Kinetically Controlled Reduction of β-Vanadyl(V) Orthophosphate: Synthesis and Characterization of New Metastable Polymorphs of Vanadium(III) Phosphate

Two thermodynamically metastable polymorphs of vanadium(III) phosphate, V^IIIPO₄-m1 and VPO₄-m2, have been obtained via reduction of β-V^VOPO₄ by moist hydrogen. The XRPD pattern of VPO₄-m1 can be assigned based on the crystal structure of β-V^VOPO₄, though with distinctly different lattice parameters (VPO₄-m1/β-VOPO₄: Pnma, a = 7.3453(12)/7.7863(5) Å, b = 6.4001(12)/6.1329(3) Å, c = 7.3196(13)/6.9673(5) Å). The XRPD pattern of VPO₄-m2 was found to be very similar to that of Fe₂(VO)(P₂O₇)(PO₄) (VPO₄-m2: P2₁/m, Z = 2, a = 8.792(4) Å, b = 5.269(2) Å, c = 10.398(6) Å, β = 112.60(4)°). The crystal structure models for VPO₄-m1 and VPO₄-m2 have been optimized by DFT calculations. Polymorph m1 contains the unprecedented butterfly shaped [V^IIIO₄] chromophore and has been further characterized by magnetic measurements, by powder reflectance spectroscopy (NIR/vis/UV), and IR spectroscopy. For six polymorphic forms of VPO₄ (m1', m1'', m2, m3, m4, and m5), DFT calculations have been performed. For the existence of VPO₄-m1', -m1'', and -m2, our experiments provide evidence. VPO₄-m3, -m4, and -m5 were obtained by structure optimization based on reduced β-VOPO₄. Their stability is predicted by the DFT calculations.

Synthesis of Porous and Highly Crystallinity Vanadium Phosphorus Oxide Catalysts by Multifunctional Biomass-Based Deep Eutectic Solvents

Deep eutectic solvents (DESs) as a novel class of ionic liquid analogues has been widely used in various fields. Herein, a multifunctional biomass-based DES composed of choline chloride (ChCl) and glucose (GLU) was synthesized and used for the modification of vanadium phosphorus oxide (VPO) catalyst, which is the sole catalyst for the selective oxidation of n-butane to maleic anhydride (MA) in industry. The DES is more green, cost-effective and efficient compared with traditional modifier (metal inorganics and organic polymers). A combination of structure and catalytic performance was investigated in detail to deeply understand its functions. Initially, this DES plays the role of structure directing agent at the stage of precursor synthesis, which can induce the formation of active plane and obtain highly crystalline precursors. Accordingly, the active phase with highly crystalline was obtained after the activation of precursors. Additionally, the DES was decomposed during the precursors activation, resulting in the formation of porous structure around 20 nm on the active plane. Besides this, the surface chemical state including the valence state and acid-base property was changed significantly due to the function of DES. All of these lead to about an 11% increase of MA mass yield, which has great significance to the high value utilization of low carbon alkanes.

Combined NMR and X-ray diffraction study of structural aspects, dynamics and charge ordering mechanism in LixVOPO4.2H2O intercalation compounds

We carried out a detailed investigation of the local ordering and dynamics of the lithium intercalation in paramagnetic Li_xVOPO₄.2H₂O (with 0 < x ≤ 1) materials. This question was addressed using a combination of X-ray diffraction, ³¹P and ⁷Li MAS NMR experiments. We first studied the structure of the fully ordered end-member of the series, Li₁VOPO₄.2H₂O, revisiting the X-ray single crystal diffraction data on the basis of the information provided by ³¹P MAS NMR. We then carried out ⁷Li MAS and exchange NMR experiments and ³¹P MAS experiments on the polycrystalline powders obtained after partial lithium insertion in VOPO₄.2H₂O phases. These experiments evidenced an unexpected ageing of the material related with lithium dynamics between the VOPO₄ layers and a V⁴⁺/V⁵⁺ charge ordering mechanism within the layers.

Effective Interlayer Engineering of Two-Dimensional VOPO4 Nanosheets via Controlled Organic Intercalation for Improving Alkali Ion Storage

Two-dimensional (2D) energy materials have shown the promising electrochemical characteristics for lithium ion storage. However, the decreased active surfaces and the sluggish charge/mass transport for beyond-lithium ion storage that has potential for large-scale energy storage systems, such as sodium or potassium ion storage, caused by the irreversible restacking of 2D materials during electrode processing remain a major challenge. Here we develop a general interlayer engineering strategy to address the above-mentioned challenges by using 2D ultrathin vanadyl phosphate (VOPO₄) nanosheets as a model material for challenging sodium ion storage. Via controlled intercalation of organic molecules, such as triethylene glycol and tetrahydrofuran, the sodium ion transport in VOPO₄ nanosheets has been significantly improved. In addition to advanced characterization including X-ray diffraction, high-resolution transmission electron microscopy, and X-ray absorption fine structure to characterize the interlayer and the chemical bonding/configuration between the organic intercalants and the VOPO₄ host layers, density functional theory calculations are also performed to understand the diffusion behavior of sodium ions in the pure and TEG intercalated VOPO₄ nanosheets. Because of the expanded interlayer spacing in combination with the decreased energy barriers for sodium ion diffusion, intercalated VOPO₄ nanosheets show much improved sodium ion transport kinetics and greatly enhanced rate capability and cycling stability for sodium ion storage. Our results afford deeper understanding of the interlayer-engineering strategy to improve the sodium ion storage performance of the VOPO₄ nanosheets. Our results may also shed light on possible multivalent-ion based energy storage such as Mg²⁺ and Al³⁺.

NMR crystallography: Applications to inorganic materials

Current developments of NMR crystallography as well as some recent applications to diamagnetic inorganic solids are presented. First, we illustrate how solid-state NMR data can be used in combination with diffraction data for the determination of the periodic part of the crystal structures, from the space group selection, to the structure determination over the refinement and validation processes. As ss-NMR, contrary to diffraction (powder and single-crystal), is not restricted to periodic boundary conditions, ss-NMR data can be used to further complete the structural description of materials, including studies of local order/disorder, etc. This illustrated through examples, which are shown and discussed in the second part of this review.