Article pubs.acs.org/EF
Two-Step Conversion of Neem (Azadirachta indica) Seed Oil into Fatty Methyl Esters Using a Heterogeneous Biomass-Based Catalyst: An Example of Cocoa Pod Husk Eriola Betiku,*,† Anietie Okon Etim,† Omoniyi Pereao,‡ and Tunde Victor Ojumu‡ †
Department of Chemical Engineering, Obafemi Awolowo University, Ile-Ife, Osun 220005, Nigeria Department of Chemical Engineering, Cape Peninsula University of Technology, Cape Town Campus, Keizersgracht and Tennant Street, Zonnebloem, Cape Town 8000, South Africa
‡
S Supporting Information *
ABSTRACT: In this study, the viability of using calcined cocoa pod husk ash (CCPHA) as a catalyst for the transesterification of neem seed oil (NSO) into biodiesel was investigated. Prior to transesterification to biodiesel, the oil was pretreated with Fe2(SO4)3 via esterification to reduce its high acid value content. The Box-Behnken design (BBD) and central composite design (CCD) of response surface methodology (RSM) were used to investigate the individual and interactive effects of the methanol/ oil ratio, catalyst amount, and reaction time on the acid value and biodiesel yield, respectively. Results of scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), and elemental analysis showed that the catalytic action of the CCPHA produced was due to its K content and microstructural development when calcined at 700 °C for 4 h. The acid value of the NSO could be reduced from 11.57 to 1.80 mg of KOH/g of oil using optimum values of the methanol/oil ratio of 2.19 (v/v), catalyst amount of 6 wt %, and reaction time of 15 min while maintaining the reaction temperature constant at 65 °C. The results confirmed that neem seed oil methyl ester (NSOME), which satisfied ASTM D6751 and EN 14214 standards, could be produced at an optimum yield of 99.3 wt % using the methanol/oil ratio of 0.73 (v/v), catalyst amount of 0.65 wt %, and reaction time of 57 min while maintaining the reaction temperature constant at 65 °C. The results of this study demonstrated the prospect of developing an heterogeneous base catalyst from cocoa pod husk (CPH) for biodiesel production, which may reduce the total cost of production.
1. INTRODUCTION Energy utilization has increased immensely in the last century because of the world’s population growth, urbanization, and industrialization. Although renewable energy is one of the world’s fastest growing energy sources, fossil fuels, with their attendant problems, will still continue to supply almost 80% of world energy use through 2040.1 The need for renewable and unconventional fuel has been on the increase with the view to ameliorate the depleting fossil fuels and the associated environmental impact. Our review of published studies revealed that tremendous efforts have been made on the search for oils other than the edible oils from plant sources to produce biodiesel with a view to address the controversial debate around the use of edible oils for biodiesel production.2 Oils, such as yellow oleander oil, neem oil, moringa oil, castor oil, rubber seed oil, mahua oil, sea mango oil, karanja oil, jojoba oil, jatropha oil, and Calophyllum inophyllum oil,3 and edibles oils, such as soybean oil,4 sesame oil,5,6 sorrel oil,7 palm kernel oil,8 and sunflower oil,9 have been investigated to show an acceptable yield for biodiesel production. Although the advantages of biodiesel lie in its renewability, nontoxicity, intrinsic lubricity, and miscibility with petro-diesel,10 its competitiveness with fossil-based fuels may only be realized if production cost is lowered.2 The use of inexpensive feedstock and/or biomass waste materials has also been highlighted as one of the important factor to consider to significantly reduce production cost, thus the reason for the increased research © XXXX American Chemical Society
interest in the use of materials such as waste oils and non-edible oils. The heterogeneous catalysis method for biodiesel production is another option that may lower production cost, with its advantages ranging from addressing problems of saponification, excess reactant consumption, environmental problem, high alcohol/oil molar ratios, additional separation costs, which are associated with homogeneous catalysis, and its recoverability, reusability, and selectivity have been reviewed widely.11−13 Heterogeneous catalysts, such as ferric sulfate, have been shown to be effective and reusable in reducing free fatty acid (FFA) of many oils.14,15 Although heterogeneous catalysts can be designed synthetically, such that they possess both acidic and basic sites necessary to effectively convert oils with high free fatty to biodiesel,2 recent studies have shown the heterogeneous catalytic potentials of some biomass materials: egg shell,16,17 plantain peels,18 banana peels,19 banana trunk,20 oil palm trunk,21 sugar cane bagasse,21 waste animal bones,22 and cocoa pod husk (CPH).2,4 In a recent study, the catalytic potential of a typical biomass waste that is abundantly common in West Africa, banana peels, in a transesterification reaction, has been linked to its potassium content and the microstructural formation achieved when calcined at 700 °C for 4 h.19 The study revealed that biodiesel Received: February 28, 2017 Revised: April 19, 2017 Published: April 25, 2017 A
DOI: 10.1021/acs.energyfuels.7b00604 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels can be produced from Bauhinia monandra seed oil at an optimum yield of 98.5 ± 0.18 wt % using catalyst loading of 2.75 wt %, methanol/oil molar ratio of 7.6:1, reaction time of 69.02 min, and temperature of 65 °C. It should be noted however that the high FFA content was first reduced from 28.76 to 0.39% in a pretreatment process using reaction conditions of methanol/FFA molar ratio of 46:1, ferric sulfate of 12 wt %, reaction time of 75 min, and temperature of 65 °C.19 It may be important at some point to develop a database of the biomass materials that may have such catalytic potential, with a view to harness them to our advantage with respect to the development of heterogeneous catalytic processes for biodiesel production. Theobroma cacao (cocoa) of the family Sterculiaceae is one of the chief tropical crops. As of 2012, West African countries contribute about 70% of the world’s cocoa production.23 Global production in 2012 was 5 003 211 MT, of which Nigeria produced 383 000 MT.23 CPH is a byproduct of the cocoa fruit with great environmental concerns. It has been reported that CPH constitutes 70−75% by weight of the fruit,24 and for 1 ton of dry cocoa beans produced, 10 tons of CPH is generated;25 this waste is significant considering the negligible amount used in the manufacture of local soap,4,26 feeding of livestock,27 and as fertilizer.28 However, its ash is rich in potassium,26,29 yielding the alkali when dissolved in water. Recent studies have shown that the potash from CPH could serve as a green catalyst for biodiesel production from soybean oil4 and waste vegetable oil.30 It should be noted in both studies that detailed information about the development and characterization of the catalyst is limited. For example, the effect of the calcination temperature on the elemental composition and the morphological and structural characterization of the calcined catalyst were also not reported. It is not clear whether the input variables for the transesterification process were optimized or the process was modeled. Also, it is unknown whether experimental design tools, such as the design of experiments (DoE) and response surface methodology (RSM), which allow for simultaneous determination of individual and interactive effects of factors affecting the biodiesel yield, were employed by the authors. These are critical to the design of a typical catalytic process that may use these biomass catalysts for biodiesel production. This work was aimed at preparing an active heterogeneous base catalyst from CPH, which was subsequently used for catalyzing the transesterification reaction of the pretreated neem seed oil (NSO) with methanol. Also, an effort was made to model the esterification process used for the pretreatment of NSO with ferric sulfate as the catalyst as a result of its high FFA content. The pretreatment study was designed using the Box-Behnken design (BBD) of RSM to investigate the individual and interactive effects of the process input variables (methanol/oil ratio, catalyst amount, and time) on the reduction of FFA of the oil. Furthermore, the transesterification process employed for the conversion of the pretreated NSO to biodiesel was also modeled. The central composite design (CCD) of RSM was used to investigate the individual and interactive effects of the process input variables (methanol/oil ratio, catalyst amount, and time) on the biodiesel yield. The process input variables investigated in both steps were optimized using the optimization tool of RSM. The quality of the neem seed oil methyl ester (NSOME) produced was also reported.
Table 1. Independent Factors Used for (a) BBD in Esterification of NSO and (b) CCD in Transesterification of Pretreated NSO (a)
coded factor levels
factor
unit
methanol/oil ratio (X1) catalyst amount (X2) time (X3) (b)
v/v wt % min
−1
0
1
1.5 2.0 2 4 10 15 coded factor levels
2.5 6 20
factor
unit
−α
−1
0
1
+α
methanol/oil ratio (X1) catalyst amount (X2) time (X3)
v/v wt % min
0.23 0.65 24.77
0.40 1.50 35
0.65 2.75 50
0.90 4.00 65
1.07 4.85 75.23
located in Zaria, Kaduna, Nigeria. The oil was properly stored in a black keg. Also, all reagents and chemicals used for this work (diethyl ether, n-hexane, ethanolic sodium hydroxide, sodium sulfate, potassium iodide, ferric sulfate, ethanol, methanol, starch, and sulfuric acid) were of analytical grade. 2.2. Bio-based Catalyst Preparation from CPH. The CPHs used for this work were collected from Aba Gbooro village within the Obafemi Awolowo University campus, Ile-Ife, Osun, Nigeria. They were washed with tap water and sun-dried for about 2 weeks until a constant weight was achieved. The dried CPHs were milled manually into fine power using porcelain mortar and pestle. Part of the resulting powder was burnt in open air, while another part of it was calcined in a muffle furnace at 300, 500, 700, 900, and 1100 °C for 4 h. The calcined cocoa pod husk ash (CCPHA) produced, which was observed coarse in nature, was homogenized using the mortar and pestle and was stored in corked plastic vessels for further analysis. 2.3. Characterization of Bio-based Catalysts. The elemental compositions and microstructures of the catalysts derived from CPH were determined using AURIGA high-resolution scanning electron microscopy (SEM) equipped with a CDU lead detector at 5 kV with a tungsten filament (Zeiss, Germany) coupled with energy-dispersive X-ray spectroscopy (EDS). It should be noted that the elemental compositions are reported as an average of three replicate data obtained at different sites on the sample microstructures obtained using SEM. The active surface functional groups present in these bio-based catalysts were investigated using a Fourier transform infrared (FTIR) spectrophotometer (Thermo-Nicolet iS10) equipped with attenuated total reflectance (ATR). The samples were recorded in the range of 4000− 400 cm−1; the baseline was corrected; and the spectra were smoothened. X-ray diffraction (XRD) patterns were recorded on a D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany) with Cu Kα radiation (λKα1 = 1.5406 Å) equipped with a LynxEye position sensitive detector (PSD) for crystalline phase determination. The Brunauer−Emmett− Teller (BET) method of adsorption of nitrogen gas was used to determine the surface area and porosity of the CCPHA with Micromeritics Instrument (ASAP 2020). The Barrett−Joyner−Halenda (BJH) method was used to find the total pore volume and average pore size of the CCPHA. 2.4. Experimental Design for the Two-Step Transesterification Process. A three-level and three-factor BBD of RSM was
Table 2. Elemental Composition of CCPHA
2. EXPERIMENTAL SECTION 2.1. Materials. The NSO used for this work was purchased from the National Research Institute for Chemical Technology (NARICT)
a
B
temperature (°C)
Ca
Ka
Mg
O
P
S
Si
300 500 700 900 1100
0.0 0.9 0.0 3.5 30.0
52.4 ± 7.3 59.3 ± 8.4 59.2 ± 5.1 50.5 ± 1.7 12.1 ± 3.1
3.2 3.1 3.0 3.0 9.3
41.5 34.2 36.2 39.6 41.4
1.1 1.2 0.8 1.2 0.7
1.6 1.4 0.7 1.3 0.5
0.6 0.0 0.3 0.9 6.6
Average of three replicates. DOI: 10.1021/acs.energyfuels.7b00604 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. SEM images of (a) raw, (b) open-air burnt CPH, and (c) calcined CPH at 700 °C samples.
Figure 2. FTIR spectra of (a) raw, (b) open-air burnt CPH, and (c) calcined CPH at 700 °C samples. employed in the modeling studies of the esterification of NSO. For this purpose, 17 experimental conditions, comprised of 12 factorial points and 5 central points, were generated and, subsequently, carried out in the laboratory. The methanol/oil ratio, catalyst amount, and reaction time were the independent factors chosen for the work. The coded and uncoded levels of the factors are shown in Table 1a, while the BBD for the experimental conditions are displayed in Table S1 of the Supporting Information. For the transesterification step, CCD, which is used for fitting complex surfaces when a quadratic model is assumed, was used for the experimental design. A fractional factorial design (five levels and three factors) was applied to produce 20 experimental conditions used to investigate the three factors (i.e., methanol/oil ratio, catalyst amount, and time) on the conversion of pretreated NSO to biodiesel.
This comprised six axial points, eight factorial points, and six central points to provide information regarding the interior of the experimental region. The coded and uncoded levels of the factors are shown in Table 1b, while the CCD for the experimental conditions are displayed in Table S2 of the Supporting Information. The experimental conditions were randomized to minimize the effects of unexpected variability in the observed responses. For both the esterification and transesterification processes, multiple regressions were used to fit the coefficients of the quadratic polynomial models of the responses. The qualities of the fits of the models were evaluated using the significance test and analysis of variance (ANOVA). The fitted quadratic response model is described by eq 1. The significant and insignificant effects of individual and interaction model terms were graphically identified using the Pareto plot of standardized effects31 C
DOI: 10.1021/acs.energyfuels.7b00604 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 3. XRD chromatograms of raw, open-air burnt CPH, and calcined CPH at 700 °C samples.
Table 3. Properties of NSOME in Comparison to Biodiesel Specification Standards
a
parameter
NSO
NSOME
ASTM D6751
EN 14214
moisture content (%) refractive index density at 25 °C (kg/m3) kinematic viscosity at 40 °C (mm2/s) acid value (mg of KOH/g of oil) iodine value (g of I2/100 g of oil) higher heating value (MJ/kg) Group I metals (Na + K) (ppm) Group II metals (Ca + Mg) (ppm) diesel index API gravity (deg) cetane number aniline point pour point (°C) cloud point (°C) flash point (°C)
0.05 1.467 891 41.5 11.57 79.31 ND ND ND 56.61 17.21 66 ND ND ND ND
0.02 NDa 887 5.3 0.5 58.96 45.88 1.9 0.62 102.36 28.20 83 358 −10 23 262
