Synthesis of porous solids

1
Synthesis of (porous) solids

• Rajiv Kumar
• Course work in Catalysis (2007)
• National Chemical Laboratory,
• Pune

2
Outline

• Lecture 1 Solution chemistry of metal ions,
  hydrolysis, condensation, complexation and
  alkoxide (sol-gel) chemistry.
• Lecture 2 Co-precipitation, Super-saturation,
  Nucleation, Growth and Growth termination
• Lecture 3 Factors influencing synthesis of
  Zeolites and related materials
• Lecture 4 Recent advances in the preparation of
  porous inorganic-organic hybrid materials.

3
Solution chemistry of metal ions

• Let us understand Water
• Behaves both as (i) Ligand (ii) Solvent.
• Water molecule is Lewis Base (via 3a1 molecular
  orbital).
• High dipolar moment (µ 1.84 Debye).
• High Dielectric constant (e 80).
• Good solvent for most of the ions breaking most
  of the polar bonds (ionic dissociation).
• Solvate both cations and anions.
• Water (Lewis base) reacts with metal cations
  (Lewis acid) as Acid-Base reaction, where OH2 ,
  OH or O2 ligands are formed.
• Hence parameters like pH, temperature and
  concentration are very important to understand
  chemical nature of (solvated) aqueous species.
• Supercritical temperature / pressure density?

4
Metal cations Partial Charge Model

• When two or more atoms, initially different in
  electro-negativity, combine then they adjust to,
  and share with, the same intermediate
  electro-negativity in the compound.
• Consequence
• For a given atom X its
• (i) electro-negativity (?x) and (ii) partial
  charge (dx) vary when the atom is chemically
  combined.
• Hence, ?x and dx must have a (linear) relation
• ?x ?x0 ?x dx (where ?x is Hardness of atom
  X)
• Hardness is related to softness as sx 1/ ?x,
  which is a measure of polarizability of the
  electron cloud around X.
• Softness increases with the radius r of X
  and then Hardness varies with 1/r.

J. Livage, Structure and Bonding, 77 (1992) 153
5
Hydrolysis of Metal Cations

• Reaction between metal cations with water where
  protons are liberated producing either hydroxy or
  oxy species.
• Formation of M?OH2 draws electron away from s
  bonding orbitals of water molecule, thereby
  weakening of OH bond takes place.
• Hence, coordinated water molecules behave as
  stronger acid (than the solvent water molecules)
  leading to spontaneous de-protonation as
• M(OH2)Nz h H2O ? M(OH)h(OH2)N-h(z-h) h
  H3O
• Where h is Hydrolysis Ratio, a measure of the
  number of protons removed from solvation sphere
  of metal cation.
• Hydrolysis Ratio (h) of a given
  M(OH)h(OH2)N-h(z-h) species mainly depends
  upon
• pH of the solution and Oxidation state of
  metal cation Mz

6
Charge-pH diagram for hydrolyzed species
Z
O2
6
OH
OH-
4
h 2N-1
2
OH2
h 1
0
0
7
14
pH
The stability order OH2 lt OH- lt O2-
7
Hydrolysis of SiIV (N4) and AlIII (N6, N4)
h
SiO2(OH)22
6
SiO(OH)3
4
Si(OH)40
Al(OH)4
Al (OH)3(OH2)30
Si(OH)3(OH2)
2
Al(OH2)63
0
0
4
8
0
4
8
12
12
pH
pH
Four hydrolyzed species HnSiO4(4n) can be
found in aq. solution based on pH. Td
coordination in whole pH range
Coordination decreases from Oh to Td as pH
increases. This change occurs around neutral pH.
8
Condensation of hydrolyzed species

• The pH modification can lead to condensation of
  M-OH species.
• Increasing pH (by adding base) condenses
  low-valent aquo-cations.
• Decreasing pH (by adding acid) condenses high
  valent oxy-anions.
• Large condensed species are obtained at/around
  Point of Zero Charge (PZC) leading to
  precipitates or gels.
• Below or above PZC, less condensed solute species
  (poly anions or poly cations) can be formed.

• Since the enthalpy change for first hydrolysis
  reaction is positive (often quite close to
  enthalpy of dissociation of water (13.3
  kcal/mole)), the tendency of metal cations to
  hydrolyze increases with temperature.
• This hydrothermal concept is commonly used in the
  synthesis of molecular sieves and mono-dispersed
  colloids / nano-sized particles.

9
Condensation of precursors containing at least
one M-OH group to form polynuclear species via
elimination of water

• Olation Nucleophilic addition of OH- on to
  hydrated metal cation.
• M-OHd- Md-OH2 ? M-OH-Md-OH2d ? M-OH-M
  H2O
• This reaction involves ol bridging H3O2-
  ligand, with a characteristic distance of about
  0.5 nm between metal atoms.
• H H H
• gtM-O HO-Mlt ? gtM-O.H-O-Mlt ? gtM-O-Mlt H2O
• H H

Oxolation Condensation of two OH groups to form
one water molecule, which is removed by giving
rise to an oxo bridge H gtM-OH HO-Mlt
? gtM-O ...H-O-Mlt ? gtM-O-Mlt H2O
Dehydration of Olate species leads to
oxo-species. Olation is generally faster than
oxolation
10
Condensation Olation and Oxolation
Olation
Oxolation
11
Complexation of cationic precursors

• Generally, anions (counter ions) are present in
  aqueous solutions of metal cations.
• Such counter ions can play important role in
  hydro-thermal chemical transformations.
• Aquo-cations and anions can interact to form a
  complex
• M(OH2)NZ aXX- ? M(X)a(OH2)N-aa(Z-aX)
  aaH2O
• Where a is dentateness of the complexing anion
  corresponding to the number of water molecules
  are replaced by anionic ligand X.
• Complexation can be described as the nucleophilic
  substitution of water molecules by anions in the
  coordination sphere of metal cation. However, in
  the presence of large excess of water molecules
  in the vicinity of complex, the reverse can also
  occur.

12
Complexation and Electro-negativity

• For stabilization of complexes, the ionic
  dissociation (by solvent water) and hydrolytic
  dissociation (via nucleophilic substitution of HX
  by water molecules) of M-X bonds have to be
  avoided.
• Both of these reactions will depend on the extent
  of electron transfer between metal and anion in
  MX bond.
• The complexing ability of an anion will, then,
  depend on the electronegativities of
• Complexed precursor MX (?MX ) and the anion X-
  (?X).
• Now, if
• ?X gt ?MX Electron density is withdrawn from
  the metal towards anion making the M-X bond more
  polar leading to ionic dissociation.
• ?X lt ?MX The electron density is pushed
  towards the metal providing the M-X bond an
  increased co-valent character and thereby making
  the M-X complex more resistant towards ionic
  dissociation (i.e. stabilization).
• ?ID (ionic dissociation) and ?HD (hydrolytic
  dissociation) ?ID ? ?X ? ?HD is the
  electronegativity range for stable complexation.

13
Complexation and pH

• Condition for complexation like ?ID ? ?X ? ?H
  can, however, be applied to a given complex
  precursor at a given pH. (Why?)
• Change in pH leads to protonation or
  deprotonation reactions.
• When pH increases, the cationic precursors get
  (i) deprotonated and (ii) their ve charge
  decreases.
• When pH decreases the anions are protonated
  giving rise to their acid forms.
• However, in both cases the M-X complex remains
  stable over a limited range of pH only.
• Hence, the optimum range of pH at which the
  complexes can be formed SHIFTS towards higher
  value, when
• Charge on MZ decreases
• Mean electronegativity of the anion decreases or
• Dentateness (a) of the complexing anion (ligand)
  increases.
• Certain anions can, then, be complexing at low pH
  at the onset of condensation process and become
  non-complexing during nucleation and crystal
  growth.

14
FeIII species as a function of pH and
concentration
pH
15
Formation of Fe3-silicate species via
Complexation
16
Alkoxides Hydrolysis, Condensation complexation
M. Ebelmen Ann Chem Phys. 16 (1846) 129 SiCl4
ROH ? Si(OR)4 4HCl

• M-OH vs. M-OR (H in metal hydroxide is replaced
  by R, an alkyl group), hence the chemistry is
  different.
• However, once hydrolyzed, the chemical reaction
  becomes similar to that in aq. solution.
• Hydrolysis gtM-OR H-OH ? gtM-OH ROH
• Condensation gtM-OR M-OH ? gtM-O-M ROH
• Complexation gtM-OR L-OH ? gtM-O-L ROH
  (Lorganic/inorganic ligand)
• Basically, these reactions start with the
  nucleophilic addition of OHd- to Md leading to
  an expansion of coordination number of the metal
  during transition state
• Then the vely charged Hd (proton) is
  transferred towards an alkoxy group leading to
  the dissociation of vely charged protonated
  ligand (ROH) from the metal.

17
Alkoxides and sol-gel chemistry

• Reactivity of metal alkoxides towards hydrolysis
  and consequent condensation mainly depends on
• Positive charge of metal atom dM and
• Ability of the metal atom to expand its
  coordination number N.
• Hence, the stability of a metal alkoxide towards
  hydrolysis decreases with increasing
  electropositive character of the metal Si(OEt)4
  gt Ti(OEt)4 ? Zr(OEt)4
• The stability of metal alkoxide towards
  hydrolysis increases with increasing chain length
  or bulkiness (and to some extent even small
  difference in the electronegative character) of
  the alkyl group Si(OMe)4 lt Si(OEt)4 lt
  Si(OPri)4
• SiIV is fourfold coordinated (Nz4) both in the
  precursor (hydrolysed or alcoholized) as well as
  in oxide form. There is no expansion of
  coordination number and thereby Si-alkoxides are
  always monomeric.

D.C. Bradley, R.C. Mehrotra and D.P. Gaur, Metal
Alkoxides, Academic press, London 1979
18
Complexation of metal alkoxides Controlled
hydrolysis and condensation

• Complexation of alkoxides by organic ligands
  (i.e.acac) leads to increased stability against
  hydrolysis.
• Complexation is used to tailor the chemical
  reactivity of highly reactive alkoxide towards
  uncontrolled hydrolysis and condensation.
• For example
• Ti(Pri)4 is highly reactive towards hydrolysis
  (moisture sensitive).
• When complexes with acetyl acetone (acac)
  (Ti/acac 1, mole/mole) highly stable
  Ti(Pri)3(acac) monomers, where Ti is fivefold
  coordinated, are formed and condensation is
  prevented.
• Zr-alkoxides also behave in the similar way.
• Bulky (secondary and tertiary) alkoxy groups tend
  to prevent oligomerization

19
Ti-alkoxides and complexes
R C2H5
AA-complex
Acac-complex
R Pri
20
Lecture - 2

• Solution chemistry of metal ions, hydrolysis,
  condensation, complexation and sol-gel chemistry.
  
• Co-precipitation / Super-saturation,
• Nucleation and Growth,
• Growth termination.
• Factors influencing synthesis of Zeolites and
  related materials.
• Recent advances in the preparation of porous
  inorganic-organic hybrid materials.

21
Precipitation

• Preferred way of precipitation is through such
  chemical reactions which produce the product with
  low solubility in the reaction medium.
• Low solubility of the product leads to quickly
  reach Super saturation state.
• For simple binary system, mentioned below,
  precipitation is also simple and
  straight-forward.
• xAy(aq.) yBx- (aq) ? AxBy(s)
  Ksp (aA)x (aB)y
• where Ksp is the solubility product constant,
  and aA and aB are the activities of cation A and
  anion B.
• Solubility (and hence precipitation) can be
  influenced by, among other parameters, the
  temperature and concentration.

22
Co-precipitation

• In complicated ternary and quaternary systems,
  the precipitation is rather more complex.
• Multiple species are precipitated
  simultaneously (hence the term
  co-precipitation).
• However, the presence of multiple species with
  different solubilities under same temperature and
  dilution, poses difficult problem.
• Possible presence of concentration gradients of
  different species during the process of
  homogeneous co-precipitation is another problem
  to be addresses.

23
Super-saturation and Precipitation

• The most important parameter for any
  precipitation is the extent or degree of
  Super-saturation (S) given by
• S aAab / Ksp
• or simply by
• S C/Ceq
• (where C and Ceq are solute concentrations at
  super-saturation and at equilibrium,
  respectively.
• Hence the difference between C and Ceq is the key
  driving force behind precipitation
• ?C C - Ceq

24
Nucleation

• Nucleation can begin in supersaturated
  solutions.
• Just at the on-set of nucleation, there exists an
  Equilibrium Critical Radius (Req) of the solute
  particles
• Req is inversely proportional to ?C (?C
  C-Ceq).
• Nuleated particles with
• R gt Req will continue to grow R lt Req will
  dissolve
• (where R is the critical radius of the solute
  particle).
• The nucleation rate (RN) is an exponential
  function of S (SC/Ceq).
• It means that RN remains negligible until a
  certain critical supersaturation degree, S, is
  reached.
• The nucleation rate (RN) is RN (dN/dt) (1/V)
  A exp(- ?Geq) / kT
• (where N is the number of Nuclei formed per unit
  time (t) and per unit volume (V) A is
  pre-exponential factor (typically ranging between
  1025 to 1056 s- m-3) and k is Boltzman constant.

25
Growth of precipitated particles

• In principle, the growth process can be limited
  by diffusion (physical parameter) or reaction
  (chemical parameter).
• However, experimental evidences are in favor of
  Diffusion Limitation influencing the growth /
  precipitation process.
• Hence, temperature as well as concentration
  gradient become very crucial for determining the
  growth rate.
• Why? Because the desired ingredient (new
  monomeric material needed for growth) is to be
  supplied to the growing particle surface via long
  distance mass transfer.
• The relationship between Growth Rate (G) and the
  supersaturation ratio, S, may be represented as
• G kG Sg
• (where kG is growth rate constant and g is growth
  order)

26
Coarsening (Ostwald Ripening) Smaller particles
are consumed by larger particles

• LSW (Lifshitz and Slyozov and Wagner) Theory
• Salient features based on complex Mathematics
• The particle size is proportional to the cube
  root of time (t).
• During diffusion controlled ripening, the number
  of solute particles increases with time, t.
• For smaller particles, the nucleation process
  must be fast and growth process must be slow.
• For narrow size distribution of small particles,
  all reactive species must be formed
  simultaneously with out any secondary nucleation
  and growth process.

Lifshitz and Slyozov (J. Phys. Chem. Solids
19(1961)35) and Wagner (Electrochem, 65(1961)
581) (LSW theory)
27
Growth Termination

• Since, thermodynamic of precipitation favor
  maximization of surface/volume ratio, the
  agglomeration of smaller particles is practically
  inevitable, particularly in the absence of
• a stabilizing / capping agent (e.g. in the
  synthesis of nano particles).
• Depletion of active ingredients (reactive
  species) below super-saturation level in a given
  reaction medium and volume (e.g. zeolite
  synthesis).
• This means that when the concentration of
  reactive species is depleted and super-saturation
  stage is disturbed, slowed down or avoided, the
  termination of growing particles (growth process)
  can occur.

28
Constructing Crystalline Microporous Zeolitic
Structures Primary, Secondary and Tertiary
Building Blocks

• Out line ..
• Zeolites Brief Structural Introduction
• Various soluble silicate and aluminosilicates
  species.
• Basic Building Units
• Primary (PBU)
• secondary (SBU)
• Tertiary (TBU)
• Basic Steps in Zeolite Synthesis

29
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30
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31
MFI Pore dimensions
SBU to MFI
32
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33
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34
Structure of Pentasil Zeolites
35
MFI (ZSM-5)
MOR (Mordenite)
BEA (Beta)
LTL (K-L)
36
Lecture 3FACTORS INFLUENCING THE SYNTHESIS OF
MICRO- AND MESO-POROUS SOLIDS

• Physical Factors
• Temperature
• Time
• Agitation / Stirring
• Aging
• Microwave heating

• Chemical Factors
• Alkalinity (pH) OH/Si
• Oxyanions as promoters
• Dilution (H2O/Si)
• Hetero metal (Fe,Ti, etc)
• Templates
• Inorganic cations (Na,K)

37
Basic steps in zeolite / molecular sieve
synthesis

• Hydrothermal synthesis of zeolites consists of
  the series of complex physico-chemical processes.
• Hydrolysis of silica / aluminum / metal source
  to form gel or solution.
• Dissolution / mineralization of the gel phase.
• Nucleation of zeolite structure (from gel or
  solution phase).
• Continued crystallization / crystal growth.
• Dissolution of any initial metastable phase.
• Crystallization / crystal growth of stable
  condensed phase

38
Factors influencing zeolite synthesis under
basic medium

• Alkalinity (OH/Si) Optimum range needed to get
  soluble silicate species (mainly Q0-Q3) required
  for nucleation and crystallization

Q0
Q1
Q2
Q3
Q4
decrease in pH

• Alkali metal ions (Na, K etc.) exhibit
  significant influence in stabilizing desired
  product. Charge balancing role in M3Silicate
  zeolites.

39
How to derive gel composition?
Take in to account various chemical
reactions Na2Si03.xH2O H2O NaOH
Si(OH)4 Na2O SiO2 H2O NaAlO2.xH2O
H2O NaOH Al(OH)3 Na2O Al2O3
H2O Al2(SO4). xH2O H2O H2SO4
Al(OH)3 H2SO4 Al2O3 H2O
OH/SiO2 molar ratio is very important
parameter. OH- Na - 2SO42- Increase in pH
during crystallization indicates the formation of
stable crystalline phase.
40
Comparision of pH and Crystallization in EU-1
Casci and Lowe Zeolites 3 (1983) 186-187
41
Alumino-silicate ring formation scheme
Si/Al gt 4
Si/Al4
Si/Al lt 4
42
Si/Al lt 4 Low silica zeolites Lowenstein rule
43
Effect of different gel compositions on synthesis
of MFI Zeolite
443
433
120
170
423
150
413
20
40
60
160
140
6
25
35
15
2
4
45
12
44
Templates (N-containing organic bases QUATS) in
zeolite synthesis

• Structure directing
• Void Space filling
• Charge balancing role.
• One template can direct more than one zeolite
  structure.
• One zeolite structure can be formed using
  different templates

45
13C-CPMAS NMR of Pyrollidine template used in
synthesizing different zeolites
As synthesized Py-ZSM-23 (10MR)
As synthesized Py-ZSM-51 (8MR)
Py
As synthesized Py-ZSM-5 (10MR)
Py-adsorbed in Calcined ZSM-5
As synthesized Py-ZSM-48 (10MR)
Py-adsorbed on SiO2
46
HETERO METAL-IONS IN ZEOLITE SYNTHESIS (BASIC
MEDIUM)

• TYPE-II
• INSOLUBLE/SPARINGLY SOLUBLE IN ALKALINE MEDIUM
  (pH gt 10)
• Examples
• M(III) Fe
• M(IV) Ti, Zr
• Rather difficult to incorporate in zeolite
  network and require special conditions.

• TYPE-I
• SOLUBLE IN ALKALINE MEDIUM (pH gt10)
• Examples
• M(III) Al, B, Ga, As
• M(IV) Si, V
• M(V) As
• Relatively easy to incorporate in zeolite
  network if ionic size permits.

47
MAIN FACTORS INFLUENCING THEINCORPORATION OF
HETERO-METALS

• IONIC SIZE (vis-à-vis Si4)
• Too small or too big ions are not compatible
• SOLUBILITY IN REACTION MEDIUM
• Insoluble/sparingly soluble metal oxides or
  hydroxides are difficult to incorporate.
• CHEMICAL NATURE TOWARDS HYDROLYSIS
• Hydrolysis of metal salts / complexes of metal
  (Ti, Zr etc.) should be controlled to avoid the
  formation of metal hydroxide or oxides.

48
Hydrolysis, condensation and complexation of
type-II metal ions/alkoxides (Fe3,Ti4)

• Salient features
• pH plays extremely important role in hydrolysis.
  
• Si4 remains in Td coordination in whole pH
  range.
• The Rate of hydrolysis of Si-alkoxides is slow
  compared to that of Ti-alkoxides or Zr-alkoxides.
• Iron forms insoluble oxides/hydroxides in basic
  pH range.

49
Formation of Ferri-silicate species via
Complexation
50
Ionic Liquids in Iono-thermal Zeolite Synthesis
Cooper et al Nature 430 (2004) 1012-1016
51
Mesoporous solids
52
Integrated Chemical Systems
Size m cm mm ?m nm Å
Macro systems
Components Cells
Integrated Chemical Systems
Macromolecules Polymers
Area of Focus
Molecules
53
Ordered Macroporous Materials
Compositions Silica, alumina, titania,
zirconia, etc. Numerous other metal
oxides Synthesis Polymer latex particle
templates - Polystyrene, (PS)
- Poly(methyl methacrylate), (PMMA) Metal
oxide precursors - Metal alkoxides,
metal salts, mixed precursors -
Solutions in alcohol Properties
Face-centered cubic arrangement of pores
Typical pore size 100 500 nm
54
Polymer Template Synthesis
75 oC, initiator
Centrifuge
Water monomer
Polymer emulsion
Decant liquid, Dry
Monomer - methyl methacrylate - styrene
Polymer - poly(methyl methacrylate), (PMMA)
- polystyrene, (PS)
Polymer particle array
55
The model (cooperative organization) for the
formation of silicatropic liquid crystal
phase/silicate-surfactant mesophases.
Precursor solutions

A

A Organic and inorganic precursor solutions
or

Micelles and isolated cationic
Inorganic silicate anions

surfactant molecules

(for example D4R oligomers)

B Preliminary interaction of the two precursor
solutions after mixing,
Ion exchange

B

or

C Multidentate interaction of the oligomeric
silicate units with the surfactant molecules
SLC assembly

C

Phase
transformation

Lamellar SLC

Hexagonal SLC


56
Mechanisms proposed for the transformation of
surfactant-silicate systems from lamellar to
hexagonal mesophases.,33 and

A
nSiO2 reaction coordinate
(B) folding of kanemite silicate sheets around
intercalated surfactant molecules formed the
hexagonal mesostructure
(A) Hexagonal mesophase obtained by charge
density matching
57
Silicate rod assembly for the formation of MCM-41
(1) and (2) involve the random ordering of
rod-like micelles and interaction with silicate
species (3) represents the spontaneous packing
of the rods and (4) is the remaining condensation
of silicate species upon final heating of the
organic/inorganic composites.
58
Liquid crystal templating mechanism for MCM-41
formation
(B) Silicate anion initiated
(A) Liquid crystal phase initiated
59
M41S Family of Mesoporous Molecular Sieves

MCM-48

MCM-50

MCM-41

(Cubic)
(Stabilized Lamellar)
(Hexagonal)
100
hkl

d(Å)

100

hkl

d(Å)

hkl

d(Å)




X-ray Diffraction Patterns


211

33.0


332

100

39.43


100

39.8


220

28.6


200

19.83

110

22.9


321

21.7

211
420

200

19.8


400

20.3


210

14.9

420

18.1

422
400

332

17.3

431
321

422

16.5

110

431

15.9
200
220
200
210
2
4
6
8
10
2
4
6
8
10
5
10
15
20
30
25
0
Degrees 2-theta
Degrees 2-theta
Degrees 2-theta



Possible Structures
Silica Sheets
TEM Images
60
Tailored Mesoporous Materials
Structure/Property Control

• Mesoporous Materials
• Tailoring Properties
• Control Shapes
• Modify for Applications
• Use as Mold/Nano-Vessel
• Explore Confinement Properties

Vary the Pore Size 1.5nm to gt10nm
Clad the Surface
Vary the Chemical Composition
Anchor Metals and Catalysts
61
Pore Diameter Controlled in MCM-41
Length of surfactant alkyl chain determines pore
diameter

60Å
20Å
10nm
10nm
Surfactant Micelle
100Å
40Å
10nm
10nm
Mesitylene Addition
Addition of solubilization agents increases pore
diameter
Hydrophobic

Micelle
Interior
Auxiliary
Organic
CH
CH
3
3
CH
CH
CH
3
3
3
Hydrophilic

CH
Exterior

3
Mesitylene
Swelled
Micelle
62

Solubilization of Added Organics Increases Pore
Size
Hydrophobic

Micelle
Interior
Auxiliary Organic
CH
CH
3
3
CH
CH
CH
3
3
3
Swelled

Hydrophilic

Mesitylene
Micelle
CH
Exterior
3
75
75
70
70
d-spacing
65
65
60
60
Pore size

XRD d
55
55

100
by Argon

spacing
50
50
physisorption
45
45
40
40
pore size
35
35
30
30
0
0.5
1.0
1.5
2.0
2.5
Moles of Mesitylene/Moles of Surfactant
63
MCM-41 Has High Benzene Sorption Capacity
100
21ºC

Total Gas Flow 190 cc/min
(C16)
80
(C14)
60
(C12)
wt
40
20
0
0
10
20
30
40
50
60
70
Vapor Pressure, torr
64
Lecture 4 Porous inorganic-organic hybrid and
organic materials Recent advances
65
Surface Modification of M-41S Materials by
Organo-Functionalization

• Mesoporous materials have high surface areas and
  easily accessible and tunable uniform sized
  pores.
• Excellent host for large guest molecules such as
  nano-sized metal particles and metal complexes.
• Organic functional groups, either on the external
  silicate surface or trapped within the channels,
  render precise control over the surface
  properties.
• Hydrolysis resistant stable anchor for metal and
  metal complexes and provide hydrophobic surface.

66
Chemical Composition of the Synthesis Gel Mixture
of Organo-Functionalized MCM-41
67
Different Routes for Surface Modification
1. Co-condensation one-pot synthesis
Surfactant removal
P. Mukherjee et al., Stud. Surf. Sci. Catal. 129
(2000) 283
68
Different Routes for Surface Modification
2. Grafting post-synthesis modification
69
13C- and 29Si-CPMAS NMR of propylthiol-MCM-41 and
aminopropyl-MCM-41
Different siloxane groups
13C CPMAS NMR
29Si CPMAS NMR
SH-MCM-41
NH2-MCM-41
70
Immobilization of Chiral Metal Catalysts on
Organo-Functionalized MCM-41 and MCM-48
71
Recycle Studies for Acetophenone Hydrogenation
72
What Do We Need To Know About Zeolites to Use
Them Effectively?

• A Zeolite must be described by more than a
  designation (e.g. ZSM-5)
• Zeolites need to be further described by...
• SiO2/Al2O3 ratio
• Crystal morphology
• Crystal size
• Phase purity
• Counterbalancing cation(s), metals or metal
  cations
• Heteroatoms present in framework
• Binder if bound

73
Counterion Lyotropy and Micelle Formation
C12H25(CH3)3NX

CMC Decreases as IO
, CI , BrO
, Br , NO

3
3
3

Aggregation Number Increases as

IO
, BrO
, CI , NO
,
Br

3
3
3

C10H21(CH3)3NX

Counterions Which Interact Weakly with H
O

2
Promote Micelle Formation
J. Phys. Chem.,
67
, 1713 (1963).

J. Colloid and Interface Science,
117
, 242 (1987).
74
Roles of the Surfactant and Inorganic Species

Surfactant



Forms molecular aggregates



Determines pore size by space filling


Concentration influences product structure
Inorganic Species



Charge balancing plays a role in surfactant


molecule organization


Oxide precursors undergo further reaction

to form extended structures
75
Periodic Mesoporous Organosilicas (PMOs)
containing tunable chemosensor in pore wall
76
Chem. Mater. 2007, 19, 5347-5354
77
Organic Mesoporous Materials
78
All organic Mesoporous Conducting Polymer
Mesoporous polyaniline synthesized by
anion templating route
79
Powder XRD patterns of different mesoporous PANI
samples. TEM images of meso-PANI
80
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81
Mesoporous cross-linked polymer MCP-1
Polyacrylate
Non-templated cross-linked Polyacrylate
Random Polyacrylate chain
Cetyltrimethylammonium bromide Surfactant
(NH4)2S2O8 Initiator
A new mesoporous cross-linked polymer containing
-CO2H groups has been synthesized by
surfactant-templating route.
82
Chem. Mater. 2007 (in press).
83
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84
FE-SEM image of MCP-1 before gold loading
FE-SEM image of MCP-1 after gold loading
85
115.8 and 132.6 ppm Signals of AA and
BA disappeared.
13C CPMAS NMR spectrum of template free MCP-1.
UV-Vis diffuse reflectance spectra of MCP-1 (a)
and GNP-MCP-1 (b).
86
Design nanomaterials employing mesoporous host
Polyaniline moieties have been grafted on the
monolayer N-propylaniline functionalized
mesoporous silica SBA-15 through in-situ
polymerization in the presence of (NH4)2S2O8.
87
J. Mater. Chem. 2007, 17, 278-284.
88
N2 adsorption/desorption isotherms and pore size
distributions for functionalized SBA-15 (A) and
grafted polyaniline in it (B).
89
Sample
Conductivity (S cm-1) Simple PANI/SBA-15
7.2 x 10-4 Grafted-PANI/SBA-15
8.5 x 10-3
90
Metal-Organic Frameworks (MOFs)

• Metal-Organic Frameworks are hybrid materials
  composed of an inorganic complex or cluster and
  an organic linker unit.
• Versatile and complex in structure as well as in
  pore size.
• Due to the wide open structure, MOFs are
  promising materials for gas separation, catalysis
  and ion exchange applications traditionally
  covered by zeolites.
• MOFs have been shown to exceed zeolite materials
  in pore size as well as in specific pore volume.
• Show the structural diversity (due to
  carboxylates, phosphonates, N-donor complexes,
  and Prussian Blue) sorptive, catalytic properties
  and dynamics of guest molecules in flexible
  Metal-Organic Frameworks.
• Thermal stability is considerably lower vis-à-vis
  zeolites.

O. Yaghi et al.,
91
MOFs
Yagi et al MMM 73 (2004) 314 MOFs A class of
solids that allows greater chemical alteration on
a periodic scale, since the methodology for
organic transformations is well-established. While
subtle changes to the coordinating organic links
often leads to new framework topologies, certain
framework types are amenable to marked
modification of the metrics and chemical nature
of these moieties. Precise structureproperty
trends can be established,and optimization of a
material may be performed in a rational manner.
As new applications for these versatile
materials are identified, a greater understanding
of the subtleties in the reticulation process
will be achieved.
92
Different classes of ligands used for MOFs
synthesis
Extended analogues of bipyridyl
4,4-bypyridyl
2,6 Naphthaline di carboxylic acid
1,3,5 TMB-tricarboxylate extended analogues
93
Thanks