The chemical absorption of CO 2 into a monoethanolamine solvent is currently the most widely accepted commercial approach to carbon dioxide capture. However, the subsequent desorption of CO 2 from the solvents is extremely energy intensive. Alternative solvents are more energy efficient, but their slow reaction kinetics in the CO 2 absorption step limits application. The use of a carbonic anhydrase (CA) enzyme as a reaction promoter can potentially overcome this obstacle. Native, engineered and artificial CA enzymes have been investigated for this application. Immobilization of the enzyme within the gas absorber or in a membrane format can increase enzyme stability and avoid thermal denaturation in the stripper. However, immobilization is only effective if the mass transfer of carbon dioxide through the liquid phase to reach the immobilization substrate does not become rate controlling. Further research should also consider the process economics of large-scale enzyme production and the long-term performance of the enzyme under real flue gas conditions.

The use of potassium carbonate as a solvent for the absorption of carbon dioxide is constrained by slow absorption kinetics, which hinders the overall rate of mass transfer. In this work, the reaction rate is promoted by the electrostatic adsorption of carbonic anhydrase (CA) onto the surface of both a porous polypropylene (PP) and a non-porous polydimethoxysilane (PDMS) hollow fiber membrane via layer-by-layer (LbL) assembly. The rate of CO 2 absorption into K 2 CO 3 is increased approximately threefold when CA is adsorbed onto the PP membrane surface, while the absorption rate of the modified PDMS membrane was slightly lower, within 70 to 90% of the PP values. The results show that the ultrathin CA films are assembled mainly on the surface of the membranes and do not penetrate into the depth of the membrane pores. The CO 2 hydration is enhanced in all cases, and the wetting of the porous PP membranes is reduced significantly by the pore blockage induced by the LbL adsorption of the polyelectrolytes.

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Carbonic anhydrase (CA) is a native enzyme that facilitates the hydration of carbon dioxide into bicarbonate ions. This study reports the fabrication of thin films of active CA enzyme onto a porous membrane substrate using layer-by-layer (LbL) assembly. Deposition of multilayer films consisting of polyelectrolytes and CA was monitored by quartz crystal microgravimetry, while the enzymatic activity was assayed according to the rates of p-nitrophenylacetate (p-NPA) hydrolysis and CO2 hydration. The fabrication of the films onto a nonporous glass substrate showed CO2 hydration rates of 0.52 ± 0.09 μmol cm(-2) min(-1) per layer of bovine CA and 2.6 ± 0.7 μmol cm(-2) min(-1) per layer of a thermostable microbial CA. The fabrication of a multilayer film containing the microbial CA on a porous polypropylene membrane increased the hydration rate to 5.3 ± 0.8 μmol cm(-2) min(-1) per layer of microbial CA. The addition of mesoporous silica nanoparticles as a film layer prior to enzyme adsorption was found to increase the activity on the polypropylene membranes even further to a rate of 19 ± 4 μmol cm(-2) min(-1) per layer of microbial CA. The LbL treatment of these membranes increased the mass transfer resistance of the membrane but decreased the likelihood of membrane pore wetting. These results have potential application in the absorption of carbon dioxide from combustion flue gases into aqueous solvents using gas-liquid membrane contactors.

A microbial carbonic anhydrase was immobilized onto the surface of hollow fiber membranes to enhance the absorption rate of carbon dioxide into an aqueous 30 wt% potassium carbonate solvent at pH 10-12. The performance of the immobilized enzyme was investigated over long time frames to determine the suitability of this approach for postcombustion carbon capture. The immobilized enzyme was able to maintain activity for up to 80 days at 25°C even when initially exposed to pH 12, however lost activity rapidly when exposed to such high pH at 50°C and subsequently was completely deactivated within 4 days. The effect of sulfur dioxide and nitric oxide that are present in post-combustion flue gas streams was investigated by contacting enzyme immobilized onto flat planar membrane coupons directly with the dry gases and with solutions containing the anions associated with the gases, namely sulfate, sulfite, nitrate and nitrite. The immobilized enzyme maintained at least 80% of its activity even in the presence of 200 mM of these anions or 500 ppm of the dry gases. Our findings indicate that the enzyme is not significantly affected by these toxic gases or their associated anions. Rather, the combination of high pH and ionic strength of the solvent at a high absorption temperature causes a reduction in the enzyme activity and affects the performance of the absorption process.