A UNIFIED SYNTHETIC APPROACH TO POROUS HYBRID SINGLE-SITE METALLOSILICATES
Martin Kejík,1 Zdeněk Moravec,1 Lucie Šimoníková,1 Aleš Stýskalík,1 Craig E. Barnes,2 Jiří Pinkas1
1Masaryk University, Department of Chemistry, CZ-61137 Brno, Czech Republic, 380095@mail.muni.cz
2University of Tennessee, Department of Chemistry, Knoxville, TN 37996-1600, USA
1. Introduction
Following on the excellent work of Ghosh,1 Clark,2 and Stýskalík et al.,3 a highly flexible non-hydrolytic sol-gel procedure for the preparation of porous hybrid silicate matrices containing well-dispersed
d-/p-block element oxide centers (sites) is presented. Firstly, a readily available spherosilicate building block (Me3Sn)8Si8O20 is reacted with a limited amount of a suitable site precursor. This leads to the
formation of sparsely linked oligomers containing uniform sites, which are fully enveloped by the spherosilicate molecules. Secondly, a dose of a divalent inert linker is introduced in order to further
cross-link the matrix to the point where it is rigid enough to retain porosity. The solvent as well as all condensation byproducts can be easily removed under vacuum to afford pure xerogels. The mild
non-hydrolytic reaction conditions allow for the incorporation of sensitive moieties. All condensation reactions are intentionally irreversible to avoid any equilibria and site rearrangement, allowing for an
approximately additive bottom-up approach. The networks are always inevitably amorphous and statistical in nature, however, a minimum site separation is enforced through the site envelopment by the
bulky spherosilicate. The aim of this work is to unify the vast pool of knowledge that has been accumulated (with various goals) on this topic and to provide a „one size fits all“ strategy for the synthesis
of sites based on elements all across the periodic table, and with as much predictive power as possible for the reactivity-structure relationships.
3. Suitable precursors
The employed condensation reactions resemble traditional proton-exchange chemistry with the difference that Me3Sn+/Me3SnCl/Me3SnR interact only weakly and thus avoid the usual pitfalls of aqueous
sol-gel chemistry. The byproducts of the condensations do not interfere with subsequent steps, they can be readily removed under vacuum, and gravimetric techniques can be used to monitor the
degree of condensation. Quantum mechanical DFT calculations revealed that the preference of Me3Sn+ for O over Cl is negligible and the primary driving factor for the condensation is the oxophilicity of
the site precursor. Me3SnOSiH3 was identified as a suitable model for the calculation of condensation thermodynamics as it was found to consistently over-estimate the reaction ΔG by 5–10 % compared
to Me3SnOSi(OSiH3)3 while providing an order of magnitude faster computation. A series of site precursors were computationally screened to determine if the condensation reaction is spontaneous and
sufficiently irreversible (Figure 1). The reactivity was found to roughly correspond to the electronegativity of the central atom. The compound Ph3SbCl2 is a borderline example and while its full
condensation was confirmed experimentally, the irreversibility of the reaction must be investigated further. The reactivity of ZnEt2/ZnCl2 was studied in depth in order to explain experimental
observations. The data shows that while it is marginally possible to create [ZnO2] sites starting from ZnEt2, the ultimate thermodynamic sink of the system is ZnCl2, therefore once a source of Cl- is
introduced (inert linker), the structure is dissolved (Figure 2). Coordination of ligands stabilizes the preferred direction of the reaction in both cases.
Figure 3 (right):
STEM-HAADF/STEM-EDS images of a single-site matrix containing [pyridine-AlO3] moieties.
Instrument: FEI Titan Themis 60-300
Figure 1:
Calculated ΔG for the condensation of selected site precursors with the spherosilicate building block at 298.15 K
Method: B3LYP-D3/cc-pVTZ+aug-cc-pVTZ-PP(Sn,Sb)//CBS
Figure 2:
Calculated relative ΔG for the condensation/coordination of the ZnEt2/ZnCl2/THF system at 298.15 K
Method: B3LYP-D3/PBE0-D3/cc-pVTZ+aug-cc-pVTZ-PP(Sn)//CBS
6. Acknowledgment
The financial support by GAČR Junior 20-03636Y is gratefully acknowledged.
CIISB research infrastructure project LM2018127 funded by MEYS CR is gratefully acknowledged
for the financial support of the measurements at the Josef Dadok National NMR Centre.
2. Synthetic strategy
+ 1.00 eq.
1.50 eq.
2-connected
3-connected
4-connected
6-connected
0.75 eq.
0.50 eq.
(Me3Sn)8Si8O20
homogeneous
solution of oligomers
or
– Me3SnCl / Me3SnR
1st STEP
– Me3SnCl
2nd STEP
PRODUCT:
hybrid microporous single-site
metallosilicate gel
SABET up to 400–800 m2 g-1
rigid
flexible
~ 0.9 nm
THF / toluene
=
„molecular silica“
60 °C–80 to 60 °C
4. Site homogeneity
In addition to the routine spectroscopic (IR, MAS NMR) characterization of products, the
uniformity of the spatial distribution of sites at the nanometer scale was for the first time
directly confirmed by STEM-EDS. The sample images (Figure 3) illustrate the typical texture
exhibited by the whole family of materials, with no apparent difference among the studied
precursors. Statistical methods are currently being applied in order to characterize the level
of chaos/correlation from the EDS data.
5. References
1. Ghosh, N. N.; Clark, J. C.; Eldridge, G. T.; Barnes, C. E.; Chem. Commun., 2004, 856–857
2. Clark, J. C.; Barnes, C. E.; Chem. Mater., 2007, 19, 3212–3218
3. Styskalik, A.; Abbott, J. G.; Orick, M. C.; Debeckere, D. P.; Barnes, C. E.; Catalysis Today, 2019,
334, 131–139
-120
-100
-80
-60
-40
-20
0
20
pyridine-AlMe3/Me3SnOSiH3
pyridine-AlCl3/Me3SnOSiH3
AlCl4-/Me3SnOSiH3
pyridine-BCl3/Me3SnOSiH3
SbCl3/Me3SnOSiH3
Ph3SbCl2/Me3SnOSiH3
PCl3/Me3SnOSiH3
SnCl4/Me3SnOSiH3
ZnEt2(THF)2/Me3SnOSiH3
ZnEt2(THF)/Me3SnOSiH3
ZnEt2/Me3SnOSiH3
ZnCl2(THF)2/Me3SnOSiH3
ZnCl2(THF)/Me3SnOSiH3
ZnCl2/Me3SnOSiH3
ZnCl3-/Me3SnOSiH3
Me3SiCl/Me3SnOSiH3
PhMe2SiCl/Me3SnOSiH3
Me2SiCl2/Me3SnOSiH3
GibbsenergychangeΔGCBS
298.15/kJmol-1
Gibbs energy change of condensation - B3LYP-D3/ΔGCBS
298.15
1st step 2nd step 3rd step 4th step