Biodiesel production from wet microalgae by using graphene oxide as solid acid catalyst

Urea-crosslinked 3D graphene oxide/MXene-SO3/cyclodextrin films for efficient and sustainable biodiesel production

Biodiesel, a renewable bio-based transport fuel consisting of alkyl esters of long-chain fatty acids, is gaining attention as an alternative to non-renewable petroleum diesel. We prepared adjustable interlayer spacing urea-crosslinked 3D graphene oxide/MXene-SO3/cyclodextrin (MGUC) film to achieve efficient and sustainable biodiesel production. Ti3C2TX (MXene) grafted sulfonic acid groups as acid-catalytic sites of the film with graphene oxide (GO) via urea cross-linking and doping cyclodextrin (CD) to construct multistage pore structures (internal pore structures generated by urea cross-linking as well as CD host-guest structures) synergistically catalyzed biodiesel production. Meanwhile, the heterogeneous carbon-based materials based on GO and MXene-S have excellent photothermal efficiency, which can achieve 95.6 % biodiesel conversion at 4 h and maintain 75.32 % activity after 8 cycles. We verified the reason for the high catalytic activity of the MGUC films, which rationalized the fast mass transfer mechanism of CD by Density functional theory (DFT)and verified the transport mechanism of the heterojunction for efficient photothermal. Therefore, this CD carbon-based solid acid film catalytic platform provides a new direction for the green and efficient preparation of bioenergy.

Nano-enabled microalgae bioremediation: Advances in sustainable pollutant removal and value-addition

Microalgae-assisted bioremediation, enriched by nanomaterial integration, offers a sustainable approach to environmental pollution mitigation while harnessing microalgae's potential as a biocatalyst and biorefinery resource. This strategy explores the interaction between microalgae, nanomaterials, and bioremediation, advancing sustainability objectives. The potent combination of microalgae and nanomaterials highlights the biorefinery's promise in effective pollutant removal and valuable algal byproduct production. Various nanomaterials, including metallic nanoparticles and semiconductor quantum dots, are reviewed for their roles in inorganic and organic pollutant removal and enhancement of microalgae growth. Limited studies have been conducted to establish nanomaterial's (CeO2, ZnO, Fe3O4, Al2O3, etc.) role on microalgae in pollution remediation; most studies cover inorganic pollutants (heavy metals and nutrients) remediation, exhibited 50-300% bioremediation efficiency improvement; however, some studies cover antibiotics and toxic dyes removal efficiency with 19-95% improvement. These aspects unveil the complex mechanisms underlying nanomaterial-pollutant-microalgae interactions, focusing on adsorption, photocatalysis, and quantum dot properties. Strategies to enhance bioremediation efficiency are discussed, including pollutant uptake improvement, real-time control, tailored nanomaterial design, and nutrient recovery. The review assesses recent advancements, navigates challenges, and envisions a sustainable future for bioremediation, underlining the transformative capacity of nanomaterial-driven microalgae-assisted bioremediation. This work aligns with Sustainable Development Goals 6 (Clean Water and Sanitation) and 12 (Responsible Consumption and Production) by exploring nanomaterial-enhanced microalgae bioremediation for sustainable pollution management and resource utilization.

Advanced nanomaterials and metal-organic frameworks for catalytic bio-diesel production from microalgal lipids - A review

Increasing energy demands require exploring renewable, eco-friendly (green), and cost-effective energy resources. Among various sources of biodiesel, microalgal lipids are an excellent resource, owing to their high abundance in microalgal biomass. Transesterification catalyzed by advanced materials, especially nanomaterials and metal-organic frameworks (MOFs), is a revolutionary process for overcoming the energy crisis. This review elaborates on the conversion of microalgal lipids (including genetically modified algae) into biodiesel while primarily focusing on the transesterification of lipids into biodiesel by employing catalysts based on above mentioned advanced materials. Furthermore, current challenges faced by this process for industrial scale upgradation are presented with future perspectives and concluding remarks. These materials offer higher conversion (>90%) of microalgae into biodiesel. Nanocatalytic processes, lack the need for higher pressure and temperature, which simplifies the overall process for industrial-scale application. Green biodiesel production from microalgae offers better fuel than fossil fuels in terms of performance, quality, and less environmental harm. The chemical and thermal stability of advanced materials (particularly MOFs) is the main benefit of the blue recycling of catalysts. Advanced materials-based catalysts are reported to reduce the risk of biodiesel contamination. While purity of glycerin as side product makes it useful skin-related product. However, these aspects should still be controlled in future studies. Further studies should relate to additional aspects of green production, including waste management strategies and quality control of obtained products. Finally, catalysts stability and recycling aspects should be explored.

Enhancing catalytic activity and pore structure of metal-organic framework-808 via ligand competition for biodiesel production from microalgal lipids at reduced temperatures

Catalysts with hierarchical porous structures and increased active defects play a crucial role in catalyzing the conversion of microalgae lipids. However, the template methods used for pore expansion and the acidification process employed to enhance activity are cumbersome and prone to deactivation. It is necessary to propose a simple and versatile synthetic approach to overcome these challenges. By modulating N,N-dimethylformamide basicity with formic acid, MOF-808 exhibited enhanced coordination of benzene-1,3,5-tricarboxylic acid to Zr-clusters, creating three types of functional defects. These defects increased pore size from 1.63 nm to 5.34 nm and enhanced catalyst acidity by 22.8%, while maintaining high porosity. The active catalytic sites were confirmed to be defect sites (exposed Zr4+) through density functional theory. Compared to regular MOF-808, catalyst MOF-808-3/1 shows enhanced hierarchical porosity and increased acidity, enabling efficient conversions at reduced reaction temperature (100 °C) and pressure (352 kPa) compared to 200 °C and 4036 kPa, respectively.

Assessing the potential of Chlorella sp. phycoremediation liquid digestates from brewery wastes mixture integrated with bioproduct production

Digestates from different anaerobic digesters are promising substrates for microalgal culture, leading to effective wastewater treatment and the production of microalgal biomass. However, further detailed research is needed before they can be used on a large scale. The aims of this study were to investigate the culture of Chlorella sp. in Digestate_M from anaerobic fermentation of brewer's grains and brewery wastewater (BWW) and to explore the potential use of the biomass produced under different experimental conditions, including diverse cultivation modes and dilution ratios. Cultivation in Digestate_M initiated from 10% (v/v) loading, with 20% BWW, obtained maximum biomass production, reaching 1.36 g L^-1 that was 0.27g L^-1 higher than 1.09 g L^-1 of BG11. In terms of Digestate_M remediation, the maximum removal of ammonia nitrogen (NH₄ ⁺-N), chemical oxygen demand, total nitrogen, and total phosphorus reached 98.20%, 89.98%, 86.98%, and 71.86%, respectively. The maximum lipid, carbohydrate, and protein contents were 41.60%, 32.44%, and 27.72%, respectively. The growth of Chlorella sp. may be inhibited when the Y(II)-F_v/F_m ratio is less than 0.4.

Tunable graphene oxide for the low-fouling electrochemical sensing of uric acid in human serum

Numerous studies have been reported to improve the selectivity of uric acid (UA) by eliminating the interference from other electroactive species that coexist in biological fluids. However, two main challenges associated with the nonenzymatic electrochemical detection of UA need to be overcome to achieve practical applications in biological samples. Those are the chemical fouling of electrodes caused by the oxidation product of UA and biofouling due to the non-specific absorption of biological macromolecules. It was found that the residual oxo-functional groups and defects on graphene played a crucial part in both electrocatalysis and anti-biofouling. Here, graphene oxide (GO) was tuned by electro-oxidation and electro-reduction and was investigated in antifouling and electrocatalytic performances for the electrochemical sensing of UA by using pristine GO, BSA bound GO, electro-reduction-treated GO and electro-oxidation-treated GO. The electro-oxidation-treated GO was explored in electrochemical sensing for the first time and exhibited the highest sensitivity and low fouling properties. Holey GO might be formed on the electrode surface by the electrochemical oxidation method in a mild and green solution without the use of an acid. The different electrode interfaces as well as the interaction with BSA were investigated by Raman spectroscopy, X-ray photoelectron spectroscopy, contact angle measurements, scanning electron microscopy, electrochemistry, and electrochemical impedance spectroscopy.

A comprehensive review on nanocatalysts and nanobiocatalysts for biodiesel production in Indonesia, Malaysia, Brazil and USA

Biodiesel is an alternative to fossil-derived diesel with similar properties and several environmental benefits. Biodiesel production using conventional catalysts such as homogeneous, heterogeneous, or enzymatic catalysts faces a problem regarding catalysts deactivation after repeated reaction cycles. Heterogeneous nanocatalysts and nanobiocatalysts (enzymes) have shown better advantages due to higher activity, recyclability, larger surface area, and improved active sites. Despite a large number of studies on this subject, there are still challenges regarding its stability, recyclability, and scale-up processes for biodiesel production. Therefore, the purpose of this study is to review current modifications and role of nanocatalysts and nanobiocatalysts and also to observe effect of various parameters on biodiesel production. Nanocatalysts and nanobiocatalysts demonstrate long-term stability due to strong Brønsted-Lewis acidity, larger active spots and better accessibility leading to enhancethe biodiesel production. Incorporation of metal supporting positively contributes to shorten the reaction time and enhance the longer reusability. Furthermore, proper operating parameters play a vital role to optimize the biodiesel productivity in the commercial scale process due to higher conversion, yield and selectivity with the lower process cost. This article also analyses the relationship between different types of feedstocks towards the quality and quantity of biodiesel production. Crude palm oil is convinced as the most prospective and promising feedstock due to massive production, low cost, and easily available. It also evaluates key factors and technologies for biodiesel production in Indonesia, Malaysia, Brazil, and the USA as the biggest biodiesel production supply.

Determination of active sites on the synthesis of novel Lewis acidic deep eutectic solvent catalysts and kinetic studies in microalgal biodiesel production

Experimental and theoretical considerations for kinetic modeling of the transesterification reaction of microalgae lipids into biodiesel were investigated using Lewis acid deep eutectic solvents (DESs) as a catalyst. The acid sites involved in the reaction were characterized using acetonitrile as a probe to understand the mechanism. DES ChCl-SnCl2 (choline chloride-tin ii chloride) showed higher catalytic activity in transesterification due to its higher acidity compared to DES ChCl-ZnCl2 (choline chloride-zinc chloride). This was illustrated by geometric optimization of the DES structures through density functional theory (DFT) which showed that the metal centers furthest from the choline moiety are the most acidic and the bond lengths of Sn-Cl were between 2.56 and 2.77 Å, and were greater than the Zn-Cl bond lengths from 2.30 to 2.48 Å, making the ChCl-SnCl2 DES more acidic and more suitable for the biodiesel production. The fatty acid methyl ester (FAME) conversion from microalgae lipid was 36.75 mg g-1 under ideal conditions (6 molar ratio methanol-lipid with 8 vol% DES dosage in methanol at 140 °C for 420 min). The activation energy is found to be 36.3 kJ mol-1 based on the pseudo-first-order reaction, in addition, the DES catalyst (ChCl-SnCl2) drove the reaction chemically and did not show mass transfer limitation. Information from this study can help to advance the development of an efficient and environmentally friendly industrial biodiesel production technology.

Graphene-based catalysts for biodiesel production: Characteristics and performance

Biodiesel is a promising alternative to reduce the dependency on fossil fuels. However, biodiesel's cost is still higher than its petroleum counterpart, hence its production process must be modified to make it economically viable. Microalgae are an alternative feedstock to replace agricultural crops for biodiesel production, and offer several advantages such as fast growth, use of non-arable land, growth in saline and wastewater, and high lipid yield. Unfortunately, biodiesel production from microalgae is very energy-intensive and costly, mainly due to the high energy consumption required for dewatering and drying. Therefore, utilizing wet microalgal biomass instead of dry biomass can be a promising solution to reduce the biodiesel production cost Furthermore, the use of heterogeneous catalysts offers high efficiency, recoverability, and reusability, and is therefore very promising from the economic and environmental perspectives. The unique characteristics of graphene-based nano-catalysts, such as their high surface area, two-dimensional structure, and functional groups, make them suitable candidates for biodiesel production. In this review, the use of graphene-based catalysts for biodiesel production is analyzed in depth, and their efficiency compared to other heterogeneous catalysts is scrutinized. Moreover, their recoverability, reusability, and economic feasibility are critically discussed, and their potential to produce biodiesel from wet microalgae is explored as a sustainable and cost-effective approach.

Aptamer-Engineered Cu2O Nanocubes as a Surface-Modulated Catalytic Optical Sensor for Lung Cancer Cell Detection

Herein, fine and homogeneous Cu₂O nanocubes are synthesized and sensitized with a hairpin-structured AS1411 aptamer for the establishment of a biosensor for lung cancer cell detection. The Apt-Cu₂O nanocubes feature a recognition function in identifying a cancer-associated surface nucleolin protein. The intrinsic reduction catalytic ability is also confirmed by the use of two benchmark substrates, methylene blue (MB) and 4-nitrophenol (4-NP). The aptamer grafting on Apt-Cu₂O nanocubes is able to greatly prevent nonspecific-protein binding and to show specificity toward the nucleolin protein. The specific binding resulting from nucleolin protein leads to less exposure of the active area of the Apt-Cu₂O nanocubes, so the catalytic ability of Apt-Cu₂O nanocubes is thus diminished. The modulated catalytic ability led to less generation of the reduced 4-AP product, and the change in absorption of 4-AP allows the quantification of the nucleolin protein with a detection limit of 0.47 nM. The as-developed biosensor is applied to the detection of nucleolin-overexpressed A549 lung cancer cells, presenting a sensitive detection limit down to 20 cells. This may be ascribed to the clustering of surface nucleolin protein in a lipid raft membrane of cancer cells, as evidenced by a notable binding of Apt-Cu₂O nanocubes on the cancer cell surface. Real human serum samples spiked with cancer cells were also investigated, and a recovery rate of 87 ± 2.4% for 20 extracted cells validates the surface-modulated Apt-Cu₂O nanocubes-based catalytic optical biosensor as a promising tool for the detection of circulating tumor cells. The establishment of the Apt-Cu₂O nanocubes may allow for further studies on their use as a potential theranostics tool for cancer therapy.

Metal-Organic Frameworks as bio- and heterogeneous catalyst supports for biodiesel production

As biodiesel (BD)/Fatty Acid Alkyl Esters (FAAE) is derived from vegetable oils and animal fats, it is a cost-effective alternative fuel that could complement diesel. The BD is processed from different catalytic routes of esterification and transesterification through homogeneous (alkaline and acid), heterogeneous and enzymatic catalysis. However, heterogeneous catalysts and biocatalysts play an essential role towards a sustainable alternative to homogeneous catalysts applied in biodiesel production. The main drawback is the supporting material. To overcome this, currently, Metal-Organic Frameworks (MOFs) have gained significant interest as supports for catalysts due to their extremely high surface area and numerous binding sites. This review focuses on the advantages of using various MOFs structures as supports for heterogeneous catalysts and biocatalysts for the eco-friendly biodiesel production process. The characteristics of these materials and their fabrication synthesis are briefly discussed. Moreover, we address in a general way basic items ranging from biodiesel synthesis to applied catalysts, giving great importance to the enzymatic part, mainly to the catalytic mechanism in esterification/transesterification reactions. We provide a summary with recommendations based on the limiting factors.

Progress on Conventional and Advanced Techniques of In Situ Transesterification of Microalgae Lipids for Biodiesel Production

Global warming and the depletion of fossil fuels have spurred many efforts in the quest for finding renewable, alternative sources of fuels, such as biodiesel. Due to its auxiliary functions in areas such as carbon dioxide sequestration and wastewater treatment, the potential of microalgae as a feedstock for biodiesel production has attracted a lot of attention from researchers all over the world. Major improvements have been made from the upstream to the downstream aspects related to microalgae processing. One of the main concerns is the high cost associated with the production of biodiesel from microalgae, which includes drying of the biomass and the subsequent lipid extraction. These two processes can be circumvented by applying direct or in situ transesterification of the wet microalgae biomass, hence substantially reducing the cost. In situ transesterification is considered as a significant improvement to commercially produce biodiesel from microalgae. This review covers the methods used to extract lipids from microalgae and various in situ transesterification methods, focusing on recent developments related to the process. Nevertheless, more studies need to be conducted to further enhance the discussed in situ transesterification methods before implementing them on a commercial scale.

Comparative study of different catalysts mediated FAME conversion from macroalga Padina tetrastromatica biomass and hydrothermal liquefaction facilitated bio-oil production

Marine macroalgae offer an endurable source of renewable biomass, which do not require cultivable area, fertilizers for cultivation for bioproducts production. In this study, marine brown macroalga Padina tetrastromatica as an alternate sustainable feedstock for the production of liquid fuels. Padina tetrastromatica biomass was collected from Mandapam; the coastal region of Rameswaram, Tamil Nadu, India. and the algal oil was extracted using sequential extractions using various solvents. Petroleum ether (PE) and dichloromethane (DCM) solvent fractions were found to have high lipids and further utilized for biodiesel production, wherein four different heterogeneous nanocatalysts (TiO₂, Bio-Fe, GO, and MgO) and commercial homogeneous catalysts (HCl and NaOH) were employed for the transesterification. High fatty acid methyl ester (FAME) recovery (92.3%) was achieved from TiO₂ mediated transesterification than the other conventional catalysts. Further, the conversion of algal biomass into bio-oil and by-products was carried out using hydrothermal liquefaction (HTL). Subsequently, the compounds were characterized by FT-IR and GC-MS analysis. The quality parameters of liquid biofuels were examined and they are in accordance with the international fuel standards. Thus, brown macroalga Padina tetrastromatica may be considered as an alternate feedstock for biofuel and other bioproducts production and TiO₂ would be a suitable catalyst for the conversion of FAME.

Synthesis and Characterization of rGO@ZnO Nanocomposites for Esterification of Acetic Acid

In the present work, the aim is to synthesize reduced graphene oxide (rGO) and zinc:reduced graphene oxide composite catalysts (ZnO:rGO) for esterification of acetic acid with n-heptanol. The physical and chemical characteristics of the rGO and rGO-metal oxide composite catalysts such as textural surface characteristics, surface morphology, thermal stability, functional groups, and elemental analysis were studied. The surface areas of rGO, ZnO_(0.5 M), and ZnO_(1 M) were recorded, respectively, at 31.72, 27.65, and 36.19 m² g^-1. Furthermore, esterification reaction parameters such as the reaction time, catalyst dosage, and reaction temperature for acetic acid were optimized to check the feasibility of rGO-metal oxide composites for a better conversion percentage of acetic acid. The optimized catalyst was selected for further optimization, and the optimum reaction parameters found were 0.1 wt % of catalyst, 160 min reaction duration, and 100 °C reaction temperature with a maximal yield of 100%. At 110 °C, the reaction conducted in the presence of 0.1 g of catalyst displayed more yield than the uncatalyzed reaction.

Synergistic effect of ultrasound and switchable hydrophilicity solvent promotes microalgal cell disruption and lipid extraction for biodiesel production

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.

Valorization of microalgae biomass into bioproducts promoting circular bioeconomy: a holistic approach of bioremediation and biorefinery

The need for alternative source of fuel has demanded the cultivation of 3rd generation feedstock which includes microalgae, seaweed and cyanobacteria. These phototrophic organisms are unique in a sense that they utilise natural sources like sunlight, water and CO₂ for their growth and metabolism thereby producing diverse products that can be processed to produce biofuel, biochemical, nutraceuticals, feed, biofertilizer and other value added products. But due to low biomass productivity and high harvesting cost, microalgae-based production have not received much attention. Therefore, this review provides the state of the art of the microalgae based biorefinery approach to define an economical and sustainable process. The three major segments that need to be considered for economic microalgae biorefinery is low cost nutrient source, efficient harvesting methods and production of by-products with high market value. This review has outlined the use of various wastewater as nutrient source for simultaneous biomass production and bioremediation. Further, it has highlighted the common harvesting methods used for microalgae and also described various products from both raw biomass and delipidified microalgae residues in order to establish a sustainable, economical microalgae biorefinery with a touch of circular bioeconomy. This review has also discussed various challenges to be considered followed by a techno-economic analysis of the microalgae based biorefinery model.

Graphitization of Coconut Shell Charcoal for Sulfonated Mesoporous Carbon Catalyst Preparation and Its Catalytic Behavior in Esterification Reaction

Here, we reported the utilization of coconut shell charcoal used for solid acid catalysts and its performance in the esterification reaction of acetic acid and methanol. The graphitization of coconut shell charcoal was carried out by the calcination and KOH activation at the temperature of 400 °C for an hour and continued at the temperature of 800 °C for an hour under nitrogen flow resulted in graphitic carbon. The effect of the addition of KOH activation was observed by varied the weight ratio of coconut shell charcoal as raw material (RM) and KOH. The selected weight ratio of RM:KOH was 1:1, 1:2, and 1:4. The resulted graphitic carbon was sulfonated by heating with the sulfuric acid to obtain a solid acid catalyst. The sulfonic time was evaluated for 5 and 10 hours. The generated particles were characterized to examine the morphology, the crystallinity, the specific surface area, the chemical bonding, and the ionic capacity using Scanning Electron Microscopy (SEM), X-Ray diffraction (XRD), nitrogen gas absorption-desorption, Fourier Transform Infrared Spectroscopy (FTIR), and titration method, respectively. The best condition for graphitization of raw material is the use of RM:KOH = 1:4, resulting in the highest surface area reaching 1259.67 m2/g and the most dominant of the sulfonic group of −SO3 bond. Furthermore, increasing the sulfonating time from 5 to 10 hours led to the increase of the yield of esterification reaction from 85% to 96.57% for graphite synthesized using RM:KOH = 1:4. Copyright © 2020 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0). 

Acidic Functionalized Nanobohemite: An Active Catalyst for Methyl Ester Production

In the biodiesel production, acidic catalysts are ideally suitable for reacting with different oil sources at various free acid levels. On the other hand, the nanocatalysts can easily be propagated in the reaction medium and provide more accessible active sites for reaction. The aim of this work was to synthesize an acidic nanocatalyst based on boehmite nanoparticles then studying it to biodiesel production from soybean oil. Up to now, no reports were found on biodiesel production by this catalyst. After the synthesis and characterization of the catalyst, using response surface methodology (RSM), the optimized conditions for transesterification were 4.87 wt.% for catalyst dosage, 13:1 for the molar ratio of methanol to oil, 60 °C for reaction temperature, and 3 h for reaction time. At the optimal point, the production yield was 99.8 %. After six consecutive use of the catalyst, the yield dropped slightly (88 %). Consequently, the catalyst can be employed efficiently several runs in the production process.

Synthesis of Acidic Heterogeneous Catalysts with High Stability Based on Graphene Oxide/Activated Carbon Composites for the Esterification of Lactic Acid

In this article, acidic heterogeneous catalysts based on graphene oxide and sulfonated biochar were prepared, characterized, and used for lactic acid esterification to form ethyl lactate. Graphene oxide was supported on activated carbon (GO/AC) to more easily filter the catalyst from the reactants. The catalysts were characterized by such methods as XRD, FT-IR, SEM, BET, and the acid-base titration. Catalytic activity was evaluated through the esterification of lactic acid. As a result, the activity of the catalysts decreased in the following order: graphene oxide > sulfonated biochar ≈ GO/AC >> activated carbon. In addition, the GO/AC catalyst showed good stability with an unchanged yield from the 3rd to the 6th recycling test. These results suggest potential applications for new acidic heterogeneous catalysts based on graphene oxide and sulfonated biochar that could replace homogeneous acids in the future.

Latest development in microalgae-biofuel production with nano-additives

BACKGROUND

Microalgae have been experimented as a potential feedstock for biofuel generation in current era owing to its' rich energy content, inflated growth rate, inexpensive culture approaches, the notable capacity of CO₂ fixation, and O₂ addition to the environment. Currently, research is ongoing towards the advancement of microalgal-biofuel technologies. The nano-additive application has been appeared as a prominent innovation to meet this phenomenon.

MAIN TEXT

The main objective of this study was to delineate the synergistic impact of microalgal biofuel integrated with nano-additive applications. Numerous nano-additives such as nano-fibres, nano-particles, nano-tubes, nano-sheets, nano-droplets, and other nano-structures' applications have been reviewed in this study to facilitate microalgae growth to biofuel utilization. The present paper was intended to comprehensively review the nano-particles preparing techniques for microalgae cultivation and harvesting, biofuel extraction, and application of microalgae-biofuel nano-particles blends. Prospects of solid nano-additives and nano-fluid applications in the future on microalgae production, microalgae biomass conversion to biofuels as well as enhancement of biofuel combustion for revolutionary advancement in biofuel technology have been demonstrated elaborately by this review. This study also highlighted the potential biofuels from microalgae, numerous technologies, and conversion processes. Along with that, the study recounted suitability of potential microalgae candidates with an integrated design generating value-added co-products besides biofuel production.

CONCLUSIONS

Nano-additive applications at different stages from microalgae culture to end-product utilization presented strong possibility in mercantile approach as well as positive impact on the environment along with valuable co-products generation into the near future.

Conversion of lipids from wet microalgae into biodiesel using sulfonated graphene oxide catalysts

Four solid acid catalysts including graphene oxide (GO), sulfonated graphene oxide (SGO), sulfonated graphene (SG), and sulfonated active carbon (SAC) were used to convert lipids in wet microalgae into biodiesel. The physiochemical properties of the catalysts were characterized with scanning electron microscope, X-ray diffraction, and thermogravimetric analysis. SGO provided the highest conversion efficiency (84.6% of sulfuric acid) of lipids to fatty acid methyl esters (FAME). Whereas SAC converted few lipids into FAME. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and elemental analysis revealed that much higher hydrophilic hydroxyl content in SGO catalyst resulted in a considerable higher conversion efficiency of lipids to FAME than that (48.6%) catalyzed by SG, although SO₃H groups (0.44mmol/g) in SGO were less than those (1.69mmol/g) in SG. Given its higher SO₃H group content than GO (0.38mmol/g), SGO had higher conversion efficiency than GO (73.1%), when they had similar hydrophilic hydroxyl contents.

Pretreatment of corn stover by solid acid for d-lactic acid fermentation

Solid acid is a new acid that is safe and green, which has been widely used in the fields of acid pickling. In this study, we adopted solid acid to pretreat corn stover and used the pretreated corn stover in the fermentation of d-lactic acid. Finally, we obtained optimal conditions for the pretreatment of corn stover by solid acid: digestion temperature of 120°C, digestion time of 80min, and solid acid concentration of 1.5%. Then adding cellulase of 30FPU/g, the conversion rate of glucose reached 71.06% after enzymatic hydrolysis for 72h. In addition, the changes of corn stover structure after pretreatment were further represented by using scanning electron microscope (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). At the same time, we used the pretreated corn stover as fermentation substrate and Lactobacillus. delbrueckii sp. bulgaricus as the starting strain to produce d-lactic acid. The yield reached 18g/L, with the optical purity being 99%e.e. This research has provided a new way to comprehensively utilizae corn stover.

Recent advances in organic reactions catalyzed by graphene oxide and sulfonated graphene as heterogeneous nanocatalysts: a review

Over the past decade, the application of carbocatalyst systems has been preferred over that of homogeneous catalytic systems because of their advantages such as physical and thermal stability of the catalysts in successive reaction runs and reusability.