Microalgal Lipid Research Articles

Obtaining eicosapentaenoic acid-enriched polar lipids from microalga Nannochloropsis sp. by lipase-catalysed hydrolysis

The aim of this work was to increase the eicosapentaenoic acid (EPA) content of saponifiable lipids (SLs) extracted from the microalga Nannochloropsis sp. These SLs are rich in polar lipids (50.2 % glycolipids [GLs] and 27.4 % phospholipids [PL] of the total SLs). The EPA enrichment was carried out by selective enzyme-catalysed hydrolysis, based on the discrimination that some lipases show towards EPA, which is hydrolysed at a lower velocity and remains in the original polar lipids in greater proportion than other fatty acids. Initially, nine enzymes and eight solvents (all permitted in the food industry) were tested as reaction media. Lipase QLM from Alcaligenes sp. and acetone proved the most effective reaction medium to concentrate EPA. Studies were carried out on the influence of the hydrolysis time, lipase QLM immobilization, its reutilization, the scaling-up of the hydrolysis reaction and the influence of the polar lipid content. The maximum EPA content (or EPA concentration factor, FC) was obtained using lipase QLM immobilized on Accurel MP 1000 (a lipase/SL ratio of 1:1 [w/w] or an immobilized lipase/SL ratio of 2.5:1 [w/w]), 50 mL acetone/g SLs, a water/SL ratio of 2.8:1 (w/w), at 40 °C, 300 rpm (in flasks with orbital shaking), and 24 h of hydrolysis time. Under these conditions, polar lipids with up to 70 % EPA (starting from a microalgal polar lipid concentrate containing 48.5 % EPA) and an EPA recovery of 92 % were achieved. These hydrolytic conditions were also applied to increase the EPA content of microalgal SLs that had an even richer polar lipid content (90.3 %). Using this substrate, polar lipids with 68.7 % EPA and an EPA recovery of 98 % were achieved.

Optimizing mixotrophic cultivation of oil-rich Tribonema minus using volatile fatty acids and glycerin: A promising approach for pH-controlling and enhancing lipid productivity

Volatile Fatty Acids (VFAs) have become a subject of significant interest recently, primarily because they can serve as a carbon source for microorganisms that produce bioactive compounds, biodegradable materials, and energy. VFAs can be produced from various waste streams via anaerobic fermentation (AF) in a cost-effective manner, providing a potential avenue for producing biofuels and biochemicals while reducing waste volume. In this study, different VFAs and glycerin individually or in combination were used as the sole carbon source in the mixotrophic cultivation of oil-rich filamentous Tribonema minus. Using different VFAs or glycerin individually caused pH fluctuation in the culture, which led to the accumulation of free ammonia, which produced protons, acidifying the environment and inhibiting microalgae growth. To minimize pH fluctuation, different proportions of organic carbon sources were combined. Under combined organic carbon sources, the growth and photosynthetic performance of T. minus were all higher than those of the single carbon source groups. When Cacetic acid:Cbutyric acid:Cglycerin was set to 0.5:0.5:1, pH in the culture remained close to neutrality, and the removal of NH4+-N and carbon reached the maximum of 81.17 and 97.79%, respectively. The microalgal biomass and lipid productivity reached the highest of 223.80 and 108.16 mg L−1 d−1, respectively. Under the optimized carbon source condition, the fatty acids and biodiesel-related qualities of T. minus were improved. This study provided a reference for the precise design of carbon sources to maximize the resource utilization of butyric acid-type AF effluent by oil-rich T. minus. These conditions also can be achieved on a large scale by adding glycerin wastes to butyric acid-type AF effluent, mainly composed of acetic and butyric acids to get the recommended ratio for microalgae cultivation.

Zeolitic imidazolate framework (ZIF-8)-derived acid-base bifunctional single-atom catalysts with Zn-Nx coordination for microalgal lipids conversion

To strengthen active sites in zeolitic imidazolate frameworks (ZIFs) to effectively convert microalgal lipids into biodiesel, acid-base bifunctional single-atom catalysts with Zn-Nx coordination were synthesized through pyrolyzing ZIF-8. The saturated coordinated Zn-N4 sites were destroyed and converted to unsaturated coordinated Zn sites and surrounding N atoms at 700 °C, which enhanced Lewis acidity and basicity, respectively. The Zn-N content in the catalyst decreased from 77.68 % to 43.26 %, while that of pyridinic N increased from 8.14 % to 36.73 %. The acidity of the catalyst increased from 0.498 to 3.979 mmol/g, whereas basicity increased from 0.219 to 0.650 mmol/g. According to density functional theory calculations, unsaturated coordinated Zn-Nx (x = 1, 2, 3) sites exhibited lower free energies and reaction energy barriers in free fatty acid esterification. The energy barriers for rate-determining steps over Zn-N4, Zn-N3, Zn-N2, and Zn-N1 sites were + 1.01 eV, +0.82 eV, +0.63 eV, and + 0.54 eV, respectively. Therefore, the optimum catalyst exhibited a high conversion efficiency of 97.9 %, which was better than that of ZIF-8 (85.4 %).

Synergetic metal-Nx and metal nanoparticles derived from Zn/Co-zeolitic imidazolate frameworks boosted conversion of microalgal lipids

In order to enhance acid-base active sites and their utilization efficiency to efficiently convert microalgal lipids into biodiesel, an acid-base catalyst was synthesized through pyrolyzing ZnCo bimetallic zeolitic imidazolate frameworks (ZIFs). Pyrolysis destroyed saturated coordinated metal-N4 sites in ZIFs and produced more unsaturated metal-Nx and metal nanoparticles, which enhanced Lewis acidity. The increased pyridinic N and unsaturated metal-Nx promoted Lewis basicity. Meanwhile, Zn doping reduced sizes of catalyst crystals and metal nanoparticles, which improved utilization efficiency of active sites. Hence, compared with pyrolyzed ZIF-67, the acidity of optimal catalyst increased from 0.205 to 1.535 mmol/g, whereas basicity increased from 0.069 to 0.405 mmol/g. DFT calculations revealed that catalytic performances of metal nanoparticles and unsaturated metal-Nx were better than that of saturated metal-N4. Therefore, the catalytic efficiency of C-Zn/Co-ZIF = 4 (molar ratio of Zn to Co was 4) catalyst was highest at 96.2%, because of the minimal catalyst crystals and metal nanoparticles.

Solid-state fermentation-based enzyme-assisted extraction of eicosapentaenoic acid-rich oil from Nannochloropsis sp

Enzymatic treatment of microalgal biomass is a promising approach for extraction of microalgal lipid, but high cost of commercially sourcing enzyme is a major drawback in industrial implementation. Present study involves extraction of eicosapentaenoic acid-rich oil from Nannochloropsis sp. biomass using low cost cellulolytic enzymes produced from Trichoderma reesei in a solid-state fermentation bioreactor. Maximum total fatty acid recovery of 369.4 ± 4.6 mg/g dry weight (total fatty acid yield of 77%) was achieved in 12 h from the enzymatically treated microalgal cells, of which the eicosapentaenoic acid content was 11%. Sugar release of 1.70 ± 0.05 g/L was obtained post enzymatic treatment at 50 °C. The enzyme was reused thrice for cell wall disruption without compromising on total fatty acid yield. Additionally, high protein content of 47% in the defatted biomass could be explored as a potential aquafeed, thus enhancing the overall economics and sustainability of the process.

Economic feasibility study for artificial lighting of microalgal flat-panel photobioreactors

Lightning belongs among critical process parameters that significantly affect growth and microalgal yield in photobioreactors. Sunshine or artificial lighting is applied to irradiate photobioreactors. Nevertheless, sunshine lighting is limited by locality and weather conditions. On the contrary, there is a typical demand for nonstop or 8000 annual working hours for industrial biorefineries using photobioreactors as an economic point of view. Thus the paper scopes to evaluate the fundamental economic feasibility of artificial lightning provided by electric energy from fossil fuels, renewable energy systems, or their mutual combinations. The economic feasibility is discussed concerning local annual sunshine hours for a model 1-ha microalgae cultivation plant. It was found that combined sunlight, solar energy- and electric energy-based artificial lighting has a significant positive effect on the economic behavior of a model biorefinery. The mutual combination of sunshine, solar energy- and electric artificial energy-based light evinces a profit increase of 2.18 for microalgal powder, 3.09 for microalgal lipids and 4.11 for microalga carotenoids production technologies all compared only to sun lighted photobioreactors. Nevertheless, capital investments in artificial lightings, solar systems, or photobioreactors, and high operating expenses represent the dominant risk factors of the discussed microalgal biorefinery. Developing cheap and reliable artificial lighting systems, solar and energy storage systems, and improving process characteristics of microalgal cultivation (process stability, high concentrated suspension in thin layer photobioreactors avoiding biofilm formation) were identified as the essential research needs and challenges to improve the economic feasibility of artificially lighted microalgal biorefinery.

Ultrafine particles of zeolitic imidazolate frameworks induced by efficient nucleation facilitated conversion of microalgal lipids

In order to improve property and utilization of active sites on catalyst surfaces, novel ultrafine particles of zeolitic imidazolate framework (ZIF-67) were synthesized for microalgal biodiesel production. The synergy of N,N-dimethylformamide (DMF) and ethanol promoted deprotonation of imidazole and efficient nucleation of ZIF-67, which reduced crystal size from ∼300 nm (using methanol as solvent) to ∼20 nm. Meanwhile, the ratio of unsaturated coordinated Co-Nx (x = 1,2,3) to saturated CoN4 increased from 0.40 to 0.57, and pyridinic N content on catalyst surface increased from 16.59% to 32.25%, which enhanced the acidity and basicity of catalyst. The total acidity of optimal ZIF-67 increased from 0.125 to 0.691 mmol/g, while total basicity increased from 0.078 to 0.137 mmol/g. Density functional theory (DFT) calculations clarified that unsaturated Co-Nx sites and pyridinic N exhibited excellent catalytic activity. Therefore, ZIF-67-DE catalyst (using DMF and ethanol as solvents) effectively converted microalgal lipids into biodiesel with a catalytic efficiency of 97.8%.

Biodiesel Production from Chlorella homosphaera by Two-Step Catalytic Conversion Using Waste Radish Leaves as a Source for Heterogeneous Catalyst.

The economic viability of algal biodiesel can be improved by enhancing the microalgal lipid accumulation and using agricultural waste as a cheap and sustainable source of catalysts. In the current study, the effect of various nitrogen concentrations on the growth and lipid of Chlorella homosphaera were investigated. Furthermore, two-step catalytic conversion was applied to convert the oil of C. homosphaera with high free fatty acids (FFA) to biodiesel using waste radish leaves as a source of a heterogeneous base catalyst. The result revealed that the maximum lipid productivity of 25.0mg L-1day-1 and lipid content of 30.83% were obtained under nitrogen-depleted and limited nitrogen conditions, respectively. The FFA was reduced from 18.79 to 0.76%, and the acid value was decreased from 37.4 to 1.52mg KOH g-1 using a 15:1 methanol to oil molar ratio (MTOR), 1.5 wt.% H2SO4, at 60°C for 150min. Under the optimized conditions, i.e., MTOR of 10:1, 3 wt.% of catalyst ratio for 120min at 60°C, the highest oil conversion of 96.61% was obtained. The physicochemical properties of the produced biodiesel were in the range of the standard specification norms for biodiesel. Hence, the proposed two-step catalytic conversion using calcined radish leaves as a heterogeneous catalyst has thus exhibited good potential for biodiesel production using algal oil with high FFA.

Efficient coupling of sulfadiazine removal with microalgae lipid production in a membrane photobioreactor

This study explored the feasibility of a coupled system for antibiotic removal and biofuel production through microalgae cultivation. Initial, batch culture experiments demonstrated that sulfadiazine (SDZ) had an inhibitory effect on Chlorella sp. G-9, and 100.0 mg L-1 SDZ completely inhibited its growth. In order to improve SDZ removal efficiency by microalgae, three membrane photobioreactors (MPBRs) with different hydraulic retention times (HRTs) were established for continuous microalgae cultivation. The efficient coupling of SDZ removal and microalgal lipid production was achieved through the gradual increment of influent SDZ concentration from 0 to 100.0 mg L-1. The reduction in SDZ ranged between 57.8 and 89.7%, 54.7–91.7%, and 54.6–93.5% for the MPBRs with HRT of 4 d, 2 d, and 1 d, respectively. Chlorella sp. Was found to tolerate higher concentrations of SDZ in the MPBR system, and the resulting stress from high concentrations of SDZ effectively increased the lipid content of microalgae for potential biodiesel production. With the increase of influent SDZ concentration from 0 to 100.0 mg L-1, the lipid content of microalgae increased by 43.5%. Chlorophyll content, superoxide dismutase activity, and malondialdehyde content of microalgae were also evaluated to explore the mechanism of microalgae tolerance to SDZ stress in MPBR.

Spirulina Cultivation Using Biogas CO2 as the Carbon Source: Preliminary Study on Biomass Growth and Productivity

Anthropogenic activities are causing a rapid increase in global carbon dioxide (CO2) emissions, which significantly contribute to global warming. Microalgae can be a sustainable solution for simultaneous wastewater treatment and sequestering CO2 through photosynthesis. The current study reports a comparative evaluation of Spirulina sp. microalgal biomass growth and lipid productivity during its cultivation supplied with air and biogas from an anaerobic digester. It was observed that there was a 4-fold increase in biomass productivity in the reactor sparged with biogas compared to air supply. The reactor sparged with biogas showed a significant increase in lipid content. This increase in biomass productivity could be attributed to the increased availability of CO2, favoring algal growth.

Microalgae lipid extraction: a novel lab-scale method within a biorefinery approach (fractioning)

Extraction and fractioning of microalgal lipids have been recurrently studied because of the market value of certain lipid families (i.e. carotenoids and xanthophylls) and the opportunity that microalgal oils represent for obtaining biofuels. Most of the methods developed for the total extraction of microalgal lipids require a subsequent separation for the different families (i.e neutral lipids, polar lipids) to be quantified. Moreover, most of them use toxic and expensive solvents. In the present study less toxic solvents that are widely used in the industry, such as hexane, acetone and, ethanol were tested in different orders for the extraction of microalgal biomass of different lipid composition and content (i.e. three different levels of nutritional stress). A comparison of the methods proposed by Bligh and Dyer, Folch and, Hara and Radin is also presented. The new method achieves microalgal total lipid extraction and separation of the lipid families simultaneously.

The growth and lipid accumulation of Scenedesmus quadricauda under nitrogen starvation stress during xylose mixotrophic/heterotrophic cultivation.

In order to conquer the block of high cost and low yields which limit to realize the commercialization of microalgal biodiesel, the mixotrophic and heterotrophic cultivation of Scenedesmus quadricauda FACHB-1297 fed on xylose was separately studied employing six forms of media: phosphorus sufficient, phosphorus restricted, and phosphorus starvation were combined with nitrogen sufficient and nitrogen starvation conditions. The maximum lipid content (about 41% of dry weight) was obtained on the 5th day (heterotrophic cultivation) and 8th day (mixotrophic cultivation) under the nitrogen starved and phosphorus sufficient (N0&P) conditions, which was about twofold in comparison to the final lipid content on the sufficient nitrogen condition (control). Under mixotrophic and heterotrophic modes, the highest lipid production was achieved in the N0&P trial, with the value of 274.96mg/L and 193.77mg/L, respectively. Xylose utilization rate of 30-96% under heterotrophic modes was apparently higher than that of 20-50% in mixotrophic modes. In contrast, phosphorus uptake rate of 100% under mixotrophic cultivation was significantly more than that of 60-90% in heterotrophic cultivation. Furthermore, under the condition of heterotrophic cultivation using xylose as a carbon source, the phosphorus had a positive impact on microalgae cell synthesis and the lipid content enhanced with the augmentation in phosphorus concentrations. We suggested that sufficient phosphorus should be supplied for obtaining higher microalgal lipid production in the lack of nitrogen under xylose heterotrophic/mixotrophic condition. This was a highly effective way to obtain efficient microalgae lipid production.

Production of microalgal biomass and lipids with superior biodiesel-properties by manipulating various trophic modes and simultaneously optimizing key energy sources

Two promising oleaginous microalgae, Scenedesmus sp. SPP and marine Chlorella sp. were cultivated under various trophic modes. Both microalgae grew best under mixotrophic/nitrogen-rich mode, indicating good metabolisms of both inorganic (CO2) and organic (glucose) carbon sources. While mixotrophic/nitrogen-starvation conditions promoted the accumulation of lipids with superior biodiesel properties. The key energy sources, including light intensity, CO2 and glucose concentrations, for mixotrophic cultivation of the selected Scenedesmus sp. SPP was simultaneously optimized by Response Surface Methodology. High light intensity combined with high CO2 retarded growth but stimulated lipid/pigment synthesis. Two-stage mixotrophic cultivation using optimal conditions for biomass (light intensity 4.8 klux, CO2 13%, glucose 1% under nitrogen-rich mode) followed by those for lipid accumulation (light intensity 6.0 klux, CO2 15%, glucose 1% under nitrogen-starvation mode) successfully enhanced biomass by 1.46 folds and lipids by 1.39 folds and also improved the biodiesel properties. These strategies may significantly contribute to the development of microalgae-based biofuel and biochemical industries.

A novel strategy to promote microalgal growth and lipid productivity by supplementation of lignin related phenolic elicitors

An environment-friendly and cost-effective approach to improving lipid productivity without sacrificing microalgal growth was established. Syringic acid (SA) and p-coumaric acid (p-CA) are two major lignin hydrolysates, widely distributed in the wastewater from downstream industries of lignin. In this study, their potential in regulating the growth and valuable metabolites biosynthesis of microalga Euglena gracilis was evaluated. The cell growth was increased by 1.63 and 1.93 times at the optimal dosage of 0.5 g·L-1 SA and 0.3 g·L-1p-CA, respectively. Cell morphology also varied under the two treatments, reflecting the changes in cellular physiological status. Moreover, increased chlorophyll a content was only observed under p-CA treatment, indicating the influences of these compounds on photosynthesis were different. Fourier-transform infrared spectroscopy analysis revealed the fluctuations in the macromolecular pools. The variation trend of carbohydrate content was different under the treatment of the p-CA and SA, demonstrating that different phenolic compounds will determine different biosynthetic pathways and the flow of carbon to metabolites in microalgal cells. However lipid biosynthesis was improved by both of the phenolic acids treatments, which would be favorable to the biofuel production since the cell biomass was not sacrificed for lipid accumulation.

Unanswered issues on decarbonizing the aviation industry through the development of sustainable aviation fuel from microalgae

Concerns have been raised about the effects of fossil fuel combustion on global warming and climate change. Fuel consumer behavior is also heavily influenced by factors such as fluctuating fuel prices and the need for a consistent and reliable fuel supply. Microalgae fuel is gaining popularity in the aviation industry as a potential source of energy diversification. Microalgae can grow in saltwater or wastewater, capture CO2 from the atmosphere and produce lipids without requiring a large amount of land. As a result, the production of oil from microalgae poses no threat to food availability. The low carbon footprint of microalgae-derived fuels has the potential to mitigate the impact of traditional aviation fuels derived from petroleum on climate change and global warming. Therefore, aviation fuels derived from microalgae have the potential to be a more environmentally friendly and sustainable alternative to conventional fuels. Gathering microalgal species with a high lipid content, drying them, and turning them into aviation fuel is an expensive process. The use of biofuels derived from microalgae in the aviation industry is still in its infancy, but there is room for growth. This study analyses the potential routes already researched, their drawbacks in implementation, and the many different conceptual approaches that can be used to produce sustainable aviation fuel from microalgal lipids. Microalgae species with fast-growing rates require less space and generate lipids that can be converted into biofuel without imperiling food security. The key challenges in algal-based aviation biofuel include decreased lipid content, harvesting expenses, and drying procedure that should be enhanced and optimized to increase process viability.

Efficient microalgal lipid production driven by salt stress and phytohormones synergistically

In this study, a novel method of coupling phytohormones with saline wastewater was proposed to drive efficient microalgal lipid production. All the six phytohormones effectively promoted microalgae growth in saline wastewater, and further increased the microalgal lipid content based on salt stress, so as to achieve a large increase in microalgal lipid productivity. Among the phytohormones used, abscisic acid had the most significant promoting effect. Under the synergistic effect of 20 g/L salt and 20 mg/L abscisic acid, the microalgal lipid productivity reached 3.7 times that of the control. Transcriptome analysis showed that differentially expressed genes (DEGs) of microalgae in saline wastewater were mainly up-regulated under the effects of phytohormones except brassinolide. Common DEGs analysis showed that phytohormones all regulated the expression of genes related to DNA repair and substance synthesis. In conclusion, synergistic effect of salt stress and phytohormones can greatly improve the microalgal lipid production efficiency.

Enhancing microalgal lipid accumulation for biofuel production.

Microalgae have high lipid accumulation capacity, high growth rate and high photosynthetic efficiency which are considered as one of the most promising alternative sustainable feedstocks for producing lipid-based biofuels. However, commercialization feasibility of microalgal biofuel production is still conditioned to the high production cost. Enhancement of lipid accumulation in microalgae play a significant role in boosting the economics of biofuel production based on microalgal lipid. The major challenge of enhancing microalgal lipid accumulation lies in overcoming the trade-off between microalgal cell growth and lipid accumulation. Substantial approaches including genetic modifications of microalgal strains by metabolic engineering and process regulations of microalgae cultivation by integrating multiple optimization strategies widely applied in industrial microbiology have been investigated. In the present review, we critically discuss recent trends in the application of multiple molecular strategies to construct high performance microalgal strains by metabolic engineering and synergistic strategies of process optimization and stress operation to enhance microalgal lipid accumulation for biofuel production. Additionally, this review aims to emphasize the opportunities and challenges regarding scaled application of the strategic integration and its viability to make microalgal biofuel production a commercial reality in the near future.

Ultrafine metal–organic framework exposed coordinatively unsaturated cobalt-nitrogen active sites for catalytic conversion of microalgal lipids to biodiesel

To improve atomic utilization of metal–organic framework for biodiesel production from microalgal lipids with high acid value, ultrafine ZIF-67 was proposed to release metal atoms normally encapsulated in the framework to form coordinatively unsaturated cobalt-nitrogen active sites Co-Nx (x < 4) on crystal surfaces. With reducing ZIF-67 crystal size from 2.3 μm to 30 nm, coordinatively saturated Co-N4 content decreased from 81.3% to 56.2%, while unsaturated Co-Nx content increased from 18.7% to 43.8% and pyridinic-N content increased from 11.6% to 32.6%. The exposed Co-Nx and pyridinic-N enhanced Lewis acid-base properties, resulting in an increased Lewis/Brønsted acidity ratio from 0.10 to 15.89 and total basicity from 0.69 to 1.89 mmol/g. Density functional theory calculation clarified adsorption energy of acetic acid on Co-N3, Co-N2, and Co-N1 sites were −0.585, −0.785, and −1.370 eV, respectively, while that on Co-N4 site was −0.186 eV. The stronger interaction between acetic acid and Co-N1 site led to more electrons transfer to increase electropositivity of carbonyl carbon from 1.379 to 1.537 eV, which facilitated the nucleophilic attack of methanol for esterification. The optimum catalyst achieved conversion efficiencies of 99.3% and 97.7% for free fatty acid and triglyceride in acidic lipids to biodiesel, respectively.

The Biorefinery of the Marine Microalga Crypthecodinium cohnii as a Strategy to Valorize Microalgal Oil Fractions

Chrypthecodinium cohnii lipids have been almost exclusively used as a source of Docosahexaenoic acid (DHA). Such an approach wastes the remaining microalgal lipid fraction. The present work presents a novel process to produce C. cohnii biomass, using low-cost industrial by-products (raw glycerol and corn steep liquor), in a 7L-bioreactor, under fed-batch regime. At the end of the fermentation, the biomass concentration reached 9.2 g/L and the lipid content and lipid average productivity attained 28.0% (w/w dry cell weight) and 13.6 mg/L h, respectively. Afterwards the microalgal biomass underwent a saponification reaction to produce fatty acid (FA) soaps, which were further converted into FA ethyl ester (FA EE). C. cohnii FA EE mixture was then fractionated, using the urea complexation method at different temperatures, in order to obtain a polyunsaturated fatty acid ethyl ester (PUFA EE) rich fraction, that could be used for food/pharmaceutical/cosmetic purposes, and a saturated fatty acid ethyl ester (SAT EE) rich fraction, which could be used as biodiesel. The temperature that promoted the best separation between PUFA and SAT EE, was −18 °C, resulting in a liquid fraction with 91.6% (w/w) DHA, and a solid phase with 88.2% of SAT and monounsaturated fatty acid ethyl ester (MONOUNSAT), which could be used for biodiesel purposes after a hydrogenation step.

Sulfamic acid–modified zeolitic imidazolate framework (ZIF-90) with synergetic Lewis and Brønsted acid sites for microalgal biodiesel production

In order to enhance acidic sites in catalysts to improve conversion efficiency of microalgal lipids into biodiesel, an efficient bifunctional catalyst with synergetic Lewis and Brønsted acid sites was synthesized by modifying zeolitic imidazolate framework (ZIF-90) with sulfamic acid (SA). The sulfamic acid that combined with ZIF-90 through imine bond (CN) provided protons and destroyed coordinated Zn-N bonds in ZIF-90, thereby enhancing the number of Brønsted and Lewis acid sites. The total acidity of optimal catalyst increased from 0.478 to 0.848 mmol/g, while ratio of Brønsted acid to Lewis acid increased from 0.32 to 0.49. The Lewis acid sites in bifunctional catalysts exhibited higher activity towards transesterification reactions of triglycerides in microalgal lipids, while Brønsted acid sites exhibited higher activity towards esterification reactions of free fatty acids. Therefore, the optimal catalyst (weight ratio of SA to ZIF-90 was 0.05) promoted conversion efficiency of microalgal lipids into biodiesel from 80.6 % to 98.3 % at 200 °C, while conversion efficiency remained 91.7 % after 6 reusability cycles.