Molybdenum(VI) Oxosulfato Complexes in MoO₃–K₂S₂O₇–K₂SO₄ Molten Mixtures: Stoichiometry, Vibrational Properties, and Molecular Structures

The structural and vibrational properties of molybdenum­(VI) oxosulfato complexes formed in MoO₃–K₂S₂O₇ and MoO₃–K₂S₂O₇–K₂SO₄ molten mixtures under an O₂ atmosphere and static equilibrium conditions were studied by Raman spectroscopy at temperatures of 400–640 °C. The corresponding composition effects were explored in the <i>X</i>_MoO₃⁰ = 0–0.5 range. MoO₃ undergoes a dissolution reaction in molten K₂S₂O₇, and the Raman spectra point to the formation of molybdenum­(VI) oxosulfato complexes. The MoO stretching region of the Raman spectrum provides sound evidence for the occurrence of a dioxo Mo­(O)₂ configuration as a core. The stoichiometry of the dissolution reaction MoO₃ + <i>n</i>S₂O₇^2– → C^2<i>n</i>– was inferred by exploiting the Raman band intensities, and it was found that <i>n</i> = 1. Therefore, depending on the MoO₃ content, monomeric MoO₂(SO₄)₂^2– and/or associated [MoO₂(SO₄)₂]_<i>m</i>^2<i>m</i>– complexes are formed in the binary MoO₃–K₂S₂O₇ molten system, and pertinent structural models are proposed in full consistency with the Raman data. A 6-fold coordination around Mo is inferred. Adjacent MoO₂²⁺ cores are linked by bidentate bridging sulfates. With increasing temperature at concentrated melts (i.e., high <i>X</i>_MoO₃⁰), the observed spectral changes can be explained by partial dissociation of [MoO₂(SO₄)₂]_<i>m</i>^2<i>m</i>– by detachment of S₂O₇^2– and formation of a MoOMo bridge. Addition of K₂SO₄ in MoO₃–K₂S₂O₇ results in a “follow-up” reaction and formation of MoO₂(SO₄)₃^4– and/or associated [MoO₂(SO₄)₃]_<i>m</i>^4<i>m</i>– complexes in the ternary MoO₃–K₂S₂O₇–K₂SO₄ molten system. The 6-fold Mo coordination comprises two oxide ligands and four O atoms linking to coordinated sulfate groups in various environments of reduced symmetry. The most characteristic Raman bands for the molybdenum­(VI) oxosulfato complexes pertain to the Mo­(O)₂ stretching modes: (1) at 957 (polarized) and 918 (depolarized) cm^–1 for the ν_s and ν_as Mo­(O)₂ modes of MoO₂(SO₄)₂^2– and [MoO₂(SO₄)₂]_<i>m</i>^2<i>m</i>– and (2) at 935 (polarized) and 895 (depolarized) cm^–1 for the respective modes of MoO₂(SO₄)₃^4– and [MoO₂(SO₄)₃]_<i>m</i>^4<i>m</i>–. The results were tested and found to be in accordance with ab initio quantum chemical calculations carried out on [MoO₂(SO₄)₃]^4– and [{MoO₂}₂(SO₄)₄(μ-SO₄)₂]^8– ions, in assumed isolated gaseous free states, at the DFT/B3LYP (HF) level and with the 3-21G basis set. The calculations included determination of vibrational infrared and Raman spectra, by use of force constants in the Gaussian 03W program

Cellulose Hydrolysis in Acidified LiBr Molten Salt Hydrate Media

We screened nine acidified molten salt hydrates (solutions with water to salt molar ratio equal or less than the coordination number of the cation) as reaction media for cellulose hydrolysis, and we found that cellulose can be efficiently hydrolyzed in LiBr acidified MSH under mild conditions (>90% yield to water-soluble products in 0.05 M H₂SO₄ at 85 °C for 30 min). The effect of various factors (temperature as well as acid and initial cellulose concentrations) on the kinetics of hydrolysis reaction was also investigated. At the lowest temperatures examined (70 and 85 °C) low amounts of degradation products have been observed, and glucose appears to be in equilibrium with its dimers and possibly other oligomers. Higher temperatures (100–115 °C) enhanced the formation of degradation products (organic acids and humins). Analysis of the kinetic data indicate that hydrolysis rates are first order in cellulose and in H₂SO₄ concentration, and the initial hydrolysis rates have an apparent activation energy ∼123 kJ/mol. X-ray diffraction, SEM, and FTIR were also used to study cellulose’s structural/morphological changes upon treatment in the LiBr MSH media, in an attempt to understand the effects of the cellulose–salt interaction. Analysis of the data indicates that the enhancement of the hydrolysis rates can be attributed to the enhancement of the acidity of reaction media through synergistic effect of dilute acid and MSH, the breaking of crystalline structure through swelling, and the interaction of the salt with cellulose chains affecting the conformation and flexibility of the glycosidic bonds

Changes in Polymorph Composition in P25-TiO₂ during Pretreatment Analyzed by Differential Diffuse Reflectance Spectral Analysis

Elucidating structural and compositional polymorph changes of multiphase semiconductors in a quick and quantitative manner is important for their manufacturing and applications in catalysis alike. Derivative peak fitting of diffuse reflectance UV–visible spectra (DPR) is presented as an inexpensive, fast, and quantitative method to estimate both the composition of a multiphase semiconductor sample as well as the band gap energies of each component semiconductor in the mixture. Degussa P25 TiO₂, a widely used catalyst support and semiconductor photocatalyst, is a mixture of anatase and rutile polymorphs. The as-received P25 samples were subjected to grinding, sieving, and calcination or a combination of these operations. Samples were analyzed via DPR, X-ray diffraction (XRD), and Raman spectroscopy to quantify the percentage of anatase and rutile polymorphs present in each sample. Compositional measurements from DPR were in good quantitative agreement with XRD and Raman analysis. The application of in situ UV–visible spectroscopy DPR allowed for the on-stream determination of the onset temperature for rutilization during calcination, which occurred just above 823 K. In situ analysis of the ground and sieved samples during calcination revealed that grinding and sieving (between 45 and 53 μm) lowered the onset of rutilization to 673 K, leading to an increase in the formation of rutile when calcined at 773 K compared to as-received P25 calcined at the same temperature. However, the same process reduced the overall extent of rutilization at a higher temperature of 973 K. Changes in the exposed facets and shapes of the anatase crystallite, as indicated by XRD, were implicated in both reducing the rutilization onset temperature as well as leading to an overall greater resistance to rutilization. The quantitative success of the DPR method in this study may lead to application in other systems of mixed semiconductors