Carbon dioxide conventional catalysts

Palladium(II) acetate micro-encapsulated in polyurea is an economical and versatile heterogeneous catalyst for a range of phosphine-free cross-coupling reactions in both conventional solvents and supercritical carbon dioxide. The catalyst can be recovered by simple filtration and recycled up to four times [93]. The potential of these materials has been demonstrated by their efficacy in Suzuld-type couplings. Investigations have centered upon carbonylation reactions to prepare aryl esters from commercially available aryl iodides. Treatment of iodo-methyl benzene tvith 3 mol% of catalyst in butanol and triethylamine at 90 °C under an atmosphere of carbon monoxide afforded butyl-methyl benzoate in an excellent yield of 89% in 16 h. 

Finally, it should be noted that Lewis acids and bases can also be used in other non-conventional media, as described in Chapter 7, e.g. fluorous solvents, supercritical carbon dioxide and ionic liquids by designing the catalyst, e.g. for solubility in a fluorous solvent or an ionic liquid, to facilitate its recovery and reuse. For example, the use of the ionic liquid butylmethylimidazolium hydroxide, [bmim][OH], as both a catalyst and reaction medium for Michael additions (Fig. 2.45) has been recently reported [151]. 

The turnover frequency (TOF = mole of product per mole of catalyst per hour) of this rapid reaction is rather high, with values up to 1400. This reaction, carried out at 50 °C in SC-CO2, is 18 times faster than in conventional tetrahydrofuran under otherwise identical reaction conditions. This formic acid synthesis can be coupled with subsequent reactions by addition of methanol or dimethylamine, this supercritical reaction system provides a highly efficient one-pot route to methyl formate and A,A-dimethylforma-mide, respectively [918]. Another example of a reaction in which carbon dioxide acts as both reactant and reaction medium is the formation of tetraethyl-2-pyranone from hex-3-yne and CO2 in the presence of an Ni(II)-diphosphane catalyst at 102 °C under supercritical reaction conditions [919]. 

Steam reforming is usually carried out in fired tubular reactors, with catalyst packed inside the tubes and fuel fired on the outside of the tubes to provide the heat of reaction. The product gas mixture contains carbon dioxide and water vapor as well as carbon monoxide and hydrogen and is conventionally known as synthesis gas or syngas. 

Hydrogenation of carbon dioxide to methanol was investigated over Cu/ZnO catalysts prepared by mechanical alloying(MA) method, which is suitable for excellent mixing of different materials to make alloys or composites. The catalytic activity increases with mechanical milling time, and methanol yield over the catalyst milled for 120 hour is about 1.5 times higher than that of conventional coprecipitated Cu/ZnO catalyst. The reason for increasing catalytic activity by MA method can be attributed to the preparation of well mixed structure of Cu and ZnO nanocrystals. 

We present results on the catalytic hydrogenation of carbon dioxide to methanol in a conventional tubular packed-bed reactor filled with a copper based catalyst. In addition, results of an alternative approach using a dielectric-barrier discharge (DBD) reactor with and without the aid of a catalyst are presented. 

The oxidative coupling of isobutene can be performed in two separate steps, coimected with reduction of catalyst and reoxidation of the reduced catalyst afterwards. The two step process leads to an improvement of DMH selectivity as compared to the conventional process. The formation of carbon dioxide requires surface lattice oxygen from tbe catalyst, while formation of DMH occurs by abstraction of protons and electrons at the catalyst surface. They are absorbed on the catalyst bulk and, finally, react to water there. Thus, the rate of carbon dioxide formation is more affected by catalyst reduction than the rate of DMH formation. 

In a further variation, the PVP-supported rhodium catalyst was used for methanol carbonylation in supercritical carbon dioxide [100]. This reaction medium has complete miscibility with CO and dissolves high concentrations of methanol and methyl iodide, while being a poor solvent for ionic metal complexes. Catalytic reaction rates up to half of those obtained in conventional liquid-phase catalysis were achieved with minimal catalyst leaching. 

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