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

Authors: 
Zhu, L., University at Buffalo, The State University of New York
Lin, H., University at Buffalo, The State University of New York
Swihart, M. T., University at Buffalo, The State University of New York

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.