XIV
h
International Conference on Molecular Spectroscopy, Białka Tatrzańska 2017
344
T9: P–18
The interaction of CO with exceptionally acidic OH groups
in high-silica Y zeolites: DFT modeling and IR experiment
Adam Stępniewski
1
, Paweł Rejmak
2
, Ewa Broclawik
1
, and Jerzy Datka
1,3
1
Jerzy Haber Institute of Catalysis PAS, Krakow, Poland, e-mail: ncstepni@cyf-kr.edu.pl
2
Institute of Physics PAS, Warszawa, Poland
3
Faculty of chemistry, Jagiellonian University, Krakow, Poland
Dealuminted zeolite Y of Si/Al=31 (USY) contains very strongly acidic homogeneous
Si–OH–Al groups. This was evidenced in low temperature IR studies of CO sorption. Huge red-
shift of stretching OH frequency caused by the interaction with CO probe (Δ
νOH...CO
= 354 cm
–1
)
was registered which can be taken as the measure of the acid strength, higher than in other
zeolites [1].
DFT modeling was applied to study the properties of Brønsted acid sites at molecular level.
Cluster and periodic models of a parent –Si(OH)Al– bridging hydroxyl and the site interacting
with CO probe were studied to select most important geometric and electronic properties
responsible for the red-shift of the OH stretch upon CO sorption. The cluster built of 18 T sites
cut from known faujasite database, with a single Al substitution (Si/Al=17) balanced by the
acidic proton at O1 (claimed most frequently populated) may be taken as a model roughly
mimicking properties of high-silica USY zeolite (optimized structure of the O–H…CO adduct
shown in Fig. 1).
Fig. 1. T18 cluster model of Brønsted acidic site in USY zeolite interacting with CO probe.
Both periodic and cluster results indicate that the CO probe may interact with the
–Si(OH)Al– bridge either in O–H…CO or in O–H…OC configuration. The C-bound geometry
is by several kJ/mol more stable than the O-bound one and may be regarded as a weak
chemisorption whereas the latter one corresponds to weak physisorption. In accord, the
calculated red shift of the OH frequency upon the interaction with CO is large for O–H…CO
complex (from 300 to 600 cm
–1
, depending on calculation method) and it is negligible for the
O–H…OC one. This may be related to temperature dependence of IR spectra in the OH range
for USY zeolite.
Keywords: ultrastable Y zeolite; acid sites; IR spectroscopy; DFT molecular modeling;
Acknowledgment
We acknowledge financial support provided by Grant No. 2015/17/B/ST5/00023 from the National Science
Centre, Poland.
References
[1] J. Datka, M. Gackowski, Ł. Kuterasiński, J. Podobiński, J. Catalysis, submitted.
XIV
h
International Conference on Molecular Spectroscopy, Białka Tatrzańska 2017
345
T9: P–19
New approach for determination of the influence of long-range order
and selected ring oscillations on IR spectra in zeolites
Andrzej Mikuła
1
, Magdalena Król
1
, and Andrzej Koleżyński
1
1
Faculty of Materials Science and Ceramics, AGH University of Science and Technology,
30 Mickiewicza Av. 30-059 Krakow, e-mail: amikula@agh.edu.pl
Vibrational spectra of zeolites are still difficult to interpret, particularly in the pseudolattice
region, where bands related to ring oscillations can be observed. By using combination of
theoretical and computational approach, a detailed analysis of these regions of spectra is
possible, however such analysis should be carried out using models with different level of
complexity and simultaneously the same theory level. On the basis of hierarchical structure of
zeolite frameworks one can expect that the total envelope of zeolite spectra should be a sum of
the spectra of structural elements that build each zeolite framework. So far, the spectra of these
aluminosilicates have not been described in this way.
In this work, we have carried out a set of ab initio calculations for chosen primary (PBU)
and secondary (SBU) building units and respective periodic zeolite frameworks built from these
units (e.g. FAU framework type). It has been found that on differential theoretical spectra
obtained after subtraction of main structural unit spectra from particular periodic one, computed
at the same theory level, there are some, strongly intensive bands which positions fit accurately
to the bands of other PBU or SBU unit that build given framework. Furthermore, the
visualisations of these bands and analogous bands in spectra of PBU/SBU units related to
particular frameworks show that they correspond to the same type of vibrations. Such results
confirm hierarchical construction of zeolite frameworks and indicate that total envelope of these
spectra is strongly related to the main SBU that builds this structure as well as the other
structural elements, mainly single 4-membered rings (S4R) and oxygen Si–O–Si(Al) bridges.
Moreover, on the basis of presented results, one can specify in the zeolite spectra the bands
related to oscillations of specific structural units, like rings as well as the conjugated vibrations
of two or more structural elements. As a confirmation of theoretical results, experimental studies
of analogous zeolite frameworks have been performed and discussed in detail as well.
Keywords: ab initio calculations; zeolite; vibrational spectra
Acknowledgment
This work was financially supported by the National Science Center of the Republic of Poland, Grant No
2015/17/B/ST8/01200.
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