(385ag) CO2 Induced Separation of Terephthalic Acid for Polyester Upcycling

Alkali decomposition of polyesters for polymer upcycling is a well-established depolymerization pathway. Particularly, decomposition of polyethylene terephthalate (PET) with sodium hydroxide has been shown to be highly productive and selective towards disodium terephthalate (Na₂TP). Traditionally, strong mineral acids (e.g. HCl or H₂SO₄) are then used to protonate and precipitate the original monomer, terephthalic acid (H₂TP) that is virtually insoluble in water, from aqueous disodium terephthalate solutions. As a potential sustainable alternative to strong acids, we are investigating the use of gaseous carbon dioxide (CO₂) as an acid switch for the recovery of terephthalic acid from aqueous disodium terephthalate solutions.

In aqueous solution, disodium terephthalate is generally dissociated as sodium (Na⁺) and terephthalate ions (TP^-2). Upon the addition of CO₂, simultaneous phase and chemical equilibrium will occur. Gaseous CO₂ dissolves as aqueous CO₂ and simultaneously hydrates (carbonic acid) and dissociates to carbonate ions (HCO₃^- and/or CO₃^-2) and aqueous protons. From a solution of Na⁺, H⁺, terephthalate, bicarbonate and/or carbonate ions, terephthalic acid precipitation occurs.

The identity and quality of the produced terephthalic acid was verified by FTIR, NMR, ICP, and optical density measurements. Terephthalic acid yields up to 94 % were verified experimentally. Low Na₂TP loadings, low temperatures, and high pressures favor the recovery of terephthalic acid. The amount of terephthalic acid that can be recovered from aqueous disodium terephthalate solutions seems to be limited in some part by a buffering effect caused by the present of the co-product, sodium bicarbonate. Modeling using equations of state and activity coefficient models is presented.