carbon nanofibers (CNFs) supported on a carbon preform were functionalized by non thermal plasma treatment (room temperature, atmospheric pressure, humid air), before being used as supports for metallic cobalt nanoparticles. It was shown that the degree of functionalization of the carbon nanofibers depends on the plasma power input, the treatment time and the CNF loading. The size of the cobalt nanoparticles generated after subsequent reduction of the Co-containing plasma treated CNF/C composites under hydrogen flow seems to be independent of the amount of supported cobalt. Changes in surface characteristics were analyzed using thermogravimetric analyses coupled to a mass spectrometer, X-ray photoelectron spectroscopy analyses and Raman spectroscopy. Transmission electron microscopy was used to complementary characterize the final size, dispersion and location of the so generated Co nanoparticles.

Surface characterization of ZnO/ZnMn{sub 2}O{sub 4} and Cu/Mn{sub 3}O{sub 4} powders obtained by thermal degradation of heterobimetallic complexes

From the selective transformation of the heterometallic (Zn-Mn or Cu-Mn) carboxylate complexes with 2,2 Prime -bipyridyl by thermal degradation at relatively low (350 Degree-Sign C) temperature, it was possible to get either well defined spinel ZnMn{sub 2}O{sub 4} over zinc oxide or well dispersed copper particles surrounded by a manganese oxide (Mn{sub 3}O{sub 4}) in a core-shell like structure. Morphology of the powder surface was examined by scanning electron microscopy with energy dispersive X-ray microanalysis (SEM/EDX). Surface composition was determined by X-ray photoelectron spectroscopy (XPS). Specific surface of the powders by nitrogen adsorption was found to be 33{+-}0.2 and 9{+-}0.06 m{sup 2} g{sup -1} for Zn-Mn and Cu-Mn samples, respectively, which is comparable to those of commercial products. - Graphical abstract: From the selective transformation of heterometallic (Zn-Mn or Cu-Mn) carboxylate complexes, it was possible to get either well defined spinel ZnMn{sub 2}O{sub 4} over zinc oxide or well dispersed copper particles surrounded by a manganese oxide (Mn{sub 3}O{sub 4}) in a core-shell like structure. Highlights: Black-Right-Pointing-Pointer Thermal degradation of heterometallic complexes results in fine disperse particles. Black-Right-Pointing-Pointer Core-shell Cu/Mn{sub 3}O{sub 4} particles are obtained. Black-Right-Pointing-Pointer ZnMn{sub 2}O{sub 4} spinel layer covers ZnO particles.