Gopinathan Sankar

Molecular-sieve catalysts for the selective oxidation of linear alkanes by molecular oxygen

Terminally oxidized hydrocarbons are of considerable interest as potential feedstocks for the chemical and pharmaceutical industry, but the selective oxidation of only the terminal methyl groups in alkanes remains a challenging task. It is accomplished with high efficiency and selectivity by some enzymes; but inorganic catalysts, although inferior in overall performance under benign conditions, offer significant advantages from a processing standpoint. Controlled partial oxidation is easier to achieve with ‘sacrificial’ oxidants, such as hydrogen peroxide, alkyl hydroperoxides oriodosylbenzene, than with molecular oxygen or air. These sacrificial oxidants, themselves the product of oxidation reactions, have been used in catalytic systems involving tailored transition-metal complexes in either a homogeneous state, encapsulated in molecular sieves or anchored to the inner surfaces of porous siliceous supports. Here we report the design and performance of two aluminophosphate molecular sieves containing isolated, four-coordinated Co(III) or Mn(III) ions that are substituted into the framework and act, in concert with the surrounding framework structure, as regioselective catalysts for the oxidation of linear alkanes by molecular oxygen. The catalysts operate at temperatures between 373 K and 403 K through a classical free-radical chain-autoxidation mechanism. They are thus able to use molecular oxygen as oxidant, which, in combination with their good overall performance, raises the prospect of using this type of selective inorganic catalyst for industrial oxidation processes.

Metal organic framework-mediated synthesis of highly active and stable Fischer-Tropsch catalysts

Depletion of crude oil resources and environmental concerns have driven a worldwide research on alternative processes for the production of commodity chemicals. Fischer-Tropsch synthesis is a process for flexible production of key chemicals from synthesis gas originating from non-petroleum-based sources. Although the use of iron-based catalysts would be preferred over the widely used cobalt, manufacturing methods that prevent their fast deactivation because of sintering, carbon deposition and phase changes have proven challenging. Here we present a strategy to produce highly dispersed iron carbides embedded in a matrix of porous carbon. Very high iron loadings (>40 wt %) are achieved while maintaining an optimal dispersion of the active iron carbide phase when a metal organic framework is used as catalyst precursor. The unique iron spatial confinement and the absence of large iron particles in the obtained solids minimize catalyst deactivation, resulting in high active and stable operation.

A new cell for the collection of combined EXAFS/XRD data in situ during solid/liquid catalytic reactions

A new experimental reaction cell which enables the recording of combined EXAFS/XRD data over a wide range of temperatures of a solid that catalyses liquid reactions in the liquid state has been designed and constructed. The cell is particularly suitable for the in situ study of both short-range and long-range structural changes during solid/liquid heterogeneous catalytic reactions. It is also suitable for the study of intercalation phenomena and may be readily adapted for certain types of electrochemical investigations. The use of this cell is exemplified by the oxidation of cyclohexene over a Zn(II)Al(III)-hydrotalcite containing intercalated cobalt phthalocyaninetetrasulphonate anions.

Bimetallic nanocatalysts for the conversion of muconic acid to adipic acidElectronic supplementary information (ESI) available: experimental details, EXAFS fit and refined EXAFS parameters. See http://www.rsc.org/suppdata/cc/b3/b300203a/

Adipic acid (2) production currently entails use and generation of environmentally harmful materials: an efficient catalyst, consisting of nanoparticles of Ru10Pt2 anchored within the pores of mesoporous silica, facilitates the production of (2) by hydrogenating muconic acid, that may be derived biocatalytically from D-glucose.

IR spectroscopic study of CD3CN adsorbed on ALPO-18 molecular sieve and the solid acid catalysts SAPO-18 and MeAPO-18

CD3CN has been used as an IR spectroscopic probe to elucidate the nature of the Bronsted and Lewis acid sites in ALPO-18, SAPO-18 and MeAPO-18 (Me = Co, Mg and Zn) microporous materials. Interaction of CD3CN with (Si)–OH–(Al) bridging hydroxy groups in SAPO-18 gives rise to CN absorption bands at 2291 and 2284 cm–1, whereas CD3CN bound to CoII(in CoAPO-18) and MgII(in MgAPO-18) centres exhibits bands at 2310–2305 and 2306–2302 cm–1, respectively. There are two types of ZnII centres in ZnAPO-18, one of which is associated with a CN band at 2316 cm–1 and the other at 2306–2303 cm–1. Interactions between CD3CN and strong/weak AlIII Lewis sites and (P)—OH groups were also observed.

Hydrotalcite-derived mixed oxides containing copper: catalysts for the removal of nitric oxide

Hydrotalcite structures containing copper, magnesium and aluminium in the metal hydroxide layers have been investigated for their potential as precursors to the formation of catalysts for the decomposition and reduction of nitrogen oxides. These CuIIMgIIAlIII hydrotalcites are catalytically active towards both the decomposition of nitric oxide and its reduction in the presence of an appropriate coreductant, such as propane. The copper centred active species, identified using EXAFS spectroscopy and combined EXAFS/XRD under operating conditions, have been found to be CuI for the decomposition and Cu0 for the reduction.

Metallocene-derived, isolated MoVI active centres on mesoporous silica for the catalytic dehydrogenation of methanol

Isolated MoVI active sites have been grafted onto the inner surfaces of MCM-41 mesoporous silica via a molybdocene dichloride precursor, to generate a catalyst which has been tested for the oxidative dehydrogenation of methanol. Mo K-edge X-ray absorption spectroscopy shows that, after calcination, at low Mo loadings (ca. 1 mol% with respect to SiO2), isolated MoO4 species are generated on the surface, whereas at higher loadings (ca. 4 mol%) there is some evidence for the formation of polymeric oxo-molybdenum species. Catalysis tests show that lower Mo loadings on the MCM-41 silica lead to higher selectivity for the production of formaldehyde, in contrast to the results of previous studies of isolated Mo species on pure silica, which report that methyl formate is the major product.

Catalytic Reaction Mechanism of Mn‐Doped Nanoporous Aluminophosphates for the Aerobic Oxidation of Hydrocarbons

In this work we apply state-of-the-art electronic-structure-based computational methods based on hybrid-exchange density functional theory to study the mechanism of the aerobic oxidation of hydrocarbons catalysed by Mn-doped nanoporous aluminophosphates (Mn-AlPOs). We compare our results with available experimental data. We show that the catalytic efficiency of Mn-AlPOs in oxidation reactions is intrinsically linked to 1) the Mn redox activity, in particular between 2+ and 3+ oxidation states, and 2) the coordinative insaturation of tetrahedral Mn embedded in AlPO frameworks, which facilitates the reaction by stabilising oxo-type radicals through the formation of Mn complexes. Our mechanism demonstrates the crucial role of both Mn(III) and Mn(II) in the reaction mechanism: Mn(III) sites undergo an initial reaction cycle that leads to the production of the alkyl hydroperoxide intermediate, which can only be transformed into the oxidative products (alcohol, aldehyde and acid) by Mn(II). A preactivation step is required to yield the reduced Mn(II) sites able to decompose the hydroperoxide intermediates; this step takes place through a transformation of the hydrocarbon into the corresponding peroxo-derivative, stabilised by forming a complex with Mn(III) and yielding at the same time reduced Mn(II) sites. Both species enter a subsequent propagation cycle in which Mn(II) catalyses the dissociation of the hydroperoxide that proceeds until the formation of the oxidative products by two parallel pathways, through alkoxy- or hydroxy-radical-like intermediates, whilst the Mn(III)-peroxo complex enables further production of the hydroperoxide intermediate.

Elucidating the genesis of Bi2MoO6 catalyst by combination of synchrotron radiation experiments and Raman scattering

Combination of in situ Raman scattering with high-resolution XRD and XAS techniques has proven to be a powerful tool to elucidate the crystal growth of gamma-Bi2MoO6 under hydrothermal conditions.

Promotion of the metal–oxide support interaction in the Ni/TiO2 catalyst. Crucial role of the method of preparation, the structure of TiO2 and the NiTiO3 intermediate

Based on in situ X-ray diffraction and EXAFS studies, it is shown that NiTiO3 formed by the reaction of TiO2(anatase) with Ni2+ during the calcination of Ni/TiO2, gives rise to Ni metal and TiO2(rutile) on reduction, a process that may be responsible for the strong interaction between the metal and the oxide support.