Heteropolyacid Catalysts Research Articles

Redox Properties of Nano-Structured <I>α</I><SUB>2</SUB>-K<SUB>6</SUB>As<SUB>2</SUB> W<SUB>18−</SUB><I><SUB>x</SUB></I> Mo<I><SUB>x</SUB></I> O<SUB>62</SUB> (<I>x</I> = 0–3) Wells-Dawson Heteropolyacid Catalysts Probed by Scanning Tunneling Microscopy

Redox Properties of Nano-Structured <I>α</I><SUB>2</SUB>-K<SUB>6</SUB>As<SUB>2</SUB> W<SUB>18−</SUB><I><SUB>x</SUB></I> Mo<I><SUB>x</SUB></I> O<SUB>62</SUB> (<I>x</I> = 0–3) Wells-Dawson Heteropolyacid Catalysts Probed by Scanning Tunneling Microscopy

Synthesis of novel urushiol-like compounds from tung oil using silica-supported phosphotungstic heteropoly acid catalyst

Tung oil, a kind of natural plant oil, has recently received increasing attention due to its high value of research and application. In this study, we report a facile approach to synthesize novel urushiol-like compounds (CAT-ME) starting from tung oil using silica-supported phosphotungstic heteropoly acid (PW/SiO2) as an effective and recyclable catalyst. To be specific, firstly, eleostearic acid glyceride, the main component of tung oil, is reacted with methanol by trans-esterification to obtain methyl eleostearate. And then Friedel-Crafts alkylation reaction of methyl eleostearate with catechol is carried out in the presence of a solid acid catalyst PW/SiO2. And the CAT-ME productive rate is about 69.3%. Furthermore, the chemical structures of the target compound CAT-ME are characterized by Fourier transform infrared spectroscopy (FTIR), ultraviolet and visible spectrophotometer (UV–vis), proton nuclear magnetic resonance spectra (1H NMR), carbon nuclear magnetic resonance spectra (13C NMR) and mass spectra. The characterization results show the designed alkylation products similar to urushiol have been synthesized successfully. Noting that urushiol (the major component of raw lacquer) can be cured by UV irradiation under air atmosphere without any photoinitiator. Thus, this present study develops a promising approach to guide the further modifications of tung oil and other unsaturated natural oils, which can improve the functional properties and expand the application fields of unsaturated natural plant oils by introducing new UV curing system.

Mechanistic Studies on the Transition Metal Oxide Catalysed Partial Oxidation of (Meth)Acrolein to the Corresponding Carboxylic Acids

The partial oxidation of unsaturated aldehydes such as acrolein and methacrolein to the corresponding carboxylic acids is of big importance for the chemical industry. The heart of these production plants is the oxidation catalyst. Acrylic acid is produced via the selective oxidation of acrolein on Mo/V/W mixed oxides with an excellent catalytic performance (S > 90 %, X > 95 %, life time >3 a). Under the same conditions the conversion of methacrolein to methacrylic acid has a poor catalytic performance (S < 35 %, X ≈ 40 %). To this day the reason for this is unclear. Hence, for the partial oxidation of methacrolein a heteropoly acid catalyst is used with a good performance (S ≈ 88 %, X ≈ 62 %), but unfortunately the lifetime is unsatisfactory (<1 a). Therefore, we have studied the gas and surface mechanism of the partial oxidation of acrolein to acrylic acid on mixed oxides with the help of in-situ experiments. The learning outcome of this study will help us to find a rational way to come up with a new catalytic system for the unsatisfactory partial oxidation of methacrolein to methacrylic acid and are summarised in a new extended mechanism.

Understanding the role of Keggin type heteropolyacid catalysts for glycerol acetylation using toluene as an entrainer

The heterogeneously catalyzed esterification (acetylation) of glycerol toward triacetin in batch mode in presence of toluene as entrainer was studied. Silicotungstic acid, tungstophosphoric acid and phosphomolybdic acid as heteropolyacids (HPAs) supported on silica, alumina or silica-alumina were used as catalysts. The course of the reaction was found to be very sensitive to the nature of the HPA as well as the support. Solid characterization by Raman spectroscopy, XRD, and pyridine-FTIR revealed that only combinations of tungsten-based HPAs and silica support were able to preserve the structure of active component throughout the preparation process, which was essential to obtain active and selective catalysts. The interaction between HPA and support was decisive for stability and dispersion of the catalytically active species. With the best performing catalyst H4SiW12O40/SiO2, selectivity to triacetin reached 71% at complete conversion within 24h. The high selectivity to triacetin is attributed the Brønsted acidic sites originated from stabilized Keggin structure and continuous removal of water during course of reaction. Toluene is able to form azeotropic mixtures with water and acetic acid and keeps the reaction temperature below the boiling point of acetic acid. Thus, water-free reaction conditions can be established. The catalyst was reusable; however, the activity and selectivity towards triacetin slightly decreased in a repetition run due to loss of active sites.

Processing Study on the Stability of Heteropoly Acid Catalyst in the Oxidation of Methacrolein to Methacrylic Acid

The oxidation of methacrolein to methacrylic acid over a heteropoly acid catalyst was studied in a pilot scale unit with tubular multisampling reactor. The catalyst performance was monitored at different aging phases during a total runtime of 195 days. Deactivation was accompanied by a loss of catalyst mass through evaporation of molybdenum oxide. With time on stream, deactivation expands from an area close to the inlet of the catalyst bed downstream to the outlet. Several causes for the deactivation were evaluated in processing experiments. Possible detrimental effects of acetic acid and carbon monoxide were excluded by cofeeding these byproducts in high concentrations. A critical parameter is the oxygen concentration because reoxidation of the catalysts is rate limiting. Low oxygen supply results in a depletion of lattice oxygen accompanied by a marked activity loss. It is suggested that continued preservation of the catalyst in the reduced state promotes structural disintegration.

Hydroxyalkylation of phenol to bisphenol F over heteropolyacid catalysts: The effect of catalyst acid strength on isomer distribution and kinetics

Hydroxyalkylation of phenol with formaldehyde to bisphenol F over heteropolyacid impregnated on clay was investigated. These catalysts displayed excellent catalytic performance for this reaction, especially that the effects of acid sites on the isomer distribution are obvious. Various solid catalysts were prepared by impregnating heteropolyacid on different kind of clay matrices, and their chemical compositions, textural properties, and acid strength of the heteropolyacid catalysts were characterized by EDX, BET, NH3-TPD, XRD, and FT-IR. Moreover, the effects of acid sites and reaction temperature on the yield and 4,4′-isomer distribution were launched by comparing the data obtained from the two kinds of catalysts. Furthermore, the kinetics of the hydroxyalkylation of phenol to BPF was established.

First Report About the Use of Micellar Keggin Heteropolyacids as Catalysts in the Green Multicomponent Synthesis of Nifedipine Derivatives

Micellar Keggin heteropolyacid catalysts were prepared using hexadecyltrimethylammonium bromide (cetyltrimethylammonium bromide—CTAB), 1-hexadecyl-pyridinium chloride, and Keggin heteropolyacids H3PMo12O40 and H4PMo11VO40 as precursors. Four catalysts were prepared (PMo12C16, PMo11VC16, PMo12C16Py, and PMo11VC16Py) and characterized by 31P NMR, FT-IR, XRD, SEM analysis and textural properties (SBET). The acidic characteristics of the catalysts were determined by potentiometric titration with n-butylamine. A series of bioactive 1,4-dihydropyridine derivatives such as nifedipine and nemadipine B were synthesized using these new materials, in a one-pot procedure in ethanol. This methodology requires a reaction time of 8 h, and a temperature of 78 °C to obtain good to excellent yields of 1,4-dihydropyridine derivatives. The micellar Keggin catalysts are insoluble in polar media, which allows easy removal of the reaction products without affecting their catalytic activity. The leaching test showed that they have an excellent stability and can be used five times as heterogeneous catalysts without appreciable loss of the catalytic activity. Using the same material, unsymmetrical 1,4-dihydropyridines such as nitrendipine can be obtained through a sequence of steps in very good yield (78 %).

The Role of Copper Exchanged Phosphomolybdic Acid Catalyst for Knoevenagel Condensation

Cu exchanged heteropolyacid catalysts were synthesized by ion exchange method and characterized using various physico-chemical techniques such as X-ray diffraction (XRD), FT-IR, Raman, BET surface area, temperature programmed desorption (TPD) of ammonia, 31P NMR, pyridine adsorbed FT-IR spectroscopy, ICP-AES and STEM analysis. XRD diffractograms shows crystallites of heteropolyacid, while FT-IR and Raman spectra indicate that the Keggin ion is retained in the catalysts. 31P NMR, ammonia TPD and pyridine adsorbed FT-IR spectra results suggest that acidity decreases once Cu was incorporated in the heteropolyacid catalyst. For the Knoevenagel condensation reaction, the Cu exchanged phosphomolybdic acid (Cu–PMA) exhibits better catalytic performance than the phosphomolybdic acid (PMA) catalyst and this is related closely with the structural and acidic properties of the catalyst.

Highly efficient preparation of HMF from cellulose using temperature-responsive heteropolyacid catalysts in cascade reaction

A series of heteropolyacid catalysts (HOCH2CH2N(CH3)3) xH3−xPW12O40 (abbreviated as ChxH3−xPW12O40, x=1, 2 and 3) had been synthesized using choline chloride and H3PW12O40 as the raw materials, which were used as heterogeneous catalysts in one-pot conversion of cellulose to 5- hydroxymethylfurfural (abbreviated as HMF) in double solvent system containing methyl isobutyl ketone (abbreviated as MIBK) and H2O. A remarkable HMF yield of 75.0% was achieved catalyzed by ChH2PW12O40 within 8h at 140°C attributed to its higher Brønsted acidity and thermoregulate property comparable to its homogeneous form H3PW12O40. To the best of our knowledge, it was almost the highest yield of HMF from cellulose by now, while the best yield of 64.0% was reported so far over AlCl3 in biphasic system. Using our biphasic reaction protocol, even raw lignocellulosic biomass straw, gave HMF yields of 27.6%. Moreover, such heteropolyacid catalysts could be recycled under simply lowering the reaction temperature to room temperature without loss of its weight, which were reused for more than 10 times.

Hydrolytic oxidation of cellulose to formic acid in the presence of Mo-V-P heteropoly acid catalysts

The reduction of oxygen-containing compounds, which is employed to produce biofuels from plant feedstock, requires a great amount of reductant that is commonly represented by molecular hydrogen obtained from non-renewable fossils. Formic acid can also serve as the reductant or hydrogen source in the production of such fuels. It is shown that formic acid can be obtained from mechanically activated microcrystalline cellulose with high yields (66%) by the one-pot catalytic process of hydrolytic oxidation in the presence of Mo-V-P heteropoly acids (HPA) possessing bifunctional (acidic and oxidizing) catalytic properties. Mechanical activation has a crucial influence on the cellulose reactivity and formic acid yields. The highest yield of formic acid was obtained by using the cellulose activated in a planetary mill, when its crystal structure was mainly crushed. The temperature of 150–160°C and the oxidizing atmosphere (20% O2 and 80% N2) are found to be the optimal reaction conditions. Pressure was shown to exert no effect on the observable kinetics of the process. A search for the optimal HPA composition demonstrates that a maximum reaction rate is observed in the presence of HPA that provides the most acidic reaction medium. A linear dependence of the reaction rate on the concentration of H+ ions and a more complicated dependence on vanadium content were revealed.

Fe4(SiW12O40)3-catalyzed glycerol acetylation: Synthesis of bioadditives by using highly active Lewis acid catalyst

In order to investigate the effect of Lewis acidic metals on the acid properties of Keggin heteropolyacids catalysts, a set of metal-exchanged HPAs (i.e. M3/nPW12O40, M3/nPMo12O40 and M4/nSiW12O40 (M=Cu, Co and Mn, n=2; M=Fe, n=3) were synthesized and their activity on glycerol esterification with acetic acid was evaluated. This procedure avoids the use of corrosive Brønsted acid catalysts. Regardless of the heteropoly anion, the activity of catalysts in terms of metal cations followed the sequence: Fe3+>Cu2+>Mn2+>Co2+. It has been found that among the HPAs, only H4SiW12O40 has the acidic protons that can be successfully exchanged with Fe3+ cations, resulting in Fe4(SiW12O40)3 salt, the most active and selective catalyst. The highest conversion (ca.99.9%) was achieved in Fe4(SiW12O40)3-catalyzed esterification reactions, along with the highest selectivity for di and triacetyl glycerol (ca.55 and 42%, respectively). Effects of temperature, stoichiometry of reactants, concentration and nature of catalysts were assessed. A kinetic study of Fe4(SiW12O40)3 or H4SiW12O40-catalyzed reactions was conducted and the activation energy determined.

Deoxygenation of Ethers and Esters over Bifunctional Pt–Heteropoly Acid Catalyst in the Gas Phase

Deoxygenation and decomposition of ethers and esters, including anisole, diisopropyl ether (DPE), and ethyl propanoate (EP), was investigated using bifunctional metal–acid catalysis at a gas–solid interface in the presence and absence of hydrogen. The bifunctional catalysts studied comprised Pt, Ru, Ni, and Cu as the metal components and Cs2.5H0.5PW12O40 (CsPW), an acidic Cs salt of Keggin-type heteropoly acid (HPA) H3PW12O40, as the acid component, with the main focus on Pt–CsPW catalyst. It was found that bifunctional metal–acid catalysis in the presence of H2 is more efficient for ether and ester deoxygenation than the corresponding monofunctional metal and acid catalysis and that metal- and acid-catalyzed pathways play different roles in these reactions. With Pt-CsPW, hydrodeoxygenation of anisole, a model for the deoxygenation of lignin, occurred with 100% yield of cyclohexane under very mild conditions (60–100 °C and 1 bar of H2). This catalyst had the highest activity in anisole deoxygenation for a...

Gas phase oxidation of 2-methyl-1,3-propanediol to methacrylic acid over heteropolyacid catalysts

Heteropolyacids with Cs, V and Cu partially oxidize 2MPDO to methacrylic acid (40% selectivity) in the gas phase at 250 °C.

Grinding and Milling: Two Efficient Methodologies in the Solvent-Free Phosphomolybdic Acid-Catalyzed and Mechanochemical Synthesis ofcis-4-Amido-N-yl-2-methyl-tetrahydroquinolines

The ball-milling process of ABB' Povarov reaction through mechano-chemical activation with phosphomolybdic acid (PMA) as heteropolyacid (HPA) catalyst, was first time explored. The developed high speed vibratory ball milling (HSVBM) green method offers simple and efficient diastereoselective synthesis of cis-2,4-disubstituted tetrahydroquinolines in good to excellent yields (56-76%).

Synthesis and characterization of Cs-exchanged heteropolyacid catalysts functionalized with Sn for carbonolysis of glycerol to glycerol carbonate

Cs exchanged heteropolyacid catalysts functionalized with various Sn contents were prepared by wet impregnation method. These catalysts were characterized by X-ray diffraction, FT-IR, Raman spectroscopy, temperature programmed desorption of ammonia and BET surface area measurements. The catalytic properties of Sn–CsPW catalysts were evaluated for the synthesis of glycerol carbonate and they exhibit an unprecedented activity for the higher glycerol conversion and selectivity towards glycerol carbonate under vacuum conditions. Sn-functionalized Cs exchanged heteropolyacid catalysts (CsPW) play a significant role in the enhancement of acidity, catalytic activity and stability. The glycerol conversion and the selectivity of carbonate formation mainly depend on the Sn content and acidity of the catalysts. Different reaction parameters such as Sn molar ratio, glycerol to urea molar ratio, reaction temperature were investigated and also optimum conditions were established. The catalyst containing molar ratio of 3:1 Sn–CsPW has shown highest conversion and glycerol carbonate selectivity.

Dehydration of Methanol to Dimethyl Ether over Heteropoly Acid Catalysts: The Relationship between Reaction Rate and Catalyst Acid Strength

Dehydration of methanol to dimethyl ether (DME) was studied at a gas/solid interface over a wide range of bulk and supported Bronsted acid catalysts based on tungsten Keggin heteropoly acids (HPA) and compared with the reaction over HZSM-5 zeolites (Si/Al = 10–120). Turnover rates for these catalysts were measured under zero-order reaction conditions. The HPA catalysts were demonstrated to have much higher catalytic activities than the HZSM-5 zeolites. A good correlation between the turnover rates and catalyst acid strengths, represented by the initial enthalpies of ammonia adsorption, was established. This correlation holds for the HPA and HZSM-5 catalysts studied, which indicates that the methanol-to-DME dehydration with both HPA and HZSM-5 catalysts occurs via the same (or similar) mechanism and the turnover rate of methanol dehydration for both catalysts is primarily determined by the strength of catalyst acid sites, regardless of the catalyst pore geometry.

ChemInform Abstract: Solvent‐Free Heteropolyacid‐Catalyzed Glycerol Ketalization at Room Temperature.

AbstractAmong the tested heteropolyacids H3PW12O40 proves to be the best suited heteropolyacid catalyst for the regioselective ketalization of glycerol to furnish selectively the 1,3‐dioxolan‐4‐methanols (III) in this solvent‐free protocol.

Tailoring the Synergistic Bronsted-Lewis acidic effects in Heteropolyacid catalysts: Applied in Esterification and Transesterification Reactions.

In order to investigate the influences of Lewis metals on acidic properties and catalytic activities, a series of Keggin heteropolyacid (HPA) catalysts, HnPW11MO39 (M = TiIV, CuII, AlIII, SnIV, FeIII, CrIII, ZrIV and ZnII; for Ti and Zr, the number of oxygen is 40), were prepared and applied in the esterification and transesterification reactions. Only those cations with moderate Lewis acidity had a higher impact. Ti Substituted HPA, H5PW11TiO40, posse lower acid content compared with TixH3−4xPW12O40 (Ti partial exchanged protons in saturated H3PW12O40), which demonstrated that the Lewis metal as an addenda atom (H5PW11TiO40) was less efficient than those as counter cations (TixH3−4xPW12O40). On the other hand, the highest conversion reached 92.2% in transesterification and 97.4% in esterification. Meanwhile, a good result was achieved by H5PW11TiO40 in which the total selectivity of DAG and TGA was 96.7%. In addition, calcination treatment to H5PW11TiO40 make it insoluble in water which resulted in a heterogeneous catalyst feasible for reuse.

SnO2–SiO2 Mesoporous Composite: A Very Active Catalyst for Regioselective Synthesis of Aromatic Ketones with Unusual Catalytic Behavior

Aromatic ketones (R-Ar-CO-R′) have been prepared directly through C–C bond formation using aromatics, aryl, and alkyl acid halides in a Friedel–Crafts reaction over a reusable and catalytically active ordered SnO2–SiO2 three-dimensional mesoporous composite (PS-4) catalyst, which was found to be superior to modified zeolites, functionalized MCM-41, and supported heteropolyacid catalysts.

Vapor Phase Dehydration of Glycerol to Acrolein Over SBA-15 Supported Vanadium Substituted Phosphomolybdic Acid Catalyst.

Vapor phase dehydration of glycerol to acrolein was investigated over heteropolyacid (HPA) catalysts containing vanadium substituted phosphomolybdic acid (H4PMo11VO40) supported on mesoporous SBA-15. A series of HPA catalysts with HPA loadings varying from 10-50 wt% were prepared by impregnation method on SBA-15 support. The catalysts were characterized by X-ray diffraction, Raman spectroscopy, Fourier Transform infrared spectroscopy, temperature-programmed desorption of NH3, pyridine adsorbed FT-IR spectroscopy, scanning electron microscopy, pore size distribution and specific surface area measurements. The nature of acidic sites was examined by pyridine adsorbed FT-IR spectroscopy. XRD results suggest that the active phase containing HPA was highly dispersed at lower loadings on the support. FT-IR and Raman spectra results confirm that the presence of primary Keggin ion structure of HPA on the support and it was not affected during the preparation of catalysts. Pore size distribution results reveal that all the samples show unimodel pore size distribution with well depicted mesoporous structure. NH3-TPD results suggest that the acidity of catalysts increased with increase of HPA loading. The findings of acidity measurements by FT-IR spectra of pyridine adsorption reveals that the catalysts consist both the Brønsted and Lewis acidic sites and the amount of Brønsted acidic sites are increasing with HPA loading. SBA-15 supported vanadium substituted phosphomolybdic acid catalysts are found to be highly active during the dehydration reaction and exhibited 100% conversion of glycerol (10 wt% of glycerol) and the acrolein selectivity was appreciably changed with HPA active phase loading. The catalytic functionalities during glycerol dehydration are well correlated with surface acidity of the catalysts.