A review of the effect of the addition of hydrogen in the selective catalytic reduction of NOx with hydrocarbons on silver catalysts

Research is progressing fast in the field of the hydrogen assisted hydrocarbon selective catalytic reduction (HC-SCR) over Ag-based catalysts: this paper is a review of the work to date in this area. The addition of hydrogen to the HC-SCR reaction feed over Ag/Al2O3 results in a remarkable improvement in NO (x) conversion using a variety of different hydrocarbon feeds. There is some debate concerning the role that hydrogen has to play in the reaction mechanism and its effect on the form of Ag present during the reaction. Many of the studies use in situ UV-Vis spectroscopy to monitor the form of Ag in the catalyst and appear to indicate that the addition of hydrogen promotes the formation of small Ag clusters which are highly reactive for NO (x) conversion. However, some authors have expressed concern about the use of this technique for these materials and further work is required to address these issues before this technique can be used to give an accurate assessment of the state of Ag during the SCR reaction. A study using in situ EXAFS to probe the H-2 assisted octane-SCR reaction has shown that small Ag particles (containing on average 3 silver atoms) are formed during the SCR reaction but that the addition of H-2 to the feed does not result in any further change in the Ag particle size. This points to the direct involvement of H-2 in the reaction mechanism. Clearly the addition of hydrogen results in a large increase in the number and variety of adsorbed species on the surface of the catalyst during the reaction. Some authors have suggested that conversion of cyanide to isocyanate is the rate-determining step and that hydrogen promotes this conversion. Others have suggested that hydrogen reduces nitrates to more reactive nitrite species which can then activate the hydrocarbon; activation of the hydrocarbon to form acetates has been proposed as the key step. It is probable that all these promotional effects can take place and that it very much depends on the reaction temperature and feed conditions as to which one is most important.

Liquid crystal tunable mm wave frequency selective surface

A frequency selective surface (FSS) which exploits the dielectric anisotropy of liquid crystals to generate an electronically tunable bandpass filter response at D Band (110-170 GHz) is presented. The device consists of two printed arrays of slot elements which are separated by a 130-mu m thick layer of liquid crystals. A 3% shift in the filter passband occurs when the substrate permittivity is increased by applying a control signal of 10 V. Measured results show that the insertion loss increases from -3.7 dB to -10.4 dB at resonance (134 GHz), thus demonstrating the potential to create a FSS which can be switched between a transmitting and a reflecting structure.

Regio- and stereo-selective dioxygenase-catalysed cis-dihydroxylation of fjord-region polycyclic arenes

Bacterial dioxygenase-catalysed cis-dihydroxylation of the tetracyclic arenes benzo[c]phenanthrene 2, and the isosteric compounds benzo[b]naphtho[1,2-d]furan 8, and benzo[b]naphtho[1,2-d]thiophene 9, has been found to occur exclusively at fjord-region bonds. The resulting cis-dihydrodiols 7, 10 and 11 were found to be enantiopure and of similar absolute configuration. cis-Dihydroxylation was also observed in the pseudo-fjord region of the 8,9,10,11-tetrahydro-precursors (12 and 13) of benzo[b]naphtho[1,2-d]furan 8, and benzo[b]naphtho[1,2-d]thiophene 9, to yield the corresponding enantiopure hexahydro cis-diols 14 and 15. A novel tandem cis-dihydroxylation and bis-desaturation of the tetrahydro-substrate, tetrahydrobenzo[b]naphtho[1,2-d]thiophene 13, catalysed by biphenyl dioxygenase, was found to yield the fjord-region cis-dihydrodiol 17 of benzo[b]naphtho[1,2-d]thiophene 9.

Hydrogenation/hydrogenolysis of disulfides using sulfided Ni/Mo catalysts

The mechanism of the hydrogenation/hydrogenolysis of dinitrodiphenyldisulfides using sulfided NiMo/ gamma Al2O3 catalysts has been examined in detail. Although two routes are possible, the major pathway involves an initial S-S bond cleavage followed by reduction of the nitro group. Importantly, the disulfide hydrogenolysis occurs in the absence of the catalyst with the role of the catalyst thought to be to activate the hydrogen and trap the cleaved intermediate as well as facilitate the reduction of the nitro group. Monitoring the mass balance throughout the reaction demonstrates the difficulty in measuring intrinsic kinetics for gas-liquid-solid reactions. Although the mass balance is restored at the end of the reaction, up to 45% of the substrate/products is found to be adsorbed on the catalyst during the reaction. (c) 2008 Elsevier B.V. All rights reserved.

Stabilization of Ti-molecular sieve catalysts used in selective sulfoxidation reactions by ionic liquids

Sulfoxidation reactions of 4,6-dimethyl-2-methylthiopyrimidine have been performed using titanosilicate catalysts in ionic liquids, dioxane and ethanol. The ionic liquid reactions showed superior reactivity compared with molecular solvents. Moreover, on examination of the recycling of the catalyst, a significant increase in the stability of catalyst was found both in terms of recycling activity and leaching of the titanium from the catalyst. The mechanism by which the ionic liquid reduces the solubilisation of the catalysts is explored.

Preparation of nanoparticulate metal catalysts in porous supports using an ionic liquid route; hydrogenation and C-C coupling

Aerogels containing palladium metal nanoparticles were prepared using an ionic liquid route and tested for activity towards hydrogenation and Heck C-C coupling reactions. (C) 2003 Elsevier B.V. All rights reserved.

Polymer-supported phosphoramidites: highly efficient and recyclable catalysts for asymmetric hydrogenation of dimethylitaconate and dehydroamino acids and esters

Several novel phosphoramidites have been prepared by reaction of the primary amines para-vinylaniline, ortho-anisidine, 2-methoxyphenyl(4-vinylbenzyl)amine, 8-aminoquinoline and 3-vinyl-8-aminoquinoline with (S)-1,1'-bi-2-naphthylchlorophosphite, in the presence of base. Rhodium(l) complexes of these phosphoramidites catalyse the asymmetric hydrogenation of dimethylitaconate and dehydroamino acids and esters giving ee values up to 95%. Soluble non-cross linked polymers of the para-vinylaniline and 3-vinyl-8-aminoquinoline-based phosphoramidites have been prepared by free radical co-polymerisation with styrene in the presence of AIBN as initiator. The corresponding [Rh(COD)](+) complexes serve as recyclable catalysts for the asymmetric hydrogenation dimethylitaconate and dehydroamino acids and esters to give ee values up to 80%. (C) 2003 Elsevier Science Ltd. All rights reserved.

Ruthenium complexes of the 1,4-bis(diphenylphosphino)-1,3-butadiene-bridged diphosphine 1,2,3,4-Me-4-NUPHOS: Solvent-dependent interconversion of four- and six-electron donor coordination and transfer hydrogenation activity

In chloroform, [RuCl2(nbd)(py)(2)] (1) (nbd = norbornadiene; py = pyridine) reacts with 1,4-bis(diphenylphosphino)-1,2,3,4-tetramethyl-1,3-butadiene (1,2,3,4-Me-4-NUPHOS) to give the dimer [Ru2Cl3(eta(4)-1,2,3,4-Me-4-NUPHOS)(2)]Cl (2a), whereas, in THF [RuCl2(1,2,3,4-Me-4-NUPHOS)(PY)(2)] (3) is isolated as the sole product of reaction. Compound 2 exists as a 4:1 mixture of two noninterconverting isomers, the major with C, symmetry and the minor with either C, or C-2 symmetry. A single-crystal X-ray analysis of [Ru2Cl3 (eta(4)-1,2,3,4-Me-4-NUPHOS)(2)] [SbF6] (2b), the hexafluoroantimonate salt of 2a, revealed that the diphosphine coordinates in an unusual manner, as a eta(4)-six-electron donor, bonded through both P atoms and one of the double bonds of the butadiene tether. Compounds 2a and 3 react with 1,2-ethylenediamine (en) in THF to afford [RuCl2(1,2,3,4-Me-4-NUPHOS)(en)] (4), which rapidly dissociates a chloride ligand in chloroform to give [RuCl(eta(4)-1,2,3,4-Me-4-NUPHOS)(en)] [Cl] (5a). Complexes 4 and 5a cleanly and quantitatively interconvert in a solvent-dependent equilibrium, and in THF 5a readily adds chloride to displace the eta(2)-interaction and re-form 4. A single-crystal X-ray structure determination of [RuCl(eta(4)-1,2,3,4-Me-4-NUPHOS)(en)][ClO4] (5b) confirmed that the diphosphine coordinates in an eta(4)-manner as a facial six-electron donor with the eta(2)-coordinated double bond occupying the site trans to chloride. The eta(4)-bonding mode can be readily identified by the unusually high-field chemical shift associated with the phosphorus atom adjacent to the eta(2)-coordinated double bond. Complexes 2a, 2b, 4, and 5a form catalysts that are active for transfer hydrogenation of a range of ketones. In all cases, catalysts formed from precursors 2a and 2b are markedly more active than those formed from 4 and 5a.