Krishnapillai Sreekumar

Bio: Krishnapillai Sreekumar is an academic researcher from Cochin University of Science and Technology. The author has contributed to research in topics: Catalysis & Diffraction efficiency. The author has an hindex of 17, co-authored 97 publications receiving 853 citations. Previous affiliations of Krishnapillai Sreekumar include National Chemical Laboratory.

Ferrospinels based on Co and Ni prepared via a low temperature route as efficient catalysts for the selective synthesis of o-cresol and 2,6-xylenol from phenol and methanol

Abstract: The alkylation of phenol with methanol is conducted over Ni 1− x Co x Fe 2 O 4 ( x =0, 0.2, 0.5, 0.8 and 1.0) type systems prepared via low temperature route. Alkylation leads to predominantly ortho methylation of phenol, yielding o -cresol and 2,6-xylenol as the products. Under optimized conditions, the total ortho selectivity was ≥94%, regardless of the catalyst composition. Only traces of anisole is formed and hardly any other xylenol or cresol isomers are detected. Phenol conversion and the individual selectivities for o -cresol and 2,6-xylenol depend strongly on the catalyst composition. Selectivity for o -cresol was maximum for NiFe 2 O 4 (i.e., when x =0), whereas upon progressive substitution of Co 2+ ions for Ni 2+ ions, the 2,6-xylenol selectivity increases with a concomitant decrease in the o -cresol selectivity. Maximum phenol conversion and 2,6-xylenol selectivity (also total ortho selectivity) were observed over CoFe 2 O 4 ( x =1). The activity and selectivity were shown to be strongly dependent on the surface acid–base properties of the system. The influences of surface acidity, cation distribution in the spinel lattice and various reaction parameters are discussed.

Clay supported titanium catalyst for the solvent free synthesis of tetrasubstituted imidazoles and benzimidazoles

Abstract: A series of clay supported metal containing catalysts were prepared and their catalytic performance was evaluated in the synthesis of tetrasubstitued imidazoles under solvent free condition. It was found that K10 supported titanium catalyst showed higher activity compared to other catalysts. The catalysts were characterized by FTIR, XRD, TG/DTA, BET surface area and SEM. The general applicability of the method was demonstrated for the synthesis of tetra- substituted imidazoles from aldehydes and amines containing various electron donating and electron withdrawing substituents. The diversity of the catalyst was studied by synthesis of benzimidazoles and quinoxalines. The mechanism of formation of the products is explained in detail. The catalyst was found to be active for three cycles.

A comparison on the catalytic activity of Zn1−xCoxFe2O4 (x = 0, 0.2, 0.5, 0.8 and 1.0)-type ferrospinels prepared via. a low temperature route for the alkylation of aniline and phenol using methanol as the alkylating agent

Abstract: Depending on the variation of the Zn2+/Co2+ ratio in the Zn1−xCoxFe2O4 (x=0, 0.2, 0.5, 0.8 and 1.0)-type ferrospinels, the systems showed different activity trends for aniline and phenol methylation using methanol as the alkylating agent. An increase in Zn2+/Co2+ ratio increased the rate of N-monomethylation of aniline, whereas, a decrease in the ratio favored the rate of ortho methylation of phenol. An attempt has been made to interpret the observed trends based on the variation of surface acid–base properties of the catalyst surface with changes in the spinel composition. The efficiency of adsorption of aniline, phenol or methanol depends not only on the catalyst surface acid–base properties but also on the polarity of the adsorbing molecules. A controlled interplay of surface acid–base properties and polarity of the respective reacting molecules determines the efficiency of a particular reaction. In the case of aniline methylation, surface basicity plays a dominating role, whereas for phenol methylation surface acidity plays a dominating role.

A New Green and Efficient Brønsted: Lewis Acidic DES for Pyrrole Synthesis

Abstract: Deep eutectic solvents (DESs) are fluids composed of different Lewis or Bronsted acids and bases, generally acknowledged as new analogues to ionic liquids (ILs), because of their similar characteristics, but with more advantages related to preparation cost, environmental impact etc. Their preparation involve the simple mixing of two components generally with moderate heating that are inexpensive, non-toxic, biodegradable and the resulting mixture is capable to overcome the drawbacks of conventional organic solvents and ILs. Chemical reactions with these materials are significantly less hazardous and they can act as catalysts as well as reaction media. Here, three new DESs based on ZrOCl2·8H2O in combination with urea, ethylene glycol and glycerol are introduced. Physicochemical properties like phase behaviour, Freezing point, density, viscosity, thermal stability and miscibility properties in common solvents are determined. In addition, a new method for the determination of acidity of DESs having both Bronsted and Lewis sites is also introduced in this work. A convenient synthesis of pyrrole through Paal–Knorr reaction is reported using a variety of amines which are used to establish the importance of this catalyst in organic reactions. The products are analysed by GC–MS, 1H NMR and 13C NMR. By comparing the three DESs, DES 1 (formed from ZrOCl2·8H2O with urea) has the lowest density, viscosity, highest acidity and thermal stability. It was shown to be an excellent green catalyst for Paal–Knorr reaction. Reusability of the catalyst was also achieved up to 4 runs, without significant loss in its catalytic activity.

A comparative study on aniline alkylation activity using methanol and dimethyl carbonate as the alkylating agents over Zn-Co-Fe ternary spinel systems

Abstract: The catalyst compositions of the Zn1 x Cox Fe2O4 (xD 0, 0.2, 0.5, 0.8 and 1.0) spiel series possessing ‘ x’ values, x0.5, are unique for selective N-monomethylation of aniline using methanol as the alkylating agent. Since dimethyl carbonate (DMC) is another potential non-toxic alkylating agent, alkylation of aniline was investigated over various Zn‐Co ferrites using DMC as the alkylating agent. The merits and demerits of the two alkylating agents are compared. Catalytic activity followed a similar trend with respect to the composition of the ferrospinel systems. DMC is active at comparatively low temperature, where methanol shows only mild activity. However, on the selectivity basis, DMC as an alkylating agent could not compete with methanol, since the former gave appreciable amounts of N,N-dimethylaniline (NNDMA) even at low temperature where methanol gave nearly 99% N-methylaniline (NMA) selectivity. As in the case of methanol, DMC also did not give any C-alkylated products. © 2000 Elsevier Science B.V. All rights reserved.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

Abstract: Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Abstract: : The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg