Fusion-Assisted Hydrothermal Synthesis and Post-Synthesis Modiﬁcation of Mesoporous Hydroxy Sodalite Zeolite Prepared from Waste Coal Fly Ash for Biodiesel Production

: Increases in biodiesel prices remains a challenge, mainly due to the high cost of conventional oil feedstocks used during biodiesel production and the challenges associated with using homogeneous catalysts in the process. This study investigated the conversion of waste-derived black soldier ﬂy (BSF) maggot oil feedstock over hydroxy sodalite (HS) zeolite synthesized from waste coal ﬂy ash (CFA) in biodiesel production. The zeolite product prepared after fusion of CFA followed by hydrothermal synthesis (F-HS) resulted in a highly crystalline, mesoporous F-HS zeolite with a considerable surface area of 45 m 2 /g. The impact of post-synthesis modiﬁcation of the parent HS catalyst (F-HS) by ion exchange with an alkali source (KOH) on its performance in biodiesel production was investigated. The parent F-HS zeolite catalyst resulted in a high biodiesel yield of 84.10%, with a good quality of 65% fatty acid methyl ester (FAME) content and fuel characteristics compliant with standard biodiesel speciﬁcations. After ion exchange, the modiﬁed HS zeolite catalyst (K/F-HS) decreased in crystallinity, mesoporosity and total surface area. The K/F-HS catalyst resulted in sub-standard biodiesel of 51.50% FAME content. Hence, contrary to various studies, the ion exchange modiﬁed zeolite was unfavorable as a catalyst for biodiesel production. Interestingly, the F-HS zeolite derived from waste CFA showed a favorable performance as a heterogeneous catalyst compared to the conventional sodium hydroxide (NaOH) homogeneous catalyst. The zeolite catalyst resulted in a more proﬁtable process using BSF maggot oil and was economically comparable with NaOH for every kilogram of biodiesel produced. Furthermore, this study showed the potential to address the overall biodiesel production cost challenge via the development of waste-derived catalysts and BSF maggot oil as low-cost feedstock alternatives.


Introduction
Biodiesel is a promising alternative renewable energy source with potential to supplement or substitute finite and highly polluting fossil fuels [1]. The utilization of biodiesel demonstrates a greener and more sustainable form of energy [2,3]. Its cost remains a major challenge vis-à-vis its counterpart, petroleum diesel [2]. Costs of oil feedstock, and purification of biodiesel products derived from the conventional homogeneous catalyst, are major contributors to the total production cost of biodiesel [3][4][5]. The drawbacks associated with conventional biodiesel production emphasize the need to explore a sustainable approach for cost competitive biodiesel production. Waste-derived black soldier fly (BSF) maggot oil has the potential to be transformed into biodiesel [6,7], and is employed as biodiesel production. Based on the above, the material is assumed to have a sufficient external surface area with available catalytic sites suitable for biodiesel conversion [39]. It could also be cheaper (with reference to material requirements) and easier to synthesize (with reference to its framework and non-requirement of structure directing agents) compared to counterpart medium and large pore zeolites [24]. Thus, in this study the HS material is investigated for its potential to counteract high biodiesel production costs caused by use of conventional zeolites or homogenous catalysts.

Results and Discussion
HS was synthesized from CFA via a fusion-assisted hydrothermal method, followed by post-modification by ion exchange of the obtained product, as described in Section 3.2. The details of the experimental procedure are given in Sections 3.2.1 and 3.2.2 respectively. This section presents and discusses the XRD, SEM-EDS, FT-IR and textural characterization of the synthesized products. thus has a small pore size (2.8 Å), resulting in a more condensed structure compared to its counterpart zeolites (MFI ZMS-5, 32 Å; FAU Na-X, 7.3 Å) [26,38] commonly explored in biodiesel production. Based on the above, the material is assumed to have a sufficient external surface area with available catalytic sites suitable for biodiesel conversion [39]. It could also be cheaper (with reference to material requirements) and easier to synthesize (with reference to its framework and non-requirement of structure directing agents) compared to counterpart medium and large pore zeolites [24]. Thus, in this study the HS material is investigated for its potential to counteract high biodiesel production costs caused by use of conventional zeolites or homogenous catalysts.

Results and Discussion
HS was synthesized from CFA via a fusion-assisted hydrothermal method, followed by post-modification by ion exchange of the obtained product, as described in Section 3.2. The details of the experimental procedure are given in Sections 3.2.1 and 3.2.2 respectively. This section presents and discusses the XRD, SEM-EDS, FT-IR and textural characterization of the synthesized products. Figure 1 presents the XRD patterns of CFA, the hydrothermal fusion assisted-derived zeolite (F-HS), and the modified ion-exchanged product (K/F-HS) The XRD results in Figure 1 show that CFA, the feedstock material obtained from South Africa, exhibited a crystalline phase of quartz (Q) and mullite (M) in major proportion. Upon fusion-assisted hydrothermal conversion of the CFA, the XRD pattern of the obtained product F-HS exhibited the peaks of a corresponding HS zeolite phase in accordance with the standard simulated pattern of sodalite [40]. However, other mineral phases The XRD results in Figure 1 show that CFA, the feedstock material obtained from South Africa, exhibited a crystalline phase of quartz (Q) and mullite (M) in major proportion. Upon fusion-assisted hydrothermal conversion of the CFA, the XRD pattern of the obtained product F-HS exhibited the peaks of a corresponding HS zeolite phase in accordance with the standard simulated pattern of sodalite [40]. However, other mineral phases such as zeolite X and quartz were detected in F-HS samples as minor phase impurities, showing that the CFA feedstock had been almost fully converted to zeolite mineral phases during the fusion-assisted hydrothermal steps. By ion exchange modification of the sample with K + Catalysts 2022, 12, 1652 4 of 16 (K/F-HS), as described in Section 3.2.2, it was observed that the intensity of all the standard and major HS zeolite peaks at 2θ = 14.14 • , 24.64, 35.1 and 43.36 • drastically decreased. The decrease in HS crystallinity could have been due to framework instability associated with the removal of charge-balancing Na + cations, as well as due to the reflux conditions prescribed for ion exchange or thermal post-synthesis calcination applied, which may have been too vigorous for HS zeolite [41]. This may have also resulted in an ineffective exchange of K + ion as a charge balancing cation during the exchange process [17,41].

Crystal Morphology
The morphology of the produced F-HS sample shows a hexagonal-cubic crystal habit (Figure 2a). After the sample was ion-exchanged, its crystal morphology altered to a more agglomerated structure with tiny crystals (Figure 2b). Intercrystalline voids between crystals was observed, which slightly narrowed after the modification process (K/F-HS). This could be due to the high temperature calcination after ion exchange during the modification process ( Figure S1 from Supplementary Materials), as well as an indication of K + ion deposits (Table 1) in the form of salts covering the surfaces of the catalyst [20,30].
conditions prescribed for ion exchange or thermal post-synthesis calcination applied, which may have been too vigorous for HS zeolite [41]. This may have also resulted in an ineffective exchange of K + ion as a charge balancing cation during the exchange process [17,41].

Crystal Morphology
The morphology of the produced F-HS sample shows a hexagonal-cubic crystal habit (Figure 2a). After the sample was ion-exchanged, its crystal morphology altered to a more agglomerated structure with tiny crystals (Figure 2b). Intercrystalline voids between crystals was observed, which slightly narrowed after the modification process (K/F-HS). This could be due to the high temperature calcination after ion exchange during the modification process ( Figure S1 from Supplementary Materials), as well as an indication of K + ion deposits (Table 1) in the form of salts covering the surfaces of the catalyst [20,30].   Table 1 presents the elemental composition of the samples as analysed by SEM-EDS. A significant proportion of Si (14.86%), Al (13.67%) and charge balancing Na + ions (11.55%) were detected in the framework of the parent HS zeolite (F-HS). The sample demonstrated a Si/Al framework ratio of 1.18 and a Na/Al ratio of 0.93. Upon ion exchange, it was observed that the percentage concentration of Si, Al and Na was reduced, with a significant increase in K + ion by 3.77% w/w. Na content declined from 11.55 (F-HS)   Table 1 presents the elemental composition of the samples as analysed by SEM-EDS. A significant proportion of Si (14.86%), Al (13.67%) and charge balancing Na + ions (11.55%) were detected in the framework of the parent HS zeolite (F-HS). The sample demonstrated a Si/Al framework ratio of 1.18 and a Na/Al ratio of 0.93. Upon ion exchange, it was observed that the percentage concentration of Si, Al and Na was reduced, with a significant increase in K + ion by 3.77% w/w. Na content declined from 11.55 (F-HS) to 8.17% (K/F-HS) w/w after the ion exchange modification process. This might suggest the partial exchange of Na + with K + ions due to the applied ion exchange procedure and possible enhanced catalyst basic strength [42]. Furthermore, the decrease in the proportion of major elements (Si, Al, Na) resulted in the decreased Si/Al and Na/Al ratio in the modified sample (K/F-HS). This is supported by the reduction in phase crystallinity and identity as  Figure 1 [41,43]. The Si/Al ratio of a typical HS zeolite structure ranges between 0.84-1 [26,44] (Table S2); which is explained better by F-HS (Si/Al = 1.18) compared to K/F-HS (Si/Al = 1.35). The reduced Na/Al ratio observed in sample K/F-HS after ion-exchange modification is evidently due to the introduction of K ions as charge balancing cations in the framework structure [3,28,45].

Fourier-Transform Infrared
The FT-IR spectra in Figure 3 depicts the structural configuration of a fusion-assisted and ion-exchanged HS zeolite sample. From the view of the spectra, it is observed that the structural configuration describing the HS zeolite phase remained intact after modification by ion-exchange. However, the broad asymmetric stretching band of T-O-T (T = Si, Al) observed between 900-1330 cm −1 (±975 cm −1 ) in F-HS significantly narrowed, and the intensity of the two sodium carbonate (CO 3 ) impurity-associated bands between 1410-1480 cm −1 , decreased in K/F-HS. The water adsorbed band observed in the spectra of F-HS at 1650 cm −1 disappeared in K/F-HS. The narrowing of asymmetric bands may correspond to decreased crystallinity observed after the ion exchange [46]. The decrease of the carbonate bands shows the removal of impurities, whereas the disappearance of water adsorption bands suggests the partial removal of water from the HS zeolite structure due to the high-temperature calcination steps after the ion-exchange process ( Figure S1). to 8.17% (K/F-HS) w/w after the ion exchange modification process. This might suggest the partial exchange of Na + with K + ions due to the applied ion exchange procedure and possible enhanced catalyst basic strength [42]. Furthermore, the decrease in the proportion of major elements (Si, Al, Na) resulted in the decreased Si/Al and Na/Al ratio in the modified sample (K/F-HS). This is supported by the reduction in phase crystallinity and identity as observed in Figure 1 [41,43]. The Si/Al ratio of a typical HS zeolite structure ranges between 0.84-1 [26,44] (Table S2); which is explained better by F-HS (Si/Al = 1.18) compared to K/F-HS (Si/Al = 1.35). The reduced Na/Al ratio observed in sample K/F-HS after ion-exchange modification is evidently due to the introduction of K ions as charge balancing cations in the framework structure [3,28,45].

Fourier-Transform Infrared
The FT-IR spectra in Figure 3 depicts the structural configuration of a fusion-assisted and ion-exchanged HS zeolite sample. From the view of the spectra, it is observed that the structural configuration describing the HS zeolite phase remained intact after modification by ion-exchange. However, the broad asymmetric stretching band of T-O-T (T = Si, Al) observed between 900-1330 cm −1 (±975 cm −1 ) in F-HS significantly narrowed, and the intensity of the two sodium carbonate (CO3) impurity-associated bands between 1410-1480 cm −1 , decreased in K/F-HS. The water adsorbed band observed in the spectra of F-HS at 1650 cm −1 disappeared in K/F-HS. The narrowing of asymmetric bands may correspond to decreased crystallinity observed after the ion exchange [46]. The decrease of the carbonate bands shows the removal of impurities, whereas the disappearance of water adsorption bands suggests the partial removal of water from the HS zeolite structure due to the high-temperature calcination steps after the ion-exchange process ( Figure S1).

Textural Properties
The textual properties of F-HS and that of the ion exchanged sample, K/F-HS, are presented in Table 2. The results show 42.2 m 2 /g mesoporous area with a total surface area of 45 m 2 /g (F-HS). The F-HS sample also revealed an average mesopore diameter between

Textural Properties
The textual properties of F-HS and that of the ion exchanged sample, K/F-HS, are presented in Table 2. The results show 42.2 m 2 /g mesoporous area with a total surface area of 45 m 2 /g (F-HS). The F-HS sample also revealed an average mesopore diameter between 12.99-15.31 nm, highlighting the mesoporous pore size distribution [47]. It may be noted in Table 2 that the total surface area of the synthesised F-HS is higher compared to HS obtained from both pure chemicals and CFA via the direct method synthesis method, as reported by Golbad et al. [44], Shipari Lapari et al. [31] and Shabani [48]. With modification by ion exchange (K/F-HS), there was a decrease in the total surface area (25.8 m 2 /g), mesoporous area (25.0 m 2 /g), as well as average pore diameter range (9.5-12.20 nm). These could be attributed to surface coverage and pore blockages due to the ion exchange deposit of K + ions [25,30], owing to the larger atomic radii of K + ions compared to Na + ions in the parent F-HS [49]. The high temperature calcination and the apparently high reflux conditions prescribed for ion exchange [25,50] over HS using a KOH alkali source in this work (Section 3.2.2), might also justify the above results. The evidence of decreased surface area and pore volumes due to the coverage of K + ions and the above-mentioned conditions also can indicate poor exchange between the Na + ions initially present in the parent F-HS structure and K + ions in the modification process. The lower resultant mesoporous area in K/F-HS suggests pore blockage and a decrease in mesoporous characteristics after ion exchange.

Pore Distribution and Isotherm Curves of Produced Catalysts
The pore size distribution curve ( Figure 4) reveals that the parent F-HS zeolite contained mesopores of 4 nm average diameter. The modified sample, K/F-HS, shows more intense and broader peaks, with pore sizes distributed around 3-5 nm (Figure 4). The modified sample further shows a broader pore size distribution between 15 and 30 nm (∼24 nm), which could imply larger mesopores with lower external surface area associated with more diffusive characteristics (less diffusion constraints) compared to the parent zeolite [33,51].
Furthermore, the N 2 adsorption isotherm curves of the samples are presented in Figure 5. Both samples show a typical isotherm representing a characteristic type III of Langmuir adsorption as classified by IUPAC [52]. The isotherm curves of the two samples commonly show a micropore filling in the region p/p o below 0.4, with a small adsorption type H 3 hysteresis loop in the range of p/p o > 0.45. It is suggested by Sotomayor et al. [47] and Thommes et al. [52] that this loop indicates mesoporosity that enhanced capillary condensation of the adsorbed nitrogen, and the loop is associated with a shallow micropore area. The narrower hysteresis shown by the modified sample (K/F-HS), signifies lower mesoporosity than the parent zeolite (F-HS), which was affirmed by the BET results in Table 2.

Biodiesel Production over CFA-Synthesised and Modified HS Zeolite
The obtained fusion F-HS zeolite and ion-exchanged zeolite (K/F-HS) was used for transesterifying waste-derived BSF maggot oil. Figure 6 shows a high biodiesel yield of 84.10% with a considerable FAME content of 64.50% over the synthesised F-HS catalyst. With a small micropore size (2.8 Å) (Table S1), HS micropores are too constrained to allow the diffusion of biodiesel into micropores, thus the non-microporous surface area (mesopores and macropores) in the nanometre size were readily accessible and allowed the rapid conversion of maggot oil to biodiesel (Table 2). Using ion exchange modified K/F-HS resulted in a biodiesel yield of 83.70% and a considerably decreased FAME content of  51.50%. The slight decline in the yield obtained using the modified K/F-HS sample could be due to decreased total surface area ( Table 2). This could be explained by lower reactant (oil-methanol) surface interaction in the process [3,20]. The decrease in FAME content using K/F-HS could also be attributed to possible poor diffusive characteristics offered by the modified sample due to smaller average pore size (Table S2) and pore blockages, as well as possible decreased acidity of the sample [28,53]. Thus, there is a high possibility that the reaction has occurred only on the external mesopore surface area (non-micropore areas) of the K/F-HS, compared to the extent of reaction that occurred over the parent F-HS zeolite [25,36,54]. The FAME content suggests good quality biodiesel obtained over the catalyst F-HS [55], compared to the quality of biodiesel obtained using K/F-HS. Nevertheless, both biodiesel samples obtained over the catalysts (Table 3) resulted in fuel quality properties that comply with the ASTMD6751 and EN14214 standard specifications [56]. It should also be noted that the fusion-assisted synthesis method for HS zeolite was associated with better catalytic performance on the basis of textural properties ( Table 2) [51]. The method also on this basis, resulted in better yield and quality biodiesel compared to the direct hydrothermal method synthesis of the catalyst [48].  Furthermore, the N2 adsorption isotherm curves of the samples are presented in Figure 5. Both samples show a typical isotherm representing a characteristic type III of Langmuir adsorption as classified by IUPAC [52]. The isotherm curves of the two samples commonly show a micropore filling in the region p/p o below 0.4, with a small adsorption type H3 hysteresis loop in the range of p/p o > 0.45. It is suggested by Sotomayor et al. [47] and Thommes et al. [52] that this loop indicates mesoporosity that enhanced capillary condensation of the adsorbed nitrogen, and the loop is associated with a shallow micropore area. The narrower hysteresis shown by the modified sample (K/F-HS), signifies lower mesoporosity than the parent zeolite (F-HS), which was affirmed by the BET results in Table 2.  Table 3. Properties of biodiesel derived from maggot oil using the fusion-synthesised and ion exchange modified HS zeolite catalyst. muir adsorption as classified by IUPAC [52]. The isotherm curves of the two samples commonly show a micropore filling in the region p/p o below 0.4, with a small adsorption type H3 hysteresis loop in the range of p/p o > 0.45. It is suggested by Sotomayor et al. [47] and Thommes et al. [52] that this loop indicates mesoporosity that enhanced capillary condensation of the adsorbed nitrogen, and the loop is associated with a shallow micropore area. The narrower hysteresis shown by the modified sample (K/F-HS), signifies lower mesoporosity than the parent zeolite (F-HS), which was affirmed by the BET results in Table 2.

Biodiesel Production over CFA-Synthesised and Modified HS Zeolite
The obtained fusion F-HS zeolite and ion-exchanged zeolite (K/F-HS) was used for transesterifying waste-derived BSF maggot oil. Figure 6 shows a high biodiesel yield of 84.10% with a considerable FAME content of 64.50% over the synthesised F-HS catalyst. With a small micropore size (2.8 Å) (Table S1), HS micropores are too constrained to allow the diffusion of biodiesel into micropores, thus the non-microporous surface area (mesopores and macropores) in the nanometre size were readily accessible and allowed the rapid conversion of maggot oil to biodiesel (Table 2). Using ion exchange modified K/F-HS resulted in a biodiesel yield of 83.70% and a considerably decreased FAME content of 51.50%. The slight decline in the yield obtained using the modified K/F-HS sample could be due to decreased total surface area ( Table 2). This could be explained by lower reactant (oil-methanol) surface interaction in the process [3,20]. The decrease in FAME content using K/F-HS could also be attributed to possible poor diffusive characteristics offered by the modified sample due to smaller average pore size (Table S2) and pore blockages, as well as possible decreased acidity of the sample [28,53]. Thus, there is a high possibility that the reaction has occurred only on the external mesopore surface area (nonmicropore areas) of the K/F-HS, compared to the extent of reaction that occurred over the parent F-HS zeolite [25,36,54]. The FAME content suggests good quality biodiesel obtained over the catalyst F-HS [55], compared to the quality of biodiesel obtained using K/F-HS. Nevertheless, both biodiesel samples obtained over the catalysts (Table 3) resulted in fuel quality properties that comply with the ASTMD6751 and EN14214 standard specifications [56]. It should also be noted that the fusion-assisted synthesis method for HS zeolite was associated with better catalytic performance on the basis of textural properties (Table 2) [51]. The method also on this basis, resulted in better yield and quality biodiesel compared to the direct hydrothermal method synthesis of the catalyst [48]. . Yield and FAME content of biodiesel derived over fusion hydrothermal and ion-exchanged HS zeolite (FAME content equivalent to ester content). Max of BD Yield (%) Max of FAME (%) Figure 6. Yield and FAME content of biodiesel derived over fusion hydrothermal and ion-exchanged HS zeolite (FAME content equivalent to ester content). Table 4 reports on the performance of F-HS zeolite and ion-exchanged K/F-HS zeolite compared to the performance of the counterpart zeolites reported in the literature for biodiesel production. The results herein reveal that both the parent F-HS and ion exchange K/F-HS resulted in better biodiesel yield performance than all zeolites reported in the literature [25,30]. In addition to biodiesel yield, these results are novel due to the properties of biodiesel obtained in each case, which attest to the outstanding performance of the HS zeolite catalyst in biodiesel production.

Techno-Economic and Environmental Impact of Hydroxyl Sodalite (F-HS) as a Heterogeneous Catalyst Compared to a Conventional Homogenous Catalyst in Biodiesel Production
Further evaluation of the performance of HS zeolite as a heterogeneous catalyst in biodiesel production was conducted on the basis of economic and environmental impact, vis-à-vis the conventional homogenous catalyst, sodium hydroxide, in the transesterification of BSF oil (Table 5 and Table S4). Introducing HS zeolite (F-HS) as heterogeneous catalyst as substitute for the conventional NaOH catalyst resulted in a decrease in cost and mass input of feedstock oil by 17% of the cost of feedstock required using NaOH conventional catalyst) per kg of biodiesel produced (Table S4). This is mostly due to the higher biodiesel yield obtained using the F-HS zeolite (84.10%) compared to the homogeneous NaOH catalyst at fixed conditions using the same feedstock oil. Furthermore, the heterogeneous catalyst F-HS, in terms of catalyst cost, was about four times more costly than the conventional NaOH homogeneous catalyst per kg of biodiesel produced in each case (i.e., $1.39 vs. $0.36/kg biodiesel) at fixed conditions (Table S4). This might imply that the use of HS zeolite was less profitable (cost-benefit) in terms of catalyst cost compared to the cost of the NaOH catalyst. However, it should be noted that the HS heterogeneous catalyst was recoverable and has the possibility of being reused for several biodiesel production cycles, unlike NaOH, which gets spent by excess washing after use [5]. Thus, this might compensate for the cost of HS in terms of catalyst cost in biodiesel production.
The environmental benefits of employing an HS zeolite heterogeneous catalyst (F-HS) as opposed to an NaOH conventional homogeneous catalyst highlights that HS zeolite obtainable from waste coal fly ash minimizes pollution in serving as a disposal management strategy of the solid waste from coal power stations (Table 5). The use of heterogeneous HS is also associated with less wastewater generation (Table S4) as opposed to the alkali wastewater disposed of during the washing of biodiesel produced in the presence of the homogeneous NaOH. The cost of the effluent treatment was estimated to be lower than that discharged from the NaOH-catalysed biodiesel production process. Table 5. Economic and environmental benefit/impact of HS heterogeneous catalyst compared to NaOH homogeneous-transesterification in biodiesel production.

•
High production cost associated with BD water washing and purification steps; and possible treatment of excess effluent • Reduced cost and volume of feedstock oil per kg of BD produced:~17% cost reduction (Table S4) -• This work • Okechukwu, Joseph, Nonso and Kenechi [5] • Reduced cost (based on volume) of process water requirement for BD purification:~20% cost reduction/kg of BD ( Figure S3) • Reduced cost (based on volume) of possible effluent (wastewater) treatment from BD purification: 19.8% cost reduction/kg of BD (Table S4) • Reduced wastewater generation (volume) and associated disposal implications from BD purification step: 19.8% reduced wastewater volume/kg BD (Table S4) • BD: biodiesel. * BSF oil was used as fixed feedstock for comparison of cases between the two catalysts.

Fusion-Assisted Hydrothermal Synthesis
The mass-mass ratio of 1:1.2 coal fly ash-NaOH mixture was crushed and then fused in a furnace at a temperature of 550 °C for 1.5 h. Thereafter, 50 mL of deionized water was added to the fused sample in a 100 mL Teflon-lined autoclave for hydrothermal reaction at 100 °C for 144 h. The obtained solid product from the reaction was filtered, washed, and dried overnight. The final product (with code name F-HS) was characterized using XRD, EDS, SEM, FT-IR and BET-BJH.

Post-Synthesis Ion-Exchange Modification
The modification of the obtained F-HS catalyst by ion-exchange using KOH as an alkali source was conducted according to the procedure previously described [25,50] (Figure S1). 1 g of the prepared catalyst (F-HS) was mixed with 1 M KOH solution in a mass ratio of 1:10 ( Figures S1 and 7). The slurry mixture was ion-exchanged by reflux at 60 °C for 24 h using a stirring speed of 800 rpm. The modified product was filtered, washed, and dried overnight at 120 °C. This was followed by calcination of the dried solid product in a muffle furnace at 550 °C for 1.5 h. The final modified solid product was assigned the code name K/F-HS.

Activity Tests of Synthesized Catalysts in Biodiesel Production from BSF Oil
The procedure for transesterification of waste-derived BSF oil for biodiesel production using the synthesized catalysts is schematically presented in Figure 7. The detailed procedure is presented in Figure S2 (Supplementary Materials).
The batch transesterification reaction was achieved in a round-bottom flask over a hotplate heating source whilst stirring at 800 rpm at a temperature of 60 °C for 1.5 h. A fixed catalyst weight of 1.5% to the weight of the oil was used, as was a methanol (MeOH) amount equivalent to MeOH-to-oil molar ratio of 15:1. The obtained product mixture  Figure S1, for BD in Figure S2).

Fusion-Assisted Hydrothermal Synthesis
The mass-mass ratio of 1:1.2 coal fly ash-NaOH mixture was crushed and then fused in a furnace at a temperature of 550 • C for 1.5 h. Thereafter, 50 mL of deionized water was added to the fused sample in a 100 mL Teflon-lined autoclave for hydrothermal reaction at 100 • C for 144 h. The obtained solid product from the reaction was filtered, washed, and dried overnight. The final product (with code name F-HS) was characterized using XRD, EDS, SEM, FT-IR and BET-BJH.

Post-Synthesis Ion-Exchange Modification
The modification of the obtained F-HS catalyst by ion-exchange using KOH as an alkali source was conducted according to the procedure previously described [25,50] (Figure S1). 1 g of the prepared catalyst (F-HS) was mixed with 1 M KOH solution in a mass ratio of 1:10 ( Figure S1 and Figure 7). The slurry mixture was ion-exchanged by reflux at 60 • C for 24 h using a stirring speed of 800 rpm. The modified product was filtered, washed, and dried overnight at 120 • C. This was followed by calcination of the dried solid product in a muffle furnace at 550 • C for 1.5 h. The final modified solid product was assigned the code name K/F-HS.

Activity Tests of Synthesized Catalysts in Biodiesel Production from BSF Oil
The procedure for transesterification of waste-derived BSF oil for biodiesel production using the synthesized catalysts is schematically presented in Figure 7. The detailed procedure is presented in Figure S2 (Supplementary Materials).
The batch transesterification reaction was achieved in a round-bottom flask over a hotplate heating source whilst stirring at 800 rpm at a temperature of 60 • C for 1.5 h. A fixed catalyst weight of 1.5% to the weight of the oil was used, as was a methanol (MeOH) amount equivalent to MeOH-to-oil molar ratio of 15:1. The obtained product mixture containing biodiesel was centrifuged for phase and catalyst separation. The biodiesel containing-phase was washed thoroughly with deionized water, dried by heating and addition of anhydrous sodium sulphate, cooled, weighed, and stored prior to characterization.
The yield of biodiesel obtained, in the case of each catalyst used, was determined using Equation (1).

Catalyst and Biodiesel Sample Characterisation
A sample characterization was conducted on both catalysts produced and derivedbiodiesel samples. The catalysts were characterized using X-ray diffraction (XRD), energydispersive X-ray spectroscopy (EDS), Fourier-transform infrared (FT-IR) and Brunauer-Emmett-Teller (BET) coupled with the Barret-Joyner-Halenda (BJH) analysis, with detailed description in Table 6. The XRD patterns of the obtained products were assigned according to the standard diffraction pattern of the sodalite zeolite structure obtained from the International Zeolite Association's (IZA) database [40]. The biodiesel samples obtained ( Figure S2) were characterized in accordance with the American Society for Testing and Materials (ASTMD6751) and the European normalization (EN14214) standards. The characterization techniques, used instruments (analytical methods) and operating conditions are presented in Table 5.

Conclusions
Hydroxy sodalite zeolite synthesised using coal fly ash waste-feedstock was successfully obtained via fusion-assisted hydrothermal synthesis. The zeolite possessed high crystalline characteristics, high mesoporous characteristics (average pore diameter range 12.99-15.31 nm) and a considerable total surface area of 45 m 2 /g. With a small micropore size (2.8 Å), HS micropores are too constrained to allow the diffusion of biodiesel into micropores; thus the high mesopore external surface area allowed for the rapid conversion of BSF maggot oil to biodiesel. The post-synthesis modification by ion exchange of the catalyst with KOH adversely affected the crystallinity and textural properties of the zeolite due to poor K + exchange. The F-HS zeolite sample proved to be highly active in biodiesel production from waste BSF maggot oil, leading to a high biodiesel yield of 84.10% and considerable quality with 64.95% FAME content. The modified sample (K/F-HS) presented reasonable activity for biodiesel production, with substandard 51.50% FAME content as compared to the parent F-HS zeolite. Thus, it was not necessary or favourable to conduct the post-modification of HS zeolite by ion exchange for biodiesel production using a KOH alkali source at high reflux conditions (temperature, agitation rate) nor to utilize a calcination step inclusive thereafter, as applied in this work. The synthesised F-HS also produced better biodiesel yield, at a lower cost of feedstock oil (~20% reduced cost per kg of biodiesel produced), and had better environmental implications of its use compared to the use of the NaOH homogenous catalyst for biodiesel production. On the basis of textural properties associated with better catalytic performance, the fusion-assisted hydrothermal synthesis of HS zeolite is deemed worthwhile for better yield and quality biodiesel in comparison to the catalyst obtained via the direct method synthesis. Thus, it is recommended to investigate whether high yield or quality in both zeolite and biodiesel products is offset by the greater energy requirement of the fusion-assisted protocol. This study has shown the valorization of waste-coal fly ash and waste-derived BSF oil feedstock into mesoporous HS zeolite catalyst and biodiesel, respectively, revealing the potential of the low-cost feedstocks to minimize biodiesel production costs and ensure sustainable production.