Heloise O. Pastore

Lamellar zeolites: an oxymoron?

“Lamellar zeolites” can be considered as an oxymoron. Zeolites, by definition, are three-dimensional structures displaying pores of a molecular dimension. Lamellar materials, contrarily, are bidimensional materials that, in some cases, can be delaminated, that is, have their lamella separated. It is exactly the combination of these properties that is so attractive in lamellar zeolites. They are zeolites whose precursors are/can be prepared in such a way as to display bidimensional organization where parts of the structure, the lamella, interact with one another by weak forces through the interactions of organic/inorganic ions and/or molecules housed in the “interlayer” spaces. From these precursors, calcination procedures lead to the 3D zeolitic structures, delamination in solutions provides individual or groups of lamella, pillarization processes conduct to mesoporous molecular sieves. Naturally, all these procedures eliminate the shape selectivity typical of zeolites in all cases of lamellar zeolites known so far, except one particular structure. However, providing that the resulting layers keep the original structure of the precursor, most of the zeolite acidity remains in the final material. In this way, pore sizes that limit the access of larger molecules to the active sites in the zeolites are not formed, the surface area of the material is increased and large molecular weight molecules are free to react in surface sites made available by the delamination or pillaring of layered zeolites. Finally, 3D fully connected zeolites can be deconstructed into layered zeolites from which all the above procedures are foreseen to provide modified solids. This recent discovery opens up a wealth of interesting structures from which to extract lamella with surface exposed active sites. Lamellar zeolites may be considered a true link between 2D and 3D solids and allow us to really determine which properties derive from one or the other structural dimensions, thus allowing us to precisely design structure-based properties for use in a range of technological applications.

Static crystallization of zeolites MCM-22 and MCM-49

Abstract Zeolites MCM-22 and MCM-49, with SiO 2 /Al 2 O 3 molar ratios (SARs) of 30 and 50, were prepared under static conditions using silica, sodium aluminate and hexamethyleneimine. An aging step before the hydrothermal treatment made it possible to obtain good-quality MCM-22 under these reaction conditions. No aging step during gel preparation gave rise to MCM-49. The aging step probably provides the reaction mixture with nuclei that grow in the static hydrothermal crystallization and whose formation conditions are extremely specific. The physico-chemical characteristics of these materials compare well with those for the same zeolites prepared under stirring, except for the morphology of the particles. The MCM-22 samples with SAR=30 are cylindrical particles with a depression in the center, while those with SAR=50 are spheres with a crack along the centerline. MCM-49, with SAR=30, crystallizes as hollow cylinders with approximately the same dimensions as MCM-22 having the same SAR.

Physicochemical Characterization and Surface Acid Properties of Mesoporous [Al]-SBA-15 Obtained by Direct Synthesis

In this work, [Al]-SBA-15 samples were prepared by three different direct synthesis methods and one postsynthesis procedure, aiming to study the influence of the preparation procedures on their structural, textural, and physicochemical features. To this aim, samples were investigated by combining different experimental techniques (XRD, N(2) physisorption, (27)Al-MAS NMR, and IR spectroscopy). All preparation methods led to the formation of aluminum-containing SBA-15 samples. Nevertheless, depending on the preparation procedure, samples exhibited different structural, textural, and surface characteristics, especially in terms of Brønsted and Lewis acid sites content. [Al]-SBA-15(1) was synthesized by the pH-adjusting method and presented the lowest surface area and pore volumes. Its surface displayed three families of medium and one family of high strength Brønsted acid sites. The Brønsted/Lewis ratio was 3.49. [Al]-SBA-15(2) and [Al]-SBA-15(3) were synthesized by prehydrolysis of the silica and the aluminum precursors. In [Al]-SBA-15(2), ammonium fluoride was used as silica condensation catalyst. These two materials presented similar surface area, pore diameters and volumes, and Brønsted acidity. The Brønsted/Lewis acid sites ratio were 3.07 and 2.15 for [Al]-SBA-15(2) and [Al]-SBA-15(3), respectively. The [Al]-SBA-15(P) obtained by postsynthesis alumination displayed surface area similar to that of [Al]-SBA-15(3), Brønsted/Lewis acid sites ratio of 2.75, and Brønsted acidity similar to that of [Al]-SBA-15(1). The presence of extra-framework aluminum oxide was identified only on [Al]-SBA-15(3) and [Al]-SBA-15(P).

Poly(phenylsilsesquioxane)s : Structural and morphological characterization

Poly(phenylsilsesquioxane) (PPSQ) polymers that were obtained from different synthetic routes were comparatively studied. The polymers were characterized by infrared and solid-state 29Si NMR spectroscopies. According to the results of X-ray diffraction and thermogravimetric analyses, the materials richest in silanol showed a less organized network and lower weight loss temperature. The morphology of the products was influenced by the preparation conditions. PPSQ, with a morphology rich in spherical particles, was achieved with an n-hexadecyltrimethylammonium bromide template in the reaction medium, whereas the morphology of this polymer obtained in the absence of the template was featureless. Small-angle X-ray scattering analyses revealed that the PPSQ samples showed a predominance of surface-fractal behavior.

CuAPSO-34 Catalysts for N2O Decomposition in the Presence of H2O. A Study of Zeolitic Structure Stability in Comparison to Cu-SAPO-34 and Cu-ZSM-5

The structural stability at high temperatures of both Cu-exchanged SAPO-34 and CuAPSO-34 with isomorphously substituted copper ions has been investigated. SAPO-34 undergoes an extensive crystallinity loss upon copper ion exchange. The CuAPSO-34 framework showed good hydrothermal stability, as reflected in its surface area and X-ray diffraction pattern after ageing treatments. This catalyst exhibited good activity in N2O decomposition even when the reaction was run in the presence of H2O or after ageing treatment under H2O vapor at 600 °C for 80 h which, conversely, led to a drastic loss of activity of Cu-ZSM-5.

Polyethylenimine-Magadiite Layered Silicate Sorbent for CO2 Capture

This paper describes the preparation of a Layered Silicate Sorbent (LSS) for CO2 capture using the layered silicate magadiite and organo-magadiite modified with polyethylenimine (PEI). The sorbents were characterized and revealed the presence of PEI as well as its interaction with CO2 at low temperatures. The thermal stability of sorbents was confirmed by thermogravimetry experiments, and the adsorption capacity was evaluated by CO2-TPD experiments. Two kinds of PEI are present in the sorbent, one exposed PEI layer that is responsible for higher CO2 adsorption because its sites are external and another one, bulky PEI, capable of low CO2 adsorption due to the internal position of sites. The contribution of the exposed PEI layer may be increased by a previous exchange of CTA(+), but the presence of the surfactant decreased the total adsorption capacity. MAG-PEI25 reached a maximum adsorption capacity of 6.11 mmol g(-1) at 75 °C for 3 h of adsorption and showed a kinetic desorption of around 15 min at 150 °C.

Catalytic DeNOx activity of cobalt and copper ions in microporous MeALPO-34 and MeAPSO-34

A combined spectroscopic and catalytic study of the NO reactivity on microporous aluminophosphates, with chabasite-related structure, CoAPO-34, CuAPO-34 and CuAPSO-34, is reported. NO and CO adsorption were monitored by FTIR spectroscopy, and revealed that Co 2+ /Co 3+ and Cu + /Cu 2+ redox couples, the sites responsible for the catalytic activity, are present in these catalysts. CoAPO-34 catalysts showed exceptionally high performances in the oxidation of NO to NO 2 , and poor activity in other DeNO x reactions. Copper-based aluminophosphates and silico-aluminophosphates, besides good performances in the NO oxidation to NO 2 , showed good activity in the N 2 O decomposition even in the presence of oxygen or water in the feed. The presence of silicon has beneficial effects both on the thermal and hydrothermal stability of the zeolitic structure, as well as on the catalytic performances of the metal- aluminophosphates.

One-step synthesis of alkyltrimethylammonium-intercalated magadiite

Cetyltrimethylammonium- and tetradecyltrimethylammonium-intercalated magadiites were prepared by direct syntheses, starting from sodium metasilicate (Na2O/SiO2 = 1.0) and nitric acid. Total substitution of sodium by cetyltrimethylammonium or tetradecyltrimethylammonium cations was not achieved in the range of surfactant: silicon molar ratios used in this study. When a phosphoniun-based surfactant replaces the ammonium surfactants in the same procedure, the result of the synthesis is a mixture of quartz and unmodified surfactant. If dodecylammonium bromide is used, an MCM-41 molecular sieve is obtained. The substitution of the silicon source by tetramethylammonium silicate or of nitric acid by hydrochloric, hydrofluoric, or acetic acids also yields MCM-41 molecular sieves, indicating that the formation of magadiite is greatly dependent on the presence of sodium cations and nitrate anions.

Synthesis and characterization of tubular aluminophosphate mesoporous materials containing framework magnesium

Abstract A series of aluminophosphates (and magnesium-aluminophosphates based on tubular molecular sieves have been synthesized using cetyltrimethylammonium bromide (CTAB) as structure-directing agent in the presence of tetramethylammonium hydroxide, at room temperature and under hydrothermal synthesis, using molar ratios of CTAB/Al=0.5 and 1.0, and 0⩽ z (=MgO/Al 2 O 3 )⩽0.015 in the gel composition. The hexagonal mesostructure of the as-synthesized aluminophosphates and magnesium-aluminophosphates was evidenced by two defined diffractions in the range between 2 θ =2° and 5°, and was obtained regardless of the amount of magnesium added to the synthesis mixture. The usual processes for removal of structure-directing agent, i.e. calcination and Soxhlet extraction with azeotropic solutions, both resulted in framework collapse, possibly due to the poor polymerization of the inorganic matrix. An alternative procedure was performed by extracting CTAB using a solution of ethanol/ n -butylamine, which showed to be efficient in extracting the majority of the organic part, without destruction of the mesostructure. After extraction, the samples exhibited only the characteristic low angle peak, typical of tubular materials, whose structure remained after further calcination. Elemental analyses confirmed the presence of magnesium in these materials. The 31 P nuclear magnetic resonance spectra of calcined sample confirmed the better condensation of the sample after thermal treatment, compared to the as-synthesized materials. The transmission electron microscopy image showed disordered arrays of tubes. These results indicates that thermally stable mesoporous magnesium-aluminophosphate materials can be obtained by using alkaline extraction and calcination as post-synthesis treatment.

Synthesis and surface properties of Ti-containing mesoporous aluminophosphates. A comparison with Ti-grafted mesoporous silica Ti-MCM-41

Abstract Mesoporous aluminophosphates (ALPO) was synthesised using cetyltrimethylammonium bromide (CTMABr) as a structure-directing agent, phosphoric acid and aluminum sulfate as sources of structure elements. Ti(IV) ions were introduced into mesoporous ALPO by grafting titanocene dichloride, [Cp 2 TiCl 2 ], onto the POH and AlOH groups exposed in the channels. FTIR spectroscopy was used to study surface hydroxyls, and by the use of NH 3 as molecular probe, to assess the surface acidity of both meso -ALPO and meso -Ti-ALPO materials. The coordination of Ti(IV) centres in Ti-ALPO material was studied using DR UV–Vis spectroscopy.