A Mixed-Valence Vanadate as Model for the Vanadium Mineral

Article pubs.acs.org/crystal

A Mixed-Valence Vanadate as Model for the Vanadium Mineral Melanovanadite: Hydrothermal Synthesis, Crystal Structure and Magnetic Properties of MgVV2VIV2O10·4H2O M. Ishaque Khan,*,† Kadir Aydemir,† James H. McNeely,† Brant Cage,† and Robert J. Doedens‡ †

Department of Biological and Chemical Sciences, Illinois Institute of Technology, Chicago, Illinois 60616, United States Department of Chemistry, University of California, Irvine, California 92697, United States

‡

ABSTRACT: A mixed-valence vanadate, MgVV2VIV2O10·4H2O (1), has been synthesized hydrothermally and characterized by single-crystal X-ray diffraction, spectroscopic and thermogravimetric analyses, and temperature dependent magnetic measurements. The compound shows magnetic properties with domination of intrasheet exchange interactions versus intersheet interactions due to large distance between vanadate sheets. It represents a synthetic analogue of the mineral melanovanadite: Ca2VV4VIV4O20·10H2O. The framework structure in 1 consists of vanadium oxide layers cross-linked by {Mg (H2O)4} groups. The vanadium oxide layers are composed of edge shared {VIVO5} square pyramids, forming {V2O8} dimers, which share corners with {VVO4} tetrahedral units. Crystal data for 1: triclinic space group P1̅ with a = 6.3232(14) Å, b = 6.3917(14) Å, c = 7.9388(17) Å, α = 88.264(19)°, β = 67.428(16)°, γ = 79.433(20)°, V = 290.97(12) Å3, Z = 1.

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INTRODUCTION The chemistry of vanadium oxides is of fundamental and practical interest. A number of vanadium oxides synthesized and characterized in recent years show rich structural features and a range of chemical and physical properties suitable for applications in such diverse areas as catalysis, medicine, magnetism, nanotechnology, cathode materials for fuel cells and batteries, chemical sensing, and materials science.1−6 Oxovanadium compounds have relevance to oxidation catalysis.7−10 An important industrial use of vanadium oxide is as a catalyst in the manufacturing of sulfuric acid by the contact process in which sulfur dioxide is oxidized to sulfur trioxide.11 Magnesium vanadate phases have attracted much attention for their application in selective oxidative catalysis.12−15 Some of these phases reported to date include MgVO 3 , MgV 2 O 4 , MgV 2 O 5 , MgV 2 O 6 , MgV 2 O 7 , and Mg2V3O8.16−21 A recent report describes the synthesis of a novel 3-D magnesium vanadium oxide open framework solid Mg7V4O16(OH)2(H2O).22 Vanadium is also known to exhibit rich geochemistry. Owing to its oxophilic behavior, it forms many oxidic species by weathering of magmatic and postmagmatic rocks.23−25 Mixed valence vanadates are common in nature. Melanovanadite, Ca2VV4VIV4O20·10H2O, is a naturally occurring mineral which belongs to the vanadium bronze family and is found in Minasragra, Peru, and Peanut Mine, Colorado, USA.26,27 During the course of our ongoing work on the design and synthesis of new types of vanadium oxide based materials,28−35 we have isolated a mixed-valence species, MgVV2VIV2O10·4H2O (1). To our knowledge, 1 is the first example of a model compound for the mineral melanovanadite. In this report, we © 2012 American Chemical Society

describe the hydrothermal synthesis and characterization of 1 by complete single crystal X-ray diffraction analysis, FT-IR spectroscopy, thermogravimetric analysis, and magnetic measurements.

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EXPERIMENTAL SECTION

Materials and Methods. Reagent grade V2O5 and Mg(BO2)2·H2O were purchased from commercial sources and were used without further purification. Hydrothermal synthesis was carried out in 23 mL Parr Teflon-lined acid digestion bombs. The FT-IR spectra were recorded as KBr pellets in the frequency range 4000−400 cm−1 with a Nexon 470 spectrometer from Thermonicolet and were analyzed by OMNIC software. TGA curves were obtained with a Mettler Toledo TGA/SDTA 851E thermogravimetric analyzer under nitrogen atmosphere over a temperature range 25−600 °C at a rate of 5 °C/min. A Varian SpectrAA 55B atomic absorption spectrometer was used for elemental analysis of Mg. Synthesis. In a typical synthesis of 1, a mixture of V2O5 (1.25 mmol), Mg(BO2)2·H2O (2.5 mmol), and H2O (278 mmol) in the molar ratio of 1:2:222, was placed in a 23 mL Teflon-lined Parr autoclave. The pH of the mixture was adjusted to 6.5 by the addition of 2.5 mL of 0.1 M HCl. This mixture was stirred at room temperature for 15 min, and the autoclave containing the mixture was heated in an electric furnace at 200 °C for 5 days. Thereafter, the furnace was turned off and the autoclave was left inside for 24 h for slow cooling to room temperature (25 °C). Dark-brown crystals were filtered from brown mother liquor (pH = 6.3), washed with distilled water, and dried in air at room temperature to give 70 mg of 1 (∼25% yield based on vanadium). Anal. calcd for 1: Mg 5.28%; found, Mg 5.13%. Received: April 5, 2012 Revised: June 4, 2012 Published: June 8, 2012 3656

dx.doi.org/10.1021/cg3004678 | Cryst. Growth Des. 2012, 12, 3656−3660

Crystal Growth & Design

Article

Prominent IR bands (KBr pellet, 4000−400 cm−1): 3420 s, 1618 s, 1424 m, 1002 vs, 960 s, 829 s, 663 m, 566 m, 469 m cm−1. Thermogravimetric analysis, Calcd: 4H2O 15.66%. Found: 16.66%; 10.36% (2.5H2O, 25−220 °C), 6.30% (1.5 H2O, 220−378 °C). Single Crystal X-Ray Diffraction Analysis. An X-ray quality crystal (red−brown prism, 0.23 × 0.20 × 0.13 mm3) of 1 was mounted on a thin glass fiber with hydrocarbon oil. A full sphere of data was collected at −180 °C on a Bruker SMART APEX II diffractometer equipped with graphite monochromatized Mo Kα radiation (λ = 0.71073 Å). The data were processed with SAINT software36 and corrected for absorption with SADABS. 37 Calculations were performed by use of the SHELXTL package.38 An X-ray crystallographic file in CIF format for the structure determination of 1 is available at Cambridge Crystallographic Data Center (CCDC 873765). Tables of atomic coordinates, bond lengths and angles, displacement parameters.

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RESULTS AND DISCUSSION Structure Determination. The synthetic procedure described in the Experimental Section produced 1 in highly crystalline form. Crystals of 1 exhibit shiny faces. They are insoluble in water and common organic solvents. The structure of 1 was determined by complete single-crystal X-ray diffraction analysis. Crystallographic data and selected bond distances are given in Tables 1 and 2, respectively. In Figure 1, the metal coordination environments, atom labels, and displacement ellipsoids are shown.

Figure 1. The asymmetric unit (and selected symmetry-equivalent atoms) in MgVV2VIV2O10·4H2O (1), showing the linkages and coordination spheres with thermal ellipsoids and atom labels.

Table 1. Crystal Data and Structure Refinement for MgVV2VIV2O10·4H2O (1) empirical formula formula wt space group crystal system a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalc (mg m−3) μ (mm−1) T (K) F(000) goodness-of-fit (GOF) on F2 R1 [I > 2σ(I)] wR2 (F2 all data)

H8 Mg O14 V4 460.13 P1̅ triclinic 6.3232(14) 6.3917(16) 7.9388(17) 88.264(19) 67.428(16) 79.433(20) 290.97(12) 1 2.626 3.239 90(2) 224 1.136 0.0156 0.0454

Figure 2. A portion of the packing diagram in the crystals of MgVV2VIV2O10·4H2O (1), showing the linkage of the layers by Mg(2+) ions. Color codes; V (orange), O (red), Mg (green).

{V2O8} dimers, which share corners with {VVO4} tetrahedral units, as shown in Figure 3. The Mg atom lies on an inversion center with a distorted octahedral environment completed by bonds to two oxygen atoms of tetrahedral VVO4 groups in adjacent layers. The coordination environment of the Mg2+ ion can be seen in Figures 1 and 2. The vanadate layers are similar in structure to those found in the naturally occurring mineral melonovanadite, Ca2VV4VIV4O20·10H2O,26 but linkages between layers differ in the two compounds. In melanovanadite, the calcium atoms have 7-fold coordination with five H2O ligands. The Ca−O interlayer linkages involve one oxygen atom from a tetrahedral VVO4 unit and one from a square pyramidal VIVO5 unit. The V−O distances in 1 are consistent with the presence of tetrahedral VV sites and square pyramidal VIV sites, as also observed in Melanovanadite and in CsV2O5,39 and in layered vanadates reported by Clearfield.40 A rather prominent supercell was observed in melanovanadite, so the diffraction pattern of 1 was carefully examined for evidence of such a feature. There were indeed a number of very weak diffraction peaks halfway between the reciprocal lattice

Table 2. Selected Bond Lengths [Å] for MgVV2VIV2O10·4H2O (1) V1−O1 V1−O2 V1−O2A V1−O3 V1−O4 V2−O1

1.9643(11) 1.9598(11) 1.9971(11) 1.9464(11) 1.6049(12) 1.7187(11)

V2−O2B V2−O3A V2−O5 Mg1−O5 Mg1−O6 Mg1−O7

1.8131(11) 1.7225(11) 1.6345(11) 2.0164(11) 2.1413(14) 2.0349(13)

Figure 2 shows a portion of the framework structure of 1. The extended structure consists of vanadium oxide layers crosslinked by Mg(H2O)4 groups. The vanadium oxide layers are composed of edge shared {VIVO5} square pyramids, forming 3657



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strength trends (2.03 and 2.14 Å for 1; 2.34−2.46 Å for melanovanadadite). The FT-IR spectra of a sample of 1 as prepared and of samples heated to 220 and 370 °C under nitrogen flow for 30 min are shown in Figure 4. All three samples had a deep-brown color. As can be seen, the infrared spectrum of as-prepared 1 has strong absorption bands at about 3400 and 1618 cm−1, which are attributed to H2O. The strong bands at 1002, 960, 829, and 663 cm−1 are due to V−O symmetric stretching and V−O−V antisymmetric stretching vibrational modes.43 As expected, heating 1 up to 220 °C reduces the amount of H2O as revealed by less pronounced H2O peaks in the infrared spectrum of the heated sample. Furthermore, the fingerprint region (1500−400 cm−1) of the spectrum shows considerable change in the position and intensities of the bands from those in the spectrum of the as-prepared sample, indicating structural changes (including V−O bonds) upon heating of 1. These structural changes are more pronounced in the sample heated to 370 °C. Water peaks almost disappeared and very broad peaks appeared in the fingerprint region, indicative of more V− O−V interactions. The magnetic properties of compound 1 were analyzed by EPR. Figure 5 displays W-band EPR spectra collected from

Figure 3. A portion of the vanadium oxide layer in MgVV2VIV2O10·4H2O (1). Color codes; V (orange), O (red).

layers in the c* direction. These peaks had a mean intensity of the order of 2% of that for the data of the primary cell and did not extend to high resolution. The number and intensity of these data were insufficient to define the structural basis for this supercell. Together with the quality and consistency of the reported structural results, these observations indicate that whatever perturbation gives rise to the additional maxima must be very minor. The thermogravimetric analysis of 1 showed that the compound has good thermal stability. Two distinct steps of weight losses were observed in TGA trace. The first step, which starts at about 60 °C and continues up to 220 °C, corresponds to loss of 2.5 molecules of water in 1. The second weight loss in the temperature range 280−378 °C is attributable to the loss of 1.5 molecules of water. As compared to 1, melanovanadite loses water at a relatively lower temperature starting from 75 °C, followed by another step of water loss in the temperature range 120−250 °C.26 This difference in the water loss pattern and temperatures can be attributed to the difference in M−OH2 (M = Mg, Ca) bond strength in the two species. The smaller ionic radius41 of Mg2+ in comparison to Ca2+ is expected to strengthen the Mg−OH2 bond, as reflected in the relative ΔH (hydration) values for the two divalent ions.42 Observed M−OH2 bond distances are consistent with these bond

Figure 5. W-band EPR powder spectra of 1. DPPH was used as a field standard, and the linewidths were determined from Lorentzian peak shape fits to the DPPH and the main peak.

powder samples at 20 and 10 K. No anisotropy was resolved in these spectra due to the large line width of the magnetic

Figure 4. FT-IR spectra of as-prepared MgVV2VIV2O10·4H2O (1) (top), 1 heated at 220 °C for 30 min under nitrogen (middle), and 1 heated at 370 °C (bottom) for 30 min under nitrogen. 3658



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Ĥ = −2JS1S2

species’ resonances. The isotropic g-value was found to be 1.983 ± 0.002, which is characteristic of VIV centers.44 Another aspect of the spectra to take into account is the increase in line width as the temperature is lowered. The peak-to-peak line width increased from 880 to 1065 G when lowering from 20 K to 10 K. This increase in line width is consistent with lowdimensional magnetic interactions. The structure of the material suggests that zero-dimensional and two-dimensional exchange should dominate over three-dimensional ordering (or ordering between the sheets), and this line width increase is consistent with the intrasheet exchange dominating over the intersheet exchange. The magnetic susceptibility of 1 vs temperature is shown in Figure 6. The inset displays μeff vs temperature, and the figure

χmiso =

(2)

2Nμ0 g 2μB2 ⎡ exp(2J /T ) ⎤ ⎥ + χ0 ⎢ kT ⎣ 1 + 3exp(2J /T ) ⎦

(3)

In eq 3, k is the Boltzmann constant, N is Avogadro’s constant, μ0 is the magnetic permeability of free space, and μB is the Bohr magneton. This model resulted in a g-value of 1.16 and a J of −2.1 K. The g-value is unrealistic, and the fit itself had a χ2 value of 1164, which is almost three orders magnitude greater than that of the high temperature Curie−Weiss fit. The molecular field model was employed to try to improve the fit.45 This model is shown in eq 4. Equation 4 also includes a term to account for a ρ amount of paramagnetic monomeric impurities in the sample, and the parameter zJ′, which describes the intermolecular coupling. χmmf = (1 − ρ)

χmiso 1 − (zJ ′/μ0 Ng 2μB2 )χmiso

+ρ

1.232 × 10−5 T

+ χ0

(4)

This parameter includes z, the number of neighbors, and J′, which is the intermolecular coupling. This fit is shown in Figure 6. The fit parameters were g = 1.91 ± 0.06, J = −4.1 K, zJ′ = −50.3 K, and p = 9.7%. The χ2 for this model was 16.82, 2 orders of magnitude better than that for the isolated dimer model. A magnetization vs field measurement was also performed at 1.9 K (not shown), and the sample never saturated up to 9 T. The magnetization at 9 T was ∼1/2 μB, far below the expected saturation value of 2 μB. These results show that there are several different exchange mechanisms operating in 1 at low temperature. In conclusion, we have successfully prepared and characterized MgVV2VIV2O10·4H2O, a synthetic analogue of the mineral Melanovanadite - Ca2VV4VIV4O20·10H2O. The magnetic measurements and modeling of magnetic properties revealed domination of intrasheet exchange interactions versus intersheet interactions due to large distance between vanadate sheets. The compound, which can be prepared in high purity and decent yield, shows thermal stability that is suitable for applications such as in selective oxidative catalysis. We are currently studying the oxidative dehydrogenation properties of the new compound.

Figure 6. Magnetic susceptibility versus temperature of powder sample of MgVV2VIV2O10·4H2O (1). Blue line is fit to the data with eq 4. Inset shows the effective magnetic moment (μeff) vs temperature corrected for the θ0 obtained from the Curie−Weiss fit.

clearly suggests that antiferromagnetic interactions are present in the system. The theoretical room temperature μeff for an uncoupled dimer with g = 1.98 is 2.4 μB, whereas Figure 6 shows the room temperature μeff to be approximately 2.0 μB. The crystal structure of 1 displays edge sharing VIV centers insulated by tetrahedral O−VV−O linkages and significantly longer bridges between sheets. Therefore, several different models were tried to fit the temperature dependence of the magnetic susceptibility, shown in Figure 6. The first model used was the Curie−Weiss model, shown in eq 1. In eq 1, C is the Curie constant, θ is the Weiss constant that accounts for exchange interactions, and χ0 is a term representing temperature independent paramagnetism and diamagnetism.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +1 312 567 3431. E-mail address: [email protected].

C = + χ0 (1) T−θ This model performed well at high temperatures but was unsatisfactory when temperatures fell below ∼15 K. The parameters from the fit were C = 8.02 × 10−6 ± 0.07 × 10−6 m3K/mol, θ = −15.3 ± 0.3 K, and χ0 = −6.42 × 10−10 ± 4.02 × 10−10 m3/mol. The χ2 of the fit was 3.1. From these values, an isotropic g-value of 1.83 was calculated. This is in poor agreement with the g-value obtained via EPR. To try to model the magnetic data more accurately, the isolated dimer model was employed. Equation 3 shows this equation. It assumes that the magnetic species are isolated dimers of S = 1/2, which interact to form an effective triplet and singlet state. The J-value is isotropic and is based on the Hamiltonian term shown in eq 2. χmCW

Notes

The authors declare no competing financial interest.

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REFERENCES

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A Mixed-Valence Vanadate as Model for the Vanadium Mineral

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