Poor-solvating property of supercritical carbon dioxide (scCO 2 ) has been a great challenge, which limits the use of CO 2 as a common ''green'' solvent. The present report describes that by increasing molar volume (v) and lowering the melting temperature, which lowers cohesive energy density or solubility parameter (d), it is possible to increase the solubility of metal-based catalysts in scCO 2 without using costly fluorinated or tailormade CO 2 -philic modifications. We have studied various chlorodistannoxanes (1) and CuÁb-diketonates (2) to support our views. The study of bio-diesel production and transesterification of hindered esters using 1 in scCO 2 shows a 2Á8-folds rate enhancement coupled with an easier catalyst and product separation than that in organic solvents. The methodology, which works at least within the range of Van der Waals sphere of interactions, can be useful to solubilizing the molecules in scCO 2 and carries great opportunity in catalysis as well as in separation science.
The Hildebrand solubility parameter (δ), which predicts the miscibility of materials, was determined for the compounds that are involved in the oxidation of p-xylene to terephthalic acid. The δ values were determined from the measured intrinsic viscosity [η] of the compounds. The following compounds (namely, methyl benzoate, p-tolualdehyde, 4-hydroxymethyltoluene, terephthalaldehyde, benzoic acid, p-toluic acid, 4-hydroxymethylbenzoic acid, phthalic acid, trimellitic acid, 4-carboxybenzaldehyde, and isophthalic acid, including p-xylene as the reactant and terephthalic acid as the main product) were studied. The predicted δ values were obtained using the group contribution method, which helped in selecting the solvents to be used for viscosity measurements. Mixtures of solvents were used to obtain δ values for phthalic acid, isophthalic acid, terephthalic acid, and trimellitic acid. It was observed that δ is apparently related to the melting points of the compounds. Three-dimensional Hansen solubility parameters were also determined for important p-xylene oxidation products for a better understanding of their solubility behavior.
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