Selective Heterogeneous Catalysts Research Articles

New heterogeneous polyoxometalate based mesoporous catalysts for hydrogen peroxide mediated oxidation reactions.

Inorganic-organic hybrid mesoporous materials were prepared by cocrystallization of a "sandwich" type polyoxometalate, [ZnWZn2(H2O)2(ZnW9O34)2]12-, and branched tripodal organic polyammonium salts, tris[2-(trimethylammonium)ethyl]-1,3,5-benzenetricarboxylate or 1,3,5-tris[4-(N,N,N-trimethylammoniumethylcarboxyl)phenyl]benzene trications. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed formation of three-dimensional perforated coral-shaped amorphous materials with the organic cations surrounding polyoxometalate anions. N2 sorption analysis showed that the hybrid materials have a BET surface area of approximately 30-50 m2 g(-1) and an average pore diameter of 36 A leading to the classification of these materials as mesoporous materials with moderate surface areas. These hybrid materials behaved as very effective and selective heterogeneous catalysts for the epoxidation of allylic alcohols and oxidation of secondary alcohols to ketones with hydrogen peroxide as oxidant. The activity and selectivity of the heterogeneous catalysts based on the hybrid materials was similar to those of homogeneous catalysts based on the same [ZnWZn1(H2O)2(ZnW9O34)2]12- polyoxometalate.

Hydroformylation of 1-hexene in supercritical carbon dioxide: characterization, activity, and regioselectivity studies.

The hydroformylation of alkenes is a major commercial process used for the production of oxygenated organic compounds. When the hydroformylation reaction is performed using a homogeneous catalyst, an organic or aqueous solvent is employed, and a significant effort must be expended to recover the catalyst so it can be recycled. Development of a selective heterogeneous catalyst would allow simplification of the process design in an integrated system that minimizes waste generation. Recent studies have shown that supercritical carbon dioxide (scCO2) as a reaction solvent offers optimal environmental performance and presents advantages for ease of product separation. In particular, we have considered the conversion of 1-hexene to heptanal using rhodium- and platinum-phosphine catalysts tethered to supports insoluble in scCO2 to demonstrate the advantages and to understand the limitations of a solid-catalyzed process. One of the historical limitations of supported catalysts is the inability to control product regioselectivity. To address this concern, we have developed tethered catalysts with phosphinated silica and controlled pore size MCM-41 and MCM-20 supports that provide improved regioselectivity and conversion relative to their nonporous equivalents. Platinum catalysts supported on MCM-type supports were the most regioselective whereas the analogous rhodium catalysts were the most active for hydroformylation of 1-hexene in scCO2.

Design, Use and Results from Combinatorial Steady-State Reactors for the Optimization of Highly Selective Heterogeneous Catalysts in Liquid Systems

AbstractA reactor was designed, built and successfully operated that allowed the simultaneous running of 96 independent reactions at steady-state. Since the problem involved the attainment of 98% in selectivity from a starting point of 93%, very precise reproducibility in selectivity was required. Because selectivity varies with time in a batch reactor, and since commercial reactors for this process are continuous, continuous reactors operated at steady-state were required. Longterm standard deviation in selectivity of <0.2% was attained. Information feedback from so many reactors allowed study of variation in catalyst composition, liquid retention times, space velocity, temperature and other variables to be accomplished in relatively short time. Comparison between heterogeneous and homogeneous catalysis in the same reactors gave insight into the underlying mechanisms determining activity and selectivity. Results from 200 uL reactors scaled very well to laboratory and pilot plant sizes.

Titanium substituted hexagonal mesoporous aluminophosphates: Highly efficient and selective heterogeneous catalysts for the oxidation of phenols at room temperature

A remarkable para-selectivity was achieved, for the first time, for the oxidation of phenol over novel titanium substituted hexagonal mesoporous aluminophosphate molecular sieves at room temperature, and that the catalysts also showed excellent activity for alkyl substituted phenols.

An Approach to the Construction of Indexed Libraries for the Combinatorial Selection of Heterogeneous Catalysts

Two series of sublibraries consisting of data for an overall 10 mixtures (five supported metal catalysts each) were constructed and tested for the selection of catalysts in terms of activity and/or selectivity in the cracking/isomerization of n-heptane.

Preparation and structural characterization of polymer-supported methylrhenium trioxide systems as efficient and selective catalysts for the epoxidation of olefins.

Novel heterogeneous compounds of methylrhenium trioxide (MTO) were prepared with poly(4-vinylpyridine) and polystyrene as polymeric supports. The wide-angle X-ray diffraction (WAXS.) analysis, performed by the application of the difference method, showed, in a representative case of the poly(4-vinylpyridine)/MTO derivatives, a slightly distorted octahedral conformation on the metal's primary coordination sphere. The Re-O and Re-C bond distances were not influenced by the polymeric nature of the ligand, while the Re-N bond distance was abnormally shorter than those previously observed for homogeneous MTO/L(n) complexes, showing a strong coordination of the rhenium atom to the support. A set of scanning electron microscopy (SEM) photographs showing the morphology of the surface of particles of poly(4-vinylpyridine)/MTO and polystyrene/MTO systems are reported. The reticulation grade of the polymer was a crucial factor for the morphology of the particles surface. Poly(4-vinylpyridine) 2% cross-linked systems were characterized by particles with very irregular shape and surface. Poly(4-vinylpyridine) 25% cross-linked systems showed particles with regular spherical shape, which morphology was similar to microcapsules obtained with polystyrene. All novel MTO compounds were efficient and selective heterogeneous catalysts for the epoxidation of olefins using environmentally friendly H2O2 as oxygen atom donor. The catalyst activity was maintained for at least five recycling experiments.

Pd/C as a Highly Active Catalyst for Heck, Suzuki and Sonogashira Reactions

A specially optimized air-stable Pd on activated carbon catalyst is demonstrated to be a highly active (TON up to 36,000), selective and convenient heterogeneous catalyst for CC couplings of aryl halides in Heck, Suzuki, and Sonogashira reactions. The Pd/C catalyst developed allows extremely low Pd concentrations (down to 0.0025 mol% for Heck coupling, 0.005 mol% for Suzuki coupling) and high conversions of aryl bromides within a few hours. Easy and complete Pd separation and recovery is possible.

Electronic factors in catalysis: the volcano curve and the effect of promotion in catalytic ammonia synthesis

The activity and selectivity of heterogeneous catalysts are determined by their electronic and structural properties. In many cases, the electronic properties are determined by the choice of both the catalytically active transition metal and promoter elements. Density functional theory is used to calculate how these two factors affect the energies of the intermediates and transition states in the ammonia synthesis reaction. We show that a linear relationship exists between the activation energy for N 2 dissociation and the binding energy of adsorbed nitrogen. The ammonia synthesis activity under industrial conditions can be determined as a function of the nitrogen–surface interaction energy by combining the calculated adsorption energy–activation energy relation with a micro-kinetic model. The result is a volcano curve and we illustrate such relationships for both the non-promoted and alkali metal promoted transition metals. We conclude that promotion is most effective for the best non-promoted catalysts and that promotion will always be essential for obtaining an optimal ammonia synthesis catalyst. Analysis of the micro-kinetic model show that the best catalysts are those with the lowest apparent activation energies and they exhibit reaction orders between two asymptotic behaviors.

An Exceptionally Stable Ti Superoxide Radical Ion: A Novel Heterogeneous Catalyst for the Direct Conversion of Aromatic Primary Amines to Nitro Compounds.

A matrix-bound superoxide radical anion, generated by treating Ti(OR)4 (R=iPr, nBu) with H2 O2 , is a selective heterogeneous catalyst for the oxidation of anilines to the corresponding nitroarenes with 50 % aqueous H2 O2 [Eq. (1)]. Yields of 82-98 % are obtained, even with anilines bearing electron-withdrawing substituents (R=NO2 , COOH).

Characterisation of active phases of a copper catalyst for methanol oxidation under reaction conditions: an in situ X-ray absorption spectroscopy study in the soft energy range

The oxidation of methanol over copper is investigated by X-ray absorption spectroscopy in the soft X-ray range under reaction conditions. This in situ method allows the surface electronic structure of the catalyst to be correlated with its performance. The correlation reveals information about the catalytic function of various oxygen species on the surface. Oxide and metastable suboxide species affect in distinctly different ways the multiple action of copper as selective or unselective heterogeneous catalyst.

Kinetic of liquid-phase reactions catalyzed by acidic resins: the formation of peracetic acid for vegetable oil epoxidation

A heterogeneous kinetic model which takes into account the complete physicochemical interaction of reactive species in a polar liquid phase with an ion-exchange resin acting both as selective sorbent and heterogeneous catalyst has been employed to analyze the peracetic acid synthesis from acetic acid and hydrogen peroxide in an aqueous solution. Model parameters were estimated using uncoupled data from phase equilibria, polymer sorption, chemical equilibrium and reaction kinetics. Activities rather than molar concentrations in the polymer phase and specific (dry weight of catalyst basis) rather than volume-based expressions were found to give the best constitutive equations for the heterogeneous reaction rate.

Argilas pilarizadas - uma introdução

The synthesis, characterization and some applications in catalysis of pillared clays are described at an introductory level. The use of x-ray diffraction, surface area measurements, thermal analysis, IR spectrophotometry and solid-state NMR in the characterization of pillared clays is briefly discussed. Pillarization followed by doping or introduction of metal clusters into clays could lead to the development of selective heterogeneous catalysts.

Mesoporous Sulfonic Acids as Selective Heterogeneous Catalysts for the Synthesis of Monoglycerides

Glycerol was esterified with an equimolar amount of lauric acid to monolaurin using zeolites, sulfonic resins, and sulfonic mesoporous materials as catalysts. The latter were obtained by immobilization of 3-mercaptopropyl groups and oxidation with H2O2. In particular, fatty acid conversions, monoglyceride yields, and selectivities obtained with mesoporous (ordered, amorphous) sulfonic catalysts were compared with those of other heterogeneous or homogeneous catalysts. When using silica gel coated with propylsulfonic acids, high reaction rates are coupled to high monoglyceride yields, e.g., 53% mono yield for a 1 : 1 glycerol : lauric acid ratio. Minimizing the autocatalytic contribution of the fatty acid reactant is a critical issue. The influence of a number of reaction parameters is investigated. The mesoporous sulfonic catalysts are also employed in the esterification of propanediols andmeso-erythritol, and for reactions with other fatty acids.

Amorphous Microporous Mixed Oxides of Tailored Polarity and their Application in Membrane Catalyzed Hydrogenation Reactions

AbstractAmorphous microporous mixed oxides (AMM) are materials characterized by a narrow micropore distribution, high porosity, definable surface polarity and homogeneous elemental distribution (absence of domains). They are prepared by a modified Sol-Gel procedure in a single preparation step. Membranes prepared from these materials are selective heterogeneous catalysts, whose microstructural properties can be adjusted to the reaction conditions. Here it is shown, how the surface polarity of the membrane affects the hydrogenation properties.

Ethene hydroformylation on Co/SiO 2 catalysts

The results from ethene hydroformylation at 173°C showed that a Co(acac)3/SiO2 catalyst prepared from Co(acac)3 precursor by gas‐phase deposition was three times as active as a catalyst prepared by impregnation from cobalt nitrate, but oxo‐selectivities were similar. The high propanal selectivities on the Co(acac)3/SiO2 seem to be related to the presence of highly dispersed active sites favouring CO insertion. As dispersion is decreased from 23 to 8% due to increasing metal content (from 5 to 16 wt%), oxo‐selectivity decreased from 39 to 25%. The activity of Co(acac)3/SiO2 remained unchanged during 68 h on stream. The gas‐phase deposition technique described here is a promising method for the preparation of active, selective and stable heterogeneous hydroformylation catalysts.

Meerwein–Ponndorf–Verley reduction of carbonyl compounds catalysed by Mg–Al hydrotalcite

Properly activated Mg–Al hydrotalcite (Mg:Al = 3:1) is found to be a highly active, selective and regenerable heterogeneous catalyst for Meerwein–Ponndorf–Verley reduction of carbonyl compounds in the liquid phase.

Room temperature Baeyer–Villiger oxidation of cyclic and linear ketones using molecular oxygen, catalysed by a novel silica-supported nickel complex

A nickel complex supported on a surface-funtionalised silica gel is an active and selective heterogeneous catalyst for the liquid phase, room temperature Baeyer–Villiger oxidation of both cyclic and linear ketones using molecular oxygen.

Oxidative Dehydrogenation of Isobutane to Isobutene: Dawson-Type Heteropolyoxoanions as Stable and Selective Heterogeneous Catalysts

Oxidative Dehydrogenation of Isobutane to Isobutene: Dawson-Type Heteropolyoxoanions as Stable and Selective Heterogeneous Catalysts

Selective Oxidation of Thioether Mustard (HD) Analogs bytert-Butylhydroperoxide Catalyzed by H5PV2Mo10O40Supported on Porous Carbon Materials

The polyoxometalate H5PV2Mo10O40(1) supported on porous carbons forms selective and recoverable heterogeneous catalysts for the rapid room temperature oxidation of thioether mustard (HD) analogs. Both selectivities (95 to 99% for sulfoxide) and rates (∼20%) vary with the carbon support at parity of all other parameters. A 99% selectivity for the sulfoxide product, even at 75% conversion of the thioether substrate, is seen for the most selective system, 10 wt% 1 on Rohm and Haas Ambersorb (1/Ambersorb). Washing for 125 h in the solvent for catalysis, toluene, results in no significant loss in catalytic activity and a recovery and reusability study at high turnover also indicates that the catalysts are very stable. The polyoxometalate/carbon catalysts can be reactivated between uses by heating to ∼80°C. The oxidation of tetrahydrothiophene (THT) to THTO bytert-butylhydroperoxide (TBHP) catalyzed by 1/Ambersorb is first order in substrate and oxidant and proportional to the concentration of active sites on the 1/Ambersorb. The activation parameters for the process are ΔH‡=11.4±0.05 kcal/mol and ΔS‡=−38.8±0.22 cal/K mol.

CATALYSIS BY SOLID STATE OXOMETALATES

Heteropoly oxometalates containing tungsten aŕe active and selective heterogeneous catalysts for the conversion of methanol. The methanol interacts with both peripheral and bulk protons to form protonated methanol which, at higher temperatures, leads to the methylation of the heteropoly anions, the precursor to the formation of carbon-carbon linkages. In contrast, the isostructural molybdenum-containing heteropoly oxometalates when supported on silica are effective for the oxidation of methane. The presence of protons is shown to be essential for the oxidation process, evidently assisting in the production of oxygen vacancies.