Hydrothermal liquefaction of microalgae using Fe3O4 nanostructures as efficient catalyst for the production of bio-oil: Optimization of reaction parameters by response surface methodology

The aim of the present work was focused on optimizing the hydrothermal liquefaction (HTL) of Spirulina platensis catalyzed by Fe3O4 nanostructures to enhance the bio-oil yield and quality of bio-oil using response surface methodology (RSM). The structural morphology and crystalline nature of the synthesized catalyst was determined using a scanning electron microscope (SEM), high resolution transmission electron microscopy (HR-TEM) and X-ray powder diffraction (XRD). Three of the vital reaction parameters such as temperature, holding time and catalyst dosage were optimized through central composite design. A maximum bio-oil yield of 32.33% was observed for the high temperature at 320 degrees C, 0.75 g of catalyst dosage and 37 min of resident time. The maximum conversion was found at a lower temperature of 272 degrees C, the bio-oil yield of 27.66% was obtained with 0.45 g of catalyst dosage and 24 min of holding time which is an energy efficient optimum condition. The maximum bio-oil yield was influenced at a lower temperature due to the high catalytic activity. While compared to higher temperatures were not much influence was observed. It clearly states that the catalyst dosage playing a critical role in the lower temperature HTL reaction. GC-MS and FT-IR analysis of the produced bio-oil exhibits significant characteristics for biofuel applications. The Fe3O4 catalyst was recyclable for up to eight repeated cycles and constant bio-oil yield for the last four cycles. It shows the excellent reproduction ability towards HTL of Spirulina sp.