Hybrid CaO/ZnFe 2 O 4 Modified with Al 2 O 3 as a Green Catalyst for Biodiesel Production from Waste Cooking Oil

Abstract


INTRODUCTION
In the last few years, energy issues have grown due to global demand worldwide.The depletion of fossil energy sources and the growth of environmental pollution were the main contributing factors to renewable energy development 1,2 .Biodiesel is a renewable alternative energy substitute for fossil fuels, it reduces greenhouse gasses and is non-toxic, and biodegradable 3 .Biodiesel is obtained through the triglyceride transesterification reaction from fatty acids such as edible feedstocks, non-edible feedstocks, and waste feedstocks such as waste cooking oils (WCO) with low-chain alcohols in the presence of a catalyst 4,5 .The use of WCO is a very efficient effort to deal with waste problems by processing them into renewable energy sources 6 .
An important factor associated with biodiesel production is the use of catalysts in synthesis.This fuel large-scale production occurs using mostly homogeneous catalysts, such as NaOH, KOH, etc.However, homogeneous catalysts are difficult to separate, corrosive, and environmentally harmful 7 .Heterogeneous catalysts are an alternative to these problems because of their favorable characteristics such as high selectivity, regeneration ease of separation from the reaction mixture, and the potential to be reused [8][9][10] .Among the heterogeneous catalysts in biodiesel synthesis, calcium oxide (CaO) exhibits high activity in transesterification 11 .Besides, it is environmentally friendly and can be synthesized from less valuable residues, namely eggshells, animal bones, snail shells, oyster shells, etc [12][13][14] .
However, the disadvantages of CaO are mainly from natural sources that have low stability causing the degradation of the structure of CaO and can dissolve in the biodiesel phase.To improve the catalytic activity and stability of calcium oxide, it is necessary to modify CaO with other metal oxides, including combining magnetic catalysts, so that can be separated by an external magnetic field 15 .Metal-ferrite nanoparticles or MFe2O4 (M=Co, Ni, Mg, Cu, etc) have many benefits including high surface area, great reusability, tunable size, high stability, and magnetic features 16,17 .Besides that, to increase the surface area the catalyst can be combined with a catalyst support, including zeolite, silica, and alumina (Al2O3) 18 .Alumina is one of the support catalysts suitable for efficient biodiesel production due to its high specific surface area, porous structure, and high stability 19 .
Several studies with heterogeneous bases CaO for transesterification in biodiesel production have been investigated, like Hybrid CaO/Al2O3 aerogelsF; Novel SrO/MgFe2O4 magnetic nanocatalysts at low temperatures 20 ; A novel robust CaO/ZnFe2O4 hollow magnetic microspheres with yeast templates 21 ; Metal loading on CaO/Al2O3 pellet catalyst 22 ; Aluminum industrial waste as a precursor of efficient CaO/Al2O3 nano-catalyst 18 ; Mg decorated CoFe2O4 nanocatalyst 23 ; Hybrid CuO/Al2O3 nanoparticles 24 .
Based on previous research, this paper combines ideas from previous research with new modifications in the preparation of the heterogeneous catalyst-based calcium oxide of chicken eggshell combined by hollow structure ZnFe2O4 using yeast cells as a biotemplate and alumina as a supported catalyst.Therefore, the purpose of this study is to synthesize a catalyst using Al2O3 as a support combined with a CaO and ZnFe2O4 composite which will be used as a catalyst in the transesterification process of waste cooking oil (WCO) into biodiesel or fatty acid methyl ester (FAME).Furthermore, the novel catalyst was characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), energy diffraction X-ray (EDX), Elemental distribution mappings by SEM, and transmission electron microscopy (TEM).

RESEARCH METHODS Materials and Instruments
The materials used in this research were chicken eggshell waste as nano-CaO source, Fe(NO3)3•9H2O, Zn(NO3)2•6H2O as ZnFe2O4 precursors, (Al(NO3)3•9H2O) as Al2O3 precursor, CH3OH were procured from Merck, and WCO as the feedstock for biodiesel, yeast was obtained from the local market.

Synthesis of nano-CaO
Nano-CaO was prepared from chicken eggshell waste using the previously reported method 25 , the collected chicken eggshells were extensively washed with distilled water and dried for 2 h in an oven at 110 °C, then ground through a ball milling process.The samples were calcined for 3 h at 900 °C with a heating rate of 10 °C/min.

Synthesis of ZnFe2O4 Hollow Structure
The synthesis of ZnFe2O4 was prepared as a previously reported method 21 .In the first stage, 2 g of yeast cells used as bio-template was dissolved in 50 ml of distilled water and stirred for 20 min to ensure complete dispersion of yeast cells.In the next stage, FeNO3•9H2O and Zn(NO3)2•6H2O were added with constant stirring at 60 °C for 1 h.Then, 25% liquid ammonia solution was added drop by drop until the pH of the solution reached between 9 and 10.The precipitate was separated, washed with distilled water, and dried in an oven at 60 °C for 12 h.

Synthesis of nano-Al2O3
Nano-Al2O3 was prepared as a previously reported method 26 .A 0.5 M Al(NO3)3•9H2O was dissolved in 40 mL of distilled water and 120 mL of 1.5 M NH4OH was added drop by drop by stirring using a magnetic stirrer for 20 min at 60 °C.The white precipitate was filtered and washed using distilled water and ethanol, dried at 100 °C for 2 h, and calcined at 550 °C for 5 h.

Synthesis of CaO-ZnFe2O4 Composite
Synthesis of CaO-ZnFe2O4 using the coprecipitation method as previously reported 21 .The ZnFe2O4 hollow nanostructures were dispersed in distilled water by ultrasonic waves.The resulting dispersion supplemented with nano-CaO was slowly stirred and adjusted with 2 M NaOH to reach a pH of 12.The mixture was stirred continuously at 600 rpm, 60 °C for 3 h.The product was separated, washed with distilled water, and dried at 60 °C.

Synthesis of CaO-ZnFe2O4/Al2O3 Nanocomposite
Nanocomposite of CaO-ZnFe2O4/Al2O3 was prepared using a slight modification of a previously reported method 27 .In the first stage, CaO-ZnFe2O4 composite was added to a solution of 10 mL of 0.25 M NaOH and slowly stirred for 1 h (mixture A).In the second stage, nano-Al2O3 in 50 mL of distilled water and slowly stirred for 1 h (mixture B).Then mixture A was added slowly to mixture B and stirred at 27 °C for 6 h.The resulting mixture was filtered and rinsed using water and ethanol, dried at 100 °C for 2 h, and calcined at 550 °C for 5 h.

Preparation of Waste Cooking Oil (WCO)
WCO preparation refers to the previous method 28 .The WCO was filtered to remove impurities with gauze, then the washing process was done with warm water with a weight ratio water to WCO of 10:1 stirred for 30 min, and left overnight so that the water and oil phases could be separated.The resulting WCO added silica gel and stirred for 3 h followed by vacuum filtration using Whatman filter paper to remove the silica gel.The WCO is stored in a tightly closed bottle.

Catalytic activity
The catalytic activity was tested as described in previous studies 29 .The experiments were done in a 100 ml glass reactor equipped with a condenser and a mechanical stirrer, this system is submerged in a water bath under controlled temperature.In a typical test, 2 wt% catalyst was added to WCO and methanol (methanol to WCO molar ratio 9:1).The mixture was refluxed by stirring at different temperatures and times.After the reaction, the catalyst was separated by an external magnet.The products are transferred to a funnel to separate the two phases of biodiesel and glycerol.Biodiesel yield was calculated in Eq (1) 30 .

Synthesis of CaO-ZnFe2O4/Al2O3 Nanocomposite
The use of Al2O3 as a catalyst support is used to increase the number of active groups, surface area, and catalytic efficiency of nanocomposites with CaO and ZnFe2O4.The role of yeast as a template in the synthesis of ZnFe2O4 is to create a hollow structure in the ZnFe2O4 that will be formed.This will cause an increase in surface area compared to ZnFe2O4 which does not have a hollow structure.

Characterization 3.2.1 FTIR Analysis
The molecular vibration of catalysts was analyzed by Fourier Transform Infrared spectroscopy (FTIR) in the range of 400-4000 cm −1 .Figure 1 shows the FTIR spectra of nano-CaO, ZnFe2O4, nano-Al2O3, CaO-ZnFe2O4 composite, and CaO-ZnFe2O4/Al2O3 nanocomposite.The FTIR spectra of nano-CaO have an intense peak at 874 cm −1 related to the stretching of the Ca-O bond and a wide peak at 512 cm −1 , which is a typical characteristic of Ca-O nanoparticles 31 .The sharp peak at 3640 cm −1 may be assigned to the OH stretching for Ca(OH)2 the due to absorption of water by CaO 22,32 (Figure 1a).The FTIR spectra of ZnFe2O4 with a yeast template have a peak around 420 cm -1 related to the stretching of Zn-O bond and around 501 cm -1 attributed to the Fe-O bond.The width peak around 3410 cm −1 could be attributed to the overlap of hydroxyl and amine  functional groups.The peak at 2926 cm −1 shows C-H asymmetric stretching and around 1491 cm −1 appointed to the amide group confirmed the presence of the protein in yeast cells 21 (Figure 1b).The FTIR spectra of nano-Al2O3 show a peak around 529 cm -1 related to the stretching of an Al-O bending vibration, which is a typical peak for nano-alumina, and around 1384 cm -1 there is the bending vibration of the hydroxyl group 33 (Figure 1c). Figure 1d shows the spectra of the CaO-ZnFe2O4 composite which has combined absorption peaks from CaO and ZnFe2O4, namely vibrations from Ca-O, Zn-O, and Fe-O.Furthermore, Figure 1e shows that the CaO-ZnFe2O4/Al2O3 nanocomposite spectra and indicate combined peaks from nano-CaO, ZnFe2O4, and nano-Al2O3.However, the nanocomposite does not show a sharp peak, possibly due to the groups being embedded in the pores of the alumina support.

XRD Analysis
XRD patterns and crystalline structures of nano-CaO, ZnFe2O4, nano-Al2O3, CaO-ZnFe2O4, and compared with CaO-ZnFe2O4/Al2O3 nanocomposites as shown in Figure 2. The characteristic peak of nano-CaO at 2θ: 17.98°, 28.56°, 34.13°, 47.17°, 50.82°, 54.22°, 64.14°, respectively.The results of the diffractogram analysis of CaO nanoparticles are in agreement with JCPDS No. 48-1467 22 (Figure 2a). Figure 2b shows the typical peaks of ZnFe2O4 at 2θ: 17.22°, 20.53°, 26.73°, 39.35°, 41.98°, 73.64°, respectively.These results are similar to previous research 21 .Figure 2c shows the typical peak of nano-Al2O3 at 2θ: 37.15°, 45.86°, 66.61°, respectively.These results are similar to previous research 19 .Figure 1d shows the diffraction pattern of the CaO-ZnFe2O4 composite which has combined peaks from CaO and ZnFe2O4.Furthermore, Figure 1e shows the diffraction pattern of CaO-ZnFe2O4/Al2O3 nanocomposite and indicates combined peaks from nano-CaO, ZnFe2O4, and nano-Al2O3.However, the nanocomposite does not show a sharp peak, possibly due to the groups being embedded in the pores of the alumina support, this is similar to the FTIR spectra.The average crystal size of CaO-ZnFe2O4/Al2O3 calculated using the Debye Scherrer equation 34 was obtained as 23.52 nm.

SEM Analysis
The surface morphology was investigated using SEM shown in Figure 3.The nano-CaO have irregular non-uniform and porous (Figure 3A), in accordance with previous research 32,35 .ZnFe2O4 hollow structure has granular irregular non-uniform, these results are similar to previous studies 21 (Figure 3B).The nano-Al2O3 has irregular non-uniform and shows pores (Figure 3C).Meanwhile, the CaO-ZnFe2O4 composite (Figure 3D)  shows that there are particles that have a granular structure arranged irregularly, indicating the presence of CaO and ZnFe2O4 for CaO-ZnFe2O4 composite, and shown in Figure 3E, the composite of CaO-ZnFe2O4 composite have combined with Al2O3 forming nanocomposite.Based on Figure 4, it can be identified that the elements that make up the CaO-ZnFe2O4/Al2O3 nanocomposite are O, Ca, Al, C, Fe, and Zn with a mass percentage of 4.2%, 22.9%, 17.6%, 8.9%, 3.7%, and 2.7%, respectively.The distribution of CaO-ZnFe2O4 on the Al2O3 surface was observed by elemental mapping (Figure 5). Figure 5A-G shows that the constituent elements are evenly distributed on the surface of CaO-ZnFe2O4/Al2O3, and CaO-ZnFe2O4 is also evenly distributed on the surface of Al2O3, which confirms the successful synthesis of the CaO-ZnFe2O4/Al2O3 nanocomposite.

BET Analysis
The N2 sorption isotherms of CaO-ZnFe2O4 and CaO-ZnFe2O4/Al2O3 (Figure 6) show typical type IV isotherms indicating the presence of mesopores for pores with diameters in the range of 2-50 nm 20 .Figure 6 displays the results of the BET surface area and pore volume for the CaO-ZnFe2O4 composite and CaO-ZnFe2O4/Al2O3 nanocomposite showing that CaO-ZnFe2O4/Al2O3 has a higher BET surface area (134.426m 2 g -1 ) and pore volume (0.204 cm 3 g −1 ) compared to CaO-ZnFe2O4 (15.314 m 2 g -1 ) and a pore volume of 0.022 cm 3 g −1 ).These surface area results indicate that the addition of alumina as a support for the CaO-ZnFe2O4 composite succeeded in increasing the surface area after it was formed into a CaO-ZnFe2O4/Al2O3 nanocomposite.

TEM Analysis
The results of TEM characterization are shown in Figure 7. Figure 7A-B shows the surface morphology of CaO-ZnFe2O4/Al2O3 with scales of 500 nm and 100 nm, respectively.The CaO-ZnFe2O4/Al2O3 nanocomposite shows that its constituent particle components have bonded one each other.The CaO-ZnFe2O4 composite with a non-uniform shape (the dark colors) attached to the Al2O3 surface (the bright colors) was observed clearly in Figure 7B. Figure 7C presents a high-resolution TEM image of CaO-ZnFe2O4/Al2O3 on a scale of 10 nm.

Catalytic Activity of CaO-ZnFe2O4/Al2O3
The catalytic activity for the conversion of waste cooking oil into biodiesel was investigated for 2 h at a temperature of 65 °C using a quantity of waste cooking oil of 5 mL, with catalyst amount of 2%, and the volume ratio of waste cooking oil: methanol of 1:9.

Effect of Al2O3: CaO-ZnFe2O4 Mass Ratio on Biodiesel Yield
The structure of alumina as catalyst support has a high surface area desired site active CaO-ZnFe2O4 to evenly spread on the pore surface, resulting in enhanced catalytic activity.Therefore, we investigated the influence of the mass ratio of Al2O3 to CaO-ZnFe2O4 in CaO-ZnFe2O4/Al2O3 nanocomposites (Figure 8).The biodiesel yield obtained using a catalyst of Al2O3/ CaO-ZnFe2O4 mass ratio 1:1; 1:2; and 1:3 is 80.43%, 93.41%, and 90.14%, respectively.This shows that at the 2:1 mass ratio, the entire surface of the Al2O3 pores is filled by the active site and is evenly distributed, resulting in efficient biodiesel production.Therefore, the Al2O3/ CaO-ZnFe2O4 with a mass ratio of 2:1 provides optimal conditions for this transesterification reaction.6) and acid-base properties of the CaO/ZnFe2O4 to the synergistic effect between CaO-ZnFe2O4 and Al2O3.

Physicochemical Properties of Biodiesel
The results of biodiesel with the best and optimal have been done to test some physicochemical properties to be a suitable substitute for fossil diesel 36,37 .Some properties of biodiesel tested in this study are acid number, density, kinematic viscosity, flash point, and cetane number.Table 1 displays that these properties are close to the required international standards as specified by the American Society for Testing Materials (ASTM) 6751 reference standard.

CONCLUSIONS
In this study, hybrid CaO/ZnFe2O4 modified with Al2O3 as a green novel catalyst for biodiesel production from WCO with high catalytic activity.The catalyst properties were investigated by FTIR, XRD, SEM, EDX, SEM-mapping, BET, and TEM analyses.The catalytic activity increased with the combination of nanoparticles effect and support catalysts obtained biodiesel yield of nano-Al2O3, nano-CaO, ZnFe2O4, CaO-ZnFe2O4, and CaO-ZnFe2O4/Al2O3 is 36.86%,67.16%, 74.83%, 86.54%, and 93.41%, respectively.The best biodiesel yield was 93.41% with a mass ratio of Al2O3 to CaO-ZnFe2O4 (2:1).The physicochemical properties (acid number, density, kinematic viscosity, flash point, and cetane number) of the produced biodiesel were within the ASTM limits, demonstrating a promising replacement with diesel.

Figure 9 . 9 .
Figure 9.Effect of type catalyst on percent yield3.3.2Effect of Catalyst Type on Biodiesel YieldSubsequently, we investigated the effect of catalyst type on the biodiesel yield as shown in Figure9.The usage of the nano-Al2O3 support catalyst achieved a biodiesel yield of 36.86%,nano-CaO catalyst of 67.16%, ZnFe2O4 of 74.83%, CaO-ZnFe2O4 of 86.54% and CaO-ZnFe2O4/Al2O3 nanocomposite of 93.41%.The biodiesel yield uses the CaO-ZnFe2O4 catalyst increases compared to the nano-CaO and ZnFe2O4.This is due to CaO-ZnFe2O4 having both acidic and basic properties, which are the base site (CaO) and acid site (ZnFe2O4).The acid-base properties of the CaO-ZnFe2O4 hollow structure could accelerate the transesterification and esterification reaction21 .Hence, CaO-ZnFe2O4/Al2O3 catalyst has the highest biodiesel yield.These indicate that the supported catalyst of the Al2O3 in nanocomposite can increase biodiesel yield due to the Al2O3 having a high surface area compared to CaO-

Table 1 .
Physical properties of biodiesel obtained in this work.