Abstract
The increasing need for sustainable processes stimulates the production and recovery of renewable organic acids. The purification of these acids is often difficult because of similar acid volatilities but can be improved through extractive distillation. Generic insights into solvent effects on the separation efficiency for close-boiling acids are however lacking. This study provides insights into the effect of acidity, the acidity difference between the acids, the hydrogen-bonding strength of the solvent, and the solvent-to-feed ratio on organic acid separation efficiency. For an acetic acid–formic acid (AA–FA) mixture, the addition of high-boiling organic acids increases the relative volatility of FA over AA significantly. The addition of a Lewis base reverses the relative volatility, which depends on the applied solvent-to-feed ratio and the Lewis base BF3 affinity. For several binary acid mixtures (such as AA–FA and monochloroacetic acid–dichloroacetic acid), where the acids have a relatively big difference in acidity (ΔpKa ≥ 1), the separation selectivity appears practically independent of the acid strength of the individual acids and increases with increasing BF3 affinity of the Lewis base. For acid mixtures with a lower ΔpKa, a lower separation selectivity is obtained, as observed for separation of the pivalic acid–butyric acid and valeric acid–2-methyl butyric acid mixtures. When one of the acids in the mixture contains a secondary ketone group (i.e., levulinic acid in a levulinic acid–octanoic acid mixture), the strongest acid based on pKa is not necessarily attracted most by the added Lewis base. This, at first sight, unexpected behavior is most likely the result of complex intra- and intermolecular interactions and is quantitatively in line with COSMO-RS-based selectivity predictions.
Highlights
Sustainability and circular economy initiatives have an increasing impact on the chemical industry, especially on how this industry needs to develop and execute its production processes
Separation of the target molecule from the solution at the required purity is often difficult and expensive. An example of such a difficult purification is the separation of acetic acid (AA) from aqueous streams containing formic acid (FA). This separation seems to be feasible via the Lenzing process[7] and the Wacker process,[8] which consist of an extraction step using trioctylphosphine oxide (TOPO) and methyl tert-butyl ether (MTBE), respectively, followed by several distillation steps
For the separation of FA and AA, other acids were selected as potential solvents based on the work reported in patent literature,[10−13] and Lewis base solvents were selected to allow a comparison of the separation of these relatively weak acids with results reported by Jongmans et al.[14] for the separation of the strong acids monochloroacetic acid (MCA) and dichloroacetic acid (DCA)
Summary
Sustainability and circular economy initiatives have an increasing impact on the chemical industry, especially on how this industry needs to develop and execute its production processes. A clear shift from oil-based raw materials to chemical building blocks from renewable sources (sustainability)[1] or from waste streams (circular economy)[2] has occurred This shift has among others resulted in a stronger focus on the production and separation of chemical building blocks with acid functionality, such as the volatile fatty acids acetic acid and butyric acid,[3,4] and the lignocellulosic-based levulinic acid (LA).[5,6] Sustainability strategies often imply that the required raw materials need to be separated from dilute solutions and/or from mixtures containing a variety of similar molecules. Separation of the target molecule from the solution at the required purity is often difficult and expensive An example of such a difficult purification is the separation of acetic acid (AA) from aqueous streams containing formic acid (FA). Some information about these Lenzing and Wacker processes can be found in patents, limited information about the separation steps, and especially about the distillation step to separate formic acid from acetic acid, is available in the open literature
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