Hydrothermal Conversion of Layered Niobate K4Nb6O17·3H2O to

Aug 30, 2017 - As shown in Figure 3 (top), K4Nb6O17·3H2O has 1D chains of corner-shared NbO6 octahedra formed along the a axis, which, in turn, form ...
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Hydrothermal Conversion of Layered Niobate K4Nb6O17·3H2O to Rare Microporous Niobate K6Nb10.8O30 Yusuke Ide*,†,‡ and Wataru Shirae‡ †

International Center for Materials Nanoarchitectonics, National Institute for Materials Science (NIMS), 1-1 Namiki Tsukuba, Ibaraki 305-0044, Japan ‡ Graduate School of Creative Science and Engineering, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo 169-8050, Japan S Supporting Information *

ABSTRACT: We report a new facile route to synthesizing K6Nb10.8O30, a rare microporous niobate. When hydrothermally treated under alkali conditions, a layered niobate, K4Nb6O17·3H2O, was converted to K6Nb10.8O30. This product had a much smaller particle size than K6Nb10.8O30, prepared by a conventional solid-state reaction, and showed enhanced adsorption properties.

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iobium oxides and niobates have been widely investigated for many applications involving catalysts, photocatalysts, piezoelectrics, and adsorbents.1 Tremendous effort has been directed toward designing the structure and morphology of these particles for better properties and performances.1,2 K6Nb10.8O30 is a unique niobate material having 1D microchannels,3 whereas the properties of K6Nb10.8O30 have lagged behind because of, at least partially, the lack of a facile synthetic method.4 We have been interested in the hydrothermal treatment of titanium dioxides and titanates, another important class of materials in many useful applications, to develop new titanium dioxide-based materials with unique structures and better functions.5 In this hydrothermal reaction, it was suggested that the products formed via dissolution/deposition or decomposition (into small segments)/assembly (of the segments) of the starting materials. Here we expand the above-mentioned hydrothermal method to a well-studied layered niobate, K4Nb6O17·3H2O,6 to facilely synthesize K6Nb10.8O30. K4Nb6O17·3H2O, prepared by a solid-state reaction between Nb2O5 and K2CO3 at 1100 °C [see the Supporting Information (SI) for more details],6a was composed of platelike particles with a lateral size of up to several micrometers, as shown in the scanning electron microscopy (SEM) image (Figure 1a). We hydrothermally treated K4Nb6O17·3H2O in the presence of benzyltrimethylammonium hydroxide (BTMA) and ammonium fluoride (NH4F) at 170 °C for 20 days (see the SI). This hydrothermal condition was slightly modified from that used in our previous studies.5 After the hydrothermal reaction, the solid product was separated, washed with water, and dried at 60 °C. As shown in Figure 1, the product was composed of rodlike particles with a length of several hundreds of nanometers and a diameter of several tens of nanometers. X-ray diffraction (XRD) revealed that the rodlike-shaped product was K6Nb10.8O30 (although only a small amount of K4Nb6O17 was contained in the product, this impurity scarcely affected the properties of K6Nb10.8O30 as © XXXX American Chemical Society

Figure 1. (a) SEM images and (b) XRD patterns of K4Nb6O17·3H2O and the hydrothermal product K6Nb10.8O30. The asterisk indicates a peak due to K4Nb6O17. The inset shows the crystal structures of both niobates. Color code: green, Nb; purple, K; red, O. Excess Nb5+ ions in the triangle channels of K6Nb10.8O30 are invisible.

described below). The full structure of K6Nb10.8O30 is shown in Figure S1 in the SI. To discuss the formation mechanism of K6Nb10.8O30, we monitored the hydrothermal process by SEM and XRD. Figure 2a shows SEM images of the hydrothermal products after reaction times of 1, 3, and 14 days. The original K4Nb6O17·3H2O platy particles were downsized, and only slight amounts of rodlike particles formed after just 1 day. The amount and size of the rodlike particles increased with increasing length of the reaction time. As shown in Figure 2b, diffraction peaks due to K6Nb10.8O30 were strengthened and those due to K4Nb6O17· 3H2O were weakened with increasing hydrothermal treatment time. These results suggest that K6Nb10.8O30 rods formed via dissolution/deposition or decomposition/assembly of K4Nb6O17·3H2O. Received: July 14, 2017

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DOI: 10.1021/acs.inorgchem.7b01796 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 3. Scheme for the formation mechanism of K6Nb10.8O30 from K4Nb6O17·3H2O based on the local structure similarity between the two crystals.

On the basis of the above experimental findings and facts, we consider a possible formation mechanism of K6Nb10.8O30 as follows: K4Nb6O17·3H2O decomposes to small segments (e.g., 1D NbO6 octahedral chains), and the segments assemble into 1D K6Nb10.8O30. In the zeolite conversion process conducted under hydrothermal conditions similar to the present hydrothermal conditions,8 one zeolite decomposes into locally ordered aluminosilicate species (referred to as “nanoparts”), whose structures are the same as the local structure of the original zeolite, and the nanoparts assemble into another zeolite. In light of this, we further performed a hydrothermal treatment of K4Nb6O17·3H2O in the presence of tetrapropylammonium hydroxide (TPA) instead of BTMA because TPA is a stronger base than BTMA (or trimethylamine derived from the decomposition of BTMA at 170 °C).9 As shown in Figure S3 in the SI, K4Nb6O17·3H2O was converted to K0.88Nb2O7.58H4.28 under these conditions, suggesting that K4Nb6O17·3H2O was dissolved and no small segments for K6Nb10.8O30 formed. This result is in good agreement with the above scenario. Remarkably, the presently synthesized K6Nb10.8O30 rodlike particles were much smaller than the K6Nb10.8O30 platy particles prepared by a conventional solid-state reaction4a (Figures 4a,b and S4 in the SI). Although the synthesis of K6Nb10.8O30 with a size and shape comparable to those of the present K6Nb10.8O30 via wet chemical methods has been reported,4b,c the procedures were relatively complicated and the aspect ratio (length/ diameter) of the conventional rods was up to 5, which was smaller than that of the present rods (up to 10). It should be noted that we could facilely synthesize K6Nb10.8O30 with enhanced properties (this point will be described below). As expected, the present K6Nb10.8O30 exhibited largely enhanced adsorption properties. N2 adsorption/desorption isotherms showed that K6Nb10.8O30 rods showed N2 adsorption at a lower partial pressure region, indicating the presence of micropores, whereas K6Nb10.8O30 plates scarcely adsorbed N2 (Figure 4c). Because the starting K4Nb6O17·3H2O had no micropore-adsorbed N2, the microporosity of the K6Nb10.8O30 rods must come from its structure. Given the fact that K6Nb10.8O30 shows the reversible insertion of Li atoms into

Figure 2. (a) Low- (left) and high-magnification (right) SEM images and (b) XRD patterns of the hydrothermal products of K4Nb6O17·3H2O obtained at reaction times of 1, 3, and 14 days. The XRD pattern of the starting K4Nb6O17·3H2O is also shown.

To obtain deeper insight into the formation mechanism, we performed a hydrothermal reaction of the raw material of K4Nb6O17·3H2O, a mixture of Nb2O5 and K2CO3, under identical conditions (see the SI). This process yielded K0.88Nb2O7.58H4.28 [K0.8H1.66Nb2O6(OH)0.54(H2O)1.04] consisting of octahedral particles7 (Figure S2 in the SI). As shown in Figure 3 (top), K4Nb6O17·3H2O has 1D chains of corner-shared NbO6 octahedra formed along the a axis, which, in turn, form a 2D structure through corner and edge sharing along the c and b axes. K6Nb10.8O30 also has such 1D chains along the c axis (as marked by dotted squares and rectangles), although they form 3D structures through corner sharing along the a axis as well as the b axis. On the other hand, such a local structure similarity is not observed between Nb2O5 and K0.88Nb2O7.58H4.28 (Figure 3, bottom). B

DOI: 10.1021/acs.inorgchem.7b01796 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Communication

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ACKNOWLEDGMENTS This work was supported in part by JSPS Kakenhi Grant 26708027.

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Figure 4. (a) SEM image of K6Nb10.8O30 plates prepared by a conventional solid-state reaction. (b) Particle-size distributions, (c) N2 adsorption/desorption isotherms, and (d) Ca2+ adsorption from water of the present and conventional K6Nb10.8O30. N2 adsorption/desorption isotherms of K4Nb6O17·3H2O are also shown in part c.

tunnels occupied by K+ ions upon Nb5+/Nb4+ redox reaction,3 K6Nb10.8O30 is likely to accommodate N2 molecules in the tunnels (Figure S1 in the SI) when substantially downsized. A similar downsizing effect of K6Nb10.8O30 was observed for the cation-exchange reaction with K+ ions. Figure 4d shows time courses of adsorption of Ca2+ ions from water on K6Nb10.8O30 rods and plates. K6Nb10.8O30 rods showed faster and highercapacity adsorption. In conclusions, we have reported the facile synthesis of a rare phase niobate, K6Nb10.8O30, via the hydrothermal treatment of a layered niobate, K4Nb6O17·3H2O. The possible formation mechanism involved the decomposition of K4Nb6O17·3H2O to small segments and the assembly of the segments into K6Nb10.8O30. The obtained K6Nb10.8O30 had a much smaller particle size compared to K6Nb10.8O30 prepared by a conventional solid-state reaction and then showed enhanced adsorption properties. A wide variety of precursors (e.g., layered niobates) with different compositions and structures are available; therefore, the present synthetic strategy will make niobium oxide/niobate materials’ design much more attractive and versatile.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01796. Experimental details and additional data (Figures S1−S4) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yusuke Ide: 0000-0002-6901-6954 Notes

The authors declare no competing financial interest. C

DOI: 10.1021/acs.inorgchem.7b01796 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry (8) (a) Inoue, T.; Itakura, M.; Jon, H.; Oumi, Y.; Takahashi, A.; Fujitani, T.; Sano, T. Synthesis of LEV Zeolite by Interzeolite Conversion Method and its Catalytic Performance in Ethanol to Olefins Reaction. Microporous Mesoporous Mater. 2009, 122, 149−154. (b) Sano, T.; Itakura, M.; Sadakane, M. High Potential of Interzeolite Conversion Method for Zeolite Synthesis. J. Jpn. Pet. Inst. 2013, 56, 183−197. (9) Sasaki, H.; Jon, H.; Itakura, M.; Inoue, T.; Ikeda, T.; Oumi, Y.; Sano, T. Hydrothermal Conversion of FAU Zeolite into Aluminous MTN zeolite. J. Porous Mater. 2009, 16, 465−471.

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DOI: 10.1021/acs.inorgchem.7b01796 Inorg. Chem. XXXX, XXX, XXX−XXX