Structured reactors for enzyme immobilization: advantages of tuning the wall morphology

A new high-porosity mullite advanced ceramic material (ACM) having an open pore structure on the micron length scale was evaluated as a monolithic support for enzyme catalyzed reactions. The hydrodynamic properties of ACM monoliths were investigated using residence time distribution (RTD) in gas–liquid Taylor flow and compared with classical cordierite monoliths. The hydrodynamic results show that the liquid phase accesses the entire open volume of the wall and exchanges rapidly with the bulk liquid in the channels. The ACM monoliths were functionalized with polyethylenimine (PEI) and with carbon prepared by different methods in order to provide adsorption sites for lactase from Aspergillus oryzae and lipase from Candida rugosa. These monolithic biocatalysts were tested for activity and stability and compared to similarly prepared biocatalysts employing classical cordierite ceramic monoliths. The use of high-porosity ACM monoliths leads to more stable and more active structured bioreactors. The highly open microstructure of ACM affords good access to catalysts deposited within the walls of a monolith and enables high enzyme loadings.     

Hydrogenation of olefins over palladium(II) complexes supported on a modified macroporous polymer bead

Gellular and macroporous polymer supports have been prepared. A modified polymer support has also been prepared by coating the internal pore wall of macroporous poly(styrene-co-divinylbenzene) with lightly crosslinked polymer containing functional groups. The supports were phosphinated and PdCl₂ was supported on them. The supports and catalysts were characterized using scanning electron microscopy and Fourier transform infrared spectroscopy. The polymer-supported Pd catalysts were used in the hydrogenation of olefins. The effects of the support structure and solvent were also studied. ©1997 SCI     

Poly(ionic liquid)-derived nanoporous carbon analyzed by combination of gas physisorption and small-angle neutron scattering

The preparation of nanoporous carbon materials and their characterization combining small-angle neutron scattering (SANS) with gas physisorption is presented. Carbon with a porous structure and tunable form is obtained here by a salt-templating approach using poly(ionic liquid) as precursor. SANS in combination with contrast matching by deuterated p-xylene was used for a separation of the scattering component deriving from the density fluctuations of the carbon matrix and the inaccessible porosity. The resulting scattering curves could be used for an unambiguous characterization of the pore structure of the materials. SANS curves measured at different partial pressure of the matching agent p-xylene were used for a differential filling of the micro- and mesopores. The analysis using the chord length distribution (CLD) was employed to determine the specific surface area and the pore size at different adsorption steps. The SANS results were in good agreement with the quenched solid density functional theory (QSDFT) analysis of the nitrogen physisorption. By the comparison of both characterization methods the pore shape could be determined. The combination of both SANS and gas physisorption is thus shown to provide a comprehensive characterization of the pore structure of the carbon monoliths throughout the entire pertinent length scale.     

芳烃与烯烃烷基化反应的催化机理进展

芳烃与烯烃的反应在有机合成中应用广泛。随着环保要求的提高,用于芳烃烷基化反应的传统催化剂逐渐被新型绿色催化剂所替代。近年来研究发现离子液体和分子筛对该反应具有高效催化作用且环境友好。本文探讨了离子液体和分子筛的酸性,总结了相应的的催化机理,对有关实验和理论研究工作进行了分析。同时揭示了离子液体和分子筛的结构对其催化性能的影响,为烷基化反应进一步研究奠定了基础。分析表明离子液体既能作为B酸,也能作为L酸起催化作用;分子筛主要作为B酸起催化作用,同时其催化性能与孔道结构、孔径大小及反应物尺寸密切相关。离子液体的稳定性较差、成本较高,而分子筛失活较快,未来需围绕提高离子液体稳定性、改进其制备方法以降低成本及改善分子筛结构以延长使用周期等方面展开研究。     

Ion transport properties of lithium ionic liquids and their ion gels

A new series of lithium ionic liquids were prepared by introducing of two electron-withdrawing trifluoroacetyl groups in borate salts containing two methoxy-oligo(ethylene oxide) groups in the structures. Successive substitution reactions of oligo-ethylene glycol monomethyl ether and trifluroacetic acid from LiBH₄ yielded the lithium salts, which were clear and colorless liquids at room temperature. The fundamental physicochemical properties, such as density, thermal property, viscosity, ionic conductivity, self-diffusion coefficients, and electrochemical stability, were measured. The lithium ionic liquids had self-dissociation ability and conducted ions even in the absence of organic solvents. New polymer electrolytes, named ‘ion gels’, were prepared by radical cross-linking reactions of a poly(ethylene oxide-co-propylene oxide)tri-acrylate macromonomer in the presence the lithium ionic liquid. An increase in the glass transition temperatures (T_g) of the ion gels was very small even with increasing lithium ionic liquid concentration, and the T_g's were lower than that of the ionic liquid itself. The ionic conductivity of the ion gels surpassed that of the lithium ionic liquid in the bulk at certain compositions.     

Stability and recycling of polymer-supported Mo(VI) alkene epoxidation catalysts

As a prelude to commissioning a reactive distillation column (RDC) for the continuous epoxidation of cyclohexene using t-butylhydroperoxide and a heterogeneous polymer-supported Mo(VI) catalyst, the long-term stability of three candidate polymer-supported Mo catalysts has been evaluated. The polymer catalysts are a polybenzimidazole-supported species, PBI.Mo, and two poly(styrene-divinylbenzene) resin-based species, Ps.AMP.Mo1 and 2, prepared in-house by amination of vinyl benzyl chloride-containing resins using 2-aminomethyl pyridine, followed by loading with Mo. The stability of each polymer catalyst was assessed by recycling a sample 10 times in small batch reactions using conditions that will form the basis of the continuous process. At the same time the loss of Mo from each support has been investigated by isolating any residue from reaction supernatant solutions, following removal of the heterogeneous polymer catalyst, and then using the residues as potential catalysts in epoxidation reactions. This is a powerful technique for identifying unambiguously loss of catalytically active Mo from the support and is not dependent on Mo analytical procedures. All three polymer catalysts are highly active and selective in 10 consecutive reactions but the supernatant solutions also contain catalytically active Mo residues. In the case of PBI.Mo and Ps.AMP.Mo2 the contribution to catalysis from homogeneous Mo species lost from the supports is all but eliminated after aging through 10 cycles, but in the case of Ps.AMP.Mo1 the contribution from leached Mo species remains significant even after similar aging. The major factor determining this differential behaviour seems to be the mole ratio of polymer-bound ligand to Mo which must be significantly above unity to provide long-term retention of Mo.     

Porous carbon xerogels with texture tailored by pH control during sol-gel process

Despite commonly accepted ideas, evaporative drying does not always completely destroy the pore texture of phenolic gel. This work shows that very porous carbon materials can be synthesized by evaporative drying and pyrolysis of aqueous resorcinol-formaldehyde gels provided that the operating variables are correctly chosen. Moreover, in this manner monoliths can be easily produced. The pore texture of the materials was studied before and after pyrolysis in order to determine which synthesis and/or pyrolysis variables have an influence on the final texture of the carbon. Results show that it is possible to tailor the morphology of these materials by varying the initial pH of the precursors solution in a narrow range. Micro-macroporous, micro-mesoporous, microporous or totally non-porous carbon materials were obtained. The specific surface area is independent from the initial pH whereas the total void volume varies from 0.4 to 1.4 cm³/g when the initial pH decreases from 6.25 to 5.45. These materials can be used as catalysts supports or for electrochemical applications, the texture control being an interesting advantage.     

Triethylsulfonium Bistriflimide as the Reaction Medium in Catalyzed and Uncatalyzed Cycloaddition [4 + 2]

Triethylsulfonium bistriflimide, [S_2.2.2][NTf₂], has been tested and compared with other ionic liquids and molecular solvents as a medium in Diels–Alder reaction between cyclopentadiene and dimethyl maleate. Triflates and chlorides of different metals have been combined with [S_2.2.2][NTf₂] and the catalytic activity of the systems formed have been determined. The effect of concentration of the catalysts in sulfonium ionic liquid and reactants on the yield and endo:exo ratio has been established. The representative catalyst—Yb(OTf)₃·xH₂O in [S_2.2.2][NTf₂] has been examined in the reaction of cyclopentadiene with various dienophiles. The use of sulfonium ionic liquids permitted recycling the catalysts. For the best four catalytic systems, the products have been isolated.     

Foam processing of poly(ethylene-co-vinyl acetate) rubber using supercritical carbon dioxide

Soft rubber foams like poly(ethylene-co-vinyl acetate) (EVA) are industrially applied in a broad range of products, including sports gear, insulation materials and drug delivery systems. In contrast to glassy polymers, few studies in literature concern the foaming of soft rubbers using supercritical carbon dioxide. In this study, open microporous matrices of EVA have been formed with CO₂. Prior to the foam expansion, sorption and swelling isotherms of CO₂ in EVA have been measured and the obtained isotherms have been correlated using the Sanchez-Lacombe equation of state. Additionally, a pressure-independent diffusion coefficient of CO₂ in EVA has been obtained from these experiments. The microporous foams have been formed by a pressure quench of the CO₂-swollen polymer matrix. Sorption pressure as well as temperature and decompression times appear to determine the pore size and bulk density of the foam. These parameters allow for a control of the foam structure of EVA rubbers.     

Synthesis and characterization of two ionic liquids with emphasis on their chemical stability towards metallic lithium

Two room temperature ionic liquids (RTILs) without acidic protons, based on different cationic species (1-n-butyl-2,3-dimethylimidazolium) (BMMI) and N-n-butyl-N-methylpiperidinium (BMP) using (CF₃SO₂)₂N⁻ (TFSI) as anion, were prepared by quaternization of their respective amines with an appropriate alkyl halide, followed by ion exchange reaction. All relevant properties of these ionic liquids, such as, thermal stability, density, viscosity, electrochemical behavior, ionic conductivity and self-diffusion coefficients for both ionic species, were determined at different temperatures. In spite of their ionic conductivity being lower than 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonylimide) (BMITFSI), the absence of an acidic proton in both compounds is crucial to maintain their chemical stability towards metallic lithium and, thereby, to make possible the safe assembly of lithium ion batteries. Both ionic liquids without acidic protons do not react with metallic lithium; on the other hand, the formation of carbene species when BMITFSI was exposed to Li was confirmed by ¹H and ¹³C nuclear magnetic resonance (NMR) and gas chromatography-mass spectrometry (GC-MS).     

Preparation of Carbon Nanotubes (CNTs)-Cordierite Monoliths by Catalytic Chemical Vapor Deposition as Catalyst Supports for Ammonia Synthesis

Tunable carbon nanotubles (CNTs)-coated monoliths as catalyst supports were prepared by catalytic chemical vapor deposition (CCVD) over deposited cobalt on cordierite. The influence of the preparation conditions such as the cobalt nitrate loading on the cordierite monoliths, the flow rate of reaction gases, reaction time and temperature on CNTs yield, thermal properties and structural features of the resulting materials were studied. The CNTs-cordierite was characterized by TEM/HREM, SEM, N₂ physisorption and TGA. The SEM showed that a relatively homogeneous mesoporous layer of CNTs covered on the surface of the cordierite monoliths. Comparing with the bare cordierite, the BET surface area and pore volume of CNTs-cordierite increased significantly. CNTs have penetrated into the cordierite substrate and led to a remarkable mechanical stability of the CNTs-cordierite monoliths against ultrasound maltreatment. The CNTs content, BET surface area, pore volume and thermal properties of CNTs-cordierite monoliths all could be changed by the variation of the synthesis conditions. Barium promoted ruthenium catalysts supported on the as-synthesized materials showed much higher activity for ammonia synthesis than their counterparts deposited on bare cordierite monoliths. Furthermore, the catalytic activity linearly increased with the BET surface area of CNTs-cordierite monoliths. The CNTs-cordierite monoliths were proved to be promising candidates as catalyst supports and the performance of catalysts supported on as-prepared materials would be easily modified by changing the growth conditions of CNTs.     

离子液体在Knoevenagel缩合反应工艺中的研究与应用进展

Knoevenagel缩合反应被广泛应用于合成香料、化工原料及药物中间体。离子液体作为反应溶剂,能促使一系列醛、酮与活泼亚甲基化合物的Knoevenagel缩合反应顺利进行。功能化离子液体作为催化剂成功催化Knoevenagel缩合反应,避免了使用额外催化剂。离子液体被认为是环境友好的"绿色"溶剂和"可设计"溶剂,为Knoevenagel缩合反应提供了高效、环保的制备途径。该文综述了离子液体在Knoevenagel缩合反应中的应用进展。     

Fabrication of activated carbons with well-defined macropores derived from sulfonated poly(divinylbenzene) networks

Poly(divinylbenzene) (PDVB) monoliths with well-defined macropores that have been sulfonated and carbonized to obtain macroporous carbon monoliths. The original macroporous PDVB networks have been synthesized by living radical polymerization accompanied by spinodal decomposition. Sulfonation prevents polymer networks from large shrinkage and weight loss during carbonization by heat-treatment in an inert atmosphere. In the case of PDVB gels sulfonated at 120 °C using conc. H₂SO₄, mesopores in the original skeletons as well as macropores are retained after carbonization. The obtained carbon monoliths are subsequently activated by CO₂, which resulted in activated carbons. The specific surface area of the obtained activated carbons reaches up to 2360 m² g⁻¹.     

Ring-opening polymerization of γ-benzyl-l-glutamate-N-carboxyanhydride in ionic liquids

This work deals with ring-opening polymerization of a representative N-carboxy-α-amino acid anhydride (NCA) in ionic liquids. The polymerization of γ-benzyl-l-glutamate-N-carboxyanhydride (BLG-NCA) with n-butylamine as an initiator in an ionic liquid ([BMI][PF₆]) proceeded as a milky white dispersion with no evidence of macroscopic precipitation. The polymerization with the primary amine under suitable conditions afforded poly(amino acid) having narrow polydispersity, molecular weights close to the theoretical values, and helical secondary structure. The polymerization rate was slightly affected by the nature of the anion and hydrophobicity of the ionic liquids, while poly(BLG)s having low polydispersities were obtained regardless of the kind of the ionic liquids. Several parameters, such as the existence of organic solvent as a co-solvent and monomer concentration, had also clear effects on the polymerization rate and/or the polydispersity of the resulting poly(BLG)s. The possible interactions between the ionic liquid and NCA monomer or the ionic liquid and the initiator were characterized using FT-IR, ¹H and ¹³C NMR measurements. The character of this polymerization process was also studied by performing kinetic investigations. We believe that this represents the first report on amine-initiated ring-opening polymerization of NCA in ionic liquid.     

Silica-Supported Ionic Liquids Prompted One-Pot Four-Component Synthesis of Pyrazolopyranopyrimidines, 3-methyl-4-aryl-4,5-dihydro-1H-pyrano[2,3-c]pyrazol-6-ones,and 1,6-diamino-2-oxo-1,2,3,4-tetrahydropyridine-3,5-dicarbonitriles

Three methods have been used for synthesis of pyrazolopyranopyrimidines, 3-methyl-4-aryl-4,5-dihydro-1H-pyrano[2,3-c]pyrazol-6-ones, and 1,6-diamino-2-oxo-1,2,3,4-tetrahydropyridine-3,5-dicarbonitrile derivatives via four-component reactions under solvent-free conditions. Silica-bonded 5-n-propyl-octahydro-pyrimido[1,2-a]azepiniumchloride (SB-DBU⁺Cl^?), silica-bonded n-propyl-4-aza-1-azoniabicyclo[2.2.2]octane chloride (SB-DABCO⁺Cl^?), and nano silica-bonded 5-n-propyl-octahydro-pyrimido[1,2-a]azepinium chloride (NSB-DBU⁺Cl^?) as silica-supported ionic liquids catalyzed these reactions. These green, heterogeneous catalysts are separated from the reactions by simple filtration. The catalysts were recovered for at least four times without significant loss of their catalytic activities.     

Carbon-based monolithic supports for palladium catalysts: The role of the porosity in the gas-phase total combustion of m-xylene

Three different carbon-based monoliths have been studied in their performance as Pd catalyst supports in the total gas-phase combustion of m-xylene at low temperatures. The first monolithic support (HPM) was a classical square channel cordierite modified with -Al₂O₃, blocking the macroporosity of the cordierite and rounding the channel cross-section, on which a carbon layer was applied by carbonization of a polyfurfuryl alcohol coating obtained by dipcoating. The other two monolithic supports were composite carbon/ceramic monoliths (MeadWestvaco Corporation, USA), microporous (WA) and a mesoporous (WB) sample.

All the catalysts have a comparable total Pd loading and very similar Pd particle size (around 5–6 nm). In sample Pd/WA the Pd is situated only in the macropores, while in the case of Pd/WB the Pd is distributed throughout the mesoporous texture. In the case of Pd/HPM, Pd particles are clearly situated at the external surface of the carbon layer.

The catalytic activities of the samples were very different, decreasing in the order: Pd/WB > Pd/WA > Pd/HPM. These results show that the carbon external surface area, the macropores and mainly mesopores, play an important role in this kind of gas-phase reactions, improving the contact between the Pd particles and the m-xylene molecules. The catalytic activity of the Pd supported on carbon-based monoliths correlates with the surface area developed in macro- and mesopores of the monolithic support.     

An activation-free method for preparing microporous carbon by the pyrolysis of poly(vinylidene fluoride)

A simple method for the preparation of microporous carbon was presented by pyrolyzing poly(vinylidene fluoride) (PVDF) at high temperature under N₂ atmosphere without activation or any other additional processes. The yield of PVDF-derived carbon is 35.0%. Its specific surface area reaches 1012 m² g with a pore volume of 0.41 cm³ g⁻¹. The carbon is microporous with unimodal pore size distribution at 0.55 nm.     

Immobilisation of chloroaluminate ionic liquids on silica materials

Two different possibilities for the preparation of immobilised ionic liquids are presented. Lewis acidic ionic liquids which have been shown to be highly active catalysts in alkylation reactions of the Friedel–Crafts type were supported on amorphous silica and MCM 41 materials. We describe the different methods used, as well as analytical data and some catalytical results achieved by these catalysts.     

Synthesizing cyclic carbonates from olefin oxides and carbon dioxide. I: Catalysis with ionic liquids

A review of catalytic systems based on ionic liquids used for promoting reactions between olefin oxides and carbon dioxide is presented. It is shown that ionic liquids based on imidazoles, pyridines, quaternary ammonium salts, and other compounds exhibit high activity in these processes and can serve as alternatives to conventional catalysts of cyclocarboxylation of olefin oxides under certain conditions. Specific features of the reactions between oxiranes and carbon dioxide in the presence of ionic liquids are considered, including reactions with the use of promoting additions based on metal salts. Mechanisms of the catalytic action of ionic liquids in syntheses of cyclocarbonates are described. Other works are critically analyzed to develop new lines in the catalysis of cyclocarboxylations of olefin oxides with ionic liquids.     

Physical adsorption and heat of immersion studies of microporous carbons

Physisorption and heat of immersion measurements have been made with two microporous carbons—a sample of charcoal cloth (BET area, 1250 m² g^?1; pore volume, 0.59 cm³ g^?1 having predominantly narrow micropores and a sample of Amoco carbon (BET area, 3700 m² g^?1; pore volume, 1.9 cm³ g^?1), having a wider distribution of micropores extending into the supermicropore range (pore width, ca. 1–2 nm). The adsorption isotherms of nitrogen and toluene are of Type 1 and reveal that the external surface of both samples is very small. The heat of immersion measurements were carried out with the aid of a Tian-Calvet microcalorimeter and with the following liquids: n-hexane, cyclohexane, neo-hexane, toluene, mesitylene and isodurene. A sample of non-porous graphitized carbon black (Vulcan 3G: BET area, 71.1 m² g^?1) was studied. Immersion calorimetry shows that in spite of the larger adsorptive capacity of the Amoco carbon, the sample of charcoal cloth has a higher adsorption affinity. The differences in the corresponding heats of immersion and in the adsorptive properties indicate that many of the pores in charcoal cloth are slit-shaped, whereas those in the Amoco carbon appear to be more cylindrical in shape.