Silica-supported Heteropoly Acid Research Articles

Confinement and surface sites control methanol adsorbate stability on MFI zeolites, SBA-15, and a silica-supported heteropoly acid

We herein investigate methanol adsorbates on a variety of heterogeneous catalysts. Systematic variation of functional groups and confinement enables new insights into nature, interactions, and stability of methanol surface species.

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.

Pd-HPW/SiO2 Bi-Functional Catalyst: Sonochemical Synthesis, Characterization, and Effect on Octahydroquinazolinone Synthesis

A Palladium-doped silica-supported heteropoly acid (HPW) (1%Pd-HPW/SiO2) bi-functional catalyst was produced using ultrasonic and conventional procedures. Both forms of catalyst were characterized with distinct analytical approaches in order to access the advantages of each one. The presence of the required functional groups in the catalyst was confirmed using FT-IR. The crystallinity of ultrasonically generated 1%Pd-HPW/SiO2 was confirmed with XRD. The existence of necessary elements in the catalyst was also suggested by XPS and EDX data. BET was used to calculate the surface area of the ultrasonically synthesized catalyst (395 m2 g−1), and it was found to be greater than that of the non-ultrasonic synthesized catalyst (382 m2 g−1). The N2 adsorption-desorption isotherm indicated mesoporous structures. The SEM morphology at a similar magnification exhibited quite different shapes. In comparison to traditional methods, ultrasonic approaches produce higher yields in less time and use less energy. Furthermore, the effect of the preparation method of the 1%Pd-HPW/SiO2 catalyst was extensively studied with respect to the synthesis of octahydroquinazolinones. Excellent product yields, a fast reaction time, and simple work-up methods are some peculiarities associated with the ultrasonically synthesized catalyst. The recycling study was also investigated and found suitable for up to four reaction cycles.

Catalytic oxidative desulfurization of benzothiophene using silica-supported heteropolyacid catalyst: activity, deactivation and regeneration of the catalyst

A series of silica-supported heteropolyacid catalysts were prepared by the hydrothermal dispersion and ultrasonic impregnation methods, using phosphotungstic acid as active component and HMS molecular sieve as support. The catalytic stability of catalysts in the oxidative desulfurization of benzothiophene has been investigated. The results show that the sulfur removal of the two catalysts decreased from 97.81 and 95.63 to 81.28 and 75.3 % after eight cycles. The fresh and spent catalysts were characterized by Fourier transform infrared spectra, N2 adsorption, X-ray diffraction and transmission electron microscopy. The main results indicate that the deactivation process of the catalysts went through two stages, i.e., the dissolution of active component is the main factor at prophase and the adsorption of N-hexadecane and sulfone is the primary reason at anaphase. Furthermore, the former is an irreversible deactivation and the latter is a reversible deactivation which can be regenerated by solvent extraction.

Hydrophobic mesoporous silica-supported heteropolyacid induced by ionic liquid as a high efficiency catalyst for the oxidative desulfurization of fuel

The hydrophobic HPMo-IL/SBA-15 material served not only as a high-efficiency catalyst but also as an adsorbent.

The Application of Silica-Supported Preyssler HPA as a Heterogeneous and Green Catalyst for the Alkylation of Benzene

Liquid-phase alkylation of benzene with 1-decene for the production of linear alkylbenzene (LAB) was investigated with silica-supported Preyssler heteropoly acid as the catalyst. Catalyst loading, Bz/C10 molar ratio, and catalyst weight percentage were investigated. Catalyst with 30% loading, Bz/C10 molar ratio of 17, and catalyst weight percentage of 2.6 showed 1-decene conversion of 100% and 2-phenyl decane selectivity of 27.2% at 80°C and 2-hr reaction.

Synthesis of 5-Acetylacenaphthene Using Silica-Supported Phosphotungstic Heteropoly Acid as an Efficient and Reusable Catalyst

To obtain new fine chemical and intermediate of functional polymer material, the acylation of acenaphthene with benzoyl chloride to 5-acetylacenaphthene catalyzed by silica-supported phosphotungstic heteropoly acid (PW/SiO2) was investigated. Pure 5-acetylacenaphthene was obtained and its structure was identified by GC/MS, FT-IR and 1H NMR spectra. The effects of experimental parameters on the catalytic acylation reaction and the possibility of reusability of PW/SiO2 catalyst were studied. Under optimun conditions, the yield and the selectivity of 5-acetylacenaphthene were up to 81.3 % and 93.6 %, respectively. PW/SiO2 shows well catalytic activity after running for 5 times. The experimental results indicate that PW/SiO2 is a recyclable and environmentally benign catalyst in the synthesis of 5-acetylacenaphthene.

Synthesis of ionones on solid Brønsted acid catalysts: Effect of acid site strength on ionone isomer selectivity

The effect of Brønsted acid site strength on the liquid-phase conversion of pseudoionone to ionone isomers (α-, β- and γ-ionone) was studied on resin Amberlyst 35W, silica-supported heteropolyacid (HPAS) and silica-supported triflic acid (TFAS). Catalyst acidity was probed by temperature-programmed desorption of NH 3 coupled with infrared spectra of adsorbed pyridine. The initial pseudoionone conversion rate followed the order: TFAS > Amberlyst 35W ≈ HPAS. Synthesis of the three ionone isomers occurred via a common cyclic carbocation intermediate formed from the activation of the pseudoionone molecule on Brønsted acid sites. Initial ionone mixtures containing a α:β:γ isomer distribution of about 40:20:40 were formed, irrespective of the acid site strength. But the ionone mixture composition changed with the progress of the reaction because γ-ionone was consecutively converted to α-ionone on HPAS and Amberlyst 35W, whereas the stronger acid sites of TFAS converted γ-ionone to β-ionone.

Transesterification of Rice Bran Oil with Methanol Catalyzed by Mg(Al)La Hydrotalcites and Metal/MgAl Oxides

Both MgAl and Mg(Al)La hydrotalcites were synthesized by an alkali-free coprecipitation method and calcined to their corresponding oxides. MgAl oxide was impregnated with different kinds of metals (M = Cs, Sr, Ba, and La). All materials were characterized by the XRD, FT-IR, BET, NH3 TPD, and SEM techniques. The catalytic activity order of the M/MgAl oxides in the transesterification of glyceryl tributyrate (a model triglyceride) with methanol was found to be Cs > Ba ≈ Sr > La. The Mg(Al)La hydrotalcite that was calcined and then rehydrated contains Brønsted base sites, exhibiting higher activity than the calcined sample with Lewis base sites. In the preparation of biodiesel from rice bran oil via a two-step catalyzed process, crude rice bran oil (11.14% FFAs) was first dewaxed/degummed and then esterified using silica-supported heteropolyacid H3PW12O40. FFAs were decreased to 0.98% at 70 °C in 4 h. This oil was then transesterified with methanol using the rehydrated Mg(Al)La hydrotalcites. At a temperature of 100 °C, a reaction time of 9 h, a methanol/oil molar ratio of 30:1, and a catalyst content of 7.5 wt %, biodiesel with 97% ester content and 78% product yield was obtained.

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.

Improved catalytic activity using water for isomerization of linear butene to isobutene over heteropolyacid catalysts

Water minimized a decrease in conversion for the isomerization of linear butene to isobutene on silica-supported heteropolyacid (H 4SiW 12O 40) catalysts while maintaining high selectivity toward isobutene. The role of water was related to surface acidic properties (number of surface acid sites and acid strength) and characteristics of hydrocarbon deposits (extent of deposits and hydrogen/carbon ratio), as determined by benzonitrile and ammonia temperature-programmed desorption as well as temperature-programmed oxidation of fresh and spent catalysts. It appears that highly unsaturated hydrocarbon deposits consisting of 8–12 carbon units covered surface acid sites, especially strong acid sites, limiting the isomerization reaction. Water in the feed was helpful in preserving the acid sites and decreasing the number of carbon units in the hydrocarbon deposits. Thus, conversion occurred more readily in the presence of water than in the absence of water.

One-pot synthesis of dihydropyrimidones using silica-supported heteropoly acid as an efficient and reusable catalyst: Improved protocol conditions for the Biginelli reaction

An efficient synthesis of 3,4-dihydropyrimidinones or thiones (DHPMs) is described, using silica-supported heteropoly acid H 3PW 12O 40/SiO 2 (PW/SiO 2) for the first time as the catalyst from an aldehyde, β-keto ester and urea or thiourea in acetonitrile. Compared to the classical Biginelli reaction conditions, this method consistently has the advantage of excellent yields, mild reaction conditions, ease of workup, survival of different functional groups, and short reaction times.

One-pot synthesis of diisobornyl ether from camphene using heteropoly acid catalysts

A novel one-pot catalytic synthesis of diisobornyl ether directly from camphene ( 1) has been developed. The reaction occurs under near ambient conditions in the presence of dissolved or silica-supported heteropoly acid H 3PW 12O 40 (PW) as a catalyst to give diisobornyl ether ( 2) with up to 90% selectivity at 50–60% conversion, along with isoborneol ( 3). Ether 2 appears to be a new compound; it was isolated as a mixture of meso and dl stereoisomers and fully characterized by MS-GC, NMR and IR spectroscopy. The supported PW catalyst can be easily separated and reused; no PW leaching from support was observed during the reaction. Conventional acid catalysts such as H 2SO 4 or Amberlyst-15 showed no activity in this reaction.

Isomerisation of α-pinene oxide over silica supported heteropoly acid H 3PW 12O 40

Silica-supported heteropoly acid H 3PW 12O 40 (PW) has been found to be a very efficient and environmentally benign solid acid catalyst for the liquid-phase isomerisation of α-pinene oxide ( 1). The reaction has been performed in cyclohexane solutions at 15–40 °C to give campholenic aldehyde ( 2) and trans-carveol ( 3) as the main products in up to 98% total yield, with 70% selectivity to more valuable product 2. Both 2 and 3 are expensive ingredients in the fragrance industry. No PW leaching has been observed during the reaction. The catalyst is easily separable from the reaction mixture and reusable without loss of its activity and selectivity.

Cyclization of (+)-citronellal to (−)-isopulegol catalyzed by H 3PW 12O 40/SiO 2

Silica-supported heteropoly acid H 3PW 12O 40 (PW) is a very efficient and environmentally benign solid acid catalyst for the liquid-phase cyclization of (+)-citronellal ( 1). The reaction has been performed in cyclohexane solutions at 15–40 °C yielding (−)−isopulegol ( 2) and (+)-neo-isopulegol ( 3) as main products in almost 100% total selectivity at 95–100% conversion, with an 80% selectivity to more valuable product 2. No PW leaching is observed during the reaction, and the catalysts can be recovered and reused several times without loss of activity. Sol–gel silica-included PW catalysts have been prepared and tested in the reaction; these showed no activity, however.

A calorimetric study of the acidity of bulk and silica-supported heteropoly acid H 3PW 12O 40

The acid strength of bulk and SiO 2-supported (10–60 wt%) H 3PW 12O 40 (PW) is characterised by differential scanning calorimetry of ammonia adsorption in a gas–solid system and by microcalorimetry of pyridine adsorption in a liquid–solid system (cyclohexane slurry). In both systems, the strength of PW in terms of the heat of adsorption of NH 3 or pyridine increases with increasing PW loading on silica, the bulk PW being the strongest acid. The temperature and the heat of NH 3 desorption are shown to indicate the acid strength, in full agreement with the heats of NH 3 adsorption. Attempts to measure the acidity of PW included in a sol–gel silica matrix have failed due to a large amount of water present in such systems.

Hydration and acetoxylation of camphene catalyzed by heteropoly acid

The liquid-phase acetoxylation and acetoxylation/hydration of camphene ( 1) in homogeneous or heterogeneous systems catalyzed by dissolved or silica-supported heteropoly acid H 3PW 12O 40 (PW) in AcOH/H 2O solutions (0–10 vol.% H 2O) have been studied. Isobornyl acetate ( 2a) and isoborneol ( 3a) are the main reaction products which have been obtained with an almost 100% total selectivity. No oligomerization of camphene has been observed. PW shows a much higher catalytic activity than conventional acid catalysts such as H 2SO 4 and Amberlyst-15. It can be recovered without neutralization and reused.

Hydration and acetoxylation of monoterpenes catalyzed by heteropoly acid

The liquid-phase hydration and acetoxylation of limonene ( 1), β-pinene ( 2) and α-pinene ( 3) catalyzed by dissolved or silica-supported heteropoly acid H 3PW 12O 40 (PW) in acetic acid and acetic acid/water solutions have been studied. All three substrates give α-terpineol ( 4) as the main product along with α-terpenyl acetate ( 5). The reaction rate increases in the order: limonene<α-pinene<β-pinene. Synthetically useful homogeneous and heterogeneous acetoxylation and hydration of 1, 2 and 3 into 4 and 5 have been developed. At room temperature under optimized conditions, 2 and 3 form a mixture of 4 and 5 with 85% selectivity at 90% substrate conversion. 1 gives 4 and 5 with 85% selectivity at 50% conversion, with the main product being ester 5 in acetic acid ( 4/ 5≈30/70 ) and alcohol 4 in HOAc/H 2O (90/10 v/v) solutions ( 4/ 5≈85/15 ). Virtually no oligomerization of monoterpenes occurs under the optimized conditions. The catalyst can be separated without neutralization and may be reused. The PW shows a much higher catalytic activity than conventional acid catalysts such as H 2SO 4 and Amberlyst-15.

Oxidation of aldehydes using silica-supported Co(II)-substituted heteropolyacid

A supported Co(II)-substituted heteropolyacid salt, Cs 6H 2P 2W 17O 61Co·OH 2, on silica was prepared and characterized by BET surface area measurements, thermal analysis (TGA/DTA) and infrared spectroscopy. The infrared spectroscopy and thermal analysis revealed that the supported heteropolyacid salt preserves Wells-Dawson structure and is stable upto 500°C. The BET result shows that by supporting the above heteropolyacid salt on silica, surface area increases. The supported heteropolyacid was used as a heterogeneous catalyst for the oxidation of aldehydes to corresponding carboxylic acids with molecular oxygen. Silica-supported heteropolyacid is an effective catalyst for the oxidation of aldehydes to carboxylic acids.

Coking and regeneration of H 3PW 12O 40/SiO 2 catalysts

The coking during propene oligomerisation over silica-supported heteropoly acid (HPA) H 3PW 12O 40 (PW) and its palladium-doped form (1.6–2.5 wt.% Pd) and subsequent catalyst regeneration have been studied. Coke formation has been found to cause rapid deactivation of the catalysts. The coked versus fresh catalysts have been characterised by 31 P and 13 C MAS NMR, XRD, XPS and TGA/TPO to reveal that the Keggin structure of the catalysts was unaffected by coke deposition in both undoped and Pd-doped PW/SiO 2. The Pd doping has been shown to affect the nature of coke formed, inhibiting the formation of polynuclear aromatics. Addition of water, methanol or acetic acid to the propene flow causes the formation of oxygenated products at the expense of propene oligomers. These additives have been found to inhibit the coking, water being the most effective inhibitor. The removal of coke from HPA catalysts has been attempted using solvent extraction, ozone treatment and aerobic oxidation. The extraction (e.g. with CH 2Cl 2) allows removing soft coke (with the TGA removal range of 170–370°C) but is unable to remove hard coke (with the TGA removal range of 370–570°C). Ozone treatment can remove both soft and hard coke at 150°C. The aerobic burning of coke on the undoped PW/SiO 2 proceeds to completion in the temperature range centred at 500–560°C, exceeding the temperature of PW decomposition. Doping the catalyst with Pd significantly decreases this temperature to allow catalyst regeneration at temperatures as low as 350°C without loss of catalytic activity.