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Low-energy synthesis of kaliophilite catalyst from circulating fluidized bed fly ash for biodiesel production
Fuel 257 (2019) 116041
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Full Length Article
Low-energy synthesis of kaliophilite catalyst from circulating fluidized bed fly ash for biodiesel production
T
⁎
Pan Yang He, Yao Jun Zhang , Hao Chen, Zhi Chao Han, Li Cai Liu College of Materials Science and Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, People’s Republic of China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Circulating fluidized bed fly ash Kaliophilite Geopolymer Heterogeneous catalysis Biodiesel
Circulating fluidized bed fly ash (CFBFA) was used to synthesize kaliophilite catalyst via a facile and low-energy two-step process: fabrication of amorphous CFBFA geopolymer and hydrothermal transformation of CFBFAbased geopolymer into kaliophilite. X-ray diffraction and scanning electron microscopy results demonstrated that CFBFA was successfully converted into kaliophilite with short prismatic crystals having size of ~1 μm. Temperature-programmed desorption of CO2 analysis results indicated the presence of abundant weak, mediumstrength and high-strength basic sites on the kaliophilite catalyst, which were respectively assigned to hydroxyl groups, K-O ion pairs, and O2− ions. When kaliophilite was used as a heterogeneous catalyst for biodiesel production, the highest biodiesel yield of 99.2% was obtained under transesterification conditions of 5 wt% catalyst concentration, methanol:canola oil mole ratio of 15:1, reaction temperature of 85 °C, and reaction time of 6 h. The kaliophilite catalyst could be easily recovered and reused for four cycles without significant deactivation. Further, a solid base catalysis mechanism for transesterification over kaliophilite catalyst was proposed.
⁎ Corresponding author at: College of Materials Science and Engineering, Xi’an University of Architecture and Technology, No. 13 Yan Ta Road, Xi’an 710055, People’s Republic of China. E-mail addresses: [email protected] (P.Y. He), [email protected] (Y.J. Zhang).
https://doi.org/10.1016/j.fuel.2019.116041 Received 12 February 2019; Received in revised form 12 August 2019; Accepted 17 August 2019 Available online 23 August 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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1. Introduction
Table 1 Chemical composition of CFBFA (wt%).
Energy is a major factor in sustainable economic development. It is imperative to develop alternative renewable energy sources because traditional non-renewable fossil fuels including coal, petroleum, and natural gas cannot satisfy increasing energy demands. Biodiesel is one such alternative. It is a clean and renewable energy source that consists of fatty acid alkyl esters, and it is an excellent option to replace fossil fuels owing to its non-toxicity and usability in unmodified diesel engines [1,2]. At present, biodiesel is usually produced from vegetable oils or animal fats by different methods, such as homogeneous catalysis, heterogeneous catalysis, enzyme catalysis, and supercritical methods [3]. Among these methods, heterogeneous base catalysis has become popular because it offers very strong competitive advantages including easy separation and reusability of catalysts [4–6]. The process of producing and using traditional energy sources also generates a large amount of polluting solid wastes. For example, pulverized coal combustion forms fly ash [7,8] and burning low-grade fuel (e.g., coal gangue, bituminous coal) on a circulating fluidized bed boiler in thermal power plants generates circulating fluidized bed fly ash (CFBFA) [9]. Around 50 million tons of CFBFA were generated each year in China [9]. CFBFA released without treatment occupies farmland and causes severe environment pollution. Therefore, it is essential to recycle CFBFA for environmental protection and sustainable development. Currently, CFBFA is mostly recycled in the form of controlled low-strength materials, non-autoclaved aerated concrete, and autoclaved bricks for construction [10,11]. CFBFA was also converted to geopolymers to enhance its value addition [9,12]. Geopolymers consist of amorphous to semi-crystalline three-dimensional inorganic networks [13]. Provis [14] et al. reported that geopolymers comprised agglomerates of nanocrystalline zeolites compacted by an amorphous gel phase and proposed that geopolymers and zeolites were interconvertible. Later studies showed that geopolymers contained nanocrystals and could be transformed into various zeolite structures through hydrothermal reactions [15–18]. These findings provide a new method to obtain zeolite or zeolite-like crystals from amorphous CFBFA-based geopolymers. Kaliophilite (KAlSiO4) is a type of feldspathoids having zeolite-like structure comprising interlinked [SiO4]4− and [AlO4]5− tetrahedra. It has the same hexagonal symmetry as nepheline; however, its unit cell (a = 27.0, c = 8.6 Å) is larger than that of nepheline (a = 10.4, c = 8.5 Å) [19]. In recent decades, kaliophilite was extensively used for dehydrogenation of ethylbenzene to styrene, hydrocarbon steam reforming for hydrogen production, ammonia synthesis, and catalytic combustion of diesel soot owing to its strongly basic potassium active sites that resist carbon deposition [20–22]. Specifically, the numerous basic potassium active sites and insolubility in vegetable oil and methanol make kaliophilite a promising heterogeneous catalyst for biodiesel production [22]. Kaliophilite was usually prepared by a solidphase reaction between flint clay (Al2O3∙2SiO2·2H2O) and KOH at 1000 °C in industry [23]. Recently, studies have reported synthesis of kaliophilite using coprecipitation, fusion, and fast sol–gel methods [23,24]. However, these methods require heat treatment (800–1200 °C), resulting in high energy consumption and production cost. To solve these problems, this study proposes transforming CFBFAbased geopolymer into kaliophilite with zeolite-like composition and framework through a low-temperature and low-energy hydrothermal method. Kaliophilite is then used as a heterogeneous base catalyst for biodiesel production, thus achieving the high-value-added reutilization of CFBFA industrial solid waste for low-cost synthesis of kaliophilite, and cost-effective production of biodiesel.
SiO2
Al2O3
CaO
Fe2O3
TiO2
K2O
Na2O
MgO
P2O5
SO3
LOI
35.14
45.35
2.87
2.61
1.82
0.34
0.08
0.23
0.12
0.54
10.90
LOI: loss on ignition. Table 2 Experimental range and levels of the independent factors. Variable
Reaction temperature Reaction time Methanol:canola oil mole ratios
Code
A B C
Unit
°C h /
Range and levels −1
0
1
75 4 12
85 6 15
95 8 18
2. Materials and methods 2.1. Materials CFBFA with true density of 2.46 × 103 kg/m3 after vibromilling 30 s was provided by Shenhua Junggar Energy Corporation, Junggar, Inner Mongolia, China. The main chemical components of CFBFA by weight percent were measured by X-ray fluorescence (XRF), as shown in Table 1. Commercial potassium water glass with original modulus of 3.13 was obtained from Usoft Chemical Technology Co., Ltd. (Shandong, China). Analytical grade potassium hydroxide (KOH) and methanol were purchased from Sinopharm Chemical Reagent Co., Ltd. Canola oil with acid value of 0.5 mg KOH/g was obtained from a local market. 2.2. Synthesis of kaliophilite The typical synthesis of kaliophilite involves the preparation of CFBFA-based geopolymer and the hydrothermal transformation of the geopolymer into kaliophilite. An alkali-activator with modulus of 1.5 was prepared by dissolving KOH in the potassium water glass. Then, 100 g of CFBFA was added to 76.8 g of the activators and stirred in a WuXi JianYi NJ-160A net paste stirrer for 5 min, and the mixture was cast in a 20 mm × 20 mm × 20 mm steel mold packed in a polyethylene film bag. The CFBFA-based geopolymer block was obtained after curing at 80 °C for 24 h. The hydrothermal transformation was performed in a 200 mL autoclave with Teflon-liner containing a geopolymer monolith and 50 mL of KOH solution at 180 °C for 24 h. The monolithic kaliophilite with compressive strength of 8 MPa was washed and dried at 105 °C for 12 h, which was crushed and screened to obtain the granular kaliophilite catalyst with particle sizes of 0.15–0.315 mm. 2.3. Characterization of kaliophilite The chemical composition of CFBFA was characterized using a Bruker S4 Pioneer analyser. X-ray diffraction (XRD) patterns of samples were recorded using a Rigaku D/MAX-2400X-ray diffractometer with Cu Kα radiation at working current of 40 mA and voltage of 40 kV. The micro-morphologies of samples were characterised using an FEI Quanta 200 scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). Fourier-transform infrared (FT-IR) spectra of samples were detected in the range from 400 to 4000 cm−1 on a Bruker Tensor 27 FT-IR spectrophotometer using a KBr disk. A nitrogen sorption experiment was performed using a Micromeritics ASAP 2020 porosimetry analyser at 77 K. The basic strength of the catalysts was evaluated by temperature-programmed desorption of CO2 (CO2-TPD) on a Micromeritics AutoChemⅡ2920 analyser. Before each TPD experiment, 0.2 g of sample was added in a U-shape quartz tube 2
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Fig. 1. XRD patterns of (a) CFBFA, geopolymer, and kaliophilite synthesized at 6 M KOH; and (b) kaliophilite synthesized under different KOH concentration.
adsorbed contaminants after each reaction. Then, the clean catalyst was dried at 105 °C for 10 h for the next cycle. In order to investigate the interactive effects of process variables on biodiesel yield, the Box-Behnken Design (BBD) experimental design based on response surface methodology (RSM) was applied with three factors and levels, that is, reaction temperature (A), reaction time (B), and methanol:canola oil mole ratios (C) under 5 wt% catalyst concentration, and the specific parameters of the independent variables were listed in Table 2. The biodiesel composition was analysed on a Shimadzu QP 2010 Ultra gas chromatography-mass spectrometry (GC–MS) system with an Agilent HP-5 MS column (30 m × 0.32 mm × 0.25 μm). N-hexane was used as solvent, and helium (99.999%) with flow rate of 1.8 mL/min and split ratio of 20:1 was used as the carrier. The oven temperature was increased from 100 to 190 °C at a rate of 10 °C/min, then increased further to 205 °C at 1 °C/min, and finally increased to 270 °C at 10 °C/ min. MS data were collected in the range of 30–600 m/z in the positive electron impact mode with ionization energy of 70 eV. Products in the mass spectra were identified using the NIST 08. LIB Mass Spectral Library. Further, the obtained biodiesel were blended with petrodiesel (meeting ASTM D975 specification) as blended fuel, and the fuel properties were analysed and evaluated according to ASTM D7467-10 standard.
Fig. 2. FT-IR spectra of CFBFA, geopolymer, and kaliophilite.
and degassed in He flow of 20 mL/min at 500 °C for 30 min. Then, CO2 adsorption was performed after the sample cooled to room temperature. The base strength of the as-prepared catalyst (H_) was determined using Hammett indicators.
3. Results and discussion 2.4. Catalyst test and product analysis 3.1. Composition, structure, and morphology of kaliophilite Heterogeneous transesterification was performed in a 250 mL flask equipped with a reflux condenser. Then, 40 g of canola oil was added to the flask with continuous magnetic stirring and the set temperature of 45–95 °C was maintained. Kaliophilite catalyst and methanol were added to this flask, and then, transesterification was conducted for set time of 0–8 h. The influences of methanol:canola oil mole ratios of 6:1 to 18:1 and catalyst concentrations of 1–6 wt% on the biodiesel yield were investigated. After the reaction finished, the kaliophilite catalyst was separated by centrifugation and the residual methanol was recycled by reduced pressure distillation. The resulting glycerol and biodiesel were separated through a separating funnel. The biodiesel yield was calculated using Eq. (1):
Y (%) = Wbio/ Woil × 100
Fig. 1(a) shows XRD patterns of the CFBFA, geopolymer, and asprepared kaliophilite at 6 M KOH. The pattern for CFBFA shows a broad diffuse hump from 15° to 35°, indicating the presence of a lot of amorphous aluminosilicate in CFBFA. Moreover, the weak diffraction peaks at 20.85°, 26.47°, 26.74°, 33.29°, and 40.90° are assigned to a handful of quartz and mullite crystals. In the XRD pattern of the geopolymer, the broad diffuse hump is typically shifted to a high diffraction angle range of 20°–40°, indicating the formation of amorphous geopolymer [25]. After hydrothermal transformation, the XRD pattern of as-synthesized kaliophilite shows a set of strong and sharp peaks at 19.76°, 20.84°, 28.87°, 34.56°, 40.74°, and 42.38°; these correspond to kaliophilite (KAlSiO4, JCPDS card No. 11-0313) [23]. Fig. 1(b) displays XRD patterns of kaliophilite synthesized under different KOH concentration. It is clear that the intensities of diffraction peaks of kaliophilite synthesized under the concentration of 6 M KOH are higher than that of kaliophilite synthesized under the concentrations of 4 M and 8 M KOH, indicating that 6 M KOH is optimal concentration.
(1)
where Y is the biodiesel yield (%); Wbio, the weight of biodiesel (g); and Woil, the weight of canola oil (g). Each test was repeated three times to obtain an average value. For reuse, the catalyst was separated from the mixture by centrifugation and washed with n-hexane to remove the 3
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Fig. 3. Morphology of (a) CFBFA, (b) geopolymer, and (c) kaliophilite and (d) EDS of kaliophilite.
In the CFBFA-based geopolymer, the absorption peak of antisymmetric stretching vibrations of SieOeSi (Al) is shifted to 1017 cm−1. In the spectrum of as-prepared kaliophilite, the peak at 985 cm−1 is attributed to asymmetric stretching vibrations of the SieOeSi bond; the peaks at 698 and 607 cm−1 are attributed to symmetric stretching vibrations of the Si (Al)eO framework; and the peaks at 561 and 480 cm−1 are attributed to bending vibrations of the SieO bond and stretching vibrations of the AleO bond, respectively [23]. XRD and FT-IR results confirm that the skeleton of the amorphous geopolymer is entirely transformed to the crystalline framework of kaliophilite. Fig. 3 shows the sizes and morphologies of (a) CFBFA, (b) geopolymer, and (c) kaliophilite and (d) EDS of kaliophilite. CFBFA shows typical particles with irregular shapes and sizes ranging from several microns to ~30 μm (Fig. 3(a)); this agrees with a previous study [27]. CFBFA-based geopolymer has homogeneous and dense microstructure (Fig. 3(b)), indicating that CFBFA dissolved and condensed to form the geopolymer binder. Kaliophilite comprises numerous prismatic crystals with size of ~1 μm (Fig. 3(c)), indicating that the amorphous geopolymer completely transforms into well-defined kaliophilite crystals. Moreover, the EDS result (Fig. 3(d)) indicates that as-prepared kaliophilite consists of Si, Al, K, and O. To further demonstrate the metallic species of Fe and Ti existed in catalyst or not, the catalyst was completely dissolved in the media of aqua regia and hydrofluoric acid. The inductively coupled plasma-optical emission spectrometer (ICP-OES) was used to detect the metallic species, and the results indicated that no any Fe and Ti species were discovered in the solution, suggesting that the metallic species from CFBFA are dissolved in the hydrothermal fluids during the transformation process. TG, DSC and DTG curves of the as-prepared kaliophilite catalyst are shown in Fig. 4. The weight loss consists of two weak stages in the TG curve. The first stage occurs at room temperature to 200 °C with the weight loss of 0.57%, corresponding to the loss of adsorbed water. The
Fig. 4. TG, DSC and DTG curves of kaliophilite catalyst. Table 3 BET surface area, average pore size and pore volume of kaliophilite catalyst. BET surface area (m2/g)
Average pore size (nm)
Pore volume (cm3/g)
3.4898
20.8854
0.0158
Fig. 2 shows the FT-IR spectra of CFBFA, geopolymer, and kaliophilite. The peaks at 3446 and 1656 cm−1 are attributed to stretching and bending vibrations of OeH, respectively, due to water adsorption on the sample surface. The spectrum of CFBFA shows three main characteristic absorption peaks at 1097, 570, and 470 cm−1 that are attributed to antisymmetric stretching vibrations of SieOeSi (Al), Al (VI)eOeSi stretching, and SieO bending vibrations, respectively [26]. 4
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Fig. 5. (a) N2 adsorption-desorption isotherm and (b) pore size distribution of kaliophilite catalyst.
kaliophilite catalyst, suggesting that more weak basic sites of hydroxyl groups are distributed in the pores and on the surface of the kaliophilite catalyst; the hydroxyl groups are considered to serve as Brønsted base sites [31]. The basic site density can be obtained by integrating the area under the curve, and the increasing area of kaliophilite represents more total basicity for CO2 desorption. Furthermore, the curve of kaliophilite shows two peaks at 590 °C and 730 °C which are assigned to the medium-strength basic sites of K-O ion pairs and high-strength basic sites of surface O2– ions, respectively [32]. The two strong basic sites have a significant Lewis character [33]. In addition, the basic strength (H_) of as-prepared kaliophilite catalyst was determined using Hammett indicators with the result of 9.8 < H_ < 15. Thus, CO2-TPD and basic strength results indicate that the kaliophilite catalyst has excellent catalytic activity of solid base for biodiesel production.
3.2. Effects of various variables on biodiesel yield As-prepared kaliophilite is used as a low-cost heterogeneous catalyst to produce biodiesel. Various factors including catalyst concentration, reaction time, reaction temperature, and methanol:canola oil molar ratio are carefully investigated to obtain the highest biodiesel yield. Transesterification is initially investigated at different catalyst concentrations of 1–6 wt% for methanol:canola oil ratio of 15:1 under reaction temperature of 95 °C for 6 h (Fig. 7(a)). The biodiesel yield is high (> 88.5%) and increases slowly with catalyst concentration. The biodiesel yields over the catalyst concentration of 5 wt% and 6 wt% are 93.1% and 92.8%, respectively. Therefore, 5 wt% is set as the optimum catalyst concentration for the transesterification reaction. Reaction temperature is one of the crucial variables influencing transesterification. The effect of reaction temperatures of 45–95 °C on biodiesel yield for catalyst concentration of 5 wt%, reaction time of 6 h, and methanol:canola oil mole ratio of 15:1 is investigated (Fig. 7(b)). As the temperature increases from 45 to 85 °C, the biodiesel yield gradually increases from 83.5% to 99.2%; however, at 95 °C, the yield decreases to 93.1%. Therefore, the optimal reaction temperature is 85 °C. Generally, the transesterification reaction temperature shows a positive association with the basic strength of the catalyst. For instance, the optimal temperature for a strongly basic catalyst like CaO and MgO is ~70 °C [34], and ETS-10 zeolite shows high catalytic activity below 140 °C [35]. The influence of reaction times of 2–8 h and beyond transesterification with 5 wt% kaliophilite catalyst and methanol:canola oil mole ratio of 15:1 at 85 °C is investigated (Fig. 7(c)). The biodiesel yield increases continuously with reaction time from 2 to 6 h, and the highest yield of 99.2% is achieved after 6 h. With increasing reaction time, the yield decreases slightly owing to glycerol formation [36]. Therefore, the optimal reaction time is 6 h.
Fig. 6. Temperature-programmed desorption of CO2 (TPD-CO2) for geopolymer and kaliophilite.
second stage appears continuously in the range of 200 °C–800 °C accompanying with the weight loss of 2.18% without significant exothermal or endothermal peaks in DSC curve. The results demonstrate that the as-prepared kaliophilite catalyst has excellent thermal stability [23]. A nitrogen adsorption-desorption isotherm is used to determine the surface area and pore size distribution of kaliophilite. Table 3 shows that kaliophilite has low Brunauer-Emmett-Teller (BET) surface area of 3.4898 m2/g, average pore size of ~20.8854 nm, and small pore volume of 0.0158 cm3/g. Fig. 5(a) shows the N2 adsorption-desorption isotherm of the kaliophilite. This isotherm has a typical type IV profile with a distinct H3 hysteresis loop according to the International Union of Pure and Applied Chemistry (IUPAC) classification, indicating the presence of inter-particle mesopores in kaliophilite [17]. Fig. 5(b) shows that the kaliophilite catalyst has a wide pore size of 7–100 nm; this is larger than the diameter of reactant molecules including triglyceride (5.8 nm) and methyl alcohol (0.43 nm) [28]. Consequently, the reactant molecules can readily adsorb on the active site in porous and kaliophilite catalyst surface, which is beneficial for the transesterification reaction [29]. CO2-TPD profiles of the CFBFA-based geopolymer and kaliophilite catalyst are recorded for assessing the basic site strength, as shown in Fig. 6. The small desorption peak at 102 °C in the curve of the geopolymer indicates the presence of some weak basic sites, corresponding to hydroxyl groups that can form a loose bond with CO2 [30]. A broader and higher peak centred at 110 °C is also seen in the profile of the 5
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Fig. 7. Effects of (a) catalyst concentration, (b) reaction temperature, (c) reaction time, and (d) methanol:canola oil mole ratio on biodiesel yield. Table 4 BBD experiments and results. Run No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Table 5 Analysis of variance for biodiesel yield.
Actual levels of variables
Biodiesel yield
A
B
C
85 85 85 75 75 85 85 75 95 85 95 85 85 75 95 95 85
6 6 6 6 6 6 8 4 4 4 8 8 4 8 6 6 6
15 15 15 12 18 15 18 15 15 12 15 12 18 15 18 12 15
99.2 99.1 99.2 93.5 96.2 99.5 96.1 88.5 89.3 90.2 92.6 95.8 93.7 97.8 91.9 95.2 99.4
Source
Sum of squares
df
Mean square
F value
P-value prob > F
Model A B C AB AC BC Residual
214.10 6.12 53.04 1.28 9.00 9.00 2.56 5.58
9 1 1 1 1 1 1 7
23.79 6.12 53.04 1.28 9.00 9.00 2.56 0.80
29.85 7.69 66.57 1.61 11.29 11.29 3.21
< 0.0001 0.0276 < 0.0001 0.2455 0.0121 0.0121 0.1162
Significant
increased from 6:1 to 15:1; however, it decreases to 95.7% when the ratio is further increased to 18:1. Therefore, the optimal methanol:canola oil mole ratio is 15:1. A previous study noted that glycerol dissolution in the additional methanol may shift the equilibrium backward, resulting in low biodiesel yield [37]. 3.3. Effects of interactive influences of three factors on biodiesel yield
The methanol:canola oil mole ratio is another crucial variable influencing transesterification. Theoretically, 3 mol of methanol is required to react with 1 mol of canola oil in transesterification, and excess methanol can improve the methyl ester yield owing to the equilibrium shifting forward in the direction of resultants. However, immoderate methanol use increases production costs because redundant methanol must be recovered and purified for reuse in the next transesterification reaction. To determine the optimal methanol:canola oil mole ratio, ratios of 6:1, 9:1, 12:1, 15:1, and 18:1 are investigated with 5 wt% kaliophilite catalyst at 85 °C for 6 h (Fig. 7(d)). The biodiesel yield increases from 87.7% to 99.2% as the methanol:canola oil mole ratio is
According to Table 2, the three-factor BBD experimental results including 17 groups are listed in Table 4, and a second-order regression Eq. (2) can be obtained to depict the interactive influences of the three variables on the biodiesel yield [38].
Biodiesel yield = 99.28 − 0.87 A+ 2.58 B+ 0.4 C− 1.5AB − 1.5AC − 0.8BC − 3.49 A2 − 3.74B2 − 1.59C2
(2)
The analysis of variance (ANOVA) is applied to evaluate the significance of the second-order model as well as the interactive influences 6
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Fig. 8. Response surfaces and contours for biodiesel yield as a function of (a) and (b) reaction temperature versus time, (c) and (d) reaction temperature versus methanol:canola oil mole ratio.
Fig. 9. (a) Reusability of kaliophilite catalyst, and (b) XRD patterns of kaliophilite catalyst before and after transesterification reaction.
Response surface analysis indicates that the interactive effects between reaction temperature (A) and reaction time (B) as well as between reaction temperature (A) and methanol:canola oil mole ratio (C) are significant.
of the three variables, and the results are summarized in Table 5. The high F-values and low probability value indicate that the second-order model is efficacious. Further, the response surface diagram and corresponding contours plots are employed to determine the interaction impacts of individual variables on the biodiesel yield. Fig. 8(a) and (b) show the interaction effects of reaction temperature and time on the biodiesel yield. The biodiesel yield approaches to the maximal value as the reaction temperature is about 85 °C for 6 h. When the reaction temperature is about 85 °C and the methanol:canola oil mole ratio is about 15:1 as shown in Fig. 8(c) and (d), the biodiesel yield is optimal.
3.4. Reusability of kaliophilite catalyst The reusability of the heterogeneous catalyst is an important factor in large-scale industrial processes. Consequently, the reusability of the as-prepared kaliophilite catalyst is detected for four cycles under the 7
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suggesting that the kaliophilite catalyst has excellent reusability. Further, there is no difference in the XRD patterns of the kaliophilite catalyst before and after these four cycles (Fig. 9(b)), indicating the excellent stability of the catalyst. 3.5. Composition and properties of biodiesel Biodiesel consists of fatty acid methyl esters, as has been commonly determined through GC–MS analysis. Fig. 10 shows GC chromatograms of biodiesel produced by catalysed interesterification under the optimal conditions, namely, kaliophilite catalyst concentration of 5 wt%, methanol:canola oil mole ratio of 15:1, reaction temperature of 85 °C, and reaction time of 6 h. A photograph of biodiesel is shown in the top left corner. Products in the mass spectra were identified using the NIST 08. LIB Mass Spectral Library. The result indicates that biodiesel mainly consist of methyl palmitoleate (C16:1), methyl palmitate (C16:0), methyl elaidate (C18:1), methyl stearate (C18:0), cis-11-eicosenoic acid, methyl ester (C20:1), methyl eicosanoate (C20:0), methyl erucate (C22:1), and methyl behenate (C22:0). Overall, biodiesel is considered to be produced successfully over the kaliophilite catalyst through interesterification. Biodiesel is usually blended with petrodiesel as a reliably source of energy, thus the properties of B10 (Blends of 10 vol% biodiesel product and 90 vol% petrodiesel) and B20 (Blends of 20 vol% biodiesel product and 80 vol% petrodiesel) blends fuel are necessary to meet ASTM D7467-10 specification [39]. Table 6 lists the physicochemical properties of the prepared B10 and B20, including acid value, viscosity, flash point, cetane number, pour point, and density. In view of that, all the parameters of the prepared blends conform to the standard of ASTM D7467-10.
Fig. 10. GC chromatogram of biodiesel.
Table 6 Fuel properties of B10 and B20 biodiesel blends. Properties
ASTM D7467-10
B10
B20
Acid value (mg KOH/g, max) Viscosity (mm2/s at 40 °C) Flash point (°C) Cetane number Pour point (°C) Density (kg/m3)
0.3 1.9–4.1 52 (min) 40 (min) −3 810–818
0.13 3.51 103 64.3 −2 813
0.18 3.81 111 67.2 −3 816
optimum conditions of 5 wt% catalyst and methanol:canola oil mole ratio of 15:1 at 85 °C for 6 h (Fig. 9(a)). Fig. 9(b) shows XRD patterns of the kaliophilite catalyst before and after transesterification to research its stability. Biodiesel yield of 90% was achieved after four cycles,
3.6. Transesterification mechanism CO2-TPD analysis results indicate that the surface and pores of the kaliophilite catalyst show abundant weak, medium-strength, and high-
Fig. 11. Proposed reaction mechanism for transesterification over kaliophilite catalyst. 8
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strength basic sites which are attributed to hydroxyl groups, K-O ion pairs, and O2− ions, respectively. These basic sites (B-) act as active centres during transesterification. Fig. 11 shows the transesterification mechanism over the kaliophilite catalyst. First, methanol adsorbs on the kaliophilite catalyst surface to form strongly basic active species of alkoxide ion (CH3O−). Second, CH3O− attacks the carbonyl carbon of the triglyceride to form a tetrahedral intermediate. Third, biodiesel (fatty acid methyl esters) is produced by the rearrangement of this tetrahedral intermediate. Fourth, an acid-base reaction between the diglyceride anion and B−-H+ of the catalyst forms diglyceride, and the catalyst is simultaneously regenerated via deprotonation. Finally, the above process is repeated two times to yield glycerol and biodiesel.
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4. Conclusions In this study, a kaliophilite catalyst was successfully synthesized from CFBFA via a low-energy and sustainable route. The as-prepared kaliophilite catalyst possessed the morphology with inter-grown pillars, high crystallinity and purity. In addition, the kaliophilite showed the basic strength (H_) of 9.8 < H_ < 15 and remarkable thermostability. When the as-prepared kaliophilite was used as a low-cost heterogeneous catalyst for biodiesel production, it showed high catalytic activity and excellent stability. Response surface analysis indicated that the interaction effects between reaction temperature (A) and reaction time (B) as well as between reaction temperature (A) and methanol:canola oil mole ratio (C) were significant. This study affords three benefits: high-value-added reutilization of CFBFA industrial by-products, low-energy synthesis of kaliophilite, and low-cost production of biodiesel. Acknowledgements This work was supported by the National Natural Science Foundation of China [grant number 21676209]; the Key Research and Development Project of Shaanxi Province [grant number 2019GY-137]; and the Cultivating Fund of Excellent Doctorate Thesis of Xi’an University of Architecture and Technology [grant number 6040318008]. References [1] Wang J, Yang L, Luo W, Yang G, Miao C, Fu J, et al. Sustainable biodiesel production via transesterification by using recyclable Ca2MgSi2O7 catalyst. Fuel 2017;196:306–13. https://doi.org/10.1016/j.fuel.2017.02.007. [2] Lai FC, Klemeš JJ, Yusup S, Bokhari A, Akbar MM. A review of cleaner intensification technologies in biodiesel production. J Cleaner Prod 2017;146:181–93. https://doi.org/10.1016/j.jclepro.2016.05.017. [3] Avhad MR, Marchetti JM. A review on recent advancement in catalytic materials for biodiesel production. Renew Sustain Energy Rev. 2015;50:696–718. https://doi. org/10.1016/j.rser.2015.05.038. [4] Shan R, Lu L, Shi Y, Yuan H, Shi J. Catalysts from renewable resources for biodiesel production. Energy Convers Manage 2018;178:277–89. https://doi.org/10.1016/j. enconman.2018.10.032. [5] Manique MC, Lacerda LV, Alves AK, Bergmann CP. Biodiesel production using coal fly ash-derived sodalite as a heterogeneous catalyst. Fuel 2016;190:268–73. https:// doi.org/10.1016/j.fuel.2016.11.016. [6] Chakraborty R, Bepari S, Banerjee A. Transesterification of soybean oil catalyzed by fly ash and egg shell derived solid catalysts. Chem Eng J 2010;165(3):798–805. https://doi.org/10.1016/j.cej.2010.10.019. [7] Yao ZT, Ji XS, Sarker PK, Tang JH, Ge LQ, Xia MS, et al. A comprehensive review on the applications of coal fly ash. Earth-Sci Rev 2015;141:105–21. https://doi.org/10. 1016/j.earscirev.2014.11.016. [8] Yao ZT, Xia MS, Sarker PK, Chen T. A review of the alumina recovery from coal fly ash, with a focus in China. Fuel 2014;120:74–85. https://doi.org/10.1016/j.fuel. 2013.12.003. [9] Qiu R, Cheng F, Huang H. Removal of Cd2+ from aqueous solution using hydrothermally modified circulating fluidized bed fly ash resulting from coal gangue power plant. J Cleaner Prod 2017;172:1918–27. https://doi.org/10.1016/j.jclepro. 2017.11.236. [10] Nguyen HA, Chang TP, Shih JY, Chen CT, Nguyen TD. Influence of circulating fluidized bed combustion (CFBC) fly ash on properties of modified high volume low calcium fly ash (HVFA) cement paste. Constr Build Mater 2015;91:208–15. https:// doi.org/10.1016/j.conbuildmat.2015.05.075.
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1021/ie501084z. [39] Pradhan P, Chakraborty R. Optimal efficient biodiesel synthesis from used oil employing low-cost ram bone supported Cr catalyst: Engine performance and exhaust assessment. Energy 2018;164:35–45. https://doi.org/10.1016/j.energy.2018.08. 181.
bleaching clay using CaO as a heterogeneous catalyst. Eur J Sci Res 2009;33(2):347–57. [38] Chakraborty R, Das S, Pradhan P, Mukhopadhyay P. Prediction of optimal conditions in the methanolysis of mustard oil for biodiesel production using cost-effective mg-solid catalysts. Ind Eng Chem Res 2014;53(51):19681–9. https://doi.org/10.
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Full Length Article
Low-energy synthesis of kaliophilite catalyst from circulating fluidized bed fly ash for biodiesel production
T
⁎
Pan Yang He, Yao Jun Zhang , Hao Chen, Zhi Chao Han, Li Cai Liu College of Materials Science and Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, People’s Republic of China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Circulating fluidized bed fly ash Kaliophilite Geopolymer Heterogeneous catalysis Biodiesel
Circulating fluidized bed fly ash (CFBFA) was used to synthesize kaliophilite catalyst via a facile and low-energy two-step process: fabrication of amorphous CFBFA geopolymer and hydrothermal transformation of CFBFAbased geopolymer into kaliophilite. X-ray diffraction and scanning electron microscopy results demonstrated that CFBFA was successfully converted into kaliophilite with short prismatic crystals having size of ~1 μm. Temperature-programmed desorption of CO2 analysis results indicated the presence of abundant weak, mediumstrength and high-strength basic sites on the kaliophilite catalyst, which were respectively assigned to hydroxyl groups, K-O ion pairs, and O2− ions. When kaliophilite was used as a heterogeneous catalyst for biodiesel production, the highest biodiesel yield of 99.2% was obtained under transesterification conditions of 5 wt% catalyst concentration, methanol:canola oil mole ratio of 15:1, reaction temperature of 85 °C, and reaction time of 6 h. The kaliophilite catalyst could be easily recovered and reused for four cycles without significant deactivation. Further, a solid base catalysis mechanism for transesterification over kaliophilite catalyst was proposed.
⁎ Corresponding author at: College of Materials Science and Engineering, Xi’an University of Architecture and Technology, No. 13 Yan Ta Road, Xi’an 710055, People’s Republic of China. E-mail addresses: [email protected] (P.Y. He), [email protected] (Y.J. Zhang).
https://doi.org/10.1016/j.fuel.2019.116041 Received 12 February 2019; Received in revised form 12 August 2019; Accepted 17 August 2019 Available online 23 August 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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1. Introduction
Table 1 Chemical composition of CFBFA (wt%).
Energy is a major factor in sustainable economic development. It is imperative to develop alternative renewable energy sources because traditional non-renewable fossil fuels including coal, petroleum, and natural gas cannot satisfy increasing energy demands. Biodiesel is one such alternative. It is a clean and renewable energy source that consists of fatty acid alkyl esters, and it is an excellent option to replace fossil fuels owing to its non-toxicity and usability in unmodified diesel engines [1,2]. At present, biodiesel is usually produced from vegetable oils or animal fats by different methods, such as homogeneous catalysis, heterogeneous catalysis, enzyme catalysis, and supercritical methods [3]. Among these methods, heterogeneous base catalysis has become popular because it offers very strong competitive advantages including easy separation and reusability of catalysts [4–6]. The process of producing and using traditional energy sources also generates a large amount of polluting solid wastes. For example, pulverized coal combustion forms fly ash [7,8] and burning low-grade fuel (e.g., coal gangue, bituminous coal) on a circulating fluidized bed boiler in thermal power plants generates circulating fluidized bed fly ash (CFBFA) [9]. Around 50 million tons of CFBFA were generated each year in China [9]. CFBFA released without treatment occupies farmland and causes severe environment pollution. Therefore, it is essential to recycle CFBFA for environmental protection and sustainable development. Currently, CFBFA is mostly recycled in the form of controlled low-strength materials, non-autoclaved aerated concrete, and autoclaved bricks for construction [10,11]. CFBFA was also converted to geopolymers to enhance its value addition [9,12]. Geopolymers consist of amorphous to semi-crystalline three-dimensional inorganic networks [13]. Provis [14] et al. reported that geopolymers comprised agglomerates of nanocrystalline zeolites compacted by an amorphous gel phase and proposed that geopolymers and zeolites were interconvertible. Later studies showed that geopolymers contained nanocrystals and could be transformed into various zeolite structures through hydrothermal reactions [15–18]. These findings provide a new method to obtain zeolite or zeolite-like crystals from amorphous CFBFA-based geopolymers. Kaliophilite (KAlSiO4) is a type of feldspathoids having zeolite-like structure comprising interlinked [SiO4]4− and [AlO4]5− tetrahedra. It has the same hexagonal symmetry as nepheline; however, its unit cell (a = 27.0, c = 8.6 Å) is larger than that of nepheline (a = 10.4, c = 8.5 Å) [19]. In recent decades, kaliophilite was extensively used for dehydrogenation of ethylbenzene to styrene, hydrocarbon steam reforming for hydrogen production, ammonia synthesis, and catalytic combustion of diesel soot owing to its strongly basic potassium active sites that resist carbon deposition [20–22]. Specifically, the numerous basic potassium active sites and insolubility in vegetable oil and methanol make kaliophilite a promising heterogeneous catalyst for biodiesel production [22]. Kaliophilite was usually prepared by a solidphase reaction between flint clay (Al2O3∙2SiO2·2H2O) and KOH at 1000 °C in industry [23]. Recently, studies have reported synthesis of kaliophilite using coprecipitation, fusion, and fast sol–gel methods [23,24]. However, these methods require heat treatment (800–1200 °C), resulting in high energy consumption and production cost. To solve these problems, this study proposes transforming CFBFAbased geopolymer into kaliophilite with zeolite-like composition and framework through a low-temperature and low-energy hydrothermal method. Kaliophilite is then used as a heterogeneous base catalyst for biodiesel production, thus achieving the high-value-added reutilization of CFBFA industrial solid waste for low-cost synthesis of kaliophilite, and cost-effective production of biodiesel.
SiO2
Al2O3
CaO
Fe2O3
TiO2
K2O
Na2O
MgO
P2O5
SO3
LOI
35.14
45.35
2.87
2.61
1.82
0.34
0.08
0.23
0.12
0.54
10.90
LOI: loss on ignition. Table 2 Experimental range and levels of the independent factors. Variable
Reaction temperature Reaction time Methanol:canola oil mole ratios
Code
A B C
Unit
°C h /
Range and levels −1
0
1
75 4 12
85 6 15
95 8 18
2. Materials and methods 2.1. Materials CFBFA with true density of 2.46 × 103 kg/m3 after vibromilling 30 s was provided by Shenhua Junggar Energy Corporation, Junggar, Inner Mongolia, China. The main chemical components of CFBFA by weight percent were measured by X-ray fluorescence (XRF), as shown in Table 1. Commercial potassium water glass with original modulus of 3.13 was obtained from Usoft Chemical Technology Co., Ltd. (Shandong, China). Analytical grade potassium hydroxide (KOH) and methanol were purchased from Sinopharm Chemical Reagent Co., Ltd. Canola oil with acid value of 0.5 mg KOH/g was obtained from a local market. 2.2. Synthesis of kaliophilite The typical synthesis of kaliophilite involves the preparation of CFBFA-based geopolymer and the hydrothermal transformation of the geopolymer into kaliophilite. An alkali-activator with modulus of 1.5 was prepared by dissolving KOH in the potassium water glass. Then, 100 g of CFBFA was added to 76.8 g of the activators and stirred in a WuXi JianYi NJ-160A net paste stirrer for 5 min, and the mixture was cast in a 20 mm × 20 mm × 20 mm steel mold packed in a polyethylene film bag. The CFBFA-based geopolymer block was obtained after curing at 80 °C for 24 h. The hydrothermal transformation was performed in a 200 mL autoclave with Teflon-liner containing a geopolymer monolith and 50 mL of KOH solution at 180 °C for 24 h. The monolithic kaliophilite with compressive strength of 8 MPa was washed and dried at 105 °C for 12 h, which was crushed and screened to obtain the granular kaliophilite catalyst with particle sizes of 0.15–0.315 mm. 2.3. Characterization of kaliophilite The chemical composition of CFBFA was characterized using a Bruker S4 Pioneer analyser. X-ray diffraction (XRD) patterns of samples were recorded using a Rigaku D/MAX-2400X-ray diffractometer with Cu Kα radiation at working current of 40 mA and voltage of 40 kV. The micro-morphologies of samples were characterised using an FEI Quanta 200 scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). Fourier-transform infrared (FT-IR) spectra of samples were detected in the range from 400 to 4000 cm−1 on a Bruker Tensor 27 FT-IR spectrophotometer using a KBr disk. A nitrogen sorption experiment was performed using a Micromeritics ASAP 2020 porosimetry analyser at 77 K. The basic strength of the catalysts was evaluated by temperature-programmed desorption of CO2 (CO2-TPD) on a Micromeritics AutoChemⅡ2920 analyser. Before each TPD experiment, 0.2 g of sample was added in a U-shape quartz tube 2
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Fig. 1. XRD patterns of (a) CFBFA, geopolymer, and kaliophilite synthesized at 6 M KOH; and (b) kaliophilite synthesized under different KOH concentration.
adsorbed contaminants after each reaction. Then, the clean catalyst was dried at 105 °C for 10 h for the next cycle. In order to investigate the interactive effects of process variables on biodiesel yield, the Box-Behnken Design (BBD) experimental design based on response surface methodology (RSM) was applied with three factors and levels, that is, reaction temperature (A), reaction time (B), and methanol:canola oil mole ratios (C) under 5 wt% catalyst concentration, and the specific parameters of the independent variables were listed in Table 2. The biodiesel composition was analysed on a Shimadzu QP 2010 Ultra gas chromatography-mass spectrometry (GC–MS) system with an Agilent HP-5 MS column (30 m × 0.32 mm × 0.25 μm). N-hexane was used as solvent, and helium (99.999%) with flow rate of 1.8 mL/min and split ratio of 20:1 was used as the carrier. The oven temperature was increased from 100 to 190 °C at a rate of 10 °C/min, then increased further to 205 °C at 1 °C/min, and finally increased to 270 °C at 10 °C/ min. MS data were collected in the range of 30–600 m/z in the positive electron impact mode with ionization energy of 70 eV. Products in the mass spectra were identified using the NIST 08. LIB Mass Spectral Library. Further, the obtained biodiesel were blended with petrodiesel (meeting ASTM D975 specification) as blended fuel, and the fuel properties were analysed and evaluated according to ASTM D7467-10 standard.
Fig. 2. FT-IR spectra of CFBFA, geopolymer, and kaliophilite.
and degassed in He flow of 20 mL/min at 500 °C for 30 min. Then, CO2 adsorption was performed after the sample cooled to room temperature. The base strength of the as-prepared catalyst (H_) was determined using Hammett indicators.
3. Results and discussion 2.4. Catalyst test and product analysis 3.1. Composition, structure, and morphology of kaliophilite Heterogeneous transesterification was performed in a 250 mL flask equipped with a reflux condenser. Then, 40 g of canola oil was added to the flask with continuous magnetic stirring and the set temperature of 45–95 °C was maintained. Kaliophilite catalyst and methanol were added to this flask, and then, transesterification was conducted for set time of 0–8 h. The influences of methanol:canola oil mole ratios of 6:1 to 18:1 and catalyst concentrations of 1–6 wt% on the biodiesel yield were investigated. After the reaction finished, the kaliophilite catalyst was separated by centrifugation and the residual methanol was recycled by reduced pressure distillation. The resulting glycerol and biodiesel were separated through a separating funnel. The biodiesel yield was calculated using Eq. (1):
Y (%) = Wbio/ Woil × 100
Fig. 1(a) shows XRD patterns of the CFBFA, geopolymer, and asprepared kaliophilite at 6 M KOH. The pattern for CFBFA shows a broad diffuse hump from 15° to 35°, indicating the presence of a lot of amorphous aluminosilicate in CFBFA. Moreover, the weak diffraction peaks at 20.85°, 26.47°, 26.74°, 33.29°, and 40.90° are assigned to a handful of quartz and mullite crystals. In the XRD pattern of the geopolymer, the broad diffuse hump is typically shifted to a high diffraction angle range of 20°–40°, indicating the formation of amorphous geopolymer [25]. After hydrothermal transformation, the XRD pattern of as-synthesized kaliophilite shows a set of strong and sharp peaks at 19.76°, 20.84°, 28.87°, 34.56°, 40.74°, and 42.38°; these correspond to kaliophilite (KAlSiO4, JCPDS card No. 11-0313) [23]. Fig. 1(b) displays XRD patterns of kaliophilite synthesized under different KOH concentration. It is clear that the intensities of diffraction peaks of kaliophilite synthesized under the concentration of 6 M KOH are higher than that of kaliophilite synthesized under the concentrations of 4 M and 8 M KOH, indicating that 6 M KOH is optimal concentration.
(1)
where Y is the biodiesel yield (%); Wbio, the weight of biodiesel (g); and Woil, the weight of canola oil (g). Each test was repeated three times to obtain an average value. For reuse, the catalyst was separated from the mixture by centrifugation and washed with n-hexane to remove the 3
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Fig. 3. Morphology of (a) CFBFA, (b) geopolymer, and (c) kaliophilite and (d) EDS of kaliophilite.
In the CFBFA-based geopolymer, the absorption peak of antisymmetric stretching vibrations of SieOeSi (Al) is shifted to 1017 cm−1. In the spectrum of as-prepared kaliophilite, the peak at 985 cm−1 is attributed to asymmetric stretching vibrations of the SieOeSi bond; the peaks at 698 and 607 cm−1 are attributed to symmetric stretching vibrations of the Si (Al)eO framework; and the peaks at 561 and 480 cm−1 are attributed to bending vibrations of the SieO bond and stretching vibrations of the AleO bond, respectively [23]. XRD and FT-IR results confirm that the skeleton of the amorphous geopolymer is entirely transformed to the crystalline framework of kaliophilite. Fig. 3 shows the sizes and morphologies of (a) CFBFA, (b) geopolymer, and (c) kaliophilite and (d) EDS of kaliophilite. CFBFA shows typical particles with irregular shapes and sizes ranging from several microns to ~30 μm (Fig. 3(a)); this agrees with a previous study [27]. CFBFA-based geopolymer has homogeneous and dense microstructure (Fig. 3(b)), indicating that CFBFA dissolved and condensed to form the geopolymer binder. Kaliophilite comprises numerous prismatic crystals with size of ~1 μm (Fig. 3(c)), indicating that the amorphous geopolymer completely transforms into well-defined kaliophilite crystals. Moreover, the EDS result (Fig. 3(d)) indicates that as-prepared kaliophilite consists of Si, Al, K, and O. To further demonstrate the metallic species of Fe and Ti existed in catalyst or not, the catalyst was completely dissolved in the media of aqua regia and hydrofluoric acid. The inductively coupled plasma-optical emission spectrometer (ICP-OES) was used to detect the metallic species, and the results indicated that no any Fe and Ti species were discovered in the solution, suggesting that the metallic species from CFBFA are dissolved in the hydrothermal fluids during the transformation process. TG, DSC and DTG curves of the as-prepared kaliophilite catalyst are shown in Fig. 4. The weight loss consists of two weak stages in the TG curve. The first stage occurs at room temperature to 200 °C with the weight loss of 0.57%, corresponding to the loss of adsorbed water. The
Fig. 4. TG, DSC and DTG curves of kaliophilite catalyst. Table 3 BET surface area, average pore size and pore volume of kaliophilite catalyst. BET surface area (m2/g)
Average pore size (nm)
Pore volume (cm3/g)
3.4898
20.8854
0.0158
Fig. 2 shows the FT-IR spectra of CFBFA, geopolymer, and kaliophilite. The peaks at 3446 and 1656 cm−1 are attributed to stretching and bending vibrations of OeH, respectively, due to water adsorption on the sample surface. The spectrum of CFBFA shows three main characteristic absorption peaks at 1097, 570, and 470 cm−1 that are attributed to antisymmetric stretching vibrations of SieOeSi (Al), Al (VI)eOeSi stretching, and SieO bending vibrations, respectively [26]. 4
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Fig. 5. (a) N2 adsorption-desorption isotherm and (b) pore size distribution of kaliophilite catalyst.
kaliophilite catalyst, suggesting that more weak basic sites of hydroxyl groups are distributed in the pores and on the surface of the kaliophilite catalyst; the hydroxyl groups are considered to serve as Brønsted base sites [31]. The basic site density can be obtained by integrating the area under the curve, and the increasing area of kaliophilite represents more total basicity for CO2 desorption. Furthermore, the curve of kaliophilite shows two peaks at 590 °C and 730 °C which are assigned to the medium-strength basic sites of K-O ion pairs and high-strength basic sites of surface O2– ions, respectively [32]. The two strong basic sites have a significant Lewis character [33]. In addition, the basic strength (H_) of as-prepared kaliophilite catalyst was determined using Hammett indicators with the result of 9.8 < H_ < 15. Thus, CO2-TPD and basic strength results indicate that the kaliophilite catalyst has excellent catalytic activity of solid base for biodiesel production.
3.2. Effects of various variables on biodiesel yield As-prepared kaliophilite is used as a low-cost heterogeneous catalyst to produce biodiesel. Various factors including catalyst concentration, reaction time, reaction temperature, and methanol:canola oil molar ratio are carefully investigated to obtain the highest biodiesel yield. Transesterification is initially investigated at different catalyst concentrations of 1–6 wt% for methanol:canola oil ratio of 15:1 under reaction temperature of 95 °C for 6 h (Fig. 7(a)). The biodiesel yield is high (> 88.5%) and increases slowly with catalyst concentration. The biodiesel yields over the catalyst concentration of 5 wt% and 6 wt% are 93.1% and 92.8%, respectively. Therefore, 5 wt% is set as the optimum catalyst concentration for the transesterification reaction. Reaction temperature is one of the crucial variables influencing transesterification. The effect of reaction temperatures of 45–95 °C on biodiesel yield for catalyst concentration of 5 wt%, reaction time of 6 h, and methanol:canola oil mole ratio of 15:1 is investigated (Fig. 7(b)). As the temperature increases from 45 to 85 °C, the biodiesel yield gradually increases from 83.5% to 99.2%; however, at 95 °C, the yield decreases to 93.1%. Therefore, the optimal reaction temperature is 85 °C. Generally, the transesterification reaction temperature shows a positive association with the basic strength of the catalyst. For instance, the optimal temperature for a strongly basic catalyst like CaO and MgO is ~70 °C [34], and ETS-10 zeolite shows high catalytic activity below 140 °C [35]. The influence of reaction times of 2–8 h and beyond transesterification with 5 wt% kaliophilite catalyst and methanol:canola oil mole ratio of 15:1 at 85 °C is investigated (Fig. 7(c)). The biodiesel yield increases continuously with reaction time from 2 to 6 h, and the highest yield of 99.2% is achieved after 6 h. With increasing reaction time, the yield decreases slightly owing to glycerol formation [36]. Therefore, the optimal reaction time is 6 h.
Fig. 6. Temperature-programmed desorption of CO2 (TPD-CO2) for geopolymer and kaliophilite.
second stage appears continuously in the range of 200 °C–800 °C accompanying with the weight loss of 2.18% without significant exothermal or endothermal peaks in DSC curve. The results demonstrate that the as-prepared kaliophilite catalyst has excellent thermal stability [23]. A nitrogen adsorption-desorption isotherm is used to determine the surface area and pore size distribution of kaliophilite. Table 3 shows that kaliophilite has low Brunauer-Emmett-Teller (BET) surface area of 3.4898 m2/g, average pore size of ~20.8854 nm, and small pore volume of 0.0158 cm3/g. Fig. 5(a) shows the N2 adsorption-desorption isotherm of the kaliophilite. This isotherm has a typical type IV profile with a distinct H3 hysteresis loop according to the International Union of Pure and Applied Chemistry (IUPAC) classification, indicating the presence of inter-particle mesopores in kaliophilite [17]. Fig. 5(b) shows that the kaliophilite catalyst has a wide pore size of 7–100 nm; this is larger than the diameter of reactant molecules including triglyceride (5.8 nm) and methyl alcohol (0.43 nm) [28]. Consequently, the reactant molecules can readily adsorb on the active site in porous and kaliophilite catalyst surface, which is beneficial for the transesterification reaction [29]. CO2-TPD profiles of the CFBFA-based geopolymer and kaliophilite catalyst are recorded for assessing the basic site strength, as shown in Fig. 6. The small desorption peak at 102 °C in the curve of the geopolymer indicates the presence of some weak basic sites, corresponding to hydroxyl groups that can form a loose bond with CO2 [30]. A broader and higher peak centred at 110 °C is also seen in the profile of the 5
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Fig. 7. Effects of (a) catalyst concentration, (b) reaction temperature, (c) reaction time, and (d) methanol:canola oil mole ratio on biodiesel yield. Table 4 BBD experiments and results. Run No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Table 5 Analysis of variance for biodiesel yield.
Actual levels of variables
Biodiesel yield
A
B
C
85 85 85 75 75 85 85 75 95 85 95 85 85 75 95 95 85
6 6 6 6 6 6 8 4 4 4 8 8 4 8 6 6 6
15 15 15 12 18 15 18 15 15 12 15 12 18 15 18 12 15
99.2 99.1 99.2 93.5 96.2 99.5 96.1 88.5 89.3 90.2 92.6 95.8 93.7 97.8 91.9 95.2 99.4
Source
Sum of squares
df
Mean square
F value
P-value prob > F
Model A B C AB AC BC Residual
214.10 6.12 53.04 1.28 9.00 9.00 2.56 5.58
9 1 1 1 1 1 1 7
23.79 6.12 53.04 1.28 9.00 9.00 2.56 0.80
29.85 7.69 66.57 1.61 11.29 11.29 3.21
< 0.0001 0.0276 < 0.0001 0.2455 0.0121 0.0121 0.1162
Significant
increased from 6:1 to 15:1; however, it decreases to 95.7% when the ratio is further increased to 18:1. Therefore, the optimal methanol:canola oil mole ratio is 15:1. A previous study noted that glycerol dissolution in the additional methanol may shift the equilibrium backward, resulting in low biodiesel yield [37]. 3.3. Effects of interactive influences of three factors on biodiesel yield
The methanol:canola oil mole ratio is another crucial variable influencing transesterification. Theoretically, 3 mol of methanol is required to react with 1 mol of canola oil in transesterification, and excess methanol can improve the methyl ester yield owing to the equilibrium shifting forward in the direction of resultants. However, immoderate methanol use increases production costs because redundant methanol must be recovered and purified for reuse in the next transesterification reaction. To determine the optimal methanol:canola oil mole ratio, ratios of 6:1, 9:1, 12:1, 15:1, and 18:1 are investigated with 5 wt% kaliophilite catalyst at 85 °C for 6 h (Fig. 7(d)). The biodiesel yield increases from 87.7% to 99.2% as the methanol:canola oil mole ratio is
According to Table 2, the three-factor BBD experimental results including 17 groups are listed in Table 4, and a second-order regression Eq. (2) can be obtained to depict the interactive influences of the three variables on the biodiesel yield [38].
Biodiesel yield = 99.28 − 0.87 A+ 2.58 B+ 0.4 C− 1.5AB − 1.5AC − 0.8BC − 3.49 A2 − 3.74B2 − 1.59C2
(2)
The analysis of variance (ANOVA) is applied to evaluate the significance of the second-order model as well as the interactive influences 6
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Fig. 8. Response surfaces and contours for biodiesel yield as a function of (a) and (b) reaction temperature versus time, (c) and (d) reaction temperature versus methanol:canola oil mole ratio.
Fig. 9. (a) Reusability of kaliophilite catalyst, and (b) XRD patterns of kaliophilite catalyst before and after transesterification reaction.
Response surface analysis indicates that the interactive effects between reaction temperature (A) and reaction time (B) as well as between reaction temperature (A) and methanol:canola oil mole ratio (C) are significant.
of the three variables, and the results are summarized in Table 5. The high F-values and low probability value indicate that the second-order model is efficacious. Further, the response surface diagram and corresponding contours plots are employed to determine the interaction impacts of individual variables on the biodiesel yield. Fig. 8(a) and (b) show the interaction effects of reaction temperature and time on the biodiesel yield. The biodiesel yield approaches to the maximal value as the reaction temperature is about 85 °C for 6 h. When the reaction temperature is about 85 °C and the methanol:canola oil mole ratio is about 15:1 as shown in Fig. 8(c) and (d), the biodiesel yield is optimal.
3.4. Reusability of kaliophilite catalyst The reusability of the heterogeneous catalyst is an important factor in large-scale industrial processes. Consequently, the reusability of the as-prepared kaliophilite catalyst is detected for four cycles under the 7
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suggesting that the kaliophilite catalyst has excellent reusability. Further, there is no difference in the XRD patterns of the kaliophilite catalyst before and after these four cycles (Fig. 9(b)), indicating the excellent stability of the catalyst. 3.5. Composition and properties of biodiesel Biodiesel consists of fatty acid methyl esters, as has been commonly determined through GC–MS analysis. Fig. 10 shows GC chromatograms of biodiesel produced by catalysed interesterification under the optimal conditions, namely, kaliophilite catalyst concentration of 5 wt%, methanol:canola oil mole ratio of 15:1, reaction temperature of 85 °C, and reaction time of 6 h. A photograph of biodiesel is shown in the top left corner. Products in the mass spectra were identified using the NIST 08. LIB Mass Spectral Library. The result indicates that biodiesel mainly consist of methyl palmitoleate (C16:1), methyl palmitate (C16:0), methyl elaidate (C18:1), methyl stearate (C18:0), cis-11-eicosenoic acid, methyl ester (C20:1), methyl eicosanoate (C20:0), methyl erucate (C22:1), and methyl behenate (C22:0). Overall, biodiesel is considered to be produced successfully over the kaliophilite catalyst through interesterification. Biodiesel is usually blended with petrodiesel as a reliably source of energy, thus the properties of B10 (Blends of 10 vol% biodiesel product and 90 vol% petrodiesel) and B20 (Blends of 20 vol% biodiesel product and 80 vol% petrodiesel) blends fuel are necessary to meet ASTM D7467-10 specification [39]. Table 6 lists the physicochemical properties of the prepared B10 and B20, including acid value, viscosity, flash point, cetane number, pour point, and density. In view of that, all the parameters of the prepared blends conform to the standard of ASTM D7467-10.
Fig. 10. GC chromatogram of biodiesel.
Table 6 Fuel properties of B10 and B20 biodiesel blends. Properties
ASTM D7467-10
B10
B20
Acid value (mg KOH/g, max) Viscosity (mm2/s at 40 °C) Flash point (°C) Cetane number Pour point (°C) Density (kg/m3)
0.3 1.9–4.1 52 (min) 40 (min) −3 810–818
0.13 3.51 103 64.3 −2 813
0.18 3.81 111 67.2 −3 816
optimum conditions of 5 wt% catalyst and methanol:canola oil mole ratio of 15:1 at 85 °C for 6 h (Fig. 9(a)). Fig. 9(b) shows XRD patterns of the kaliophilite catalyst before and after transesterification to research its stability. Biodiesel yield of 90% was achieved after four cycles,
3.6. Transesterification mechanism CO2-TPD analysis results indicate that the surface and pores of the kaliophilite catalyst show abundant weak, medium-strength, and high-
Fig. 11. Proposed reaction mechanism for transesterification over kaliophilite catalyst. 8
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strength basic sites which are attributed to hydroxyl groups, K-O ion pairs, and O2− ions, respectively. These basic sites (B-) act as active centres during transesterification. Fig. 11 shows the transesterification mechanism over the kaliophilite catalyst. First, methanol adsorbs on the kaliophilite catalyst surface to form strongly basic active species of alkoxide ion (CH3O−). Second, CH3O− attacks the carbonyl carbon of the triglyceride to form a tetrahedral intermediate. Third, biodiesel (fatty acid methyl esters) is produced by the rearrangement of this tetrahedral intermediate. Fourth, an acid-base reaction between the diglyceride anion and B−-H+ of the catalyst forms diglyceride, and the catalyst is simultaneously regenerated via deprotonation. Finally, the above process is repeated two times to yield glycerol and biodiesel.
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4. Conclusions In this study, a kaliophilite catalyst was successfully synthesized from CFBFA via a low-energy and sustainable route. The as-prepared kaliophilite catalyst possessed the morphology with inter-grown pillars, high crystallinity and purity. In addition, the kaliophilite showed the basic strength (H_) of 9.8 < H_ < 15 and remarkable thermostability. When the as-prepared kaliophilite was used as a low-cost heterogeneous catalyst for biodiesel production, it showed high catalytic activity and excellent stability. Response surface analysis indicated that the interaction effects between reaction temperature (A) and reaction time (B) as well as between reaction temperature (A) and methanol:canola oil mole ratio (C) were significant. This study affords three benefits: high-value-added reutilization of CFBFA industrial by-products, low-energy synthesis of kaliophilite, and low-cost production of biodiesel. Acknowledgements This work was supported by the National Natural Science Foundation of China [grant number 21676209]; the Key Research and Development Project of Shaanxi Province [grant number 2019GY-137]; and the Cultivating Fund of Excellent Doctorate Thesis of Xi’an University of Architecture and Technology [grant number 6040318008]. References [1] Wang J, Yang L, Luo W, Yang G, Miao C, Fu J, et al. Sustainable biodiesel production via transesterification by using recyclable Ca2MgSi2O7 catalyst. Fuel 2017;196:306–13. https://doi.org/10.1016/j.fuel.2017.02.007. [2] Lai FC, Klemeš JJ, Yusup S, Bokhari A, Akbar MM. A review of cleaner intensification technologies in biodiesel production. J Cleaner Prod 2017;146:181–93. https://doi.org/10.1016/j.jclepro.2016.05.017. [3] Avhad MR, Marchetti JM. A review on recent advancement in catalytic materials for biodiesel production. Renew Sustain Energy Rev. 2015;50:696–718. https://doi. org/10.1016/j.rser.2015.05.038. [4] Shan R, Lu L, Shi Y, Yuan H, Shi J. Catalysts from renewable resources for biodiesel production. Energy Convers Manage 2018;178:277–89. https://doi.org/10.1016/j. enconman.2018.10.032. [5] Manique MC, Lacerda LV, Alves AK, Bergmann CP. Biodiesel production using coal fly ash-derived sodalite as a heterogeneous catalyst. Fuel 2016;190:268–73. https:// doi.org/10.1016/j.fuel.2016.11.016. [6] Chakraborty R, Bepari S, Banerjee A. Transesterification of soybean oil catalyzed by fly ash and egg shell derived solid catalysts. Chem Eng J 2010;165(3):798–805. https://doi.org/10.1016/j.cej.2010.10.019. [7] Yao ZT, Ji XS, Sarker PK, Tang JH, Ge LQ, Xia MS, et al. A comprehensive review on the applications of coal fly ash. Earth-Sci Rev 2015;141:105–21. https://doi.org/10. 1016/j.earscirev.2014.11.016. [8] Yao ZT, Xia MS, Sarker PK, Chen T. A review of the alumina recovery from coal fly ash, with a focus in China. Fuel 2014;120:74–85. https://doi.org/10.1016/j.fuel. 2013.12.003. [9] Qiu R, Cheng F, Huang H. Removal of Cd2+ from aqueous solution using hydrothermally modified circulating fluidized bed fly ash resulting from coal gangue power plant. J Cleaner Prod 2017;172:1918–27. https://doi.org/10.1016/j.jclepro. 2017.11.236. [10] Nguyen HA, Chang TP, Shih JY, Chen CT, Nguyen TD. Influence of circulating fluidized bed combustion (CFBC) fly ash on properties of modified high volume low calcium fly ash (HVFA) cement paste. Constr Build Mater 2015;91:208–15. https:// doi.org/10.1016/j.conbuildmat.2015.05.075.
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