(57h) Doping Polybenzimidazole (PBI) with Phosphoric Acid to Enhance Membrane H2/CO2 Separation Properties

Membranes
that permeate H₂ and reject CO₂ at temperatures above 150
°C are of great interest for low cost CO₂ capture in pre-combustion
processes. The current leading polymeric material for this application is poly[2,2’-(m-phenylene)-5,5’-bisbenzimidazole]
(PBI), which exhibits good thermal stability, strong size-sieving ability and good
H₂/CO₂ selectivity. In this study, we demonstrate that H₂/CO₂
selectivity in PBI can be significantly enhanced by doping with phosphoric acid
(H₃PO₄). H₃PO₄ can bond strongly
with imidazole rings in PBI, and the complexes are thermally stable up to 200 °C
under vacuum. We prepare a series of H₃PO₄ doped PBI
samples by immersing PBI thin films (12 µm) in solutions containing H₃PO₄
and methanol. The H₃PO₄ concentration can be varied from
0.05 wt.% to 1.0 wt.% to achieve different doping levels in the PBI films (0.3
- 1.0, defined as the molar ratio of H₃PO₄ to PBI
repeating unit). Increasing the doping level increases H₂/CO₂
selectivity and decreases H₂ permeability. For example, pure PBI exhibits pure-gas H₂
permeability of 27 Barrers and H₂/CO₂ selectivity of 14 at
150 °C, while the PBI
with a doping level of 0.44 exhibits H₂ permeability of 6.1 Barrers
and H₂/CO₂ selectivity of 59 at 150 °C. As the doping level increases
to 1.0, the PBI shows an impressive H₂/CO₂ selectivity of
136, though the H₂ permeability decreases to 1.3 Barrers at 150 °C. This performance is above the
upper bound in the Robeson’s plot for H₂/CO₂ separation. This
presentation will also discuss the effect of H₃PO₄ doping
on structural changes such as free volume, as well as CO₂ sorption
and diffusion, and the structure/property relationships in these H₃PO₄
doped PBI thin films.