Theoretical and experimental study of ion-exchange process on zeolites from 5-1 structural group
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In this work the DFT calculations were carried out by means of Gaussian09 program for chosen model cluster—mor Composite Building Unit of zeolites structures. Vibrational frequencies computations and infrared spectra were obtained subsequently for above model. The results of calculations have been compared with IR (MIR and FIR range) spectroscopic studies of zeolites belonging to 5-1 structural group (mordenite and ferrierite) after heavy metal cations (Ag+, Zn2+, Cd2+, Pb2+, and Cr3+) immobilization. Changes in intensities and positions of the bands corresponding to the characteristic ring and Me–O vibrations have been observed. These rings occur in pseudomolecular complexes (built of [SiO4] and [AlO4] tetrahedra) which constitute the secondary building units and form zeolite framework. Based on the results of DFT method calculations, normal vibrations of the 5-1 unit terminated by different cations (Na+, K+) have been visualized. Obtained results have been applied for interpretation of experimental spectra of selected zeolites. The most significant changes have been determined in the region of pseudolattice vibrations (800–500 cm−1), as well as Me–O vibrations (below 400 cm−1). It was proven those cations’ sorption causes changes in the experimental spectra of zeolites. Sorption has been conducted on monoionic forms of mordenite and ferrierite. Based on the results of the sorption/desorption experiments, the proportion of ion-exchange to chemisorption in the process and the effective cation exchange capacity of the individual samples have been estimated. Results of AAS studies have been compared with those obtained by vibrational (IR and Raman) spectroscopy. Changes in intensities and positions of the bands corresponding to the characteristic ring vibrations, due to the immobilization of heavy metal ions, have been observed.
KeywordsFT-IR spectra Zeolite Mordenite Ferrierite Sorption
Due to the specific chemical and physicochemical properties resulting in very wide use in many areas of chemistry and chemical technology, structures of zeolites are the subject of numerous studies (including the use of vibrational spectroscopy), which aim is to link the macroscopic characteristics of zeolite materials with their internal build.
As mentioned, zeolites have a lot of applications, among which the most important is their use in the process of heavy metal cations immobilization. Cations can be immobilized on aluminosilicates by two mechanisms: ion-exchange and chemisorption . Ion-exchange process leads to creation of new kind of bonds and small deformation of the initial zeolite structure, so it is possible to observe changes in the IR-spectra of zeolites which are result of heavy metal cations immobilization. Previous studies [2, 3] showed that incorporation of cations into the zeolite structure results mainly in changes of the intensity of the bands associated with ring-opening (RO type) vibrations as well as modification of bands due to Me–O vibrations in FIR spectra (below 400 cm−1).
Interpretation of infrared spectra of zeolites often is difficult and in such situations computational methods can be helpful. However, regardless of the method used to interpret the spectra of zeolites the basic problem is the choice of a fragment of the crystal structure. First, it cannot be a single tetrahedron, because such units are present in all the structures of silicates and aluminosilicates, not only in zeolites. Unit cell is also not useful, because its size can be up to hundreds of atoms . While, systems of tetrahedra forming so-called Secondary Building Units (SBU), such as single and double rings, can be such model units. For such fragments of framework, theoretical models in the form of isolated pseudo-molecules terminated by different cations can be built .
Mordenite’s structure is a framework containing chains of five-membered rings of linked silicate and aluminate. These building units form structure with straight 12-membered ring channels (6.7 × 7.0 Å) (parallel to ) and crossed 8-membered ring channels (2.9 × 5.7 Å) (parallel to ). Similarly the ferrierites structure is characterized by 2-dimensional channel system, which are formed by 10- (3.4 × 5.5 Å) and 8-membered (3.4 × 4.8 Å) ring channels [6, 7]. Isolated rings with such a large diameter are not sufficiently stable for the calculation and during geometry optimization assume a linear conformation. On the other hand, the SBU of this group of minerals has the lowest possible symmetry. In addition, in the case of this unit, due to a different number of tetrahedral atoms in 5-membered ring and distinct ring deformation in 5-1 unit it is hard to talk about typical RO vibrations. However, based on ours previous works , presence of vibrations that are characteristic for all SBUs can be expected, such as it was in the case of 4-4-1 units in relation to the structure of clinoptilolite. On the other hand, the cations depending on the location in the zeolite framework occupy different crystallographic sites, and thus show characteristic vibrational modes in the FIR spectra . But cations position determination based on the IR spectrum is difficult, especially in the case of high-silica zeolites, for which the non-tetrahedral cation concentration is low.
The aim of this work is to examine the usefulness of quantum–mechanical calculated methods coupled with founded model for the interpretation of experimental spectra as well as the influence of non-tetrahedral cations on shape of their envelopes. Additionally, the results of heavy metal cations (Cu2+, Zn2+, Cd2+, Pb2+ and Cr3+) sorption studies on mordenite and ferrierite are described.
2.1 Computational details
Based on results obtained by Gaussian, the vibrational energy distribution (VED) was calculated. From the formulas for potential energy of a point hooked on the spring (E p = kx 2 /2) and the frequency of its vibration (ω = (k/m) ½) new formula for potential energy can be derived: E p = ½mx 2 ω 2. Following the assumption that the deflection is a maximum (calculated amplitude), it will be an expression for the total energy. The frequency in the formula is a constant (in mode all of the atoms have the same frequency), so total energy of this material point is proportional to the expression mA 2, where m—mass of the atom and A—deflection of this atom from the equilibrium position, calculated by Gaussian. Based on above assumption, for VED calculations the fallowing procedure was used. Expression mA 2 was calculated for each mode of normal for every atom. Each type of atom has been separately summed and calculated sums to 100 % have been standardized. Each normal mod using the three types of atoms with the largest proportions have been described.
2.2 Experimental details
Physicochemical parameters of starting zeolites
Nominal cation form
Surface area (m2/g)
Sorption of heavy metals was also carried out to test the effect of the presence of extra-framework cations on the envelope of the spectra. Selected cations were introduced into mordenite (MOR) and ferrierite (FER) structure from aqueous solutions of nitrates: AgNO3, Zn(NO3)2·6H2O, Cd(NO3)2·4H2O, Pb(NO3)2 and Cr(NO3)3·9H2O (POCH) respectively. The starting concentration of the metals was in the range 1 × 10−3 to 20 × 10−3 mol/dm3. A suspension of the zeolite in water (20 g/dm3) was shaken with the appropriate metal salt solution for 24 h at 25 °C and centrifuged. All the sorption experiments were done in triplicates for each ion concentration. Atomic absorption spectroscopy (Philips PU-9100×) was used to determine the concentration of metal cations in the solutions before and after the sorption experiments. pH was controlled. After the ion-exchange process the samples were triply washed with distilled water. To desorb the exchanged metals, the mineral samples were flooded with 1 M NaCl or CH3COONH4 aqueous solution and the process was carried out similarly as in the case of sorption. Samples selected for spectroscopic studies were dried at 80 °C for several days.
Results of AAS studies have been compared with those obtained by FT-IR spectroscopy, from which the influence of the sorbed cations on zeolite’ structure has been determined. Infrared spectra were measured on a Bruker VERTEX 70v vacuum spectrometer. They were collected in the mid and far infrared region, after 256 scans at 2 cm−1 resolution. Samples were prepared using the standard KBr (Merck) and polyethylene (Merck) pellets methods for MIR and FIR spectra, respectively.
3 Results and discussion
The bands at 1067 and 1022 cm−1 in the spectrum of Na-mor unit correspond to the asymmetric stretching vibrations of Si–O(Si) bridges and, as for most silicates, can be found as the most intense bands in the experimental spectra. It is worth noting that the analyzed structures are classified as highly siliceous zeolites (in this case Si/Al ratio is 26 for modrenite and 40 for ferrierite), hence almost total absence of bands in the spectra derived from Si–O–Al bridges. Next group of bands in the computed spectrum (i.e. intensive maxima at 943, 910 and 815 cm−1) are assigned to vibrations of bonds associated with the presence of terminal oxygen (Si–O−) and cannot be found in the spectra of real structures. Other groups of bands with lower intensities correspond to symmetric stretching (at 682 and 586 cm−1 in theoretical spectrum) and bending vibrations (at 519, 425 and 359 cm−1). The above results show that even in the case of simple cluster model, the obtained results agree well with related literature [10, 11] and thus allow good description of experimental data.
One of the results of the carried out calculations is the information about the vibrational energy distribution (VED). It has been assumed that the characteristic vibrations of the mor unit are normal vibrations, related primarily to the symmetric stretching (or less bending) vibrations of Si–O(Si), so during the implementation of these vibrations, silica atoms and/or bridging oxygen atoms the most move from the equilibrium sites. Therefore, in order to recognize characteristic ring-like vibrations, only those in which the percentage of energy vibration of the Si and bridging O atoms is the greatest should be taken into account. Characteristic vibrations of the Me–O type (Figs. 4b, 5b), present in the FIR range, have been identified in a similar manner. In this case, the maximum share of vibration energy of non-tetrahedral cations, i.e. Na+ and K+, was taken as major criterion.
It is expected, that metal cations sorption will affect the characteristic bands corresponding to the vibrations of mor structural units. Comparison of computed IR and Raman spectra of analyzed unit terminated by Na+ and K+ ions was shown in Fig. 6. Terminal ions differing in the atomic mass cause a considerable change in the frequency of vibrations characteristic for a given unit. It can be noticed that Na+ → K+ cation exchange has generally resulted in band shift slightly towards lower wave numbers. The decrease in intensity is also visible in the spectra (Fig. 6) but they are not so significant. In contrast, in the Raman spectra clear decrease in the intensity of bands in the pseudolattice vibration range is evident, which suggest, that ion-exchange could influence the deformation degree of the initial structure.
In real structures SBUs are more rigid due to the incorporation into a spatial framework of tetrahedra (single unit has more degrees of freedom than the whole structure). Is expected, that interpretation of vibrational spectra based on calculated for periodic models will be more clear, unambiguous and better matching to the experimental data, so calculations of this type are planned (e.g. by the use of Crystal). Additionally, presented model is useful mainly in interpretation of MIR spectra but they are not sufficient for interpretation of the whole FIR spectrum, due to the lack of proper description of lattice vibration. Therefore, taking into account the influence of the translational symmetry of crystal lattice could be theoretically studied for more accurate characterization of this range of the spectra. However, it should be noted that such calculations are extremely time-consuming due to the high unit cell size (up to several thousands of atoms).
The spectra calculated for chosen model were used in detailed analysis and interpretation of the experimental ones, collected for the mordenite and ferrierite after different cations sorption.
Ion-exchange properties of zeolites are connected with the presence of non-compensated negative charges and the presence of surface functional groups in crystalline lattice discontinuity points. This is a reversible process accompanied by the formation of weak bonds between the zeolite and metal ions (van der Waals bonds) . Chemisorption process results the formation of stable inner-sphere complexes. This is due to the fact that functional groups of aluminosilicates network (mainly OH) form strong chemical bonds with the ion without hydration layer . In zeolites the ion-exchange process usually dominates over chemisorption, however not in all cases. Zeolites used in this study have a high Si/Al molar ratio (such as all pentasils). It follows from this a large share of chemisorption, which does not require negative charge like ion exchange and takes place in statistically random positions in the structure. Thus, changes in the spectra, especially in the MIR range, are unlikely. It is especially visible in the case of Cr3+ ions—these ions form the aqua complexes, which, due to its large size, have hindered diffusion in the skeleton (hence the low value of the sorption). Simultaneously at high pH (zeolites are characterized by high pH of surface), chromium tend to precipitate in the form of hydroxide. In the case chromium ions, no effect associated with the ion-exchange process is visible in the MIR spectrum (Fig. 8)—described in the manuscript band at 733 and 560 cm−1 not change. The situation is somewhat different in the case of silver ions, which do not hydrolyze in an aqueous environment and, therefore, their binding with the zeolite structure will be stronger. Confirmation can be found in the FIR spectrum, in which band at 96 cm−1 can be assigned to the Ag–O vibrations (Fig. 9).
It has been found, that based on DFT calculations of vibrational spectra of small structural fragment (mor unit) models a characteristic vibrations can be identify and interpretation may be carried over to experimental spectrum.
MIR range spectra for zeolite measured after the sorption process show changes caused primarily by the ion-exchange process. The presence of heavy metal cations in the zeolite structure causes the changes in the pseudolattice range of IR spectra. The bands, which are related to characteristic ring vibrations, become more intensive—in the case of mordenite, the band at about 560 cm−1 can be defined as a sorption indicator. Vibrational spectroscopy can be used in the studies of sorption properties.
An introduction of heavy metal cations into the structure of zeolites has caused the considerable modification of bands due to Me–O vibrations in FIR spectra. Cation exchange in the structure of both analyzed zeolite structures cause changes the intensity and position of bands in the range 300–60 cm−1. In this case, systematic changes connected with the type of cation have been revealed, however this changes can not be fully interpreted, due to the nature of the research model.
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