Paul Ratnasamy

Structural studies on NiO-CeO2-ZrO2 catalysts for steam reforming of ethanol

Abstract The influence of Ce/Zr ratio on the redox behavior of Ni in a series of NiO-CeO2-ZrO2 catalysts was investigated using in situ electron paramagnetic resonance (EPR), diffuse reflectance UV-visible (DRUV-visible) and X-ray photoelectron spectroscopy (XPS). At all concentrations, a small amount of Ni (species I) substitutes in the fluorite lattice. Superparamagnetic, nanosize Ni crystallites (species II) were found in samples with 1–5xa0wt.% NiO and ferromagnetic, larger Ni crystallites (species III) were detected in samples with 20xa0wt.% or more NiO when contacted with hydrogen. Ce promoted the reduction of Ni. The reducibility of Ni decreased in the order: I>III>II. At steam reforming conditions (in the presence of H2+H2O+hydrocarbon/alcohol at 773xa0K), the extent of Ni reduction varies in the order: H2+alkane>H2+alcohol>H2 alone. Catalytic activity and especially stability in the steam reforming of bio-ethanol (containing 5xa0ppm S) correlates with the type III Ni species and is influenced by both the Ni-content and the Ce/Zr ratio in the support. A catalyst of composition NiO (40xa0wt.%)-CeO2 (30xa0wt.%)-ZrO2 (30xa0wt.%) maintained its activity for more than 500xa0h without deactivation.

Ferrisilicate analogs of zeolites

The present review deals with ferrisilicate analogs of zeolites, most of them belonging to the medium (10-ring) and large (12-ring) pore systems. The ionic radii of Si 4+ , Al 3+ and Fe 3+ are 0.039, 0.057 and 0.067 nm, respectively

Selective oxidation over copper and manganese salens encapsulated in zeolites

Abstract Copper and Mn(III)(X 2 -salen) complexes, where salen 2− =N,N′-ethylenebis(salicylideneaminato) and X=H, Cl, Br or (NO 2 ), encapsulated in the cavities of zeolites NaX and NaY, have been synthesized and characterized by various physicochemical measurements. Samples obtained by synthesizing the complexes `in situ in the cavities of the zeolite by the `flexible ligand method contain a higher concentration of the complex than those obtained by synthesizing the zeolites around the preformed copper complex. Substitution of the aromatic hydrogen atoms of the salen ligand by electron-withdrawing groups like -Cl, -Br and NO 2 has two major effects: (1) retention and concentration of the copper complex in the zeolite cavities is enhanced (due to the larger size of the substituents); and (2) the spectral properties of the encapsulated complex are modified. Cyclic voltammetric data indicate that the zeolite matrix facilitates the reduction of Mn(III) to Mn(II), suggesting that it behaves like an electron-withdrawing substituent. The rates of decomposition of H 2 O 2 and tert-butyl hydroperoxide as well as the selective, low-temperature oxidation of phenol, styrene and para-xylene are all enhanced by electron-withdrawing substituents on the salen ligand.

Zeolite-encapsulated manganese(III)salen complexes

Abstract Manganese(III) complexes of [ N , N ′-ethylenebis(salicylidene-aminato)] (salen), [ N , N ′-ethylenebis(5-chloro-salicylidene-aminato)] (Cl 2 Salen), [ N , N ′-ethylenebis(5-bromo-salicylidene-aminato)] (Br 2 Salen) and [ N , N ′-ethylenebis(5-nitro-salicylidene-aminato)] [(NO 2 ) 2 Salen] have been encapsulated in the supercages of zeolite X by the zeolite synthesis method. The catalysts have been characterized by FTIR, UV–Vis and EPR spectroscopic techniques, XRD, SEM, thermal and elemental analysis, as well as nitrogen adsorption and cyclic voltammetric studies. The extent of encapsulation of the Mn(III)Salen complexes in zeolite X varies with the nature of the substituent group on the aromatic ring. While bromo groups enhance encapsulation, substitution with –NO 2 groups decreases the amounts of Mn(III) complexes encapsulated in the cavities of the zeolites. Cyclic voltammetric data indicate that the zeolite matrix facilitates the reduction of Mn(III) to Mn(II), suggesting that it behaves like an electron-withdrawing substituent. The aerobic oxidation of styrene to benzaldehyde, styrene oxide and phenylacetaldehyde over these catalysts is also reported.

Direct conversion of methane to methanol

Abstract Methane has been converted to a mixture of methanol and formaldehyde at ambient conditions with high activity (TON above 100) and selectivity (CO2 less than 5%) using phthalocyanine complexes of Fe and Cu encapsulated in zeolites as catalysts and O2/tert-butyl hydroperoxide as oxidants.

Zeolite-encapsulated copper (X2-salen) complexes

Abstract Copper (X 2 –salen) complexes, where salen 2− ue605 N , N ′-ethylenebis(salicylideneaminato) and X=H, Cl, Br or (NO 2 ), encapsulated in the cavities of zeolites NaX and NaY, have been synthesized and characterized by various physicochemical measurements. Samples obtained by synthesizing the complexes in situ in the cavities of the zeolite by the `flexible ligand method contain a higher concentration of the complex than those obtained by synthesizing the zeolites around the pre-formed copper complex. Substitution of the aromatic hydrogen atoms of the salen ligand by electron withdrawing groups like –Cl, –Br and –NO 2 has two major effects: (1) retention and concentration of the copper complex in the zeolite cavities is enhanced (due to the larger size of the substituents) and (2) the electronic and spectral properties of the encapsulated complex are modified. The rates of decomposition of H 2 O 2 over these encapsulated, substituted copper salens approach those of natural catalase enzymes and correlate with the rates of oxidation of phenol.

Oxidation of para-xylene over zeolite-encapsulated copper and manganese complexes

Abstract Salen, saltin and salcyhexen complexes of copper and manganese, encapsulated in the cavities of zeolite NaX have been investigated as catalysts for the aerobic oxidation of para -xylene in the absence of added halogen promoters and using tertiary -butyl hydroperoxide as the initiator at low temperatures. Significant conversion levels (upto 50–60%) are attained. The major products include toluic acid, toluyl aldehyde and toluyl alcohol. The zeolite-encapsulated complexes did not undergo any colour change during the reaction and could be easily separated and reused many times. In contrast, the neat complexes, while they were active in the first cycle, were completely destroyed during the run and changed colour. They, however, gave lower conversions compared to the encapsulated catalysts. Conversion increases when electron-withdrawing substituents (like Cl, Br and NO 2 ) are substituted in the aromatic ring.

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.

Selective oxidation of phenols using copper complexes encapsulated in zeolites

Abstract The oxidation and hydroxylation of phenols to dihydroxy aromatic compounds have been investigated using phthalocyanines and substituted (chloro- and nitro-) phthalocyanines of copper encapsulated in zeolites (X and Y). H 2 O 2 has been used as oxidant. At ambient conditions, the encapsulated copper chloro- and nitro phthalocyanine complexes, unlike their unsubstituted analogs, are able to oxidise, selectively, phenol to catechol and hydroquinone. The catalytic efficiency (turnover number) of the copper atoms are higher in the encapsulated state compared to that in the “neat” complex. The solid catalysts have been characterised by IR, UV, ESR and ESCA spectroscopy. The influence of various process parameters (like temperature, reaction time, substrate/oxidant ratio, catalyst weight etc.) on the conversion and product distribution is also illustrated and discussed.

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.