Carbon dioxide, supercritical hydrogenation reaction

In recent years, attempts have been made to make use of the advantages of the supercritical carbon dioxide in chemical reactions. The first technical examples concerning the use of carbon dioxide in a pilot-plant scale chemical reaction were heterogeneous catalyzed hydrogenation and radical polymerization [38-42]. Meanwhile, hydrogenation reactions have been scaled up in a 1000 t/a commercial multipurpose plant. 

Catalytic hydrogenation in supercritical carbou dioxide has been studied. The effects of temperature, pressure, and CO2 concentration on the rate of reaction are important. Hydrogenation rates of the two double bonds of an unsaturated ketone on a commercial alumina-supported palladium catalyst were measured in a continuous gra-dient-less internal-recycle reactor at different temperatures, pressures, and C02-to-feed ratios. The accurate control of the organic, carbon dioxide, and hydrogen feed flow rates and of the temperature and pressure inside the reactor provided reproducible values of the product stream compositions, which were measured on-line after separation of the gaseous components (Bertucco et al., 1997). 

High catalytic activities have been achieved by the PYRPHOS- [18], PPCP [20], BICHEP- [21], Et-DuPHOS-Rh [19] complexes among others, allowing the reaction with a substrate-to-catalyst molar ratios (S/C) as high as 50,000. With a [2.2]PHANEPHOS-Rh complex, the reaction proceeds even at -45°C [27], Supercritical carbon dioxide, a unique reaction medium, can be used in the DuPHOS and BPE-Rh-catalyzed hydrogenation [43], A highly lipophilic counteranion such as tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF) or trifluoromethanesulfonate is used to enhance the solubility of the cationic Rh complexes. Under the most suitable reaction conditions of 102 atm of carbon dioxide, 1 atm of hydrogen, and 22°C, a-amino acid derivatives are produced with up to 99.7% ee. 

The influence on the activation enCTgy by formation of a water containing activated complex is different in the two steps of reaction pathway [40]. As a consequence the decomposition of formic acid proceeds differently in the gas phase and in supercritical water. In the gas phase nearly complete reaction to CO and v/ater is observed. In supercritical water formic acid decomposes nearly completely to carbon dioxide and hydrogen [39,40]. This is shown experimentally [39] and by calculations [40]. The increase of the water gas shift reaction by water is measured in pure wato, when the density of water is raised above 0.35 g/cm [41]. This is a rafher high water density, thCTefore it is not clear wheth very small amounts of water are able to lead to the measured difference in the CO content (Figure 2a) by influencing the formic acid decomposition or not. 

Another interesting application of high-pressure tubes is the in-situ investigation of reactions in supercritical solvents such as carbon dioxide. For example, the iridium-catalyzed enantioselective hydrogenation of imines was investigated in a sapphire tube at 313 K [32]. 

Supercritical and Compressed Carbon Dioxide as Reaction Medium and Mass Separating Agent for Hydrogenation Reactions... 

Keywords Asymmetric Hydrogenation m Carbon Dioxide m Carbonylation m Dimethylformamide Enantioselectivity m Formic Acid m Homogeneous Hydrogenation n Palladium Catalysts Radical Reactions m Ruthenium Catalysts m Supercritical Fluids m Solvent Replacement... 

Very recently Jackson reported the use of supercritical carbon dioxide (SCCO2) in this tandem reaction, lowering the required total pressures of carbon monoxide and hydrogen [54]. 

Amorphous fluoropolymers have many applications in the areas of advanced materials where they are used in applications requiring thermal and chemical resistance. Their manufacture is hindered by their low solubility in many solvents. Many fluoropolymerizations cannot be carried out in hydrocarbon solvents because the radical abstraction of hydrogen atoms leads to detrimental side reactions. Chlorofluorocarbons (CFCs) were thus commonly used, but their use is now strictly controlled due to their ozone depleting and greenhouse gas properties. Supercritical carbon dioxide is a very attractive alternative to CFCs and it has been shown that amorphous fluoropolymers can be synthesized by... 

To study the effect of the Ru/Al Oj catalyst on hydrogen yield for refomung of glucose in supercritical water, the experiments were compared to reactions with and without catalytic runs imder identical conditions. Typical product distributions are shown in Table 6.9 for experiments with and without a Ru/Al Oj catalyst at 973 K with 1 wt.% glucose feed (Byrd et al., 2007). There was a significant reduction in carbon monoxide and methane yields in the presence of the catalyst. The main products of the reaction were hydrogen, methane, carbon dioxide, and carbon monoxide. The low carbon monoxide yield (0.1% by vol.) indicates that the water-gas shift reaction approaches completion. 

The three-phase catalytic hydrogenation of an unsaturated ketone using supercritical carbon dioxide as a solvent was studied in order to simulate the performance of a semi-industrial trickle-bed reactor. It is shown that supercritical CO2 strongly increases the reaction rate (Devetta et al., 1999). 

Heterogeneously catalyzed hydrogenation reactions can be run in batch, semibatch, or continous reactors. Our catalytic studies, which were carried out in liquid, near-critical, or supercritical C02 and/or propane mixtures, were run continuously in oil-heated (200 °C, 20.0 MPa) or electrically heated flow reactors (400 °C, 40.0 MPa) using supported precious-metal fixed-bed catalysts. The laboratory-scale apparatus for catalytic reactions in supercritical fluids is shown in Figure 14.2. This laboratory-scale apparatus can perform in situ countercurrent extraction prior to the hydrogenation step in order to purify the raw materials employed in our experiments. Typically, the following reaction conditions were used in our supercritical fluid hydrogenation experiments catalyst volume, 2-30 mL total pressure, 2.5-20.0 MPa reactor temperature, 40-190 °C carbon dioxide flow, 50-200 L/h ... 

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