Materials with controlled porosity

IUPAC definition of porous materials: Microporous: < 2 nm Mesoporous : 2-50 nm Macroporous: > 50 nm Molecular dimension Shape-selectivity Molecular sieves

porosity is controlled by the crystalline structure microporous ZEOLITES porosity is controlled by the crystalline structure crystalline aluminosilicates used as catalysts for oil refining, petrolchemistry (e. g. catalytic cracking), and organic synthesis in the production of fine chemicals MFI MOR

mesoporous SOL-GEL SYNTHESIS Synthetic approach: use of surfactant in the synthesis batch to form large pores MESOPORES AMORPHOUS SILICA WALLS C. T. Kresge et al., Nature, 1992, 359, 710-712

This procedure is an example of nanotechnology, because structures of nanometric size are obtained surfactant molecules self-assembly spontaneously porosity is controlled by synthesis conditions, e.g. the length of the chain and the possible presence of swelling aromatics

M41S* * Mobil materials Temperature: 20-150 °C Surfactant: cationic (R’3RN+), neutral (EO20–PO70–EO20, RNH2), anionic (ROSO3-) R = CnH2n+1, n = 8-22 pH: acidic (HCl), basic (NaOH),  7 Surfactant removal: CALCINATION (500-600 °C) EXTRACTION ( < 100 °C) M41S* * Mobil materials

The symmetry of the mesophase may affect the particles morphology

High-resolution microscopy reveals the ordered array of uniform pores

The mesophase symmetry may be revealed by X-ray diffraction at low angle cubic hexagonal lamellar ill-defined

The mesoporosity may be revealed by N2 adsorption isotherm classification of hysteresis loops proposed by De Boer Adsorption isotherm of nitrogen on MCM-41 with 4.0 nm pores at 77 K

Not only silicas… ► other sol-gel oxides (Al2O3, TiO2,…) ► replica materials by casting

Catalytic and sensing behavior not dealt with in the present talk. Ordered mesoporous silicas used as: catalyst sensors hosts for immobilized molecules (enzymes) hosts for mobile molecules (drug release) Catalytic and sensing behavior not dealt with in the present talk.

Two topics ► formation of active species at the inner surface of pores: functionalization with organic moieties ► deposition of active species on the inner surface of pores: immobilization of enzymes

In silica-based materials the surface may be covered by Silanols (Si-OH) Si-O-Si bridges Physisorbed water ► The amount and distribution of these species affect the hydrophobic/hydrophilic properties of the surface ► Si-OH species are reactive functionalities for the incorporation of catalytic active species D. Brunel, A. Cauvel, F. Di Renzo, F. Fajula, B. Fubini, B. Onida and E. Garrone, New J. Chem., 24 (2000) 807.

Introduction of active species

Frequently used alkylsilanes for anchoring molecules Frequently used alkylsilanes for anchoring molecules. These functional groups can also be used as active catalytic sites by themselves.

Organic BASES anchored to MCM-41 surface Post-synthesis grafting Brunel et al. Catalysis Today 2629 (2002) 1–14

Catalytic cycle during Knoevenagel condensation using MCM-41-grafted primary amine M. Laspéras, T. Llorett, L. Chaves, I. Rodriguez, A. Cauvel, D. Brunel, Stud. Surf. Sci. Catal. 108 (1997) 75.

co-condensation route to surface tailoring: R-Si(OR)3 (RO)4Si + R remove template H2O+ surfactant

Organic ACID anchored to MESOPOROUS SILICA surface X = -SO3H, -COOH MOLECULAR SPECIES WHICH MAY BE INVESTIGATED BY VIBRATIONAL SPECTROSCOPY (VIBRATIONAL MODES!)

Infrared spectroscopy Dosage of ammonia: study of the acidity and accessibility of –COOH species 7COOH-SBA-15 outgasssed at 473 K - C=O stretching mode peak decreases two band appears at 1550 cm-1 and 1407 cm-1, due respectively to the asymmetric (νas) and symmetric (νs) stretching vibration of the carboxylate group –COO- Sonia Fiorilli et al., submitted, 2004

NH3 strips the proton from carboxylate anion –COO- and Infrared spectroscopy Dosages of ammonia: study of the acidity and accessibility of –COOH species difference spectra with respect to background. - a broad absorsorption due to NH4+ deformation vibration at around 1456 cm-1 NH3 strips the proton from –COOH to give carboxylate anion –COO- and ammonium ion (NH4+) Sonia Fiorilli et al., submitted, 2004

► formation of active species at the inner surface of pores: functionalization with organic moieties ► deposition of active species on the inner surface of pores: immobilization of enzymes

Horseradish peroxidase S. Inagaki and coworkers, Chem. Mater. 2000, 12, 3301-3305 Toyota Central R&D Laboratories Horseradish peroxidase

1,2-diaminobenzene oxidation, TBHP as oxidant

► the loading efficiency shows clear correlation with the porosity of silica support; ► when the average mesopores size just matches the molecular diameters of the enzyme, immobilized HRP exhibits the peak activity in an organic solvent and the best stability; ► enzyme immobilization with FSM-16 or MCM-41 having a suitable mesopore size would be useful for and applicable to industrial processes and other applications, especially certain environmentally useful enzymatic reactions such as the decomposition of lignin or dioxins.

A. Corma, Chemical Review, 1997, Vol. 97, No. 6 A. Taguchi, F. Schüth, Microporous and Mesoporous Materials, 77 (2005), available on web

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