The Promise of Spirocyclic Polymers of Engineered Mobility in Membrane-Based Liquid Separations

The initial separation of crude oil components is an energy-intensive process, consuming approximately 1,100 terawatt-hours (TWh) per year, equivalent to around 1% of humanity’s total energy expenditure. This staggering energy consumption primarily arises from the thermal distillation method employed, which separates compounds based on their boiling points. Such a process is not only energy-intensive but also environmentally unsustainable. Membrane-based separations offer a promising alternative by differentiating molecules based on size, shape, or polarity. However, the practical implementation of membrane-based separations has been hampered by the scarcity of solution-processable polymers that can maintain the required permeance and selectivity without undesirable swelling, plasticization, or dissolution in organic mixtures.

Polymers of intrinsic microporosity (PIMs) have been at the forefront of gas separation technologies due to their high free volumes and interconnected pores. While PIMs have shown promise in organic solvent separations, they face limitations when dealing with small molecules (molecular weight < 600 Da) due to polymer dilation under such conditions. The spirobifluorene aryl diamine (SBAD) family of polymers, introduced to address these issues, exhibited reduced swelling compared to PIM-1. However, their performance was limited to light shale crude separation at low stage cuts.  A recent study published in the prestigious journal Nature Materials, led by Professors Ryan Lively and M. G. Finn from the Georgia Institute of Technology, brings a ray of hope by introducing a novel class of polymers, known as “DUCKY” materials, designed to revolutionize the landscape of liquid separations. In their quest to address the shortcomings of existing membrane materials, the authors introduced a groundbreaking class of polymers, referred to as “DUCKY” materials. These polymers, aptly named after the copper-catalyzed azide-alkyne cycloaddition reaction (CuAAC), are designed to be soluble in common organic solvents, scalable, and capable of membrane-based fractionation of various crude oil feeds. These materials represent a significant departure from traditional PIMs, emphasizing the importance of spirocyclic building blocks.

The synthesis of DUCKY polymers involves azide- and alkyne-containing monomers that can be prepared efficiently and at scale. Gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) spectroscopy were employed to characterize the resulting polymers. Notably, DUCKY polymers exhibited high molecular weights and solubility in various organic solvents, making them suitable for solution-phase processing into membranes. Their step-growth polymerization process was efficient and amenable to large-scale production. The authors evaluated the separation performance of the thin-film composite membranes fabricated from DUCKY polymers in molecular separations. These membranes outperformed traditional PIM-1 in solute rejection, particularly for molecules with molecular weights around 500 g/mol. DUCKY-9, in particular, exhibited high permeance and selectivity in the separation of a 12-component mixture representing lower-boiling components of crude oil. Moreover, these membranes demonstrated excellent selectivity for molecules within the diesel boiling range.

One of the most promising aspects of DUCKY materials is their potential to revolutionize crude oil fractionation. These materials, especially DUCKY-9, showed remarkable enrichment of light hydrocarbons in Arabian light crude oil at low stage cuts. Additionally, they exhibited impressive removal of impurities such as sulfur, nitrogen, and heavy metals, surpassing the performance of traditional polyimide membranes.

The study also explored the separation of atmospheric tower bottoms (ATBs), a challenging distillation cut in oil refining. DUCKY-10 proved to be better suited for this task, offering substantial enrichment of valuable components and reducing sulfur and metal impurities. A deep dive into the molecular properties of DUCKY polymers revealed their unique characteristics. They exhibited a balance between chain mobility and structural rigidity, allowing for dynamic connections of microporous void spaces without excessive swelling that would compromise selectivity. Molecular modeling and pore analysis shed light on their separation performance, highlighting the importance of considering both kinetic and thermodynamic factors.

In conclusion the new developed DUCKY materials represents a significant advancement in the field of membrane-based liquid separations. Their synthesis, scalability, and remarkable performance in separating complex hydrocarbon mixtures offer a glimpse into the future of energy-efficient and environmentally sustainable separation processes. The ability to tailor these materials for specific applications and their compatibility with various feed compositions make them a promising candidate for addressing the global energy and resource challenges.