Synthesis and Structure Determination of a New Microporous Zeolite with Large Cavities Connected by Small Pores

Abstract

A new small-pore germanosilicate zeolite, named as ITQ-49, has been synthesized using a new ditetraalkylphosphonium dication as an organic structure-directing agent, and its structure has been solved by direct methods applied to the powder X-ray diffraction pattern of the calcined solid. This new zeolite crystallizes in the space group Immm with cell parameters a = 19.6007(8) Å, b = 18.3274(7) Å, and c = 16.5335(6) Å. The pore topology of ITQ-49 consists of large, nonspherical cavities that are connected to each other through small eight-membered-ring windows, resulting in a unidirectional small-pore zeolite that has a relatively large adsorption capacity. Also, ITQ-49 contains double four-membered-ring units where Ge is preferentially located, and fluoride anions are placed inside these units.

Full text

Synthesis and Structure Determination of a New Microporous Zeolite
with Large Cavities Connected by Small Pores
Manuel Herná
ndez-Rodríguez, Jose L. Jordá
, Fernando Rey,*and Avelino Corma*
Instituto de Tecnología Química (UPV-CSIC), Universidad Polité
cnica de Valencia −Consejo Superior de Investigaciones
Científicas, Av. de los Naranjos s/n, 46022 Valencia, Spain
*
SSupporting Information
ABSTRACT: A new small-pore germanosilicate zeolite,
named as ITQ-49, has been synthesized using a new
ditetraalkylphosphonium dication as an organic structure-
directing agent, and its structure has been solved by direct
methods applied to the powder X-ray diffraction pattern of
the calcined solid. This new zeolite crystallizes in the space
group Immm with cell parameters a= 19.6007(8) Å, b=
18.3274(7) Å, and c= 16.5335(6) Å. The pore topology of
ITQ-49 consists of large, nonspherical cavities that are
connected to each other through small eight-membered-
ring windows, resulting in a unidirectional small-pore
zeolite that has a relatively large adsorption capacity. Also,
ITQ-49 contains double four-membered-ring units where
Ge is preferentially located, and fluoride anions are placed
inside these units.
Zeolites are crystalline microporous materials, mainly
constituted by oxides of silicon and/or other elements
(e.g., Al, B, Ti, Ge, Sn) in tetrahedral coordination. The well-
defined size and distribution of the structural channels in each
different zeolitic framework type confer to these materials
multiple applications in processes such as gas adsorption,
separation, catalysis, and encapsulation or controlled release of
molecules, among others, some of them with industrial
applications.
1−9
This is the major force for studying novel
methods for the preparation of new zeolitic structures, since it
would give the possibility of obtaining tailored materials with
the most appropriate channel system for each specific
application. To date, up to 201 different zeolitic structures
have been accepted by the International Zeolite Association,
10
and this number is still increasing.
To obtain novel structures, several approaches have been
attempted. The incorporation of fluoride anions replacing
hydroxyl anions as the silica-mobilizing agent in the synthesis
gel has been found very often to drive the reaction toward the
formation of double four-membered-ring (D4R) units in the
zeolites obtained under those conditions.
11−21
In the same way,
the isomorphic substitution of Si with Ge presents a stronger
directing effect, favoring the formation of not only the D4R
units
20−30
but also the previously elusive double three-
membered-ring (D3R) units,
29,30
opening the doors to new
families of zeolites. Nonetheless, the most important approach
for obtaining new zeolites is the use of tetraalkylammonium
organic cations, which in the zeolite field are generally called
organic structure-directing agents (OSDAs).
20,21,31
Recently, the employment of P-containing OSDAs such as
tetraalkylphosphonium and phosphazene cations instead of the
tetraalkylammonium cations typically used for zeolite syntheses
has been described. These uncommon OSDAs have given
several new extra-large-, large-, and medium-pore zeolitic
structures.
30,32−35
To date, however, no small-pore zeolites
have been synthesized using tetraalkylphosphonium as the
OSDA.
In this report, we describe the synthesis and crystal structure
of ITQ-49, a new small-pore zeolite containing large non-
spherical cavities accessible by a one-directional system of eight-
membered-ring (8R) channels, that has been synthesized using
ditetraalkylphosphonium dications [obtained from butane-1,4-
diylbis(tri-tert-butylphosphonium) hydroxide] as the OSDA
along with Ge and F−as inorganic structure directing agents
(ISDAs). Details of the synthesis are described in the
Supporting Information (SI).
Elemental analysis of the as-made zeolite ITQ-49, as well as
13C and 31P magic-angle-spinning (MAS) NMR spectroscopy
(see the SI) indicated that the OSDA dications remain intact
within the zeolitic channels, with their positive charges being
balanced by the fluoride ions, which are located in the small
D4R cages, as indicated by the presence of resonance bands at
−9 and −20 ppm in the 19F MAS NMR spectrum of the as-
made material
22,36
(see the SI). Chemical analysis showed a Si/
Ge ratio of 4.7, very similar to that used in the synthesis gel.
The phosphorus content corresponds to a (Si + Ge)/P ratio of
21.6 in the as-made material as well as in the final calcined solid.
This indicates that the P-containing OSDA gives rise to the
formation of phosphate-like species during calcination, instead
of volatile phosphines as occur in zeolites with larger pore
apertures. These phosphate-like species were further identified
by means of 31P MAS NMR spectroscopy (see the SI).
Scanning electron microscopy indicated an average crystal size
of ∼0.2 μm, precluding structure determination using single-
crystal X-ray diffraction.
Powder X-ray diffraction (PXRD) data were collected on a
PANalytical X’Pert PRO diffractometer in the Bragg−Brentano
geometry using Cu Kαradiation, as described in the SI. Prior to
the measurement, in order to remove all of the organic content,
the sample was calcined in situ at 923 K for 5 h under a
continuous flow of dry air in an Anton Parr XRK-900 reaction
chamber attached to the diffractometer.
Received: June 20, 2012
Published: August 1, 2012
Communication
pubs.acs.org/JACS
© 2012 American Chemical Society 13232 dx.doi.org/10.1021/ja306013k |J. Am. Chem. Soc. 2012, 134, 13232−13235

The PXRD pattern was then indexed using the program
TREOR,
37
which revealed an orthorhombic unit cell with a=
19.6005 Å, b= 18.3272 Å, and c= 16.5333 Å. Analysis of the
systematic extinctions suggested as possible extinction symbol
I, corresponding to the space groups I222 (No. 23); I212121
(No. 24); Imm2, Im2m,orI2mm (No. 44), or Immm (No. 71).
Integrated intensities were extracted by a LeBail analysis of the
diffraction pattern using the program FULLPROF.
38
The
crystal structure was solved using the program FOCUS.
39
As a
first attempt, the groups with the lowest and highest
symmetries (I222, I212121, and Immm) were tested. Only the
group with highest symmetry, Immm, gave a satisfactory result,
consisting in nine independent T (Si or Ge) positions. Next,
the positions of 17 bridging O atoms were calculated using the
program KRIBER,
40
and the coordinates of all of the atoms
were optimized geometrically using the program DLS-76.
41
After a subsequent Rietveld refinement using FULLPROF, this
structure proved to be the right one. The PXRD pattern of the
refined structure is shown in Figure 1. The residuals of the
refinement were Rwp = 0.097, Rexp = 0.041, RB= 0.057, and RF=
0.093. Refinements in lower symmetries did not provide any
improvement in the residuals.
The structure of ITQ-49 can be constructed using four
building units (Figure 2). The main cage is a large
4854687281101unit, which is fused with a second cage by
sharing of a 10-membered-ring (10R) window, giving rise to a
“peanutlike”large cavity (Figure 2, top right). The structure is
completed with nonaccessible 486474,4
45463, and 46cages. The
“peanutlike”cavities are connected to the neighboring ones by
sharing of the 8R apertures, forming straight lobular channels
along the baxis that are accessible through the 8R windows
with a pore aperture of 5.0 Å ×3.6 Å. However, the entrance to
the cavities through the 8R windows is shifted by b/2 for each
channel relative to its closest neighbors. When the entrance to
ITQ-49 zeolite is viewed along the [010] direction, half of the
cavities offer an 8R pore aperture, while the other half present a
10R cup corresponding to a hemicavity (Figure 3, bottom).
The presence of the odd seven-membered-ring (7R) units is
very unusual in zeolites, and only other four structures, ZSM-18
(MEI),
42
SSZ-58 (SFG),
43
SSZ-23 (STT),
44
and VPI-8
(VET)
45
present this type of ring. In fact, ITQ-49 is the first
small-pore zeolite having 7R units in its structure. Each 7R
window has a pore opening of 3.7 Å ×2.2 Å, which is too small
to allow diffusion of any relevant molecule for catalysis or
separation. Thus, this topology must be considered as a
unidirectional 8R zeolite rather than a bidirectional 7R ×8R
zeolite.
The structure of zeolite ITQ-49 is fully consistent with the
adsorption properties. The micropore volume estimated from
the N2adsorption isotherm at 77 K is 0.18 cm3g−1. The
average micropore aperture calculated from the high-resolution
Ar adsorption isotherm at 87 K is 6.1 Å, which is much larger
than that obtained from crystallographic data. However, this
discrepancy can be explained considering that the 8R windows
give access to very large cavities, resulting in overestimated pore
aperture values, as observed previously for other well-known
zeolites.
46
The use of Ge and F−as ISDAs during zeolite crystallization
leads to the presence of D4R units in the ITQ-49 framework.
Notoriously, the D4R cages in this structure possess the lowest
symmetry reported to date for this building unit, with six
independent T sites (Figure 2). This allowed us to perform a
detailed study of preferential Ge occupation among the
different T sites.
Figure 1. Rietveld refinement of the PXRD pattern of ITQ-49 calcined
at 923 K. Data points show the observed PXRD pattern; the line along
these points is the calculated pattern, with the difference profile at the
bottom. The vertical tick marks below the pattern give the positions of
the Bragg reflections. The inset highlights the agreement in the 2θ
region from 15 to 75°.
Figure 2. Building units of ITQ-49, indicating the constituting T
atoms. Left, from top to bottom: 4854687281101,4
86474,4
45463, and 46
units. Right: top, “peanutlike”cavity with 7R and 8R entrances;
bottom, relative location of the cages. Oxygen atoms have been
omitted for clarity.
Journal of the American Chemical Society Communication
dx.doi.org/10.1021/ja306013k |J. Am. Chem. Soc. 2012, 134, 13232−1323513233

Rietveld refinement of the T-site occupancies as a mixture of
Si and Ge indicated a total Si/Ge ratio of 4.7, in good
agreement with the chemical analysis. The preferential
occupation of Ge at T6 sites (Si/Ge = 2.0) is clear, with
smaller Ge occupancies at T1, T2, T7, and T5 (Si/Ge = 3.6,
3.4, 3.5 and 5.2, respectively); all of these sites are located in the
D4R cage. On the other hand, there is a clear preferential
occupation of Si at T8 sites (Si/Ge = 9.2), which are also
placed in the D4R units. This value is very close to that
observed in T sites not in the D4R cage. This is clear evidence
that there is a large chemical anisotropy inside the otherwise
highly symmetrical D4R cage.
The occupancies of Ge at T9 sites (Si/Ge = 12.9),
corresponding to the first neighbor of the D4R unit, and at
T3 (Si/Ge = 8.3) and T4 (Si/Ge = 10.9) sites in a single four-
membered ring are also low. All of these values are in good
agreement with the behavior previously described for other
zeolitic silicogermanates.
22−30
Therefore, we can conclude that the use of new
ditetraalkylphosphonium dications as the OSDA combined
with the use of Ge and F−as ISDAs during zeolite
crystallization has resulted in a new unidirectional small-pore
zeolite, which we have named ITQ-49.
■ASSOCIATED CONTENT
*
SSupporting Information
Syntheses of the OSDA and the zeolite; characterization by
chemical analysis, thermal and textural analysis, solid-state MAS
NMR spectroscopy, and PXRD; atomic coordinates of ITQ-49
and a table of interatomic distances and angles; and a CIF file of
atomic coordinates. This material is available free of charge via
the Internet at http://pubs.acs.org.
■AUTHOR INFORMATION
Corresponding Author
acorma@itq.upv.es; frey@itq.upv.es
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
The authors are thankful for financial support by the Spanish
MICINN (MAT2009-14528-C02-01 and PLE2009-0054) and
Consolider Ingenio 2010-Multicat. M.H-R. acknowledges the
FPI Program of the Spanish MICINN for a predoctoral
fellowship. The authors thank Dr. A. Vidal and T. Blasco for
NMR assistance and useful discussions.
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