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 spacewithin their structure” is inherent , porous materials are difficultto make naturally.But Einstein say “ in the middle of difficulty lies opportunity”The above statement was made true by the Mobil scientist in theyear of 1992 by successfully synthesizing the Mesoporousmaterials (MCM-41 and MCM-48) by using soft templatestrategy.This opened a new area of materials called Mesoporousmaterials, and still lot of work are fascinating in this field.
What are Porous materials Most generalized definition of porous materials is continuous andsolid network material filled through voids. A material can be recognized as porous if itsinternal voids can be filled with gases.The history of porous materials began with the zeolites havingaluminoisilicates framework which was synthesized by the use singletemplate molecule with small pore.
Classification of porous materials Depending on pore size Depending on building frameworkMicropoorus Mesoporous Macroporous<2nm 2-50 nm >50nm ZSM-5 MCM-41 Sponge Purely inorganic Organic Inorganic Purely hybrid organic 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 templatemethod.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 natureTO4 networks TO4,TO5 and TO6 networksSi and Al are four connected by covalent Si and Al are 2 or three connected ,morebond, less surface hydroxyl groups surface hydroxyl groupsHydrophobic surface area Hydrophilic surface areaHigh hydrothermal stability Less hydrothermal stabilityCrystalline walls and are thick Amorphous walls and are thinSynthesis temp is high (80 – 300 °C) Synthesis temp is low (-10 to 120°C)Long crystallization time Formation rates are fastAqueous media is required for Non –aqueous solvents and non polarcrystallization solvents can be usedSynthesis of zeolite is carried out in Synthesis pH rang is from 0 to 12neutral /weakly acid media
Synthesis of mesoporous materialsSoft template (endo template) Hard template (exo template, nano casting)• Uses soft templates like organic •Uses inorganic materials like silica, carbon etcmolecules . •Tedious work up, hard to get good morphology and costly•Good shape , Size and morphology. CMK-1 simple• SBA-15 ,MCM-41 etc Meso-silica CMK-1
Synthesis of mesoporous materials using soft template strategy1. 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 •Excellent solubility •Excellent solubility•High critical Micelle con. • repulsion between the • High critical micelle temp.•acidic and basic media anionic surfactant is more. •Acidic / basic media• toxic and expensive • non-toxic and cheap
2. Formation of micelle• At a Low surfactant concentration will favorarrangement on the surface.• As the concentration increases surface being morecrowded as result molecule arrange in to micelles.• At certain concentration the surface is completelyloaded and any further addition leads to theMicelle arrengment.This conc. is known as CMC.• Beyond the CMC self assembly of micelle occurs tofrom 3D and 2D rod like arrays.• Different template have the different CMC.• To get ordered materials – 0 to20 mg/L
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 lV=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 throughintermediate ions Acidic-Medium Basic -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 regularitys of products. Reorganization, growth and crystallization 80 -150 °C is temperature is usually used. High temperature will lead to the disorder and decomposition ofsurfactants. 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 Light irradiation Calcination•Slow heating rate. Solvent Extraction • Microwave• N2 -1 hr • Solvent ethanol /THF •Ultraviolet rays O2- 4-6 hr – 2 °C/min • small HCl is added O3 and O• O2-1 °C/min • Surfactant can be reused• No surfactant recover• low Surface silanol groups• Not good for low thermal stable materials•P-123 -550 °C• CTAB -350 °C
Application Chemical Catalysis1. Mesoporous Materials as catalystAl-MCM-41 ,B-MCM-41, Sn-SBA-15 - AcidicK- AlMCM-41 , Cs-AlMCM-41 -basicTiMCM-41 ,V-MCM-41 -Redox
2.Mesoporous Materials as catalyst support • Noble metal supported catalyst supports Au-McM-41 , Pt- CMK-3 • Metal oxide supported catalysis Fe2O3- MCM-41 ,RuO2-SBA-15 • Metal complex supported catalysts Mn-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
1) Sustained Drug Release SystemVirtually any drug within a size range compatible with the size of the meso-channels has potential to be encapsulated and further deliver intracellularly. M. Vallet-Regi, et al. “A New Property of MCM-41: Drug Delivery System”, Chem. Mater. 2001, 13, 308-311.
2. Mesoporous Materials for Bone Tissue Engineering
Mesoporous Bioactive scaffoldsScaffolds are needed that can act as temporary templates for bone regenerationand actively stimulate vascularized bone growth so that bone grafting is nolonger 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 tostimulate osteoprogenitor cells.
Pioneers in Mesoporous Materials Galen Stucky SBA-15 Fudan university FDU-11Ryong RyooKIT -6Hard templateCMK materials P.Selvum IITM-56, NCCR-56
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 Thank you