Original article revista.iq.unesp.br | Vol. 47 | special issue 2 | 2022 | 30 Eclética Química Journal, vol. 47, special issue 2, 2022, 30-36 ISSN: 1678-4618 DOI: 10.26850/1678-4618eqj.v47.2SI.2022.p30-36 K4Nb6O17 layered hexaniobate: revisiting the proton-exchanged reaction Mariana Pires Figueiredo1 , Vera Regina Leopoldo Constantino1+ 1. University of São Paulo, Institute of Chemistry, São Paulo, Brazil. +Corresponding author: Vera Regina Leopoldo Constantino, Phone: +551130919152, Email address: vrlconst@iq.usp.br ARTICLE INFO Article history: Received: August 30, 2021 Accepted: November 10, 2021 Published: August 17, 2022 Section Editor: Assis Vicente Benedetti Keywords 1. intercalation compounds 2. acidic niobate 3. ion-exchange rection 4. thermal analysis ABSTRACT: The layered hexaniobate of K4Nb6O17 composition and its derivatives comprise nanostructured materials that exhibit suitable properties for application in catalysis, electrochemistry, and energy, for instance. The exchange of K+ cations to obtain the acidic or protonic niobate form is the main route to originate appropriate precursors to promote the hexaniobate exfoliation, yielding a dispersion of thin layers (2D particles) that can be scrolled under exclusive conditions. Hexaniobate presents two regions (I and II), being the former considered more accessible than region II. In this work, the proton exchange efficiency of the K4Nb6O17 was investigated by thermogravimetric analysis coupled to mass spectrometry (TGA-MS) and metal analysis by inductively coupled plasma spectroscopy (ICP). The products of thermal decomposition profile of the HxK(4-x)Nb6O17 phase were isolated at defined temperature values and characterized by X-ray diffractometry and Raman spectroscopy. The cation exchange percentages obtained by TGA-MS (68.0%) and by quantification of deintercalated K+ by ICP (64.0%) are similar and endorse that region II can also be modified and, consequently, contribute to the exfoliation process. http://revista.iq.unesp.br/ojs/index.php/ecletica/index https://doi.org/10.26850/1678-4618eqj.v47.2SI.2022.p30-36 mailto:vrlconst@iq.usp.br https://orcid.org/0000-0002-9389-2057 https://orcid.org/0000-0001-9276-7329 Original article revista.iq.unesp.br 31 Eclética Química Journal, vol. 47, special issue 2, 2022, 30-36 ISSN: 1678-4618 DOI: 10.26850/1678-4618eqj.v47.2SI.2022.p30-36 1. Introduction The interest in layered hexaniobates is motivated by their physicochemical properties and the possibility of application as precursors for the development of nanostructured materials to attend catalysis (Wei and Nakato, 2009), electrochemistry (Elumalai et al., 2021), and energy (Maeda et al., 2009) fields, for instance. Such materials present general formula A4Nb6O17‧nH2O (with A = K+, Rb+, or Cs+) and orthorhombic unit cell: a = 7.83 Å, b = 32.21 Å, and c = 6.46 Å (Gasperin and Le Bihan, 1980; 1982). The distorted octahedrons [NbO6] units are bounded by the edges and the vertices, generating layers with structure called ReO3-type deficient double strand (Bizeto et al., 2006; Rao and Raveau, 1998), as shown in Fig. 1. The three-dimensional structure is formed by the layers stacking in a face-to-face arrangement. Due to the high amount of oxygen atoms coordinated to niobium atoms, hexaniobate presents high layer charge density, which can make the intercalation process by ion exchange reaction (a topotactic reaction) a challenging process. The electroneutrality of the negatively charged layers is maintained by alkali metal cations present in the interlayer region. Figure 1. Schematic representation of the layered hexaniobate structure. One of the most prominent characteristics of A4Nb6O17‧nH2O material is the interlayer differentiation, which can be explored to tune its chemical and physical properties. Reported for the first time by Gasperin and Le Bihan (1980), the so-called interlayer I and II regions of the hexaniobate matrix (Fig. 1) present different cations array and behaviors in relation to the hydration process. Ions in the interlayer I can be hydrated, which can made it more accessible than interlayer II. Consequently, it is supposed that region II is not involved in ion exchange reactions (Kimura et al., 2014; Shiguihara et al., 2010). However, for the K4Nb6O17 hexaniobate, it is reported that K+ can be exchanged by multivalent cations in region I but cations exchange in region II is limited to monovalent cations (Müller- Warmuth and Schöllhorn, 1994). The exchange of cations to obtain the acidic or protonic niobate form aims to produce precursors able to encapsulate bulky species, such as tetrabutylammonium ion, and support the osmotic swelling facilitating the exfoliation process (Shiguihara et al., 2007). Dispersed exfoliated nanosheets can curl themselves resulting in nanoscrolls with variable diameters, formed according to adjusted experimental conditions. Hence, the reactivity of hexaniobate regarding the layer’s separation is affected by the protonation level. Exfoliated nanosheets are investigated for the preparation of thin films targeting coatings or hybrid self-assembled multifunctional systems with a great interest for sensors, among several other applications (Bizeto et al., 2009). Considering the most studied hexaniobate composition, K4Nb6O17, the phase resulting from the exchange of K+ ion by H+ (or hydronium ion, H3O+) is frequently represented by the H2K2Nb6O17 formula (Liu et al., 2017; Shiguihara et al., 2010; Silva et al., 2018). However, other works (Guo et al., 2020; Li et al., 2016) have considered proton-exchange values greater than 50%, although chemical elemental quantifications are not provided. In another case, the isolated acid phase was generically expressed as HxK(4-x)Nb6O17 (Hu et al., 2014). There is no agreement regarding the extension of H+ intercalation and the quantification of the exchange extension is rarely reported. Thus, the investigation concerning the proton exchange reaction involving the K4Nb6O17 phase is of interest and required to tune hexaniobate properties. In this work, the K4Nb6O17 proton exchange reaction is revisited. Thermogravimetric analysis coupled to mass spectrometry (TGA-MS) was used to assess the proton exchange efficiency of K4Nb6O17, and the result was compared to the deintercalated amount of K+ obtained by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-AES). Additionally, X-ray diffraction (XRD) and Raman spectroscopy were employed to characterize the acidic hexaniobate treated at defined temperature values to better assign the thermal events and, consequently, the extension of the proton exchange. 2. Experimental 2.1 Reagents Niobium(V) oxide (CBMM – 99,98%, optic degree), potassium carbonate (Merck – p.a.), and nitric acid (Synth – P.A.) were used as received. http://revista.iq.unesp.br/ojs/index.php/ecletica/index https://doi.org/10.26850/1678-4618eqj.v47.2SI.2022.p30-36 Original article revista.iq.unesp.br 32 Eclética Química Journal, vol. 47, special issue 2, 2022, 30-36 ISSN: 1678-4618 DOI: 10.26850/1678-4618eqj.v47.2SI.2022.p30-36 2.2 Synthesis of K4Nb6O17 phase The potassium layered hexaniobate was synthetized by the ceramic method. Nb2O5 and K2CO3 solids (with K2CO3 in 10 mol% excess related to stoichiometric proportions) were manually ground together, added to a platinum crucible, and heated at 1000 °C in two steps of 5 h. The rate of heating was maintained in 15 ºC min–1. The solid was washed with deionized water by centrifugation cycles for the removal of residual K2O and then dried at 80 ºC for 24 h. 2.3 Preparation of HxK(4-x)Nb6O17 phase by ion exchange reaction The HxK(4-x)Nb6O17 phase was obtained under mild conditions suspending 1 g of K4Nb6O17‧nH2O in 40 mL of 6 mol L–1 HNO3 solution under stirring at 60 °C. After 24 h, the supernatant and the deionized water portions used to wash the solid were transferred to a filter from Millipore Express® plus with 0.22 μm pore diameter. All liquid collected as a filtrate was added to a 250 mL flask and the volume was completed with deionized water; later, the amount of K+ cation was quantified by chemical analysis. 2.4 Thermal treatment of HxK(4-x)Nb6O17 sample Four portions of the HxK(4-x)Nb6O17 sample were separately added to crucibles and thermal treated at 250, 300, 350, or 400 °C for 30 min. 2.5 Instruments XRD patterns of K4Nb6O17 and HxK(1-x)Nb6O17 powdered samples were recorded in a Rigaku MiniFlex equipment, using Cu anode (1.518 Å), scan range 1.5- 70°(2θ), and scan step of 0.015° (2θ)/2s. Thermogravimetric analysis coupled to mass spectrometry (TGA-MS) was performed in a Netzsch thermalanalyzer model TGA/DSC 490 PC Luxx coupled to an Aëolos 403 C mass spectrometer, using platinum crucible, from room temperature to 600 °C, and heating rate of 10 °C min–1 under synthetic air flow of 50 mL min–1. Raman spectra were recorded in a Bruker FT-Raman (laser wavelength = 1064 nm), at 50 mW and 512 scans, resolution of 4 cm–1, and gain of 32. ICP-AES analysis of potassium element from the supernatant of the ion exchange reaction was performed in an Spectro Analytical Instruments equipment at the Central Analítica of Instituto de Química (Universidade de São Paulo – USP). The synthesis of the K2Nb6O17 phase and the thermal treatment of HxK(4-x)Nb6O17 sample were proceeded in a Carbolite RHF 1500 furnace. 3. Results and discussion Figures 2a and 2b show the XRD patterns of K4Nb6O17 and HxK(4-x)Nb6O17 samples, respectively, displaying narrow and intense peaks related to the basal spacing, which profiles agree with the literature (Bizeto and Constantino, 2004; Madaro et al., 2011; Nassau et al., 1969). For the K4Nb6O17 precursor, no reflections concerning the Nb2O5 nor K2O reagents were identified. The positions of the (02n0) peaks are in agreement with the tri-hydrated potassium layered hexaniobate. (Bizeto and Constantino, 2004; Nassau et al., 1969; Shiguihara et al., 2007). Basal spacing (d(040)) is equal to 9.36 Å. After the exchange of K+ ions by H+ cations, the (040) peak is displaced towards higher angles region in agreement with the intercalation of hydrated H+/H3O+ cations, conducting to the decrease in the basal spacing to 7.97 Å, as shown in Fig. 2b. Figure 2. XRD patterns of hydrated (a) K4Nb6O17 and (b) HxK(4-x)Nb6O17 samples. The dehydroxylation process of the HxK(4-x)Nb6O17 phase was investigated by heating the sample at different temperature values. Figure 3 shows the XRD patterns of the HxK(4-x)Nb6O17 sample and the thermal treated material at 250, 300, 350, and 400 °C. The increase in the temperature promotes a progressive reduction in the intensity of the (040) peak, already evident for the sample heated at 250 °C, as well as the decrease in the d(040) basal spacing, indicating the partial collapse of the layered structure because de dehydroxylation process. http://revista.iq.unesp.br/ojs/index.php/ecletica/index https://doi.org/10.26850/1678-4618eqj.v47.2SI.2022.p30-36 Original article revista.iq.unesp.br 33 Eclética Química Journal, vol. 47, special issue 2, 2022, 30-36 ISSN: 1678-4618 DOI: 10.26850/1678-4618eqj.v47.2SI.2022.p30-36 Figure 3. XRD patterns of HxK(4-x)Nb6O17 sample and the products isolated after the thermal treatment at 250, 300, 350, and 400 °C. Figure 4 shows the Raman spectra of HxK(4-x)Nb6O17 and the thermal treated samples. For the nonthermal treated sample, the bands in the 200 – 300 cm–1 region are attributed to the bending of the Nb–O–Nb bond. Slightly-distorted octahedra originate the bands in the 500 – 700 cm–1 range, while the bands between 850 and 1000 cm–1 are assigned to highly-distorted [NbO6] units, in which the terminal Nb–O groups present double bond character (Bizeto et al., 2010). Increasing the temperature value, the band around 940 cm–1 progressively decreases in intensity because of the condensation of the terminal Nb-O groups. Bands at around 545 and 666 cm–1 are shift towards lower energy region with heat intensification, what can be related to difications in the [NbO6] octahedra and with the formation of new Nb–O–Nb bonds. Shifts observed for the sample treated at 250 °C in comparison to the original material indicate that dehydroxylation already started at about this temperature value, in agreement with XRD results. Figure 5 presents TGA-DTG (A) and DSC-MS (B) curves of the HxK(4-x)Nb6O17 sample in which four main mass loss events are observed (Fig. 5a). The first and second endothermic events occurring from room temperature up to 130 °C and in the 130 – 230 °C range are related to the release of superficially adsorbed and intercalated water molecules (Fig. 5b, MS curve of the fragment m/z = 18). The third (at about 230 – 310 °C) and fourth (in 310 – 600 °C range) mass loss events can be related to the dehydroxylation of the dehydrated matrix by the condensation of -OH terminal groups originated from the bond established between the interlayer H+ and the niobyl (Nb=O) group, promoting the formation of new chemical bonds. The thermal decomposition of the HxK(4-x)Nb6O17 material can be expressed by Eq. 1 (Bizeto and Constantino, 2004). Table 1 presents the detailed mass loss percentages from TGA results (Fig. 5) related to the HxK(4-x)Nb6O17 sample, as well as the number of mols of released water molecules in the four mass loss events. For all samples, the third mass loss event is the less pronounced and initiates at around 230 °C. Figure 4. Raman spectra of HxK(4-x)Nb6O17 sample and the products isolated after the thermal treatment at 250, 300, 350, and 400 °C. http://revista.iq.unesp.br/ojs/index.php/ecletica/index https://doi.org/10.26850/1678-4618eqj.v47.2SI.2022.p30-36 Original article revista.iq.unesp.br 34 Eclética Química Journal, vol. 47, special issue 2, 2022, 30-36 ISSN: 1678-4618 DOI: 10.26850/1678-4618eqj.v47.2SI.2022.p30-36 Figure 5. TGA (solid line) and DTG (dashed line) (a) and DSC (solid line) and MS (dashed line) (b) curves of HxK(4-x)Nb6O17 sample. Table 1. DTG peaks, mass loss percentages for the respective temperature ranges of TGA mass loss events for the HxK(4-x)Nb6O17 material and the amount of released water molecules. Temperature range (°C) DTG peak (°C) Δ% mass Amount of released water (mol) 25-130 88 1.37 0.71 130-230 166 1.52 0.78 230-310 256 0.84 0.44 310-600 347 1.80 0.92 Table 2 shows the proton exchange percentages calculated from TGA (Tab. 1), considering only the fourth or both third and fourth mass loss events, and the exchange percentages obtained from ICPAES analyses of potassium, considering the Eq. 1. 𝐻𝑥𝐾4−𝑥𝑁𝑏6𝑂17 ∆ ⃗⃗ ⃗⃗ ⃗⃗ ⃗⃗ ⃗⃗ 𝑥 2 𝐻2𝑂 + (𝑥 − 1)𝑁𝑏2𝑂5 + (2 − 𝑥 2 )𝐾2𝑁𝑏4𝑂11 (1) Table 2. Proton exchange percentages calculated from TGA and ICP-AES data 4° TGA mass loss event (%) 3° and 4° TGA mass loss event (%) ICP – AES (%) 45.8 68.0 64.0 From results displayed in Tab. 2, considering only the fourth mass loss event for the calculation of the proton exchange percentage, the value is close to 50% (45.8%). Bizeto and Constantino (2004) calculated the exchange percentage for the HxK(4-x)Nb6O17 sample prepared by suspending the K4Nb6O17 phase in 6 mol L–1 HNO3 solution at 60 °C for three days. In that work, it was considered that the dehydroxylation occurs from 315 °C. Thus, the last TGA mass loss event was considered for the calculation of ion exchange percentage, resulting in about 50%. However, when considered here both the third and the fourth events corresponding to the dehydroxylation, proton exchange percentage was much higher (68.0%) and closer to that one observed from ICP- AES quantification (64.0%). Thus, regions I and II were hydrated and could undergo the exchange of K+ by H+ ion. In this work, the hexaniobate dehydroxylation process was studied for a more precise determination of its starting temperature, shown to be below 300 °C. Accordingly, the quantification of K+ cations by ICP- AES endorsed the necessity to consider both third and fourth mass loss events for the determination of the extension of the proton exchange reaction from TGA analysis. Moreover, results presented here indicate that region II can also be hydrated in a meaningful amount and experience the intercalation of H+ cations. Acid phase obtained under mild experimental conditions reported in this study can be expressed as H2.7K1.3Nb6O17 or H2.6K1.3Nb6O17, calculated from TGA and ICP-AES, respectively. 4. Conclusions The XDR patterns confirmed the formation of the K4Nb6O17‧3H2O and HxK(4-x)Nb6O17 phases. From ICP- AES results, proton exchange was equal to 64.0%. Comparatively, results from TGA indicated 45.8% and 68.0% of proton exchange when considered hydroxylation event above 230 or 310 °C, respectively. These analytical data, combined to the XRD and Raman spectroscopic analyses of the acid hexaniobate heated from 250 to 400 °C, indicate that dehydroxylation process starts around 230 °C. Region II can also be hydrated and have K+ cations exchanged by H+. TGA- MS is a suitable tool to determine the level of protonation of hexaniobate, assisting the investigation of experimental approaches to improve the niobate exfoliation, such as ultrasonication, because exfoliation depends on the reactivity of the acidic intermediate phase. Authors’ contribution Conceptualization: Constantino, V. R. L. Data curation: Figueiredo, M. P.; Constantino, V. R. L. Formal Analysis: Figueiredo, M. P. Funding acquisition: Constantino, V. R. L. http://revista.iq.unesp.br/ojs/index.php/ecletica/index https://doi.org/10.26850/1678-4618eqj.v47.2SI.2022.p30-36 Original article revista.iq.unesp.br 35 Eclética Química Journal, vol. 47, special issue 2, 2022, 30-36 ISSN: 1678-4618 DOI: 10.26850/1678-4618eqj.v47.2SI.2022.p30-36 Investigation: Figueiredo, M. P. Methodology: Figueiredo, M. P.; Constantino, V. R. L. Project administration: Constantino, V. R. L. Resources: Constantino, V. R. L. Software: Not applicable. Supervision: Constantino, V. R. L. Validation: Figueiredo, M. P. Visualization: Figueiredo, M. P. Writing – original draft: Figueiredo, M. P. Writing – review & editing: Figueiredo, M. P.; Constantino, V. R. L. Data availability statement All data sets were generated or analyzed in the current study. Funding Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). Grant No: 305446/2017-7. Acknowledgments The authors are grateful to the Laboratório de Espectroscopia Molecular Hans Stammreich (Institute of Chemistry – USP) for the Raman spectra recording. References Bizeto, M. A; Constantino, V. R. L.; Structural Aspects and Thermal Behavior of the Proton Exchanged Layered Niobate K4Nb6O17. Mater. Res. Bull. 2004, 39 (11), 1729–1736. https://doi.org/10.1016/j.materresbull.2004.05.001 Bizeto, M. A.; Christino, F. P.; Tavares, M. F. M.; Constantino, V. R. L. Aspectos estruturais relacionados ao processo de troca iônica no niobato lamelar K4Nb6O17. Quim. Nova. 2006, 29 (6), 1215-1220. https://doi.org/10.1590/S0100-40422006000600013 Bizeto, M. A.; Shiguihara, A. L.; Constantino, V. R. L. Layered niobate nanosheets: building blocks for advanced materials assembly. J. Mater. Chem. 2009, 19 (17), 2512– 2525. https://doi.org/10.1039/b821435b Bizeto, M. A.; Leroux, F.; Shiguihara, A. L.; Temperini, M. L. A.; Sala, O.; Constantino, V. R. L. 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