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ABSTRACT: Direct ring hydroxylation of aromatics, namely, anisole, toluene, and benzylchloride, using TS-1 catalyst, a MFI-type titanium silicate molecular sieve, dilute H2O2 as oxidizing agent, and water as reaction medium, was investigated under solid−liquid−liquid (SLL)-type triphase conditions. The aim was to study the relative influence of commonly used organic solvents, under solid−liquid (SL)-type biphase reaction conditions, as compared to that of water, on the conversion and product selectivity. Under solvent-free triphase conditions, the ring hydroxylation of anisole, toluene, and benzylchloride leads to significant enhancement in the conversion, turn-over frequency, and para-selectivity. However, in the case of benzyl chloride, no ring hydroxylation took place under biphase conditions (using acetone as solvent) contrary to significant ring hydroxylation (ca. 60% H2O2 efficiency) under triphase aqueous medium conditions. The simple product recovery (phase separation of organic and aqueous layers) and use of water as reaction medium are other advantages. Since TS-1 is relatively hydrophobic, the reaction is hindered in the presence of organic solvents as they compete favorably with the substrate for diffusion inside the TS-1 channels. However, in the presence of water, the organic substrate competes favorably with water, leading to increased conversion and para-selectivity.
10/2007;
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ABSTRACT: [27Al]-1H REAPDOR (Rotational Echo Adiabatic Passage DOuble Resonance) NMR experiments distinguish structurally dissimilar Brønsted acid sites located in main 8 and 12 membered-ring channels of the zeolite mordenite.
Chemical Communications 09/2003; · 6.17 Impact Factor
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ABSTRACT: High-resolution solid state 29Si, 27Al magic angle spinning (MAS) and 27Al triple quantum magic angle spinning (3Q-MAS) NMR spectroscopic techniques were used to characterize highly crystalline ultrastable Y (USY) zeolites having different framework Si/Al ratios, obtained by multistep-temperature-programmed (MSTP) steaming at different temperatures. The 29Si MAS NMR spectra of high silica USY show, in addition to the regularly ordered Q4(Si) framework sites (δ ≈ −105 ppm), the presence of distorted tetrahedral environments (δ ≈ −111 ppm), generated by the healing of the framework vacancies created by dealumination. Further, 27Al MAS experiments show the presence of aluminum in four (AlIV), five (AlV), and six (AlVI) coordinations, whereas the multiplicity within AlIV and AlVI is revealed by 27Al 3Q-MAS experiments. Two different tetrahedral and octahedral Al environments are resolved in the samples subjected to increasing steaming severity. Quantification of all the Al environments is provided based on isotropic chemical shifts (δCS) and second-order quadrupole interaction parameters (PQ), which were derived by a graphical analysis of 3Q-MAS data. In high silica USY materials prepared by MSTP method, the two AlIV species resolved by 3Q-MAS experiments could be assigned to Al environments which are tetrahedrally connected to silicons in the original and healed regions of faujasite framework.
05/2002;
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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,-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.
Catalysis Letters 01/1996; 40(1):47-50. · 2.24 Impact Factor
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ABSTRACT: New MFI type arseno-silicate molecular sieves (As(V)-MFI), synthesized for the first time, can be crystallized at lower temperature (70–90C) in 1–3 days under hydrothermal conditions. Interestingly, these As(V)-MFI molecular sieves can be synthesized even faster than the As-free silicalite-1 under identical conditions. Further, the crystallization becomes faster with the increase in As content of the reaction mixture, contrary to the observations made on all the other high silica molecular sieves, so far. These microporous As(V)-silicates exhibit significant catalytic activity in (i) phenol hydroxylation using 30% aq. H2O2 (ca. 55% H2O2 efficiency) and (ii) oxidative dehydrogenation of 2-butanol and benzyl alcohol using air under fixed bed vapor phase reaction conditions exhibiting ca. 75–85% selectivity towards 2-butanone and benzaldehyde+benzoic acid, respectively.
Catalysis Letters 08/1995; 35(3):327-334. · 2.24 Impact Factor
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ABSTRACT: Multinuclear solid-state NMR spectroscopy, employing 29Si MAS,27Al MAS/3Q-MAS and (47,49)Ti wide-line experiments, has been used for the structural characterization of titanium substituted ultra-stable zeolite Y (Ti-USY). 27Al MAS experiments show the presence of aluminum in four (Al(IV)), five (Al(V)), and six (Al(VI)) coordination, whereas the multiplicity within Al(IV) and Al(VI) is revealed by 27Al 3Q-MAS experiments. Two different tetrahedral and octahedral Al environments are resolved and their isotropic chemical shifts (delta(CS)) and second-order quadrupole interaction parameters (P(Q)) have been determined by a graphical analysis of the 3Q-MAS spectra. The emergence of signal with higher intensity at -101 ppm in the 29Si MAS spectrum of Ti-USY samples indicates the possible occurrence of Q4(3Si,1Ti) type silicon environments due to titanium substitution in the faujasite framework. High-field (11.74T) operation, using a probehead specially designed to handle a large sample volume, has enabled the acquisition of 47,49Ti static spectra and identification of the titanium environment in the zeolite. The chemical shielding and electric field gradient tensors for the titanium environment in the zeolite have been determined by a computer simulation of the quadrupolar broadened static 47,49Ti NMR spectra.
Solid State Nuclear Magnetic Resonance 24(2-3):184-95. · 1.71 Impact Factor
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ABSTRACT: Under solvent-free triphase conditions (solid catalyst TS-1, organic substrate and aqueous H2O2, S-L-L) a significant enhancement in the reaction rate (ca by 15–25 times) during the oxidation of benzene, compared to that obtained under conventional biphase system using an organic solvent (solid TS-1 catalyst in the presence of a cosolvent to homogenize the organic and aqueous layers, S-L), was observed. Time-dependent studies in the hydroxylation of benzene over TS-1/H2O2system indicated that while in the presence of a cosolvent (like acetone, acetonitrile, or methanol) a long induction period was observed; in solvent-free conditions the induction period was almost absent. The effect of various reaction parameters such as benzene/H2O2molar ratio, mode of addition of H2O2, reaction temperature, catalyst concentration, stirring speed, and the dilution level on benzene conversion and phenol selectivity was also studied. Around 85±5 mol% H2O2utilization for oxygenated benzene products (reaction time=2 h) as well as phenol selectivity could be obtained under the present solvent-free, triphase conditions (at 333 K and benzene to H2O2mole ratio=1–3). The corresponding values under similar reaction conditions, except the presence of organic solvent (reaction time=8 h), in conventionally used biphase system were 17±3 and 85±5, respectively. Further, the present solvent-free method offers distinct advantages of easier workup and being environmentally safer, because of the absence of organic solvents. Competitive adsorption experiments suggest that under triphase condition benzene competes favorably with water for diffusion inside the TS-1 channels. However, under biphase conditions benzene faces strong competition with organic solvents for diffusion inside relatively hydrophobic TS-1 channels resulting in low conversion.
Journal of Catalysis. 178(1):101-107.
