Francesco Di Renzo

Microporosity and connections between pores in SBA-15 mesostructured silicas as a function of the temperature of synthesis

Imaging of the platinum replica of the porous structure and low-pressure argon adsorption allowed us to elucidate the complicated porous structure of SBA-15. These techniques enabled us to draw a coherent picture of the evolution of the SBA-15 precursor mesophase as a function of the synthesis temperature. TEM of the platinum replicas has been unable to show bridges between the structural mesopores of SBA-15 synthesized at low temperature, whereas mesoporous bridges are clearly observed for samples formed at higher temperature. Argon adsorption has evidenced the ultramicroporosity of the materials formed at low temperature, as well as its evolution to secondary porosity with diameters greater than 1.5 nm under more severe hydrothermal treatment.

Study of the state of aluminium in zeolite-β

Abstract The modifications of the state of aluminium when introducing protons into zeolite-β have been investigated by combining several analytical techniques. Low temperature (350°C) thermolysis of ammonium cations or treatment with dilute nitric acid provokes the development of a27Al NMR signal at 0 ppm attributable to aluminium in octahedral symmetry. Calcination at 550°C generates NMR invisible aluminium and a strongly acidic hydroxyl group vibrating at 3780 cm−1 in the infra-red spectra. The neutralization of the protons by reaction with a solution of pyridine in ethanol at room temperature or with gaseous ammonia at 100°C allows for all the aluminium atoms of the solid to recover a tetrahedral symmetry. The AlIV → AlVI transformation can be also reversed by replacing the protons by sodium or potassium cations. It is then concluded that the octahedrally coordinated and NMR invisible aluminium must be regarded as inherent parts of the framework of the zeolite. Their formation is explained by a distortion of aluminic sites caused by the high electron affinity of the proton.

Micelle-templated silicates as a test bed for methods of mesopore size evaluation

MCM-41-type micelle-templated silicates have been used as standard adsorbents to test usual methods of pore size evaluation in the diameter range 40–80 A. Pore sizes evaluated by XRD lattice parameters and void fraction have been used as standard values. Methods of pore size evaluation based on the Kelvin equation underevaluate the diameter of cylindrical pores by nearly 20%, unless the effect of the curvature radius of the adsorbent surface is taken into account. The Kelvin method modified according to Tolman or Broekhoff and de Boer accurately fits the experimental data. The 4V/S method also underestimates diameters by nearly 20% when a value aN2=16.2 A2 is used for the area covered by an adsorbed N2 molecule in the BET surface area determination. The use of results from the cited literature, indicating aN2=13.5 A2 on the surface of hydroxylated silica, allows the 4V/S method to fit standard diameters. The empirical application to mesopores using the method of Horvath and Kawazoe is also discussed.

Transition-metal ligands bound onto the micelle-templated silica surface

Abstract This paper reviews recent works on the design of immobilized transition-metal ligands on solid supports. After an overview of some results concerning ligand anchorage on polymeric supports and encapsulation of transition-metal complexes inside layered or zeolitic minerals, the grafting of ligands onto the silicic wall surface of micelle-templated silicas (MTS) is reported. MTS silicas featuring a regular mesoporous system of pore-monodispersed size and exhibiting larger pores than zeolites, provide a new opportunity to allow anchorage of organic moieties through the silanation procedure. Mn(III) Salpr and t Salpr complexes bound onto the MTS surface are active in epoxidation reaction using PhIO as oxygen donor. Anchorage of (1 R ,2 S )-ephedrine has been also investigated with the aim to obtain benefit from the MTS structure effect. These new supported chiral catalysts are active in alkylation of benzaldehyde with diethylzinc although less enantioselective than the corresponding homogeneous catalyst. The effect of dispersion of active sites and of surface passivation has been investigated and discussed in terms of the nature of the support surface.

Aerogel materials from marine polysaccharides

Hydrocolloid-forming polysaccharides are natural polyelectrolytes able to form stable hydrogels largely used in the food and pharmaceutical industry. Gelling polysaccharides derived from seaweeds or wastes of the seafood industry include polymers with several functional groups: alginates (carboxylic groups), carrageenans (sulfonic groups) and chitosan (amino groups). This article deals with suitable methods to prepare dry materials which retain the dispersion of the polymer hydrogel, namely polysaccharide aerogels. The materials whose properties are herewith described satisfy most of the appropriate requirements for heterogeneous catalysts and supports: they are stable in most organic solvents, present a high surface area and diverse accessible surface functionalities. Their application as catalysts, catalyst supports or adsorbents provide a new opportunity to obtain useful materials from one of the less energy-intensive sources of biomass.

Thermal and mechanical stability of micelle-templated silica supports for catalysis

Thermal and mechanical properties of hexagonal micelle-templated silica (MTS) (like MCM-41, SBA-3, SBA-15) are critical features for their application as catalysts and catalyst supports. Care should be taken in the interpretation of catalytic results which strongly depend on MTS stability. A decrease in conversion of more than 60% in a reaction of n-hexadecane isomerization has been observed by compressing MTS beyond a critical crushing pressure. Stability data have already been reported in literature, but they are extremely scattered, and can only be understood if the synthesis conditions which control the properties of MTS are taken into account. We show in this study that thermal stability is strongly dependent on wall thickness, a property which can be controlled by adjusting the alkalinity and the surfactant/silica ratio in the synthesis. The crushing of MTS under uniaxial compression follows the same mechanism as macroscopic hexagonal honeycomb structure. Their crushing strength varies as the square of the ratio between wall thickness and cell parameter.

Zeolites as tailor-made catalysts: Control of the crystal size

Abstract The size of zeolite crystals – a property which influences the efficiency of catalysts and the selectivity for several reactions – can be controlled by a proper choice of crystallization conditions. A review of the influence of chemical and physical parameters on zeolite crystal size is given.

A 28-year-old synthesis of micelle-templated mesoporous silica

Abstract A procedure for the preparation of low-density silica described in a patent filed in 1969 [V. Chiola, J.E. Ritsko, C.D. Vanderpool, US Patent 3 556 725, 1971] leads to the formation of solids having all the properties of MCM-41, the ordered mesoporous silica disclosed in 1991 [J.S. Beck, C.T.-W. Chu, I.D. Johnson, C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.W. Vartuli, WO Patent 91/11390, 1991].

Adsorption, intrusion and freezing in porous silica: the view from the nanoscale

This review presents the state of the art of molecular simulation and theory of adsorption, intrusion and freezing in porous silica. Both silica pores of a simple geometry and disordered porous silicas which exhibit morphological and topological disorders are considered. We provide a brief description of the numerical models of porous silicas available in the literature and present the most common molecular simulation and theoretical methods. Adsorption in regular and irregular pores is discussed in the light of classical theories of adsorption and capillary condensation in pores. We also present the different evaporation mechanisms for disordered systems: pore blocking and cavitation. The criticality of fluids confined in pores, which is still the matter of debate, is then discussed. We review theoretical results for intrusion/extrusion and freezing in silica pores and discuss the validity of classical approaches such as the Washburn-Laplace equation and Gibbs-Thomson equation to describe the thermodynamics of intrusion and in-pore freezing. The validity of the most widely used characterization techniques is then discussed. We report some concluding remarks and suggest directions for future work.

Thioether oxidation by hydrogen peroxide using titanium-containing zeolites as catalysts

Abstract Titanium substituted ZSM-5 (TS-1) and BEA (Ti-beta) are investigated as catalysts for the selective oxidation of thioethers with diluted H2O2, in a protic solvent, at 303 K. The performances of TS-1 and Ti-beta samples are analyzed in relation with structure of thioethers, nature of solvent and shape selectivity effect of catalysts. A mechanism for organic sulfides oxidation on Ti-containing zeolites is proposed.