Surfactant‐Encapsulated Polyoxometalates as Immobilized Supramolecular Catalysts for Highly Efficient and Selective Oxidation Reactions

A type of interesting immobilized supramolecular catalysts based on surfactant‐encapsulated polyoxometalates has been developed for oxidation reactions. Through a sol‐gel process with tetraethyl orthosilicate, hydroxyl‐terminated surfactant‐encapsulated polyoxometalate complexes have been covalently and uniformly bound to a silica matrix with unchanged complex structure. The formed hybrid catalysts possess a defined hydrophobic nano‐environment surrounding the inorganic clusters, which is conducive to compatibility between the polyoxometalate catalytic centres and organic substrates. The supramolecular synergy between substrate adsorption, reaction, and product desorption during the oxidation process has been found to have an obvious influence on the reaction kinetics, with the activity of the catalyst being greatly improved. The supramolecular catalysts performed effectively in the selective oxidation of several different kinds of organic compounds, such as alkenes, alcohols, and sulfides, and the main products were the corresponding epoxides, ketones, sulfoxides, and sulfones. More significantly, the catalyst could be easily recovered by simple filtration, and the catalytic activity was well retained for at least five cycles. Finally, the present strategy has proved to be a general route for the fabrication of supramolecular hybrid catalysts containing common polyoxometalates suitable for various purposes.     

Nearly monodisperse silica microparticles form in silicone (pre)elastomer mixtures

The formation of silica from a tetraalkoxysilane in a sol-gel process usually requires a highly polar, typically aqueous, medium that aids in the hydrolysis of the silane and leads to electrostatic stabilization of the growing silica particles. Formation of such silica particles in a hydrophobic medium is much more challenging. We report the formation of silica microspheres within silicone oils (hydroxy-terminated poly(dimethylsiloxane), HO-PDMS) during elastomer cure using atmospheric humidity in a one-pot and one-step synthesis. Using tetraethyl orthosilicate (TEOS) as both cross-linker and silica precursor, and aminopropyl-terminated dimethylsiloxane oligomer (AT-PDMS) as a catalytic surfactant, silica particles of low polydispersity formed near or at the air interface of the elastomer: the presence of a hydrophilic polymer, poly(ethylene glycol) (PEG), had an indirect effect on the particle formation, as it assisted with water transmission into the system, which resulted in particle formation over a wider range of parameters and facilitated silicone elastomer cure further away from the air interface. Depending on the relative humidity during cure, the sizes of particles presenting at the air interface varied from ~6-7 μm under ambient conditions (20-30%RH) to ~7-9 μm at high relative humidity (90% RH). The origin of the controlled particle synthesis is ascribed to the relative solubility of the catalyst and the efficiency of water permeation through the silicone matrix. AT-PDMS preferentially migrates to the air interface, as shown by ninhydrin staining, where it both catalyzes alkoxysilane hydrolysis and condensation, and stabilizes the growing silica particles prior to aggregation. Since reactions in the presence of this catalyst are slow, TEOS can migrate from within the pre-elastomer body to the interface faster than water can penetrate the silicone, such that the main locus of hydrolysis/condensation leading both to silica formation and elastomer cross-linking is at the air interface.     

Characterization of sol-gel immobilized lipases

The immobilization of lipases within a chemically inert hydrophobic sol-gel support, which is prepared by polycondensation of hydrolyzed tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) or iso-butyltrimethoxysilane (iso-BTMS), results in heterocatalysts. The heterocatalysts so prepared showed a dramatically enhanced catalytic activity and stability as measured by the hydrolysis and transesterification of soybean oil. The lipase/sol-gel materials were characterized by nitrogen adsorption to determine their specific surface area. Solid state NMR was used to reveal the degree of cross-linking of the sol-gel materials. Scanning electron microscopy and atomic force microscopy were used to observe the morphology of the biocatalysts. Transmission electron microscopy and confocal microscopy were used to investigate the enzyme distribution within the sol-gel materials. The characterization studies showed that the most active lipase-containing sol-gel was a non-porous amorphous material with enzyme randomly distributed throughout the sol-gel material. The activity of the immobilized enzyme did not correlate to the degree of cross-linking or the specific surface area of the sol-gel materials. The highly retained activity of the immobilized enzyme was more likely attributed to the conformational changes of the enzyme during the immobilization, which result in enzyme's fixation in a more favorable conformation and to the lipophilic environment of the hybrid matrix structure which facilitates the transport of the hydrophobic substrate to the active sites.     

Spreading of Surfactant Solutions over Hydrophobic Substrates

The spreading of surfactant solutions over hydrophobic surfaces is considered from both theoretical and experimental points of view. Water droplets do not wet a virgin solid hydrophobic substrate. It is shown that the transfer of surfactant molecules from the water droplet onto the hydrophobic surface changes the wetting characteristics in front of the drop on the three-phase contact line. The surfactant molecules increase the solid-vapor interfacial tension and hydrophilize the initially hydrophobic solid substrate just in front of the spreading drop. This process causes water drops to spread over time. The time of evolution of the spreading of a water droplet is predicted and compared with experimental observations. The assumption that surfactant transfer from the drop surface onto the solid hydrophobic substrate controls the rate of spreading is confirmed by our experimental observations. Copyright 2000 Academic Press.     

Continuous process for singlet oxygenation of hydrophobic substrates in microemulsion using a pervaporation membrane

Chemically generated singlet oxygen (1O2, 1Deltag) is able to oxidize a great deal of hydrophobic substrates from molybdate-catalyzed hydrogen peroxide decomposition, provided a suitable reaction medium such as a microemulsion system is used. However, high substrate concentrations or poorly reactive organics require large amounts of H2O2 that generate high amounts of water and thus destabilize the system. We report results obtained on combining dark singlet oxygenation of hydrophobic substrates in microemulsions with a pervaporation membrane process. To avoid composition alterations after addition of H2O2 during the peroxidation, the reaction mixture circulates through a ceramic membrane module that enables a partial and selective dewatering of the microemulsion. Optimization phase diagrams of sodium molybdate/water/alcohol/anionic surfactant/organic solvent have been elaborated to maximize the catalyst concentration and therefore the reaction rate. The membrane selectivity towards the mixture constituents has been investigated showing that a high retention is observed for the catalyst, for organic solvents and hydrophobic substrates, but not for n-propanol (cosurfactant) and water. The efficiency of such a process is illustrated with the peroxidation of a poorly reactive substrate, viz., beta-pinene.     

A new mesoporous micelle-templated silica route for enzyme encapsulation

A new mesoporous micelle-templated silica (MTS) route for enzyme encapsulation is presented. The pore structure is given by a new association oflecithin (double chain surfactant) and dodecylamine as cosurfactant. To enhance and to well protect the enzyme activity, lactose was loaded in the synthesis. The mixed-micelles give after the addition of tetraethyl orthosilicate a well-ordered mesoporous material with a spongelike rigid structure stable after calcination at 550 degrees C. The size of the pores lies between 30 and 40 A, matching well with the size of the lipases. The activity of this heterogeneous catalyst was tested in the hydrolysis of the ethylthiodecanoate. These new biocatalysts were very active, more than hydrophobic sol-gel materials and commercially available sol-gel encapsulated lipase. This new MTS synthesis route allows one to encapsulate in one-step various enzymes, even those that are very fragile.     

Dependence of enantioselectivity on the distribution of a chiral hydrogenation catalyst between an aqueous and a micellar phase: investigations using pulsed field gradient spin echo NMR spectroscopy

The enantioselectivity obtained from rhodium complex catalyzed hydrogenations conducted in water can often be increased considerably by the addition of amphiphiles. At present the reasons for this increase in selectivity are not fully understood. The application of pulsed field gradient spin echo NMR (PGSE-NMR) spectroscopy to determine the average diffusion coefficients of the catalysts in both known and novel examples of asymmetric hydrogenation shows definitively that the increase in enantioselectivity is coupled with an aggregation of the catalyst to the micelles. This aggregation or solubilization of the catalyst in the micelles leads to the formation of a new colloidal phase in the aqueous solution. This phase has stronger hydrophobic properties, and thus the hydrogenation is more comparable to those conducted in a hydrophobic or less polar organic solvent. In the case of anionic amphiphiles, which form amphiphilic salts with the cationic catalyst, the embedment of the catalyst complex into the micelle is generally complete. The whole hydrogenation then takes place exclusively inside the micelles, leading to high enantioselectivity. If the catalyst is not completely embedded into the micelle, for example in the cases of nonionic or cationic surfactant solutions, the solubility of the substrate plays an important role. For soluble substrates the hydrogenation of the substrate occurs predominately in the aqueous phase itself, leading to very poor enantioselectivities. In these cases, only the use of a large excess of amphiphile, far above the critical micelle concentration (cmc), will lead to higher enantioselectivities due to a shift of the equilibrium towards the micellar bonded forms of catalyst and substrate. In contrast, poorly soluble substrates exhibit a high tendency to be incorporated into micelles, which leads to much higher enantioselectivities if the cmc of the surfactant is small enough. Changes in the cmc of amphiphiles caused by their aggregation with catalysts could also be estimated. The variation in selectivity observed for the catalysts containing seven-membered, flexible chelate rings is apparently due to changes in their conformation in the less polar micellar medium, and this effect is also seen in organic solvents. As expected, catalysts containing smaller chelate rings show this effect to a considerably lower extent since they are conformationally more rigid.     

Insertion of zeolite in sol-gel made catalytic material: an EPR study

Incorporation of zeolite to catalyst base-supports generally endows them with improved catalytic properties; especially those related to selectivity and activity. These effects are due to the stronger than normal acidity of zeolites and their molecular sieve effect. Such properties alter notoriously the way in which metals incorporate into a crystalline sol-gel catalytic matrix. Most noticeable effects on the surface as per EPR analysis occur upon incorporation of copper as metal catalyst when ZSM-5 zeolite is added, then when adding β-zeolite and finally when using Y-zeolite. This revised version was published online in June 2006 with corrections to the Cover Date.     

Sol-gel encapsulated horseradish peroxidase: a catalytic material for peroxidation

This study addresses the viability of sol-gel encapsulated HRP (HRP:sol-gel) as a recyclable solid-state catalytic material. Ferric, ferric-CN, ferrous, and ferrous-CO forms of HRP:sol-gel were investigated by resonance Raman and UV-visible methods. Electronic and vibrational spectroscopic changes associated with changes in spin state, oxidation state, and ligation of the heme in HRP:sol-gel were shown to correlate with those of HRP in solution, showing that the heme remains a viable ligand-binding complex. Furthermore, the high-valent HRP:sol-gel intermediates, compound I and compound II, were generated and identified by time-resolved UV-visible spectroscopy. Catalytic activity of the HRP:sol-gel material was demonstrated by enzymatic assays by using I(-), guaiacol, and ABTS as substrates. Encapsulated HRP was shown to be homogeneously distributed throughout the sol-gel host. Differences in turnover rates between guaiacol and I(-) implicate mass transport of substrate through the silicate matrix as a defining parameter in the peroxidase activity of HRP:sol-gel. HRP:sol-gel was reused as a peroxidation catalyst for multiple reaction cycles without loss of activity, indicating that such materials show promise as reusable catalytic materials.     

Selenium-mediated micellar catalyst: an efficient enzyme model for glutathione peroxidase-like catalysis

Mimicking the properties of the selenoenzyme glutathione peroxidase (GPx) has inspired great interest. In this report, a selenium-containing micellar catalyst was successfully constructed by the self-assembly of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) with benzeneseleninic acid (PhSeO2H) through hydrophobic and electrostatic interaction in water. The selenium-containing micellar catalyst demonstrated substrate specificity for both 3-carboxy-4-nitrobenzenethiol (ArSH, 2) and cumene hydroperoxide (CUOOH), and their complexation was confirmed by UV and fluorescence spectra. More importantly, it demonstrated high GPx activity in two assay systems. It is about 126 times more effective than the well-known GPx mimic ebselen in the classical coupled reductase assay system; however, by using hydrophobic substrate ArSH (2) as an alternative of glutathione (GSH, 1), the micellar catalyst exhibited remarkable 500-fold and 94 500-fold rate enhancements compared with that of PhSeO2H and PhSeSePh.     

Preparation and properties of polybenzoxazole-silica nanocomposites via sol-gel process

A novel organic/inorganic hybrid material has been prepared through the sol-gel process. A high temperature polymer, polybenzoxazole (PBO), was chosen as the organic phase due to its inherent low dielectric constant and low water absorption. The inorganic phase was generated via sol-gel reaction from a silica precursor, phenyltriethoxysilane (PTEOS). Due to the hydroxyl groups in the PBO precursor backbone and the water release during the cyclization of the precursor, the sol-gel reaction proceeded without the addition of water and any catalyst. After curing at 350 °C, we obtained the PBO/silica nanocomposites. From TEM and SEM photographs, the silica particles dispersed in the PBO matrix were nano-sized. With an addition of 100 wt% of PTEOS, the T_g of PBO was increased 35 °C. The dielectric constant of the hybrid materials increased with the increasing amount of PTEOS.     

Electrochemical Systems Based on Sol-Gel Silica Matrix Impregnated with Organic Solvent

Two electrochemical systems based on sol-gel silica matrix impregnated with organic solvent were prepared and studied. The first one is composed of tetramethylorthosilicate based material filled with ferrocene solution in polar solvent: propylene carbonate. Electrodes are immersed in this solid electrolyte during all stages of sol-gel process. Despite of the lack of the extra added salt, by using ultramicroelectrode, undistorted electrochemical signal corresponding to the electrooxidation of the ferrocene was obtained. Its diffusion coefficient within the sol-gel matrix depends on the time elapsed after gelation and it is not much below that in salt solution in the same solvent. The second system is based on graphite dispersion in hydrophobic sol-gel silicate matrix. This material was filled with mixture of liquid butylferrocene and hexadecane. After immersion in aqueous salt solution it serves as working electrode. The electrochemical signal corresponding to the electrooxidation of the butylferrocene within organic phase was obtained. Probably the electrode process occurs at three phase (carbon/organic phase/aqueous phase) junction and it is accompanied by anion transfer through the liquid-liquid interface.     

Study of cryostructuring of polymer systems. 32. Morphology and physicochemical properties of composite poly(vinyl alcohol) cryogels filled with hydrophobic liquid microdroplets

Cryogenic treatment (freezing at −20°C for 12 h followed by defrosting at a rate of 0.03°C/min) of decane, dodecane, or tetradecane emulsions in a poly(vinyl alcohol) solution (80 g/l) is employed to prepare composite cryogels containing microdroplets of liquid hydrophobic fillers entrapped into a macroporous hydrogel matrix. The effects of the type of a hydrocarbon, the degree of filling, and the addition of a surfactant (decaethylene glycol cetyl ether) on the physicomechanical properties, heat endurance, and morphology of the composites are studied. It is shown that, an increase in the content of liquid hydrophobic fillers within some range of their volume fraction enhances the rigidity of corresponding cryogels. Incorporation of the nonionic surfactant into the initial emulsions results in a complex dependence of the rigidity of the resulting composite cryogels on surfactant concentration and variations in the morphology of pores in the gel phase. At the same time, the heat endurance of all examined composite cryogels weakly depends on the type and concentration of the hydrocarbon fillers, as well as the presence of surfactant additives.     

High-temperature electrocatalysis using thermophilic P450 CYP119: dehalogenation of CCl4 to CH4

The use of a thermophilic cytochrome P450, CYP119, in electrocatalytic dehalogenations of C1 halocarbon solvents is studied. Temperature stable enzyme-modified electrodes were constructed using sol-gel and polymeric surfactant approaches. CYP119 deposited in a dimethyldidodecylammonium poly(p-styrene sulfonate) (DDAPSS) film has good retention of electrochemical activity up to 80 degrees C. At potentials approaching the FeII/I couple, the CYP119/DDAPSS films demonstrate high catalytic dehalogenations of the C1 chloromethanes CCl4, CHCl3, and CH2Cl2. Product analysis identified mixtures of sequentially dechlorinated products up to methane; no evidence for radical-coupled products was observed. The yield of methane from the CYP119-catalyzed reduction of CCl4 is increased 35-fold from 25 degrees C to 55 degrees C. In combination with the lack of C2 products, the facility of an overall eight-electron reductive dehalogenation suggests that the substrate is constrained within the protein during electrocatalytic turnover.     

金催化顺丁烯二酸酐的选择加氢反应

采用溶胶-凝胶法制备了一系列担载纳米金催化剂,用以催化顺丁烯二酸酐(简称顺酐,MA)的选择加氢反应．模板剂的引入改变了催化剂载体的结构,从而提高了其催化活性和对加氢产物丁二酸酐(SA)以及丁二酸二乙酯(DFAS)的选择性．同时考察了反应温度、溶剂、催化剂制备方法对顺酐选择加氢反应的影响以及催化剂的重复使用性能．实验结果表明,以焙烧处理除去模板剂十八胺的Au／SiO2-O(C)为催化剂时,顺酐选择加氢制取丁二酸酐和一步加氢酯化制取丁二酸二乙酯的效果最佳,其转化率以及产物选择性均大于99．5％．     

纯水中有氧条件下Pd/C催化的无配体Suzuki反应简

报道一种在纯水中Pd/C催化的高效Suzuki反应体系,该体系无需除氧,且无外加配体和添加剂,底物普适性广泛,含有亲水或疏水基团的溴代芳烃都能被高效活化. 此外,对部分杂环芳烃与芳基硼酸的偶联反应也有较好的催化效果. 该催化剂可高效循环使用三次,且性能无明显下降.     

担载纳米金催化乙醇选择氧化制乙酸乙酯

采用溶胶-凝胶法制备了一系列担载纳米金催化剂，用以催化乙醇选择氧化反应，模板剂的引入改变了催化剂载体的结构，从而提高了其催化活性和对乙酸乙酯的选择性，同时考察了反应温度、时间、以及催化剂制备方法对乙醇选择氧化反应的影响，实验结果表明使用以焙烧处理除去模板剂十八胺的Au／SiO2-O(C)为催化剂时，乙醇选择氧化一步制取乙酸乙酯的效果最佳，其选择性最高可达88．1％。     

Gemini surfactants at the air/water interface: a fully atomistic molecular dynamics study

Gemini surfactants typically consist of two single-chain surfactants chemically linked by a spacer molecule. We report herein the results of fully atomistic molecular dynamics (MD) simulations of a series of Gemini surfactants: CsH2s-alpha,omega-bis(C12H25N+(CH3)2Cl-), at the air/water interface with s = 3, 4, 6, 12, 14, and 16, at values of the initial surface area per surfactant AS = 70 A2, 77 A2, 95 A2, 151 A2, 133 A2, and 103 A2, respectively. The AS values employed were obtained from surface tension and neutron reflection experiments at the respective cmc of each surfactant. The Gemini surfactant corresponding to s = 3 was also simulated at AS = 105 A2, which is the experimentally derived value of surface area per surfactant at 1/10th of cmc. Only the surfactants with s = 12 and 14 and the surfactant with s = 3 at AS = 105 A2 gave a stable monolayer at the air/water interface. In other cases, we observe movement of some surfactant molecules from the air/water interface into the aqueous phase, resulting in a stable primary monolayer of surfactants at the air/water interface and a small concentration of surfactant molecules below it. The latter form aggregates, with their hydrophobic chains in the core. The density profiles along the normal to the interface are compared with the ones obtained from neutron reflection experiments. The MD simulations confirm the bending of the spacer toward the hydrophobic chains as the spacer length is increased and the spacer becomes more hydrophobic. The simulations have helped to shed light on the low-resolution picture which emerges from experimental analyses.     

Unusual wetting dynamics of aqueous surfactant solutions on polymer surfaces

Static and dynamic contact angles of aqueous solutions of three surfactants--anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethylammonium bromide (DTAB), and nonionic pentaethylene glycol monododecyl ether (C(12)E(5))--were measured in the pre- and micellar concentration ranges on polymer surfaces of different surface free energy. The influence of the degree of substrate hydrophobicity, concentration of the solution, and ionic/nonionic character of surfactant on the drop spreading was investigated. Evaporation losses due to relatively low humidity during measurements were taken into account as well. It was shown that, in contrast to the highly hydrophobic surfaces, contact angles for ionic surfactant solutions on the moderately hydrophobic surfaces strongly depend on time. As far as the nonionic surfactant is considered, it spreads well over all the hydrophobic polymer surfaces used. Moreover, the results obtained indicate that spreading (if it occurs) in the long-time regime is controlled not only by the diffusive transport of surfactant to the expanding liquid-vapor interface. Obviously, another process involving adsorption at the expanding solid-liquid interface (near the three-phase contact line), which goes more slowly than diffusion, has to be active.     

On autophobing in surfactant-driven thin films

Recent experiments (Afsar-Siddiqui, A. B.; Luckham, P. F.; Matar, O. K. Langmuir 2004, 20, 7575-7582) on the spreading of aqueous droplets containing cationic surfactants over thin aqueous films supported by negatively charged substrates demonstrated trends in the spreading behavior with either increasing surfactant concentration or increasing film thickness. Although the substrate is initially hydrophilic and the droplet spreads, surfactant adsorption at the substrate renders it hydrophobic leading to droplet retraction. We generate a model here using lubrication theory that allows the effect of the surfactant on the wettability to be taken into account. Our numerical results show that due to basal adsorption of surfactant at the interface, the initially hydrophilic solid substrate is rendered hydrophobic. This then drives droplet retraction and dewetting, which is in agreement with the experimentally observed trends.