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With the rapid industrialization, heavy reliance on fossil energy has increased the concentration of CO₂ in the atmosphere. Therefore, carbon capture and storage (CCS) is essential nowadays to mitigate the CO₂ emission thus reducing its potential impacts on climate change. One of the primary target is to capture CO₂ from the coal-fired power station flue gas. Currently, amine-based solutions are commonly applied to absorb CO₂ from flue gas due to their high CO₂ absorption capacity. However, the strong bonding between solvents and CO₂ requires undesired parasitic energy to strip CO₂ from the solvent. The attempt to apply the amine-functionalized porous solid absorbents for CO₂ capture provides a promising alternative to the amine-solution due to their good absorptive capacity and easy regeneration. However, the large scale preparation of such functionalized solid is still challenging and careful optimization of synthesis parameters are required.

The use of benign solvents for CO₂ capture such as water and potassium carbonate with far lower heat for desorption can limit the energy penalty to strip CO₂ from the solvent. However, these solvents exhibit much lower CO₂ hydration kinetics thus requiring larger footprint and higher flow rate, which could offset the benefits of easier desorption. In order to overcome the rate-limiting step of the CO₂ hydration, a biomimetic approach by using enzymes such as carbonic anhydrase (E.C.4.2.1.1) has been recently proposed by a number of researchers. CA possesses very high turnover numbers up to 1×10^6Ìýs^-1 making it the fastest catalyst for CO₂ hydration. However, the short lifetime and low stability of the free enzymes have limited their usefulness especially under the harsh operating conditions of the carbon capture process from flue gases. With these concerns, the preparation of biocatalytic materials based on enzyme immobilization has become a common strategy to improve the stability of the enzymes and increase the possibility for enzyme recovery and reuse.

In this work, we proposed a series of biocatalytic gas liquid membrane contactors to promote the CO₂Ìý³ó²â»å°ù²¹³Ù¾±´Ç²Ô:

Biocatalytic Janus membranes for CO₂Ìý°ù±ð³¾´Ç±¹²¹±ô

In this work, a novel hydrophilic-superhydrophobic Janus membrane was prepared via coating a single sided hydrophilic carbon nanotubes (CNTs) on a fluorosilane treated superhydrophobic membrane support; carbonic anhydrase was then immobilized onto the CNTs coating layer to form the final biocatalytic membrane. The Janus biocatalytic membrane was applied in a gas-liquid membrane contactor for CO₂ conversion where the hydrophilic side was facing the liquid phase and the superhydrophobic side oriented towards the gas phase. Such a configuration ensured the immobilized CA remains hydrated and minimized the CO₂ diffusion length in solvent. Improved CO₂ hydration efficiency was observed with the Janus membrane when compared with equal amount of free CA. Together with the good stability and easy biocatalytic membrane regeneration, great potential can be realized with this type of Janus biocatalytic membrane for the application of such membranes not only in a gas-liquid membrane contactor configuration for CO₂ capture from flue gas but also other types of enzymatic gas-liquid membrane contactors.

TiO₂ functionalized biocatalytic nanoparticle and membrane

Preparation of TiO₂ based biocatalytic naonparticles and membranes via CA immobilization facilitates the reuse of the enzyme and could be potentially integrated in a gas-liquid membrane contactor for high efficient CO₂ capture. In this work, different immobilization protocols were compared based on CA loading, activity and stability. For biocatalytic nanoparticles, over 80% activity recovery and 163 mg/g support was achieved. Repeated reuse and recovery of the biocatalytic nanoparticles over twenty cycles showed only modest loss in activity. For the biocatalytic membranes, the nanostructure of the titania coating and the pH values during immobilization were examined to optimize the biocatalytic performance. Biocatalytic membranes prepared at pH 6 with two cycles of sol-gel coating were able to immobilize 700 µg CA/cm²Â nominal membrane area. The CO₂ hydration efficiency of the biocatalytic nanoparticles and membranes were examined, and only marginal loss of catalytic efficiency was observed when compared with their free CA counterpart, indicating good potential to apply such biocatalytic nanoparticles and membranes for CO₂Ìý³¦´Ç²Ô±¹±ð°ù²õ¾±´Ç²Ô.

Janus Membranes with Asymmetric Wettability for Fine Bubble Aeration 

A superaerophobic-hydrophobic Janus membrane is fabricated by single-sided polydopamine/polyethyleneimine co-deposition. The Janus membrane significantly promotes the fine bubble generation and gas/liquid mass transfer. Its application is exemplified by high efficient dopamine oxidization and enzymatic CO2 hydration. The Janus membrane aerator shows great potentials in many gas-involved energy generation and environmental remediation processes.

Digital pictures and schematic diagrams of bubbling processes for a) nascent membrane and b) Janus membrane, in which the gas flow rate is 100 mL/min; c) dissolved oxygen concentration with time during an oxygen bubbling process; d) UV-vis absorbance of dopamine solutions at 420 nm after bubbled by nascent and Janus membranes, respectively.

Biocatalytic nanoparticle membrane contactor for CO₂Ìý°ù±ð³¾´Ç±¹²¹±ô

The biocatalytic gas-liquid membrane contactor is a combination of the traditional membrane contactor and the biocatalytic reaction process. It can effectively promote the CO₂ hydration rate with relatively benign solvents. The key challenge for its application is the stability and reusability of the biocatalysts. In this work, we immobilized carbonic anhydrase onto TiO₂ nanoparticles and then suspended the biocatalytic nanoparticles within the liquid absorbents. Both pristine PP and superhydrophobic modified PP membrane were applied to construct the membrane contactor.

This work further investigated the performance of the biocatalytic membrane contactor under various industrial-related conditions, including a wide range of pH, different buffer conditions and temperature. Then the reusability of the biocatalytic nanoparticles were demonstrated with 10 cycles of CO₂ hydration test. The mass transfer coefficients were calculated to understand the effect of biocatalytic nanoparticles on the CO₂ transport within the membrane contactor. Finally, a much higher biocatalytic CO₂ hydration rate was obtained with a flat sheet membrane contactor, which had much lower membrane resistance. The results in this work suggest the biocatalytic membrane contactor is a promising candidate for CO₂ capture from flue gas, biogas and natural gas.