Heteropoly Acid Loading Research Articles

Solid acid catalysts comprising heteropoly acids supported on SiO2, TiO2 and ZrO2: A microcalorimetric investigation of catalyst acidity and new insight into the mechanism of alcohol dehydration over HPA

Keggin-type heteropoly acids (HPAs) have strong Brønsted acidity and are widely used as acid catalysts. The aim of this work is the systematic characterisation of the acid strength of HPA catalysts comprising H3PW12O40 and H4SiW12O40 supported on SiO2, TiO2 and ZrO2 with 10–100% HPA loading and testing their activity in dehydration of alcohols to gain new mechanistic insight regarding the role of bulk-type and surface-type HPA catalysis in this reaction. The HPA catalysts were prepared by impregnation from an aqueous solution and characterised using BET, XRD, TGA, DRIFTS and ICP–OES. The acid strength of HPA catalysts was determined by NH3 adsorption microcalorimetry and found to decrease in the order HPA/SiO2 > HPA/TiO2 > HPA/ZrO2. For each type of HPA catalyst, the acid strength increased with increasing HPA loading. This can be attributed to HPA-support interaction reducing the strength of HPA proton sites at low HPA loadings. The activity of HPA catalysts was tested in the gas-phase dehydration of MeOH and i-PrOH in a fixed-bed reactor. Evidence was obtained that alcohol dehydration over HPA catalysts in a steady-state flow system occurs via the surface-type rather than bulk-type catalysis as suggested hitherto. This follows from a strong correlation between the reaction rate and the number of surface proton sites in HPA catalysts.

From CO2 to DME: Enhancement through Heteropoly Acids from a Catalyst Screening and Stability Study.

The direct synthesis of dimethyl ether (DME) via CO2 hydrogenation in a single step was studied using an improved class of bifunctional catalysts in a fixed bed reactor (T R: 210-270 °C; 40 bar; gas hourly space velocity (GHSV) 19,800 NL kgcat -1 h-1; ratio CO2/H2/N2 3:9:2). The competitive bifunctional catalysts tested in here consist of a surface-basic copper/zinc oxide/zirconia (CZZ) methanol-producing part and a variable surface-acidic methanol dehydration part and were tested in overall 45 combinations. As dehydration catalysts, zeolites (ferrierite and β-zeolite), alumina, or zirconia were tested alone as well as with a coating of Keggin-type heteropoly acids (HPAs), i.e., silicotungstic or phosphotungstic acid. Two different mixing methods to generate bifunctional catalysts were tested: (i) a single-grain method with intensive intra-particular contact between CZZ and the dehydration catalyst generated by mixing in an agate mortar and (ii) a dual-grain approach relying on physical mixing with low contact. The influence of the catalyst mixing method and HPA loading on catalyst activity and stability was investigated. From these results, a selection of best-performing bifunctional catalysts was investigated in extended measurements (time on stream: 160 h/7 days, T R: 250 and 270 °C; 40 bar; GHSV 19,800 NL kgcat -1 h-1; ratio CO2/H2/N2 3:9:2). Silicotungstic acid-coated bifunctional catalysts showed the highest resilience toward deactivation caused by single-grain preparation and during catalysis. Overall, HPA-coated catalysts showed higher activity and resilience toward deactivation than uncoated counterparts. Dual-grain preparation showed superior performance over single grain. Furthermore, silicotungstic acid coatings with 1 KU nm-2 (Keggin unit per surface area of carrier) on Al2O3 and ZrO2 as carrier materials showed competitive high activity and stability in extended 7-day measurements compared to pure CZZ. Therefore, HPA coating is found to be a well-suited addition to the CO2-to-DME catalyst toolbox.

Recyclable Clay-Supported Heteropolyacid Catalysts for Complete Glycolysis and Aminolysis of Post-consumer PET Beverage Bottles

In this investigation, the use of phosphotungstic acid (PWA) and phosphomolybdic acid (PMA) as well as Zn2+ containing kaolin and bentonite explored for chemical recycling of post-consumer poly(ethyleneterephthalate) (PET) wastes have been explored. The clay supported catalysts containing 5wt% of the metals and heteropolyacids (HPAs) synthesized using wet impregnation method. Nitrogen adsorption and desorption studies, SEM–EDX mapping, powder XRD, FTIR and XPS analysis have evaluated effect of metal ions and HPAs loading on the surface area, pore volume, elemental composition and crystalline nature. Total surface area of BET increased with a loading of 5 wt% of Zn2+, PWA and PMA on kaolin and bentonite, while the pore volume and pore diameter remain unchanged. SEM and EDAX mapping images showed that the heteropolyacids crystals are well dispersed on the surface and occupied interlayer spaces of the clay support. SEM–EDX showed that bentonite showed a better loading of PWA and PMA compared to kaolin. PET waste water bottles collected from the local market used for the chemical recycling process. The aminolysis reaction using Zn2+ and PWA loaded on bentonite showed complete depolymerisation of PET wastes to produce 87–98% of BHETA. The glycolysis reaction using the above catalysts showed complete depolymerisation at 180–210 °C and yielded 78–90% of BHET. When comparing the clay, bentonite performed well in terms of heteropolyacid loading and afforded a higher yield of BHET and BHETA because of higher loading of Zn and HPA, as supported by SEM–EDX and XPS. We also examined reusability of the catalysts for glycolysis.

Selective dehydration of 1-butanol to butenes over silica supported heteropolyacid catalysts: Mechanistic aspect

Butenes are considered as important olefinic building block to produce fuels/fuel additives and commodity chemicals. In the present investigation, selective dehydration of 1-butanol to butenes was studied in a continuous-flow fixed-bed reactor using various silica-supported heteropolyacid (HPA) catalysts such as phosphotungstic acid (PTA), silicotungstic acid (STA), phosphomolybdic acid (PMA), and silicomolybdic acid (SMA) as the solid acid catalysts. The physicochemical properties of these HPA were determined by BET, powder XRD, FTIR, NH3-TPD, and Py-FTIR. The acid strength and Brønsted/Lewis (B/L) acid ratio were increased with higher loading of HPA on silica. The nature of HPA (addenda and hetero atom) and loading of HPA are important factors for the dehydration of 1-butanol and selectivity towards butenes. PTA and STA showed superior catalytic activity than PMA and SMA. The reaction temperature and WHSV also strongly affected the butanol conversion and selectivity of butenes. The selectivity of di-n‑butyl ether decreases with the rising temperature from 523 K to 623 K. The isomerization of 1-butene leading to the formation of other butene isomers depends on the HPA loading, temperature, and WHSV. The presence of molybdenum addendum atom in PMA and SMA promotes dehydrogenation and hydrogenation, leading to the formation of various light hydrocarbons. The 20PTA/SiO2 catalyst afforded 99.8% selectivity towards butenes at quantitative conversion of 1-butanol, whereas the 20STA/SiO2 catalyst gave nearly 97.0% conversion of 1-butanol and 99.9% butenes selectivity at 673 K, 37.4 h−1 of WHSV.

Direct Synthesis of Dimethyl Ether from Syngas on Bifunctional Hybrid Catalysts Based on Supported H3PW12O40 and Cu-ZnO(Al): Effect of Heteropolyacid Loading on Hybrid Structure and Catalytic Activity

The performance of bifunctional hybrid catalysts based on phosphotungstic acid (H3PW12O40, HPW) supported on TiO2 combined with Cu-ZnO(Al) catalyst in the direct synthesis of dimethyl ether (DME) from syngas has been investigated. We studied the effect of the HPW loading on TiO2 (from 1.4 to 2.7 monolayers) on the dispersion and acid characteristics of the HPW clusters. When the concentration of the heteropoliacid is slightly higher than the monolayer (1.4 monolayers) the acidity of the clusters is perturbed by the surface of titania, while for concentration higher than 1.7 monolayers results in the formation of three-dimensional HPW nanocrystals with acidity similar to the bulk heteropolyacid. Physical hybridization of supported heteropolyacids with the Cu-ZnO(Al) catalyst modifies both the acid characteristics of the supported heteropolyacids and the copper surface area of the Cu-ZnO(Al) catalyst. Hybridization gives rise to a decrease in the copper surface area and the disappearance of the strong acidic sites typical of HPW nanocrystals, showing all hybrids similar acid sites of weak or medium strength. The activity of the hybrids was tested for direct DME synthesis from syngas at 30 bar and 250 °C; only the hybrids with HPW loading higher than 1.4 monolayers showed activity for the direct synthesis of DME, showing that the sample loaded with 2.7 monolayers of heteropolyacid had higher activity than the reference hybrid representative of the most widely applied catalysts based on the combination of Cu-ZnO(Al) with HZSM-5. In spite of the high activity of the hybrids, they show a moderate loss in the DME production with TOS that denotes some kind of deactivation of the acidity function under reaction conditions.

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 carbonylation of dimethyl ether catalyzed by supported heteropoly acids: The role of Brønsted acid properties

The carbonylation of dimethyl ether (DME) to methyl acetate (MA) is a vital step in a direct route of ethanol production from syngas. Heteropoly acids (HPAs) as a kind of solid acid catalysts can facilitate the reaction. To improve the accessibility of Brønsted acid sites, we prepared a series of supported HPAs catalysts by wetness impregnation on SBA-15. The impact of the HPAs loading and composition on the textural structure and acidic properties was investigated by comparing with the bulk HPAs. Taking acidity analysis (Pyridine-IR and TPD) and catalytic measurement results, we explored the effect of the amount and strength of Brønsted acid site on this reaction. The observations demonstrated that more and stronger acid sites are prone to MA formation.

Kinetic and mechanistic study of the synthesis of ionone isomers from pseudoionone on Brønsted acid solids

The kinetics of the liquid-phase synthesis of α-, β- and γ-ionones from pseudoionone was studied on Brønsted acid solids. Four silica-supported tungstophosphoric acid catalysts containing different heteropolyacid loadings, as well as a silica-supported triflic acid sample and a commercial resin (Amberlyst 35W) were tested in a batch reactor at 343–383K under autogenous pressure.The final composition of the ionone isomer mixture depended on the catalyst acidic properties and operational conditions. The reaction pathways leading to the three ionone isomers were elucidated by postulating a heterogeneous Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic model. First order rate expressions, participation of a single Brønsted acid site in each reaction step and a cationic cyclic intermediate shared by the three ionone isomers were the main model assumptions. It was found that α-, β- and γ-ionones form directly from pseudoionone by cyclization. However, the final concentration of α- and β-ionones is enhanced in consecutive pathways involving the isomerization of γ-ionone. The relative importance of the isomerization steps and the selective formation of α- or β-ionone depend on the Brønsted acid site strength and reaction temperature.

Tungstophosphoric acid immobilised in SBA-15 as an efficient heterogeneous acid catalyst for the conversion of terpenes and free fatty acids

Alkoxylation of α−pinene, β−pinene and limonene was performed in the presence of SBA-15-occluded tungstophosphoric acid (HPW). The HPW was immobilised in SBA-15 using the sol-gel method. The catalysts were characterised by N2 adsorption, FT-IR, Raman spectroscopy, X-Ray diffraction, ICP-AES and TEM. A series of catalysts with different heteropolyacid loadings ranging from 1.8 to 19.3 wt. % were prepared. PW4-SBA-15 (with 10.8 wt. %) exhibited the highest catalytic activity for the alkoxylation of α-pinene with ethanol. An approximately 53% selectivity to α-terpinyl ethyl ether was observed over the PW-SBA-15 catalysts. PW4-SBA-15 was also used as a catalyst for the alkoxylation of other terpenes, including β-pinene and limonene. The PW4-SBA-15 catalyst exhibited high catalytic stability for the alkoxylation of α-pinene with ethanol.PW4-SBA-15 was also used as a catalyst for the esterification of free fatty acids (i.e., palmitic, stearic and oleic acids) with ethanol. Good catalytic activity was observed for the PW4-SBA-15 catalyst with the different substrates used in the esterifications.

Hydrogenolysis of glycerol to propanediols over heteropolyacids promoted AgCu/Al2O3 catalysts

Various amounts and different types of heteropolyacids promoted 5Ag15Cu/Al2O3 catalysts were prepared by impregnation method and analyzed through many techniques. The synthesized catalysts were evaluated for hydrogenolysis of glycerol to propanediols. The results demonstrated that heteropolyacids loading facilitated the reduction, promoted the dispersion, enhanced the acidity, and increased Broensted acid sites of the AgCu catalysts, which benefited the generation of 1,3-propanediol. Compared with phosphotungstic acid and phosphomolybdic acid, silicotungstic acid promoted AgCu catalyst had a better performance for 1,3-propanediol due to the better Cu dispersion and higher Broensted acidity. In addition, when the reaction was performed at 220 °C under 3.5 MPa for 8 h, the chosen 5Ag15Cu-10HSiW/Al2O3 achieved a 69.6% glycerol conversion with 35.6% 1,2-propanediol selectivity and 21.5% 1,3-propanediol selectivity.

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.

Metal–acid bifunctional catalysts for selective hydrogenolysis of glycerol under atmospheric pressure: A highly selective route to produce propanols

Metal–acid bifunctional catalysts containing platinum and heteropolyacids (HPA) in supported on zirconia have been employed for the selective vapor phase hydrogenolysis of glycerol to propanols (1-propanol+2-propanol) under normal atmospheric pressure. The Pt–HPA/ZrO2 catalysts (HPAs such as silicotungstic acid (STA), phosphotungstic acid (PTA), phosphomolybdic acid (PMA) and silicomolybdic acid (SMA)) were prepared by impregnation method and the calcined catalysts were characterized by means of X-ray powder diffraction, FTIR, Raman spectroscopy, BET surface area, CO chemisorption and NH3-Temperature-programmed desorption methods. The characterization results reveal the presence of well dispersed Pt metallic phase and Keggin structure of HPAs on support. The catalytic performance during glycerol hydrogenolysis is well correlated with Pt dispersion and the acidic properties of catalysts. Among the tested catalysts, it was found that Pt–PTA/ZrO2 exhibited excellent selectivity to propanols (98%) with total glycerol conversion at 230°C. A detailed study was made on the effect of various reaction parameters such as the influence of reaction temperature, reduction temperature, glycerol concentration, hydrogen flow rate, feed flow rate and HPA loading to unveil the optimized reaction conditions. Reaction temperature was observed to have a significant effect on the glycerol conversion and product selectivity. The spent catalysts were also characterized by the X-ray diffraction, NH3-TPD and SEM analysis to investigate the changes in catalytic performance and causes of catalyst deactivation.

One-pot hydrothermal preparation of SBA-15 supported Keggin-type 12-tungstophosphoric heteropolyacid mesoporous material and its bifunctional catalytic performance

In this work, a series of 12-tungstophosphoric acid (HPW) catalysts supported on mesoporous molecular sieves SBA-15 were prepared by a one-pot hydrothermal method and their physical and chemical properties were characterized by various techniques such as fourier transform infrared spectroscopy, powder X-ray diffraction, nitrogen adsorption–desorption and transmission electron microscopy. It has been found that all the catalysts retained the mesopore structure of SBA-15 and the primary Keggin-type structure of heteropoly acid. The surface areas and pore volumes of HPW-SBA-15 decreased with the increase of HPW heteropoly acid loadings. HPW units were mostly encapsulated inside the mesopores of SBA-15. The prepared catalysts showed excellent multifunctional catalytic properties. Their bifunctional catalytic performances were examined by photo-catalytic degradation of rhodamine-B dye and microwave-catalytic synthesis of isoamyl acetate. The sample with 20 wt% loading of 12-tungstophosphoric acid was found to be more active than the other catalysts under the photo and microwave-catalytic reaction conditions. Accordingly, reasons for the catalytic activity difference were simply explicated by the effect of HPW concentration on the physicochemical characteristic of HPW-SBA-15 catalysts such as surface areas, porosity, and mesostructure.

Selective dehydrogenation of propane to propene with O2–H2 on bifunctional Pt-H3PMo12O40 catalysts

Properties of Pt/SiO2 catalysts modified by a heteropoly acid (HPA) were studied in oxidative dehydrogenation of propane with an O2 and H2 mixture. The catalysts were characterized with N2 adsorption, IRS, XPS, and HAADF-STEM coupled with EDX, and the possibility for Pt and HPA to be highly dispersed and well-mixed on the silica surface has been shown. In the presence of O2 and H2, propene could be obtained at temperatures as low as 150°C, but heating to 200°C provided much better selectivity (up to 94–96% at propane conversion of 2–4%). The process probably proceeded through oxidation of propane with peroxo species to C3-alcohols on Pt sites, followed by dehydration of the alcohols on the acid sites of HPA. The catalytic properties were strongly dependent both on the feed composition (the C3H8/H2/O2/N2 ratio) and on the composition of catalysts, optimum HPA loading being 20–30wt.% at 0.2–1wt.% of Pt. Under optimal operating conditions, a space time yield of propene was at a level of 420gkgcat−1h−1, without a tendency to decline with time on stream.

PolyPOM Hybrid Membranes from Practical Chemistries with very High Proton Conductivities

The heteropoly acids (HPA) are a protogenic moiety that could potentially enable proton exchange membranes that operate in fuel cells under hotter and drier conditions. We now report the incorporation of HPAs into polymers based on either trifluorovinyl ether (TFVE) monomers or functionalized Dyneon{trade mark, serif} materials. The proton conductivity of the TFVE polymers depends strongly on co-monomer chemistry, where long methylene chains appear to be better but does not increase linearly with respect to HPA loading. Some of these polymers show phase changes between crystalline states under drier conditions and an amorphous state when the polymer is wetter. All of the polymers have very low swelling at high RHs. The Dyneon{trade mark, serif} materials have very high proton conductivties for relatively low loadings of HPA, but their cnductivity is strongly dependent on film processing. The strength of these films mimics their parent polymer. All of these materials are amenable to mass manufacture.

Catalytic Performance of Nano-SiO2-Supported Preyssler Heteropolyacid in Esterification of Salicylic Acid with Aliphatic and Benzylic Alcohols

An efficient and environmentally benign procedure for the catalytic esterification of salicylic acid with aliphatic alcohols, C n H 2 n +1 OH ( n = 1–5) and benzylic alcohols, RC 6 H 4 CH 2 OH (R = H, NO 2 , OCH 3 , Br, Cl, Me) was developed using nano-SiO 2 -supported Preyssler heteropolyacid both under thermal conditions and microwave irradiation. Silica nanostructures were obtained through a sol-gel method and were characterized by transmission electron microscopy and powder X-ray diffraction. The effects of various parameters such as solvent type, molar ratio of substrates, Preyssler heteropolyacid loading on silica, catalyst amount, temperature, and reaction time were studied and the optimum conditions were obtained. It has been found that the catalyst with 30 wt% loading is highly active and shows high yields in esterification reactions. The use of nano-SiO 2 -supported Preyssler heteropolyacid coupled with microwave irradiation allows a solvent-free, rapid (3 min), and high-yielding reaction. This catalyst can be easily recovered and reused for many times without a significant loss in its activity. An environmentally benign procedure for the catalytic esterification of salicylic acid with alcohols was developed in the presence of nano-SiO 2 -supported Preyssler heteropolyacid both under thermal conditions and microwave irradiation.

Acetylation of glycerol over heteropolyacids supported on activated carbon

Abstract The acetylation of glycerol was carried out over dodecatungstophosphoric acid (PW) supported on activated carbon, being the monoacetin, diacetin and triacetin the reaction products. A series of catalysts, with different heteropolyacid loading (from 3.5 to 6.5 wt.%), were prepared. It was observed that the catalytic activity increases with the amount of PW immobilized on the activated carbon, being the PW2_AC (with 4.9 wt.%) the most active sample. However, at high loading of heteropolyacid on the activated carbon, a decrease on the catalytic activity was observed, which can be probably explained due to some hindrance in the activated carbon porous system. All catalysts exhibited good values of selectivity to the diacetin. Catalytic stability of the PW2_AC was also studied by performing consecutives batch runs with the same catalyst sample. After the third batch, it was observed that the catalytic activity stabilized.

Catalytic oxidation of methacrolein to methacrylic acid over silica-supported 11-molybdo-1-vanadophosphoric acid with different heteropolyacid loadings

Catalytic oxidation of methacrolein (MAL) to methacrylic acid (MAA) over SiO 2-supported H 4PMo 11VO 40 with different H 4PMo 11VO 40 loadings was investigated. H 4PMo 11VO 40/SiO 2 showed high activity in comparison with unsupported H 4PMo 11VO 40, and 3.3 mol% H 4PMo 11VO 40/SiO 2 (50 wt.% H 4PMo 11VO 40) had the highest activity, which was five-times larger than that of unsupported H 4PMo 11VO 40 due to high dispersion of H 4PMo 11VO 40 on SiO 2, as determined by temperature-programmed desorption of benzonitrile. On the other hand, the supported catalysts were less selective toward the formation of MAA. From X-ray diffraction and Raman spectroscopy, it was determined that H 4PMo 11VO 40 decomposed to form MoO 3 on SiO 2 during the catalytic reaction. Since SiO 2-supported MoO 3 and unsupported MoO 3 had only very low selectivity toward the formation of MAA in the oxidation of MAL, it was concluded that the formation of MoO 3 caused the decrease in the catalytic performance of the supported catalysts.

Ionone synthesis by cyclization of pseudoionone on silica-supported heteropolyacid catalysts

The liquid-phase cyclization of pseudoionone to ionones (α, β and γ isomers) was studied on silica-supported heteropolyacid (HPA) catalysts containing between 18.8 and 58.5% HPA. The HPA used was tungstophosphoric acid (H 3PW 12O 40). The catalyst surface and structural properties were thoroughly characterized by several techniques. The density, chemical nature and strength distribution of the surface acid sites were determined by adsorbing and monitoring by temperature-programmed desorption and infrared spectroscopy probe molecules such as NH 3 and pyridine. The pseudoionone conversion to ionones increased linearly with the Brønsted acid site density until the HPA loading approached to the monolayer saturation coverage. For higher HPA contents, the pseudoionone adsorption and conversion were hampered because of spatial constraints that diminished the reactant accessibility to the proton active sites. The highest ionone yield, 79%, was obtained on a 58.5 wt% HPA/SiO 2 catalyst at 383 K and is comparable to the best values reported in literature for the homogeneously catalyzed reaction using sulfuric acid. The ionone isomer distribution was modified by varying both the temperature and the reaction time. A reaction mechanism was postulated in which ionone isomers (α, β and γ) are primary products, but γ-ionone is isomerized to α-ionone while β-ionone is not converted in the other isomers.

A revision for the synthesis of β-enaminones in solvent free conditions: efficacy of different supported heteropoly acids as active and reusable catalysts

Catalytic performance of different supported heteropoly acids (HPAs) in the synthesis of β-enaminones and β-enamino esters from various substituted amines and β-diketones or β-keto esters has been studied. In order to improve the yield and reaction time, the effect of various solvents, HPA loading, different amount of the catalyst and solventless conditions have been investigated. 40% (w/w) PW/SiO2 was found to be a very efficient and reusable catalyst in comparison with others, under solvent free conditions. In this eco-friendly approach, the usage of organic solvents and hazardous reagents were minimized. Also, a high yield of products within short reaction times were obtained.