Parasuraman Selvam

We report on the synthesis and the characterization of a novel cobalt trimesate metal-organic framework, designated as KCL-102. Powder X-ray diffraction pattern of KCL-102 is dominated by a reflection at 10.2◦ (d-spacing = 8.7 Å), while diffuse reflectance UV-Vis spectroscopy indicates that the divalent cobalt centers are in two different coordination geometries: tetrahedral and octahedral. Further, the material shows low stability in humid air, and it transforms into the well-known phase of hydrous cobalt trimesate, Co3 (BTC)2·12H2O. We associated this transition with the conversion of the tetrahedral cobalt to octahedral cobalt.

The conversion of solar energy into fuel has gained significant interest, particularly in photocatalytic water splitting, and the materials that efficiently generate hydrogen from water or aqueous solution using solar irradiation are highly desired for the hydrogen economy. Photocatalysts made of N-doped TiO2 are frequently utilized for breaking of water molecules in the process of generating hydrogen. To achieve this target, a unique defect-induced nitrogen-doped highly organized 2D-hexagonal periodic mesoporous titania, TiO2-xNy with a well-crystallized framework is synthesized in a reproducible way using structure-directing agents, e. g., F108, F127, P123, and CTAB. The nitrogen is incorporated into these samples through a facile method involving the calcination of templated materials in an air. A systematic characterization of the resulting ordered mesoporous titania employing a battery of experimental techniques indicates the presence of considerable amounts of intrinsic defects, viz., trapped electrons in oxygen vacancy and/or Ti3+ centres via nitrogen-doping in the titania matrix. These defects in turn promote the charge separation of photogenerated excitons, and therefore exhibit excellent photocatalytic activity for the hydrogen evolution reaction as compared to commercial titania such as Aeroxide®P-25. The superior activity of the N-doped mesoporous TiO2 is attributed to the synergistic effect of facile charge migration with high carrier density, unique phase composition (bronze and anatase), slow recombination of photo-induced excitons, and enhanced absorbance from ultra-violet to the visible region.

Glyceric acid is one of the important intermediates of glycerol oxidation. However, it is extremely difficult to achieve high glyceric acid selectively as the reaction follows a complex pathway with the formation of a variety of undesired products. In this context, periodic mesoporous carbon-supported gold catalysts show promise for selective formation of the targeted product. Therefore, in this investigation, an effort has made to study the effect of particle size and shape of a series of carbon supported with varied sized/shaped gold catalysts on the liquid-phase oxidation of glycerol. Further, the role played by the surface of gold-nanoparticles has prompted us to investigate in detail the isotropic (nanocube/nanosphere) and anisotropic (nanoslabs/nanorods) gold supported onto both activated/amorphous carbon (Au/AC) and ordered mesoporous carbon (Au/CMK-3) towards glycerol oxidation. It was observed that the CMK-3 supported gold nanocube exhibited the highest yield towards glyceric acid with an excellent stability in terms of recyclability/reusability.

Creating new, environmentally friendly catalytic systems for producing C-C bonds from renewable feedstock is a challenging objective in synthetic chemistry. Traditional methods often use expensive noble metals, organic or organometallic reagents, or toxic halides with co-catalysts or additives. However, borrowing hydrogen (BH) or hydrogen auto-transfer (HAT) technology offers a simple and cost-effective alternative for creating C-C bonds using readily available alcohols as alkylating agents. This study shows that commercial TiO2 nanoparticles can act as a heterogeneous catalyst for C-alkylation reactions via a hydrogen-borrowing mechanism. The TiO2 catalyst was effective in producing high yields of C-alkylated products from acetophenones and primary alcohols, and reusability tests showed that the catalyst was robust and stable. The TiO2 catalyst was also capable of catalyzing multistep reactions to produce quinolines from 2-aminobenzyl alcohol and ketones with yields of up to 96.0%. Control experiments showed that the reaction pathway involved borrowing hydrogen. The use of this heterogeneous catalyst offers several advantages over traditional homogeneous catalysts, including lower cost, greater functional group tolerance, etc.

Metal-free carbonaceous materials are an emerging class of heterogeneous catalysts for sustainable chemistry. However, designing such catalytic materials with unique properties for specific organic transformations remains a challenge due to an inadequate understanding of their active sites. Herein, we report our studies on the use of nitrogen-doped ordered mesoporous carbons as a biomimetic novel heterogeneous catalyst for the borrowing-hydrogen or hydrogen-auto-transfer class of cascade reactions. These redox neutral processes have gained prominence owing to their high atom economy and sustainability. Our experimental investigations, supported by computer modelling, show that the local nitrogen environment, i.e., the benzannulated pyridine substructures on the edges/defects of the carbon matrices, play a critical role as catalytic hydrogen shuttles in borrowing-hydrogen reactions mimicking the role of the NAD(P)+/NAD(P)H redox couple.

We report here the synthesis of nano-zeolite, viz., ZSM-5 with MFI topology, having a unique brain-coral morphology, designated as n-ZSM-5. These nanoscopic structures (250-300 nm) are in turn formed by self-organization of uniform nanoparticles of zeolite nanospheres of sizes 10-40 nm. Such a remarkable crystal architecture endows the material with improved physico-chemical properties, viz., enhanced surface area, distributed acid sites, mesoporosity and reduced diffusion resistance, which make n-ZSM-5 a promising solid acid catalyst. For comparison, bulk or conventional ZSM-5, referred to as c-ZSM-5, was also synthesized and its performance for tertiary butylation of phenol was evaluated. Density functional theory modelling studies provide an adequate description of the variation of acid strength observed in temperature-programmed desorption of ammonia experiments.

We report the adsorption equilibrium, kinetics and thermodynamic studies of endo-1,4-β-D-xylanase adsorption onto ordered mesoporous matrices of silica, carbon, and zirconia. Rapid adsorption of the enzyme was observed within 5 min, and the equilibrium was attained in 60 min. The adsorption profiles were fitted to a pseudo-second-order kinetic equation indicating a tendency towards chemisorption and that the kinetics improved with temperature. Besides, it was found that the adsorption equilibrium data followed a typical Langmuir model and that the activation energy was determined using the Arrhenius equation. Likewise, the computed thermodynamic parameters revealed that the adsorption is favourable, spontaneous, and endothermic.

A highly organized hexagonal mesoporous iron phosphate framework structure with the designation HMI-41 was successfully synthesized for the first-time in a reproducible way using imidazolium-based ionic liquid as structure directing agent. The unique templating properties of ionic liquid generated a highly ordered well-crystallized mesoporous matrix having high surface area (445 m2 g−1), thicker pore wall (2.1 nm) and narrow pore size distribution (3.1 nm). The presence of active sites within a tetrahedral framework structure made the novel HMI-41 catalyst highly effective for phenol hydroxylation in an acidic medium with hydrogen peroxide as the oxidant. The catalyst exhibited outstanding performance, achieving an impressive 80% hydroquinone selectivity and 21% phenol conversion with a hydroquinone-to-catechol ratio of seven, which is the highest value ever reported.

High-quality 2D-hexagonal ordered mesoporous titania with a well-crystallized framework structure and a high surface area was successfully synthesized, in a reproducible way, using Pluronic F127, Pluronic P123, Synperonic F108 and CTAB as structure-directing agents, and the resulting nanostructured matrix is designed as TMF-127, TMP-123, TMF-108 and TMC-016/TMC-036, respectively. The periodic array of the pore structure of these mesoporous materials is described by combining small-angle X-ray diffraction, high-resolution transmission electron microscopy and nitrogen sorption techniques. All these materials show significant amounts of intrinsic defects, viz., electrons trapped in oxygen vacancy and/or Ti3+ centres, by tuning the pore structure. These defects in-turn promote the charge separation of photogenerated excitons, and therefore exhibit excellent photocatalytic activity for the degradation of famotidine (TMP-123: DE100 = 75 min; P-25: DE100 = 120 min), and 4-chlorophenol (TMP-123: DE95 = 180 min; P-25: DE60 = 180 min). The superior activity of the mesoporous titania over the fumed titania (P-25) is ascribed to: (i) light absorption extending into the visible region, (ii) low charge-transfer resistance and high carrier density, and (iii) intrinsic Ti3+ defects, as deduced from DRUV-Visible, photo-electrochemical (Nyquist and Mott-Schottky) and EPR studies, respectively.

Nanoscale materials of carbon, silica and zirconia were used to immobilize a recombinant endo-1, 4-β-D-xylanase (XynC) of B. subtilis KCX006. The adsorption of endo-1, 4-β-D-xylanase on nanomaterials of carbon, silica and zirconia followed the pseudo-second-order kinetic model. The activation energies for adsorption of endoxylanase on carbon, silica and zirconia nanomaterials were 9.94 kJ mol−1, 40.44 kJ mol−1 and 16.33 kJ mol−1 respectively. The recovered activity (RA) of endoxylanase immobilized on carbon, silica and zirconia nanomaterials was in the range of 52%–92%. The endoxylanase immobilized on zirconia nanoparticles showed maximum RA. All immobilized endoxylanase showed optimum activity at pH 6.6 similar to that of free/soluble endoxylanase. But compared to free endoxylanase, all immobilized endoxylanase had broad optimum temperature range (50–65 °C) for catalytic activity. The Michaelis-Menten constant (Km) increased for all immobilized endoxylanase due to substrate diffusion limit. The endoxylanase immobilized on above nanomaterials was used repeatedly for XOS production from xylan. All immobilized endoxylanase produced X2–X6 and substituted XOS similar to free endoxylanase from beechwood xylan and extracted crude xylans from sorghum and sugarcane bagasse. The endoxylanase immobilized on above nanoparticles did not lose activity after five batches of repeated use. The results showed that endoxylanase immobilized on carbon, silica and zirconia matrices would be useful for production of XOS by enzyme recycling.