Hydrogen Purification Using Poly(ionic liquid)-Based Mixed Matrix Membranes

Hydrogen (H₂) purification is a critical step in hydrocarbon fuel processing for clean H₂ production, while simultaneously mitigating carbon dioxide (CO₂) emissions in electricity, power and fuel production schemes. The main challenge is to remove CO₂ and other impurities from syngas streams, leaving an enriched H₂ stream for clean and efficient energy generation. The development of efficient and cost-effective technologies for the purification of H₂ is of great importance.

Membranes are considered a very promising technology that shows clear advantages over the already existing technologies, namely low energy and operating costs, reduced environmental impact and small scale of the equipment.

In this regard, the main objective of this work is the development of a class of poly(ionic liquid)-based mixed matrix membranes (MMMs), containing a room temperature ionic liquid (RTIL) and a metal organic framework (MOF) as a filler, for the CO₂/H₂ separation from syngas streams. These membranes combine the high CO₂ affinity and high thermal stability of RTILs with the high permeability and selectivity of MOFs. Moreover, poly(ionic liquids) (PILs) combine the macromolecular structure of conventional polymers with the unique characteristics of ionic liquids, serving as a suitable polymeric matrix for (MMMs).

The MOFs studied in this work were MIL-53, Cu₃(BTC)₂ and ZIF-8. A pyrrolidinium-based PIL and an RTIL composed by the same structural anion [Tf₂N]^- were chosen.

PIL-RTIL composite membranes were first prepared by blending a PIL with a free RTIL. MOF-containing MMMs were then prepared, with different degrees of MOF incorporation (10, 20 and 30 wt.%), and their potential towards CO₂/H₂ separation was evaluated. The prepared membranes were characterized by Scanning Electron Microscopy (SEM); Thermogravimetric analysis to evaluate their thermal stability; Mechanical properties, through puncture tests and Fourier Transform Infrared (FTIR) spectroscopy in order to understand the interactions established between the polymer and MOF. In addition, single gas permeation experiments were carried out for CO₂ and H₂, at 30ºC and 0.7 bar, in order to determine MMMs permeabilities and ideal selectivity.

SEM images of the prepared membranes revealed a dense structure with no visible defects or voids, indicating a good compatibility between the PIL, RTIL and MOF particles. Moreover, it was also revealed a good dispersion of the MOF in the polymeric matrix. The single gas permeation results demonstrated an enhancement in CO₂ permeability with an increase in MOF loading, for all MMMs when compared with the pure PIL-RTIL composite membrane. In fact, the MMMs studied surpass the known permeability/selectivity trade-off correlation, thus proving that these membranes may be considered a potential option for application in CO₂/H₂ gas separation.