Fabrication and characterization of highly selective porous polymeric materials for CO2 capture

The rapid increase in energy demand worldwide while fossil fuel remains its largest source has led to search for cost effective and energy efficient materials capable of selectively capture CO₂, released by fossil fuels. In CO₂ capture technology, post combustion carbon capture (PCC) is the best practicable capture technology as it can easily be retrofitted to existing plant without any crucial modifications. Ideal adsorbents for CO₂ capture are expected to feature some distinct hallmarks, among which comprise acceptable CO₂ uptake and high preferential CO₂/N₂ selectivity, low regeneration energy, stable cyclic adsorption capacity, stability under operating conditions, economic viability and less environmental toxicity. Most importantly in PCC, high CO₂ selectivity at low pressures is a key factor in adsorbents selection to attain an acceptable separation efficiency, which will not require additional CO₂ purification that can results in an increased capital and operational costs. In addition, the synthesised adsorbents production scalability, low regeneration energy and less environmental burden are other key factors that have been neglected. These key factors are the gap that this thesis addresses.

This work aimed at synthesis and characterisation of porous polymeric materials with high preferential selective for CO₂. The life cycle assessment of the polymers synthesised were also carried out. In the first stage, amide-based molecularly imprinted polymer (MIP) adsorbent was fabricated for post combustion CO₂ capture by the simple bulk polymerization method using methacrylamide as the functional monomer and oxalic acid as the template. The dynamic CO₂ adsorption capacities were investigated in a fixed bed adsorption column. The FTIR and XPS spectra revealed many -NH₂ functionality distributed on the MIPs surface, enhancing the CO₂ uptake capacity. From the dynamic CO₂ uptake assessment at 15/85 CO₂/N₂, the MIP with the highest concentration of template possessed the highest CO₂ capture capacity (0.40 mmol/g at 313 K and 0.15 bar partial pressure, SBET 258 m²/g), while the non-imprinted counterpart exhibited the lowest (0.34 mmol/g, SBET 250 m²/g) under the same condition. A significant difference was found in the capturing capacity of the template and non-template counterpart.

In the second stage, hypercrosslinked poly[methacrylamide-co-(ethylene glycol dimethacrylate)] porous polymeric particles with high CO₂-philicity, referred to as HCP-MAAMs, were synthesised for low pressure CO₂ capture. The polymers were inherently nitrogen-enriched and exhibited a high affinity towards selective CO₂ capture at low pressures, which is a key factor in PCC technology. The presence and density of NH₂ moieties within the polymer network were determined using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The thermogravimetric analysis (TGA) showed that the polymers were thermally stable up to 515–532 K. The maximum CO₂ adsorption capacity at 273 K was 1.56 mmol/g and the isosteric heat of adsorption was 28–35 kJ/mol. An increase in the density of amide groups within the polymer network resulted in a higher affinity towards CO₂ at low pressure. At a CO₂:N₂ ratio of 15:85, CO₂/N₂ selectivity at 273 K was 52 at 1 bar and reached 104 at ultra-low CO₂ partial pressure. The specific heat requirement was superior to most of conventional solvents such as MDEA-PZ and K₂CO₃. More so, the separation efficiency is high with purity of 90% and above, thus, does not require additional CO₂ purification.

In the third stage, the effect of crosslinker, solvent volume and initiator was probed on the HCP-MAAMs. The maximum CO₂ uptake reported at 273 K and 1 bar CO₂ partial pressure was up to 1.45 mmol g^-1 and the isosteric heat of adsorption was 27-35 kJ/mol. At 1 bar total pressure, 273 K, and a CO₂:N₂ molar ratio of 15:85, the polymer with the lowest crosslinking density and the smallest SBET (HCP-MAAM-2C) exhibited unprecedented CO₂/N₂ selectivity of 394, thus one of the highest reported so far in literature. In addition, the life cycle assessment revealed a lower environmental impact of HCP-MAAMs when compared to molecularly imprinted polymer particles produced from oil-in-oil emulsions by suspension polymerisation reported by another researcher. Thus, it can be said that HCP-MAAMs are promising materials for post-combustion CO₂ capture owing to their high selectivity, ease of regeneration, environmentally friendly fabrication process, and low regeneration energy requirement.

In the third stage, comparison was carried out for three polymeric adsorbents (HCP-MAAm-2, HCP-AAm and HCP-TAAm) fabricated for CO₂ adsorption using three different monomers, methacrylamide, acrylamide and triallylamine; crosslinked with ethylene glycol dimethacrylate (EGDMA) by bulk polymerization techniques. The effect of their functional group on CO₂ selectivities, CO₂ uptake, and their thermal stability was also reported. It was deduced from their surface spectra, via FTIR and XPS that the polymeric adsorbents fabricated from the monomer of methacrylamide and acrylamide performed better for CO₂ capture than triallylamine which could been attributed to the less reactivity of the latter been a tertiary polymer. Thermal gravimetric analysis revealed high temperature stability up to 300 °C. However, the polymer synthesis from triallylamine exhibited the lowest isosteric heat which could have been as a result of low affinity for CO₂ as it a tertiary polymer. The life cycle assessment (LCA) showed that poly(EGDMA-co-MAAM) had the lowest environmental impact due to the highest CO₂ uptake capacity.

In the last stage, polyacrylamide was synthesised without crosslinker. The polyacrylamide was electrospun to produced polyacrylamide nanofibers (PANF). To evaluate the optimum condition for electrospinning, polymer concentration, applied voltage, flow rate and tip-to-collector distance ratio effect on the fibres morphology and diameters were studied. The CO₂ and N₂ sorption uptake of the nanofiber was also investigated at 273 K. The nanofiber has low affinity for CO₂, which could have been because of the absence of crosslinking agent, thus revealing the importance of crosslinker in polymer synthesis for CO₂ capture. Also, another likely possible solution to enhance the CO₂ uptake could have been by carbonisation of the electrospun fibres at high temperatures.

The findings in this thesis indicates that a lot of parameters must be put into consideration during the formulation and synthesis procedure. With the correct formulation, high CO₂ capture with excellent selectivity, non-toxic and low regeneration energy ideal for PCC technology ideal for commercial and industrial purpose is attainable. More so, HCP-MAAM-2 was fabricated in large quantity, tested and approved for commercial purpose with our industrial partner.