Joan Fraga-Dubreuil

Organic reactions in high-temperature and supercritical water

This paper describes various examples of organic reactions carried out in high-temperature water (HTW) that have been performed at the University of Nottingham using either batch-type reactors or continuous-flow systems. Homogeneously and heterogeneously catalyzed partial oxidations of aromatic compounds, in situ hydrogenations, heterocyclic reactions, hydrolysis (HYD), saponification (SAP), and deuteration reactions are presented.

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.

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.

Rapid and clean synthesis of phthalimide derivatives in high-temperature, high-pressure H2O/EtOH mixtures

A variety of phthalimide derivatives have been synthesised effectively in high-temperature, high-pressure H2O/EtOH mixtures (HTHP-H2O/EtOH) as the solvent. This clean method is based on the condensation of o-phthalic acid and amines and affords phthalimide compounds as pure crystals in most cases, because of the dehydrating effect and change in solvation properties of H2O/EtOH at high pressures and temperatures. After conducting a series of model reactions, it was found that a mixture, 1/1 v/v H2O/EtOH, was appropriate for obtaining good yields combined with high purity of the phthalimides. Moderate to excellent yields were obtained depending on the nature of the amine. Aromatic amines generally afforded higher yields than aliphatic ones except for 3-hydroxypropylamine, where a yield up to 95% was obtained.

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.

Selective aerobic oxidation of para-xylene in sub- and supercritical water. Part 3: effects of geometry and mixing in laboratory scale continuous reactors

In this paper we report a strong dependence of the observed performance of the catalyst on the geometry and the configuration of laboratory scale reactors in the continuous aerobic oxidation of p-xylene in supercritical water. Small differences, such as the length of the feed pipes protruding into the reactor, have a very large effect on the observed yields and selectivities as well as on the reproducibility of the results. Different reactor designs also exert an influence on the perceived catalyst performance. We demonstrate that these effects are consistent with the relative efficiency of mixing of the reactant streams in the different reactors. The overall conclusion is that caution is required when comparing sets of data derived from studying such reactions even in apparently similar experimental arrangements.

A dramatic switch in selectivity in the catalytic dehydrogenation of 4-vinylcyclohexene in high pressure steam; a cautionary lesson for continuous flow reactions

We report an investigation of the continuous oxidative dehydrogenation of 4-vinylcyclohexene (VCH) in high pressure steam. Oxidative dehydrogenation reactions such as this are often limited by unselective or total oxidation. We find that these side-reactions not only occur in this case, but can also have a major influence on the selectivity; a small increase in flow rate results in a complete switch in selectivity of the reaction. Our results suggest that styrene (ST) is formed as the initial product but that unless the H2 is sequestered, ST may then be hydrogenated to yield ethylbenzene (EB). The observation of periodic temperature spikes near the surface of the catalyst bed indicate cycles of propagating flames occur in a relatively small volume of the reactor, leading to total oxidation of some VCH and removal of O2, which would otherwise sequester the H2. These flames give rise to the large variations in the observed product selectivity. We suggest that these observations may not be restricted to this reaction system and reactor configuration, and may occur in other situations where small-scale continuous flow oxidation reactors are used.