(755a) Polybenzimidazole Based Membranes for Organic Solvent Nanofiltration (OSN)

Polybenzimidazole Based Membranes for Organic Solvent Nanofiltration (OSN)

 Ruiyi Liu, Irina Valtcheva, Piers Gaffney, Patrizia Marchetti, Andrew Livingston

Barrer Centre, Department of Chemical Engineering, Imperial College London

Organic liquids are ubiquitous in chemical science based industries, which range in scale from refining to pharmaceutical production. It is generally accepted that 40-70% of capital and operating costs in these industries are dedicated to separations; and a substantial fraction of this cost is related to processing of organic liquids. Membrane technology utilised in Organic Solvent Nanofiltration (OSN) has the potential to provide game changing alternatives to conventional concentration and purification technologies such as evaporation, liquid extraction, adsorption and chromatography. The membranes must offer resistance to organic environments, attractive selectivities and permeabilities. Ideally they should also be resistant to physical aging under use.

A good polymer candidate which could overcome these challenges is 2,2–(m-phenylene)–5,5–bibenzimidazole (PBI), which belongs to the class of polybenzimidazoles and possesses thermal, mechanical and chemical stability towards organic solvents, acids and bases. The imidazole ring of PBI provides a reaction site which can be used for chemical modification (crosslinking) of the polymer chains. The polymer used for this study is commercially available Celazole PBI (~30 000 Da) in N,N-dimethylacetamide (DMAc) solution. Membranes were prepared by casting solutions of PBI with different concentrations on polypropylene non-woven fabric, followed by phase inversion in a water bath. In order to improve the chemical resistance, the polymer was modified in a post treatment step using crosslinking with 3 wt% p-α,α’-dibromoxylene (DBX) in organic solvent. The reactions were carried out at 70-80 °C for 24 h, and the chemical modification was verified by FTIR, microanalysis and x-ray photoelectron spectroscopy (XPS).

The chemical conditions of three commercial applications were chosen to demonstrate the performance of the obtained PBI membranes in extreme pH conditions – peptide and oligonucleotide synthesis, and carbon capture and storage (CCS). The challenging conditions are as follows: a) 20 % v/v piperidine/DMF (peptides); b) 3 % v/v dichloroacetic acid/acetonitrile (oligonucleotides); and c) 20 % v/v monoethanol amine/water (CCS). In order to assess the membrane stability in such media, first a dip test was performed. Small pieces of membrane were placed in the acidic and basic solutions for several months. No visible degradation of the polymer or colour change of the transparent solutions was observed. Filtration experiments were conducted in a crossflow set up using a range of polystyrene oligomers dissolved in solvent to determine MWCO. The polystyrene solution was then replaced by one of the basic or acidic solutions which were used to treat the membranes for several hours. Finally the polystyrene solution was filtered again through the membranes in order to evaluate whether the extreme pH change degraded them or not.

PBI membranes showed good performance after the acid and basic treatment in terms of rejection. However for the oligonucleotides application, residual cross-linking agent decorating the membrane with unreacted functional groups interfered with the oligo synthesis. This was overcome by a post-crosslinking functionalization which also improved permeance and allowed tailoring of the permeation properties.