Recycling chromatography

Multidimensional chromatography has been called column switching, coupled-column chromatography, recycle chromatography, and mode sequencing, among other terms. 

Other modes of operation, including recycle and flow reversal schemes and continuous chromatography, are discussed in Ganetsos and Barker (Preparative and Production Scale Chromatography, Marcel Dekker, New York, 1993). 

We further synthesized unsymmetrical MiniPHOS derivatives 13b (Scheme 13) [30]. Thus, enantioselective deprotonation of l-adamantyl(dimethyl)phos-phine-borane (74, R = 1 -Ad), followed by treatment with ferf-butyldichlorophos-phine or 1-adamantyldichlorophosphine, methylmagnesium bromide and bo-rane-THF complex afforded the optically active diphosphine-boranes 82b as a mixture with the corresponding raeso-diastereomer. Enantiomerically pure unsymmetrical MiniPHOS-boranes 82b were obtained by column chromatography on silica gel or separation by recycling preparative HPLC. 

Both recycled and non-recycled essential oils met the standards for specific gravity, angular rotation and refractive index of the Food Chemicals Codex [22] and United States Pharmacopeia [23] for cold-pressed lemon peel oil. Results of gas chromatography analysis show the same compounds and levels in both systems, specially for the oxygenated compounds as citral (neral and geranial). Therefore, recycling the aqueous discharges to the extractor does not cause undesirable modifications. 

The recycle reactor is shown schematically in Figure 1. It consists of a catalytic or electrocatalytic reactor unit with a bypass loop, a recycle pump and a molecular sieve trap unit. The latter comprises one or two packed bed columns in parallel each containing 2-10 g of Linde 5A molecular sieve pellets. On line gas chromatography (Shimadzu 14A) was used for the analysis of CH4, O2, CO, CO2, C2H4 and C2H6 in the reactants and products. 

Hydrolytic Kinetic Resolution (HKR) of epichlorohydrin. The HKR reaction was performed by the standard procedure as reported by us earlier (17, 22). After the completion of the HKR reaction, all of the reaction products were removed by evacuation (epoxide was removed at room temperature ( 300 K) and diol was removed at a temperature of 323-329 K). The recovered catalyst was then recycled up to three times in the HKR reaction. For flow experiments, a mixture of racemic epichlorohydrin (600 mmol), water (0.7 eq., 7.56 ml) and chlorobenzene (7.2 ml) in isopropyl alcohol (600 mmol) as the co-solvent was pumped across a 12 cm long stainless steel fixed bed reactor containing SBA-15 Co-OAc salen catalyst (B) bed ( 297 mg) via syringe pump at a flow rate of 35 p,l/min. Approximately 10 cm of the reactor inlet was filled with glass beads and a 2 pm stainless steel frit was installed at the outlet of the reactor. Reaction products were analyzed by gas chromatography using ChiralDex GTA capillary column and an FID detector. 

Quinones, I., Grill, C.M., Miller, L., and Guiochon, G., Modeling of separations by closed-loop steady-state recycling chromatography of a racemic pharmaceutical intermediate,. Chromatogr. A, 867, 1, 2000. 

Dawkins, J. V., Forrest, M. J., and Shepherd, M. J., Separation of vinyl chloride oligomers by recycle high performance size exclusion chromatography, /. Liq. Chromatogr., 13, 3001, 1990. 

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