Nawal Kishor Mal

Photocontrolled reversible release of guest molecules from coumarin-modified mesoporous silica

Since the discovery of MCM-41 more than ten years ago, many investigations have explored the suitability of hexagonal mesoporous silicas for potential practical applications. These range from catalysis and optically active materials to polymerization science, separation technology and drug delivery, with recent successes in the fabrication of hybrid mesoporous organosilicas expected to open up further application possibilities. Because the pore voids of this class of materials exhibit relatively narrow pore size distributions in the range of 2–4 nm in diameter, mesoporous silicas can selectively include organic compounds and release them continuously at a later stage. The functionalization of MCM-41 pore voids with photoactive derivatives provides influence over the materials absorption behaviour, but full control over the release process remains difficult. Here we show that the uptake, storage and release of organic molecules in MCM-41 can be regulated through the photocontrolled and reversible intermolecular dimerization of coumarin derivatives attached to the pore outlets. Successful functionalization requires uncalcined MCM-41 still filled with the template molecules that directed the formation of its pores, to ensure that coumarin derivatives attach preferentially to the pore outlets, rather than their inside walls. We find that this feature and the one-dimensional, isolated nature of the individual pores allow for efficient and reversible photocontrol over guest access to the materials interior.

Hydroxylation of phenol over Sn-silicalite-1 molecular sieve : solvent effects

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.

Synthesis of tin-silicalite molecular sieves with MEL structure and their catalytic activity in oxidation reactions

Abstract Tin-containing, medium pore molecular sieves (Si/Sn ratios above 40) with MEL structure have been syntheszed hydrothermally and characterized by a variety of techniques. Changes in unit cell volume (XRD) account for about 20% of Sn 4+ ions in possible tetrahedral positions. 119 Sn MAS-NMR spectra, however, indicate that these are mostly in octahedral coordination. Framework IR spectra show an absorption at 970 cm −1 which is associated with Si O Sn vibration. Despite the uncertainty of the environment of the Sn 4+ ions in the silicalite structure, these tin-silicates are found to be quite active in the hydroxylation of phenol and toluene with aqueous H 2 O 2 as oxidant. The product distribution suggests that most of the Sn 4+ ions are located within the channels of the MEL structure.

In-situ polymerization of grafted aniline in the channels of mesoporous silica SBA-15

Monolayer N-propylaniline functionalized mesoporous silica SBA-15 (MFMS) has been successfully used for the grafting of polyaniline moieties through in-situ polymerization in the presence of ammonium peroxodisulfate (APS) as oxidant. For comparison, siliceous SBA-15 has been used to prepare non-grafted polyaniline (PANI) nanocomposites. The SBA-15, MFMS, and PANI nanocomposites were thoroughly characterized through powder XRD, TEM, SEM, FTIR, TG-DTA and N2 sorption measurements. The powder XRD pattern and TEM image of MFMS show a hexagonal (p6mm) structure. The FTIR, TG-DTA and N2 sorption studies confirm the presence of lattice bound N-propylaniline groups in MFMS material. The decrease in mesopore volume of MFMS from 0.89 to 0.61 ml g−1 after polymerization indicates that the polymers are well packed inside the channels of SBA-15. The thermal analysis further reveals that covalently grafted polyaniline decomposes at much higher temperature than the non-grafted analogue. The increased electrical conductivity of grafted polyaniline compared to non-grafted composites is attributed to the presence of more counter ions and the increased intensity of the “electronic-like absorption’ band at 1155 cm−1 of the former. The grafted PANI exhibits electrorheological effects with an increased shear viscosity under an applied electric field compared with the non-grafted polyaniline nanocomposite.

Sn-ZSM-12, a new, large pore MTW type tin-silicate molecular sieve: synthesis, characterization and catalytic properties in oxidation reactions

A new large pore tin-silicate analogue of zeolite ZSM-12 (MTW topology) with Si/Sn molar ratio >70 has been synthesized hydrothermally using a new template, hexamethylene bis(benzyl dimethyl ammonium hydroxide). This material exhibits an expansion in unit cell volume (XRD), an IR band at 970 cm−1 and a charge transfer band at 205 nm in the UV-Vis region indicating the presence of Si-O-Sn units with Sn4+ centers in Td configuration. Sn-ZSM-12 catalyzes the oxidation of phenol,m-cresol andm-xylene using dilute H2O2 as an oxidizing agent.

Oxidation of ethylbenzene over Ti-, V- and Sn-containing silicalites with MFI structure

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.

Synthesis of Al-free Sn-containing molecular sieves of MFI, MEL and MTWtypes and their catalytic activity in oxidation reactions

Al-free Sn-silicalites with MFI, MEL and MTW structures have been synthesizedhydrothermally and characterized by XRD, FT-IR, 119 Sn MAS-NMR and sorption techniques. The unit cell volume expansion depends on the manner Sn atoms are linked to the silicalite network. Framework IR spectra show an absorption at 970 cm −1 which is associated with Si-O-Sn vibration. SnO x units may be linked through an edge with SiO 4 tetrahedra and can be described as structural defects. Sn-silicalites are active in the oxidation of phenol, toluene, m-cresol and m-xylene with aqueous H 2 O 2 as oxidant leading to products of both aromatic hydroxylation and oxidation of the methyl substituent. The product distribution in general and the ability of Sn-MTW-silicalite to oxidize bulkier naphthalene and 2-methylnaphthalene suggest that the Sn 4+ ions are located within the channels of the molecular sieves.

Iron oxide nanoparticles stabilized inside highly ordered mesoporous silica

Nanosized iron oxide, a moderately large band-gap semiconductor and an essential component of optoelectrical and magnetic devices, has been prepared successfully inside the restricted internal pores of mesoporous silica material throughin-situ reduction during impregnation. The samples were characterized by powder XRD, TEM, SEM/EDS, N2 adsorption, FT-IR and UV-visible spectroscopies. Characterization data indicated well-dispersed isolated nanoclusters of (Fe2O3)n, within the internal surface of 2D-hexagonal mesoporous silica structure. No occluded Fe/Fe2O3 crystallites were observed at the external surface of the mesoporous silica nanocomposites. Inorganic mesoporous host, such as hydrophilic silica in the pore walls, directs a physical constraint necessary to prevent the creation of large Fe2O3 agglomerates and enables the formation of nanosized Fe2O3 particles inside the mesopore

Synthesis of hexagonal and cubic super-microporous niobium phosphates with anion exchange capacity and catalytic properties

Super-microporous hexagonal niobium phosphate synthesized using neutral surfactant (S0I0 mechanism) and cubic structure with cationic surfactant (S+X-I+); the hexagonal niobium phosphate possesses an excellent anion exchange capacity (6.3 mmol g-1) and high product selectivity towards 4-naphthaquinone (88.6%) in the oxidation of 1-naphthol in presence of aqueous H2O2.

Synthesis of a novel mesoporous tin phosphate, SnPO4

Novel mesoporus tin phosphates have been synthesized using alkyltrimethylammonium bromide (alkyl = C8-C18) as surfactant; the structure of the materials is stable at 500 degrees C. UV-VIS spectra show tetrahedral coordination of tin.