Solvent-free one step aminolysis and alcoholysis of low-quality triglycerides using sodium modified CaO nanoparticles as a solid catalyst

Abstract

Here, we report the solvent-free one step preparation of fatty acid amides and fatty acid alkyl esters from low quality triglycerides using a solid catalyst. A series of Na⁺ modified CaO has been prepared in the form of nanoparticles by a modified incipient-wetness impregnation method, and used as a solid catalyst for aminolysis and alcoholysis of triglycerides. The prepared catalyst 3-NaC/CaO-450 (3 wt% Na impregnated in CaO) was found to exist in nanoparticle form (particle size = 25 nm) as suggested by TEM, DLS and powder XRD studies, have a surface area of 10.9 m² g⁻¹ (BET method) and found to have the basic strength of 15.0 < H_ < 18.4 as supported by the Hammett indicator test. The catalyst was found to complete the aminolysis (5 wt% catalyst; 6 : 1 molar ratio of diethanolamine/oil; 110 °C) with excellent recyclability and is also equally efficient for the alcoholysis (5 wt% catalyst; 12 : 1 molar ratio of methanol/oil; 65 °C) of used cotton seed oil.

---

1. Introduction

Renewable raw materials are going to play a significant role for upgrading sustainable green chemistry. Renewable raw materials viz., vegetable oils and fat share the great proportion of the current consumption in the chemical industry.¹ The derivatives of vegetable oils have numerous benefits viz., they are renewable, nontoxic, biodegradable and environmentally friendly. These derivatives have potential industrial applications such as fatty acid methyl esters (FAMEs) which are a desired substitute for fossil fuels² and fatty acid amides (FAA) and have attracted attention due to their biological activities and industrial applications in surfactants, lubricants, cosmetics, shampoos, foam control agents, fungicides, corrosion inhibitors, water repellents and detergents.^3–5 Due to low reactivity and thermal properties they are also used for the synthesis of anti-block and anti-slip additives in polyethylene films.⁶ Fatty acid amides also considered to increase the ignition quality characteristic of the fuel as it increases the cetane number of the fuel.^7,8

FAMEs mostly obtained by the alcoholysis (transesterification) reaction of the naturally occurring triglycerides (vegetable oils or animal fat) with short chain alcohols (e.g., MeOH and EtOH) in the presence of homogeneous catalysts (for example NaOH, KOH, NaOMe, KOMe, HCl and H₂SO₄)^9–11 or heterogeneous catalysts (for example Li/CaO, Zn/CaO).^12,13

On the other hand due to their numerous applications, fatty acid amides produced at industrial scale from fatty acids by reaction with anhydrous ammonia at high temperature (∼200 °C) and high pressure (345–690 kPa).¹⁴ Fatty acid amides can also be prepared from fatty acids or fatty acid alkyl esters derived from fatty acids through treatments with different amine compounds.^15,16 Enzymatic catalyzed amidation from the fatty acids or their esters has also been reported.^14,17,18 Triglycerides or vegetable oils can be converted into their fatty acid amides via amidation using ammonia or amine compounds.^19,20 The fatty acid amides were prepared by refluxing palm oil and urea using sodium ethoxide as a homogeneous catalyst in the presence of ethanol.²¹ Monosubstituted fatty acid amides were synthesized by reacting primary amine with vegetable oil, tallow, and fish oil at the boiling point of amine for longer reaction time.^22,23 Ethylenediamine has also been used for the preparation of fatty acid amides by using fatty acids at 180–185 °C under nitrogen environment and with continuous removal of water.²⁴ However, high temperature and long reaction time causes self-condensation of diethylamine that results to form N,N′-bis(2-hydroxyethyl) piperazine or morpholine instead of FAA.^25,26

The preparation processes of FAMEs and FAA utilized mostly the homogeneous catalysts, however, homogeneous catalysts have several disadvantages like, non-recyclability, contaminated product and deactivation by high moisture (>0.3 wt%) and free fatty acid (>0.5 wt%) contents in feedstock.^27,28 Also, the use heterogeneous catalyst especially in case of FAA synthesis, such as lipase, require high temperature and high pressure. Thus, the whole process become tedious, lengthy and requires large amounts of water and energy. To overcome these drawbacks, solid catalysts could be more advantageous as these are reusable, less corrosive, easy to separate from the reaction mixture^27,29 and effective even when moisture and FFA contents are high in feedstocks.¹²

Present study demonstrated the preparation of Na⁺ modified CaO in nano-particle form and its characterization by powder XRD, SEM, TEM, Hammett indicators and surface area measurement studies. The prepared catalyst has been used as solid catalyst and found as fully efficient for the one step solvent free aminolysis and alcoholysis of used cotton seed oil with diethanolamine (DEA) and methanol, respectively.

2. Experimental section

2.1 Materials and methods

The chemicals used in present study viz., sodium carbonate, lithium carbonate, potassium carbonate, calcium oxide, methanol (99.8%), methyl laurate (>98%) and diethanolamine (99.8%) were purchased from Sigma-Aldrich, USA.

The free fatty acids (FFAs) value, iodine, and the saponification value of the mutton fat (MF), soybean oil (SO), virgin cotton seed (CSO), used cottonseed seed oil (UCSO), castor oil (CO), karanja oil (KO), and jatropha oil (JO) were determined by following the methods as reported in literature³⁰ and the moisture content was determined by the Karl Fisher titrimetric method (Table 1).

Table 1 The chemical analysis of vegetable oils used as feedstock in present study

S. No.	Feedstock	Free fatty acid value (wt%)	Moisture content (wt%)	Saponification value (mg KOH per g)	Iodine value
1	Used cotton seed oil	2.2	0.40	190.3	101.3
2	Karanja oil	4.1	0.35	184	96.6
3	Jatropha oil	8.4	0.38	195.2	101.3
4	Virgin cotton seed oil	0.1	0.31	188.4	102.4
5	Soybean oil	0.2	0.27	189.6	125.1
6	Mutton fat	1.3	0.32	192.1	46.3
7	Castor oil	2.0	0.34	180.2	90.5

---

2.2 Preparation of catalyst

The modified incipient-wetness impregnation method³¹ was employed to prepare nanocrystalline sodium metal ion impregnated calcium oxide. In a typical preparation, CaO slurry (10 mg/40 mL ethanol) was sonicated for 1 h then 10 mL of alkali metal (Na₂CO₃ or Li₂CO₃ or K₂CO₃) solution in ethanol of desired concentration was added drop wise into CaO slurry and stirred for 2 h at room temperature. The concentration of the sodium carbonate was also varied to obtain Na⁺ loading in the range of 1 to 5 wt% in CaO. The mixture was stirred for 3 h, then dried and calcined at varying temperature of 250–650 °C for 12 h. The solid thus obtained was characterized by powder XRD, BET surface area measurement, Hammett indicator test, DLS, SEM and TEM techniques.

The prepared catalysts were designated as xx-NaC/CaO-T, where xx represents the sodium concentration (wt%) in CaO and T represents calcinations temperature. For example 3-NaC/CaO-450 represent the catalyst prepared by impregnating 3 wt% of Na⁺ (using Na₂CO₃) in CaO and calcined at 450 °C.

2.3 Aminolysis reactions

Aminolysis reactions of used cotton seed oil, mutton fat or methyl laurate and FAMEs derived from used cotton seed oil and mutton fat have been performed with diethanolamine (DEA) in the presence of 3-NaC/CaO-450 catalyst as shown in Schemes 1–3. All aminolysis reactions were performed in a 100 mL two necks round bottom flask fitted out with a water-cooled condenser, magnetic stirrer and oil bath. Vegetable oil was mixed with diethanolamine to maintain the diethanolamine to oil molar ratio of 6 : 1, in the presence of 5 wt% catalyst (3-NaC/CaO-450) at 110 °C in a typical aminolysis reaction as shown in Scheme 1. Aminolysis reactions of used cotton seed oil have been carried out by varying one parameter at a time in order to institute the best suited reaction conditions for the complete aminolysis. After the finishing point of the reaction, the final reaction mixture was filtered through ordinary filter paper and was dissolved in 60 mL hexane and placed in a separatory funnel. The lighter upper hexane layer was separated from the heavier lower glycerol layer, washed with water, dried with sodium sulfate. Finally, the hexane was rotary-evaporated to yield the pure fatty acid amide (FAA) derivative.

Scheme 1 Aminolysis of cotton seed oil in the presence of NaC/CaO catalyst.

Scheme 2 Aminolysis of used cotton seed oil derived FAMEs in the presence of NaC/CaO catalyst.

Scheme 3 Aminolysis of methyl laurate in the presence of NaC/CaO catalyst.

In order to monitor the continuous progress of the reaction, the samples from the reaction mixture have been withdrawn after every 10 min, and subjected to FTIR analysis.

To test the efficacy of NaC/CaO catalyst with other substrates, same catalyst has also been employed for catalyzing the aminolysis reactions of used cotton seed oil derived FAMEs (Scheme 2) and commercially available methyl laurate (Scheme 3) using diethanolamine to FAMEs or methyl laurate molar ratio of 4 : 1 in the presence of 5 wt% catalyst (NaC/CaO) at 110 °C.

The reaction mixture was filtered through ordinary filter paper after the completion of the reaction, washed with water and organic layer was dried over sodium sulphate. The FAA so obtained was further characterized by FTIR (Fig. S2, ESI†) and ¹H-NMR (Fig. S3, ESI†) techniques. The amide product obtained from the amidation of the methyl laurate was also characterized by mass spectrometry (Fig. S4, ESI†) besides FTIR and proton NMR studies. The FTIR technique has also been used to monitoring of reaction and quantification of the amide products formed by following the literature reported procedure.⁷

FAA derived from used cotton seed oil: yield > 99%. FTIR (cm⁻¹): 3397 (ν_OH), 1615 (ν_C=O); ¹H-NMR (CDCl₃, δ ppm): 5.3 (m, –CH=CH–), 3.77 (m, –CH₂OH), 3.46 (m, –NCH₂–), 2.7 (m, –CH=CH–CH₂–CH=CH–), 2.31 (m, –CH₂–CO–), 2.0 (m, –CH₂–(CH₂)_n–CH–), 1.6–1.25 (m, –(CH₂)_n–), 0.95 (m, –CH=CH–CH₃), 0.87 (m, –CH₂–CH₃).

FAA derivative of used cotton seed oil derived FAMEs: yield > 99%. FTIR (cm⁻¹): 3398 (ν_OH), 1615 (ν_C=O); ¹H-NMR (CDCl₃, δ ppm): 5.3 (m, –CH=CH–), 3.78 (m, –CH₂OH), 3.48 (m, –NCH₂–), 2.7 (m, –CH=CH–CH₂–CH=CH–), 2.3 (m, –CH₂–CO–), 2.0 (m, –CH₂–(CH₂)_n–), 1.6–1.25 (m, –(CH₂)_n–), 0.95 (m, –CH=CH–CH₃), 0.87 (m, –CH₂–CH₃).

FAA derivative of mutton fat derived FAMEs: yield > 99%. FTIR (cm⁻¹): 3398 (ν_OH), 1615 (ν_C=O); ¹H-NMR (CDCl₃, δ ppm): 5.3 (m, –CH=CH–), 3.75 (m, –CH₂OH), 3.47 (m, –NCH₂–), 2.3 (m, –CH₂–CO–), 2.0 (m, –CH₂–(CH₂)_n–), 1.6–1.25 (m, –(CH₂)_n–), 0.87 (m, –CH₂–CH₃).

FAA derivative of methyl laurate (N,N-diethanoldodecylamide): yield > 99%. FTIR (cm⁻¹): 3397 (ν_OH), 1615 (ν_amide–C=O); ¹H-NMR (CDCl₃, δ ppm): 3.8 (m, –CH₂OH), 3.5 (m, –NCH₂–), 2.3 (m, –CH₂–CO–), 1.6–1.25 (m, –(CH₂)_n–), 0.87 (m, –CH₂–CH₃); EI-MS (m/z) (intensity (%), fragment): 287.2 (3, M), 270.3 (100, M–H₂O), 227.22 (10, M–CH₃(CH₂)₃), 175.2 (10, M–CH₃(CH₂)₇), 132.2 (12, M–CH₃(CH₂)₁₀), 114.2 (5, M–CH₃(CH₂)₁₀OH).

2.4 Alcoholysis reaction

In order to show the effectiveness of the 3-NaC/CaO-450 for catalyzing the reactions other than aminolysis, same catalyst has also been used for the transesterification or alcoholysis of variety of triglycerides (virgin cotton seed oil, used cotton seed oil, soybean oil, castor oil, karanja oil, jatropha oil and mutton fat) with methanol as shown in Scheme 4. All transesterification reactions were carried out in a 100 mL, two neck round bottom flask fitted with a water-cooled condenser, oil bath and a magnetic stirrer. In a typical transesterification reaction, triglyceride was mixed with methanol (in 12 : 1 molar ratio with respect to triglyceride) with 5 wt% of 3-NaC/CaO-450 and heated at 65 °C, till the completion of reaction.

Scheme 4 Transesterification of triglyceride (cotton seed oil) in the presence of NaC/CaO catalyst.

The reaction mixture was filtered to remove the catalyst after the completion of the reaction, rotary evaporated to recover the excess methanol and finally kept in separating funnel for 12 h to separate the lower glycerol layer from upper FAMEs layer. FAMEs thus obtained, were further analyzed and quantified by ¹H-NMR (Fig. S5, ESI†) by following the literature reported procedure¹² as given below:

Yield = {2I_(methoxy)/3I_(methylene)} × 100

where I_(methoxy) and I_(methylene) are the areas of the methoxy and methylene protons, respectively, in ¹H NMR spectra of FAMEs.

FAMEs derived from used cotton seed oil: H-NMR (CDCl₃, δ ppm): 5.31 (m, –CH=CH–), 3.6 (s, –OCH₃), 2.7 (m, –CH=CH–CH₂–CH=CH–), 2.31 (m, –CH₂–CO–), 2.0 (m, –CH₂–CH=CH–), 1.6–1.25 (m, –(CH₂)_n–), 0.94 (m, –CH=CH–CH₃), 0.86 (m, –CH₂–CH₃).

FAMEs derived from karanja oil: ¹H-NMR (CDCl₃, δ ppm): 5.33 (m, –CH=CH–), 3.6 (s, –OCH₃), 2.72 (m, –CH=CH–CH₂–CH=CH–), 2.3 (m, –CH₂–CO–), 2.01 (m, –CH₂–(CH₂)_n–), 1.61–1.25 (m, –(CH₂)_n–), 0.96 (m, –CH=CH–CH₃), 0.87 (m, –CH₂–CH₃).

FAMEs derived from jatropha oil: ¹H-NMR (CDCl₃, δ ppm): 5.3 (m, –CH=CH–), 3.61 (s, –OCH₃), 2.73 (m, –CH=CH–CH₂–CH=CH–), 2.31 (m, –CH₂–CO–), 2.03 (m, –CH₂–(CH₂)_n–), 1.61–1.25 (m, –(CH₂)_n–), 0.97 (m, –CH=CH–CH₃), 0.85 (m, –CH₂–CH₃).

FAMEs derived from soybean oil: ¹H-NMR (CDCl₃, δ ppm): 5.31 (m, –CH=CH–), 3.6 (s, –OCH₃), 2.76 (m, –CH=CH–CH₂–CH=CH–), 2.3 (m, –CH₂–CO–), 2.04 (m, –CH₂–(CH₂)_n–), 1.6–1.25 (m, –(CH₂)_n–), 0.98 (m, –CH=CH–CH₃), 0.88 (m, –CH₂–CH₃).

FAMEs derived from castor oil: ¹H-NMR (CDCl₃, δ ppm): 6.48 (–OH) 5.52 (m, –CH₂–CH=CH–), 5.38 (m, –CH=CH–), 3.6 (s, –OCH₃), 3.61 (m, –CH–OH), 2.31 (m, –CH₂–CO–), 2.21 (m, –CH₂–CH=CH–), 2.02 (m, –CH₂–(CH₂)_n–), 1.61–1.30 (m, –(CH₂)_n–), 0.97 (m, –CH=CH–CH₃), 0.87 (m, –CH₂–CH₃).

FAMEs derived from mutton fat: ¹H-NMR (CDCl₃, δ ppm): 5.35 (m, –CH=CH–), 3.6 (s, –OCH₃), 2.31 (m, –CH₂–CO–), 2.03 (m, –CH₂–(CH₂)_n–), 1.61–1.25 (m, –(CH₂)_n–), 0.86 (m, –CH₂–CH₃).

3. Results and discussion

3.1 Catalyst preparation and characterization

3.1.1 BET surface area and Hammett indicator test. Being less toxic and less expensive CaO has been selected as basic support for the preparation of the sodium impregnated CaO catalyst. The catalytic activity of such catalyst was found to depend on the basic strength and surface area of the catalyst³² and same has been modified by impregnating calcium oxide with 1–5 wt% of sodium carbonate. The basic strength (H_) of the commercially available CaO was found to be in the range of 9.8–10.1. The basic strength of CaO was further amplified initially from 10.1–11.1 to 11.1–15.0 as the sodium ion concentration in CaO was increased from 1 to 3 wt% at calcinations temperature 250 °C and increased to 15.1–18.4 as calcination temperature was increased to 450 °C (Table 1). A further increase in Na⁺ concentration was not found to increase the basic strength of NaC/CaO material whereas increase in calcinations temperature (650 °C) leads to decrease in the basic strength. The enhancement of the basic strength of CaO upon sodium ion impregnation with increase in calcinations temperature could be due to the materialization of strong basic sites on the CaO surface.¹²

The surface area of commercially available calcium oxide and 3-NaC/CaO-450 was measured by BET method and the same was found to be increased from 3.9 m² g⁻¹ to 10.9 m² g⁻¹ after impregnating the CaO with 3 wt% of sodium as shown in Table 2. The calcinations temperature has direct impact on surface area and it was found to increase from 8.6 m² g⁻¹ to 10.9 m² g⁻¹ as calcinations temperature increases from 250 to 450 °C. Although, further increase in calcinations temperature (650 °C) reduces surface area to 7.8 m² g⁻¹ which could be due to the sintering of particles. The impregnation of lithium and potassium also caused the increase in surface area from 3.9 to 9.5 and 9.6, respectively.

Table 2 Comparison of BET surface area, particle size (Powder XRD) and basic strength of CaO with prepared catalysts

S. No.	Catalyst type	BET surface area (m^2 g^−1)	Particle size (Debye Scherrer) nm	Basic strength (H_)
1	CaO	3.9	105	9.8 < H_ < 10.1
2	1-NaC/CaO-450	10.8	32	10.1 < H_ < 11.1
3	2-NaC/CaO-450	10.7	28	11.1 < H_ < 15.0
4	3-NaC/CaO-450	10.9	28	15.0 < H_ < 18.4
5	4-NaC/CaO-450	10.8	27	15.0 < H_ < 18.4
6	5-NaC/CaO-450	10.9	29	15.0 < H_ < 18.4
7	3-NaC/CaO-250	8.6	35	11.1 < H_ < 15.0
8	3-NaC/CaO-650	7.8	41	11.1 < H_ < 15.0
9	3-LiC/CaO-450	9.5	70	15.0 < H_ < 18.4
10	3-KC/CaO-450	9.6	39	15.0 < H_ < 18.4

---

3.1.2 Powder XRD crystallography studies. In order to study the effect of calcinations temperature, sodium ion concentration and different alkali metal ions on the structure of CaO, powder XRD patterns are compared in Fig. 1. Presence of peaks at 2θ values of 37.46°, 53.9°, 32.23°, and 67.38° in CaO, exhibited the characteristic reflections of calcium oxide cubic form (JCPDS card no. 821691) as shown in Fig. 1A. Low intensity peaks at 29.37° and 64.23° indicates the presence of minor amount of calcium carbonate (JCPDS 881811) as calcite in the CaO. The powder XRD pattern of Na⁺ doped CaO calcined at 250 °C (3-NaC/CaO-250) shows the conversion of CaO cubic form to Ca(OH)₂ hexagonal form as supported by the presence of peaks at 34.18°, 18.02°, 47.19°, 50.76°, 28.61° and 62.59° (JCPDS 841276) as shown in Fig. 1A. Increase in the calcination temperature to 450 °C, 3-NaC/CaO-450 shows the existence of both the cubic (CaO = 37.8°, 54.53°, 32.68°, 68.13°) and hexagonal (Ca(OH)₂ = 34.61°, 47.63°, 51.36°, 18.61°, 29.23°, 63.18°) phases. However, calcination at more higher temperature (650 °C) diminished the hydroxide moieties and hence, 3-NaC/CaO-650 exists only in cubic phase (CaO) as shown by the peaks at 37.79°, 54.55°, 32.68° and 69.12°. The similar XRD patterns have been observed for the CaO doped with Li⁺ and K⁺ as shown in Fig. 1B.

Fig. 1 Comparative powder XRD patterns of (A) CaO with 3-NaC/CaO-250, 3-NaC/CaO-650 and (B) 3-NaC/CaO-450, 3-LiC/CaO-450 and 3-KC/CaO-450 (* = calcium oxide, ♦ = calcium hydroxide and Δ = calcium carbonate).

The absence of the diffraction patterns of Na₂CO₃, Li₂CO₃ and K₂CO₃ could be either due to its high degree of dispersion on the CaO surface. Debye–Scherrer method³³ was used for the particle size determination of all the prepared catalysts using powder XRD data. Particle sizes of the 3-NaC/CaO-450 (28 nm) were found to be significantly lower than that of pure CaO (105 nm) as shown in Table 1. The change of Na⁺ concentration (1–5 wt%) in CaO was not found to alter the structure (Fig. S1, ESI†) and particle size of NaC/CaO significantly (Table 2). The decrease in the particle size and presence of both oxide (CaO) and hydroxide moieties (Ca(OH)₂) could lead to higher activity of 3-NaC/CaO-450 towards the aminolysis and transesterification reactions in comparison to pure CaO.

3.1.3 SEM and TEM analysis. SEM analysis of 3-NaC/CaO-450 shows that it has hexagonal and irregular shaped cluster of 50–60 nm sized particles as shown in Fig. 2A. TEM studies of the same particles shows that these are the clusters of further smaller spherical and elongated shaped particles with an average particle size of ∼25 nm (Fig. 2B). Particle size distribution analysis (dynamic light scattering analysis, DLS) of CaO and 3-NaC/CaO-450 showed that they have average particle size of 100 nm and 25 nm, respectively, which is similar with XRD and TEM analysis (Fig. 2C and D).

Fig. 2 (A) SEM, and (B) TEM images of 3-NaC/CaO-450. (C) Particle size distributions of CaO and (D) 3-NaC/CaO-450.

The characterization studies of 3-NaC/CaO-450 reveals that it is not only exist in nano particle form but also possess higher basic strength and larger surface area than pure CaO and hence, expected to show higher activity.

3.2 Aminolysis reaction

The prepared alkali metal impregnated CaO has been used as catalyst for the aminolysis of a variety of feedstocks (cotton seed oil, used cotton seed oil, mutton fat, methyl laurate, and FAMEs derived from used cotton seed oil and mutton fat) with diethanolamine. Meanwhile, used cotton seed oil (2.2 wt% FFAs) has been selected for optimizing the parameters to achieve the complete aminolysis.

3.2.1 Optimization of reaction parameters for the aminolysis. To test the effect of impregnated alkali metal ions on the activity of the catalyst, three different catalysts viz., 3-NaC/CaO-450, 3-KC/CaO-450 and 3-LiC/CaO-450 were prepared. The reaction proceeds at faster rate in the case of 3-NaC/CaO-450 (0.5 h) while other catalyst shows relatively lower activity (1 h and 1.5 h for 3-KC/CaO-450 and 3-LiC/CaO-450, respectively) towards aminolysis of UCSO as summarized in Fig. 3A. Being highest in activity towards aminolysis reaction 3-NaC/CaO-450 was selected for further studies. Also, the turnover frequency (TOF) was found to be 39.6, 19.8 and 13.2 h⁻¹ for 3-NaC/CaO-450, 3-KC/CaO-450 and 3-LiC/CaO-450, respectively, suggested that 3-NaC/CaO-450 possess better catalytic activity as compared to other prepared catalysts.

Fig. 3 Effect of (A) impregnated alkali metal ion and (B) calcination temperature on the time required and turnover frequency (TOF) for complete aminolysis of used cotton seed oil. (Reaction conditions; methanol: feedstock = 6 : 1 (m/m), catalyst amount = 5 wt%, temperature = 110 °C).

In order to determine the optimum calcination temperature for the better catalytic activity, 3 wt% Na doped CaO was calcined in the temperature range of 250–650 °C. The reaction time for complete conversion (98 ± 2%) to FAA was found to decrease from 1 h to 0.5 h with the increase in calcination temperature of NaC/CaO from 250 to 450 °C. Moreover, NaC/CaO at 450 °C calcination temperature has higher values of BET surface area and basic strength, thus allowing a higher concentration of Lewis base at the catalyst surface. However, a further increase in calcination temperature (650 °C) was found to increase the reaction time to 0.75 h for complete conversion to FAA yield as shown in Fig. 3B. The lesser activity at higher calcination temperature (>450 °C) could be attributed to the reduction in surface area as well as basic strength of the catalyst due to the sintering of the NaC/CaO particles (Table 2). The turnover frequency was found to be maximum when calcinations temperature was 450 °C (39.6 h⁻¹) and also suggested that 450 °C is optimum calcinations temperature for better rate of reaction. Thus 3 wt% Na doped CaO calcined at 450 °C (3-NaC/CaO-450) was found to be more efficient than its other counterparts, and hence, employed for optimizing the reaction parameters for the aminolysis and transesterification of UCSO.

To determine the optimal sodium ion concentration in CaO to achieve the best catalytic activity, a series of NaC/CaO catalysts were prepared by varying the amount of sodium from 1–5 wt%. The aminolysis of used cottonseed oil was performed with diethanolamine (DEA/oil = 6 : 1; m/m) at 110 °C in the presence of 5 wt% catalyst. FAA yield was found to increase as the amount of sodium ion in CaO were increased from 1 to 3 wt% (Fig. 4A). However, further increase in Na⁺ ion concentration does not have an influence on the reaction rate and hence, 3-NaC/CaO-450 catalyst was selected for optimizing the other parameters to attain the minimum time for the complete aminolysis of used cotton seed oil.

Fig. 4 Effect of (A) sodium ion concentration, (B) catalyst concentration, (C) reaction temperature and (D) DEA/oil molar ratio on the complete aminolysis of used cotton seed oil. (Reaction time = 0.5 h).

A series of aminolysis of used cottonseed oil with DEA (1 : 6, m/m) at 110 °C were performed in the presence of 3-NaC/CaO-450 by varying its amount from 1 to 10 wt% (catalyst/oil) to find the optimal catalyst concentration. The reaction rate of aminolysis (FAA yield) increases as catalyst concentration was increased from 1 to 5 wt%. A further increase in catalyst concentration does not alter the reaction rate significantly and hence, the aminolysis reactions were performed with a 5 wt% catalyst concentration for optimization the other parameters (Fig. 4B).

In order to find the optimum temperature, a series of aminolysis reactions were conducted in the presence of 5 wt%, (catalyst/used cottonseed oil) 3-NaC/CaO-450 catalyst from 30 °C to 130 °C. The fatty acid amide conversion increases as temperature of the reaction was increased to 110 °C as shown in Fig. 4C. Further increase in reaction temperature does not affect the FAA yield significantly and hence, all aminolysis reactions have been carried out at 110 °C. Also, the complete aminolysis (98 ± 2% m/m) of used cotton seed oil has been possible at room temperature (35 °C) though needed longer reaction duration (∼2 h).

The optimum DEA/UCSO molar ratio has been determined by performing the reactions with 3 : 1 to 8 : 1 molar ratios at 110 °C using 5 wt% of NaC/CaO catalyst. The rate of formation of FAA increases as diethanolamine/oil molar ratio was increased from 3 : 1 to 6 : 1 (Fig. 4D). Further increase in diethanolamine/oil molar ratio does not influence the rate of aminolysis significantly.

3.2.2 Effect of variety of feedstocks. The catalytic efficacy of the prepared NaC/CaO catalyst towards the aminolysis of substrates other than UCSO was also tested. Same reaction has been performed with methyl laurate (ML), mutton fat (MF), used cotton seed oil (UCSO), virgin cotton seed oil (CSO), soybean oil (SO), FAMEs of MF and FAMEs of UCSO and the prepared catalyst was found to be efficient for the complete aminolysis of all the feedstocks used. In case of methyl laurate, FAMEs of MF and FAMEs of UCSO, less molar ratio of DEA/feedstock (4 : 1) was required though the reaction time was same for the complete aminolysis of all feedstocks (Table 3).

Table 3 Comparison of catalytic activity of prepared catalyst for the complete aminolysis of different feedstocks. (Conditions; catalyst amount = 5 wt% of feedstock, temperature = 110 °C, reaction time = 0.5 h, FAA yield = 98 ± 2%)

S. No.	Type of feedstock	DEA : feedstock molar ratio
1	CSO	6 : 1
2	SO	6 : 1
3	UCSO	6 : 1
4	MF	6 : 1
5	FAMEs (UCSO)	4 : 1
6	FAMEs (MF)	4 : 1
7	ML	4 : 1

---

3.2.3 Reusability and homogeneous contribution of the catalyst. Reusability is one of the significant features of a heterogeneous catalyst for its industrial application. In order to test the reusability of 3-NaC/CaO-450, it was recuperated from the reaction mixture by filtration, washed with hexane and dried at 100 °C. The recovered catalyst was used for the six catalytic runs under the same experimental condition and regeneration method. The catalyst was capable to complete (98 ± 2%, m/m) the aminolysis of used cotton seed oil for six successive cycles as shown in Fig. 5. However, the reaction time increases steadily after every successive run, indicating there is a gradual loss of the activity. The decline in catalytic activity after every cycle could be due to the fractional leaching of sodium from the calcium oxide support or due to the partial deactivation of the catalytic sites.

Fig. 5 Reusability studies of the 3-NaC/CaO-450 catalyst towards the aminolysis of used cotton seed oil. (Reaction conditions; catalyst amount = 5 wt% of feedstock, temperature = 110 °C, reaction time = 0.5 h, FAA yield = 98 ± 2%).

The catalyst reusability study shows the steady loss of the catalytic activity which could be due to the partial leaching of the active species from the catalyst support. In order to reckon the homogeneous contribution involved in the catalyst activity, the catalyst, 3-NaC/CaO-450 catalyst (500 mg, equivalent to 5 wt% of oil used later for the aminolysis), has been refluxed with diethanolamine for 0.5 h at 110 °C and then catalyst was separated from diethanolamine by filtration. Diethanolamine thus obtained was mixed with used cotton seed oil to maintain amine/oil molar ratio of 6 : 1, and reaction mixture was stirred at 110 °C for 0.5 h. Under these experimental conditions no conversion of oil to FAA has been found. This experiment clearly ruled out the possibility of aminolysis due to any leached species, and also supports that solid catalyst is accountable for the entire catalytic activity.

Furthermore, the loss of catalytic activity was examined by ICP (Inductively Coupled Plasma Spectrometry), powder XRD, FTIR and TEM analysis of fresh and recycled NaC/CaO. The ICP analysis suggested leaching of Ca metal ions but in negligible low amount (less than 5 ppm) whereas there was no measureable loss of Na metal ions. The FTIR analysis shows no change in the fresh and recycled NaC/Cao (Fig. S6B, ESI†). The powder XRD analysis of fresh and recycled catalyst shows the presence of new unknown phase peaks which suggested the distortion of structure after reaction (Fig. S6A, ESI†). On the other hand TEM analysis shows the aggregation of particles which could be due the multiple reuse of NaC/CaO (Fig. S7, ESI†). On the basis of all these studies, the distortion of structure of NaC/CaO (hexagonal phase of CaO) could be the most possible reason for the loss of catalytic activity for aminolysis.

3.3 Alcoholysis reaction

In order to show the efficacy of NaC/CaO for the reaction other than aminolysis, a series of reactions have been performed for the complete alcoholysis (>98% yield) of variety of triglycerides.

3.3.1 Optimization of different reaction parameters for alcoholysis. A series of transesterification reactions of UCSO with methanol (12 : 1 molar ratio) at 65 °C were carried out in the presence of nanocrystalline 3-NaC/CaO-450 by varying its concentration from 1 to 10 wt% (catalyst/oil) in order to find the optimum catalyst concentration. The yield of FAMEs was found maximum (98 ± 2%) as catalyst concentration was increased from 1 to 5 wt%. Further increase in catalyst concentration does not have an effect on yield of FAMEs significantly (Fig. 6A). Other parameters of the transesterification reactions were further optimized with catalyst concentration of 5 wt% (catalyst/oil).

Fig. 6 Effect of (A) catalyst concentration, (B) reaction temperature, (C) methanol/oil ratio and (D) moisture content on the FAMEs yield. (Reaction conditions; catalyst amount = 5 wt% of feedstock, temperature = 65 °C, reaction time = 0.5 h).

The prepared catalyst was used for a series of transesterification reactions of UCSO by varying the reaction temperature from 35 to 75 °C. The FAMEs yield was reached to 98% as temperature of the reaction was increased from 35 °C to 65 °C. Further increase in reaction temperature does not affect the activity of catalyst (Fig. 6B) and hence, further all transesterification reactions have been performed at 65 °C.

The effect of methanol/oil molar ratio on transesterification reaction is one of the most important parameter which affects the FAMEs yield as well as cost of the production. Minimum 3 : 1 molar ratio of methanol to oil required for the complete conversion of vegetable oil to FAMEs. However, transesterification being a reversible reaction, usually carried out with high amount of methanol to shifts the equilibrium in forward direction to achieve the maximum FAMEs yield in short reaction time. To find out the optimum methanol amount in present study, the transesterification reactions of UCSO were performed with 3 : 1 to 18 : 1 molar ratios (methanol/oil) at 65 °C using 5 wt% of 3-NaC/CaO-450 solid catalyst. The complete transesterification (98 ± 2% yield) was observed as methanol/oil molar ratio was increased from 3 : 1 to 12 : 1 (Fig. 6C). The transesterification reactions for optimization the other parameters were further studied with 12 : 1 of methanol/oil molar ratio.

Presence of water in feedstock always inherited the reaction progress and more than 0.3 wt% leads to the saponification instead of transesterification in the presence of homogenous catalyst.¹² UCSO used in present study were found to have 0.4 wt% moisture contents and transesterification reaction of the same using homogeneous catalyst (NaOH or KOH) leads to the soap formation. However, complete conversion was observed when same reaction catalyzed by nanocrystalline 3-NaC/CaO-450. In order to determine the moisture resistance of the 3-NaC/CaO-450, the transesterification reactions of UCSO were performed in the presence of 0.4–10.4 wt% (water/oil) water. The solid catalyst was found to be effective for the complete transesterification of UCSO in 5.5 h even in the presence of 10.4 wt% of moisture contents (Fig. 6D). Addition of higher water content (12.4 wt%) decreases the FAMEs yield (61%) and further increase leads to the soap formation.

3.3.2 Effect of different alcohols and feedstocks with varying free fatty acid contents. To test the efficiency of the 3-NaC/CaO-450 towards the transesterification of UCSO with alcohols of varying carbon chains (1–4 carbon), a 12 : 1 ethanol to oil molar ratio in presence of 5 wt% catalyst at 65 °C has been employed. The catalyst was found to be effective for the complete alcoholysis and yielded 99% respected fatty acid alkyl ester (Fig. 7A). The TOF was found to be 39.6, 23.8, 15.8 and 11.3 h⁻¹ for methanol, ethanol, propanol and butanol, respectively. The higher reaction duration of 50 min, 75 min and 105 min with ethanol, propanol and butanol, respectively, in comparison to methanolysis reaction (30 min) suggests that the inherent carbon chain length of the alcohols influenced catalyst activity. The influence on NaC/CaO activity with increasing carbon chain length could be attributed to the reduction in acidic strength of alcohols and change in alcohol solubility.

Fig. 7 Effect of (A) the alcohol carbon chain length towards the alcoholysis of UCSO and (B) the FFAs on the alcoholysis of a variety of feedstock. (Reaction conditions; catalyst concentration = 5 wt%, methanol/oil molar ratio = 12 : 1 and reaction temperature 65 °C. Acronyms; MeOH = methanol, EtOH = ethanol, PrOH = 1-propanol, BtOH = 1-butanol, SO = soybean oil, MF = mutton fat, CSO = cotton seed oil, UCSO = used cotton seed oil, CO = castor oil, KO = karanja oil and JO = jatropha oil).

Presence of FFAs in feedstock is also critical like moisture content and leads to saponification if FFAs is more than 0.5 wt% (in case of homogeneous catalyst).²⁷ UCSO used in present study has 2.2 wt% FFAs and transesterification reaction of the same using NaOH or KOH as homogenous catalyst leads to the saponification instead of transesterification. However, 3-NaC/CaO-450 was found efficient and yielded the complete conversion of oil to FAMEs in 0.5 h. The optimal FFA tolerance of the prepared catalyst towards the transesterification reactions was observed using variety of natural feedstocks having different amount of FFAs (wt%) like, animal fat (1.3), virgin soybean oil (0.2), cotton seed oil (0.11), used cotton seed oil (2.2), castor oil (2.0), karanja oil (4.1) and jatropha oil (8.4). The solid catalyst was found effective for the complete transesterification of all the feedstocks used (having upto 8.4 wt% FFAs) as shown in Fig. 7B. However, high FFAs affect the catalytic activity as reaction time increases with increase in FFAs content (karanja and jatropha oil).

4. Conclusions

Solid catalyst Na₂CO₃ impregnated CaO was prepared in nanoparticles form (particle size = 25 nm) by very simple method and used for solvent free one step aminolysis and transesterification reactions of a variety of feedstocks. NaC/CaO was found to complete the aminolysis and alcoholysis of UCSO using diethanolamine and methanol, respectively, in 0.5 h at 110 °C and 65 °C, respectively. The catalyst was found to be equally efficient for the aminolysis and alcoholysis of other feedstocks. The turnover frequency was found to be 39.6 h⁻¹ for both the aminolysis and alcoholysis reactions of used cotton seed oil using NaC/CaO. The catalyst was efficient even in the presence of moisture content up to 10.4% and also completes the transesterification of UCSO with long chain (1–4 carbon) alcohols. The catalyst shows excellent reusability and used for six successive catalytic cycles for the aminolysis of UCSO.

Acknowledgements

We are thankful to Center for University-Wide Research Facilities, Chonbuk National University, Jeonju, South Korea for powder XRD, FE-SEM, TEM and DLS analysis.

Notes and references

1. B. Subhasree, R. Baskar, R. Laxmi Keerthana, R. Lijina Susan and P. Rajasekaran, Food Chem., 2009, 115, 1213–1220.
2. R. Luque, L. Herrero-Davila, J. M. Campelo, J. H. Clark, J. M. Hidalgo, D. Luna, J. M. Marinas and A. A. Romero, Energy Environ. Sci., 2008, 1, 542–564.
3. U. Biermann, W. Friedt, S. Lang, W. Lühs, G. Machmüller, U. O. Metzger, M. R. Gen Klaas, H. J. Schäfer and M. P. Schneider, in Biorefineries-Industrial Processes and Products, Wiley-VCH Verlag GmbH, 2008, pp. 253–289,  DOI:10.1002/9783527619849.ch25.
4. T. M. Kuo, H. Kim and C. T. Hou, Curr. Microbiol., 2001, 43, 198–203.
5. H. Maag, J. Am. Oil Chem. Soc., 1984, 61, 259–267.
6. K. J. Liu, A. Nag and J.-F. Shaw, J. Agric. Food Chem., 2001, 49, 5761–5764.
7. R. Alcantara, J. Amores, L. Canoira, E. Fidalgo, M. J. Franco and A. Navarro, Biomass Bioenergy, 2000, 18, 515–527.
8. S. Stournas, E. Lois and A. Serdari, J. Am. Oil Chem. Soc., 1995, 72, 433–437.
9. J. M. Encinar, J. F. González, J. J. Rodríguez and A. Tejedor, Energy Fuels, 2002, 16, 443–450.
10. U. Rashid, F. Anwar, B. R. Moser and S. Ashraf, Biomass Bioenergy, 2008, 32, 1202–1205.
11. P. Sun, J. Sun, J. Yao, L. Zhang and N. Xu, Chem. Eng. J., 2010, 162, 364–370.
12. D. Kumar and A. Ali, Energy Fuels, 2013, 27, 3758–3768.
13. R. S. Watkins, A. F. Lee and K. Wilson, Green Chem., 2004, 6, 335–340.
14. M. C. de Zoete, A. C. Kock-van Dalen, F. van Rantwijk and R. A. Sheldon, J. Mol. Catal. B: Enzym., 1996, 1, 109–113.
15. N. P. Awasthi and R. P. Singh, Eur. J. Lipid Sci. Technol., 2009, 111, 202–206.
16. S. H. Feairheller, R. G. Bistline Jr, A. Bilyk, R. L. Dudley, M. F. Kozempel and M. J. Haas, J. Am. Oil Chem. Soc., 1994, 71, 863–866.
17. W. E. Levinson, T. M. Kuo and C. P. Kurtzman, Enzyme Microb. Technol., 2005, 37, 126–130.
18. M. J. J. Litjens, M. Sha, A. J. J. Straathof, J. A. Jongejan and J. J. Heijnen, Biotechnol. Bioeng., 1999, 65, 347–356.
19. J. Rawlins, M. Pramanik and S. Mendon, J. Am. Oil Chem. Soc., 2008, 85, 783–789.
20. B. K. Sharma, A. Adhvaryu and S. Z. Erhan, J. Agric. Food Chem., 2006, 54, 9866–9872.
21. E. A. J. Al-Mulla, W. M. Z. W. Yunus, N. A. B. Ibrahim and M. Z. A. Rahman, J. Oleo Sci., 2009, 58, 467–471.
22. A. Bilyk, R. Bistline Jr, G. Piazza, S. Feairheller and M. Haas, J. Am. Oil Chem. Soc., 1992, 69, 488–491.
23. E. Jordan Jr and W. Port, J. Am. Oil Chem. Soc., 1961, 38, 600–605.
24. A. L. McKenna and W. C. C. H. C. Division, Fatty amides: synthesis, properties, reactions, and applications, Witco Chemical Corporation, Humko Chemical Division, 1982.
25. R. Bistline, J. Hampson and W. LinField, J. Am. Oil Chem. Soc., 1983, 60, 823–828.
26. R. G. Bistline, E. W. Maurer, F. D. Smith and W. M. Linfield, J. Am. Oil Chem. Soc., 1980, 57, 98–103.
27. E. Lotero, Y. Liu, D. E. Lopez, K. Suwannakarn, D. A. Bruce and J. G. Goodwin, Ind. Eng. Chem. Res., 2005, 44, 5353–5363.
28. C. R. Venkat Reddy, R. Oshel and J. G. Verkade, Energy Fuels, 2006, 20, 1310–1314.
29. A. Demirbas, Prog. Energy Combust. Sci., 2007, 33, 1–18.
30. D. Kumar and A. Ali, Energy Sources, Part A, 2014, 36, 1093–1102.
31. N. Degirmenbasi, N. Boz and D. M. Kalyon, Appl. Catal., B, 2014, 150–151, 147–156.
32. G. V. Smith and F. Notheisz, in Heterogeneous Catalysis in Organic Chemistry, ed. G. V. S. Notheisz, Academic Press, San Diego, 1999, pp. 1–28,  DOI:10.1016/b978-012651645-6/50001-9.
33. S. B. Qadri, E. F. Skelton, D. Hsu, A. D. Dinsmore, J. Yang, H. F. Gray and B. R. Ratna, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 60, 9191–9193.

---

Footnote

† Electronic supplementary information (ESI) available: Materials and methods, spectrum (XRD, FT-IR, mass and ¹H-NMR), Fig. S1–S7. See DOI: 10.1039/c6ra13446g

---