(501e) Evaluating Sustainable Alternatives for Membrane Separation Processes: The Case of Poly Hydroxy Alkanoates (PHA)

Membrane technology is widely accepted to be a competitive alternative to traditional separation techniques, aiming to improve the overall sustainability of chemical processes. Still, most of the materials involved in the production of membranes cannot be considered sustainable, since they are fossil-based, and the possibilities to recycle them are somehow limited. This work aims to introduce a family of bio-based and biodegradable polymers as a sustainable alternative to materials traditionally used as membranes for gas separation.

Polyxydroxyalkanoates (PHAs) are a family of linear optically active semi-crystalline polyesters produced by bacterial fermentation, known for their fully renewable origin, biodegradability in many environments and physiological biocompatibility [1]. The gas transport properties of these materials are still scarcely characterized experimentally, and their determination is complicated by a number of uncertainty sources, such as a time-dependent degree of crystallinity. In this study, we aim at evaluating the structure-property relationships of different homo- and copolymers of the PHA family through molecular simulations, in order to gain information about their applicability in the membrane gas separation field. In particular, three homopolymers and two copolymers of the PHA family were considered:

• poly(3-hydroxybutyrate) (P3HB);
• poly(3-hydroxyvalerate) (P3HV);
• poly(4-hydroxybutyrate) (P4HB);
• poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV);
• poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (PHBB).

Molecular models of each material were simulated using Molecular Dynamics (MD), obtaining amorphous density and solubility parameter values that were successfully validated with experimental data found in literature [2, 3]. Other useful descriptors, such as accessible free volume and surface, were then evaluated and correlated to the chemical composition of different polymers.

The characterization of transport properties of the selected polymers was then initiated by evaluating sorption and diffusion values for two gases, CH₄ and CO₂, by means of Widom insertion method and MD simulations. The results were compared with experimental values, obtained through sorption tests, performed on PHBV with 8% of 3-hydroxyvalerate monomers, purchased from Merck-Sigma. The simulations showed how the transport properties in these materials are governed by the solubility-driven selectivity, throughout the whole composition range.

Furthermore, molecular simulations were used to obtain pressure-Volume-Temperature (pVT) data for the selected polymers that were fitted by different Equations of State. The parameters obtained from such a fitting were then used to extend the gas sorption predictions, giving a complete picture of transport properties for PHA family.

Molecular simulations provide a powerful screening method to identify the optimal biopolymer formulation for the desired gas separation process, e.g., CO₂/N₂, O₂/N₂ and CO₂/CH₄, allowing to minimize the experimental effort and to introduce new sustainable materials to the market.

References:

[1] K. Sudesh, H. Abe, Y. Doi, “Synthesis, structure and properties of polyhydroxyalkanoates: biological polyesters”, Prog. Polym. Sci., 2000, 25, 1503-1555.

[2] H. Mitomo, N. Morishita, Y. Doi, “Structural changes of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) fractionated with acetone-water solution”, Polymer, 1995, 36, 2573-2578.

[3] H. Mitomo, W.-C. Hsieh, K. Nishiwaki, K, Kasuya, Y. Doi, “Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) produced by Comamonas acidovorans”, Polymer, 2001, 42, 3455-3461.