The emission of carbon dioxide gas (CO2) has significantly increased over the past few decades due to rapid industrialization and fossil fuel usage. This has resulted in unpredictable environmental changes such as climate change and global warming, raising concerns amongst environmentalists, stakeholders and policymakers. With the indispensable need to reduce the emission of greenhouse gases, stringent measures have been formulated. Most of these measures aim at creating a green and sustainable economy by converting CO2 into useful products that can be recycled.
Hydrogeneration is an appealing route for CO2 utilization as it facilitates its conversion to high-value chemicals such as methanol and formate. This process uses specific catalysts to accelerate the reactions. Despite the availability of several catalysts, numerous drawbacks like low conversion rates and poor selectivity have limited their applications. Recently, enzymatic catalysts have emerged as potential candidates for efficient CO2 hydrogenation owing to their remarkable advantages. Unfortunately, their application is limited by the poor solubility of CO2 under normal pressure. To this end, carbonic anhydrase has been used to enhance enzyme-catalyzed CO2 conversion. Nevertheless, its strengthening effect remains unsatisfactory.
Previously, the authors successfully applied the polyethyleneimine (PEI) with high CO2 affinity in in-situ enzymatic hydrogenation and achieved a high initial reaction rate, an indication that PEI-modified materials are potential candidates for enhancing the enzymic conversion of CO2. Motivated by these results, Beijing Institute of Technology researchers: Professor Wenfang Liu, Tingting Zhai (Master student), Caihong Wang(Master student), et. al. together with Dr. Yanzi Wang from Shenzhen University investigated the feasibility of dopamine/polyethylenimine – modified silica (PDA/PEI-SiO2) as strengthening materials for enhancing the enzymatic conversion of CO2 to formate. The work is currently published in the research journal, ACS Sustainable Chemistry and Engineering.
In their approach, silica microspheres with exceptional mobility, flexibility, and mechanical stability were first modified by co-deposition of PEI and dopamine (DA). They were then characterized using various characterization techniques such as Fourier Transform Infrared, SEM and TEM. The resultant PDA/PEI-SiO2 was used as an enhancement material in the hydrogenation of CO2 to formate. The authors studied the effects of the newly introduced modification conditions on the enzymic reaction as well as the influence of immobilized CA on the formate synthesis. Finally, the reusability of the immobilized CA and PDA/PEI-SiO2 were investigated to establish their feasibility.
The authors observed an increase in the initial reaction rate from 13.4 to 27.2 times that of the blank system and an increase in the loading amount of PDA/PEI-SiO2 from 0.1 to 0.4 g. This was attributed to the vital role played by introduced amino groups on the PDA/PEI-SiO2 surface. Furthermore, PDA/PEI-modified SiO2 was used as a support to CA, thereby resulting in improved synesthetic effects. This included an acceleration of the formate synthesis to 48.6 times with a low PDA/PEI-SiO2-CA loading amount of 0.2 g. Compared to PDA/PEI-SiO2 that retained only 56.1% after being used once, PDA/PEI-SiO2-CA demonstrated better reusability, retaining up to 95.1% activity after use 10 cycles, owing to the introduction of the CA that improved the carbon dioxide desorption and effective utilization of the amino groups.
In a nutshell, the authors reported, for the first time, the modification of silica microspheres through co-deposition of DA and PEI to produce PDA/PEI-SiO2, a better strengthening material for dehydrogenation of CO2 to formate. An improvement in the enzymatic reaction rate and synergistic effects was reported and was higher than all the previously reported values. An improvement in the reusability was also reported, especially for PDA/PEI-SiO2-CA attributed to the enhanced synergistic effects. Based on the results, the authors explained their method provided a high-efficient and straightforward strategy for intensification of enzymatic CO2 conversion processes.
Zhai, T., Wang, C., Gu, F., Meng, Z., Liu, W., & Wang, Y. (2020). Dopamine/Polyethylenimine-Modified Silica for Enzyme Immobilization and Strengthening of Enzymatic CO2 Conversion. ACS Sustainable Chemistry & Engineering, 8(40), 15250-15257.