Topic 5 - Synthesis by Sol-gel Methods (MJC)

Synthesis of catalyst by sol-gel methods. Synthesis with alcoxide, xerogels, aerogels, and cryogels

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Universidad de Antioquia

Universidad de Antioquia

Environmental catalysis Group

Present by: Manuel J Cano

Professors: PhD Aída L Villa & PhD Lina M Gonzalez

2015

Outline

Block 1: Definitions- Sol- Precursor- Gel- Alkoxides

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Block 2: Sol-Gel Process- Definition.- Key steps.- Reactions for gel formation.- General example

Block 3: Type of porous materials from sol-gel:

- Aerogel.

- Xerogel

- Cryogel freeze-dryer.

Definitions: Sol

• Sol: Liquid suspension of solid particles. For a sol to exist, the solid particles must be small enough for the forces responsible of dispersion to be greater than those of gravity.

- Size: from 1 nm to 1000 nm.- Production: By the hydrolysis

and partial condensation of a precursor such as an inorganic salr or a metal alkoxide (further condensation produces a gel).

- Precursor: a salt or an organometallic compound.

3Catalyst preparation: science and engineering. Regalbuto, John. New York: CRC, 2007.Preparation of Solid Catalysts. Edited by G. Ertl, H. Knozinger, J. Weitkamp. Wiley VCH, 1999.Introduction to the Sol-Gel Processing. Pierre Alain. London. Kluwer Academic Publishers. 1998

Precursors

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Metal saltMetal Alkoxide

Metal alkoxides M(OR)n, are derivatives of alcohols, ROH (Ex. Aluminum ethoxideAl(OC2H5)3 which are

- Easily accessible, inexpensive organic compounds and available in high purity.

- Extremely weak as acids- Easily removable via hydrolysis and thermal

treatment, leaving high purity hydrated oxides.

Historically, these have been employed in sol-gel process, which readily undergo catalyzed hydrolysis and condensation to form nanoscale oxide or hydroxide particles.

The general formula of a metallic salt is MmXn where M is the metal, X an anionic group (Ex. Aluminum chloride AlCl3).

The metal salts include chlorides, acetates, nitrates, sulfides and so on. Chlorides, nitrates and sulfides have high solubility in water or organic solvents.

The sol-gel process with metal salts can be realized by chelating the metal ions by organic ligands.

Introduction to the Sol-Gel Processing. Pierre Alain. London. Kluwer Academic Publishers. 1998Sol-Gel Science and Technology Processing Characterization and Applications. Edited by Sumio Sakka. Kluwer Academic Publishers. 2004

Alkoxides

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• In theory all metal oxide can be synthesized by sol-gel process.

• Porous oxide materials used in heterogeneous catalysis as catalyst support or precursors of catalyst supports can be prepared by sol-gel process.

• One of the best starting material for sol-gel preparations of metal oxides are the class of metalorganic compound known as metal alkoxides.

• General formula:

M(OR)x

Where: M is a metal (Si, Zr, Al, Ti,…) and R is an alkyl group (most often –CH3, -C2H5, -C3H7 or –C4H9) and x is the valence state of the metal (for instance: x=4 with Si, Zr and Ti, and x=3 with Al).

Catalyst preparation: science and engineering. Regalbuto, John. New York: CRC, 2007.

Definitions: Gel

• Gel: is a diphasic material made out of a liquid entrapped in the porosity of a solid one. It is a porous 3-D interconnected solid network that expands in a stable way throughout a liquid medium and is limited by the size of the container.

• It can be considered as a macromolecule (or polymer) of a volume equal to that of the liquid mixture that has given birth to it.

• Polymer: is a group of molecules whose structure can be generated through repetition of one or a few elementary units.

- Synthesis: Obtained by a hydrolysis step followed by a condensation reaction. The destabilization of a sol into a gel by modifying the pH of the dispersion.

6Catalyst preparation: science and engineering. Regalbuto, John. New York: CRC, 2007.Preparation of Solid Catalysts. Edited by G. Ertl, H. Knozinger, J. Weitkamp. Wiley VCH, 1999.Introduction to the Sol-Gel Processing. Pierre Alain. London. Kluwer Academic Publishers. 1998

Sol-Gel Process

• Sol-gel processing is one of the routes for the preparation of porous materials by their solidification from a true solution phase.

• There are four key steps:- Formation of a gel.

- Aging of a gel.

- Removal of solvent.

- Heat treatment.

7Preparation of Solid Catalysts. Edited by G. Ertl, H. Knozinger, J. Weitkamp. Wiley VCH, 1999.

Handbook of Heterogeneous Catalysis. Gerhard Eartl, Helmut Knozinger, Ferdi Schuth, Jens Weitkamp. 2008 Wiley – VCH Verlag GmbH & Co. KGaA.


Metal Alkoxides and formation of the gel.

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• Metal alkoxides are popular precursors because they are rapidly hydrolyzed to the corresponding hydroxide, except the most thoroughly studied among them, silicon tetraethoxide (TEOS) Si(OC2H5)4, which requires an acid or basis catalyst for hydrolysis.

• In the hydrolysis, the hydroxyl ion becomes attached to the metal atom as follow:

• R is an alkyl group and therefore ROH is an alcohol. Depending on the amount of water and catalyst present, hydrolysis may go to completation, so that all of OR groups are replaced by OH:

• Or stop while the metal is only partially hydrolyzed, M(OR)x-n(OH)n

• Two partially or completely hydrolyzed molecules can link together in a condensation reaction.


Condensation Reaction

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• By definition, condensation liberates a small molecule, such as water or alcohol.

• This kind of reaction can continue to build larger and larger silicon or aluminum or titanium or zirconium containing molecules by a polymerization process in alcoholic medium.


Formation of the Gel

• Hydrolysis:

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• Condensation:

• Sol-gel chemistry with metal alkoxides can be described in terms of two classes of reactions:

• These two reactions capture the key phenomenological idea that a three-dimensional gel network comes from the condensation of partially hydrolyzed species.

• Any parameters that affect either or both of these reactions could affect the properties of the product. It has been pointed out that the important variables are the relative rates of hydrolysis and condensation.

Preparation of Solid Catalysts. Edited by G. Ertl, H. Knozinger, J. Weitkamp. Wiley VCH, 1999.

Gelation

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• Gelation occurs by cross-linking of the colloidal particles according to the reaction:

• Both hydrolysis and condensation conditions set the primary structures of the gels and thereby influence further steps.

• Alkoxides are vey sensitive to water, so hydrolysis is possible for a large variety of compounds.

Aerogels and Related Porous Materials. Gesser, H. Goswami, P. Chem. Rev 89 765-788 1989.

General Example

12Catalyst preparation: science and engineering. Regalbuto, John. New York: CRC, 2007.

Dependence of rates for hydrolysis and gelation with regard to pH

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• a) This figure shows rates of hydrolysis and condensation of tetrathylorthosilicate Si(OC2H5)4, which is the most widely studied precursor, vary as a function of pH.

Preparation of Solid Catalysts. Edited by G. Ertl, H. Knozinger, J. Weitkamp. Wiley VCH, 1999.

Handbook of Heterogeneous Catalysis. Gerhard Eartl, Helmut Knozinger, Ferdi Schuth, Jens Weitkamp. 2008 Wiley – VCH Verlag GmbH & Co. KGaA.

Aerogels and Related Porous Materials. Gesser, H. Goswami, P. Chem. Rev 89 765-788 1989.

• Acid catalysis leads to a more polymeric form of gel with linear chains as intermediates.

• Base catalysis yields colloidal gels.

Aerogels

• Aerogels: They are ultra-porous solid materials exhibiting large pore volumes (porosities >90%), high surface areas (hundreds of m2/g), and low bulk densities.

• They can be directly prepared as powders, thin films or monoliths, opaque, translucent or transparent to light.

• They are obtaining through combination of sol-gel chemistry and subsequent drying under supercritical conditions with respect to the liquid phase filling their porosities.


Aerogels

• Drying the gel by evaporating under ambient pressure gives rise to capillary pressure that causes shrinkage of the network.

• Typical solvents: methanol, ethanol and CO2.

• In a supercritical fluid a single phase exist, drying in SC conditions avoid the L-G interface during vaporization, hence preventing collapse of solid network by capillary forces.

• The solid structure undergoes only a few modifications during this operation , and the highly porous and aerated product is called an aerogel.

15Applied heterogeneous catalysis. J.F. Le Page. Paris 1987.Heterogeneous Catalytic Materials. Solid State Chemistry, Surface Chemistry and Catalytic Behaviour. Elsevier. B.V 2014.

• Aerogels: This type of drying consists of eliminating the capillary forces either by operating at temperatures higher than the critical temperature of water, or by vaporizing under vacuum at low temperatures of (-50 to 5 ºC).

Surface Tension Coefficients


• The gel is apparently a one-phase piece of solid semielastic material.

• In fact, it is a two-phase solid made out of a liquid entrapped in the porosity of a solid one.

• The pores are filled by the liquid phase, which is always characterized by its never-zero-surface tension coefficient and which is at the origin of the formation of a concave meniscus in the pore between the liquid and the vapor phases.

• The greater surface tension coefficient, the greater the capillary forces. These can exert forces attaining 100 tons/cm on the walls of the pore during their emptying steps.

Aerogels – Supercritical Drying Process

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• A simple evaporation cannot preserve the porosity developed in the gel when it is still wet.

• In order to avoid the development of the capillary gradient stresses responsible for the collapse of the porosity of the dry solid, one must operate a process where the surface tension of the liquid phase is zeroed.

• Supercritical drying process:- Pressurization combined with heating to attain SC conditions.

- Continuous flushing with fresh SC fluid to remove the solvents.

- Depressurization is used to remove the fluid phase. Heating and depressurization must be slow enough to avoid shear stress and consequent cracks in the nanostructure.

Catalyst preparation: science and engineering. Regalbuto, John. New York: CRC, 2007.Heterogeneous Catalytic Materials. Solid State Chemistry, Surface Chemistry and Catalytic Behaviour. Elsevier. B.V 2014.


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• Hydrogels can never be supercriticallydried with respect to water because their critical constants are so high that all gels are peptized and recrystallized, leading to very poor textual properties.

• Consequently, before supercritical extraction of the wet aqua (hydo)-gel, water must be exchanged with a more “gentle” liquid medium.

• This is the reason why an organic medium is usually used for the sol-to-gel step giving alcogels when alcohols are the medium or acetogels when acetone is chosen.



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• An autoclave is necessary because during the heating of the gels, they must be continuously covered by the liquid until the critical conditions are met, where it transforms in a fluid phase without the building of a liquid-vapor interface.

- The gel is placed in the autoclave with a sufficient amount of liquid and flushed by a nitrogen flow in order to extract all traces of water contained in air

- Close autoclave and heat at supercritical T, generally 10ºC higher than the Tc.

- After equilibration the outlet valve is opened to reach ambient pressure.

- When P = Patm the autoclave is again flushed by N2.

- The cooling of the autoclave is performed (under nitrogen) and the lid is opened when the temperature is the ambient one.


Example 1 – Aerogels preparation.

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• Silica aerogels are low density solid state porous materials derived from gel in which the liquid component of the gel has been replaced by air..

• The properties of silica aerogels are highly dependent on the post-treatment steps like gel washing, gel aging and gel drying.

• Gel aging effect on the physical and microstructural properties of methyltrimethoxysilane CH3Si(OCH3)3 based silica aerogels were studied.

Example 1: Aerogel

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• Two acid-base catalyzed sol gel process was followed to prepared the alcolgel.

Step 1: Metanol as solvent.

• Molar ratio was varied MeOH/MTMS from 7 to 35.

• Acidic water (oxalic acid 0.001 M) was added, drop by drop, while constant stirring for 15 min.

Step 2: After 24 h basic water (ammonium hydroxide 10 M) was added to the alcosol, drop by drop while stirring for 15 min.

Example 1

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• Two acid-base catalyzed sol gel process was followed to prepared the alcolgel.

Step 3: The homogeneous alcosols obtained were transferred to airtight Borosilglass test tubes. Gelation took place at 27ºC.

Step 4: Aging (0 to 5 days)

Step 5: Supercritical drying process – Autoclave T=265ºC (80ºC/h) P=10 MPa. Metanolgets transformed into a supercritical fluid which was evacuated slowly.

Example 1

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Effect of MeOH/MTMS molar ratio (M) on gelation time:

- Time for gelation increased from 1 to 10 h with an increase in M value from 7 to 35.

- For lower M value, the spacing between the reacting silica species is less which leads to the formation of 3-D silica network at short time.

- For high M values, the spacing between reacting species increases which decrease rate of condensation reaction and hinders the formation of gel network.

Example 1

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Effec of gel aging on the bulk density and volume shrinkage:

- The bulk density and the volume shrinkage of the silica aerogels decrease with an increase of aging period up to 2 days.

- Aging period up to 5 day resulted in move volume shrinkage and high bulk density.

Example 1

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Effec of gel aging on volume shrinkage:

- At gel point, the network (SI-O-Si bonds) forms sufficiently enough to prevent the solvent flow from pores of the network.

- Chemical reaction responsible for the gelation will continue long after the gel point, during the aging process and form Si-O-Si bonds.

- Aging periods < 2 days: Most of the Si atoms have unreacted –OR and –OH groups giving rise to very delicate gel which constrict easily during the supercritical drying process because the poor stability in the network structure and turned into high density aerogels. This brings out changes in other properties like porosity.

Example 1

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- Aging periods > 2 days: The terminal –OR and –OH groups condense more and more, and the gel network become more hybrid with an increase in the aging time, the stability of the network is very less because of excess condensation. This excess condensation makes the network more brittle and hence the poor stability.

- After drying process more shrinkage of the gel occurs which automatically leads to the high density aerogels.

Example 1

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- Optimum for 2 days: The 2 days of aging period of an alcogel is found to be optimum aging period for low density aerogels where the condensation of the hydroxyl species takes place to the appropriate extent with efficient bond strength.

- The shrinkage was found minimal for 2 days of gel aging process and went on increasing on both the sides of aging period scale.

Example 1

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Effect of gel aging on the chemical bondings present in the aerogels:- The effect of gel aging on the chemical

bounds in the aerogels was studied by FTIR.

- There is an increase in the intensity of C-H and Si-C bonds and decrease in the intensity of O-H bonds for 2 days of aging period.

- An increase in the intensity of O-H bonds for higher days of gel aging periods (3-5 days) is due to fact that polycondensation of silica species increases with increase in gel aging period.

Stretching

Ben

din

g

Xerogels – Ordinary drying

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• Xerogels: This is a kind of dry gel obtaining by ordinary drying.

This type of drying can be done a conventional drying oven (100 – 200º C).

- The constituent particles of the gel experiment a very large capillary tension, which decreases as the diameter of the pores increase. Two opposing phenomena occur:

- Capillary force tends to bring the particles closer together.

- The chemical bonds binding the particles and sometimes the repulsions of electrostatic charges tend to maintain the structure in its initially state.

- Generally the capillary forces prevail and the drying operations are accompanied by a contraction of the structure of the gel that varies inversely with the diameter of the pores.

Applied heterogeneous catalysis. J.F. Le Page. Paris 1987.

Xerogels – Ordinary drying

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• The capillary forces cause shrinkage of the gel network. The resulting dried gel is often reduced in volume by a factor of 5 to 10 compared to the original wet gel.

• The average dimensions of the pores and the thickness of the skeleton depend upon the structure that exists at the gel point and the extent of the collapse or distortion of this structure that results from drying.

• Typical surface areas for xerogelsrange from 500 to 900 m2/s, while aerogel surface areas may exceed 1000 m2/g.

Applied heterogeneous catalysis. J.F. Le Page. Paris 1987.Sol-Gel Science: The Physics and Chemistry of Sol-Gel ProcessingC. J. Brinker and G. W. Scherer, Academic Press, San Diego, Calif, USA, 1990.

Example 2. Xerogels vsAerogels

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• WO3/ZrO2 systems were prepared by sol-gel technique using different procedures for the solvent extraction: evaporation in vacuum at room temperature (xerogel) and extraction in supercritical conditions (aerogel).

• Two reaction were investigated under mild conditions: i) acylation of veratrole with acetic anhydride; ii) acylation of anisole with benzoic anhydride.

Example 2. Xerogels vsAerogels

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• WO3/ZrO2 (ZW) present high catalytic activity and stability in process requiring medium-high acidity, such as acylation and isomerization reactions.

• The physicochemical properties of ZW catalysts and, in particular, their acidity strength are strictly correlated to the synthetic procedure and mainly to the relative amount of each oxide.

• Usually ZW materials are prepared by impregnation of zirconium oxyhydroxide with ammonium metatungstate and subsequent high temperature oxidation or by co-precipitation of the zirconium and tungsten precursors.

• But this conventional mixed oxides synthesis techniques do not usually produce homogeneous materials with high surface area.

• Sol-gel synthesis allows the strict control of the physicochemical properties and of the homogeneity of the final materials. This method stabilizes the surface area and the pore volume of the catalysts with respect to traditional synthetic procedures.

Example 2. Xerogels vsAerogels - Experimental

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1. Gel

Mixing metal alcoxide precursors Zr(OC3H7)4 & W(OC3H7)6

+ Mixture of i-PrOH + Water in presence of HNO3.

2.1 Xerogel (XZW) 2.2 Aerogel (AZW)

Removing the solvent from the wet gel under vacuum (10-2 Torr) at room temperature.

Supercritical elimination of the solvent (i-PrOH) in Autoclave at 250 ºC and 4.5x104 Torr.

2. Drying

3. Calcination Flowing air (30 mL/min) for 3 h at 800 ºC.

Example 2. Xerogels vsAerogels - Results

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• i) Acylation of veratrole with acetic anhydride

• ii) Acylation of anisole with benzoic anhydride.

2 – methoxybenzophenone

4 – methoxybenzophenone (desired product)

3, 4 – dimethoxy acetophenone

• Acylation of aromatic compound leads to intermediates for the production of dyes, pharmaceuticals, agrochemical and fragrances.

• Acylation of veratrole leads to the synthesis of p-acetoveratrole. Precursor of a component in an insecticide formulation (verbutin)

• Acylation of anisole leads to the synthesis of p-acetoanisoleprecursor of a sun protector (Parsol).


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• WO3: Nominal concentration 15% for all samples.

• AZW: Aerogel sample.

• XZW: Xerogel sample.

• WO3: A commercial WO3 oxide (99% Aldrich) was used as reference.

• AZ: Simply zirconia prepared by sol-gel (Aerogel).

• B-Zeolite: Used as reference (Commercial).

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Example 2. Xerogels vsAerogels – Recycling test

• The catalysts were recovered and reused for three runs. • The catalysts deactivation in these reaction is likely due to the presence of

organic species adsorbed on the surface (indicated by the brown color assumed by the exhausted catalysts), which are difficult to remove by simply washing with solvents.

• Therefore, for all these catalytic systems a regeneration step by thermal treatment was required.

• After 3 h of reaction, the catalysts were filtered from the slurry, washed either with anisole or veratrole, dried at 110 ºC overnight and regenerated by thermal treatment in air at 450 ºC for 90 min.

• Also the β-zeolite was recovered and regenerated but already in the first reuse it is turned out to be completely inactive.

• In the case AZW system the catalytic activity was restored almost completely after calcination

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Example 2. Xerogels vsAerogels – Recycling test

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Example 2. Xerogels vsAerogels – Conclusions

• WO3/ZrO2 catalysts were synthesized using sol-gel method. • All samples were found to be active in acylation reaction of veratrole and

anisole with good conversions and yields.• The synthetic parameters and, in particular, the solvent extraction step causes

some important variation in the physic-chemical features of the systems researched and in their catalytic activity.

• The best performance was achieved by the aerogel sample obtained by the supercritical extraction of the solvent that prevents the collapse of the porous network of the wet gel and it allows to obtain expanded structure with unusual porous texture.

• These more porous primary particles are more stable during high temperature calcination treatment and lead to final materials with higher surface area, different pore organization and optimal density of W/Zr.

• The Aerogel sample plays a central role especially in the recycling, allowing the complete restoration of the catalytic activity, by a proper calcination.

Cryogels – Freeze Dryer

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• Cryogels: Another way of avoiding the presence of the liquid-vapor is freeze drying, in which the pore liquid is frozen into a solid that subsequently sublimes this one under vacuum to give an aerogel-like material called a cryogel.

• Frezee-drying was introduced into aerogel processing as early as 1981 in Japan. Represents a safer and more economical alternative to supercritical drying, and allows water as a solvent.

• Cryogels Synthesis :

• Preparation of cryogels consists of three independent, but mutually related steps:

• i) Gel formation.

• ii) Drying

• iii) Heat treatment – Calcination


Cryogels – Example 3

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• Work objective:

• Develop an economical method that permits preparation of larger size samples of high surface area alumina and chromia based catalysts.

Cryogels – Example 3. Synthesis.

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• Gel precursors: Nitrates because, in comparison with other inorganic salts, they have the lowest decomposition temperatures.

1) Aqueous solutions of precursor nitrates of desired composition were prepared.

2) Gel precipitation: using aqueous ammonia or urea. (Avoid organic agents because might require high temperature calcination for complete removal).

3) Single component pH values were kept at 5 or 8, but multi-component gels (NiO/Al2O3, La2O3/Al2O3) required the pH between 7 and 8.

4) Excess liquid remove (f.i: 1/2 of the total volume is removed before frezing)5) Washed gels were frozen either by pouring or spraying into liquid nitrogen. 6) Remove of liquid nitrogen and dry the frozed material under vacuum.

Cryogels – Example 3. Results.

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• Similar to aerogels, cryogels have very low bulk densities, high specific volumes which result from vey low shrinkage. Cryogels are lighter than xerogels, having large volumes of interparticle porosity.

• Cryogels can have very low bulk densities, high specific surface areas, and a relatively large volume of micropores.

• The preparation process involving freeze-drying as an alternative to supercritical drying, has several advantages: wide range of low cost precursors, low cost solvent (water), safety, (freeze) dry gel free of organic residues, simplifying thereby the final heat treatment.

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Cryogels – Example 3. Conclusions.

Topic 5 - Synthesis by Sol-gel Methods (MJC)

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