P. Ratnasamy

Catalytic properties of crystalline titanium silicalites I. Synthesis and characterization of titanium-rich zeolites with MFI structure

Crystalline titanium silicalites with the MFI structure and high titanium content (Si/Ti=10) have been prepared using improved synthesis procedures. The monoclinic lattice symmetry of silicalite-1 changes to orthorhombic on introduction of titanium. The titanium silicalites retain their orthorhombic symmetry even after calcination in air. On progressive incorporation of titanium, (i) the unit cell dimensions and volume (from XRD) increase linearly, (ii) the crystal size decreases progressively, (iii) the intensity of the framework IR band at 960 cm−1 increases, (iv) a band around 47000 cm−1 appears in the electronic spectra due to charge transfer transitions involving Ti(IV) sites, (v) the hydrophilicity of the zeolite (from water adsorption measurements) increases, (vi) the adsorption capacity for cyclohexane increases, (vii) the 29Si MAS NMR lines exhibit progressive broadening, and finally (viii) the catalytic activity in the hydroxylation of phenol to catechol and hydroquinone also increases markedly. Based on these observations, it is concluded that a significant fraction of titanium in these samples is situated in framework positions. The catalytic properties of these well-defined titanium-modified zeolites will be described in more detail in the subsequent parts of this series.

Catalytic properties of crystalline titanium silicalites III. Ammoximation of cyclohexanone

The ammoximation of cyclohexanone to cyclohexanone oxime over the titanium silicalite TS-1 with NH3 and H2O2 is reported. The ammoximation takes place at conversions exceeding 90% with selectivities greater than 99%. The influence of various catalyst and process parameters on the oximation reaction is reported. By-products from condensation reactions become significant at low concentrations of H2O2 and NH3. Possible mechanisms for the formation of the oxime and by-products are reported.

Hydroxylation of phenol over ts-2, a titanium silicate molecular sieve

Abstract The influence of different process parameters on the hydroxylation of phenol with H 2 O 2 over the titanium silicate molecular sieve TS-2 has been investigated. Apart from the primary products vis. , p - and o -dihydroxybenzenes, the corresponding quinones are also formed. Higher Ti contents in the catalyst (TS-2) and higher catalyst concentrations lower the formation of the secondary oxidation products. Solvents have an influence on the relative amounts of the two dihydroxybenzenes in the product. H 2 O 2 efficiency of up to 74% is obtainable at ∼ 28% phenol conversion to dihydroxybenzenes.

Catalytic properties of titanium silicalites: IV. Vapour phase beckmann rearrangement of cyclohexanone oxime

The transformation of cyclohexanone oxime into caprolactam has been studied over the titnaium silicalite TS-1 and the related MFI analogs, ZSM-5 and Silicalite-1. The incorporation of Ti increases the yield of caprolactam, besides lowering the deactivation rate. Both oxime conversion and selectivity for the lactam increase with increasing Ti content in the zeolite. The influence of temperature and feed rate (WHSV) on the yield of caprolactam over TS-1 has been investigated at different times on stream.

Reactive oxo-titanium species in titanosilicate molecular sieves: EPR investigations and structure–activity correlations

The structure of the reactive oxo-titanium species (hydroperoxo-, peroxo-, and superoxo-titanium) in titanosilicate molecular sieves (TS-1, Ti-β, amorphous Ti–SiO2, and TiMCM-41), generated on interaction with aqueous H2O2 or urea–H2O2 adducts, was investigated by electron paramagnetic resonance (EPR) and diffuse reflectance UV–visible (DRUV–visible) spectroscopies and magnetic susceptibility measurements. Two types of superoxo-titanium species (A and B) were identified in TS-1 and amorphous Ti–SiO2. An additional oxo species, A′, was also detected over Ti-β. TiMCM-41 generated, mainly, the B-type species. DRUV–visible spectroscopy and magnetic susceptibility measurements suggest the coexistence of both the superoxo- and hydroperoxo-titanium species. The A-type oxo-titanium species originate from the framework Ti sites having a tetrapodal (SiO)4Ti structure located inside the pores, while the B-type species originate predominantly from tripodal (SiO)3Ti(OH) structures, also located in the framework, probably at the external surface. The local structural environment of the Ti4+ ions, solvent, and temperature influence the concentration of these oxo-titanium species. The concentration of the A-type species decreased in the order: TS-1>Ti-β>amorphous Ti–SiO2. The B type varied as follows: TiMCM-41>amorphous Ti–SiO2>Ti-β>TS-1. In the oxidation of styrene, the A-type species correlate with styrene oxide formation, while the presence of B-type species lead to nonselective products. Experimental evidence is given for the involvement of the hydroperoxo-titanium species in olefinic epoxidations (styrene to styrene oxide) and the superoxo-titanium in aromatic hydroxylations (phenol to hydroquinone and catechol).

Ammoximation of cyclohexanone over a titanium silicate molecular sieve, TS-2

The catalytic activity and selectivity of the titanium silicate molecular sieve, TS-2 (having ZSM-11 structure), in the ammoximation of cyclohexanone and other carbonyl compounds with H2O2 and NH3 is reported in this paper. The studies were carried out at different temperatures and reactant concentrations and with different catalysts. TS-2 produces cyclohexanone oxime with high selectivities, while silicalite-2 and SiO2 produce large quantities of the condensation product, peroxydicyclohexylamine. Similarly, when the concentrations of cyclohexanone and H2O2 are larger than that of NH3, the formation of the byproducts is favoured. Based on the above results, mechanisms for the formation of cyclohexanone oxime and peroxydicyclohexylamine are presented.

Vapour phase Beckmann rearrangement of cyclohexanone oxime over MEL type zeolites

The vapour phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam is reported over different MEL zeolites viz., silicalite-2, titanium silicate-2 (TS-2), ZSM-11 and alumino-titanium-silicate (Al-TS-2) and fumed silica. TS-2 exhibits 100% conversions with lactam selectivities close to 90%. The titanium containing analogs are also more resistant to deactivation than the parent zeolites. The conversion of cyclohexanone oxime and selectivity for ε-caprolactam increase with temperature. The selectivity for ε-caprolactam increases with the duration of run at all the temperatures.

Pd-SAPO-31, an efficient, heterogeneous catalyst for Heck reactions of aryl chlorides

Pd-SAPO-31 exhibits high activity for Heck reactions of aryl chlorides. These catalysts with activities superior to most known solid catalysts can be recovered and reused with negligible loss in activity.

Chemoselective oxidation of organic compounds having two or more functional groups

The chemoselective oxidation by aqueous H 2 O 2 of organic molecules containing two or more oxidizable functions has been studied over TS-1 and VS-1 molecular sieves. The substrates included allyl alcohol, methallyl alcohol, acrolein, methacrolein, allyl chloride and methallyl chloride. The substituent groups influence the reaction rates and product functional group chemoselectivity by both electronic and steric effects.

Synthesis and characterization of a crystalline vanadium silicate with MEL structure

Synthesis of a novel crystalline vanadium silicate having the MEL structure (silicalite-2) is reported. Tetraethyl orthosilicate and vanadyl sulfate were the raw materials and tetrabutylammonium hydroxide was the additive. The influence of various synthesis parameters such as the SiV molar ratio, concentration of the organic base, and dilution levels on the crystallization of vanadium silicate has been investigated. The kinetics of crystallization has been followed using X-ray powder diffraction, SEM, framework i.r., and e.s.r. spectroscopic techniques. Under optimum conditions of synthesis, vanadium is incorporated as V4+ in a progressive manner in the MEL framework during the crystallization process. After calcination in air at 773 K, most of the vanadium (as V5+) is still intact within the silicalite structure.