Supported Heteropoly acids

1
Polyoxometalates Why are they unique ?
Mr. P. Indra Neel Ph. D. Seminar - I
2
Contents

• Introduction
• Properties of polyoxometalates
• Potentials of polyoxometalates
• Challenges in exploitation
• Catalytic applications
• Conclusion

3
Polyoxometalates

• General formula of polyoxometalates
• XxMmOy(-q)
• X hetero atom, B, Al, Si, Ge and P, As
  (or)
• Fe, Mn, Co, Cu and Zn 
• M addenda atoms are attached to hetero atoms
  through oxygen atoms
• Mo, W, V, Ta, Nb, Os
• -q charge varies from -3 to -28
• Criteria for addenda atoms
• High charge (5 or 6) and small size
• Ionic radius 0.53 Å lt r lt 0.70 Å
• Expandable coordination number from 4 to 6
• Ability to form double bonds with unshared
  oxygen atoms

4
Heteropoly acids Unique features

• Strong Bronsted acids
• Presence of great number of water molecules
• Multifunctionality
• Structural mobility
• Can function as homogeneous as well as
  heterogeneous catalysts
• Easy alteration of chemical composition
• Efficient oxidants
• Insoluble in non-polar solvents
• Environmentally benign

5
Conductivity values of hydrates of HPW and its
alkali salts
Ubavka b. Mioc, Marija R. Todorovic, Snezana M.
Uskokovic-Markovic, Zoran P. Nedic and Nada S
Bosnjakovic, J. Serb. Chem. Soc. 65(5-6), 2000,
399
6
Conventional acids vs Solid acid catalysts
Sulphuric acid Most
widely used catalyst in chemical industry
Other industrial catalysts AlCl3, BF3,
TiCl4, HF Drawback
Acid strength cannot be modulated or
altered Operational difficulties
corrosive
toxic
effluent disposal
product
separation Solid Acid Catalysts
natural clays minerals
cation exchange resins

supported acids like H3PO4/alumina
mixed metal
oxides
heteropoly acids
zeolites

7
Why Solid acids?

• Acid strength can be modulated
• High catalytic activity and selectivity
• Nature of acid sites is known
• Modification can be confirmed
• Do not corrode the reactors
• Repeated use is possible
• Separation is easy
• Safe to handle

8
Potentials of Heteropoly acids

• Structure of Heteropoly acids
• Pseudo liquid phase behaviour
• Types of reactions promoted by Heteropoly acids

9
Types of Heteropoly compounds
10
Structures of different Heteropolyanions
Keggin
Dawson
Anderson
Waugh
Silverton
11
Structure of Keggin anion
M3O13
PO4
H3XM12O40
12
Where is the proton ?
(a)
(b)
(c)
Proton sites in heteropoly acids (a) HPA in
solution (b) solid PW hexahydrate (c) solid
dehydrated PW
Ivan V. Kozhevnikov, Catal. Rev. Sci. Eng. 37(2),
1995, 311
13
Hierarchical Structure of Heteropoly compounds
W3O13 triplet
Primary structure
PW12O403- Keggin anion, ?1nm
Unit cell of Cs2.5H0.5PW12O40
Secondary structure
nonporous nanocrystallite (primary particle,
?10nm)
Tertiary structure
Porous aggregate of nanocrystallites
secondary particle 0.1-0.5 ?m
Heteropoly compounds are defined at molecular
level
Mokoto Misono, Chem. Commun., 2001, 1141
14
Three types of catalysis for solid heteropoly
compounds
reactant
product
reactant
product
reactant
product
(b) Pseudo liquid or Bulk
type (I)
(a) Surface type
(c ) Bulk type (II)
Mokoto Misono, Chem. Commun., 2001, 1141
15
Evidence for pseudoliquid phase

• Rate increased at first with
• increasing ethanol pressure
• The amount increased from 0.4 to 8
• molecules per anion pressure
• increased 0.4 to 60 kPa
• The amount correspond to 4- 80
• times the monolayer formation
• Ethylene ethanol/proton is low
• Ether ethanol/proton is high

Amount of absorbed ethanol molecule anion-1
Log (rate/ molg-1h-1)
Log (p/ kPa)
K1
k1
C2H5OH H C2H5OH2
C2H4 H2O
(1) C2H5OH C2H5OH2
(C2H5OH)2H (C2H5)2O H2O
(2) C2H5OH (C2H5OH)2H
(C2H5OH)3H ( not reactive)
(3)
K2
k2
Makoto Misono, Toshio Okuhara and Tatsumi Ichiki,
J. Am. Chem. Soc. 109, 1987, 5535
16
Types of reactions promoted by heteropoly acids
17
Acid Catalyzed reactions over heteropoly acids
Reaction
Catalyst Remarks
(1)
H3PW12O40/MCM-41 Liquid phase
reflux 20 Wt H4SiW12O40/MCM-41
H3/MCM-41 gt ?-zeolite H3PW12O40/MCM-41
T 343 K, S 91
(Shape
selective) H3PW12O40/SiO2
T 293 K , Y 75
(2)
(3)
Toshio Okuhara, Noritaka Mizuno and Makoto
Misono, Applied Catalysis A General 222, 2001,
63
18
Reaction
Catalyst Remarks
(4)
Cs2.5H0.5PW12O40 /SiO2 T 333 K,
activity
Cs2.5/SiO2 H-ZSM-5 Cs2.5H0.5PW12O40
T 353 K, activity

Cs2.5 gt
H-ZSM-5 Cs2.5H0.5PW12O40
T 353 K, activity
Cs2.5 gt H-ZSM-5,

H2SO4 Cs2.5H0.5PW12O40
T 298 K Cs2.5H0.5PW12O40
T 373 K, S 100

(5)
(6)
(7)
Pinacol rearrangement of
(8)
19
Challenges ahead

• Improving thermal stability
• Enhancement of surface area
• Evaluation of surface acidity

20
Thermal stability

• What is the need to improve the thermal
  stability ?
• How to improve the thermal stability ?
• Increasing the ionicity - by changing the
  counter ion

21
Thermogravimetric analysis of H3PW12O40. nH2O and
NH4PW12O40. nH2O
100
100
95
95
90
Weight /(?W/?T)
Weight /(?W/?T)
90
85
85
80
80
333 413 493 573 653 733
813 893 973
333 413 493 573 653 733
813 893 973
Temp (K)
Temp (K)
HPW
NH4PW
H - 0.52 Å
NH4 - 1.70 Å

• Repulsion between anions decreases
• Smaller cations high mobility
• Larger cation contact with more oxygen atoms
  of periphery anions

H. Hayashi and J. B. Moffat, J. Catal., 83, 1983,
192
22
Effect of counter cation on thermal stability
A. Cs2.5H0.5PW12O40
B. H3PW12O40
812 cm-1
985 cm-1
883 cm-1
1080 cm-1
(a)
( a)
(b)
Absorbance
(b )
(c)
(c )
Wavenumber/ cm-1
Changes in IR spectra of heteropoly compounds
after pretreatment at (a) 773, (b) 673 and (c)
573 K
Yutae Na, Toshio Okuhara and Makoto Misono, J.
Chem. Soc. Faraday Trans., 91(2), 1995, 367-373
23
Surface area

• Why to improve specific surface area ?
• What are the methods available ?

• Bulk Heteropoly acids 2-10 m2/g
• Monovalent salts of heteropoly acids a maximum
  of 180 m2/g

Another alternative Disperse on suitable
carrier
24
Supported Heteropoly acids
Role of support

• Enhance the availability of active sites for the
  reactants
• Improve dispersion
• Reduce sintering
• Acts as heat sink
• Life time of the catalyst increases
• Always superior to respective unsupported
  systems
• More economical

25

• Key issues in supported heteropoly acids
• Retention of polyanion structure
• Better dispersion
• Firm fixation
• Choice of Support
• High surface area
• Relatively Inert
• Thermal stability
• Different supports employed
• ?-Al2O3, MgO, ZrO2 , TiO2, Zeolites, Clays
• Mixed oxidesZrO2-TiO2, ZrO2-Al2O3, SiO2-Al2O3
• Activated Carbon, Mesoporous silica (MCM-41)

26
Synthesis of MCM-41
Molar composition SiO2 9.0 EtOH 0.20 CTAB
160 H2O
Sodium meta silicate (10.6 g) H2O (60 g)
CTAB (3.36 g) EtOH (20 g)
pH 11
Stirring for 3 h
Gel
Transferred to autoclave
Temp. 140 ºC, 12 h
Precipitate
Filtered, washed and dried
MCM-41 assynthesised
MCM-41
27
XRD patterns of (a) SiMCM-41 and PW/SiMCM-41 with
different loadings (b) 9 wt (c) 17 wt (d) 23
wt (e) 29 wt and (f) 33 wt
Afshin Ghanbari-Siahkali, Andreas Philippou, John
Dwyer and Michael W. Anderson Applied Catalysis
A General 192, 2000, 57-69
28
Adsorption data on SiMCM-41 and PW/SiMCM-41
29
Schematic representation of PW interaction with
the silanol groups (Si-OH) with in the pores of
SiMCM-41 materials
Afshin Ghanbari-Siahkali, Andreas Philippou, John
Dwyer and Michael W.Anderson, Applied Catalysis A
General, 192, 2000, 57
30
-16.2 ppm
-16.6 ppm
-11.0 ppm
-14.9 ppm
-13.5 ppm
-11.7 ppm
-12.1 ppm
31P MAS NMR spectra of (a) hydrated PW (b)
dehydrated PW (c)PW/SiMCM-41 (9 wt) and (d)
PW/SiMCM-41 (23 wt)
Afshin Ghanbari-Siahkali, Andreas Philippou, John
Dwyer and Michael W. Anderson Applied Catalysis
A General 192, 2000, 57
31
FT-IR spectra of (a) pure PW, (b) 23 wt and (c )
17 wt PW/SiMCM-41
32
Activity of PW/SiMCM-41 as a function of PW
loading for 1,3,5-TIPB conversion at 300 ºC
33
Conversion of 1,3,5-TIPB at 300 ºC over PW and
PW/SiMCM-41 catalysts
34
Determination of strength of solid
acid (1) Amine titration method using
indicators gives the sum of the amounts of
both Bronsted and Lewis acid (2) Gaseous base
adsorption method (TPD method) acid amount
under actual working conditions as a catalyst can
be determined (3) Inert gas adsorption
method (4) In-situ FT-IR spectroscopy (5)
Acidity from catalytic activity the
dehydration of isopropyl alcohol the
isomerization of butene
A good
measure of acidity
Fairly good correlations are found
35

• Solid Acid
• solid on which colour of a basic indicator
  changes (or)
• solid on which a base is chemically adsorbed
• Solid acids are
  graded acids
• Strength of Solid Acid
• It is the ability of the surface to convert an
  adsorbed
• neutral base into its conjugate acid.
• What is the scale to be adopted for these
  solids ?
• Whether pH or pka or some other scale and
  why ?
• For proton transfer from surface to adsorbate
  
• H0 pKa logB/BH
• For electron pair transfer from the adsorbate to
  the surface
• H0 pKa logB/AB

36
Basic indicators used for the measurement of acid
strength
37
Acid strength measured by Hammett indicators
() Acid colour of indicator was observed (-)
not at all observed
Toshio Okuhara, Toru Nishimura, Hiromu Watanabe
and Makoto Misono J. Mol. Cat., 74, 1992, 247
38
H0 values - liquid acids vs solid acids
Liquid
Solid
39
TPD spectra of ammonia on supported HPA
B
A
HPA content wt
Desorption intensity
Temp ?C
A H3PW12O40/SiO2
B H4SiW12O40/SiO2
Yusuke Izumi, Ren Hasebe and Kazuo Urabe, J.
Catal., 84, 1983, 402
40
TPD of Ar Evaluation of surface acidity

• TPD of NH3
• acid sites may be decomposed because of high
  desorption temperatures
• accurate acid strengths of solid super acids
  cannot be evaluated
• Argon
• completely inert towards super acids at room
  temperature
• shows acid-base like interaction with acid
  sites at low temperature
• acquires induced dipole when it interacts with
  a strong dipole
• interaction strength depends on the acid
  strengths

Hiromi Matsuhashi and Kazushi Arata, Chem.
Commun., 2000, 387
41
Argon TPD Systematic procedure
Solid acid 15 - 40 mg
Glass sample tube
Pretreated in vacuum for 2 h
Sample exposed to 6.7 KPa Ar at room temp.
followed by cooling to 113 K
Adsorption carried at 113 K for 10 min
Excess of Ar removed by evacuation at 113 K
TPD in the temp. range of 113K - 223 K
Programmed rate 2 K/min
Desorbed Ar was detected by a mass spectrometer
and an ionization gauge
42
Argon TPD profiles of solid acids
138.9 K
139.5 K
139.8 K
136 K
132.8 K
131.5 K

• Desorption peaks 120-170 K

Hiromi Matsuhashi and Kazushi Arata, Chem.
Commun., 2000, 387
43
Solid acids and activation energies of Ar
desorption

• An apparent activation energy of desorption is
  calculated by
• 2lnTm- lnß Ed/RTm
  const
• The order of activation energies of desorption
  is as follows
• SO42-/ZrO2 gtCs2.5H0.5PW12O40 gtH-Mordenite
  H-ZSM-5 gt H-Y gt SiO2-Al2O3

The order of activation energy of
desorption reflects the acid strength of the
solid acid sites
44
Catalytic Applications
Synthesis of diphenyl methane
Significance Important
intermediate in spices, pharmaceuticals
and other fine chemicals
Commercial method Friedel-crafts
reaction with benzyl chloride and
benzene Conventional catalysts
Aluminium-amalgum, AlCl3, ZnCl2 Solid acid
catalysts HY Zeolite
H ZSM 5
Sulfated ZrO2
Substrates
Benzene and formalin Reaction
Condensation of Benzene and
formalin Catalyst
HPW Features Very
attractive , Economical , only by-product is
water
Zhaoyin Hou and Toshio Okuhara, Chem. Commun.,
2001, 1686
45
Conversion and selectivity of diphenyl methane
synthesis from benzene and formalin
Reaction conditions benzene 40 cm3 (450
mmol), formalin 6.72 cm3 (HCHO 90 mmol,
H2O 222 mmol,
methanol 18.2 mmol), 160 ºC, 2 h
46
Acetolysis of Cyclic Ethers
HPA
1,4 diacetoxy butane
Tetrahydrofuran AcOH Ac2O
Acetolysis of tetrahydrofuran at 333 K THF 0.5
ml AcOH 1-10 ml Ac2O 0-9 mlCatalyst (HPW)
50 mg
Yusuke Izumi, Kuniko Iida, Kyoko Usami and
Toshiko Nagata, Applied Catalysis A General,
256, 2003, 199
47
Catalytic activity for THF acetolysis at 333 K
THF 0.5 ml AcOH 5.0 ml Ac2O 5.0 ml
Catalyst 50 mg Temp 333 K
48
Acetolysis of 1,4 - dioxane
HSiW
1, 4 dioxane (CH3CO)2O CH3COOH
1, 2 diacetoxy ethane
Acetolysis of 1,4 dioxane at 363 K 1,
4-dioxane 9 ml, AcOH 1.0 ml Ac2O 1.0 ml
catalyst (HSiW) 200 mg
49
Conclusions

• Among the available solid acids, heteropoly
  systems are unique in the sense that they not
  only have free protons like the mineral acids but
  in proper protective environment for modulated
  release but also possess other acid sites like
  any other solid acids.
• The geometrical and architectural features of
  the heteropoly compounds make them unique in the
  sense they are amenable to generation of a
  variety of systems where the acidity and
  reactivity can be modulated.
• Even though there are a variety of techniques
  available to monitor the surface acidity of solid
  acid catalysts, Argon adsorption measurements
  have proved to be particularly appealing for the
  determination of acidity of heteropoly compounds.

50

• Heteropoly compounds are unique in a sense that
  they can in addition to promoting acid catalyzed
  reactions, promote oxidation reactions.
• The oxidation activity exhibited by them can be
  modulated. Hence they can be used as highly
  selective partial oxidation catalysts and will
  prove themselves superior to many other available
  partial oxidation catalysts.
• There are a variety of reactions that heteropoly
  compounds can promote and this versatile feature
  of heteropoly compounds may be exploited in
  future in unconventional areas like providing
  protective function in organic synthesis for the
  production of low volume high value specific
  pharmaceuticals.

51
Thank You
52
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53
Hydration of Olefins First commercial process
based on HPA catalysis RCHCH2 H2O
RCH(OH)CH3
Hydration of isobutene Used for the separation
of isobutene from the C4
hydrocarbon stream produced
by cracking
Over all rate v k1 olefinH3O k2
olefin H3O HPAn-
I. V. Kozhevnikov, Chemical Reviews, 98, 1998, 178
54
Preparation of H3PMo12O40 .nH2O
MoO3 (144g) 1400 ml H20
H3PO4 (9.57g)
Vigorous stirring , 3 h, under reflux
Yellow solution
Cooled, filtered
Mother liquor
Evaporative boiling, 3-4 h
Yellow Crystals, H3PMo12O40 .nH2O
J. C. Bailor, in Inorganic Synthesis. ed. H. S.
Booth, Vol. 1 (McGraw Hill, 1939) p. 132
55
Preparation of H3PW12O40. nH2O
Na2WO4, 50.0 g
Na2HPO4, 8.0 g
Dissolved in 75.0 ml boiling water
Conc. HCl 25 wt, 40.0 ml added drop wise
H3PW12O40. nH2O precipitated
Extracted with ether
H3PW12O40 .nH2O
J. C. Bailor, in Inorganic Synthesis. ed. H. S.
Booth, Vol. 1 (McGraw Hill, 1939) p. 132
56
An outline of the sample cooling system for Ar TPD
35
57

• TGA and (ii) DTA curves of (a) 9 wt (b) 17 wt
  (c) 23 wt (d) 29 wt
• (e) 33 wt PW on SiMCM-41 and (f) pure PW

58
Dissociation constants of Heteropoly acids vs
mineral acids at 25 ºC
Heteropoly acids are stronger than conventional
mineral acids
Ivan V. Kozhevnikov, Catal. Rev. Sci. Eng. 37,
1995, 311
59
Icosahedron 12 vertices 20 faces (equilateral
triangles) 30 sides
60
References

• 1. Yusuke Izumi, Kuniko Iida, Kyoko Usami and
  Toshiko Nagata,
• Applied Catalysis A General 256 (2003) 199
• 2. Zhaoyin Hou and Toshio Okuhara, Chem. Commun.,
  (2001) 1686
• 3, Toshio Okuhara, Noritaka Mizuno and Makoto
  Misono,
• Applied Catalysis A General 222 (2001) 63
• 4. Mokoto Misono, Chem. Commun., (2001) 1141
• 5. Hiromi Matsuhashi and Kazushi Arata, Chem.
  Commun., (2000) 387
• Afshin Ghanbari-Siahkali, Andreas Philippou, John
  Dwyer, Michael W. Anderson,
• Applied Catalysis A General 192 (2000) 57
• 7. Yutae Na, Toshio Okuhara and Makoto Misono,
  J. Chem. Soc. Faraday Trans., 91 (1995) 367
• 8. Ivan V. Kozhevnikov, Catal. Rev.-Sci. Eng.,
  37 (1995) 311