zeolite-based catalyst obtained from waste chicken eggshell and coal fly ash for biodiesel production

Fuel 267 (2020) 117171

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A CaO/zeolite-based catalyst obtained from waste chicken eggshell and coal fly ash for biodiesel production

T

Stefan M. Pavlovića, Dalibor M. Marinkovića, Milan D. Kostićb, Ivona M. Janković-Častvanc, ⁎ Ljiljana V. Mojovićc, Miroslav V. Stankovića, Vlada B. Veljkovićb,d, a

University of Belgrade, Institute of Chemistry, Technology, and Metallurgy, Njegoseva 12, 11001 Belgrade, Serbia University of Niš, Faculty of Technology, Bulevar oslobodjenja 124, 16000 Leskovac, Serbia c University of Belgrade, Faculty of Technology and Metallurgy, Karnegijeva 4, 11000 Belgrade, Serbia d The Serbian Academy of Sciences and Arts, Knez Mihailova 35, 11000 Belgrade, Serbia b

ARTICLE INFO

ABSTRACT

Keywords: Biodiesel Fly ash Zeolitic material Eggshell Methanolysis Kinetic modeling

The present paper is focused on the development of a new environment-friendly methanolysis catalyst completely based on waste materials: lignite coal fly ash and chicken eggshells. A novel catalyst based on CaO supported on a fly ash-based zeolitic material (CaO/FA-ZM) was obtained from a cancrinite-sodalite group zeolite-like material (vishnevite type) and active CaO by alkali activation in a new miniature autoclave reactor system and hydration-dehydration. Agitation by rotation of the entire reaction mixture led to a more homogeneous zeolitic product and saved both time and energy. The obtained catalyst structure corresponds to gismondine and the crystallographic modification of calcium silicate (α’-dicalcium silicate) with deposited CaO. The characteristics of the synthesized catalyst were determined using ED-XRF, XRD, FT-IR, SEM, Hg-porosimetry, N2-physisorption, LDPSA, and Hammett indicators. The CaO/FA-ZM catalyst exhibited a high activity (97.8% of FAME for only 30 min) and stability (a negligible drop in activity in five consecutive cycles) in the methanolysis reaction under the optimal reaction conditions (temperature of 60 °C, methanol/oil molar ratio of 6:1, and catalyst concentration of 6 wt%). A kinetic study was performed using two different mechanisms: the irreversible pseudo-first-order reaction mechanism in two regimes (heterogeneous and homogeneous) and the changing mechanism combined with the triacylglycerol mass transfer limitation. Both models showed a satisfactory agreement between the experimental and predicted values of conversion degree (R2 > 0.93), confirming their validity for the CaO-based heterogeneously catalyzed methanolysis. The values of the activation energy calculated for both mechanisms were 67.17 and 58.03 kJ mol−1, respectively.

1. Introduction The increasing global demand for energy and serious environmental issues (global warming, climate change, biodiversity concern, and different types of pollution) caused by the use of fossil fuels have urged extensive research activities focused on the development of alternative fuels, such as biodiesel, and resulting solid wastes valorization [1]. Biodiesel can be classified as a less toxic, biodegradable, renewable, and environment-friendly energy source obtained mainly by the methanolysis of different triacylglycerol (TAG)-based feedstocks [2]. Although the homogeneous biodiesel production is commonly used in industrial processes, numerous problems (regenerability, reusability, and separability) have led to research and development of heterogeneously

acid and base-catalyzed processes [3]. Since recently, many researchers have increased their attention to the conversion of waste raw materials into valuable catalytically active materials through biodiesel production [4,5], such as coal and biomass fly ashes (FA), shells (eggs, oysters, and clams) [6–11], bones [12,13] and lime [14]. FA is a by-product of coal-based industries, such as thermal power plants. Because of enormous annual global production and negative environmental impacts (water, soil, and air pollution), it is necessary to find a way for its practical and economic valorization [15,16]. In this context, considering the appropriate chemical composition and morphological properties [6,17,18], FA can be used as a potential material for the synthesis of different polycrystalline meso- and microporous aluminosilicate minerals known as zeolites, which can be used as

Corresponding author at: University of Niš, Faculty of Technology, Bulevar oslobodjenja 124, 16000 Leskovac, Serbia. E-mail addresses: [email protected] (S.M. Pavlović), [email protected] (D.M. Marinković), [email protected] (I.M. Janković-Častvan), [email protected] (L.V. Mojović), [email protected] (M.V. Stanković), [email protected] (V.B. Veljković). ⁎

https://doi.org/10.1016/j.fuel.2020.117171 Received 6 September 2019; Received in revised form 3 January 2020; Accepted 21 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.

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catalytic materials [7,19,20], support for different active catalytic components [21], adsorbents, and ion exchangers [22–25]. The change in certain process parameters, such as synthesis temperature, concentration of alkali agents and mineralizers (NaOH, KOH, Ca(OH)2, CaCl2, NaCl, etc.), synergism of alkali agents [26], reaction time, aging period, and seed adding leads to the formation of different zeolite-like materials with differently defined structural and morphological properties (analcime, cancrinite, chabazite, Ca-zeolites, faujasite, NaP1, sodalite, X, Y, A zeolites, etc.) [27,28]. Many studies, targeting on the valorization of ashes, FA, and CaO from natural sources in catalytic transesterification of oily feedstocks, indicated an insufficient FAME yield or conversion [6,9,13], fast deactivation in consecutive cycles [6,7,10,14], prolonged reaction time [6,9,11,12] or more severe reaction conditions [8,12]. These drawbacks can be related to textural and morphological limitations affecting mass transfer in complex threephase systems. For instance, CaO from waste shells and bones is less active if it is loaded onto the chemically untreated FA than onto the previously treated FA [12]. The untreated FA is not appropriate support due to its extremely low specific surface area and porosity [8,12]. FA treatment to produce zeolite structures leads to the improvement of specific surface area, which is adequate for the uniform distribution of active catalytic species. Although some synthesis procedures can be used for FA conversion into high-quality zeolites, such as NaP1, NaX or 3A with high specific surface area, it leads to the formation of a micromesopore system, which is not adequate for the reaction with large organic molecules, such as TAGs [6,7]. It is well known that neat alkaliand alkaline earth-based catalysts intensively leach into the reaction mixture [3]. An optimized hydrothermal synthesis can be used to prevent this leaching by building a sufficiently stable bond between catalytically active CaO and FA or Fa-based zeolite. Thereby, new zeolite forms can be obtained (heulandite, epistilbite, gismondine, philipsite, and wairakite). CaO plays a dual role, acting simultaneously as a mineralizer agent and an active catalytic site. However, the above-mentioned synthesis is multi-staged and time-consuming [7]. Besides that, these processes result in multi-phase zeolitic materials attributed to the incomplete conversion of the glassy phase from FA and the rheology of the slurry causing inadequate or ineffective mixing which forms an inhomogeneous reaction mixture [29,30]. The most commonly used contact agitation (via different impellers) causes a shearing effect, harmful to stability and purity of the zeolitic product [31]. Consequently, the obtained final product might have an uneven chemical/ mineralogical composition and extremely uneven physical parameters, primarily the shape and size of pores, particle size, and insufficiently developed specific surface area, which is not desirable. This study aimed at obtaining a new low-cost and active catalytic material, completely based on solid wastes, such as lignite coal FA and chicken eggshells, in a timesaving two-stage process. In the first stage, FA was converted into a zeolite-like material using, for the first time, a rotating miniature custom-made autoclave reactors system as a specific pressurized system for activation with NaOH. This reactor allows adequate agitation by rotating the entire reaction mixture, instead of contact mixing using a stirrer, thus enabling significant saving of time and energy and obtaining a homogeneously structured product. In the second stage, the gismondine and crystallography modification of the calcium-silicate supported CaO catalyst was obtained by the hydration-dehydration method. The synthesized catalyst was characterized and used for the methanolysis of sunflower oil. In addition, the effects of the influential reaction parameters (temperature, methanol/oil molar ratio, and catalyst concentration) on the formation of methyl esters were analyzed. Finally, the reaction mechanism was tested supposing the two kinetic models.

fired thermal power plant (Morava, Svilajnac, Serbia). Raw chicken eggshells were gathered in the household. The alkali activation was done using NaOH (analytical grade) as a fusion agent. The used feedstock for biodiesel production was commercial sunflower oil (Dijamant a.d., Zrenjanin, Serbia). Methanol (99.99%, GC quality, Acros Organics) was used as a reagent. 2.2. Catalyst preparation In order to remove unburned carbon, crude FA was calcined at 850 °C for 2 h. Thereafter, the calcined sample was subjected to acidification with 6 M HCl at 80 °C and a solid-to-liquid ratio of 1:5 for 6 h. After filtration and washing until neutral pH and drying at 110 °C overnight, the FA sample was alkali-activated and hydrothermally crystallized in an alkaline medium under the following conditions: concentration of alkali reagent of 6.25 M (NaOH aqueous solution), temperature of 260 °C, and solid-to-liquid ratio of 1:5 for 4 h in the rotating miniature autoclave reactor system. The system consisted of an oven with a rotating shaft with four small-volume reactors (autoclaves) (Fig. 1). The zeolitic material synthesized from FA, referred to as FAZM, was filtered, washed until pH of 9 in order to remove residual NaOH, which contribute to the latent catalyst activity, and dried at 110 °C overnight. Prior to thermal and chemical activation, raw chicken eggshells were washed with distilled water to remove impurity and interference material, dried in a hot air oven at 110 °C to evaporate adhering water, and ground to a fine powder (labeled as ES-R), which was calcined in a muffle furnace at 900 °C for 2 h to obtain a sample containing CaO form (designated as ES-900). The final eggshells/FAbased catalytic material was obtained by the hydration-dehydration method [32]. An appropriate quantity of the ES-900 powder was slowly added to an aqueous 10 wt% FA-ZM stock suspension that was magnetically stirred at 700 rpm. Hydration was conducted at 60 °C for 6 h. The hydration product was filtered, dried at 110 °C overnight and calcined at 650 °C for 4 h (ramping rate: 5 °C·min−1), resulting in the CaO/FA-ZM catalyst, which was kept in a desiccator until the catalytic tests. 2.3. Samples characterization The chemical composition of the raw and synthesized samples was analyzed using an X-ray fluorescence analysis equipment (EDX-8000 energy dispersive X-ray fluorescence spectrometer). The identification of a crystalline structure was carried by X-ray diffractometry (XRD, Bruker D8 Endeavor diffractometer) over the angular range of 10-90° (2θ) at a scanning rate 1° min−1 with a step size of 0.02°, using CoKα radiation (λ = 0.178896 nm). FT-IR spectra were recorded using a Shimadzu IRAffinity-1 Fourier transform infrared spectrophotometer (Attenuated Total Reflection-MIRacle 10) in the wavenumber range of 4000–400 cm−1 using 64 scans at 8 cm−1 resolution. The surface morphology was analyzed by a field emission scanning electron microscope (Tescan MIRA3 XMU). Prior to imaging, the dried powder samples were sputter-coated with a thick, uniform layer of Au/Pd alloy. The specific surface area was calculated from the nitrogen adsorption/ desorption isotherms obtained at −196 °C in an ASAP instrument (Micromeritics ASAP 2020) using the BET-equation. Hg-porosimetry measurements were performed in the fully automated conventional apparatus Carlo Erba 2000 porosimeter (pressure range: 0.1–200 MPa; pore size (diameter) range: 7.5–15000 nm). Data acquisition was carried out using the Milestone 200 software package. The powder particle size was assessed by a Cilas 1090 laser diffraction particle size analyzer (LDPSA). The base strength (H_)/basicity were determined using the Hammett indicators: bromothymol blue (H_ = 7.2), thymol blue (H_ = 8.9), thymolphthalein (H_ = 9.9), thymol violet (H_ = 11.0), and 2, 4-dinitroaniline (H_ = 15.0). Approximately 0.1 g of the sample was added to 4.5 mL of a solution of Hammett indicators diluted in methanol and left to equilibrate for 2 h with periodical shaking after

2. Materials and methods 2.1. Materials Waste FA was collected from the electrostatic precipitators of a coal2

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Fig. 1. Rotating miniature autoclave reactor system: (1) oven; (2) temperature and rotating controller; (3) rotating shaft attachment; (4) autoclaves holder; and (5) autoclaves arrangement.

which no further color changes were noticed. The base strength of the catalyst was reported as being stronger than the weakest applied indicator that exhibited a color change, but weaker than the strongest indicator that showed no color change [33]. The basicity of the solid base catalysts was measured by suspension titration using a 2 mmol L−1 benzoic acid in anhydrous ethanol. Elemental analysis of the Ca leached into the crude biodiesel phase was conducted biaxial optical emission spectrometry using an inductively coupled plasma spectrometer (ICP OES, axial Thermo Scientific iCAP 6500 Duo ICP, Thermo Fisher Scientific). Total mineralization of the samples was conducted in highpurity quartz vessels using wet microwave digestion in a closed system (Advanced Microwave Digestion System, ETHOS 1, Milestone).

samples of FAME were diluted with a mixture of n-hexane and 2-propanol (5:4 v/v) in a ratio of 1:200 and filtered through a 0.45 μm pore size membrane filter. 2.5. Kinetic study The previous kinetic studies of the CaO-based catalyzed methanolysis reactions have confirmed the sigmoidal FAME content profile, indicating two controlling factors, namely mass transfer and chemical reaction, which are dominant in the initial and the later stage of the reaction, respectively [14,34–38]. To describe the kinetics of the overall process, the two kinetic models were tested and compared. The first model (Model 1) supposes the irreversible pseudo-first-order rate laws for both the initial heterogeneous regime, where the TAG mass transfer resistance controls the reaction rate and the later pseudo-homogeneous regime, where the chemical reaction limits the overall reaction [34]. The second model (Model 2) proposes the changing mechanism combined with TAG mass transfer limitation and uses only an equation to describe the reaction rate during the whole reaction [36]. These kinetic models, described in detail elsewhere [14], are represented by Eqs. (1a and b) (Model 1) and Eq. (2) (Model 2):

2.4. Experimental setup for biodiesel production The sunflower oil methanolysis reaction was carried out in a 500 mL three-necked spherical glass reactor coupled to a reflux condenser and equipped with a magnetic stirrer. The reactor was immersed in a constant temperature glycerol bath placed on a hotplate (Heidolph MR HeiStandard) coupled with a temperature controller. The reaction mixture was magnetically stirred (850 rpm). Catalytic tests were performed under different reaction conditions, changing the concentration of the catalyst (2, 4, or 6 wt% referred to the initial oil weight), reaction temperature (30, 40, 50, or 60 °C) and methanol/oil molar ratio (6:1, 12:1, or 18:1). The desired amounts of methanol and catalyst were loaded to the reactor and thermostated to the required temperature while agitated (30 min). Separately, sunflower oil was thermostated at the same temperature. After thermostating, the stirrer was turned off and the oil was added to the reactor. Thereafter, the stirrer was switched on at a stirring rate of 850 rpm and the reaction was timed. Samples of the reaction mixture were taken from the reactor during the reaction (0.5, 1, 1.5, 2, 3, 4, 5, 6, and 7 h) and centrifuged at 10000 rpm for 10 min (EBA 21, Hettich Zentrifugen) to separate the fatty acid methyl ester (FAME) phase from the rest of the reaction mixture.

ln

ln(1

xA) = kc a·t

(1a)

ln(1

xA) = kapp,1· t + C1

(1b)

1

xA = 3kapp,2·cA0· t + C2 xA

(2)

where xA is the TAG conversion degree, kca is the volumetric TAG mass transfer coefficient, kc is the TAG mass transfer coefficient, a is the average specific interfacial area, cA0 is the initial TAG concentration, kapp,1 is the apparent pseudo-first-order reaction rate constant that includes the catalyst concentration, kapp,2 is the apparent reaction rate constant, t is the reaction time, and C1 and C2 are the integration constants. The significance of both models was statistically evaluated based on the mean relative percentage deviation (MRPD) and the coefficient of determination (R2).

2.4.1. FAME analysis (HPLC method) The formed FAME was analyzed by the modified Holčapek HPLC method described elsewhere [34]. Before the HPLC analysis, the 3

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Table 1 Chemical composition of the raw and synthesized samples determined by ED XRF. Sample

SiO2

Al2O3

CaO

Fe2O3

K2 O

MgO

Na2O

22.59 19.21 1.28 n.d. 12.70

7.48 2.07 93.08 96.21 49.98

6.28 3.84 n.d.a n.d. 1.86

1.60 0.13 0.10 0.08 0.05

2.19 0.98 1.18 2.08 1.22

0.24 23.42 2.55 0.70 10.09

(wt. %) FA FA-ZM ES-R ES-900 CaO/FA-ZM a

59.18 49.63 0.76 n.d. 24.11

n.d. – not detected.

3. Results and discussion 3.1. Chemical composition (ED XRF) Table 1 presents the chemical composition of raw and synthesized samples in terms of oxide contents. FA comprises SiO2 and Al2O3 as major oxides. According to the standard specification ASTM C618 [39], this sample falls within class F as the total content of SiO2 + Al2O3 + Fe2O3 was higher than 80 wt%, while the content of CaO is 7.48 wt%. The high content of SiO2 makes it a suitable aluminosilicate precursor for synthesizing zeolites [7]. The contents of SiO2 and Al2O3 in the FA-ZM sample were lower than in FA, which indicated that non-reacting silicates and aluminates were retained in the alkaline solution. The loss of Al2O3 was lower than that of SiO2. The molar Si/Al ratios in the FA and FA-ZM samples were 2.31 and 2.28, respectively indicating that SiO2 and Al2O3 were utilized properly during the zeolitization. The high content of sodium in the FA-ZM sample indicated the conversion of FA into the Na+ form of the zeolitic material by capturing Na+ ions to balance the negative charge on the aluminates in aluminosilicate framework structures. Chemical analysis of the ES-R and ES-900 samples showed that both samples contained a high percentage of Ca and small quantities of other metals. Additionally, the molar Si/Al ratio of the CaO/FA-ZM catalyst was 1.94 with a high content of the active form of CaO. 3.2. XRD analysis Fig. 2 depicts the crystalline phase composition of the tested samples. The experimental X-ray diffractogram of the FA sample revealed the typical reflections of crystalline phases for quartz, mullite, hematite, and anorthite (inset in Fig. 2). After the FA zeolitization, numerous diffraction peaks of the new crystalline phase with a zeolite-like structure corresponding to the cancrinite-sodalite group, vishnevite hexagonal prismatic crystals (PDF#46–1333) had appeared (Fig. 2a) [40]. Prior to the final catalyst synthesis, powdered eggshells were calcined, whereby only lime (PDF#37-1497) could be identified (inset in Fig. 2b). After the hydration-dehydration, the peaks of the dried samples (Fig. 2b) could be assigned to the new phases corresponding to portlandite (PDF#44–1481), calcium aluminum hydroxide carbonate hydrate (PDF#36-377), and the Ca zeolite-like structure, whereas the vishnevite structure was partially retained. A part of calcium was hydrated and implemented in the new structure, whereas the other part was deposited as calcium-hydroxide. Calcination at 650 °C led to the conversion of calcium-hydroxide into calcium-oxide, the destruction of unstable calcium aluminum hydroxide carbonate hydrate and the formation of the structures with diffraction lines that could be attributed to lime, gismondine (GIS type) (PDF#20–452), and α’-dicalcium silicate (PDF#36-642) (Fig. 2c) formed through the reaction of CaO and SiO2 [41]. Sharma et al. [42] reported on zeolite P, as a synthetic analog to the GIS-type zeolites [43–45], whereas Kazemian et al. [46] used highly Si-containing fly ash for the synthesis of the P-type zeolites. The obtained catalyst (Fig. 2c) contained particularly intense and

Fig. 2. XRD patterns of (a) FA-ZM (inset: FA); (b) CaO/FA-ZM-dried (inset: ES-R and ES-900); and (c) CaO/FA-ZM catalyst. (α-CS-α’-dicalcium-silicate; A-anorthite; C-calcite; Ca-calcium aluminium hydroxide carbonate hydrate; G – gismondine; H – hematite; L - lime; M – mullite; P – portlandite; Q – quartz; and V – vishnevite).

differentiated lime peaks, which were also visible on the diffractograms of the calcined eggshells (Fig. 2b), and which undoubtedly represented a catalytically active species [36]. The presence of the other calciumsilicate phases formed during the appropriate treatment of CaO·SiO2 mixture led to the formation of the catalytically inactive calcium4

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Table 2 Main features of texture from Hg-porosimetry and N2-physisorption measurements, basic strength, and basicity. Sample

Hg-porosimetry a

FA ES-R ES-900 FA-ZM CaO/FA-ZM

3

Vtotal (cm /g)

0.718* 0.172 0.449 1.885 2.142

N2-physisorption b

P (vol %)

48.1* 29.8 55.2 82.9 87.8

c

2

SAHg (m /g)

18.1* 0.8 1.5 21.1 18.7

d

2

Basic characteristics e

BETSA (m /g)

2.1 0.2 0.7 21.9 13.5

3

Vmicro (cm /g)

0.0010 0.0002 0.0004 0.0073 0.0050

f

3

Vmeso (cm /g)

Basic strength (H_)

Basicity (mmol/g)

0.0022 0.0009 0.0007 0.0603 0.0545

H_ < 7.2 H_ < 7.2 11.0 < H_ < 15.0 7.2 < H_ < 8.9 11.0 < H_ < 15.0

– – 15.93 0.36 23.21

a

Total cumulative volume. Porosity. c Specific surface area from Hg-measurements. d Surface area from N2 multipoints. e Micropore volume. f Mesopore volume. * No reliable. b

(about of 83 cm−1) towards lower frequencies (956 cm−1) in the FAZM spectrum, which was a good indication of the reaction between the glassy components of FA and the alkali activator (NaOH) to form a zeolitic material. A sharp band with high intensity at 956 cm−1 was assigned to the frequencies of T-O (T = Si, Al) bond in the tetrahedron along with the lines that bonded the tetrahedron oxygen atoms (TO4)4with the central Si or Al atoms [9]. This demonstrated that an obvious change in the microstructure took place during alkali activation resulting in the formation of zeolitic material with different microstructure. The large shift towards low wavenumbers might be attributed to the partial replacement of SiO4 species by AlO4 that changed the local chemical environment of the Si-O bond. A larger shift indicated a higher degree of Al penetration from the glassy part of FA into the (SiO4)4- skeleton as observed analogously in zeolites [50]. The absorption bands in the region of 420–500 cm−1 of the FA-ZM spectrum were assigned to the internal tetrahedron Si/Al-O bending vibrations. When loaded with CaO, three new bands at 1415 cm−1, 871 cm−1, and 543 cm−1 were clearly observed. The band at 1415 cm−1 of the CaO/ FA-ZM catalyst was due to the asymmetric stretch of CO32– groups [51]. The new band at 871 cm−1 was attributed to the out of plane bend vibration mode for CO32–. The external linkage band appearing at 543 cm−1 could be related to the presence of a double ring in the framework structures and was observed in all the zeolitic structures that contained double 4 (D4R) and double 6 (D6R) rings [52]. In the inset spectrum of raw eggshells (ES-R, inset Fig. 3), the major absorption bands occurred at 1407 cm−1 due to the asymmetric stretch of CO32–, at 871 cm−1 caused by the out of plane bend vibration mode for CO32– at 712 cm−1 due to Ca-O bond and the in of plane bend vibration mode for CO32– molecules [53]. The inserted spectrum of eggshell calcined at 900 °C (ES-900, inset Fig. 3) did not show many dissimilarities from that of non-calcined eggshells. Upon heat treatment at 900 °C, eggshells started to lose carbonate and the adsorption band intensities of CO3-2 molecules decreased.

silicate compounds with different thermal stability and suitable properties specific for catalyst support [47]. An interaction between the catalytically active lime phase and the resulting calcium-alumino-silicate phase gave a big contribution to the increment of the supported catalyst surface, cumulative volume, and porosity (Table 2), and thus restraining agglomeration and stabilized the catalyst surface. Therefore, the CaO/Fa-ZM catalyst is expected to achieve very high catalytic activity and improved stability compared to the neat CaO catalyst. 3.3. FT-IR analysis Structural information concerning the vibrations of chemical bonds in the molecular units was obtained by FT-IR spectroscopy (Fig. 3). The main feature of the FA spectrum was a wide band centered at around 1038 cm−1 associated with an asymmetric flexible stretch mode of an internal Si-O vibration and may be attributed to the presence of quartz. The broad bands centered at 776 cm−1 and 595 cm−1 were associated with the Si-O-Si and Si-O-Al symmetric stretching vibrations corresponding to quartz and mullite, respectively [6]. A spectral band centered at 463 cm−1 was associated with the structure of insensitive internal tetrahedral Si/Al-O bending vibrations [48]. The framework vibrations common to zeolitic materials [49] could be identified in the spectrum of the FA-ZM sample (Fig. 3). The FA-ZM spectrum showed interesting differences when compared with the FA spectrum. Broadband at 1038 cm−1 in the FA spectrum had become sharper and shifted

3.4. SEM micrography The SEM micrographs of all samples are shown in Fig. 4. The micrograph of FA (Fig. 4a) indicates a spherical shape with a smooth surface. In Fig. 4b, the inner surface of the FA sphere prevails large pores (2–5 μm), filled with smaller spheres ranging from 0.5 to 2 μm. The amorphous aluminosilicate FA structure was dissolved and transformed into the needle-like structure (Fig. 4c and d), which could be attributed to the vishnevite-type cancrinite-sodalite zeolite group. During alkali activation, the alkali agent penetrated through the FA spheres and formed an aluminosilicate gel, which was later transformed into a zeolite crystal-like structure [28,45]. The ES-R and ES-900 micrographs (Fig. 4e and f) indicated a compact structure. However, the ES-R particles had sharp edges, whereas the ES-900 obtained by

Fig. 3. FT-IR spectra of FA, FA-ZM, and CaO/FA-ZM (inset: ES-R and ES-900). 5

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Fig. 4. SEM micrographs of raw materials and products during catalyst synthesis (a) and (b) FA; (c) and (d) FA-ZM; (e) ES-R; (f) ES-900; (g) and (h) CaO/FA-ZM.

calcination presented the rod-like particles ranging from 1.6 μm to 3.8 μm as it was previously confirmed in similar researches [54]. The final catalyst (Fig. 4g and h) presents large calcium-silicate and Ca zeolite-like aggregates and irregularly sized crystallites with the deposit, which could be attributed to CaO, as confirmed by XRD analysis. 3.5. Textural and basicity properties The main textural features (pore volume, porosity, and surface area) of all samples based on Hg-porosimetry and N2-physisorption data, as well as basic strength and basicity, are presented in Table 2 and Figs. 5 and 6. Hg porosimetry. The cumulative volume intrusion and pore size distribution (PSD) curves for the tested samples are shown in Fig. 5. In the case of ES-R and ES-900 samples having a monomodal PSD (inset in Fig. 5), the intruded volume increased with increasing the pressure with Fig. 6. N2-adsorption–desorption isotherms of CaO/FA-ZM (inset: FA-ZM).

a sharp uptake for pore diameters centered about 1550 nm. However, for the FA sample, the cumulative volume intrusion had no sharp volume uptake (even at 200 MPa), which might mean that the sample densification due to its low compressive strength was possible upon pressurization (no mercury extruded out; extrusion step not shown). Besides that, in the small pore range (< 1 μm), the FA sample could be crushed upon mercury penetration (see the resulting a rather broad PSD and corroborated by electron microscopy, Fig. 4b) Therefore, for this sample, no reliable PSD could be achieved by the Hg-porosimetry measurement [55]. For the CaO/FA-ZM catalyst, the mercury volume uptake was not as sharp as for the FA-ZM sample (Fig. 5). Moreover, the porous structure of the CaO/FA-ZM catalyst was multimodal with large (around 3.1 μm), medium (centered about 1.0 μm) or small (essentially < 100 nm) pore diameters. The scanning electron micrographs of the same sample (Fig. 4g and h) showed a high porosity with heterogeneous pore sizes, confirming the presence of large, medium or

Fig. 5. Cumulative volume intrusion and pores size distribution of FA-ZM −1 and CaO/FA-ZM −2 (inset: FA-3; ES-R – 4; and ES-900-5). 6

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small pores. N2 physisorption. The nitrogen adsorption experiments gave precise and reliable surface areas through the multipoint BET analysis (Table 2). The FA, ES-R and ES-900 samples had a small specific surface area, which agreed with the reported data [7,8]. The larger BET surface areas of the FA-ZM and CaO/FA-ZM samples (Table 2) predicting likely superior catalytic activity might be associated with the zeolitization and catalyst preparation method. Alkali activation of FA produced a zeolitelike material (FA-ZM) with the developed surface area and mesoporosity as a result of the phase transformation confirmed by the XRD analysis. The CaO/FA-ZM catalyst kept the high porosity and BET surface area originated from the FA-ZM sample (Table 2) with a mildly expressed multimodal porous structure having abroad small pore range caused by the formation of different silicate forms and phases during the hydration-dehydration and calcination treatments. From the results presented in Table 2 and Figs. 5 and 6, it was obvious that calcination increases the pore diameter in the mesopore and macropore regions (12.2–18.6 nm and 472–3246 nm, respectively) regions, reducing the micropore volume that affected the BET surface area of the CaO/FA-ZM catalyst. In accordance to the IUPAC classification [56], the obtained adsorption-desorption isotherms for the FA-ZM and CaO/FA-ZM samples (Fig. 6) corresponded to mesoporous materials (Type IV) whereas both hysteresis could be classified as the H3 type (non-rigid aggregates of plate-like particles (Fig. 4g and h) with a defined network of mesoand macropores). Particle size distribution. As can be seen in Fig. 7, the particle size distribution curves for the FA; FA-ZM, and CaO/FA-ZM samples were uniform. The particle size of FA was significantly reduced after alkali activation (from 65.0 μm to 36.4 μm) because the initial FA structure was totally destroyed (Fig. 4a and d) and converted into the uniformly graded zeolite particles (Fig. 4e and f). The further synthesis process additionally reduced the particle size, so the average particle size of the CaO/FA-ZM catalyst was 17.7 μm. The parameters of zeolitization and hydration-dehydration play the main role in particle formation. If the process parameters (low pressure, temperature, and alkali agent concentration) are less rigorous, the FA destruction is gradual while the particle size firstly increases due to the deposition of the newly formed product on it. For similar systems [57], further activation leads to the formation of the reduced final particles (13.5–26 μm). Basic strength and basicity. The basic strength and basicity of all samples are given in Table 2. The basic strength was as follows: CaO/ FA-ZM > ES-900 > FA-ZM > ESR. The FA and ES-R samples exhibited a neutral pH reaction (H_ ≤ 7.2) due to their aluminosilicate and carbonate nature, respectively whereas the FA-ZM sample exhibited a mildly basic strength (7.2 < H_ < 8.9), unsuitable for

Fig. 8. The variation of the reaction mixture composition with the progress of the sunflower oil methanolysis over the CaO/FA-ZM catalyst (temperature of 60 °C, methanol/oil molar ratio of 12:1, and catalyst concentration of 4 wt%).

methanolysis reaction. Converting carbonate into the oxide form (ES900), the basic strength expectedly increased (H_ = 11.0–15.0). The CaO/FA-ZM catalyst exhibited the highest basicity, higher by 45.7% than calcined eggshells and by two orders of magnitude than the FA-ZM material. It was attributed to the uniform distribution of unreacted CaO on the surface of the zeolite-like structure, which was significantly higher than that of the ES-900 sample. 3.6. Catalytic test and kinetic study The changes of the TAG, diacylglycerol (DAG), monoacylglycerol (MAG), and FAME contents in the esters phase during the sunflower oil methanolysis reaction are shown in Fig. 8. The neat zeolite carrier showed no catalytic activity under the applied reaction conditions. As was expected, the FAME content varied sigmoidally with time. In the initial stage of the reaction, the FAME formation rate increased slowly, accelerated rapidly and finally, reached the equilibrium concentration. On the other hand, the TAG content decreased adequately. The MAG and DAG contents passed through their maximum, but their contents were very low throughout the reaction. 3.6.1. Influence of reaction parameters Fig. 9 shows the change of FAME content during the sunflower oil methanolysis under various reaction conditions. The reaction variables were temperature (30–60 °C), methanol/oil molar ratio (6:1–18:1), and catalyst concentration (2–6 wt%). The traditional experimental optimization was carried out by maintaining the two reaction parameters constant while varying the third parameter. Influence of temperature. The influence of the reaction temperature on FAME content was studied at the methanol/oil molar ratio of 12:1, the catalyst concentration of 4 wt%, and the reaction time of 7 h (data for 30 °C are not shown). Higher reaction temperature increased the reaction rate and shortened the reaction time. After 2 h, the FAME concentrations at 40 °C, 50 °C, and 60 °C were 5.9%, 20.9%, and 95.0%, respectively showing a noticeable influence of temperature in the initial stage of reaction (Fig. 9a). The overall positive effect of higher reaction temperature was also confirmed by comparing the reaction times required to reach the reaction equilibrium FAME concentrations. Namely, the equilibrium FAME contents at the reaction temperature of 60 °C, 50 °C, and 40 °C were reached for about 2 h, 4 h, and 7 h, respectively. At the reaction temperature of 30 °C, the methanolysis was very slow, and the equilibrium FAME concentration was achieved in about 24 h.

Fig. 7. Particle size distribution of FA, FA-ZM, and CaO/FA-ZM. 7

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improved mass transfer [11,13,35,60]. Accordingly, in the range of 30–60 °C, the reaction temperature of 60 °C was considered most suitable for the sunflower oil methanolysis. Influence of molar ratio. Fig. 9b shows the effect of the methanol/oil molar ratio on FAME content at the reaction temperature of 60 °C and the catalyst concentration of 4 wt%. The highest FAME concentration was obtained at the methanol/oil molar ratio of 6:1. With the methanol/oil molar ratio of 12:1 or 18:1, the equilibrium was achieved for a longer time (Fig. 9b). The observed adverse effect of a higher methanol/oil molar ratio on the reaction rate might be due to limited access of TAG molecules to the catalytically active sites on the catalyst surface occupied by methanol molecules in the initial reaction stage [61]. It seemed that this assumption was supported by the longer initial period of the reaction at the methanol/oil molar ratios of 12:1 and 18:1, as can be seen in Fig. 9b. Generally, the methanol/oil molar ratio of 6:1 ensures FAME content higher than 98% in many methanolysis reactions [62,63]. Influence of catalyst concentration. Fig. 9c presents the influence of the catalyst concentration on FAME content at the reaction temperature of 60 °C and the methanol/oil molar ratio of 12:1. With increasing the catalyst concentration the initial lag period was shortened and the equilibrium conversion was also achieved in a shorter time. A higher catalyst loading had a significant positive effect on the reaction rate in the early stage of the reaction by increasing the number of available active sites on the catalytic surface. Optimal reaction parameters. Analysis of the influence of the key reaction parameters on the reaction rate and FAME content showed the following optimal conditions: temperature of 60 °C, methanol/oil molar ratio of 6:1, and catalyst concentration of 6 wt%. Under these optimal conditions, higher equilibrium FAME content was obtained in a shorter time, 97.8% and 99.1% in 30 min and 2 h, respectively. 3.6.2. Kinetic study of biodiesel production over CaO/FA-ZM catalyst To quantify the influence of the reaction parameters on FAME content in the methanolysis over the CaO/FA-ZM catalyst, the experimental results were interpreted in terms of the kinetics in accordance with the proposed Models 1 and 2. Previously, the influence of the internal diffusion was estimated based on the Thiele modulus (Th) for the TAG mass transfer through the reaction mixture [34]. Since the Thvalues were 0.006 and 0,022, respectively (i.e. Th ≪ 0.4), the internal diffusion resistance was negligible. The volumetric TAG mass transfer coefficient, kca, and the apparent pseudo-first-order rate constants, kapp,1, and kapp,2, were determined from the dependencies of –ln(1-xA) and ln(xA/(1−xA)) on time using Eqs. (1 and 2), respectively. As can be seen in Fig. 10, there was a satisfactory fit to the pseudo-first-order rate laws and the changing mechanism combined with the TAG mass transfer limitation, which was supported by a very high R2 > 0.93 for the developed models. The calculated rate constants for the models at different temperatures are given in Table 3. All the rate constants increased with increasing the reaction temperature, which was ascribed to the improved hydrodynamic conditions and TAG mass transfer because of lower viscosity and better miscibility of the reactants. On the other hand, the better activity at a lower methanol/oil molar ratio was explained by the fine distribution of methanol in the multiphase system without the formation of the methanol shelter around the catalyst particles that would hinder the TAG mass transfer [34]. The obtained values of the rate constants corresponded to the previously reported ones for similar systems with CaO-based catalysts, which varied from 0.03 to 0.09 min−1 for the Model 1 [35] and from 0.025 to 0.063 L mol−1 min−1 for the Model 2 [36]. The activation energy of the sunflower oil methanolysis over the CaO/FA-ZM catalyst was determined using the Arrhenius equation (Fig. 11):

Fig. 9. Variations of FAME content with time at different (a) temperatures (40, 50, and 60 °C), (b) methanol/oil molar ratio (6:1, 12:1, and 18:1), and (c) catalyst concentrations (2, 4, and 6 wt%).

The present results were similar to those reported in the previous studies on biodiesel production from oil sources, where the final FAME contents higher than 90% were obtained [34,37,58,59]. The positive effect of temperature on the methanolysis reaction rate was explained by its endothermic nature, the improved miscibility of the immiscible reactants, and the reduced viscosity of the reaction mixture that 8

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Fig. 10. Dependences of (a) –ln (1-xA) and (b) ln (xA/(1-xA)) versus time during the sunflower oil methanolysis over the CaO/FA-ZM catalyst (circle − 40 °C; square − 50 °C; triangle − 60 °C, experimental data – solid symbols, and sigmoidal fit – open symbols).

kapp,1 = 2.34·109·exp

8078.9 T

(3a)

kapp,2 = 3.35·107·exp

6979.5 T

(3b)

for the Models 1 and 2, respectively. In order to verify the proposed kinetic models, the predicted TAG conversion degree and experimental data are shown in Fig. 12a and b. A good agreement between the models and the experiments was confirmed by the MRPD-values of ± 12.5% and 11.9% for the Models 1 and 2, respectively. The model 2 was slightly more accurate, probably due to its applicability in the whole reaction period. The calculated values of the activation energy were 67.17 and 58.03 kJ mol−1 for the models1 and 2, respectively. These values were in the range of the reported values for the methanolysis over various CaO-based catalysts (16–140 kJ mol−1). They were particularly in good agreement with the values reported for the doped, supported, and mixed CaO catalysts but lower than the values for the systems using neat CaO (Ea = 100 kJ mol−1) [64]. The lower activation energy was ascribed to better efficiency of the CaO/FA-ZM catalyst, compared to neat CaO, due to its specific chemical and structural properties, such as high basicity and satisfactory pore structure reducing the TAG mass transfer limitations [65]. Under the comparable reaction conditions, the CaO/FA-ZM catalyst appeared to be more efficient than the other CaO-based catalysts. A high conversion (> 95%) was provided for about 1 h, 1.5 h, 2 h, 2.5 h and 4 h with the CaO/FA-ZM catalyst (present work), quicklime [36], flower-like nano CaO [66], nanocrystalline K-CaO catalyst [67], and palm oil kernel shell biochar catalyst [14], respectively.

Fig. 11. The Arrhenius equation curves for the sunflower oil methanolysis over CaO/FA-ZM catalyst.

catalyst concentration of 6 wt%) and the reaction time of 3 h. After the reaction was completed, the solid catalyst was separated from the reaction mixture by filtration and reused without any pretreatment. The CaO/FA-ZM catalyst was reused five times with a negligible decrease in catalytic activity, with FAME content from 99.2% to 97.9% achieved in the first and fifth cycles, respectively. The known drawback of CaObased catalysts is calcium leaching during the reaction, which can be overcome by adequate purification [14]. The obtained calcium content in the crude FAME phases in the successive reactions decreased from 1170.4 mg kg−1 in the first cycle to 629.9 mg kg−1 in the fifth cycle. However, this leaching did not reduce the catalytic activity of the CaO/

3.6.3. Catalyst stability and reusability Catalyst reusability was tested under the optimal reaction conditions (temperature of 60 °C, methanol/oil molar ratio of 6:1, and Table 3 Values of kinetic model parameters. Temperature (°C)

40 50 60

Methanol/oil molar ratio

12:1

Catalyst concentration (wt. %)

4

Model 1

Model 2

kca (min−1)

R2

kapp,1 (min−1)

R2

kapp2 (L mol−1 min−1)

R2

0.0007 0.0015 0.00236

0.966 0.974 0.917

0.0132 0.0375 0.0617

0.986 0.946 0.985

0.0074 0.0125 0.0283

0.947 0.996 0.996

9

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Fig. 12. The comparison of (a) the irreversible pseudo-first kinetic model and (b) kinetic model which including mass transfer resistance (line) and experimental data (symbols) during sunflower oil methanolysis over CaO/FA-ZM catalyst at different temperatures (● −40 °C; ■ −50 °C; and ▲ −60 °C).

The used catalyst was characterized by FT-IR and SEM methods in order to determine the morphological and structural changes on the surface of the catalyst used in five consecutive cycles. The FT-IR spectra (Fig. 14) of the used catalyst showed that the surface functional groups were preserved, whereby the additional spectral lines appeared. The low-intensity spectral lines between 1100 and 1300 cm−1 could be assigned to the vibration of the C-CH2-O group, the stretching of the C–H bond, and the asymmetric stretching of C-O-C bond. The previous band at 1415 cm−1, attributed to CaO, was overlapped with more intense bands slightly shifted to higher wavenumbers, which might be associated with an asymmetric stretching of the C–H bond and an asymmetric bending of the same functional group. A sharp and intense line at 1746 cm−1 corresponded to the C = O vibrations was an especial characteristic of esters. The intense dual-band with peaks at 2854 cm−1 and 2924 cm−1 originated from the C–H vibration of methylene groups and the stretches and compression of the methyl group. All these spectral lines are the characteristics of FAME molecules and confirm their presence on the surface of the used catalyst [37]. The spectral lines at wavenumbers higher than 3000 cm−1, especially the low-intensity wide knee, could be assigned to the vibration of the hydrogen bond of the hydroxyl group that likely did not originate from the adsorbed air moisture; they could be associated with the presence of residual glycerol or some glyceride compounds [69]. The analysis of the SEM micrographs of the used catalyst (Fig. 15) revealed that its surface became polished compared to the fresh ones. Irregularly sized crystallites and particles distinctive at the surface of

Fig. 13. Leaching of the neat CaO (ES-900) and CaO/FA-ZM catalyst in consecutive cycles.

FA-ZM catalyst over five repeated cycles. The neat CaO (ES-900 sample), obtained from the same eggshells as the CaO/FA-ZM catalyst, exhibited more than four times higher leaching in the crude FAME phases under the same reaction conditions (Fig. 13). Significantly lower calcium leaching from the CaO/FA-ZM catalyst, compared to the ES900 was ascribed to good incorporation and stabilization of the active centers on the carrier surface, as indicated by the material characterization. The stability of the present catalyst is comparable with some other CaO-based catalysts obtained from natural sources, but its reusability has proven superior. For instance, the CaO-based catalyst derived from palm kernel shell biochar was active for only 3 consecutive cycles, with Ca leaching going up to 722 mg kg−1 (after 4 h of reaction) [14]. The natural calcite obtained in a similar manner, i.e. hydrated-dehydrated and calcined, showed nearly ten times higher leaching than the present CaO/FA-ZM catalyst, 456 mg L-1 (after 1 h of reaction) versus 48.49 mg L-1 (after 3 h of reaction), respectively [32]. The neat CaO catalyst obtained by applying hydration-dehydration and calcination leached 825 mg kg−1 (after 2 h reaction); moreover, it was active for only 2 consecutive cycles [38]. Investigating the solubility of the neat CaO in the modeled biodiesel-methanol-glycerol mixture, Granados et al. found even higher values, up to 600 mg L−1 [68]. The recent promising results of crude biodiesel purification with a high Ca content with the use of a relatively simple procedure employing methanol containing anhydrous sodium carbonate at boiling temperature reduced the concerned drawback caused by the present catalyst leaching [14].

Fig. 14. FT-IR spectra of fresh and used (after the fifth cycle) CaO/FA-ZM catalyst. 10

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Fig. 15. SEM micrographs of used (after the fifth cycle) CaO/FA-ZM catalyst.

the fresh catalyst looked agglomerated, and in some areas even had grainy appearance after the reaction. Sharp-edged particles were no longer present on the surface of the used catalyst. This new smooth appearance along with the FT-IR evidence led to the conclusion that certain molecules of the reaction mixture were deposited onto the surface of the used catalyst. Interestingly, major changes of the catalyst surface were not found when the contact time of the material with the reaction mixture was increased through consecutive cycles. The only change in the appearance of the surface was in slightly larger agglomerates after the fifth cycle (Fig. 15b), compared to those after the first cycle (Fig. 15a), which was due to a larger amount of the deposited molecules over time. However, the reaction products deposited to the catalyst surface did not affect its catalytic activity adversely during the five successive cycles.

including its low cost and positive impact on the environment by reduction of FA amount at landfills. CRediT authorship contribution statement Stefan M. Pavlović: Investigation, Software, Visualization, Writing - original draft. Dalibor M. Marinković: Conceptualization, Writing original draft, Project administration. Milan D. Kostić: Formal analysis. Ivona M. Janković-Častvan: Formal analysis. Ljiljana V. Mojović: Resources. Miroslav V. Stanković: Supervision, Validation. Vlada B. Veljković: Methodology, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper

4. Conclusion A novel heterogeneous CaO/FA-ZM catalyst was successfully prepared from waste sources. The lignite FA and chicken eggshells were used as feedstocks in the catalyst synthesis in a two-stage process, which was characterized and used in the methanolysis of sunflower oil. In the first stage, FA was converted into the cancrinite-sodalite zeolitelike material (vishnevite type) by alkali activation under the hydrothermal conditions, using a new miniature custom made rotating autoclave reactor system. The characteristic of this reactor is the rotation of the entire reaction mixture to achieve better contact, thereby achieving a homogeneously structured product while saving time and energy. In the second stage, hydration-dehydration of the zeolite-like material and calcined eggshell led to the formation of Ca-zeolite (gismondine type), crystallographic modification of Ca-silicate (α’-dicalcium silicate), and CaO. In the methanolysis reaction, the CaO/FA-ZM catalyst exhibited a remarkable performance under the optimal reaction conditions (temperature of 60 °C, methanol/oil molar ratio of 6:1, and catalyst concentration of 6 wt%), providing the FAME content of 97.8% in only 30 min, with a negligible decrease of its activity in five consecutive cycles without any pretreatment. The two tested kinetic models were shown valid for the methanolysis reaction over CaO/FA-ZM catalyst, as confirmed by high R2values (> 0.93) and an excellent agreement between the predicted and experimental values of FAME content. Both models indicated relatively low values of the activation energy (67.17 and 58.03 kJ mol−1). The use of the CaO/FA-ZM catalyst obtained completely from waste, besides the superior catalytic performance, has many advantages

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