High Substrate Conversion Research Articles

Catalytic oxidation of alcohols with polymer-supported ruthenium complex under mild conditions

Polymer-supported ruthenium-containing complex PS–Phen–Ru was synthesized (where PS = chloromethyl polystyrene resin, Phen = 1,10-phenanthroline) and was characterized by FT-IR, ICP, and XPS. The supported complex was used to catalyze the oxidation of primary aliphatic alcohols as well as aromatic alcohols in the presence of iodosylbenzene. The oxidations were carried out in acetonitrile solution, affording the corresponding aldehydes or ketones in high substrate conversion and high selectivities under mild reaction conditions. The catalyst can be easily prepared and can be recycled.

Optimal design of continuously stirred membrane reactors in series using Michaelis-Menten kinetics with competitive product inhibition: theoretical analysis

Analytical expressions were derived for the optimal design (based on the minimum of the volume of the total number of reactors) of N continuously stirred membrane reactors (CSMRs) performing the enzyme-catalyzed reaction described by Michaelis-Menten kinetics with competitive product inhibition. The influence of membrane selectivity for both substrate and product on the total dimensionless residence time of the reactors (overall volume) was determined. The optimal design of N CSMRs (variable volume reactors) was compared with equal volume membrane reactors required to achieve the same degree of substrate conversion. The effect of kinetic and operating parameters on the performance of membrane reactors was determined. Optimization results show that membrane reactors are superior to continuously stirred tank reactors (CSTRs) in series at a high substrate rejection coefficient and low product rejection coefficient, high substrate conversion and using a small number of reactors. Also a high dimensionless Michaelis-Menten constant, high dimensionless inhibition constant and low substrate concentration in the feed to the first reactor improved the performance of the membrane reactors vs. CSTRs in series. The reduction in total volume of the optimal membrane reactors compared to CSTRs in series was up to 86% for the conditions in this work. A comparison between the optimum and equal volume design of membrane reactors in series showed no major difference in total volume between the two design criteria at a practical range of operating conditions. A volume reduction up to 16% was observed for the conditions in this work.

Highly efficient Heck olefin arylation in the presence of iminophosphine–palladium(0) complexes

The Heck coupling of aryl bromides with olefins such as styrene or butyl acrylate is efficiently catalysed by the iminophosphine–palladium(0) complex [Pd(dmfu)(P-N)] (dmfu = dimethyl fumarate; P-N = 2-(PPh 2)C 6H 4-1-CH NC 6H 4OMe-4) ( 1) in polar solvents. With activated aryl bromides such as 4-bromoacetophenone turnover numbers of up to 20,000 can be achieved at 140 °C in 2 h. The presence of electron-donating groups leads to decreased reaction rates, nevertheless, high substrate conversions can be obtained in reasonable reaction times. Kinetic studies indicate that complex 1 is only a precursor of the actual catalytic species. Experiments aimed to demonstrate the intervention of metallic palladium did not lead to conclusive findings.

Analysis of acetic acid productivity in a continuous two‐stage bioreactor with cell recycling

AbstractThe performance of a two‐stage system with cell recycling and fresh feed at each stage is studied numerically for continuous acetic acid production. In this system, both filtrate and bleed broth from the first stage are supplied to the second fermenter. At high substrate conversions, this configuration is found to provide higher acetic acid productivities than either a previous configuration where only the first‐stage bleed broth is fed to the second stage or a single recycle chemostat at the same bleed ratios. Copyright © 2005 Society of Chemical Industry

Comparison of Lactic Acid Productivities at High Substrate Conversions in a Continuous Two-stage Fermenter With Cell Recycle Using Different Kinetic Models

The steady-state performance of a two-stage recycle fermenter with two different types (I and II) of configurations for staging was investigated numerically for the continuous production of lactic acid. In Type I the bleed broth withdrawn from the first stage is supplied to the second fermenter, and in Type II both filtrate and bleed broth from the first stage are introduced to the second stage. Using four different kinetic models taken from the literature, the effects of operating parameters on the overall lactic acid productivities and the overall substrate conversions are examined. At moderate conversions, productivities for Type I operation are found to be higher than those for Type II and for the single-stage system. In the case of high conversions close to complete consumption, Type II operation is more efficient than the Type I and single-stage systems. For 99% conversion and 40 kg m−3 substrate feed concentration, productivities for Type II are predicted to be 14–74% higher compared to those for the single-stage system at the same bleed ratios.

The effect of vanadium sources on the synthesis and catalytic activity of VMCM-41

Mesoporous VMCM-41 was synthesized hydrothermally using various vanadium sources, viz, tetravalent vanadium such as vanadyl sulfate and vanadyl acetylacetonate, as well as pentavalent vanadium like sodium vanadate and ammonium vanadate. The influence of different vanadium sources on the framework substitution of vanadium, as well as their catalytic activity for the oxidation of cyclohexane, was investigated. Among the different vanadium stocks, the tetravalent vanadium sources showed maximum vanadium incorporation in the silicate framework of MCM-41. As a consequence, these catalysts gave much higher substrate conversion and excellent product selectivity. On the other hand, the catalysts prepared from pentavalent vanadium sources showed lower activity owing to the smaller amounts of vanadium in the matrix. Although the activity of the catalyst slightly decreased after first recycle as due to leaching of small amounts of active vanadium species, it however remained nearly the same even after several recycles. This was further confirmed by washing experiments wherein non-framework vanadium ions were removed upon ammonium acetate treatment; the washed catalysts showed a similar activity to those of the recycled catalysts. Thus, recycled/washed VMCM-41 behaves truly as heterogeneous catalyst. Furthermore, the influence of pore size of the catalyst was tested for the oxidation of bulkier substrate, viz, cyclododecane.

Mesoporous VMCM-41: highly efficient and remarkable catalyst for selective oxidation of cyclohexane to cyclohexanol

Liquid phase oxidation of cyclohexane was carried out under milder reaction conditions over mesoporous VMCM-41 molecular sieve catalysts using aqueous hydrogen peroxide as oxidant, acetic acid as solvent, and methyl ethyl ketone as initiator. The catalysts showed high substrate conversion and excellent product (cyclohexanol) selectivity. Although the activity of the catalyst slightly decreased after first recycle, owing to leaching of small amount of non-framework vanadium ions, it, however, remained nearly same thereafter. This observation was further confirmed by washing experiments where the non-framework vanadium ions were removed upon ammonium acetate treatment. Further, the washed catalyst also showed an activity similar to that of the recycled catalyst. Thus, the recycled or washed VMCM-41 behaves truly as heterogeneous catalyst. This observation was complemented and confirmed by both filtrate and quenching studies. The effects of reaction time, temperature, Si/V molar ratio, and catalyst concentration on the catalyst performance were examined in order to optimize the conversion of cyclohexane and selectivity of cyclohexanol. However, the use of strong oxidizing agent, e.g., tertiary butyl hydroperoxide, resulted in the formation of cyclohexanone as the major product. In addition, the use of solvents like methanol, dioxan and acetone showed lower activity.

Catalytic activity of highly ordered mesoporous VMCM-48

Mesoporous VMCM-48 molecular sieves with Si/V (molar) ratios of 25–200 were synthesized hydrothermally and systematically characterized by various analytical and spectroscopic techniques, viz., XRD, N 2 sorption, TG-DTA, TEM, EPR, 51V MAS-NMR, FT-IR, and DRUV-VIS. XRD studies suggest that the substitution of vanadium occurs in the silicate framework structure of MCM-48. TEM and ED investigations confirm the highly ordered cubic structure of VMCM-48. EPR and 51V NMR studies indicate the presence of pentavalent vanadium ions in tetrahedral framework positions, while DRUV-VIS spectra show their existence in two different environments. The catalytic activity of these well-characterized (both in the calcined and the washed forms) materials was evaluated for cyclohexane oxidation under mild reaction conditions. All these catalysts gave high substrate conversion and excellent product (cyclohexanol) selectivity. Furthermore, unlike many other vanadium-based heterogeneous catalysts reported, the mesoporous VMCM-48 catalysts show minimal leaching of the active vanadium species. This was confirmed by washing, recycling and quenching experiments where only a small amount of vanadium ions leaches out in the case of calcined samples, while vanadium was not detected for the washed ones. Finally, the catalytic activity of VMCM-48 was also compared with mesoporous VMCM-41 as well as microporous VS-1 catalyst. The results indicate that the former showed superior activity to those of the latter two, such superiority could, however, be directly related to the amount of vanadium incorporated in MCM-48 being larger than the amounts in MCM-41 and MFI structures.

Tertiary butylation of phenol over mesoporous MeMCM-48 and MeMCM-41 (Me = Ga, Fe, Al or B) solid acid catalysts

Mesoporous Na-GaMCM-48 molecular sieves having a silicon-to-gallium (molar) ratio of 30–90 were synthesized hydrothermally, and the structure, coordination geometry of the gallium, acidic properties, and catalytic activity were systematically investigated using a number of analytical and spectroscopic techniques. XRD, TEM and N 2 sorption investigations indicate an MCM-48 structure with highly ordered mesoporosity. 71Ga MAS-NMR studies reveal that gallium substitutes isomorphously in the mesoporous silicate framework of MCM-48. The NH 3-TPD profiles suggest the presence of high concentrations of moderate-to-strong Brønsted acid sites in H-GaMCM-48. The catalytic performance of this protonated catalyst was evaluated for the t-butylation of phenol. The results indicate that the H-GaMCM-48 catalysts are highly active for the chosen reaction, and show much higher substrate conversion than many other catalyst systems. However, compared to the analogous H-GaMCM-41, the H-GaMCM-48 shows a slight decrease in p- t-butyl phenol selectivity owing to the formation of 2,4-di- t-butyl phenol. On the other hand, the deactivation is very minimal on account of the three-dimensional pore system of the MCM-48 structure compared to the one-dimensional pore opening of the MCM-41 structure.

Mesoporous H-GaMCM-48: A remarkable solid acid catalyst for tertiary butylation of phenol

A mesoporous gallosilicate (GaMCM-48) molecular sieve having a silicon-to-gallium (molar) ratio of 60 was synthesized hydrothermally, and the structure, coordination geometry of gallium, acidic properties, and catalytic activity were investigated systematically employing various analytical and spectroscopic techniques. XRD, TEM, and N 2 sorption investigations show a MCM-48 structure with highly ordered (meso)porosity. 71Ga MAS-NMR studies reveal that gallium substitutes isomorphously in the mesoporous silicate framework of MCM-48. Further, NH 3-TPD profiles suggest the presence of a high concentration of moderate-to-strong Brønsted acid sites in H-GaMCM-48. The catalytic performance of this protonated catalyst was evaluated for the t-butylation of phenol reaction. The results indicate that the H-GaMCM-48 catalyst is highly active for the chosen reaction and shows a much higher substrate conversion than many other catalyst systems. However, compared to the analogous H-GaMCM-41, the H-GaMCM-48 shows only a slight decrease in p- t-butyl phenol selectivity owing to the formation of 2,4-di- t-butyl phenol. On the other hand, the deactivation is very minimal on account of the three-dimensional pore system of MCM-48 structure compared to the one-dimensional pore opening of the MCM-41 structure.

Assessment of the suitability of imidazolium ionic liquids as reaction medium for base-catalysed reactions Case of Knoevenagel and Claisen–Schmidt reactions

Abstract For the Knoevenagel condensation of benzaldehyde and malononitrile in 1-butyl-3-methylimidazolium hexafluorophosphate [(bmim)PF6] to which alkali metal hydroxide in ethanol had been previously added, the product is formed with high selectivity and the ionic liquid could be reused for at least five runs without the need of additional base. Low to moderate mass balances were observed in the first runs until the ionic liquid becomes saturated and the mass balances become almost complete. For the Claisen–Schmidt condensation of acetophenone with benzaldehyde, besides the expected conjugated ketone formed with high selectivity, minor amounts of ethyl benzoate are also formed. Reuse of the ionic liquid require additional amounts of base. Acid–base titration shows that most of the base (up to 80%) may disappear by reaction with the ionic liquid in the absence of any reagent. Therefore, addition of extra amounts of base is necessary in order to achieve high substrate conversion in subsequent runs and the reactions become not catalytic. This raises the point that imidazolium ionic liquids are only suitable for a small range of base catalysed reactions, and that imidazolium liquids substituted at the 2-position will be more suited as medium for based-catalysed reactions.

Lipase-catalyzed resolution of 4-aryl-substituted β-lactams: effect of substitution on the 4-aryl ring

Pseudomonas cepacia lipase (PS-30) was used in hydrolytic resolution of 3-acetoxy-4-aryl-substituted azetidin-2-ones (>97% ee). Twenty-three β-lactam substrates with varied substituents at the C-4 center of the ring were synthesized and subjected to lipase-PS catalyzed hydrolysis in phosphate buffer (pH 7.2, 0.2 M) at 25°C. The reactions occurred with high enantioselectivity and substrate conversion. The effect of substitution on the C-4 aryl ring on lipase hydrolytic activity was dependent upon the steric and electronic nature of the substituent and its position on the aryl ring. The stereopreference of the lipase PS-30 for the (3 S,4 R) enantiomer was rationalized using a known active site model. Absolute stereochemistry of the enantiomers was established using single crystal X-ray crystallographic techniques.

Acetohydroxyacid synthase: a new enzyme for chiral synthesis of R-phenylacetylcarbinol.

We have found that acetohydroxyacid synthase (AHAS) is an efficient catalyst for the enantiospecific (> or =98% enantiomeric excess) synthesis of (R)-phenylacetylcarbinol (R-PAC) from pyruvate and benzaldehyde, despite the fact that its normal physiological role is synthesis of (S)-acetohydroxyacids from pyruvate and a second ketoacid. (R)-phenylacetylcarbinol is the precursor of important drugs having alpha and beta adrenergic properties, such as L-ephedrine, pseudoephedrine, and norephedrin. It is currently produced by whole-cell fermentations, but the use of the isolated enzyme pyruvate decarboxylase (PDC) for this purpose is the subject of active research and development efforts. Some of the AHAS isozymes of Escherichia coli have important advantages compared to PDC, including negligible acetaldehyde formation and high conversion of substrates (both pyruvate and benzaldehyde) to PAC. Acetohydroxyacid synthase isozyme I is particularly efficient. The reaction is not limited to condensation of pyruvate with benzaldehyde and other aromatic aldehydes may be used.

Monoterpenes oxidation in the presence of Y zeolite-entrapped manganese(III) tetra(4-N-benzylpyridyl)porphyrin

Under homogeneous conditions and as a Y zeolite supported complex, manganese(III) tetra(4-N-benzylpyridyl)porphyrin (MnTBzPyP) is an active catalyst in the epoxidation of (R)-(+)-limonene and α-pinene, and in the hydroxylation of carvacrol and thymol, using H2O2/ammonium acetate, at room temperature and atmospheric pressure. A combined study on the characterization and catalytic performance of MnTBzPyP–NaY suggests that its preparation led to a successful inclusion of MnTBzPyP inside the zeolite during the hydrothermal synthesis. Unfortunately, irreversible catalyst deactivation occurs in the presence of H2O2. Changes in the crystalline structure of MnTBzPyP–NaY during the oxidative transformations are accompanied by leaching of the porphyrin complex to the reaction solution, accounting for the nearly complete loss of activity when the catalyst is re-used. Higher substrate conversions were achieved with the zeolite-impregnated complex. The iron analogue is inactive, under the applied reaction conditions.

Synthesis, characterization, and catalytic properties of chromium-containing hexagonal mesoporous aluminophosphate molecular sieves

Chromium-containing hexagonal mesoporous aluminophosphate molecular sieve materials were synthesized, characterized and evaluated for the oxidation of cyclohexane under mild conditions. The catalysts showed remarkable activity for the chosen reaction with very high substrate conversion and excellent product selectivity. In addition, recycling studies over the spent catalysts as well as studies on quenched solutions under the reaction conditions indicated no or very meager loss in activity; thus, the materials behaved truly as heterogeneous catalysts. This observation was well supported by ICP-AES analysis of the filtrates obtained from the washed catalysts where no leaching of active chromium species was detected. Furthermore, the reaction carried out using these filtrates confirmed the absence of active metal ions as it exhibited only a marginal activity.

Mesoporous (Cr)MCM-41: A Mild and Efficient Heterogeneous Catalyst for Selective Oxidation of Cyclohexane

Liquid-phase oxidation of cyclohexane was carried out at moderate reaction conditions over mesoporous (Cr)MCM-41 molecular sieves using acetic acid as solvent, hydrogen peroxide as oxidant, and methyl ethyl ketone as initiator. Under these conditions, the catalysts showed high substrate conversion and excellent product selectivity. Although the activity of the catalyst decreased after the first recycle, owing to leaching of nonframework chromium ions, it, however, remained nearly the same thereafter. This observation was further confirmed by washing experiments where the nonframework chromium ions were removed upon ammonium acetate treatment or under reaction conditions. Further, the washed catalyst also showed a activity similar to that of the recycled catalyst. The recycled or washed (Cr)MCM-41, therefore, behaves truly as a heterogeneous catalyst, and thus it forms the first example of a nonleaching chromium-based catalyst reported so far. The use of a stronger oxidizing agent, e.g., tertiary butyl hydroperoxide, resulted in the formation of cyclohexanone as the major product. On the other hand, the use of solvents like methanol, dioxan, and acetone showed lower conversion. Likewise, the use of initiators such as acetone, cyclohexanone, and acetaldehyde also resulted in lower activity.

Mesoporous (Cr)MCM-41: A Mild and Efficient Heterogeneous Catalyst for Selective Oxidation of Cyclohexane

Liquid-phase oxidation of cyclohexane was carried out at moderate reaction conditions over mesoporous (Cr)MCM-41 molecular sieves using acetic acid as solvent, hydrogen peroxide as oxidant, and methyl ethyl ketone as initiator. Under these conditions, the catalysts showed high substrate conversion and excellent product selectivity. Although the activity of the catalyst decreased after the first recycle, owing to leaching of nonframework chromium ions, it, however, remained nearly the same thereafter. This observation was further confirmed by washing experiments where the nonframework chromium ions were removed upon ammonium acetate treatment or under reaction conditions. Further, the washed catalyst also showed a activity similar to that of the recycled catalyst. The recycled or washed (Cr)MCM-41, therefore, behaves truly as a heterogeneous catalyst, and thus it forms the first example of a nonleaching chromium-based catalyst reported so far. The use of a stronger oxidizing agent, e.g., tertiary butyl hydroperoxide, resulted in the formation of cyclohexanone as the major product. On the other hand, the use of solvents like methanol, dioxan, and acetone showed lower conversion. Likewise, the use of initiators such as acetone, cyclohexanone, and acetaldehyde also resulted in lower activity.

Catalytic Oxidation of Alkylaromatics over Mesoporous (Cr)MCM-41

Liquid-phase catalytic oxidation of certain alkylaromatics over mesoporous (Cr)MCM-41 molecular sieves in the presence of chlorobenzene and tertiary butylhydroperoxide showed high substrate conversion and excellent product selectivity. The high catalytic activity of the materials is attributed to the complete oxidation of chromium ions to hexavalent chromium and the absence of pentavalent chromium in the mesoporous matrix.

Analysis of the performance of a two-stage fermentor with cell recycle for continuous ethanol production using different kinetic models

The performance of a two-stage fermentor with cell separators after each stage is studied numerically for continuous ethanol production. In order to reuse viable cells in the two-stage system, the bleed broth withdrawn from the first fermentor is introduced to the second one. Four different kinetic models with product and cell rate inhibitions are taken from the literature. Steady-state ethanol productivity at high substrate conversions is examined under various operating parameters. With the cell inhibition model, an increase in overall ethanol productivity is found not to be very significant. The other models with different types of product inhibitions gave overall higher ethanol productivities than in a single chemostat with recycle. Also, the total fermentor volume of the two-stage system was always found to be smaller than that of a single chemostat with recycle.

Esters and N, N-dialkylamides of 2-(trifluoromethyl)acrylic acid (TFMAA) through Pd-catalysed carbonylation of fluorinated unsaturated substrates

Esters and amides of 2-(trifluoromethyl)acrylic acid (TFMAA) have been synthesised by two different routes involving CO-chemistry. The alkoxycarbonylation of 2-bromo-3,3,3-trifluoropropene was carried out in the presence of PdCl 2(PPh 3) 2. High substrate conversions were obtained, but the yield in acrylic esters was generally low because the desired unsaturated esters reacted further adding a molecule of alcohol to the C–C double bond. The carbonylation of 2-bromo-3,3,3-trifluoropropene in the presence of secondary amines produced the corresponding unsaturated amides in high yields; the addition of the amine to the C–C double bond also occurred, but this side reaction was minimised by using secondary cyclic amines such as morpholine or piperidine. Alternatively, the acrylic esters can be obtained by hydromethoxycarbonylation of 3,3,3-trifluoropropyne using the catalytic system Pd(OCOCH 3) 2/2-pyridyldiphenylphosphine/CH 3SO 3H. In this process the most important side product is the isomeric crotonic ester. The regioselectivity of the reaction can be controlled to a great extent by a suitable choice of the solvent.