Green synthesis of biodiesel from microalgae cultivated in industrial wastewater via microbubble induced esterification using bio-MOF-based heterogeneous catalyst

Abstract

Conventional homogeneous catalysts are non-biodegradable, requiring complicated downstream separation, further diminishing the overall economics. In this paper, a new heterogeneous catalyst, a metal-organic framework modified by biomaterials (Bio-MOF), complexed with an ionic liquid ([HMIM][HSO4]/Bio-MOF), has been synthesized and integrated with microbubble mediated mass transfer technology to overcome the challenges of low conversion. The ionic liquid 1-methylimidazole hydrogen sulfate provides enhanced reactivity, while the Bio-MOF provides more active sites for the reaction. Detailed kinetic analysis suggests the biodiesel production reaction occurs on the vapor/liquid (microalgae oil) interface. The results revealed a higher conversion of (molar ratio of oil: methanol =1: 15, catalyst loading = 0.5 wt% of MO, temperature = 70 ºC) 92 ± 4% was achieved in 30 min relative to acid-catalyzed biodiesel production. Along with higher reactivity and larger surface area provided by the catalysts, the unprecedented higher conversion and rate of reaction is attributed to the “local” excess of alcohol present at the interface and simultaneous removal of water from the reactor. The results of activation energy (7.9 kJ mol−1), enthalpy (4.92 kJ mol−1), entropy (−287 J mol−1), and Gibbs free energy (103.58 kJ mol−1) show that current process required less energy is required for biodiesel production. The catalyst was recycled seven times and showed high stability with little reduction in its activity over these seven runs. The study demonstrates that this newly developed catalyst provides a high reaction rate and conversion of biodiesel production from microalgae oil using a heterogeneous catalyst by integrating it with microbubble-mediated mass transfer.