Comparison of mass transfer effects in the heterogeneously catalysed hydrogenation of phenyl acetylene in heptane and an ionic liquid

The selective heterogeneous catalytic reduction of phenyl acetylene to styrene over palladium supported on calcium carbonate is reported in both an ionic liquid and a molecular solvent. By using a rotating disc reactor in conjunction with results from a stirred tank reactor it is possible, for the first time, to disentangle the mass transfer contributions in the ionic liquid system. For both heptane and 1-butyl-3-methyl imidazolium bis{(trifluoromethyl)sulfonyl}imide, the reaction in the rotating disc reactor is dominated by reaction in the entrained film on the disc compared with very limited reaction in the bulk liquid. The lower reaction rate obtained in the ionic liquid compared with the organic solvent is shown to be due to the slow transport of the hydrogen dissolved in the liquid. It is clear from the results presented herein that, although the hydrodynamics of similar reactors used for biological treatment of wastewater are well understood, on using a more viscous fluid and higher rotation speeds necessary for fine chemical catalysis these simple relationships breakdown.

Cathepsin L1, the major protease involved in liver fluke (Fasciola hepatica) virulence.

The secretion and activation of the major cathepsin L1 cysteine protease involved in the virulence of the helminth pathogen Fasciola hepatica was investigated. Only the fully processed and active mature enzyme can be detected in medium in which adult F. hepatica are cultured. However, immunocytochemical studies revealed that the inactive procathepsin L1 is packaged in secretory vesicles of epithelial cells that line the parasite gut. These observations suggest that processing and activation of procathepsin L1 occurs following secretion from these cells into the acidic gut lumen. Expression of the 37-kDa procathepsin L1 in Pichia pastoris showed that an intermolecular processing event within a conserved GXNXFXD motif in the propeptide generates an active 30-kDa intermediate form. Further activation of the enzyme was initiated by decreasing the pH to 5.0 and involved the progressive processing of the 37 and 30-kDa forms to other intermediates and finally to a fully mature 24.5 kDa cathepsin L with an additional 1 or 2 amino acids. An active site mutant procathepsin L, constructed by replacing the Cys26 with Gly26, failed to autoprocess. However, [Gly26]procathepsin L was processed by exogenous wild-type cathepsin L to a mature enzyme plus 10 amino acids attached to the N terminus. This exogenous processing occurred without the formation of a 30-kDa intermediate form. The results indicate that activation of procathepsin L1 by removal of the propeptide can occur by different pathways, and that this takes place within the parasite gut where the protease functions in food digestion and from where it is liberated as an active enzyme for additional extracorporeal roles.

syn-Benzene dioxides: chemoenzymatic synthesis from 2,3-cis-dihydrodiol derivatives of monosubstituted benzenes and their application in the synthesis of regioisomeric 1,2- and 3,4-cis-dihydrodiols and 1,4-dioxocins

cis-2,3-Dihydrodiol metabolites of monosubstituted halobenzenes and toluene have been used as synthetic precursors of the corresponding 3,4-cis-dihydrodiols. Enantiopure syn-benzene dioxide intermediates were reduced to the 3,4-cis-dihydrodiols and thermally racemised via the corresponding 1,4-dioxocins. The syn-benzene dioxide-1,4-dioxocin valence tautomeric equilibrium ratio was found to be dependent on the substituent position. The methodology has also been applied to the synthesis of both enantiomers of the 1,2-(ipso)- and 3,4-cis-dihydrodiols of toluene. This chemoenzymatic approach thus makes available, for the first time, all three possible cis-dihydrodiol regioisomers of a monosubstituted benzene.

The energetics of tetrahydrocarbazole aromatization over Pd(111): A computational analysis

The carbazole moiety is a component of many important pharmaceuticals including anticancer and anti-HIV agents and is commonly utilized in the production of modern polymeric materials with novel photophysical and electronic properties. Simple carbazoles are generally produced via the aromatization of the respective tetrahydrocarbazole (THCZ). In this work, density functional theory calculations are used to model the reaction pathway of tetrahydrocarbazole aromatization over Pd(111). The geometry of each of the intermediate surface species has been determined and how each structure interacts with the metal surface addressed. The reaction energies and barriers of each of the elementary surface reactions have also been calculated, and a detailed analysis of the energetic trends performed. Our calculations have shown that the surface intermediates remain fixed to the surface via the aromatic ring in a manner similar to that of THCZ. Moreover, the aliphatic ring becomes progressively more planer with the dissociation of each subsequent hydrogen atom. Analysis of the reaction energy profile has revealed that the trend in reaction barriers is determined by the two factors: (i) the strength of the dissociating ring-H bond and (ii) the subsequent gain in energy due to the geometric relaxation of the aliphatic ring. (c) 2008 American Institute of Physics.

Quantitative DRIFTS investigation of possible reaction mechanisms for the water-gas shift reaction on high-activity Pt- and Au-based catalysts

The present work emphasizes the importance of including a full quantitative analysis when in situ operando methods are used to investigate reaction mechanisms and reaction intermediates. The fact that some surface species exchange at a similar rate to the reaction product during isotopic transients is a necessary but not sufficient criterion for participation as a key reaction intermediate. This is exemplified here in the case of highly active low-temperature water-gas shift (WGS) catalysts based on gold and platinum. Operando DRIFTS data, isotopic exchanges, and DRIFTS calibration curves relating the concentration of formate species to the corresponding DRIFTS band intensity were combined to obtain a quantitative measure of the specific rate of formate decomposition. Despite displaying a rapid isotopic exchange rate (sometimes as fast as that of the reaction product CO2), the concentration of formates seen by DRIFTS was found to account for at most only 10% of the CO2 produced under the experimental conditions reported herein. These new results obtained on Au/CeZrO4 and Pt/CeO2 preparations (which are among the most active low-temperature WGS catalysts reported to date), led to the same conclusions regarding the minor role of IR-observable formates as those obtained in the case of less active Au/Ce(La)O-2 and Pt/ZrO2 catalysts. (c) 2007 Elsevier Inc. All rights reserved.

On the complexity of the water-gas shift reaction mechanism over a Pt/CeO2 catalyst: Effect of the temperature on the reactivity of formate surface species studied by operando DRIFT during isotopic transient at chemical steady-state

The present report investigates the role of formate species as potential reaction intermediates for the WGS reaction (CO + H2O -> CO2 + H-2) over a Pt-CeO2 catalyst. A combination of operando techniques, i.e., in situ diffuse reflectance FT-IR (DRIFT) spectroscopy and mass spectrometry (MS) during steady-state isotopic transient kinetic analysis (SSITKA), was used to relate the exchange of the reaction product CO2 to that of surface formate species. The data presented here suggest that a switchover from a non-formate to a formate-based mechanism could take place over a very narrow temperature range (as low as 60 K) over our Pt-CeO2 catalyst. This observation clearly stresses the need to avoid extrapolating conclusions to the case of results obtained under even slightly different experimental conditions. The occurrence of a low-temperature mechanism, possibly redox or Mars van Krevelen-like, that deactivates above 473 K because of ceria over-reduction is suggested as a possible explanation for the switchover, similarly to the case of the CO-NO reaction over Cu, I'd and Rh-CeZrOx (see Kaspar and co-workers [1-3]). (c) 2006 Elsevier B.V. All rights reserved.

One-pot multistep synthetic strategies for the production of fenpropimorph using an ionic liquid solvent

The one-pot synthesis of the fungicide fenpropimorph has been achieved using two different synthetic strategies in an ionic liquid. The first pathway consists of a Heck coupling followed by reductive amination; the second pathway consists of an aldol condensation followed by hydrogenation/reductive amination. Homogeneous and heterogeneous palladium catalysts have been utilised in the ionic liquid to provide a catalyst/solvent system that is suitable for recycling and process optimisation.

Utilisation of ionic liquid solvents for the synthesis of Lily-of-the-Valley fragrance {beta-Lilial (R); 3-(4-t-butylphenyl)-2-methylpropanal}

The Heck arylation of 2-methylprop-2-en- I -ol in ionic liquids and organic solvents is reported using a range of homogeneous and heterogeneous palladium catalysts. Higher activity is observed in the ionic liquid media compared with N-methyl pyrrolidinone and under solventless conditions. The ionic liquid-catalyst system may be recycled easily with little loss in activity, although significant palladium leaching from the heterogeneous catalyst was observed. In the case of Trans-bis(2,3-dihydro-3-methylbenzothiazole-2-ylidene)diiodopalladium (11) reported to be highly active for this transformation, significant induction petiods were observed indicating that nanoparticles may be responsible for the catalysis. Using the ionic liquid Heck reaction, a recyclable synthesis for the fragrance beta-Lilial((R)) has been developed. (c) 2004 Elsevier B.V. All rights reserved.

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.

Alternating copolymerisation of styrene and carbon monoxide in ionic liquids

Room temperature ionic liquids have been used as solvents for the palladium-catalysed copolymerisation of styrene and carbon monoxide. The behaviour of various ionic liquids, the nature and concentration of palladium catalyst, and the reusability of the catalyst-ionic liquid system are discussed. The effects of cation, anion and alkyl chain length of the ionic liquids on the reaction are also addressed. The yield of the polyketone in the ionic liquid systems is enhanced over conventional solvents studied under similar conditions.

First-principles study of oxygenates on Co surfaces in Fischer-Tropsch synthesis

Extensive density function theory calculations are performed to study the mechanism of the formation of aldehyde and alcohol on Co surfaces in Fischer-Tropsch synthesis, a challenging issue in heterogeneous catalysis. Three possible pathways for the production of formaldehyde and methanol on flat and stepped Co(0001) surfaces are investigated: (i) CO + 4H -> CHO + 3H -> CH2O + 2H -> CH3O + H -> CH3OH; (ii) CO + 4H -> COH + 3H -> CHOH + 2H -> CH2OH + H -> CH3OH; and (iii) the coupling reactions of CH2 + O -> CH2O and CH3 + OH -> CH3OH. It is found that these pathways are generally favored at step sites, and the preferred mechanism is pathway (i) via CHO. Furthermore, the three traditional chain growth mechanisms in Fischer-Tropsch synthesis are semi quantitatively compared and discussed. Our results suggest that the two mechanisms involving oxygenate intermediates (the CO-insertion and hydroxycarbene mechanisms) are less important than the carbene mechanism in the production of long chain hydrocarbons. However, the CO-insertion mechanism may be responsible for the production of long-chain oxygenates.

On the need to use steady-state or operando techniques to investigate reaction mechanisms: An in situ DRIFTS and SSITKA-based study example

The nature of the surface species formed at the surface of 2 wt.% Pt/CeO2 catalyst during the forward water-gas-shift (WGS, CO + H2O -> CO2 + H-2) and the reverse reaction (RWGS) were essentially identical. More, the surface concentration of formate, carbonate and carbonyl species was similar in each case. The presence of well-resolved IR bands allowed an unequivocal relative quantitative analysis of each species, avoiding the use of the carboxylate stretching region (1600-1200 cm(-1)). However, the quantitative analysis in the case of an isotopic study was complicated due to the overlapping of the various isotope bands, yet this problem could be overcome by integrating the high-wavenumber part of the bands. The reactivity of the surface species formed under RWGS conditions was followed under two different gaseous streams. Firstly, the reactivity of these intermediates were followed under an inert gas (i.e., At), in which case carbonates were essentially stable and less reactive than formates. Secondly, the reactivity of the same surface species was followed when switching to the corresponding C-13-labelled feed (i.e., (CO2)-C-13 + H-2), in which case carbonates were exchanged significantly faster than formates. While carbonates species have been reported as reaction intermediate under reaction conditions, the increased stability or surface poisoning by these carbonates in the absence of reaction mixture was highlighted. Ultimately, this work re-emphasises the need to use steady-state conditions if the true operando reactivity of the adsorbates and structure of the solid are to be determined. (c) 2005 Elsevier B.V. All rights reserved.

An investigation of alternative catalytic approaches for the direct synthesis of hydrogen peroxide from hydrogen and oxygen

Catalytic systems for the direct production of hydrogen peroxide from hydrogen and oxygen are investigated, and the factors which make a successful process identified. The use of low metal loadings, an organic co-solvent (such as ethanol) and reduced palladium as the catalytic metal all lead to good activity and selectivity. (C) 2002 Elsevier Science B.V. All rights reserved.

A review of the selective reduction of NOx, with hydrocarbons under lean-burn conditions with non-zeolitic oxide and platinum group metal catalysts

Research on the selective reduction of NOx with hydrocarbons under lean-burn conditions using non-zeolitic oxides and platinum group metal (PGM) catalysts has been critically reviewed. Alumina and silver-promoted alumina catalysts have been described in detail with particular emphasis on an analysis of the various reaction mechanisms that have been put forward in the literature. The influence of the nature of the reducing agent, and the preparation and structure of the catalysts have also been discussed and rationalised for several other oxide systems. It is concluded for non-zeolitic oxides that species that are strongly adsorbed on the surface, such as nitrates/nitrites and acetates, could be key intermediates in the formation of various reduced and oxidised species of nitrogen, the further reaction of which leads eventually to the formation of molecular nitrogen. For the platinum group metal catalysts, the different mechanisms that have been proposed in the literature have been critically assessed. It is concluded that although there is indirect, mainly spectroscopic, evidence for various reaction intermediates on the catalyst surface, it is difficult to confirm that any of these are involved in a critical mechanistic step because of a lack of a direct quantitative correlation between infrared and kinetic measurements. A simple mechanism which involves the dissociation of NO on a reduced metal surface to give N(ads) and O(ads), with subsequent desorption of N-2 and N2O and removal of O(ads) by the reductant can explain many of the results with the platinum group metal catalysts, although an additional contribution from organo-nitro-type species may contribute to the overall NOx reduction activity with these catalysts.

Chemoenzymatic synthesis of the trans-dihydrodiol isomers of monosubstituted benzenes via anti-benzene dioxides

Enantiopure cis-2,3-dihydrodiols, available from dioxygenase-catalysed cis-dihydroxylation of monosubstituted benzene substrates, have been used as synthetic precursors of the corresponding trans-3,4-dihydrodiols. The six-step chemoenzymatic route from cis-dihydrodiol precursors, involving acetonide, tetraol, dibromodiacetate and diepoxide intermediates, and substitution of vinyl bromide and iodide atoms, has been used in the synthesis of ten trans-dihydrododiol derivatives of substituted benzenes. The general applicability of the method has been demonstrated by its use in the synthesis of both enantiomers of the trans-1,2- and 3,4-dihydrodiol derivatives of toluene.