H3PW12O40 Catalysts Research Articles

Efficient conversion of carbohydrates and biomass into furan compounds by chitin/Ag co-modified H3PW12O40 catalysts

The development of heterogeneously functional catalysts is of great significance in selective catalysis for biomass valorization. Herein, we report for the first time the applicability of chitin/Ag co-modified H3PW12O40 composites (Chx-AgPW) as catalysts for producing valuable furans from carbohydrates and biomass. The as-prepared Ch15-AgPW catalyst exhibited the best catalytic performance, due to the dual Brǿnsted-Lewis acidity and appropriate chitin content. When the amount of Ch15-AgPW catalyst was 0.25 mmol, an excellent 5-hydroxymethylfurfural (HMF) yield of 78.3% could be achieved at 150 °C for 3 h with nearly 100% glucose conversion in the H2O/MIBK (VH2O:MIBK = 4:6) solvent system. Furthermore, high-yield HMF could also be obtained from other carbohydrates such as fructose (85.3%), sucrose (72.3%), cellobiose (65.6%), starch (52.7%) and inulin (43.5%). Additionally, corn stover, rice straw and sugarcane bagasse under typical reaction conditions simultaneously produced furfural and HMF in yields of 64.6%–72.2% and 18.5%–25.6%, respectively. Combined with the characterization, mechanism study demonstrated that the surface functional groups in chitin were conductive to rapid absorption of the substrates; the Brǿnsted acid sites from heteropoly anion (H2[PW12O40]-) promoted the initial depolymerization of biomass and the final dehydration to furans; the introduction of Ag+ favored the isomerization of intermediates. After reaction finished, the Ch15-AgPW catalyst could be easily recovered by filtration and effectively reused at least eight cycles without substantial loss in catalytic activity. This work provides a novel strategy to synthesize stable heterogeneous catalysts with tunable acidity for producing bio-based fuels and chemicals from renewable resources.

A highly active and stable organic-inorganic combined solid acid for the transesterification of glycerol under mild conditions

Solid acid catalyst plays a crucial role in the petroleum refinery industry and bio-refinery technology. In this work, p-phenolsulfonic acid (PSA) was successfully grafted onto the surface of KH560-modified zirconium phosphate (K-ZrP) in a facile routine. The structure and property of this organic-inorganic combined solid acid PSA/K-ZrP-x were characterized via XRD, FTIR, 13C solid-state NMR, TG, N2 adsorption-desorption, SEM, pyridine-adsorption FTIR and XPS technologies. The characterization results showed that KH560 can bond with ZrP and promote the grafting of PSA on the surface of K-ZrP via the condensation reaction between its epoxy ring and the phenolic hydroxyl group in PSA. Consequently, PSA/K-ZrP-2 exhibited excellent performance and stability in the transesterification between glycerol and methyl acetate among the tested H3PW12O40, Amberlyst-45, HBEA, HZSM-5, ZrP, AlCl3 and FeCl3 catalysts. The calculated conversion of glycerol reached 81.3% with a 97.9% selectivity for monoacetin (MAG) and diacetin (DAG) with a 2.2% dosage of [H+] at 100 °C for 4 h. The highest specific activity of PSA/K-ZrP-2 reached 24028.2 mg-glycerol/g-cat/h in a short reaction time (at 0.17 h), and it could be recycled five times without obvious deactivation.

Assessing the dispersion of supported H3PW12O40 catalysts: No longer a hurdle thanks to in situ IR upon pyridine adsorption

Assessing the dispersion of supported H3PW12O40 catalysts: No longer a hurdle thanks to in situ IR upon pyridine adsorption

Phosphotungstic acid anchored on amine-functionalized MIL-101: An effective catalyst for the direct esterification of 1-butene to sec-butyl acetate

Abstract This article proposes a novel strategy for enhancing the dispersion and stability of supported H3PW12O40 (HPW) via the charge interaction between HPW anions and amino groups originating from ethylenediamine (ED) grafted on the unsaturated metal sites of MIL-101. Due to its larger specific surface area and pore volume as well as stronger interactions between HPW and the carrier, the as-prepared catalyst exhibited a higher sec‑butyl acetate (SBAC) yield for the direct esterification of olefins than did the catalyst prepared by the encapsulation of HPW in the mesoporous cages of MIL-101. In addition, the HPW content incorporated in the as-prepared catalyst was optimized, and the results showed that among the series of catalysts, the HPW/ED-grafted MIL-101 catalyst with 40 wt% HPW had the highest activity and good stability for olefin esterification. This investigation sheds light on the conversion of 1-butene into high-value SBAC via the amine-functionalized MIL-101-supported HPW catalysts.

Assessing the dispersion of supported H3PW12O40 catalysts: No longer a hurdle thanks to in situ IR upon pyridine adsorption

Pyridine is since a long time used as an infrared (IR) molecular probe to characterize the acidity of solids, in particular of heteropolyacids (HPAs). The latter are metal-oxygen-clusters nowadays widely used in acid catalysis. Indeed, thanks to their exceptionally strong Brönsted acidity, they allow operating chemical reactions under significantly milder conditions than required by conventional catalysts. In the case of H3PW12O40, the most acidic Keggin-type HPA, it is well-known that the exposure to pyridine leads to an unusual IR spectrum. Indeed, the band at about 1540 cm−1 associated to the ν19b pyridinium ring stretch mode is split in two. Up to now, this phenomenon was studied with the only aim of understanding its origin, which was proposed to be a tunneling effect related to a frustrated rotation of the pyridinium cation within the solid bulk of H3PW12O40. Here, we demonstrate for the first time that it can actually be used as an analytical tool, namely to probe the degree of dispersion of supported H3PW12O40 units, a key parameter dictating their catalytic performance. We support H3PW12O40 on TiO2 P25 (hydrophilic) and TiO2 T805 (hydrophobic), so yielding samples with different degrees of dispersion, as qualitatively assessed through X-ray photoelectron spectroscopy. Through in situ Fourier-transform IR spectroscopy upon pyridine adsorption, we show that the ν19b pyridinium vibration band only and systematically splits in the presence of agglomerated Keggin units. More precisely, the higher the fraction of agglomerated Keggin units, the more marked the band splitting. So, the latter appears as a fingerprint of agglomerated H3PW12O40. Furthermore, following an easy treatment, the IR spectra allow quantitatively evaluating the fractions of dispersed vs. agglomerated Keggin units within the samples, which is very difficult to achieve through any other technique.

Preparation and properties of zirconia nanotube-supported 12-tungstophosphoric acid catalyst

Zirconia nanotube-supported H3PW12O40 (HPW) catalysts were obtained by loading HPW onto zirconia nanotube arrays which were prepared through anodization of zirconium foil in the mixture of formamide and glycerol (volume ratio=1:1) containing 1wt% NH4F and 1wt% H2O. The samples were characterized through scanning electron microscope, X-ray diffraction, infrared spectra and thermogravimetric analysis. Various factors affecting the catalytic activities have been investigated. The catalysts, prepared through pretreating the nanotube carrier at 400°C, followed by loading with 35wt% HPW and calcining at 200°C, possess high catalytic activities in the synthesis of fatty acid ethyl ester. Under the optimal reaction conditions, the conversion percentages of lauric acid, oleic acid and stearic acid are all higher than 98.5%.

Catalytic dehydration of glycerol to acrolein over M2.5H0.5PW12O40 (M = Cs, Rb and K) phosphotungstic acids: Effect of substituted alkali metals

Catalytic conversion of glycerol into value-added chemicals, particularly acrolein via acid-catalyzed dehydration route has received much attention due to the potential uses of acrolein. This work reports the synthesis of various alkaline metal substituted phosphotungstic acid (H3PW12O40, HPW) catalysts, namely M2.5H0.5PW12O40 (M=Cs, Rb and K) using a controlled precipitation method. A systematic structural, morphology, and chemical characterization were conducted using various analytical techniques. XRD studies revealed that the incorporation of alkaline metals in H3PW12O40 leads to decreased crystallite size and enhanced lattice strain. N2 adsorption–desorption studies show that the specific surface area of H3PW12O40 is significantly improved from 5 to 82 (K2.5H0.5PW12O40), 103 (Rb2.5H0.5PW12O40), and 94m2/g (Cs2.5H0.5PW12O40). XRD, Raman, and FT-IR studies confirm the Keggin structure of all the alkaline metal substituted HPW catalysts. The acidity strengths estimated by NH3-TPD analysis were obtained in the following order: H3PW (2654.91μmole/g)>K2.5H0.5PW (1060.10μmole/g)>Rb2.5H0.5PW (762.08μmole/g)>Cs2.5H0.5.5PW (461.81μmole/g). Although alkaline metal substituted H3PW12O40 catalysts exhibit higher specific surface area and smaller crystallite size compared to parent H3PW12O40 low glycerol conversions were found for substituted H3PW12O40 catalysts. As well, the parent H3PW12O40 catalyst shows an excellent acrolein selectivity (95%) which is much higher than that of Cs2.5H0.5.5PW (81.9%) and very close to the selectivities obtained over Rb2.5H0.5PW (95.1%) and K2.5H0.5.5PW (95.6%) catalysts. The catalytic performance of H3PW12O40andM2.5H0.5PW12O40 materials is directly proportional to their acidic strengths, indicating that the catalyst acidity is a key factor for achieving better results in glycerol dehydration.

3D graphene aerogel anchored tungstophosphoric acid catalysts: Characterization and catalytic performance for levulinic acid esterification with ethanol

3D graphene aerogel (GA) was synthesized from graphene oxide (GO) using a hydrothermal procedure combined with freeze-drying. Then, they were used to assemble the supported H3PW12O40 (HPW) catalysts. The optimization of the reaction parameters for levulinic acid esterification with ethanol was performed and the catalytic performance of the HPW/GA catalysts was comparatively investigated. The results showed that these catalysts had abundant porous structure. In particular, they were found to be efficient in the synthesis of ethyl levulinate, which is promising in reducing the consumption of petroleum-derived fossil fuels.

Support enhanced α-pinene isomerization over HPW/SBA-15

A family of mesoporous SBA-15 supported H3PW12O40 (HPW) catalysts were synthesized by wet-impregnation and compared with fumed silica analogues for the solventless isomerization of α-pinene under mild conditions. Structural and acidic properties of supported HPW materials were characterized by powder XRD, HRTEM, XPS, TGA, N2 porosimetry, DRIFTS, and ammonia and propylamine chemisorption and TPD. The high area, mesoporous SBA-15 architecture facilitates the formation of highly dispersed (isolated or low dimensional) HPW clusters and concomitant high acid site densities (up to 0.54mmolg−1) relative to fumed silica wherein large HPW crystallites are formed even at low HPW loadings. α-Pinene exhibits a volcano dependence on HPW loading over the SBA-15 support due to competition between the number and accessibility of acid sites to the non-polar reactant, with the superior acid site accessibility for HPW/SBA-15 conferring a 10-fold rate enhancement with respect to HPW/fumed silica and pure HPW. Monocyclic limonene and terpinolene products are favoured over polycyclic camphene and β-pinene by weaker polyoxometallate analogues over SBA-15.

Immobilizing heteropolyacids on zirconia-modified silica as catalysts for oleochemistry transesterification and esterification reactions

A new method of chemical immobilization of Keggin heteropolyacids (HPAs) was suggested. H3PW12O40, H4SiW12O40, and H3PMo12O40 were immobilized on the silica which was previously grafted with zirconium butoxide. The immobilization method promoted strong interaction HPA-support and yielded 25wt.% of well-dispersed HPAs, so increasing the density of acid sites. The catalysts were active in the reaction of transesterification of methyl stearate with n-butanol and esterification of oleic acid with trimethylolpropane. We demonstrate that, contrary to the immobilized H3PMo12O40, the H3PW12O40 and H4SiW12O40-based catalysts are stable toward leaching in a non-polar oleic acid medium. A discussion on circumventing the leaching in non-polar versus polar media is proposed in terms of interaction strength HPA-support. The stronger interaction (i.e., better resistance for leaching) between the support and H3PW12O40 (or H4SiW12O40) is referred to the lower difference of electronegativity between Zr and W and the lower polarizability of the bonds Zr–O–W compared to Zr–O–Mo.

Metal-organic frameworks with high tungstophosphoric acid loading as heterogeneous acid catalysts

High loadings of 12-tungstophosphoric acid (H3PW12O40) were incorporated within highly porous metal-organic framework MIL-101. The samples have been characterized using different techniques such as XRD, TEM, FT-IR, TGA, N2 adsorption and Raman spectroscopy. XRD and TEM did not show any aggregation of the H3PW12O40 nanocrystals on the surface of MIL-101 crystals up to 70wt%. Similarly, N2 sorption–desorption measurements reveal that samples with H3PW12O40 loading up to 70wt% retain their mesoporosity. The incorporation of H3PW12O40 crystals gradually increases both the surface acidity and the acid strength of the H3PW12O40/MIL-101 catalysts up to 70wt% loading. The catalytic performances of the H3PW12O40/MIL-101 solid catalysts were tested in three acid-catalyzed organic transformations, namely, the Pechmann, Esterification and Friedel–Crafts acylation reactions. In the three reactions, the catalytic activity attains the maximum value at 70wt% of H3PW12O40 loading, thus demonstrating direct correlation with dispersion, mesoporosity and acid strength. The unique attributes of MIL-101 and a well-dispersed level of H3PW12O40 prohibit the conglomeration and deactivation of H3PW12O40, which leads to the enhancement of the catalytic properties. The H3PW12O40 catalysts supported on MIL-101 are potentially promising heterogeneous catalysts for acid-catalyzed organic transformations in environmental friendly processes, to supersede the use of conventional homogeneous H3PW12O40 catalysts.

TiO2-supported heteropoly acids for low-temperature synthesis of dimethyl ether from methanol

The production of dimethyl ether via methanol dehydration with TiO2-supported H3PW12O40 (HPW) or H4SiW12O40 (HSiW) catalysts has been studied. Both supported heteropoly acids (HPAs) exhibit superior performances in terms of methanol conversion rate and selectivity toward dimethyl ether at significantly lower temperatures than benchmark Al2O3- and ZSM-5-based catalysts. Both supported and unsupported heteropoly acids have strong Brønsted acid sites; however, the TiO2-supported HPAs have a lower amount of acid sites than the bulk HPA. In addition, upon HPA deposition on TiO2, the water of crystallization molecules, located between neighboring Keggin structures, becomes adsorbed more weakly, so they can be easily replaced by methanol molecules. As a result of the higher accessibility of methanol to the active acid sites of the supported heteropoly acids, both H3PW12O40/TiO2 and H4SiW12O40/TiO2 exhibit higher normalized rates for methanol conversion and total DME production than unsupported HPAs.

H3PW12O40/MCM-41 Mesoporous Solid Acids as Promising Catalysts for the Alkenylation of p-Xylene with Phenylacetylene

Alkenylations of p-xylene with phenylacetylene over various zeolites like HY, H-Beta, and H-ZSM-5 have been performed. Effects of the types of zeolite and pretreatment conditions, such as ion exchange times, calcination temperature, and HCl concentration for dealumination, on the catalytic properties have been studied. Moreover, the HY, H-Beta, and MCM-41 supported both H3PW12O40 and H3SiW12O40 solid acid catalysts, which were employed to promote the alkenylation reaction. It has been found that H3PW12O40/MCM-41 mesoporous solid acid could become a promising catalyst for alkenylation of p-xylene. The possible mechanism of alkenylation of p-xylene with phenylacetylene over solid acid catalysts is also proposed.

Nano Fe2O3, Clinoptilolite and H3PW12O40as Efficient Catalysts for Solvent-Free Synthesis of 5(4H)-Isoxazolone under Microwave Irradiation Conditions

A quick and solvent-free approach involving the exposure of neat reactants to microwave irradiation in conjunction with the use of clinoptilolite, H3PW12O40 and Fe2O3 nanoparticle catalysts is described. In this work, condensation of hydroxylamine hydrochloride, sodium acetate, acetoacetic or benzoyl acetic ethyl ester and appropriate aldehydes by employing catalysts gave 5(4H)-isoxazolone only in one step. Catalyst amount, temperature effects and catalysts reusability were monitored. Among the catalysts, Fe2O3 nanoparticles had better performance than other catalysts from viewpoint of yield and reaction time. The present protocol offers several advantages, such as short reaction time, reasonable yield, mild reaction condition and recycling catalysts with a very easy workup.

Compensation effect in isopropanol dehydration over heteropoly acid catalysts at a gas–solid interface

Kinetics of isopropanol dehydration at a gas–solid interface over a range of bulk and supported Brønsted acid catalysts based on H3PW12O40 (HPW) Keggin-type heteropoly acid was studied in a continuous flow fixed-bed reactor in the temperature range of 55–135°C and isopropanol partial pressure of 0.94–5.52kPa. The bulk catalysts included HPW itself and its Cs acid salts CsnH3−nPW12O40 (CsnH3−nPW). The supported catalysts comprised of 15wt.% HPW supported on SiO2, TiO2, ZrO2 and Nb2O5. Under the conditions studied, the reaction was found to be of zero order in isopropanol. The turnover rate decreased in the order HPW>Cs2.5H0.5PW≈Cs2HPW>HPW/SiO2>HPW/TiO2>HPW/Nb2O5≈HPW/ZrO2, which is in line with the acid strength of these catalysts. The true activation energies E and pre-exponential factors A obtained from zero-order kinetics, were found to exhibit a compensation linear relationship lnA=mE+c. Moreover, the bulk and supported catalysts exhibited different compensation plots. This is suggested to be due to the different chemical structure of Brønsted acid sites in these catalysts, resulting in differing reaction mechanisms. The bulk catalysts possess strong surface H+ sites located on peripheral (bridging) oxygen atoms in the Keggin unit, whereas supported HPW catalysts have weaker H+ sites probably located on oxygen atoms of support. Consequently, on the bulk catalysts, isopropanol dehydration is suggested to occur via E1 elimination pathway, whereas on the supported catalysts it might also involve a contribution of E2 elimination pathway.

Direct alkenylation of aromatics with phenylacetylene over supported H3PW12O40 catalysts as a clean and highly efficient approach to producing α-arylstyrenes

Phosphotungstic acid (PTA) catalysts supported on MCM-41 prepared via a wet impregnation method assisted by vacuum with heating (IMPVH) were first employed for direct alkenylation of different aromatics with phenylacetylene to synthesize α-arylstyrenes. N2 adsorption–desorption, FT-IR, X-ray diffraction (XRD), and NH3 temperature-programmed desorption (NH3 TPD) characterization techniques were used to reveal the relationship between the catalyst’s nature and properties. The results demonstrate that the fabricated 25wt.% PTA/MCM-41 catalyst exhibits outstanding catalytic performance, remarkably better than that on HY zeolite. It is also found that the catalytic properties of the catalysts are strongly dependent on PTA dispersity, the nature of the acid sites, the preservability of PTA Keggin structure, and the mesopore architecture, notably affected by PTA loading and calcination temperature. The results for catalytic stability illustrate that more than 99% of maximum conversion can be obtained, and more than 92% conversion can be maintained for up to 540min time on stream. We find that the decrease in catalytic activity, along with the long reaction time, is mainly ascribable to deactivation by coke deposition. The spent catalyst can be refreshed, and 97.1% conversion can be obtained over the regenerated catalyst. This approach is also highly efficient for extra-substituted benzene, polycyclic aromatics, and even heteroaromatics, suggesting that the method presented in this paper can be a green and highly efficient synthesis protocol for α-arylstyrenes.

Sustainable production of acrolein: Preparation and characterization of zirconia-supported 12-tungstophosphoric acid catalyst for gas-phase dehydration of glycerol

ZrO2-supported H3PW12O40 (HPW) catalysts were used to catalyze the dehydration of glycerol to produce acrolein at 315 °C under a high space velocity (GHSVGlycerol = 400 h−1). The catalysts were prepared by varying the loading of HPW on two ZrO(OH)2 supports of different preparations, followed by calcination in flowing nitrogen. The use of alcogel-derived ZrO(OH)2-AN for the support, compared with conventional hydrogel-derived ZrO(OH)2-CP, produced HPW/ZrO2 catalysts that showed higher surface areas and better catalytic performance for the selective formation of acrolein from glycerol dehydration. Independent of the preparation history of ZrO2, the Keggin-anion density (HPW nm−2) at the catalyst surface appeared as a key to the selectivity and mass-specific activity of HPW for acrolein production. Acrolein selectivity as high as 70 mol% was obtained over the catalysts having the intermediate densities (0.18–0.65 HPW nm−2), at which most of the Keggin HPW remained intact after calcination up to 650 °C. At higher densities, destruction of the Keggin HPW would occur during the calcination, which led to lower acrolein selectivity. High selectivity (71 mol%) and high yield (≥54%) for acrolein production were found sustainable over the best performing HPW/ZrO2-AN catalyst for reaction times up to 10 h.

Hydroisomerization of n-Heptane Over Cr Promoted Pt-bearing H3PW12O40 Catalysts Supported on Dealuminated USY Zeolite

The Pt-bearing H3PW12O40 (PW) catalysts, promoted by Cr or La, supported on dealuminated USY zeolite were prepared and their catalytic activities were evaluated in the hydroisomerization of n-heptane with an atmospheric fixed-bed reactor. The catalysts were characterized by XRD, BET, IR and H2-chemisorption. The results showed that the dealuminated USY support retained the Y zeolite structure and the PW well kept its Keggin structure in the supported PW catalysts and the doping of Cr or La into the catalysts enhanced the dispersion of Pt on the catalyst surface. The Pt-bearing PW catalysts doped with Cr or La, especially Cr, exhibited much higher catalytic activity and selectivity than the catalysts without dopants. Both the conversion and selectivity were discussed in relation with physicochemical properties of catalysts.

Hydroisomerization of n-heptane over bimetal-bearing H3PW12O40 catalysts supported on dealuminated USY zeolite

The bimetal-bearing (CePt or LaPt) 12-tungstophosphoric acid (H3PW12O40 (PW)) catalysts supported on dealuminated USY zeolite (DUSY) were prepared by impregnation and characterized by XRD, BET, IR, and H2-chemisorption. Their catalytic activities were tested in the hydroisomerization of n-heptane with a continuous atmospheric fixed-bed reactor. After the steam treatment combined with the acid leaching, as well as the supporting with PW and the bimetals, the DUSY support retains the Y zeolite porosity and the PW well keeps its Keggin structure in catalysts. The doping of Ce into the catalysts enhances the dispersion of Pt on the catalyst surface. The Pt-bearing PW catalysts doped with Ce or La, especially Ce, exhibit much higher catalytic activity and selectivity than the catalysts without dopants at lowered reaction temperatures. At the optimal reaction conditions, i.e., the reaction temperature of 250°C and WHSV of 1.4 h−1, the catalyst with a Pt loading of 0.4%, PW loading of 10% and a molar ratio of Ce to Pt of 15:1 shows a conversion of n-heptane of 70.3% with a high selectivity for isomerization products of 94.1%.

Structure and Catalytic Performance of H3PW12O40/SiO2 Prepared by Several Methods

Abstract Silica-supported H3PW12O40 catalysts were prepared by the impregnation method, sol-gel technique, and sol-gel technique using ionic liquids as the template, respectively, to solve the problems of H3PW12O40 such as low surface area, difficult separation, and reuse. The catalysts were characterized by means of Fourier transform infrared spectroscopy, X-ray diffraction, N2 adsorption, and NH3 temperature-programmed desorption. Their catalytic performance for the nitration of benzene was evaluated. The results revealed that H3PW12O40 in all the catalyst samples kept its Keggin structure. The specific surface area of the H3PW12O40/SiO2 catalyst prepared by the impregnation method was 475.2 m2/g, and the yield of benzene over it decreased from 82.4% to 70.7% after 4 runs. The specific surface area of the H3PW12O40/SiO2 catalyst prepared by the sol-gel technique was 498.6 m2/g, and the nitration of benzene over it decreased from 85.1% to 79.6% after 4 runs. The H3PW12O40/SiO2 catalyst prepared by the sol-gel technique using [emim]BF4 as the template had mesoporous structure and a specific surface area of 558.5 m2/g, and the nitration of benzene over this catalyst decreased from 84.7% to 79.9% after 4 runs. The three H3PW12O40/SiO2 catalyst samples showed higher stability and activity in the nitration of benzene.