M. D’Amorea, C.Bisioa, G. Talaricob, M. Cossia, L.
Marchesea.
aDipartimento di Scienze e Tecnologie Avanzate,
Università del Piemonte Orientale “A. Avogadro”, Via Bellini 25/G, 15100 Alessandria, leonardo.marchese@mfn.unipmn.it
bDipartimento di Chimica, Università di Napoli
“Federico II”, Complesso di Monte S. Angelo, Via Cintia, 80126 Napoli O O Gd3+ N N N O O O O N O CH3CONH O O H OH OH OH O O O O N N N O O O O N O O Gd 3+
Lamellar aluminophosphates modified by intercalation of alkylamines are key intermediates in the synthesis of
aluminophosphates (ALPOs) and silicoaluminophosphates (SAPOs) molecular sieves and
can be also used as host framework in the fields of nanocomposites. However, until the CAL-n family of microporous silicoaluminophosphate molecular sieves
was presented1, there were no good candidates as layered
reactants, that is lamellar aluminophosphates and silicoaluminophosphates with structures similar to the hydrated layered silicates. It is from the work of Cheng et
al.2, who reported the synthesis of the layered
aluminophosphate ALPO-ntu (whose empirical formula
is AlPO2(OH)2[NH2(CH2)xCH3], x=3, 5, 7), employing
amines as structure directing agents, that the perspective to obtain ALPOs and SAPOs molecular sieves using
layered materials as precursors became possible1. In
addition, layered aluminophosphates, especially those organically modified by intercalation of different n- alkylamines, have the assets of a larger interlayer distance and organophilic galleries between the sheets to host either organic molecules or polymer chains. Moreover, the interaction between organic molecules and inorganic aluminophosphate may influence the acidity of sites present on the surfaces and drive the interfacial phenomena in polymer composites.
The experimental data available on this class of layered aluminophosphates were not sufficient to fully resolve both their local and long range structural features. For this reason computational approaches are proposed to model structure and surface properties of these materials thus obtaining useful insights on their chemical behaviour. Reliable models may also help a better understanding of experimental data. The acidity of sites on ALPO layers and the interactions between organic molecule and the inorganic substrate have been specifically addressed.
Since all experimental (X-ray powder diffraction
patterns, IR, 27Al and 31P MAS NMR) data relative to the
alkylamine-aluminophosphates are in agreement with a kanemite-like structure, ab-initio computational models of butylamine-ALPO-kanemite were built starting from a
reasonable structure of silica kanemite3.
The computational tools to simulate these solids were based on density functional theory (DFT) employing localized basis sets: both cluster and periodic models were taken into account, and IR spectra were simulated for optimised structures. Results were compared with the IR experimental spectrum of butylamine-ALPO- kanemite. The harmonic vibrational spectra were computed (PBE0/6-31 G(d), LANL2DZ level) for all the cluster minima, to identify the chemical species present in the material and support the interpretation of the experimental data.
Computations on cluster of ALPO surface show that the P-OH moiety is markedly more acidic than Al-OH and
the structure of relative anion is stabilized by PO-···H-O-
Al hydrogen bond. The results are interpreted in terms of an acid-base reaction between protons P-OH groups in the inorganic layer and the butylamine molecules. The actual position of the positive butylamonium heads was determined by a bidimensional PBC calculation on
the ALPO layer in presence of an equivalent layer of NH3
molecules close to the surface. During the optimisation
all the surface P-OH groups reacted with NH3 molecules
and were quickly deprotonated forming NH4+ ions: as a
consequence, the Al-OH groups rotated and the intramolecular H-bond observed in the isolated anion
were lost in favour of the stronger interaction with NH4+.
In the AlPO-NH4+ periodic layer the ammonium ion was
substituted by CH3(CH2)3NH3+ ion: the optimised
structure shows tridentate butylammonium ions i.e. the ion interacting via three hydrogen bonds, one with oxygen atom on Al and the other two with oxygen atoms on phosphorous in the layer, these H bonds are not equivalent (Fig. 1).
Fig. 1. Detail of optimized structure
(
PBC/PBE/6-21G level)ofa layer of butyl-ALPO-kan
This position of butylamonium ion on surface leads to a
downward shift of the -NH3+ stretching frequencies,
which were found in the region 3200-3000 cm-1 (Fig. 2),
significantly lower than the isolated NH vibration. Al-OH groups in Butyl-ALPO-Kan point on lattice oxygens in a well ordered network of H-bonds, and give a sharp peak
at 3584 cm-1 in the IR spectrum.
The IR spectrum presents also bands around 2550 cm-1
and 2090 cm-1, which derive from both strong H-bonds in
R-NH3+···-OP complexes and combination of rotations of
RNH3+ with the bendings at 1635 cm-1 and 1555 cm-1.
The positions of the bands strongly depends upon the
strength of hydrogen bonds between protons in R-NH3+
groups and both PO- and basic surface oxygens. The
shape of the combination bands derives from the interactions with vibrational states of the ν (NH) band of hydrogen bonded groups. A Fermi resonance between fundamental stretching modes and the overtones and/or combinations, which pronounce an Evans window around
2180 cm-1, is in fact clearly observable.
3500 3000 2500 2000 1500 0,3 0,4 0,5 0,6 0,7 0,8 0,9 3075 1555 1635 n-Butyl AlPO Kanemite 3584 Ab so rb an ce Wavenumber [cm-1]
Fig.3: IR spectrum of butylammonium intercalated in ALPO- kan.
1. (a): Pastore, H.O.; Coluccia, S. and Marchese, L.; Annu. Rev.
Mater. Res.,2005, 35, 351; (b): Pastore, H.O.; Martins, G.A.V.;
Strauss,M.; Pedroni, L.G.; Superti, G.B.; de Oliveira, E.C.; Gatti , G. and Marchese, L.; Micropor. Mesopor. Mater., (in press).
2.Cheng, S.; Tzeng, J.; HSU,B.; Chem. Mater. 1997, 9, 1788- 1796.
3.Garvie, L.A.J. ; Devouard, B.; Groy, T.L.; Camara, F. ; Buseck; P.R.; American Mineralogist, 1999, 84, 1170-1175.