Paul A. Hamley

Selective partial oxidation in supercritical water: the continuous generation of terephthalic acid from para-xylene in high yield

The continuous selective oxidation of p-xylene (p-X) by O2 (generated by thermal decomposition of aqueous H2O2) catalyzed by MnBr2 in supercritical H2O at ca. 400 oC is reported for the first time. The selectivity for terephthalic acid (TA) is >90%. Compared to existing industrial processes, the reaction has the potential for a significant increase in energy efficiency and a substantial reduction in waste. This reaction is significant because the presence of H2O lowers the catalytic activity of MnBr2 in the conventional route to TA via oxidation of p-X in CH3COOH.

In situ generation of hydrogen for continuous hydrogenation reactions in high temperature water

The thermal decomposition of HCO2H or preferably, HCO2X (X = Na or NH4) can be used to generate H2 for the continuous hydrogenation of aromatic and cyclic aldehydes, ketones and nitroaromatics in high temperature pressurised water (HTPW). This means that hydrogenation reactions can be carried out in exactly the same equipment as has previously been used for selective oxidation in HTPW, thus facilitating relatively simple application of these reactions for non-specialists.

Simultaneous continuous partial oxidation of mixed xylenes in supercritical water

In this paper we show that a mixture of xylenes can be simultaneously oxidised in supercritical water (scH2O) in a continuous mode to a mixture of the corresponding carboxylic acids in high combined yield, despite the differences in reactivity of the xylene isomers in conventional oxidation. The single phase environment in scH2O together with the effect of higher temperatures should increase the reaction rate for each of these oxidation reactions and thus reduce the reactivity differences between the components of the C8 refinery mixture. Such a process should lead to a considerable reduction in the overall energy input for the oxidation of xylenes. The process in scH2O described here could simplify the downstream purification processes to a simple crystallization process. This is commercially important, because the purification process can be as expensive as the reaction producing the product. Furthermore, the oxidation of mixed xylenes could avoid the need not only for downstream purification, but also for the upstream separation of the xylene isomers. The use of high temperature water also offers significant cost advantages through enhanced energy recovery, due to a higher process temperature. Finally, the process totally eliminates the use of organic solvents.

On the role of the pore filling medium in photoluminescence from photochemically etched porous silicon

Porous silicon thin films created under laser illumination in fluoride solutions without biasing have been studied by a variety of techniques to investigate the film structure and photoluminescence (PL). The use of ultrathin silicon wafers allows us to perform plan view transmission electron microscopy studies without recourse to thinning procedures that might adversely affect the film structure. Supercritically dried samples are compared to air dried samples and clearly demonstrate the deleterious effects of air drying on film structure. PL studies were performed (a) while the sample is submerged in aqueous HF, (b) in Ar after rinsing in ethanol, and (c) in air after rinsing in ethanol. The wavelength of light used to fabricate the film is found to correlate strongly with the peak PL wavelength when measured in solution. Little correlation is found in Ar or in air. Exposure to air can change the PL spectrum dramatically on a time scale of just seconds. We demonstrate that samples can exhibit essentially ...

Catalytic selective partial oxidations using O2 in supercriticalwater: the continuous synthesis of carboxylic acids

A variety of different alkylaromatic compounds having a range of alkyl groups (methyl, ethyl and isopropyl), varying aromatic groups (benzene, naphthalene and pyridine), and largely different ring electron densities (toluene and 3,4-dimethoxytoluene) have been successfully oxidized selectively to aromatic aldehyde and carboxylic acids in sub- and supercriticalwater using a continuous reactor under a wide range of experimental conditions. The estimated difference in initial reactivities of these substrates is approximately 10 000. Selected yields include toluene → benzoic acid (83%), isopropylbenzene → benzoic acid (46%), ethylbenzene → benzoic acid (68%), 2,6-dimethylnaphthalene → 2,6-naphthalenedicarboxylic acid (25%; co-oxidized with p-xylene) and 3,4-dimethoxytoluene → 3,4-dimethoxybenzoic acid (60%). 2-Nitrotoluene gave very poor yields, consistent with previous attempts to oxidize this substance homogeneously. For the first time, the reactivities and carboxylic acid yields of all three isomers of methylpyridine (picoline) have been determined. The initial reactivities are 3-isomer > 2-isomer > 4-isomer, the thermal decarboxylation rates are 2-isomer ≫ 3-isomer > 4-isomer, and the best yields are 3-isomer > 4-isomer > 2-isomer, with maximum yields of 50, 33, and 18 mol% respectively.

The continuous synthesis of ε-caprolactam from 6-aminocapronitrile in high-temperature water

Caprolactam (CPL) is a widely used chemical intermediate for the production of Nylon-6. However, existing synthetic routes in industry have severe drawbacks. The development on the synthesis of CPL from 6-aminocapronitrile (ACN), using near- and supercritical water as the solvent, reactant and catalyst, is described in this paper. The two-step reaction (hydrolysis and cyclization) to produce CPL is combined in a single process, by using a continuous-flow system. Effects of pressure, temperature, residence time and the concentration of ACN were studied. The high-temperature high-pressure environment possesses unique properties which result in very efficient catalysis. The overall CPL yield reaches 90% within a short residence time (<2 min).

Selective aerobic oxidation of para-xylene in sub- and supercritical water. Part 2. The discovery of better catalysts

An extensive and systematic study has been carried out on the catalytic effect of more than 20 elements on the aerobic oxidation of p-xylene to terephthalic acid in super- and subcritical water. Reactions have been performed in a continuous reactor under catalyst unsaturated conditions. Reaction product, by-products and intermediates have been quantified as well as the burn (the amount of CO2 originating from total oxidation of p-xylene). CuBr2 has been found to be a superior catalyst to MnBr2, which has been widely used in the literature for this reaction in water at high temperatures. At catalyst unsaturated conditions (i.e. with low concentrations of catalyst), MnBr2 gives a terephthalic acid yield of 36.1% whereas CuBr2 enhances this value to 55.6%. A strong synergistic effect has been found between CuBr2 and other metals and sources of bromide. Indeed, we show that Cu/Co/Br, Cu/Co/NH4/Br and other mixtures give better results than CuBr2 reaching a terephthalic acid yield of 70.5% for the four component catalyst. The compositions of the catalyst as well as the reactor temperature have been optimized and their effects on the analyzed compounds are discussed. A substantial amount of additional data is included in the electronic supplementary information.

Synthesis, X-ray crystal structure and photochemistry of (η5-pentamethylcyclopentadienyl)(dicarbonyl)(dihydrido) rhenium in cyclohexane and liquid xenon solutions and in low temperature media at about 12 K

An improved synthesis of ( η 5 -C 5 Me 5 )Re(CO) 2 (H) 2 has been devised (yield 88%) via ( η 5 -C 5 Me 5 )Re CO) 3 and ( η 5 -C 5 Me 5 )-Re(CO) 2 (Br) 2 . An X-ray crystallographic determination has shown that the hydrido ligands occupy trans positions, in confirmation of IR and NMR measurements. The properties and reactions of trans -( η 5 -C 5 Me 5 )Re(CO) 2 (H) 2 and related compounds ( η 5 -C 5 Me 5 )Re-(CO) 2 (X)(Y) (X  H, Me; Y  H, Me, Cl) are described. The solution photoctemistry of trans -( η 5 -C 5 Me 5 )Re CO 2 (H) 2 ) in cyclohexane at 298 K and in liquid xenon at 200 K, including studies under D 2 , indicate that the primary photoproduct is the cis isomer and that trans to cis interconversion, which can be reversed thermally, is an intramolecular process. Photochemical studies of ( η 5 -C 5 Me 5 )Re-(CO) 2 N 2 ) in liquid Xe under H 2 and D 2 pressures at 200 K gave cis -( η 5 -C 5 Me 5 )Re(CO) 2 and cis -( η 5 -C 5 Me 5 )Re(CO) 2 (D) 2 , respectively. Matrix isolation studies at about 12 K, including 13 CO labelling, confirmed that the photoisomerisation process is an intramolecular process since no ejected CO is observed and no 13 CO uptake occurred. subsequent photolysis affords H2 and CO ejection yielding ( η 5 -C 5 Me 5 )Re(CO) 2 and ( η 5 -C 5 Me 5 )Re(CO)(H) 2 , respectively . In N 2 and CO matrices the subsequent photolysis also yielded ( η 5 -C 5 Me 5 )Re(CO) 2 ( N 2 ) and ( η 5 -C 5 Me 5 )Re(CO) 3 but in CH 4 matrices there was no evidence of CH photoactivation to yield ( η 5 -C 5 Me 5 )Re(CO) 2 (CH 3 )(H). Reversals from cis to trans could not be observed for gas matrices but was observed at near ambient temperatures for the cis isomer produced at about 12 K in Nujol mull media.

Selective aerobic oxidation of para-xylene in sub- and supercritical water. Part 1. Comparison with ortho-xylene and the role of the catalyst

The selective, continuous, aerobic oxidations of para-xylene (pX) and ortho-xylene (oX) were performed in an identical fashion in supercritical water. The xylenes were oxidized without a catalyst and with hydrobromic acid, cobalt(II) and manganese(II) bromide catalysts. The conversions and yields to phthalic acid (OA) from oX were always significantly higher than those for terephthalic acid (TA) from pX. The formation of CO2 was significantly higher for pX than oX despite the higher conversions to oX. These results are unexpected because the literature teaches that thermal and catalytic decarboxylation is much higher for OA than TA. The superior yields from oX are consistent with a lower steady-state concentration of hydroxyl radicals, OH˙ due to the internal, concerted attack of the peroxides with the oX methyl group. This mechanism forms the phthalide directly from o-tolualdehyde (oTOL) which is consistent with the observation that ortho-toluic acid (OTA) is much lower in oX than para-toluic acid, PTA, in pX oxidation. This mechanism also lowers the steady-state concentration of aromatic acids consistent with the observed lower benzoic acid and CO2 yields. Overall, the results suggest that the metal catalysts can play more than one role, thereby opening up the opportunity for discovering new catalytic synergies which are explored in our next paper, Part 2 of this series.

Dihydride versus dihydrogen complex; the photochemical reaction of [(η5;-C5H5)M(CO)4](M = V, Nb and Ta) with hydrogen in solution at both cryogenic and room temperatures

UV photolysis of [cpM(CO)4](cp =η5-C5H5; M = V, Nb and Ta) and H2has been studied by IR spectroscopy both in n-heptane solution at 25 °C and in liquid Xenon solution at –110 to –45 °C; for V, the reaction leads to formation of the non-classical complex [cpV(CO)3(η2-H2)] while, for Ta, the dihydride [cpTa(CO)3H2] is formed; for Nb, there is a temperature dependent equilibrium between the classical [cpNb(CO)3H2] and non-classical [cpNb(CO)3(η2-H2)] compounds, which presents an unusual opportunity to study the formation of these species using nanosecond Time-resolved IR spectroscopy (TRIR).