Direct Transesterification: From Seeds to Biodiesel in One-Step Using Homogeneous and Heterogeneous Catalyst

Abstract

Biodiesel is a renewable alternative biofuel and is an option to diversify the conventional fossil fuels. Moreover, biodiesel is nontoxic, biodegradable, and biomass renewable diesel fuel and its combustion produces low amount of CO, CO2, hydrocarbon, and particulate matter. It can be produced through transesterification reaction. The most common method is homogeneous transesterification process using basic catalyst as NaOH. However, this route has drawbacks as long timespans, saponification reaction, a large amount of solvent, and a large amount of water to neutralize the methyl esters to eliminate the catalyst. This chapter presents the direct transesterification as a green and sustainable alternative method to improve the benefits of conventional transesterification. The direct transesterification is a one-step process to obtain biodiesel from seed crops in presence of a catalyst. Jatropha curcas L. and Ricinus communis have been evaluated as non-edible seeds feedstocks. Also, various acid and basic homogeneous and heterogeneous catalysts have been investigated. Results shown that heterogeneous direct transesterification yields ~99% with 5 wt% catalyst in 4 h without n-hexane for oil extraction or water for purify the biodiesel. Heterogeneous direct transesterification is a promising method of obtaining biodiesel as methanol acts as a reactant and as a solvent.

1. Introduction

The production of biofuels from renewable biomass resources is an attractive way to mitigate CO₂ emissions and alleviate the shortage of fossil fuels. Biofuels have emerged as one of the most promising renewable energy sources, offering one alternative to substitute for petroleum-based diesel fuel. Biomass offers an alternative solution, low-cost solution to a drop-in transportation fuel for blending with conventional diesel. The most common biofuels are based on vegetable oil for fatty acid methyl esters (FAMEs) biodiesel. However, the use of edible oils for bio-based fuels production may create controversy. The biodiesel production from edible oil crops has been questioned in several fields, and their negative effects by its high cost and their destruction of soil cultivation are discussed. Recently, agro-industrial waste and sustainably managed non-food feedstocks have been considered as a new generation of fatty acid methyl esters (FAMEs) or advanced biodiesel. For example, non-edible oilseed crops such as Ricinus communis oil and J. curcas L.. R. communis are a new option for biodiesel production because they are not edible, due to their high oil content, easy propagation, resistance, rapid growth, and adoption in wide agroclimatic conditions [1, 2, 3, 4, 5, 6, 7], and it provides commercially viable alternative to edible oils. These oils are not suitable for human consumption [8], and therefore, its production does not interfere with the food industry or harvesting lands.

Biodiesel from renewable biomass resources is an attractive way to mitigate CO₂ emissions and alleviate the shortage of fossil fuels [4, 5, 6, 9, 10]. Biodiesel can be produced via esterification or transesterification process. Triglycerides (TG) from vegetable oils and animal fats react with short-chain alcohols and acid or base catalysts to render fatty acid methyl esters (FAMEs) and glycerol as by-product [9, 10]. Every triglyceride molecule reacts with three equivalents of alcohol to produce glycerol and three fatty acid (methyl) ester molecules. Methanol is the most common alcohol due to its low price, high activity, and green chemistry metrics. Nowadays, the biodiesel is produced industrially by homogeneous transesterification due to the high yields, lower cost, and the short reaction time. The most common homogeneous catalysts used are NaOH and KOH as basic catalyst and H₂SO₄ as an acid catalyst. However, homogeneous acid and base catalysts present many disadvantages such as long timespans, a large amount of solvent, reactors and engine manifolds corrosion, the difficulty of phase separation, their removal from the resulting biodiesel from the reaction, the neutralization and purification of the biofuel at the end of the reaction, and the excessive use of water to removal emulsions and soaps for these processes. The base catalysts cause saponification, a competition reaction, and decrease the yield of the biodiesel obtained. The acid catalysts increase the biodiesel yield, but stable conditions are needed, or the reaction is affected and slows down the transesterification. As by-product, glycerol has significant value to the industry. However, the recovered glycerol is normally impure due to the presence of salts, soaps, monoglycerides, and diglycerides, and the glycerol purification process consequently adds an additional cost [11, 12].

On the other hand, the use of heterogeneous catalysts represents another strategy to obtain biofuels. These catalysts seek to make the production of biodiesel greener, generating less aggressive residues to the environment and reducing manufacturing costs. In transesterification reaction with heterogeneous catalyst, the reagents and the catalyst are in different phases. Usually, the catalyst is solid; meanwhile, the reagents are in liquid phase. The use of heterogeneous catalysts would result in simpler, cheaper separation processes, a reduced water effluent load as well as lower energy consumption and cleaner operating conditions [9].

Furthermore, catalyst would not have to be continuously added and would be easier to reuse. In general, the saponification reaction is not common with heterogeneous catalyst, and the purity of the products increases because the catalyst is more selective. There would be no neutralization products, enhancing the purity of the glycerol by-product [11, 13]. Several solid base and acid catalysts are explored in transesterification reaction because they offer enhanced process, avoid the quenching steps, and allow continuous operation. Solid acid as heterogeneous catalyst in transesterification reaction of oils into biodiesel renders lower activity, also it is necessary for higher reaction temperatures, and they are lower in comparison with base catalyst [14]. In contrast to solid bases catalysts, solid acids can esterify free fatty acids (FFAs) though to FAME. Some examples of acid heterogeneous catalysts are ZrO₂, TiO₂, SnO₂, zeolites, etc., whereas the basic heterogeneous catalysts most commonly used are CaO, MgO, SrO. The yields obtained with these basic catalysts were 97%, 92%, and 98%, respectively [2, 3, 15, 16]. Some of these catalysts are used to obtain biodiesel by means of used cooking oil, algae, and different oils such as palm oil [13, 15, 16, 17, 18, 19]. CaO/${\mathrm{TiO}}_2$ using canola oil results in a yield of 96.9% [18]. Another alternative is the use of biomass, for example, papaya seed ash as a catalyst, as it contains metal oxides such as ${\mathrm{K}}_2\mathrm{O}$, MgO, and CaO having a yield of 95.6% [19]. Besides, it was shown that sodium zirconate (Na₂ZrO₃) exhibited interesting catalytic properties as a basic heterogeneous catalyst to produce biodiesel via a soybean oil transesterification reaction, which led to good purity [20, 21, 22]. Furthermore, the alteration of the chemical composition as a possible increment of active basic sites, incorporating cesium to the Na₂ZrO₃ catalyst by a simple impregnation method, improves the yield of reaction due to its basic character. The catalytic system Cs-sodium zirconate (cesium-impregnated sodium zirconate) was considered as modified in transesterification reaction of soybean oil and jatropha oil for biodiesel production [23]. Results showed outstanding FAME conversion, from soybean oil, of 98.8% using 1 wt% of catalyst in just 15 min. These catalysts can operate at low concentrations and in short reaction times due to its high basicity and low solubility [20, 23]. Recently, other ceramic materials such as sodium silicate (Na₂SiO₃) [24] and sodium zincsilicate (Na₂ZnSiO₄) [25] have been evaluated with high yields to produce biodiesel ∼99% and ∼ 98%, respectively.

However, these conventional processes consist of two steps: first, the oil extraction and then transesterification of triglycerides in the presence of the catalyst. Both steps generate drawbacks that promote a more expensive biodiesel production. Therefore, for both environmental and economic reasons, there is increasing interest in new alternatives to produce biofuels. It will face new challenges as a technology to overcome the feasible routes to biofuels. Hence, green chemistry presents a new synthesis route to obtain products from conventional or traditional methods favoring the sustainable process, avoiding hazardous and toxic substances (e.g., hexane or dichloromethane) to extract vegetable oils, and favoring the energy efficiency. As a new alternative, the direct transesterification represents an innovative and in situ route to obtain biodiesel from non-edible oils-seed in one step in the presence of homogeneous and heterogeneous catalysts [21, 22].

2. Direct transesterification of seeds

It is well known that biodiesel is conventionally produced by transesterification of vegetable oils using an alcohol and homogeneous or heterogeneous catalysts. The conventional transesterification of oils is based on two steps: 1) the oil extraction and then 2) the transesterification of triglycerides (Figure 1) [15, 16, 26, 27]. In the first step, there are several basic methods for obtaining oils, such as chemical extraction, supercritical fluid extraction, steam distillation, mechanical extraction. The second step is the transesterification of triglycerides from homogeneous or heterogeneous catalysts. Both catalysts could be acid or basic. For example, biodiesel production through the conventional transesterification from soybean, sunflower, and Cynara cardunculus oils (edible oils) and Jatropha and R. communis oil (non-edible oils), with methanol or ethanol using acidic (HCl, H₂SO₄) and basic (NaOH, KOH) homogeneous catalysts, has been reported [5, 26, 27, 28].

Figure 1.

Conventional and direct transesterification of seed oil.

An alternative to the conventional method is the direct transesterification (Figure 1). Figure 1 shows the comparison between conventional (two steps) and direct transesterification (one step). Direct transesterification is a novel green one-pot synthesis to produce biodiesel from non-edible seeds using homogeneous or heterogeneous catalysis. In this in situ one-step process, the extraction and transesterification take place simultaneously in one vessel. The one-step process is more economical and efficient than conventional method [21, 22]. Recently, a few studies have demonstrated that biodiesel production can be achieved from the direct transesterification of seed oils or microalgae biomass with homogeneous catalysts [28, 29, 30, 31, 32, 33, 34]. Although both alkaline and acid homogeneous catalysts were suitable for the reaction, most studies use a basic catalyst (sodium hydroxide, potassium hydroxide, or sodium methoxide) due to reduced corrosiveness, lower amount of catalyst, and reaction time. Also, biodiesel production through the homogeneous direct transesterification from edible oils such as soybean, sunflower, and Cynara cardunculus seeds, and non-edible oils such as Jatropha curcas L and Ricinus communis seeds, with methanol or ethanol, has been reported [1, 5, 28, 31]. FAME yields are very high and comparable with conventional homogeneous as well as heterogeneous catalysis (∼99%).

Recently, heterogeneous direct transesterification represents a new alternative in biodiesel production; due to these, solid catalysts have many advantages over liquid catalyst used in homogeneous direct transesterification. The heterogeneous catalyst exhibits easy separation, lower energy consumption, and cleaner operation. Using solid catalyst could be eliminated the contaminated waste, the formation of soaps, and the emulsification of products that are generated in the homogeneous acid and basic transesterification [21, 22]. It is important to note that the heterogeneous reaction can proceed without polluting or hazardous solvents, for example, without n-hexane for oil extraction. Thus, methanol as a reactant acts as a solvent to carry out the reaction and obtain the methyl esters. This important solvent change increased in seven times the greenness of the heterogeneous reaction in comparison with the conventional method (two-step). Besides, the water consumption to purify the biodiesel is not necessary. Direct transesterification shows the environmental benefits related to solvents and energy consumption [21, 22].

2.1 Direct transesterification of J. curcas and R. communis using homogeneous catalyst

2.1.1 Seed-oil extraction

J. curcas L. is a plant oil with more than 3500 species. It is native to Mesoamerica, covering northern Mexico and Central America. Jatropha seed contains a high percentage of vegetable oil that can be used in biodiesel production [28, 30, 35]. Another vegetable oil used for biodiesel production is the R. communis, which is native to tropical Africa and is considered a highly invasive species in Some Asian and European countries [5, 21, 22]. J. curcas L. and R. communis seeds oils are suitable for biodiesel production because they are highly available seeds and neither their fruits nor plants are edible. These seeds are widely distributed in several places as a weed in urban and agricultural areas, and therefore, they have a great capacity for adaptation that allows them to be cultivated in all tropical and subtropical regions, although it is typical in semiarid regions [32, 33, 34, 35]. Both species are considered toxic plants in the human. For example, jatropha seed contain toxic compounds known as phorbol esters, while in the R. communis seed, there is an albumin known as ricin. These toxic compounds can cause diarrhea, rapid breathing, tumor promotion in humans, etc., in high concentrations [36]. The extraction of the oil is carried out by several methods such as chemical extraction, supercritical fluid extraction, steam distillation, mechanical extraction, solvent extraction, CO₂ extraction, maceration, enfleurage, among others [30, 37, 38]. Also, seed oils can be obtained through the Soxhlet extraction method [37]. These methods of extraction require an excessive energy consumption so in the most cases the cost of the final product becomes more expensive. In addition, the extraction of the vegetable oil involved the use of toxic solvents and water, which makes the process less environmentally friendly.

Table 1 shows the results of jatropha and R. communis oils extraction from the seed and the shell, using n-hexane and methanol as solvents. These seeds were provided by the State of Morelos in Mexico. The oil content of jatropha and R. communis seeds corresponded to yields ranging from 48 to 52% and from 50 to 52%, respectively (entries 1–4), while the oil content of the shell was only <7% (entries 5 and 6). It is important to know that the quantity of the seed oils helps to quantify the amount of biodiesel in the direct transesterification of seeds.

Entrya	Jatropha curcas L.	R. communis	Solvent	Yield %
1	Seed	—	n-hexane	52
2	Seed	—	Methanol	48
3	—	Seed	n-hexane	52
4	—	Seed	Methanol	50
5	Shell	—	n-hexane	5
6		Shell	n-hexane	7

Table 1.

Ricinus communis and jatropha oils extraction.

^a

Soxhlet extraction method for 6 hours.

The amount of vegetables oils extracted agreed with the values reported in the literature, which ranges from 40 to 60% for jatropha oil [1, 28, 36, 37, 38, 39] and ranges from 40 to 56% for R. communis oil [39] by the chemical oil extraction method (Soxhlet extraction). Therefore, the percentage of the vegetable oil extraction in both seeds with n-hexane and methanol is similar. Therefore, the methanol is an interesting option to carry out the reactions as a reactant and solvent.

Vegetable oils (triglycerides) contain mainly mixture of triglycerides (TAG), with a different composition of the alkyl chains depending on their origin. Figure 2 shows the jatropha (Figure 2A) and R. communis (Figure 2B) oil compositions that were calculated according to electrospray ionization mass spectrometry (ESI-MS) analysis [40]. Jatropha oil mainly contains oleic (O), linoleic (L), and linolenic (L_n) triglycerides. The most abundant [TAG + Na]⁺ ions are of m/z 905 (C₅₇H₁₀₂O₆), 901 (C₅₇H₉₈O₆), and 899 (C₅₇H₉₆O₆) that correspond to LOO, LLL, and L_nLL, respectively (Figure 2A). Other triglycerides of m/z 907 (C₅₇H₁₀₄O₆), 903 (C₅₇H₁₀₀O₆), and 897 (C₅₇H₉₄O₆) are attributed to OOO, OLL, and L_nL_nL in less amount. Diglyceride as LL (m/z 639, C₃₉H₆₈O₅) was found. The free fatty acids (FFAs) contained in Jatropha oil were detected by ESI(−)-MS. Figure 2A shows the spectrum of ESI(−)-MS that displays mainly ions corresponding to the deprotonated molecules [FFA-H]⁻ from oleic (m/z = 281, C₁₈H₃₄O₂), linoleic (m/z = 279, C₁₈H₃₂O₂) acids as the most abundant. In the same way, R. communis oil contains several lipids such as tri-, di-glycerides, and FFA. The [TAG+Na]⁺ ion most abundant is the ricinoleic TAG (RRR) with m/z = 955 ion. Diglycerides such as RR (m/z, C39:2) and ricinoleic acid (m/z = 297, C18:0) were detected (Figure 2B).

Figure 2.

TAG and FFA fingerprints of jatropha (A) and Ricinus communis (B) oils obtained by ESI(+)-MS and ESI(−)-MS ionization technique.

2.1.2 Conventional and direct transesterification processes

The production of biodiesel from vegetable oils and fats can be carried out by several routes (pyrolysis, microemulsion, transesterification, etc.). The most used conversion method is from the transesterification of triglycerides in the presence of homogeneous basic catalysts (NaOH and KOH) with methanol. It has been reported that by this synthesis route, the conversion of oil to biodiesel is 99.99%. One of the disadvantages at the industrial level is the recovery of the catalyst and the emulsions and soaps that are obtained during the reaction. Moreover, this conventional process consists of two steps: First, it is necessary extract the vegetable oil from different seeds prior to the transesterification, commonly the oil is extracted with a hazardous solvent or mechanical extraction with high energy consumption and then, in a second step, subsequently transesterification of triglycerides in the presence of the catalyst. These disadvantages promote excessive energy consumption and high operational and biodiesel production costs. Therefore, a new strategy for obtaining biodiesel is proposed. Direct transesterification is a method (in situ or one-pot) for transforming seed oils (biomass) to free acid alkyl esters and glycerin, in the presence of a short-chain alcohol and the catalyst. This one-step process is more economical and efficient than conventional method. Figure 1 shows the comparison between conventional (two steps) and direct transesterification (one step).

Table 2 shows the conventional and direct transesterification reactions of jatropha and R. communis oils using acidic (HCl) (entries 1 and 9) and basic (NaOH) homogeneous catalysts.

Entry	Seed	Catalyst	Catalyst/oil (wt. %)	[CH3OH]/[Oil]b	Time (h)	Solvent	FAME Yieldc (%)
1a	Jatropha	HCl	31.00	327:1	6	—	98.70
2a	Jatropha	NaOH	1.20	9:1	3	—	99.99
3	Jatropha	NaOH	0.30	9:1	9	n-hexane	45.24
4	Jatropha	NaOH	0.30	16:1	9	n-hexane	54.53
5	Jatropha	NaOH	1.20	9:1	9	n-hexane	81.16
6	Jatropha	NaOH	1.20	16:1	9	n-hexane	99.99
7	Jatropha	NaOH	1.20	65:1	9	methanol	99.99
8	Jatropha	NaOH	2.00	16:1	9	n-hexane	99.99
9a	R. communis	HCl	31.00	327:1	6	—	99.10
10a	R. communis	NaOH	1.20	9:1	3	—	99.99
11	R. communis	NaOH	0.30	9:1	9	n-hexane	48.29
12	R. communis	NaOH	0.30	16:1	9	n-hexane	53.00
13	R. communis	NaOH	1.20	16:1	9	n-hexane	99.99
14	R. communis	NaOH	1.20	37:1	9	methanol	97.80
15	R. communis	NaOH	2.00	16:1	9	n-hexane	99.99

Table 2.

Biodiesel from conventional and direct transesterification of jatropha and Ricinus communis seed oils using homogeneous catalysts.

^a

Conventional transesterification.

^b

Molar ratio of methanol to oil.

^c

Isolated fatty methyl esters, FAME.

The Jatropha and R. communis oils were transesterified in methanol (CH₃OH) using acidic catalyst (HCl/CH₃OH, 5 v/v %) that corresponds to (HCl/oil, 31 wt. %); the oil conversion to fatty acid methyl esters (FAME’s) was 98.70% and 99.10%, respectively (entries 1 and 9). In the same way, these non-edible oils were transesterified using basic catalyst (NaOH/CH₃OH, 5 wt. %) that corresponds to (NaOH/oil, 1.20 wt. %) with 99.99% of conversion to FAME.

It has been reported that the use of acidic homogeneous catalysts can catalyze esterification and transesterification simultaneously; however, it is not sensitive to the free fatty acids (FFA) content in the oils, the oil conversion to FAME needs high catalyst concentration, long reaction times, high molar ratio of alcohol to oil, and the catalyst separation is difficult in comparison with the base-catalyzed process [41, 42, 43, 44, 45, 46, 47]. In Table 2, the vegetable oil transesterification using the acidic catalyst (HCl) was conducted by conventional reflux for 6 hours, while for the basic catalyst (NaOH), the reaction remained at 50°C for 3 hours (entry 10). Therefore, the direct transesterification of seed oils was carried out from the basic homogeneous catalyst, NaOH. The solvent extraction step that is required in the conventional process but not in direct transesterification, and is usually the most capital and running cost-intensive [46]. Few studies have demonstrated the FAME production achieved from the in situ transesterification using a homogeneous catalyst, also reactive extraction, of several seed oils such as soybean [30], sunflower [1, 31], Jatropha [22, 28], R. communis [21], microalgae (Schizochytrium limacinum, Chlamydomonas, and Chlorella) [32, 33], and others. In this method, oil-bearing seeds are ground and then reacted directly with the alcohol and catalyst (basic homogeneous and heterogeneous catalysts), thereby eliminating the timespans and a large amount of solvent for the oil extraction.

In particular, the homogeneous direct transesterification of the jatropha and R. communis seed oils in hexane or methanol as solvents, in the presence of NaOH basic catalyst, was studied. Table 2 shows the results of the jatropha (entries 3–8) and R. communis (entries 11–15) seed oil conversion to free acid methyl esters (FAMEs, biodiesel) to different conditions of reaction (ratio of catalyst to oil, molar ratio of CH₃OH to oil, and time). The effect of several factors that are type of solvent, catalyst concentration, temperature, reaction time, methanol-oil ratio, and particle size has been investigated to optimize the direct transesterification of seed oils for achieving maximum oil yield [33, 48, 49, 50]. Jatropha and R. communis seed oil conversion to FAME from direct transesterification was optimized using the following parameters: NaOH homogeneous catalyst amount, the ratio of methanol/oil, and the effect of reaction, which are described below.

Figure 3 shows the FAME yield obtained from homogeneous direct transesterification of jatropha and Ricinus communis seeds. Figure 3A shows the FAME yield versus catalyst amount, and Figure 3B shows the relation between FAME yield versus molar ratio [CH₃OH]/[oil].

1. Effect of NaOH amount

   Figure 3A shows the results of direct transesterification reactions of jatropha and Ricinus communis seeds using methanol and NaOH as catalyst with ratio of NaOH/oil = 0.3 wt. % to 1.2 wt. %. The molar ratio [CH₃OH]/[oil] = 16:1, time = 9 hours were maintained constant using hexane as solvent (Table 2, entries 4 and 12). The catalyst amount affects the yield of the FAME products for both seeds. Low catalyst concentration (NaOH/oil = 0.3% by weight) reached the maximum yield for jatropha and R. communis seed oil of 54.53% and 53.00%, respectively. The conversion increased as the amount of catalyst increased from 0.5 wt. % to 0.8 wt. %, varying for the jatropha seed from 79.5% to ∼97.7% and for the R. communis seed from 88.45 to 98.2%, respectively. Then, the maximum yield reached an equilibrium. The optimum catalyst amount for both seed oils was at 1.2 wt. % of catalyst, and the FAME yield was ∼99.99%. An increment in catalyst amount (2 wt. %) does not affect the oil conversion to FAME.

2. Effect of methanol-oil ratio

   Figure 3B shows the plot of methanol-oil ratio versus FAME yield of the direct transesterification of jatropha and Ricinus communis using a molar ratio NaOH/oil = 1.2 wt. % (Table 2, entries 5–7, 13, and 14). The methanol-to-oil molar ratio varied for jatropha and R. communis seed oils within the range of 9:1–65:1. The maximum oil conversion to FAME products (99.99%) in both seed oils was obtained at the methanol-to-oil molar ratio of 16:1. Table 2 shows the results of these reactions when n-hexane is used as solvent (entries 5, 6, and 13). The excess methanol in the direct transesterification (methanol-to-oil molar ratio of 37:1 and 65:1) is used as a solvent, and the conversion of FAME was 99.99% (Table 2, entries 7 and 14).

3. Influence of the reaction time

   Figure 4 shows the kinetic curves of FAME yields versus reaction time of the jatropha and Ricinus communis seeds. According to the previous results, the optimum catalyst was 1.2 wt. % of NaOH and a molar ratio [CH₃OH]/[oil] = 16:1. The oil conversion to FAME jatropha and FAME R. communis was increased 65% and 64% from 0.5 hours, respectively. Over the period from 1 to 8 hours, FAME yield from jatropha seed was increased with values ranging from 71 to 98% and for R. communis seed was from 68.5% to 98.4%, respectively. The reactions reached equilibrium after 9 hours with the maximum conversion of 99.99% in both cases. The maximum conversion to FAME jatropha and FAME R. communis is observed in the ¹H-NMR spectra (Figure 5).

Figure 3.

FAME yield for the direct transesterification of jatropha and Ricinus communis seed oil versus catalyst amount (A) and ratios [CH₃OH]/[oil] (B).

Figure 4.

Reaction time influence on FAME yields (direct transesterification) from Jatropha and Ricinus communis seeds.

Figure 5.

¹H-NMR (400 MHz, CDCl₃) spectra of jatropha FAME (A) and Ricinus communis FAME (B).

Figure 5 shows the ¹H-NMR spectra of FAME products obtained from jatropha seeds (A) and FAME products obtained from Ricinus communis (B). The signal arising in 3.66 ppm region corresponds to the protons of CH₃-O-. The signals observed at 5.39–5.30 ppm (A) and 5.59–5.53 ppm to 5.42–5.34 pp. (B) represent the protons of vinyl group (HC=CH) contained in the unsaturated fatty acids. The FAME obtained from jatropha; the spectrum (A) shows an intense signal at 2.79 ppm compared with the spectrum (B) that corresponds to protons of -CH₂ group bonded to carbon-carbon double bond in the methyl linoleate chain. The FAME R. communis is composed of methyl ricinolate chains. The spectrum (B) shows the signal of the hydroxyl group (-CH-OH) that is observed at 3.65–3.59 ppm.

The composition of the Jatropha and R. communis biodiesel products obtained from direct transesterification of seed oil using NaOH/MeOH catalysts was determined from electrospray ionization mass spectrometry (ESI-MS) technique. The biodiesel composition was detected as (FAME + Na]⁺ ions. Figure 6A shows the chromatogram of fatty acid methyl esters (FAMEs) from the seed of Jatropha curcas L. We can see that the oleic- (C18:1, 30.00%) and linolenic (C18:2, 48.30%) methyl esters compounds are the most abundant, respectively. The palmitic- (C16:1), stearic- (C18:0), and eicosanoic methyl esters (m/z = 349, C20:0) signals are shown in low ratio. Figure 6B shows the chromatogram of FAMEs from the seed of R. communis. The most abundant signal corresponds to the ricinoleic methyl ester (C18:1-OH, 91.50%), followed by FAMEs in smaller amounts such as linoleic- (C18:2), oleic- (C18:1), stearic- (C18:0), eicosanoic- (C20:0), and dihydroxystearic (C18:0-(OH)2) methyl esters, respectively.

Figure 6.

ESI (+)-MS of fatty acid methyl esters (FAME) from (A) Jatropha curcas and (B) Ricinus communis seeds.

2.2 Direct transesterification of J. curcas and R. communis using heterogeneous catalyst

An enhancement of conventional transesterification is direct transesterification. In this method, to produced biodiesel, the biomass is reacted directly with the short-chain alcohol and the catalyst in one step. To overcome some drawbacks of the homogeneous direct transesterification reaction, heterogeneous catalysts have been used. The heterogeneous in situ or direct transesterification is expected to be an effective FAME production process with low cost and minimal environmental impact because of simplifying the production under mild reaction conditions. It is a cheaper method than conventional because the extraction of the triglycerides and the transesterification are done in situ at the same time decreasing the time to obtaining biodiesel and the excessive use of different resources. However, there are few studies were reported with solid heterogeneous catalyst in direct transesterification.

Direct transesterification has been reported using different heterogeneous catalysts. As acid heterogeneous catalyst, the CT 269 ion-exchange resin was reported to obtain biodiesel from microalgae [51]. The optimum conditions of this reaction were 95°C, and mass ratio catalyst/biomass equals to 0.52:1. On the other hand, the heterogeneous basic catalyst reported was LiOH-pumice [52] obtained by acid treatment and wet impregnation. The highest yield obtained was 47% with 20 wt% of catalyst at 80°C in 3 hours of reaction time and a relation methanol/biomass 12 mL/g. The biomass used in this reaction was Chlorella sp. microalgae. The same microalgae were used to produce biodiesel using carbon-dot functionalized strontium oxide [53]. The reaction was carried out with microwave radiation with dried microalgae mixed with chloroform, methanol, and 0.3 g of the catalyst. The temperature of the reaction was 60° C, the conversion of the lipids into FAME’s was 97 wt%, and the maximum yield was 45.5%. CaO obtained from eggshell waste was evaluated in direct transesterification from A. obliquus microalgae. The eggshell needed a previous treatment to obtain the CaO as catalyst and obtained FAMEs. The biomass was mixed with the catalyst and methanol with a ratio 10:1 wt/vol; the temperature of the reaction was 70°C for a period of 1–5 h. About 86.41% was the yield reported using 1.7% (w/w) [54]. Biodiesel was produced from palm kernel by in situ transesterification using CaO as catalyst. The biomass was mixed with methanol and CaO. The reaction was carried out at 65° C for 3 hours. The oil content in the biomass was 33.08%. In this case, the size of the catalyst was very important, and the optimal size was <1 mm approximately [55]. In another study, strontium oxide as catalyst was evaluated using castor and jatropha seeds with microwave and ultrasound irradiation. The yield obtained with castor seeds was 57.2% of the total weight, and the conversion into FAMEs was 99.9%. With jatropha seeds, the yield was 41.1% with a conversion of the oil into FAMEs of 99.7% by microwave irradiation. By ultrasound irradiation, the yields were 48.2% and 32.9% from castor and jatropha seed, respectively [32].

Recently, Na₂ZrO₃ was evaluated as heterogeneous catalyst using J. curcas L. and R. communis seeds. This ceramic material exhibits interesting catalytic properties as basic catalysts and their low solubility and high stability. For heterogeneous direct transesterification, the seeds were ground and mixed with methanol and the catalyst with a molar ratio 1:65 (oil/methanol) at 65°C. At the first reactions, n-hexane was added to the reaction to favor the vegetable oil extraction. The presence of n-hexane in the reactions using 10 wt% of Na₂ZrO₃ as catalyst decreases the conversion of FFAs in 76% for Jatropha Curcas L. and 66% for R. communis seeds [21, 22]. This is caused by the decrement in the contact area between the reaction products caused by the non-polar solvent despite the increase to 10 wt% of the catalyst. The ideal conditions using only methanol as reactant and solvent were 5 wt% of catalyst at 65° C for 8 hours obtaining conversion to FAME of 99.9% in the first cycle and 72.5% after 5 cycles of reusing the catalyst. Figure 7 shows the kinetic curves of the FAME yield of in situ transesterification reaction of Jatropha seeds. The use of methanol as reactant and solvent in transesterification reaction represents an environmental benefit due to methanol being environmentally friendly, and it can be a replacement for n-hexane or dichloromethane, which are toxic and hazard solvents [21].

Figure 7.

Kinetic curves of the FAME conversion efficiency of in-situ transesterification reaction of Jatropha seeds and methanol heterogeneous catalysts.

As with jatropha seeds, the heterogeneous direct transesterification of Ricinus communis seed was performed with Na₂ZrO₃ in the presence of methanol. The yield obtained with optimal conditions (65° C, 5 wt%) was ∼99.9% just in 4 hours. Figure 8 discloses the FAME yield versus the reaction time. The selectivity between the catalysts reduced the reaction time and increased the yield and conversion. The hydroxyl group, -OH, of the ricinoleic methyl ester interacts with the high surface basicity of the solid catalyst, increasing their catalytic activity (Figure 5B shows the 1H-NMR spectra of the FAME R. communis, similar spectra). In this reaction n-hexane was substitute by methanol. The toxicity and environmental hazard decreased. Methanol as solvent increased the greenness of heterogeneous process, and the products obtained show high purity.

Figure 8.

kinetic curves of the FAME conversion efficiency of in situ transesterification reaction of Ricinus communis seeds and methanol heterogeneous catalysts.

In addition, some reusing reactions are being carried out to determine the selectivity and stability of the catalyst. The heterogeneous direct transesterification (one-step process) using Na-based catalyst is a promising alternative for more sustainable, cleaner, and efficient FAME production.

After a few reusing reactions, a reduction in FAME conversions was observed. This may be attributed to the adsorption of organic matters from the biomass and the glycerol on the catalyst surface, which generates a low contact area between the oil and the Na₂ZrO₃.

In recent years, heterogeneous acid catalysts have been reported in biodiesel production and are an emerging field of research. But before producing competitive biodiesel, it is necessary to improve some aspects of catalyst as the stability of acid sites and the control of the surface properties.

3. Conclusions

Direct transesterification reaction from non-edible crops was successfully evaluated using both homogeneous and heterogeneous catalysts, from Jatropha curcas L and R. communis seeds, in one step. However, heterogeneous direct transesterification provides a new alternative and promising method to obtain high-purity biofuels. No toxic organic solvent was used to extract the oil from seeds, or no water was used to neutralize the products. Furthermore, methanol acts as reactant and solvent in the in situ reaction. This method improved the FAMEs production because the heterogeneous catalyst produces maximum conversion of ∼99.9% with 5 wt% of catalyst at 65°C. For R. communis, the optimal conditions were reached in 4 hours of reaction time due to the -OH group. In addition, the reuse and the stability of the solid catalyst in the direct transesterification reactions with yields of>72.5% in fifth cycle were evaluated. However, it is necessary investigate the optimal condition to increase the yield of FAME by favoring the contact between the biomass and the catalyst. Finally, heterogeneous direct transesterification has potential environmental and energy benefits in comparison with the conventional biodiesel production method.

4. Transesterification future research direction

The processes for the biodiesel production must be carried out in a sustainable way using third- and fourth-generation feedstocks. Currently, our work group is focused on the development of green processes for the preparation of biodiesel through green routes that imply incorporating principles of sustainable chemistry. These clean processes incorporate the principles of a) atomic efficiency, b) use of renewable raw materials, c) preparation of heterogeneous catalysts, d) prevention of waste generation, and e) heterogeneous design reactors.

1. According to the results discussed in this chapter (Section 2), direct transesterification (in situ) is a promising alternative for obtaining biodiesel, this method simplifies the conventional process and eliminates the oil extraction stages. With this chemical synthesis, we are reducing the use of solvents and making the process of obtaining biodiesel more efficient and environmentally friendly in short times.

2. The use of raw materials or biomass considered as waste generates an added value to the transesterification process. For example, according to data from the United Nations Food and Agriculture Organization, more than 6.5 million tons of avocado are produced, where Mexico contributes 34% with approximately 2.1 million tons. In this research, the avocado stone is incorporated as a raw material to produce biodiesel, considered a renewable resource.

3. It is being proposed to prepare heterogeneous catalysts from new synthesis methods or using biomass waste as precursor, for example, eggshell waste because of the high content of CaO. There is great interest in these types of catalysts for environmental, energy, and economic reasons. On the other hand, studies are still being carried out on its reuse in the direct transesterification cycles. Also, it is important to investigate the interaction between seeds and the catalyst.

4. Biomass as residual by-product of direct transesterification reactions of the oil seeds is a value-added product. One proposal for its use is to obtain biopolymers; that is, these biopolymers are expected to have applications in the manufacture of bags, and that they can replace conventional plastics.

5. One of the technological challenges in biodiesel production is the design of heterogeneous catalysts. Thus, the scaling of heterogeneous processes is a key for the implementation of direct transesterification. The use of heterogeneous catalysts would result in simpler, cheaper separation processes as well as capital and energy costs.

Acknowledgments

Authors are grateful to Centro de Nanociencias y Micro y Nanotecnología del Instituto Politécnico Nacional (IPN) for their assistance in ESI-MS and 1H-NMR analyses. Also, the authors greatly appreciated the support from SECTEI CM-059/2021, SIP-IPN 2104 (modulo 20220625) projects and BEIFI scholarship. Finally, A. Martínez Ponce acknowledges the financial support from the DGAPA-UNAM PAPIME PE106522 project.