Solid MgO Research Articles

Melting of B1‐Phase MgO From Simultaneous True Radiative Shock Temperature and Sound Speed Measurements to 250 GPa on Samples Preheated to 2300 K

AbstractTo refine the melting curve, equation of state, and physical properties of MgO we performed plate impact experiments spanning 170–250 GPa on MgO single crystals, preheated to 2300 K. A controlled thermal gradient in 20 mm long samples enabled radiative temperature (3%–4%) and rarefaction overtake observations (yielding sound speed 2%) close to the hot Mo driver with a free surface below 2000 K that minimized evaporation. Ta flyers were launched by two‐stage light‐gas gun up to 7.6 km/s and sample radiance was recorded with a 6‐channel (500–850 nm) pyrometer. Shock front reflectivity was measured at 198 and 243 GPa using 50/50 sapphire beam‐splitters. Most experiments show monotonic increases of shock temperature with pressure, from (168 GPa, 7100 K) to (243 GPa, 9400 K), in good agreement with predictions of our MgO B1 phase equation of state. Measured sound speeds are parallel to but 10% higher than model predictions for bulk sound speed of solid B1 MgO, confirming ductile behavior of preheated MgO. Two experiments, at 238 and 246 GPa, showed anomalously low shock temperature and sound speed, suggesting melting. Using reported MgO melting data up to 120 GPa and our data at 232–246 GPa, we constructed a maximum‐likelihood Simon‐Glatzel fit. At Earth's core‐mantle boundary pressure (135 GPa), our best‐fit interpolated MgO melting point is K. Our proposed melting line falls within the envelope of theoretical predictions but does not completely agree with any particular model curve. Our results reduce the uncertainty on MgO melting temperature at Earth's core‐mantle boundary by a factor of 17 and provide an anchor for extension to multicomponent systems.

Efficient and fast arsenate removal from water by in-situ formed magnesium hydroxide

MgO nanoparticles have good As-adsorption capacity in treating As-contaminated wastewater but suffer from high production cost. In this study, instead of using pre-formed MgO nanoparticles, we found that in-situ formed Mg(OH)2 from MgCl2 and NaOH reaction exhibited super high arsenate (As(V)) removal efficiency. Only 1.5 mmol/L of in-situ formed Mg(OH)2 could remove more than 95% As(V) within 10 min to make the As contaminated water (10 mg-As(V)/L) meet the municipal wastewater treatment standard, whereas MgO nanoparticles failed. The Mg-As sludge has an amorphous crystal structure while no Mg(OH)2 phase could be observed. As(V) existed uniformly within the sludge which was confirmed by elemental mapping. A precipitation-adsorption-coagulation mechanism might exist, which could relieve the restriction of limited surface area of solid MgO adsorbents. This study not only reveals an applicable method for efficient removal of trace level As(V) from water but also implies the huge potential of in-situ formed adsorbents in water treatment.

Multieffect Preoxidation Strategy to Convert Bituminous Coal into Hard Carbon for Enhancing Sodium Storage Performance.

Preoxidation is an effective strategy to inhibit the graphitization of coals during carbonization. However, the single effect of the traditional preoxidation strategy could barely increase surface-active sites, hindering further enhancement of sodium storage. Herein, a multieffect preoxidation strategy was proposed to suppress structural rearrangement and create abundant surface-active sites. Mg(NO3)2·6H2O helps to introduce oxygen-containing functional groups into bituminous coal at 450 °C, which acted as a cross-linking agent to inhibit the rearrangement of carbon layers and promote structural cross-linking during the subsequent thermal carbonization process. Besides, the residue solid decomposition product MgO would react with carbon to create surface-active sites. The obtained coal-based hard carbon contained more pseudographitic domains and sodium storage active sites. The optimized sample could deliver an excellent capacity of 287.1 mAh g-1 at 20 mA g-1, as well as remarkable cycling stability of capacity retention of 96.1% after 200 cycles at 50 mA g-1, and notable capacity retention of 88.9% after 1000 cycles at 300 mA g-1. This work provides an effective and practical strategy to convert low-cost bituminous coal into advanced hard carbon anodes for sodium-ion batteries (SIBs).

Microwave-supported one-pot reaction for the synthesis of 5-alkyl/arylidene-2-(morpholin/thiomorpholin-4-yl)-1,3-thiazol-4(5H)-one derivatives over MgO solid base

Abstract In this study, we synthesized 5-alkyl/arylidene-2-(morpholin/thiomorpholin-4-yl)-1,3-thiazol-4(5H)-one through a one-pot reaction using all starting materials in a microwave oven. The presence of the solid base MgO catalyzed the reaction in the eco-friendly solvent of ethanol as a green chemistry approach owing to the noticeable advantages of short reaction times to save energy and less toxic starting materials for environmental friendliness. This indicates that the one-pot reaction makes the process simpler, in which the reaction time (1 h) is shorter than that of conventional methods (10 h). The yield of the reactions reached 55–76% for 18 final products consisting of 17 derivatives of morpholine or thiomorpholine with various aldehydes and one extended moiety of the primary amine, of which 13/18 final compounds were new. The purification procedure was performed without using polluting solvents. The structures were confirmed using IR, 1H-NMR, 13C-NMR, and MS analyses.

Removal behavior of Al–Ti–O inclusions in aluminum deoxidized molten steel containing titanium by microporous magnesium foam ceramic filters

Al–Ti–O inclusions in aluminum deoxidized and titanium (Ti)-alloyed steel aggravate the submerged nozzle clogging and instability of the continuous casting process, lowering the cleanliness of casting billet. In the present investigation, the removal behavior of the oxide inclusions in aluminum deoxidized IF steel and Ti-alloyed welding wire steel by microporous/fused MgO foam ceramic filter (PMF/FMF) and the improvement of the steel cleanliness were comparatively investigated. It is found that MgO foam ceramic filters immersed in Al deoxidized steel effectively reduced its number density of Al2O3 inclusions and total oxygen (TO) content, by use of forming Al–Mg–O spinel interface reaction layer between the filters and tested steel. Furthermore, the cleanliness increase of Ti-alloyed steel immersed by the filters, due to formation of a more complete and continuous Al–Mg–Ti–O spinel layer on their surface, was bigger compared with that in Ti-free steel, with sporadically distributed Al–Mg–O spinel of interface layer on the surface of filters immersed in Ti-free steel. This is ascribed to the higher Ti content and some liquid inclusion of MgO–Al2O3-TiOX in Ti-alloyed steel, enhancing the reaction of the MgO of aggregates in the filters with Al, Ti, and O and deposition of other solid MgO–Al2O3-TiOX on the surface of the filter, respectively. In addition, the better purification efficiency of the PMF on tested steel compared with the FMF, was due to its higher porosity, roughness and more liquid phase components of the matrix in PMF, which improve the reaction of filter with liquid steel, and raise its adsorption capacity with Al2O3-based inclusions in the steel, forming Al–Mg–O spinel interface layer to removal the oxide inclusions.

Modelling Selective CO2 Absorption and Validation via Photosynthetic Bacteria and Chemical Adsorbents for Methane Purification in Anaerobic Fermentation Bioreactors.

This study delves into advanced methane purification techniques within anaerobic fermentation bioreactors, focusing on selective CO2 absorption and comparing photosynthetic bacteria (PNSB) with chemical adsorbents. Our investigation demonstrates that MgO-Mg(OH)2 composites exhibit remarkable CO2 selectivity over CH4, substantiated through rigorous bulk and surface modelling analyses. To address the challenges posed by MgCO3 shell formation on MgO particles, hindering CO2 transport, we advocate for the utilisation of MgO-Mg(OH)2 composites. In on-site experiments, these composites, particularly saturated MgO-Mg(OH)2 solutions (S2), achieved an astonishing 100% CO2 removal rate within a single day while preserving CH4 content. In contrast, solid MgO powder (S3) retained a mere 5% of CH4 over a 10 h period. Although PNSB (S1) exhibited slower CO2 removal, it excelled in nutrient recovery from anaerobic effluent. We introduce a groundbreaking hybrid strategy that leverages S2's swift CO2 removal and S1 PNSB's nutrient recovery capabilities, potentially resulting in a drastic reduction in bioreactor processing time, from 10 days when employing S1 to just 1 day with the use of S2. This represents a remarkable efficiency improvement of 1000%. This pioneering strategy has the potential to revolutionise methane purification, enhancing both efficiency and sustainability. Importantly, it can be seamlessly integrated into existing bioreactors through an additional CO2 capture step, offering a promising solution for advancing biogas production and promoting sustainable waste treatment practices.

Two level deacidification mathematical model for the description of transport of solid alkaline particles and diffusion of ions in a treated acid paper

Industrial progress in papermaking in the early nineteenth century led to the technologies that resulted in more acidic papers, which was caused mainly by the exploitation of alum (KAl(SO4)2) and rosin as sizing agents. The papers prepared by such technologies have degraded more quickly. From the 1930s various deacidification and preservation processes with basic agents have been developed. The most widespread deacidification process is with the aerosol (spray system) consisting of microscale particles MgO and perfluoroheptane (PFH) as a carrier (the so-called Bookkeeper process). The shortcomings of this process are the low dissolution of solid MgO particles and the transport to the interior of acidic paper. We have developed a theoretical two-level model of the Bookkeeper process suitable for prediction of deacidification extent. The model involves both the dissolution/reaction of the solid particles and transport of solvated ions and solid particles inside the bulk of paper. The developed model coincides with the results of the performed deacidification experiment. The model is also in good agreement with the Lucas–Washburn equation, which is usually used for the description of the penetration of a deacidifying agent into the paper.

Equation of state of MgO up to 345 GPa and 8500 K

The accurate equation of state (EOS) of MgO can be used as a pressure scale, but the EOS currently used has too much uncertainty to serve as a pressure scale, especially at high-pressure and high-temperature conditions. Here we present a complete EOS of solid MgO up to 345 GPa, 8500 K, and its derived pressure-volume-temperature (P-V-T) EOS form, which can be used as a pressure scale. In this EOS, the free energy is expressed as a three-parameter function based on a quasi-Debye thermodynamic model, which establishes a relatively accurate phonon thermodynamic description by determining the phonon dispersion relation and phonon density of states from the volume-dependent sound velocity. The three parameters are determined by a global optimization method, which minimizes the sum of the weighted variances between the physical quantities calculated by the thermodynamic model and the relevant experimental values within the unified EOS framework. To improve the accuracy of the EOS, the parameters are constrained by room-temperature sound velocities and shock compression data, including P-V-T data and sound velocities up to 265 GPa along the principal Hugoniot of MgO measured by us, the P-V-T data at 174--203 GPa, and partial P-V-T data of MgO preheated to 1850 K by others. The derived EOS of MgO can well reproduce not only the above data used to define the parameters, but also the shock compression data and the measured isobaric specific heat not used in the parameter optimization.

Effects of pressure on hydrogen diffusion behaviors in MgO.

Hydrogen, as the smallest atom and a key component of water, can penetrate into materials in various forms (e.g., atoms, molecules), which has significant effects on their properties; hence, the diffusion behavior of hydrogen has aroused widespread attention. One of the major compositions in the Earth's interior is MgO. Thus, the diffusion behavior of hydrogen in MgO under high pressure is vital for understanding the water cycle in the Earth's interior. However, the hydrogen diffusion behavior in MgO under high pressure is still poorly understood. Herein, the hydrogen diffusion behaviors in MgO with increasing pressure are systematically investigated in the framework of first-principles methods. Our results show that separated H atoms tend to converge to form H2 molecules, and H2 molecules tend to gather together. The energy barriers of both H and H2 diffusion in MgO increase with pressure. Notably, our results illustrate that hydrogen prefers to diffuse in solid MgO in its molecular state even under high pressure. Furthermore, the attempt frequency of hydrogen in MgO increases with temperature, while it decreases with pressure. This study will deepen our understanding of hydrogen diffusion behavior in MgO under high pressure and provide guidance for studies on particle diffusion in solid materials under extreme conditions.

Kinetic Salt Effect on Base-Catalyzed Isomerization of d-Glucose into d-Fructose.

Isomerization of d-glucose (Glc) into d-fructose (Fru) is of great importance for food sector as well as for valorization of lignocellulosic biomass. Soluble and solid bases exhibit high catalytic activity for the isomerization. Here, we report a salt effect on the base-catalyzed aqueous-phase Glc-Fru isomerization. Addition of soluble salts (Na2 SO4 , NaNO3 , K2 SO4 , and NaCl) results in an increased apparent reaction rate (factors of 1.5 to 6). The salt effect was observed both in the presence of soluble base NaOH at constant pH value and solid bases MgO, Li3 PO4 , and Mg-Al hydrotalcite. Apparent activation energy and UV absorption spectra were not significantly influenced by addition of salts. Potentiometric titration showed that the acidity constants of the saccharides increase in the presence of electrolytes. Since the rate of the isomerization depends on the thermodynamic acidity constant of Glc, the isomerization is accelerated by the presence of electrolytes.

Electrochemistry Isolates Elemental Phosphorus from Mg3(PO4)2

Phosphorus (P) is an essential raw material for many value-added chemicals in modern industry, yet the natural minerals to extract P are depleting. As an alternative, municipal wastewater is a promising secondary source of P. Recovery of P from wastewater by chemically induced precipitation as insoluble metal phosphates is a typical practice. Mg3(PO4)2 is a commonly found phosphate in sewage sludge or incinerated sewage sludge ash. Here we try to extract elemental P from Mg3(PO4)2 by electrochemistry. By using molten CaCl2 as the solvent, PO4 3− in the Mg3(PO4)2 crystal can be quickly leached into the melt as free ions. The presence of Mg2+ in the melt boosts the solubility and dissolution rate of PO4 3− significantly. High-purity elemental P can be isolated from free PO4 3− in the melt by electrolysis. This work demonstrates that Mg3(PO4)2 is a possible precursor to prepare elemental P by electrochemistry, though the negative influence of Mg2+ of forming solid MgO on the electrode surface during electrolysis needs to be addressed.

Synthesis of Mg-Al LDH and its calcined form with natural materials for efficient Cr(VI) removal

Layered double hydroxide (LDH) has been extensively studied due to its excellent capacity of heavy metal adsorption. However, the synthesis of LDH is always complex and requiring pure chemical reagents. In this study, two solid minerals, MgO and metakaolin (MK), derived from abundant natural resources, were used as raw materials to synthesize Mg-Al LDH via a facile one-pot method. The compositions and morphologies of synthetic products and its calcined form(C-LDH) were characterized by X-ray diffraction (XRD), Thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Cr(VI) adsorption by the LDH and C-LDH was systematically studied including the effect of contact time, initial pH value, temperature, liquid/solid ratio, and initial Cr(VI) concentration. The adsorption kinetics and isotherms were modelled, and thermodynamic parameters were determined. Results showed that the LDH and C-LDH exhibited high adsorption capacities (23.5–33.2 mg/g and 33.2–38.9 mg/g, respectively) within a wide pH range of 1.75–6.0, and maintained a higher adsorption capacity in all cases afterwards. The pseudo-second-order kinetic model fitted the kinetic data well in which the Langmuir model described the adsorption isotherms the best. The adsorption process was endothermic and spontaneous. The adsorption mechanisms may include anion exchange, redox reaction, and electrostatic adsorption. Moreover, the effect of the synthesis conditions (i.e., alkali concentration, temperature and liquid/solid ratio) on the adsorption characteristics were discussed. And XPS, UV-Vis Spectrophotometer (UV-Vis) and FTIR analysis confirmed Cr(VI) adsorption on the LDH and C-LDH surface, followed by its subsequent reduction to Cr(III). It has been revealed that the LDH/C-LDH synthesized in this work exhibited higher Cr(VI) removal capacities compared to the ones reported in the literature, demonstrating the significant potential of synthesizing high performance absorbents by low-cost natural materials.

Co-axial electrospun hollow MgO nanofibers for efficient removal of fluoride ions from water

Porous and hollow metal oxide nanomaterials receive more attraction towards adsorption of various contaminates from water due to extremely high surface area. Co-axial electrospinning is an advance method to easily fabricate long length hollow nanotubes/nanofibers. Herein, hollow MgO nanofibers were synthesized using co-axial electrospinning method followed by post annealing. The co-axial electrospinning was performed by maintaining applied voltage (15 kV), needle tip to collector distance (12 cm) and flow rate of core and shell solutions (0.3 and 0.5 mL/h respectively). The formation, morphologies, bonding and surface area of the prepared nanofibers was analysed by XRD, FESEM, TEM and BET analytical techniques. The inner pore diameter of hollow MgO nanofibers was 32 nm. The surface area was found to be 527 m2/g which is much higher as compared to solid MgO nanofibers. The prepared hollow MgO nanofibers were used for the adsorption of fluoride ions from water. The effect of dose, contact time, pH and concentration were analysed. The fluoride adsorption process follows Pseudo-second-order kinetic and Langmuir isotherm models. The maximum adsorption capacity of hollow MgO was found to be 294 mg/g which is almost three-time greater than that of solid MgO nanofibers (92 mg/g). All the co-existing ions have showed negligible effect on fluoride adsorption except CO32- and PO43- ions. The adsorption energy of fluoride adsorbed MgO was calculated. The adsorption energy is found to be − 6.9004 eV. The negative value of adsorption energy indicates the feasibility of adsorption process.

Implementation and Validation of Constrained Density Functional Theory Forces in the CP2K Package.

Constrained density functional theory (CDFT) is a powerful tool for the prediction of electron transfer parameters in condensed phase simulations at a reasonable computational cost. In this work we present an extension to CDFT in the popular mixed Gaussian/plane wave electronic structure package CP2K, implementing the additional force terms arising from a constraint based on Hirshfeld charge partitioning. This improves upon the existing Becke partitioning scheme, which is prone to give unphysical atomic charges. We verify this implementation for a variety of systems: electron transfer in (H2O)2+ in a vacuum, electron tunnelling between oxygen vacancy centers in solid MgO, and electron self-exchange in aqueous Ru2+–Ru3+. We find good agreement with previous plane-wave CDFT results for the same systems, but at a significantly lower computational cost, and we discuss the general reliability of condensed phase CDFT calculations.

Synthesis of highly active solid base catalysts of SrO-ZrO2 and SrO-SiO2 by solid–liquid interface reactions of hydrated strontium hydroxide with metal alkoxides

Highly dispersed SrO in amorphous ZrO2 or SiO2, SrO–ZrO2, and SrO–SiO2 mixed oxides were synthesized by applying the solid–liquid interface reaction of Sr(OH)2•8 H2O in the solid phase with an equal number of moles of Zr(OC3H7)4 or with twice the number of Si(OC2H5)4 moles dissolved in 2-propanol. After heat treating at 723 K, the SrO–ZrO2 catalyst exhibited a higher activity than the conventional solid base MgO catalyst for the base-catalyzed retroaldol reaction of diacetone alcohol. The catalytic activity of SrO–ZrO2 was higher by a factor of 1.5 than that of MgO prepared by the thermal decomposition of hydrated magnesium oxalate (MgC2O4•2H2O). By contrast, the highest activity of SrO–SiO2 obtained by thermal activation at 673 K was much lower than that of MgO. The active SrO–ZrO2 catalyst was composed of catalytically inactive species of SrZrO3, and small amounts of Sr(OH)2 and SrCO3. The expected active species of SrO was not detected in powder X-ray diffraction analysis in any sample. The formation of SrO in small crystal size dispersed in amorphous ZrO2 was strongly expected. Similar results were obtained in the SrO–SiO2 sample. An active site formed on SrO–ZrO2 surface is proposed.

Highly efficient catalytic ozonation for ammonium in water upon γ-Al2O3@Fe/Mg with acidic-basic sites and oxygen vacancies

Catalytic ozonation has prospects in the advanced treatment of nitrogen removal, and solid base MgO can efficiently catalyze the ozonation of ammonium nitrogen. However, it is necessary to improve the problem of easy loss, difficult recovery, and low percentage of gaseous products. Here, MgO, amorphous Fe2O3, and γ-Al2O3 were selected as doping components and supports, respectively, to prepare γ-Al2O3@Fe/Mg composite catalysts with abundant acidic-basic sites and oxygen vacancies. The results show that γ-Al2O3@Fe/Mg5 can efficiently catalyze the ozonation of ammonium nitrogen (98.73%) with 67.82% gaseous product selectivity under the conditions of initial pH = 7, catalyst dosage of 112.88 g/L, and ozone dosage of 2.4 mg/min. The doping of Fe2O3 and MgO with a weaker lattice oxygen binding energy improves the gaseous product selectivity. The mechanism of ammonium nitrogen removal for γ-Al2O3@Fe/Mg5 is revealed, especially the intrinsic contribution of acidic-basic sites and oxygen vacancies. The pH and active sites play different roles in ozone decomposition for NH4+ removal. Surface hydroxyl protonation on basic sites and oxygen vacancies and electron transfer on acidic sites are responsible for ozone decomposition to hydroxyl radicals. Moreover, γ-Al2O3@Fe/Mg5 exhibits good stability, few leaching ions, and can be settled in water for easy recovery. This study suggests that γ-Al2O3@Fe/Mg5 is a good candidate for the catalytic ozonation of ammonium nitrogen.

Melt Migration and Chemical Differentiation by Reactive Porosity Waves

AbstractMelt transport across the ductile mantle is essential for oceanic crust formation or intraplate volcanism. However, mechanisms of melt migration and associated chemical interaction between melt and solid mantle remain unclear. Here, we present a thermo‐hydro‐mechanical‐chemical (THMC) model for melt migration coupled to chemical differentiation. We consider melt migration by porosity waves and a chemical system of forsterite‐fayalite‐silica. We solve the one‐dimensional (1D) THMC model numerically using the finite difference method. Variables, such as solid and melt densities or MgO and SiO2 mass concentrations, are functions of pressure (P), temperature (T), and total silica mass fraction (). These variables are pre‐computed with Gibbs energy minimization and their variations with evolving P, T, and are implemented in the THMC model. We consider P and T conditions relevant around the lithosphere‐asthenosphere boundary. Systematic 1D simulations quantify the impact of initial distributions of porosity and on the melt velocity. Larger perturbations of cause larger melt velocities. An adiabatic or conductive geotherm cause fundamentally different vertical variations of densities and concentrations, and an adiabatic geotherm generates higher melt velocities. We quantify differences between melt transport (considering incompatible tracers), major element transport and porosity evolution. Melt transport is significant in the models. We also quantify the relative importance of four porosity variation mechanisms: (a) mechanical compaction and decompaction, (b) density variation, (c) compositional variation, and (d) solid‐melt mass exchange. In the models, (de)compaction dominates the porosity variation. We further discuss preliminary results of 2D THMC simulations showing blob‐like and channel‐like porosity waves.

Role of metal (Pt)–support (MgO) interactions in base-free glucose dehydrogenation

The solid base support MgO instead of a homogeneous base increases the efficiency of Pt-catalysed glucose dehydrogenation and avoids catalyst poisoning due to alkali metal ions.

Fabrication of porous forsterite-spinel-periclase ceramics by transient liquid phase diffusion process for high-temperature thermal isolation

Porous forsterite-spinel-periclase ceramics with low thermal conductivity were synthesized via a transient liquid phase diffusion process by using pre-synthesized pellets and fused magnesia powder. The effects of sintering temperature on the pore formation, phase composition, sintering behavior, and properties of the resulting porous ceramics were investigated. The pre-synthesized pellets had a porous structure and contained a large amount of cordierite and enstatite. During the sintering progress, the pellets were converted into a transient liquid phase, which diffused into the solid MgO matrix. The liquid phase diffusion reaction promoted forsterite and spinel formation, which resulted in the in-situ formation of large pores. At elevated temperatures, the liquid phase disappeared and a large number of well-developed grains were simultaneously precipitated from the liquid phase. Porous ceramics with thermal conductivities of 0.42–0.48 W/(m·K) and refractoriness under load values of 1588 °C and 1624 °C were obtained after sintering at 1600 °C for 3 h.

Base Catalysis of Sodium Salts of [Ta6−xNbxO19]8− Mixed-Oxide Clusters

The solid base catalysis of sodium salts of Lindqvist-type metal oxide clusters was investigated using a Knoevenagel condensation reaction. We successfully synthesized the sodium salts of Ta and Nb mixed-oxide clusters Na8−nHn[(Ta6−xNbx)O19]·15H2O (Na-Ta6−xNbx, n = 0, 1, x = 0–6) and found them to exhibit activity for proton abstraction from nitrile substrates with a pKa value of 23.8, which is comparable to that of the conventional solid base MgO. The Ta-rich Na-Ta6 and Na-Ta4Nb2 exhibited high activity among Ta and Nb mixed-oxide clusters. Synchrotron X-ray diffraction (SXRD) measurements, Fourier-transform infrared (FT-IR) spectroscopy, and X-ray absorption spectroscopy (XAS) revealed the structure of Na-Ta6−xNbx: (1) The crystal structure changed from Na7H[M6O19]·15H2O to Na8[M6O19]·15H2O (M = Ta or Nb) by the anisotropic expansion of the unit cell with an increase in Ta content; (2) Highly symmetrical Lindqvist [Ta6−xNbxO19]8− was generated in Na-Ta4Nb2 and Na-Ta6 because of the symmetrical association of Na+ ions with [Ta6−xNbxO19]8− in the structure. DFT calculation revealed that the Lindqvist structures with high symmetry have large NBO charges on surface oxygen species, which are strongly related to base catalytic activity, whereas the composition hardly affects the NBO charges. The above results showed that the Brønsted base catalysis was sensitive to the symmetry of the Lindqvist [Ta6−xNbxO19]8− structure. These findings contribute to the design of solid base catalysts composed of anionic metal oxide clusters with alkaline-metal cations.