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Synthesis of Biodiesel from the Oily Content of Marine Green Alga Ulva fasciata.

Byline: Abdul Majeed Khan and Noureen Fatima

Summary: The present study is focused on the chemical transformation of oils derived from the marine green alga Ulva fasciata Delile to biodiesel. The transesterification of algal oil was performed with a variety of alcohols using Na metal and NaOH as catalysts. Transesterification of algal oil by mechanical stirring yielded significant biodiesel within an hour at 60 C with NaOH and at room temperature with Na metal. In addition, microwave irradiated transesterification produced significant amount of biodiesel with NaOH and Na metal within 1-5 minutes. However, reaction of sodium metal in microwave oven was highly exothermic and uncontrollable that could also damage the radiation source. The reactivity order of alcohols was found to be methanol greater than ethanol greater than benzyl alcohol greater than 1-propanol greater than 1-butanol greater than 1-pentanol greater than 1-hexanol greater than 2-propanol. Isopropyl alcohol was found to be least reactive due to steric hindrance.

Benzyl alcohol was found to be more reactive than 1-propyl alcohol due to the electron withdrawing effect of benzene ring. The highest % conversion of FAME and FAEE were found to be 97% and 98% respectively using Na metal through mechanical stirring. Biodiesel production was confirmed by thin layer chromatography (TLC).

Furthermore, the fuel properties including density, kinematics viscosity, high heating value, acid value, free fatty acid (%), cloud point and pour point of U. fasciata oil and all the esters were determined and compared with the standard limits of biodiesel. Fatty acid methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 1-pentyl and 1-hexyl esters showed the fuel properties within the biodiesel standard limits therefore all of them were considered as the substitute of biodiesel. On the other hand, the fuel properties of benzyl ester were found to be above the limits of biodiesel specifications and thus it could not be considered as biodiesel. This research article will be helpful to overcome the current challenges of energy crisis, global warming and environmental pollution.

Keywords: Ulva fasciata, oily content, Transesterification, Biodiesel, Mechanical stirring, Microwave irradiations.

Introduction

The depletion of fossil fuels, increasing demand of energy and global pollution issues has increased the interest in renewable substitutes of energy. Biodiesel is the environmental friendly, renewable and clean burning fuel. It is an alternate to diesel fuel and chemically it is the mixture of alkyl esters of long chain fatty acids [1. 2]. It can be used as a successful transportation fuel by blending in any percentage with petro-diesel or without blending. It is free from sulphur and aromatic compounds and emits lower CO, CO2 and hydrocarbons on combustion as compared to the diesel fuel [3-5].

Biodiesel is generally produced by the transesterification of vegetable oils or animal fats in the presence of methanol [6, 7]. Other alcohols like ethanol, 1-propanol, 2-propanol and 1-butanol have also been reported for biodiesel production [8]. Transesterification is usually carried out by mechanical stirring in acidic or basic medium that intermix the reactants rapidly [9]. Acidic medium requires high temperature and more time to complete the reaction whereas base catalysed transesterification required low temperature and shorter time [10]. However, the base catalysed reactions produce excess amount of soap that makes the separation of biodiesel difficult [4]. The reaction is highly sensitive to water that cause the hydrolysis of esters to form free fatty acids [11]. Thus, reaction must be performed in anhydrous conditions. The microwave irradiated transesterification of triglycerides looks attractive due to the direct provision of heat energy to the reactants and fast rate of reaction [12].

Biodiesel production has been reported from various sources including vegetable oils and animal fats. However, the use of these sources in biodiesel production can affect the food supply. Algae can be used as an alternative source to produce biodiesel [13-16]. Algal biomass absorbs carbon dioxide and light for its growth hence they reduce greenhouse gases from the environment by consuming CO2. There are two main groups of algae namely macroalgae and microalgae. Both marine macroalgae and microalgae have been reported for the production of biodiesel [9, 14-16]. Most of the macroalgae species have less than 5% oily content while microalgae containing relatively high yield of oil which was found to be within the range of 30-70% [17-18].

The marine macroalga Ulva fasciata has been utilized in the present work for the production of biodiesel. U. fasciata grows in both intertidal regions of rocky shores and deep water regions [19]. It is chemically composed of carbohydrates, proteins, essential amino acids, vitamins, lipids, minerals and other compounds [20, 21]. A detailed study on the fatty acid composition of the oily contents of U. fasciata has already been reported which showed that it consists of many fatty acids [22].

Experimental

Materials

Different materials like dichloromethane, n- hexane, charcoal, column silica gel, TLC cards, ethyl acetate, Na metal, NaOH, methanol, ethanol, 1- propanol, 2-propanol, 1-butanol, 1-pentanol, 1- hexanol, benzyl alcohol, chloroform and toluene were used for the experimental purpose. All the materials used in the experiments were of analytical grade.

Collection and Pre-treatment of Alga

U. fasciata (Fig. 1) was collected from the Buleji Coast, Karachi during Oct., 2010. It was identified by Prof. Dr. Mustafa Shameel (late), Department of Botany, and University of Karachi. The voucher specimen number (KUH-SW-C1324) of the alga was deposited in the herbarium of the Department of Botany, University of Karachi, Pakistan. The alga was dried under shadow for 10 days and then under sunlight for 6 days. Dry alga was crushed by hands and then grinded into the powder form (3.7 Kg) by Chopper of Black and Decker Company (FX3508).

Dried powder (3.7 Kg) of U. fasciata was soaked in dichloromethane (5 litres) for one week. The extract was separated using cotton cloth and the soaking process was repeated thrice. The extract was concentrated by rotary evaporator under reduced pressure that produced the crude oil. The concentrated oily content was dissolved in n-hexane and mixed with the activated charcoal in order to remove the pigments. The charcoal was separated by filter paper and the oily content was passed through the column chromatography for further purification. The slurry of oily content with silica gel (20 g) was prepared and then it was loaded on the silica gel (200 g, mesh size: 100-200) column. Initially, the column was run with n-hexane and a number of fractions were collected. Now, the solvent system was changed to n-hexane: ethyl acetate (9:1). The fractions were compared by thin layer chromatography (TLC) examination and the related fractions were mixed together.

The process of column chromatography resulted in the removal of pigments and impurities that yielded light yellow coloured oil.

Transesterification of Algal Oil

Algal oil was treated with a series of alcohols in the presence of Na metal and NaOH to produce biodiesel. Two techniques, namely mechanical stirring and microwave irradiations were used to perform the reactions. All the reagents were made completely anhydrous before performing the experiments. In order to absorb the moisture, silica gel was added to oil and alcohols. All the reagents were placed in the desiccators to avoid the contact of moisture.

Mechanical Stirring

Na metal (0.1 g) and methanol (10 ml) were taken in a reaction flask (50 ml capacity) and stirred on the hot plate stirrer (Lab Tech (R) Daihan Labtech Co., Ltd) to form sodium methoxide. Now, silica gel (0.1 g) was added in the flask in order to absorb the moisture. The hot oil (5 g) was added drop-wise in the flask. The reaction mixture was stirred for 1 hour without heating. After the completion of reaction, the products were separated through separating funnel.

The upper layer contains biodiesel while the lower layer contains glycerol, soap and un-reacted oil. Furthermore, biodiesel was washed with hot water in order to remove the glycerol and soap present in biodiesel layer. Likewise, algal oil was transesterified using ethanol, 1-propanol, 2-propanol, 1-butanol, 1- pentanol, 1-hexanol and benzyl alcohol to produce the corresponding esters i.e. FAME, FAEE, FAP1E, FAP2E, FAB1E, FAPen1E, FAH1E and FABnE. All the fatty acid alkyl esters (FAAE) were also prepared by the same process using NaOH (0.1 g) and heating at 60 C. FAP2E was not formed in significant amount in 1 hour. Therefore, reaction time was increased up to 5 hours to achieve the maximum yield.

Microwave Irradiations

Na metal (0.1 g) and methanol (10 ml) were stirred in a reaction flask (50 ml capacity) to form sodium methoxide on the hot plate stirrer (Lab Tech (R) Daihan Labtech Co., Ltd) then algal oil (5 g) and silica gel (0.1g as water absorbent) were added in the flask. The reaction mixture was refluxed in the modified microwave oven (fitted with a reflux condenser, DW131-HP, 2.45 A- 109 hertz frequency and 900 Watt microwave output power) for 2 minutes. After the completion of the reaction, the flask was allowed to cool down and the biodiesel i.e. fatty acid methyl ester (FAME) was separated from by-products through separating funnel. Furthermore, biodiesel was separated from soap and glycerol by hot water. In the same way, sodium methoxide was prepared by reacting NaOH (0.1 g) with methanol which was treated with algal oil for 5 minutes to produce FAME.

Purification and Identification

A series of biodiesels was purified by column chromatography using silica gel (200 g, mesh size: 100-200) as stationary phase and n-hexane : chloroform : toluene (7:2:1) as mobile phase. The slurry of biodiesel was made with silica gel (20 g) and then loaded in the column. The column was run with the mobile phase and the eluted fractions were compared by TLC examination then the related fractions were mixed together.

Characterization

The gas chromatography (GC) of U. fasciata oil was performed using gas chromatogram (Shimadzu, Model No. GC-8A, Japan). The biodiesels were confirmed by TLC examination using n-hexane: chloroform: toluene (7:2:1) as mobile phase and silica gel as stationary phase. TLC spots were visualized by iodine vapours. In addition, the fuel properties including density, kinematics viscosity, high heating value, acid value, free fatty acids (%), cloud point and pour point of U. fasciata oil and biodiesel were determined and the results were compared with the standard limits for biodiesel [23, 24].

Results and Discussion

U. fasciata contains 30 g of oil per kg of dry algal powder. The algal oil was analysed by TLC examination that showed the presence of unsaturated fatty acids, triglycerides, terpenoids, steroids and other compounds. The oil was purified by column chromatography that separated the impurities and pigments of oil. The GC spectrum of U. fasciata oil showed the presence of several fatty acids within the retention time range of 6-48 min (Fig. 2). The density, kinematics viscosity and acid values of algal oil were found to be high and above the standard limits of biodiesel. Therefore, the algal oil cannot be directly used in the diesel engine and it must be converted to its esters having fuel properties within the standard limits of biodiesel.

Sodium alkoxides were separately prepared with sodium metal and NaOH then they were treated with different alcohols to form corresponding esters (Fig. 3, 4). The mechanical stirring was found to be effective method of transesterificaton that rapidly intermixed reactants during the reaction. In this method, Na metal produced all the esters at room temperature within an hour whereas NaOH produced all the esters at 60 C. Hot oil was used during the reactions just to increase the rate of reaction. The highest % conversion of FAME and FAEE were found to be 97% and 98% respectively using Na metal through mechanical stirring.

On the other hand, microwave irradiated transesterification of algal oil using NaOH produced 89% FAME in 5 minutes while Na metal reacted vigorously and completed the reaction within 2 minutes and showed 96% conversion of triglycerides to biodiesel. The reaction was highly exothermic and uncontrollable in microwave oven and even it could damage the radiation source of microwave oven. In addition, due to high reactivity, Na metal formed soap in large quantity that made it difficult to separate the soap from biodiesel. Due to this limitation of Na metal in microwave irradiated reactions, other esters were not prepared in the modified microwave oven (Fig. 5).

As the transesterification reaction is highly sensitive to the presence of water, therefore all the reagents were made anhydrous before conducting experiments. In addition, the reactions were performed in the presence of silica gel which played dual character i.e. it absorbed the moisture as well as it played a role of catalyst. Furthermore, the reagents were placed in the desiccators to avoid the contact of moisture. The reactivity of alcohols slightly decreased with the increase in the chain of alkyl group. Benzyl alcohol was found to be reactive due to the electron withdrawing effect and formed the sodium benzyloxide rapidly.

The steric hindrance of secondary alcohol i.e. 2-propyl alcohol decreased its reactivity. It did not form considerable amount of product in 1 hour therefore the reaction was repeated and performed for 5 hours which produced 90% biodiesel. The reactivity order of alcohols was observed to be methanol greater than ethanol greater than benzyl alcohol greater than 1-propanolgreater than 1-butanol greater than 1-pentanol greater than 1-hexanol greater than 2- propanol. The TLC examination of biodiesel in comparison with the algal oil confirmed the production of FAAE which showed the lower spot of the un-reacted fatty acid and upper spot of biodiesel. Rf values of the regular series of the esters i.e. methyl to hexyl esters and benzyl ester were found to be 0.24, 0.24, 0.28, 0.28, 0.28, 0.30, 0.33 and 0.25 respectively (Fig. 6). The conversion % of oil to FAAE has been shown in Table-1.

The fuel properties like density, kinematics viscosity, acid value, cloud point and pour point of methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 1-pentyl and 1-hexyl esters were found to be within the biodiesel standard limits. It provided the evidence that all these esters can be used as biodiesel. The single drawback of using higher alcohols for producing biodiesel is their high cost. Thus, further research is required to be conducted to reduce the cost of biodiesel produced from long chain alcohols in order to make it feasible to use their esters as biodiesel on the commercial basis.

The fuel properties of benzyl ester were out of the range of the biodiesel specifications and this was due to the structural difference of benzyl esters from other esters. Therefore, benzyl ester cannot be considered as biodiesel. The relative decrease in high heating value, acid value and free fatty acid (%) of FAAE was observed after the transesterification of algal oil (Table 1).

Table-1: The detailed study of conversion (%) and fuel properties of oil and fatty acid alkyl esters (FAAE).

###Conversion###Kinematics###Cloud###Pour

###Density###High heating value###Acid value###Free fatty acid

###Samples###(%)###viscosity###point###point

###(g / cm3)###(kJ/g)###(mg KOH / g sample)###(%)

###Na metalNaOH###(mm2 / s)###(C)###(C)

###(0.0317 A- Density) +###VKOH A- NKOH A- 56.1 /###0.503 A- Acid

###---###---###---###---

###38.053###Wsample###value

BD standards [25-###0.86 -

###---###---###1.9 6.0###----###0.80 max###---###-3 to 12###-15 to 16

###26]###0.90

###Oil###---###---###0.91###28.24###38.95###9.78###4.91934###4###-6

###FAME###97###83###0.88###3.68###38.17###0.51###0.25653###-1###-2

###FAEE###98###78###0.88###4.31###38.19###0.51###0.25653###-2###-4

###FAP1E###70###65###0.89###4.31###38.19###0.53###0.26659###0###-4

###FAP2E###39###35###0.86###3.83###38.17###0.59###0.29677###- 0.5###-9

###FAB1E###75###51###0.87###4.78###38.20###0.65###0.32695###-1###-6

###FAPen1E###73###55###0.86###4.93###38.21###0.60###0.30180###0.5###0

###FAH1E###70###44###0.87###5.51###38.23###0.64###0.32192###-2###-5

###FABnE###80###70###0.96###9.24###38.35###2.16###1.08648###-4###-10

Conclusions

The marine macroalga Ulva fasciata Delile contained 30 g of oily contents per kg of algal powder. This alga was used to prepare biodiesel using a number of alcohols by the transesterification reaction. Transesterification by mechanical stirring produced FAAE within an hour by heating with NaOH and without heating with Na metal. Microwave assisted transesterification was found to be successful with NaOH while Na metal as a catalyst was not effective as it started bumping vigorously during the reaction. However, it gave very fast conversion. All the reactions were conducted in anhydrous conditions due to the sensitivity of reaction to the presence of water.

The reactivity of alcohols decreased with the increase in the carbon chain. The production of FAAE was confirmed by their TLC examination in comparison with the algal oil in n-hexane : chloroform : toluene (7:2:1) system and iodine vapours were used to visualize the oil and FAAE spots. The fuel properties of FAME, FAEE, FAP1E, FAP2E, FAB1E, FAPen1E and FAH1E resembled the standard limits for biodiesel while FABnE did not match the criteria. Therefore, all these esters except FABnE can be used as the substitute of biodiesel. Further optimizations are required to reduce the cost of biodiesel produced by long chain alcohols. However, short chain alcohols are better than long chain alcohols for diesel engine.

Acknowledgements

The authors are greatly thankful to the Higher Education Commission of Pakistan for the provision of scholarship to Noureen Fatima through Indigenous Ph.D. 5000 Fellowship Program (1173083-PS7-208, 50018488).

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