Showing papers by "National Chemical Laboratory published in 1996"
TL;DR: Manipulation of the GI gene by site-directed mutagenesis holds promise that a GI suitable for biotechnological applications will be produced in the foreseeable future.
491 citations
300 citations
TL;DR: In this article, a polycrystalline nickel-zinc ferrite powder was obtained by chemical coprecipitation involving the corresponding metal nitrates and sodium hydroxide.
178 citations
TL;DR: It is concluded that either only one metal ion is needed for catalysis, or that a second metal-binding site can only exist in the presence of substrate and/or other domains of the protein.
173 citations
TL;DR: In this article, the authors show that the crystallization rate of zeolites and related molecular sieves can be accelerated by a factor of as much as about five by the addition of various 'promoter' ions, such as perchlorate, phosphate, nitrate, sulphate and carbonate.
Abstract: ZEOLITES and other microporous molecular sieves1–4 exhibit useful catalytic, absorption and ion-exchange properties. Most synthetic zeolites are made by hydrothermal methods, which generally suffer from the drawback of long crystallization times5. Here we show that the crystallization rate of zeolites and related molecular sieves can be accelerated by a factor of as much as about five by the addition of various 'promoter' ions, such as perchlorate, phosphate, nitrate, sulphate and carbonate. By using enhanced-sensitivity 29Si NMR studies to monitor soluble silicate species during crystallization, we show that the promoter ions enhance the rate of formation of the oligomeric (mainly tetrameric) silicate ions responsible for nucleation and growth of the crystals.
154 citations
TL;DR: In this article, the adsorption of methane, ethane, ethylene, and carbon dioxide on silicalite-l was investigated under similar conditions using a gravimetric adaption apparatus.
140 citations
TL;DR: In this paper, the selectivity and selectivity of the LaNiO3 perovskite has been investigated in the context of the synthesis of methane to syngas.
122 citations
TL;DR: In this paper, a NiO−CaO catalyst was used to transform methane to syngas with 100% selectivity for both CO and H2 under different temperature and space velocities.
Abstract: Steam reforming, CO2 reforming, and simultaneous steam and CO2 reforming of methane to CO and H2 over NiO−CaO catalyst (without any prereduction treatment) at different temperatures (700−850 °C) and space velocities (5000−70 000 cm3·g-1·h-1) are investigated. The catalyst is characterized by XRD, XPS, and temperature-programmed reduction (TPR). The catalyst showed high activity/selectivity in both the steam and CO2 reforming reactions and the simultaneous steam and CO2 reforming. In the CO2 reforming, the coke deposition on the catalyst is found to be very fast. However, when the CO2 reforming is carried out simultaneously with the steam reforming, the coke deposition on the catalyst is drastically reduced. By the simultaneous CO2 and steam reforming (at ≥800 °C and space velocity of about 20 000−30 000 cm3·g-1·h-1), methane can be converted almost completely to syngas with 100% selectivity for both CO and H2. The H2/CO ratio in products can be varied between 1.5 and 2.5 quite conveniently by manipulating...
113 citations
TL;DR: The UV absorption spectrum of the compounds released by enzyme treatment, and after alkali extraction, showed a characteristic peak at 280 nm, indicating the presence of lignin in the released coloring matter.
92 citations
TL;DR: Facile one to one transesterification of ketoesters catalysed by solid superacid (sulphated SnO2) is described.
91 citations
TL;DR: SAPO-11 is the best catalyst for the reaction among the investigated catalysts, ALPO 4 -11 on silica carrier, silynated zeolite and Silynated SAPO 11.
Abstract: SAPO-11 exhibited a high caprolactam yield for the vapour phase Beckmann rearrangement of cyclohexanone oxime to caprolactam. At 663 K, the oxime conversion is 99,9% with a caprolactam selectivity around 85.7%. The effect of silicon content of the SAPO-11 on the catalytic efficiency is compared with aluminosilicate having MFI structure. SAPO-11 is the best catalyst for the reaction among the investigated catalysts, ALPO 4 -11 on silica carrier, silynated zeolite and silynated SAPO-11. Acid sites with medium strength ( Si / Al > 10 000) are very active in the rearrangement reaction.
TL;DR: In this article, it was shown that acetone and acetonitrile probably coordinate strongly with Sn4+ centers preventing the solvolysis of the SiOSn units and the formation of peroxo intermediate.
Abstract: Tin-silicalite-1 (MFI) prepared by hydrothermal synthesis has been used as catalyst in the hydroxylation of phenol with aqueous H2O2. Isolated Sn4+ ions which are probably attached to the defect silanols are active in this reaction. At optimum conditions, a H2O2 efficiency of 70% and a ortho to para product ratio of 1.6 have been achieved. The solvent used has a strong influence on the activity. The UV-Vis spectral studies indicate that acetone and acetonitrile probably coordinate strongly with Sn4+ centers preventing the solvolysis of the SiOSn units and the formation of peroxo intermediate. Methanol probably causes the cleavage of SiOSn bond to form SiOH and SnOMe species. Water was found to be an efficient solvent. The formation of EPR active radical ion in presence of H2O2 and H2O and its attenuation in presence of acetone, acetonitrile and methanol support the above conclusion.
TL;DR: In this article, the titanium silicate molecular sieve TS-1 has been found to be an efficient catalyst for Baeyer-Villiger (BV) oxidation of cyclic and aromatic ketones under triphase conditions.
Abstract: Titanium silicate molecular sieve TS-1 has been found to be an efficient catalyst for Baeyer-Villiger (BV) oxidation of cyclic and aromatic ketones under triphase conditions (solid catalyst along with two immiscible liquid reactants in the absence of any cosolvent), using dilute H2O2. Reactions studied are the oxidation of cyclohexanone and acetophenone. Whereas in the case of cyclohexane,e-caprolactone, a BV product along with hydroxy and diketones is formed, in the case of acetophenone, phenyl acetate, a BV product along witho- andp-hydroxyacetophenone is formed. The acidic nature of titanium peroxo species, stabilised by water, is proposed to be responsible for such reactions under triphasic reaction conditions.
TL;DR: Titrimetric analysis, 51V liquid state NMR, and ESR data indicate that both V5+ (mostly HVO42-) and V4+ species (VO(OH)3-) are present in precursor gels used in the synthesis of V-MEL molecular sieves.
Abstract: Titrimetric analysis, 51V liquid state NMR, and ESR data indicate that both V5+ (mostly HVO42-) and V4+ species (VO(OH)3-) are present in precursor gels used in the synthesis of V-MEL molecular sieves. 29Si NMR studies of the gels indicate that the V5+ species interact with the silicate species. Two different V species have been identified in as-synthesized V-MEL samples. The concentration of the two species depends on the vanadium input and the synthesis atmosphere (air or He). The first one is incorporated in the lattice and is not extracted by a solution of 1 N NH4OAc after calcination. The second one is a loosely bound V species. The former species is V5+ in a tetrahedral (Td) environment (lattice), and the latter species is V4+ in an octahedral (Oh) environment. The V4+ species transforms into V5+ species (two types) with a symmetrical tetrahedral (Td) environment of the orthovanadate type during calcination in the presence of air. The first species (V5+ incorporated in the lattice) is insensitive to...
TL;DR: In this article, the kinetics of biphasic hydroformylation of 1-octene was studied in the presence of ethanol cosolvent, using water-soluble Rh/TPPTS as a catalyst.
Abstract: The kinetics of biphasic hydroformylation of 1-octene has been studied in the presence of ethanol cosolvent, using water-soluble Rh/TPPTS as a catalyst. The rate was found to be first order with catalyst and olefin concentrations and fractional order with hydrogen concentration. The rate versus CO concentration passed through a maximum, indicating a negative order dependence at higher pressures. These trends have been interpreted on the basis of a well-known hydroformylation mechanism. A rate equation derived assuming the addition of olefin to the active catalyst as a rate-determining step was found to fit the data satisfactorily. The kinetic parameters were evaluated for different temperatures.
Journal Article•
TL;DR: In this article, a quasi-steady flow is computed for any momentary impeller position, without requiring any empirical input/adjustable parameter, which can be used as a general purpose design tool for screening various mixer configurations and to evolve an optimum stirred vessel design.
Abstract: The ability to numerically simulate the flow in baffled, stirred vessels is fast becoming vital to their optimal design. Most of the past attempts have adopted a black box treatment to the impeller swept region, requiring experimentally-based input. More recent efforts are based on the computation of the full time varying flow field within and outside the impeller swept region. An intermediate approach has been developed in this paper, in which a quasi-steady flow is computed for any momentary impeller position. The method captures almost all the significant details of the flow both within and outside the impeller without requiring any empirical input/adjustable parameter. The method was applied to the flow generated by an axial impeller which is the most widely used impeller in the process industries. The case of a fully baffled vessel with standard pitched blade turbine was simulated using a FLUENT code. The time-averaged momentum transport equations were solved along with a turbulence model. The time derivative terms in the full transport equations were formulated in terms of spatial derivatives for the impeller swept region. The impeller rotation was simulated in terms of appropriate source terms at the blade surfaces. The model predictions were compared with the published experimental data obtained using the laser Doppler anemometer. It must be emphasized again that all the predictions were obtained by specifying just an impeller geometry, location and tip speed without requiring any boundary conditions near the impeller. The influence of impeller clearance on the generated flow was also correctly simulated. The approach can be used as a general purpose design tool for screening various mixer configurations and to evolve an optimum stirred vessel design.
TL;DR: In this article, an approach to introduce conformational constraint and chirality into the PNA backbone by bridging the ethylenediamine and glycine components of the same unit by a methylene group is described.
TL;DR: The influence of synthesis in acidic or alkaline media on the incorporation of vanadium in the MFI (ZSM-5) lattice has been studied in this paper, where it was found that the sample prepared from alkaline medium contains Si-OH groups from defect centers while such groups are absent in the sample obtained from acidic medium.
TL;DR: In this paper, the effect of O 2 and H 2 pretreatments on the initial activity and product selectivity and their variation with time on stream (0 − 180 min) of H-gallosilicate (H-GaMFI), H-GaAlMFI, and Ga/H-ZSM-5 catalysts (with bulk Si/(Al + Ga) = 34.5 ± 2.5) in the propane aromatization (at 600°C) has been investigated, using a microreactor operated in both the pulse and continuous reaction
TL;DR: In this paper, Mn-, Co-, or Fe-stabilized ZrO2 catalysts for low-temperature combustion of methane were shown to have activity comparable to that of noble metal catalysts.
Abstract: Activity comparable to that of noble metal catalysts is shown by the Mn-, Co-, or Fe-stabilized ZrO2 catalysts for the low-temperature combustion of methane. The incorporation of the transition metal into the ZrO2 bulk structure not only stabilizes the cubic (fluorite) modification, but also increases the reactivity of the lattice oxygen atoms drastically.
TL;DR: The results of the larvicidal activity indicate that the cuticle degrading enzyme complex of M. verrucaria has a good potential for the control of mosquitoes.
Abstract: Myrothecium verrucaria produced a high activity of extracellular insect cuticle degrading enzymes, chitinases, proteinases and lipases. Both first (I) instar and fourth (IV) instar larvae of a mosquito, Aedes aegypti, a vector of yellow fever and dengue, were susceptible to crude culture filtrate (100% mortality within 48 h at 170 mg/l) . The supplementation of purified M. verrucaria endo-chitinase with commercial lipase decreased the lethal time (LT50) from 48 h to 24 h for I instar and 120 h to 96 h for IV instar. Our results of the larvicidal activity indicate that the cuticle degrading enzyme complex of M. verrucaria has a good potential for the control of mosquitoes.
TL;DR: P pH-dependent UV difference spectra suggest that N3 protonation is not a requirement for triplex formation by sp-ODN and interstrand interaction of conjugated spermine more than compensates for loss in stability due to absence of a single Hoogsteen hydrogen bond.
Abstract: Oligodeoxynucleotide (ODN) directed triplex formation has therapeutic importance and depends on Hoogsteen hydrogen bonds between a duplex DNA and a third DNA strand. T*A:T triplets are formed at neutral pH and C+*G:C are favoured at acidic pH. It is demonstrated that spermine conjugation at N4 of 5-Me-dC in ODNs 1-5 (sp-ODNs) imparts zwitterionic character, thus reducing the net negative charge of ODNs 1-5. sp-ODNs form triplexes with complementary 24mer duplex 8:9 show foremost stability at neutral pH 7.3 and decrease in stability towards lower pH, unlike the normal ODNs where optimal stability is found at an acidic pH 5.5. At pH 7.3, control ODNs 6 and 7 carrying dC or 5-Me-dC, respectively, do not show any triple helix formation. The stability order of triplex containing 5-Me-dC-N4-(spermine) with normal and mismatched duplex was found to be X*G:C approximately X*A:T > X*C:G > X*T:A. The hysteresis curve of sp-ODN triplex 3*8:9 indicated a better association with complementary duplex 8:9 as compared to unmodified ODN 6 in triplex 6*8:9. pH-dependent UV difference spectra suggest that N3 protonation is not a requirement for triplex formation by sp-ODN and interstrand interaction of conjugated spermine more than compensates for loss in stability due to absence of a single Hoogsteen hydrogen bond. These results may have importance in designing oligonucleotides for antigene applications.
TL;DR: In this paper, the authors investigated the properties of three metallo-silicates, TS-1, VS-1 and Sn-silicalite-1 (MFI structure) using H2O2 as oxidant at 333-353 K.
Abstract: The oxidation of ethylbenzene has been carried out over TS-1, VS-1 and Sn-silicalite-1 (MFI structure) using H2O2 as oxidant at 333–353 K. The major products were 1-phenylethanol and acetophenone arising from the oxidation of the side chain. Aromatic ring hydroxylation leads to the formation of ortho-andpara-hydroxy ethylbenzene as a minor side reaction. The differences in the product selectivities could be explained on the basis of the reaction intermediates. Among the three metallo-silicates, Sn-silicalite-1 is found to be the most active with an H2O2 efficiency of about 60 mol.%. The influence of various reaction parameters on the conversion and selectivity of ethylbenzene over Sn-silicalite-1 is reported.
TL;DR: In this article, natural kaolinitic clay possessing transition metals such as Fe and Ti in its lattice has been found to catalyze efficiently the chemoselective acetalization and thioacetalization of variety of carbonyl compounds with ethane-1,2-diol and ethane -1, 2-dithiolrespectively.
TL;DR: In this paper, various dialkoxides of zinc, magnesium and boron were examined for the enantioselective addition of diethylzinc to aryl aldehydes.
Abstract: Various dialkoxides of zinc, magnesium and boron were examined for the enantioselective addition of diethylzinc to aryl aldehydes. Only zinc based reagent catalysed the reaction effectively providing up to 89% ee of the product.
TL;DR: In this paper, an efficient and convenient conversion of oximes to corresponding carbonyl compounds with sodium perborate in glacial acetic acid was reported, where the conversion was carried out in a single step.
TL;DR: In this article, natural kaolinitic clay possessing transition metals such as Fe, Ti in its lattice has been found to catalyze efficiently the protection of a variety of alcohols with 2,3-dihydro-4H-pyran (DHP) and hexamethyldisilazane (HMDS) at ambient conditions.
TL;DR: An efficient in vitro propagation protocol by organogenesis was developed for Withania somnifera (L.) Dunal and 1600 plantlets on an average can be produced from a single leaf.
TL;DR: In this paper, a review of halogenation reactions over zeolite catalysts is presented, where product selectivity depends on many reaction parameters like solvent, temperature and halogenating agent in addition to the composition and structure of the zeolites catalyst.
Abstract: Halogenation reactions over zeolite catalysts are reviewed. Product selectivity depends on many reaction parameters like solvent, temperature and halogenating agent in addition to the composition and structure of the zeolite catalyst. Even though, zeolites, in general, are unstable in the presence of the halogenoacids liberated during halogenation, recent developments, like the use of L-zeolite, may lead to the industrial use of zeolites in halogenation reactions.