Polystyrene and silica gel supported AlCl3 as highly chemoselective heterogeneous Lewis acid catalysts for Friedel–Crafts sulfonylation of aromatic compounds

Crosslinked polystyrene supported AlCl₃ (Ps–AlCl₃) and silica gel supported AlCl₃ (SiO₂–AlCl₃) have shown to be mild and chemoselective heterogeneous Lewis acid catalysts for the sulfonylation of different kinds of aromatic compounds with p-toluene sulfonyl chloride (TsCl) and benzene sulfonyl chloride (PhSO₂Cl) as sulfonylating agents. These solid acid catalysts are stable (as bench top catalysts) and can be easily recovered and reused without appreciable change in their efficiency.     

Comparison of AlEt2Cl and ZnCl2 supported on silica gel as catalysts of Diels-Alder reactions. Influence of the nature of the dienophile

Non-modified silica gel, silica gel treated with AlEt₂Cl, and ZnCl₂ supported on silica gel are tested as catalysts in the Diels-Alder reactions of cyclopentadiene (1) with methyl acrylate (2a), (-) -menthyl acrylate (2b) and acrylonitrile (2c). In general, supported catalysts are more efficient than non-modified silica gel. Two different kinds of silica gel are tested and the amount of aluminium incorporated when they are treated with AlEt₂Cl changes from one to another, which influences the chemical yield but not the selectivity of the reactions. In general, aluminium catalysts are more efficient than ZnCl₂-supported ones in the reactions of acrylates, but the latter solid is a better catalyst in the reactions of acrylonitrile (2c). Differences in acid strength and hardness of the catalytic sites are invoked to account for these results.     

Influence of the support composition on the hydrogenation of acrolein over Ag/SiO2–Al2O3 catalysts

The gas phase hydrogenation of acrolein over supported silver catalysts has been investigated with a focus on the influence of the support acidity. Acidity has been varied by preparing silver catalysts supported on silica/alumina supports with varying SiO₂/Al₂O₃ ratio. After the catalytic experiments the Ag catalysts exhibit similar particle sizes, as revealed with TEM (transmission electron microscopy). The acidity of the samples was estimated using TPD of adsorbed ammonia which gives the total acidity of the samples, furthermore by IR of adsorbed pyridine to identify the Brønsted and Lewis acidity. No Brønsted acidity was found, and the Lewis acidity showed a clear dependence on the support composition. It is shown that a high total acidity and a high amount of strong Lewis acid sites on the catalysts cause a low conversion of acrolein and low selectivity to allyl alcohol. The interaction of silver with the support or effects of the metal–support perimeter are discussed as possible reasons for this behaviour.     

Conversion of solvent-refined coal to liquid products in the presence of Lewis acids

The formation of liquid products from a solvent-refined coal (SRC) was studied using Lewis acid catalysts in the presence of either benzene or cyclohexane as an extracting solvent. The soluble products were characterized by elemental analysis, ¹H-n.m.r., and, in some experiments, by gel permeation chromatography. Only ZnCl₂ and SnCl₂, the weakest Lewis acids examined, enhanced the dissolution of SRC over that observed in the absence of a catalyst. Increases in solubility were accompanied by increases in $\text{H}\text{C}$ and aliphatic character and by decreases in average molecular weight. These changes are ascribed to the ability of ZnCl₂ and SnCl₂ to promote depolymerization and hydrogenation of the SRC. Alkylation with isopropanol was also found to enhance the solubility of SRC in benzene and cyclohexane.     

Synthesis of mixed silica–titania by the sol–gel method using polyethylenimine: porosity and catalytic properties

By means of the sol–gel method, hybrid xerogels of silica, titania and silica–titania mixed networks of different composition with polyethylenimine (PEI) were formed. After removal of PEI followed by calcination, mesoporous oxides of high surface area with monomodal and narrow porosity distribution were obtained. The surface area of the mixed oxides decreases with increasing titania content, but pore size remains almost constant when a PEI:(SiO₂ + TiO₂) ratio of 1 is kept constant in the xerogel precursor. By doubling and tripling the proportion of PEI, the surface area increases by 15% and 30%, respectively, but pore size remains constant, indicating the formation of a larger number of PEI domains of similar size in the hybrid. Isopropanol decomposition reactions were carried out using the mixed oxides as catalysts, and it was found that the networks contain Lewis acid sites but lack Brønsted acid sites.     

Oxidation of Benzene to Maleic Anhydride in a Fluidized Bed Reactor

In this project, the selective oxidation of benzene to maleic anhydride (MAN) was studied. Gas phase catalytic oxidation of benzene was carried out in a laboratory scale fluidized bed reactor on six different types of catalysts, which have different compositions. Effects of temperature, flow rates of benzene and air and catalyst type on the reaction selectivity were investigated at atmospheric pressure. The experiments were performed over a temperature range of 325 to 400 °C, a space‐time (W/F_A0) range from 11.28 × 10⁵ to 31.9 × 10⁵ g s mol^–1, and benzene/air mole ratio changes between 0.0109 and 0.0477. It was seen that conversion of benzene to MAN increased with increasing temperature for the catalysts supported by silica gel, aluminum oxide and titanium oxide. From the results it was found that conversion increased with increasing flow rate of air. When the comparison of the catalysts were made, it could be said that catalysts supported by silica gel showed higher MAN conversions. So it can be concluded that catalysts supported by silica gel were more suitable catalysts for benzene oxidation to MAN in a fluidized bed reactor.     

Catalytic performance of zinc containing ionic liquids immobilized on silica for the synthesis of cyclic carbonates

Cycloaddition of carbon dioxide and epoxides was investigated using zinc halide based Lewis acidic ionic liquids (ILs) as catalysts. ILs such as 1-butyl-3-methylimidazolium bromide (BMImBr), 1-butylpyridinium bromide (BPyBr), tetra-n-butylammonium bromide (TBABr) were mixed with zinc halide and supported on silica gel to produce heterogeneous catalysts. Catalytic reaction tests demonstrated that the incorporation of zinc ions can significantly enhance the catalytic activity of the silica-supported ILs for the cycloaddition of CO₂ to epoxides in solvent-free conditions. BPyBr-ZnCl₂/SiO₂ showed the highest propylene carbonate yield of 98% when the reaction was carried out with 0.5 g of catalyst at 120 °C at 1.89 MPa of CO₂ pressure for 4 h. The immobilized zinc containing IL catalyst could be reused for at least four cycles without any considerable loss of its activity.     

Enhanced esterification of carboxylic acids with ethanol using propylsulfonic acid-functionalized natural rubber/hexagonal mesoporous silica nanocomposites

Propylsulfonic acid-functionalized natural rubber (NR)/hexagonal mesoporous silica (HMS) nanocomposites (NR/HMS-SO₃H) with different acid contents were prepared via an in situ sol–gel process, and then applied as heterogeneous acid catalysts in the esterification of model carboxylic acids and palm fatty acid distillate (PFAD) with ethanol. The NR/HMS-SO₃H composites exhibited a wormhole-like framework with enhanced wall thickness, high mesoporosity, and enhanced hydrophobicity. The NR/HMS-SO₃H composites exhibited a superior catalytic performance compared to a commercial Nafion/silica composite solid acid catalyst (SAC-13) and conventional propylsulfonic acid-functionalized HMS (HMS-SO₃H). The NR/HMS-SO₃H catalyst can be regenerated and reused in the esterification.     

Preparation and acidity studies of silica-alumina catalysts

The surface acidic strength distribution in a series of silica-alumina catalysts prepared by coprecipitation and stepwise precipitation techniques has been measured by the butylamine adsorption method. The amount of silica in the finished catalysts ranged from 86.0 to 96.9%. A linear relationship between the pH of the final gel (i.e. pH values of the solution in equilibrium with the gel material before washing) and X[=Al₂O₃/(SiO₂+Al₂O₃)] was observed up to a certain level of pH; for coprecipitated catalysts it was 6.0 whereas for those precipitated stepwise the limit of pH was 7.0. Coprecipitated silica-aluminas were more enriched with weak acid sites or Bronsted sites (pK_a = +6.8) and their concentration reached a maximum at about 87% silica. Stepwise precipitated silica-aluminas, on the other hand, have a greater population of strong acid sites, thus making them useful for cracking reactions.     

Silica Gel- and MCM-41-Supported MoS2 Catalysts for HDS Reactions

MCM-41- and silica gel-supported MoS₂ catalysts were prepared. MCM-41 was synthesized and impregnated with precursor, then activated to obtain the active phase. The sol–gel method was used for providing the SiO₂ support as well as for including the catalyst precursors in one single step of preparation. Such catalysts have applications particularly in hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) processes. A comparison of the activities of the catalysts was made. The catalytic activity results showed the method of preparation used in this study was successful in producing very efficient catalysts for the HDS of dibenzothiophene (DBT).A higher selectivity for direct C–S bond cleavage was observed for the MoS₂ catalyst supported on SiO₂ by the sol–gel method. X-ray diffraction studies showed that the catalysts were poorly crystallized with a very weak intensity of the (002) line of 2H-MoS₂.     

Effect of acid type in WO X clusters on the esterification of ethanol with acetic acid

Tungsten oxide clusters supported on silica (WO _X /SiO₂) with different W loading levels and the effect of acid type on the esterification of acetic acid with ethanol were examined. The catalysts were characterized using various techniques (XRD, Raman spectroscopy, NH₃-TPD and FT-IR) to investigate the crystallinity and the nature of the acid sites. The change in the composition of two tungsten oxide species (polytungstate and crystalline WO₃) leads to the change of Lewis acid to Br?nsted acid ratio. Importantly, the ratio of the two different acid types has a substantial effect on the catalytic activity. The fraction of Lewis acid to total acid sites rapidly changed from 23% to 77% due to the presence of crystalline WO₃. Where the Lewis acid sites accounted for 55% of the total acid sites, the WO _X /SiO₂ catalyst showed the highest catalytic activity among the prepared catalysts.     

Methoden zur Phosphatidanalytik am Beispiel der Lipide aus Seren und Eigelb

Techniques of Phospholipid Analyses Applied to Sera and Egg Yolk Thin layer (TLC) and gas-liquid-chromatography (GLC) were used to analyse phospholipids and diglycerides prepared from phospholipids. TLC of lipids is carried out on silica gel and for diglycerides on silica gel impregnated with 10% AgNO₃. Fatty acids of individual lipids were identified by GLC. The recovery of lipids was evaluated after chromatography of phospholipids and diglycerides and after digestion with phospholipase-C from Bacillus cereus. Two techniques of acetylation with two different catalysts are compared. The calculations were referred to phosphorus or glycerol in lipids. The precise amount of phospholipids was determined after charring as inorganic phosphorus. Diglycerides were assayed after saponification by an enzymatic determination of glycerol. The reliability of some lipid techniques could be shown. The values of phospholipids for egg yolk and sera are in correspondence to literature.     

Activity of modified SnO2 catalysts for acid-catalysed reactions

The effect of Lewis and Brønsted acid modification on the catalytic behaviour of SnO₂ was studied. Modified AlCl₃, FeCl₃, ZnCl₂ and H₂SO₄ were used for this purpose. Catalytic activity was tested for Friedel–Crafts alkylation, acylation and Pechmann condensation. It was observed that the alkylation activity of SnO₂ was improved upon treatment with Lewis acids and Brønsted acids. However the acylation activity was observed only when SnO₂ was treated with H₂SO₄. Moderate improvement in the activity for Pechmann condensation reaction was observed in the case of all the modified catalysts. It was inferred that this method of modification resulted in an increase in acid strength as well as acidity of the parent oxide catalyst. © 1998 SCI     

Isobutane/butene alkylation over supported heteropoly acid catalysts: I. Influence of the structure of silica

Catalysts consisting of heteropoly acids (HPAs) supported on different silica and mesoporous molecular sieves have been prepared by impregnation and the sol–gel method, respectively, and their catalytic behavior in fixed‐bed alkylation of isobutane with butene has been investigated. The activity, selectivity and stability of the supported‐HPA catalysts could be correlated with the surface acidity of the catalysts, the structure of supports as well as the time on stream (TOS). In the fixed‐bed reactor, the acidity of the heteropoly acid is favorable to the formation of dimerization products (C ₈ ⁼ ); especially, the pore size of supports was seen to have an important effect on activity and product distribution of the catalysts. Contrary to the traditional solid‐acid catalysts, the supported‐HPA catalysts own an excellent stability for alkylation, which makes it possible for these supported catalysts to replace the liquid‐acid catalysts used in industry. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sol–Gel Assisted Preparation of Chromia–Silica Catalysts for Non-Oxidative Dehydrogenation of Propane

Preparation of continuous silica gel in the presence of dissolved Cr(NO₃)₃ leads to a hard high surface area (710 m²/g) microporous xerogel. Formation of the gel in the pore structure of mesoporous Pore-Expanded MCM-41 (PE-MCM-41) and macroporous commercial Cab–O–Sil silica results in retention of the mesoporous structure in the case of PE-MCM-41, and the formation of a powder with a broad pore size distribution in the case of Cab–O–Sil silica. Comparison of the catalytic activities in non-oxidative dehydrogenation of propane revealed a linear correlation of the initial conversion with the surface area for all three samples. The sample prepared from PE-MCM-41 through a sol–gel assisted procedure was the most active, particularly with respect to the catalysts prepared by simple wet impregnation with chromium nitrate.     

Amination of Polyethylene Glycol to Polyetheramine over the Supported Nickel Catalysts

Surface nickel (NiO _x ) species, surface NiAl _x O _y compound, and NiO crystallites are present on the Ni/Al₂O₃ catalysts, and the ratio of these nickel species is dependent on the nickel loading. Surface nickel interacts with the TiO₂ support to form a surface nickel titanate compound (NiTiO _x ) which has a lower reducibility. The weak interaction between the surface nickel and the silica support results in the formation of NiO crystallites on the SiO₂ surface. The Ni/Al₂O₃ and Ni/TiO₂ catalysts contain new surface Lewis acid sites and the amount of surface Lewis acid sites increases with increasing nickel concentration. The Ni/SiO₂ catalysts have no sign of the presence of the surface Lewis acid sites. Only the Ni/Al₂O₃ catalysts have shown the ammonia adsorption at temperature of 200°C. Supported nickel on alumina catalysts possess the highest amination conversion, and the amine yield increases with increasing nickel loading up to 15% and starts to level off. By comparing amination catalysis with quantitatively TPR studies of the H₂ consumed of the Ni/Al₂O₃ catalysts, it appears that the dispersed nickel species are the active sites for amination. In addition, the amination product is mainly the secondary amine due to the presence of water.     

Phenol alkylation with isobutene — influence of heterogeneous Lewis and/or Brønsted acid sites

Acidic solid catalysts with different types of acidity were used to study the liquid-phase alkylation of phenol with isobutene. A phosphonium ionic liquid immobilized on silica type carrier exhibiting pure Lewis acidity, Amberlyst 15 with pure Brønsted acidity as well as WO₃/ZrO₂ with both types of acid sites were used for this study. The active sites are postulated based on pyridine-FT-IR and NH₃-TPD studies, BET analyses, MAS NMR and XRD measurements. The different properties of the chosen catalysts are mirrored in the product distribution of the reaction mixture. It was found that WO₃/ZrO₂ is a very active and selective catalyst for the production of 2,4-di-tert-butylphenol under mild reaction conditions.     

On the acidity of liquid and solid acid catalysts. Part 2. A thermodynamic and kinetic study for acid-catalysed nitrations

Solid acids prepared by adding sulfuric acid on silica gel have been used as catalysts in the nitration of nitrobenzenes and their properties have been tested by kinetic studies at 25°C. Nitration rates in concentrated aqueous solutions of sulfuric acid were also analysed and the catalytic efficiencies of sulfuric acid in liquid and solid phase were compared by using kinetic data of analogous compounds. The results show that the solid acid samples exhibit nitrating properties very similar to those observed in concentrated aqueous solutions of sulfuric acid (range of 90 wt%). The relationship between nitration rates and effective concentration of electrophilic species [NO ₂ ⁺ ], determined by studying the protonation–dehydration equilibrium of nitric acid in strong acids (HNO₃ + H⁺ ⇌ H₂O + NO ₂ ⁺ ), was tested to better understand the acidity properties of medium. This revised version was published online in November 2006 with corrections to the Cover Date.     

Uses of Ozone in a Three-Phase System for Water Treatment: Ozone Adsorption

A new technique of using ozone for water treatment is presented. This new technique consists of using a three-step-process composed firstly of ozone adsorption on an appropriate adsorbent, secondly water treatment, and thirdly regeneration of the adsorbent. Results regarding ozone adsorption (the first step) are presented in this paper. Different types of silica gel and a type of TiO₂ have been tested for ozone adsorption. It was found that the physical characteristics of the silica gel affect its capacity for ozone. Titanium dioxide has shown ozone decomposition instead of adsorption as it contains Lewis acid sites. An exponential decrease of the silica gel capacity with its moisture content has been found. Linear isotherms in the range of ozone concentrations less than 100?g/m³ NTP have been found. A particle diffusion model with linear equilibrium isotherm has been used to model the breakthrough curves in fixed bed columns.     

Comparison of Lewis acids of different hardness supported on silica gel as catalysts of Diels–Alder reactions of (E)-2-cyanocinnamates

The behaviour of several Lewis acids, derived from zinc, aluminium and titanium, supported on silica gel, as catalysts of the Diels-Alder reactions of cyclopentadiene with methyl, (1R,2S,5R)-menthyl, and (R)-pantolactone (E)-2-cyanocinnamates has been compared. The best catalytic activities were observed with the zinc catalysts. The extent and even the direction of the asymmetric induction changes for the same chiral auxiliary depending on the catalyst used. Ab initio theoretical calculations, carried out on model dienophile-catalyst intermediates, show that the coordination of the softer zinc derivatives at the nitrogen atom is thermodynamically favoured, whereas coordination to the carbonyl oxygen atom is preferred for the harder aluminium catalysts. However, in both cases the most reactive intermediate comes from the coordination of the Lewis acid to the carbonyl group of the dienophile in the s-trans conformation. This revised version was published online in August 2006 with corrections to the Cover Date.