Dawson Heteropolyacid Research Articles

Tailored Brønsted and Lewis surface acid sites of the phosphotungstic Wells Dawson heteropoly-acid

The synthesis of transition metals Wells Dawson heteropoly salts along with their elemental composition, water content, structure and specific surface area were thoroughly investigated. Additionally, the use of a molecular probe such as pyridine allowed the quantification of Lewis and Brønsted surface acid sites. The transition metals counter-cations provide both Lewis and Brønsted acid sites. The concentration of Brønsted acid sites is influenced by the crystallization water associated with the heteropoly anion.Additionally, the dispersion of the phosphotungstic Wells Dawson heteropolyacid over transition metal oxides evidenced the presence of both Brønsted and Lewis surface acid sites. The concentration of these sites is related to the surface dispersion of the crystals of the HPA over the oxide supports and the degree of hydration of the HPA.

Local Structure of Supported Keggin and Wells–Dawson Heteropolyacids and Its Influence on the Catalytic Activity

Keggin [PW12O40]3– and Wells–Dawson [P2W18O62]6– heteropolyanions are nanosized transition-metal-oxygen clusters belonging to the heteropolyacids (HPAs) family. They are widely used as catalysts du...

Efficient N-acylation of sulfonamides using cesium salt of Wells–Dawson heteropolyacid as catalyst: Synthesis of new N-acyl sulfonamides and cyclic imides

N-acylation of substituted sulfonamides with different anhydrides in the presence of Cesium salt of Wells–Dawson heteropolyacid (Cs5HP2W18O62) as an efficient and reusable catalyst was investigated for the first time. Cs5HP2W18O62 was used with a catalytic amount in water as a green solvent. At room temperature, a series of N-acylsulfonamides were synthesized, while under refluxing conditions, new cyclic imides containing sulfonyl group were obtained. Atom-economy, high yields, easy work-up, as well as simple catalyst recovery and reusability are the key features of this procedure.

Local structure of the supported Keggin and Wells–Dawson heteropolyacids and its consequence on their (photo)catalytic activity

Local structure of the supported Keggin and Wells–Dawson heteropolyacids and its consequence on their (photo)catalytic activity

Investigation of linear alkylbenzene synthesis using nanotitania-supported Dawson heteropolyacid as catalyst by statistical design approaches

Alkylation of benzene with 1-decene for production of linear alkylbenzene in the presence of nanotitania (n-TiO2)-supported Dawson heteropolyacids was studied. Various operating parameters including benzene to 1-decene molar ratio, catalyst weight percent (wt%), catalyst loading (wt%), calcination temperature, reaction time, and temperature influenced the reaction yield. Fractional factorial design was employed to screen these parameters, and significant parameters modeled by a central composite design. Finally, the best conditions were defined by using nonlinear Nelder–Mead optimization to reach maximum reaction yield.

Molecular structure and thermal stability of the oxide-supported phosphotungstic Wells-Dawson heteropolyacid.

We present, for the first time in the literature, a systematic study of the molecular structure of the Wells-Dawson heteropolyacid H6P2W18O62·24H2O (HPA) dispersed on TiO2, SiO2, ZrO2 and Al2O3. The heteropolyacid-based materials were synthesized through a conventional impregnation method (in aqueous and ethanol media) at a loading that corresponds to the theoretical "monolayer" coverage (dispersion limit loading). The combination of Raman and infrared studies demonstrates the presence of crystals of HPA (regardless of the nature of the medium used during the synthesis) suggesting that the dispersion limit loading was greatly exceeded. In situ temperature programmed spectroscopy analyses demonstrated that the Raman shift of the distinctive W[double bond, length as m-dash]O Raman mode of the phosphotungstic Wells-Dawson heteropolyacid is sensitive to the local environment, that is, the amount of water molecules associated with the structure. Moreover, the aqueous based species associated with such structures are recognizable through infrared spectroscopy.

Infrared and Raman Investigation of Supported Phosphotungstic Wells- Dawson Heteropolyacid

The combination of both infrared and Raman spectroscopy are powerful tools to obtain information of catalytic materials at a molecular level. The present investigation shows, a systematic study “in situ” about the effects of the temperature in the molecular structure of the supported phosphotungstic Wells-Dawson heteropolyacid (HPA). The infrared and Raman analyses of the samples were performed under in situ conditions from RT to 500 oC range in flowing helium. The oxide supported heteropolyacid was synthesized through a conventional impregnation method (in aqueous) at theoretical monolayer coverage. These studies provided evidences on the dehydration of the HPA upon in situ calcination. Moreover, the in situ investigation allows establishing the surface molecular structure or the thermal stability of the HPA. Additionally, the presence of crystals of HPA over TiO2 suggests that the “theoretical” monolayer loading should be re-evaluated. Keywords: HPA, IR in situ, “Theoretical” Monolayer, Thermal Stability, Raman in situ, Wells-Dawson.

Electrochemical and UV–visible spectroscopy studies of K6As2W18 − xMoxO62 (x = 0–3) Wells–Dawson heteropolyacid catalysts for oxidative dehydrogenation of benzyl alcohol

Redox behaviors of molybdenum-substituted Wells–Dawson tungstoarsenate heteropolyacid catalysts (α-K6As2W18−xMoxO62 (x=0–3)) were probed by electrochemical and UV-visible spectroscopy measurements. Reduction potential of K6As2W18−xMoxO62 (x=0–3) catalysts shifted to more positive potential with increasing molybdenum content. UV–visible spectroscopy analysis was conducted as a simple diagnostic method for reduction potential. Absorption edge energy determined by UV–visible spectroscopy decreased with increasing reduction potential. Gas-phase oxidative dehydrogenation of benzyl alcohol was carried out as a model reaction. Yield for benzaldehyde increased with increasing electrochemical reduction potential and with decreasing UV–visible absorption edge energy of the catalysts.

Amine-functionalized nanosilica-supported Dawson heteropolyacid: an eco-friendly and reusable photocatalyst for photodegradation of malachite green

Dawson heteropolyacid, H6[P2W18O62], was chemically anchored to the amine-functionalized nanosilica surface in different loadings. The synthesized nanocomposite was characterized by FT-IR spectroscopy and N2 adsorption experiments. The preservation of the structure of the Dawson and dispersion of the cluster on the support were verified on the basis of the spectroscopic data. The photocatalytic degradation efficiency of the synthesized nanocomposites was studied using an aqueous solution of malachite green dye as a model water pollutant. The mineralization of dye contents was investigated in different loadings from 10 to 30 % at room temperature. Decolorization was observed in shorter time using 30 % loading. The photocatalyst was reusable after the filtration and remained active even after being washed with hot water, which confirm its attachment firmly to the surface of amine-functionalized nanosilica.

Phenol photodegradation process assisted with Wells–Dawson heteropolyacids

This work aims to investigate the catalytic properties of the heteropolyacids (HPAs) within photocatalytic process. Photocatalytic degradation of phenol in aqueous solution was performed using Wells–Dawson HPAs as catalysts under UV-illuminated condition at 254 nm. The effects of various parameters such as, concentration of catalyst, initial pH, and initial phenol concentration on the performance of photocatalytic process have been investigated to obtain the efficient and durable catalysts and determine the better reaction conditions. The kinetics and the efficiency of phenol degradation were determined by various analytical techniques, such as HPLC and TOC. The results show that the photocatalytic degradation efficiency increases with decreasing pH and the optimum pH is 2 for both catalysts. The photodegradation follows pseudo-first-order kinetics with rate constants k = 8.56 × 10−2 and 9.94 × 10−2 min−1 for iron- and vanadium-substituted HPAs, respectively.

Redox properties of α2-K8P2W17(M·OH2)O61 (M = MnII, ZnII, FeII, CoII, and NiII) Wells–Dawson heteropolyacids probed by scanning tunneling microscopy and tunneling spectroscopy

Abstract Scanning tunneling microscopy (STM) and tunneling spectroscopy investigations of α 2 -K 8 P 2 W 17 (M·OH 2 )O 61 (M = Mn II , Zn II , Fe II , Co II , and Ni II ) Wells–Dawson heteropolyacids (HPAs) were conducted. HPAs formed self-assembled and well-ordered arrays on graphite surface. Negative differential resistance (NDR) phenomena were observed in the tunneling spectra of the HPAs. NDR peak voltage of the HPAs appeared at less negative voltage with increasing reduction potential and with decreasing UV–visible absorption edge energy. HPAs were then applied to the electro-oxidation of methanol as a redox mediator. The oxidation activity for residual intermediates in the reaction increased as NDR peak voltage of HPA appeared at less negative voltage.

Synthesis, characterization of cesium and cobalt substituted wells–Dawson heteropolyoxotungstates salts and their photocatalytic applications

Heteropoly compounds in the solid state are ionic crystals (sometimes amorphous) consisting of large polyanions, cations, water of crystallization, and other molecules. Heteropolyacids (HPAs) have several advantages as catalysts. On the one hand, they have a very strong Bronsted acidity, especially the cobalt and cesium salts; on the other hand they are exhibiting fast reversible multielectron redox transformations under mild conditions. The cobalt and cesium salts of wells- Dawson HPAs were synthesized and characterized using elemental analysis and spectroscopic techniques ( 31 P-NMR, FT-IR). The wells-Dawson anions possess the ability to accept or release electrons through an external potential or upon exposure to UV radiation (photochemical reactions). The catalytic tests of these salts were investigated on phenol degradation where the UV photodegradation of acidified aqueous solutions (pH = 2) were studied in a batch photoreactor under ambient temperature and continuous circulation of phenol solution. The results reveal high catalytic activity for two HPAs, the best catalyst is the salt of cesium; where the presence of cesium improves significantly both the photcatalytic activity and the selectivity to oxalic acid.

Dawson Heteropolyacid: A Green, Eco-Friendly, and Reusable Catalyst for One-Pot Synthesis of 4-Aminoquinazolines

A new multicomponent synthesis of 4-aminoquinazolines from the reaction of anthranilonitrile, acylchlorides, and ammonium acetate in the presence of catalytic amounts of Dawson-type heteropolyacids (HPAs) is reported. The catalytic performance of different forms of Dawson-type HPAs, H6 P2M18O62 (M = WVI, MoVI) and mixed addenda forms (M = CoII, CuII, NiII, MnIII) were compared and in all cases, maximum of yield was observed by using H6P2W18O62 as catalyst. The plausible mechanism was estimated. In all cases, the Dawson catalyst was easily recovered and recycled with retention of their initial structure and activity. The effects of reaction conditions have been studied and the results showed yields of reaction are good to excellent.

ChemInform Abstract: Solvent‐Free Synthesis of Functionalized Pyridine Derivatives Using Wells—Dawson Heteropolyacid as Catalyst.

AbstractThe condensation of 3‐formylchromones, β‐ketoesters, and ammonium acetate under conditions A) gives highly functionalized pyridines (III).

Etherification of n-butanol to di-n-butyl ether over H3PMo12−W O40 (x= 0, 3, 6, 9, 12) Keggin and H6P2Mo18−W O62 (x= 0, 3, 9, 15, 18) Wells–Dawson heteropolyacid catalysts

Abstract Etherification of n-butanol to di-n-butyl ether was carried out over H3PMo12 − xWxO40 (x = 0, 3, 6, 9, 12) Keggin and H6P2Mo18 − xWxO62 (x = 0, 3, 9, 15, 18) Wells–Dawson heteropolyacid (HPA) catalysts. Acid strength of H3PMo12 − xWxO40 Keggin and H6P2Mo18 − xWxO62 Wells–Dawson HPA catalysts was determined by NH3-TPD (temperature-programmed desorption) measurements. The correlations between desorption peak temperature (acid strength) of the HPA catalysts and catalytic activity revealed that conversion of n-butanol and yield for di-n-butyl ether increased with increasing acid strength of the catalysts, regardless of the identity of HPA catalysts (without HPA structural sensitivity).

Wells–Dawson heteropolyacid as reusable catalyst for sustainable synthesis of flavones

The behavior of Wells–Dawson (H 6P 2W 18O 62·24H 2O, WD) acid, both bulk and supported on silica, for the cyclodehydration reaction of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione to obtain flavone was studied in heterogeneous conditions, using toluene as solvent. Catalytic experiences with bulk and supported catalysts with different WD acid loadings and reuse of the catalysts were done. The catalytic activity of supported catalysts was higher than that of the bulk catalyst, and their activity was almost constant after three reaction cycles. The following reaction conditions: reflux toluene and 1% mmol of WD supported on silica (0.4 g WD acid by gram of silica) were used for the preparation of five flavones. Yields above 85% were obtained in 4–5 h.

Scanning Tunneling Microscopy Study of H6+x P2Mo18−x V x O62 (x = 0–3) Wells–Dawson Heteropolyacid Catalysts: Correlation of NDR Peak Voltage with Reduction Potential and Oxidation Catalysis

Scanning tunneling microscopy (STM) and tunneling spectroscopy studies were carried out to examine the redox properties of vanadium-containing H6+x P2Mo18−x V x O62 (x = 0, 1, 2, 3) Wells–Dawson heteropolyacid (HPA) catalysts. The HPAs formed two-dimensional well-ordered monolayer arrays on a graphite surface and exhibited a distinctive current–voltage behavior called negative differential resistance (NDR). The NDR peak voltages of H6+x P2Mo18−x V x O62 HPAs were correlated with reduction potentials determined by temperature-programmed reduction and with catalytic activity for oxidative dehydrogenation of isobutyraldehyde to methacrolein. The NDR peak voltage of H6+x P2Mo18−x V x O62 appeared at less negative voltage with increasing reduction potential and oxidation catalysis. Negative differential resistance (NDR) peak voltage of H6+x P2Mo18−x V x O62 determined by scanning tunneling microscopy appeared at less negative voltage with increasing reduction potential and with increasing oxidation catalysis.

ChemInform Abstract: Wells—Dawson Heteropolyacid Supported on Silica: A Highly Efficient Catalyst for Synthesis of 2,4,5‐Trisubstituted and 1,2,4,5‐Tetrasubstituted Imidazoles.

AbstractThe multicomponent cyclocondensation of benzil with aromatic aldehydes, an amine and ammonium acetate (or ammonium acetate alone) is efficiently catalyzed by SiO2‐supported Wells—Dawson heteropolyacid to give the polysubstituted imidazole derivatives (IV) and (VI) in high yields.

ATR-FTIR Study of the Decomposition of Acetic Anhydride on Fosfotungstic Wells–Dawson Heteropoly Acid Using Concentration-Modulation Excitation Spectroscopy

The decomposition of acetic anhydride in liquid phase on a fosfotungstic Wells–Dawson heteropoly acid (HPA) was investigated by in situ ATR-FTIR spectroscopy. Transient and concentration-modulation excitation spectroscopy (MES) experiments in combination with phase-sensitive detection (PSD) were used to monitor the solid–liquid interface. The MES method is based on the periodic variation of a parameter of the reaction media such as, the reactant concentration. That periodic alteration causes varying infrared signals of the surface adsorbed species that are subsequently demodulated with the PSD methodology. Thus, the separation of the static signals from the changing ones is achieved, and species with different response can be observed in the spectra. Using MES-PSD coupled with ATR-FTIR, acetic anhydride was observed to decompose to acetic acid, acetate and acyl [CH3C(O)+] species, involving Bronsted acid sites of the HPA catalyst. The CH3C(O)+ is a very unstable intermediate species and it is the key intermediate in the Friedel–Crafts acylation reactions. Moreover, the acetate groups are spectator species since remain strongly adsorbed on the catalyst surface and do not further react.

Acylating Capacity of the Phosphotungstic Wells−Dawson Heteropoly Acid: Intermediate Reactive Species

The mechanism of adsorption and decomposition of acetic anhydride over phosphotungstic Wells−Dawson heteropoly acid (HPA) was investigated through infrared spectroscopy with especially dedicated in situ cells. Both gaseous and liquid acetic anhydride dissociatively chemisorbs on the heteropoly acid, producing the intermediate acyl CH3CO+ species, adsorbed acetate species, and acetic acid. The former is known as the key intermediate species in the Friedel−Crafts acylation reaction. Moreover, the consumption of the Brønsted acid sites observed during the adsorption and decomposition of acetic anhydride over the HPA indicates that those sites are the active ones of the genesis of the acyl intermediate species. Surface acetate species and acetic acid strongly adsorb on the active acid sites deactivating the heteropoly acid.