Biodiesel production from Silybum marianum L. seed oil with high FFA content using sulfonated carbon catalyst for esterification and base catalyst for transesterification

Highlights

• PET was converted to activated carbon and then sulfonated to prepare carbon acid catalyst.

• Carbon acid catalyst was used for esterification of high acid value Silybum marianum L. seed oil.

• Biodiesel was obtained with 96.98% efficiency.

Abstract

In this research work, waste of polyethylene terephthalate (PET) was converted into activated carbon and the latter was used in the preparation of a carbon acid catalyst. Waste of PET was converted into activated carbon via carbonization and steam activation, then the activated carbon was sulfonated using fuming sulfuric acid in order to produce the carbon acid catalyst. The prepared carbon acid catalyst was tested for esterification of high acid value non-edible oil, Silybum marianum L. seed oil (SMSO) via optimized protocol. Amount of the carbon acid catalyst, methanol to oil molar ratio, temperature and time were the experimental variables optimized. Esterification of SMSO with methanol using the prepared carbon acid catalyst reduced its parent acid value (20.0 mg KOH/g) to the acceptable limits for base-catalyzed transesterification (<2.0 mg KOH/g) using 6.0% w/w of the catalyst, 15:1 methanol to oil molar ratio, 68 °C reaction temperature and 180 min of reaction. The performance of the catalyst was reduced gradually during its recycling and reached to 60.0% at the 5th cycle. Kinetics of esterification of SMSO using the prepared carbon acid catalyst followed pseudo first order kinetics, and the activation energy was found to be 70.98 kJ/mol. The esterified oil was converted to biodiesel through optimized base-catalyzed transesterification with methanol. Biodiesel with (96.98% yield and purity of 96.69% w/w) yield was obtained using 0.80% KOH w/w, 6:1 methanol to oil molar ratio, 60 °C reaction temperature, 75 min of reaction and 600 rpm rate of stirring. The biodiesel properties were within the recommended biodiesel standards as prescribed by ASTM D 6751 and EN 14214. Transesterification of the esterified oil was found to fellow first order kinetics, and the activation energy was calculated to be 17.92 kJ/mol.

Introduction

The fast depletion of conventional petro sources as well as the environmental concerns represented by the global warming phenomena make the need to find renewable energy sources is gaining considerable interest worldwide. Biodiesel (BD) can be produced from either vegetable oils or animal fats through transesterification reaction with alcohol using a suitable catalyst. Transesterification process can be catalyzed using various catalysts such as an alkali, acid or enzyme catalyst. However, the high cost of the feedstock oils used for BD production raised its cost of production. Consequently, cheaper feedstocks such as non-edible oils, animal oils or fats and waste cooking oils were used for BD production in order to reduce its cost of production [1], [2], [3], [4], [5], [6], [7], [8].

The high free fatty acids (FFAs) content of an oil or fat makes its conversion into BD via direct transesterification process useless, because FFAs deactivate the base catalyst and causes formation of soaps. As a result, FFAs level should be reduced to the acceptable limits (<1.0 wt.%) prior to transesterification process either by esterification of FFAs with methanol using an acid catalyst or esterification of FFAs with glycerol in order to reduce the FFA content to the acceptable limits for base-catalyzed transesterification. However, these processes raises the production cost as well as it produces huge amount of the polluted water [1], [2], [9]. To do so, FFAs are converted to fatty acid methyl ester (FAME) Sulfuric acid (H₂SO₄) is usually used as a catalyst for esterification of high FFAs level oils with methanol, due to its high activity and low cost. However, esterification by using minerals acids such as H₂SO₄ associated with many drawbacks such as the use of specialized acid resistant reactors to overcome corrosion problems as well as the need to multiple washing steps after the reaction, in order to remove the unreacted H₂SO₄ [10], [11]. As a result, heterogeneous acid catalysts such as ion exchanges (Amberlyst), zeolites, hetropolyacids, WO₃/ZrO₂, H₃PW₁₂O₄₀·6H₂O and sulfonated carbon catalysts were widely used for esterification of high FFAs oils, to overcome problems associated with homogenous acid catalysis [10], [11]. Heterogeneous acid catalysts can easily be removed from the reaction mixture without using water, neutralization step is not required and thus, it can be potentially reused. Moreover, heterogeneous acid catalysts can catalyze both esterification and transesterification reactions simultaneously, allowing lower cost feedstocks to be processed [10].

Due to their good thermal and mechanical stability as well as their chemical inertness, carbon-based solid acids were considered ideal catalysts and the most promising solid acid catalysts for many reactions. These catalysts could be prepared through sulfonation of incompletely pyrolysed biomass such as starch, sucrose, glucose, glycerol or bio-char [1], [2], [10], [11]. Two methods were used for synthesis of carbon catalysts, the first includes direct pyrolysis of the carbon precursor followed by sulfonation, whereas the second includes activation of the carbonaceous material produced via pyrolysis in order to increase its surface area followed by sulfonation. Many agents were used for sulfonation of carbon precursors such as H₂SO₄, fuming sulfuric acid, SO₃ gas, ClSO₃H and 4-benzenediazoniumsulfonate [1], [2], [10], [11]. Carbon acid catalysts were widely used as catalysts for variant applications. Guo et al. [12] used lignin- derived carbon catalyst for synthesis of BD from acidified soybean soapstock. Dawodu et al. [13] found that carbon acid catalysts produced from glucose and Calophyllum inophyllum seed cake were very effective toward conversion of non-edible oil with high free fatty acid into BD. Maneechakr et al. [14] investigated BD production from waste cooking oil via one-step process using a novel sulfonic functionalized carbon spheres derived from cyclodextrin and found that the prepared catalyst can catalyze transesterification of waste cooking oil with shorter reaction time. Shuit et al. [15] reported that carbon nanotubes appear to be a promising catalyst support for biodiesel production. Kansedo and Lee [16] studied optimization and kinetic of heterogeneous transesterification of non-edible sea mango (Cerbera odollam) oil using sulfated zirconia as a solid acid catalyst. Carbon solid catalysts were not only used for BD production, but for variety of applications. Tao et al. [17] tested sulfonated carbon produced from biomass waste as a solid catalyst for esterification of glycerol. Lui et al. [18] found that sulfonated magnetic carbon nanotube arrays were effective solid acid catalysts for the hydrolyses of polysaccharides in crop stalks. Finally, Guo et al. [19] used hierarchical porous carbon derived from sulfonated pitch for electrical double layer capacitors.

The increasing consumption of plastics resulted in accumulation of huge amounts of the solid wastes. Consumption of polyethylene terephthalate (PET) has increased in particular due to its use as an alternative to the glass that is used in soft drink bottles and containers. Approximately, 20% of the total solid wastes are PET. The disposal of PET waste together with its low bio- and photo-degradability represented a serious challenge for industrial countries world-wide. Incineration, hydrolysis and recycling are the most common methods to eliminate PET waste. Due to PET is a type of carbon rich wastes; its use as a precursor in the activated carbon preparation was widely investigated by many authors [20], [21], [22].

Silybum marianum L. is a wild annual plant which belongs to the Asteraceae family. It is naturally grows and also cultivated worldwide. It can also found at various regions of Asia such as Iraq, Iran and Syria. In Iraq, it could grow at mild climatic regions such as Mosul and Erbil Governorates. In China, it was also cultivated at some provinces such as Guangdong, Hubei, Shanxi and Qinghai. The seeds of this plant are used as a source of many flavone compounds. The extracted oil can be used as a cure for many diseases including Viral Hepatitis and Cirrhosis. Some researchers reported that the pharmacologically active component of the extract (silymarin) is made of isomeric mixture of flavonolignans, silychristin, silydianin, diastereoisomers silybin and isosilybin. It was reported by several authors that oil content of Silybum marianum L. varies from 28 to 45% w/w of seeds. Also in silymarin industrial oil production, the oil is considered a byproduct and not much utilized [7], [23]. In Iraq, this plant grows naturally. Besides, harvesting of this plant begins by the end of May. However, annual production of Silybum marianum L. is not known yet. In addition, oil is extracted from the seed using the cold press method. The main use of SMSO is as a medication. Synthesis of carbon catalysts from waste polymers was not reported in the literature. Moreover, no literature was reported on the use of the carbon acid catalyst produced from waste polymers in esterification of oils with high FFAs content. Besides, no literature was reported about esterification of high acid value Silybum marianum L. seed oil using carbon acid catalyst.

The objectives of the present study are: (a) to convert PET waste into high surface area activated carbon (b) sulfonation of the latter to produce carbon acid catalyst contains a polar group, i.e. sulfonic group (c) esterification of high acid value Silybum marianum L. seed oil using the prepared carbon acid catalyst in order to reduce its acid value to the acceptable limits for base-catalyzed transesterification (d) to optimize the esterification parameters such as amount of carbon acid catalyst, methanol to oil molar ratio, temperature and time (e) investigation kinetic of esterification using the prepared carbon acid catalyst (f) synthesis of biodiesel from SMSO esterified using the prepared carbon acid catalyst via optimized base – catalyzed transesterification (g) determination of properties of both the esterified oil and biodiesel via ASTM standard methods and (h) investigation the transesterification kinetic of the esterified oil as well.