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An interest in biodiesel as an alternative fuel for diesel engines has been increasing because of the issue of petroleum depletion and environmental concerns related to massive carbon dioxide emissions. Researchers are strongly driven to pursue the next generation of vegetable oil-based biodiesel. Oleaginous microalgae are considered to be a promising alternative oil source. To commercialize microalgal biodiesel, cost reductions in oil extraction and downstream biodiesel conversion are stressed. Herein, starting from an investigation of oil extraction from wet microalgae, a review is conducted of transesterification using enzymes, homogeneous and heterogeneous catalysts, and yield enhancement by ultrasound, microwave, and supercritical process. In particular, there is a focus on direct transesterification as a simple and energy efficient process that omits a separate oil extraction step and utilizes wet microalgal biomass; however, it is still necessary to consider issues such as the purification of microalgal oils and upgrading of biodiesel properties. [ABSTRACT FROM AUTHOR]

Microalgal lipid with a high free fatty acid (FFA) content was directly extracted from Chlorella vulgaris, using SDBS, in an acid-catalyzed hot-water extraction process. The total fatty acid content of C. vulgaris was 296.0 mg/g cell. Under the 1.0% sulfuric acid, 0.4% SDBS conditions, the FFA content of the lipid increased to 96.7%, and the lipid-extraction yield was 248.4 mg/g cell. Under the 2.0% sulfuric acid, 0.2% SDBS conditions, the FFA content of the lipid was 96.1%, and the lipid-extraction yield was 266.0 mg/g cell. Whereas the FAME content of the microalgal lipid extracted by hexane-methanol was 76.4% at the 10.0% sulfuric acid concentration, the FAME content of the high-FFA microalgal lipid was increased to 70.1% at a sulfuric acid concentration of only 0.1%. By combined sulfuric acid/SDBS treatment, high-FFA microalgal lipid was extracted in large yields; moreover, the amount of catalyst was remarkably reduced in the esterification of FFA. [ABSTRACT FROM AUTHOR]

Highlights: [•] Chlorophyll removal from microalgal lipid by hypochlorous acid. [•] Decrease in unsaturated fatty acid content of microalgal lipid by hypochlorous acid. [•] Hypochlorous acid produced from NaClO solution and NaCl electrolysis. [Copyright &y& Elsevier]

• Novel bifunctional heterogeneous catalyst was synthesized for biodiesel production. • IL/IRMOF-3 exhibited synergistic catalysis of acid-base and Brønsted-Lewis acid sites. • The new bond formed between IL and IRMOF-3 strengthened catalysts recyclability. • The conversion efficiency of microalgal lipids to FAME with IL/IRMOF-3 was 98.2% In order to enhance catalytic active sites to improve microalgal biodiesel production, a novel efficient bifunctional catalyst was synthesized by modifying metal–organic framework (IRMOF-3) with Brønsted acidic ionic liquid (1-butylsulfonate-3-methylimidazolium bisulfate [BSO 3 HMIm][HSO 4 ], IL). It was determined that IL was immobilized on IRMOF-3 framework through chemical bonding formed by an acid-base reaction between sulfonic acid group of IL and amino group of IRMOF-3. The bifunctional catalyst exhibited acid–base synergistic catalysis as well as a synergistic effect of Brønsted acid and Lewis acid, which efficiently catalyzed esterification and transesterification of microalgal lipids. The introduction of [BSO 3 HMIm][HSO 4 ] on IRMOF-3 framework promoted the acidity of the catalyst from 8.91 to 10.76 mmol/g and increased strong/weak acidity ratio from 0.27 to 0.85, resulting an increased catalytic conversion efficiency of 98.2% under optimal reaction conditions. Due to the strong ionic bond, the catalyst showed good reusability with the catalytic efficiency up to 85.5% after six cycles. [ABSTRACT FROM AUTHOR]

Highlights: [•] TiO2 nanoparticles entrapped in aminoclay matrix were synthesized. [•] Microalgae harvesting efficiency was achieved ∼85% at 3.0g/L injection. [•] ∼95% of microalgae cells was disrupted by photocatalysis within 3h. [•] Integration of harvesting/lipid-extraction steps was demonstrated. [ABSTRACT FROM AUTHOR]

[Display omitted] • First time use of ultrasound-assisted TEPDA to extract lipids from wet microalgae. • TEPDA dissociated into cations to promote cell disruption and lipid extraction. • Ultrasound-assisted TEPDA reduced lipid extraction time of SHS from 24 h to 2 h. • Ultrasound-assisted TEPDA achieved the lipid extraction efficiency of 98.2% To facilitate the lipid extraction from Nannochloropsis oceanica with thick cell wall using switchable hydrophilicity solvent, ultrasound-assisted N, N, N', N'-tetraethyl-1,3-propanediamine (TEPDA) was used to effectively destruct the cell wall. TEPDA cations were adsorbed on the cells via electrostatic force and formed the electron-donor–acceptor (EDA) complex with the hydroxyl groups in cellulose. This broke the hydrogen-bonding interactions between cellulose chains and stripped them from cell wall, thus reducing the cell wall thickness from 141 nm to 68.6 nm. Moreover, TEPDA cations neutralized the negatively charged phospholipid bilayers, decreasing the cell surface zeta potential from −27.5 eV to −14.1 eV. The local electrostatic equilibrium led to cell membrane leakage. The ultrasound promoted the stripping of the cellulose chains at a power intensity of 0.5 W/mL and frequency of 20 kHz, achieving the lipid extraction efficiency of 98.2% within 2 h at a volume ratio of 1:4 of wet microalgae to TEPDA. [ABSTRACT FROM AUTHOR]

Abstract: In this study, the effects of enzymatic hydrolysis on lipid extraction from microalga (Chlorella vulgaris) were investigated prior to biodiesel production. The initial fatty acids content of C. vulgaris was 87.6 mg/g cell. The microalgal cell walls were hydrolyzed by cellulases and then their lipid fractions were extracted using various organic solvents such as hexane, methanol, and chloroform. Optimal pH and temperature for the enzymatic hydrolysis were pH 4.8 and 50 °C, respectively, and the maximal hydrolysis yield was 85.3%, which was achieved after 72 h. After the enzymatic hydrolysis, the lipid extraction yield by the organic solvents was improved compared to when there was no enzymatic hydrolysis process, by 1.29–1.73-fold depending on the solvents used. The total fatty acid methyl ester (FAME) productivity through the enzymatic hydrolysis was higher than when there was no enzymatic hydrolysis, by 1.10–1.69-fold depending on the solvents used. When lipid was extracted from the C. vulgaris after the enzymatic hydrolysis in chloroform-methanol solution, FAME productivity was 59.4 mg FAME/g cell. [Copyright &y& Elsevier]

Highlights: [•] Comparison of lipid extractions for Chlorella sp. with low- and high-lipid content. [•] Lipid extractions of wet microalgae biomasses by four organic-nanoclays. [•] ∼100% conversion of the extracted lipid’s fatty acids to biodiesel. [•] One of most effective lipid extraction in wet microalgal biomass. [ABSTRACT FROM AUTHOR]

Abstract: In microalgae-based biorefinement, one of the highest practical priorities is to reduce the costs of downstream processes. As one potential solution, microalgae harvesting by organoclays has received particularly keen research interest. In the present study, cationic charged aluminum- and magnesium-backboned organoclays were synthesized and solubilized in aqueous solution due to their high-density of amino sites. Each, within 30min of its injection into 1.7g/L-concentration microalgal feedstocks, effected harvesting efficiencies of almost 100% at concentrations above 0.6g/L while maintaining a neutral pH. Conclusively, organoclays, if recycled efficiently, can be uniquely effective microalgae harvesting agents. [Copyright &y& Elsevier]

• Switchable solvent TEPDA was used first to extract lipids from wet microalgae. • Lipid extraction of TEPDA was 1.85x that of chloroform, and 3.33x that of n-hexane. • TEPDA dissociated into surfactant to promote cell disruption and lipid extraction. Switchable solvent N, N, N', N'-tetraethyl-1,3-propanediamine (TEPDA) was proposed to extract lipids from wet Nannochloropsis oceanica with a 5% higher extraction efficiency than chloroform–methanol. It was found that TEPDA acted mainly as an organic solvent to soften and dissolve lipids, while a small amount of TEPDA was dissociated into tertiary amine ion, i.e., (C 2 H 5) 2 N-(CH 2) 3 -NH+(C 2 H 5) 2. This cation acted as a surfactant to promote cell disruption and lipid separation. With moisture increasing from 0 to 84 wt%, more TEPDA was dissociated into cationic surfactant to induce local rearrangement of phospholipid bilayers in cell membranes through electrostatic interaction, resulting in the fractal dimension of disrupted cells increased from 1.49 to 1.66. Accordingly, the yield of fatty acid methyl ester (FAME) through transesterification of lipids extracted with TEPDA increased by 9%, while FAME yield from lipids extracted with chloroform and n-hexane decreased by 41% and 65%, respectively. [ABSTRACT FROM AUTHOR]