Synthesis of nano-CaO catalyst with SiO2 matrix based on palm shell ash as catalyst support for one cycle developed in the palm biodiesel process

Available online 5 April 2023

2666-0164/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Case Report

Synthesis of nano-CaO catalyst with SiO ₂ matrix based on palm shell ash as catalyst support for one cycle developed in the palm biodiesel process

R. Manurung

^*

, S.Z.D.M. Parinduri, R. Hasibuan, B.H. Tarigan, A.G.A. Siregar

Department of Chemical Engineering, Faculty of Engineering, Universitas Sumatera Utara, Medan, 20155, Indonesia

A R T I C L E I N F O Keywords:

Catalyst Nano-CaO Palm shell ash Biodiesel Support

A B S T R A C T

In this research, waste chicken eggshells and palm oil mill boiler ash were utilized as heterogeneous solid base catalysts in biodiesel synthesis. The utilization of solid waste as a catalyst source can increase economic value and is an environmentally friendly process. This research aimed to investigate the effectiveness of catalyst for biodiesel synthesis using palm oil. Eggshell-nano-CaO was impregnated with various palm ash concentrations (10–50 wt%) and various calcination temperatures (600^◦C-900 ^◦C), followed by drying and calcination. Biodiesel synthesis using palm oil was conducted at 60 ^◦C, 15:1 methanol ratio, and 500 rpm. It was also conducted with various catalyst concentrations (2–5 wt%) and reaction times (60–180 min). The average particle size of egg- shell-nano-CaO is 24.98 nm. At 800 ^◦C, the calcination temperature has the highest surface area of 21.483 m²/g with an average pore size of 5.331 nm and a pore volume of 0.057 cc/g. In addition, catalyst has high basicity, which is 8 mmol/g. The maximum yield of 97% methyl ester content was obtained with 20 wt% palm ash concentration and 800 ᵒC calcination temperature. Heterogeneous catalysts derived from waste chicken eggshells and palm shell ash have high potential as solid-base catalysts in biodiesel synthesis.

1. Introduction

Increased human population growth and industrial growth will also increase energy consumption (Reshad et al., 2013). states that high energy consumption causes restrictions on fossil fuels, necessitating the use of alternative energy to meet energy consumption [1,2]. Biodiesel is an alternative fuel that can be used as a substitute for fossil fuels (Ong et al., 2014). One widely used biodiesel production technology is the use of heterogeneous catalysts. Heterogeneous catalysts have several ad- vantages, including high catalyst activity in the transesterification re- action, being more stable, and being more environmentally friendly [3].

Some examples of heterogeneous catalysts in transesterification re- actions include TiO₂, Zn, Na₂SO₃, ZrO₂/Al₂O₃, CaO, MgO, Zeolite, and others [4,5]. Catalyst synthesis has an important role because it can determine the characteristics of the catalyst produced. To produce a catalyst with high reactivity for the transesterification reaction, several stages of heterogeneous catalyst synthesis are carried out. The stages of catalyst synthesis depend on the catalyst’s raw material and the type of catalyst compound to be used. One of catalyst synthesis processes is impregnation process (Zul et al., 2021).

CaO is a basic heterogeneous catalyst that has higher catalyst

reactivity than heterogeneous acid catalysts. However, CaO has poor stability in the transesterification reaction, resulting in the release of Ca²⁺ions. This can be overcome by carrying out a wet impregnation process of CaO catalyst support to inhibit the release of Ca²⁺ions in the reaction [6–8]. One of the catalyst support components that is often used and has great potential because it has a large surface area, a large pore volume, and good thermal stability is silica (Pandiangan et al., 2015 [7];

Lani et al., 2019). Several studies on the synthesis of CaO/SiO2 catalysts using various catalyst materials, including egg shells and rice bran treated with used cooking oil, have been reported. Lani et al. (2019) reported that methanol: oil ratio of 20:1, a temperature of 60C, and a time of 2 hours produced a biodiesel yield of 84% with a catalyst surface area of 9.47 and 11.57 m2/g. In addition, Putra et al. [6] studied CaO/SiO2 catalysts from eggshells and clay with a biodiesel yield of 91%. As well as Chen et al. [4], we have also studied the CaO/SiO2

catalyst from egg shells and Na2SiO3, which produces a biodiesel yield of 90.2%.

In this study, researchers will examine the synthesis of nano-CaO catalysts from eggshells impregnated with SiO2 material from oil palm shell ash. Researchers emphasize the use of catalyst raw materials that are sourced from waste and are environmentally friendly. This is useful

* Corresponding author.

E-mail address: renita.manurung@usu.ac.id (R. Manurung).

Contents lists available at ScienceDirect

Case Studies in Chemical and Environmental Engineering

journal homepage: www.sciencedirect.com/journal/case-studies-in-chemical- and-environmental-engineering

https://doi.org/10.1016/j.cscee.2023.100345

Received 19 February 2023; Received in revised form 23 March 2023; Accepted 25 March 2023

Case Studies in Chemical and Environmental Engineering 7 (2023) 100345

in lowering production costs, increasing the value of goods, and addressing the problem of waste in the environment. One of the efforts to make the palm biodiesel production process in one cycle with the catalyst production used is the use of palm kernel shell ash obtained from a palm oil mill. In this study, egg shells were synthesized using the C–H-D method, which produced CaO with a small particle size (nano- CaO). Ozor et all. (2023) reported that nano-CaO catalyst using sol-gel method can produce a particle size of 48 nm and after being modified with silica, catalyst can produce yield of 97.8% at ratio methanol:oil of 9:1, 3 hours, and 80 ^◦C. As a result, one of the efforts made in utilizing solid waste as a heterogeneous catalyst source is the wet impregnation process of nano-CaO compounds produced from the C–H-D and SiO2

methods sourced from palm shell ash to produce higher catalytic activity and lower production costs for biodiesel production. This research is aimed at obtaining nano-CaO impregnated with palm ash as a catalysts with the best characteristics that can be applied in the transesterification process of palm oil to produce biodiesel.

2. Material and methods 2.1. Material

Palm oil was obtained from a local market in Medan, Indonesia. The raw material characterization of palm oil is shown in Table 1. In general, palm oil contains approximately 50% saturated fatty acids, 40%

monounsaturated fatty acids, and 10% polyunsaturated fatty acids.

Fatty acid composition of palm oil was shown on Table 2. For catalyst production, waste chicken egg shells were collected from local restau- rants in Medan City, Indonesia, while palm kernel shell ash was obtained from a palm oil mill in Deli Serdang District, Sumatera Utara, Indonesia.

Methanol and citric acid were purchased from Sigma Aldrich and used as received.

2.2. Catalyst preparation and characterization

Chicken eggshell waste was collected and washed with distilled water until clean. Then, it was dried at 60 ^◦C in the oven. Eggshells were manually ground with a mortar and sieved through a 500-m micro-sieve.

The powder was then washed again with hot distilled water to remove organic and inorganic compounds from the surface. Then, it was dried in an oven at 120 ^◦C for 24 hours. Then it was ground and crushed with blender and sieved through a 100-mesh sieve to produce a homogeneous

particle size. It was calcined in furnace at 900 ^◦C for 3 h. It would convert CaCO3 to CaO. After that, eggshell-CaO was hydrolyzed by soft water with 60 ^◦C temperature reaction for 6 hours. To separate the mixture from the liquid, it was filtered through filter paper. The filtrate was dried at 120 ^◦C for 24 hours and calcined at 870 ^◦C for 3 hours to produce CaO become nano-CaO [9]. The synthesis nano-CaO from waste eggshell were denoted as eggshell-nano-CaO.

Fig. 1 shows leaching diagram process of palm shell ash using citric acid. Citric acid used to remove impurities and produce silica material from palm shell ash. It was dissolved in distilled water with a concen- tration of 2%. 10–30 g of palm shell ash were mixed in 500 ml of citric acid solution. Then the mixture was stirred with a magnetic stirrer for 60 minutes at 70 ^◦C. Then the mixture was rinsed with distilled water to remove citric acid and then filtered. Palm ash was dried in an oven for 60 minutes at 60 ^◦C, then calcined at 700 ^◦C for 3 hours (Pa et al., 2010).

Palm ash as matrix catalyst, was prepared by wet impregnation.

Impregnation process with the following procedure: In 100 mL of distilled water, 5 g of CaO are dissolved. Then, palm ash was added to the mixture with varied weight ratio of 10–50 wt% CaO. The mixture 700 ^◦C for 3 hours e was then mixed for 4 hours in an 80 ^◦C reflux condenser circuit. Then the mixture was filtered and dried in the oven at 150 ^◦C for 2 hours. Then, it was calcined at 600–900 ^◦C for 4 hours. The resulting of impregnation process were denoted as catalyst.

Eggshell-nano-CaO, palm ash, and catalyst were characterized through the PSA (Particle Analyzer) for particle distribution size and the FESEM (Field Emission Scanning Electron Microscope) for catalyst morphology.

XRD and FTIR were used to study functional group analysis. The scan- ning electron microscope with energy dispersive X-ray (JEOL JSM- 6510LA) was used to study the morphology of the catalyst and deter- mine the compound content of the catalyst. The elemental and com- pound content of the palm kernel shell ash extracted sample was determined using X-ray fluorescence. BET-BJH for pore catalyst char- acterization used Quantachrome TouchWin v1.22. And quantitative Table 1

Raw material characterization of palm oil.

Properties Unit Value

Free Fatty Acid % 1.324

Moisture Content % 0.15

Specific gravity % 0.925

Acid value % 0.8

Saponification value mgKOH 210

Table 2

Fatty acid composition of palm oil.

Fatty acid Unit Value

Lauric acid (C12:0) % 0.177

Myristic acid (C14:0) % 0.798

Palmitic acid (C16:0) % 36.451

Palmitoleic acid (C16:1) % 0.169

Stearic acid (C18:0) % 3.897

Oleic acid (C18:1) % 45.601

Linoleic acid (C18:2) % 12.183

Linolenic acid (C18:3) % 0.215

Arachidic acid (C20:0) % 0.355

Eicosenoic acid (C20:1) % 0.154

Fig. 1. Palm shell ash leaching process.

R. Manurung et al.

basicity is done by the titration method using Hammet indicators, which are phenolphthalein and bromomethyl blue. The quantitative determi- nation of basicity was done by titration with benzoic acid [10]. The dispersions of solid (0.15 g) in a methanol solution of Hammett in- dicators (2 mL, 0.1 mg/mL) were stirred for 25 min and then titrated with a methanol solution of benzoic acid (0.01 M) to determine the total basicity [11].

2.3. Transesterification reaction

Transesterification was carried out in a 25-mL three-neck round- bottom flask connected with a reflux condenser, a magnetic stirrer (500 rpm), and a hot plate. Heterogeneous catalyst commonly use large of methanol ratio. Lani et all (2019) report 15:1 is the best methanol ratio used. So, methanol molar ratio 15:1 was used in this study and was added with catalyst amounts that varied in the range of 2–3.5 wt%. The mixture was stirred continuously for 2 hours to establish the methoxide compound. The flash was then filled with 25 mL of palm oil and heated to 60ᵒC while a methanol-catalyst mixture was added in parallel. The transesterification reaction was conducted for a period ranging from 60 to 150 minutes. After the reaction, the mixture was allowed to cool to room temperature. The catalyst was separated from the mixture by a vacuum filter. The liquid mixture was then placed in a separator funnel for 24 hours to separate the biodiesel, glycerol, and remaining reactants into two layers. The bottom layer contained methanol and glycerol, while the top layer contained biodiesel products. Biodiesel was washed with hot water (60 ^◦C) until the washing water was clean. Biodiesel product was heated at 100 ^◦C to remove any remaining water. The dried biodiesel was weighed and analyzed. The percentage yield of biodiesel

was determined according to equation (1).

Yield biodiesel(%) =weight of biodiesel(g)

weight of palm oil(g)×methyl ester content(%) (1)

2.4. Properties of biodiesel

A biodiesel product was analyzed for the content of methyl ester using the Shimadzu-2010 gas chromatography. Biodiesel was evaluated according to the European Standard Method (EN14214) for fuels derived from vegetable oils [12]. A density meter (Anton Paar DMA 35 Basic) was used to measure the density of biodiesel, while the viscosity was measured using a rotational viscometer (Cole-Parmer EW-98965-40). The acid-base titration method using phenolphthalein was used to determine the acid value. The fatty acid composition was determined using gas chromatography (GC-2010, Shimadzu).

3. Result and discussion 3.1. Characterization of catalyst

3.1.1. Eggshell-nano-CaO Particle Size Analyzer

Eggshell-_nano-CaO (which was produced from raw egg shells) was characterized by a PSA (Particle Size Analyzer) to determine the particle size and particle size distribution of nano-CaO. The average size of Eggshell-_nano-CaO particles is 24.98 nm. Comparison Eggshell-_nano-CaO and Eggshell-CaO can be seen on Fig. 2. The Eggshell -CaO particle size distribution graph shows that the particle diameter is in the range of 3–11 μm. The highest diameter of Eggshell -CaO particles is in the range Fig. 2. Morphology and particle distribution of (a) CaO (b) Nano-CaO.

Case Studies in Chemical and Environmental Engineering 7 (2023) 100345

of 7–8 μm. The particle diameter distribution of Eggshell-nano-CaO is in the range of 50–250 nm. In the graph, it can be seen that the highest distribution of particle diameter sizes in Eggshell-nano-CaO is in the size range of 50–100 nm. The C–H-D process is known to reduce the particle size of CaO due to hydration and calcination processes. Calcination temperatures above 800 ^◦C can change CaCO3 compounds (which are contained in chicken eggshell powder) into CaO compounds and carbon dioxide compounds [9]. In addition, the C–H-D process can increase the surface area and alkalinity of the catalyst. This is due to the CO2 release from carbonate compounds during the calcination process, which can increase the surface area, as well as the release of H2O compounds from the surface during the hydration process, which can break the crystal structure along with the formation of additional base sites on the cata- lyst. Yoosuk et al. (2010) have reported and proven this. The reactions that occur in the C–H-D process can be seen as follows: (Yoosuk et al., 2010; Gupta and Virendra 2018).

Chicken eggshell (CaCO3) → CaO +CO2

CaO +H2O → Ca(OH)2

Ca(OH)₂ → CaO +H₂O During hydrolysis, the compound CaO is converted to Ca(OH)2. This hydrolysis process will affect the crystallinity of the resulting catalyst (Yoosuk et al., 2010). The hydrolysis process, which use water, can reduce the crystal size and crystallinity of CaO, resulting in smaller particle sizes (Yoosuk et al., 2010). The release of carbon dioxide can lead to the formation of nano-CaO, which is calcined at 900 ^◦C for 3 hours [13].

3.1.2. Silica compound-X-ray fluorescence

The results of leached palm ash using citric acid were tested for analysis of the compound content using XRF (X-Ray Fluorescence). The silica content and other compounds before and after extraction can be seen in Table 3. The table shows that the silica content of palm shell ash is 95.9%. Meanwhile, some literature states that the silica content in palm shell ash before the leaching process is 39–46% (Utama et al., 2018; [14]. High silica contents in palm shell ash prove that palm shell ash has the potential to utilize silica from palm oil waste originating from boiler combustion.

3.1.3. Functional group-fourier transform infrared spectroscopy

The results of functional group analysis on eggshell-nano-CaO, palm ash, and catalyst using FTIR can be seen in Fig. 3 (a). The absorption peak at 570 cm⁻¹ identifies the CaO group on the eggshell-_nano-CaO graph, and the weak absorption peaks at 1423 cm⁻¹ and 858 cm⁻¹ identify the asymmetric bond of the CO32− group. In addition, the peak of 3640 cm⁻¹ identifies the asymmetry of the hydroxyl group (O–H). CaO has high active surface ability to absorb atmospheric air and CO2, water and CO2 can be absorbed on the surface, while CaO would converted into Ca(OH)₂ and CaCO₃. This was also reported by Pandit and Fulekar, as well as Gupta and Virendra. The peak at 2885 cm⁻¹ identified the C–H bond. The peaks of 1094 cm⁻¹ and 1056 cm⁻¹ are characteristic of the peaks in alcohols with C–O bonds.

The analysis results of palm ash functional groups can be seen to have a strong absorption peak at 788 cm⁻¹ which identifies the Si–O–Si group. In addition, the peak of 1032 cm⁻¹ also identifies the Si–O–Si group. The peaks of 1465 cm⁻¹ and 1678 cm⁻¹ identified Si––O groups and O–H hydroxyl groups. This hydroxyl group is due to the absorption of H2O on the silica surface [5,15].

Catalyst, after modified with palm ash, functional groups were seen.

The absorption peak of 3641 cm⁻¹ identified the absorption of O–H groups, and the absorption peak of 1451 cm⁻¹ identified the C––O symmetry stretch in CO₃²⁻; this was obtained from the nano-CaO spec- trum (Lani et al., 2019). By comparing the three spectra, there is an absorption peak of 1157 cm⁻¹ which identified the stretching vibration of the Si–O–C bond in the Ca2SiO4 compound. In addition, the vibration peak at 965 cm⁻¹ can identify Si–O–Ca bonds caused by the impreg- nation process of eggshell-nano-CaO with palm ash [4,16]. Meanwhile, there are Si–O–Si bonds at the absorption peak of 1054 cm⁻¹, which are obtained from unbounded silica during the impregnation process. On the other hand, the absorption peaks of 2885 cm⁻¹, 1678 cm⁻¹, and 885 cm⁻¹ identify carbonate ion strains (Lani et al., 2019).

Table 3

Element and compound content of oil palm shell ash.

Element

(%) Before

leaching After

leaching Compound

(%) Before

leaching After leaching

Al 0.63 0.357 Al2O3 1.19 0.35

Si 18.21 85.799 SiO2 39.02 95.9

P 2.25 1.31 P2O5 5.16 1.57

S 0.36 1.126 SO3 0.9 1.47

Ca 6.71 0.887 CaO 9.39 0.65

Rb 0.03 0.066 Rb2O 0.03 0.04

Sr 0.36 0.022 SrO 0.04 0.01

Zr - 0.018 ZrO2 - 0.01

Fig. 3. FTIR spectra of (a) Nano-CaO, SiO2, and nano-CaO/SiO2 (B) nano-CaO/SiO2 on variation of calcination temperature.

R. Manurung et al.

Fig. 3(b) shows the results of the FTIR analysis with various calci- nation temperature. At the absorption peak of 3645 cm⁻¹ which is shown in the four spectra of the variation. The –OH hydroxyl group is identified by the absorption peak caused by the formation of Ca(OH)2 as a result of atmospheric air absorption on the surface of CaO. The C––O group (symmetrical stretch) peak on CO32− is also visible at the absorp- tion peaks of 2890 cm⁻¹ and 880 cm⁻¹. Meanwhile, the absorption peak of the C––O asymmetric stretching group in CO32− is 1430 cm⁻¹ (Lani et al., 2019). The graph shows that the absorption peaks of the asym- metric C––O stretch groups in CO32- are stronger at 600 and 700^◦ Celsius than at 800 and 900^◦Celsius. High calcination temperature can be converted CaCO3 into CaO, reducing the content of the asymmetric stretching group C––O in CO₃²⁻ on the catalyst. From the four spectra, you can see the Si–O–Ca bond at the absorption peak of 966 cm⁻¹ [4, 16]. This proves that there is a reaction between nano-CaO and SiO2 in the impregnation process. In addition, the absorption peak of 1134 cm⁻¹ was seen at a calcination temperature of 800 ^◦C, which identified the Si–O–C bond in the Ca2SiO4 compound. This proven that at a calcination temperature of 800 ^◦C, the absorption peaks are higher at the peaks of the Si–O–C and Si–O–Ca bonds, and the absorption peaks are lower at the asymmetry peaks of C––O in CO32−.

3.1.4. Morphology catalyst-surface emission spectroscopy

Surface morphology analysis was carried out using FESEM for egg- shell-nano-CaO and SEM for eggshell-CaO, palm ash, and catalyst. FESEM and SEM results can be seen in Fig. 2. In Fig. 4 (a), the surface morphology of eggshell-CaO with a magnification of 10,000X has porous particles with the oval, spherical, and irregular structures. In Fig. 4 (b), the surface morphology of eggshell-nano-CaO with a magnification of 250,000X has particle shape that is not much different from eggshell-CaO. The surface morphology of eggshell-nano-CaO is similar as reported by Niju et al. and Kirubakaran & Arul. In addition, the particles appear stacked. This is due to the hygroscopic nature of the CaO compound, which easily absorbs water vapor from the air during the sample de- livery process for analysis, causing agglomeration in the CaO sample, which appears to be piled up in the test results. Several researchers have also reported the same thing, including Ozor et al. [8], Niju et al. [17], and Kirubakaran and Arul [18]. Fig. 4 (c) shows the surface morphology of SiO2 at 1000×magnification with round and oval porous particles.

Fig. 4 (d) shows the surface morphology of catalyst at 1500×magnifi- cation with round and oval porous particles (see Fig. 5).

The catalyst with varying amounts of palm ash was also tested for the compound content using SEM-EDX, which can be seen in Table 4. In the table, it can be seen that the content of SiO2 compounds tends to increase with an increase in the amount of palm ash. Meanwhile, CaO compounds tend to decrease with increasing amounts of palm ash. The decrease in the content of CaO compounds is in line with the decrease in the total alkalinity of the catalyst with the addition of silica. This is because CaO is alkaline, so the addition of silica will reduce the content of CaO compounds and the alkalinity of the catalyst (Lani et al., 2019). Lani et al. (2019) also reported from the results of the XRD analysis that with the addition of silica, more intense peaks were seen, with decreasing CaO. Therefore, the CaO content of the catalyst decreases with the addition of palm ash, as evidenced by the lower alkalinity of the catalyst.

3.1.5. Pore characterization-nitrogen adsorption-desorption

Table 5 shows the results of the surface area, pore volume, and Fig. 4. Surface Morphology (a) eggshell -CaO (b) eggshell-nano-CaO (c) palm ash (d) catalyst.

Table 4

Catalyst Compounds with Variations in the Amount of palm ash.

Compound 10% 20% 30% 40% 50%

C 2.81 1.73 6.24 4.15 5.06

MgO 1.49 1.02 0.74 0.85 0.82

SiO2 11.81 39.84 48.16 45.14 52.54

CaO 80.54 53.35 42.06 45.97 38.66

CuO 1.71 1.58 1.8 2.38 1.61

ZnO 0 1.01 1 1.5 1.31

ZrO2 1.64 1.47 0 0 0

Table 5

Surface area and pore volume of catalyst.

Characterization Calcination Temperature

600 ^◦C 700 ^◦C 800 ^◦C 900 ^◦C surface area (m²/g) 14.411 20.627 21.483 16.032

pore volume (cc/g) 0.043 0.053 0.057 0.043

average pore size (nm) 5.961 5.149 5.331 5.326

Case Studies in Chemical and Environmental Engineering 7 (2023) 100345

average pore size of the catalyst with 20 wt% palm ash at various calcination temperatures 600 ^◦C, 700 ^◦C, 800 ^◦C, and 900 ^◦C. The sur- face area of the catalyst increased from 14.411 m²/g to 21.032 m²/g as the calcination temperature increased from 600 ^◦C to 800 ^◦C. However, at calcination temperature of 900 ^◦C, something unexpected happened, and the surface area decreased by 16.032 m²/g. Ho et al. [19] have also reported the same thing. This decrease can occur because, at high temperatures, the atoms in small particles can diffuse and join together to form larger particles [5]. High calcination temperatures can reduce the morphological size of the catalyst and increase its surface area [19].

The average pore size will be smaller at high calcination temperatures, resulting in a large surface area. Meanwhile, the pore volume will in- crease with a high calcination temperature (comparable to the outside surface). At a calcination temperature of 800 ^◦C, it has the highest surface area of 21.483 m²/g, an average pore size of 5.331 nm, and a volume of 0.057 cc/g. This proves that the calcination temperature can affect the formation of pores in the catalyst. Porous materials are well suited for catalytic activity as well as increased adsorption and energy storage. In addition, it can provide more catalyst-active sites on the surface for high-reaction conversion [13].

The porosity of the catalyst was also analyzed using adsorption and desorption with the N2 method. The graph of nitrogen adsorption- desorption isotherms on catalyst with various calcination tempera- tures is shown in Fig. 6. The mesoporous structure is indicated by the presence of hysteresis loops on the isotherm curve, similar to the type II

isotherm according to the International Union of Pure and Applied Chemistry (IUPAC) classification. Type II isotherms are S-shaped or sigmoid curves and are generally found in materials with larger pores than micropores. The graph shows that the hysteresis loops are closely spaced and the adsorption and desorption are almost parallel. It shows pores with a regular geometry. This is similar to what has been reported by Yoosuk et al. (2010). The isotherm graph shows capillary conden- sation at the initial stage (at a relative pressure of 0.5), with nitrogen condensation occurring in the internal and second mesopores at a higher partial pressure (P/Po =0.8), which is associated with the presence of porosity between the particles. This shows the ability or absorption to enter triglycerides into the pores of the catalyst in the transesterification reaction [20].

3.1.6. Compound group-X-ray Fluoresces

Fig. 7 shows the results of the XRD analysis on eggshell-_nano-CaO, palm ash, and catalyst. In the eggshell-nano-CaO sample, it shows that the peak characterization of CaO has a large content at 2θ =32.24^◦, 37.4^◦, and 53.88^◦[19]. Due to CaO exposure to air, the Ca(OH)2 peak can be seen at 2θ =18.06^◦, 28.64^◦, 34.06^◦, 47.14^◦, and 50.8^◦. The same peak has also been reported by Lani et al. The palm ash sample shows a peak at 2θ

=20.62^◦, 26.44^◦, and 35.94^◦, which is the SiO2 peak, while the peak at 2θ =21.84^◦, 31.32^◦, 35.94^◦, and 45^◦is the SiO2.xH2O peak. This can be caused by the less-than-optimal calcination process, which leaves water compounds that bind to SiO2. Meanwhile, in the catalyst samples, peaks Fig. 5. Nano-CaO/SiO2 catalyst with variation in amount of silica (wt%) (a) 10; (b) 20; (c) 30; (d) 40; (e) 50.

R. Manurung et al.

were seen at 2θ =25.84^◦and 45.66^◦, which represented the formation of Ca2SiO4. This occurred due to the reaction of CaO with silica in the impregnation process [16]. This was also proven in functional group analysis with FTIR. The CaO peak was seen to decrease in the catalyst sample compared to the eggshell-nano-CaO sample at 2θ =32.24^◦and

37.44^◦. The Ca(OH)₂ peak has the following coordinates: 2θ =17.94^◦, 28.58^◦, 34.14^◦, 47.24^◦, 50.76^◦, and 53.94^◦.

3.1.7. Basicity catalyst-hammet indicator titration and yield biodiesel correlation

Table 6 shows the alkalinity of the catalyst at various palm ash concentrations (10–50 wt%) and various calcination temperature with the correlation of yield biodiesel. Analysis of this basicity by titration using the Hammett indicator. Titration using the Hammett indicator provides qualitative information on the alkaline nature of the solid Fig. 6.N2 adsorption/desorption graph at calcination variation temperature (a) 600 ^◦C, (b) 700 ^◦C, (c) 800 ^◦C and (d) 900 ^◦C.

Fig. 7. XRD Pattern of (a) eggshell-nano-CaO, palm ash and catalyst.

Table 6

Catalyst basicity on palm ash content variation and calcination temperature variation with yield biodiesel correlation.

Sample Total Basicity (mmol/

g) Yield Biodiesel

(%)

eggshell-nano-CaO 2.4 53,0

palm ash 0.6 –

palm ash variation (wt%) 10 3.2 51,09

20 8.0 71,77

30 2.4 62,93

40 2.0 64,87

50 1.5 63,95

calcination temp. variation

(ᵒC) 600 3.7 42,90

700 3.9 62,80

800 8.0 71,77

900 2.8 68,06

Case Studies in Chemical and Environmental Engineering 7 (2023) 100345 catalyst. The Hammett indicators used were bromothymol blue (pKa =

7.2) and phenolphthalein (pKa =9.8). Depending on the amount of silica, both of these indicators can change color when mixed with all of the catalyst samples at different temperatures. The addition of bromo- thymol blue to the five samples can change the color from yellow to green. This is similar to the addition of phenolphthalein to the five samples, which can change the color from colorless to red. The nano- CaO sample can also change the color of the two Hammet indicators, but the palm ash sample can only change the bromothymol blue indi- cator. The color change in the Hammet indicator indicates a more alkaline than the Hammet indicator (as can be seen from the pKa value) [10].

In the table, it can be seen that the total basicity of eggshell-nano-CaO is 2.4 mmol/g and palm ash is 0.6 mmol/g, which is lower than the total basicity of catalyst after modified with palm ash. The highest total alkalinity of the catalyst with 20 wt% concentration palm ash which was 8 mmol/g. With an increase in palm ash concentration from 30 wt% to 50 wt%, total basicity decreased from 8 mmol/g to 1.5 mmol/g. This reduction in total basicity is caused by the active side of the nano-CaO surface being coated with Si compounds, as well as the addition of palm ash concentration [4]. This is also evidenced that the higher total basicity of eggshell-nano-CaO (2.4 mmol/g) compared to catalyst, which has 40 wt% and 50 wt% palm ash concentration (2.0 mmol/g and1.5 mmol/g). Palm ash which content with SiO2 has the lowest basicity due to its nature as a catalyst support [12].

The addition of high concentrations of palm ash can reduce the alkalinity of the catalyst. The basicity level of the catalyst correlates with the catalytic activity of the catalyst because a catalyst with a low base strength will result in a decrease in the catalytic activity of the catalyst [5]. In addition, Lani et al. (2019) also reported that increasing the amount of silica would increase the surface area to 11.54 m²/g, but adding high silica would decrease the surface area to 3.14 m²/g.

Therefore, it can be proven that the eggshell-nano-CaO and palm ash impregnation process can increase the catalytic activity, as seen from the total alkalinity of the catalyst, which is higher than that of egg- shell-nano-CaO and palm ash. But high addition palm ash (that is too high;

up to the optimum point, 20 wt%) would decrease catalytic activity, as seen from the decrease in total alkalinity.

The catalyst with 20 wt% palm ash produced the highest biodiesel yield of 71.76% with high total alkalinity. The higher the amount of palm ash, would produce lower the total alkalinity of the catalyst, so the biodiesel yield tends to decrease by up to 63.95%. Yield biodiesel had same pattern with the total alkalinity of the catalyst. The decrease in the alkalinity of the catalyst, which reduces the biodiesel yield produced by adding the amount of palm ash, is also the same as what has been re- ported by Abdullah et al. [20], where the addition of too small and too large amounts of K2CO3 and CuO at impregnation process will produce low biodiesel yields.

Table 6 shows the total basicity of the catalyst with calcination temperature of 600–900 ᵒC. The two Hammet indicators used, changed color when mixed with all catalyst samples at various calcination tem- peratures. At 800 ^◦C, the calcination temperature has the highest total basicity of 8 mmol/g. However, at 900 ^◦C, it has low total basicity as well as low yield biodiesel. This reduction is comparable to the results of the catalyst surface area and peak groups in the previous discussion. The high calcination temperature will reduce the surface area and total ba- sicity, which can be seen in the low absorption peak groups on Si–O–C in Ca2SiO4 compounds, and will produce low yields. Therefore, the surface area of the catalyst and its total basicity are important factors in determining the catalytic activity of a catalyst.

3.2. Effect of biodiesel yield results on variation of catalyst amount and reaction time

In the previous discussion, catalyst with 20 wt% concentration palm ash produced high biodiesel yield due to its high catalytic activity.

Therefore, this biodiesel application uses catalyst under these condi- tions. Fig. 8 shows the effect of reaction time and the amount of catalyst on the yield biodiesel. The graph shows the yield of biodiesel with several catalyst concentrations of 2 wt% to 3.5 wt% and reaction time of 60 minutes–150 minutes. The fixed variables are the methanol-to-oil ratio (15:1), the reaction temperature of 60 ^◦C, and the stirring speed of 500 rpm. With a 2 wt% catalyst and a reaction time of 60–150 mi- nutes, the yield of biodiesel increases from 54.7% to 72.8%. This increasing yield is because, with a small amount of catalyst, it takes a long time to produce high biodiesel yields. Generally, reactions using heterogeneous catalysts are slow at the initial stage, and longer reaction times are required to obtain profitable products because the reaction mixture is a three-phase system (Lani et al., 2019). Therefore, at a 2 wt%

catalyst amount with 150 minutes reaction time is required to produce the highest yield of biodiesel.

The catalyst amount of 2.5 wt% produces an increasing yield bio- diesel with increased reaction time of up to 120 minutes. The increase in biodiesel yield occurred from 70.1% (60 minutes reaction time) to 72.4% (120 minutes reaction time). At 150 minutes of reaction time, the biodiesel yield decreased to 70.5%. This is because the trans- esterification reaction is reversible; a long reaction time (the reaction has reached equilibrium) will shift the equilibrium towards the product, which will convert the methyl ester back into fatty acids and form soap.

Ho et al. [19] and Yoosuk et al. (2016) have reported the same.

Therefore, at a catalyst amount of 2.5 wt% and a reaction time of 120 minutes, high-yield biodiesel is produced.

At reaction times ranging from 60 to 150 minutes, catalyst concen- trations of 3 wt% and 3.5% show the same trend in biodiesel yield. The trend of decreasing biodiesel yield occurred from 74.4% to 72.3% (3 wt

% catalyst amount) and from 72.2% to 47.3% (3.5 wt% catalyst amount) with increasing reaction time. This could be because a larger amount of catalyst can lead to a thicker reaction mixture, which causes increased mass transfer resistance between the reactants and the catalyst and subsequently results in a decreased yield of biodiesel (Lani et al., 2019).

This is also proven by the lower biodiesel yield produced at a 3.5 wt%

catalyst amount compared to 3 wt% catalyst amount at the same reac- tion time. At the high catalyst amounts of 3 wt% and 3.5 wt%, it takes 60 minutes to produce a high biodiesel yield. Therefore reaction time and amount of catalyst are influence to produce high biodiesel yields by considering the reversible transesterification reaction and mass transfer resistance between the reactants and catalyst. The highest biodiesel yield was 74.4% with the addition of 3 wt% catalyst and a reaction time of 60 minutes.

The biodiesel yield was fluctuate with adding 0.5 wt% catalyst and tends to be random. It was due to CaO has high active surface ability to absorb atmospheric air and CO2, water and CO2 can be absorbed on the surface, while CaO would converted into Ca(OH)₂ and CaCO_3. And its affected to catalytic activity.

Fig. 8.Effect of reaction time and amount of catalyst on biodiesel yield.

R. Manurung et al.

3.3. Physicochemical characteristics of biodiesel

The physicochemical properties of palm biodiesel under operating conditions of 3 wt% catalyst amount and 60 minutes of reaction time were analyzed by the EN14214 standard method for biofuels shown in Table 7. Under these operating conditions, biodiesel was produced with the highest yield of 74.4% and 97.47% methyl ester content. The vis- cosity and density of the biodiesel produced were 867.49 kg/m3 and 4.41 cSt, respectively. All properties are within the range according to the EN14214 standard. Palm biodiesel with a solid base catalyst, nano- CaO/SiO2, can be categorized as biodiesel fuel.

4. Conclusion

Heterogeneous catalysts derived from waste chicken eggshells and palm oil mill boiler ash have high potential as solid-base catalysts in biodiesel synthesis. The wet impregnation process of eggshell-nano-CaO

and palm ash from solid waste can produce high catalytic activity, as seen in the alkalinity level of the catalyst, which is influenced by the amount of palm ash and the calcination temperature. A catalyst with 20 wt% palm ash concentration and 800 ᵒC calcination temperature pro- duced the best characteristics with surface area of 21.843 m²/g, volume of 0.057 cm³/g, alkalinity of 8 mmol/g, and pore radius of 5.331 nm.

With biodiesel yield produced was 71.77%. The 3 wt% amount of catalyst and 60-min reaction time used in this study resulted in higher biodiesel yields. Biodiesel properties such as density, viscosity, and methyl ester content are specified with EN14214.

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.

Data availability

The data that has been used is confidential.

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Table 7

Physicochemical properties of palm biodiesel.

Characterization Unit Standard EN14214 Value

Methyl Ester % 96.5 97.47

Density kg/m³ 860–900 868

Viscosity cSt 3.5–5 4.23

monoglyceride % max. 0.8 0.34

Diglyceride % max. 0.2 0

Glycerol % <0.24 0.22