Alkoxide Catalyst Research Articles

ChemInform Abstract: New Early Transition‐Metal Complexes as Homogeneous Catalysts for Olefin Oxidation

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Synthesis and Catalysis of Ti-MCM-41 Materials with Organic–Inorganic Hybrid Network

Abstract Ti-Containing MCM-41 materials were organically modified by employing an organosilane with a bridging organic group between two Si, and employed as catalysts for olefin oxidation using H2O2 as oxidant. The incorporation of a bridging organic group resulted in an increase in d spacing, in contrast to the modification with organosilane having terminal alkyl group, which provides decreased d spacing. High hydrophobicity of organically bridged Ti-MCM-41 materials led to high selectivity to epoxide by depressing allylic oxidation.

The role of XAFS in the in situ and ex situ elucidation of active sites in designed solid catalysts.

New mesoporous and microporous catalysts based on silica, aluminophosphates, or aluminophosphates containing one or other of the heteroatoms Ti, Co, Mn, Fe and Cr, are ideally suited for study by X-ray absorption fine structure (XAFS). The information derived from X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) elucidates the nature of the catalytically active site, generally (but not invariably) under in situ conditions. This, in turn, provides new insights into the mechanism of the catalysis and suggests methods of improving the performance of the original catalyst. In this way, significant advances have recently been made in designing catalysts for the selective oxidation of alkanes, for the epoxidation of alkenes and for the dehydration of alcohols to yield olefins. Combined with density functional theory (DFT) computations, XAFS studies have also yielded fresh insights into the architecture of nanoparticle catalysts, such as Ru12Cu4C2 supported on mesoporous silica.

Enantioselective borane reduction of aromatic ketones catalyzed by chiral aluminum alkoxides

The asymmetric borane reduction of prochiral ketones with an alkoxide catalyst prepared in situ from aluminum tri- iso-propoxide and ( R)-binaphthol was examined. Using these conditions, alcohols were obtained in high yield and e.e.'s of up to 83%.

On the nature of isolated active sites in open-structure catalysts for the aerial oxidation of alkanes

Unique insights into structural aspects of ternary and quaternary oxide phases of selective oxidation catalysts containing bismuth and molybdenum are obtainable via high-resolution transmission electron microscopy. Some of these are summarised in relation to the facile loss of structural oxygen during the course of catalytic action. Recent advances in scanning transmission electron microscopy that promise to elucidate the structures of new catalysts are briefly discussed. The paper deals mainly with in situ studies (using X-ray absorption fine structure) of single-site oxidation catalysts and, in particular, with regio- and shape-selective oxidation of n-alkanes using CoIII(MnIII)-AlPO-18 and AlPO-36 catalysts.

Stereoselective ring‐opening polymerization of rac‐lactide with a single‐site, racemic aluminum alkoxide catalyst: Synthesis of stereoblock poly(lactic acid)

Stereoselective ring‐opening polymerization of rac‐lactide with a single‐site, racemic aluminum alkoxide catalyst: Synthesis of stereoblock poly(lactic acid)

Molecular Design of Single-Site Metal Alkoxide Catalyst Precursors for Ring-Opening Polymerization Reactions Leading to Polyoxygenates. 1. Polylactide Formation by Achiral and Chiral Magnesium and Zinc Alkoxides, (η3-L)MOR, Where L = Trispyrazolyl- and Trisindazolylborate Ligands

Single-site achiral and chiral C3-symmetric complexes LMOR, where M = Mg and Zn, L = an η3-trispyrazolyl- or η3-trisindazolyl-borate ligand and R = Et, tBu, Ph, or SiMe3, have been synthesized and employed in the ring-opening polymerization of l-, rac-, and meso-lactide in CH2Cl2 at 25 °C and below. The polymerization occurs by acyl cleavage and gives rise to polylactide, PLA, with PDI of 1.1−1.25 up to 90% conversion. Studies of the kinetics of polymerization reveal first order behavior in both lactide and metal catalyst. For L = tris(3-tert-butylpyrazolyl)borate, (tBupz)3BH, polymerization of ∼500 equiv of l-lactide proceeds to 90% conversion within 1 h and 6 d for the magnesium and zinc catalysts, respectively. The zinc complexes are, however, more tolerant to air and moisture and solid samples where R = SiMe3 are persistent in air for several days. The rate of polymerization is also significantly influenced by the nature of the η3-L spectator ligand. Chiral C3-symmetric catalysts, where L = tris(indaz...

Microporous Zirconia–Silica Mixed Oxides Made by Sol–Gel as Catalysts for the Liquid-Phase Oxidation of Olefins with Hydrogen Peroxide

The preparation of a series of microporous zirconia–silica mixed oxides by sol–gel is reported. These have been characterized by BET methods, thermogravimetric analysis, X-ray diffraction analysis, UV–vis spectroscopy, and TPD of ammonia. The materials have high surface areas; they are amorphous and possess only mild surface acidity. They have been tested in the oxidation of different substrates with hydrogen peroxide, particularly the oxidation of cyclohexene. The catalysts are moderately active and produce mainly products arising from oxirane ring opening, particularly when the reactions are carried out in the absence of solvent. The use of a less hydrophilic reaction medium and/or the partial methylation of the silica surface tend to increase the reaction productivity and reduce hydrogen peroxide consumption.

Polymer-bound schiff-base complex catalyst for effective oxidation of olefins with molecular oxygen

Polymer-bound Schiff-base ligand (PS–SalPhe) was prepared from polystyrene-bound salicylaldehyde and phenylalanine, and its complex (PS–SalPhe–M) (M = Co, Mn) was also synthesized. The polymer ligand and its complex were characterized by infrared spectra, small area X-ray photoelectron spectroscopy, and ICP–AES. In the presence of the complex, cyclohexene can be effectively oxidized by molecular oxygen without a reductant. The major products of the reaction are 2-cyclohexen-1-ol (—OH), 2-cyclohexen-1-one (CO), and 2-cyclohexen-1-hydroperoxide (—OOH ), which is different from the typical oxidation of cyclohexene. The mechanism of cyclohexene oxidation is also discussed. Long-chain linear aliphatic olefins, such as 1-octene, 1-decene, 1-dodecene, and 1-tetradecene, can be directly oxidized by molecular oxygen catalyzed by PS–SalPhe–M (M = Mn, Co), which yields the 1,2-epoxy alkane. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1138–1143, 2000

Polymer-bound schiff-base complex catalyst for effective oxidation of olefins with molecular oxygen

Polymer-bound Schiff-base ligand (PS–SalPhe) was prepared from polystyrene-bound salicylaldehyde and phenylalanine, and its complex (PS–SalPhe–M) (M = Co, Mn) was also synthesized. The polymer ligand and its complex were characterized by infrared spectra, small area X-ray photoelectron spectroscopy, and ICP–AES. In the presence of the complex, cyclohexene can be effectively oxidized by molecular oxygen without a reductant. The major products of the reaction are 2-cyclohexen-1-ol (—OH), 2-cyclohexen-1-one (CO), and 2-cyclohexen-1-hydroperoxide (—OOH ), which is different from the typical oxidation of cyclohexene. The mechanism of cyclohexene oxidation is also discussed. Long-chain linear aliphatic olefins, such as 1-octene, 1-decene, 1-dodecene, and 1-tetradecene, can be directly oxidized by molecular oxygen catalyzed by PS–SalPhe–M (M = Mn, Co), which yields the 1,2-epoxy alkane. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1138–1143, 2000

Stereoselective ring-opening polymerization ofrac-lactide with a single-site, racemic aluminum alkoxide catalyst: Synthesis of stereoblock poly(lactic acid)

The polymerization of racemic lactide with a racemic aluminum alkoxide catalyst is reported. Microstructural analysis of the polymer produced with 1 H NMR spectroscopy revealed that an isotactic stereoblock poly(lactic acid) formed, where each enantiomerically pure block contained an average of 11 lactide monomer units. The melting point of this polymer, 179°C, was higher than that of the enantiomerically pure polymer, consistent with the cocrystallization of the enantiomeric blocks of the polymer. The mechanism of the polymer formation is currently unknown, although a polymer exchange pathway, where living chain ends switch between metal centers to produce diastereomeric active species, is proposed.

Amino acid Schiff base complex catalyst for effective oxidation of olefins with molecular oxygen

The amino acid Schiff base manganese complex (Sal–Phe–Mn) was prepared with l-phenylalanine, salicylaldehyde and Mn(OAc) 2·4H 2O. The ligand and the complex were characterized by the infrared spectra, small area X-ray photoelectron spectroscopy, and ICP-AES. In the presence of the manganese complex, cyclohexene was effectively oxidized by molecular oxygen without reductant. The major products of the reaction were 2-cyclohexen-1-ol (–OH), 2-cyclohexen-1-one (CO) and 2-cyclohexen-1-hydroperoxide (–OOH), which was different with typical oxidation of cyclohexene. The influence of reaction temperature and additive for oxidation had been studied. The selectivity of 2-cyclohexen-1-hydroperoxide varied with reaction time and different additives. The mechanism of cyclohexene oxidation had also been discussed.

Concerning the Mechanism of Ring-Opening Polymerization by Coordinate Catalysts: The 2D Homonuclear J-Resolved Spectrum of Poly(cyclohexene oxide)

The use of 2D homonuclear J-resolved 1H NMR spectroscopy in the examination of the methine region of poly(cyclohexene oxide) formed by polymerization of cyclohexene oxide employing the Union Carbide calcium−amide−alkoxide catalyst reveals that the majority of the ring opening occurs by backside attack.

Discovery of Novel Catalysts for Alkene Epoxidation from Metal-Binding Combinatorial Libraries

New lead structures for olefin oxidation catalysts have been obtained from a combinatorial library of 5760 metal-ligand complexes (see the microscopy picture). Iron complexes led to clean epoxide product formation using H2 O2 as the terminal oxidant. Parallel libraries were used to determine ligand features important for high catalytic activity and to identify enantioselective catalyst structures (see the Equation).

Theoretical Studies of the Factors Controlling Insertion Barriers for Olefin Polymerization by the Titanium-Chelating Bridged Catalysts. A Search for More Active New Catalysts

In search of more active new catalysts, density functional theory was used to predict insertion barriers for ethylene polymerization for a variety of unknown Ti-chelating bridged alkoxide catalysts, [YR‘XR‘Y]TiCH3+, where X, Y = O, S, Se, Te, and R‘ = C6H4, C2H2, C2H4 with and without substituents. The use of ligands having donating and bridging atoms that are capable of donating electron density to the cationic metal center decreases the insertion barriers. For [(C6H4O)X(C6H4O)]TiCH3+, both the olefin coordination energy, X = S(21.4 kcal/mol) > Se(19.2) >Te(16.6), and migratory insertion barrier, X = S(6.4) > Se(5.9) > Te(5.7), decrease with the increasing donating capability of the bridging atom X to the metal center, i.e., via X = S < Se < Te. The oxygen bridge, however, gives the lowest insertion barrier (4.5 kcal/mol) in this group. The role of the phenyl group was explored by replacing it by C2H2 and C2H4 moieties. Having conjugation through the X−CC−Y moiety in these complexes turns out to be very ...

Enhanced propoxylation of alcohols and alcohol ethoxylates

AbstractAn alkoxylation catalyst consisting of a calcium/aluminum complex partially neutralized in an alcohol ethoxylate base was found to be substantially more active than a conventional potassium hydroxide catalyst in the propoxylation of alcohols and alcohol ethoxylates. With the Ca/Al alkoxide catalyst, propoxylation was faster, required less catalyst, and proceeded at lower temperatures. The latter is desirable since it could help reduce the formation of allyl alcohol and allyl alcohol propoxylate by‐products. The two catalysts produced different oligomer distributions, particularly at higher temperatures, which were found to have a marginal impact on surface chemistry.

Catalytic Membrane Sensors. A Thin Film Modified H2 Resistive Sensor for Multi-Molecular Detection

A so-called “catalytic membrane sensor” (CMS) is being developed to impart selectivity and reactivity to the surface of an existing sensor by modifying it with a series of thin films. The proposed “sandwich-type” modification involves deposition of a catalyst layer between two size selective sol-gel layers on a Pd/Ni resistive H2 sensor. The role of the catalyst is to convert organic materials to H2 and organic by-products by a dehydrogenation mechanism. The roles of the membranes are to impart chemical specificity by molecular sieving of the analyte and the converted product streams as well as to control access to the underlying Pd/Ni sensor. The “sandwich” modification will mediate the sensor response and avoid potential poisoning effects. Ultimately, an array of these CMS elements encompassing different catalysts and membranes will further enable improvements in selectivity and specificity via pattern recognition methodologies. This report details the synthesis of the various thin film solutions (i.e., catalyst precursors, sol-gel solution), the generation of the catalyst through ion-exchange, a comparison of the double alkoxide versus standard catalyst precursors, and the processing required to generate a CMS. This paper also details the novel and surprising experimental data concerning the increased selectivity and durability observed for the sol-gel modified H2 gas sensing resistors. The utility of the CMS component as a multi-molecular detector will be reported later.

Fast formation of high‐purity methyl esters from vegetable oils

AbstractExperiments have confirmed that the base‐catalyzed methanolysis of vegetable oils occurs much slower than butanolysis because of the two liquid phases initially present in the former reaction. For the same reason, second‐order kinetics are not followed. The use of a cosolvent such as tetrahydrofuran or methyl tertiary butyl ether speeds up methanolysis considerably. However, like one‐phase butanolysis, one‐phase methanolysis initially exhibits a rapid formation of ester, but then slows drastically. Experiments show that the half‐life of the hydroxide catalyst is too long to explain the sudden slowing of the reaction. Similarly, lower rate constants for the methylation of the mono‐ and diglycerides are not a reasonable explanation. Instead the cause has been identified as the fall in polarity which results from the mixing of the nonpolar oil with the methanol. This lowers the effectiveness of both hydroxide and alkoxide catalysts. Increasing the methanol/oil molar ratio to 27 in the one‐phase system raises the polarity such that the methyl ester content of the ester product exceeds 99.4 wt% in 7 min. This has obvious implications for the size of new methyl ester plants as well as the capacity of existing facilities.

Sol–gel bismuth–molybdenum–titanium mixed oxides I. Preparation and structural properties

Bismuth molybdenum titanium oxides, potential catalysts for the partial oxidation of olefins, were prepared via the sol–gel route. Either Bi(NO 3) 3·5H 2O and (NH 4) 6Mo 7O 24·4H 2O, or BiCl 3 and MoOCl 4 were used as precursors together with Ti-isopropoxide. One sample was dried by semicontinuous extraction with supercritical CO 2, affording an aerogel, the others were heated in vacuo resulting in xerogels. For comparison, two bismuth molybdenum oxides on a titania support, and a bismuth molybdate were prepared. The resulting materials were characterized by ICP-AES, N 2 physisorption, XRD, XPS, FT Raman, and UV–vis spectroscopies. The sol–gel derived materials were X-ray amorphous and possessed a high surface area after drying, and a marked mesoporosity in case of the aerogel. The morphology changed upon calcination in O 2 at 773 K, resulting in diminished surface area and a loss of micropores. The bismuth molybdenum oxide phases of the materials containing more than 50 wt% titania remained X-ray amorphous after calcination. The Bi- and Mo-content, the type of Bi- and Mo-precursors and their prehydrolysis, as well as the drying method had a major influence on the structural properties, and the surface and bulk composition of the aerogel and xerogels. Application of Bi-chloride and Mo-chloride precursors can result in significant deviation from the desired bulk composition, likely due to incomplete hydrolysis and evaporation during the subsequent drying and calcination steps.

On the Structure of the Active Site of Ti-Silicalite in Reactions with Hydrogen Peroxide: A Vibrational and Computational Study

Ti-Silicalite (TS-1) is an excellent catalyst for the selective oxidation of alkenes to epoxides, of NH3to hydroxylammine and of the ammoximation of cyclohexanone, using H2O2as oxidant. This paper reports spectroscopic (IR/Raman) and computational (HF, cluster models) studies on the complexes formed upon interaction of the Ti catalytic centre with water and hydrogen peroxide. Evidence is presented of the formation of unstable hydroperoxidic species upon interaction with neutral aqueous H2O2solutions, which can be converted into a more stable ionic complex in a basic environment. The vibrational spectra of these complexes have been recorded in appropriately designed low-temperature experiments on solution-soaked powders bearing some resemblance with the catalytic reactor environment. The assignment of the spectra was confirmed byab initiocalculations and by parallel experiments on the structurally similar Ti-free silicalite.