XIV
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International Conference on Molecular Spectroscopy, Białka Tatrzańska 2017
346
T9: P–20
The structure of geopolymers – theoretical studies
Andrzej Koleżyński
1
, Magdalena Król
1
, and Mikołaj Żychowicz
2
1
Faculty of Materials Science and Ceramics, AGH University of Science and Technology,
30 Mickiewicza Av. 30-059 Krakow, Poland, e-mail: andrzej.kolezynski@agh.edu.pl
2
Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Polandxxx
The structure of geopolymers consists of (similar to zeolites) polymeric Si–O–Al framework
(chains of [Si
O4
] and [AlO
4
] tetrahedra with oxygen shared corners forming 3D network [1]) with the
main distinction that unlike zeolites, it is amorphous in room temperature and undergoes
crystallization in temperature exceeding 500°C. It is commonly assumed that geopolymerization is a
process in which small molecules (oligomers) combine together forming 3D network. As a result,
dense, amorphous, semi-crystalline structure is obtained, which is very difficult to describe
accurately. One can try to model such amorphous structure using molecular dynamics methods
typically applied for glasses (with some success, at least in some cases), or starting from big, periodic
superstructures and destroying randomly some of the structural components using predefined
algorithms, but these methods are extremely time consuming and the results are usually less than
satisfactory. On the other hand, since geopolymers are built from small oligomers, being a kind of
building blocks (in analogy to secondary building units in zeolites) and their structural identity
remains mostly intact after geopolymerization, one can try different approach using classical and
quantum mechanical methods, namely to start with oligomers and create consecutively more and
more complicated periodic models (within sufficiently big unit cell) mimicking amorphousness via
local disorder while maintaining translational symmetry for making the model computationally
tractable and checking the viability of resulting model structures by e.g. comparison of calculated
vibrational spectra with respective one obtained from experiment.
In this work, we present the results of a series of ab initio DFT calculations (geometry
optimization and vibrational spectra calculation) using Crystal14 program [2, 3], carried out for a few
model oligomers with different SiO2/Al2O3 ratio and their selected combinations. Based on the
optimized oligomer structures obtained in above calculations, model periodic structures of
“amorphous” geopolymer were prepared, their geometry optimized and vibrational spectra calculated,
using GULP software [4, 5] and classical molecular mechanics approach.
Simultaneously, a samples of geopolymer were synthesized in order to compare theoretical
spectra with the experimental ones. Metakaolin, obtained as a result of calcination of natural kaolin
clay at 700°C for 2 h, was activated with alkaline solutions of sodium silicate (as technical sodium
water glass) and sodium hydroxide. The first were used as a secondary source of silicon (used for the
regulation of the chemical composition of the reaction system) and the second as a mineralizing
agent. The geopolymer slurry was molded, sealed and cured at 80°C for 24 h. Finally samples were
dried at room temperature. The spectra in the mid infrared (4000–400 cm
–1
) were measured on Bruker
VERTEX 70v vacuum FT-IR spectrometer using the standard KBr pellets methods. They were
collected in after 128 scans at 4 cm
–1
resolution. For all the spectra, the linear baseline correction has
been carried out.
The calculated spectra were compared with respective experimental ones and discussed in details.
Acknowledgment
This work was financially supported by the National Science Center of the Republic of Poland, Grant No
2015/17/B/ST8/01200
References
[1] J. Davidovits, J. Therm. Anal. 37(8) (1991) 1633.
[2] R. Dovesi, R. Orlando, E. Alessandro, C.M. Zicovich-Wilson, C. Bartolomeo, S. Casassa, L. Maschio, M.
Ferrabone, M. De La Pierre, P. D'Arco, Y. Noel, M. Causa, M. Rerat, B. Kirtman, Int. J. Quantum Chem.
114 (2014) 1287.
[3] R. Dovesi, V.R. Saunders, C. Roetti, R. Orlando, C.M. Zicovich-Wilson, F. Pascale, B. Civalleri, K. Doll,
N.M. Harrison, I.J. Bush, P. D'Arco, M. Llunel, M. Causa, Y. Noel, CRYSTAL14 User's Manual, 2014.
[4] J.D. Gale, JCS Faraday Trans. 93 (1997) 629.
[5] J.D. Gale, A.L. Rohl, Mol. Simul. 29 (2003) 291.
XIV
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International Conference on Molecular Spectroscopy, Białka Tatrzańska 2017
347
T9: P–21
Vibrational spectra of various cationic forms of zeolite Y
Andrzej Koleżyński
1
, Magdalena Król
1
, and Andrzej Mikuła
1
1
Faculty of Materials Science and Ceramics, AGH University of Science and Technology,
30 Mickiewicza Av. 30-059 Krakow, Poland, e-mail: andrzej.kolezynski@agh.edu.pl
Zeolites are a group of materials characterized by microporous aluminosilicate frameworks with
numerous applications like e.g. ion-exchange and sorption processes or catalysis. In the latter
application, very important role play various cationic forms of faujasite. However, due to various
Si/Al ratio and resulting local ordering, detailed analysis of their structural properties is not an easy
task. A common tool used in structural analysis of zeolite type materials is vibrational spectroscopy,
unfortunately due to their complex structure detailed and reliable analysis of the properties and shape
of vibrational spectra is a very difficult task which usually requires additional support from theoretical
calculations. Since unit cell size of a typical zeolite is quite big, containing hundreds (in case of
zeolite Y) or more atoms, it is still rare to find theoretical works reporting results of ab initio
calculations of vibrational spectra. In the past, the usual approach was based on the assumption that
due to hierarchical structure of typical zeolites, vibrational spectrum can be analyzed using theoretical
results obtained for isolated structural elements (primary and secondary building blocks) and this
approach turned to be quite successful. However, if one want to analyze (very often) subtle effects of
extra framework cation exchange, sorption processes of small molecules or processes undergoing
during catalysis on structural changes and vibrational spectra properties, definitely more accurate
periodic models has to be used in calculations. The problem is, however, that such models are still
beyond practical reach of quantum mechanical methods (besides simplest cases, like e.g. silicalite
frameworks for less complicated zeolites. like faujasite) and an feasible alternative is (before more
powerful computers will be at our disposal) the use of classical molecular mechanics methods – the
crude approximations ignoring electronic interactions, but in case of zeolites, able to provide
reasonable results.
In this work, we present the results of ab initio calculations carried out for a simple silicalite
model of faujasite framework using Crystal14 program [2] together with a series of calculations
performed for model periodic structures of various cationic forms of zeolite Y (with silica-to-alumina
ratio 5:1) using GULP software [3] and classical molecular mechanics approach. In all cases full
geometry optimization (unit cell size and atomic positions) was done and vibrational spectra
calculated. In order to confirm the validity of obtained results, samples of respective cationic forms of
faujasite have been synthesized and vibrational spectra measured.
In the experimental part of this work sodium forms of zeolite Y (Zeolyst) were used as starting
material. Sorption of various cations on the sodium forms of zeolite Y was studied using aqueous
solutions of metal nitrates. A suspension of the zeolite in water (20 g/dm3) was shaken with the
appropriate metal salt solution for 24 h at 80°C and centrifuged. After the ion-exchange process the
samples were triply washed with distilled water and then dried at 80°C for several days. Phase
compositions of studied materials were confirmed using X-ray diffraction (XRD). Infrared spectra of
different form of zeolites were measured using Bruker VERTEX 70v vacuum spectrometer. They
were collected in the region of 4000–60 cm
–1
after 256 scans at 2 cm
–1
resolution using standard KBr
and polyethylene pellets methods for MIR and FIR spectra, respectively.
The calculated spectra were compared with the experimental ones and their properties analyzed in
details and presented.
Acknowledgment
This work was financially supported by the National Science Center of the Republic of Poland, Grant No
2015/17/B/ST8/01200
References
[1] J. Čejka, H. van Bekkum, Zeolites and Ordered Mesoporous Materials: Progress and Prospects, Studies in
Surface Science and Catalysis, Elsevier, Prague, 2005.
[2] R. Dovesi et al., Int. J. Quantum Chem. 114 (2014) 1287.
[3] J.D. Gale, JCS Faraday Trans., 93, 629 (1997); J.D. Gale and A.L. Rohl, Mol. Simul. 29 (2003) 291.
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