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A new metallate phase of V2O5 crystalline microstructure achieved in a facile route: Synthesis, characterization, and measurement in catalytic reactions
Journal of Colloid and Interface Science 438 (2015) 122–129
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
A new metallate phase of V2O5 crystalline microstructure achieved in a facile route: Synthesis, characterization, and measurement in catalytic reactions Xiao-Xi Zhang a, Ji-Xiao Wang a, Na Xing a, Xi-Tong Ma a, Xiao-Dong Feng a, Yong-Heng Xing a,⇑, Zhan Shi b a b
College of Chemistry and Chemical Engineering, Liaoning Normal University, Huanghe Road 850#, Dalian City 116029, PR China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China
a r t i c l e
i n f o
Article history: Received 28 May 2014 Accepted 14 September 2014 Available online 13 October 2014 Keywords: Vanadium pentoxide Solution method Bromination catalytic reaction Catalytic oxidation
a b s t r a c t Experiencing a series of complicated changes, abundant orange crystals of novel metallic phase of vanadium pentoxide were obtained by a mild chemical method, the formula of which is defined as [V3(l3-O)2(l1-OH)O5]H2O. Differ from the synthesis methods of vanadium oxide published, we have adopted a simple solution method that mixed starting materials are refluxing in the system of ethanol–water under a relatively lower temperature. Symmetry of the crystals is Monoclinic, with cell unit dimensions: a = 4.9978(10) Å, b = 8.4273(17) Å, c = 7.8669(16) Å, b = 96.44(3)° and space group of P21/m. The structure of the complex was characterized by elemental analysis, IR, UV–vis spectroscopy and single-crystal diffraction analysis. Powder X-ray diffraction (PXRD) was used to detect the purity of the crystals, and crystal morphology was detected by the scanning electron microscope (SEM). In addition, in order to extend application of oxidovanadium complexes, bromination catalytic activity about the complex in a single-pot reaction of the conversion of phenol red to bromophenol blue in a mixed solution of H2O–DMF at a constant temperature of 30 ± 0.5 °C with a buffer solution of NaH2PO4ANa2HPO4 (pH = 5.8) was evaluated firstly, indicating that the complex can be considered as a potential functional model of bromoperoxidase, in the meantime, we have conducted the bromination catalytic reaction to simulate and measure the changes in reaction process indirectly. Besides, catalytic oxidation activity of the complex is also evaluated in the oxidation of cyclohexane (Cy) and cyclopentane with hydrogen peroxide promoted under mild conditions, showing potential catalytic activity of the complex by comparing TON (total turnover number) ratios of CyO/CyOH (CyO is the abbreviation of cyclohexanone and CyOH represents cyclohexanol) in the oxidation results. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction In the family of 3d transition metal complexes, vanadium oxides have received considerable attentions owing to their outstanding properties in physical and chemical research field [1–3], for example: electrical, optical, magnetic and environmental area [4–16]. Numerous researchers have also studied their behaviors when the oxides were designed as cathode material for rechargeable Li+ battery [17], or have reported their excellent performance as catalysts in oxidation reactions [18–20] to further drive for practical capacities of vanadium oxides. It is to be noted that the interest in catalytic oxidation carrying on with the presence of metallic vanadium complex as catalyst has increased recently [21,22]. ⇑ Corresponding author. E-mail address: [email protected] (Y.-H. Xing). http://dx.doi.org/10.1016/j.jcis.2014.09.032 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
By checking the literatures [23], there are mainly two factors influencing the catalytic performance of vanadium oxides, one is the oxidation states of the vanadium ion, and the other is the configuration of the molecule. As far as we know, vanadium usually adopts valence states from +3 to +5 in the complexes; different valence state and various coordination environment determine the geometry of vanadium oxides, by sharing corners, edges and faces, leading to variable structural configurations of vanadium oxides (tetrahedron, trigonal bipyramid, square pyramid, regular octahedron, and distorted octahedron), which is the source of abundant catalytic agents. Until now, several kinds of geometries of V O systems have been reported [24–33], characters of molecular structures, natures of the complexes and meaningful studies of the properties are involved among those researches. Vanadium Oxides have been prepared by a variety of synthetic ways in amorphous or crystalline forms [34–37]; for example, Popuri et al. [38]
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have provided a rapid way to obtain VO2 using a hydrothermal process with V2O5 and citric acid as precursors; Wu et al. [30] used an available pathway to accomplish paramontroseite VO2 by a simple chemical reaction of sodium orthovanadate (Na3VO412H2O) and thioacetamide (TAA). Besides, V2O5 is also a widely studied hotspot topic, since it is a common material for synthesis and catalysis system. Vanadium oxides possess different elemental composition forms, and various coordination polyhedra, which build up the tremendous structural diversity of vanadium oxides and bring the unique electrochemical and optical properties of vanadium oxides [39–42]. Traditional synthetic approach for vanadium oxides is hardly to reach, high cost and strict experimental conditions, sometimes high temperature and means of calcination are needed. While in our work, we have easily achieved abundant crystals of vanadium pentoxide in a mild condition refluxing in water–alcohol system at a relatively lower temperature, the method is facile and environmental. To our knowledge, it is found that molecular structure of the as-prepared complex and synthetic method has not been reported so far. Here, the new vanadium oxide is characterized by elemental analysis, IR spectra, UV–vis spectroscopy, single-crystal X-ray diffraction, SEM and Powder X-ray diffraction. In order to extend application of the vanadium oxide, we tested firstly for its catalytic activity both in bromination catalytic system with phenol red as organic substrate and in catalytic oxidation with hydrogen peroxide promoted. The results indicate that the complex can be considered as a potential functional model of bromoperoxidase (V-BrPO), and a promising catalytic agent used for obtaining clean energy source. In the meantime, the transformation mechanism from NH4VO3 to vanadium pentoxide is proposed; furthermore, we make full use of catalytic bromination reaction to track the species in solution during the synthetic reaction in situ. 2. Experimental section 2.1. Materials and methods All the other chemicals used were of analytical grade and without further purification. Elemental analyses for C, H and N were carried out on a Perkin Elmer 240C automatic analyzer, and content of V was measured by a Plasma-Spec(I)-AES model ICP spectrometer. Infrared spectra were recorded on a JASCO FT/IR-480 spectrometer in the range of 200–4000 cm 1 with pressed KBr pellets. UV–vis absorption spectra were recorded on a JASCO V-570 spectrometer (200–2500 nm, in the form of a solid sample). X-ray powder diffraction (PXRD) patterns were obtained on a Bruker Advance-D8 equipped with Cu Ka radiation, in the range 5° < 2h < 60°, with a step size of 0.02° (2h) and an count time of 2 s per step. The SEM samples were performed with a SU8000 field emission scanning electron microscope. A 30% aqueous solution of hydrogen peroxide was used as primary oxidant in the cyclohexane oxidation reaction. The products of cyclohexane (Cy) oxidation were analyzed by a GC-9790 series gas chromatograph equipped with a flame ionization detector (FID) and a capillary column (PG2000, column length: 30 m; internal diameter: 0.25 mm).
the solution was transferred into pyxides with refluxing for 3 h at a temperature of 80 °C. The orange bulk crystals were obtained in ca. 68% yield based on V(V). Anal. calc. for H3O9V3: H, 1.00; V, 51.00. Found: H, 0.70; V, 50.17%. IR (KBr, m, cm 1): 3412, m(OAH); 1629, 1418, d(OAH); 1007, m(V@O); 625, ms(VAOAV); 475, d(VAOAV). UV–vis (kmax, nm): 406(LMCT), 790(d–d⁄). 2.3. X-ray single crystal structural determination The crystal was mounted on glass fibers for X-ray measurement. Reflection data were collected at room temperature on a Bruker AXS SMART APEX II CCD diffractometer with graphite– monochromated Mo Ka radiation (k = 0.71073 Å) and a x scan mode. All the measured independent reflections (I > 2r(I)) were used in the structural analyses, and semi-empirical absorption corrections were applied using SADABS program [43]. The structure was solved by the direct method and refined using SHELXL-97 [44]. The non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms of the lattice water were found in the difference Fourier map. The hydrogen atom of hydroxyl group was not located. Crystal data, details of the data collection and the structure refinement of complex are given in Table 1, the parameters of selected bond lengths, bond angles and hydrogen bonds are listed in Table 2. Figs. S1 and S2 present the infrared spectra and UV–vis spectra of the as-prepared complex, respectively. 2.4. Measurement of the bromination activity in solution Bromination reaction activity test for the product was carried out in a mixed solution of H2OADMF at a constant temperature of 30 ± 0.5 °C. The complex was dissolved in the addition of 25 mL H2OADMF mixture (DMF: 1 mL; H2O: 24 mL). The solutions used for the kinetic measurements were maintained at a constant concentration of H+ (pH 5.8) by the addition a buffer solution of NaH2PO4ANa2HPO4 [45]. The reactions were conducted in the addition of a phenol red solution. Solutions of the complex with five different concentrations were confected in five different
Table 1 Crystallographic data for the complex.*
2.2. Preparation of [V3(l3-O)2(l1-OH)O5]H2O NH4VO3 (0.1 mmol, 0.0117 g), triflusal (0.05 mmol, 0.0124 g) and some carboxylic acids as auxiliary ligands (1,3,5-Benzenetricarboxylic acid, 1,2,4,5-Benzenetetracarboxylic acid, salicylic acid, benzenesulfonic acid, malic acid, aminoacetic acid and so on; 0.1 mmol) were dissolved in 15 mL CH3CH2OHAH2O system (the ratio of volume is 2:1), instantaneously giving an opaque orange solution which was stirred at room temperature for 3 h. Then,
Formula
H3O9V3
M (g mol 1) Crystal system Space group a (Å) b (Å) c (Å) a (deg) b (deg) c (deg) V (Å3) Z Dcalc (g cm 3) Crystal size (mm) F(0 0 0) l(Mo Ka)/mm 1 Reflections collected Independent reflections (I > 2r(I)) Rint Parameters D(q) (e Å 3) Goodness of fit h (°) Ra wR2a
299.84 Monoclinic P2(1)/m 4.9978(10) 8.4273(17) 7.8669(16) 90 96.44(3) 90 329.24(12) 2 3.025 0.28 0.17 0.11 288 4.188 3200 804 0.0417 62 0.525, 0.633 1.166 3.55–27.44 0.0272(0.0290)b 0.0750(0.0787)b
*a b
R = R||Fo| |Fc||/R|Fo|, wR2 = [R(w(Fo2 Based on all data.
Fc2)2/R(w(Fo2)2)]1/2; [Fo > 4r(Fo)].
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Table 2 parameters about selected bond for the complex. Bond lengths (Å) V1–O1 V2–O3
1.602(2) 1.8358(19)
V1–O2 V2–O4
1.7404(19) 2.284(3)
V1–O3 V2–O5
2.003(2) 1.600(3)
Bond angles (°) O1–V1–O2 V1–O3–V2 O5–V2–O4
106.28(10) 108.17(10) 174.22(12)
O1–V1–O3 O4–V2–O3
111.19(9) 73.68(7)
O2–V1–O3 O5–V2–O3
142.21(9) 102.67(9)
Hydrogen bonds (Å) D–H A O1W–H1WA O5 O1W–H1WB O1
d(D–H)/Å 0.85 0.85
d(H A)/Å 2.31 2.20
d(D A)/Å 2.924(5) 2.958(4)
\D–H A/° 129 149
cuvettes and then the cuvettes were placed in a constant temperature water bath for 5 min. And the changes of spectral data were recorded using a 721 UV–vis spectrophotometer every 5 min. Finally, the resulting data during the reaction were collected and fitted with the help of the curve-fitting software in the Microsoft Excel program. In addition, the synthetic reaction for the complex was refluxed in a round-bottom flask convenient for sampling 0.3 mL at a time and every 10 min was taken as an interval, the sample solution was diluted into 5 mL (DMF 0.5 mL and H2O 4.2 mL for addition) and sealed in a dark and 4 °C condition, immediately. The solutions extracted from the reaction system were also tested in the bromination catalytic reaction as above with a fixed concentration, and the samples must be detected as soon as possible. The bromination of phenol red was monitored by the measurement of the absorbance at 592 nm for reaction aliquots which were extracted at specific time points and diluted into a phosphate buffer with pH = 5.8. It is hypothesized that the rate of this reaction can be described by the rate equation of ‘‘dc/dt = kc1xc2yc3z’’, evolving from that the equation ‘‘log(dc/dt) = log k + xlog c1 + ylog c2 + zlog c3’’ is calculated, and which is corresponding to ‘‘ log(dc/dt) = xlog c1 – b (b = log k + ylog c2 + zlog c3)’’, thereinto, k is the reaction rate constant; c1, c2 and c3 represent the concentrations of the complex, KBr and phenol red, respectively; while x, y, z are the corresponding reaction orders. According to Lambert–Beer’s law, A = e d c, which is differential, dA/dt = e d(dc/dt), where A is the measurable absorbance of the resultant; e is molar absorption coefficient, which of bromophenol blue is measured as 14,500 M 1 cm 1 at 592 nm; d is the light path length of sample cell (d = 1). When the measurable absorbance data were plotted versus reaction time, a line was obtained and the reaction rate of the complex (dA/dt) was received by the slope of this line. By changing the concentration of the complex in the reaction system, a series of dA/dt data were tracked. The reaction rate constant (k) can be obtained according to a plot of log (dc/dt) versus log c1 and was fitted using the curve-fitting software in the program Microsoft Excel by generating a least squares fit to a general equation of the form ‘‘y = mx b’’, in which ‘‘m’’ is the reaction order of the complex [V3(l3-O)2(l1-OH)O5]H2O in this reaction and ‘‘b’’ is the intercept of the line. In the experiment, considering that the reaction orders of KBr and phenol red (y and z) are 1 in the light of the literatures [46]; c2 and c3 are known as 0.4 and 10 4 mol/L, respectively. Based on the equation of ‘‘b = log k + ylog c2 + zlog c3’’, we can compute and get the reaction rate constant (k).
of acetonitrile was stirred under atmospheric pressure, the required amounts of n(Cy): n(catalyst)=10,000:1, n(H2O2): n(catalyst)= 15,000:1, n(HNO3): n(catalyst)= 2000:1. The reaction was terminated by adding 1.5 mL of diethyl ether. The extracted reaction mixture was analyzed by GC equipped detector by internal standard method with 0.03 g (3.26 10 4 mol) of methylbenzene as internal standard. The identification of the oxidation products was performed by comparison of their retention times with that of the commercial CyOH and CyO. 3. Results and discussion 3.1. Synthesis Our original intention was to synthesize some complexes with triflusal and other auxiliary carboxylic acid ligands. However, we have obtained some orange crystals of a new metallate phase of V2O5, which was characterized by element analysis, IR, UV–vis and X-ray. Whether the molecular configuration or the synthetic method we used is found to be different from literatures. In order to gain further insight into the effect of various experimental conditions on the crystal formation for the oxovanadium complex, we have conducted experiment to trap the species in the solution and found that the formation process of the complex is complicated. On the basis of experimental data, we have deduced the formation mechanism as below (illustrated in Scheme 1) [47]. In this work, the transformation procedure we deduced from NH4VO3 to product [V3(l3-O)2(l1-OH)O5]H2O is achieved through the changes of the species of vanadate in aqueous solution. Under the unique condition which is ethanol–water solution containing triflusal and other auxiliary ligands, the starting material NH4VO3 was added into the ethanol–water system and ionized (a), being activated by H+, which was further turned into a form of VO3 (b); and then, free water molecule attacked the electropositive V5+ center of VO3 as a nucleophile, forming an intermediate (c) by the combination of VO3 and H2O, that is a key reaction step as we assumed and identical to the result we captured in bromination catalytic reaction of phenol red; in short time, the transition intermediate (c) has turned to be a stable state [VO2(OH)2] as shown in Scheme 1(d); in solution, it easily occurs that two [VO2(OH)2] molecules collide to form a dimer, and two water molecules were eliminated in that procedure, V2O26 (f) was obtained at the same time; at last, after a dehydration reaction between O2 of V2O26 anion and H+, the conversion was accomplished.
2.5. Experimental set up for catalytic oxidation
3.2. Structural description and spectroscopy
The catalytic experiment was carried out in a sealed glass vial, placed in a water bath of 4 °C with magnetic stirring. In the experiment, the reaction solution mixed with 0.0004 g catalyst and 3 mL
The molecular structure of vanadium pentoxide is depicted in Fig. 1. The principal bond distances and angles for the complex are summarized in Table 2.
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Scheme 1. The transformation mechanism from NH4VO3 to product deduced in reaction solution.
Fig. 1. (a) The coordination structure of the complex; (b) the fragment of the 1D-chain structures; (c): the two-dimensional packing network of the complex (all H atoms expect for the hydrogen bonds are omitted for clarity). Symmetry transformation used to generate equivalent atoms for complex. #1: x, y, z; #2: x, 0.5 y, z; #3: 1 + x, 1 + y, z; #4: x, 1 + y, 1 + z.
X-ray single crystal analysis reveals that the complex is crystallized in the monoclinic system with P2(1)/m space group. An asymmetry unit for the complex [V3(l3-O)2(l1-OH)O5]H2O consists of one and a half vanadium atoms, one l3-O oxygen (O3), a half terminal hydroxyl oxygen atom (O5), two and a half terminal oxygen atoms (O1, O2, O4), and a half lattice water (O1W). Thereinto, O1W is found to be disordered with occupancies 0.5. The bond lengths of VAO are in the range of 1.601(3)–2.285(3) Å, the angles of OAVAO is in the range of 73.67(8)–174.02(12)° and the angle of VAOAV is 108.20(10)°, respectively. Moreover, the contact distance between V(1) and V(2) is 3.1106(8) Å [30]. In structure of the complex, V(1) atom is coordinated by two l3-O oxygen atoms (O3, O3#1, #1: x, y, z) and two terminal oxygen atoms (O1, O2) to form a geometry configuration of distorted tetrahedron; in the mean time, V(2) atom is coordinated by two l3-O oxygen atoms (O3, O3#2, #2: x, 0.5 y, z), one terminal hydroxyl oxygen atom (O5) and one terminal oxygen atom (O4) with the molecular configuration of a distorted tetrahedron (Fig. 1). Viewing the architecture of selfassembling, two V(1) atoms, two l3-O oxygen atoms and two terminal oxygen atoms are constructing a building block V2O6 and then forming a 1D-chain structure via a VO2 moiety. At last, by the hydrogen bond interaction between oxygen atoms coming from V2O5 moiety and water molecule, namely, O1W#4AH1WA#4 O5 (2.918(5) Å, 129, #4: x, 1 + y, 1 + z) and O1W#4AH1WB#4 O1#2 (2.956(4) Å, 149°, #2: x, 0.5 y, z; #4: x, 1 + y, 1 + z), the complex is connected to form a twodimensional connection network. From the IR spectra (Fig. S1) of the as-prepared vanadium pentoxide, broad and strong absorption appearing at 3412 cm 1
indicates the presence of the m(OAH) of the water molecule, and sharp absorption bands observed at 1629 cm–1, 1418 cm–1 are features of d(OAH) of water molecule. The vibration of V@O interaction is presented at 1007 cm–1. The peak at 625 cm–1 is the symmetric stretching vibrations of VAOAV, and the peak at 475 cm–1 can be assigned to the banding vibration of VAOAV. The electronic absorption spectra (Fig. S2) of the complex were recorded at room temperature in the form of the solid sample. The bands at 406 and 790 nm are assigned to d–d⁄ transitions of the central metal vanadium of the vanadium oxide. The powder X-ray diffraction data of the complex were obtained and compared with the simulated single-crystal diffraction data (Fig. 2 as below). No change was observed under the present temperature and duration conditions influencing, the phase of the complex is considered as purity owning to the agreement of the peak positions. The different intensity may be due to the preferred orientation of the powder sample. As depicted above, the peaks in PXRD support the message which was delivered from SEM images. Fig. 3 shows SEM images of [V3(l3-O)2(l1-OH)O5]H2O complex, the morphology of the crystalline microstructure is like a chock with its length approximately up to 100 lm, the width and the thickness of which are 50 lm and 40 lm, respectively. 3.3. Functional mimics of the vanadium haloperoxidases 3.3.1. Mimicking bromination reaction of the complex As is known to all that with the presence of H2O2 and bromide, oxidovanadium complexes can mimic a reaction in which
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Intensity
a b
Scheme 2. The reactive bromination process from phenol red to bromophenol blue.
10
20
30
40
50
2.8
2 θ (° )
592nm
vanadium haloperoxidases could catalyze the bromination reaction of organic substrates. For example, the bromination of trimethoxybenzene [48], benzene, salicylaldehyde and phenol as substrates by the VO2+ segment [49], and the bromination of phenol red by [VO(O2)H2O]+ and correlative species [50]. Here, we have investigated the bromination reaction activity of as-prepared vanadium pentoxide taking phenol red as an organic substrate, which is shown by the conversion of phenol red to bromophenol blue. The reaction is rapid and stoichiometric, producing the halogenated product by the reaction of oxidized halogen species with the organic substrate, and the reactive process is drawn in Scheme 2 [51,52]. The solution of vanadium pentoxide was added to the standard reaction of bromide in phosphate buffer with phenol red tracking for oxidized bromine, visible color change from yellow to blue of the solution can be observed during the experiment. As is shown in Fig. 4, a decrease in absorbance of the peak at 443 nm due to the loss of phenol red and an increase in the absorbance of the peak at 592 nm characteristic of the bromophenol blue recorded by the electronic absorption spectra, which shows that the complex is of comparative catalytic activity. What is interesting is that the peaks at 443 nm and 592 nm are splitted during the last stage of the bromination oxidation in the electronic absorption spectra, which is similar to the results reported [53,54]. 3.3.2. Kinetic studies of mimicking bromination reaction A series of dA/dt data for the complex were obtained (Fig. 5) to explore kinetic study of mimicking bromination reaction by changing the concentration. According to the data, the plot of log(dc/dt) versus –log c for the complex is depicted as a straight line with a slope of 1.02 and an intercept of 2.2629, as shown in Fig. 6. The value of slope confirmed that the reaction order is first-order reaction
Absorbance/A
2.4
Fig. 2. PXRD powder patterns: (a) the experimental PXRD for complex; (b) the simulated PXRD pattern calculated from single-crystal structure of complex.
2.0 1.6
443nm
1.2 0.8 0.4 0.0
400
500
600
Wavelength/nm Fig. 4. Oxidative bromination of phenol red catalyzed by product. Spectral changes at 10 min intervals. The reaction mixture contained phosphate buffer (pH 5.8), KBr (0.4 mol L 1), phenol red (10 4 mol L 1) and the complex (0.1 lmol L 1).
Fig. 5. The measurable absorbance dependence on time for the complex. Conditions used: pH = 5.8, c(KBr) = 0.4 mol/L, c(H2O2) = 1 mmol/L, c(phenol red) = 10 4 mol/L. c(complex /mmol/L) = a: 1.93 10 2; b: 3.87 10 2; c: 5.80 10 2; d: 1.16 10 1; e: 1.35 10 1.
dependence on vanadium. On account of the equation of ‘‘b = log k + ylog c2 + zlog c3’’, the reaction rate constant k is reckoned by the concentrations of KBr and phenol red (c2 and c3), the
Fig. 3. SEM photographs of complex [V3(l3-O)2(l1-OH)O5]H2O.
- log (dc/dt)
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- log c Fig. 6. log(dc/dt) dependence of –log c for the complex in DMFAH2O at 30 ± 0.5 °C (c is the concentration of the product [V3(l3-O)2(l1-OH)O5]H2O; Conditions used: c(phosphate buffer) = 50 mmol/L, pH = 5.8, c(KBr) = 0.4 mol/L, c(phenol red) = 10 4 mol/L.
reaction orders of KBr and phenol red (y and z), and the intercept of the straight line (b). In the experiment, the reaction orders of KBr and phenol red (y and z) are considered as 1 according to the literature [55,56], c2 and c3 are known as 0.4 mol/L and 10 4 mol/L, respectively; therefore, the reaction rate constant (k) for the complex can be calculated as 1.36 102 (mol/L) 2 s 1, the value of that is close to which of the traditional V2O5 through the data of contrastive experiment, so we have obtained a criterion to estimate the brominated catalytic activity for as-prepared vanadium pentoxide, the related data and curves in the presence of traditional V2O5 are drawn in Supplementary material (Figs. S3–S5). It is concluded from the upwards data and we found that the reaction order of the vanadium pentoxide in bromination reaction is close to 1, showing the first-order dependence on vanadium. Along with the increase of concentration of vanadium pentoxide, the catalytic rate is accelerated, correspondingly. The cycle catalytic brominated reaction mechanism for the complex is illustrated in Scheme 3. One V@O double bond of the vanadium pentoxide is attacked by H2O2, thereinto, H+ of H2O2 and electronegative O which belongs to vanadium pentoxide get combined easily, while O2H is linking with the center of vanadium ion, hence an intermediate is formed (step a); Subsequently, after a dehydration reaction, free radical of V2 O6 is achieved (step b); In the solution, Br is firstly oxidized by the V2 O6 radical and they get connected (step c); Then with the presence of H+, catalyst vanadium pentoxide is regained to participate in the next catalytic reaction circle, and the oxidant HOBr can be further applied to catalyze the organic substrate phenol red (step d) [53,54,57]. Anyway, catalytic reaction rate is mainly dependent on the production of intermediate radical species.
Scheme 3. The cycle catalytic brominated reaction mechanism.
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3.3.3. Synthetic reaction procedure detection by bromination reaction A series of data of absorbance dependence on time for the complex are achieved, all the trends of different time slice in catalytic bromination reaction are almost consistent, and the sample is extracted after 40 min of the synthetic reaction and illustrated in Fig. 7 for an example. When starting materials were just dissolving into the wateralcohol system, the first point was recorded, the absorbance of which was comparatively high and that presented ionizing course of NH4VO3 activated by H+ in the solution (point 1); after a period of stable time, the absorbance dropped sharply to reach a nadir drawn in the figure, we inferred that some organic species is attacking the V5+ center of the intermediate during the reaction, and the bromination reaction was affected by the factor of sterichindrance, resulting in a large fall in electronic absorption (point 4); followed with the next point, namely, the absorbance recorded in 40 min for the synthetic process, the trend of the curve went up substantially to a top peak because of the active transition intermediate produced at this time, so the catalytic activity was enhanced. At last, the levels of absorbance changed in a small range, the influence of ionic transformation does not work much on the catalytic bromination reaction. The series diversification of the absorbance in Fig. 7 is accordant to the transformation procedure we deduced from NH4VO3 to vanadium pentoxide in Scheme 1, based on condition controlled and instrument interfered, we have tracked the intermediate species during the synthetic reaction by the method of bromination catalytic study, and the result is practicable.
Fig. 7. The measurable absorbance dependence of time for the synthetic procedure detection of the complex in catalytic bromination reaction.
Fig. 8. The TON ratios of CyO/CyOH depending on the reaction time promoted by complex and standard material V2O5. Conditions: n(Cy):n(catalyst) = 10,000:1, n(H2O2):n(catalyst) = 15,000:1, n(HNO3):n(catalyst) = 2000:1, catalyst (0.0004 g), CH3CN (3 mL), 40 °C.
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o
Scheme 4. Main products observed during the catalytic oxidation of Cy.
3.3.4. Catalytic activity studies To compare the catalytic oxidative activities between the complex and V2O5 (the standard material), both of them are used as catalysts in the Cy oxidation with H2O2 as primary oxidant, when keeping the fixed molar ratio of Cy/catalyst (10,000:1), H2O2/catalyst (15,000:1), HNO3/catalyst (2000:1), and catalyst (0.0004 g) in 3 mL of acetonitrile at 40 °C. The main products are displayed in Scheme 4. The turnover number (TON) ratios of CyO/CyOH catalyzed by the complex and V2O5 depending on different reaction time are shown in Fig. 8. Compared to the standard material V2O5, the ratios of CyO/CyOH promoted by the complex are all higher in the whole reaction process, especially at 5 and 8 h, which are nearly twice and four times, respectively. This indicates that the complex we synthesized is superior to the standard material of V2O5 as catalysts applied in the Cy oxidation.
4. Conclusion In conclusion, a new metallic phase V2O5 is crystallized in a facile way in water- alcohol system at a temperature of 80 °C, its formula is defined as [V3(l3-O)2(l1-OH)O5]H2O by single-crystal diffraction analysis, elemental analysis, IR and UV–vis spectroscopy are also performed to further ensure the structure of the complex. PXRD characterization reveals that bulk of the complex is obtained in purity and SEM data indicates that the micro-particles of crystals are chock-like with an average length of about 100um. In order to study the properties of the complex, we have tested the catalytic activity of the complex in the bromination reaction with phenol red as organic substrate and oxidation of cyclohexane for the first time, the results certificate that the as-prepared vanadium pentoxide is a potential functional catalytic model, and it can be applied in further environmental catalytic research. Moreover, we have detected the species varieties in the synthetic reaction by the method of bromination reaction with phenol red, the data of absorbance recorded by electronic spectra deliver the message indirectly, and which matches our propose on formation mechanism. Acknowledgments We acknowledge Natural Science Foundation of China (No. 21371086); College of Chemistry, Jilin University, Changchun 130012, P.R. China (Grant No. 2013-05); and Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, P.R. China (Project No. 1210908-06-K) for financial assistance.
Appendix A. Supplementary material The infrared spectra and UV-vis spectra of the as-prepared complex are listed in Figs. S1 and S2, respectively. And the data and curves in the presence of traditional V2O5 are given in Supplemantary material (Figs. S3–S5).
Detail data of the complex has been deposited with the FIZ Karlsruhe ICSD database. Copies of this information may be obtained free of charge. Further details of the crystal structure investigations may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49 7247 808 666; e-mail: crysdata(at)fiz-karlsruhe(dot)de, http://www.fiz-karlsruhe. de/request_for_deposited_data.html) on quoting the deposition number CSD-427774 for the complex [V3(l3–O)2(l1– OH)O5]H2O. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.jcis.2014.09.032. References [1] V.E. Henrich, P.A. Cox, The Surface Science of Metal Oxides, Cambridge University Press, Cambridge, 1994. [2] J. Schoiswohl, M. Sock, S. Surnev, M.G. Ramsey, F.P. Netzer, G. Krasse, J.N. Andersen, Surf. Sci. 555 (2004) 101. [3] S. Surnev, M.G. Ramsey, F.P. Netzer, Prog. Surf. Sci. 4 (2003) 73. [4] H. Fei, X. Liu, Y. Lin, M. Wei, J. Colloid Interface Sci. 428 (2014) 73. [5] A.A. Taha, A.A. Hriez, Y. Wu, H. Wang, F. Li, J. Colloid Interface Sci. 417 (2014) 199. [6] F.J. Quites, C. Bisio, R. Cássia, G. Vinhas, R. Landers, L. Marchese, H.O. Pastore, J. Colloid Interface Sci. 368 (2012) 462. [7] H.-E. Wang, D.-S. Chen, Y. Cai, R.-L. Zhang, J.-M. Xu, Z. Deng, X.-F. Zheng, Y. Li, I. Bello, B.-L. Su, J. Colloid Interface Sci. 418 (2014) 74. [8] J. Xie, C. Wu, S. Hu, J. Dai, N. Zhang, J. Feng, J. Yang, Y. Xie, Phys. Chem. Chem. Phys. 14 (2012) 4810. [9] Y.L. Cheaha, N. Guptaa, S.S. Pramanab, V. Aravindanb, G. Weea, M. Srinivasana, J. Power Sources 196 (2011) 6465. [10] X. Zhou, C. Cui, G. Wu, H. Yang, J. Wu, J. Wang, G. Gao, J. Power Sources 238 (2013) 95. [11] D. Yu, Y. Qiao, X. Zhou, J. Wang, C. Li, C. Chen, Q. Huo, J. Power Sources (2014), http://dx.doi.org/10.1016/j.jpowsour.04.099(2014. [12] J.A. Bennett, J.E. Pander, M.A. Neiswonger, J. Electroanal. Chem. 654 (2011) 1. [13] A.G. Kong, Y.J. Ding, P. Wang, H.Q. Zhang, F. Yang, Y.K. Shan, J. Solid State Chem. 184 (2011) 331. [14] A. Pana, D. Liu, X. Zhou, B.B. Garcia, S. Liang, J. Liu, G. Cao, J. Power Sources 195 (2010) 3893. [15] Y.S. Kim, M.Y. Song, E.S. Park, S. Chin, G.-N. Bae, J. Jurng, Appl. Biochem. Biotechnol. 168 (2012) 1143. [16] C. Wu, F. Feng, Y. Xie, Chem. Soc. Rev. 42 (2013) 5157. [17] D. Vernardou, M. Apostolopoulou, D. Louloudakis, N. Katsarakis, E. Koudoumas, J. Colloid Interface Sci. 424 (2014) 1. [18] Y.-L. Hu, X.-B. Liu, D. Fang, Catal. Sci. Technol. 4 (2014) 38. [19] B.M. Weckhuysen, D.E. Keller, Catal. Today 2811 (2002) 1. [20] C. Karunakaran, S. Senthilvelan, J. Colloid Interface Sci. 289 (2005) 466. [21] E. Lodyga-Chruscinska, G. Micera, E. Garribba, Inorg. Chem. 50 (2011) 883. [22] W. Plass, Coord. Chem. Rev. 255 (2011) 2378. [23] S. Surnev, M.G. Ramsey, F.P. Netzer, Prog. Surf. Sci. 73 (2003) 117. [24] D.B. McWhan, Phys. Rev. B 10 (1974) 490. [25] B.L. Chamberland, J. Solid State Chem. 7 (1973) 377. [26] Y. Oka, S. Sato, T. Yao, N. Yamamoto, J. Solid State Chem. 598 (1998) 594. [27] F. Theobald, R. Cabala, J. Bernard, J. Solid State Chem. 438 (1976) 431. [28] L. Liu, F. Cao, T. Yao, Y. Xu, M. Zhou, B. Qu, B. Pan, C. Wu, S. Wei, Y. Xie, New J. Chem. 36 (2012) 619. [29] Y. Wang, Z. Zhang, Y. Zhu, Z. Li, R. Vajtai, L. Ci, P.M. Ajayan, ACS Nano 2 (2008) 1492. [30] C. Wu, Z. Hu, W. Wang, M. Zhang, J. Yang, Chem. Commun. 33 (2008) 3891. [31] Z. Gui, R. Fan, X.H. Chen, Y.C. Wu, J. Solid State Chem. 157 (2001) 250. [32] Y. Gao, L. Bai, W. Li, H. Luo, P. Jin, J. Ceram. Soc. Jpn. 116 (2008) 395. [33] W.G. Menezes, D.M. Reis, T.M. Benedetti, M.M. Oliveira, J.F. Soares, R.M. Torresi, A.J.G. Zarbin, J. Colloid Interface Sci. 337 (2009) 586. [34] F.N. Dultsev, L.L. Vasilieva, S.M. Maroshina, L.D. Pokrovsky, Thin Solid Films. 255 (2006) 255. [35] S. Pavasupreea, Y. Suzukia, A. Kitiyanana, S. Pivsa-Artb, S. Yoshikawa, J. Solid State Chem. 178 (2005) 2152. [36] J.H. Kim, Y.C. Hong, H.S. Uhm, Surf. Coat. Technol. 201 (2007) 5114. [37] Y.S. Lin, C.W. Tsai, P.W. Chen, Solid State Ionics 179 (2008) 290. [38] S.R. Popuri, M. Miclau, A. Artemenko, C. Labrugere, A. Villesuzanne, M. Pollet, Inorg. Chem. 52 (9) (2013) 4780. [39] N. Shash, Ionics 19 (2013) 1825. [40] N.C. Vieira, W. Avansi, A. Figueiredo, C. Ribeiro, V.R. Mastelaro1, F.E. Guimarães, Nanoscale Res. Lett. 7 (2012) 310. [41] J. Bao, X. Zhang, L. Bai, W. Bai, M. Zhou, J. Xie, M. Guan, J. Zhou, Y. Xie, J. Mater. Chem. A (2014), http://dx.doi.org/10.1039/C3TA15293F. [42] M. Li, X. Wu, L. Li, Y. Wang, D. Li, J. Pan, S. Li, L. Sun, G. Li, J. Mater. Chem. A 2 (2014) 4520. [43] G. M. Sheldrick, SADABS, Program for Empirical Absorption Correction for Area Detector Data, University of Göttingen: Göttingen, Germany, 1996. [44] G.M. Sheldrick, SHELXS 97, Program for Crystal Structure Refinement, University of Göttingen, Göttingen, Germany, 1997.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
A new metallate phase of V2O5 crystalline microstructure achieved in a facile route: Synthesis, characterization, and measurement in catalytic reactions Xiao-Xi Zhang a, Ji-Xiao Wang a, Na Xing a, Xi-Tong Ma a, Xiao-Dong Feng a, Yong-Heng Xing a,⇑, Zhan Shi b a b
College of Chemistry and Chemical Engineering, Liaoning Normal University, Huanghe Road 850#, Dalian City 116029, PR China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China
a r t i c l e
i n f o
Article history: Received 28 May 2014 Accepted 14 September 2014 Available online 13 October 2014 Keywords: Vanadium pentoxide Solution method Bromination catalytic reaction Catalytic oxidation
a b s t r a c t Experiencing a series of complicated changes, abundant orange crystals of novel metallic phase of vanadium pentoxide were obtained by a mild chemical method, the formula of which is defined as [V3(l3-O)2(l1-OH)O5]H2O. Differ from the synthesis methods of vanadium oxide published, we have adopted a simple solution method that mixed starting materials are refluxing in the system of ethanol–water under a relatively lower temperature. Symmetry of the crystals is Monoclinic, with cell unit dimensions: a = 4.9978(10) Å, b = 8.4273(17) Å, c = 7.8669(16) Å, b = 96.44(3)° and space group of P21/m. The structure of the complex was characterized by elemental analysis, IR, UV–vis spectroscopy and single-crystal diffraction analysis. Powder X-ray diffraction (PXRD) was used to detect the purity of the crystals, and crystal morphology was detected by the scanning electron microscope (SEM). In addition, in order to extend application of oxidovanadium complexes, bromination catalytic activity about the complex in a single-pot reaction of the conversion of phenol red to bromophenol blue in a mixed solution of H2O–DMF at a constant temperature of 30 ± 0.5 °C with a buffer solution of NaH2PO4ANa2HPO4 (pH = 5.8) was evaluated firstly, indicating that the complex can be considered as a potential functional model of bromoperoxidase, in the meantime, we have conducted the bromination catalytic reaction to simulate and measure the changes in reaction process indirectly. Besides, catalytic oxidation activity of the complex is also evaluated in the oxidation of cyclohexane (Cy) and cyclopentane with hydrogen peroxide promoted under mild conditions, showing potential catalytic activity of the complex by comparing TON (total turnover number) ratios of CyO/CyOH (CyO is the abbreviation of cyclohexanone and CyOH represents cyclohexanol) in the oxidation results. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction In the family of 3d transition metal complexes, vanadium oxides have received considerable attentions owing to their outstanding properties in physical and chemical research field [1–3], for example: electrical, optical, magnetic and environmental area [4–16]. Numerous researchers have also studied their behaviors when the oxides were designed as cathode material for rechargeable Li+ battery [17], or have reported their excellent performance as catalysts in oxidation reactions [18–20] to further drive for practical capacities of vanadium oxides. It is to be noted that the interest in catalytic oxidation carrying on with the presence of metallic vanadium complex as catalyst has increased recently [21,22]. ⇑ Corresponding author. E-mail address: [email protected] (Y.-H. Xing). http://dx.doi.org/10.1016/j.jcis.2014.09.032 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
By checking the literatures [23], there are mainly two factors influencing the catalytic performance of vanadium oxides, one is the oxidation states of the vanadium ion, and the other is the configuration of the molecule. As far as we know, vanadium usually adopts valence states from +3 to +5 in the complexes; different valence state and various coordination environment determine the geometry of vanadium oxides, by sharing corners, edges and faces, leading to variable structural configurations of vanadium oxides (tetrahedron, trigonal bipyramid, square pyramid, regular octahedron, and distorted octahedron), which is the source of abundant catalytic agents. Until now, several kinds of geometries of V O systems have been reported [24–33], characters of molecular structures, natures of the complexes and meaningful studies of the properties are involved among those researches. Vanadium Oxides have been prepared by a variety of synthetic ways in amorphous or crystalline forms [34–37]; for example, Popuri et al. [38]
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have provided a rapid way to obtain VO2 using a hydrothermal process with V2O5 and citric acid as precursors; Wu et al. [30] used an available pathway to accomplish paramontroseite VO2 by a simple chemical reaction of sodium orthovanadate (Na3VO412H2O) and thioacetamide (TAA). Besides, V2O5 is also a widely studied hotspot topic, since it is a common material for synthesis and catalysis system. Vanadium oxides possess different elemental composition forms, and various coordination polyhedra, which build up the tremendous structural diversity of vanadium oxides and bring the unique electrochemical and optical properties of vanadium oxides [39–42]. Traditional synthetic approach for vanadium oxides is hardly to reach, high cost and strict experimental conditions, sometimes high temperature and means of calcination are needed. While in our work, we have easily achieved abundant crystals of vanadium pentoxide in a mild condition refluxing in water–alcohol system at a relatively lower temperature, the method is facile and environmental. To our knowledge, it is found that molecular structure of the as-prepared complex and synthetic method has not been reported so far. Here, the new vanadium oxide is characterized by elemental analysis, IR spectra, UV–vis spectroscopy, single-crystal X-ray diffraction, SEM and Powder X-ray diffraction. In order to extend application of the vanadium oxide, we tested firstly for its catalytic activity both in bromination catalytic system with phenol red as organic substrate and in catalytic oxidation with hydrogen peroxide promoted. The results indicate that the complex can be considered as a potential functional model of bromoperoxidase (V-BrPO), and a promising catalytic agent used for obtaining clean energy source. In the meantime, the transformation mechanism from NH4VO3 to vanadium pentoxide is proposed; furthermore, we make full use of catalytic bromination reaction to track the species in solution during the synthetic reaction in situ. 2. Experimental section 2.1. Materials and methods All the other chemicals used were of analytical grade and without further purification. Elemental analyses for C, H and N were carried out on a Perkin Elmer 240C automatic analyzer, and content of V was measured by a Plasma-Spec(I)-AES model ICP spectrometer. Infrared spectra were recorded on a JASCO FT/IR-480 spectrometer in the range of 200–4000 cm 1 with pressed KBr pellets. UV–vis absorption spectra were recorded on a JASCO V-570 spectrometer (200–2500 nm, in the form of a solid sample). X-ray powder diffraction (PXRD) patterns were obtained on a Bruker Advance-D8 equipped with Cu Ka radiation, in the range 5° < 2h < 60°, with a step size of 0.02° (2h) and an count time of 2 s per step. The SEM samples were performed with a SU8000 field emission scanning electron microscope. A 30% aqueous solution of hydrogen peroxide was used as primary oxidant in the cyclohexane oxidation reaction. The products of cyclohexane (Cy) oxidation were analyzed by a GC-9790 series gas chromatograph equipped with a flame ionization detector (FID) and a capillary column (PG2000, column length: 30 m; internal diameter: 0.25 mm).
the solution was transferred into pyxides with refluxing for 3 h at a temperature of 80 °C. The orange bulk crystals were obtained in ca. 68% yield based on V(V). Anal. calc. for H3O9V3: H, 1.00; V, 51.00. Found: H, 0.70; V, 50.17%. IR (KBr, m, cm 1): 3412, m(OAH); 1629, 1418, d(OAH); 1007, m(V@O); 625, ms(VAOAV); 475, d(VAOAV). UV–vis (kmax, nm): 406(LMCT), 790(d–d⁄). 2.3. X-ray single crystal structural determination The crystal was mounted on glass fibers for X-ray measurement. Reflection data were collected at room temperature on a Bruker AXS SMART APEX II CCD diffractometer with graphite– monochromated Mo Ka radiation (k = 0.71073 Å) and a x scan mode. All the measured independent reflections (I > 2r(I)) were used in the structural analyses, and semi-empirical absorption corrections were applied using SADABS program [43]. The structure was solved by the direct method and refined using SHELXL-97 [44]. The non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms of the lattice water were found in the difference Fourier map. The hydrogen atom of hydroxyl group was not located. Crystal data, details of the data collection and the structure refinement of complex are given in Table 1, the parameters of selected bond lengths, bond angles and hydrogen bonds are listed in Table 2. Figs. S1 and S2 present the infrared spectra and UV–vis spectra of the as-prepared complex, respectively. 2.4. Measurement of the bromination activity in solution Bromination reaction activity test for the product was carried out in a mixed solution of H2OADMF at a constant temperature of 30 ± 0.5 °C. The complex was dissolved in the addition of 25 mL H2OADMF mixture (DMF: 1 mL; H2O: 24 mL). The solutions used for the kinetic measurements were maintained at a constant concentration of H+ (pH 5.8) by the addition a buffer solution of NaH2PO4ANa2HPO4 [45]. The reactions were conducted in the addition of a phenol red solution. Solutions of the complex with five different concentrations were confected in five different
Table 1 Crystallographic data for the complex.*
2.2. Preparation of [V3(l3-O)2(l1-OH)O5]H2O NH4VO3 (0.1 mmol, 0.0117 g), triflusal (0.05 mmol, 0.0124 g) and some carboxylic acids as auxiliary ligands (1,3,5-Benzenetricarboxylic acid, 1,2,4,5-Benzenetetracarboxylic acid, salicylic acid, benzenesulfonic acid, malic acid, aminoacetic acid and so on; 0.1 mmol) were dissolved in 15 mL CH3CH2OHAH2O system (the ratio of volume is 2:1), instantaneously giving an opaque orange solution which was stirred at room temperature for 3 h. Then,
Formula
H3O9V3
M (g mol 1) Crystal system Space group a (Å) b (Å) c (Å) a (deg) b (deg) c (deg) V (Å3) Z Dcalc (g cm 3) Crystal size (mm) F(0 0 0) l(Mo Ka)/mm 1 Reflections collected Independent reflections (I > 2r(I)) Rint Parameters D(q) (e Å 3) Goodness of fit h (°) Ra wR2a
299.84 Monoclinic P2(1)/m 4.9978(10) 8.4273(17) 7.8669(16) 90 96.44(3) 90 329.24(12) 2 3.025 0.28 0.17 0.11 288 4.188 3200 804 0.0417 62 0.525, 0.633 1.166 3.55–27.44 0.0272(0.0290)b 0.0750(0.0787)b
*a b
R = R||Fo| |Fc||/R|Fo|, wR2 = [R(w(Fo2 Based on all data.
Fc2)2/R(w(Fo2)2)]1/2; [Fo > 4r(Fo)].
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Table 2 parameters about selected bond for the complex. Bond lengths (Å) V1–O1 V2–O3
1.602(2) 1.8358(19)
V1–O2 V2–O4
1.7404(19) 2.284(3)
V1–O3 V2–O5
2.003(2) 1.600(3)
Bond angles (°) O1–V1–O2 V1–O3–V2 O5–V2–O4
106.28(10) 108.17(10) 174.22(12)
O1–V1–O3 O4–V2–O3
111.19(9) 73.68(7)
O2–V1–O3 O5–V2–O3
142.21(9) 102.67(9)
Hydrogen bonds (Å) D–H A O1W–H1WA O5 O1W–H1WB O1
d(D–H)/Å 0.85 0.85
d(H A)/Å 2.31 2.20
d(D A)/Å 2.924(5) 2.958(4)
\D–H A/° 129 149
cuvettes and then the cuvettes were placed in a constant temperature water bath for 5 min. And the changes of spectral data were recorded using a 721 UV–vis spectrophotometer every 5 min. Finally, the resulting data during the reaction were collected and fitted with the help of the curve-fitting software in the Microsoft Excel program. In addition, the synthetic reaction for the complex was refluxed in a round-bottom flask convenient for sampling 0.3 mL at a time and every 10 min was taken as an interval, the sample solution was diluted into 5 mL (DMF 0.5 mL and H2O 4.2 mL for addition) and sealed in a dark and 4 °C condition, immediately. The solutions extracted from the reaction system were also tested in the bromination catalytic reaction as above with a fixed concentration, and the samples must be detected as soon as possible. The bromination of phenol red was monitored by the measurement of the absorbance at 592 nm for reaction aliquots which were extracted at specific time points and diluted into a phosphate buffer with pH = 5.8. It is hypothesized that the rate of this reaction can be described by the rate equation of ‘‘dc/dt = kc1xc2yc3z’’, evolving from that the equation ‘‘log(dc/dt) = log k + xlog c1 + ylog c2 + zlog c3’’ is calculated, and which is corresponding to ‘‘ log(dc/dt) = xlog c1 – b (b = log k + ylog c2 + zlog c3)’’, thereinto, k is the reaction rate constant; c1, c2 and c3 represent the concentrations of the complex, KBr and phenol red, respectively; while x, y, z are the corresponding reaction orders. According to Lambert–Beer’s law, A = e d c, which is differential, dA/dt = e d(dc/dt), where A is the measurable absorbance of the resultant; e is molar absorption coefficient, which of bromophenol blue is measured as 14,500 M 1 cm 1 at 592 nm; d is the light path length of sample cell (d = 1). When the measurable absorbance data were plotted versus reaction time, a line was obtained and the reaction rate of the complex (dA/dt) was received by the slope of this line. By changing the concentration of the complex in the reaction system, a series of dA/dt data were tracked. The reaction rate constant (k) can be obtained according to a plot of log (dc/dt) versus log c1 and was fitted using the curve-fitting software in the program Microsoft Excel by generating a least squares fit to a general equation of the form ‘‘y = mx b’’, in which ‘‘m’’ is the reaction order of the complex [V3(l3-O)2(l1-OH)O5]H2O in this reaction and ‘‘b’’ is the intercept of the line. In the experiment, considering that the reaction orders of KBr and phenol red (y and z) are 1 in the light of the literatures [46]; c2 and c3 are known as 0.4 and 10 4 mol/L, respectively. Based on the equation of ‘‘b = log k + ylog c2 + zlog c3’’, we can compute and get the reaction rate constant (k).
of acetonitrile was stirred under atmospheric pressure, the required amounts of n(Cy): n(catalyst)=10,000:1, n(H2O2): n(catalyst)= 15,000:1, n(HNO3): n(catalyst)= 2000:1. The reaction was terminated by adding 1.5 mL of diethyl ether. The extracted reaction mixture was analyzed by GC equipped detector by internal standard method with 0.03 g (3.26 10 4 mol) of methylbenzene as internal standard. The identification of the oxidation products was performed by comparison of their retention times with that of the commercial CyOH and CyO. 3. Results and discussion 3.1. Synthesis Our original intention was to synthesize some complexes with triflusal and other auxiliary carboxylic acid ligands. However, we have obtained some orange crystals of a new metallate phase of V2O5, which was characterized by element analysis, IR, UV–vis and X-ray. Whether the molecular configuration or the synthetic method we used is found to be different from literatures. In order to gain further insight into the effect of various experimental conditions on the crystal formation for the oxovanadium complex, we have conducted experiment to trap the species in the solution and found that the formation process of the complex is complicated. On the basis of experimental data, we have deduced the formation mechanism as below (illustrated in Scheme 1) [47]. In this work, the transformation procedure we deduced from NH4VO3 to product [V3(l3-O)2(l1-OH)O5]H2O is achieved through the changes of the species of vanadate in aqueous solution. Under the unique condition which is ethanol–water solution containing triflusal and other auxiliary ligands, the starting material NH4VO3 was added into the ethanol–water system and ionized (a), being activated by H+, which was further turned into a form of VO3 (b); and then, free water molecule attacked the electropositive V5+ center of VO3 as a nucleophile, forming an intermediate (c) by the combination of VO3 and H2O, that is a key reaction step as we assumed and identical to the result we captured in bromination catalytic reaction of phenol red; in short time, the transition intermediate (c) has turned to be a stable state [VO2(OH)2] as shown in Scheme 1(d); in solution, it easily occurs that two [VO2(OH)2] molecules collide to form a dimer, and two water molecules were eliminated in that procedure, V2O26 (f) was obtained at the same time; at last, after a dehydration reaction between O2 of V2O26 anion and H+, the conversion was accomplished.
2.5. Experimental set up for catalytic oxidation
3.2. Structural description and spectroscopy
The catalytic experiment was carried out in a sealed glass vial, placed in a water bath of 4 °C with magnetic stirring. In the experiment, the reaction solution mixed with 0.0004 g catalyst and 3 mL
The molecular structure of vanadium pentoxide is depicted in Fig. 1. The principal bond distances and angles for the complex are summarized in Table 2.
X.-X. Zhang et al. / Journal of Colloid and Interface Science 438 (2015) 122–129
125
Scheme 1. The transformation mechanism from NH4VO3 to product deduced in reaction solution.
Fig. 1. (a) The coordination structure of the complex; (b) the fragment of the 1D-chain structures; (c): the two-dimensional packing network of the complex (all H atoms expect for the hydrogen bonds are omitted for clarity). Symmetry transformation used to generate equivalent atoms for complex. #1: x, y, z; #2: x, 0.5 y, z; #3: 1 + x, 1 + y, z; #4: x, 1 + y, 1 + z.
X-ray single crystal analysis reveals that the complex is crystallized in the monoclinic system with P2(1)/m space group. An asymmetry unit for the complex [V3(l3-O)2(l1-OH)O5]H2O consists of one and a half vanadium atoms, one l3-O oxygen (O3), a half terminal hydroxyl oxygen atom (O5), two and a half terminal oxygen atoms (O1, O2, O4), and a half lattice water (O1W). Thereinto, O1W is found to be disordered with occupancies 0.5. The bond lengths of VAO are in the range of 1.601(3)–2.285(3) Å, the angles of OAVAO is in the range of 73.67(8)–174.02(12)° and the angle of VAOAV is 108.20(10)°, respectively. Moreover, the contact distance between V(1) and V(2) is 3.1106(8) Å [30]. In structure of the complex, V(1) atom is coordinated by two l3-O oxygen atoms (O3, O3#1, #1: x, y, z) and two terminal oxygen atoms (O1, O2) to form a geometry configuration of distorted tetrahedron; in the mean time, V(2) atom is coordinated by two l3-O oxygen atoms (O3, O3#2, #2: x, 0.5 y, z), one terminal hydroxyl oxygen atom (O5) and one terminal oxygen atom (O4) with the molecular configuration of a distorted tetrahedron (Fig. 1). Viewing the architecture of selfassembling, two V(1) atoms, two l3-O oxygen atoms and two terminal oxygen atoms are constructing a building block V2O6 and then forming a 1D-chain structure via a VO2 moiety. At last, by the hydrogen bond interaction between oxygen atoms coming from V2O5 moiety and water molecule, namely, O1W#4AH1WA#4 O5 (2.918(5) Å, 129, #4: x, 1 + y, 1 + z) and O1W#4AH1WB#4 O1#2 (2.956(4) Å, 149°, #2: x, 0.5 y, z; #4: x, 1 + y, 1 + z), the complex is connected to form a twodimensional connection network. From the IR spectra (Fig. S1) of the as-prepared vanadium pentoxide, broad and strong absorption appearing at 3412 cm 1
indicates the presence of the m(OAH) of the water molecule, and sharp absorption bands observed at 1629 cm–1, 1418 cm–1 are features of d(OAH) of water molecule. The vibration of V@O interaction is presented at 1007 cm–1. The peak at 625 cm–1 is the symmetric stretching vibrations of VAOAV, and the peak at 475 cm–1 can be assigned to the banding vibration of VAOAV. The electronic absorption spectra (Fig. S2) of the complex were recorded at room temperature in the form of the solid sample. The bands at 406 and 790 nm are assigned to d–d⁄ transitions of the central metal vanadium of the vanadium oxide. The powder X-ray diffraction data of the complex were obtained and compared with the simulated single-crystal diffraction data (Fig. 2 as below). No change was observed under the present temperature and duration conditions influencing, the phase of the complex is considered as purity owning to the agreement of the peak positions. The different intensity may be due to the preferred orientation of the powder sample. As depicted above, the peaks in PXRD support the message which was delivered from SEM images. Fig. 3 shows SEM images of [V3(l3-O)2(l1-OH)O5]H2O complex, the morphology of the crystalline microstructure is like a chock with its length approximately up to 100 lm, the width and the thickness of which are 50 lm and 40 lm, respectively. 3.3. Functional mimics of the vanadium haloperoxidases 3.3.1. Mimicking bromination reaction of the complex As is known to all that with the presence of H2O2 and bromide, oxidovanadium complexes can mimic a reaction in which
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Intensity
a b
Scheme 2. The reactive bromination process from phenol red to bromophenol blue.
10
20
30
40
50
2.8
2 θ (° )
592nm
vanadium haloperoxidases could catalyze the bromination reaction of organic substrates. For example, the bromination of trimethoxybenzene [48], benzene, salicylaldehyde and phenol as substrates by the VO2+ segment [49], and the bromination of phenol red by [VO(O2)H2O]+ and correlative species [50]. Here, we have investigated the bromination reaction activity of as-prepared vanadium pentoxide taking phenol red as an organic substrate, which is shown by the conversion of phenol red to bromophenol blue. The reaction is rapid and stoichiometric, producing the halogenated product by the reaction of oxidized halogen species with the organic substrate, and the reactive process is drawn in Scheme 2 [51,52]. The solution of vanadium pentoxide was added to the standard reaction of bromide in phosphate buffer with phenol red tracking for oxidized bromine, visible color change from yellow to blue of the solution can be observed during the experiment. As is shown in Fig. 4, a decrease in absorbance of the peak at 443 nm due to the loss of phenol red and an increase in the absorbance of the peak at 592 nm characteristic of the bromophenol blue recorded by the electronic absorption spectra, which shows that the complex is of comparative catalytic activity. What is interesting is that the peaks at 443 nm and 592 nm are splitted during the last stage of the bromination oxidation in the electronic absorption spectra, which is similar to the results reported [53,54]. 3.3.2. Kinetic studies of mimicking bromination reaction A series of dA/dt data for the complex were obtained (Fig. 5) to explore kinetic study of mimicking bromination reaction by changing the concentration. According to the data, the plot of log(dc/dt) versus –log c for the complex is depicted as a straight line with a slope of 1.02 and an intercept of 2.2629, as shown in Fig. 6. The value of slope confirmed that the reaction order is first-order reaction
Absorbance/A
2.4
Fig. 2. PXRD powder patterns: (a) the experimental PXRD for complex; (b) the simulated PXRD pattern calculated from single-crystal structure of complex.
2.0 1.6
443nm
1.2 0.8 0.4 0.0
400
500
600
Wavelength/nm Fig. 4. Oxidative bromination of phenol red catalyzed by product. Spectral changes at 10 min intervals. The reaction mixture contained phosphate buffer (pH 5.8), KBr (0.4 mol L 1), phenol red (10 4 mol L 1) and the complex (0.1 lmol L 1).
Fig. 5. The measurable absorbance dependence on time for the complex. Conditions used: pH = 5.8, c(KBr) = 0.4 mol/L, c(H2O2) = 1 mmol/L, c(phenol red) = 10 4 mol/L. c(complex /mmol/L) = a: 1.93 10 2; b: 3.87 10 2; c: 5.80 10 2; d: 1.16 10 1; e: 1.35 10 1.
dependence on vanadium. On account of the equation of ‘‘b = log k + ylog c2 + zlog c3’’, the reaction rate constant k is reckoned by the concentrations of KBr and phenol red (c2 and c3), the
Fig. 3. SEM photographs of complex [V3(l3-O)2(l1-OH)O5]H2O.
- log (dc/dt)
X.-X. Zhang et al. / Journal of Colloid and Interface Science 438 (2015) 122–129
- log c Fig. 6. log(dc/dt) dependence of –log c for the complex in DMFAH2O at 30 ± 0.5 °C (c is the concentration of the product [V3(l3-O)2(l1-OH)O5]H2O; Conditions used: c(phosphate buffer) = 50 mmol/L, pH = 5.8, c(KBr) = 0.4 mol/L, c(phenol red) = 10 4 mol/L.
reaction orders of KBr and phenol red (y and z), and the intercept of the straight line (b). In the experiment, the reaction orders of KBr and phenol red (y and z) are considered as 1 according to the literature [55,56], c2 and c3 are known as 0.4 mol/L and 10 4 mol/L, respectively; therefore, the reaction rate constant (k) for the complex can be calculated as 1.36 102 (mol/L) 2 s 1, the value of that is close to which of the traditional V2O5 through the data of contrastive experiment, so we have obtained a criterion to estimate the brominated catalytic activity for as-prepared vanadium pentoxide, the related data and curves in the presence of traditional V2O5 are drawn in Supplementary material (Figs. S3–S5). It is concluded from the upwards data and we found that the reaction order of the vanadium pentoxide in bromination reaction is close to 1, showing the first-order dependence on vanadium. Along with the increase of concentration of vanadium pentoxide, the catalytic rate is accelerated, correspondingly. The cycle catalytic brominated reaction mechanism for the complex is illustrated in Scheme 3. One V@O double bond of the vanadium pentoxide is attacked by H2O2, thereinto, H+ of H2O2 and electronegative O which belongs to vanadium pentoxide get combined easily, while O2H is linking with the center of vanadium ion, hence an intermediate is formed (step a); Subsequently, after a dehydration reaction, free radical of V2 O6 is achieved (step b); In the solution, Br is firstly oxidized by the V2 O6 radical and they get connected (step c); Then with the presence of H+, catalyst vanadium pentoxide is regained to participate in the next catalytic reaction circle, and the oxidant HOBr can be further applied to catalyze the organic substrate phenol red (step d) [53,54,57]. Anyway, catalytic reaction rate is mainly dependent on the production of intermediate radical species.
Scheme 3. The cycle catalytic brominated reaction mechanism.
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3.3.3. Synthetic reaction procedure detection by bromination reaction A series of data of absorbance dependence on time for the complex are achieved, all the trends of different time slice in catalytic bromination reaction are almost consistent, and the sample is extracted after 40 min of the synthetic reaction and illustrated in Fig. 7 for an example. When starting materials were just dissolving into the wateralcohol system, the first point was recorded, the absorbance of which was comparatively high and that presented ionizing course of NH4VO3 activated by H+ in the solution (point 1); after a period of stable time, the absorbance dropped sharply to reach a nadir drawn in the figure, we inferred that some organic species is attacking the V5+ center of the intermediate during the reaction, and the bromination reaction was affected by the factor of sterichindrance, resulting in a large fall in electronic absorption (point 4); followed with the next point, namely, the absorbance recorded in 40 min for the synthetic process, the trend of the curve went up substantially to a top peak because of the active transition intermediate produced at this time, so the catalytic activity was enhanced. At last, the levels of absorbance changed in a small range, the influence of ionic transformation does not work much on the catalytic bromination reaction. The series diversification of the absorbance in Fig. 7 is accordant to the transformation procedure we deduced from NH4VO3 to vanadium pentoxide in Scheme 1, based on condition controlled and instrument interfered, we have tracked the intermediate species during the synthetic reaction by the method of bromination catalytic study, and the result is practicable.
Fig. 7. The measurable absorbance dependence of time for the synthetic procedure detection of the complex in catalytic bromination reaction.
Fig. 8. The TON ratios of CyO/CyOH depending on the reaction time promoted by complex and standard material V2O5. Conditions: n(Cy):n(catalyst) = 10,000:1, n(H2O2):n(catalyst) = 15,000:1, n(HNO3):n(catalyst) = 2000:1, catalyst (0.0004 g), CH3CN (3 mL), 40 °C.
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o
Scheme 4. Main products observed during the catalytic oxidation of Cy.
3.3.4. Catalytic activity studies To compare the catalytic oxidative activities between the complex and V2O5 (the standard material), both of them are used as catalysts in the Cy oxidation with H2O2 as primary oxidant, when keeping the fixed molar ratio of Cy/catalyst (10,000:1), H2O2/catalyst (15,000:1), HNO3/catalyst (2000:1), and catalyst (0.0004 g) in 3 mL of acetonitrile at 40 °C. The main products are displayed in Scheme 4. The turnover number (TON) ratios of CyO/CyOH catalyzed by the complex and V2O5 depending on different reaction time are shown in Fig. 8. Compared to the standard material V2O5, the ratios of CyO/CyOH promoted by the complex are all higher in the whole reaction process, especially at 5 and 8 h, which are nearly twice and four times, respectively. This indicates that the complex we synthesized is superior to the standard material of V2O5 as catalysts applied in the Cy oxidation.
4. Conclusion In conclusion, a new metallic phase V2O5 is crystallized in a facile way in water- alcohol system at a temperature of 80 °C, its formula is defined as [V3(l3-O)2(l1-OH)O5]H2O by single-crystal diffraction analysis, elemental analysis, IR and UV–vis spectroscopy are also performed to further ensure the structure of the complex. PXRD characterization reveals that bulk of the complex is obtained in purity and SEM data indicates that the micro-particles of crystals are chock-like with an average length of about 100um. In order to study the properties of the complex, we have tested the catalytic activity of the complex in the bromination reaction with phenol red as organic substrate and oxidation of cyclohexane for the first time, the results certificate that the as-prepared vanadium pentoxide is a potential functional catalytic model, and it can be applied in further environmental catalytic research. Moreover, we have detected the species varieties in the synthetic reaction by the method of bromination reaction with phenol red, the data of absorbance recorded by electronic spectra deliver the message indirectly, and which matches our propose on formation mechanism. Acknowledgments We acknowledge Natural Science Foundation of China (No. 21371086); College of Chemistry, Jilin University, Changchun 130012, P.R. China (Grant No. 2013-05); and Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, P.R. China (Project No. 1210908-06-K) for financial assistance.
Appendix A. Supplementary material The infrared spectra and UV-vis spectra of the as-prepared complex are listed in Figs. S1 and S2, respectively. And the data and curves in the presence of traditional V2O5 are given in Supplemantary material (Figs. S3–S5).
Detail data of the complex has been deposited with the FIZ Karlsruhe ICSD database. Copies of this information may be obtained free of charge. Further details of the crystal structure investigations may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49 7247 808 666; e-mail: crysdata(at)fiz-karlsruhe(dot)de, http://www.fiz-karlsruhe. de/request_for_deposited_data.html) on quoting the deposition number CSD-427774 for the complex [V3(l3–O)2(l1– OH)O5]H2O. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.jcis.2014.09.032. References [1] V.E. Henrich, P.A. Cox, The Surface Science of Metal Oxides, Cambridge University Press, Cambridge, 1994. [2] J. Schoiswohl, M. Sock, S. Surnev, M.G. Ramsey, F.P. Netzer, G. Krasse, J.N. Andersen, Surf. Sci. 555 (2004) 101. [3] S. Surnev, M.G. Ramsey, F.P. Netzer, Prog. Surf. Sci. 4 (2003) 73. [4] H. Fei, X. Liu, Y. Lin, M. Wei, J. Colloid Interface Sci. 428 (2014) 73. [5] A.A. Taha, A.A. Hriez, Y. Wu, H. Wang, F. Li, J. Colloid Interface Sci. 417 (2014) 199. [6] F.J. Quites, C. Bisio, R. Cássia, G. Vinhas, R. Landers, L. Marchese, H.O. Pastore, J. Colloid Interface Sci. 368 (2012) 462. [7] H.-E. Wang, D.-S. Chen, Y. Cai, R.-L. Zhang, J.-M. Xu, Z. Deng, X.-F. Zheng, Y. Li, I. Bello, B.-L. Su, J. Colloid Interface Sci. 418 (2014) 74. [8] J. Xie, C. Wu, S. Hu, J. Dai, N. Zhang, J. Feng, J. Yang, Y. Xie, Phys. Chem. Chem. Phys. 14 (2012) 4810. [9] Y.L. Cheaha, N. Guptaa, S.S. Pramanab, V. Aravindanb, G. Weea, M. Srinivasana, J. Power Sources 196 (2011) 6465. [10] X. Zhou, C. Cui, G. Wu, H. Yang, J. Wu, J. Wang, G. Gao, J. Power Sources 238 (2013) 95. [11] D. Yu, Y. Qiao, X. Zhou, J. Wang, C. Li, C. Chen, Q. Huo, J. Power Sources (2014), http://dx.doi.org/10.1016/j.jpowsour.04.099(2014. [12] J.A. Bennett, J.E. Pander, M.A. Neiswonger, J. Electroanal. Chem. 654 (2011) 1. [13] A.G. Kong, Y.J. Ding, P. Wang, H.Q. Zhang, F. Yang, Y.K. Shan, J. Solid State Chem. 184 (2011) 331. [14] A. Pana, D. Liu, X. Zhou, B.B. Garcia, S. Liang, J. Liu, G. Cao, J. Power Sources 195 (2010) 3893. [15] Y.S. Kim, M.Y. Song, E.S. Park, S. Chin, G.-N. Bae, J. Jurng, Appl. Biochem. Biotechnol. 168 (2012) 1143. [16] C. Wu, F. Feng, Y. Xie, Chem. Soc. Rev. 42 (2013) 5157. [17] D. Vernardou, M. Apostolopoulou, D. Louloudakis, N. Katsarakis, E. Koudoumas, J. Colloid Interface Sci. 424 (2014) 1. [18] Y.-L. Hu, X.-B. Liu, D. Fang, Catal. Sci. Technol. 4 (2014) 38. [19] B.M. Weckhuysen, D.E. Keller, Catal. Today 2811 (2002) 1. [20] C. Karunakaran, S. Senthilvelan, J. Colloid Interface Sci. 289 (2005) 466. [21] E. Lodyga-Chruscinska, G. Micera, E. Garribba, Inorg. Chem. 50 (2011) 883. [22] W. Plass, Coord. Chem. Rev. 255 (2011) 2378. [23] S. Surnev, M.G. Ramsey, F.P. Netzer, Prog. Surf. Sci. 73 (2003) 117. [24] D.B. McWhan, Phys. Rev. B 10 (1974) 490. [25] B.L. Chamberland, J. Solid State Chem. 7 (1973) 377. [26] Y. Oka, S. Sato, T. Yao, N. Yamamoto, J. Solid State Chem. 598 (1998) 594. [27] F. Theobald, R. Cabala, J. Bernard, J. Solid State Chem. 438 (1976) 431. [28] L. Liu, F. Cao, T. Yao, Y. Xu, M. Zhou, B. Qu, B. Pan, C. Wu, S. Wei, Y. Xie, New J. Chem. 36 (2012) 619. [29] Y. Wang, Z. Zhang, Y. Zhu, Z. Li, R. Vajtai, L. Ci, P.M. Ajayan, ACS Nano 2 (2008) 1492. [30] C. Wu, Z. Hu, W. Wang, M. Zhang, J. Yang, Chem. Commun. 33 (2008) 3891. [31] Z. Gui, R. Fan, X.H. Chen, Y.C. Wu, J. Solid State Chem. 157 (2001) 250. [32] Y. Gao, L. Bai, W. Li, H. Luo, P. Jin, J. Ceram. Soc. Jpn. 116 (2008) 395. [33] W.G. Menezes, D.M. Reis, T.M. Benedetti, M.M. Oliveira, J.F. Soares, R.M. Torresi, A.J.G. Zarbin, J. Colloid Interface Sci. 337 (2009) 586. [34] F.N. Dultsev, L.L. Vasilieva, S.M. Maroshina, L.D. Pokrovsky, Thin Solid Films. 255 (2006) 255. [35] S. Pavasupreea, Y. Suzukia, A. Kitiyanana, S. Pivsa-Artb, S. Yoshikawa, J. Solid State Chem. 178 (2005) 2152. [36] J.H. Kim, Y.C. Hong, H.S. Uhm, Surf. Coat. Technol. 201 (2007) 5114. [37] Y.S. Lin, C.W. Tsai, P.W. Chen, Solid State Ionics 179 (2008) 290. [38] S.R. Popuri, M. Miclau, A. Artemenko, C. Labrugere, A. Villesuzanne, M. Pollet, Inorg. Chem. 52 (9) (2013) 4780. [39] N. Shash, Ionics 19 (2013) 1825. [40] N.C. Vieira, W. Avansi, A. Figueiredo, C. Ribeiro, V.R. Mastelaro1, F.E. Guimarães, Nanoscale Res. Lett. 7 (2012) 310. [41] J. Bao, X. Zhang, L. Bai, W. Bai, M. Zhou, J. Xie, M. Guan, J. Zhou, Y. Xie, J. Mater. Chem. A (2014), http://dx.doi.org/10.1039/C3TA15293F. [42] M. Li, X. Wu, L. Li, Y. Wang, D. Li, J. Pan, S. Li, L. Sun, G. Li, J. Mater. Chem. A 2 (2014) 4520. [43] G. M. Sheldrick, SADABS, Program for Empirical Absorption Correction for Area Detector Data, University of Göttingen: Göttingen, Germany, 1996. [44] G.M. Sheldrick, SHELXS 97, Program for Crystal Structure Refinement, University of Göttingen, Göttingen, Germany, 1997.
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