Levulinate production from algal cell hydrolysis using in situ transesterification

doi:10.1016/j.algal.2017.06.024

Algal Research

Volume 26, September 2017, Pages 431-435

https://doi.org/10.1016/j.algal.2017.06.024 Get rights and content

Highlights

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Levulinic acid ester can be produced by in situ transesterification of microalgae.
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Carbohydrate-rich biomass is favorable for producing levulinic acid ester.
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More than 40 mol% of glucose were converted to levulinic acid ester under 130 °C and 15 v% of sulfuric acid condition.

Abstract

In situ transesterification (direct conversion) of microalgal cells is a promising method to produce biodiesel from microalgae because it integrates the oil extraction and conversion process in one step. Not only biodiesel but also a few biochemicals can be produced through this process because both lipids and carbohydrates are converted under acidic conditions. Levulinic acid ester (levulinate) is one of the byproducts of in situ transesterification, which can be used as an additive in fuels or fragrances. This study investigated the effect of cell composition and reactive variables on the productivity of levulinic acid ester. The cell compositions of microalgal strains were compared between Nannochloropsis and Chlorella, and more levulinate was produced from carbohydrate-rich Chlorella cells. Both reaction temperature and acid concentration highly affected the levulinate yield, whereas the type of alcohols did not have much influence on the yield. Consequently, more than 40 mol% glucose inside the cell was converted to levulinate with a 15 v% sulfuric acid concentration at 130 °C.

Introduction

Commercialization of microalgal biodiesel is still challenging because of high cost and intensive energy consumption for cultivation and downstream processing [1], [2]. According to a previous study, more than 75% of the total consumed energy for the whole process is used in the extraction and conversion steps, even if the process omits the cell drying step [3]. Wet in situ transesterification can be an alternative solution for this problem. It is a promising biodiesel (fatty acid alkyl ester, FAAE) production process in which alcohol and cells react directly eliminating the oil extraction step and cell drying step [4], [5], [6], [7]. This method can reduce capital cost because it simplifies the process and improves the biodiesel yield compared to the sequential two-step process of lipid extraction and conversion [8]. However, in situ transesterification also has a critical problem in that it uses an excessive amount of alcohol relative to the lipid [9]. Even if alcohol can be recycled by distillation, its high vaporization heat requirement makes the in situ process difficult for commercialization.

To compensate for the high cost and energy consumption, much research on producing extra value-added products from microalgal cells are ongoing such as producing coloring agents and antioxidants [10], [11], [12]. Without any extra processes, in situ transesterification can produce not only biodiesel but other value-added chemicals. Its acidic condition hydrolyzes carbohydrates to other chemicals. It was reported that ethyl levulinate (EL), ethyl formate (EF) and diethyl ether (DEE) were found as the byproducts of in situ transesterification using ethanol and chloroform as the reactant and solvent, and sulfuric acid as the catalyst [13]. Among these products, levulinate, the esterified form of levulinic acid, is a highly valuable product that can be used as a fuel additive, or fragrance additive [14], [15], [16]. Levulinate as a fuel additive has several merits. A prior study reported that blending of levulinate to diesel improved the fuel lubricity [17]. Furthermore, it is a carbohydrate-derived material from the cell; therefore, more fuel can be generated than the original lipid contents. Levulinate can be produced from hexoses such as fructose, glucose, galactose, and mannose. Two pathways for levulinate production from glucose are shown in Fig. 1. A hexose molecule from the carbohydrate in the biomass is converted to 5-(hydroxymethyl)-2-furaldehyde (HMF) through dehydration under acidic condition. Then, HMF is immediately rehydrated and becomes levulinic acid. Finally, levulinic acid is esterified with an alcohol molecule, and one levulinate molecule is generated. Another pathway starts from the etherification between the hexose and alcohol into methyl α-d-glucopyranoside (MGP). It degrades into 2-(dimethoxymethyl)-5-(methoxymethyl)furan (DMF), and DMF yields levulinate directly. These two routes are dominant in water and alcohol respectively [14], [18].

This study demonstrated the effect of the algal cell composition on the yield of hydrolysis products in wet in situ transesterification. Variables including the reaction temperature and acid catalyst concentrations were investigated for enhancing the productivity of levulinate from in situ transesterification of wet microalgal biomass. The series of experiments focused on finding dominant factors that affect levulinate productivity according to the carbohydrate composition. Two kinds of algal cells, Nannochloropsis gaditana and Chlorella sp. were used, and sulfuric acid was chosen as the catalyst for transesterification and cell hydrolysis. Two different alcohols of methanol or ethanol were used as both solvent and reactant to compare the productivity of methyl levulinate (ML) and ethyl levulinate (EL).

Section snippets

Comparison of the two microalgal cell compositions

First, two microalgal strains were chosen and compared. One was Chlorella sp. KR-1 from the Korea Institute of Energy Research (KIER), and the other was Nannochloropsis gaditana purchased from AlgaSpring (Netherlands). Their compositions were classified as lipids, carbohydrates, ash and proteins. Lipids were measured from the recovered mass using the Folch method (methanol-chloroform (1:2 v/v)) after a 1 h sonication pretreatment [19]. The recovered organic phase was separated by the addition of

Biodiesel production

The FAAE yields with respect to various reaction temperatures, reaction times and two different alcohols are shown in Fig. 2. 38 g/L of Chlorella cells were used, and the acid concentration was 2.5 v%. Both the FAME and FAEE yields reached maximum values, which were about 90% at temperatures higher than 100 °C within 1 h. At 1 h, the FAME yields were 81.2%, 88.1%, and 88.7% under 100 °C, 115 °C and 130 °C respectively. The yield of FAEE, at the same time, were 84.9%, 85.1%, 83.3% under 100 °C, 115 °C and

Conclusion

The yield of levulinic acid ester (levulinate) in wet in situ transesterification was investigated by first identifying the carbohydrate composition and contents of two different strains of microalgal cells. The levulinate yield was enhanced by the higher carbohydrate contents in Chlorella more than in Nannochloropsis at temperatures higher than 100 °C. The productivity of levulinate was highly affected by both the reaction temperature and acid concentration but not by the type of alcohol.

Funding

This work was supported by the Advanced Biomass R&D Center (ABC) as the Global Frontier Project funded by the Ministry of Science, ICT and Future Planning (ABC-2010-0029728).

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Levulinate production from algal cell hydrolysis using in situ transesterification

Highlights

Abstract

Introduction

Section snippets

Comparison of the two microalgal cell compositions

Biodiesel production

Conclusion

Funding

Biotechnol. Adv.

Biotechnol. Adv.

Bioresour. Technol.

Bioresour. Technol.

Bioresour. Technol.

Bioresour. Technol.

Bioresour. Technol.

Process Biochem.

J. Biosci. Bioeng.

Biotechnol. Adv.

Bioresour. Technol.

Biomass Bioenergy