Sustainable Biodiesel Production from Turkish Coffee Waste Oil: A Comparative Study with Homogeneous and Heterogeneous Catalysts

Abstract

Biodiesel is a renewable fuel obtained from vegetable or animal oils and a good alternative to fossil fuels. Since the raw material cost constitutes much of the total biodiesel production cost, cheaper waste oils are potential substitutes for vegetable oils in biodiesel production. Coffee is the product with the second-highest trade volume in the world after oil, at approximately 1.5–2 million tons annually, and results in a huge amount of waste. Recycling such waste into fuels is a promising way to solve the waste problem and this waste is potential raw material for biodiesel production. In this study, biodiesel was produced from the oil extracted from Turkish coffee waste, which has approximately 10–15% oil. The molar ratio of methanol to Turkish coffee waste oil (12, 15, 20), catalyst concentration (1, 1.5, 2 wt.%), and time (60, 120 min.) were the studied parameters. Potassium hydroxide and ion exchange resin were used as catalysts in the experiments. The highest biodiesel yield was obtained with potassium hydroxide catalyst, while the results obtained by using ion exchange resin may be improved. After the parametric study was completed for biodiesel production, the physical and chemical properties of the produced biodiesel were compared with the international biodiesel standards. The values of properties were at an acceptable level and are suitable for improvement.

1. Introduction

In recent years, energy consumption has increased significantly due to the growth in the population and the change in lifestyles. The increase in energy demand has resulted in the depletion of fossil fuel resources, which are typically limited, and their use raises serious environmental concerns. So, the urgent need for alternative and renewable energy sources has appeared. Renewable energy is necessary to reduce foreign dependency in meeting the energy needs of countries and to ensure sustainable energy use. Today, approximately 20% of the energy consumed worldwide is obtained from renewable energy sources [1,2,3,4,5].

Biodiesel, which is a sub-category of biomass energy, is an alternative fuel obtained from vegetable or animal-origin oils. It is easier to produce biodiesel than other renewable energy sources. Biodiesel is a renewable, non-toxic, and easily biodegradable liquid fuel, and has recently shown promise in being used in diesel engines separately or with conventional diesel fuel. It may also reduce carbon monoxide, unburned hydrocarbons, particulate matter, and sulfur dioxide found in exhaust emissions [6,7,8]. Biodiesel is a type of fuel that is released as a result of the reaction of oils with a short-chain alcohol with the help of a catalyst. Oils to be used as raw materials by the transesterification method, which is the most common method in biodiesel production, are esterified with a monohydric alcohol (ethanol, methanol), in the presence of a catalyst (acidic, basic catalysts, and enzymes), giving biodiesel (fatty acid methyl esters) and glycerol as the main products. By removing glycerol during the transesterification reaction, the oil is thinned and its properties are brought closer to diesel fuel. The theoretically transesterification reaction for biodiesel production is given in Figure 1 below:

Biodiesel is generally produced from edible vegetable oils. It has been reported that the raw material cost constitutes approximately 80% of the total biodiesel production cost. Unused and virgin oils of high quality make biodiesel more expensive than diesel fuel and also cause the prices of vegetable oil to increase. Therefore, a low cost of biodiesel-production raw materials is required [9,10,11,12,13].

Coffee has been consumed for over a thousand years and takes its name from the city of Kaffa in today’s Ethiopia. It is the second-most important raw material that shapes the world economy after oil and it is estimated that coffee is consumed by 80% of the world’s adult population. The report of the International Coffee Organization (ICO) published in June 2023 shows that coffee consumption has shown an increasing trend over the years. According to the report, although there are some decreases in coffee production, the amount of consumption is constantly increasing. It states that worldwide coffee consumption increased by 1.8% between 2016/2017 and 2019/2020 [14,15,16]. Turkish coffee is known as the method of preparing and cooking coffee discovered by the Turks. Traditional Turkish coffee is a different culture with its unique taste, smell, foam, preparation, cooking, presentation, tools, and equipment. Coffee consumption in Turkey in 2020/2021 was approximately 106 million kg and is increasing every year, with 1.2 kg of coffee per person per year. In Turkey, Turkish coffee accounts for 40% of outdoor beverage preference, whereas this value is around 65% in homes [15,16,17,18].

The chemical composition of coffee waste depends on a variety of factors, including plant species, place of cultivation, age of the coffee plant, climate, and soil conditions. In previous studies, it has been determined that coffee waste contains 10–20% fat in terms of weight and 80–90% of the weight of the oil is glycerides, these consisting of free fatty acids (FFAs). Assuming that the oil in coffee waste is 16% by weight, the use of the coffee waste as biodiesel raw material produces 0.9 million tons of biodiesel which equals 3.5% (when the total world fuel supply is considered to be 26 million tons). Therefore, coffee waste has become an interesting alternative to traditional biodiesel feedstocks and also reduces local solid waste generation as it does not directly compete with food crops for farmland [19,20,21].

Commercial production of biodiesel is based on the use of strongly basic (i.e., NaOH and KOH) or acidic solutions (H₂SO₄) as catalysts for its transesterification with alcohols (especially methanol). Homogeneous catalysts are often corrosive to process equipment and can lead to hydrolysis of oils and fatty acid methyl esters, depending on the water content of the feedstock. The cost of biodiesel can still be higher today than petroleum-based diesel. Using waste oils as raw materials for biodiesel production also has the potential to solve the problem of waste oil disposal. But, if waste oils contain more than 1 wt% free fatty acids, soap formation occurs under homogeneous alkaline catalysis. The use of heterogeneous catalysts simplifies biodiesel production and separation processes. Because they can be easily separated from the reaction mixture and allow multiple uses. The main reasons for the increasing interest in ion exchange resins are that they are non-corrosive and insensitive to free fatty acids. Due to their high concentration, they can catalyze the reaction under mild reaction conditions.

The aim of this study is to evaluate Turkish coffee waste as a raw material for biodiesel production. There are studies about biodiesel production from coffee waste oil. These studies are generally carried out by using filter coffee waste oil as feedstock. Although the origin of Turkish coffee and the other coffee types (such as filter coffee) is the same, since the cooking and brewing methods are different, the chemical compositions of their waste may differ. The main novelty of this study is that the oil was extracted from only Turkish coffee waste, which has not been observed in the literature.

Table 1. Studies on biodiesel production using filter coffee waste oil.

Methanol/Oil	Catalyst	Catalyst wt (%)	Time (min)	Biodiesel Yield (%)	Reference
30	KOH	4	180	97.11	[19]
4:1	H2SO4/KOH	1	120	--	[22]
--	Microalgae	1.1	132	20	[23]
6:1	H2SO4/NaOH	20:1/1.5	90	--	[24]
6:1	H2SO4/NaOH	1	120	73.4	[25]
5:1–9:1	H2SO4/KOH	1.5	60–150–240	55–85	[26]
9:1	NaOH	1	60	92	[27]
6:1	NaOH	1	--	81	[28]

shows the previous studies carried out by using filter coffee waste oil as feedstock in biodiesel production. They mostly used acidic or basic homogeneous catalysts with different alcohol/oil ratios and catalyst weights. In this study, biodiesel was produced by using the traditional homogeneous-type potassium hydroxide catalyst and heterogeneous-type ion exchange resin (Amberlyst 15) catalyst. There are many studies with ion exchange resins being used as a catalyst in transesterification reactions, but there is no study reported in the literature with any type of coffee waste extracted oil. Some studies focusing on biodiesel production by using ion exchange resins as catalysts found in the literature are given in

Table 2. Studies on biodiesel production using ion exchange resins as catalyst.

Oil	Catalyst	Reference
Mustard oil	Amberlyst 15	[29]
Waste cooking oil	Amberlyst 15	[30]
Soybean oil	Amberlyst 15 Wet microalgae	[31]
Oleic acid	Amberlyst 15	[32]
Pongamia oil	Indion 810	[33]
Wastewater sludge	Amberlyst 15, 36; IR120 IR120	[34]
Waste cooking oil	Purolite D5081, CT-122, CT-169, CT-175, CT-275, Diaion PA306s	[35]
Waste cooking oil	Purolite D5081, Novozyme 435	[36]
Waste cooking oil	Diaion PA306S	[37]

. Finally, the physicochemical properties of the produced biodiesel were compared with international standards, and its suitability for commercial production was discussed.

2. Materials and Methods

The flowchart of the experimental study is given in Figure 2. It shows biodiesel production starting from the collection of waste. After drying the coffee waste, the oil was extracted and Turkish coffee waste oil was obtained. Then, by using the required transesterification reaction conditions, which are explained below, biodiesel was produced.

2.1. Chemicals

Turkish coffee waste was collected from homes and local cafes. The main chemicals, methanol (CH₃OH) and potassium hydroxide (KOH), were purchased from Merck Co., NJ, USA. The cation exchange resin in the H+ form, Amberlyst 15 (A15), was obtained from Rohm and Haas Co, Philadelphia, USA. The Soxhlet process was applied for the extraction process in which petroleum ether was used as an organic solvent. The Soxhlet device temperature was kept at 65–70 °C. The oil was separated from the organic solvent using a rotary vacuum evaporator. The yield was calculated on a dry weight basis. The experimental results showed that the oil content of Turkish coffee waste was 16.8 ± 0.1% (on a dry weight basis). The fatty acid profile of the oil obtained by extraction is given in

Table 3. Chemical composition of Turkish coffee waste oil.

Fatty Acid Composition		(wt%)
Name	Formula
Oleic acid	C18H34O2	40.78
Linoleic acid	C18H32O2	34.63
Myristic acid	C14H28O2	12.5
Palmitic acid	C16H32O2	3.98
Stearic acid	C18H38O2	3.33
Eicosenoic acid	C24H48O2	1.98
11-Eicosenoic acid	C20H38O2	1.32
Behenic acid	C22H44O2	0.73
Tetracosanoic acid	C24H48O2	0.45
Others		0.3

. This extraction process and fatty acid analysis was carried out at Tübitak, Bursa Test and Analysis Laboratory (BUTAL).

2.2. Acid Catalyzed Pretreatment

The acid value of oils was measured by using the standard titration method. The FFA content of the coffee waste oil was determined by using the standard titration method [12,38]. The FFA content of the extracted oil was obtained at a value of 4.12% and it indicated the necessity of an esterification reaction first. This is the step before the main transesterification process [22,26]. Esterification was carried out using a ratio of 20:1 M with sulphuric acid (H₂SO₄) as a catalyst. As a result of this reaction, the percentage of FFA was lowered to 1.8%.

2.3. Transesterification Experiments

The transesterification reactions of oil with methanol were carried out in a three-necked, refluxed 250 mL batch reactor at 65 °C. A 600 rpm stirring speed was applied using a magnetic stirrer. Catalyst amounts of 1%, 1.5%, and 2% of the weight of the oil and reaction times of 60 min and 120 min were the studied parameters. The methanol/oil molar ratios were 12:1, 15:1, and 20:1. The studied parameters of the experiments are shown in

Table 4. The parameters studied in the transesterification experiments.

Catalyst Type	Methanol/Oil Ratio (Molar)	Catalyst Amount (wt%)	Time (min)
KOH-Amberlyst 15	12–15–20	1	60
			120
		1.5	60
			120
		2	60
			120
			120

. All experiments were made three times, and the results were averaged. While calculating the biodiesel yield, the following formula was used [39,40]:

$$Biodieselyield\left(\%\right)={\displaystyle \frac{weightofbiodiesel}{weightofoil}}\ast 100$$

2.4. Biodiesel Tests

After biodiesel is produced, characterization tests must be performed. The purpose of these tests is to determine whether the biodiesel obtained complies with the specified standards. The characterization of fuel properties determines the quality of the fuel. In this study, the experiments were repeated for the biodiesel obtained as a result of the experiment that gave the highest efficiency result for both catalyst types. These experiments had a molar ratio of 20 and the catalyst quantity of 2% with a reaction time of 60 min for the KOH catalyst; the molar ratio was 20, the catalyst quantity 2%, and the reaction time 120 min for the A15 catalyst. The biodiesels obtained as a result of these studies were subjected to analysis. The chemical composition of the Turkish coffee waste oil biodiesel when using both catalysts analyzed at Bursa Tubitak Butal is given in

Table 5. Chemical composition of biodiesel produced from Turkish coffee waste oil produced by using two type of catalysts.

Fatty Acid Composition		(wt%)
Name	Formula	KOH	A15
Linoleic acid	C18H32O2	33.75	32.96
Palmitic acid	C16H32O2	28.16	26.75
Myristoleic acid	C14H26O2	14.75	15.09
Oleic acid	C18H34O2	7.85	8.28
Stearic acid	C18H38O2	5.09	6.04
Lignoceric acid	C24H46O2	4.19	4.85
Arachidic acid	C20H40O2	2.14	1.99
Myristic acid	C14H28O2	1.96	2.05
Behenic acid	C22H44O2	1.01	0.92
Lauric acid	C12H24O2	0.99	0.89
Others		0.11	0.18

. Measurements of the properties were performed according to standards [12].

It is an important advantage that biodiesel obtained from vegetable and animal oils by the transesterification method does not contain aromatics and sulfur. However, it has an oxygen content of around 10–15% of its mass. Biodiesel is a renewable, biodegradable, non-toxic fuel and has combustion characteristics similar to diesel fuel. For this reason, it is preferable to use it in engines directly or by mixing it with diesel fuel. Biodiesel also has some negative fuel properties such as a low volatility, high density, high viscosity, and high pour point.

3. Results and Discussion

3.1. Biodiesel Yield

3.1.1. Effect of Methanol/Oil Ratio

The investigation of each parameter was carried out by keeping the other parameters constant. One of the most important parameters affecting biodiesel is the methanol/oil molar ratio. Stoichiometrically, three moles of fatty acid methyl ester (biodiesel) are formed as a result of the reaction of three moles of methanol and one mole of triglycerides. But it is known that more volumes of methanol are required to drive the reaction forward and reach maximum yield [41]. Additionally, excessive use of alcohol helps reduce the viscosity of the reaction medium in cases where the density difference between the reactants is large. This serves to increase the contact surface between reactants. The molar ratio of methanol to oil values were 12, 15, and 20 in this study. The results of the experiments showed that the biodiesel yield increased as the mole ratio of methanol to oil was increased (Figure 3 and Figure 4). The maximum biodiesel yield obtained was 91.12% with a KOH catalyst at 2 wt% and a methanol oil ratio of 20 for 60 min., and a yield of 90.34% was obtained for a reaction time of 120 min.

3.1.2. Catalyst Type

In this study, it was observed that the biodiesel yield values obtained with Amberlyst 15 catalyst were approximately 8–13% lower than the KOH catalyst for all studied reaction parameters. According to the literature, basic catalysts are the catalysts that give better efficiency than the acid catalysts in biodiesel production [42]. Regarding the efficiency as compared with the disadvantages explained above, the applicability of ion exchange resins has been investigated and it has been observed that a non-negligible high efficiency has been achieved.

3.1.3. Reaction Time

Since energy consumption in the biodiesel production process is directly related to time, it is important to determine a sufficient reaction time. As seen in Table 1, biodiesel production studies have been carried out between a range of 30 min and 9 h. In the previous studies focused on finding the optimum reaction time for biodiesel production, it was stated that 120 min was the optimum reaction time for the waste oil [43]. In another study, it was observed that the optimum reaction time for base and acid catalysts are generally within the range of 1–2 h to obtain the maximum biodiesel yield [3]. In this study, reaction time was evaluated as 60 min and 120 min. It was found that the effect of contact time did not cause a significant change in biodiesel yield when compared to the effect of other parameters (Figure 3 and Figure 4).

3.1.4. Effect of Catalyst Amount

It is known that the effect of the amount of catalyst is important in biodiesel production reactions and if catalyst is not used, it is very likely that the reaction will not occur. However, if the amount of catalyst is too high in the mixture, it may negatively affect the effect of mixing and cause the reactants to not interact sufficiently with the catalyst. This may lead to a decrease in reaction efficiency. On the contrary, if a smaller amount of catalyst is used than required, the desired reaction efficiency value will not be reached. Therefore, the appropriate amount of catalyst needs to be investigated. The literature listed in Table 1 shows that different amounts of catalysts between 1 and 4% were studied. Goh et al. (2020) studied KOH at a quantity of 4% and found a biodiesel yield higher than 97% [19]. In this study, studies were carried out at 1%, 1.5%, and 2% catalyst ratios by weight, keeping the methanol/oil molar ratio (at 12, 15, and 20) and time constant (at 60 and 120 min). For both catalyst types, biodiesel yield increased when the catalyst weight was increased from 1% to 1.5% and 1.5 to 2% (Figure 3 and Figure 4).

3.2. Biodiesel Characteristics

There exist some standards for physical and chemical properties of synthesized biodiesel in order to increase the commercial value of biodiesel [44,45,46,47]. These properties were determined for this experimental study and compared with the literature values in

Table 6. Physicochemical properties of standard limits [48,49], waste coffee oil biodiesel from references, and Turkish coffee oil biodiesel produced by using KOH and A15 catalysts.

Property	Limits	Ref. [22]	Ref. [25]	Ref. [19]	Biodiesel
					KOH	A15
Density (kg/m3)	860–900	895	891.5	886.2	878.6	899.4
Kinematic viscosity (40 °C) (mm2/s)	3.5–5	5.16	5.26	4.16	4.85	5.03
Saponification value (mg KOH/g oil)	---	209.54	---	---	184	190.7
Acid value (mgKOH/g oil)	<0.5	0.5	0.78	1.1	0.2	0.23
Iodine value (Wijs g/100 g oil)	<120	85.89	73.41	---	116	128
Peroxide value (meq g/kg)	2.1–4.1	---	---	---	2.8	3.1

. The analyses were carried out at Bursa Uludag University and Bursa Tubitak Butal by ASTM D 6751 and EU methods [12].

The appearance of Turkish coffee waste oil was a dark brown color and highly viscous with a sweet scent. The values specified in Table 6 are the physical and chemical results of biodiesel obtained by using the oil obtained from Turkish coffee waste. The results are in accordance with the references and the standard values in the literature for both catalysts. Although they provide the standard values, the biodiesel values obtained with the conventional catalyst KOH are more acceptable than A15. This result also brings into consideration the need to develop studies conducted with ion exchange resins.

The density of vegetable oils generally varies between 860 and 920 kg/m³, although it varies depending on the oil type. The density of the resulting biodiesel decreases and can reach the level of diesel fuel. In this study, the densities of the biodiesel samples were found within the fuel standards. Viscosity is, similarly to density, a characteristic property of biodiesel. High viscosity can cause poor atomization of the fuel, poor combustion, clogging of the injectors, and deterioration of the lubricating oil. Viscosity varies depending on temperature. The viscosity value of the produced biodiesel was between 3.5 and 6 mm²/s at 40 °C.

Since the raw materials used in biodiesel production show acidic properties, the pH value of the obtained biodiesel samples should be between 0 and 7. This is an indication that the fuel has acidic properties. Fuels are between desired pH values. Acid value should be measured before and after the experiments. Since higher acid values may cause corrosion, the determined standard limits are important. The results of the analyses showed that the biodiesel obtained by using two catalysts have acid values lower than those of the standards. The saponification value is the amount in milligrams of KOH required to saponify 1 g of oil. Also, there are acceptable values for saponification values. The iodine number varies depending on the properties of vegetable oils and the number of double bonds. Fuels with high iodine numbers may cause blockages in the injector holes or damage to the combustion chamber. The level of iodine and peroxide numbers are within the set limits.

4. Conclusions

The biggest obstacle to the commercial application of biodiesel is the cost of production, especially the cost of raw materials. For this reason, studies in recent years have focused on obtaining biodiesel from waste oils. Since coffee is the second largest commercial product in the world, its waste is high enough to produce biodiesel in a cheaper way. In this study, obtaining biodiesel from the waste of Turkish coffee was attempted, it being consumed at a very high rate, especially in Turkey, and whose waste is much greater than filter coffee because of its preparation technique.

Here, the oil was extracted from spent coffee grounds using the Soxhlet extraction method. In the literature, the effect of the solvents used in the extraction method on the yield and chemical and physical properties of the extracted oil has been studied. This topic could also be studied in more detail.

During the study, the effect of the methanol/oil ratio, catalyst type, catalyst amount, and reaction time were the studied parameters. The biodiesel yield values were found to increase with an increase in the methanol/oil ratio. The studied catalysts were the conventional basic catalyst KOH and a different catalyst, an ion exchange resin, Amberlyst 15. Although KOH gives better results than A15, the values reached by using A15 cannot be ignored in future studies for coffee waste oil. The two-step transesterification process has been found to be highly effective for converting coffee waste oil to biodiesel. In the first step, the FFA level of the extracted oil is reduced by acid-catalyzed esterification, while in the second step, the remaining triglycerides are transesterified with methanol using a catalyst to produce biodiesel and glycerol. The results showed that catalyst amount increases the biogas yield positively between 1 and 2% wt. The reaction times applied to the experiments were 60 and 120 min. These times were chosen according to the literature, but the results did not show a significant change. It was observed that the properties of biodiesels were within the specified standards.