Speciation and location of Co2+, Mn2+ and Ni2+ in the extraframework positions of the dehydrated zeolite matrix of ferrierite structure were studied in detail using FTIR spectroscopy of antisymmetric T–O–T vibrations of the zeolite framework. Me2+–ferrierites were prepared by the ion exchange of the NH4– and Na–zeolite forms and by impregnation of the NH4 form. Bare Me2+ occupies all three known cationic sites in dehydrated cationic zeolite. The wavenumbers of bands of individual cations in individual sites were identified. At low Me2+ loadings (Me2+/Al < 0.15), Me2+ replaces two protonic sites and exclusively bare Me2+ is present in dehydrated samples. Sets of such samples were employed for the estimation of extinction coefficients of Co2+, Mn2+ and Ni2+ in cationic sites. These coefficients differ for individual cations but are the same for a cation at different sites. Ion exchange to the NH4 form allows preparation of samples with maximum possible loading of bare Me2+ only for Co2+. In the case of Mn2+, exchange to the Na-parent zeolite or impregnation is required for this purpose while samples with maximum loading by bare Ni2+ can be prepared only by impregnation.
Zeolites are key materials for chemical production (heterogeneous catalysis, gas separation and purification). Zeolites are crystalline microporous aluminosilicate molecular sieves formed by SiO4 and AlO4− tetrahedra sharing their corners , . The isomorphous substitution of silicon by aluminum in the zeolite framework introduces a negative charge which has to be balanced by extraframework positively charged species. The extraframework protons, cations or cationic species can play the role of active sites for both acid- and redox-catalyzed reactions. The huge variability of extraframework active sites in zeolites: Brønsted and Lewis acid sites, cation basic sites, redox cation and oxidic sites, makes it possible to tune their properties to the requirements of the catalytic reactions. This, together with large internal surface, high mechanical and thermal stability and rather simple and economic large-scale production results in the fact that zeolites represent currently the widest and most important group of industrial catalysts , . They are used as catalysts in a wide range of acid-catalyzed reactions for the transformations of hydrocarbons in the petrochemical industry and synthesis of fine chemicals , . Metallozeolites with metal ion cationic species represent redox catalysts for NO/NOx elimination from diesel exhausts and N2O abatement . Recently, zeolites were reported to be promising catalysts in the utilization of biomass , conversion of methane to oxygen-containing products and utilization of carbon dioxide , , , .
Introduction of transition metal ions (Me2+) to the zeolite matrix led to formation of several types of counter Me ion/oxo species: bare cations coordinated to oxygen atoms of the framework, cation-oxo species, bi(poly)nuclear cation-oxo species, metal (ion (oxo)) clusters and metal oxide species. Formation of these species depends on the zeolite topology, distribution of Al atoms in the zeolite matrix, metal loading and preparation procedure. Mostly, bare cations in extraframework cationic positions (α-, β- and γ-sites) were proposed as the active sites for redox reaction , . Depending on the conditions of catalyzed reaction, isolated cation-oxo species, bi(poly)nuclear cation-oxo species, metal (ion (oxo)) clusters and metal oxide species can perform redox catalytic cycles , , , , . Recently, the highly effective active sites with two bare cooperating Fe cations in β cationic sites in ferrierite (FER) for N2O decomposition were proposed , , , , . In recent years, significant developments have occurred in the field of the characterization of solids, connected predominantly with synchrotron-based methods as e.g. EXAFS/XANES and related methods, high-resolution XRD methods and X-ray tomography. However, the classic approach based on FTIR spectroscopy still shows significant potential in the study of bare cations in zeolites , . Based on the FTIR spectroscopy results, cations can be analyzed using the perturbation of the antisymmetric T–O–T vibrations of the zeolite framework by the presence of bare cations (960–880 cm−1) or adsorption of the weak base d3-acetonitrile as a probe of Me2+. This contribution will focus on the employment of the T–O–T vibrations for characterization of the bare cations in ferrierite. The bonding of the cation (with preferred Me2+–OFRAMEWORK distance) in the cationic site results in the perturbation of the T–O–T angle of only a limited number of T–O–T framework structures adjacent to a metal ion , , , .
The cation-specific FTIR spectra in the region of T–O–T vibrations have already been identified for mono-, di- and trivalent cations embedded in various zeolite matrices , , , , , , , . The positions of the band characterized bare cations located in α, β and γ positions in zeolites are site-specific and thus provide a unique and independent method to evaluate the respective occupation of the cation positions. It was shown that the intensities of the individual T–O–T bands increased linearly with the cation concentration and thus enabled semiquantitative evaluation of the cations anchored to the zeolite framework. The detailed analysis of the T–O–T region could also provide information on the formation of complexes between the cation and the external ligand, i.e. NO or NH3 , . The formation of a ligand–cation complex can be reflected in a shift of the T–O–T bands to higher wavenumbers , , , , . This finding represents an obstacle for the quantitative analysis of the bare cations in zeolites because the bands corresponding to bare Me2+ cations and ligand–Me2+ complexes could overlap. Moreover, cationic species that were not visible by FTIR spectroscopy were recently reported , , , , . However, characterization of well-defined bare cations in the region of T–O–T vibrations by FTIR spectroscopy could offer a good basis for studying cations’ behavior under redox conditions.
In this paper, a revision of the quantitative analysis of the siting of Co2+ in the ferrierite matrix, with an emphasis of the possible presence of other than bare Co2+ in the Co–zeolites, is made. The results for Co–ferrierite then serve as the comparison for the quantitative analysis of Mn2+ and Ni2+ in ferrierite. Particular attention is paid to the siting and distribution of bare Me2+ in the ferrierites with a maximum loading of Me2+ as crucial for the formation of binuclear Me2+ cationic structures unique for ferrierite and suggested as highly active sites in redox reactions.
Commercially supplied ferrierite Si/Al 8.5 (Tosoh Corporation, Japan) was exchanged (3×24 h at room temperature (RT), 100 ml of a solution per 1 g of the zeolite) with either 1 M NaNO3 or 1 M NH4NO3, washed with distilled water and dried at ambient temperature. NH4–FER and Na–FER were further exchanged to obtain Co2+, Ni2+ and Mn2+ forms of ferrierite with various metal loadings (Table 1). For the impregnation method, NH4–FER was exclusively used as the starting material. Co–FER were prepared by ion exchange with 0.025–0.1 M solution of Co(NO3)2·6H2O at RT (for details, see Table 1). Mn–FER and Ni–FER were prepared by ion exchange using 0.005–0.33 M Mn(NO3)2·4H2O or Ni(NO3)2·6H2O at RT at 60°C or 80°C (Table 1). After the ion exchange, the zeolites were thoroughly washed with distilled water and dried in air at RT.
||Parameters of ion exchange
|Me/Al||Me mmol/g||Solution||mol/l||Time h/repetition||Temperature K|
aData from .
To obtain higher content of Ni2+ and Mn2+ in ferrierite, a dry impregnation method was used . Parent NH4– and H–FER were granulated to obtain a fraction of 0.3–0.6 mm grains and dehydrated at 120°C for 4 h. The corresponding amount of Mn(NO3)2·4H2O or Ni(NO3)2·6H2O was dissolved in water and added dropwise to the zeolite to obtain Ni2+ and Mn2+ loading ranging from 0.5 to 2.0 wt%. Prepared samples were dried in static air for 24 h at RT. Then, zeolites were calcined in air at 450°C for 4 h. The color change from light gray to dark gray was observed. The results of the elemental analysis of metal-containing zeolites carried out using X-ray fluorescence (XRF) are collected in Table 1.
FTIR spectra of the samples were collected on a Nicolet 6700 spectrometer equipped with a liquid nitrogen cooled detector with a resolution of 2 cm−1. The samples were prepared as self-supporting pellets (10 mg/cm2). Prior to the measurement, samples were evacuated under dynamic vacuum (10−3 Pa) at 450°C (temperature ramp 4°C/min) for 3 h. The Origin 8.1 software (OriginLab, Northampton, MA, USA) was used for data processing. FTIR spectra were simulated using previously established parameters characterizing cations in zeolites , , , .
Me2+ (Co2+, Mn2+ and Ni2+) siting in ferrierite
The effect of the binding of countercation in the dehydrated zeolites on the FTIR spectra of antisymmetric T–O–T vibrations of the zeolite framework (980–880 cm−1) is depicted in Fig. 1. The FTIR spectrum of the H–FER in the region of T–O–T vibrations did not exhibit any bands. The coordination of cations to the Al atoms in the ferrierite ring results in the perturbation of T–O–T vibrations, which is reflected in the formation of new complex bands between 980 and 880 cm−1 (see Figs. 1 and 2). Simulations of the spectra provided the following T–O–T bands: 942, 918 and 885 cm−1 for Co–ferrierite, 940, 918 and 879 cm−1 for Ni–ferrierite and 953, 928 and 902 cm−1 for Mn–ferrierite. The latter bands are observed only at the highest Me2+ loadings. The position of the first two bands for Co– and Mn–ferrierites fits those already reported, while those for Ni–ferrierite only slightly differ as is shown in Table 2 . The wavenumber corresponding to the third band for Ni–ferrierite and Mn–ferrierite was not reported before, while the value obtained for Co–ferrierite is in agreement with previous findings , .
|Mn||953 (953)||928 (927)||902||30.4|
|Co||942 (943)||918 (917)||885||38.5|
|Ni||940 (943)||918 (913)||879||35.8|
However, these bands were attributed to the shifted antisymmetric T–O–T stretching vibrations of the zeolitic lattice induced by binding of bare Me2+ in cationic positions, the shift of the T–O–T vibrations to higher wavenumbers due to the weakening of the ligation of the cation to the framework by the formation of the Me2+–ligand complex was reported . Thus, FTIR spectra of bare Me2+–ferrierites in the region of OH vibrations (see Fig. 3) were analyzed and confirmed the presence of bands exclusively attributed to skeletal OH groups. A narrow band at 3747 cm−1 reflects terminal silanol groups (Si–OH) with weak acidity , , . The narrow band at 3601 cm−1 corresponds to the unperturbed and a broad low-intensity absorption between 3600 and 3350 cm−1 to the perturbed framework Brønsted bridging groups (Al–OH–Si) , , . No band between 3600 and 3700 cm−1 attributable to the Me2+–OH complexes , , ,  was present in the spectra of Me2+–zeolites with Me2+ loadings below the maximum, see Fig. 3. Nevertheless, besides well-known Me2+–OH species balancing only one framework Al atom, also monovalent [Me3+O2−]+ complexes were recently suggested to be possibly formed in zeolites; [Co3+O2−]+ complexes were reported for Co-beta and Co-SSZ-13 zeolites , , . These monovalent species are not reflected in the FTIR or UV–vis spectra. Thus, the only way for the unambiguous analysis of monovalent and divalent Me2+ species in dehydrated zeolite is a quantitative study of Brønsted bridging groups (terminal Si–OH groups of weak acidity that are unable to accommodate cations). This analysis is concluded in Fig. 4. At least up to Me/Al=0.15, two Brønsted Si–OH–Al groups are consumed for the accommodation of one Me2+. This clearly demonstrated that below this metal ion loading, exclusively β bare Me2+ cations in α and are present in the zeolite. The deviation from this ratio at higher metal loadings (Me/Al>0.15) indicates that other species than bare divalent cations should be present in highly metal-loaded samples. In Co–ferrierite with highest Co loading, Co/Al 0.40, 78% of introduced cations should correspond to monovalent species (this is the upper limit because also uncharged Co–oxo species should be formed), for maximum exchanged Mn–ferrierite (Mn/Al 0.21) 70%. These results confirmed that analysis of coordination of the cation in the vicinity of one or two Al atoms is essential for the investigation of cation siting and distribution in the zeolite .
The above results indicate that both low- and medium-loaded ferrierite (Me/Al<0.25) exhibited a band in the FTIR spectrum in the region of perturbed T–O–T vibrations reflecting the siting of bare Me2+. Thus, the bands at 942, 918 and 885 cm−1 of Co–ferrierite, 940, 918 and 879 cm−1 of Ni–ferrierite and 953, 928 and 902 cm−1 of Mn–ferrierite reflect bare Me2+ located in α-, β- and γ-sites of the ferrierite framework, were assigned to respective cation positions. Note that previous studies did not exclude the formation of monovalent cationic complexes. Using ab initio calculations, the location of cationic sites and the local arrangement of cationic α- and β-sites were already predicted (Fig. 5) , , , , , , , . In contrast to the cations in the α- and β-sites, the detailed structure of the γ-site cannot be suggested due to the unknown location of Al atoms in this site (α-site is formed by two Al atoms, the T1, and β-site by two Al in the T2 sites) . The suggested attribution of cationic sites to the T–O–T band is further confirmed by the analysis of cation siting in highly loaded zeolites. In this case, cation siting is directly controlled by the distribution of Al atoms (2 Al atoms in one six-membered ring) in individual cationic sites. This point will be discussed in the subsequent section.
Quantitative analysis of Me2+ (Co2+, Mn2+ and Ni2+) siting in ferrierite
The exclusive presence of bare cations in samples with Me/Al<0.15 allows estimation of extinction coefficients of cations in individual cationic sites serving for the quantitative analysis of Me2+ siting in the zeolite. The effect of metal loading on the intensity of the T–O–T vibration for Me–ferrierites is depicted in Fig. 6. For Me/Al<0.15, there is a linear dependence of the intensity of T–O–T vibrations on the metal loading although several spectral components reflect siting of Me2+ in the α- and β-sites. This suggests that extinction coefficients of Me2+ in the α- and β-sites are similar, although the geometry of the sites and cation coordinations are different , , . However, the variability of the occupation of cationic sites in samples with low metal loading is rather low as follows from Fig. 7. Cations for the Me–FER with low metal loading (Me/Al 0.15) predominantly occupied α and β cationic sites (see Fig. 7). It was calculated on the basis on the experimental data, Figs. 6 and 7, that differences in extinction coefficient values up to 25% caused observable change on the level of 5% in linear dependence of T–O–T vibrations.
Nevertheless, the recent analysis of the Co siting in the zeolite SSZ-13  demonstrated the changes in the occupation of individual sites while the total intensity of perturbed T–O–T vibrations was linear with the content of bare Co2+. It can be suggested that the extinction coefficients of Me2+ located in different cationic sites of ferrierite are the same within experimental error. The extinction coefficients obtained from the linear regression of the concentration dependence of the intensity of T–O–T vibration on metal loading in the concentration region below Me/Al=0.15 are presented in Table 2. However, while similar extinction coefficients were suggested for the cation in different sites, Table 2 shows significant differences between the extinction coefficients of different cations (up to 25%). Without an explanation of this discrepancy, quantitative analysis of the concentration of bare Me2+via perturbed T–O–T vibrations is unacceptable. The arrangement of occupied cationic sites represents the result of the interplay between the cation inducing the deformation, coordination and flexibility of the empty cationic site in zeolites. Although the arrangement of cationic sites differs, there is no reason to expect different capability of the cation to perturb the cationic site, which was suggested to be its intrinsic property (e.g. hydration heat or hydration diameter was suggested) , . Thus, the arrangements of cationic sites with the cation can be suggested to be a result of similar “strength” of the perturbation (cation specific) on different starting rings.
The exact relationship between the perturbation of T–O–T vibration and corresponding T–O–T wavenumber is not known, however, their correlation (evidenced by the similar changes of T–O–T bands with the cation) can be discussed. The observed differences in wavenumbers of perturbed T–O–T vibrations of α-, β- or γ-sites represent differences in the parent T–O–T vibrations of empty sites, thus the shift of vibrations due to the perturbation caused by cations can be suggested as similar. This assumption permits explaining the rather low effect of cation (Co, Mn or Ni) on the position of bands of individual cationic sites, which changed by less than 14 cm−1. In contrast, the differences in the position of the most intense bands α and β for particular cations differs by more than 22 cm−1 (e.g. for Co–ferrierite: α 942 cm−1 and β 918 cm−1). The changes in the positions of the bands of α- and β-sites for Co, Ni or Mn do not influence significantly the perturbations of the T–O–T vibrations. This suggests that the value of extinction coefficients for the cation at different cationic sites should be similar. The abovementioned differences of extinction coefficients of different cations clearly demonstrated that they can be employed only for the analysis of the siting of the given cation.
Speciation of Me2+ (Co2+, Mn2+ and Ni2+) in ferrierite
It is well known that besides bare Me2+ cations, other cationic species could be introduced to the zeolites. The type of the present cationic species in dehydrated zeolites is the result of two complex processes – metal ion introduction and zeolite dehydration/activation. Only in the case of the exclusive presence of bare Me2+ cations in the zeolite and occupation of all cationic sites for divalent cations, the concentration of bare Me2+ cations and their distribution in individual sites (α, β and γ) is given by the Al distribution in the zeolite framework. As was previously proved, only Al pairs (two Al atoms in 6- or 8-membered rings of ferrierite) in cationic positions (α, β or γ) stabilized bare Me2+. In the investigated ferrierite matrix, 62% of Al atoms are located as Al pairs , , , . Approximately 70% of Al pairs are situated in the β-site and 20% in the α-site , . Cations located in both β and α cationic sites are suggested as active sites for redox processes due to their facility in the changes of oxidation state and the open coordination sphere (structure published , , ).
Figure 8 depicts the concentration of bare Me2+ (Me/Al) and their location in β-, α- and γ-sites in dehydrated MeH–ferrierites prepared using the ion exchange method. For all concentration ranges, Co2+ and Ni2+ in ferrierite are predominantly located in the β-site while the concentrations of Mn2+ in the α- and β-sites of ferrierite are comparable. This indicates a higher stabilization energy for Co2+ and Ni2+ in the β-site than in the α-site. The α-site in ferrierite is easily accessible through the main 10-member ring channel while access to the β-site is limited by the 8-membered ring. Bare Me2+ in α- and β-sites of ferrierite are exclusively present in Me/Al<0.15 (Fig. 6). However, for Ni2+ and Mn2+, the maximum concentration of bare Me2+ prepared using the ion exchange method is rather low (maximum Me/Al values are 0.09 for NiH– and 0.21 for MnH–ferrierites). The analysis of the distribution of Al atoms in studied ferrierites revealed that the limit for bare divalent cations is Me/Al 0.31 . Among studied Me–ferrierites prepared by ion exchange, only CoNH4–ferrierite provides a highly loaded sample (Co/Al 0.40). However, this ion exchange degree is above the maximum ion exchange capacity of the parent ferrierite for bare Me2+ (see above), thus the formation of other species than bare Me2+ is expected.
To reach higher concentrations of bare cations in the ferrierite, two different preparation methods for Ni and Mn cations were applied: (i) ion exchange to the Na–ferrierite and (ii) impregnation of the NH4–ferrierite. While ion exchange of the Na form of parent ferrierite allows to reach maximum ion exchange capacity for divalent cation for Co2+ (0.32)  and Mn2+ (Me2+/Al 0.32), the ion exchange degree for Ni2+ is still low (Ni/Al 0.17). The high Ni loading (Ni/Al 0.29 and 0.36) in ferrierite was reached by impregnation of parent ferrierite by Ni(NO3)2. Analogously, in the case of Mn–ferrierite, impregnation resulted in high manganese loading in ferrierite, Mn/Al 0.38. The FTIR spectra of nickel and manganese ferrierite with high metal loading prepared using the impregnation method are presented in Fig. 9. The concentration of bare Me2+ cations and their distribution between individual cationic sites for maximum exchanged Me–ferrierites are given in Fig. 10. The ion exchange of Co2+ to the NaK–ferrierite results in maximum possible loading of bare Co2+ (CoBARE/Al 0.32), exclusively . Thus, ion exchange from both NH4–ferrierite and Na–ferrierite represents a simple but powerful method for the preparation of Co–ferrierites with bare Co2+. Location of Co cations in cationic sites in both maximum exchanged samples (Co/Al 0.31), which is controlled by the distribution of Al atoms in ferrierite, confirmed that 70–75% of Co2+ is present in the β-site, 15–20% is located in the α-site and 10% of Co2+ is accommodated in the γ-site . Ion exchange of Ni2+ to the NH4– and Na–ferrierite (Ni/Al 0.08 and 0.17, respectively) provides the exclusive presence of bare Ni2+ with the prevailing location of Ni2+ in the β-site. Nevertheless, the total amount of bare Ni2+ is rather low (NiBARE/Al 0.17 for the ion exchange method from Na–ferrierite). The introduction of Ni2+ to the NH4–ferrierite by impregnation provides significantly higher Ni loading in the sample (Ni/Al 0.36), but still less than 60% of Ni cations were present in the form of bare Ni2+ (NiBARE/Al 0.20). Among them, 60% of Ni2+ are located in the β-site, 15% in the α-site and 25% in the γ-site. It indicates that the impregnation method leads to the increasing presence of Ni2+ in the γ-site. It was shown that the cations in the γ-site with pseudo-octahedral coordination and closed coordination sphere , ,  are protected against transformation to oxo species and formed in the overexchanged samples. Ion exchange of Mn2+ to the Na–ferrierite, as well as impregnation, provides highly loaded samples with Mn/Al 0.32 and 0.38, respectively. Both samples contain a high amount of bare Mn2+ (Mn/Al 0.32 and 0.38), however, Mn–ferrierite prepared using the impregnation method shows that ca 20% of Mn species is present in a form other than bare Mn2+.
The above results show that Co–, Mn– and Ni–ferrierites with predominantly bare cations can be prepared not only for low Me loaded samples but also for samples with the maximum occupation of cationic sites for bare divalent cations. Such samples contain the maximum concentration of bare Me2+ – the most frequent type of catalytic centers in the reactions catalyzed by metallozeolites. Moreover, these highly loaded with bare Me2+ samples guarantee the formation of the unique type of the active site exclusively connected with the ferrierite topology – the pair of cooperating bare Me2+ , . In the case of Co2+ and Mn2+, the ion exchange procedure provides samples highly loaded with bare Me2+ located opposite to each other. In the case of Ni2+, ion exchange does not result in high enough incorporation of the metal ions to the zeolite and the impregnation procedure has to be employed.
FTIR spectroscopy of perturbed antisymmetric T–O–T vibrations represents a powerful tool for the (semi)quantitative analysis of the bare Me2+, their siting and distribution in the dehydrated metallozeolites, as was demonstrated for the ferrierite matrix. Nevertheless, proper estimation of extinction coefficients allowing this analysis requires a combination of the FTIR spectroscopy of T–O–T vibrations with a supplementary method for the analysis of residual cations in parent zeolite. This can be done in the case of NH4–zeolites by the quantitative analysis of residual Brønsted bridging Al–OH–Si sites. Only samples with exclusive presence of bare Me2+ (i.e. ions replacing two Brønsted acid sites) can be employed for the estimation of the extinction coefficients of bare Me2+.
Extinction coefficients for bare Ni2+ and Mn2+ located in the ferrierite zeolite matrix were established and the coefficient for Co2+ was examined and they were employed for the quantitative analysis of Me2+ speciation in ferrierite.
Co2+, Ni2+ and Mn2+ occupy previously reported α-, β- and γ-sites for extraframework cations in the ferrierite matrix. Wavenumbers of antisymmetric T–O–T vibrations reflecting Co2+, Ni2+ and Mn2+ in the α- and β-sites were confirmed, while they were estimated for cations Ni2+ and Mn2+ in the γ-sites. It was proven and explained that the extinction coefficients of the cations in individual sites exhibited similar values and can be taken for quantitative analysis.
Co–, Ni– and Mn–ferrierites with maximum possible loading of bare Co2+, Ni2+ and Mn2+ in cationic sites were successfully prepared by the ion exchange or impregnation methods. Such samples guarantee not only a high concentration of bare Me2+ as active sites in ferrierite catalysts but also the formation of specific types of ferrierite active sites–pairs of bare Me2+.
A collection of invited papers based on presentations at the 13th International Conference on Solid State Chemistry (SSC-2018), Pardubice, Czech Republic, September 16–21, 2018.
Funding source: Grant Agency of the Czech Republic
Award Identifier / Grant number: # 17-00742S
Award Identifier / Grant number: RVO: 61388955
Funding source: Ministry of Education
Award Identifier / Grant number: LM2015039
Funding statement: This work was supported by the Grant Agency of the Czech Republic, Funder Id: http://dx.doi.org/10.13039/501100001824, under projects # 17-00742S and the RVO: 61388955. Chemical analyses of samples were provided in the frame of CATPRO (Ministry of Education, Youth and Sports, ref. no. MSMT-1000/2016, Funder Id: http://dx.doi.org/10.13039/501100001823, under Project No. LM2015039), which has been integrated into the National Program for Sustainability I of the Ministry of Education, Youth and Sports of the Czech Republic through the project Development of the UniCRE Centre, Project Code LO1606.
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