Role of gel aging in template-free synthesis of micro and nano-crystalline sodalites

 
Online version is available on http://research.guilan.ac.ir/csm
 
CSM   
Chemistry of Solid Materials
Vol. 2 No. 1 2014
 
 
[Research]
 
 
 
Role of gel aging in template-free synthesis of micro and nano-
crystalline sodalites
 
S. M. Pourali
1
 , A. Samadi-Maybodi
2
*
1
Ph.D Student, Analytical Division, Faculty of Chemistry, University of Mazandaran,
Babolsar, Iran  
2
Professor, Analytical Division, Faculty of Chemistry, University of Mazandaran,
Babolsar, Iran
* Corresponding authour's Email: samadi@umz.ac.ir
 
Article history:
(Received: 17 Aug 2014, Revised: 16 Sep 2014, Accepted: 12 Oct 2014)
ABSTRACT
A  facile  effective  stirring  aging  at  room  temperature  prior  to  conventional  hydrothermal
treatment was employed in the template-free synthesis of micro- and nano-crystalline sodalites
with  two  different  initial  gel  compositions.  The  effect  of  initial  Si/Al  molar  ratio,  NaOH
concentration and stirring aging time were investigated on the morphology and particle size of
the  synthesised  sodalites.  The  results  revealed  that  applying  various  stirring  aging  time  can
change  the  proportion  of  the  contaminant  phase  associated  with  sodalite  and  alter  the
morphology of the sodalite crystals from wool ball-like consisting of nano-threads to cabbage-
like with  nano-leaves,  as well  as  size  distribution  of  nanocrystalline  sodalite. The  size  of  the
nanocrystals was in the range of 50-80 nm as observed by FE-SEM and the yield was as high as
about 15%.  
 
Keywords:  Nanocrystalline  Sodalite,  Wool  ball-like,  Cabbage-like,  Morphology,  Argon
isotherm.
1. INTRODUCTION
Sodalites,  a  class  of  microporous
aluminosilicates  with  general  formula
of M8[ABO4]6X2,  are  built  of  a  six-
membered ring (6R) with a pore size of
0.28 nm, and the maximum diameter of
the  void  included  in  the  framework  of
0.63  nm  [1].  So  far,  sodalite  crystals
attracted  considerable  attention  due  to
their  widespread  application  in
hydrogen storage [2-4], semiconductors
[5, 6] and pigment occlusion [7]. Nano-
sized  and micro-sized  sodalite  crystals
could be easily synthesised  in presence
of  organic  templates  [8-12]  or
structure-directing  agents  (SDAs)  [13]
through hydrothermal  treatment. Nano-
sized  sodadlites  have  been  obtained
using  SDAs  by  solid-solid
transformation of Al2O3 pillared clay in
the  alkaline  solution  [14,  15],  direct
solid  transformation  of  preformed
silicate nanocrystals  [16], and washing
sodium  aluminosilicate  solution  by
NaOH  solution  to  remove  the
amorphous  phase  [17].  However,
removal  of  SDAs  often  leads  to
irreversible  aggregation  of  the
nanocrystals  and  decrease  in
crystallinity.  The  yield  of  zeolite
nanocrystals  by  this  approach  is S. M. Pourali, A. Samadi-maybodi /CSM Vol.2 No.1, 2014 pp.21-31
22                              
 
normally  lower  than ~ 10%, calculated
based  on  silicon  source  used  [18].
Furthermore,  SDAs  tend  to  alter  Si/Al
molar  ratio  of  the  final  products,  thus
remarkably  affect  their  application
[19].  The  organic  additives  are  non-
recyclable  and  their  application  is
costly  and  requires  calcination  which
results  in  the  production  of  CO2  and
NOx pollution problems [20]. Okubo T.
et  al.  reported  the  hydrothermal
synthesis  of  sodalite  nanocrystals
without  using  any  organic  additives  at
low temperature [17]. Undoubtedly, the
morphology  of  zeolites  is  originally
related  to  the  framework  type and also
closely  related  to  the  micropore  size,
crystal  size  and  shape,  and  directly
affects  the  physicochemical  properties
of zeolites. Morphological properties of
zeolites  are  particularly  important  in
catalytic applications where the particle
shape can have a dramatic effect on the
product  distribution  due  to  the
differences  in  rates  of  transport/
diffusion  and  reaction. Recently,  great
efforts  have  been  directed  toward
designing  zeolites  with  desired
functions by  applying nanotechnology.
Thus,  there has been a great  interest  in
developing  synthetic  approaches  to
control crystal  size and morphology of
zeolites [21-23].   
 
2. EXPERIMENTAL
2.1 Reagents and Materials
Sodium aluminate, sodium metasilicate
(H2O 43%, Na2O 29%, SiO2 28%) and
sodium hydroxide were purchased from
Merck  (AR  grade)  and  used  without
further  purification.  Double  distilled
water  was  used  throughout  the
experiments.
2.2 Synthesis
In  a  typical  procedure,  sodium
aluminate, sodium hydroxide and water
were  placed  in  a  plastic  beaker  and
heated  to 70 °C while stirred  to ensure
achievement  of  a  homogeneous
mixture,  and  then  cooled  to  room
temperature.  Sodium  metasilicate  was
dissolved in water, heated to 70 °C and
stirred  to  obtain  a  homogeneous
mixture,  then  cooled  to  room
temperature.  Afterwards,  silicate
solution  was  added  slowly  to  the
aluminate  solution  under  vigorous
stirring. Subsequently, the gel was aged
at  room  temperature  for  certain  time,
according  to  Table  1,  with  vigorous
stirring  prior  to  conventional
hydrothermal  treatment  to  get  a
homogenous  gel-mix.  Finally,  the  gel
was  transferred  into  a  Teflon-lined
stainless-steel  autoclave,placed  in  an
air-oven  maintained  at  the  required
temperature.  At  the  end,  the
precipitates  were  recovered  by
centrifugation,  washed  with  double
distilled water and dried  in air. The gel
composition  (molar  ratio  of  the  used
chemicals),  synthesis  conditions,  and
resulting  solid  phases  are  presented  in
Table  1.For  FE-SEM  and  argon
adsorption-desorption  isotherm  studies
on  the  samples,  the  synthesized
products  were  calcined  at  600  °C  for
2h.  
 
 
 
Table 1. Synthesis conditions of the samples
 
Duration    Temperature  
Stirring aging (samples
labeled as)   
H2O    Na2O   SiO2   Al2O3  
Gel
Composition
48 h   100 °C  
0, 20 min, 72 h (S1, S2,
S3)   
267   6    10   1   a   
48 h   100 °C   
0, 1 h, 2.5 h (S4, S5,
S6)  
50   2.1   3.8   1   b  S. M. Pourali, A. Samadi-maybodi /CSM Vol.2No.1, 2014 pp.21-31
23
 
2.3 Characterization
Powder  X-ray  diffraction  (XRD)
patterns  of  the  as-synthesized  samples
were  recorded on  a GBC MMA X-ray
difractometer using Cu Kα radiation of
wavelength  0.154178  nm  at  35.4  KV
and  28  mA.  Diffraction  data  were
recorded  between  5  and  50º2θwith  a
scanning  speed  of  5º/min.The  FT-IR
spectrum  was  recorded  on  a  Bruker
Tensor 27 Spectrometer.
Field-emission  scanning  electron
microscopy  (FE-SEM)  images  were
provided  using  a  HITACHI,  S-4160
field-emission  electron  microscope
operating  at  15  kV  for  indicating  the
morphology of the samples.  
The specific surface area was evaluated
using  the  Brunauer–Emmett–Teller
(BET)  method,  and  the  pore  size
distribution  was  calculated  from
desorption  branches  of  argonisotherms
applying  the  Barrett–Joyner–Halenda
(BJH)  method  (Quantachrom  Nova
2000e, USA).
 
3. RESULTS AND DISCUSSION  
Figure 1 shows the XRD patterns of the
zeolites  S1  and  S4prepared  by  two
different  starting  gel  compositions (a
and  b,  Table  1)  under  same  synthetic
condition. As  it can be seen, Figure 1a
and 1b  shows  mixed  phases  of
analcime  and  NaP-1  zeolites  prepared
under  no  stirring  aging.  With  gel
composition  a,  having  higher  Si/Al
molar  ratio  (5),  zeolite NaP-1 was  the
dominant  phase  while  for  gel
composition  b,  having  lower  Si/Al
molar  ratio  (1.9),  both  phases  showed
high degree of crystallinity.   
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1. The XRD patterns of samples S1 and S4 prepared with gel compositions a and b under
no stirring aging.
 
 
 
To  further  investigate  the  phase
transformation  and  morphology
changes  of  both  gel  compositions,
different  stirring  aging  times  were
employed  prior  to  conventional
hydrothermal  reaction  and  the  results
were  followed  by XRD  and  FE-SEM. S. M. Pourali, A. Samadi-maybodi /CSM Vol.2 No.1, 2014 pp.21-31
24                              
 
First,  evolution  of  gel  composition  a
was studied under two different stirring
aging time of 20 min and 72 h (S2 and
S3,  respectively).  Fig.  2  illustrates  the
XRD patterns of the samples S2 and S3
with  S2  indicating  the  formation  of  a
new  phase  of  sodalite  octahydrate
zeolite.  It  also  showed  a  significant
decrease  in  analcime  phase  comparing
to  S1  with  no  stirring  aging  and  a
moderate decrease in NaP-1 phase.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 2. The XRD patterns of the samples S2 and S3 prepared under 20 min and 72 h stirring
aging, from gel composition a.
 
 
 
On  the  other  hand,  under  prolonged
stirring  aging  time  of  72  h,  the  XRD
pattern  of  S3  (Figure  2)  showed  a
dramatic  change  in  phase  dominancy
with  sodalite  octahydrate  being  the
dominant phase and NaP-1 zeolite as a
remaining  impurity  phase  and  no
analcime  peak  was  observed.  This
decrease in NaP-1 and analcime phases
after  adding  to  aging  step  is  the  result
of  their  consumption  toward  the
formation  of  a  more  pure  and  stable
phase  of  sodalite  octahydrate.  After
confirming  the  phase  formation  by
XRD  analysis,  the  morphology  and
particle size distribution were observed
by FE-SEM (Figure 3).  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 S. M. Pourali, A. Samadi-maybodi /CSM Vol.2No.1, 2014 pp.21-31
25
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 3. FE-SEM images of the samples S2 and S3 prepared under 20 min and 72 h stirring
aging, using gel composition a.
 
 
 
As  it  can  be  seen  in  Figure  3,  sample
S2  has  a  wool  ball-like  morphology
and  spheres  of  sodalite  aggregate
together. The average size of each wool
ball  is ~ 3 μm with  threads having  the
width  range  of  80-120  nm.  Also,  the
presence  of  diamond-like  NaP-1
crystals  is  observable  which  is  in
agreement with its XRD pattern (Figure
2,  S2).  The  size  of  each  diamond  is
about 2μm,  which  are  marked  with
arrows.  However,  under  prolonged
aging of 72 h  (S3),  the morphology of
the  wool  ball-like  sodalite  changed  to
well-shaped  cabbage-like  structures
with an average size of 3 μm and leaves
in  the  range  of  50-80  nm.  Figure  4
illustrates  another  schematic  image  of
cabbage-like  (S3)  crystals  at  higher
magnifications  showing  mainly  nano-
leaves  which  are  the  nano-features  of
the  microcrystals  of  sodalites.
Furthermore, it illustrates that the nano-
leaves  are  connected  to  each  other
through  an  axis,  which  results  in  the
hierarchical cabbage-like morphology.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 S. M. Pourali, A. Samadi-maybodi /CSM Vol.2 No.1, 2014 pp.21-31
26                              
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 4. Nano-leaves of cabbage-like microcrystalline sodalites at 1000X magnification.
 
Secondly, evolution of gel composition
b  was  studied  under  two  different
stirring aging of 1 h and 2.5 h (labeled
as  S5  and  S6,  respectively).  Figure  5
shows  the XRD  patterns  of mentioned
samples  with  S5  representing  pure
sodalite octahydrate phase. The  lack of
analcime and NaP-1 phases in the XRD
patterns  implied  the  transformation  of
these  unstable  phases  to  a more  stable
phase  of  sodalite  which  is  also  well-
known  to  be  preferentially  formed  at
high  NaOH  concentrations  (Table  1)
[24].  The  obtained  products  showed
high  sodalite  crystallinity.  With
increase in aging time to 2.5 h, not only
the  peaks  associated  to  sodalite
octahydrate  were  observed  but  also  a
new  phase  emerged  which  is  marked
with asterisk (Figure 5, S6). The newly
appeared  peaks  can  be  well-identified
as  cancrinite.  Due  to  the  structural
similarity  between  sodalite  and
cancrinite,  they  can  be  synthesised
under  similar  reaction  conditions  [25].
Also,  the higher Na2O/H2O molar ratio
of the gel composition b (0.04) than gel
composition  a  (0.02)  favors  the
formation  of  cancrinite  phase  in  the
former mixture after enough aging time
(Table  1)  [26].  In  addition,  a  slight
decrease  in  peak  intensity  and  a  bit
broadening  can  be  seen  in  the  XRD
pattern  (Figure  5,  S6)  suggesting  the
consumption of sodalite phase  in  favor
of  cancrinite  and  a  change  in  size
distribution of the crystals. These XRD
results can clearly demonstrate that 1 h
of  stirring  aging  time  is  enough  to
obtain  pure  sodalite  octahydrate  phase
and  further  aging  time  can  lead  to
formation  of  disfavored  new  phase  of
cancrinite.  Also,  Debye-Scherrer
formula  was  applied  to  calculate  the
particle size of  this pure sodalite phase
which resulted in~ 30-40 nm (S5).
 
 
 
 
 
 
 
 
 
 S. M. Pourali, A. Samadi-maybodi /CSM Vol.2No.1, 2014 pp.21-31
27
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.5. The XRD patterns of sodalite octahydrate samplesof S5 and S6, prepared under 1 h and
2.5 h stirring aging from gel composition b, respectively. The cancrinite phase is denoted with
asterisk.
 
The Fourier  transform  infrared (FT-IR)
spectrum  of  pure  nano-sodalite  (S5)  is
shown in Fig. 6. The absorption band at
~  3500  cm-1
can  be  assigned  to  the
stretching  vibration  of  structural
hydroxyl group (OH-
) of silicate  lattice
and the strongest vibration at ~990 cm-1
 
is  due  to  the  asymmetric  stretching
mode  of  the  tetrahedrally  coordinated
Si  [14,  27].  The  other  ʻfingerprintʼ
absorptions were  also  observed  at  524
and  465  cm-1
 which  correspond  to  the
symmetric  stretching mode  of  internal
SiO2  tetrahedra,  and  the  structure-
intensitive  T-O  bending  mode  of
tetrahedral  TO4  units  (T=  Al  and  Si),
respectively.  A  diagnostic  feature  of
the sodalite formation is the appearance
of  a  new  absorption  peak  at  434cm-1
 
due  to  the  formation  of  single  four-
membered  ring  (S4R)  of  sodalite  unit
[28].  
The symmetric stretching vibrations (υs
(T-O))  in  the  670-730  cm-1
  region  are
due  to  the symmetric stretch of T-O-T.
Since  these  vibrations  are  sensitive  to
the  mass  of  the  anion  and  cation
included  in  sodalite  cage,  the  peak
positions give an  invaluable diagnostic
for  encapsulated  molecules  such  as
NaOH and H2O [17, 29]. A sharp peak
corresponding to the water deformation
mode  at  ~  1650  cm-1
 was  appeared  in
the  spectrum.  Due  to  the  strong
hydrogen  bonding,  the  central  O…H
stretching  vibration  is  expected  to  be
broader  and  shift  to  lower  numbers.
Therefore,  the  broad  peak  centered  at
absorption 1400 cm-1
can  tentatively be
assigned  to  this  stretching  vibration  of
O…H [17].
 
 
 
 
 
 
 
 
 S. M. Pourali, A. Samadi-maybodi /CSM Vol.2 No.1, 2014 pp.21-31
28                              
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.6. The FT-IR spectrum of sodalite octahydate sample S5, prepared from gel composition b
under 1 h of stirring aging.
 
 
 
Figure 7 illustrates the morphology and
particle size of  the samples S5 and S6.
The FE-SEM  image of S5 shows well-
shaped  spherical  crystals  of  sodalite
octahydrate with size distribution range
of  50-80  nm  which  is  in  good
agreement  with  Debye-Scherrer
calculation.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
The yield of the product based on SiO2
was  calculated  to be  about 15%. Also,
the FE-SEM image of the mixed phases
of  sodalite  and  cancrinite  shows
nanosphere crystals  in  the range of 40-
60 nm (Fig. 7, S6). The insets show the
higher magnification images.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 S. M. Pourali, A. Samadi-maybodi /CSM Vol.2No.1, 2014 pp.21-31
29
 
 
Fig.7. The FE-SEM images of samples S5 and S6, prepared from gel composition b under 1 h
and 2.5 h of stirring aging, respectively.
 
 
The  surface  texture  of  the  synthesised
nanocrystalline  sodalite  (S5)  was
studied by argon adsorption-desorption
isotherm.  It  should  be  noticed  that  the
maximum  window  in  the  sodalite
structure  is 6R with a size of 0.28 nm.
Therefore, N2  adsorption-desorption
isotherm  is  not  useful  to  characterise
the  internal  micropore  [30]  and  only
can roughly show the BET surface area
coming  from  external  surface  of  SOD
particles  [8]. Figure 8 shows  the argon
adsorption-desorption  isotherm  (a)  and
BJH method (b) analysis of sample S5.
Figure  8a  displays  type  II  adsorption-
desorption  isotherm,  being  almost
reversible.  This  type  of  isotherm
represents  unrestricted  monolayer–
multilayer  adsorption  [31]. The  results
showed  that  the  BET  surface  area
(SBET)  of  nanozeolite  sodalite  is  79.78
m2
g-1
. The  t-plot analysis  showed  that
the  obtained  sample  had  very  low
micropore  volume  (<  0.002  cm3
 g-1
)
and external  surface area very close  to
the SBET value which  indicated  that  the
formation of microporous  impurities  in
the  sodalite  sample  was  negligible.
This  type  of  isotherm  represents
unrestricted  monolayer–multilayer
adsorption  [32-34].  Furthermore,  the
average pore diameter of 1.14 nm was
obtained by BJH method (Figure 8b).
 
 S. M. Pourali, A. Samadi-maybodi /CSM Vol.2 No.1, 2014 pp.21-31
30                              
 
 
 
Fig.8. Argon adsorption-desorption isotherm (a) and BJH method diagram (b) of
nanozeolite sodalite sample (S5)
 
 
 
4. CONCLUSION
In  this work,  a  simple  effective  aging
process  was  employed  to  control
morphology  and  size  distribution  of
sodalite  by  using  two  different  initial
gel compositions. The  results  indicated
that  stirring aging at  room  temperature
prior  to  hydrothermal  treatment  can
result in different morphologies in each
gel  mixture  as  well  as  reduction  in
particle  size. With  a  Si/Al molar  ratio
of  5and  20  min  of  aging  time,
microcrystals  of  sodalite  with  wool
ball-like  morphology  was  observed
which  had  nano-threads  (80-120  nm)
and  altered  to  cabbage-like  micro-
crystals  with  50-80  nm  leaves  by
increasing in aging time to 72 h. On the
other hand, sodalite nanocrystals of 50-
80  nm were  observed  in  high  yield  (~
15%) with Si/Al molar ratio of 1.9 and
1  h  aging  time while  prolonged  aging
time  of  2.5  h  led  to  formation  of
cancrinite  as  a  new  phase.  The
synthesized  nanosodalite  exhibited
small  particles  which  were  evidenced
by  the  broadening  of  XRD  peaks,
Debye-Scherrer  calculation,  FE-SEM
images  and  argon  adsorption-
desorption  isotherm.  Therefore,  the
stirring  aging  process  is  promising  for
simple  synthesis  of  micro-  and  nano-
crystalline sodalites.  
 
Acknowledgements  
The  authors  would  like  to  thank Mrs.
Mehrnoosh  Sadeghi-Pari  from  Tehran
University, Nano-electronic Laboratory
for  her  infinite  patience  in  FE-SEM
measurements.
 
REFERENCES
[1]  K.L. Moran, W.T.A. Harrison, T.E.
Gier, X. Bu, D. Herren, H. Eckert,
G.D.  Stucky,  Chem.  Mater.  8,
1930 (1996).
[2]   X.C. Xu, Y. Bao, C.S. Song, W.S.
Yang,  J.  Liu,  L.W.  Lin,
Microporous  Mesoporous  Mater.
75, 173 (2004).
[3]  A. Julbe, J. Motuzas, F. Cazevielle,
G.  Volle,  C.  Guizard,  Sep.  Purif.
Technol. 32, 139 (2003).
[4]   J.C.  Buhl,  T.M.  Gesing,  C.H.
Ruscher, Microporous Mesoporous
Mater. 80, 57 (2005). S. M. Pourali, A. Samadi-maybodi /CSM Vol.2No.1, 2014 pp.21-31
31
[5]   A.  Stein,  M.  Meszaros,  P.M.
Macdonald,  G.A.  Ozin,  G.D.
Stucky, Adv. Mater. 3, 306 (1991).
[6]  A. Stein, G.A. Ozin, G.D. Stucky,
J. Am. Chem. Soc. 112, 904 (1990)
[7]   D.  Arieli,  D.E.W.  Vaughan,  D.
Goldfarb,  J.  Am. Chem.  Soc.  126,
5776 (2004).
[8]    J.F. Yao, L.X. Zhang, H.T. Wang,
Mater. Lett. 62, 4028 (2008)
[9]   S.  Mintova,  S.Y.  Mo,  T.  Bein,
Chem. Mater. 13, 901 (2001)
[10]  S. Mintova, N.H. Olson, T. Bein,
Angew.  Chem.,  Int.  Ed.  38,  3201,
(1999).
[11]   S.  Mintova,  N.H.  Oslon,  V.
Valtchev,  T.  Bein,  Science  283,
958  (1999).
[12]   S.  Mintova,  V.  Valtchev,  Stud.
Surf. Sci. Catal.125, 141 (1999)
[13]   B.J.  Schoeman,  J.  Sterte,  J.E.
Otterstedt, Zeolites 14, 208 (1994)
[14]   S.R. Lee, M. Park, Y.S. Han, J.H.
Choy,  J.  Phys.  Chem.  Solids  65,
421 (2004).
[15]  S.R. Lee, M. Park, Y.S., Han, J.H.
Choy,  Chem.  Mater.  15,  4840
(2003).
[16]    J.F.  Yao,  H.T.  Wang,  K.R.
Ratinac, S.P. Ringer, Chem. Mater.
18, 1394 (2006).
[17]  W. Fan, K. Morozumi, R. Kimura,
T. Yokoi, T. Okubo, Langmuir 24,
6952 (2008).
[18]  L. Tosheva, V.P. Valtchev, Chem.
Mater. 17, 2494 (2005).
[19]  C.M. Zimmerman, A. Singh, W.J.
Koros,  J.  Membr.  Sci.  137,  145
(1997).
[20]   Z.  Ghasemi,  H.  Younesi,  H.
Kazemian, Can. J. Chem. Eng. 89,
601 (2011).
[21]   A.  Kuperman,  S.  Nadimi,  S.
Oliver,  G.A.  Ozin,  J.M.  Graces,
M.M.  Olken,   Nature  365,  239
(1993).
[22]   S.  Feng,  T.  Bein,  Science  265,
1839 (1994).
[23]  F. Shi, X. Chen, L. Wang, J. Niu,
J. Yu, Z. Wang, X. Zhang, Chem.
Mater. 17, 6177 (2005).
[24]  J. Jiang, X.G.L. Feng, C. Duanmu,
Y.  Jin,  T.  Hu,  J.  Wu,  Powder
Technol, 217, 298 (2012).
[25]  M.T. Weller, J. Chem. Soc.Dalton
Trans, 4227 (2000).
[26]  F. Ocanto, R. Àlvarez, B. Moy, M.
Brikgi, C. Urbina de Navarro, C.F.
Linares,  J.  Mater.  Sci.  43,  190
(2008).
[27]B. Herreros, J. Klinowski,  J. Chem.
Soc. Faraday 91, 147 (1995)
[28]   D.  Li,  H.Y.  Zhu,  K.R.  Ratinac,
S.P.  Ringer,  H.  Wang,
Microporous  Mesoporous  Mater.
126, 14 (2009).
[29]   G.  Engelhardt,  J.  Felsche,  P.
Sieger,  J.  Am.  Chem.  Soc.  114,
1173  (1992).
[30]  E.M. Flanigen, H. Khatami, H.A.
Szymanski,  Am.  Chem.  Soc.
Washington, DC, (2009).
[31]   K.S.W.  Sing,  D.H.  Everett,
R.A.W.  Haul,  L.  Moscow,  R.A.
Pierotti,  J.  Rouqérol,  T.
Siemieniewska,  Pure  Appl. Chem.
57, 603 (1985).
[32]  N. Hiyoshi, Appl. Catal. A 41, 164
(2012).
[33] C. Grader, J.C. Buhl, Microporous
Mesoporous  Mater.  171,  110,
(2013).
[34] J.C. Buhl, K. Schuster, L. Robben,
Microporous  Mesoporous  Mater.
142, 666 (2011).