Kinetics of phenol hydrogenation over supported palladium catalyst

Kinetics of vapor phase hydrogenation of phenol to cyclohexanone over Pd/MgO system has been studied in a flow microreactor under normal atmospheric pressure. The reaction rate is found to be negative order with respect to the partial pressure of phenol and has increased from −0.5 to 0.5 with increasing temperature (473 to 563 K). The apparent activation energy (E_a) of the process is found to be close to 65 kJ per mol. On the basis of kinetic results a surface mechanism is proposed.     

An approximation algorithm for finding long paths in Hamiltonian graphs

In this paper we present a polynomial-time algorithm that finds paths of length Ω((logn/loglogn)²) in undirected Hamiltonian graphs, improving the previous best of Ω(logn).     

Impedance and dielectric studies of ferroelectric SrBi2Nb2O9 ceramics

This paper reports the dielectric and impedance characteristics of ferroelectric SrBi₂Nb₂O₉ (SBN) ceramics in the 100 Hz-1 MHz frequency range at various temperatures (300-823 K). A strong low frequency dielectric dispersion (LFDD) associated with an impedance relaxation has been found to exist in these ceramics in the temperature range 573-823 K. The Z″ of the AC complex impedance showed two distinct slopes in the frequency range 100 Hz-1 MHz suggesting the existence of two dispersion mechanisms. This non-ideal behavior has been explained on the basis of the expression, Z*=R₀/(1+(iω/ω₁)^m+(iω/ω₂)ⁿ) [J. Phys. Chem. Solids 53 (1992) 1] where ω₁ and ω₂ characterize the lattice response and the charge carrier behavior, respectively. The exponents m and n were obtained from the curve fitting. The exponent n was found to exhibit a minimum at the Curie temperature, T_c (723 K) whereas the m was temperature independent.     

Adsorption of Pb(2+) ions on nanosized gamma-Fe(2)O(3): formation of surface ternary complexes on ligand complexation

Adsorption of Pb²⁺ ions on the combustion derived nanosized γ-Fe₂O₃ and its thiourea complex composite is reported. The adsorbents upon adsorption of Pb²⁺ ions are characterised by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray microanalysis and infrared spectroscopy techniques. The eluent is characterised by atomic absorption spectroscopy for the estimation of Pb²⁺ ions. The reduction in the amount of lead after adsorption was estimated to be around 50% in case of complex composite adsorbent and around 15% in case of the γ-Fe₂O₃ adsorbents. Orthorhombic PbSO₄ precipitated out from the eluent and is reported with a model reaction. Adsorption of lead onto the complex composite is explained through the formation of a surface tertiary complex. The advantage of employing a thiourea-γ-Fe₂O₃ complex composite as solid adsorbent for the adsorption of heavy metal pollutants is envisaged in the present investigation.     

Studies on hydrogen ferrite phase in γ-Fe2O3 synthesised from ferrous fumarate half hydrate precursor

Thermal techniques viz., TG/DTA/DSC methods have been employed in understanding the formation and decomposition of hydrogen ferrite phase (H-F) as a metastable intermediate in the conversion of -Fe₂O₃·xH₂O, to -Fe₂O₃. Magnetic hysteresis measurements carried out at liquid N₂ and at room temperature for the vacancy ordered -Fe₂O₃ and its H-F phase also supplement these studies.The author thanks prof. M.R. Udpa, IIT, Madras for supplying TG/DTA data on Stanton Redcroft Instrument and to the Head, Department of Chemistry, S.P. University, Vallabh Vidyanagar for providing the facilities.     

Alkylation of catechol with methanol to guaiacol over sulphate-modified zirconia solid acid catalysts

A series of sulphate-promoted ZrO₂ solid acid catalysts with different contents of SO₄ ²⁻ were calcined at 450°C in air for 4 h and tested for the liquid-phase alkylation of catechol to guaiacol in a fixed-bed down-flow reactor. The 5 wt.% SO₄ ²⁻ on ZrO₂ showed the best conversion (82%) and selectivity for guaiacol (84%) at 200°C and 1 bar pressure. A smooth correlation was observed between the catalytical activity and surface acidity of sulphated zirconia. Based on our results, a surface mechanism is proposed.     

Synthesis of homoceramides, novel ceramide analogues, and their lack of effect on the growth of hippocampal neurons

The synthesis of a new series of D-erythro-homoceramide analogues is described. Several synthetic approaches were investigated. Homoceramides can be successfully synthesized from L-homoserine as chiral building block and a protected Weinreb-amide as a key intermediate. The synthesis of short-chain analogues with a heptyl side chain, as well as with a phenyl residue in the sphingoid part (instead of the naturally occurring tridecyl side chain), was effected. The homoceramides 15-17 and 24 were investigated for their potential to reverse the inhibitory effect of fumonisin B(1) on axonal growth. Unfortunately, none of the tested compounds showed any biological activity due to their lack of metabolism to glucosylhomoceramide.     

Raney nickel reductions—VIII A synthesis of 1:2-benzanthracene and its 3′-methyl and 4-methyl derivatives

1:2-Benzanthracene and the 3′-methyl derivative have been prepared by Raney nickel reduction of the sulphuric esters of the leuco derivatives of 1:2-benzanthraquinone and 4′-chloro-3′-methyl-1:2-benzanthraquinone, followed by dehydrogenation. 3-Hydroxy-1:2-benzanthraquinone was methylated in the 4-position by formaldehyde, sodium hydrosulphite and sodium hydroxide solution (the Marschalk reaction). Simultaneous reduction of the nuclear hydroxyl and quinone groups was effected by Raney nickel reduction of the trisulphuric ester of 3:9:10-trihydroxy-4-methyl-1:2-benzanthracene, and the resultant hexahydro-4-methyl-1:2-benzanthracene was dehydrogenated to 4-methyl-1:2-benzanthracene. The preparation of 3:4:9:10-dibenzopyrene from Mayvat brilliant red AF by Raney nickel reduction of the sulphuric ester of the leuco derivative and subsequent dehydrogenation is described.     

Specific determination of cysteine and penicillamine through cyclization to 2-thioxothiazolidine-4-carboxylic acids

A very simple and highly specific method for the determination of cysteine and penicillamine is presented. Treatment with 1,1'-thiocarbonyldiimidazole in slightly basic solutions converts cysteine rapidly and quantitatively to a very stable derivative, 2-thioxothiazolidine-4-carboxylic acid, which is not formed by thiols or amines. The cyclic derivative has a characteristic UV spectrum with a maximum at 272 nm and it can be quantified by one of two ways. (1) When only inorganic ions and common additives are present, a spectrophotometer or a plate reader capable of handling multiple samples is sufficient to estimate cysteine in the concentration range of 2-150 muM. Penicillamine is determined similarly by cyclization to 5,5-dimethyl-2-thioxothiazolidine-4-carboxylic acid. The method is also applicable to derivatives of cysteine modified only at the carboxyl group. (2) To determine cysteine in complex mixtures, a liquid chromatograph connected to a UV detector is used. The elution is rapid with well-separated peaks for the thiazolidine derivatives. The detection limit is 2 pmole of cysteine or penicillamine per injection and the detector response is linear up to 1 nmole. The usefulness of the method is demonstrated by determining cysteine and penicillamine in capsules and by measuring cysteine in a dietary supplement.     

Characterisation of selected hypnotic drugs and their metabolites using electrospray ionisation with ion trap mass spectrometry and with quadrupole time-of-flight mass spectrometry and their determination by liquid chromatography-electrospray ionisation-ion trap mass spectrometry

The electrospray ionisation-ion trap mass spectrometry (ESI-MSⁿ) of selected hypnotic drugs, i.e. zopiclone, zolpidem, flunitrazepam and their metabolites have been investigated. Sequential product ion fragmentation experiments (MSⁿ) have been performed in order to elucidate the degradation pathways for the [M+H]⁺ ions and their predominant fragment ions. These MSⁿ experiments show certain characteristic fragmentations in that functional groups are generally cleaved from the ring systems as neutral molecules such as H₂O, CO, CO₂, NO₂, amines and HF. When an aromatic entity is present in a drug molecule together with a nitrogen-containing saturated ring structure as with zopiclone and its N-desmethyl metabolite fragmentation initially occurs at the latter ring with the former being resistant to fragmentation. The structures of fragment ions proposed for ESI-MSⁿ can be supported by electrospray ionisation-quadrupole time-of-flight mass spectrometry (ESI-QTOF-MS).These molecules can be identified and determined in mixtures at low ng/ml concentrations by the application of liquid chromatography (LC)-ESI-MSⁿ which can be used for their analysis in saliva samples.This paper includes a tabulation of mass losses/signals at low m/z values for these hypnotic drugs and many others in recent publications which will be of value in the characterisation of drug metabolites of unknown structure and also natural product pharmaceuticals isolated from plants, etc.