Copper–Organic Cationic Ring with an Inserted Arsenic–Vanadium

Communication pubs.acs.org/IC

Copper−Organic Cationic Ring with an Inserted Arsenic−Vanadium Polyanionic Cluster for Efficient Catalytic CrVI Reduction Using Formic Acid Xue-Li He, Yun-Ping Liu, Kai-Ning Gong, Zhan-Gang Han,* and Xue-Liang Zhai College of Chemistry & Material Science, Hebei Normal University, Shijiazhuang, Hebei 050024, China S Supporting Information *

Our recent research has focused on the synthesis of POMbased supramolecular assemblies.14 Such hybrid systems can potentially combine the advantages of inorganic oxo−metal clusters and organic molecules, as well as benefit from the close interaction and synergistic effect between two moieties. By the reaction of NH4VO3, NaAsO2, Cu(Ac)2, and 1,3-di-4-pyridylpropane (bpp) at 160 °C for 5 days, a doubly interpenetrated metal−organic framework, [Cu4(bpp)4][β-As8V14O42(H2O)] (1), was constructed, in which the [β-As8V14O42(H2O)]4− {As8V14} polyanionic clusters (10.397 Å × 10.324 Å) are captured in large eight-membered cationic rings of [Cu4(bpp)4]4+ (18.508 Å × 14.647 Å) through multiple weak noncovalent interactions (CCDC 1029008, Table S1, and thermogravimetric (TG) and X-ray diffraction (XRD) data in Figures S1 and S2 in the Supporting Information, SI). Figure 1

ABSTRACT: Polyanionic cluster [β-As8V14O42(H2O)]4− is well embedded in a large porous eight-membered cationic ring of the copper ligand, giving a stable host− guest supramolecular system. The assembly exhibits an efficient heterogeneous catalytic performance for the reduction of CrVI using formic acid at ambient temperature.

P

olyoxometalates (POMs) have been an important subject because of their potential applications in various fields.1−3 The excellent features of structural integrity and catalytic activity allows them be useful in many industrial processes.4 However, some inherent drawbacks of these kinds of inorganic materials, such as high solubility in aqueous solution and low stability of classical polyanions, limit the scope of their practical applications. To overcome these problems and improve the possibility of catalyst recovery and recycling, great efforts have been devoted to doping them onto a solid support (such as TiO2, SiO2, graphene, etc.).5 A further advance in this field is to construct host−guest hybrid assembly systems of POM−metal−organic framework (MOF).6−8 Such hybrid systems facilitate the catalytic process upon changing from homogeneous to heterogeneous and could be used in environmental cleanup, such as the removal of CrVI from wastewater. CrVI is a ubiquitous toxic pollutant in the aquatic environment,9,10 and removal from water is required at any cost. In contrast to highly water-soluble and toxic CrVI, CrIII is much less soluble in water and is nontoxic and relatively inert to humans. Therefore, changes in the oxidation state have a dramatic effect on chromium solid/solution partitioning and subsurface migration rates. The reduction reaction by many inorganic and organic reductants occurs in acid solution, including, but not limited to, FeII/Fe0, H2S/HS−, alkanes, alcohols, aldehydes, ketones, and aliphatic and aromatic acid,11 etc. Formic acid (FA) has also been employed for reducing CrVI to CrIII (see eq 1). The most benefit is in the fact that its oxidation product is CO2. So, there will be no secondary pollution and also no remnants of any harmful substances. So far, only platinum (Pt) and palladium (Pd) nanoparticles (NPs) have been reported to be active for the reduction of CrVI to CrIII using FA.12,13 The pursuit of more environmentally friendly and economical catalysts is still the most important work as well as a challenge.

Figure 1. Mixed ball-and-stick, polyhedral, and space-filling view showing the host−guest system, in which the polyanionic [βAs8V14O42(H2O)]4− cluster is encapsulated in a large eight-membered cationic ring of {[Cu4(bpp)4]4+}.

presents the host−guest system of anion and cation in crystal 1. The assembly exhibits an efficient heterogeneous catalytic performance for the reduction of CrVI using FA at ambient temperature. In reported structures, the {As8V14} cluster usually presents two conformations: α and β type (see Figure S3 in the SI).15 Structural analysis shows that the polyanion in 1 belongs to the β{As8V14} structure. In 1, a large porous metal−organic cationic molecular ring of {[Cu4(bpp)4]4+} is built on reduced CuI ions as nodes and flexible bpp ligands as connectors. Every cationic ring is peripherally bridged by four [As8V14O42]4− anions via covalent

Cr2O7 2 − + 8H+ + 3HCOOH ↔ 3CO2 + 2Cr 3 + + 7H 2O

Received: December 19, 2014

(1) © XXXX American Chemical Society

A

DOI: 10.1021/ic5030444 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry Cu−O bonding interactions. In turn, each {As8V14} cluster in a quadridentate ligand covalently coordinates to four copper centers coming from four different [Cu4(bpp)4]4+} rings (see Figure S4 in the SI). As shown in Figure 2a, the types of linkages

Figure 3. UV−vis spectral evolutions with time during the reduction of Cr2O72− by FA at 25 and 50 °C in the presence of 1. The insets are the pseudo-first-order kapp plots of −ln(Ci/C0) with time for the above reaction.

reduction of CrVI. It may be noted here that the reaction proceeds through a clear isosbestic point at 313 nm, showing that the redox reaction is proceeding smoothly without forming multiple products.19 The addition of excess sodium hydroxide to the resulting solution led to the formation of a green solution, indicating that CrVI had been reduced to CrIII.13,20 We can determine the reaction rate if we assume that it is first-order because of excess FA. The apparent rate constant value is calculated from the slope of plot −ln(Ci/C0) against time as 1.83 × 10−2 min−1 at room temperature. The transformation of the CrVI ion is 72% in 70 min. It is also evident that the effect of temperature enhancement on the catalytic ability is significant under similar conditions. The same catalytic experiment is finished in only a short 30 min. The apparent rate constant value at 50 °C is raised to 4.3 × 10−2 min−1, approximately 3 times greater than that achieved at room temperature. It should be emphasized that these are apparent rate constants used for comparative purposes among these materials and will depend on the reaction conditions employed. In contrast to the precious metal Pd and Pt NPs as catalysts, crystal 1 is cheaper and easier to prepare. Considering that crystal 1 may be partially dissolved in this case, the simple catalytic comparative experiments were also completed in a similar fashion with the use of free bpp, V2O5, CuO, and V2O5 + CuO, respectively. The ratio of metal oxides is calculated according to their corresponding values in 1. As shown in Figure S5 in the SI, the experimental results indicated that these oxides and the free bpp ligand have no effect on the reduction. Through the above-mentioned experimental results, one can draw a conclusion that compound 1 does not decompose but just partially dissolves in the solution during the reaction process, which is probably affected by high temperature. Furthermore, one {As8V14}-containing compound, Cd4(5,5′mbpy)10[As8V14O42(H2O)]2·2H2O (2; mbpy = 5,5′-dimethyl2,2′-dipyridyl; CCDC 1043240), had also been tested on the catalytic reduction reaction.21 Crystal 2 shows a 1D chainlike structure in which the {As8V14} clusters are bridged by Cd-mbpy subunits (Figure S6 in the SI). As shown in Figure 4, crystal 2 only presents a weak catalytic performance for this reduction under reaction conditions similar to those of crystal 1. Therefore, one can speculate that the host−guest system and composite structure of 1 play important roles in the catalytic transformation from CrVI to CrIII. Because the reduction step is an electron-transfer process, reported work also confirmed that the catalysis reaction occurred through electron transfer at the noble metal NP surface.22 Polyanion {As8V14} as a nanosized cluster has a large delocalized π bond located at the pseudospherical surface, which might be more conducive to electron transfer in a much faster manner.

Figure 2. (a) 2D sheets showing the coordination environments of cationic [Cu4(bpp)4]4+ and anionic [As8V14O42]4− in 1. (b) 2D sheets are highlighted in two kinds of colors to present the 2-fold interpenetrated network. (c) Topological scheme showing the 2D net of the Schläfli notation (44,62). The two kinds of topological nodes are {[Cu4(bpp)4]4+} (small ball) and {As8V14} (big ball). For clarity, the guest water molecules are not shown.

among anions and cations form a 2D wavelike sheet. For the sake of illustration, if {[Cu4(bpp)4]4+} and {As8V14} are set as two kinds of topological nodes, the complete Schläfli notation for an infinite 2D net is (44,62). It is interesting to find that two independent and identical 2D sheets are corrugated so that each is able to pass through the other an infinite number of times, presenting a 2-fold interpenetrated metal−organic topological network. Every {β-As8V14} anion precisely locates at the center of a large eight-membered circular {[Cu4(bpp)4]4+} host frame coming from the other composite sheet (see Figure 2b). The interpenetrating 2 × {Cu4(bpp)4-As8V14}∞ networks are thus formed. The complete topology Schläfli symbols (see Figure 2c) are (4,62) for the copper cation and (64,102) for the {As8V14} anion. Only nonconvalent interactions exist between the anionic cluster and its surrounding macrocycle, forming an attractive host−guest system. Crystallographically independent copper sites [Cu(1) and Cu(2)] exhibit the same T-shaped coordination geometries. Bond-valence-sum analyses indicate that both Cu(1) and Cu(2) are of the +1 oxidation state.16 The T-shaped coordination configurations also conform to CuI centers. It is significant to consider the factor of the metal combining with the flexible ligand, which can provide useful information to prepare novel POM-based hybrid structures. Reported works have confirmed that the vanadium oxide clusters are good catalysts for many reactions.17 It has recently been reported that the highly dispersed Pt and Pd NPs inside MIL-101 pores exhibited catalytic performances for the reduction of CrVI in the presence of excess FA, whereas Au@ MIL-101 and Rh@MIL-101 catalysts were inactive.18 Motivated by the discovery, we have tested the catalytic activity of crystal 1 with the reduction of CrVI contamination. The reaction does not occur in the absence of a catalyst at room temperature (25 °C) and 50 °C. However, the reduction rate increases dramatically with the addition of 1 in the solution. As shown in Figure 3, with increasing reaction time, the absorption of CrVI at 348 nm decreases successively, accompanied by a change in the color of the solution and slowly fading until colorless, indicating the B

DOI: 10.1021/ic5030444 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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This might explain the reason why the pure oxide and those mixtures are inactive for the kind of reduction. Also, it shows that the UV spectra are basically unchanged when only crystal 1 is added to the CrVI solution in the absence of HCOOH, as shown in Figure S7 in the SI, indicating that adsorption of Cr6+ to {As8V14} does not occur markedly and is unnecessary for the following reduction. The catalyst can be easily separated after the reaction and reused several times. The catalytic reaction was performed multiple times using the same sample. It is found that the conversion rates of CrVI become lower with an increase of the repeated cycles (see Figure 4b), which can be ascribed to the fact that the crystals are sparingly soluble in water and some of them might be lost with multiple cycles. In addition, a possible reason is that the surface of the crystal sample adsorbs a lot of relatively insoluble Cr3+, which could impede the contact of 1 and Cr6+, leading to reduction of the catalytic activity. The IR spectra of 1 before and after the catalytic reduction may be used to prove the stability of the crystal material (see Figure S8 in the SI). In conclusion, a doubly interpenetrated host−guest metal− organic framework has been constructed and characterized. Polyoxovanadate anions were captured in the large porous eightmembered cationic macrocycle of {[Cu4(bpp)4]4+}. The crystal has the potential to replace costly Pd and Pt NPs used in the environmental remediation for CrVI-containing wastewater. This kind of POM-based system is cheaper and is easily obtained; in particular, they can be structurally designed to optimize their property and application in more environmentally friendly catalytic reactions. It is expected to construct diverse new MOF materials with different types of active POMs for their wide applications in the future.

ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data in CIF format, details of experiments, and additional figures, tables, UV−vis spectra, IR, XRD, and TG curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 4. (a) Catalytic conversion rates C = (C0 − Ci)/C0 × 100% of CrVI in the presence of 1 and 2, respectively. (b) Four consecutive cycles using the same sample of 1.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Grant 21341003) and the Natural Science Foundation of Hebei Province (Grant B2011205035). C

DOI: 10.1021/ic5030444 Inorg. Chem. XXXX, XXX, XXX−XXX