Silica-supported Heteropolyacids Research Articles

Dehydration of Isopropanol over Silica-Supported Heteropoly Acids

Dehydration of i-PrOH is used in academic research as a test reaction to probe the acid properties of solid acid catalysts. Also, it has practical importance for the utilization of surplus acetone produced by the Hock process for the combined manufacturing of phenol and acetone as well as for the production of propene from renewable resources and waste. This study demonstrates the excellent performance of polyoxometalate acid catalysts comprising silica-supported Keggin-type heteropoly acids H3PW12O40 and H4SiW12O40 for gas-phase i-PrOH-to-propene dehydration at ambient pressure. These catalysts show similar efficacies, giving an i-PrOH conversion and propene selectivity of 96.8 and 99.7% for 25%HPW/SiO2 and 97.1 and 99.4% for 25%HSiW/SiO2 in a fixed-bed reactor at 120 °C, a relevant-to-practice i-PrOH partial pressure of 15 kPa and a contact time W/F = 27 g h mol−1 (GHSV = 900 mL g−1 h−1). The catalysts are stable, resisting deactivation for at least 24 h time on stream. The HPA/SiO2 catalysts are superior to aluminosilicate zeolites such as H-mordenite, HZSM-5 and HY for i-PrOH-to-propene dehydration in terms of i-PrOH conversion, propene selectivity and catalyst stability.

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.

The influence of surface coverage on the catalytic activity of silica-supported heteropolyacids

A series of heteropolyacid (HPA)—silica composites were prepared from H3PMo12O40 (HPM) and H4PVMo11O40 (HPVM) by supporting on two types silica supports: Aerosil–Degussa and Romsil–Ramnicu Valcea with different active phase concentrations. The structure and texture of these HPA/silica composites were studied by XRD, FT-IR spectroscopy and nitrogen physisorption at 77 K. The effect of the support on the thermal stability was investigated by TG-DTG and DTA. By the deposition of heteropolyacids on silica type supports, an important decrease of thermal stability was observed on Romsil types and a small decrease on Aerosil types. By comparing with initial supports, the specific surface areas of HPAs deposited on all supports decreased with the active phase concentration. The catalytic properties of the prepared samples were studied by using a reactant pulse method at different temperatures. The ethanol conversion proceeds by two main pathways: an oxide hydrogenation reaction on redox catalytic centers with acetaldehyde formation, and a dehydration reaction on acidic centers with ethylene and diethyl ether formation. The reaction rates of ethanol conversion and acetaldehyde formation increased on supported HPA compared to bulk HPA. The favorable effect of HPAs deposition on silica supports for the oxidehydrogenation pathway to acetaldehyde results from the examination of apparent activation energies Ea and reaction rate values. Evidence of decreasing Ea was obtained for acetaldehyde formation in the case of silica Romsil-supported HPM (39.6 kJ/mol) comparatively with bulk HPM (56.6 kJ/mol). For all reaction temperatures, the reaction rates of acetaldehyde formation are higher for supported samples than unsupported ones.

N-Butane Isomerization over Silica-Supported Heteropolyacids: Study of Some Parameters

Isomerization of n-butane was studied on silica-supported heteropolyacids. The activity depends strongly on the solvent used for the preparation and on the polyoxometalate loading. This behavior was explained by different proportions of isolated and bulk heteropolyacids. Silicotungstic acid is inherently more selective than phosphotungstic acid. In presence of platinum and hydrogen the catalysts are stable and highly selective (more than 97%) to isobutane. A study as a function of temperature shows that there is an optimal temperature for performing the reaction.

Progress of the Application of Mesoporous Silica-Supported Heteropolyacids in Heterogeneous Catalysis and Preparation of Nanostructured Metal Oxides

Mesoporous silica molecular sieves are a kind of unique catalyst support due to their large pore size and high surface area. Several methods have been developed to immobilize heteropolyacids (HPAs) inside the channels of these mesoporous silicas. The mesoporous silica-supported HPA materials have been widely used as recyclable catalysts in heterogeneous systems. They have shown high catalytic activities and shape selectivities in some reactions, compared to the parent HPAs in homogeneous systems. This review summarizes recent progress in the field of mesoporous silica-supported HPAs applied in the heterogeneous catalysis area and preparation of nanostructured metal oxides using HPAs as precursors and mesoporous silicas as hard templates.

Highly efficient catalyst for the decarbonylation of lactic acid to acetaldehyde

The gas phase decarbonylation of lactic acid was performed over various silica-supported heteropolyacids. The obtained performances were, by far, higher than those previously described in the literature. In particular, the best results were obtained for silicotungstic acid-based catalysts, which showed very high yields of acetaldehyde (81–83%) at high lactic acid conversion (up to 91%).

Methoxylation of α-pinene over heteropolyacids immobilized in silica

The methoxylation of α-pinene was studied using heteropolyacids immobilized on silica as catalysts, at 60 °C, being the α-terpinyl methyl ether the main product. Tungstophosphoric acid (PW), molybdophosphoric acid (PMo), tungstosilicic acid (SiW) and molybdosilicic acid (SiMo) were immobilized on silica by sol–gel method. It was observed that the catalytic activity of the silica-supported heteropolyacids decreases in the series: PW2_S > SiW_S > PMo_S > SiMo_S. A series of PW immobilized on silica with different PW loading were prepared. It was observed that the catalytic activity increases with the amount of PW immobilized on silica. However, at high amount of PW on silica, a decrease of the catalytic activity was observed. Good values of selectivity to α-terpinyl methyl ether (about 60% near complete conversion) were obtained with all catalysts. Catalytic stability of the PW2_S was evaluated by performing consecutive batch runs with the same catalyst sample. After the third batch it was observed a stabilisation of the initial activity. A kinetic model was developed assuming that the α-pinene is consumed according to the parallel reaction network. It was observed that the kinetic model fits the experimental concentration data quite well.

Dehydration of glycerol in gas phase using heteropolyacid catalysts as active compounds

Different silica-, alumina-, and aluminosilicate-supported heteropolyacid catalysts were prepared using phosphomolybdic acid H 3PMo 12O 40⋅ xH 2O, phosphotungstic acid H 3PW 12O 40⋅ xH 2O, silicotungstic acid H 4SiW 12O 40⋅ xH 2O, and ammonium phosphomolybdate (NH 4) 3PMo 12O 40⋅ xH 2O as precursor compounds. The as-synthesised solids were characterised by nitrogen adsorption, XRD, TG/DTA, Raman spectroscopy, and TPD of ammonia. Silica-supported heteropolyacids are rather well crystallised, whereas alumina-supported samples are X-ray amorphous. Investigations using Raman spectroscopy of calcined samples and TG/DTA revealed that molybdenum-containing heteropolyacids tend to decompose partly close to 400 °C into molybdates and MoO 3 whereas tungsten-containing samples are stable. This makes in particular tungsten-based materials interesting acid catalysts for the dehydration of glycerol in the gas phase. In particular, the influence of selected support materials, catalyst loading, and temperature on acrolein formation was studied at standardised reaction conditions (10% by weight of glycerol in water, 225–300 °C, modified contact time 0.15 kg h mol −1). Surprisingly, alumina is found to be superior to silica as support material with regard to catalyst activity and selectivity. Nevertheless, tungsten based heteropolyacids showed outstanding performance and stability. Acrolein was always the predominant product with maximum selectivity of 75% at complete conversion over silicotungstic acid supported over alumina and aluminosilicate.

Synthesis of Ionones by Cyclization of Pseudoionone on Solid Acid Catalysts

Ionone synthesis (α, β and γ isomers) by pseudoionone cyclization was studied on zeolite HBEA, Amberlyst 35W, SiO2–Al2O3, and unsupported and silica-supported heteropolyacids (HPA/SiO2). Ionone formation was preferentially promoted on strong Bronsted acid sites. A 79% ionone yield was obtained on 58.5 wt.% HPA/SiO2 after 1.5 h of reaction at 383 K and 250 kPa. This value is similar to the best yields reported for the homogeneously-catalyzed reaction using sulfuric acid.

Isomerization of n-Hexane over Silica-Supported Heteropolyoxometallates Promoted by Pt-Ce Oxides

Abstract Catalysts based on Keggin-type heteropolyacids and reduced Pt-Ce oxides supported on silica were prepared for their use in isomerization of n-hexane in the presence of hydrogen. These mixed oxides are either used alone or mixed with silica-supported heteropolyacids. The catalyst after oxidation, reduction, and catalytic tests was characterized by X-ray diffraction, Fourier transform infrared spectroscopy, Fourier transform Raman, thermogravimetry, and N 2 adsorption. The objective of the present study is to select the catalyst having the best isomer (branched) yield under optimal operating conditions (reaction temperature, reduction temperature, flow duration, etc.). The results show that the presence of the reduced oxides is not only beneficial for eliminating coke position but also effective for maintaining the structure of silica-supported heteropolyacids during reaction. Moreover, the better activity and the higher selectivity for isomers can be found with these samples.

Gasoline desulfurization by catalytic alkylation over silica-supported heteropolyacids: From model reaction to real feed conversion

In FCC gasoline, the alkylation of sulfur compounds with olefins present in the feed, followed by a distillation to eliminate them, could be a good alternative for the desulfurization. Acidic materials (zeolites, supported acids) were screened for the alkylation of 3-methylthiophene (3MT) with 2-methyl-2-butene (2M2B), used as model reaction. Results showed that the silica-supported 12-phosphotungstic (HPW) and 12-silicotungstic (HSiW) acids are the best catalysts among zeolites and other supported acids. The performance of 40HPW/SiO 2 was then studied in a real feed of FCC gasoline. The production of heavier sulfur compounds was obtained with high conversion, and this catalyst still showed some activity after 50 h of reaction, as compared to the silica-supported phosphoric acid (SPA-11), used as a reference. Moreover the deactivation could be reduced by increasing the reaction temperature.

Production of acrolein from glycerol over silica-supported heteropoly acids

Dehydration of glycerol to produce acrolein was performed over several solid acids. Supported heteropoly acids were effective as a catalyst for the dehydration of glycerol. The catalytic activity depended on the types of heteropoly acid and on the size of the mesopores in the silica support. Silicotungstic acid supported on silica with mesopores of 10 nm showed the highest catalytic activity with the acrolein selectivity of >85 mol% at an ambient pressure and 275 °C.

Mechanical mixtures of Me (Ni, Pd) Ce oxides and silica-supported heteropolyacids: Role and optimal concentration of each active species in n-hexane isomerization

Catalytic properties of silica-supported heteropolyacids (HPA) in a mechanical mixture with reduced Me-Ce oxides (Me = Ni, Pd) in n-hexane isomerization are studied. The role of each component of the mixed oxides (Ce and, typically, Ni and Pd) and their optimum content has been illuminated: cerium is not only beneficial for eliminating or preventing coke deposition but is also effective for maintaining the Keggin structure of the highly-organized HPA during the reaction and probably allows a better dispersion of the second metal species. Nickel and palladium, present as Ni0 and Pd0, reinforce the activation of the alkane, which is difficult to obtain by means of a direct attack by an acid, and, thus, enhance noticeably the activity of the catalyst. The best mechanical mixtures are obtained with 30–70 wt % NiCeO-HPW/SiO2 and 50–50 wt % Pd0.1CeO-HPW/SiO2. These mixtures have the highest efficiency for a Ni/(Ni + W) atomic ratio of 0.66 and a Pd/(Pd + W) ratio of 0.40, respectively. Finally, the conversion of n-hexane is in the order HPW > HSiW > HBW, which seems to be consistent with the order of their acid strength as per the literature, but the isomerization selectivity appears to be slightly higher on HSiW.

Mechanical mixtures of PdCeO and HPA/SiO 2: structure and reactivity for n-hexane isomerisation

New reduced Ce–Pd oxides mechanically mixed with silica-supported heteropolyacids (HPA) such as H 3PW 12O 40 and H 4SiW 12O 40 were prepared, characterized and tested for n-hexane hydroisomerisation. X-ray diffraction (XRD) and Fourier-transform infrared (FT-IR) show that the silica-supported HPA keep their Keggin structure after reaction. Mechanical mixtures of mixed Pd–Ce oxides and supported heteropolyacids have a beneficial effect for the n-hexane isomerisation to dibranched alkanes and sensible decrease of the cracking is observed.

Silica‐Supported Heteropolyacids Readily Induce Cyclodimerization of Styrenes and Stilbenes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Friedel-Crafts acylation and related reactions catalyzed by heteropoly acids

The Friedel-Crafts acylation of anisole (AN) with acetic anhydride (AA) and the Fries rearrangement of phenyl acetate in the liquid phase catalyzed by bulk and silica-supported heteropoly acids (HPA), mainly H 3 PW 12 O 40 (PW), have been studied. In anisole acylation, PW exhibits very high activity, yielding up to 98% para and 2-4% ortho isomer of methoxyacetophenone (MOAP) at 90-110°C and an AN/AA molar ratio of 10-20. The reaction appears to be heterogeneously catalyzed; no contribution of homogeneous catalysis by HPA was observed. PW is almost 100 times more active than the zeolite H-Beta, which is in agreement with the higher acid strength of HPA. The PW catalyst is reusable, although gradual decline of activity was observed due to the coking of the catalyst. The acylation is inhibited by the product because of adsorption of MOAP on the catalyst surface. In contrast to anisole, the acylation of toluene with HPA is far less efficient than that with H-Beta. Evidence is provided that the activity of HPA in toluene acylation is inhibited by preferential adsorption of acetic anhydride on the catalyst. It is demonstrated that PW is a very efficient and reusable catalyst for the Fries rearrangement of phenyl acetate in homogeneous or heterogeneous liquid-phase systems at 100-150°C.

Friedel–Crafts Acylation Catalysed by Heteropoly Acids

The Friedel–Crafts acylation of anisole (AN) with acetic anhydride (AA) in liquid phase catalysed by bulk and silica-supported heteropoly acids (HPA), mainly H3PW12O40 (PW), has been studied. The PW exhibit very high activity, yielding up to 98% para and 2–4% ortho isomer of methoxyacetophenone (MOAP) at 90–110°C and an AN/AA molar ratio of 10–20. Catalyst pretreatment is essential; the activity passes a maximum at a pretreatment temperature of 150°C. The acylation of anisole appears to be heterogeneously catalysed; no contribution of homogeneous catalysis by HPA was observed. PW is almost a factor of 100 more active than the zeolite H-beta, which is in agreement with the higher acid strength of HPA. The PW catalyst is reusable, although gradual decline of activity was observed due to the coking of the catalyst. The acylation is inhibited by product because of adsorption of MOAP on the catalyst surface. The ratio of adsorption coefficients of MOAP and anisole has been found to be 37 at 90°C. Anisole acylation is first order in acetic anhydride, the order in catalyst is 0.66, and the apparent activation energy is 41 kJ/mol in the temperature range of 70–110°C. In contrast to anisole, the acylation of toluene with HPA is far less efficient than that with H-beta. Evidence is provided that the activity of HPA in toluene acylation is inhibited by preferential adsorption of acetic anhydride on the catalyst.

Isomerization of n-Hexane over Silica-Supported Heteropoly Acids Promoted by the Reduced Ce–Ni Oxides

The structure and catalytic properties of silica-supported heteropoly acids promoted by the reduced Ce–Ni oxides are first studied by using chemical analysis XRD, FT-Raman, XPS, EPR, TG, surface area measurements, and microreactor test. It is found that silica-supported heteropoly acids have isomerization activity, but are very easy to deactivate by coke deposition. With the promotion of the reduced Ce–Ni oxides, however, the better activity and the higher selectivity to isomers could be obtained, suggesting that the reduced Ce–Ni oxides really have hydrogenating/dehydrogenating functions. Moreover, it is found that the presence of the reduced Ce–Ni oxides is not only beneficial for eliminating the coke deposition, but also effective for maintaining the structure of silica-supported heteropoly acids during reaction. The effect of the composition of the mechanical mixtures of silica-supported heteropoly acids and the Ce–Ni oxides on the catalytic properties has been explored.

Silica-supported heteropoly acids promoted by Pt/Al2O3 for the isomerization of n-hexane

Silica-supported heteropoly acids promoted by Pt/Al2O3 present efficient catalytic activity for the isomerization of n-hexane, and among them 50 wt% HSiW/SiO2 promoted by Pt/Al2O3 shows the best catalytic performance, which might be closely related to the maintenance of the surface structure of heteropoly acids during reaction.

Silica-supported heteropolyacids as catalysts in alcohol dehydration reactions

Catalysts based on Keggin-type heteropolyacids supported on silica were prepared for their use in dehydration of alcohols in liquid phase. Species present in the catalysts were characterized by diverse physicochemical techniques, disclosing that both molybdophosphoric and tungstophosphoric acids are found with their primary Keggin structure intact. Their catalytic behavior was studied in the dehydration of alcohol of interest for the production of intermediary compounds for fine chemical industries. Specifically, the dehydration of 1,2-diphenylethanol, 1-(3,4-dimethoxyphenyl)-2-phenylethanol and cholesterol was studied. It was obtained a process for the direct dehydration of cholesterol. It was found that the catalysts are very active and selective, also allowing an easy separation from the reaction medium. The same catalysts were used several times without appreciable loss of catalytic activity and after the reaction they showed physicochemical properties similar to those of the original catalysts.