Fossil fuels, which are recognized as unsustainable sources of energy, are continuously consumed and decreased with increasing fuel demands. Microalgae have great potential as renewable fuel sources because they possess rapid growth rate and the ability to store high-quality lipids and carbohydrates inside their cells for biofuel production. Microalgae can be cultivated on opened or closed systems and require nutrients and CO 2 that may be supplied from wastewater and fossil fuel combustion. In addition, CO 2 capture via photosynthesis to directly fix carbon into microalgae has also attracted the attention of researchers. The conversion of CO 2 into chemical and fuel (energy) products without pollution via this approach is a promising way to not only reduce CO 2 emissions but also generate more economic value. The harvested microalgal biomass can be converted into biofuel products, such as biohydrogen, biodiesel, biomethanol, bioethanol, biobutanol and biohydrocarbons. Thus, microalgal cultivation can contribute to CO 2 fixation and can be a source of biofuels. This article reviews the literature on microalgae that were cultivated using captured CO 2 , technologies related to the production of biofuels from microalgae and the possible commercialization of microalgae-based biofuels to demonstrate the potential of microalgae. In this respect, a number of relevant topics are addressed: the nature of microalgae (e.g., species and composition); CO 2 capture via microalgae; the techniques for microalgal cultivation, harvesting and pretreatment; and the techniques for lipid extraction and biofuel production. The strategies for biofuel commercialization are proposed as well.

The as-synthesized amine-functionalized MCM-41 material was prepared through direct synthesis by co-condensation of tetraethyl orthosilicate (TEOS) with 3-aminopropyl triethoxysilane (APS) at different molar ratios and a pH of approximately 13 for CO2 capture under various CO2 concentrations, temperatures, and moistures. The prepared as-synthesized APS-functionalized MCM-41 (as-APS/MCM) possessed nitrogen content up to 3.46 mmol N/g and CO2 adsorption capacities up to 1.18 mmol/g under 15% CO2 in N2 at 35 °C and 1.74 mmol/g under pure CO2 at 25 °C. The CO2 adsorption capacity was 73% higher than the APS-grafted calcined MCM-41 prepared by postmodification. Because the CO2 adsorption capacity of the as-APS/MCM was found to come mainly from the coated APS rather than the incorporated APS, prehydrolysis of TEOS and post-treatment including template removal and APS neutralization were not required. Dynamic adsorption–desorption cycles revealed that the as-APS/MCM possessed high thermal stability for CO2 capture.

Polyethylenimine-containing mesoporous silica powders (PEI-MSP) were prepared through one-pot synthesis from mixtures of tetraethyl orthosilicate and PEI in water/ methanol. This proposed route required only 4% of reaction time and energy and used 50% of chemical reagents used in two-step synthesis of PEI-loaded SBA-15 (PEI/SBA) prepared through impregnation, thereby decreasing the cost of the resulting adsorbents. The PEI component in the one-pot route functioned not only to provide amine active sites for CO 2 capture but also as a basic catalyst and a pore-structuredirecting agent. The pelletized PEI-MSP using our proposed binder solution possessed high mechanical strength, durability, and CO 2 adsorption capacity and recovery, satisfying industrial requirements and reducing the pressure drop in practical use. Dynamic adsorption−desorption cycles of PEI-MSP powders and pellets revealed high thermal stabilities. Therefore, this one-pot synthetic route is promising for preparing PEI-MSP as appropriate adsorbents for CO 2 capture when using a temperature swing adsorption technology.

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