Materials with controlled Materials with controlled

porosityporosity

IUPAC definition of porous materials:

- Microporous: < 2 nm

- Mesoporous : 2-50 nm

- Macroporous: > 50 nm Shape-selectivity

Molecular sieves

Molecular dimension

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

C. T. Kresge et al., Nature, 1992, 359, 710-712

Synthetic approach: use of surfactant in the synthesis batch to form large pores MESOPORES

AMORPHOUS SILICA WALLS

SOL-GEL SYNTHESISmesoporous

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

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

hexagonal

ill-defined

cubic

lamellar

Adsorption isotherm of nitrogen on MCM-41 with 4.0 nm pores at 77 K

classification of hysteresis loops

proposed by De Boer

The mesoporosity may be revealed by N2 adsorption isotherm

Not only silicas…

► other sol-gel oxides (Al2O3, TiO2,…)

► replica materials by casting

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. These functional groups can also be used as active catalytic sites by themselves.

Organic BASES anchored to MCM-41 surface

Brunel et al. Catalysis Today 2629 (2002) 1–14

Post-synthesis grafting

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:

RR

RR

RR

RR

RR

RR

RR

remove

template

H2O+surfactant

R-Si(OR)3

(RO)4Si+

X = -SO3H, -COOH

Organic ACID anchored to MESOPOROUS SILICA surface

MOLECULAR SPECIES WHICH MAY BE INVESTIGATED BY VIBRATIONAL SPECTROSCOPY (VIBRATIONAL MODES!)

2000 1800 1600 1400

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

background last dosage of ammonia

ab

sorb

an

ce

wavenumber (cm-1)

7COOH-SBA-15 outgasssed at 473 K

Infrared spectroscopyInfrared spectroscopy

Dosage of ammonia: study of the acidity and accessibility of –COOH species

- - C=OC=O stretching mode peak stretching mode peak decreasesdecreases

- two band appears at two band appears at 1550 cm1550 cm-1-1 and and 1407 cm1407 cm-1-1, due respectively to , due respectively to

the asymmetric (the asymmetric (ννasas) and symmetric ) and symmetric

((ννss) stretching vibration of the ) stretching vibration of the

carboxylate group carboxylate group –COO–COO--

Sonia Fiorilli et al., submitted, 2004

Infrared spectroscopyInfrared spectroscopy

Dosages of ammonia: study of the acidity and accessibility of –COOH species

1800 1600 1400

-0.3

0.0

0.3

0.6

abso

rban

ce

wavenumber (cm-1)

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

S. Inagaki and coworkers, Chem. Mater. 2000, 12, 3301-3305Toyota 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 andapplicable 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

The end