Immobilization of Potassium-Based Heterogeneous Catalyst over Alumina Beads and Powder Support in the Transesterification of Waste Cooking Oil

Abstract

In this work, the beads and powder potassium hydroxide (KOH) and potassium carbonate (K₂CO₃) supported on alumina oxide (Al₂O₃) were successfully prepared via incipient wetness impregnation technique. Herein, the perforated hydrophilic materials (PHM) made from low-density polyethylene (LDPE) was used as the catalyst reactor bed. The prepared catalysts were investigated using TGA, XRD, BET, SEM-EDX, TPD, FTIR while spent catalysts were analyzed using XRF and ICP-AES to study its deactivation mechanism. The catalytic performance of beads and powder KOH/Al₂O₃ and K₂CO₃/Al₂O₃ catalysts were evaluated via transesterification of waste cooking oil (WCO) to biodiesel. It was found that the optimum conditions for transesterification reaction were 1:12 of oil-to-methanol molar ratio and 5 wt.% of catalyst at 65 °C. As a result, the mesoporous size of beads KOH/Al₂O₃ and K₂CO₃/Al₂O₃ catalysts yielded 86.8% and 77.3% at 2 h’ reaction time of fatty acids methyl ester (FAME), respectively. It was revealed that the utilization of PHM for beads K₂CO₃/Al₂O₃ increase the reusability of the catalyst up to 7 cycles. Furthermore, the FAME produced was confirmed by the gas chromatography-mass spectroscopic technique. From this finding, beads KOH/Al₂O₃ and K₂CO₃/Al₂O₃ catalysts showed a promising performance to convert WCO to FAME or known as biodiesel.

1. Introduction

One of the most promising and safest means to overcome the global environmental issue is by replacing conventional diesel with biodiesel generated from edible and non-edible plant oils or animal fats [1,2]. Theoretically, biodiesel has low sulfur content than petrol diesel and could reduce hazardous gas emissions, such as SO₂, NO₂, CO_2, and particulate matter, generally produced by diesel engines [3]. Nowadays, scientists believed that the utilization of WCO as the biodiesel feedstock could reduce environmental pollutions.

Esterification and transesterification processes are the common techniques to produce biodiesel. Both techniques generally assist by heterogeneous or homogeneous catalyst either acidic or basic. The homogeneous catalytic reaction is defined as the catalyst that has the same phase as a solvent and is found to have several drawbacks, such as (i) corrosion of container pipelines [4], (ii) wastewater pollutions, and (iii) weak catalyst recovery [5]. The heterogeneous catalyst was introduced because of easy preparation, affordability and promising high biodiesel conversion. The chemical characteristic of the catalyst can be acidic or basic depending on the method used. Example of acid catalysts used are such as sulfonated D-glucose [6], MF-SO₃H [7], and SiO₂(CH₂)₃NHSO₃H [8]; while, for basic catalyst used such as KOH/NaY [9], Ca/Ba/Al₂O₃ [10], MgO/Al₂O₃ [11], and K₃PO₄/Ac [12]. The chemical characteristic of the catalyst is important to know to match with the feedstock used.

Recently, the powdered heterogeneous catalysts such as RHC/K₂O/Fe [13] and KOH/Al₂O₃ [14] have been utilized in conversion of biodiesel from WCO. However, the powder catalyst has drawbacks such as easy deactivation of catalyst upon exposure to the reaction medium [15], incompatibility with the large batch of the reactor [16], and weak catalyst recovery that leads to difficulties in recycling the catalyst [17]. The use of heterogeneous beads catalysts has been considered. The beads catalyst has an advantage in terms of its different geometrical shapes that create various pores, allowing the optimum hydrodynamic and contact between reactant, catalyst, and solvent to increasing the catalyst’s performance [18].

Several heterogeneous beads catalysts are reported compatible with biodiesel production [19,20], such as K₂CO₃/Al₂O₃ and KOH/Al₂O₃ due to their simple preparation, low cost, and high-yield capacity of biodiesel production [21,22]. The reversible chemisorption on both K₂CO₃/Al₂O₃ and KOH/Al₂O₃ was reported to be stable at higher temperature and pressure [23,24,25], making the essential salt suitable as a catalyst for the reactions. The alkaline-catalyzed transesterification process is generally adopted for biodiesel production because alkaline metal alkoxides and hydroxides are the most effective transesterification catalysts compared to the acid catalysts. However, for economic reasons, hydroxides are more often used.

This research aims to improve and study the binding strength between K₂CO₃ and KOH onto Al₂O₃ beads support and Al₂O₃ powder with a simple, environmental-friendly, and green preparation technique by using perforated hydrophilic material (PHM) as reactor container for beads catalyst. To the best of our knowledge, this is the first reported paper to discuss the utilization of PHM reactor for the protection and stability of the catalyst. This study also focused on the effect of alumina beads-supported K₂CO₃ and KOH catalysts on the transesterification of WCO with low free fatty acid (FFA) numbers. Besides, the PHM reactor made from low-density polyethylene (LDPE) for reactor container was introduced to reduce the catalyst damage and deactivation due to physical contact with the stirrer. The PHM reactor was designed as a porous catalyst container that helps the catalyst maintain its shape before and after the reaction. The solution can easily pass through the container via a tiny hole. The effects of the catalyst preparation, reaction conditions, catalyst performance, and catalyst reusability, were also investigated and discussed in this paper.

2. Results and Discussion

2.1. Thermogravimetric Analysis

In this work, to determine the optimum calcination temperature for the catalyst, the thermal gravimetric analysis (TGA) was carried out; thus, both powdered and beads K₂CO₃/Al₂O₃ and KOH/Al₂O₃ catalysts were analyzed. As shown in Figure 1, results of all prepared catalysts ranging between 32 and 1000 °C showed four stages of decomposition phase, which are: (1) Evaporation of water molecules for all catalyst in between 32 °C and 100 °C [14,26]; (2) commencement of dehydroxylation of OH^- molecules [27]; and (3) observation of broadband of K₂CO₃/Al₂O₃ catalyst, indicating the decomposition of KHCO₃ [28].

At stage 3, the 25% decomposition of KAl(CO₃)(OH)₂ occurred for beads K₂CO₃/Al₂O₃ ranging from 300 to 400 °C [29]. While the last decomposition phase at stage 4, represents the successful activation of the catalyst as a result of the formation of K₂O and K₂O₂ has been reported in XRD (Figure 2). The optimum calcination temperature of 700 °C was chosen based on the average of 850 °C and 650 °C, the temperatures at which for both powder and beads Al₂O₃ treated with KOH showed the formation of K₂O₂ (4KOH + O₂ → 2K₂O₂ + 2H₂O) and additional active species for K₂CO₃ showed the formation of K₂O (2K₂CO₃ + O₂ → 2CO₂→O₂ + K₂O) species on the Al₂O₃ [18]. For the K₂CO₃/Al₂O₃ powder catalyst (Figure 1c), the desorption peak is very small because the layer is compressed in one thermogram. The effect of decomposition rate between powder and beads catalyst is not much different, so the decomposition of K₂CO₃/Al₂O₃ powder catalyst can be referred to K₂CO₃/Al₂O₃ beads catalyst. A study from Sharikh et al. [30] also stated that by using K₂CO₃ catalyst, the suitable calcination temperature was at 700 °C because of the activation of the catalyst in the range of 680 °C to 1000 °C. Meanwhile, starting from 700 °C, the powder and beads Al₂O₃ treated with K₂CO₃ showed the formation of K₂O with the simultaneous loss of carbon dioxide [31]. The catalyst studied showed good strength and stability at high temperatures making the hydrophobic characteristic of the catalyst more efficient toward biodiesel production.

2.2. XRD Analysis of K₂O₂/Al₂O₃ and K₂O/Al₂O₃ Catalysts

The crystallinity and phase transition of powder Al₂O₃, beads Al₂O₃, powder KOH/Al₂O₃, beads KOH/Al₂O₃, powder K₂CO₃/Al₂O₃, and beads K₂CO₃/Al₂O₃ samples were studied by using XRD. As shown in Figure 2, there are ten peaks observed at 2θ = 26°, 35°, 38°, 44°, 53°, 58°, 62°, 67°, 69°, and 78° which corresponded to the Al₂O₃ (JCPDS file 51-0769). The intensity of the support (beads and powder) increase after KOH and K₂CO₃ are deposited onto the surface of the support. In general, all catalysts possessed high crystallinity, indicated by the high intensity and sharp peaks. Based on (Figure 2c,f), all catalysts show same plot for the presence of K₂O₂ at 2θ = 31°, 33°, and 34° (JCPDS file 50-05241) except for K₂CO₃/Al₂O₃ beads (Figure 2f) which show additional K₂O₂ spotted at 2θ = 43° (JCPDS file 50-05241). For K₂O, it was found that the species spotted at 2θ = 22°, 39°, 40°, 41°, and 42° for KOH/Al₂O₃ powder (Figure 2c) and another K₂O spotted at 2θ = 47°, 48°, 49°, and 50° for K₂CO₃/Al₂O₃ beads (Figure 2f) (JCPDS file 50–1327). The intensity peaks of beads KOH/Al₂O₃ and powder K₂CO₃/Al₂O₃ seem to decrease compared to others due to the hindrance of the higher amount of K disperse on the surface of the catalyst [29]. In this study, powder KOH/Al₂O₃ and beads K₂CO₃/Al₂O₃ catalyst was successfully converted to desired species (K₂O₂ and K₂O) for the reaction.

2.3. Catalyst Surface Characteristics

Pore size distribution curves and nitrogen adsorption-desorption isotherm graph for powder Al₂O₃, beads Al₂O₃, powder KOH/Al₂O₃, beads KOH/Al₂O₃, powder K₂CO₃/Al₂O₃, and beads K₂CO₃/Al₂O₃ samples are shown in Figure 3. The BET surface area, total pore volume and average pore diameter are shown in

Table 1. Physical properties of alumina-based K₂CO₃ and KOH catalyst with its mass content and density of active sites.

Catalyst	Physical Properties			Mass of Element (%)			Density of Active Site (mmol/g)
	Surface Area (m2/g) 1	Pore Volume (cm3/g) 2	Pore Size (nm) 2	K	O	Al	Weak (<160 °C)	Moderate (160–400 °C)	Strong (>400 °C)
powder Al2O3	2.50	0.01	5.03	0.00	0.00	99.98	2.36	-	-
beads Al2O3	282.10	0.44	6.24	0.00	0.00	99.98	0.34	0.20	0.45
powder KOH/Al2O3	2.40	0.01	5.16	22.50	45.26	32.24	N/A	N/A	N/A
beads KOH/Al2O3	133.40	0.35	7.09	25.37	41.48	33.15	0.15	0.08	2.38
powder K2CO3/Al2O3	1.506	0.0069	5.437	18.96	43.27	37.77	N/A	N/A	N/A
beads K2CO3/Al2O3	199.94	0.474	6.512	28.41	45.44	26.15	0.00	0.17	3.94

¹ BET surface area; ² estimated from Barrett, Joyner, and Halenda (BJH) model.

. It was observed that the BET surface area of all catalysts decreases as compared to Al₂O₃ powder and beads support. Surface area for powder KOH/Al₂O₃ and K₂CO₃/Al₂O₃ decrease from 2.45 m²/g to 2.36 m²/g and 1.51 m²/g, respectively. For beads catalyst, surface area for KOH/Al₂O₃ and K₂CO₃/Al₂O₃ decrease from 282.10 m²/g to 133.37 m²/g and 199.94 m²/g, respectively. It was suspected that the surface of catalyst support was covered by potassium compounds, and the pore of the catalyst was blocked [32]. M.A Mohammed et al. [33] also stated that the reduction of the surface area might be due to the sintering effect that led to particle growth and crystallization. On the other hand, the pore sized of KOH/Al₂O₃ and K₂CO₃/Al₂O₃ catalyst increased, as observed in Figure 3.

All four catalysts and supports that have narrow pore size distribution with diameter in between ~5 and ~7 nm are displayed in Figure 4c–f and summarized in Table 1. Pore sizes for Al₂O₃ powder and beads support were 5.03 and 6.24 nm, respectively and the broad peak of pore distribution is displayed in Figure 3a,b. When KOH and K₂CO₃ are immobilized onto the surface of the Al₂O₃ powder support, the pore size increases to 5.15 nm and 5.44 nm, with both catalysts seem to have similar pore distribution with Al₂O₃ powder support (Figure 3c,e). For Al₂O₃ beads support, the better pore distribution was spotted after KOH and K₂CO₃ were immobilized onto the surface of the Al₂O₃ beads support (Figure 3d,f). Beads KOH/Al₂O₃ and K₂CO₃/Al₂O₃ seem to have wider pore distribution and better pore size making the catalyst much effective and efficient. As shown in Figure 3 all catalyst displayed type V (based on IUPAC classification) isotherms for the typical H₃ hysteresis loop for powder catalyst and H₄ hysteresis loop for beads catalyst, validating the existence of mesoporous characteristics (2–50 nm based on IUPAC) [34] in the channel. Based on the isotherm plot in Figure 3, if the adsorption-desorption reaction occurs in relative pressure (p/p°) of 0.4 to 0.6 is generally attributed to small pores [35]. The KOH/Al₂O₃ powder, KOH/Al₂O₃ beads, K₂CO₃/Al₂O₃ powder, and K₂CO₃/Al₂O₃ beads samples show a hysteresis effect in the 0.4 to 0.6 relative pressure region, which is characteristic of small pore texture. Powder catalyst has small pore texture and small surface area while beads catalyst shows smaller pore texture and large surface area based on the hysteresis loop [36]. Beads catalysts seem to be the best catalyst as the surface area, pore size, and pore distribution are better than powder catalysts.

2.4. Scanning Electron Microscope—Energy Dispersive X-ray Analysis

Figure 4 shows the SEM images of Al₂O₃, KOH/Al₂O₃, and K₂CO₃/Al₂O₃ catalyst with the EDX mapping of potassium (K) and oxygen (O). It was observed that the morphology of alumina-based K₂CO₃ and KOH catalysts exhibited a relatively tight irregular plate structure. For powder K₂CO₃/A₂O₃ (Figure 4e), the appearance of a large oval shape-like fibrous plate on the surface is due to the agglomeration of the catalyst in addition to the exfoliation of its surface. Compared to the beads catalyst (Figure 4d,f), the irregular plate image can be clearly observed and showed a good morphological structure with higher surface area as recorded by BET data in

Table 2. The reusability study for beads catalyst with and without PHM.

Catalyst	Reusability
	1st	2nd	3rd	4th	5th	6th	7th	8th
K2CO3/Al2O3 (PHM)	77.4	70.2	66.7	55.7	43.6	40.2	19.5	0
K2CO3/Al2O3	72.1	0	0	0	0	0	0	0
KOH/Al2O3 (PHM)	86.8	70.6	0	0	0	0	0	0
KOH/Al2O3	73.5	0	0	0	0	0	0	0

.

Saba et al. [28] stated that the strong basic properties of the catalyst of K₂CO₃/Al₂O₃ have a high potential to absorb moisture resulting from particle agglomeration.

This observation is consistent with the data in Table 1, where the structure is mesoporous with a high surface area. Based on the mapping images of the EDX, the distribution of elements K and O revealed significant dispersion on the surface of the Al₂O₃ support material. It was observed that the distribution of K and O on the beads Al₂O₃ has better uniformity as compared to powder catalyst. This finding is in good agreement with the research reported by A. Islam et al. [34] where they reported that the potassium element has good dispersion on the surface of the beads Al₂O₃ catalyst.

2.5. Temperature Programmed Desorption-Carbon Dioxide Analysis

The strength and proportion of the different basic sites of the catalyst need to be evaluated by thermal desorption of CO₂ (CO₂-TPD). Figure 5 shows the TPD-CO₂ profile of calcined material for beads Al₂O₃, KOH/Al₂O₃, and K₂CO₃/Al₂O₃ with the amount of CO₂ desorbed (mV) as a function of the CO₂ desorption temperature (°C). The higher CO₂ desorption peak indicates the highest catalyst basicity because the strength of the surface bond reflects the temperature at which the CO₂ is desorbed, resulting in peaks that can be correlated to three different types of active sites. The three different types of strength actives sites reported by Junior et. al. and Shan et. al. [18,37] were weak sites (CO₂ desorbed <160 °C), medium sites (CO₂ desorbed 160 °C < x < 400 °C), and strong site (CO₂ desorbed >400 °C). Then, according to the attained desorption temperature, beads Al₂O₃ show medium desorption peak range 90 °C to 384 °C (highest peak at 270 °C). The peak was assigned by the presence of SiO₂, where it was used as the binder to keep the sphere structure of supports and enhanced basic properties of catalyst [38]. For beads, KOH/Al₂O₃ catalyst shows a strong and broad desorption peak range 350 °C to 800 °C (peak at 536 °C). This broadband indicated the presence of covalent bonding between CO₂ and K/Al₂O₃ [32]. However, the K₂CO₃/Al₂O₃ has broad peaks from 350 °C to 850 °C, peaked at 764 °C with two small shoulders at 524 °C and 630 °C. Within that range, between 350 and 580 indicates low strength, in between 580 and 697 indicates medium strength, and 700 and 860 indicates high strength [39,40,41]. Furthermore, it was observed that K₂CO₃/Al₂O₃ has 3.94 mmol/g of basic sites, and KOH/Al₂O₃ has 2.38 mmol/g of basic sites. From this data, K₂CO₃/Al₂O₃ has higher basic sites and is predicted to be a high performance of catalyst for biodiesel production. TPD-CO₂ findings were in agreement with BET data in Table 2 in regards to the increased basicity due to the higher surface area (higher active sites) [41]. Despite the similarity between the physical properties of beads and powder catalysts, beads catalyst remains a better choice.

2.6. Transesterification Reaction

Further analysis on the sample was conducted by using GC-MS to calculate the actual pure FAME content from the impure biodiesel collected from transesterification reaction where the percentage of ester content was obtained. Methyl heptadecanoate (C17) was used as the internal standard in the analysis to calculate the ester content based on the peak area of FAME species in the sample relative to the standard [42]. The ester content was calculated using formula (2) as mentioned in Section 2.6 (EN14103). Sample from beads K₂CO₃/Al₂O₃ catalyst at 1 h reaction time was tested for GCMS qualitative analysis, and from the result in Figure 6, eight peaks were observed for the entire compound present in the biodiesel, of which only seven corresponded to the methyl ester group [43]. The identified FAME was analyzed against the standard peak from pure biodiesel sample (internal standard) of myristate methyl ester, palmitate methyl ester, stearate methyl ester, oleate methyl ester, linoleate methyl ester, linolenate methyl ester, and heptadecanoic acid methyl ester. All peaks that appeared (except for 9-hexadecanoic acid) were verified against different esters and confirmed using the mass spectroscopy (MS) library programmed. Furthermore, a standard sample (100% pure biodiesel) was tested and used as a benchmark for the selection of the right peaks to calculate the ester contained in the collected biodiesel [42].

The results of biodiesel yield are shown in Figure 7. The experiments were carried out using a catalyst containing 30% KOH/70% Al₂O₃ and 30% K₂CO₃/70% Al₂O₃. The optimum condition for the transesterification reaction was 1:12 of oil to methanol ratio, 5 wt% of catalyst loading, 65 °C of reaction temperature, and 1 to 4 h reaction time. According to the results, powder KOH/Al₂O₃ catalyst shows 66.7% biodiesel yield at 3 h reaction time, and powder K₂CO₃/Al₂O₃ catalyst shows 72.28% biodiesel yield at 4 h reaction time. In comparison, beads catalyst exhibited the highest biodiesel yield which beads KOH/Al₂O₃ catalyst shows 86.8% biodiesel yield at 2 h reaction time and powder K₂CO₃/Al₂O₃ catalyst shows 77.3% biodiesel yield at 2 h reaction time. This result is coherent with BET and TPD results where beads catalyst provides larger surface area (refer to Table 1) that increases many active sites deposited on the surface and increasing the basicity of the catalyst. The lack of performance of powder catalyst can be divided into two, which are either the catalyst is easily coated by glycerol as the catalyst and glycerol at the bottom of the container or direct contact between the catalyst and magnetic stirrer resulting in deactivation of the catalyst [44,45]. Moreover, a study from [34] stated that increasing the reaction time will ultimately increase the effect of the leaching of the powder catalyst. The catalyst that can produce biodiesel more than 70% can be considered a good catalyst, as industrial production in Malaysia using homogeneous catalysts can produce biodiesel up to 65% to 85% per batch. Beads catalysts for both K₂CO₃ and KOH remain a better choice for increased biodiesel production compared to the powder catalyst. In comparison, research studied by Shan et. al. [37], produce biodiesel from palm oil catalyzed by palygorskite-supported K₂CO₃ resulting in 75–85% biodiesel yield at 2 h reaction time.

2.7. Reusability Testing for Beads KOH/Al₂O₃ and K₂CO₃/Al₂O₃

K₂CO₃/Al₂O₃ and KOH/Al₂O₃ (beads and powder) catalysts that reveal the highest yield in Section 2.6 then underwent reusability testing to obtain the stability information of the catalyst after a few cycles of reaction. Beads formed of both catalysts were proceed at 2 h reaction time while for powder K₂CO₃/Al₂O₃ catalyst at 4 h reaction time and KOH/Al₂O₃ catalyst at 3 h reaction time. The catalyst recovered after the reaction was washed and reactivated at 700 °C for 2 h. By washing and reactivating the catalyst, the impurities that remained on the catalyst’s surface were removed, and the active sites were exposed for another reaction. The reusability testing was performed up to the 7th cycle for K₂CO₃/Al₂O₃ beads catalyst and 2nd cycle for KOH/Al₂O₃ beads catalyst by maintaining the amount of catalyst for every cycle in order to cover up the loss of the catalyst amount during the separation and reactivation process. From the experiment, the mass for the spent catalyst remains unchanged and Figure 8 reveals that there was a slight decrease in the pattern in the biodiesel yield. However, the conversions were still considered as high until the second cycle. For KOH/Al₂O₃ beads and powder catalyst, the reusability of the catalyst stops at the third cycle because the catalyst becomes soggy and ruptures due to the hygroscopic characteristic of the material [14,46]. The K₂CO₃/Al₂O₃ catalyst shows tremendous performance in converting the biodiesel. The strong and stable structure makes the catalyst very useful until the last cycle. In the end, the reason for declining biodiesel conversion might be due to the leaching of K₂O₂ and K₂O from Al₂O_3, resulting in the loss of active sites from the surface of the catalyst [47,48].

The reusability study for comparison between beads catalyst with and without using PHM is tabulated in Table 2. PHM shows an excellent contribution by preventing the occurrence of cracks coupled with maintaining the shape of the catalyst until 7th catalyst cycle. The disadvantages without using PHM that has been studied in this project are (i) the beads catalyst have high potential to crack due to the direct contact with the magnetic stirrer and (ii) the catalyst leaches out from the support after a few catalytic cycles. A previous study conducted by [31] used K₃PO₄ supported-alumina beads and revealed that the reusability of the catalyst was up to the 2nd cycle, as leaching effect resulting from the crushed beads catalyst becomes apparent at the 3rd cycle.

2.8. Identification of Post-Reaction Compounds on the Catalyst Surface Using Fourier-Transform Infrared Spectroscopy

The beads K₂CO₃/Al₂O₃ and beads KOH/Al₂O₃ catalysts were chosen and favored over others due to their high biodiesel production and reusability. Thus, further investigation was carried out to identify the compounds present in the spent catalyst using FTIR and compared with fresh beads K₂CO₃/Al₂O₃ and KOH/Al₂O₃ catalysts. The catalysts showed sample adsorption of wavelength, with the observation of higher adsorption intensity of beads KOH/Al₂O₃ catalyst compared to beads K₂CO₃/Al₂O₃ catalyst. This implies that the catalyst can easily degenerate after yielding low biodiesel for few consecutive cycles and then become unrecyclable. This may be attributed to the presence of an undesired compound on the surface of the catalyst after the reaction. The compound probably binds permanently to the surface of the catalyst and inhibit the performance of active sites. The catalyst may also be leached out from the support, thereby reducing the catalyst performance.

Figure 9 shows the FTIR spectrum of the fresh and spent catalyst after 2nd cycle for KOH/Al₂O₃ and 7th consecutive cycle for K₂CO₃/Al₂O₃, resulting in the degeneration of the potential catalyst. From the FTIR spectrum, the fresh catalyst for KOH/Al₂O₃ and K₂CO₃/Al₂O₃ shows straight lines indicating that there is no species/impurities found on the surface of the catalyst compared to the spent catalyst. For the spent catalyst, it was presented that the strong hydroxyl (v-OH) stretching vibration of the surface of the hydroxyl group that attached to the catalyst can be observed at the wavelength range of 3500 to 3200 cm⁻¹ [44]. For sp³ and sp³ aliphatic hydrocarbon chain (v-CH) for stretching vibration can be observed at a wavelength of 3000 to 2820 cm⁻¹ and 2750 cm⁻¹, respectively [49,50]. The carbonyl group (v-C=O) stretching vibration of 1735 cm⁻¹ and continue adsorption at wavelength of 1600 cm⁻¹ due to aliphatic alkene structure (v-C=C) were caused by fatty acids methyl ester [51,52]. The hydroxyl (v-OH) bending vibration occurs at 1550 caused by H₂O adsorbed from the air [44]. The vibration of a cyclic compound such as benzene rings (v-C=C) and the major absorption peak at the wavenumber of 1407 cm⁻¹ can be assigned to the asymmetric stretching of carbonate (v-CO₃^2-) group observed in the K₂CO₃ calcined at 700 °C. The resonance of the carbon bond atom with an oxygen atom (v-CO, v-C=C, and v-COC) can be observed at a range of 1600 to 1200 cm⁻¹ and continue at a range of 1100 to 1000 cm⁻¹, respectively [53,54,55]. The lower frequency basically shows the properties of the metal oxide where the range 1000 to 700 cm⁻¹ indicates the strong bending of a carbonyl compound with metal ions (v-CO-X⁺) where X represents the potassium ion (K) [37,56]. As shown in Figure 9, the spent catalyst shows the formation of a new peak that might be coming from the impurities or side product at the end of the reaction. Surprisingly, as can be seen from the catalyst’s figure, the potential beads catalyst retains its shape and can overcome the mass loss problem from the catalyst caused by cracking or flushing away of the catalyst together with the reagent used in the reaction. This statement is supported by the TGA result in Figure 1, where the catalyst shows good stability even calcined at higher temperatures. The next observation found was the catalyst changes its color from white to dark orange gradually in every cycle. The spent catalyst changes its color because of the poisoning effect by the side product of the reaction [27,57,58], which is glycerol. During the separation process, glycerol has a dark red color and the side product was located at the bottom of the flask mixed with beads catalyst [59] through the PHM. At the end of 7th catalytic cycle, the physical appearance of the catalyst shows a crack layer for some beads due to the vibration and collision with a magnetic stirrer which reduces the catalyst’s performance [60]. Moreover, the K₂O and K₂O₂ have difficulties from water molecules (H₂O) during the reaction because the catalyst reacts with sorbed water from the sample by physical adsorbtion [34] and causes the bending of OH radical (>1650 cm⁻¹) and formation of glycerol carbonate (1800–1000 cm⁻¹) [14].

2.9. XRF and ICP-AES Analysis

Spent beads K₂CO₃/Al₂O₃ catalyst were chemically analyzed by means of X-ray fluorescent spectrometry (XRF) as mentioned in

Table 3. XRF analysis results of used beads K₂CO₃/Al₂O₃ catalyst in the transesterification process.

Element Analyzed	Fresh (%)	1st Cycle (%)	4th Cycle (%)	8th Cycle (%)
Potassium, (K)	0.21	0.97	4.3	6.3
Aluminum, (Al)	0.00	0.02	0.05	2.4
Others	0.03	1.03	1.52	2.0

. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) was carried out to study the element present in the biodiesel liquid, and the data are shown in

Table 4. Chemical analysis by ICP-AES of biodiesel liquid sample by using beads K₂CO₃/Al₂O₃ catalyst in the transesterification process.

Element Analyzed	1st Cycle (%)	4th Cycle (%)	8th Cycle (%)
Potassium, (K)	1.0	3.3	6.4
Aluminum, (Al)	0.0	0.1	1.5

. The XRF and ICP-AES data were obtained only for 1st, 4th, and 8th cycles. According to the results of analyses, it was detected that the leaching happens on the surface of the catalyst—the number of elements that leach out from the surface does not exceed 7%. The previous study also reported that using potassium-based catalyst resulted in the lowest percent of leaching [61] and is easy to recover as the catalyst has defined shape and is larger in size. These data have an agreement with ICP-AES where the catalyst performance starts to drop at the 8th cycle.

3. Materials and Methods

3.1. Materials

The WCO was obtained from Pasir Gudang, Johor, Malaysia. The pre-treated process started when the WCO bottle was heated first on a hot water bath to dissolve all saturated fat that was deposited at the bottom of the bottle. After that, WCO was filtered to remove traces such as suspended solids and WCO was transferred to the beaker and it was heated at 100 °C on a hotplate for 1 h to remove traces of water, followed by preliminary analysis; acid value (AV) (ASTM D664/D8045-17e1), saponification value (SV) (ASTM D94), free fatty acids content (FFA) (ASTM D5555-95), viscosity (ASTM D445/D446), and moisture content (ASTM D5556-19). Meanwhile, the beads alumina oxide (Al₂O₃) (particle size < 5 mm), and powder Al₂O₃ (particle size < 1mm), the analytical grade (AR) methanol (CH₃OH, purity 99%), phenolphthalein (C₂₀H₁₄O₄, purity 99%), and methyl heptadecanoate (C₁₈H₃₆O₂, purity 99%) as the internal standard for biodiesel were purchased from Sigma-Aldrich (Selangor, Malaysia). The potassium carbonate (K₂CO₃, purity 99%), potassium hydroxide (KOH, purity 99%), sodium hydroxide (NaOH, purity 99%), and hydrochloric acid (HCl) (32% concentration, purity 97%) were obtained from Merck (Selangor, Malaysia) chemical company. Meanwhile, the container for the catalyst in the reaction medium (Figure 10) was made from low-density polyethylene (LDPE) of perforated hydrophilic materials (PHM).

3.2. Preparation of K₂CO₃ and KOH/Al₂O₃ Catalysts

In this work, both beads and powder catalysts were prepared by incipient wetness impregnation technique. Briefly, K₂CO₃ and KOH salt solutions were added dropwise to water suspension containing powder-Al₂O₃ in the separate beaker. The mixture was stirred and heated at 100 °C for 5 h, followed by the ageing process at 90 °C for 24 h, and calcined at 700 °C for 3 h with ramping 15 °C min⁻¹ to produce powdered K₂CO₃/Al₂O₃ and KOH/Al₂O₃ catalysts. The same procedure was applied to prepare K₂CO₃/Al₂O₃ and KOH/Al₂O₃ catalysts using beads Al₂O₃ as support.

3.3. Catalyst Characterization of K₂CO₃/Al₂O₃ and KOH/Al₂O₃ Catalysts

The TGA-Pyris 2012 thermogravimetric analysis assisted by thermal gas detector GSA7 from Perkin Elmer (Selangor, Malaysia) was used to determine the thermal decomposition of the prepared catalyst. The analysis was carried out in a static air atmosphere at a heating rate of 15 °C min⁻¹. The weight lost and the derivative thermogravimetry (DTG) of the sample against the temperature was plotted.

While the LabX XRD-6100 PANalytical, X-ray diffraction (XRD) analyzer was used for the identification and characterization of the internal structure, bulk phase, and the composition crystallinity phases of the catalyst, the analysis was carried out utilizing X’Pert PRO Theta from PANalytical diffracted beam monochromator, using step scan mode with range 0°−90° of theta (θ) with the scan rate of 2° min⁻¹.

For the quantity, distribution, and strength of the catalyst’s active sites, the prepared catalysts were examined using Thermo Finnigan (Selangor, Malaysia), TPDRO 1100 series of carbon dioxide-temperature programmed desorption CO₂-TPD. The sample was first treated with N₂ and heated up to 150 °C for 1 h to remove all the particulates and moisture, followed by the absorption of CO₂ gas at 10 cc min⁻¹ for 1 h at ambient temperature. The second treatment of the sample was done to remove excess CO₂ gas by flowing the N₂ gas through the samples for another 1 h. Finally, the sample is ready for analysis, and was heated from 50 °C to 1000 °C with a heating rate at 10 °C min⁻¹, and the He gas was flowed through the system to carry the CO₂ gas detached from the sample before being detected by the thermal conductivity detector. Finally, the intensity of detected gas molecules was plotted versus the temperature.

N₂ adsorption/desorption isotherm analysis was performed using Micro metrics ASAP 2010 (Selangor, Malaysia) to determine the catalyst surface area, average pore sizes, pore type, and total pore volumes. Prior to the analysis, the sample was degassed and heated for 24 h at 120 °C to remove moisture and other particulate matter. The N₂ gas was allowed to enter the sample reactor to fill the tube with N₂ molecules. The reactor containing catalyst was a dip into the liquid N₂ to ensure the physisorption attachment of the molecules on the surface of the catalyst. The data isotherms were recorded against the pressure detected to determine the pore volume and surface area of the K₂CO₃ and KOH/Al₂O₃ catalysts.

For the morphological and elemental composition analyses, the TESCAN VEGA3 scanning electron microscopy (SEM), Novatiq scientific Singapore (Selangor, Malaysia) equipped with energy-dispersive X-ray spectroscopy (EDX) was utilized. Before analysis, the sample powder was placed on the sample holder and coated with Au using a TESCAN coater.

To study the stability and the strength of catalyst structure, the spent catalyst was analyzed by Fourier transform infrared spectroscopy (FTIR) model Nicolet Is5 from Thermo Fisher (Selangor, Malaysia). It was used to record the remaining active species present on the catalyst after few cycles of the reaction. The spent catalyst was analyzed by Supermini bench-top sequential wavelength dispersive X-ray fluorescence (XRF) spectrometer in helium (He) atmosphere. Normalized contents of a dominant element were used as a reference and compared to samples recovered after transesterification reaction. The contents were calculated from XRD intensities and analytical apparatus factory calibration data for certain elements. All samples before XRF analysis were ground, sieved to particle size (x < 0.011 mm), and dried at 250° for 6 h. The contents of the main components on the biodiesel liquid were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The ICP-AES measurements were performed on a Prodigy 7 (Selangor, Malaysia) spectrometer and carried out under optimized experimental con.

3.4. Transesterification Reaction of WCO

The transesterification process was adapted with slight modification from previous research reported by Abdullah et al. [25]. About 10 g of the WCO was added to a 50 mL two-necked round bottom flask, heated to 65 °C and subsequently added with 4.32 g methanol with oil to methanol molar ratio of 1:12. Later, 30 wt.% of beads catalyst was added to the PHM before the reaction started. Figure 10 showed a schematic image for the open-system reactor, which was used in this experiment.

The mixture of the product was separated by the centrifugation process to recover the spent catalyst. The final product was observed as three layers with the top layer containing biodiesel, the middle layer containing glycerol, and the bottom layer was the catalyst. For the beads catalyst, after the reaction ended, the catalyst was separated by removing PHM, followed by centrifugation process. As the top layer found was biodiesel, the biodiesel needs to wash up by hot water by using a separating funnel several time to remove impurities such as excess glycerol and methanol. After that, the biodiesel was transferred into the beaker and heated up at 100 °C for 2 to 5 min to remove excess water from the washing process. Finally, the biodiesel was collected for the determination of FAME yield using 7860B Agilent gas chromatography-mass spectroscopy (GC-MS). The injector and detector temperature were programmed at 240 °C. Helium was used as the carrier gas with a flow rate of 19.2 mL min⁻¹. The Agilent J&W (Selangor, Malaysia) column temperature was set at 150 °C with ramping rate 15 °C min⁻¹.

3.5. Determination of Ester Content

Table 5. Equations to calculate the biodiesel conversion, ester content, and biodiesel yield.

Formula	Equation	Nr.
Conversion (%) *	$\mathrm{d}=\frac{\mathrm{A}}{\mathrm{Mass} \mathrm{of} \mathrm{WCO}}\times 100\%$	(1)
Ester content (%) From EN 14103	$\mathrm{c}=\frac{\mathrm{TA}-\mathrm{AEI}}{\mathrm{AEI}}\times \frac{\mathrm{CEI}\times \mathrm{VEI}}{\mathrm{m}}\times 100$	(2)
Mass of pure biodiesel from biodiesel conversion (g)	$\mathrm{Y}=\mathrm{c}\times \mathrm{A}$	(3)
Pure biodiesel yield (%)	$\mathrm{FAME} \mathrm{yield}, \%=\frac{\mathrm{Y} \left(\mathrm{g}\right)}{\mathrm{mass} \mathrm{WCO} \left(\mathrm{g}\right)}\times 100\%$	(4)

* Conventional technique to estimate the biodiesel conversion.

shows the formula to calculate the biodiesel from the conversion until the pure result, and Equation (1) shows the classical technique to calculate biodiesel where A refers to the mass of the product obtained divided by the mass of the WCO [62]. However, due to the presence of impurities stated in Section 2.5, this formula cannot reflect the true estimate of the collected biodiesel [63]. Hence, Equations (2)–(4) suggested the most accurate formula to determine the exact amount of FAME in the product.

Therefore, to obtain the actual mass of the pure biodiesel, the GC peaks area of FAME present in the product was determined by adding the internal standard of methyl heptadecanoate and comparing with the biodiesel standard. Thus, the FAME yield or percentage of ester content was determined based on Equations (2)–(4) as indicated by Agilent standard method EN14103. Where c refers to the ester content in biodiesel sample, TA for the total area, AEI for the area of internal standard, CEI for the concentration (mg mL⁻¹) of methyl heptadecanoate solution, VEI for the volume (mL) of methyl heptadecanoate solution, and m for the mass of the sample.

4. Conclusions

K₂CO₃ and KOH supported onto Al₂O₃ beads and powder successfully converted a waste cooking oil to biodiesel via transesterification process. With an optimum reaction condition of catalyst concentration of 30%, oil to methanol ratio of 1:12, catalyst loading of 5 wt%, and reaction temperature of 65 °C, the following yields of pure biodiesel were obtained: 86.8% for beads KOH/Al₂O₃ catalyst at 2 h reaction time, 77.4% for beads K₂CO₃/Al₂O₃ catalyst at 2 h, 66.7% for powder KOH/Al₂O₃ catalyst at 3 h, and 72.3% for powder K₂CO₃/Al₂O₃ catalyst at 4 h. In terms of biodiesel yield, KOH/Al₂O₃ catalyst shows the highest biodiesel yield collected, but K₂CO₃/Al₂O₃ catalyst offers better reusability (up to 6 times for K₂CO₃/Al₂O₃ powder catalyst and 7 times for K₂CO₃/Al₂O₃ beads catalyst) by using PHM as the reactor container. Based on the result, in between powder and beads catalysts, beads catalyst can be considered a better alternative to powder heterogeneous and normal homogeneous catalyst for biodiesel production as it can be easily recovered, can be reuse back without mass loss of the catalyst, and proved suitable for an industrial-scale application. Overall, beads K₂CO₃/Al₂O₃ catalyst shows better performance and PHM was suitable as the reaction container in the transesterification process, as it maintains the original shape and mass of beads catalyst.