Anthony A. Clifford

Solubilities of solids and liquids of low volatility in supercritical carbon dioxide

A table is given of the compounds of low volatility, whose experimental solubilities in supercritical carbon dioxide have been published up to the end of 1989, with the temperature and pressure ranges of the experimental measurements, the experimental method, and references to the source of data. The data for pure compounds, which were presented in tabular form in the original publications, are shown in a series of figures along with correlation lines for each isotherm. The method of correlation was to fit the experimental data for each isotherm, in the form of the natural logarithm of the product of mole fraction and pressure, to a linear function of density above a pressure of 100 bars. The constants obtained from the fitting procedures are given in a table. Procedures for estimating, from these constants, the solubilities of the compounds at temperatures and pressures different from those of the experimental data are suggested.

A model for dynamic extraction using a supercritical fluid

Abstract An idealized model is suggested for the analysis of the results of extraction in a flow system by a supercritical fluid from a matrix which contains small quantities of extractable materials, such that the extraction is not limited by solubility. The model is that of diffusion out of a homogeneous spherical particle into a medium in which the extracted species is infinitely dilute. The effect of matrix shape and size variation and also solubility limitations on the model, which occur in real systems, are discussed. Predictions of the model are compared with experimental results for systems which are mostly physically unlike the ideal model. However, important features of the behavior of these systems are found to accord with the predictions of the model. The use of the model gives an insight useful for the design of industrial extraction processes and also provides an extrapolation method for obtaining quantitative analytical extraction results relatively quickly.

Supercritical fluid extraction in environmental analysis

Abstract Supercritical fluid extraction (SFE), usually with carbon dioxide and often with a modifier, is a rapid, selective and convenient method for sample clean-up in environmental analysis. Three inter-related factors influence analyte recovery in SFE: solubility in the fluid, diffusion through the matrix and adsorption in the matrix. SFE may be coupled on-line to various analytical methods: gas, liquid and supercritical fluid chromatography. A wide range applications of SFE from environmental samples is described: hydrocarbons, chlorobenzenes and chlorobiphenyls, dioxins and chlorinated pesticides, herbicides and ionic surfactants. Organic compounds may be concentrated from air and water and extracted from adsorbents by SFE. Direct SFE from water is also possible.

Capillary electrochromatography using columns packed with a supercritical fluid carrier

SummaryColumns for capillary electrochromatography may be prepared by packing reversed phase silica-based material using a supercritical fluid carbon dioxide carrier. Procedures for the in-situ manufacture of frits and UV detection windows, and the wetting of columns are described. The columns were employed in two commercial instruments (and a home-built system), and their properties investigated during the separation of standard mixtures of test compounds. The columns are highly efficient and durable, with reduced plate heights of 1.0–1.4. The repeatability of retention time, peak area and peak height was measured. The influence of applied voltage and column temperature and of electrokinetic injection parameters was explored, along with other practical considerations.

USE OF FLUORINATED PALLADIUM SOURCES FOR EFFICIENT PD-CATALYSED COUPLING REACTIONS IN SUPERCRITICAL CARBON DIOXIDE

Abstract Commercially available palladium sources [Pd(OCOCF 3 ) 2 and Pd(F 6 -acac) 2 ] containing fluorinated ligands are superior to non-fluorinated sources [Pd(OCOCH 3 ) 2 and Pd 2 (dba) 3 ] for carrying out a variety of palladium catalysed coupling reactions in supercritical carbon dioxide. Significantly reduced temperatures and catalyst loadings are possible, and also the range of phosphine ligands which can be used is increased, including those that are often considered to be poor ligands for coupling reactions under conventional conditions (PCy 3 , PBu 3 ).

A simple solvent selection method for accelerated solvent extraction of additives from polymers

A simple solvent selection procedure for accelerated solvent extraction (ASE®) of polymers is described using Hildebrand solubility parameters. A series of extractions with a solvent with a solubility parameter several Hildebrand units (MPa1/2 ) different from the polymer are carried out at increasing temperatures until maximum extraction is reached. A second solvent with a solubility parameter close to that of the polymer is then added incrementally at the optimum temperature found from the initial experiments, until a maximum in the extraction rate is reached. This method is used to determine optimum conditions for ASE® of additives from ground polypropylene (PP), poly(vinyl chloride) (PVC) and nylon. Complete extractions were possible of Irganox 1010 (pentaerythrityl tetrakis (3,5-di-tert-butyl-4-hydroxyphenyl) propionate) from freeze ground PP in 18 min (propan-2-ol–cyclohexane (97.5∶2.5) at 140 °C), and dioctyl phthalate (DOP) from PVC in 13 min (hexane–ethyl acetate (60∶40) at 170 °C). ASE® extraction from PVC pellets was possible in reasonable time (1 h) without grinding or cutting the polymer. The results were not significantly different from those obtained by conventional solvent extraction methods. From nylon, 96% extraction of the dimer in 19 min (hexane–ethanol (60∶40) at 170 °C) was possible, compared to the total amount extracted by exhaustive ASE®. The behaviour of poly(ethylene terephthalate) (PET) and poly(methyl methacrylate) (PMMA) was observed with different solvents at high temperature to determine likely optimum extraction conditions. These were ethyl acetate at 190 °C for PET and hexane–ethyl acetate (70∶30) at 150 °C for PMMA.

Suppression of double bond isomerisation in intramolecular Heck reactions using supercritical carbon dioxide

The modified reagent system [Pd(OCOCF3)2/P(2-furyl)3/EtNiPr2] gives enhanced conversions compared with standard conditions [Pd(OAc)2/PPh3/NEt3] for intramolecular Heck cyclisations, and when used in scCO2 results in the suppression of double bond isomerisation to minimal levels, which is otherwise a serious competing side reaction in conventional solvents.

The Effect of solubility on the kinetics of dynamic supercritical-fluid extraction☆

Abstract A model is developed for dynamic supercritical-fluid extraction, which includes the effects of both solvation by the fluid and diffusion out of the matrix. The model is consistent with many of the features of available experimental evidence. It predicts the forms of both typical ‘pressure threshold’ and kinetic curves for dynamic extraction.

Reactions in Supercritical Fluids

The reasons for carrying out chemical reactions in supercritical fluids are discussed. Most of these motivations relate to the physical behaviour exhibited in these media, in particular phase behaviour, diffusion rates and ‘anomalies’, and solvation effects. Examples are given of the use of phase and diffusion behaviour. The transition-state theory is developed for supercritical fluids and used to explain solvation effects in chemical reactions in the media. The possibilities of product control in supercritical fluids via diffusion and solvation effects are outlined. Finally the use the continuously varying solvent properties available in supercritical fluids for the fundamental study of reaction mechanisms is reviewed. Because of their environmental advantages, discussions focus upon the use of carbon dioxide and water as fluids.

Reactions in Unusual Media

This review provides an introduction to three of the most well-developed solvent replacement strategies currently under investigation for synthetic chemistry: Ionic liquids, fluorous phase techniques, and supercritical carbon dioxide. They are all fascinating reaction media, and have considerable potential for use in pharmaceutical synthesis. However, this has to be balanced with problems and limitations of the new methods. This review aims to provide an overall account of recent advances in the use of unusual media for synthetic chemistry, with an emphasis on highlighting potential benefits, but also limitations, of each of the methods described.