Optimization of biomass durian peel as a heterogeneous catalyst in biodiesel production using microwave irradiation

: The present study investigated biodiesel production from the transesteri ﬁ cation of palm oil with methanol using calcined biomass durian peel (BDP) as a heterogeneous catalyst assisted by microwave irradiation. Characterization of the calcined BDP showed that K 2 O is the main compound with a concentration of 86.15 wt%. The e ﬀ ect of three independent variables of catalyst weight (3 – 12 wt%), reaction time (1 – 10 min), and power of microwave (180 – 900 W) was used to determine the optimum condition on biodiesel production using the response surface method-based on the Box – Behnken design experiment. The optimum biodiesel conversion of 97.3% was achieved under experimental parameters of catalyst concentration of 12 wt%, reaction time of 9 min, and microwave power of 180 W. The catalyst concentration and reaction time have signi ﬁ cant e ﬀ ects on biodiesel conversion.


Introduction
Increasing global concern about dwindling petroleum resources leading to an uncontrolled escalation of petroleum prices is resulting in a worldwide concerted effort to find a suitable energy source alternative [1].In addition, the demand for renewable energy has increased rapidly as a result of increasing public awareness of environmental issues [2,3].Biodiesel is an alternative fuel because it is made from renewable sources with lower pollutant emissions [4].In addition, biodiesel could be used directly without requiring any modification in diesel machines [2].Commercially, biodiesel is produced from the transesterification of oils/lipids using an alkaline homogeneous catalyst [5,6].However, biodiesel production currently has several problems such as the homogeneous catalyst cannot be reused, requires an expensive separation step, and produces a lot of waste water due to the use of a homogeneous catalyst [5].To solve the problems, heterogeneous substances that have different phases with the reactants have been studied and explored as catalysts in biodiesel production.Heterogeneous catalyst offers some advantages such as being reusable, less sensitive to free fatty acid and water, non-corrosive, and high selectivity [7].Some studies have proved that heterogeneous substances could catalyze transesterification reactions producing biodiesel with high yield [8][9][10][11][12][13][14].However, the catalytic activity of heterogeneous catalysts is lower than that of homogeneous catalysts; hence, the reaction time is relatively longer [15].Recently, bio-waste materials that contain potassium oxide as the main compounds have similar catalytic activity with homogenous basic catalysts [16][17][18].A biodiesel yield of 99% was achieved in 25 min reaction time at 65°C using calcined Brassica nigra plant which contains 56.13% potassium ion [19].Interestingly, this bio-waste catalyst could facilitate transesterification reaction at room temperature conditions.Tarigan et al. reported that a biodiesel conversion of 95.4% could be achieved in a reaction time of 30 min at room temperature using waste passion fruit peel as a solid catalyst [17].Furthermore, microwave and homogenizers could intensify biodiesel production using heterogeneous catalysts hence diminishing reaction time [11,16].The heterogeneous catalyst derived from calcined elephant-ear tree pods have reported could catalyze transesterification reaction and produce biodiesel conversion of 99% in 5.88 min under microwave irradiation [11] while a homogenizer device require 10 min reaction time to achieved similar result using palm bunch as a catalyst [16].Durian is a famous and exotic fruit particularly in Southern Asia.The durian fruit consists of 50-65% flesh and seed and 45-55% peels [20].Durian production in Indonesia reached 1.58 million tons in 2022 in which 0.79 million ton is the peel [21].The utilization of biomass durian peel (BDP) is limited and usually is simply discharged without any treatment that could pollute the environment.No references could be found in the literature related to the utilization of BDP as a heterogeneous catalyst in transesterification reactions under microwave irradiation.
Therefore, this study aims to determine the performance of BDP as a heterogeneous catalyst in the transesterification palm oil to biodiesel under microwave irradiation.The optimum biodiesel conversion was determined based on parameters of catalyst weight, reaction time, and microwave power using the response surface method.The Box-Behnken model was used to investigate the optimum reaction conditions at a 95% confidence level.

Materials
The BDP was collected from a local durian shop in Medan, Indonesia, cut into small chunks, and heated in an oven until dry.The dried BDP was crushed using a coffee grinder (ACE Kris, 150 W, Indonesia) and sieved to form a powder.The BDP powder was heated in a muffle furnace (Thermolyne Thermo Fisher Scientific model FB1310M-33, MA, USA) at 600°C for 4 h and stored in a desiccator before use.The calcination temperature of 600°C was chosen based on the previous published research [5].Furthermore, the metal oxides as the main components in the calcined BPD were composed at this temperature [5,10].The X-ray fluorescence (XRF) (Rigaku Supermini200, TX USA) was used to determine the elemental composition of the calcined BDP.A Household microwave (ACE Kris, Indonesia) is modified and is equipped with a condenser and magnetic stirrer.The input and output powers of the microwave were 1,400 and 900 W, respectively.All chemicals used in this study were purchased from a local chemical distributor in Medan, Indonesia, and were used as received.

Box-Behnken experimental design
The Box-Behnken design experiment with three parameters was applied to determine the optimum condition for microwave-assisted transesterification of palm oil to biodiesel using calcined BDP.The range of three-level reaction conditions is shown in Table 1.The ratio molar of palm oil to methanol was fixed at 1:12.The relationship between the variables with biodiesel conversion and yield was calculated using a second-order polynomial equation as shown in Eq. 1 to explain the effects of variables on linear, quadratic, and interaction terms.
where Y is the biodiesel conversion or yield; X i and X j are the independent factors; and 0, i, ii, and ij are the intercept, linear, and quadratic interaction coefficients, respectively.

Microwave-assisted transesterification of palm oil using BDP as a catalyst
5.8 mL of palm oil and 2.8 mL of methanol were added with an investigated amount of calcined BDP in a 100 mL roundbottom flask equipped with a condenser and magnetic stirrer.The flask was placed in the microwave and the reaction time was counted when the microwave was started based on investigated time and power.After completion, the mixture was separated using centrifugation at 5,000 rpm for 10 min.The biodiesel product in the top layer was then separated and stored in a desiccator for conversion analysis using gas chromatography (GC) (Shimadzu type 2010, Japan).

Conventional transesterification of palm oil using BDP as a catalyst
The palm oil was also transesterified using BPD as a catalyst in the batch reflux method for comparison.Palm oil (5.8 mL), methanol (2.8 mL based on the ratio of 1:12), and 0.63 g of calcined BDP were placed in a 100 round-bottom flask connected to a reflux condenser with a magnetic hotplate stirrer set to 500 rpm.The reaction was conducted for 10 min, and the biodiesel product was collected after centrifugation followed by the evaporation of leftover methanol and stored in a desiccator for GC analysis.

Results and discussion
The fatty acids profile of the palm oil is shown in Table 2 which is dominated by oleic acid and palmitic acid of 43.88 and 38.41%, respectively.The XRF analysis showed that calcined BDP consists of K 2 O, P 2 O 5 , and SO 3 as the main compound with a concentration of 86.15, 6.98, and 3.25 wt%, respectively.The transesterification of palm oil with methanol under microwave irradiation was conducted using calcined BDP as a heterogeneous catalyst.The effect of catalyst weight, reaction time, and microwave power on biodiesel conversion was studied in the Box-Behnken design experiment to determine the optimum condition.
The experimental and predicted biodiesel conversions are depicted in Table 3.
Based on the Box-Behnken model, the quadratic regression was quantified using Eq. 1, where A, B, and C represent the catalyst weight (wt%), reaction time (min), and microwave power (W), respectively.The relationship between the observed and predicted biodiesel conversion shows good linearity with a correlation coefficient (R 2 ) of 89% (Eq.2).The effect of linear factors, i.e. catalyst concentration and reaction time, was found to be highly significant on biodiesel conversion with values P < 0.0118 and P < 0.0131, respectively.

Effect of interaction parameters
The 3D response surface was used to determine the individual and cumulative effect of the parameters and the mutual interaction between the parameter and the dependent parameter.The resulting surface response 3D plots of biodiesel conversion as a function of two independent variables, (1) catalyst concentration and reaction time, (2) catalyst concentration and microwave power, and (3) reaction time and microwave power are shown in Figure 1a-c, respectively.As shown in Figure 1a, increasing catalyst concentration and reaction time up to a certain value has increased the biodiesel conversion.However, the conversion slightly decreases when both parameters rise.This is presumably due to the high catalyst amount in the mixture generating high viscosity in the reaction mixture and agglomeration of the catalyst leading to poor dispersion of the BDP catalyst [5,22,23].This result is in agreement with Niju et al. in the utilization of banana pseudostem as a catalyst in the transesterification of Madhuca indica oil to biodiesel [24].Similarly, Tarigan et al. in the transesterification of palm oil to biodiesel using waste banana peels as a catalyst observed an increasing biodiesel conversion up to a maximum of 97% and remained steady after that [5]. Figure 1b depicts the interaction effect of catalyst concentration and microwave power on biodiesel conversion.With the increase in catalyst concentration up to a specified level, biodiesel increases.In contrast, the biodiesel conversion was decreased with increasing microwave power.Furthermore, interaction parameters of reaction time and microwave power as shown in Figure 1c also have a similar graph pattern in which increasing reaction time increases the biodiesel conversion while increasing microwave power could decrease it.

Validation of the experimental model
The response surface method assists in identifying the combination of input parameter settings that provide reaction conditions with optimum biodiesel conversion.The optimum values of the independent parameters were achieved considering the lower and higher values of catalyst concentration, reaction time, and microwave power.Based on that, the predicted optimum biodiesel conversion of 98.6% could be achieved under reaction conditions of catalyst concentration of 12 wt%, reaction time of 9 min, and microwave power of 180 W. The experimental conversion based on that reaction condition is 97.3%.This shows that the predicted and experimental biodiesel conversions are in excellent agreement.Biomass durian peel for biodiesel production  3

Comparison BDP catalyst using microwave and reflux method
As a comparison, the calcined BDP catalyst was tested for its catalytic activity using the conventional reflux method.
The reflux reaction was performed under similar reaction conditions to the microwave method.The biodiesel conversion of 63% was achieved after 9 min of reaction time.This conversion was lower by 35% compared with microwaveassisted transesterification of palm oil to biodiesel using calcined BDP as a heterogeneous catalyst.The higher conversion using microwave irradiation was attributed to the  high-frequency rotation of reactants due to the constantly changing magnetic and electric fields [25,26].This result is in agreement with the previously published result of the transesterification of soybean oil to biodiesel using nanopowder calcium oxide as a catalyst under microwave irradiation [27].Hsiao et al. showed that a biodiesel conversion of 53.5% could be achieved at 15 min of reaction time using microwave irradiation compared to 22.1% using the conventional reflux method [27].In addition, some researchers concluded that microwave is more energy efficient than the conventional reflux method [28][29][30].

Comparison of biodiesel production using biomass catalyst in different methods
Calcined biomass-derived agricultural waste has been used as a heterogeneous catalyst in biodiesel production and showed catalytic activity comparable with a homogeneous base catalyst.As shown in Table 4, the biodiesel conversion of >90% was achieved using calcined biomass as a catalyst.Potassium in the form of potassium carbonate or potassium oxide is the main component of the biomass ash.However, the concentration of potassium did not exert a significant influence on the conversion of biodiesel.Notably, the passion fruit peel, which exhibits the lowest potassium concentration according to Table 4, demonstrated a biodiesel conversion rate that was comparable to that of the BDP, which possesses the highest concentration of potassium ions.The parameters governing the reactions, such as the molar ratio of oil to methanol and the weight of the catalyst, appear to have negligible influence on the biodiesel conversion.The molar ratio employed varied between 1:6 and 1:30, while the catalyst weight spanned from 2 to 12 wt%.Interestingly, calcined passion fruit and banana peel could catalyze the transesterification of palm oil to biodiesel at room temperature and resulted in a biodiesel conversion of 95.4% and 97.7%, respectively, after a reaction time of 30 min [5,17].In contrast, the current study demonstrates that the calcined BDP can serve as an effective catalyst for biodiesel production within a reaction time of 9 min using microwave irradiation.Therefore, in terms of reaction time, microwave irradiation-assisted biodiesel production using a calcined biomass is capable of reducing reaction time by 80% and 70% in comparison to ultrasound and homogenizer methods, respectively.

Biodiesel properties
The fatty acid composition of the oil primarily exerts a significant influence on several critical characteristics of biodiesel such as cetane number, density, viscosity, oxidative stability, pour point, and cloud point [31].Those properties could be predicted using a reliable and accurate equation which has been reported elsewhere [32][33][34].Table 5 shows the predicted palm oil biodiesel properties which were compared with waste cooking oil-biodiesel and international standards (ASTM D6751 and EN 14214).As expected, the palm oil-biodiesel satisfies the international standard for all the physicochemical properties.The palm oil-biodiesel has a higher cetane number than the standard and WCO-biodiesel which represent its ability to burn within the engine [1].This outcome can be attributed to the high amount of saturated fatty acid (SFA) in palm oil.However, the high level of SFA has a significant effect on the cold flow properties and oxidation stability.The cloud and pour point properties of palm oil-biodiesel are 15.2 and 9.7°C, respectively, which thereby limiting its use to ambient temperatures above freezing.However, the oxidation stability property of palm oil-biodiesel showed higher than the minimum standards presenting a potential for extended storage [35].

Conclusions
Microwave-assisted transesterification of palm oil to biodiesel was conducted using BDP as a heterogeneous catalyst.The calcined BDP contains K 2 O as the main compound with a concentration of 86.15 wt%.The response surface method based on the Box-Behnken design experiment was used to determine the optimum reaction condition.The optimum biodiesel conversion of 98.6% was predicted achieved using a catalyst concentration of 12 wt%, reaction time of 9 min, and microwave power of 180 W. The experimental result obtained at the optimized condition was found to be very close to that predicted value.The catalyst concentration and reaction time were observed to have a significant effect on biodiesel conversion.The result showed that microwave and calcined BDP could enhance biodiesel production in a short reaction time.

Figure 1 :
Figure 1: 3D surface plots describing the response surface for biodiesel conversion as a function of (a) catalyst weight and reaction time, (b) catalyst weight and microwave power, and (c) reaction time and microwave power.

Table 1 :
Reaction parameters and operating levels

Table 2 :
The fatty acids profile of palm oil

Table 3 :
Experimental design based on the Box-Behnken model and their observed and predicted responses

Table 4 :
Summary of biodiesel production using biomass catalyst in different methodsBiomassPotassium concentration (wt%) Method and reaction conditions (ratio molar oil: methanol, catalyst weight, temperature, and

Table 5 :
Comparison biodiesel properties