Lewis acid metal ionexchanged MAPO36: Characterisation and catalytic performance

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Lewis acid metal ion-exchanged MAPO-36
Characterisation and catalytic performance

• S. Vishnu Priya
• Research Scholar
• Department of Chemistry
• Anna University
• Chennai - 600 025

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Zeolites and Zeotype molecular sieves

• Zeolites are crystalline aluminosilicates having
  three dimensional framework
• Primary buildings units are SiO4 and AlO4
  tetrahedra (known as TO4)
• These TO4 are shared by a common oxygen atom in
  corners to form secondary building unit (SBU).
• These SBUs are arranged in a specific
  geometrical pattern to form a definite crystal
  structure and uniform pore size.
• Zeotypes are polymorphs of zeolites having AlO4
  and PO4 as primary building units

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Template concept for AlPO-n synthesis
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Generation of Bronsted acid sites in AlPOs
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AlPO-36 molecular sieve

• Unidimensional with 12-ring elliptical channel.
• Having a free aperture
• Dimension - 6.5 and 7.4 Å
• Can be readily synthesised using one type of
  template, namely, tripropylamine
• AlPO-36 is a highly template specific material
  and the metal-template interaction influences the
  structure formation
• Isomorphous substitution of the following cations
  are already reported in the literature Si4,
  Co2, Zn2, Mn2, Mg2, Cr6, Ti4 and V5.
• These materials are tested towards various
  catalytic reactions of industrial importance.

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Hydrothermal synthesis of MAPO-36

• Sources
• Aluminium isopropoxide
• Phosphoric acid
• Magnesium nitrate
• Tripropylamine
• Gel composition 1.83Pr3N0.92Al2O30.17MgO1P2O5
  80H2O

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Ion-exchange of MAPO-36
Refluxed for 6h
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Physico-chemical characterisation

• XRD
• SEM
• TGA
• TPD (ammonia)

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XRD
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Contd.,

• XRD patterns of MAPO-36 coincide with the one
  already reported in the literature
• XRD patterns of ion-exchanged MAPO-36 molecular
  sieves also show similar features as that of
  MAPO-36.
• Hence there is no structural degradation during
  ion-exchange.
• There are no patterns corresponding to
  non-framework metal oxide in the XRD patterns of
  ion-exchanged MAPO-36 molecular sieves.
• Hence the metal oxides even if present are in low
  amount they are below the detectable limit of
  XRD.

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Thermogravimetric analysis of ion-exchanged
MAPO-36
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• Thermograms show initial weight loss below 150ºC
  due to desorption of water.
• The second minute weight loss between 200 and
  400ºC is due to decomposition of M(OH)3 formed
  during ion-exchange.
• But non-framework M2O3 is not detected in the XRD
  analysis.
• Hence it may be present in a trace amount and the
  grain size is not sufficient enough to be
  detected by XRD.
• The third weight loss between 500 and 600ºC is
  assigned to the decomposition of charge
  compensating M(OH)2 to MO species.
• The thermogram of ZnMAPO-36 differs from others
  and hence Zn2 does not produce ZnO species.

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• The existence of M(OH)2 species is based on the
  Plank-Hirschler mechanism
• Plank-Hirschler mechanism is not applicable to
  the metal ions with a 2 oxidation state.
• Fe3, La3 and Ce3 with their high ionic
  potential can readily undergo hydrolysis to form
  M(OH)2 which then become MO during calcination.
  

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Scanning electron microscope pictures of MAPO-36
and ZnMAPO-36
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Contd.,

• SEM picture of MAPO-36 shows spherical particles
  of various sizes.
• There are pits on the surface of particles and
  the presence of such pits in the SEM picture of
  MAPO-36 is already reported.
• Aggregation of fine needles in an organized way
  to provide spherical sponge like morphology of
  different sizes.
• Destruction of small amount of spherical
  morphologies into small particles in irregular
  shape occurs during ion-exchange.
• The pH of the medium used for ion-exchange and
  stirring speed may be the reason for destruction
  of bigger particles into smaller ones.

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TPD (ammonia) of MAPO-36 and ion-exchanged
MAPO-36
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TPD (ammonia) desorption values
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• Desorption of ammonia up to 200ºC indicates the
  presence of weak acid sites.
• Desorption of ammonia above 500ºC indicates the
  presence of strong acid sites respectively.
• Lewis acid metal ions, weak Bronsted acid sites
  and defective sites are included as weak acid
  sites.
• Decrease in the number of strong acid sites and
  increase in the number of weak acid sites are
  observed in FeMAPO-36.
• This clearly indicates that only part of strong
  acid sites are ion-exchanged.
• When M3 ions are ion-exchanged there will be
  loss of three protons in the framework.

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• However, there may be splitting of coordinated
  water molecules around M3 ions to form two
  protons and M(OH)2.
• These two newly formed protons will be taken up
  by two oxygen bridges of the framework.
• These two protons may not be as acidic as that of
  actually exchanged protons.
• The charge of a metal ion significantly
  influences the ion-exchange.
• This kind of selective ion-exchange could be
  useful for some specific chemical transformations
  which require only weak acid sites.

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tert-Butylation of phenol and its importance

• tert-Butylation of phenol is taken as a model
  reaction to test the catalytic activity of Lewis
  acid metal ion-exchanged MAPO-36.
• tert-Butylation of phenol is an industrially
  important reaction.
• Commercial catalysts are H2SO4, H3PO4,
  ion-exchanged resins, etc.,
• 2-terbutylphenol is used as a raw material for
  the production of antioxidants and agrochemicals.
• 4-tert-butylphenol is used in the production of
  phenol-formaldehyde resins which are applied in
  binders, lacquers, varnishes, etc.
• 2, 4-tert-butylphenol is used in the production
  of ultraviolet absorbents, heat stabilisers for
  polymeric materials etc.,
• The major drawbacks of the homogeneous system are
  hazardous nature and tedious work-up procedure.
• In order to overcome the above said problem a
  variety of solid acid catalysts are employed in
  tert-butylation of phenol.

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Phenol tert-butylation over MO sites
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Phenol tert-butylation over Bronsted acid sites
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• Alkylation of phenol occurs through dissociative
  chemisorption of phenol on MO and the resulting
  phenolic proton is transferred to its
  neighbouring bridging oxygen.
• tert-Butyl alcohol is chemisorbed on the protonic
  sites to form tert-butyl cation which then
  undergoes electrophilic attack at the o- or p-
  position of the adjacent chemisorbed phenol
  yielding 2-TBP or 4-TBP.
• Selectivity to 4-TBP was expected to increase
  with increase in temperature, decrease in the
  selectivity was observed at 350 and 400 ºC.
• The decrease in the selectivity at higher
  temperatures is largely due to its conversion to
  polyalkylated phenolics.

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• MAPO-36 is found to be the least active among the
  catalysts. This observation clearly establishes
  that the reaction is largely controlled by Lewis
  acid sites.
• Fe, La and CeMAPO-36 showed higher 4-TBP
  selectivity than ZnMAPO-36.
• Hence, Fe, La and CeMAPO-36 with more number of
  weak acid sites could be considered as better
  catalysts than others.
• The conversion was found to be high at 12 than
  other feed ratios.
• The less conversion with 13 is due to
  suppression of chemisorption of phenol in the
  presence of excess tert-butyl alcohol.

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Time on stream
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• The conversion decreases gradually with increase
  in time on stream and at the end of 6 h stream
  only 25 conversion is observed.
• This is due to enhanced initial activity on the
  Lewis acid sites, followed by deactivation at the
  expense of rapid process of accumulation of
  reaction products and intermediates.
• Thus the deactivation of the catalyst at longer
  hours of time on stream may be due to
  accumulation of reaction products on the active
  sites and blocking of Lewis acid sites.
• 2,4-Di-TBP selectivity increases with increase in
  time on stream due to conversion of mono
  alkylated products to 2,4-di-TBP.

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Conclusion

• Formation and presence of MO species in the in
  MAPO-36 molecular sieve is found to be active
  for specific chemical transformations.
• Ce3 ion-exchanged catalyst is found to be the
  better catalyst towards tert-butylation of
  phenol.
• Ion-exchanged MAPO-36 catalysts are also tested
  towards Beckmann rearrangement, toluene
  disproportionation and acylation of toluene.
• Ion-exchanged catalysts found to be the better
  catalysts than parent MAPO-36 catalyst.

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Acknowledgements

• Prof. V. Murugesan
• Prof. M. Palanichamy
• Prof. (Retd.) Banumathi Arabindoo
• Prof. S. Nanjundan, HOD Chemistry
• Department of Science and Technology (DST)

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Thank you