materials with controlled porosity. iupac definition of porous materials: - microporous: < 2 nm -...
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Materials with controlled Materials with controlled
porosityporosity
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IUPAC definition of porous materials:
- Microporous: < 2 nm
- Mesoporous : 2-50 nm
- Macroporous: > 50 nm Shape-selectivity
Molecular sieves
Molecular dimension
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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
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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
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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
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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
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The symmetry of the mesophase may affect the particles morphology
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High-resolution microscopy reveals the ordered array of uniform pores
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The mesophase symmetry may be revealed by X-ray diffraction at low angle
hexagonal
ill-defined
cubic
lamellar
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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
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Not only silicas…
► other sol-gel oxides (Al2O3, TiO2,…)
► replica materials by casting
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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.
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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
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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.
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Introduction of active species
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Frequently used alkylsilanes for anchoring molecules. These functional groups can also be used as active catalytic sites by themselves.
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Organic BASES anchored to MCM-41 surface
Brunel et al. Catalysis Today 2629 (2002) 1–14
Post-synthesis grafting
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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.
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co-condensation route to surface tailoring:
RR
RR
RR
RR
RR
RR
RR
remove
template
H2O+surfactant
R-Si(OR)3
(RO)4Si+
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X = -SO3H, -COOH
Organic ACID anchored to MESOPOROUS SILICA surface
MOLECULAR SPECIES WHICH MAY BE INVESTIGATED BY VIBRATIONAL SPECTROSCOPY (VIBRATIONAL MODES!)
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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
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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
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► 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
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S. Inagaki and coworkers, Chem. Mater. 2000, 12, 3301-3305Toyota Central R&D Laboratories
Horseradish peroxidase
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1,2-diaminobenzene oxidation, TBHP as oxidant
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► 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.
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• 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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The end