Paul B Webb | PubMed Health

PubMed2004

Ionic liquids and eutectic mixtures as solvent and template in synthesis of zeolite analogues.

The challenges associated with synthesizing porous materials mean that new classes of zeolites (zeotypes)-such as aluminosilicate zeolites and zeolite analogues-together with new methods of preparing known zeotypes, continue to be of great importance. Normally these materials are prepared hydrothermally with water as the solvent in a sealed autoclave under autogenous pressure. The reaction mixture usually includes an organic template or 'structure-directing agent' that guides the synthesis pathway towards particular structures. Here we report the preparation of aluminophosphate zeolite analogues by using ionic liquids and eutectic mixtures. An imidazolium-based ionic liquid acts as both solvent and template, leading to four zeotype frameworks under different experimental conditions. The structural characteristics of the materials can be traced back to the solvent chemistry used. Because of the vanishingly low vapour pressure of ionic liquids, synthesis takes place at ambient pressure, eliminating safety concerns associated with high hydrothermal pressures. The ionic liquid can also be recycled for further use. A choline chloride/urea eutectic mixture is also used in the preparation of a new zeotype framework.

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PubMed2004

Continuous flow homogeneous catalysis using supercritical fluids.

The continuous flow hydroformylation of 1-octene catalysed by Rh/[RMIM][Ph(2)PC(6)H(4)SO(3)](R = 1-propyl, 1-pentyl or 1-octyl) dissolved only in the steady state reaction mixture and using scCO(2) as a transport vector for both substrates and products gives rates up to 160-240 catalyst turnovers h(-1) with low rhodium leaching over a 12 h period at a total pressure of 125-140 bar.

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PubMed2003

Continuous flow hydroformylation of alkenes in supercritical fluid-ionic liquid biphasic systems.

A process for the hydroformylation of relatively low volatility alkenes (demonstrated for 1-dodecene) in a continuous flow system is described. The catalyst is dissolved in an ionic liquid while the substrate and gaseous reagents are transported into the reactor dissolved in supercritical CO(2), which simultaneously acts as a transport vector for aldehyde products. Decompression of the fluid mixture downstream yields products which are free of both reaction solvent and catalyst. The use of rhodium complexes of triaryl phosphites leads to ligand degradation through reaction of the ionic liquid with water and subsequent attack of the released HF on the phosphite. Sodium salts of sulfonated phosphines are insufficiently soluble in the ionic liquids to obtain acceptable rates, but replacing the sodium by a cation similar to that derived from the ionic liquid, allows good solubility and activity to be obtained. The nature of the ionic liquid is very important in achieving high rates, with 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amides giving the best activity if the alkyl chain is at least C(8). Catalyst turnover frequencies as high as 500 h(-1) have been observed, with the better rates at higher substrate flow rates. Rhodium leaching into the product stream can be as low as 0.012 ppm, except at low partial pressures of CO/H(2), when it is significantly higher. Oxygen impurities in the CO(2) feed can lead to oxidation of the phosphine giving higher rates, lower selectivities to the linear aldehyde, increased alkene isomerization and greater leaching of rhodium. However, it is found that under certain process conditions, the supercritical fluid-ionic liquid (SCF-IL) system can be operated continuously for several weeks without any visible sign of catalyst degradation. Comparisons with commercial hydroformylation processes are provided.

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