Extended structural materials composed of transition-metal-substituted arsenicniobates and their photocatalytic activity

Abstract

Two novel transition-metal-cluster-containing arsenicniobates, [Cu(en)₂(H₂O)][Cu(en)₂]₄{AsNb₉V₇O₄₄}·8H₂O (1) and [Cu(en)₂(H₂O)][Cu(en)₂]₄H{AsNb₈V₈O₄₄}·11H₂O (2), were designed and synthesized by reaction of K₇HNb₆O₁₉·13H₂O, Cu(Ac)₂·H₂O, Na₂VO₃, As₂O₃, and en molecules. Structural analysis revealed that compounds 1 and 2 contained the structural similar [AsNb₉V₇O₄₄]¹⁰⁻ and [AsNb₈V₈O₄₄]¹¹⁻ polyoxoanions, respectively, which are obtained by inserting a {NbV₇} or {V₈} ring into the tetravacant polyoxoniobate [AsNb₈O₃₆]²⁷⁻. Polyoxoanions in 1 and 2 could be regarded as the Keggin polyoxoniobates [AsNb₉V₃O₄₀] or [AsNb₈V₄O₄₀] capped by four VO₅ units. The high-nuclear vanadium clusters were firstly introduced into the Keggin-type arsenicniobates. Further, polyoxoanions in 1 and 2 were connected by the metal–organic linkers into the 1D chain and 2D layer-like framework, respectively. Photocatalytic studies indicated that compounds 1 and 2 are both active for photocatalytic degradation of organic dye in aqueous solution, and they are stable and easily separated from the photocatalytic system for reuse as well.

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Introduction

Polyoxometalates (POMs) are an outstanding class of nanosized metal-oxo clusters with wide structural diversity and O-enriched surfaces. They have received much attention due to their fascinating properties and potential applications in many different fields.¹ Among the POM family, polyoxotungstates, polyoxomolybdates, and polyoxovanadates are the dominant subclasses as they can be easily synthesized by simple acidification and exist with a wide pH range.^2,3 Polyoxoniobates (PONs) attracted more and more attention recently as they possess of potential applications in various fields such as photocatalysis, nuclear-waste treatment, and base-catalyzed decomposition of biocontaminants.⁴ To date, many isopolyoxoniobates, such as [Nb₆O₁₉]⁸⁻ and [Nb₁₀O₂₈]⁶⁻, [Nb₂₀O₅₄]⁸⁻, [H₉Nb₂₄O₇₂]¹⁵⁻, [HNb₂₇O₇₆]¹⁶⁻, [Nb₃₂O₉₆H₂₈]⁴⁻, and their transition-metal Ti-, Cr-, V- and Cu-containing derivatives were synthesized.^5–7 Compared to the isopolyoxoniobate chemistry, the development of heteropolyniobate (HPON) is still in its infancy stage. In 2002, the first HPON {[Ti₂O₂][SiNb₁₂O₄₀]}¹²⁻, where the Keggin-type HPON was connected into an infinite one-dimensional (1D) chain, was reported by Nyman et al.⁸ Inspired by this pioneering work, several Keggin-type HPONs, C-type HPONs and their derivations were prepared subsequently.⁹ These HPON compounds were obtained by introducing Si, Ge and V into the HPON structure [TNb₁₂O₄₀] (T = Si, Ge and V).¹⁰ As well known, Keggin and Wells-Dawson-type POMs are the most widely recognized and thoroughly studied and, they have been widely used as the precursors and building units for constructing functional materials.^11,12 Introduction of novel heteroatom to the typical POMs will not only increase the variety of POM chemistry, but also can supply new candidates for design and synthesis of function materials. Arsenic, located below the phosphorus and right the germanium in the periodic table of elements, has similar chemical properties or atomic radius with them.¹³ The introduction of As into the HPONs provided a new type of the PON family, and a new candidates for design and synthesis of function materials. This will be a promising field for construction new PON compounds. Up to date, the research of arsenicniobate chemistry is still rarely explored.^14,15 We have long-term committed to the research of PONs, and a series of P, Ge, V and Si-centered PONs [XNb₈V^IV₈O₄₄] (X = P and V), [PNb₁₂O₄₀(VO)₆], [GeNb₁₂V^IV₂O₄₂], [SiNb₁₂V^IV₂O₄₂] were designed and synthesized.¹⁶ Here, the arsenic atom as heteroatom was introduced to the PON structure resulting in two Keggin-type polyoxoanions [AsNb₈V₄O₄₀] and [AsNb₉V₃O₄₀] capped by four VO₄ units, which were further connected by [Cu(en)₂]²⁺ metal–organic groups into the 1D chain and 2D layer-like framework, respectively. Photocatalytic studies indicate that compounds 1 and 2 are both active for photocatalytic degradation of organic dye in aqueous solution.

Experimental section

Materials and methods

All chemicals were commercially purchased and used without further purification. K₇HNb₆O₁₉·13H₂O was prepared according to the literature and its identity was confirmed by IR spectrum.¹⁷ IR spectrum was recorded in the range of 400–4000 cm⁻¹ on an Alpha Centaurt FT/IR Spectrophotometer using KBr pellets. Elemental analyses for Cu, V, and Nb were performed with a Leaman inductively coupled plasma (ICP) spectrometer, and C and N analyses were obtained on a Perkin-Elmer 2400 CHN elemental analyzer. TG analyses were performed on a Perkin-Elmer TGA7 instrument in flowing N₂ with a heating rate of 10 °C min⁻¹.

Synthesis of 1

A 2 mL aqueous solution containing 0.040 g Cu(OAc)₂·H₂O (0.200 mmol) was dropwise added to a 4 mL aqueous solution of K₇HNb₆O₁₉·13H₂O (0.055 g, 0.040 mmol), As₂O₃ (0.02 g, 0.100 mmol) and NaVO₃ (0.049 g, 0.400 mmol). Then, 1 mL 1,2-diaminoethane was added dropwise to the mixture. Subsequently, 1 M HCl were used to adjust the pH value of the mixture to 10.50. Then, the mixture was transferred to a Teflon-lined stainless steel autoclave (23 mL) and kept at 160 °C for 5 days. When the mixture was slowly cooled to room temperature, brown crystals were obtained and washed with distilled water (yield 40.6% based on Nb). Elemental analysis calcd (%) for 1: Cu 10.40, V 11.69, Nb 27.42, C 7.86, N 9.17; found: Cu 10.62, V 11.32, Nb 27.26, C 7.62, N 9.58.

Synthesis of 2

K₇HNb₆O₁₉·13H₂O (0.055 g, 0.040 mmol) was added in 4 mL aqueous solution of As₂O₃ (0.02 g, 0.100 mmol) and NaVO₃ (0.049 g, 0.400 mmol) resulting in solution A. 2 mL aqueous solution containing 0.040 g Cu(OAc)₂·H₂O (0.200 mmol) was dropwise added to solution A. Then, 1 mL 1,2-diaminoethane was added dropwise to the mixture. The pH value of the mixture was adjusted to 10.80 with 1 M HCl solution, and it was then transferred to a Teflon-lined stainless steel autoclave (23 mL) and kept at 160 °C for 5 days. When the mixture was slowly cooled to room temperature, brown crystals were obtained and washed with distilled water (yield 38.2% based on Nb). Elemental analysis calcd (%) for 2: Cu 10.35, V 13.31, Nb 24.27, C 7.83, N 9.13; found: Cu 10.56, V 13.82, Nb 24.49, C 7.26, N 9.39.

X-ray crystallography

Single crystals of compounds 1 and 2 were glued onto top of the glass fiber. Data was collected on a Rigaku R-AXIS RAPID IP diffractometer at 293 K using graphite monochromated Mo K_α radiation (λ = 0.71073 Å) and IP techniques. The structures of compounds 1 and 2 were both solved by direct methods and refined by full-matrix least-squares methods on F² using the SHELXTL-97 crystallographic software package.¹⁸ The hydrogen atoms attached to organic ligands were fixed in calculated positions. Further details of the X-ray structural analysis are given in Table 1 ESI.†

Table 1 Crystal data and structure refinement for 1 and 2^a

	1	2
a Note. R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = ∑[w(Fo^2 − Fc^2)^2]/∑[w(Fo^2)^2]^1/2.
Empirical formula	C20H98N20O53AsNb9V7Cu5	C20H105N20O56AsNb8V8Cu5
Fw	3052.5	3065.5
Crystal system	Monoclinic	Triclinic
Space group	C2/c	P1̄
a (Å)	25.852(5)	12.907(3)
b (Å)	27.121(5)	14.153(3)
c (Å)	24.729(5)	14.277(3)
β (°)	109.49(3)	80.34(3)
V (Å^3)	16 345(6)	2300.9(8)
Z	8.00	1.00
T (K)	293(2)	293(2)
Λ (Å)	0.71073	0.71073
R1, wR2 [I > 2σ(I)]	0.0707, 0.1748	0.0826, 0.2230
R1, wR2 (all data)	0.1058, 0.1932	0.1012, 0.2378

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Photocatalytic experiments

Photocatalytic performance of compounds 1 and 2 were investigated with photodegradation of RhB and MB solution. 10 mg 1 or 2 were added to 100 mL of 10.0 mg L⁻¹ RhB or MB solution. The obtained suspensions were magnetically stirred in the dark for about 30 min to ensure the equilibrium of the working solution. The solution was then exposed to UV irradiation from a 125 W Hg lamp with a distance of 3–5 cm between the liquid surface and the lamp, kept stirring during irradiations.

Results and discussion

Synthesis

During the past decades, hydrothermal synthesis has been proven to be a useful synthetic technique in the preparation of novel POM-based materials. In a specific hydrothermal process, many factors can affect the formation and growth of crystalline products, such as the initial concentration of reactants, pH value, reaction time and temperature. Compound 1 was prepared by the hydrothermal reaction of K₇HNb₆O₁₉·13H₂O, Cu(Ac)₂·H₂O, Na₂VO₃, As₂O₃, and en. Parallel experiments showed that the pH value of the reaction system play a significant role in the formation of compound 1, and the optimized pH value should be in the range from 10.3 to 10.6. It is difficult to prepare compound 1 with a lower pH value than 10.3. Compound 2 was prepared by the similar synthetic procedure with that of 1 except that the pH value of the reaction system of 2 should be in the range of 10.7 to 11.0.

Crystal structure

Compound [Cu(en)₂(H₂O)][Cu(en)₂]₄{AsNb₉V₇O₄₄}·8H₂O (1) is composed of an isolated {AsNb₉V₇} cluster, decorated by five [Cu(en)₂]²⁺, and charge-balanced by one [Cu(en)₂]²⁺ group. As shown in route I in Scheme 1, the {AsNb₉V₇} cluster in compound 1 was composed of a {AsNb₈} unit, which captured a {NbV₇} ring into the tetravacant PON [AsNb₈] resulted in the anion [AsNb₉V₇]. Alternatively, the tetravacant building block [AsNb₈] has sixteen activated oxygen atoms, which could further coordinate with other metal cations. As shown in Scheme 1 (route II), the [AsNb₈] unit captured one {NbO₆} and three {VO₆} units into a Keggin-type {AsNb₉V₃} PON. The Keggin polyoxoanion was then capped by four VO₅ units resulting in a tetra-capped Keggin-type HPN {AsNb₉V₇}. Further, the {AsNb₉V₇} modified by one [Cu(en)₂(H₂O)]²⁺ and two [Cu(en)₂]²⁺ groups via the Cu–O–V/Nb bonds (Fig. 1a). And, the {AsNb₉V₇} anion was connected by two [Cu(en)₂]²⁺ into a one-dimensional chain through V–O–Cu bonds, and the V–O and Cu–O distances are in the range of 1.618(9)–1.620(9) Å and 2.537–2.751 Å, respectively (Fig. 1c).

Scheme 1 Schematic view of the polyoxoanion structure in 1.

Fig. 1 (a) Ball-and-stick view of the structural unit of 1; (b) polyhedral representation of the {V₈} cluster in 2; (c) structure of the 1D chain composed of the {AsNb₉V₇} units and [Cu(en)₂]²⁺ groups in 1.

Crystal structure analysis reveals that compound [Cu(en)₂(H₂O)][Cu(en)₂]₄H{AsNb₈V₈O₄₄}·11H₂O (2) consists of a {AsNb₈V₈} cluster decorated by three [Cu(en)₂]²⁺ groups and charge-balanced by one [Cu(en)₂(H₂O)]²⁺ unit. In compound 2, the polyoxoanion {AsNb₈V₈} is a {V₈}-cluster-containing HPN. As shown in Fig. 2, the {AsNb₈V₈} can be described as a {AsV₈} cluster coordinated by two {Nb₄O₁₈} units. The {AsV₈} cluster is composed of two kinds of VO_x (x = 5 or 6) polyhedra around one AsO₄ tetrahedron. Bond valence calculations indicated that the V centers both in the VO₅ and VO₆ groups are in the IV valence, and the V–O lengths the VO₅ and VO₆ groups are in the range of 1.617(9)–1.960(11) Å and 1.633(8)–2.461(14) Å, respectively (Table S2†). The {Nb₄O₁₈} fragment is a dimmer consisting of two [Nb₂O₁₀]¹⁰⁻ units obtained by decomposition of the Lindqvist cluster [Nb₆O₁₉]⁸⁻. Two {Nb₄O₁₈} moieties capped on the {AsV₈} cluster to give the {AsNb₈V₈} anion cluster (Fig. 2c), which has been observed in the V- and P-centered PONs.^16a However, it was first observed in the arsenicniobate compounds. Further, the {AsNb₈V₈} cluster was connected by [Cu(en)₂]²⁺ groups into a two-dimensional (2D) porous network through V–O–Cu bonds, and the V–O and Cu–O distances are in the range of 1.633(8)–1.644(9) Å and 2.473–2.523 Å, respectively. As shown in Fig. 4d, each {AsNb₈V₈} cluster was connected with four neighboring polyoxoanions by [Cu(en)₂]²⁺ linkers, resulting in a (4,4)-connected 2D framework with 14.16 × 14.26 Å holes. The 2D frameworks were connected with each other by Cu–O–Nb weak interactions to 3D framework with the Cu–O lengths of 2.929 Å and Nb–O lengths of 1.732(10) Å. In the 3D framework, there are one dimensional channels with the size of 7.5 Å × 7.2 Å along the b-axis (Fig. 3). The [Cu(en)₂(H₂O)]²⁺ groups are located in the channels of the 3D framework, balancing the charge of the framework.

Fig. 2 (a) Ball-and-stick representation of the {Nb₄} unit both in 1 and 2; (b) polyhedral representation of the {AsV₈} unit in 2; (c) polyoxoanion {AsNb₈V₈} consists of a {AsV₈} cluster and two {Nb₄} units in 2; (d) 2D framework of 2 constructed from the polyoxoanion {AsNb₈V₈} and [Cu(en)₂]²⁺ groups.

Fig. 3 3D porous framework of 2 along the b-axis.

Fig. 4 Photocatalytic degradation of MB (a) and RhB (b) by compound 1; photocatalytic degradation of MB (c) and RhB (d) by compound 2.

Thermal analyses

To study the thermal stability of compounds 1 and 2, the thermogravimetric (TG) analysis was carried out on these two compounds. As shown in Fig. S3,† the TG curve of compound 1 shows three step weight losses in the range of 50 to 600 °C. The first weight loss of 5.54% occurs between 50 and 200 °C, corresponding to the loss of lattice water molecules (cal. 5.31%). The other two weight loss of 9.91% is from 200 to 400 °C and 6.86% from 400 to 600 °C, corresponding to the loss of the ligands in the 1D framework (cal. 19.66%). The TG curve of compound 2 shows a weight loss of 6.20% between 50 and 200 °C (Fig. S4†), corresponding to the loss of lattice water molecules (cal. 7.05%). The other two weight loss of 9.7% is from 200 to 400 °C and 8.06% from 400 to 600 °C, corresponding to the loss of the ligands in the 3D framework (cal. 19.57%).

IR spectra

The IR spectrum of compounds 1 and 2 display similar characteristic vibration patterns, resulting from the HPONs in the region of 400–1100 cm⁻¹, is shown in Fig. S5 and S6.† The terminal M = O_t (M = V and Nb) vibrations appear at 953 and 893, 807 cm⁻¹ for 1, 951, 895, and 811 cm⁻¹ for 2, respectively.^10,16 The characteristic peaks at 648, 604, 490 cm⁻¹ for 1, 649, 604, and 477 cm⁻¹ for 2 are assigned to the bridging M–O_b–M vibrations, respectively. Bands at 1040 cm⁻¹ are assigned to As–O_c vibration of 1 and 2.^10,16 The characteristic absorption bands of the organic groups appear at 1102–1585 cm⁻¹ and 1098–1583 cm⁻¹ for 1 and 2, respectively.

Photocatalytic property

In the past decades, photocatalysts have been widely studied to decompose waste organic dyes so as to purify water resources. Methylene blue (MB) and rhodamine B (RhB) are typical dye molecules to evaluate the activity of photocatalysts used for the purification of waste water.^19,20 POMs have regarded as one type of new potential photocatalysts in the degradations of organic dyes.²⁰ However, POM salts are usually water-soluble and may induce secondary pollution if they are directly used as photocatalysts in aqueous solution, and they are difficult to be isolated for reuse. Up to date, much attention has been paid to the construction of novel heterogeneous POM-based photocatalysts.²⁰ Herein, the heterogeneous photocatalytic performance of compounds 1 and 2 towards degradation of MB and RhB under UV irradiation were investigated. In the presence of compound 1, the MB and RhB degrades from 100% to 6.3% and 19.8% during 2.5 h, the degradation rate curve of 1 is nearly linear, corresponding to about 37.5% and 32.1% per hour (Fig. 4a and b). For compound 2, MB and RhB degrades to 7.8% and 11.8% during 2.5 h and the degradation rate is 36.9% and 35.3% per hour (Fig. 4c and d). A control photolysis experiment was also performed under the same conditions by using Cu(en)₂ as the catalyst or without any catalyst. After 2.5 h photocatalytic process, the decolorization rate of MB solution can only achieve 9.8% and 4.5%, respectively, and the decolorization rate of RhB solution can achieve 6.0% and 3.9%, respectively (Fig. S7†). These results reveals that compounds 1 and 2 are active photocatalysts for the degradation of MB and RhB. As shown in Fig. S8 and S9,† the PXRD pattern of as-synthesized samples is in good agreement with that of the simulated one from the single crystal data, which proves the phase purity of the bulk materials. Further, the PXRD pattern of the photocatalyst after the photocatalytic reaction also shows similar peaks with that of the as synthesized sample, revealing that the photocatalyst is stable in the photocatalytic reaction. Additionally, we found that the catalytic activity of compounds 1 and 2 was maintained in the recycled experiments, no significant loss in photocatalytic activity was observed (Fig. S10 and S11†).

Conclusion

In conclusion, we reported two extended structures based on transition-metal-cluster-substituted arsenicniobate clusters. Compounds 1 and 2 contain a similar travacant PON [AsNb₈O₃₆]²⁷⁻ clusters, which captured the {V₈} and {NbV₇} rings forming the transition-metal-cluster-substituted polyoxoarsenicniobates. In this work, the As atom was introduced into the heteropolyoxoniobate system, which indicated a promising filed for constructing new PON-based materials. Photocatalytic studies revealed that compounds 1 and 2 are active for photocatalytic degradation of dye molecules.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21101022), and Science and Technology Development Project Foundation of Jilin Province (20150520001JH).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1047305 and 1047306. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra08262e

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