Detection of structural elements of different zeolites in nucleating solutions by electrospray ionization mass spectrometry

Article abstract of DOI:10.1002/anie.200803007

Building blocks in solution: Characteristic structural elements of the germanium-containing zeolites ZSM-5, polymorph C of zeolite Beta, andzeolite A (see picture) are identified in solution immediately before nucleation. The conditions of the reaction direct the polycondensation of silicate towards specific structures at the solution stage. (Figure Presented).

Full text of DOI:10.1002/anie.200803007

Communications
DOI: 10.1002/anie.200803007
Zeolites
Detection of Structural Elements of Different Zeolites in Nucleating
Solutions by Electrospray Ionization Mass Spectrometry**
Bernd Bastian Schaack, Wolfgang Schrader, and Ferdi Schüth*
Zeolitic materials are crystalline silicates, formed by corner-
ammonium hydroxide (TPAOH) as organic template, and
subsequent hydrothermal treatment. The speciation in solu-
tion was followed by ESI-MS. As TEOS hydrolyses immedi-
ately, the first mass spectrum was recorded one minute after
combining the reactants. This spectrum is dominated by peaks
corresponding to monomeric silicon (Figure 1a) or germa-
sharing TO tetrahedra (T= Al, Si) with a regular array of
4
microporous channels and/or cavities. This pore system, in
combination with reactive intra- or extraframework species, is
responsible for their unique properties and thus for their use
[
1]
in catalysis, chemical separation, and adsorption. Their
synthesis is directed towards a specific structure by the
reaction conditions, the presence of structure-directing
[
2]
agents, often amines or organic ammonium salts, or the
introduction of specific heteroelements. For instance, zinc or
germanium lead to the formation of new materials with a high
[
3]
fraction of three-ring (3R) or double four-ring (D4R) units,
[
4]
respectively, which are known to induce very open networks
[
5]
with large pore diameters.
The formation of zeolites from solution occurs through a
complex sequence of numerous reactions that are partly
running in parallel. During these early steps, the final
[
6]
structure of the zeolite can be strongly influenced. However,
because of analytical limitations, little is known about the
species occurring during these steps. Thus, various nucleation
mechanisms have been proposed and are still controversially
[
7]
discussed in the literature.
In previous investigations we have introduced electro-
spray ionization mass spectrometry (ESI-MS) as a powerful
tool to investigate speciation and time-dependent oligomer
[8,9]
distributions in silica solutions.
We have now for the first
time succeeded in the identification of characteristic struc-
tural elements of three germanium-containing zeolites, that is,
ZSM-5 (structure code MFI), polymorph C of zeolite Beta
(
BEC), and zeolite A (LTA), in the solutions immediately
before nucleation.
Zeolite ZSM-5 and its pure silica form silicalite-1 (MFI)
consist of two intersecting channel systems, limited by
[
10]
elliptical ten-ring (10R) openings. Its germanium-contain-
ing, aluminum-free form was obtained by adding tetraethyl-
orthosilicate (TEOS) to an aqueous solution, with germanium
dioxide (GeO ) as germanium source and tetrapropyl-
2
[*] B. B. Schaack, Dr. W. Schrader, Prof. Dr. F. Schüth
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany)
Fax: (+49)208-306-2995
E-mail: schueth@mpi-muelheim.mpg.de
Figure 1. Time-dependent mass spectra recorded during the synthesis
of ZSM-5 from an aqueous solution with the following molar compo-
sition: 0.962SiO :0.038GeO :0.36TPAOH:20H O. a) After combining
the reactants, b) 2 h after mixing and heating to 958C, c) after 4 h, and
d) after 4.5 h. Displayed species: the monodehydroxylated 3R, 4R, and
[
**] The authors acknowledge partial financial support by the Leibniz
program of the Deutsche Forschungsgemeinschaft (DFG), in
addition to the basic funding provided by the Max-Planck-Gesell-
schaft (MPG). We thank Dr. C. Weidenthaler and Dr. C. W. Lehmann
for their support in the X-ray analyses as well as Ms. S. Palm for her
support in the EDX analyses.
2
2
2
5R units (a); a series of species starting from the monodehydroxylated
D5R unit (b,c), enlarged by formally attaching a SiO molecule (Dm/
2
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803007.
z 60 Da). e) Structure of zeolite ZSM-5 (MFI topology, taken from the
International Zeolite Association (IZA) structure database).
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Angewandte
Chemie
nium hydroxo species,their dimers, and monocyclic species.
Satellite signals with a mass reduced by 18 Da, caused by the
elimination of one water molecule, were observed as well (see
Table S1 in the Supporting Information for a complete list of
species observed). The occurrence of dehydroxylated species,
which are only stable in the gas phase of the spectrometer, is
ately. Thus, the first mass spectrum was recorded one minute
after combining the reactants (Figure S4 in the Supporting
Information). It shows a multitude of species, up to approx-
imately 1400 Da. The main signals at m/z 95 and 141 can be
attributed to monomeric silicon and germanium hydroxo
À
species, MO H3 . As during the synthesis of ZSM-5, satellite
4
[11]
characteristic for all the obtained mass spectra.
signals with a mass reduced by 18 Da, caused by the
elimination of one water molecule, were observed as well.
The fact that species with higher masses were obtained in the
first spectrum is in line with studies reporting an enhanced
With increasing reaction time and by heating the reaction
mixture to 958C, species with higher masses start to occur.
The resulting widespread peak distribution is characterized
by signals with a mass difference of 60 Da, which indicates a
series of species emerging from each other by formally adding
[14]
crystallization rate with increasing amount of germanium.
With increasing reaction time and after heating the system
to the reaction temperature (1458C) the intensity of species
with higher masses increases, while the intensity of species
with smaller masses decreases. As in the synthesis of ZSM-5,
after four hours the intensities are shifted to species with
smaller m/z values, caused again by depolymerization as a
result of the reduced silicate concentration in solution.
Simultaneously performed DLS measurements showed that
exactly at this time colloidal particles, too large to be detected
by ESI-MS, start to grow in solution (Figure S3 in the
Supporting Information). The analysis of the species distri-
bution gives new insight into the nucleation of polymorph C.
Figure 2a shows the ESI mass spectrum recorded imme-
diately before particles start to grow in solution (see Figure S4
in the Supporting Information for the full series of time-
dependent mass spectra). The low mass range is characterized
by monomeric, dimeric, and monocyclic species (see Table S2
in the Supporting Information for a complete list of species).
Units with the 4R species as basic structure, enlarged by
condensing monomers to the corners of the square, were
observed as well. Consecutive satellite peaks with a mass
difference of 46 Da reveal that these units contain up to three
germanium species in place of silicon atoms. The middle mass
range is dominated by D4R units, enlarged by attaching
monomers to the corners of the cube, which contain up to
three germanium atoms.
The 4R and D4R species are structural elements of
polymorph C of zeolite Beta, but rather unspecific ones,
because they also occur in many other zeolites. However,
connected D4R species bridged by one silicate unit are rather
characteristic for zeolite Beta (Figure 2b), and can be
attributed to signals occurring in the higher mass range for
units containing up to six germanium atoms (see enlarged part
of Figure 2a). Although this assignment is not absolutely
certain, the isotopic patterns of the silicon and germanium
give very precise information on the sum formula. There are
no alternative consistent progressions of silicate species other
than the one starting with two silicon-bridged D4R units. In
particular, no indications of directly linked D4R units were
observed in this system, in contrast to the synthesis of
zeolite A (see below). The final solid was determined to be
the polymorph C of zeolite Beta by XRD (Figure S5 in the
Supporting Information) with a Si/Ge ratio of 2.4, slightly
lower than that of the solution species. In a control experi-
ment, the same synthesis composition was used, but TEAOH
was replaced by TPAOH. This led to formation of the MFI
structure, and the solution prior to formation of the solid
SiO . No mixed species containing silicate oligomers and the
2
organic template were detected under these conditions. Two
possible structures can be attributed to the unit which
represents the starting point of this series: either a double
five-ring (D5R) or a dehydroxylated D4R unit enlarged by
the attachment of two monomers to the corners of the cube.
However, no signal corresponding to a D4R unit enlarged by
one monomer was observed in solution, and thus it is highly
probable that the basic unit is the D5R species. These do not
occur in the structure of MFI as such. Nevertheless, the D5R
species at least includes a major structural element of MFI-
type zeolites, a five-membered ring (Figure 1e), which could
be incorporated into the growing zeolite by a similar
mechanism to that observed for the exchange of larger units
[
9]
between D4R and D3R species.
Furthermore, satellite peaks with an enhanced mass of
6 Da prove that in all the occurring species one silicon atom
4
at most is exchanged against germanium (most probably
because of the low germanium concentration in the synthesis
solution). After 4 h the intensities are shifted again to species
with smaller m/z values (Figure 1d). Simultaneously per-
formed dynamic light scattering (DLS) measurements have
shown that exactly at this time colloidal particles, too large to
be detected by ESI-MS, start to grow in solution (Figure S1 in
the Supporting Information). The mass spectrum recorded
after this time represents the remaining solution, with a
decreased silicate concentration that leads to depolymeriza-
tion and silicate species at lower masses. The final solid was
proven to be ZSM-5 by X-ray diffraction (XRD; Figure S2 in
the Supporting Information).
To determine whether the presence of characteristic
structural elements immediately before nucleation is a more
general phenomenon, two other systems were studied in
detail. Zeolite Beta is known to be a highly intergrown hybrid
of two distinct, but closely related structures, polymorphs A
[12]
and B, which usually occur in a ratio of 60:40.
A third
polymorph (C) with a three-dimensional pore topology, which
has large linear channels defined by 12-membered rings, was
for a long time predicted to exist as well (structure code:
BEC). Its structure is formed by connecting D4R and four-
ring (4R) units by O–Si–O bridges. As germanium is known to
increase the stability of these units, the synthesis of poly-
morph C was carried out successfully by using framework
[
13]
substitution of silicon against germanium.
Upon adding the silicon source (TEOS) to an aqueous
solution containing GeO and the organic template (tetra-
2
ethylammonium hydroxide, TEAOH), it dissolves immedi-
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Communications
Figure 2. a) ESI mass spectrum recorded during the synthesis of polymorph C of zeolite Beta immediately before the first particles start to grow
in solution. Molar composition of the synthesis mixture: 0.67SiO :0.33GeO :0.5TEAOH:0.5HF:37.5H O. b) Structure of polymorph C of zeolite
Beta (BEC topology, taken from the IZA structure database).
2
2
2
contained predominantly the 5R species, thus supporting the
important role of the organic template.
Another zeolite containing D4R units is zeolite A (struc-
ture code: LTA). It can be built up by linking sodalite cages
time and after hydrothermal treatment at 1508C. After four
hours the intensity of the species in the upper mass range
starts to decrease again. As the reaction mixture contains less
water than the synthesis gel of zeolite Beta (polymorph C),
the system has a viscosity too high to perform DLS measure-
ments simultaneously. Nevertheless, the shift of the species to
lower masses can be taken as indication of colloidal particles
in solution, as in the other two systems.
Data analysis of the species occurring during the synthesis
of zeolite A revealed the presence of oligomers rather similar
to those in the synthesis of polymorph C. The most significant
difference was observed in the upper mass range (see
enlarged part of Figure 3a): the signals can most straightfor-
wardly be attributed to species representing D4R units that
are directly linked over siloxane bonds (see Table S3 in the
Supporting Information for a complete list of species). Their
corresponding satellite signals (Dm/z 46 Da), together with
their isotopic pattern, prove that these species occur with up
to six incorporated germanium atoms as well. These directly
linked cubes are fingerprints of the zeolite A structure
(b cages), related by translation along the cube axis, through
D4R units. Hence, the zeolite A structure can be formed by
linking D4R units over siloxane bonds directly. Its aluminum-
free synthesis was developed by using the structure-directing
effect of supramolecular self-assembled organic molecules in
combination with an isomorphic exchange of framework
[
15]
silicon against germanium. On adding the silicon source
TEOS) to an aqueous solution containing GeO and the
(
2
organic template, it dissolves immediately and results in a
clear solution.
The first spectrum (Figure 3a) was recorded one minute
after combining the reactants (see Figure S6 in the Supporting
Information for time-dependent mass spectra). In line with
the synthesis of zeolite Beta polymorph C, species with higher
masses, up to 1400 Da, but with low intensities were observed
as well. Their intensities increase with increasing reaction
Figure 3. a) ESI mass spectrum recorded during the synthesis of zeolite A immediately before the first particles start to grow in solution. Molar
composition of the synthesis mixture: 0.67SiO :0.33GeO :0.5ROH:0.5HF:20H O. b) Structure of zeolite A (LTA topology, taken from the IZA
2
2
2
structure database).
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Angewandte
Chemie
(
Figure 3b). Furthermore, their compositions are in the range
of the silicon-to-germanium ratio of the final solid (Si/Ge =
.8). The final product of the synthesis was proven to be
details). Samples were taken continuously and analyzed by ESI-MS.
The obtained solid was washed with distilled water and characterized
by XRD and EDX analysis.
2
ZSM-5 was obtained from an aqueous synthesis mixture with the
following composition: 0.962SiO :0.038GeO :0.36TPAOH:20H O.
zeolite A by XRD (Figure S7 in the Supporting Information).
In summary, we have for the first time found clear
evidence for the presence of different characteristic structural
elements of zeolites in the prenucleating solution, obtained by
using different templates and different synthetic conditions.
This proves that the conditions of the reaction direct the
polycondensation of silicate towards specific structures at the
solution stage. While we would not claim, based on our
results, that zeolite formation necessarily proceeds through
the assembly of structural elements already present in
solution, this certainly could be a viable scenario. It has
become clear that the synthesis conditions for specific zeolites
favor the formation of species characteristic of these struc-
tures already in solution. Whether they assemble directly,
exchange structural fragments, or depolymerize again to
feed the silicate pool is still unclear. The necessary ingredients
for zeolite nucleation by the assembly of structural subunits
are, however, certainly in place, and the findings revealed
herein can be used to guide future work on this important
problem, as long as the systems are not too viscous to be
injected into the mass spectrometer. In addition, the findings
may present a new method for screening zeolite synthesis
systems for novel structures. For this, one would need to vary
the conditions and simultaneously monitor the species in the
synthesis solution, for instance, by using a tubular reactor with
continuously changing reactant composition. If new and
interesting oligomeric species are detected, full-scale zeolite
synthesis experiments could be initiated around the param-
eters discovered in the mass spectrometric study. Such
programs have now been launched in our laboratory.
2
2
2
[16]
The synthesis was carried out in a polypropylene vessel at 958C.
Samples were taken continuously and analyzed by ESI-MS and DLS.
The obtained solid was washed with distilled water and characterized
by XRD.
Received: June 23, 2008
Revised: July 31, 2008
Published online: October 20, 2008
Keywords: hydrothermal synthesis · mass spectrometry ·
.
nucleation · silicates · zeolites
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(EDX) analysis.
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Angew. Chem. Int. Ed. 2008, 47, 9092 –9095
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 9095