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Biodiesel production from palm oil using sulfonated graphene catalyst
Accepted Manuscript Biodiesel production from palm oil using sulfonated graphene catalyst Medy C. Nongbe, Tchirioua Ekou, Lynda Ekou, Yao Kouassi Benjamin, Erwan Le Grognec, François-Xavier Felpin PII:
S0960-1481(17)30024-1
DOI:
10.1016/j.renene.2017.01.024
Reference:
RENE 8463
To appear in:
Renewable Energy
Received Date: 23 May 2016 Revised Date:
23 December 2016
Accepted Date: 10 January 2017
Please cite this article as: Nongbe MC, Ekou T, Ekou L, Benjamin YK, Le Grognec E, Felpin F-X, Biodiesel production from palm oil using sulfonated graphene catalyst, Renewable Energy (2017), doi: 10.1016/j.renene.2017.01.024. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Biodiesel production from palm oil using sulfonated graphene catalyst
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Medy C. Nongbe,a,b Tchirioua Ekou,b Lynda Ekou,b Yao Kouassi Benjamin,c Erwan Le
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Grognec,a François-Xavier Felpina,d*
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a
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de la Houssinière, 44322 Nantes Cedex 3, France
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b
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Milieu, 02 BP 801 Abidjan 02, Côte d’Ivoire
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Université de Nantes, UFR des Sciences et des Techniques, CNRS UMR 6230, CEISAM, 2 rue
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Université Nangui Abrogoua, Laboratoire de Thermodynamique et de Physico-Chimie du
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c
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Synthèse, de l'Environnement et des Energies Nouvelles, Bp 1093 Yamoussoukro, Côte d’Ivoire
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d
Institut National Polytechnique Houphouët BOIGNY, Laboratoire de Procédés Industriels, de
Institut Universitaire de France, 1 rue Descartes, 75231 Paris Cedex 05, France
*Corresponding author. Tel: (+33) 251 125 422. Email: [email protected]
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Abstract
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In this study we report on sulfonated graphene, obtained by chemical exfoliation of inexpensive
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graphite and functionalization of the resulting graphene sheets with benzene sulfonic acid, as a
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new active catalyst for the transesterification of palm oil with methanol into biodiesel. The
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experimental conditions for obtaining fatty acid methyl esters were carefully optimized through
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the evaluation of several parameters including the catalyst loading, temperature, reaction time
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and methanol-to-oil molar ratio. Of particular relevance, we noticed that an excess of methanol
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was required to allow high transesterification yield, but an excessive dilution proved to be
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deleterious for the reaction yield due to lower interactions between the reactants and the catalyst.
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The heterogeneous catalyst showed a high thermal robustness and was successfully recycled
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without significant erosion of the reaction yield. Our catalytic system yields biodiesel with a high
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purity (>98%) after a single filtration and do not produce aqueous waste.
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Keywords
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Palm oil / Biodiesel / Transesterification / Heterogeneous Catalyst / Sulfonated graphene
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1. Introduction
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The depletion of fossil fuel reserves associated to the increasing worldwide energy demand,
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environmental concerns regarding the emission of pollutants and greenhouse gases, represent a
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complex equation that will only be solved with the use of alternative energies.[1]
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In 2009, European and North America regulations established new emission standards
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requiring the use of ultra-low-sulfur diesel with a maximum sulfur content of 10 ppm. Biodiesel,
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obtained by transesterification of triglycerides to fatty acid methyl esters (FAMEs) with
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methanol, is arguably one of the most promising non-petroleum-based renewable fuel (Scheme
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1).[2-4] It offers environmentally favorable emission profiles as it reduces carbon dioxide
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emissions by almost 70% compared to petroleum-based diesel and is free from sulfur and
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polycyclic aromatic hydrocarbons.[5-7] Sulfur is known to increase lubricity and its elimination
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from conventional fuel can be balanced with the use of 1-2% of biodiesel as additive without
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requiring engine modification.[8]
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Scheme 1. General strategy for the production of biodiesel from triglycerides
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The transesterification of triglycerides, originating from animal fats or vegetable oils such as
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rapeseed, soybean, sunflower and palm oils, with methanol occurs under either basic or acid
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conditions.[9] Industrially, the current biodiesel production is carried out in a batch or flow mode
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with homogeneous base catalysts including sodium hydroxide and sodium methylate.[10] Base
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catalysis is usually preferred over acid catalysis as it displays higher reaction rates and lower
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corrosion issues. However, base catalysis suffers from the formation of sodium soap as side
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product and large amount of wastewater from the neutralization step, requiring costly steps of
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refining.
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The use of heterogeneous catalysts addresses, at least in part, these issues since it allows
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simplifying isolation procedures through a simple filtration and does not produce wastewater in a
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large extent.[11-16] Considerable effort has been expended on designing new heterogeneous
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acid[17, 18] and basic catalysts.[19] Metal oxides, and particularly CaO,[20, 21] are certainly the
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most studied class of solid catalysts as they are inexpensive and contain highly basic active sites.
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However, these catalysts suffer from limited robustness and extensive leaching in the aqueous
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phase due to their partial solubility in MeOH. In order to avoid the formation of soap and
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enhance the catalyst robustness, we considered that developing a heterogeneous acid consisting
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of highly accessible active acidic sites strongly anchored to a stable and insoluble platform would
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constitute an efficient strategy. This led us to consider graphene as a robust support for sulfonic acid functions. Graphene is
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one of the most stimulating material, discovered in 2007 by Gein and Novoselov,[22] resulting in
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an impressive mass of discoveries in a short timeframe, in scientific area as diverse as electronic,
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catalysis and medicine.[23-26] Graphene consists of sp2-hybridized carbons forming a very
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peculiar two-dimension honeycomb structure of one-atomic thickness with an exceptional
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mechanical resistance and large theoretical surface area of 2630 m2 g-1.[27, 28] As part of a
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research program devoted to the use of graphene in catalysis, we recently reported the graphene-
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promoted acetalization of glycerol[29] and described a straightforward and scalable synthesis of
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sulfonated graphene (GR-SO3H) starting from graphite as inexpensive material.[30] In this report
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we unveil the high efficiency of GR-SO3H for the production of biodiesel from palm oil as well
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as its remarkable robustness allowing successive reuses without a significant erosion of its
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catalytic activity.
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2. Materials and methods
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2.1. Materials
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Graphite and sulfanilic acid were purchased from Sigma-Aldrich. All reagents and solvents were used as received without further purification.
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2.2. Extraction of palm oil
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Palm fruits used in this study were gathered in the area of Yamoussoukro (Ivory Coast).
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Freshly gathered palm fruits were thoroughly washed to remove impurities and insects. The finest
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fruits selected for this study were dried in an oven at 100 °C for 2 hours. The dried fruits were cut
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into four pieces lengthwise and the pulp was separated from the seeds. The pulp was Soxhlet
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extracted in refluxing hexane for 12 hours. Volatiles were removed under vacuum overnight
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giving the palm oil as a yellow-orange liquid which partially crystallized on standing at room
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temperature.
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2.3. Preparation of graphene (GR)
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GR was prepared according to our previously reported procedure.[30] Graphite powder (2 g)
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was mixed with sulfuric acid (4.6 wt. equiv., 5 mL), potassium persulfate (0.5 wt. equiv., 1 g) and
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phosphorus pentoxide (0.5 wt. equiv., 1 g). The resulting mixture was heated at 80 °C for 2
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hours. Then, the solid was filtered through a fritted glass and washed with deionized (DI) water
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(100 mL), MeOH (200 mL) and Et2O (200 mL). The resulting black paste (2.15 g, 108% in mass)
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was dried at 40 °C under vacuum overnight. The resulting material (2.15 g) was mixed with
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sulfuric acid (43.9 wt. equiv., 54 mL) at 0 °C and potassium permanganate (3.5 wt. equiv., 7.46
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g) was then added by portion with careful attention. The reaction mixture was stirred at 35 °C for
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2 hours and then the reaction mixture was cooled to 0 °C, followed by the addition of 30%
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hydrogen peroxide (2.84 wt. equiv., 5.9 mL) in DI water (80 mL). The solid was centrifuged
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(4000 rpm, 20 minutes) and the liquid phase was removed. DI water (50 mL) was added and the
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resulting suspension was centrifugated (4000 rpm, 15 minutes); this step being performed three
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times. The same procedure was carried out with MeOH and Et2O. The resulting brown solid
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graphene oxide (GO), (3.54 g, 164% in mass) was dried at 40 °C under vacuum overnight.
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GO (500 mg) was sonicated in DI water (500 mL) for 2 hours. A solution of 5% Na2CO3 (12
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mL) was added in order to increase the pH up to 9-10. Then, 64% hydrazine hydrate (41.2 wt.
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equiv., 20 mL) was added to the suspension and the mixture was refluxed for 24 hours. The
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solution was cooled down to room temperature and filtered through Millipore membrane (nylon
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filter 0.45 µm, 47 mm) and washed with 1N HCl (100 mL) and acetone (300 mL). GR (black
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powder, 284 mg, 57% in mass) was dried at 40 °C under vacuum overnight.
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2.4. Preparation of sulfonated graphene (GR-SO3H)
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GR (273 mg) was sonicated in DI water (40 mL) for 2 hours. Then, sodium nitrite (3.47 wt.
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equiv., 947 mg) and sulfanilic acid (2.89 wt. equiv., 789 mg) were added to the resulting solution,
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allowing the formation of the diazonium salt in situ, and the reaction was conducted at 25 °C for
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24 hours. The solution was filtered through Millipore membrane (nylon filter 0.45 µm, 47 mm)
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and washed with 1N HCl (100 mL) and acetone (300 mL). The black powder (352 mg, GR-
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SO3H) was dried at 40 °C under vacuum overnight. The loading of –SO3H was calculated to be
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1.75 mmol SO3H.g-1 by elemental analysis.
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2.5. Transesterification of palm oil in batch
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In a typical experiment, GR-SO3H (25 mg), palm oil (250 mg) and MeOH (236 µL) were
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placed in a sealed tube and vigorously stirred at 100 °C for 14 hours. The resulting mixture was
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filtered through Millipore membrane (nylon filter 0.45 µm, 25 mm) and the solid was thoroughly
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washed with acetone. The filtrate was concentrated under vacuum and analyzed by 1H NMR. The
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yield of the FAMEs formed was calculated by 1H NMR on a Bruker AVANCE 300 MHz 6
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spectrometer in CDCl3 using the following formula: %FAME = (2ICH3 x 100)/(3ICH2) where ICH3
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refers to the integration of the protons from the methyl ester groups of FAMEs at 3.63 ppm and
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ICH2 refers to the integration of protons α to the carbonyl groups of both triglycerides and FAMEs
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at 2.27 ppm.
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2.6. Recycling procedure
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In a typical experiment, GR-SO3H (25 mg), palm oil (250 mg) and MeOH (236 µL) were
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placed in a sealed tube and vigorously stirred at 100 °C for 14 hours. The resulting mixture was
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filtered through Millipore membrane (nylon filter 0.45 µm, 47 mm) and the solid was thoroughly
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washed with acetone. The GR-SO3H catalyst was subsequently treated with 1N HCl (20mL) and
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MeOH (20 mL) to remove any trace of glycerol and then dried under vacuum for 6 hours. More
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than 97% of the catalyst initially introduced was recovered for the next reuse.
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3. Result and discussions
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The production of biodiesel by transesterification of triglycerides with methanol has been
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envisaged with a variety of raw materials originating from animal fats or vegetable oils.[31, 32]
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While rapeseed and soybean oils have been preferred as renewable energy source in Europe and
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North America, the use of palm and jatropha oils for biodiesel production participates to a new
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economic activity in West Africa and South-East Asia. The palm oil used in this study was
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extracted from palm trees that are endemic plantations in Ivory Coast. The production of palm oil
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supports more than two millions of Ivoirians, and its transformation to biodiesel represents an
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important economic interest for the country.[33]
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3.1. Catalyst preparation and characterization
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The acid catalyst used in this study consisted in graphene sheets functionalized with
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benzenesulfonic acid functions (GR-SO3H). It was prepared following a scalable procedure
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recently developed in our laboratory (Scheme 2).[30] In short, inexpensive graphite was oxidized
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into graphene oxide (GO) following a two-step approach. After being exfoliated under ultrasound
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into isolated sheets, GO was reduced to graphene (GR) by hydrazine. The functionalization of
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GR with 4-diazoniumbenzenesulfonic acid under free-radical conditions provided GR-SO3H with
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an acid group density of ca. 1.75 mmol H+ per g, as determined by both elemental analysis and
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temperature-programmed desorption using ammonia. The covalent anchoring of benzenesulfonic
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acid groups onto the graphene surface was also secured by X-ray photoelectron spectroscopy
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(XPS). The full scan spectrum of GR-SO3H shows the presence of sulfur atoms in the S2s and
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S2p region at 232.1 and 168.1 eV respectively (Figure 1a). The peak at 168.1 eV can be ascribed
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to sulfonic acid functions and no other sulfur species, including sulfuric esters, can be detected.
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The high resolution spectrum of the C1s region shows a high content of sp2 carbons with a
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binding energy at 284.4 eV and a very low occurrence of oxygenated species suggesting that the
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honeycomb structure of graphene has not been altered by the functionalization step (Figure 1b).
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The negligible amount of carboxylic acid functions detected by XPS ruled out their possible
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contribution to the catalytic activity of GR-SO3H. This observation also suggests that the
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catalytic activity of GR-SO3H is the result of only one kind of active sites, e.g., sulfonic acid
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functions. This result is confirmed by 31P solid-state NMR of GR-SO3H after chemisorption of
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Et3P(O) which displays a single peak at 78.5 ppm, confirming the presence of only one type
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of acid at the surface of graphene and ruling out the presence of sulfuric esters.
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Transmission electron microscopy (TEM) image of GR-SO3H reveals the high transparency and
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the wrinkled paper-like structure of the material accounts for the successful exfoliation of
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graphene sheets in a random fashion.
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1. K2S2O8, P2O5 H2SO4, 80 °C, 2 h
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Oxidation-Exfoliation
O O Graphene oxide (GO) SO3H
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Scheme 2. Preparation of sulfonated graphene (GR-SO3H)
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Reduction SO3H
SO3H Sulfonated graphene (GR-SO3H) SO3H
Functionalization
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reflux, 24 h
1.75 mmol H+/g
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OH
SO3H
NaNO2, H2O 25 °C, 24 h Graphene (GR)
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NH2-NH2, H2O
Graphite
H2N
HO2C
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HO2C
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Fig. 1. Survey scan (a) and high resolution of C1s spectra of GR-SO3H. (c) TEM image of GR-
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SO3H.
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3.2. Optimization of reaction conditions on the transesterification of palm oil
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3.2.1. Effect of reaction temperature
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Investigations on the transesterification of palm oil with methanol using GR-SO3H started
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with the evaluation of the temperature on the reaction outcome. Heterogeneous catalysis is often
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caught between a low activity requiring a high reaction temperature and a moderate stability of
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the catalyst upon prolonged heating. Therefore, the temperature is an important parameter
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requiring a delicate compromise between reaction conversion and catalyst stability. The
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transesterification of triglycerides requires at least three molar equivalents of methanol but for
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determining the influence of the temperature on the production of FAMEs from palm oil, we
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empirically fixed the molar MeOH-to-oil ratio at 12:1 ensuring a fair conversion even at room
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temperature. The temperature-yield relationship summarizing our investigations is depicted in
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Fig. 2a. In the presence of GR-SO3H (10 wt.-%), the reaction rate was sluggish at 25 °C and a
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modest yield (30%) was recorded after 14 h of stirring. While warming the mixture to 50 °C did
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not significantly improved the biodiesel production (37%), the methanolysis significantly
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increased at 80 °C (68%) before reaching a plateau at 100 °C (78%). An increase of the reaction
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temperature to 120 °C did not modify the mixture composition and could be deleterious for the
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catalyst stability.
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3.2.2. Effect of catalyst loading
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With these promising results in hand, we optimized the catalyst loading which determines the
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number of active sites available in GR-SO3H for the transesterification process as depicted in Fig.
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2b. The influence of graphene, regardless of the presence of sulfonic acid functions, might not be
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negligible since the high hydrophobicity of graphene might induce a positive hydrophobic effect
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with lipophilic substrates. At low catalyst loading, a modest yield was calculated and working
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with 10 wt.-% of GR-SO3H was required to reach good reaction yield. Increasing the catalyst 11
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loading at 20 wt.-% was not recommended since it marginally impacted the reaction yield while
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it excessively increased the cost of the process and led to stirring issues due to a thick reaction
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mixture.
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3.2.3. Effect of reaction time
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The reaction time is an important parameter for esterification reactions as prolonged heating
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can cause the reaction to reverse and degrade the heterogeneous catalyst. The catalytic activity
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was explored with reaction time ranging from 1 h to 24 h and the results were summarized in Fig.
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2c. After 1 h of stirring at 100 °C, half of the palm oil was transesterified by MeOH into
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biodiesel. Then, the transesterification gently evolved until the chemical equilibrium was reached
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after ca. 14 h of stirring and no evolution of the chemical composition was observed upon
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extended reaction times, up to 24 hours. While heterogeneous catalysts usually display reduced
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activity and require longer reaction times with respect to their homogeneous counterpart due to
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less efficient mass transfer, the high activity observed after only 1 hour suggests that the low
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mass transfer might be positively balanced by confinement and hydrophobic effects.
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3.2.4. Effect of methanol-to-oil ratio
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In order to drive the transesterification forward the formation of FAMEs, we evaluated the
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influence of the methanol-to-oil molar ratio. In general, high volume of methanol shift the
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chemical equilibrium to the formation of FAMEs, but this paradigm can be challenged by
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dilution effects that strongly influence the interaction between the triglyceride and the active sites
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of GR-SO3H (Fig. 2d). Surprisingly, we observed that using a stoichiometric amount of MeOH 12
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was not detrimental for the reaction yield with respect to the use of a 4-fold excess (12:1 molar
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ratio). By contrast, the reaction yield significantly increased up to 98% upon using a 20:1
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methanol-to-oil molar ratio, leading to a nearly quantitative conversion of palm oil to biodiesel.
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These spectacular results suggest that the increase of the conversion is not directly related to the
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methanol-to-oil molar ratio but rather to the more efficient stirring and mass transfer in more
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diluting conditions. As expected, excessive dilution with molar ratio ranging from 24:1 to 36:1
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negatively affected the transesterification efficiency due to lower interactions between the
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reactants and the catalyst and the low solubility of palm oil in MeOH.
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Fig. 2. Effect of (a) temperature, (b) catalyst loading, (c) time and (d) MeOH-to-oil molar ratio
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on palm oil transesterification with MeOH.
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3.2.5. Background reactions
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To ascertain the exact role of GR-SO3H on the transesterification process, background
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reactions were carried out with the optimized parameters (Table 1). When GR-SO3H was omitted
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from the mixture of palm oil and MeOH, a marginal conversion of triglycerides to biodiesel was
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observed (entry 2, 8% yield). An additional experiment with pristine graphene (GR) as catalyst
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confirmed that the high transesterification rate obtained with GR-SO3H essentially resulted from
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the contribution of sulfonic acid functions but the hydrophobicity of GR slightly increased the
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reaction yield with regard to the catalyst-free reaction (entry 3, 14% yield).
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Entrya Catalyst
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GR-SO3H
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GR
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Yield of FAMEs (%) 98
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Table 1. Effect of catalysts on palm oil transesterification
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Reaction conditions: MeOH:oil = 20:1, 10 wt.-% catalyst, 14 h, 100 °C.
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The conversion of the transesterification reaction and the purity of the biodiesel obtained from
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the process developed in this study can be easily followed by 1H NMR in CDCl3 (Fig. 3). The
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spectrum of palm oil showed characteristic peaks of triglycerides bearing both saturated and
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unsaturated chains with signals of the olefinic protons and glycerol backbone at 5.31-5.35 and 14
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4.10-4.32 ppm respectively. Upon transesterification, the formation of the biodiesel was easily
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followed with the apparition of a singlet at 3.63 ppm attributed to the methyl esters of fatty acids
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concomitantly to the disappearance of the protons of the glycerol backbone. The reaction yield
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was calculated by integration of the acyl protons at 2.27 ppm and the methoxy groups at 3.63
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ppm, following the formula disclosed on the experimental section.
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Fig. 3. 1H NMR of (a) palm oil and (b) biodiesel.
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The composition of biodiesel obtained from the transesterification of palm oil was determined
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by gas chromatography following the European standard method EN 14103. The biodiesel was
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obtained in high purity (>98%) and mainly consisted of oleate, palmitate, linoleate and stearate 15
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methyl esters and the proportion of saturated versus unsaturated methyl ester was calculated to be
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42.5 and 57.5 respectively.
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Table 2. FAMEs composition of biodiesel.
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0.56
Palmitate (C16:0)
34.43
Stearate (C18:0)
6.18
Oleate (C18:1)
49.22
Linoleate (C18:2)
7.03
Linolenate (C18:3)
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Arachidate (C20:0)
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Eicosenate (C20:1)
0.16
Behenate (C22:0)
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Laurate (C12:0)
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Fatty acid methyl ester Composition (Wt.-%)
The synthetic usefulness of a heterogeneous catalytic system is intimately linked to the
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robustness of the catalyst, allowing, in the most favored scenarios, successive reuses without
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compromising the catalytic activity. We examined the recycling properties of GR-SO3H on the
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production of FAMEs from palm oil (Fig. 4). Our results revealed that the catalytic activity of
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GR-SO3H remained impressively stable upon successive reuses since the reaction yield
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marginally decrease with the catalyst recycled in 4 consecutive runs. It must be pointed out that
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all recycling experiments were carried out under identical experimental conditions. In other
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words, neither the reaction time (14 h), nor the reaction temperature (100 °C), nor the methanol-
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to-oil molar ratio (20:1) were modified to maintain the catalytic activity of GR-SO3H at a such
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high level. These results suggest that GR-SO3H was not altered upon reuses and neither leaching
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of sulfonic acid functions,[34-36] nor the formation of inactive sulfonate esters,[37, 38] occurred
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in our reaction conditions.
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Fig. 4. Recycling studies.
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4. Conclusion
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In summary, a highly active sulfonated graphene catalyst (GR-SO3H) was developed for the
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transesterification of palm oil with methanol into fatty acid methyl esters. The catalyst was
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obtained by chemical exfoliation of inexpensive graphite followed by the functionalization of the
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as-prepared graphene with benzenesulfonic acid functions. The catalytic activity of GR-SO3H
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and the effect of experimental conditions on the transesterification level were deeply studied.
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This optimization studies revealed that the catalyst performed remarkably well at 100 °C with a
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methanol-to-oil molar ratio of 20:1, producing biodiesel with 98% yield. The outstanding
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catalytic properties of GR-SO3H were ascribed to a good acid density, high hydrophobicity, and
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excellent thermal stability. The recycling studies only showed a marginal erosion of the reaction
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yield suggesting that neither leaching of sulfonic acid functions, nor the formation of inactive
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sulfonate esters occurred under the optimized conditions. We believe that GR-SO3H is an
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appealing catalyst for the production of biodiesel from palm oil. We are currently, developing a
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continuous flow approach of this technology that could certainly be more suitable for production
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in West Africa.
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Acknowledgements
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We gratefully acknowledge the University of Nantes, the “Centre National de la Recherche
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Scientifique” (CNRS), the “Région Pays de la Loire” in the framework of a “recrutement sur
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poste stratégique”. F.-X.F. is a member of the “Institut Universitaire de France” (IUF). We
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acknowledge Christine Labrugère (PLACAMAT, University of Bordeaux), Julie Hémez
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(CEISAM, University of Nantes) and Denis Loquet for XPS, TEM and GC analyses respectively.
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Dr. Nicolas Oger and Dr. Franck Rataboul (IRCELYON) are gratefully acknowledged for fruitful
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discussions.
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References
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Sulfonated graphene catalyzes the transesterification of palm oil into biodiesel The biodiesel production is achieved with 98% yield The catalyst is recyclable without significant erosion of its activity No aqueous waste is produced during the process Simple and practical experimental conditions
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S0960-1481(17)30024-1
DOI:
10.1016/j.renene.2017.01.024
Reference:
RENE 8463
To appear in:
Renewable Energy
Received Date: 23 May 2016 Revised Date:
23 December 2016
Accepted Date: 10 January 2017
Please cite this article as: Nongbe MC, Ekou T, Ekou L, Benjamin YK, Le Grognec E, Felpin F-X, Biodiesel production from palm oil using sulfonated graphene catalyst, Renewable Energy (2017), doi: 10.1016/j.renene.2017.01.024. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Biodiesel production from palm oil using sulfonated graphene catalyst
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Medy C. Nongbe,a,b Tchirioua Ekou,b Lynda Ekou,b Yao Kouassi Benjamin,c Erwan Le
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Grognec,a François-Xavier Felpina,d*
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a
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de la Houssinière, 44322 Nantes Cedex 3, France
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b
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Milieu, 02 BP 801 Abidjan 02, Côte d’Ivoire
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Université de Nantes, UFR des Sciences et des Techniques, CNRS UMR 6230, CEISAM, 2 rue
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Université Nangui Abrogoua, Laboratoire de Thermodynamique et de Physico-Chimie du
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c
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Synthèse, de l'Environnement et des Energies Nouvelles, Bp 1093 Yamoussoukro, Côte d’Ivoire
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d
Institut National Polytechnique Houphouët BOIGNY, Laboratoire de Procédés Industriels, de
Institut Universitaire de France, 1 rue Descartes, 75231 Paris Cedex 05, France
*Corresponding author. Tel: (+33) 251 125 422. Email: [email protected]
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Abstract
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In this study we report on sulfonated graphene, obtained by chemical exfoliation of inexpensive
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graphite and functionalization of the resulting graphene sheets with benzene sulfonic acid, as a
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new active catalyst for the transesterification of palm oil with methanol into biodiesel. The
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experimental conditions for obtaining fatty acid methyl esters were carefully optimized through
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the evaluation of several parameters including the catalyst loading, temperature, reaction time
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and methanol-to-oil molar ratio. Of particular relevance, we noticed that an excess of methanol
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was required to allow high transesterification yield, but an excessive dilution proved to be
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deleterious for the reaction yield due to lower interactions between the reactants and the catalyst.
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The heterogeneous catalyst showed a high thermal robustness and was successfully recycled
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without significant erosion of the reaction yield. Our catalytic system yields biodiesel with a high
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purity (>98%) after a single filtration and do not produce aqueous waste.
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Keywords
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Palm oil / Biodiesel / Transesterification / Heterogeneous Catalyst / Sulfonated graphene
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1. Introduction
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The depletion of fossil fuel reserves associated to the increasing worldwide energy demand,
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environmental concerns regarding the emission of pollutants and greenhouse gases, represent a
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complex equation that will only be solved with the use of alternative energies.[1]
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In 2009, European and North America regulations established new emission standards
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requiring the use of ultra-low-sulfur diesel with a maximum sulfur content of 10 ppm. Biodiesel,
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obtained by transesterification of triglycerides to fatty acid methyl esters (FAMEs) with
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methanol, is arguably one of the most promising non-petroleum-based renewable fuel (Scheme
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1).[2-4] It offers environmentally favorable emission profiles as it reduces carbon dioxide
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emissions by almost 70% compared to petroleum-based diesel and is free from sulfur and
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polycyclic aromatic hydrocarbons.[5-7] Sulfur is known to increase lubricity and its elimination
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from conventional fuel can be balanced with the use of 1-2% of biodiesel as additive without
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requiring engine modification.[8]
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Scheme 1. General strategy for the production of biodiesel from triglycerides
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The transesterification of triglycerides, originating from animal fats or vegetable oils such as
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rapeseed, soybean, sunflower and palm oils, with methanol occurs under either basic or acid
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conditions.[9] Industrially, the current biodiesel production is carried out in a batch or flow mode
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with homogeneous base catalysts including sodium hydroxide and sodium methylate.[10] Base
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catalysis is usually preferred over acid catalysis as it displays higher reaction rates and lower
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corrosion issues. However, base catalysis suffers from the formation of sodium soap as side
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product and large amount of wastewater from the neutralization step, requiring costly steps of
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refining.
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The use of heterogeneous catalysts addresses, at least in part, these issues since it allows
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simplifying isolation procedures through a simple filtration and does not produce wastewater in a
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large extent.[11-16] Considerable effort has been expended on designing new heterogeneous
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acid[17, 18] and basic catalysts.[19] Metal oxides, and particularly CaO,[20, 21] are certainly the
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most studied class of solid catalysts as they are inexpensive and contain highly basic active sites.
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However, these catalysts suffer from limited robustness and extensive leaching in the aqueous
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phase due to their partial solubility in MeOH. In order to avoid the formation of soap and
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enhance the catalyst robustness, we considered that developing a heterogeneous acid consisting
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of highly accessible active acidic sites strongly anchored to a stable and insoluble platform would
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constitute an efficient strategy. This led us to consider graphene as a robust support for sulfonic acid functions. Graphene is
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one of the most stimulating material, discovered in 2007 by Gein and Novoselov,[22] resulting in
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an impressive mass of discoveries in a short timeframe, in scientific area as diverse as electronic,
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catalysis and medicine.[23-26] Graphene consists of sp2-hybridized carbons forming a very
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peculiar two-dimension honeycomb structure of one-atomic thickness with an exceptional
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mechanical resistance and large theoretical surface area of 2630 m2 g-1.[27, 28] As part of a
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research program devoted to the use of graphene in catalysis, we recently reported the graphene-
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promoted acetalization of glycerol[29] and described a straightforward and scalable synthesis of
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sulfonated graphene (GR-SO3H) starting from graphite as inexpensive material.[30] In this report
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we unveil the high efficiency of GR-SO3H for the production of biodiesel from palm oil as well
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as its remarkable robustness allowing successive reuses without a significant erosion of its
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catalytic activity.
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2. Materials and methods
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2.1. Materials
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Graphite and sulfanilic acid were purchased from Sigma-Aldrich. All reagents and solvents were used as received without further purification.
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2.2. Extraction of palm oil
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Palm fruits used in this study were gathered in the area of Yamoussoukro (Ivory Coast).
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Freshly gathered palm fruits were thoroughly washed to remove impurities and insects. The finest
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fruits selected for this study were dried in an oven at 100 °C for 2 hours. The dried fruits were cut
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into four pieces lengthwise and the pulp was separated from the seeds. The pulp was Soxhlet
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extracted in refluxing hexane for 12 hours. Volatiles were removed under vacuum overnight
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giving the palm oil as a yellow-orange liquid which partially crystallized on standing at room
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temperature.
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2.3. Preparation of graphene (GR)
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GR was prepared according to our previously reported procedure.[30] Graphite powder (2 g)
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was mixed with sulfuric acid (4.6 wt. equiv., 5 mL), potassium persulfate (0.5 wt. equiv., 1 g) and
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phosphorus pentoxide (0.5 wt. equiv., 1 g). The resulting mixture was heated at 80 °C for 2
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hours. Then, the solid was filtered through a fritted glass and washed with deionized (DI) water
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(100 mL), MeOH (200 mL) and Et2O (200 mL). The resulting black paste (2.15 g, 108% in mass)
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was dried at 40 °C under vacuum overnight. The resulting material (2.15 g) was mixed with
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sulfuric acid (43.9 wt. equiv., 54 mL) at 0 °C and potassium permanganate (3.5 wt. equiv., 7.46
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g) was then added by portion with careful attention. The reaction mixture was stirred at 35 °C for
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2 hours and then the reaction mixture was cooled to 0 °C, followed by the addition of 30%
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hydrogen peroxide (2.84 wt. equiv., 5.9 mL) in DI water (80 mL). The solid was centrifuged
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(4000 rpm, 20 minutes) and the liquid phase was removed. DI water (50 mL) was added and the
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resulting suspension was centrifugated (4000 rpm, 15 minutes); this step being performed three
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times. The same procedure was carried out with MeOH and Et2O. The resulting brown solid
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graphene oxide (GO), (3.54 g, 164% in mass) was dried at 40 °C under vacuum overnight.
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GO (500 mg) was sonicated in DI water (500 mL) for 2 hours. A solution of 5% Na2CO3 (12
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mL) was added in order to increase the pH up to 9-10. Then, 64% hydrazine hydrate (41.2 wt.
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equiv., 20 mL) was added to the suspension and the mixture was refluxed for 24 hours. The
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solution was cooled down to room temperature and filtered through Millipore membrane (nylon
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filter 0.45 µm, 47 mm) and washed with 1N HCl (100 mL) and acetone (300 mL). GR (black
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powder, 284 mg, 57% in mass) was dried at 40 °C under vacuum overnight.
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2.4. Preparation of sulfonated graphene (GR-SO3H)
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GR (273 mg) was sonicated in DI water (40 mL) for 2 hours. Then, sodium nitrite (3.47 wt.
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equiv., 947 mg) and sulfanilic acid (2.89 wt. equiv., 789 mg) were added to the resulting solution,
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allowing the formation of the diazonium salt in situ, and the reaction was conducted at 25 °C for
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24 hours. The solution was filtered through Millipore membrane (nylon filter 0.45 µm, 47 mm)
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and washed with 1N HCl (100 mL) and acetone (300 mL). The black powder (352 mg, GR-
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SO3H) was dried at 40 °C under vacuum overnight. The loading of –SO3H was calculated to be
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1.75 mmol SO3H.g-1 by elemental analysis.
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2.5. Transesterification of palm oil in batch
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In a typical experiment, GR-SO3H (25 mg), palm oil (250 mg) and MeOH (236 µL) were
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placed in a sealed tube and vigorously stirred at 100 °C for 14 hours. The resulting mixture was
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filtered through Millipore membrane (nylon filter 0.45 µm, 25 mm) and the solid was thoroughly
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washed with acetone. The filtrate was concentrated under vacuum and analyzed by 1H NMR. The
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yield of the FAMEs formed was calculated by 1H NMR on a Bruker AVANCE 300 MHz 6
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spectrometer in CDCl3 using the following formula: %FAME = (2ICH3 x 100)/(3ICH2) where ICH3
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refers to the integration of the protons from the methyl ester groups of FAMEs at 3.63 ppm and
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ICH2 refers to the integration of protons α to the carbonyl groups of both triglycerides and FAMEs
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at 2.27 ppm.
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2.6. Recycling procedure
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In a typical experiment, GR-SO3H (25 mg), palm oil (250 mg) and MeOH (236 µL) were
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placed in a sealed tube and vigorously stirred at 100 °C for 14 hours. The resulting mixture was
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filtered through Millipore membrane (nylon filter 0.45 µm, 47 mm) and the solid was thoroughly
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washed with acetone. The GR-SO3H catalyst was subsequently treated with 1N HCl (20mL) and
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MeOH (20 mL) to remove any trace of glycerol and then dried under vacuum for 6 hours. More
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than 97% of the catalyst initially introduced was recovered for the next reuse.
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3. Result and discussions
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The production of biodiesel by transesterification of triglycerides with methanol has been
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envisaged with a variety of raw materials originating from animal fats or vegetable oils.[31, 32]
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While rapeseed and soybean oils have been preferred as renewable energy source in Europe and
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North America, the use of palm and jatropha oils for biodiesel production participates to a new
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economic activity in West Africa and South-East Asia. The palm oil used in this study was
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extracted from palm trees that are endemic plantations in Ivory Coast. The production of palm oil
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supports more than two millions of Ivoirians, and its transformation to biodiesel represents an
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important economic interest for the country.[33]
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3.1. Catalyst preparation and characterization
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The acid catalyst used in this study consisted in graphene sheets functionalized with
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benzenesulfonic acid functions (GR-SO3H). It was prepared following a scalable procedure
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recently developed in our laboratory (Scheme 2).[30] In short, inexpensive graphite was oxidized
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into graphene oxide (GO) following a two-step approach. After being exfoliated under ultrasound
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into isolated sheets, GO was reduced to graphene (GR) by hydrazine. The functionalization of
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GR with 4-diazoniumbenzenesulfonic acid under free-radical conditions provided GR-SO3H with
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an acid group density of ca. 1.75 mmol H+ per g, as determined by both elemental analysis and
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temperature-programmed desorption using ammonia. The covalent anchoring of benzenesulfonic
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acid groups onto the graphene surface was also secured by X-ray photoelectron spectroscopy
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(XPS). The full scan spectrum of GR-SO3H shows the presence of sulfur atoms in the S2s and
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S2p region at 232.1 and 168.1 eV respectively (Figure 1a). The peak at 168.1 eV can be ascribed
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to sulfonic acid functions and no other sulfur species, including sulfuric esters, can be detected.
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The high resolution spectrum of the C1s region shows a high content of sp2 carbons with a
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binding energy at 284.4 eV and a very low occurrence of oxygenated species suggesting that the
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honeycomb structure of graphene has not been altered by the functionalization step (Figure 1b).
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The negligible amount of carboxylic acid functions detected by XPS ruled out their possible
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contribution to the catalytic activity of GR-SO3H. This observation also suggests that the
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catalytic activity of GR-SO3H is the result of only one kind of active sites, e.g., sulfonic acid
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functions. This result is confirmed by 31P solid-state NMR of GR-SO3H after chemisorption of
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Et3P(O) which displays a single peak at 78.5 ppm, confirming the presence of only one type
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of acid at the surface of graphene and ruling out the presence of sulfuric esters.
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Transmission electron microscopy (TEM) image of GR-SO3H reveals the high transparency and
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the wrinkled paper-like structure of the material accounts for the successful exfoliation of
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graphene sheets in a random fashion.
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1. K2S2O8, P2O5 H2SO4, 80 °C, 2 h
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Reduction SO3H
SO3H Sulfonated graphene (GR-SO3H) SO3H
Functionalization
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reflux, 24 h
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OH
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NaNO2, H2O 25 °C, 24 h Graphene (GR)
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Graphite
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Fig. 1. Survey scan (a) and high resolution of C1s spectra of GR-SO3H. (c) TEM image of GR-
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SO3H.
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3.2. Optimization of reaction conditions on the transesterification of palm oil
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3.2.1. Effect of reaction temperature
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Investigations on the transesterification of palm oil with methanol using GR-SO3H started
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with the evaluation of the temperature on the reaction outcome. Heterogeneous catalysis is often
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caught between a low activity requiring a high reaction temperature and a moderate stability of
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the catalyst upon prolonged heating. Therefore, the temperature is an important parameter
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requiring a delicate compromise between reaction conversion and catalyst stability. The
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transesterification of triglycerides requires at least three molar equivalents of methanol but for
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determining the influence of the temperature on the production of FAMEs from palm oil, we
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empirically fixed the molar MeOH-to-oil ratio at 12:1 ensuring a fair conversion even at room
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temperature. The temperature-yield relationship summarizing our investigations is depicted in
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Fig. 2a. In the presence of GR-SO3H (10 wt.-%), the reaction rate was sluggish at 25 °C and a
214
modest yield (30%) was recorded after 14 h of stirring. While warming the mixture to 50 °C did
215
not significantly improved the biodiesel production (37%), the methanolysis significantly
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increased at 80 °C (68%) before reaching a plateau at 100 °C (78%). An increase of the reaction
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temperature to 120 °C did not modify the mixture composition and could be deleterious for the
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catalyst stability.
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3.2.2. Effect of catalyst loading
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With these promising results in hand, we optimized the catalyst loading which determines the
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number of active sites available in GR-SO3H for the transesterification process as depicted in Fig.
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2b. The influence of graphene, regardless of the presence of sulfonic acid functions, might not be
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negligible since the high hydrophobicity of graphene might induce a positive hydrophobic effect
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with lipophilic substrates. At low catalyst loading, a modest yield was calculated and working
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with 10 wt.-% of GR-SO3H was required to reach good reaction yield. Increasing the catalyst 11
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loading at 20 wt.-% was not recommended since it marginally impacted the reaction yield while
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it excessively increased the cost of the process and led to stirring issues due to a thick reaction
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mixture.
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3.2.3. Effect of reaction time
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The reaction time is an important parameter for esterification reactions as prolonged heating
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can cause the reaction to reverse and degrade the heterogeneous catalyst. The catalytic activity
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was explored with reaction time ranging from 1 h to 24 h and the results were summarized in Fig.
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2c. After 1 h of stirring at 100 °C, half of the palm oil was transesterified by MeOH into
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biodiesel. Then, the transesterification gently evolved until the chemical equilibrium was reached
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after ca. 14 h of stirring and no evolution of the chemical composition was observed upon
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extended reaction times, up to 24 hours. While heterogeneous catalysts usually display reduced
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activity and require longer reaction times with respect to their homogeneous counterpart due to
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less efficient mass transfer, the high activity observed after only 1 hour suggests that the low
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mass transfer might be positively balanced by confinement and hydrophobic effects.
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3.2.4. Effect of methanol-to-oil ratio
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In order to drive the transesterification forward the formation of FAMEs, we evaluated the
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influence of the methanol-to-oil molar ratio. In general, high volume of methanol shift the
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chemical equilibrium to the formation of FAMEs, but this paradigm can be challenged by
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dilution effects that strongly influence the interaction between the triglyceride and the active sites
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of GR-SO3H (Fig. 2d). Surprisingly, we observed that using a stoichiometric amount of MeOH 12
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was not detrimental for the reaction yield with respect to the use of a 4-fold excess (12:1 molar
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ratio). By contrast, the reaction yield significantly increased up to 98% upon using a 20:1
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methanol-to-oil molar ratio, leading to a nearly quantitative conversion of palm oil to biodiesel.
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These spectacular results suggest that the increase of the conversion is not directly related to the
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methanol-to-oil molar ratio but rather to the more efficient stirring and mass transfer in more
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diluting conditions. As expected, excessive dilution with molar ratio ranging from 24:1 to 36:1
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negatively affected the transesterification efficiency due to lower interactions between the
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reactants and the catalyst and the low solubility of palm oil in MeOH.
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Fig. 2. Effect of (a) temperature, (b) catalyst loading, (c) time and (d) MeOH-to-oil molar ratio
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on palm oil transesterification with MeOH.
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3.2.5. Background reactions
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To ascertain the exact role of GR-SO3H on the transesterification process, background
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reactions were carried out with the optimized parameters (Table 1). When GR-SO3H was omitted
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from the mixture of palm oil and MeOH, a marginal conversion of triglycerides to biodiesel was
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observed (entry 2, 8% yield). An additional experiment with pristine graphene (GR) as catalyst
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confirmed that the high transesterification rate obtained with GR-SO3H essentially resulted from
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the contribution of sulfonic acid functions but the hydrophobicity of GR slightly increased the
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reaction yield with regard to the catalyst-free reaction (entry 3, 14% yield).
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Entrya Catalyst
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GR-SO3H
2
--
3
GR
a
Yield of FAMEs (%) 98
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Table 1. Effect of catalysts on palm oil transesterification
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Reaction conditions: MeOH:oil = 20:1, 10 wt.-% catalyst, 14 h, 100 °C.
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The conversion of the transesterification reaction and the purity of the biodiesel obtained from
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the process developed in this study can be easily followed by 1H NMR in CDCl3 (Fig. 3). The
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spectrum of palm oil showed characteristic peaks of triglycerides bearing both saturated and
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unsaturated chains with signals of the olefinic protons and glycerol backbone at 5.31-5.35 and 14
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4.10-4.32 ppm respectively. Upon transesterification, the formation of the biodiesel was easily
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followed with the apparition of a singlet at 3.63 ppm attributed to the methyl esters of fatty acids
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concomitantly to the disappearance of the protons of the glycerol backbone. The reaction yield
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was calculated by integration of the acyl protons at 2.27 ppm and the methoxy groups at 3.63
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ppm, following the formula disclosed on the experimental section.
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Fig. 3. 1H NMR of (a) palm oil and (b) biodiesel.
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The composition of biodiesel obtained from the transesterification of palm oil was determined
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by gas chromatography following the European standard method EN 14103. The biodiesel was
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obtained in high purity (>98%) and mainly consisted of oleate, palmitate, linoleate and stearate 15
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methyl esters and the proportion of saturated versus unsaturated methyl ester was calculated to be
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42.5 and 57.5 respectively.
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Table 2. FAMEs composition of biodiesel.
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0.56
Palmitate (C16:0)
34.43
Stearate (C18:0)
6.18
Oleate (C18:1)
49.22
Linoleate (C18:2)
7.03
Linolenate (C18:3)
0.15
Arachidate (C20:0)
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Eicosenate (C20:1)
0.16
Behenate (C22:0)
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Laurate (C12:0)
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Fatty acid methyl ester Composition (Wt.-%)
The synthetic usefulness of a heterogeneous catalytic system is intimately linked to the
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robustness of the catalyst, allowing, in the most favored scenarios, successive reuses without
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compromising the catalytic activity. We examined the recycling properties of GR-SO3H on the
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production of FAMEs from palm oil (Fig. 4). Our results revealed that the catalytic activity of
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GR-SO3H remained impressively stable upon successive reuses since the reaction yield
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marginally decrease with the catalyst recycled in 4 consecutive runs. It must be pointed out that
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all recycling experiments were carried out under identical experimental conditions. In other
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words, neither the reaction time (14 h), nor the reaction temperature (100 °C), nor the methanol-
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to-oil molar ratio (20:1) were modified to maintain the catalytic activity of GR-SO3H at a such
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high level. These results suggest that GR-SO3H was not altered upon reuses and neither leaching
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of sulfonic acid functions,[34-36] nor the formation of inactive sulfonate esters,[37, 38] occurred
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in our reaction conditions.
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Fig. 4. Recycling studies.
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4. Conclusion
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In summary, a highly active sulfonated graphene catalyst (GR-SO3H) was developed for the
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transesterification of palm oil with methanol into fatty acid methyl esters. The catalyst was
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obtained by chemical exfoliation of inexpensive graphite followed by the functionalization of the
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as-prepared graphene with benzenesulfonic acid functions. The catalytic activity of GR-SO3H
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and the effect of experimental conditions on the transesterification level were deeply studied.
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This optimization studies revealed that the catalyst performed remarkably well at 100 °C with a
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methanol-to-oil molar ratio of 20:1, producing biodiesel with 98% yield. The outstanding
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catalytic properties of GR-SO3H were ascribed to a good acid density, high hydrophobicity, and
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excellent thermal stability. The recycling studies only showed a marginal erosion of the reaction
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yield suggesting that neither leaching of sulfonic acid functions, nor the formation of inactive
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sulfonate esters occurred under the optimized conditions. We believe that GR-SO3H is an
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appealing catalyst for the production of biodiesel from palm oil. We are currently, developing a
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continuous flow approach of this technology that could certainly be more suitable for production
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in West Africa.
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Acknowledgements
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We gratefully acknowledge the University of Nantes, the “Centre National de la Recherche
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Scientifique” (CNRS), the “Région Pays de la Loire” in the framework of a “recrutement sur
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poste stratégique”. F.-X.F. is a member of the “Institut Universitaire de France” (IUF). We
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acknowledge Christine Labrugère (PLACAMAT, University of Bordeaux), Julie Hémez
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(CEISAM, University of Nantes) and Denis Loquet for XPS, TEM and GC analyses respectively.
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Dr. Nicolas Oger and Dr. Franck Rataboul (IRCELYON) are gratefully acknowledged for fruitful
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discussions.
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Sulfonated graphene catalyzes the transesterification of palm oil into biodiesel The biodiesel production is achieved with 98% yield The catalyst is recyclable without significant erosion of its activity No aqueous waste is produced during the process Simple and practical experimental conditions
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