Mesoporous Materials – Synthesis and Applications

Vijaykumar S.Marakatti,PPISR, Bangalore.

Outline of the talk

Introduction to porous materials

Classification of porous materials

Synthesis mechanism of mesoporous materials

Applications of mesoporous materials

Introduction“the overwhelming tendency for solids to minimize void space within their structure” is inherent , porous materials are difficult to make naturally.

But Einstein say “ in the middle of difficulty lies opportunity”

The above statement was made true by the Mobil scientist in the year of 1992 by successfully synthesizing the Mesoporous materials (MCM-41 and MCM-48) by using soft template strategy.

This opened a new area of materials called Mesoporous materials, and still lot of work are fascinating in this field.

What are Porous materials

The history of porous materials began with the zeolites having aluminoisilicates framework which was synthesized by the use single template molecule with small pore.

Most generalized definition of porous materials is continuous and solid network material filled through voids.

A material can be recognized as porous if its internal voids can be filled with gases.

Classification of porous materials

Depending on pore size Depending on building framework

Micropoorus Mesoporous Macroporous

<2nm 2-50 nm >50nm

Purely inorganic Organic Inorganic hybrid

Purely organic

ZSM-5 MCM-41 Sponge

Silica MOF organic porous polymers

Mesoporus Materials Meso a Greek prefix – “ in between ” - micro and macro porous system Mesoporous materials may be ordered or disordered.

They possess high Surface area -400 -1000 m2/g Large pore volume High stability -500 -600 °C

They are usually synthesized by the use of Soft template method.

Ex: MCM-41,SBA-15,FDU-11,IITM-56 etc.

Difference between the zeolites and mesoporous materials

Zeolites Mesoporous MaterialsHighly crystalline Periodic arrangement with amorphous in

nature

TO4 networks TO4,TO5 and TO6 networks

Si and Al are four connected by covalent bond, less surface hydroxyl groupsHydrophobic surface area

Si and Al are 2 or three connected ,more surface hydroxyl groups Hydrophilic surface area

High hydrothermal stability Less hydrothermal stability

Crystalline walls and are thick Amorphous walls and are thin

Synthesis temp is high (80 – 300 °C)Long crystallization time

Synthesis temp is low (-10 to 120°C)Formation rates are fast

Aqueous media is required for crystallization

Non –aqueous solvents and non polar solvents can be used

Synthesis of zeolite is carried out in neutral /weakly acid media

Synthesis pH rang is from 0 to 12

Synthesis of mesoporous materials

Soft template (endo template)

• Uses soft templates like organic molecules .

•Good shape , Size and morphology. simple• SBA-15 ,MCM-41 etc

Hard template (exo template, nano casting)

• Uses inorganic materials like silica, carbon etc

• Tedious work up, hard to get good morphology and costly

CMK-1

Meso-silica

CMK-1

Synthesis of mesoporous materials using soft template strategy

1. Surfactants.2. Formation of Micelles.3. Inorganic precursor .4. Interaction of Micelles with inorganic precursor.5. Hydrothermal treatment followed by separation and drying6. Removal of template.7. Proposed mechanism8. Characterization of material.

1. Surfactant/ Template/Structure directing agent. Large organic molecules(High molecular weight) with both hydrophilic and hydrophobic groups.

Depending upon charge they can be classified as

Cationic Anionic Non ionic/neutral•Excellent solubility•High critical Micelle con.•acidic and basic media• toxic and expensive

•Excellent solubility• repulsion between the anionic surfactant is more.

•Excellent solubility• High critical micelle temp.•Acidic / basic media• non-toxic and cheap

2. Formation of micelle• At a Low surfactant concentration will favor arrangement on the surface.

• As the concentration increases surface being more crowded as result molecule arrange in to micelles.

• At certain concentration the surface is completely loaded and any further addition leads to the Micelle arrengment.This conc. is known as CMC.

• Beyond the CMC self assembly of micelle occurs to from 3D and 2D rod like arrays.

• Different template have the different CMC.

• To get ordered materials – 0 to20 mg/L

Different type of surfactant arrangement

What Makes them to show different type of arrangements ? 1. Critical micelle concentration Low micelle conc. are good to get ordered materials.

2. Packing parameter (g) g = V / ao l

V=Total volume of surfactant hydrophobic chains+ co-solvnet.ao= Effective hydrophilic head group area at the aq. micelle surface.l = kinetic surfactant tail length.

g <1/3 = cubic and 3 D hexagonal,1/3<g>1/2 = 2D hexagonal1/2<g>2/3 = cubicg=1 = Lamellar

3. The hydrophilic and hydrophobic Volume ratio (VH/VL)

3.Inorganic precursor and pHInorganic Precursor silica depends upon pHBasic synthesis (pH =9.5 to 12.5)-Polymerization and cross Linkage of silicate species are reversibleSilica gel, colloidal sol, Water glass , TEOS etc.Acidic synthesis(pH= 1 to 2 )- Irreversible.- Slow hydrolysis TEOS is preferred.

4. Interaction of Micelles with inorganic precursor.

Direct interaction of Surfactant with inorganic precursor

Basic -Medium Acidic-Medium

Interaction of Surfactant with inorganic precursor through intermediate ions

Basic -MediumAcidic-Medium

Interaction of non ionic Surfactant with inorganic precursor through intermediate ions.

Hydrogen bonds favors most of times

5. Hydrothermal treatment followed by separation and drying.

Method to improve mesoscopic regularity's of products.

Reorganization, growth and crystallization

80 -150 °C is temperature is usually used.

High temperature will lead to the disorder and decomposition of surfactants.

Separation – filtration or centrifugation.

Washing –alcohol or water. Basic media needs through washing.

Drying at room temperature is good.

6. Removal of template. Removal of template will give rise to mesoporosity

Different ways by which template can be removed

Calcination•Slow heating rate.• N2 -1 hr O2- 4-6 hr – 2 °C/min• O2-1 °C/min• No surfactant recover• low Surface silanol groups • Not good for low thermal stable materials•P-123 -550 °C• CTAB -350 °C

Solvent Extraction• Solvent ethanol /THF• small HCl is added• Surfactant can be reused

• Microwave•Ultraviolet rays O3 and O

Light irradiation

p6mmIa3d,

Pm3n

Im3mFd3m

Fm3m.

MCM-48

FDU-2SBA-16

KIT-5

SBA-1 and 6

SBA-15

Proposed mechanisms for synthesis mesoporous materials

1. Silicate rod assembly

2 or 3 monolayer's of silicates species first deposits on isolated surfactant miceller rods

2. Cooperative self assembly

Low concentration Surfactant

3. True liquid crystal templating route

High concentration Surfactant

Complete synthesis summary

7. Characterization of mesoporous materials1. Low angel XRD 0.5 -5 -2theta - orederness / disorder.

2. N2 sorption measurements –BJH method

Surface area/pore diameter /pore volume

3. TEM images Order ,Morphohology , Wall thickness

Applications

Application Chemical Catalysis1. Mesoporous Materials as catalyst

Al-MCM-41 ,B-MCM-41, Sn-SBA-15 - Acidic K- AlMCM-41 , Cs-AlMCM-41 -basicTiMCM-41 ,V-MCM-41 -Redox

2.Mesoporous Materials as catalyst support• Noble metal supported catalyst supportsAu-McM-41 , Pt- CMK-3• Metal oxide supported catalysis Fe2O3- MCM-41 ,RuO2-SBA-15• Metal complex supported catalystsMn-Salen –SBA-15

3. Application in environmental catalysisImmobilization of enzyme on Mesoporous Materials as catalysts Enzymes are excellent biocatalysts with high selectivity and efficiency in environment field, chemical and pharmaceutical industries are limited by their poor stability and chemical sensitivity

4. Good adsorbents

5.Applications in Biomedical field

1) Sustained Drug Release System

M. Vallet-Regi, et al. “A New Property of MCM-41: Drug Delivery System”, Chem. Mater. 2001, 13, 308-311.

Virtually any drug within a size range compatible with the size of the meso-channels has potential to be encapsulated and further deliver intracellularly.

2. Mesoporous Materials for Bone Tissue Engineering

Mesoporous Bioactive scaffoldsScaffolds are needed that can act as temporary templates for bone regeneration and actively stimulate vascularized bone growth so that bone grafting is no longer necessary.

Bioactive glass is an ideal material because it rapidly bonds to bone anddegrades over time, releasing soluble silica and calcium ions that are thought to stimulate osteoprogenitor cells.

1 Day

3 Day

7 Day

6.Mesoporous Polymers

Pioneers in Mesoporous Materials

Galen Stucky SBA-15

Ryong RyooKIT -6Hard templateCMK materials

Fudan universityFDU-11

P.SelvumIITM-56, NCCR-56

Thank you

References:“On the Controllable Soft-Templating Approach to Mesoporous Silicates”Ying Wan and Dongyuan Zhao, chemical reviews, Volume 107, Number 7 .“Soft templating strategies for the synthesis of mesoporous materials: Inorganic,organic–inorganic hybrid and purely organic solids”Nabanita Pal, Asim Bhaumik, Advances in Colloid and Interface Science 189–190 (2013) 21–41