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Biodiesel synthesis via metal oxides and metal chlorides catalysis from marine alga Melanothamnus afaqhusainii
Biodiesel synthesis via metal oxides and metal chlorides catalysis from marine alga Melanothamnus afaqhusainii Abdul Majeed Khan, Noureen Fatima PII: DOI: Reference:
S1004-9541(15)00465-6 doi: 10.1016/j.cjche.2015.12.015 CJCHE 455
To appear in: Received date: Revised date: Accepted date:
2 June 2015 2 September 2015 6 September 2015
Please cite this article as: Abdul Majeed Khan, Noureen Fatima, Biodiesel synthesis via metal oxides and metal chlorides catalysis from marine alga Melanothamnus afaqhusainii, (2015), doi: 10.1016/j.cjche.2015.12.015
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ACCEPTED MANUSCRIPT Biotechnology and Bioengineering Biodiesel synthesis via metal oxides and metal chlorides catalysis from marine alga Melanothamnus afaqhusainii
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Abdul Majeed Khan* , Noureen Fatima
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Research Laboratory of Bioenergy, Department of Chemistry, Federal Urdu University of Arts,
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Science and Technology, Gulshan-e-Iqbal Campus, University Road, Karachi-75300, Pakistan
Abstract This research article demonstrates the most comprehensive comparative catalytic study of different metal oxides and metal chlorides towards the methanolysis of triglycerides of marine red
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macroalga Melanothamnus afaqhusainii. CaO was found to be the most reactive metal oxide that yielded 80% biodiesel while ZnCl2 was the most reactive metal chloride that produced 60% biodiesel by mechanical stirring for 6 hours at 100-110 °C. The overall reactivity order of the catalysts was
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found to be CaO > MgO > PbO2 > ZnCl2 > TiCl4 > PbO > HgCl2 > ZnO > AlCl3 > SnCl2 > TiO2 whereas, CaCl2, MgCl2, Al2O3, HgO, PbCl2, MnO2, MnCl2, Fe2O3 and FeCl3 were found to be nonreactive for transesterification of trigylcerides. In addition, a detailed study of the screening of mobile
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phases and spraying reagents was conducted which showed that petroleum ether : chloroform :
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toluene (7:2:1) is the best mobile phase, whereas iodine crystals/silica gel is the best visualizing agent for the thin layer chromatography (TLC) examination of biodiesel. Biodiesel production was
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confirmed by comparative TLC examination. It was further supported by the determination of fuel properties of biodiesel, which were found to be similar to the standard limits of American Society for Testing and Materials (ASTM).
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Keywords biomass, methanolysis, triglycerides, biodiesel, catalysts *Corresponding author. E-mail address: [email protected] Article history: Received 2 June 2015 Received in revised form 2 September 2015 Accepted 6 September 2015 Available online xxxx
1 INTRODUCTION Energy is the basic human survival need. The current energy requirements are fulfilled by the consumption of fossil fuels (coal, petroleum and natural gas) which are being depleted with the passage of time [1]. In addition, the burning of fossil fuels
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ACCEPTED MANUSCRIPT emits toxic and greenhouse gases which are resulting in global warming and pollution [2-4]. The limited supply and the non-renewable mode of fossil fuels have urged to search for the viable alternatives of energy that can overcome these issues [1, 5, 6].
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Biodiesel is the green alternative of petrodiesel that emits a lesser amount of pollutants
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like CO, CO2, SO2 and un-burnt hydrocarbons. It is the mixture of fatty acid alkyl
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esters produced by the transesterification of edible or non-edible oils with the short chain alcohols [7-10].
Transesterification takes place in the presence of homogenous or heterogeneous
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catalysts. The homogenous catalysts are highly reactive and provide single phase for the reactants, but the separation of catalyst and byproducts from biodiesel is difficult.
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On the other hand, heterogeneous catalysts are also very reactive, non-corrosive, avoid soap formation and easily separated at the end of the reaction. There are different kinds
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of heterogeneous catalysts, including the metal oxides, solid bases, solid acids and
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enzymes etc [11-15]. Various metal oxides such as BaO, SrO, CaO, ZnO, TiO2, ZrO2, CeO2, MgO, CaO/SiO2, BaO/SiO2 and MgO/SiO2 have been reported for biodiesel
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synthesis [16-19]. The metal chlorides like ZnCl2, AlCl3, SnCl2 and several transition metal complexes of Sn+2, Zn+2, Pb+2 and Hg+2 have also been reported to be the reactive heterogeneous Lewis acid catalysts for the transesterification reaction [9, 20,
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21].
The use of non-edible oils is preferred as the source of biodiesel production,
because the consumption of vegetable oils or animal fats for this purpose can cause food starvation [22]. Various species of marine macro-algae and micro-algae have been investigated for biodiesel synthesis [23-26]. In this article, marine macroalga Melanothamnus afaqhusainii has been utilized for biodiesel production. M. afaqhusainii belongs to the class Rhodophyta which is reddish brown in color with height up to 80 cm. This alga has a small disk like holdfast with small rhizoidal projections and it is strongly attached to the rocks [27, 28]. A number of metals including Mg, Fe, Mn, Cu, Ni, Zn, Cr, Pb, Co and Cd are reported from M. afaqhusainii [29]. The fatty acid composition of this alga has also been reported [30].
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ACCEPTED MANUSCRIPT This research article represents the conversion of marine red macroalga M. afaqhusainii oil to biodiesel in the presence of a number of metal oxides and metal
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chlorides as catalysts.
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2 EXPERIMENTAL 2.1 Materials
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Different reagents that have been used for the synthesis of biodiesel and its analysis including dichloromethane, ethyl acetate, n-hexane, silica gel, methanol,
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ethanol, petroleum ether, chloroform, toluene, diethyl ether, acetic acid, acetone, CaO, MgO, Al2O3, HgO, PbO, PbO2, TiO2, MnO2, Fe2O3, ZnO, CaCl2, MgCl2, AlCl3, HgCl2, PbCl2, SnCl2, TiCl4, MnCl2, FeCl3, ZnCl2, conc. H2SO4, iodine crystals,
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analytical grade.
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vanillin, ceric sulphate, ninhydrin, silica gel and K2Cr2O7. All these chemicals were of
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2.2 Collection and Pre-treatment The marine red macroalga M. afaqhusainii was collected from the Bulleji coast, Karachi (Fig. 1). The alga was identified by Prof. Dr. Mustafa Shameel (late) and its
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voucher specimen (KUH-SW-R1461) was deposited in the herbarium of the Department of Botany, University of Karachi. It was dried, crushed and then ground by the chopper of Black and Decker Company (FX3508) to obtain the powdered alga (12 kg). The alga was soaked in dichloromethane : ethyl acetate : n-hexane (1:2:3) for three weeks. The solvent was separated using a rotary evaporator under reduced pressure to obtain the concentrated oily contents. The oil was purified by passing it through the column chromatography using silica gel as stationary phase while n-hexane as mobile phase.
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Figure 1 Pictorial representation of marine red alga Melanothamnus afaqhusainii
2.3 Transesterification
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Algal oil (5 g), methanol (10 ml) and CaO (0.5 g) were taken in an Erlenmeyer flask. The mixture was stirred and refluxed on the hot plate stirrer (Lab Tech
®
Daihan
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Labtech Co., Ltd) at 100-110 °C for 18 hours. The reaction was monitored with the
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TLC examination each over 6 hours of biodiesel production using petroleum ether : chloroform : toluene (7:2:1) as mobile phase and iodine crystals/silica gel as
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visualizing agent. When the reaction completed, two distinct layers were observed, the upper layer contains biodiesel i.e. fatty acid methyl ester (FAME) while the lower layer contains glycerol, un-reacted oil and catalyst. Both the layers were separated by
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separating funnel. Similar method was used to synthesize FAME using a number of other metal oxides, including MgO, Al2O3, HgO, PbO, PbO2, TiO2, MnO2, Fe2O3, ZnO and metal chlorides CaCl2, MgCl2, AlCl3, HgCl2, PbCl2, SnCl2, TiCl4, MnCl2, FeCl3 and ZnCl2. The biodiesel was purified by column chromatography using the same solvent system as used for TLC examination. The percentage yield of the biodiesel was calculated using the formula: Biodiesel yield = Biodiesel produced (after column chromatography) ×100% Oil used
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2.4 Reusability of Catalysts After the completion of the transesterification reactions with different heterogenous catalysts including metal oxides and metal chlorides, the reaction
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ACCEPTED MANUSCRIPT mixtures were kept at room temperature for one hour to cool down the contents. Then, it was filtered to separate out the solid catalysts from the product. The recovered catalyst on the filter paper (85%-90% of original used catalyst) was washed for three
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times with the mixture of n-hexane : chloroform (1:1, 20 ml) to remove oily contents
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and biodiesel traces. The catalysts were dried for one hour at 100 °C in the oven. The
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weight of the catalysts so recovered was found to be 5% -10% less than the originally taken catalysts. These catalysts were re-used for the transesterification of algal oil to check their catalytic activity.
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2.5 Screening of Mobile Phases and Visualizing Agents for TLC The TLC of biodiesel was run with different combinations of polar and non-
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polar mobile phases to check the best system that separates the products efficiently. Mobile phases used in the analysis were petroleum ether, petroleum ether : chloroform
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: toluene (8:1:1 and 7:2:1), n-hexane : diethyl ether : acetic acid (8:2:0.1), n-hexane :
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ethyl acetate : acetic acid (9:1:0.1), n-hexane : diethyl ether : acetone (8:2:0.2), petroleum ether : ethyl acetate : acetone (9:0.5:0.5) and petroleum ether : diethyl ether :
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chloroform (4.5:4.5:1). The analytical grade petroleum ether having boiling point range 40-60°C was used as mobile phase for column and thin layer chromatography.
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After running the TLC in these mobile phases, it was visualized by a number of spraying reagents like H2SO4 (10%) in ethanol, iodine crystals/silica gel, vanillin (15 g) in ethanol (250 ml) and conc. H2SO4 (2.5 ml), H2SO4 (10%) in ceric sulphate, FeCl3 in methanol : H2O (1:1), ninhydrin (0.2 g) in ethanol (100 ml), K2Cr2O7 (0.5%) in conc. H2SO4 (0.2 ml) and UV-lamp (254 nm) in order to select the best one for biodiesel identification.
2.6 Analysis The oil and biodiesel were characterized by a number of fuel properties that include density, dynamic viscosity, kinematics viscosity, acid value, cloud point and pour point. These properties were compared to the ASTM standard limits of biodiesel. In addition, the oil and biodiesel were also analyzed by thin layer chromatography.
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ACCEPTED MANUSCRIPT 3 RESULTS AND DISCUSSION The dry weight of marine macroalga M. afaqhusainii produced 1.3% oily
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content that was purified by the column chromatography. The oil was transesterified to
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biodiesel i.e. fatty acid methyl ester (FAME) by mechanical stirring using a series of metal oxides and metal chlorides as catalysts at 100-110 °C. The reactions were
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continued for 18 hours to achieve the maximum biodiesel yield. The reaction progress was checked after the intervals of six hours. Metal oxides were found to be more
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reactive than the corresponding metal chlorides. Generally, metal oxides act as basis, whereas metal chlorides act as Lewis acids. The mechanism showed that the metal
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oxides formed the complexes both with the methanol and the carbonyl group of a triglyceride molecule [31]. This complex formation favored the transesterification reaction. CaO gave the highest yield (80%) of biodiesel after 6 hours at 100-110 °C. It
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is due to the highest basic character of CaO. The reactivity of metal oxides decreased in
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the order of CaO > MgO > PbO2 > PbO > ZnO > TiO2 whereas, Al2O3, HgO, MnO2
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and Fe2O3 were found to be non-reactive at the experimental conditions followed. All the metal oxides were easily recovered after completion of the reactions and re-used for catalysis. A number of solvent systems were investigated to select the best mobile
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phase for TLC examination of biodiesel. Petroleum ether : chloroform : toluene (7:2:1) was selected as the most suitable mobile phase. Furthermore, different visualizing agents were also used to sort out the reagents that give the most prominent spot of biodiesel. The investigation showed that iodine crystals/silica gel is the suitable candidate to identify the biodiesel. The Rf value of FAME was calculated to be 0.35 (Table 1, Figs. 2-4).
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Table 1 Comparison of metal oxides and metal chlorides for the catalysis of biodiesel production Color of catalysts
Color of complexes
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Types of metals
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Catalysts
FAME yield/%
Catalytic activity
6 hours
12 hours
18 hours
80
84
89
Excellent
CaO
Alkaline earth metal (II A)
White
White
2.
CaCl2
Alkaline earth metal (II A)
White
White
00
00
00
Nil
3.
MgO
Alkaline earth metal (II A)
White
White
76
80
83
Excellent
4.
MgCl2
Alkaline earth metal (II A)
White
White
00
00
00
Nil
5.
Al2O3
Boron family (III A)
White
White
00
00
00
Nil
6.
AlCl3
Boron family (III A)
Yellow
Yellow
25
31
35
Moderate
7.
HgO
Transition metal (II B)
Dark orange
Dark orange
00
00
00
Nil
8.
HgCl2
Transition metal (II B)
White
Light pink
42
46
52
Moderate
9.
PbO
Carbon family (IV A)
Orange
Orange
50
61
66
Good
10.
PbO2
Carbon family (IV A)
Black
Black
66
70
73
Good
11.
PbCl2
Carbon family (IV A)
White
White
00
00
00
Nil
12.
SnCl2
Carbon family (IV A)
White
White
15
19
25
Poor
13.
TiO2
Transition metal (IV B)
White
White
8
13
20
Poor
14.
TiCl4
Transition metal (IV B)
Light yellow liquid
Dark purple
53
63
68
Good
15.
MnO2
Transition metal (VII B)
Black
Black
00
00
00
Nil
16.
MnCl2
Transition metal (VII B)
Pink (Light pink on drying)
Light pink
00
00
00
Nil
17.
Fe2O3
Transition metal (VIII B)
Reddish brown
Reddish brown
00
00
00
Nil
18.
FeCl3
Transition metal (VIII B)
Brown (Black on drying)
Brown
00
00
00
Nil
19.
ZnO
Transition metal (VIII B)
White
White
39
44
50
Moderate
20.
ZnCl2
Transition metal (VIII B)
White
Red
60
66
70
Good
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Figure 2 General chemical reaction for biodiesel production using metal oxide and metal chloride catalysts
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Figure 3 Mechanism of biodiesel production using metal oxides
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Figure 4 TLC examination of FAME catalyzed by different metal oxides The metal chlorides are less reactive than metal oxides due to their acidic
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behavior. However, they also produced a significant yield of biodiesel i.e. 60%, 53% and 42% biodiesel was produced using ZnCl2, TiCl4 and HgCl2 respectively after 6 hours at 100-110 °C. The mechanism of reaction showed that the acidic site (free metal orbitals) of metal chloride interact the oxygen atom of the carbonyl group of triglycerides to make the complex, which activated the reactive site (carbonyl carbon) of triglyceride that enhanced the rate of attack of methanol to form the desired product [21]. The reactive transition metal chlorides like HgCl2, TiCl4 and ZnCl2 made colored complexes while the other complexes of metal chlorides were found to be colorless. The overall reactivity order of the metal chlorides as observed by the TLC examination was observed to be ZnCl2 > TiCl4 > HgCl2 > AlCl3 > SnCl2 while CaCl2, MgCl2, PbCl2, MnCl2 and FeCl3 were found to be non-reactive for this reaction at the operating conditions. The separation of few metal chlorides was slightly difficult as they were 10
ACCEPTED MANUSCRIPT miscible in the un-reacted alcoholic phase. The product was confirmed by the same
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TLC examination system as used for metal oxide catalysts (Table 1, Figs. 2, 5 and 6).
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Figure 5 Mechanism of transesterification of triglycerides catalyzed by metal chlorides
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Figure 6 TLC examination of FAME catalyzed by different metal chlorides In most of the cases, the catalysts for biodiesel synthesis are purchased from the
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market that increases the production cost of biodiesel synthesis on large scale. Therefore, if the already used catalysts will be re-used in the next coming batch of biodiesel synthesis then the cost of the biodiesel production will be reduced to a significant level. The re-usability of these catalysts has been confirmed experimentally and all the metal oxides (CaO, MgO, PbO2, PbO, ZnO and TiO2) and metal chlorides (ZnCl2, TiCl4, HgCl2, AlCl3 and SnCl2) showed the same reactivity towards the next coming batches of biodiesel synthesis as for the previous one. These catalysts were found to be highly stable even at high temperature as it is evident from their calcination at 800-1000 oC for the preparation of nanoparticles. The TLC examination system to analyze biodiesel was finalized after screening a wide variety of mobile phases and TLC spraying reagents and techniques. When only
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ACCEPTED MANUSCRIPT petroleum ether as mobile phase was used for this purpose, it did not separate the biodiesel from fatty acids. The presence of acetone in most of the cases absorbed moisture that badly disturbed the separation. Petroleum ether : diethyl ether :
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chloroform (4.5:4.5:1) displayed the spots of fatty acids and biodiesel but overlapping
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with each other. Petroleum ether : chloroform : toulene (7:2:1) was selected as the best
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mobile phase system because it separated the biodiesel spot from other spots very clearly (Table 2). The UV lamp at 254 nm was only helpful to detect the conjugated fatty acids, but biodiesel spots could not be visualized. The presence of H2SO4 in few
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spraying reagents damaged the TLC strip due to the moisture content. The iodine crystals/silica gel was found to be the most effective spraying reagent for biodiesel
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identification on laboratory scale as well as on a commercial scale. The iodine crystals not only detected the fatty acids, but also the biodiesel spots were very clear while the
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silica gel was added in order to absorb the moisture during the TLC examination
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(Table 3).
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Table 2 Screening of mobile phases for thin layer chromatography (TLC) of biodiesel Merits
Demerits
1.
Petroleum ether
No any significant merit
Less polar for biodiesel separation
2.
Petroleum ether : chloroform : toluene (8:1:1)
Good separation
No demerit
3.
Petroleum ether : chloroform : toluene (7:2:1)
Excellent separation
4.
n-Hexane : diethyl ether : acetic acid (8:2:0.1)
No any significant merit
5.
n-Hexane : ethyl acetate : acetic acid (9:1:0.1)
No any significant merit
Highly polar and could not separated the fatty acids and biodiesel
6.
n-Hexane : diethyl ether : acetone (8:2:0.2)
No any significant merit
Highly polar and showed overlapping of fatty acids and methyl esters
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Petroleum ether : ethyl acetate : acetone (9:0.5:0.5)
No any significant merit
Acetone absorbed the moisture rapidly which badly disturbed the separation.
8.
Petroleum ether : diethyl ether : chloroform (4.5 : 4.5 : 1)
No any significant merit
Overlapping of biodiesel and fatty acids was observed
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Mobile phases
Less polar and showed the presence of moisture
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No demerit
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Table 3 Screening of TLC visualizing spraying reagents and techniques Merits
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H2SO4 (10%) in ethanol.
Colorless spots of biodiesel observed
2.
Iodine crystals + silica gel
Rapid appearance of visible biodiesel spots
No any significant demerit
3.
Vanillin (15 g) + ethanol (250 ml) +
Colorless spot of biodiesel observed
TLC was damaged due to aqueous H2SO4.
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Spraying reagents
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S. No.
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conc. H2SO4 (2.5 ml).
Demerits TLC was damaged due to aqueous H2SO4.
Ultravoilet lamp (254 nm)
UV is only helpful to detect the conjugated fatty acids
Biodiesel spots were not found to be UV active
5.
Ceric sulphate in H2SO4 (10%)
It gave brown spots of biodiesel upon heating
TLC was damaged due to aqueous H2SO4
6.
FeCl3 in methanol : H2O (1:1)
No any significant merit
Fatty acid and biodiesel spots were not appeared and TLC strip became brownish yellow
7.
Ninhydrin (0.2 g) in ethanol (100 ml)
No any significant merit
No any spot appeared
8.
K2Cr2O7 (S0.5%) in conc. H2SO4 (0.2 ml)
No any significant merit
No spot was appeared and it also damaged the TLC strip due to the presence of aqueous H2SO4
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these properties decreased and found to be in accordance with the ASTM standard
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limits for biodiesel which provided the strong evidence of biodiesel production (Table
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Table 4 Fuel properties of biodiesel in comparison with the oil and ASTM standard limits Fuel properties -3
Dynamic viscosity
Kinematics viscosity 2
-1
/mPa·s
/mm ·s
Oil
0.92
24.17
26.27
FAME
0.87
3.20
3.67
ASTM standard limits [11,32]
0.875 to 0.9
---
1.9 to 6.0
Cloud point
Pour point
Acid value
/°C
/°C
/mg KOH·(g sample)-1
3
0
18.2
-1
-2
0.75
-3 to 12
-15 to 16
0.8 max
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/g·cm
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Density
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Samples
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ACCEPTED MANUSCRIPT 4 CONCLUSIONS Biodiesel was synthesized from the oily content of marine red macro-alga M.
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afaqhusainii using various heterogeneous catalysts including metal oxides and metal
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chlorides. The metal oxides produced higher yield of biodiesel and thus showed more reactivity as compared to the metal chlorides. The reactivity order of metal oxides was
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found to be CaO > MgO > PbO2 > PbO > ZnO > TiO2 and that of metal chlorides was ZnCl2 > TiCl4 > HgCl2 > AlCl3 > SnCl2. All the catalysts were easily recovered at the
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end of reaction and re-used in the next coming batch of biodiesel synthesis where they showed similar reactivity and stability. In addition, petroleum ether : chloroform :
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toluene (7:2:1) was selected as the best mobile phase and iodine vapors/silica gel as visualizing agent for biodiesel. This system displayed clear spots of both fatty acids and biodiesel. The fuel properties of biodiesel such as density, kinematics viscosity,
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cloud point, pour point and acid value matched the criteria of ASTM standard limits of
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biodiesel. This research article will be helpful to synthesize the biodiesel on a
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commercial scale from any cheapest source like waste cooking oil, animal fats, jatropha oil etc. In fact, this research will be valuable to overcome the energy crisis, global warming, pollution, biodiversity and instability of global economy.
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ACKNOWLEDGEMENTS The authors are greatly thankful to the Higher Education Commission of
Pakistan for providing the Ph.D. scholarship (117-3083-PS7-208, 50018488) to Noureen Fatima under Indigenous Ph.D. 5000 Fellowship Program.
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Yoo, S.J., Lee, H.S., Veriansyah, B., Kim, J., Kim, J.D., Lee, Y.W., “Synthesis of biodiesel from rapeseed oil using supercritical methanol with metal oxide catalysts”, Bioresour. Technol., 101, 8686-8689 (2010).
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Venkatesh, Shamshuddin, S.Z.M., Shyamsundar, M., Vasanth, V.T., “Biodiesel synthesis from Pongamia pinnata oil over modified CeO2 catalysts”, J. Mex. Chem. Soc., 58 (4), 378385 (2014). Mohadesi, M., Hojabri, Z., Moradi, G., “Biodiesel production using alkali earth metal oxides
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Lewis acid-catalyzed transesterification”, Fuel, 88, 560-565 (2009).
Casas, A., Ramos, M.J., Rodríguez, J.F., Pérez, Á., “Tin compounds as Lewis acid catalysts for esterification and transesterification of acid vegetable oils”, Fuel Process. Technol., 106,
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biodiesel production from waste cooking oil”, Appl. Energ., 104, 683-710 (2013). Maghraby, D.M.El., Fakhry, E.M, “Lipid content and fatty acid composition of Mediterranean macro-algae as dynamic factors for biodiesel production”, Oceanologia, 57, 86-92 (2015). Khan, A.M., Hussain, M.S., “Production of biofuels from marine macroalgae Melanothamnus
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afaqhusainii and Ulva fasciata”, J. Chem. Soc. Pak., 37 (2), 371-379 (2015). Sánchez, A., Maceiras, R., Cancela, A., Rodríguez, M., “Influence of n-hexane on in situ
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transesterification of marine macroalgae”, Energies, 5, 243-257 (2012). Mata, T.M., Martins, A.A., Caetano, N.S., “Renew. Microalgae for biodiesel production and other applications: a review”, Sust. Energ. Rev., 14, 217-232 (2010). (27)
Afaq-Husain, S., Shameel, M., “Further investigations on the red alga Melanothamnus
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Khan, A.M., “Phytochemical and structural studies on the chemical constituents of Taxus wallichiana, Tanacetum gracile, Jolyna laminarioides and other marine algae”, Ph. D. Thesis, H. E. J Research Institute of Chemistry, University of Karachi, Pakistan (2000). Retrieved from http://eprints.hec.gov.pk/979/1/711.html.htm.
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GRAPHICAL ABSTRACT
The graphical abstract shows the comparison of yields of biodiesel produced from Melanothamnus afaqhusainii catalyzed by metal oxides and metal chlorides. CaO was found to be the most potential catalyst from investigated metal oxides series whereas ZnCl2 was found to be the most active catalyst from metal chloride series.
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S1004-9541(15)00465-6 doi: 10.1016/j.cjche.2015.12.015 CJCHE 455
To appear in: Received date: Revised date: Accepted date:
2 June 2015 2 September 2015 6 September 2015
Please cite this article as: Abdul Majeed Khan, Noureen Fatima, Biodiesel synthesis via metal oxides and metal chlorides catalysis from marine alga Melanothamnus afaqhusainii, (2015), doi: 10.1016/j.cjche.2015.12.015
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ACCEPTED MANUSCRIPT Biotechnology and Bioengineering Biodiesel synthesis via metal oxides and metal chlorides catalysis from marine alga Melanothamnus afaqhusainii
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Abdul Majeed Khan* , Noureen Fatima
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Research Laboratory of Bioenergy, Department of Chemistry, Federal Urdu University of Arts,
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Science and Technology, Gulshan-e-Iqbal Campus, University Road, Karachi-75300, Pakistan
Abstract This research article demonstrates the most comprehensive comparative catalytic study of different metal oxides and metal chlorides towards the methanolysis of triglycerides of marine red
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macroalga Melanothamnus afaqhusainii. CaO was found to be the most reactive metal oxide that yielded 80% biodiesel while ZnCl2 was the most reactive metal chloride that produced 60% biodiesel by mechanical stirring for 6 hours at 100-110 °C. The overall reactivity order of the catalysts was
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found to be CaO > MgO > PbO2 > ZnCl2 > TiCl4 > PbO > HgCl2 > ZnO > AlCl3 > SnCl2 > TiO2 whereas, CaCl2, MgCl2, Al2O3, HgO, PbCl2, MnO2, MnCl2, Fe2O3 and FeCl3 were found to be nonreactive for transesterification of trigylcerides. In addition, a detailed study of the screening of mobile
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phases and spraying reagents was conducted which showed that petroleum ether : chloroform :
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toluene (7:2:1) is the best mobile phase, whereas iodine crystals/silica gel is the best visualizing agent for the thin layer chromatography (TLC) examination of biodiesel. Biodiesel production was
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confirmed by comparative TLC examination. It was further supported by the determination of fuel properties of biodiesel, which were found to be similar to the standard limits of American Society for Testing and Materials (ASTM).
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Keywords biomass, methanolysis, triglycerides, biodiesel, catalysts *Corresponding author. E-mail address: [email protected] Article history: Received 2 June 2015 Received in revised form 2 September 2015 Accepted 6 September 2015 Available online xxxx
1 INTRODUCTION Energy is the basic human survival need. The current energy requirements are fulfilled by the consumption of fossil fuels (coal, petroleum and natural gas) which are being depleted with the passage of time [1]. In addition, the burning of fossil fuels
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Biodiesel is the green alternative of petrodiesel that emits a lesser amount of pollutants
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like CO, CO2, SO2 and un-burnt hydrocarbons. It is the mixture of fatty acid alkyl
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esters produced by the transesterification of edible or non-edible oils with the short chain alcohols [7-10].
Transesterification takes place in the presence of homogenous or heterogeneous
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catalysts. The homogenous catalysts are highly reactive and provide single phase for the reactants, but the separation of catalyst and byproducts from biodiesel is difficult.
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On the other hand, heterogeneous catalysts are also very reactive, non-corrosive, avoid soap formation and easily separated at the end of the reaction. There are different kinds
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of heterogeneous catalysts, including the metal oxides, solid bases, solid acids and
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enzymes etc [11-15]. Various metal oxides such as BaO, SrO, CaO, ZnO, TiO2, ZrO2, CeO2, MgO, CaO/SiO2, BaO/SiO2 and MgO/SiO2 have been reported for biodiesel
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synthesis [16-19]. The metal chlorides like ZnCl2, AlCl3, SnCl2 and several transition metal complexes of Sn+2, Zn+2, Pb+2 and Hg+2 have also been reported to be the reactive heterogeneous Lewis acid catalysts for the transesterification reaction [9, 20,
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The use of non-edible oils is preferred as the source of biodiesel production,
because the consumption of vegetable oils or animal fats for this purpose can cause food starvation [22]. Various species of marine macro-algae and micro-algae have been investigated for biodiesel synthesis [23-26]. In this article, marine macroalga Melanothamnus afaqhusainii has been utilized for biodiesel production. M. afaqhusainii belongs to the class Rhodophyta which is reddish brown in color with height up to 80 cm. This alga has a small disk like holdfast with small rhizoidal projections and it is strongly attached to the rocks [27, 28]. A number of metals including Mg, Fe, Mn, Cu, Ni, Zn, Cr, Pb, Co and Cd are reported from M. afaqhusainii [29]. The fatty acid composition of this alga has also been reported [30].
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ACCEPTED MANUSCRIPT This research article represents the conversion of marine red macroalga M. afaqhusainii oil to biodiesel in the presence of a number of metal oxides and metal
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chlorides as catalysts.
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2 EXPERIMENTAL 2.1 Materials
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Different reagents that have been used for the synthesis of biodiesel and its analysis including dichloromethane, ethyl acetate, n-hexane, silica gel, methanol,
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ethanol, petroleum ether, chloroform, toluene, diethyl ether, acetic acid, acetone, CaO, MgO, Al2O3, HgO, PbO, PbO2, TiO2, MnO2, Fe2O3, ZnO, CaCl2, MgCl2, AlCl3, HgCl2, PbCl2, SnCl2, TiCl4, MnCl2, FeCl3, ZnCl2, conc. H2SO4, iodine crystals,
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analytical grade.
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vanillin, ceric sulphate, ninhydrin, silica gel and K2Cr2O7. All these chemicals were of
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2.2 Collection and Pre-treatment The marine red macroalga M. afaqhusainii was collected from the Bulleji coast, Karachi (Fig. 1). The alga was identified by Prof. Dr. Mustafa Shameel (late) and its
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voucher specimen (KUH-SW-R1461) was deposited in the herbarium of the Department of Botany, University of Karachi. It was dried, crushed and then ground by the chopper of Black and Decker Company (FX3508) to obtain the powdered alga (12 kg). The alga was soaked in dichloromethane : ethyl acetate : n-hexane (1:2:3) for three weeks. The solvent was separated using a rotary evaporator under reduced pressure to obtain the concentrated oily contents. The oil was purified by passing it through the column chromatography using silica gel as stationary phase while n-hexane as mobile phase.
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Figure 1 Pictorial representation of marine red alga Melanothamnus afaqhusainii
2.3 Transesterification
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Algal oil (5 g), methanol (10 ml) and CaO (0.5 g) were taken in an Erlenmeyer flask. The mixture was stirred and refluxed on the hot plate stirrer (Lab Tech
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Daihan
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Labtech Co., Ltd) at 100-110 °C for 18 hours. The reaction was monitored with the
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TLC examination each over 6 hours of biodiesel production using petroleum ether : chloroform : toluene (7:2:1) as mobile phase and iodine crystals/silica gel as
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visualizing agent. When the reaction completed, two distinct layers were observed, the upper layer contains biodiesel i.e. fatty acid methyl ester (FAME) while the lower layer contains glycerol, un-reacted oil and catalyst. Both the layers were separated by
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separating funnel. Similar method was used to synthesize FAME using a number of other metal oxides, including MgO, Al2O3, HgO, PbO, PbO2, TiO2, MnO2, Fe2O3, ZnO and metal chlorides CaCl2, MgCl2, AlCl3, HgCl2, PbCl2, SnCl2, TiCl4, MnCl2, FeCl3 and ZnCl2. The biodiesel was purified by column chromatography using the same solvent system as used for TLC examination. The percentage yield of the biodiesel was calculated using the formula: Biodiesel yield = Biodiesel produced (after column chromatography) ×100% Oil used
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2.4 Reusability of Catalysts After the completion of the transesterification reactions with different heterogenous catalysts including metal oxides and metal chlorides, the reaction
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times with the mixture of n-hexane : chloroform (1:1, 20 ml) to remove oily contents
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and biodiesel traces. The catalysts were dried for one hour at 100 °C in the oven. The
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weight of the catalysts so recovered was found to be 5% -10% less than the originally taken catalysts. These catalysts were re-used for the transesterification of algal oil to check their catalytic activity.
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2.5 Screening of Mobile Phases and Visualizing Agents for TLC The TLC of biodiesel was run with different combinations of polar and non-
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polar mobile phases to check the best system that separates the products efficiently. Mobile phases used in the analysis were petroleum ether, petroleum ether : chloroform
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: toluene (8:1:1 and 7:2:1), n-hexane : diethyl ether : acetic acid (8:2:0.1), n-hexane :
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ethyl acetate : acetic acid (9:1:0.1), n-hexane : diethyl ether : acetone (8:2:0.2), petroleum ether : ethyl acetate : acetone (9:0.5:0.5) and petroleum ether : diethyl ether :
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chloroform (4.5:4.5:1). The analytical grade petroleum ether having boiling point range 40-60°C was used as mobile phase for column and thin layer chromatography.
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After running the TLC in these mobile phases, it was visualized by a number of spraying reagents like H2SO4 (10%) in ethanol, iodine crystals/silica gel, vanillin (15 g) in ethanol (250 ml) and conc. H2SO4 (2.5 ml), H2SO4 (10%) in ceric sulphate, FeCl3 in methanol : H2O (1:1), ninhydrin (0.2 g) in ethanol (100 ml), K2Cr2O7 (0.5%) in conc. H2SO4 (0.2 ml) and UV-lamp (254 nm) in order to select the best one for biodiesel identification.
2.6 Analysis The oil and biodiesel were characterized by a number of fuel properties that include density, dynamic viscosity, kinematics viscosity, acid value, cloud point and pour point. These properties were compared to the ASTM standard limits of biodiesel. In addition, the oil and biodiesel were also analyzed by thin layer chromatography.
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ACCEPTED MANUSCRIPT 3 RESULTS AND DISCUSSION The dry weight of marine macroalga M. afaqhusainii produced 1.3% oily
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content that was purified by the column chromatography. The oil was transesterified to
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biodiesel i.e. fatty acid methyl ester (FAME) by mechanical stirring using a series of metal oxides and metal chlorides as catalysts at 100-110 °C. The reactions were
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continued for 18 hours to achieve the maximum biodiesel yield. The reaction progress was checked after the intervals of six hours. Metal oxides were found to be more
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reactive than the corresponding metal chlorides. Generally, metal oxides act as basis, whereas metal chlorides act as Lewis acids. The mechanism showed that the metal
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oxides formed the complexes both with the methanol and the carbonyl group of a triglyceride molecule [31]. This complex formation favored the transesterification reaction. CaO gave the highest yield (80%) of biodiesel after 6 hours at 100-110 °C. It
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is due to the highest basic character of CaO. The reactivity of metal oxides decreased in
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the order of CaO > MgO > PbO2 > PbO > ZnO > TiO2 whereas, Al2O3, HgO, MnO2
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and Fe2O3 were found to be non-reactive at the experimental conditions followed. All the metal oxides were easily recovered after completion of the reactions and re-used for catalysis. A number of solvent systems were investigated to select the best mobile
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phase for TLC examination of biodiesel. Petroleum ether : chloroform : toluene (7:2:1) was selected as the most suitable mobile phase. Furthermore, different visualizing agents were also used to sort out the reagents that give the most prominent spot of biodiesel. The investigation showed that iodine crystals/silica gel is the suitable candidate to identify the biodiesel. The Rf value of FAME was calculated to be 0.35 (Table 1, Figs. 2-4).
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Table 1 Comparison of metal oxides and metal chlorides for the catalysis of biodiesel production Color of catalysts
Color of complexes
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Types of metals
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Catalysts
FAME yield/%
Catalytic activity
6 hours
12 hours
18 hours
80
84
89
Excellent
CaO
Alkaline earth metal (II A)
White
White
2.
CaCl2
Alkaline earth metal (II A)
White
White
00
00
00
Nil
3.
MgO
Alkaline earth metal (II A)
White
White
76
80
83
Excellent
4.
MgCl2
Alkaline earth metal (II A)
White
White
00
00
00
Nil
5.
Al2O3
Boron family (III A)
White
White
00
00
00
Nil
6.
AlCl3
Boron family (III A)
Yellow
Yellow
25
31
35
Moderate
7.
HgO
Transition metal (II B)
Dark orange
Dark orange
00
00
00
Nil
8.
HgCl2
Transition metal (II B)
White
Light pink
42
46
52
Moderate
9.
PbO
Carbon family (IV A)
Orange
Orange
50
61
66
Good
10.
PbO2
Carbon family (IV A)
Black
Black
66
70
73
Good
11.
PbCl2
Carbon family (IV A)
White
White
00
00
00
Nil
12.
SnCl2
Carbon family (IV A)
White
White
15
19
25
Poor
13.
TiO2
Transition metal (IV B)
White
White
8
13
20
Poor
14.
TiCl4
Transition metal (IV B)
Light yellow liquid
Dark purple
53
63
68
Good
15.
MnO2
Transition metal (VII B)
Black
Black
00
00
00
Nil
16.
MnCl2
Transition metal (VII B)
Pink (Light pink on drying)
Light pink
00
00
00
Nil
17.
Fe2O3
Transition metal (VIII B)
Reddish brown
Reddish brown
00
00
00
Nil
18.
FeCl3
Transition metal (VIII B)
Brown (Black on drying)
Brown
00
00
00
Nil
19.
ZnO
Transition metal (VIII B)
White
White
39
44
50
Moderate
20.
ZnCl2
Transition metal (VIII B)
White
Red
60
66
70
Good
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Figure 2 General chemical reaction for biodiesel production using metal oxide and metal chloride catalysts
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Figure 3 Mechanism of biodiesel production using metal oxides
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Figure 4 TLC examination of FAME catalyzed by different metal oxides The metal chlorides are less reactive than metal oxides due to their acidic
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behavior. However, they also produced a significant yield of biodiesel i.e. 60%, 53% and 42% biodiesel was produced using ZnCl2, TiCl4 and HgCl2 respectively after 6 hours at 100-110 °C. The mechanism of reaction showed that the acidic site (free metal orbitals) of metal chloride interact the oxygen atom of the carbonyl group of triglycerides to make the complex, which activated the reactive site (carbonyl carbon) of triglyceride that enhanced the rate of attack of methanol to form the desired product [21]. The reactive transition metal chlorides like HgCl2, TiCl4 and ZnCl2 made colored complexes while the other complexes of metal chlorides were found to be colorless. The overall reactivity order of the metal chlorides as observed by the TLC examination was observed to be ZnCl2 > TiCl4 > HgCl2 > AlCl3 > SnCl2 while CaCl2, MgCl2, PbCl2, MnCl2 and FeCl3 were found to be non-reactive for this reaction at the operating conditions. The separation of few metal chlorides was slightly difficult as they were 10
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TLC examination system as used for metal oxide catalysts (Table 1, Figs. 2, 5 and 6).
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Figure 5 Mechanism of transesterification of triglycerides catalyzed by metal chlorides
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Figure 6 TLC examination of FAME catalyzed by different metal chlorides In most of the cases, the catalysts for biodiesel synthesis are purchased from the
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market that increases the production cost of biodiesel synthesis on large scale. Therefore, if the already used catalysts will be re-used in the next coming batch of biodiesel synthesis then the cost of the biodiesel production will be reduced to a significant level. The re-usability of these catalysts has been confirmed experimentally and all the metal oxides (CaO, MgO, PbO2, PbO, ZnO and TiO2) and metal chlorides (ZnCl2, TiCl4, HgCl2, AlCl3 and SnCl2) showed the same reactivity towards the next coming batches of biodiesel synthesis as for the previous one. These catalysts were found to be highly stable even at high temperature as it is evident from their calcination at 800-1000 oC for the preparation of nanoparticles. The TLC examination system to analyze biodiesel was finalized after screening a wide variety of mobile phases and TLC spraying reagents and techniques. When only
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ACCEPTED MANUSCRIPT petroleum ether as mobile phase was used for this purpose, it did not separate the biodiesel from fatty acids. The presence of acetone in most of the cases absorbed moisture that badly disturbed the separation. Petroleum ether : diethyl ether :
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chloroform (4.5:4.5:1) displayed the spots of fatty acids and biodiesel but overlapping
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with each other. Petroleum ether : chloroform : toulene (7:2:1) was selected as the best
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mobile phase system because it separated the biodiesel spot from other spots very clearly (Table 2). The UV lamp at 254 nm was only helpful to detect the conjugated fatty acids, but biodiesel spots could not be visualized. The presence of H2SO4 in few
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spraying reagents damaged the TLC strip due to the moisture content. The iodine crystals/silica gel was found to be the most effective spraying reagent for biodiesel
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identification on laboratory scale as well as on a commercial scale. The iodine crystals not only detected the fatty acids, but also the biodiesel spots were very clear while the
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silica gel was added in order to absorb the moisture during the TLC examination
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Table 2 Screening of mobile phases for thin layer chromatography (TLC) of biodiesel Merits
Demerits
1.
Petroleum ether
No any significant merit
Less polar for biodiesel separation
2.
Petroleum ether : chloroform : toluene (8:1:1)
Good separation
No demerit
3.
Petroleum ether : chloroform : toluene (7:2:1)
Excellent separation
4.
n-Hexane : diethyl ether : acetic acid (8:2:0.1)
No any significant merit
5.
n-Hexane : ethyl acetate : acetic acid (9:1:0.1)
No any significant merit
Highly polar and could not separated the fatty acids and biodiesel
6.
n-Hexane : diethyl ether : acetone (8:2:0.2)
No any significant merit
Highly polar and showed overlapping of fatty acids and methyl esters
7.
Petroleum ether : ethyl acetate : acetone (9:0.5:0.5)
No any significant merit
Acetone absorbed the moisture rapidly which badly disturbed the separation.
8.
Petroleum ether : diethyl ether : chloroform (4.5 : 4.5 : 1)
No any significant merit
Overlapping of biodiesel and fatty acids was observed
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Mobile phases
Less polar and showed the presence of moisture
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No demerit
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Table 3 Screening of TLC visualizing spraying reagents and techniques Merits
1.
H2SO4 (10%) in ethanol.
Colorless spots of biodiesel observed
2.
Iodine crystals + silica gel
Rapid appearance of visible biodiesel spots
No any significant demerit
3.
Vanillin (15 g) + ethanol (250 ml) +
Colorless spot of biodiesel observed
TLC was damaged due to aqueous H2SO4.
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Spraying reagents
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conc. H2SO4 (2.5 ml).
Demerits TLC was damaged due to aqueous H2SO4.
Ultravoilet lamp (254 nm)
UV is only helpful to detect the conjugated fatty acids
Biodiesel spots were not found to be UV active
5.
Ceric sulphate in H2SO4 (10%)
It gave brown spots of biodiesel upon heating
TLC was damaged due to aqueous H2SO4
6.
FeCl3 in methanol : H2O (1:1)
No any significant merit
Fatty acid and biodiesel spots were not appeared and TLC strip became brownish yellow
7.
Ninhydrin (0.2 g) in ethanol (100 ml)
No any significant merit
No any spot appeared
8.
K2Cr2O7 (S0.5%) in conc. H2SO4 (0.2 ml)
No any significant merit
No spot was appeared and it also damaged the TLC strip due to the presence of aqueous H2SO4
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these properties decreased and found to be in accordance with the ASTM standard
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limits for biodiesel which provided the strong evidence of biodiesel production (Table
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Table 4 Fuel properties of biodiesel in comparison with the oil and ASTM standard limits Fuel properties -3
Dynamic viscosity
Kinematics viscosity 2
-1
/mPa·s
/mm ·s
Oil
0.92
24.17
26.27
FAME
0.87
3.20
3.67
ASTM standard limits [11,32]
0.875 to 0.9
---
1.9 to 6.0
Cloud point
Pour point
Acid value
/°C
/°C
/mg KOH·(g sample)-1
3
0
18.2
-1
-2
0.75
-3 to 12
-15 to 16
0.8 max
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ACCEPTED MANUSCRIPT 4 CONCLUSIONS Biodiesel was synthesized from the oily content of marine red macro-alga M.
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afaqhusainii using various heterogeneous catalysts including metal oxides and metal
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chlorides. The metal oxides produced higher yield of biodiesel and thus showed more reactivity as compared to the metal chlorides. The reactivity order of metal oxides was
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found to be CaO > MgO > PbO2 > PbO > ZnO > TiO2 and that of metal chlorides was ZnCl2 > TiCl4 > HgCl2 > AlCl3 > SnCl2. All the catalysts were easily recovered at the
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end of reaction and re-used in the next coming batch of biodiesel synthesis where they showed similar reactivity and stability. In addition, petroleum ether : chloroform :
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toluene (7:2:1) was selected as the best mobile phase and iodine vapors/silica gel as visualizing agent for biodiesel. This system displayed clear spots of both fatty acids and biodiesel. The fuel properties of biodiesel such as density, kinematics viscosity,
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cloud point, pour point and acid value matched the criteria of ASTM standard limits of
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biodiesel. This research article will be helpful to synthesize the biodiesel on a
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commercial scale from any cheapest source like waste cooking oil, animal fats, jatropha oil etc. In fact, this research will be valuable to overcome the energy crisis, global warming, pollution, biodiversity and instability of global economy.
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ACKNOWLEDGEMENTS The authors are greatly thankful to the Higher Education Commission of
Pakistan for providing the Ph.D. scholarship (117-3083-PS7-208, 50018488) to Noureen Fatima under Indigenous Ph.D. 5000 Fellowship Program.
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GRAPHICAL ABSTRACT
The graphical abstract shows the comparison of yields of biodiesel produced from Melanothamnus afaqhusainii catalyzed by metal oxides and metal chlorides. CaO was found to be the most potential catalyst from investigated metal oxides series whereas ZnCl2 was found to be the most active catalyst from metal chloride series.
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