Synthesis of Hybrid Phosphomolybdates and Application as Highly

Sep 4, 2018 - Comparison of catalytic ability of three crystals to reduce CrVI using formic acid as reductant, found that crystal 1 was effectively ac...
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Synthesis of Hybrid Phosphomolybdates and Application as Highly Stable and Effective Catalyst for the Reduction of Cr(VI) Xing Xin, Xuerui Tian, Haitao Yu, and Zhangang Han* College of Chemistry and Material Science, Hebei Normal University, Shijiazhuang, Hebei 050024, PR China

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S Supporting Information *

ABSTRACT: Three hybrid phosphomolybdates were successfully synthesized by hydrothermal method and characterized by a series of physicochemical analysis techniques. Xray single-crystal structural analysis revealed that three compounds with the reductive polyanionic clusters (MoV) were wrapped by protonated organic component bpp cations (bpp = 1,3-bi(4-pyridyl)propane) through the complex supramolecular hydrogen bonding network. They also have similar molecular formulas: (H2bpp)3[PbM(H2O)2]2H6{M[Mo6O12(OH)3(HPO4)3(PO4)]2}2·nH2O (M = Fe in 1, Zn in 2, n = 4; or M = Mn in 3, n = 6). The oxidation states of all Mo centers in these polyanions are in the form of +5, presenting clusters with the higher negative charge. The feature showed that they were easy to be modified by transition metal and organic moieties, so as to form a high-dimensional structure and produce functional materials with specific properties. Comparison of catalytic ability of three crystals to reduce CrVI using formic acid as reductant, found that crystal 1 was effectively active to this redox reaction. The conversion of CrVI can reach 99% after 120 min of heating in 55 °C water bath, and the conversion of above 95% can still be achieved after 5 recycles of applications.

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others.6,23 With the fast development of economies, its discharge increases stepwise and has also become to a more and more critical environmental problem.24 The traditional handling methods are to use sodium bisulfite or hydrazine hydrate as reducer to reduce the toxic CrVI to the relatively safe CrIII. However, the effective catalysts are noble metal Pt/Pd nanoparticles. The exploration for new catalytic systems and catalysts still remain to be a challenging task. New kinds of catalysts for this redox reaction need to be gradually explored and found so that the reducing process of CrVI can be simply but efficiently carried out.25−27 In our previous work, we also found the hourglass-type phosphomolybdates were catalytically active for the reduction of CrVI to CrIII under a mild condition.28,29 As the continuous work, here three analogical hybrids were synthesized by using the flexible bpp (bpp = 1,3-bi(4-pyridyl)propane) as a special organic binder. They were (H2bpp)3[PbM(H2O)2]2H6{M[Mo6O12(OH)3(HPO4)3(PO4)]2}2·4H2O (M = Fe (1) and Zn (2)) and (H2bpp)3[PbMn(H2O)2]2H6{Mn[Mo6O12(OH)3(HPO4)3(PO4)]2}2·6H2O (3). Different from those previous researches, the existing {P4Mo6} polyanions were continually modified by host group metal lead element. These compounds had been characterized by various methods and tested its activity in catalysis reduction of CrVI. The results

INTRODUCTION Polyoxometalates (POMs) have the clear and tunable crystal structures, more importantly, many interesting physical and chemical properties for different desires in a variety of fields, such as analysis, catalytic, pharmaceutical, and materials science.1−5 Recently, with the improvements of test methods and cognitive level, the various POM compounds with novel structure and special properties have been synthesized.6−8 Through combination with POM and organic moiety, some advantages of the inorganic and organic components can be integrated together, which provides more possibilities for the synthesis of new crystals with the desired performance.9−12 As a special member of POM family, hourglass-type {M[P4MoV6O31]2}n− (abbr. {P4Mo6}) polyanion consists of two [P4MoV6O31]12‑ subunits bridged by one single metal M center.13,14 This type of structure shows more pendant oxygen atoms than common spherical structures, which implies the possibility of more active sites in a catalyst system.15−17 Moreover, the value states of all Mo atoms in this structure are +5, which can easily translate between reduced and oxidation states in a redox catalysis process. The polyanionic structures can be well-maintained even through multiple reactions, so these special physicochemical properties ensure that they can be used as molecular catalysts in redox reactions.18−22 Hexavalent chromium (CrVI) ion is a common pollutant with high toxicity and is produced in various fields, such as metallurgy, tanning, dyeing, and anticorrosion, among © XXXX American Chemical Society

Received: May 15, 2018

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DOI: 10.1021/acs.inorgchem.8b01321 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystal Analytical Data of Compounds 1−3 compounds empirical formula formula weight wavelength crystal system space group a (Å) b (Å) c (Å) α, β, γ (deg) volume (Å3), Z density (calculated) absorption coefficient F(000) goodness-of-fit on F2 final R indices [I > 2σ(I)]a R indices (all data)

1 C39H94Mo24N6O132P16Pb2Fe4 6195.06 0.71073 Å monoclinic C2/c 22.2894(8) 21.8950(8) 16.6547(8) 90, 96.732(4), 90 8071.9(6), 2 2.544 Mg/m3 4.490 mm−1 5860 1.047 R1 = 0.0413, wR2 = 0.1091 R1 = 0.0501, wR2 = 0.1165

2 C39H94Mo24N6O132P16Pb2Zn4 6221.04 0.71073 Å monoclinic C2/c 22.2177(10) 21.8656(10) 16.6843(9) 90, 96.708(5), 90 8049.8(7), 2 2.567 Mg/m3 4.738 mm−1 5892 1.071 R1 = 0.0898, wR2 = 0.2446 R1 = 0.1023, wR2 = 0.2698

3 C39H98Mo24N6O134P16Pb2Mn4 6227.46 0.71073 Å monoclinic C2/c 22.4440(7) 21.9632(6) 16.6794(5) 90, 96.790(3), 90 8164.3(4), 2 2.529 Mg/m3 4.395 mm−1 5896 1.055 R1 = 0.0413, wR2 = 0.1064 R1 = 0.0486, wR2 = 0.1117

R1 = ∑∥F0| − |Fc∥/∑|F0|; wR2 = {∑[w(F02 − Fc2)2]/∑[w(F02)2]}1/2.

a

corrected by anisotropy. The crystal data of compounds 1−3 are shown in Table 1. Preparation of Carbon Paste Electrodes Which Doped by Crystals 1−3. The carbon paste electrodes (CPEs) which modified by crystals 1−3 (named CPE-1, -2, and -3) were made by the following process: 10 mg of crystal 1 (or 2 or 3) and 100 mg of graphite powder were mixed in agate mortar, carefully grinding to make them uniform. After two drops of liquid paraffin was added, the mixture was filled into the inner diameter of 3 mm plastic tube with that 2 mm diameter copper wire was inserted from the other end. The compacted fillers were put in shaded place for about 3 days. After the liquid paraffin was naturally evaporated, the electrode surface was gently polished with weighing paper. CPE-2 and CPE-3 were made by same method except that compound 1 was replaced by compounds 2 and 3, respectively. Experiment of Cr(VI) Reduction Reaction. The redox reaction of K2Cr2O7 and HCOOH catalyzed by compounds 1−3 were conducted in 80 mL beakers. The chromate-containing wastewater was prepared by 500.0 mL of water and 53.0 mg of K2Cr2O7. Before each catalytic experiment, the prepared solution (50.0 mL) was stirred at room temperature for 1 h to make the static saturation sorption of solid catalyst, then added 0.5 mL HCOOH as reduction. A sample of 3.0 mL was taken to test UV spectrum at each 0.5 h. The initial concentrations of K2Cr2O7 and HCOOH are 3.55 × 10−4 M and 3.83 × 10−1 M, respectively.

showed that crystal 1 is more efficient than the others, nearly all of CrVI ion can be reduced in mere 120 min under water bath temperature of 55 °C. Combined with our previous research, the inner reaction mechanism can be discussed and proved.

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EXPERIMENTAL SECTION

Materials and Measurements. All chemical reagents were using without any second purification and obtained through commercial channels. UV−vis spectra were measured with U3010 UV−visible spectrophotometer (Shimadzu). Thermogravimetric (TG) analyses were carried out with a PerkinElmer Pyris Diamond TG/DTA Instruments. Powder X-ray diffractions (XRD) were carried out on Bruker AXS D8 Advance diffractometer. Cyclic voltammetry (CV) were performed using CHI660e Electrochemical workstation (Shanghai Chenhua). Energy-dispersive spectroscopy (EDS) was measured by a cold field-emission scanning electron microscope (S4800). Syntheses of 1−3. Compound 1 was synthesized by mixing the following materials and stirring for about 0.5 h at room temperature. They were FeCl2·4H2O (0.08 g, 0.38 mmol), bpp (0.03 g, 0.15 mmol), PbCl2 (0.05 g, 0.18 mmol), Na2MoO4·2H2O (0.24 g, 0.99 mmol), H3PO4 (0.5 mL, 7.50 mmol), and H2O (8 mL, 0.44 mol). After the pH was modulated to 1.2 by using H3PO4, the solution was putted into a Teflon-lined autoclave (20 mL) and kept at 160 °C for 5 days. Through cooling with the rate of 8 °C/h, the wine rodlike crystals were filtered, washed by distilled water, and dried at a cool dry place (yield: 48.6% based on Mo). Elemental anal. calcd for C39H94Mo24N6O132P16Pb2Fe4 (%): C, 7.56; H, 1.53; N, 1.36. Found (%): C, 7.57; H, 1.53; N, 1.36. The syntheses of compounds 2 and 3 were similar to those of 1, except that the raw material FeCl2·4H2O was replaced by ZnO (0.05 g, 0.614 mmol) or MnCl2·4H2O (0.08 g, 0.385 mmol). After the same procedure, black crystals were obtained (yield: 34.9% and 23.7% based on Mo, respectively). Elemental anal. calcd for C39H94Mo24N6O132P16Pb2Zn4 (%): C, 7.53; H, 1.52; N, 1.35. Found (%): C, 7.55; H, 1.51; N, 1.34. Elemental anal. calcd for C39H98Mo24N6O134P16Pb2Mn4 (%): C, 7.52; H, 1.52; N, 1.35. Found (%): C, 7.54; H, 1.50; N, 1.34. X-ray Crystallography. The block crystal was selected and adhered to the top of the glass fiber, measured on a sample table. The crystal diffraction point data were collected at the temperature of 296(2) K by using a ray of Mo K ray (λ = 0.71073 Å). The crystal structure data were analyzed by SHELEXL program, modified by the full matrix least-square method, and all non-hydrogen atoms were

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RESULTS AND DISCUSSION Synthesis. All Mo in the {P4Mo6} polyanionic cluster are +5 oxidation states, which is different from the raw material Na2MoO4 and the other classical POMs. Thus, to say, {P4Mo6}-type cluster is unique to the larger POM family. The reaction conditions are important in the processes of synthesis, and all experimental parameters have to get optimized. The synthetic methods, raw material ratios, and reaction parameters for compounds 1−3 were nearly same, but the raw material of the center metal M was changed. Singlecrystal structural analysis indicated that these compounds have similar structural features. This work clearly demonstrated that the reciprocal replacement of components is possible in metal−oxygen cluster. Phosphoric acid not only provides enough phosphorus in synthesis, but also modulates the pH to its optimal value. bpp also plays two kinds of roles: one acts as reducing agent to transform all Mo(VI) into reduced states Mo(V); another acts as the counterbalance cation to stabilize the crystal structure. The reason to use ZnO as Zn source is B

DOI: 10.1021/acs.inorgchem.8b01321 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry because that the water absorption of zinc chloride is very strong, which is difficult for accurate weighing. The simplified synthetic route can be seen in Scheme 1. Scheme 1. Schematic Diagram of Synthetic Methods of Compounds 1−3

Structure Analysis. Crystal analytical data revealed that compounds 1−3 are of homomorphism each other and have the similar structure features. The hourglass-like metal−oxygen clusters, {PbM[P4Mo6O31]2}n−, are connected and arranged in the same method. The difference is in the central metal atom M (Fe in 1, Zn in 2 and Mn in 3). The basic unit [P4Mo6O31]12− is formed by six {MoO6} octahedra and four {HnPO4} tetrahedra via corner- and edge-shared modes. A planar six-element annular {Mo6} structure is constructed by six {MoO6} octahedra in with the Mo···Mo average distances of 2.592 or 3.505 Å. One {PO4} occupies the central site of {Mo6} ring, and the other three {PO4} are distributed evenly around the ring (see Figure 1a). The Mo−O bond lengths vary from 1.665(7) to 2.281(5) Å; the P−O bond lengths vary from 1.507(5) to 1.589(9) Å. The corresponding bond angles of O−P−O range from 106.6(3) to 112.1(3)°. The bridged metal M is six-coordinated by six O donors from two {P4Mo6} subunits, forming a quasi-hourglass structure with the two half units around it (see Figures. 1b,c). The bond valence sums (BVSs) of compounds 1−3 show that the valence states of all P and Mo atom are +5, and the central atom M is the +2 oxidation state (Table S1−S8). One homologues analog was once reported by our group in 2015,21 in which the {Mn(P4Mo6O31)2} clusters were linked by the double {Mn2} cationic fragments into 1D chain-like arrangement (Figure 2a). The clear difference between compounds 1−3 and the above structure is that one manganese atom of {Mn2} cationic fragments is replaced by one lead atom, which changes the {Mn2} to the {PbMn} miscellaneous structure (Figure 2b). The inorganic {Mn(P4Mo6O31)2} clusters were also changed into a 2D layer-like structure due to the linkages of {PbMn} fragments (Figure 2c). In the double metal {PbMn} group, the Pb(1) atom is linked by 6 oxygen atoms (two O(2), two O(5), and two O(16) from two-sided {Mn(P4Mo6O31)2} clusters), and the distances of

Figure 2. (a) Reported {Mn2[Mn(P4Mo6O31)]2} structure; (b) ORTEP view showing the {PbM[M(P4Mo6O31)]2} cluster in compounds 1−3; (c) polyhedral diagram along in the ab plane showing the different connections of M(2) between adjacent four polyanionic units; (d) 2D structure showed the repeated arrangement of {PbM[M(P4Mo6O31)]2} in compounds 1−3.

Pb−O between them are 2.515(5), 2.423(4) and 2.804(4) Å, respectively. The Pb atoms undertake the responsibility to bridge polyanionic clusters into a twisted 1D inorganic structure. Owing to the bigger size of Pb atom than the Mn in {Mn2} unit, M was pressed out the original 1D rank. In the coordination environment of M(2) octahedron, four equatorial oxygen atoms respectively come from four {Mn(P4Mo6O31)2} clusters (Figure 2c). The remaining two vertexes are occupied by two water ligands. Therefore, metal M(2) atoms link {Mn(P4Mo6O31)2} clusters into a 2D inorganic layer-like structure. The bpp cations and crystal water molecules accommodate the voids among inorganic layers, and have complex hydrogen bonding interactions with polyanions (Figure 2d). For clarity, the {M(P4Mo6)2}n− and [PbM(H2O)2]4+ can be set as two kinds of topological nodes A and B. As seen in Figure 3a, the twisted 1D chains are drawn in red, and the linkages between the chains are identified in yellow. The adjacent purple and yellow spheres are arranged alternately so that the twisted 1D chains are connected at the same intervals to form a 3D-wave network structure (Figure 3b). Organic

Figure 1. (a) Clear ball−stick structure of {P4Mo6O31} half unit; (b) mixed ball−stick and polyhedral view of {M(P4Mo6O31)2} subunit; (c) hourglass-type structure, and all oxygen atoms are omitted for clarity. C

DOI: 10.1021/acs.inorgchem.8b01321 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. (a) 2D structure showing the linking methods between adjacent one-dimensional chains (all O atoms and part P atoms are omitted to facilitate observation) and its simplified topological scheme; (b) 3D topological structure of inorganic moiety in compounds 1−3; (c) space-filling view along the b axis showing the stacking manner of inorganic and organic moieties.

Figure 4. (a) IR spectra and (b) X-ray diffraction patterns of compounds 1−3.

Thermogravimetric analysis of compounds 1−3 were measured from 25 to 800 °C in N2 atmosphere. Results showed that this kind of hybrids contains two weight loss steps: loss of crystalline water and the partial organic molecules were lost before 300 °C; and the others bpp were lost after 300 °C (Figure S2). The weight loss of compound 1 between 25 and 549 °C was 9.89% (theoretical value: 11.95%), compounds 2 and 3 in this range were 10.76 and 12.16%, respectively (theoretical value: 12.03 and 12.55%). All their experimental values were below their theoretical values, indicating that organic bpp in the crystal has not been completely lost in the temperature range. Cyclic Voltammograms of CPEs 1−3. In order to explore the intrinsic mechanism of the reaction, the cyclic voltammograms of 1−3 are tested and discussed. The threeelectrode system was composed of: modified CPE as working electrode, platinum wire electrode as counter electrode, and Ag/AgCl electrode reference electrode. The electrolyte is 1 M H2SO4, and the test range was from −200 to 500 mV. As shown in Figure S3, it can be found that all of these compounds exhibit 3 groups of redox peaks (labeled as I−I′, II−II′, and III−III′). The positions of redox peaks III−III′, II− II′, and I−I′ in 1−3 are basically consistent with each other and correspond to electron gain or loss in the oxidation− reduction reactions of Mo element from small to large

H2bpp cations accommodate the regular holes, and the inorganic−organic hybrid structures are formed by electrostatic force, which can effectively reduce the solubility of polyanionic clusters in water and improve their structural stability (see Figure 3c). FTIR, XRD, and TG Analyses. The IR spectra of compounds 1−3 were measured from 400 to 4000 cm−1, as shown in Figure 4a, three compounds shows the similar characteristic absorption bands, which means they contain the same chemical bonds or groups. The characteristic bands between 680 and 750 cm−1 are ascribed to ν(Mo−O−Mo) vibrations; peaks around 960 cm−1 are attributed to ν(Mo−Ot) vibrations. The ranges of ν(P−O) are about 1050 cm−1, and the strong peaks at 1506 and 1635 cm−1 are assigned to the ν(C−C) and ν(C−N) vibrations of organic bpp. The broad bands at 3200 to 3700 cm−1 are ν(C−H) and ν(O−H) vibrations, which also mean that there are complex hydrogenbonding interactions contained in these compounds. The above characteristic absorption peaks are fully consistent with crystal structures. Besides, XRD data were shown in Figure 4b. Compounds 1−3 have strong diffraction peaks in the angel range of 2θ = 7−8°, it indicates that they have similar structural features. The calculated and experimental XRD contrast data of compounds 1−3 can be seen in Figure S1. D

DOI: 10.1021/acs.inorgchem.8b01321 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. Successive UV−vis spectra of CrVI-containing solution catalyzed by crystals 1−3, and the change of residual ratio with reactive time.

Figure 6. Conversions of Cr(VI) catalyzed by crystal 1 with (a) 55 °C but different dosages of catalyst; (b) 50 mg of catalyst but different temperature.

number.20 Taking the redox potentials of peaks II−II′ of compounds 1−3, we plot charts with the corresponding scan rates. Results showed that it has a linear relationship between scanning velocity and peak intensity, and there was no obvious change in peak positions during 50−170 mV s−1, which indicated that the redox process was stable and reversible, and the rate-determining step of the redox reaction was the adsorption step. Catalytic Properties of Compounds 1−3. In order to compare with the previous experimental data,29 the default experimental conditions were fixed in 50 mL of system solution, 20 mg of crystal as catalyst, and water bath temperature of 55 °C. Results showed that 1 was more effective than 2 or 3 to catalytic reduction of CrVI. The reductive percentage was 60%, but compounds 2 and 3 are only 18 and 19% in 180 min (Figure 5), respectively. Compounds 1−3 crystallized in the same structures and had the similar components, and the only difference was in the central atom M, indicating that the bridged metal M plays an important role in regulating catalytic performance of crystal

materials. Here 1 was taken as an example to optimize the parameters of catalysis reaction. It was shown in Figure 6a that with the increasing dosage of crystals from 20 to 50 mg, the redox reaction rates were also enhanced obviously. When the dosage was 50 mg, nearly all CrVI could be reduced in the short time of 120 min. If it was continually increased to 60 mg, then the effect was not further improved accordingly. In contrast, the reaction rate became a little slower than that of 50 mg of crystal. A possible reason was that the interactions among solid crystal particles made them easily agglomerated when the excessive catalysts were put in reaction system, which decreased the contact area between catalysts and reactants (Figure 6a), and the successive UV−vis spectra in Figure 6 can be seen in Figures S4−S5. On the basis of 50 mg catalyst, the influence of the experimental temperature on the conversion of CrVI was further investigated. We carried out four water bath heating experiments with the different temperatures. As seen in Figure 6b. The data showed that the parameter of 55 °C should be the optimal temperature in the reaction time of 120 min. When the reaction temperature was kept at 65 °C, it must be pointed out that E

DOI: 10.1021/acs.inorgchem.8b01321 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 7. (a) Recycling tests used crystal 1 as catalyst for the reduction of CrVI under same conditions; (b) XRD patterns of crystal 1 after recycling experiments, respectively.

Figure 8. (a) Pseudo-first-order reaction fitting lines of the conversions of CrVI under different temperatures with crystal 1 as catalyst. (b) Control experiments showing the influence on the conversions of Cr(VI) catalyzed by 1 with the different adding order of reagents.

follows pseudo-first-order kinetics in the presence of excess formic acid, the overall catalytic reaction could be described as 6 steps, and the rate-determining step is the surface reaction step (the adsorbed HCOOH was oxidized on the surface of POM, and translated into the adsorbed CO2 and H2O, the POM is reduced to POM− at the same time).29 As shown in Figure 8a, by fitting the CrVI conversions catalyzed by crystal 1 to reaction time at different temperatures, good fitting results can be obtained, proving that the catalytic redox reaction used crystal 1 as catalyst fully conforms to this mechanism. The order to add reagents also had an effect on the reaction rate. As shown in Figures 8b and S9, by stirring 1 or HCOOH in K2Cr2O7 solution for 1 h, then adding HCOOH or crystal 1, leads to the similar effect on the redox reaction. However, if stirring 1 in HCOOH−H2O solution for 1 h, then adding solid K2Cr2O7 to prepare a solution with similar concentration, the reductive rate was obviously slower than the above two experimental conditions. Thus, the reaction mechanism may be describe as following 7 steps (Scheme 2), and the absorbed steps 1−4 are important in reactions. A comparative analysis found that the 3D structure based on {M[P4Mo6 O31]2} 22− clusters by double-bridged [PbM(H2O)2]4+ subunits linked into became more stable. This change will not only affect the catalytic performance of crystal catalyst to a certain extent but also effectively reduce the agglomeration effect. It can be seen in Table 2, the reduction ability of compound 1 is weaker than that of the pure bpp{Fe[P4Mo6]2}n− under the same experimental conditions. However, due to the weak agglomeration effect, the catalytic performance of 1 is still increasing even when the dosages is increased to 50 mg, which makes all CrVI can be reduced in a short time of 2 h. It is also interesting to note that the

the rate of reduction was faster in the beginning 90 min than those low temperatures; however, the phenomenon of crystal dissolution occurred at the same time, in which the catalysts were lost in solution and could not be recovered and reused. In order to explore the recoverable utilization of catalyst 1, the repeated tests were carried out under the same experimental parameters. After each experiment, the solution was completely recovered, filtered, and washed to collect the remaining solids. Figure 7a showed that the catalytic reduction of Cr(VI) was still maintained ca. 96% in cycle 5, meaning that the performance of catalyst are highly reproducible (UV−vis and IR spectra can be seen in Figure S6). Therefore, the stability of catalyst can be improved by introducing Pb into the connecting unit. By using IR and XRD to determine whether catalyst 1 was changed after catalysis, the results were presented in Figures 7b and S5, the characteristic diffraction and absorption peaks of 1 can be mapped one-to-one in the graph, illustrating that crystal structures have not obvious change. Besides, EDS data for crystals after cycles 1−5 (Figure S7 and Table S7) showed that no Cr element was attached on the surface of solid crystal particles even after 5 cycles. On the basis of the above research, the optimal conditions were set as 50 mg of crystal as catalyst and a water bath temperature of 55 °C. Furthermore, the catalytic performances of compounds 2 and 3 were also tested in this condition. As be seen in Figure S8, the results showed that both of them got enhancements to some degree. When increased the dosages of crystals 2 or 3 from 20 to 50 mg, the reductive percentages were enhanced from 18 and 19% to 40 and 54% in 180 min, respectively. Discussion on the Catalysis Reaction. The reduction reaction for Cr(VI) to Cr(III) by using {P4Mo6} as catalyst F

DOI: 10.1021/acs.inorgchem.8b01321 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Accession Codes

Scheme 2. Proposed Mechanism for the POM-Mediated Reduction of Cr(VI) and HCOOH (HA)a

CCDC 1842803−1842805 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Corresponding Author

(1) HA + CrVI → X (HCOOH and CrVI are combined into X). (2) X + * → X* (X is adsorbed onto the surface of 1). (3) HA + * → HA* (HCOOH is transferred into intermediate). (4) CrVI + * → CrVI* (CrVI is transferred into intermediate). (5) HA* + POM → CO2 + POM− (surface reaction, the rate-determining step)

a

*E-mail: [email protected]; [email protected]. ORCID

Zhangang Han: 0000-0002-9758-6735 Notes

The authors declare no competing financial interest.

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Table 2. Conversions of Reduce Cr(VI) to Cr(III) by Using Different Materials As Catalysts at 55 °C catalyst (mg)

conversion

catalyst (mg)

conversion

none 1 (20) 2 (20) 3 (20) 1 (50) 2 (50) 3 (50)

0% 60% 18% 19% 99% (in 2 h) 40% (in 3 h) 54% (in 3 h)

MoO3 (20) bpp (20) PbNi (20)22 PbCd (20)22 Fe[P4Mo6]2 (20)29 Mn[P4Mo6]2 (20)29 Zn[P4Mo6]2 (20)29

0% 0% 29% 19% 83.8% 0% 0%

ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (Grant 21871076), and the Hebei Natural Science Foundation of China (Grants B2016205051 and B2015205116).

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reduction ability of {M[P4Mo6]2} with Zn or Mn as the central elements is also changed slightly after being modified by lead and can reach 54 or 40% respectively at the dosage of 50 mg.

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CONCLUSIONS Three cases of supramolecular phosphomolybdates were synthesized by hydrothermal synthesis. The difference in the structural features of 1−3 only lies in the replacement of different central transition metals, which is beneficial to explore their catalysis reaction mechanism at a molecular level. Crystal 1 showed the excellent catalytic activity for CrVI reduction at 55 °C used HCOOH as a reductant, suggesting that the catalytic activity of crystal materials is closely related to their central metals. In addition, it is found that the isolated {M[P4Mo6O31]2}22− anionic units can be connected by [PbM(H2O)2]4+ cationic fragments into a more stable 3D structure, which can effectively reduce the agglomeration effect of solid particles. Of course, the introduction of lead will inevitably lead to a secondary pollution, which is also a problem that needs to be considered and addressed. Although it can be coprecipitated with reduced trivalent chromium ion by adjusting pH, it is better to find a new group that can play the same connecting role. This work is significant for the design and synthesis of crystal catalyst. Our group is also currently undertaking further work to design and synthesis compounds with desired structures and higher catalytic activity.

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AUTHOR INFORMATION

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01321. CV, TGA, and XRD curves (PDF) G

DOI: 10.1021/acs.inorgchem.8b01321 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.8b01321 Inorg. Chem. XXXX, XXX, XXX−XXX