Showing papers on "Phosphotungstic acid published in 2019"

Magnetic field alignment of stable proton-conducting channels in an electrolyte membrane.

Abstract: Proton exchange membranes with short-pathway through-plane orientated proton conductivity are highly desirable for use in proton exchange membrane fuel cells. Magnetic field is utilized to create oriented structure in proton exchange membranes. Previously, this has only been carried out by proton nonconductive metal oxide-based fillers. Here, under a strong magnetic field, a proton-conducting paramagnetic complex based on ferrocyanide-coordinated polymer and phosphotungstic acid is used to prepare composite membranes with highly conductive through-plane-aligned proton channels. Gratifyingly, this strategy simultaneously overcomes the high water-solubility of phosphotungstic acid in composite membranes, thereby preventing its leaching and the subsequent loss of membrane conductivity. The ferrocyanide groups in the coordinated polymer, via redox cycle, can continuously consume free radicals, thus helping to improve the long-term in situ membrane durability. The composite membranes exhibit outstanding proton conductivity, fuel cell performance and durability, compared with other types of hydrocarbon membranes and industry standard Nafion® 212. Proton exchange membranes with short-pathway through-plane proton conductivity are attractive for proton exchange membrane fuel cells. Here the authors align proton conducting channels orthogonal to the plane of composite proton exchange membranes using a magnetic field for improved fuel cell performance.

In situ spectroscopy-guided engineering of rhodium single-atom catalysts for CO oxidation

Abstract: Single-atom catalysts have recently been applied in many applications such as CO oxidation. Experimental in situ investigations into this reaction, however, are limited. Hereby, we present a suite of operando/in situ spectroscopic experiments for structurally well-defined atomically dispersed Rh on phosphotungstic acid during CO oxidation. The identification of several key intermediates and the steady-state catalyst structure indicate that the reactions follow an unconventional Mars-van Krevelen mechanism and that the activation of O2 is rate-limiting. In situ XPS confirms the contribution of the heteropoly acid support while in situ DRIFT spectroscopy consolidates the oxidation state and CO adsorption of Rh. As such, direct observation of three key components, i.e., metal center, support and substrate, is achieved, providing a clearer picture on CO oxidation on atomically dispersed Rh sites. The obtained information are used to engineer structurally similar catalysts that exhibit T20 values up to 130 °C below the previously reported Rh1/NPTA. Single-atom catalysts have been studied for CO oxidation, but experimental in situ investigations are limited. Here, the authors use a suite of in situ/operando spectroscopy to identify key intermediates and define design principles to enhance the CO oxidation activity of atomically dispersed Rh on heteropoly acids.

Heteropolyacid supported on Zr-Beta zeolite as an active catalyst for one-pot transformation of furfural to γ-valerolactone

Abstract: A novel bifunctional catalyst that enables an efficient one-pot conversion of furfural into γ-valerolactone (GVL) has been developed by anchoring heteropolyacid (HPA) on Zr-Beta zeolite The catalysts were prepared by a post-synthesis procedure, which consists of the dealumination of Al-Beta, incorporation of Zr into the beta framework through solid-state ion-exchange and impregnation of the HPA Zr-Beta is used as a Lewis acid catalyst to catalyze the transfer hydrogenation of furfural and levulinic acid/ester using 2-propanol as a hydrogen donor To deal with the inability of Zr-Beta to catalyze the hydrolytic ring-opening of furans toward GVL, phosphotungstic acid (HPW) and silicotungstic acid (HSiW) were introduced to the Zr-Beta as BrOnsted acid sites The characterization of the catalysts using XRD, UV–vis and XPS as well as TPD of ammonia and FT-IR spectroscopy of the adsorbed pyridine revealed that the HPA/Zr-Beta possesses both isolated Lewis and BrOnsted acid sites When they were applied to the one-pot cascade conversion of furfural, the initial activity of the HPA/Zr-Beta toward GVL production were 2–3 times greater than that for Zr-Beta due to the enhanced hydrolytic ring-opening of the furans promoted by the added BrOnsted acidity Especially, HPW loaded Zr-Beta demonstrated a remarkable GVL yield of ∼70% at 433 K after 24 h due to its high thermal stability and stronger BrOnsted acidity, and its activity far surpasses that of the conventional Sn-Al-Beta zeolite (∼40%) Overall, this study demonstrates that an incorporation of HPA into Lewis acid Sn- or Zr-Beta zeolites is an effective strategy to create isolated Lewis and BrOnsted acid sites within a single catalyst, thereby allowing the selective cascade catalysis for the cost-effective production of high-value chemicals

Electron-hole interactions in choline-phosphotungstic acid boosting molecular oxygen activation for fuel desulfurization

Abstract: Many studies have been conducted regarding the separation behavior of carriers (electrons and holes) because of involving the generation of superoxide radicals (O2− ), hydroxyl radicals (HO ), and hydrogen peroxide (H2O2) in the photocatalytic process of heteropolyacids. Instead, relatively little attention has been focused on the potential Coulomb interactions between photogenerated electrons (e−) and holes (h+). Herein, choline-phosphotungstic acid (Ch3-HPW) was synthesized via one-step acid-base neutralization reaction method, and characterized. The electronic excited state analysis of Ch3-HPW showed that the formation of singlet oxygen (1O2) was related to the electron-hole interactions in the photocatalytic process of ground state molecular oxygen (3O2) activation. Subsequently, a facile fuel photocatalytic oxidative desulfurization and extraction system was established on the basis of Ch3-HPW, air, and acetonitrile (MeCN), to better understand the 3O2 activation in specific applications. The main photocatalytic reaction conditions affecting the desulfurization process, including the amount of Ch3-HPW, the volume ratio of MeCN to model oil, the initial S-concentration, air/N2 bubbling, sulfur compounds, and fuel composition, were systematically investigated under UV radiation. The sulfur removal for model oil and straight-run gasoline in the system were 99.6% and 89.9%, respectively. The results of radical scavenger experiments, electron spin-resonance (ESR) spectroscopy, and density functional theory (DFT) calculations further demonstrated that 1O2, H2O2, and h+ played important roles in the oxidation of sulfur-containing compounds. A new method was developed for the desulfurization of liquid fuels using green and inexpensive O2 in this work to promote the development of photocatalytic process of exciton-involved HPA-based photocatalysts.

Highly Crystalline Mesoporous Phosphotungstic Acid: A High-Performance Electrode Material for Energy-Storage Applications.

Abstract: Heteropoly acids (HPAs) are unique materials with interesting properties, including high acidity and proton conductivity. However, their low specific surface area and high solubility in polar solvents make them unattractive for catalytic or energy applications. This obstacle can be overcome by creating nanoporosity within the HPA. We synthesized mesoporous phosphotungstic acid (mPTA) with a spherical morphology through the self-assembly of phosphotungstic acid (PTA) with a polymeric surfactant as stabilized by KCl and hydrothermal treatment. The mPTA nanostructures had a surface area of 93 m2 g-1 and a pore size of 4 nm. Their high thermal stability (ca. 450 °C) and lack of solubility in ethylene carbonate/diethyl carbonate (EC/DEC) electrolyte are beneficial for lithium-ion batteries (LIBs). Optimized mPTA showed a reversible capacity of 872 mAh g-1 at 0.1 A g-1 even after 100 cycles for LIBs, as attributed to a super-reduced state of HPA and the storage of Li ions within the mesochannels of mPTA.

Synthesis and characterization of Keggin-type polyoxometalate/zirconia nanocomposites—Comparison of its photocatalytic activity towards various organic pollutants

Abstract: Heteropolyacid supported ZrO2 nanocomposites has been prepared through wet impregnation method with the intention to improve the photodegradation performance under UV light. The material consist of mixed monoclinic and cubic ZrO2 phase and orthorhombic Keggin type 12- phosphotungstic acid(PTA) with different ratios. The phase structure, chemical composition and oxidation state of the elements tungsten (W), zirconia (Zr) were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). In addition, Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, Diffuse Reflectance Ultraviolet–Visible Spectroscopy (DRS-UV–Vis) were carried out to confirm the presence of functional group and optical absorption properties. Surface area of the synthesized composite was analyzed by BET analysis and morphology was confirmed using high resolution scanning electron microscopy (HR-SEM) and high resolution transmission electron microscopy (HR-TEM) and the results reveal that PTA is chemically bonded to zirconia support which confirms that the sample was synthesized successfully. The heterogeneous photocatalytic activity of the composites were studied by using aqueous solution of various organic pollutants such as methylene blue (MB), methyl orange (MO), rhodamine B (Rh-B), crystal violet (CV), bromo cresol green (BCG) dyes, 4- Nitrophenol and 2,4- Dichlorophenoxy acetic acid under UV light illumination. It is found that each dye needs different specific ratio of nanocomposite to degrade efficiently at a given time interval.

Preparation of core-shell PW12@TiO2 microspheres and oxidative desulfurization performance.

Abstract: A polyoxometalate-based microsphere catalyst has been prepared through the one-step template method using phosphotungstic acid as the core and TiO2 as the shell, denoted as PW12@TiO2. Multiple characterisation methods namely FT-IR, XRD, XPS, Raman, SEM and TEM were used to characterize the resultant materials, and results indicate that the phosphotungstic acid was encapsulated into the TiO2 phase as the core to form the core-shell structure. The resultant composites were used as catalysts for the oxidative desulfurization of a model oil with H2O2 as oxidant and acetonitrile as solvent. Catalyst PW12@TiO2 exhibited good catalytic activity, with 99.9% S-removal of dibenzothiophene after 60 min under the optimum conditions. Leaching and recycling experiments revealed that the PW12@TiO2 catalyst has excellent recyclability, and there was no significant decrease in S-removal after seven cycles under identical reaction conditions, which could be attributed to the fabrication of the core-shell structure, thus inhibiting the loss in the active sites.

Phosphotungstic acid immobilized nanofibers-Nafion composite membrane with low vanadium permeability and high selectivity for vanadium redox flow battery.

Abstract: A novel composite vanadium-blocking proton exchange membrane for vanadium redox flow battery (VRB) was designed and constructed by immobilizing phosphotungstic acid (PWA) on Nano Kevlar Fibers (NKFs) via a solution-casting method. The proton/vanadium selectivity of the composite membrane is greatly improved by incorporating the complex formed by NKFs and PWA into the Nafion matrix. Simple tuning of the complex doping quantity results in different composite membranes with superior vanadium barrier properties (namely, a minimum vanadium permeability of 2.46 × 10-7 cm2 min−1). This coupled with proton conductivities reaching 0.061 S cm−1 at room temperature indicates a proton/vanadium selectivity of 2.48 × 105 S min cm−3, which is 6.3 times higher than that of recast Nafion (0.34 × 105 S min cm−3). When tested in a VRB, the performance of the single cell assembled with the composite membrane greatly outperforms that with recast Nafion at current densities ranging from 40 to 100 mA cm−2.

Surface Functionalization of Magnetite Nanoparticles with Sulfonic Acid and Heteropoly Acid: Efficient Magnetically Recoverable Solid Acid Catalysts.

Abstract: New magnetically recoverable solid acid catalysts for acid-catalyzed reactions were designed via the surface chemical functionalization of silica-coated magnetite nanoparticles (SCMNPs) with sulfonic acid groups. First, the SCMNPs were covalently functionalized with 3-aminopropyl groups to achieve Amp-SCMNPs. Then, reaction of the Amp-SCMNPs with 1,4-butane sultone followed by acidification with phosphotungstic acid (HPW) or diluted sulfuric acid produced magnetically recoverable solid acid catalysts, HPW-ampsul-SCMNPs and H-ampsul-SCMNPs, respectively. Both catalysts were characterized by various physicochemical analyses such as Fourier transform infrared (FT-IR) and inductively coupled plasma-optical emission (ICP-OES) spectroscopies, vibrating sample magnetometry (VSM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and energy-dispersive X-ray (EDX) analyses. Finally, the catalytic activities of the prepared catalysts were examined in the esterification of acetic acid with butanol and acetalization of benzaldehyde with ethylene glycol. Excellent catalytic efficiencies were obtained in both cases. The catalysts were consecutively recovered and reused five times without significant loss of their activities.

A nano-reactor based on PtNi@metal–organic framework composites loaded with polyoxometalates for hydrogenation–esterification tandem reactions

Abstract: Tandem catalysis (i.e., a process in which a desirable product is synthesized by a one-step process consisting of sequential reactions) has attracted intensive attention owing to its sustainable green and atom-economical characteristics. In this process, the utilization of a high-efficiency multifunctional catalyst is key. However, different functional sites integrated within the catalyst are required to be rationally designed and precisely engineered to guarantee the synergy between the catalytic reactions. Herein, a novel kind of hydrogenation-esterification tandem catalyst with metal/acid (alloy/polyoxometalates) active sites integrated within the metal-organic frameworks (MOFs) was prepared by a facile self-sacrificial template route. In this tandem catalyst, the MOF cavities served as tandem reactors, the PtNi alloy sites encapsulated within the MOF material acted as hydrogenation sites, and the solid phosphotungstic acid embedded in the MOF cavities provided esterification sites. This well-designed tandem catalyst showed outstanding activity and selectivity towards the one-step synthesis of amino-ester-type anesthetics (e.g., benzocaine) owing to the synergistic catalysis of the metal and acid sites. Clearly, this novel tandem catalyst simplifies the traditional industry process and provides a new method to rationally construct new tandem catalysts.

High proton conductivity polybenzimidazole proton exchange membrane based on phosphotungstic acid-anchored nano-Kevlar fibers

Abstract: A novel polybenzimidazole (PBI) proton exchange membrane based on phosphotungstic acid (PWA)-anchored nano-Kevlar fibers (NKFs) has been successfully prepared by solution casting. Employing NKFs instead of frequently used oxide support as anchoring agent has effectively conquered the problem of the poor interface compatibility between the inorganic component and the polymer, resulting in the excellent dispersion of PWA in the matrix and providing favorable conditions for the formation of consecutive proton transport channels. The as-obtained PBI/NKFs@PWA membrane exhibits proton conductivities as high as 0.029 and 0.051 S cm−1 at 20 and 80 °C without extra humidity, respectively. And benefit from the undetectable leakage of PWA, the proton conductivity retention could achieve 93.16% within 500 h. Simultaneously, the methanol barrier property of the hybrid membrane is far beyond Nafion, indicating a membrane selectivity of 12.08 × 104 S (s cm−3), which is 8.21 times higher than that of Nafion 115. The hybrid membrane allowed for sufficient proton conductivity, robust stability, lower methanol permeability, as well as low cost compared with Nafion, shows great potential for direct methanol fuel cell applications.

Aluminum doped solid acid with suitable ratio of Brønsted and Lewis acid sites synthesized by electric-flocculation of phosphotungstic acid via hydrothermal treatment for producing 5-hydroxymethylfurfural from glucose

Abstract: Novel solid acid catalyst PWAl-200 was synthesized by a new green method of electric-flocculation of phosphotungstic acid (PW) with aluminum as electrodes to transform glucose to 5-hydroxymethylfurfural (5-HMF). Through electric-flocculation, PW was deposited in the hydrated aluminum received from electrolysis. Then the obtained floc was treated by hydrothermal process to get solid acid. Characterization results showed that after electric-flocculation, the electron-withdrawing influence of terminal W O in the PW on hydrated aluminum formed Lewis (L) acid sites on these six-coordinated aluminum. The H+ derived from PW could supply Bronsted (B) acid sites. In the further hydrothermal treatment, hydrated aluminum dehydrated to produce reformed six-coordinated aluminum. It was linked to heteropoly anions ligands through oxygen bridges, producing more L acid center. Besides, four-coordinated aluminum formed from the hydrated aluminum. The positive charge it produced increased L acid sites due to the strong effect of nearby tungsten species, further adjusting to a moderate ratio of Lewis/Bronsted (L/B) on solid acid for catalytic synthesis 5-HMF from glucose. The highest yield was 61.7% at 170 ℃ for 4 h and the catalyst could be recycled for four times and tend to stabilize.

Green Oxidation of Cyclohexanone to Adipic Acid over Phosphotungstic Acid Encapsulated in UiO-66

Abstract: A very stable catalyst, phosphotungstic acid (PTA) encapsulated in metal–organic framework UiO-66, was prepared by a simple one-pot solvothermal method. Characterization results show that UiO-66 is quite stable in the catalyst preparation process, and PTA is encapsulated in the cavities of UiO-66 with good dispersity. The as-synthesized composite material exhibited good catalytic activity and excellent reusability for the green oxidation of cyclohexanone to adipic acid (AA). Under mild reaction conditions, the isolated yield of AA was as high as 80.3% without the introduction of any organic solvent or phase transfer agent. The excellent immobilization effect of UiO-66 for PTA is mainly because UiO-66 has a well matched window size to confine PTA molecule in its nanocages.