Preferential orientations of structure directing agents in zeolites

Article abstract of DOI:10.1039/c5dt02558c

The local structure of as-synthesised silicalite-1 zeolites is modified using asymmetric R(Pr)₃N⁺ structure directing agents. Using multi-nuclear NMR (¹H, ¹³C, ¹⁴N, ¹⁹F, ²⁹Si), we show for the first time the ability of these cations to adopt preferential orientations at the zeolite channels' crossing.

Full text of DOI:10.1039/c5dt02558c

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Preferential orientations of structure directing
agents in zeolites†
Cite this: Dalton Trans., 2015, 44,
16680
Eddy Dib, Antoine Gimenez, Tzonka Mineva and Bruno Alonso*
Received 6th July 2015,
Accepted 23rd August 2015
DOI: 10.1039/c5dt02558c
www.rsc.org/dalton
The local structure of as-synthesised silicalite-1 zeolites is
In connection with these studies, we investigate the effect
modified using asymmetric R(Pr)3N+ structure directing agents. of small modifications of TPA+ on the local structure, defined
Using multi-nuclear NMR (1H, 13C, 14N, 19F, 29Si), we show for the here at the scale of the zeolite crystal cell. These small modifi-
first time the ability of these cations to adopt preferential orien- cations are a sort of chemical perturbation that may also give
tations at the zeolite channels’ crossing.
access to new information on the role of TPA+ in the formation
of the MFI topology. From previous 13C solid-state NMR
studies,13 and notably from the different 13C chemical shifts
for carbons Cα and Cγ, it appears that the four propyl chains
do not have the same conformation inside the as-synthesised
zeolite. Therefore, one interesting approach is to replace one
propyl chain by another shorter or longer chain, and to check
the effect on the as-synthesised zeolite. This can give interest-
ing insights if we are able to determine the orientation at the
crossing of the channels of the asymmetric SDA: Et(Pr)3N+ (tri-
propylethylammonium, TPEA+) and Bu(Pr)3N+ (butyltripropyl-
ammonium, BTPA+). For a single molecule, it will be one of
the various statistically possible orientations, or one specific
and fixed orientation.
In order to look carefully at these effects, we focus on the
most ordered MFI-type zeolite, the silicalite-1, formed using the
fluoride route.14 Modifications of the local order or local struc-
ture for silicalite-1 can be related to 13C and 29Si solid-state
NMR spectrum modifications14,15 and also to strong variations
in 14N quadrupolar interaction parameters as recently shown.16
Using this series of SDA (TPA+, TPEA+, BTPA+) within a stan-
dard fluoride route procedure (see ESI 1–3†), we obtained a
series of highly crystalline silicalite-1 zeolites presenting the
typical crystal morphology and the X-ray diffraction patterns of
MFI-type zeolites (see ESI 4 and 5†). The 29Si{1H} CP-MAS
(Cross Polarization Magic Angle spinning) spectrum recorded
when using TPA+ (TPA-MFI) is typical for the silicalite-1
(Fig. 1).15,17 By replacing TPA+ by one of the considered asym-
metric SDA, the spectrum resolution is almost preserved. We
observe an upfield or downfield displacement for most of the
29Si peaks, suggesting specific local modifications of the silica
skeleton, probably arising from variations in SiOSi angles
between the interconnected SiO4 tetrahedrons. The expla-
nation for these variations in 29Si chemical shifts for each
specific Td site is not straightforward (see table in ESI 6†). But
The combination of interesting pore channel’s topology and
pore surface’s functionality within crystalline solids is a key
factor for the success of zeolites in a large number of fields
among which are heterogeneous catalysis, adsorption and ion-
exchange processes.1–3 These textural and structural properties
depend on the synthesis conditions, especially on the interplay
between structure directing agents (SDA) and zeolite building
units.4,5 The design of specific SDA to reach the desired pro-
perties is therefore an important issue that still drives many
research studies.
Zeolites with MFI topology (e.g. ZSM-5, silicalite-1, TS-1) are
made of straight and sinusoidal 10 MR channels with pore
diameters in the 5.0–5.5 Å range.6 Tetrapropylammonium
(TPA+) is a well-known and efficient SDA for these zeolites.7 It
is spatially located at the crossing between the two kinds of
channels (straight and sinusoidal) with each of its propyl
chains pointing towards each channel direction.8 Other tetra-
alkylammonium SDA have also been explored, including small
symmetric9 or asymmetric10 molecules, n-alkyltrimethyl-
ammonium surfactants,11 small diquats or triquats9 made up
of two or three ammonium groups. And recently, the use of
long alkyl chain diquat surfactants has allowed the formation
of layered MFI zeolites, giving rise to a significant increase in
their catalytic properties by reducing the crystallite size to the
nanometer range.12
Institut Charles Gerhardt Montpellier – UMR 5253 CNRS/UM/ENSCM, 8 rue de
l’Ecole Normale, 34296 Montpellier Cedex 5, France. E-mail: bruno.alonso@enscm.fr;
Fax: +33 4671 63470; Tel: +33 4671 63443
†Electronic supplementary information (ESI) available: Synthesis procedures,
complementary solid-state multi-nuclear NMR data, SEM, XRD and TGA results.
See DOI: 10.1039/c5dt02558c
16680 | Dalton Trans., 2015, 44, 16680–16683
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for TPA-MFI, −127 ppm for TPEA-MFI and for BTPA-MFI).
Besides, the location of F− anions in the same kind of cage
indicates that SDA cations are probably located at the same
crossing positions as the spatial distributions of both charged
species are related through electrostatic interactions.19
1H NMR MAS spectra present relatively well resolved peaks
in the 0–4 ppm range assigned to the alkyl groups of the
organic molecules (see ESI 6†). The very low amplitude of the
well-known peak related to SiO−⋯HOSi groups20 (located at ca.
10 ppm) shows the almost lack of defects in the samples. From
2D 1H–1H dipolar double quantum–simple quantum corre-
lation experiments (see ESI 6†), we conclude that each methyl
or methylene CHx group in one chain is spatially close to the
other CHx groups of the same chain, indicating that the
chains are not fully extended. In the particular case of
1
TPEA-MFI, for which the H peaks of the two sorts of chains
are more easily distinguished, the results are also consistent
with the absence of close contacts between chains of different
nature. This fully agrees with the location of SDA molecules at
the crossing of channels with the pending chains located at
the beginning of each channel, as is known for TPA+.
In order to investigate the SDA orientations we have
recorded 13C solid-state NMR CP-MAS spectra (Fig. 2). In the
case of TPA-MFI as-synthesised zeolites, there are two distinct
signals for the Cγ(Pr) positions with a 1 : 1 signal area ratio.
They are assigned to the location of the propyl chains in the
two different types of channels: straight and sinusoidal
(δiso(13C) = 11 and 10 ppm resp.).21 When one propyl chain is
replaced by another chain, the signal area ratio between these
two peaks can be: (i) preserved (1 : 1) if there is a statistical dis-
tribution of the alkyl chains within the different channels, or
(ii) modified (1 : 2 or 2 : 1) if the alkyl chains have specific
locations as a function of their length. Interestingly, it is this
Fig. 1 29Si{1H} (black) and 29Si{19F} (gray) NMR CP-MAS spectra of the
as-synthesized zeolites (ν0 29Si = 79.6 MHz; νMAS = 5.0 kHz). The inset
presents the correlation between the length of the modified chain and
the average chemical shift of the whole 29Si{1H} (black filled circles) and
29Si{19F} (clear circles) NMR CP-MAS spectra.
it is however interesting to note that these variations clearly
depend on the nature of the used SDA. In addition, the
average value of the 29Si chemical shift varies gradually with
the length of the modified alkyl group (inset in Fig. 1). These
observations are confirmed when inspecting the 29Si{19F}
CP-MAS spectra obtained with 19F polarisation transfer
(Fig. 1). In this case, the averaged 29Si chemical shift for the Td
sites around the F− counter anion varies even more strongly
with respect to the length of the alkyl group.
The environment and location of fluorides in the silica skel-
eton can be explored using 19F NMR. The resulting 19F spectra
for the three samples are very similar although not strictly
identical. And the estimated chemical shift anisotropy para-
meters (δiso, ΔCSA and ηCSA) for the main signal (92 to 97% in
area)‡ have similar values (see ESI 6†). They are thus assigned
to F− counter-anions located in the same kind of cage
[415262].18 In these cages, F− anions are subjected to a
dynamic exchange between penta-coordinated Si sites that can
be slightly affected by the geometrical variations of the silica
skeleton. This may also explain the small variations observed
in the 29Si chemical shift of the signal corresponding to the
Fig. 2 13C{1H} NMR CP-MAS spectra (ν0 13C
= 75.5 MHz; νMAS =
exchange between the SiO4/2 and SiO4/2F− species (−125 ppm 5.0 kHz).
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latter case that occurred here. This means that each asym- interactions is similar, and indeed we found CQ values in the
metric tetraalkylammonium cation has a specific orientation range 50–55 kHz by spectrum modelling (see ESI 6†). These
at the crossing between the zeolite’s channels. Moreover, we values are different from those obtained in the case of the syn-
clearly observe an influence of the length of the alkyl chain. thesized SDA bromide salts themselves (see ESI 6†). For this
When a propyl chain is replaced by a smaller ethyl chain series of zeolites, the similarity between the electric field gradi-
(TPEA-MFI) the signal of Cγ(Pr) in the straight channel is ents at N atoms measured by 14N NMR comes from a rough
decreased, and this is explained by the location of the ethyl invariability of the three factors previously identified.21–23
chain in this type of channel. On the contrary, in the case of Indeed, we have shown here the preserved location of F−
BTPA-MFI, the signal of Cγ(Pr) in the sinusoidal channel is counter anions in the [415262] cages (vide supra). And we
decreased, and the butyl chain is thought to be located in this hypothesise that CNC angle distributions are not dramatically
other type of channel.
affected by the use of asymmetric R(Pr)3N+ cations, although
If we now consider the signals for the Cα(Pr) positions, some different hindered vibrations may exist and could explain
there are two signals in a 1 : 3 ratio for TPA-MFI. Here, the the slightly different 14N NMR SSB envelope for BTPA-MFI.
chemical shifts’ differences cannot be easily explained by the
These observations, in agreement with the above presented
effect of the channel’s nature (straight or sinusoidal), but poss- 29Si NMR data, indicate that the zeolite skeleton is able to
ibly by the existence of a more distorted angular configuration adapt to the geometry of the organic molecule in order to
for the methylene groups attached to the nitrogen (e.g. vari- reach the more stable state as a function of the intermolecular
ations in NCC and dihedral NCCC angles) that gives rise to a interactions (electrostatic, hydrophobic and vdW) occurring
signal at a higher chemical shift (65.8 ppm). The disappear- between the organic and inorganic species (SDA, silicates, F−)
ance of this peak in the 13C spectrum of TPEA-MFI (and not during zeolite formation.
BTPA-MFI) along with the localisation of the ethyl chain in the
However, the flexibility of the zeolite skeleton in the pres-
straight channel (vide supra) leads us to conclude that this ence of the asymmetric SDA does not explain why these
more distorted angular configuration corresponds to one of cations adopt specific orientations at the crossing between
the propyl chains lying in the straight channel.
straight and sinusoidal channels. And more precisely, why are
Additional information on the local order in zeolites can be ethyl chains occupying the straight channels, and butyl chains
gained from 14N NMR spectroscopy. In particular, the 14N the sinusoidal ones? We could envisage that the main reason
quadrupolar interaction, characterised by the quadrupolar is a difference in steric constraints: the bulkiest chains will
coupling constant CQ and the quadrupolar asymmetry para- preferentially be located in the largest channels. But this argu-
meter ηQ, is very sensitive to conformational changes around ment is not strong enough because: (1) there are no big differ-
the N (CNC angle distribution) as well as to the mobility22 and ences in channels’ dimensions (from crystallographic data:25
to the spatial distribution of the charges (cations and counter N–N distances between TPA+ are always around 10.5 Å, and
anions).23,24 The 14N NMR spectra of the as-synthesized zeo- pore openings are similar); (2) this does not explain the prefer-
lites (Fig. 3) show similar extents for the spinning sideband ence of the smallest ethyl chains for a specific channel. There-
(SSB) patterns. This means that the intensity of quadrupolar fore, we think that the reason could lie in the differences in
energy between conformational states. As discussed above,
TPA+ adopts a specific conformation inside ordered MFI-zeo-
lites that leads to a characteristic 13C spectrum where Cα(Pr)
signals are split into two with a 1 : 3 ratio. The signal corres-
ponding only to one Cα(Pr) atom (δiso = 65.8 ppm) is possibly
the signature of an angular distortion of the propyl chain
occurring at this α position. The presence of angular distor-
tions certainly originates in the mismatch existing between the
energetically preferred Td geometry at the N site and the less
symmetric geometry formed by the four channels at their
crossing. Also, this distortion has an energetic cost for the
system that could be reduced when the propyl chain is
replaced by a shorter ethyl chain. Under these conditions, the
related 13C signal disappears from its specific location, as
observed. On the contrary this distortion could lead to an
incremented energy when the propyl chain is replaced by the
butyl chain.
In the series of samples studied here, the size of the used
organic molecule varies. But what will happen if we modify the
organic molecule, adding for example a chemical function,
Fig. 3 14N NMR spectra of the as-synthesized zeolites (ν0(14N)
43.3 MHz; νMAS = 2.0 kHz).
=
while preserving its size? To answer this question, we recently
synthesized EtOHTPA-MFI zeolites using EtOHTPA+ (2-hydroxy-
16682 | Dalton Trans., 2015, 44, 16680–16683
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tripopylammonium cation) as SDA (other parameters being
equal). Interestingly a distribution of 29Si chemical shifts is
observed for the resulting silicalite-1 (see ESI 7†), but the
average value of the 29Si chemical shifts is the same as for
TPA-MFI. This indicates clearly that there is an increase in the
local disorder (also observed by the existence of a distribution
in 14N quadrupolar parameters) but that the average positions
of the Td sites are the same as in TPA-MFI. In other words, the
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Conclusions
The present study is the first demonstration that preferential
orientations of the organic SDA molecules occur when asym-
metric R(Pr)3N+ molecules are used in the synthesis of MFI type
zeolites. In particular, ethyl chains will be located in the straight
channels when using TPEA+, while butyl chains will be in the
sinusoidal channels when using BTPA+. From multinuclear
NMR data, we deduce that: (1) the spatial distribution of
charges is preserved in the medium/long-range (14N, 19F NMR)
under the effect of electrostatic interactions; (2) the zeolite silica
skeleton adapts locally to the geometry of the molecule
(29Si NMR) so as to minimise the short-range intermolecular
interaction; (3) and the SDA molecule adopts a specific orien-
tation (13C NMR) so as to minimise the conformational energy.
Further work based on experimental and theoretical data is cur-
rently in progress to better define and understand the location,
conformations and dynamics of these SDA.
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Acknowledgements
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The authors would like to thank P. Gaveau, C. Biolley and
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‡Other minor peaks are observed at around −80 ppm and −120 ppm and
assigned to structure defects and residual NH4F resp.
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This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 16680–16683 | 16683