Showing papers on "Phosphotungstic acid published in 2017"

MOF catalysts in biomass upgrading towards value-added fine chemicals

Abstract: The development of new synthetic routes from biomass sources towards already existing molecules, which are then called bio-based molecules, or the transformation of biomass into new building blocks and materials will be of great impact. The review presents a critical comparison between metal–organic frameworks (MOFs) and other catalysts (e.g. zeolites) for biomass transformation and valorization to platform chemicals: cellulose hydrolysis to glucose, fructose or sorbitol; fructose, glucose or maltose to 5-hydroxymethylfurfural (5-HMF); sucrose to methyl lactate; furans, levulinic acid, lignin or vanillin as feedstock; triglycerides to esters and glycerol. For example, in the case of cellulose hydrolysis as well as glucose isomerization MOF-based catalysts could not compete with zeolites and sulfonated carbon which display significantly higher activity. In DMSO, MIL-101Cr-SO3H-15% and NUS-6(Hf) are among the best heterogeneous catalysts reported so far for the conversion of fructose into 5-HMF. For the glucose-to-5-HMF transformation MIL-101Cr-SO3H is only a low-to-medium activity catalyst for 5-HMF while mesoporous tantalum phosphate as well as Sn montmorillonite display significantly higher activities. On the other hand, MIL-101Cr-SO3H preferentially transformed glucose to 5-HMF over levulinic acid while the catalysts Amberlyst-15 and sulfuric acid gave mostly levulinic acid. For levulinic acid conversion to ethyl levulinate UiO-66Zr catalysts can compete with other heterogeneous catalysts for the levulinic esterification reaction. For active MOF catalysts open metal sites (coordinatively unsaturated sites) are important as the activity increases with the amount of missing linkers. The two MOFs MIL-101Cr and UiO-66 and their derivatives are used in many studies. These MOFs did not only act as catalysts themselves but also served as hosts or support to embedded catalytic species, e.g., phosphotungstic acid (PTA), ruthenium and palladium nanoparticle (Ru-NP, Pd-NP) or poly(N-bromomaleimide) catalysts. For the conversion of vanillin into 2-methoxy-4-methylphenol the selectivity of Pd@UiO-66Zr-NH2 was quantitative compared with other supported Pd catalysts (selectivity 48%). Further, MOFs were used as precursors for decomposition and carbonization due to their high porosity and uniformly distributed metal centers to yield catalytically active metal–carbonaceous materials with high thermal and chemical stability. For example, metal nanoparticles supported on nanoporous carbon (M/NC) were synthesized by carbonization and carbothermal reduction of Ru, W, V, and Ti metal precursors loaded in IRMOF-1 or IRMOF-3. Fe–Co-based MOF-derived catalysts are a highly efficient system for the conversion of 5-HMF to 2,5-diformylfuran. In water-containing reactions, the water stability of MOFs is of high importance.

Strategies for enhancing the catalytic performance of Metal-Organic Frameworks in the fixation of CO2 into cyclic carbonates

Abstract: Metal–organic frameworks (MOFs) with accessible Lewis acid sites are finding increasing application in the field of heterogeneous catalysis. However, the structural instability of MOFs when they are exposed to high temperature and/or high pressure often limits their applicability. In this study, two strategies were applied to achieve a MOF catalyst with high stability, activity and selectivity in the reaction of CO2 with styrene oxide to produce styrene carbonate. In the first approach, a MOF with linkers with high connectivity as MIL-100(Cr) was studied, leading to promising activity and recyclability in consecutive catalytic runs without loss of activity. In the second strategy, a MOF with linkers with lower connectivity but with encapsulated Keggin phosphotungstic acid (MIL-101(Cr)[PTA]) was prepared. However, the activity of this catalyst decreased upon reuse as a consequence of deterioration of the MOF. Further investigations were dedicated to the enhancement of the catalytic performance of MIL-100 and included the variation of the metal centre as well as the type and loading of organic salt acting as nucleophile source. This allowed tuning the nature of the organic halide to the specific porous structure of MIL-100(Cr) to prevent diffusion limitations. The best catalytic performance was obtained for MIL-100(Cr) in combination with EMIMBr ionic liquid, which gave very high styrene carbonate yield (94 %) with complete selectivity after 18 h of reaction at mild temperature (60 °C).

Hydroisomerization of n-Hexane Using Acidified Metal–Organic Framework and Platinum Nanoparticles

Abstract: Exceptionally high surface area and ordered nanopores of a metal-organic framework (MOF) are exploited to encapsulate and homogeneously disperse a considerable amount of phosphotungstic acid (PTA). When combined with platinum nanoparticles positioned on the external surface of the MOF, the construct shows a high catalytic activity for hydroisomerization of n-hexane, a reaction requiring hydrogenation/dehydrogenation and moderate to strong Bronsted acid sites. Characterization of the catalytic activity and acidic sites as a function of PTA loading demonstrates that both the concentration and strength of acidic sites are highest for the catalyst with the largest amount of PTA. The MOF construct containing 60% PTA by weight produces isoalkanes with 100% selectivity and 9-fold increased mass activity as compared to a more traditional aluminosilicate catalyst, further demonstrating the capacity of the MOF to contain a high concentration of active sites necessary for the isomerization reaction.

Atomically Dispersed Rhodium on Self-Assembled Phosphotungstic Acid: Structural Features and Catalytic CO Oxidation Properties

Abstract: Atomically dispersed Rh catalysts supported on phosphotungstic acid (PTA) with Rh loading up to 0.9 wt % were prepared through a self-assembly method. Rh stays exclusively as single atom species coordinated to six oxygen atoms, plausibly located at the 4-fold hollow site on one phosphotungstic acid (PTA) molecule together with a chemically adsorbed O2 molecule. The catalyst is active in CO oxidation affording turnover frequencies between 0.2 and 1.7 s–1 from 165 to 195 °C, with an apparent activation energy of 127 kJ/mol. The catalyst is highly stable, well maintaining the Keggin structure of PTA as well as the single-atom identity of Rh after three catalytic cycles (50 to 400 °C). CO activation is mainly achieved on Rh via the formation of dicarbonyl species, while oxygen activation and transfer mainly occur through PTA. The proposed catalytic cycle consists of an alternation of Rh(CO)23+ species and Rh(CO)21+ species on PTA, during which CO transforms into CO2 with oxygen activation being rate-determining.

Heteropolyacid supported on amine-functionalized halloysite nano clay as an efficient catalyst for the synthesis of pyrazolopyranopyrimidines via four-component domino reaction

Abstract: An efficient heterogeneous hybrid catalyst was developed by functionalization of halloysite clay nanotubes by γ-aminopropyltriethoxysilane and then immobilization of a Keggin type heteropolyacid, phosphotungstic acid. This hybrid catalyst was characterized by SEM/EDX, FTIR and XRD and its catalytic activity for the synthesis of pyrazolopyranopyrimidine derivatives via four-component domino reaction of barbituric acid, hydrazine hydrate, ethyl acetoacetate and benzaldehyde was investigated. The results indicated that the hybrid system can promote the reaction to afford the desired products in high yields and short reaction times. The superior catalytic activity of this system was confirmed when compared with those reported, previously. Moreover, this novel heterogeneous catalyst was found to be easily separable and recyclable. It was re-used at least three times with negligible loss of activity. Significantly, this protocol can be extended to nonconventional green heating sources such as microwave and ultrasonic irradiations.

Esterification of Levulinic Acid Using ZrO2-Supported Phosphotungstic Acid Catalyst for Ethyl Levulinate Production

Abstract: Ethyl levulinate (EL) is a versatile bio-based chemical with various applications such as fragrance and flavoring agents and also fuel blending component. EL can be produced through catalytic esterification of levulinic acid (LA) with ethanol. Herein, a series of zirconia (ZrO2)-supported phosphotungstic acid (HPW) (HPW/Zr) catalyst, 15-HPW/Zr, 20-HPW/Zr, and 25-HPW/Zr, were prepared, characterized, and tested for EL production. The physicochemical properties of catalysts were characterized using Fourier-transformed infrared (FTIR) spectroscopy, x-ray diffraction (XRD), field emission scanning electron microscope (FESEM), N2 physisorption, and ammonia temperature-programmed desorption (NH3-TPD). The effect of reaction parameters: reaction time, temperature, catalyst loading, and molar ratio of LA to ethanol was inspected on LA conversion and EL yield. The catalyst with high surface area and high acidity seemed suitable for EL production. Among the catalysts tested, 20-HPW/Zr exhibited the highest EL yield of 97.3% at the following conditions: 150 °C, 3 h, 1.0 g of 20-HPW/Zr and 1:17 M ratio of LA to ethanol. The 20-HPW/Zr could be reused for at least four times with insignificant decrease in the EL yield. This study demonstrates the potential of ZrO2-supported HPW for bio-based alkyl levulinate production at mild process conditions.

Cellulose nanocrystals with different length-to-diameter ratios extracted from various plants using novel system acetic acid/phosphotungstic acid/octanol-1

Abstract: The novel system [acetic acid/phosphotungstic acid (H3PW12O40)/octanol] was proposed for catalytic solvolysis of cellulose and for obtaining cellulose nanocrystals (CNC). Several alternative experiments involving mixtures with different compositions were carried out; reaction time was also varied. CNC particles from cotton, linen, softwood and hardwood cellulose were prepared in the experiments performed for 40 min in the presence of 0.25 mol% of the heteropolyacid. CNC samples were characterized by transmission electron microscopy and atomic force microscopy, X-ray diffraction analysis, and thermogravimetric analysis. It was established that the resulting nanoparticles had high crystallinity and rod-like shape; their length varied from 160 to 400 nm (cotton CNC had the shortest length, and linen CNC had the longest length), and CNC thickness ranged from 6 to 10 nm. Thermal stability of CNC was lower than that of initial celluloses and decreasing in the following sequence: cotton > softwood > hardwood > linen cellulose.

In Situ Formed Phosphoric Acid/Phosphosilicate Nanoclusters in the Exceptional Enhancement of Durability of Polybenzimidazole Membrane Fuel Cells at Elevated High Temperatures

Abstract: Most recently, we developed a phosphotungstic acid impregnated mesoporous silica (PWA-meso-silica) and phosphoric acid doped polybenzimidazole (PA/PBI) composite membrane for use in high temperature fuel cells and achieved exceptional durability under a constant current load of 200 mA cm(-2) at 200 degrees C for over 2700 h. In this work, the fundamental role of PWA-meso-silica in enhancing the stability of the PA/PBI membrane has been investigated. The microstructure, the PA uptake, swelling ratio, mechanical property and conductivity of PA/PBI/PWA-meso-silica composite membranes depend on the loading of PWA-meso-silica. The results indicate that the optimum limit of PWA-meso-silica loading in the PA/PBI membranes is 15 wt%. Detaled analysis indicates that the mesoporous structure of the PWA-meso-silica framework disintegrates, forming phosphosilicate phases within the PBI polymeric matrix during fuel cell operation at 200 degrees C. The in situ formed phosphosilicates can immobilize a significant amount of PA, forming PA/phosphosilicate nanoclusters that possess high proton conductivity (e.g., 7.2 x 10(-2) S cm(-1) at 250 degrees C) and stability and substantially inhibits acid leaching out of themembrane. The substantially reduced acid leaching also alleviates the excess acid in the catalyst layer, reducing the detrimental effect of excess acid on the agglomeration of Pt catalysts especially in the cathode catalyst layer. These phenomena are responsible for the exceptional stability in proton conductivity as well as the significantly reduced agglomeration of Pt nanoparticles in the anode and cathode catalyst layers of PA/PBI/PWA-meso-silica composite membrane fuel cells. (c) 2017 The Electrochemical Society. All rights reserved.

Proton conductivity and morphology of new composite membranes based on zirconium phosphates, phosphotungstic acid, and silicic acid for direct hydrocarbon fuel cells applications

Abstract: The effect of Phosphotungstic acid (PWA) on the proton conductivity and morphology of zirconium phosphate (ZrP), porous polytetrafluoethylene (PTFE), glycerol (GLY) composite membrane was investigated in this work. The composite membranes were synthesized using two approaches: (1) Phosphotungstic acid (PWA) added to phosphoric acid and, (2) PWA + silicic acid were added to phosphoric acid. ZrP was formed inside the pores of PTFE via the in situ precipitation. The membranes were evaluated for their morphology and proton conductivity. The proton conductivity of PWA–ZrP/PTFE/GLY membrane was 0.003 S cm−1. When PWA was combined with silicic acid, the proton conductivity increased from 0.003 to 0.059 S cm−1 (became about 60% of Nafion’s). This conductivity is higher than the proton conductivity of Nafion–silica–PWA membranes reported in the literature. The SEM results showed a porous structure for the modified membranes. The porous structure combined with this reasonable proton conductivity would make these membranes suitable as the electrolyte component in the catalyst layer for direct hydrocarbon fuel cell applications.

(Zn, Ni, Cu)-BTC Functionalized with Phosphotungstic Acid for Adsorptive Desulfurization in the Presence of Benzene and Ketone

Abstract: A novel composite adsorbent, PTA@(Zn, Ni, Cu)-BTC, was prepared by (Zn, Ni, Cu)-BTC doped with different amounts of phosphotungstic acid (PTA) using an impregnation method. Apparatus like nitrogen adsorption–desorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), inductively coupled plasma optical emission spectroscopy (ICP-OES), and NH3-temperature programmed desorption (NH3-TPD) are employed to characterize our obtained adsorbents. The adsorption properties of as-prepared adsorbents for the dibenzothiophene (DBT) were evaluated by means of fixed-bed breakthrough experiments. Unlike conventional MOFs (metal–organic frameworks), the PTA@(Zn, Ni, Cu)-BTC exhibited high DBT selectivity adsorption performs with a superior uptake capacity compared with those previously reported in the literatures. Additionally, PTA@(Zn, Ni, Cu)-BTC showed a remarkable stability in the presence of benzene and acetone, maintaining about 95% initial uptake capacity after eight...

Cellulose nanocrystals prepared in H3PW12O40-acetic acid system

Abstract: Cellulose nanocrystals (CNC) were prepared by destruction of cotton cellulose using phosphotungstic acid in acetic acid medium. The study shows the influence of the heteropolyacid concentration, pre-activation, and hydrogen peroxide addition on the size of CNC particles. CNC were characterized using the methods of dynamic light scattering, FTIR-spectroscopy, thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction, scanning and transmission electron microscopy, and potentiometric titration. CNC particles have highly-crystalline structure and rod-like morphology. Freeze-dried CNC hydrosols form lamellar or fibrous agglomerates, depending on the initial concentration of the CNC sol. Potentiometric titration results show increase in surface activity index and offset of pKa values for CNC active centers in comparison to initial cellulose material.

Synthesis, Characterization, and Application of Silica-Supported Copper-Doped Phosphotungstic Acid in Claisen–Schmidt Condensation

Abstract: An efficient, novel, and recyclable silica-supported copper-doped phosphotungstic acid (CuPTA/SiO2) has been synthesized by an impregnation method. The catalyst has been characterized by FTIR, XRD, SEM/EDX, ICP–AES, EPR, XPS, and NH3-TPD analyses. The catalytic application of the catalyst has been explored for Claisen–Schmidt condensation. The catalyst could be recycled for six runs without any appreciable loss in catalytic activity. The catalytic activity of the recovered catalyst after six runs was confirmed by FTIR, XRD, SEM/EDX, ICP–AES, and EPR analyses. The Claisen–Schmidt products were obtained in excellent yield in a shorter time period.