Intracellular microalgae lipids can be extracted by a variety of methods, such as mechanical crushing extraction, chemical extraction, enzymatic extraction, supercritical carbon dioxide (SCCO2) extraction [46], micro­wave extraction [47], etc. Microalgae lipids in the form of triglycerides or fatty acids can be converted to biodiesel through transesterification/ (esterification for fatty acids) reactions after the extraction [48]. In order to achieve efficient reaction, the choice of catalyst is very important. The traditional liquid acid and alkali catalyst are called homogeneous catalysts because they act in the same liquid phase as the reaction mixture. Due to their simple usage and less time required for lipids conversion, the ho­mogeneous catalysts dominate the biodiesel industry. However, the trans­esterification catalyzed by homogeneous catalysts needs high purity feed­stock and complicated downstream processing [49], so high efficiency and low pollution catalysts such as solid acid catalysts, solid alkali catalysts, enzyme catalyst, supercritical catalyst systems and ionic liquid catalysts are receiving increasing attentions. Krohn et al. [50] studied the catalytic process using supercritical methanol and porous titania microspheres in a fixed bed reactor to catalyze the simultaneous transesterification and esterification of triglycerides and free fatty acids to biodiesel. The process was able to reach conversion efficiencies of up to 85%. Patil et al. [51] re­ported a process involving simultaneous extraction and transesterification of wet algal biomass containing about 90% of water under supercritical methanol conditions.