Heteropolyacid Catalysts Research Articles

A density functional theory study on the mechanism of Dimethyl ether carbonylation over heteropolyacids catalyst

Dimethyl ether (DME) carbonylation is considered as a key step for a promising route to produce ethanol from syngas. Heteropolyacids (HPAs) are proved to be efficient catalysts for DME carbonylation. In this work, the reaction mechanism of DME carbonylation was studied theoretically by using density functional theory calculations on two typical HPA models (HPW, HSiW). The whole process consists of three stages: DME dissociative adsorption, insertion of CO into methoxyl group and formation of product methyl acetate. The activation barriers of all possible elementary steps, especially two possible paths for CO insertion were calculated to obtain the most favorable reaction mechanism and rate-limiting step. Furthermore, the effect of the acid strength of Bronsted acid sites on reactivity was studied by comparing the activation barriers over HPW and HSiW with different acid strength, which was determined by calculating the deprotonation energy, Mulliken population analyses and adsorption energies of pyridine.

Preparation and testing of ceasium Brønsted ion-exchanged Al-SBA-15 supported heteropoly acid as heterogeneous catalyst in the fructone fragrancy synthesis

Here we report a new ceasium Bronsted ion-exchanged Al-SBA-15 (Cs/AS15) supported heteropoly acid (HPA) H3PW12O40 (HPA-Cs/AS15) material which can be prepared and used as heterogeneous catalyst for the fructone fragrancy synthesis. It was demonstrated that Cs+ was more effective than NH4+ as counter cation of ion-exchanged AS15 support for preparing HPA/AS15 catalyst with higher acidity and HPA leaching stability. HPA-Cs/AS15 material showed high acidity with the maximum NH3 desorption peak appeared at 759.6 °C in the NH3-TPD spectrum, stronger than pure HPA (635.7 °C) and the ammonium Bronsted ion-exchanged Al-SBA-15 supported HPA (HPA-NH4/AS15) (559.1 °C) materials. The HPA leaching stability test showed that the HPA content of HPA-Cs/AS15 catalyst reduced only by 2.55% after five washing times, much less than that of HPA-NH4/AS15 catalyst (49.22%). The higher acidity helped improve the HPA-Cs/AS15 catalyst performance in the fructone fragrancy synthesis. The ethyl acetoacetate conversion over this catalyst was 94.82%, better than those over HPA-NH4/AS15 (93.49%) and pure HPA (89.52%) catalysts, although HPA-Cs/AS15 catalyst had lower HPA content (23.16 wt%) than HPA-NH4/AS15 (24.28 wt%). In addition, it was shown that the reaction conditions such as ethyl acetoacetate:ethanediol reactant molar ratio of 1:1.5, ethanediol used as the diol reactant, and toluene used as the solvent, the fructone fragrancy synthesis reaction over HPA-Cs/AS15 catalyst reached the highest ethyl acetoacetate conversion (95.58%). The toluene’s boiling temperature of 110.6 °C can promote the dispersion and contact abilities of the reactants with HPA molecules presented inside the pores of HPA-Cs/AS15 catalyst.

Converting formaldehyde-methylethylketone adduct to a nonlinear C5 1,3-diol over Pt-ceria catalysts for isoprene production

Isoprene has outmost importance in the manufacture of elastomer. Herein, an alternative approach for isoprene production from readily available chemicals is reported: condensation of methyl ethyl ketone and formaldehyde, followed by the hydrogenation and dehydration reaction. The focus was paid on the hydrogenation reaction over Pt-ceria catalysts giving 2-methyl-1,3-butanediol, where the influence of Ce species on the catalyst structure was studied, and the structure-activity relationship was established. It deduced that the presence of CeO2 appeared to be essential for the catalyst structure (e.g., dispersion, morphology) and the enhanced catalytic performance (e.g., activating H2, facilitating the reaction), these benefited from the presence of the oxygen vacancies on CeO2 surface. In addition, a relatively weak Pt-support interaction gave larger Pt particles which resulted in a higher yield. Finally, the application of the as-synthesized 2-methyl-1,3-butanediol in isoprene production was performed in the presence of the heteropolyacid catalysts, affording isoprene in ∼80 % yield.

Review: cascade reactions for conversion of carbohydrates using heteropolyacids as the solid catalysts

Conversion of biomass into renewable chemicals and fuels has received significant attention as an approach to enhance global sustainability. Biomass conversion, especially carbohydrate conversion, commonly involves cascade reactions. During these cascade reactions, various activity sites on carbohydrates may have different reactivities that enable us to control the final products through adjusting these activity sites. Heteropolyacids (HPAs), one of the major solid acid catalysts, are extensively used in biomass conversion owing to their numerous excellent physicochemical properties, such as strong Bronsted acidity, oxidizability, structural versatility, and easy design. Accordingly, this review aims to outline recent breakthroughs in obtaining platform chemicals from carbohydrates by cascade reactions involving heteropolyacid catalysts. Moreover, the relationship between the final products and the structure of heteropolyacid catalysts is discussed and insights will help us to design more efficient solid acid catalysts for conversion of biomass to more sustainable chemical products.

Understanding reaction kinetics, deprotonation and solvation of brønsted acidic protons in heteropolyacid catalyzed synthesis of biorenewable alkyl levulinates

In search of a ‘descriptor’ for Brønsted acid-catalyzed biorenewable transformations in a complex reaction environment, two concepts related to the reactivity of Brønsted acid catalysts are explored. A simple reaction involving the esterification of levulinic acid in three different alcohol mediums (ethanol, 1-propanol, and 1-butanol) is experimented with two different Keggin heteropolyacid (HPA) catalysts to synthesize alkyl levulinates. On the same HPA catalyst, and different solvent medium, apparent activation energies of the esterification reaction are observed to increase by an average of ~5 kJ/mol on increasing the alkyl chain length of the alcohol medium by one carbon. Obtained apparent activation energies are corresponding with the solvation energies of the Brønsted proton in the respective alcohol medium. In contrast, on changing the HPA catalyst and keeping the same alcohol medium, the apparent activation energies are observed to differ by an average of ~19 kJ/mol. This directly correlates with the difference (~20 kJ/mol) in the vapor phase deprotonation energies (DPE) of the two HPA catalysts. Thus, in the solvent environment, DPE values and the degree of solvation of the Brønsted acidic protons are describing the reactivity of the HPA catalysts.

Novel strategies for glucose production from biomass using heteropoly acid catalyst

Abstract Bioethanol and direct glucose fuel cells pledged clean energy to the world. Cellulose depolymerization for glucose production has been a successful approach in bioethanol production. Heteropoly acids (HPAs) are strong Bronsted solid acid catalysts for biomass hydrolysis. Keggin type HPAs, namely, Silicotungstic acid (HSiW), Phosphotungstic acid (HPW), and Phosphomolybdic acid (HPMo), were used for the hydrolysis of lignocellulosic biomass to glucose. Five different biomass feedstocks, namely, miscanthus, sugarcane leaves, switchgrass, sunflower seeds, and bamboo leaves, were examined for the feasibility of total reducing sugar (TRS) yield through the composition analysis and catalytic biomass hydrolysis. Sunflower seeds contained the maximum holocellulose with 90.6%, and switchgrass contained the least i.e., 77.63%. Among the five biomass tested, switchgrass resulted in the highest TRS (5.77 wt/dry wt. %) with HPMo catalyst at a catalyst to biomass ratio of 30:100 (wt./wt. %), a reaction temperature of 120 °C for 3 h. The reaction parameters for depolymerization were optimized for all three HPAs, and the optimized conditions were 3 h and 120 °C. HPMo showed maximum TRS yield (5.77 wt/dry wt.%) among the three HPAs at 30:100 catalyst to biomass ratio. However, a catalyst to biomass ratio of 20:100 (wt./wt.%) was economical (5.25 wt/dry wt.%) for commercial application.

Effects of support and reaction pressure for the synthesis of dimethyl ether over heteropolyacid catalysts

Dimethyl ether (DME) is an advanced second-generation biofuel produced via methanol dehydration over acid catalysts such as γ-Al2O3, at temperatures above 240 °C and pressures above 10 bar. Heteropolyacids such as tungstosilicic acid (HSiW) are Brønsted acid catalysts with higher DME production rates than γ-Al2O3, especially at low temperatures (140–180 °C). In this work, we show that the performance of supported HSiW for the production of DME is strongly affected by the nature of the support. TiO2 and SiO2 supported HSiW display the highest DME production rates of ca. 50 mmolDME/h/gHSiW. Characterization of acid sites via 1H-NMR, NH3-isotherms and NH3-adsrobed DRIFT reveal that HSiW/X have Brønsted acid sites, HSiW/TiO2 showing more and stronger sites, being the most active catalyst. Methanol production increases with T until 200 °C where a rapid decay in methanol conversion is observed. This effect is not irreversible, and methanol conversion increases to ca. 90% by increasing reaction pressure to 10 bar, with DME being the only product detected at all reaction conditions studied in this work. The loss of catalytic activity with the increasing temperature and its increasing with reaction pressure accounts to the degree of contribution of the pseudo-liquid catalysis under the reaction conditions studied.

Catalytic oxidation of 5-hydroxymethylfurfural into 2,5-diformylfuran using V-containing heteropoly acid catalysts

2,5-Diformylfuran (DFF) is an attractive and challenging compound that can be synthesized with good selectivity by the oxidation of bio-based 5‐hydroxymethylfurfural (5-HMF) using specific catalysts. Vanadium-containing heteropoly acids (PMoV HPAs) are highly active oxidation catalysts whose activity can be tuned by changing vanadium(V) (VV) content. This study reports the preparation of a number of PMoV HPAs with different VV amount and type of outer-sphere cation and investigating their catalytic performance in 5-HMF-to-DFF conversion. The characterization of synthesized HPAs by simple instrumental methods (pH-measurement, potentiometry, and titration analysis) and NMR revealed retaining Keggin structure and increasing oxidation potential with rising VV amount. The complete optimization of reaction conditions allowed one to reach a DFF yield of 92% using Co2H6P3Mo18V7O84 at 110 °C in water/MIBK for 90 min under atmospheric pressure. The described method of 5-HMF oxidation is based on using effective soft oxidants that can be easily recycled and reused at least five times without significant loss of catalytic activity.

Highly efficient conversion of cellulose into 5-hydroxymethylfurfural using temperature-responsive ChnH5-nCeW12O40 (n = 1–5) catalysts

The design of stable heteropolyacid (HPA) catalysts with strong Brønsted acidity and mild Lewis acidity is crucial for the efficient conversion of renewable biomass to value-added chemicals. Firstly, a novel strategy was proposed to synthesize the Ce-based HPA of H5CeW12O40. Secondly, the H5CeW12O40 was combined with ChCl to form a series of temperature-responsive HPA catalysts (ChnH5-nCeW12O40, n = 1–5), which allowed the catalysts to be applied as soluble catalysts at a temperature >80 °C and be easily separated as an insoluble catalyst at a temperature <80 °C. In the H2O/DMSO/MIBK biphasic system, one-pot production of 5-hydroxymethylfurfural (5-HMF) from cellulose was achieved with an excellent yield of 67.5% over ChH4CeW12O40 catalysis at 140 °C for 6 h, and other carbohydrates were converted, including fructose (83.1%), glucose (74.2%), sucrose (75.8%), cellobiose (70.2%) and starch (68.7%). Such high efficiency was attributed to the homogeneous catalysis at high temperature, the strong Brønsted acidity of [H5-nCeW12O40]n−, and the appropriate Lewis acidity from Ce(III). Moreover, the temperature-responsive property and the high stability were beneficial for recycling without significant structural change and activity loss.

Supported heteropolyacids catalysts for the selective hydrocracking and isomerization of n-C16 to produce jet fuel

In the process of two-stage hydrogenation from bio-oil to jet fuel, the selective cracking and isomerization of long-chain alkanes is the key step. The n-C16 was used as the model reactant to produce jet fuel and the supported metal-acid bifunctional HPA/MCM-41 catalyst was prepared. The acid strength and amount of the catalyst could be modified by adjusting the type (HPW, HSiW, HPMo) and loading of HPA. The highest yield of jet fuel (≈ 20 wt.%) was obtained over Pt/40 wt.% HPMo/MCM-41 (WHSV = 1 h−1), which possessed more B acid sites but few strong acid sites. Besides, it possesses a higher yield than the previous work and favorable catalytic stability of 100 h. It was further explored that the reaction path net of hydrocracking and isomerization of n-C16 was a cascade reaction from n-C16 to single-branched isomer, further to multi-branched isomer and then to the cracking product.

Production of a Renewable Functionalized 1,3-Diene from Furfural–Acetone Adduct over Supported Heteropolyacid Catalysts

The transformation of renewable feedstocks into high-value chemicals is a sustainable and eco-friendly energy technology. In this work, a functionalized 1,3-diene (1-(2-furyl)-1,3-butadiene, F-dien...

Dehydration of methanol and ethanol over silica-supported heteropoly acids in the gas phase: Surface-type versus bulk-type catalysis mechanism

Dehydration of MeOH to dimethyl ether and EtOH to diethyl ether and ethene was studied at the gas-solid interface in the presence of bulk and SiO2-supported Keggin heteropoly acids (HPAs) H3PW12O40 (PW) and H4SiW12O40 (SiW) as catalysts. The catalysts were prepared by HPA impregnation from water and MeOH. Their acid strength, texture and structural integrity was characterised using NH3 adsorption calorimetry, BET, XRD and DRIFT spectroscopy. The strength of acid sites in HPA/SiO2 catalysts increased monotonously with HPA loading. In the dehydration of MeOH and EtOH, the turnover reaction rate for PW catalysts was higher than for SiW catalysts in agreement with their acid strength. HPA catalysts prepared from water and MeOH had a very close acid strength and showed similar activities in alcohol dehydration. The steady-state catalyst activity was found to correlate with the density of catalyst proton surface sites rather than with the HPA loading. This indicates that alcohol dehydration occurred via a mechanism of surface-type HPA catalysis at the gas-solid interface rather than a bulk-type (pseudo-homogeneous) catalysis.

Genesis of active phase in MoW/Al2O3 hydrotreating catalysts monitored by HAADF and in situ QEXAFS combined to MCR-ALS analysis

Alumina supported MoW hydrotreating catalyst was synthesized by using bimetallic H4[SiMo3W9O40] Keggin heteropolyacid (HPA). Catalysts based on monometallic H4[SiMo12O40] and H4[SiW12O40] and their mixture were also studied. Genesis of the active phase was studied during atmospheric gas sulfidation by H2S/H2 of the catalysts by in-situ Quick X-ray absorption spectroscopy (XAS) and High-Angle Annular Dark-Field (HAADF) imaging. The combination of different chemometric tools such as Principal Component Analysis (PCA) and Multivariate Curve Resolution with Alternating Least Squares (MCR-ALS) allowed to determine the number of intermediate species, their chemical nature and concentration profiles during sulfidation. It was found that tungsten sulfidation using bimetallic HPA precursor started at lower temperature, compared to W sulfidation in the monometallic and in the mixture of monometallic HPA catalysts. Simultaneous sulfidation of Mo and W atoms in case of the bimetallic molecular precursor can govern the formation of mixed MoWS2 phase, which formation during activation was evidenced by HAADF.

Catalytic glycerol dehydration-oxidation to acrylic acid

ABSTRACT This article provides a comprehensive and critical review of the latest studies on catalytic glycerol dehydration-oxidation to acrylic acid. The two-bed catalytic system in one or two reactors involves glycerol dehydration to acrolein and subsequent oxidation of acrolein to acrylic acid. Zeolites, metal oxides, heteropoly acids, and phosphates are effective in the dehydration of glycerol to acrolein. Mo–V–O catalysts appear active in the acrolein oxidation to acrylic acid. The glycerol can be completely converted to acrolein with 98% selectivity. In such a two-step process, the step of catalytic dehydration is thought to be critical. A few recent studies reveal that the conversion of glycerol to acrylic acid in two reactors can be also achieved via allyl alcohol as intermediate. For the one-bed catalytic glycerol oxydehydration to acrylic acid, a single catalyst must possess both active acid sites and active redox sites. Mo–V–O, W–V–O, Mo–V–W–O, W–V–Nb–O oxide catalysts, and heteropoly acid catalysts are particularly promising. Currently, a 60% yield of acrylic acid can be achieved over H0.1Cs2.5(VO)0.2(PMo12O40)0.25(PW12O40)0.75 at 340°C. However, all the catalysts rapidly deactivate due to coking. Coking usually occurs during the glycerol oxydehydration step. Optimizing reaction conditions such as increasing water and oxygen feeding, lowering reaction temperature, tuning the catalysts by finely doping, adjusting the surface acidity and enlarging pores of the solid catalysts can inhibit coking to some extent by slowing the deactivation of catalyst. Yet coking over catalysts is a major obstacle when conducting glycerol oxydehydration on a large scale. We suggest that future work should place an emphasis on revealing the essence of coking, further designing coking-resisting catalysts, and developing an efficient reaction and separation system.

Investigation of the acrolein oxidation on heteropolyacid catalysts by transient response methods

Isotopic assessment by oxygen, water and acrolein labeling delivers more detailed mechanistic insights.

Unraveling the role of the lacunar Na7PW11O39 catalyst in the oxidation of terpene alcohols with hydrogen peroxide at room temperature

The Na7PW11O39 lacunar salt was the most active among Keggin lacunar salts (Na8SiW11O39 and Na7PMo11O39), a saturated salt (Na7PW12O40) and heteropolyacid catalysts.

One-pot synthesis of formic acid via hydrolysis–oxidation of potato starch in the presence of cesium salts of heteropoly acid catalysts

Solid bifunctional catalysts based on cesium salts of V-containing heteropoly acids (CsHPA: Cs3.5H0.5PW11VO40, Cs4.5H0.5SiW11VO40, Cs3.5H0.5PMo11VO40) and Cs2.5H0.5PMo12O40 were used for studying one-pot hydrolysis–oxidation of potato starch to formic acid at 413–443 K and 2 MPa air mixture. It was shown that the optimum process temperature that prevents formic acid from destruction is 423 K. The studies were focused on the influence of the composition of heteropoly anions on the yield and selectivity of formic acid. Using W–V–P(Si) CsHPA results in the product overoxidation compared to Mo–V-containing CsHPA. The activity of Cs–PMo was significantly lower compared to Cs–PMoV. This may indicate that vanadium plays an important role in the oxidation process. The most promising catalyst was Cs3.5H0.5PMo11VO40 which provided the maximum yield of formic acid equal to 51%. Cs3.5H0.5PMo11VO40 was tested during nine cycles of starch hydrolysis–oxidation to demonstrate its high stability and efficiency.

NH3-SCR of NO with novel active, supported vanadium-containing Keggin-type heteropolyacid catalysts

Supported vanadium-substituted Keggin polyoxometalates (POMs) were applied as catalysts for the selective catalytic reduction of NO using NH3 as reductant (NH3-SCR).

Synthesis of functionalized dihydroimidazo[1,2-A]pyridines and 4-thiazolidinone derivatives from maleimide, as new class of antimicrobial agents

New dihydroimidazo[1,2-a]pyridines and 4-thiazolidinone derivatives have been synthesized by condensation reaction of substituted maleimide with 2,3-diaminopyridine or thiosemicarbazone under neutral or acidic medium in low and good yields. Structures and purity of these new products were confirmed by HRMS, 1H, 13C NMR, IR spectroscopy and crystal X-ray analysis. Optimization of reaction conditions for the preparation of dihydroimidazo[1,2-a]pyridine was studied by using a Keggin-type heteropolyacid catalyst. All the synthesized compounds were evaluated in vitro for their antibacterial and antifungal activities against six pathogenic bacteria and a strain of yeast. Compounds 4d, 7b, and 7d were more potent than the reference drug, against Candida albicans IPA (200). Compounds 4a, 4b, 7a, and 7b possessed the highest inhibitory effect or bactericidal activity against SARM.

Optimization of process variables for esterification of bio-oil model compounds by a heteropolyacid catalyst

In this work, the catalytic upgrading of a bio-oil model compound via esterification with ethanol was studied using 12-tungstosilicic acid as the catalyst. The response surface method was used to investigate and optimize the process variables, which include the ethanol to acid molar ratio, catalyst loading, reaction temperature, and reaction time for maximum conversion of the organic acid for esterification under atmospheric conditions. The effect of high He pressure on acid conversion for esterification was also studied. The maximum acid conversion was almost 90%, which can be achieved at reaction temperature of 77 °C, ethanol to acid molar ratio of 5:1, catalyst loading of 4.0% w/w, and reaction time of 8.3 h. Pressurizing the reaction with inert He did not have any effect for acid conversion at this optimum condition and had a slight negative effect under the studied conditions. After upgrading, the esterified products were found to improve the heating value from 17.6 to 23.2 MJ/kg and appeared to enhance fuel properties.