Darbha Srinivas

Synthesis of Cyclic Carbonates from Olefins and CO2 over Zeolite-Based Catalysts

Metal phthalocyanine complexes (MPc; M = Cu2+, Co2+, Ni2+ and Al3+) encapsulated in zeolite-Y exhibit high catalytic activity for the cycloaddition of CO2 to epichlorohydrin and propylene oxide yielding the corresponding cyclic carbonates. The catalysts could be separated easily from the reaction mixture and reused with little loss in activity. These environmentally benign catalysts are also more efficient than either the “neat” complexes or those obtained by supporting them on solids like silica.

Benzylic oxidation with H2O2 catalyzed by Mn complexes of N,N′,N″-trimethyl-1,4,7-triazacyclononane: spectroscopic investigations of the active Mn species

Abstract Mn complexes of N,N′,N″-trimethyl-1,4,7-triazacyclononane (Mn–tmtacn) exhibit good catalytic activity, at ambient temperatures, for the benzylic oxidation of aromatics selectively to the corresponding alcohol and carbonyl compounds with H2O2 as oxidant in the presence of carboxylate buffers. The active Mn species in the reaction medium was investigated by UV–visible (UV–Vis), FT-IR–attenuated total reflectance (FT-IR–ATR) and electron spin resonance (ESR) spectroscopic techniques. The studies revealed the formation of terminal oxo- and μ-oxo-Mn(IV)–tmtacn complexes during the reaction. The oxo-manganese complexes and the nature of the carboxylic acid play an important role in the activation of the benzylic C–H bond.

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.

Redox and catalytic chemistry of Ti in titanosilicate molecular sieves: an EPR investigation

An EPR study of Ti 3+ in titanosilicate molecular sieves, TS-1, TiMCM-41, ETS-10 and ETS-4 is reported. Ti 4+ is reduced to Ti 3+ by dry hydrogen above 673 K. Ti ions in TS-1 and TiMCM-41 are located in tetragonally elongated T d and those of ETS-10 and ETS-4 in a tetragonally compressed O h geometric positions. Reduction at 873 K revealed the presence of two non-equivalent Ti 3+ sites in TS-1 and TiMCM-41. Ti 4+ ions in a tetrahedral geometry are more difficult to reduce than in an octahedral symmetry. The effects of cation exchange and Pt impregnation, on the geometry and reducibility of titanium in ETS-10, are also examined. Interaction of a tetrahedrally coordinated Ti 3+ with O 2 or H 2 O 2 results in a diamagnetic titanium(IV) hydroperoxo species. Under the same conditions, an octahedrally coordinated Ti 3+ forms a paramagnetic titanium(IV) superoxo species. The higher catalytic activity of TS-1 and TiMCM-41 in selective oxidation reactions is probably a consequence of the formation of the hydroperoxy species on their surface during the catalytic reaction. The presence of Pt in the vicinity of Ti enables the use of H 2 and O 2 (instead of H 2 O 2 ) to generate the active hydroperoxy site. The absence of formation of titanium hydroperoxy species in ETS-4 and ETS-10 is the cause of their inactivity in selective oxidation reactions.

Redox behavior and selective oxidation properties of mesoporous titano- and zirconosilicate MCM-41 molecular sieves

Abstract Mesoporous titano- and zirconosilicate molecular sieves, Ti-MCM-41 and Zr-MCM-41, respectively, with Si/M ratios in the range from 11 to 96 (M=Ti or Zr), have been synthesized by the hydrothermal method and characterized by XRD, XRF, N 2 adsorption and diffusive reflectance UV–Vis (DRUV–Vis), FT-IR and electron spin resonance (ESR) spectroscopic techniques. The redox behavior and selective oxidation properties of these materials have been investigated. ESR of samples reduced with LiAlH 4 (298 K) and H 2 (673–873 K) reveals two types of metal ion species: species I ′ located inside the pore walls and species I ′′ located at the pore surface. The reduced species I ′′ are highly reactive towards oxygen and form M(O 2 −· ) radicals. The M(O 2 −· ) radicals were also observed when the samples were reacted with aqueous H 2 O 2 or tert -butylhydroperoxide (TBHP). ESR studies reveal that Ti-MCM-41 is easier to reduce and reoxidize than Zr-MCM-41. The DRUV–Vis spectra are consistent with a monoatomic dispersion of the metal ions. In the samples with high metal loading the presence of a nanocrystalline metal oxide phase cannot be ruled out. Both Ti-MCM-41 and Zr-MCM-41 catalyze the hydroxylation of 1-naphthol with aqueous H 2 O 2 and the epoxidation of norbornylene with TBHP.

EPR spectroscopy of copper and manganese complexes encapsulated in zeolites

Abstract The structural basis for the enhanced catalytic activities of copper and manganese Schiff base complexes encapsulated in zeolite-Y is investigated by EPR spectroscopy. The study provides an unequivocal evidence for the encapsulation of complexes inside the supercages of zeolite-Y. The EPR spectroscopy distinguishes the encapsulated complexes from the “neat” and surface-adsorbed metal complexes. Neat complexes showed broad EPR spectra corresponding to nearest neighbour spin–spin interactions whereas the zeolite-encapsulated metal complexes showed well resolved metal hyperfine features similar to the spectra in dilute frozen solutions. The spin Hamiltonian parameters reveal a distorted square pyramidal geometry and an increase in the in-plane covalency of metal–ligand bond as a consequence of encapsulation. The observed changes in the molecular electronic structure are correlated to the enhanced catalytic activity of the encapsulated metal complexes.

Efficient, direct synthesis of dimethyl carbonate from CO2 using a solid, calcined zirconium phenylphosphonate phosphite catalyst

Direct synthesis of dimethyl carbonate (DMC) from CO2 and methanol with 100 mol% selectivity using a solid, reusable, hydrophobic, calcined zirconium phenylphosphonate phosphite catalyst is reported for the first time. DMC yield as high as 26 mmol g−1 catalyst correlated with the concentration of acid and base sites is obtained.

Novel, benign, solid catalysts for the oxidation of hydrocarbons

The catalytic properties of two classes of solid catalysts for the oxidation of hydrocarbons in the liquid phase are discussed: (i) microporous solids, encapsulating transition metal complexes in their cavities and (ii) titanosilicate molecular sieves. Copper acetate dimers encapsulated in molecular sieves Y, MCM-22 and VPI-5 use dioxygen to regioselectively ortho-hydroxylate l-tyrosine to l-dopa, phenol to catechol and cresols to the corresponding o-dihydroxy and o-quinone compounds. Monomeric copper phthalocyanine and salen complexes entrapped in zeolite-Y oxidize methane to methanol, toluene to cresols, naphthalene to naphthols, xylene to xylenols and phenol to diphenols. Trimeric μ3-oxo-bridged Co/Mn cluster complexes, encapsulated inside Y-zeolite, oxidize para-xylene, almost quantitatively, to terephthalic acid. In almost all cases, the intrinsic catalytic activity (turnover frequency) of the metal complex is enhanced very significantly, upon encapsulation in the porous solids. Spectroscopic and electrochemical studies suggest that the geometric distortions of the complex on encapsulation change the electron density at the metal ion site and its redox behaviour, thereby influencing its catalytic activity and selectivity in oxidation reactions. Titanosilicate molecular sieves can oxidize hydrocarbons using dioxygen when loaded with transition metals like Pd, Au or Ag. The structure of surface Ti ions and the type of oxo-Ti species generated on contact with oxidants depend on several factors including the method of zeolite synthesis, zeolite structure, solvent, temperature and oxidant. Although, similar oxo-Ti species are present on all the titanosilicates, their relative concentrations vary among different structures and determine the product selectivity.

The structural basis for the enhanced oxygenase activity of copper acetate dimers encapsulated in zeolites

The oxygenase mimicking activity of copper acetate dimers in the regioselective ortho-hydroxylation of L-tyrosine to L-dopa is enhanced on encapsulation in zeolite Y. The structure and magnetic properties of the catalytic active site were characterized by EPR spectroscopy. The spectra of this “zeozyme” reveal the presence of (1) copper acetate dimers in the supercages and (2) isolated Cu(II) ions in the sodalite cages of the zeolite. There are significant differences in the EPR spectra of the “neat” and encapsulated complexes: on encapsulation in zeolite, the Cu–Cu exchange coupling constant, −, increases to 310 from 259 cm−1 for the “neat” complex (i.e., by about 19.7%). Simultaneously the Cu–Cu separation in the dimer, estimated indirectly from the exchange coupling constant, shortens to 2.40 Å in the encapsulated state from 2.64 Å in the “neat” complex. There is, hence, a relatively greater overlap of the metal orbitals of the dimer copper atoms inside the restricted confines of the zeolite cages. The consequent, enhanced, trans axial lability of the phenolate and dioxygen ligands promotes the catalytic oxygenase activity of copper acetate dimers on encapsulation in zeolites. A causal relationship between changes in the structural features of an active site on encapsulation in the zeolite and the corresponding catalytic activity has, thus, been established.

A novel, zeolite-encapsulated μ3-oxo Co/Mn cluster catalyst for oxidation of para-xylene to terephthalic acid

Trinuclear, μ3-oxo mixed metal acetato complexes, [CoMn2(μ3-O)(MeCO2)6(py) 3] (py = pyridine) encapsulated in zeolite HY, exhibit high catalytic efficiency in the selective aerial oxidation of para-xylene to terephthalic acid; interestingly, the formation of 4-carboxybenzaldehyde, the worrisome impurity in the conventional process, is suppressed significantly over these solid catalysts.