Heteropolyacid Catalysts Research Articles

One-Pot Four-Component Dakin-West Synthesis of β-Acetamido Ketones Catalyzed by a Vanadium-Substituted Heteropolyacid

The multicomponent condensation of an aryl aldehyde, acetyl chloride, acetonitrile, and enolizable ketone as one-pot synthesis of β-acetamido ketones in high yields was investigated using commercial, non-corrosive, and environmentally benign Keggin and Wells-Dawson heteropolyacid catalysts. The best catalyst was H 5 PW 10 V 2 O 40 . The methodology used simple experimental conditions, and the short reaction times and high yields indicate it is a useful strategy for the large scale synthesis of β-acetamido ketones. An efficient one-pot method for the synthesis of β-acetamido ketones by H 5 PW 10 V 2 O 40 as an environmentally benign and recyclable catalyst is described. The catalyst afforded short reaction times and high to excellent yields.

A Brønsted–Lewis-surfactant-combined heteropolyacid as an environmental benign catalyst for esterification reaction

Abstract A Bronsted–Lewis-surfactant-combined heteropolyacid (HPA) catalyst (C 16 TA)H 4 TiPW 11 O 40 (C 16 TA = cetyltrimethyl ammonium) had been designed and used as a water-tolerantly heterogeneous catalyst in esterification of free fatty acid with a higher conversion (94.7%) and excellent efficiency (91.8% yield) due to its acidic properties and structures. This micellar polyoxometalate catalyst was stable during the reaction and can be recycled by a simple separation process.

Transesterification of used vegetable oils with a Cs-doped heteropolyacid catalyst in supercritical methanol

In this study, used vegetable oils (VO) were transesterified in supercritical methanol with a Cs-doped heteropolyacid (HPA) catalyst so as to produce biodiesel. The stability of the prepared catalysts was determined by measuring the amount of dissolved heteropoly anions. Cs2.5PW12O40 exhibited the highest stability in supercritical methanol. Kinetic parameters were determined at the initial stage of the reaction to examine the reaction rate of the process. The effects of the reaction parameters, including the molar ratio of methanol to oil, reaction pressure, temperature, and time, on the content of fatty acid methyl esters (FAMEs) were also investigated in order to obtain optimum reaction conditions. The content of FAME reached 92% under the optimum reaction conditions of 260°C, 20MPa, a molar ratio of methanol to oil of 40, and a reaction time of 40min in the presence of 3% catalyst. The results showed that supercritical methanol compensated for the low reaction rate of solid acid catalytic transesterification, while the Cs-doped HPA catalyst mitigated the harsh operation conditions of the supercritical methanol process.

Recovery and reuse of heteropolyacid catalyst in liquid reaction medium through reversible encapsulation in Cu3(BTC)2 metal–organic framework

The solvent dependent solubility of HKUST-1 metal–organic framework (MOF) with encapsulated Keggin 12-tungstophosphoric acid (HPW@Cu3(BTC)2) was exploited to liberate homogeneous heteropolyacid catalyst in a reaction medium and to recover the catalyst by encapsulation in the MOF upon addition of hexane. The application of the new Keggin type heteropolyacid catalyst recovery concept was demonstrated in the esterification of acetic acid with 1-propanol and salicylic acid with ethanol.

Direct synthesis of hydrogen peroxide using Au–Pd-exchanged and supported heteropolyacid catalysts at ambient temperature using water as solvent

The direct synthesis of hydrogen peroxide from molecular H2 and O2 represents a green and economic alternative to the current anthraquinone process used for the industrial production of H2O2. In order for the direct process to compete with the anthraquinone process, there is a need for enhanced H2O2 yields and H2 selectivity in the process. We show that Au–Pd-exchanged and supported Cs-containing heteropolyacid catalysts with the Keggin structure are considerably more effective in achieving high H2O2 yields in the absence of acid or halide additives than previously reported catalysts. The Au–Pd-exchanged Cs-heteropolyacid catalysts also show superior H2O2 synthesis activity under challenging conditions (ambient temperature, water-only solvent and CO2-free reaction gas). Au plays a crucial role in achieving the improved performance of these heteropolyacid-based catalysts. The heteropolyacid limits the subsequential hydrogenation/decomposition of H2O2.

Solid-acid and ionic-liquid catalyzed one-pot transformation of biorenewable substrates into a platform chemical and a promising biofuel

A wide variety of polymeric carbohydrate-rich weed species were directly converted to a platform chemical, 5-hydroxymethylfurfural (HMF), and a promising next-generation biofuel, 5-ethoxymethyl-2-furfural (EMF), with homogeneous and heterogeneous catalysts under mild reaction conditions. Bronsted acidic IL catalysts, [DMA]+[CH3SO3]− and [NMP]+[CH3SO3]−, were found to be effective enabling maximum 58 and 52 wt% HMF yields, respectively, from foxtail weed. Strong Lewis acidic silica supported heteropolyacid (HPA-SiO2) catalyst was also effective producing a maximum 32 wt% HMF from the same weed substrate. Both IL catalysts were effective for high-purity EMF production from HMF and weeds. HMF was quantitatively converted to EMF in 2 h. In the case of weed substrates, EMF was formed as the major product. The ratio of EMF and ethyl levulinate (EL) in the isolated product was 7 : 1. To address the sustainability issue and potential industrial application opportunity of the current method, a larger scale experiment under conventional heating demonstrated to produce 55 wt% HMF in 4 h. Most importantly, the spent catalyst and the solvent system were efficiently recycled for four consecutive catalytic cycles without a significant loss in yield.

A large sample volume magic angle spinning nuclear magnetic resonance probe for in situ investigations with constant flow of reactants

A large-sample-volume constant-flow magic angle sample spinning (CF-MAS) NMR probe is reported for in situ studies of the reaction dynamics, stable intermediates/transition states, and mechanisms of catalytic reactions. In our approach, the reactants are introduced into the catalyst bed using a fixed tube at one end of the MAS rotor while a second fixed tube, linked to a vacuum pump, is attached at the other end of the rotor. The pressure difference between both ends of the catalyst bed inside the sample cell space forces the reactants flowing through the catalyst bed, which improves the diffusion of the reactants and products. This design allows the use of a large sample volume for enhanced sensitivity and thus permitting in situ(13)C CF-MAS studies at natural abundance. As an example of application, we show that reactants, products and reaction transition states associated with the 2-butanol dehydration reaction over a mesoporous silicalite supported heteropoly acid catalyst (HPA/meso-silicalite-1) can all be detected in a single (13)C CF-MAS NMR spectrum at natural abundance. Coke products can also be detected at natural (13)C abundance and under the stopped flow condition. Furthermore, (1)H CF-MAS NMR is used to identify the surface functional groups of HPA/meso-silicalite-1 under the condition of in situ drying. We also show that the reaction dynamics of 2-butanol dehydration using HPA/meso-silicalite-1 as a catalyst can be explored using (1)H CF-MAS NMR.

Transesterification of Used Cooking Oils Catalyzed by CsTPA/SBA15 Catalyst System in Biodiesel Production

An environmentally beneficial, facile and clean method for the production of Biodiesel from Used Cooking Oils have been developed. Owing to corrosion and re-usability problems associated with the alkali catalysts like NaOH and KOH, a novel Heteropolyacid Cesium substituted do-decatungstophosphoric acid (CsTPA) on SBA 15 was synthesized. A process for the production of Biodiesel from Used Groundnut Oil(UGO), Used Sunflower Oil(USO) and Used Palm Oil(UPO) has been studied and the reaction conditions that affect the yield and purity of product esters like the concentration of catalyst, methanol:oil molar ratio, reaction time and temperature were optimized. Performance of the catalyst was compared with that of the conventional alkali catalysts and it was found that the proposed heteropolyacid catalyst system gave higher yield of 91.5% for UPO. The catalyst activity did not vary much even after reusing the recovered catalyst system for up to three runs. The fuel properties for the Used Cooking Oil esters produced were found to be in agreement with ASTM standards. Comparative performance and emission study of UCOs with diesel was carried out to check the effect of CO, CO2 and NOx.

Liquid phase dehydration of glycerol to acrolein catalyzed by silicotungstic, phosphotungstic, and phosphomolybdic acids

Catalytic activities of silicotungstic, phosphotungstic, and phosphomolybdic acids in the liquid phase dehydration of glycerol to acrolein were investigated in a semi-batch reactor. The acidities of the heteropolyacid catalysts were evaluated by NH 3-TPD. The effect of the acidity of the heteropolyacid catalyst on the conversion of glycerol and the yield of product was discussed. Silicotungstic acid exhibited high catalytic activity in the dehydration of glycerol to acrolein. The maximum acrolein yield of 78.6% was achieved when glycerol was completely converted at the reaction temperature of 300 °C with the mole ratio of silicotungstic acid to glycerol of 1 × 10 −4:1. The catalytic activities of the heterpolyacids toward the formation of acrolein were in an order of silicotungstic acid > phosphotungstic acid > phosphomolybdic acid, revealing that the dehydration of glycerol to acrolein was affected by the acidity and the stability of the heteropolyacids. Hydroxyacetone and acetic acid were also detected with the yields of less than 10%, respectively. In the liquid phase dehydration of glycerol to acrolein catalyzed by silicotungstic acid, semi-batch reaction technique is a potentially practical method in viewpoint of high acrolein yield, low catalyst consumption, and energy saving.

Immobilization of 12-tungstophosphoric acid on LaSBA-15 and its catalytic activity for alkylation of o-xylene with styrene

12-Tungstophosphoric heteropoly acid (HPW) catalysts supported on La-incorporated mesoporous molecular sieves (LaSBA-15) with various acid loadings were prepared by impregnation, and their physicochemical properties were characterized by elemental analysis, X-ray diffraction, nitrogen adsorption, FT-IR spectroscopy, diffuse reflectance UV–vis spectra and NH 3 temperature-programmed desorption. It was found that LaSBA-15 could accommodate discrete HPW due to its large pore volume and high surface area, and meanwhile the Keggin structure of heteopolyaniona could be retained. Moreover, these catalysts were investigated in the alkylation of o-xylene with styrene. Results showed that 30%HPW/LaSBA-15 catalyst had the highest strength of acidic sites and the best catalytic activity under optimum conditions. Compared with 30% HPW/SBA-15 catalyst, the 30%HPW/LaSBA-15 one had better regeneration, which was due to the strong interaction between the HPW and the support in the presence of La, thus prevent HPW leaching from the support.

Scanning tunneling microscopy and tunneling spectroscopy studies of niobium-containing H6+x P2W18−x Nb x O62 (x=0, 1, 2, 3)Wells-Dawson heteropolyacid catalysts to probe their redox property and oxidation catalysis

Niobium-containing H6+xP2W18−xNbxO62 (x=0, 1, 2, 3) Wells-Dawson heteropolyacids (HPAs) were investigated by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) in order to elucidate their redox properties. The HPAs formed two-dimensional well-ordered monolayer arrays on graphite surface and exhibited a distinctive current-voltage behavior called negative differential resistance (NDR) in their tunneling spectra. NDR peak voltage measured on HPA molecule was correlated with reduction potential and absorption edge energy determined by electrochemical method and UV-visible spectroscopy, respectively. NDR peak voltage of H6+xP2W18−xNbxO62 Wells-Dawson HPAs appeared at less negative voltage with increasing reduction potential and with decreasing absorption edge energy. Oxidative dehydrogenation of isobutyraldehyde was also carried out as a model reaction to probe oxidation catalysis of the HPAs. The trend of NDR peak voltage of H6+xP2W18−xNbxO62 Wells-Dawson HPAs was well consistent with the trend of yield for methacrolein.

An efficient and green synthetic protocol for the preparation of bis(indolyl)methanes catalyzed by H 6P 2W 18O 62·24H 2O, with emphasis on the catalytic proficiency of Wells-Dawson versus Keggin heteropolyacids

A simple, atom economy, and a highly well-organized green protocol has been developed for the synthesis of bis(indolyl)methanes. Wells-Dawson diphosphooctadecatungsticacid (H 6P 2W 18O 62·24H 2O) was employed as a proficient, a highly water tolerant, and green heteropolyacid catalyst for the electrophilic substitution reactions of indole with various aldehydes and ketones to afford the corresponding bis(indolyl)methane derivatives under solvent-free condition. Presumably, discrepancies observed in the reactivity patterns of various heteropolyacids would be explained concerning differences in the acidity, structural diversity, and dissimilarities in the approach of the substrates to the bulk of the heteropolyacid catalysts. It is confirmed that the identity of the catalyst was retained almost completely during the catalytic reaction.

Synthesis and Characterization of Silicotungstic Acid Nanoparticles via Sol Gel Technique as a Catalyst in Esterification Reaction

Glycerol monooleate (GMO) as an esterification product of oleic acid and glycerol is highly potential as an anti-friction substance in the engine lubricant. The purpose of this work is to study the synthesis, characterization and catalytic performance of solid heteropoly acid catalysts, namely silicotungstic acid bulk (STAB) and STA-silica sol gel (STA-SG). The activity and selectivity of STAB and STA-SG in the esterification reaction have been investigated and compared to the homogeneous catalyst i.e. sulphuric acid (H2SO4). The synthesized catalysts were characterized by BET, XRD, TEM, XPS and TPD-NH3. BET analyses shown that the STA-SG catalyst is very high in surface area compared to STAB of 460.11 m2/g and 0.98 m2/g, respectively. From the XPS analyses, there was a significant formation of W-O-Si, W-O-W and Si-O-Si bonding in STA-SG compared to that in STAB. The main species of O1s (90.74 %, 531.5 eV) followed by other O1s peak (9.26 %, 532.8 eV) were due to the presence of W-O-W and W-O-Si bonds, respectively. In addition, the ease of separation for STA-SG catalyst was attributed to its insoluble state in the product phase. The esterification products were then analysed by FTIR and HPLC. Both the H2SO4 and the STAB gave high conversion of 100% and 98%, while lower selectivity of GME with 81.6% and 89.9%, respectively. On the contrary, the STA-SG enabled a conversion of 94%, while significantly higher GME selectivity of 95% rendering it the more efficient solid acid catalyst.

Esterification of camphene over heterogeneous heteropoly acid catalysts: Synthesis of isobornyl carboxylates

Silica supported H 3PW 12O 40 (PW), the strongest heteropoly acid in the Keggin series, is an active and environmentally friendly solid acid catalyst for liquid-phase esterification of camphene, a renewable biomass-based substrate, with C 2, C 4 and C 6 short-chain fatty acids. The reaction provides isobornyl carboxylates, useful as fragrances, in virtually 100% selectivity and 80–90% yield. The reaction is equilibrium-controlled and occurs under mild conditions with a catalyst turnover number of up to 3000. The use of hydrocarbon solvent prevents PW from leaching to allow easy catalyst recovery. The catalyst can be reused several times without loss of activity and selectivity.

Nanostructured H<SUB>3+<I>x</I></SUB>PW<SUB>12−<I>x</I></SUB>Nb<SUB><I>x</I></SUB>O<SUB>40</SUB> (<I>x</I> = 0–3) Keggin Heteropolyacid Catalysts

Nanostructured H(3+x)PW(12-x)NbxO40 (x = 0, 1, 2, 3) Keggin heteropolyacid (HPA) catalysts were investigated by scanning tunneling microscopy (STM) and tunneling spectroscopy to probe their redox property and oxidation catalysis. STM image showed that the HPAs formed two-dimensional well-ordered monolayer arrays on graphite surface. In tunneling spectra of the HPAs deposited on graphite, they exhibited a distinctive current-voltage behavior referred to as negative differential resistance (NDR). NDR peak voltage measured atop HPA molecule was then correlated with reduction potential and absorption edge energy determined by electrochemical method and UV-visible spectroscopy, respectively. It was revealed that NDR peak voltage of the HPAs appeared at less negative voltage with increasing reduction potential and with decreasing absorption edge energy. In order to correlate NDR peak voltage of H(3+x)PW(12-x)NbxO40 Keggin HPAs with oxidation catalysis, oxidative dehydrogenation of isobutyraldehyde to methacrolein was carried out as a model reaction. NDR peak voltage of the HPAs appeared at less negative voltage with increasing yield for methacrolein.

Screening of New Additives of Active-Carbon-Supported Heteropoly Acid Catalyst for Friedel–Crafts Reaction by Gaussian Process Regression

Activity of heteropoly acid catalyst supported on active carbon for Friedel–Crafts reaction was predicted by Gaussian process regression (GPR), using the five main principal components of physicoch...

Screening of new additives to heteropoly acid catalyst for Friedel–Crafts reaction by microwave heated HTS and by Gaussian process regression

Activity of heteropoly acid (HPA) catalyst for Friedel–Crafts reaction was promoted by Pt addition of which effect was discovered by means of microwave heated high-throughput screening (HTS) and Gaussian process regression (GPR). In the screening, activities of Na, Mg, Mn, Zn, Pd, Cs, Pr and W promoted HPA were measured, and every activity test using microwave irradiation required only 150 s. The results and physicochemical properties of these 8 elements were used to construct regression models by a radial basis function network (RBFN), a support vector machine, and GPR. The regression model by GPR predicted that Pt is an effective additive, which promotes the activity, and the activity was experimentally verified to be 8 times higher than that of the unpromoted HPA catalyst. The performance of the regression model by GPR was superior to those by RBFN or by SVM because an excellent effect of Pt addition was discovered only by GPR. In addition to the extrapolative prediction, advantages of GPR model are that the performance and accuracy of the regression model are increased by using expected improvement which can suggest the additional experiments necessary for the improvement of the regression model.

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).

Clean production of glucose from polysaccharides using a micellar heteropolyacid as a heterogeneous catalyst

Hydrolysis of polysaccharides into glucose was performed over a micellar heteropolyacid (HPA) catalyst [C16H33N(CH3)3]H2PW12O40 (abbreviated as C16H2PW). This micellar heterogeneous catalyst exhibited high selectivity and activity in the hydrolysis of polysaccharides, such as starch and cellulose. Hydrolysis velocities of starch and cellulose were 0.019 and 0.006g/h at 120°C and 170°C, respectively. The results demonstrated that C16H2PW is an effective catalyst for the production of glucose and can be reused.

H6GeMo10V2O40•16H2O nanoparticles prepared by hydrothermal method: a new and reusable heteropoly acid catalyst for highly efficient acetylation of alcohols and phenols under solvent-free conditions

A new Keggin-type heteropoly acid, namely decamolybdodivanadogermanic acid (H6GeMo10V2O40•16H2O), with nanosized particles (5-8 nm), has been synthesized by a hydrothermal method and characterized by elemental analysis, thermogravimetric analysis (TGA), powder X-ray diffractometry (XRD), Fourier-transform infrared spectroscopy (FTIR), UV-Visible spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and potentiometric titration. H6GeMo10V2O40•16H2O revealed high catalytic activity for acetylation of various alcohols and phenols with acetic anhydride at room temperature (298 ± 2 K) and under solvent-free conditions. The catalyst can be easily recovered and used repeatedly for five cycles with a slight loss of activity. The catalytic activity of H6GeMo10V2O40•16H2O was higher than that of other Keggin-type heteropoly acids, such as H3PW12O40, H3PMo12O40 and H4SiW12O40.