Modulating the selectivity for CO and butane oxidation over heterogeneous catalysis through amorphous catalyst coatings

The preparation of porous films directly deposited onto the surface of catalyst particles is attracting increasing attention. We report here for the first time a method that can be carried out at ambient pressure for the preparation of porous films deposited over 3 mm diameter catalyst particles of silica-supported Pt-Fe. Characterization of the sample prepared at ambient pressure (i.e., open air, OA) and its main structural differences as compared with a Na-A (LTA) coated catalyst made using an autoclave-based method are presented. The OA-coated material predominantly exhibited an amorphous film over the catalyst surface with between 4 and 13% of crystallinity as compared with fully crystallized LTA zeolite crystals. This coated sample was highly selective for CO oxidation in the presence of butane with no butane oxidation observed up to 350 degrees C. This indicates, for the first time, that the presence of a crystalline membrane is not necessary for the difference in light off temperature between CO and butane to be achieved and that amorphous films may also produce this effect. An examination of the space velocity dependence and adsorption of Na+ on the catalysts indicates that the variation in CO and butane oxidation activity is not caused by site blocking predominantly, although the Pt activity was lowered by contact with this alkali.

Behavior of the heterogeneous electron-transfer rate constants of arenes and substituted anthracenes in room-temperature ionic liquids

We report the anodic oxidation of several arenes and anthracenes within room-temperature ionic liquids (RTILs). In particular, the heterogeneous electron-transfer rates (k(0)) for substituted anthracenes and arenes are also investigated in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C(2)mim][NTf2]) and found not to obey the outer-sphere Marcus-type behavior of these compounds in contrast to the behavior in traditional organic solvents,in particular the predictions for k(0) with molecular size and solvent static dielectric constant. To obtain the electron-transfer rate for 9-phenylanthracene, the dimerization and heterogeneous electron-transfer kinetics of its electrogenerated radical cations is studied in [C(2)mim][NTf2] and eight other RTILs and are both found to be largely independent of the solution viscosity.

Efficient heterogeneous asymmetric catalysis of the Mukaiyama aldol reaction by silica- and ionic liquid-supported Lewis acid copper(II) complexes of bis(oxazolines)

Lewis acid complexes based on copper(II) and an imidazolium-tagged bis(oxazoline) have been used to catalyse the asymmetric Mukaiyama aldol reaction between methyl pyruvate and 1-methoxy-1-tri-methylsilyloxypropene under homogeneous and heterogeneous conditions. Although the ees obtained in ionic liquid were similar to those found in dichloromethane, there was a significant rate enhancement in the ionic liquid with reactions typically reaching completion within 2 min compared with only 55% conversion after 60 min in dichloromethane. However, this rate enhancement was offset by lower chemoselectivity in ionic liquids due to the formation of 3-hydroxy-1,3-diphenylbutan-1-one as a by-product. Supporting the catalyst on silica or an imidazolium-modified silica using the ionic liquid or in an ionic liquid-diethyl ether system completely suppressed the formation of this by-product without reducing the enantioselectivity. Although the heterogeneous systems were characterised by a drop in catalytic activity the system could be recycled up to five times without any loss in conversion or ee.

Utilization of the three-dimensional volcano surface to understand the chemistry of multiphase systems in heterogeneous catalysis

CO hydrogenation is used as a model system to understand why multiphase catalysts are chemically important in heterogeneous catalysis. By including both adsorption and subsequent surface reactions, kinetic equations are derived with two fundamental properties, the chemisorption energies of C and O (Delta H-C and Delta H-O, respectively). By plotting the activity against Delta H-C and Delta H-O, a 3-D volcano surface is obtained. Because of the constraint between Delta H-C and Delta H-O on monophase systems, a maximum can be achieved. However, if multiphase systems are used, such a constraint can be released and the global maximum may be achieved.

Electrochemistry of phenol in bis{(trifluoromethyl)sulfonyl}amide ([NTf2](-)) based ionic liquids

The electrochemistry of phenol and 4-tert-butyl-phenol is described in [C(2)mim][NTf2] and [C(4)mpyrr][NTf2] ionic liquids. Oxidation of phenol and phenolate is observed at E-p(a) = +1.64 and +0.24 V vs. Ag in both ionic liquids. On the cathodic sweep at a potential of -2.05 P 02 V vs. Ag under an oxygen atmosphere, the production of O-2(2-) dianions triggers the formation of phenolate anions which undergo chemical oxidation to the phenoxyl radical. The phenoxyl radical then reacts with the [NTf2](-) anion of the ionic liquid to form the corresponding phenyl triflate molecule. (c) 2005 Elsevier B.V. All rights reserved.

A mechanistic study of the electro-oxidation of bromide in acetonitrile and the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide at platinum electrodes

The oxidation of bromide has been investigated by linear sweep and cyclic voltammetry at platinum electrodes in the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, ([C(4)mim][NTf2]), and the conventional aprotic solvent. acetonitrile, (MeCN). Similar voltammetry was observed in both solvents, despite their viscosities differing by more than an order of magnitude. DigiSim(R) was employed to simulate the voltammetric response. The mechanism is believed to involve the direct oxidation of bromide to bromine in a heterogeneous step, followed by a homogenous reaction to form the tribromide anion: 2Br(-) --> Br-2 + 2e(-)

Bronsted-Evans-Polanyi relation of multistep reactions and volcano curve in heterogeneous catalysis

Multistep surface processes involving a number of association reactions and desorption processes may be considered as hypothetical one-step desorption processes. Thus, heterogeneous catalytic reactions can be treated kinetically as consisting of two steps: adsorption and desorption. It is also illustrated that the hypothetical one-step desorption process follows the BEP relation. A volcano curve can be obtained from kinetic analysis by including both adsorption and desorption processes.