Synthesis of ethylene glycol monomethyl ether monolaurate catalysed by KF/NaAlO₂ as a novel and efficient solid base

Abstract

A novel solid base catalyst of NaAlO₂ modified with KF (x-KF/NaAlO₂) was prepared using a wet-impregnation method and used for the synthesis of ethylene glycol monomethyl ether monolaurate (EGMEML) via transesterification of ethylene glycol monomethyl ether (EGME) and methyl laurate (ML). The catalyst was characterized using the Hammett indicator method, X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, and scanning electron microscopy with an energy dispersive spectrometer. The effect of the reaction parameters such as the amount of KF loading, molar ratio of EGME to ML, dosage of the catalyst, reaction time and temperature on the yield of EGMEML was investigated. These characterizations led to a conclusion that the reaction between NaAlO₂ and KF mainly generates fluoroaluminates, which act as the main active sites for the transesterification. The catalyst shows excellent catalytic activity and good stability. The highest yield of 91% was obtained over 30%-KF/NaAlO₂ at an EGME/ML molar ratio of 3.0, a catalyst amount of 5 wt%, and a reaction time of 4 h at 120 °C. A yield of 80% was obtained after use for three consecutive rounds without reactivation. Furthermore, a desirable yield of 88.0% of a novel biodiesel of ethylene glycol methyl ether soybean oil monoester was obtained with 30%-KF/NaAlO₂ as a catalyst. Moreover, it was found that the reaction follows second-order kinetics, the activation energy (E_a) of the reaction of EGME with ML equals 56.54 kJ mol⁻¹, and the thermodynamic parameters of activation were evaluated based on an activation complex theory of the reaction, and the following data were obtained: ΔG^‡ > 0, ΔH^‡ > 0 and ΔS^‡ < 0, indicating the unspontaneous and endergonic nature of the reaction of EGME with ML. A Koros–Nowak test was conducted and the results confirmed that the diffusion limitations did not affect the catalytic activity. Finally, a few of the physicochemical properties of the EGMEML as biodiesel were determined, and the values were within those of European standards.

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1 Introduction

Energy is an indispensable factor for humans to preserve economic growth and maintain the standard of living.¹ In recent years, due to the transportation and the basic industry sectors, the increase in energy demand has been met using fossil resources (crude oil, natural gas and coal, principally).² Hence, recent interest in alternative sources for petroleum-based fuels has been stimulated. Among many possible resources, biodiesel obtained by the use of oils of plant origin like vegetable oils and tree borne oil seeds has drawn more attention.³ This kind of fuel is biodegradable and non-toxic, and has similar calorific value compared to petroleum diesel.⁴ Moreover, biodiesel can be mixed with petroleum diesel with the two fuel components allowed in any proportion.⁵ For instance, as inherently biodiesel possesses better lubricity, the lubricity of diesel can be improved with the addition of biodiesel.⁶ Recently, a kind of novel biodiesel was synthesized by the transesterification reaction of FAMEs with short chain glycol ethers.^7,8 Compared with the conventional biodiesel, novel biodiesel not only has the advantages of traditional biodiesel, but also attempts the introduction of ether groups into the molecules, and the amount of oxygen contained can be largely improved. So, engine-out exhaust emissions and combustion are effectively reduced.⁹ Guo et al.¹⁰ reported the synthesis of ethylene glycol monomethyl ether palm oil monoester (EGMMEPOM) through the transesterification of a refined palm oil and ethylene glycol monomethyl ether. It was found that EGMMEPOM has a maximal reduction of smoke and NO_x by 26.3% and 14.0% and CO and HC are also significantly reduced compared with diesel fuel. Gao et al.¹¹ disclosed the production of ethylene glycol n-propyl ether palm oil monoester (EGPEPOM) through transesterification of refined palm oil and ethylene glycol n-propyl ether with sodium as the catalyst. In comparison to diesel fuel, the emissions of smoke, NO_x, and brake-specific CO and HC of EGPEPOM decreased by 37.5, 23.7, 66.6 and 27.1%, respectively. Zhang et al.¹² revealed the manufacture of ethylene glycol monobutyl ether palm oil monoester (EGMEPM) via transesterification of palm oil and ethylene glycol monobutyl ether (EGME). Smoke, CO, HC, and NO_x emissions were reduced by 57%, 40%, 75%, and 25% compared to diesel fuel, respectively. Therefore, the novel biodiesel has been regarded as an excellent clean energy.

Homogeneous or heterogeneous catalysts are usually used to promote the transesterification reaction rate.¹³ Although homogeneous catalysts have a high catalytic efficiency in a short time, they also have several drawbacks such as soap formation, catalytic efficiency reduction caused by the consumption of the catalyst, the increase of viscosity and gel formation.¹⁴ Moreover, removal of the homogeneous base catalyst after the reaction creates a large amount of caustic wastewater, contaminating the environment and increasing the production costs.¹⁵ The use of heterogeneous solid base catalysts effectively solved the issues in transesterification. Moreover, heterogeneous solid base catalysts can be easily separated from the reaction mixture, conveniently regenerated, and the corrosion of the reactor could be decreased by some degree, leading to a much safer, cheaper and more environment-friendly operation.¹⁶ In recent years, our group has been devoted to the study of solid base catalysts applied in the production of novel biodiesels. Fan et al.¹⁷ used calcined sodium silicate as a catalyst for the synthesis of novel biodiesel from soybean oil methyl ester and ethylene glycol monomethyl ether, and a maximum yield above 90.0% was obtained. The catalyst preparation process is simple and the separation process becomes easy after the reaction. Chen et al.¹⁸ synthesized KF-modified Ca–Al hydrotalcite and used it as a catalyst to produce a novel biodiesel of ethylene glycol monomethyl ether monolaurate, and the effect of the structure and preparation method of the catalyst on the catalytic activity were discussed. However, developing excellent activity and handleability of the solid base catalysts is still a challenge in the transesterification.

Sodium aluminate is an important commercial inorganic chemical due to the versatility of its technological applications. Sodium aluminate is mainly applied to effective water treatment systems. It is also used in the production of paper, paint pigments, alumina-containing catalysts, dishwasher detergents, ingot molds, molecular sieves, concrete, and so on.¹⁹ Normally, NaAlO₂ is water soluble and shows strong basicity in water, but is insoluble in alcohol. Cobas et al.²⁰ studied the isomerization of a galactooligosaccharides mixture by the action of sodium aluminate. Under optimal conditions, the isomerization yield was >60%. Mutreja et al.²¹ evaluated sodium aluminate as a basic catalyst for the transesterification of waste mutton fat with methanol. The conversion to biodiesel could reach 97%, and the sodium ion concentration in the unwashed biodiesel obtained was found to be 5.6 ppm. Bai et al.²² developed the one-pot synthesis of glycidol from glycerol and dimethyl carbonate using NaAlO₂ as a solid base catalyst. It showed a high conversion of glycerol and selectivity to glycidol. The conversion of glycerol and the selectivity to glycidol remained at 87.1% and 68.7% as the catalysts were used for the next run. Cherikkallinmel et al.²³ used sodium aluminate from waste aluminium source as a catalyst for the transesterification of Jatropha oil. A maximum yield of 99% was obtained at 65 °C. The catalyst kept a high catalyst activity after calcinating at 650 °C. Wan et al.²⁴ employed sodium aluminate as a heterogeneous catalyst for the production of biodiesel from soybean oil by transesterification. The yields were 66.2%, 62.9%, and 61.4% as the catalyst was used for three cycles.

In order to prepare a base catalyst with high efficiency and stability for the transesterification, KF loading sodium aluminate solid base catalyst was synthesized using the immersion method, and used in the production of ethylene glycol monomethyl ether monolaurate and soybean oil monoester novel biodiesel in this paper. The Hammett indicator method, X-ray diffraction (XRD), thermogravimetric analysis (TG-DSC), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy and energy dispersive spectrometry (SEM-EDS) were performed to characterize the structure of the catalyst, in an attempt to explain the correlation between the structure and activity of the catalyst. The different reaction parameters such as temperature, reactant ratio, reaction time and amount of catalyst were optimized. A desirable yield of 88.0% was obtained as the catalyst was used for the production of ethylene glycol methyl ether soybean oil monoester. At the same time, the reusability and stability of the catalyst were investigated. Koras–Nowak criterion tests demonstrated that the mass transport phenomenon did not affect the catalytic activity. Finally, the rate constant and the activation energy were determined from a kinetic study. The values of the thermodynamic parameters of the Gibbs free energy of activation (ΔG^‡), enthalpy of activation (ΔH^‡) and entropy of activation (ΔS^‡) were investigated using the activation complex theory.

2 Experimental

2.1 Materials

KF·2H₂O, NaAlO₂, methanol, methyl laurate and ethylene glycol monomethyl ether were of analytical grade, and all reagents were purchased from Aladdin, China.

2.2 Catalyst characterizations

The basic strength of the sample (H₋) was determined using Hammett indicator. About 20.0 mg of the sample was shaken with 5.00 mL methanol and two or three drops of Hammett indicator–methanol solution (0.1%, w/w) and then left to equilibrate for 2 h when no further color changes were observed.²⁵ The Hammett indicators used and the corresponding H₋ values are listed in Table 1. Alkaline determination was measured using a benzoic acid titration method, using 0.02 mol L⁻¹ benzoic acid-anhydrous ethanol solution as a titrant, until the basic color of the indicator adsorbed on the surface of the solid alkali just disappeared.

Table 1 Colors and H₋ value of some Hammett indicators

Indicator	Basic color	Acid color	H−
Bromthymol blue	Yellow	Blue	7.2
Phenolphthalein	Pink	Colorless	9.8
2,4-Dinitroaniline	Red	Yellow	15.0

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Powder XRD diffraction was performed on a Bruker D8 Advance (Germany) diffractometer, using Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 50 mA. The scanning speed was 3° min⁻¹ and the scanned area ranged from 2θ = 5–80°.

Thermogravimetric analysis (TG-DSC) was carried out using NETZSCH STA F3. 10 mg of the sample was used and alumina was used as a reference. TG and DSC curves were obtained from 25 °C to 800 °C under a nitrogen atmosphere with a heating rate of 10 °C min⁻¹.

FT-IR spectra were recorded on an AVATAR370 spectrometer in the range of 4000–400 cm⁻¹, with 4 cm⁻¹ resolution. The KBr pellet technique was applied for preparing samples. All measurements were conducted at room temperature.

Morphology of the samples was observed with SEM using a Rigaku S-4300 spectrometer (Japan). The voltage was 20 kV and the vacuum degree of the sample room was better than 10⁻⁴ Pa.

2.3 Catalyst preparation

The catalyst was prepared using the wet impregnation method. Initially, NaAlO₂ was activated at 400 °C for 4 h. Then, 1.00 g NaAlO₂ was immersed in 20 mL KF methanol solution with a certain amount of KF·2H₂O and stirred for 4 h at room temperature. Methanol was removed at 80 °C. The precursor was dried at 80 °C overnight and stored in a vacuum desiccator. Prior to the reaction, the precursor was activated in a muffle furnace at 400 °C for 4 h. For convenience, the catalysts were designated as x-KF/NaAlO₂, where x stands for the KF loading mass percentage. 30%-KF/NaAlO₂ indicates 30 wt% of KF with respect to NaAlO₂.

2.4 Catalytic activity measurements

The transesterification of ethylene glycol monomethyl ether with methyl laurate was carried out in a 50 mL three-necked glass round bottom flask, equipped with a long condenser connected to water circulation and a thermometer. The desired amount of ethylene glycol monomethyl ether, methyl laurate and catalyst were added into the flask. The reaction was carried out on a hot plate with a magnetic controlled stirrer until it reached the desired reaction temperature and time. After that, the reaction was stopped by cooling the reactor to room temperature and the catalyst was separated from the liquid phase by filtration. The supernatant liquid was purified in a rotary vacuum evaporator to remove excess EGME and the byproduct, methanol. All experiments were performed under atmospheric pressure.

2.5 Product analysis

Analysis of the reaction products was conducted with an SP 6890 gas chromatograph equipped with a flame ionization detector and a OV-101 column (30 m × 0.52 mm, film thickness 0.5 μm). The GC oven temperature was maintained at 210 °C for 1 min and then increased to 230 °C at a rate of 10 °C min⁻¹ and held for 1 min, and ramped at 10 °C min⁻¹ up to 260 °C, and the oven temperature was kept at 260 °C throughout the analysis. The injector temperature was fixed at 280 °C and the detector temperature was at 280 °C. The GC was connected to a ChemStation which recorded the peak areas and retention times in the chromatogram. The yield (Y) was calculated by the following eqn (1).where m₁ is the product actual mass (g), m₂ is the theoretical calculated mass of the target product (g) and w is the mass concentration of the target product determined by GC.

The turnover frequency (TOF) was calculated by the following eqn (2).

where m_cat is the mass amount of the catalyst (g), mol_actual is the mole amount of target product, t is the desired reaction time (min), and f_m is the amount of basic sites of the 30%-KF/NaAlO₂ (mmol g⁻¹).

3 Results and discussion

3.1 Characterization of catalyst

The thermal stability of NaAlO₂ and the as-prepared KF/NaAlO₂ were examined by TG-DSC experiments as shown in Fig. 1. Three major weight losses for NaAlO₂ appeared aligned with the three endothermic peaks in the relevant DSC profile with the transition temperatures at 78 °C, 143 °C and 200 °C, respectively (Fig. 1A). The first weight loss of 2.9% is assigned to the elimination of water, the second weight loss of 3.7% is due to desorption of CO₂ from the basic sites,²² and the last loss of 3.0% is attributed to the decomposition of the crystal water of NaAlO₂. After loading KF (Fig. 1B), the fresh KF/NaAlO₂ sample presents three main weight losses in the temperature ranges of 100–150 °C, 150–200 °C and 320–370 °C. The first weight loss of 3.7% is assigned to the elimination of water adsorbed on the surface, and the second weight loss of 5.6% is due to the removal of CO₂. Compared with that of NaAlO₂, the new endothermic peak at 330 °C may be assigned to the formation of new compounds. Based on these results all the catalysts were activated at 400 °C.

Fig. 1 TG-DSC curves of support precursor (A) and 30%-KF/NaAlO₂(B).

XRD patterns of NaAlO₂ and KF/NaAlO₂ were obtained to determine the change of the crystal structure of the catalysts as shown in Fig. 2A. NaAlO₂ calcinated at 400 °C exhibited the typical diffraction peaks of NaAlO₂ at 2θ values of 20.7°, 21.2°, 21.2°, 30.3°, 33.2°, 34.3° and 34.9° (JCPDS file 33-1200). With an increase in the loading amount of KF, the intensities of the diffraction peaks of NaAlO₂ decreased, meanwhile, new peaks appeared. Among them, the peaks of 30.7° and 41.5° belong to that of Na₅Al₃F₁₄ (JCPDS file 30-1144), and the peaks of 29.7°, 36° and 42° correspond to that of K₃AlF₆ (ref. 26) (Fig. 2B). These demonstrate that there is a strong interaction between KF and NaAlO₂. The results of the XRD patterns are in agreement with those of TG-DSC. When the KF amount was up to 40%, another new phase of KF (JCPDS file 36-1458) obviously came out, as well as the intensities of the diffraction peaks of NaAlO₂, Na₅Al₃F₁₄ and K₃AlF₆ decreasing, which may result from the excess of KF covering the active sites.²⁷

Fig. 2 XRD patterns (A) and their enlarged view (B) of KF/NaAlO₂ catalysts. NaAlO₂ (a); 10%-KF/NaAlO₂ (b); 20%-KF/NaAlO₂ (c); 30%-KF/NaAlO₂ (d); 40%-KF/NaAlO₂ (e).

The investigation of KF/NaAlO₂ was conducted via IR spectroscopy as shown in Fig. 3A. It can be seen that there are two bands at 3440 and 1640 cm⁻¹, which are assigned to the stretching and bending vibrations of the physically adsorbed water, respectively.²⁸ With the increase of KF loading, the intensities of the absorption bands at 3440 and 1640 cm⁻¹ were enhanced. This indicates that the adsorbed O–H band could not entirely belong to water molecules in the air but is partly assigned to surface hydroxyl groups formed by alkaline activity sites. The two bands at 1367 cm⁻¹ and 1440 cm⁻¹ are attributed to carbonate species.²⁹ The existence of the CO₃²⁻ species is due to the exposure of the catalyst in air for FT-IR analysis.³⁰ As shown in Fig. 3B, the peak at 811 cm⁻¹ corresponds to the formation of O–O triangular species bonds, and the peaks at 617 and 558 cm⁻¹ belong to the vibrations of the Al–O bond.²² With the KF loading amount increased, both the vibrations of the Al–O bond and O–O triangular species bonds existed but decreased, which indicates that the loading KF acted with NaAlO₂. The result is consistent with that of XRD.

Fig. 3 FT-IR patterns of the catalysts (A) and their enlarged view (B). NaAlO₂ (a); 10%-KF/NaAlO₂ (b); 20%-KF/NaAlO₂ (c); 30%-KF/NaAlO₂ (d); 40%-KF/NaAlO₂ (e).

The morphological structure unit of NaAlO₂ is a rodlike profile which is arranged irregularly (Fig. 4A), and the rod displays a tetrahedron structure and a smooth surface (Fig. 4B). After loading KF, the catalyst still kept the rodlike structure, and some new crystalline granules on the surface were observed (Fig. 4C). These particles on the rods could be the formed new materials of Na₅Al₃F₁₄ and K₃AlF₆, which is in line with the results of the XRD, TG-DSC and FT-IR. Fig. 5 shows the element mappings of 30%-KF/NaAlO₂, confirming the presence of K, Al, F, Na and O and their homogeneous distribution on the surface of the catalyst.

Fig. 4 SEM images of the NaAlO₂ support (A), the enlargement of the NaAlO₂ support (B) and fresh 30%-KF/NaAlO₂ catalyst (C).

Fig. 5 SEM image of 30%-KF/NaAlO₂ catalyst (A) and elemental mapping images of Na (B), Al (C), O (D), K (E) and F (F).

Table 2 summarizes the basic strength and basicity of x-KF/NaAlO₂. The catalysts could change the color of phenolphthalein (H₋ = 9.8) from colorless to purple, but failed to convert 2,4-dinitroaniline (H₋ = 15.0) from yellow to mauve. Therefore, their basic strength could be tentatively denoted as 9.8 < H₋ < 15.0.³¹ As shown in Table 2, the yield of 68% of EGMEML was obtained as NaAlO₂ was directly used as a catalyst for the transesterification, and it was found the mixture after the reaction formed a gel, which caused a difficulty in the separation of the catalyst from the reaction mixture. When the amount of KF loading increased to 10%, the basicity of the catalyst decreased obviously and the EGMEML yield changed slightly compared with that of NaAlO₂ as a catalyst. With a further increase in the amount of KF loading, the yield EGMEML first increased then decreased; at a load of KF of 30%, the highest yield of EGMEML was obtained. The results aligned with the change of the total basicity.

Table 2 Basic strength and basicity of x-KF/NaAlO₂ catalysts

Catalyst	Basic strength (H−)	Basicity (mmol g^−1)	Yield^a (%)
a Reaction conditions: amount of catalyst 5.0%, reaction time 4 h, molar ratio of EGME/ML of 3.0, reaction temperature of 120 °C.
NaAlO2	9.8 < H− < 15.0	1.0	68
10%-KF/NaAlO2	9.8 < H− < 15.0	0.8	66
20%-KF/NaAlO2	9.8 < H− < 15.0	1.4	76
30%-KF/NaAlO2	9.8 < H− < 15.0	2.2	90
40%-KF/NaAlO2	9.8 < H− < 15.0	1.9	85

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After the loading of KF, the activity of the catalysts may be associated with the formation of basic active sites generated from the preferential attack of the F⁻ ions of KF on the element aluminum of NaAlO₂. In this case, the active sites thus generated correspond to the fluoride ions themselves and to the negatively charged oxygen atoms located in their vicinity, and another dominant factor is the presence of K⁺ and Na⁺ cations highly distributed around the active sites and generating the catalyst’s basicity.^32–34

A further increase in the loading amount of KF to 40% led to a decrease of the basicity and resulted in a decrease in the EGMEML yield. It is very likely that the higher amount of KF loading may result in the covering of basic sites by the excess KF and/or an agglomeration of the KF phase occurring during calcination.²⁸ The fact is that the highest yield of novel biodiesel was obtained using 30%-KF/NaAlO₂ as a catalyst for the transesterification.

3.2 Influence of the reaction parameters

The effect of the KF mass content of x-KF/NaAlO₂ on the catalytic performance was investigated as shown in Fig. 6. After loading KF, the yield of EGMEML improved obviously. With the increase of the KF amount from 10% to 30%, the EGMEML yield changed from 66% to 91%. This may be related to the number of catalyst surface active sites. The more the amount of KF loaded, the more active sites are generated. Thus, the yield of EGMEML increases. However, further increasing of the loading amount of KF to 40% means that the yield is slightly reduced, which may be due to an excess amount of KF covered active sites. This is consistent with the results of the Hammett indicator. So, the best KF loading amount of 30% was chosen.

Fig. 6 Influence of KF loading amount on the yield of EGMEML. Reaction condition: molar ratio of EGME/ML of 3.0, amount of catalyst of 5%, reaction time of 4 h, and reaction temperature of 120 °C.

The presence of catalyst is necessary for the transesterification reaction to proceed.⁴ The effect of the catalyst amount on the EGMEML yield was investigated as shown in Fig. 7A. With the augmentation in the amount of the catalyst from 2 to 5%, there was an increase in yield from 60% to 91%, which may be due to more catalytic sites available for the reactants.³⁵ With a further increase in the amount of the catalyst from 5 to 6%, a slight decrease of the EGMEML yield was observed. The reason was possibly due to the high viscosity of the slurry causing a mixing problem involving the reactants, products and solid catalyst.³⁶ Hence, the amount of catalyst of 5% was selected for optimizing the other reaction conditions.

Fig. 7 Influence of reaction conditions on the yield of EGMEML: (A) molar ratio of EGME/ML of 3.0, reaction time of 4 h, and reaction temperature of 120 °C; (B) the amount of catalyst 5%, reaction time 4 h, reaction temperature 120 °C; (C) the molar ratio of EGME/ML of 3.0, amount of catalyst of 5%, and reaction temperature of 120 °C; (D) molar ratio of EGME/ML of 3.0, amount of catalyst of 5% and reaction time of 4 h.

As transesterification of EGME with ML is a reversible reaction, a molar ratio higher than the stoichiometric ratio of EGME and ML is required to shift the equilibrium of the reaction towards EGMEML production. The behavior of ML transesterification with EGME at different molar ratios of ML to EGME is shown in Fig. 7B. ML and EGME in a 1 : 3 molar ratio resulted in the highest yield of 91%. When the molar ratio of EGME/ML is above 3, the excess amount of EGME may cause the reaction system dilution and reaction rate reduction,³⁷ which resulted in a decrease of the yield of EGMEML, as well as an increase of the cost for the excess EGME and its recovery.³⁸ Thus, ML and EGME in a 1 : 3 molar ratio were used for optimizing other parameters.

In order to evaluate the effect of the reaction time on the EGMEML yield, reactions were allowed to proceed for varying durations of 2 to 6 h and the obtained results are shown in Fig. 7C. At the beginning of EGMEML synthesis, the EGMEML yield increased rapidly from 65% to 90%. The highest transesterification efficiency was reached at a reaction time of 4 h. After 4 h, the yield was not significantly increased. This indicates that the reaction reached the equilibrium state.

The effect of various reaction temperatures during the transesterification reaction was also studied with 30%-KF/NaAlO₂ as a catalyst over the range of 90–130 °C as shown in Fig. 7D. With the increase of reaction temperature, the yield of EGMEML largely improved. The highest EGMEML yield of 91% was procured at a reaction temperature of 120 °C, as the viscosity of the reactants reduced and the interactions of the reactant molecules were sped up by higher amounts of energy.⁴ As the temperature further increased to 130 °C, the yield decreased clearly, which may be due to the EGME vaporization and the formation of bubbles, limiting the reaction on the three-phase interface.¹⁸

3.3 Transesterification of different FAMEs with EGME

Several fatty acid methyl esters (FAMEs) with different carbon numbers were selected as raw materials in the production of various ethylene glycol monomethyl ether fatty acid esters with 30%-KF/NaAlO₂ as the catalyst. Table 3 shows the yield for the various FAMEs, such as methyl oleate, methyl stearate, methyl palmitate, methyl linoleate and methyl laurate as raw materials in the reaction. A maximum yield of 91.0% was obtained for the transesterification of ML and EGME. The results show that 30%-KF/NaAlO₂ is an effective heterogeneous catalyst for the reaction of various FAMEs with EGME.

Table 3 Yields of different FAMEs in the transesterification reaction

a Reaction conditions: molar ratio of EGME/FAME of 3.0, mass amount of catalyst of 5 wt%, reaction time of 4 h, and reaction temperature of 120 °C.
Reactant	methyl oleate	methyl stearate	methyl palmitate	methyl linoleate	methyl laurate
Yield^a (%)	85.1	81.0	83.7	85.9	91.0

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3.4 Production of the novel biodiesel ethylene glycol monomethyl ether soybean oil monoester

Based on the optimized conditions of the transesterification of ML with EGME, 30%-KF/NaAlO₂ was used as a catalyst in the production of a novel biodiesel of ethylene glycol monomethyl ether soybean oil monoester from the raw material of methyl soybean oil ester biodiesel and EGME. As expected, a novel biodiesel yield of 88.0% was obtained when the soybean biodiesel/EGME molar ratio was 1 : 6, the reaction temperature was 120 °C, the mass amount of the catalyst was 5%, and the reaction time was 4 h. It can be concluded that 30%-KF/NaAlO₂ is an efficacious catalyst for the preparation of soybean oil-based novel biodiesel.

3.5 Comparison of the catalytic activity of KF/NaAlO₂ with other basic catalysts

In order to demonstrate the high catalytic activity of KF/NaAlO₂, some common base catalysts were used for the transesterification. The results of the transesterification catalyzed by KF, NaF, KOH, etc. are summarized in Table 4.

Table 4 Comparison of the catalytic activity of KF/NaAlO₂ with other basic catalysts for the production of novel biodiesel

Catalyst	Catalyst amount (%)	Reaction temperature (°C)	EGME/ML	Reaction time (h)	EGMEML yield (%)
NaF	5	120	3	4	0.8
KF	5	120	3	4	4.3
KOH	5	120	3	4	70
CaO	5	120	3	4	61
NaAlO2	5	120	3	4	68
30%-KF/NaAlO2	5	120	3	4	91

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It is obviously observed from Table 4 that KF/NaAlO₂ exhibits the highest activities among the catalysts, the catalytic activity of NaF is the weakest, and those of NaAlO₂, KOH and CaO are medium. However, KOH as a catalyst in post-processing generated a lot of waste water from the experimental process, and NaAlO₂ was difficult to separate from the product mixture because of the gelation, which is same as that of CaO. The yield of EGMEML with KF/NaAlO₂ as a catalyst is almost same as that of calcinated Na₂SiO₃¹⁷ and KF/HTL-M¹⁸ in the transesterification of ML with EGME. Therefore, KF/NaAlO₂ could be an efficient potential solid base catalyst in the production of novel biodiesel.

3.6 Reusability and stability of KF/NaAlO₂

FT-IR spectroscopy of the fresh catalyst, used and unwashed catalyst, as well as used and washed catalyst are shown in Fig. 8. It was found that the yield of 77% for EGMEML was obtained when the catalyst used once was directly used in the next round without washing or calcination. When the used catalyst was washed with acetone, catalytic activity largely improved, and the yield of EGMEML increased to 84%. All the characteristic absorption bands of the used catalyst are the same as that of the fresh catalyst. However, for the unwashed catalyst, three new bands at 1631–1731 cm⁻¹, 2850 cm⁻¹ and 2920 cm⁻¹ appeared, which was attributed to C=O and C–H stretching vibrations, respectively.³⁹ After the catalyst washing with acetone three times, the new bands disappeared. The phenomena implies that the surface of the used catalyst may be covered by a few organic oligomers.

Fig. 8 FT-IR of fresh catalyst (a), catalyst used and unwashed (b) and catalyst used and washed (c).

Fig. 9 shows SEM micrographs of 30%-KF/NaAlO₂ fresh catalyst (A) and catalyst used for the third round (B). It can be found that the aspect of the particles and agglomerates of the fresh 30%-KF/NaAlO₂ and the used catalyst are similar, meaning that the surface roughness and rodlike structure remained unchanged. The basic strength and basicity of the used 30%-KF/NaAlO₂ were also determined and listed in Table 5. It showed that the catalyst that was used for three rounds kept the same basic strength as the fresh catalyst, but the basicity clearly decreased. EGMEML yields of 90, 84, and 80% were obtained as 30%-KF/NaAlO₂ was used for three consecutive reactions.

Fig. 9 SEM micrographs of 30%-KF/NaAlO₂ of the fresh catalyst (A) and the catalyst used for the third round (B).

Table 5 Basic strength and basicity of used 30%-KF/NaAlO₂ catalysts

Catalyst	Basic strength (H−)	Basicity (mmol g^−1)	Yield^a (%)
a Reaction conditions: the amount of catalyst 5.0%, reaction time 4 h, molar ratio of EGME/ML of 3.0, reaction temperature of 120 °C.
First round	9.8 < H− < 15.0	2.2	90
Second round	9.8 < H− < 15.0	1.8	84
Third round	9.8 < H− < 15.0	1.6	80

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The analysis of EDS for the surface of the fresh 30%-KF/NaAlO₂ catalyst and recycled 30%-KF/NaAlO₂ catalyst were carried out and the results were listed in Table 6. Each datum in Table 6 was the average value determined from three points on the catalyst surface. It can be seen from Table 6 that the content of K, Na and F decreased and the basicity and yield decreased after the catalyst was used for three consecutive rounds compared with that of the fresh catalyst. This may be caused by the active sites running off on the surface of the catalyst during the recycling process, leading to the loss of basicity, and thereby the catalytic activity of the catalyst decreases.

Table 6 Elemental composition of NaAlO₂ and 30%-KF/NaAlO₂

Sample	Na	Al	O	K	F	C
NaAlO2	25.34	23.73	44.42	0	0	6.50
30%-KF/NaAlO2	17.39	13.84	34.46	13.49	18.40	2.42
30%-KF/NaAlO2 used for the third round	13.44	13.52	36.14	11.55	16.56	8.79

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In order to ensure the heterogeneous nature of the catalyst, a leaching test was performed under optimized reaction conditions. After reacting for 1.0 h, the catalyst was removed using centrifugation and the reactants were heated again for an additional 4.0 h. As can be seen from Fig. 10, no significant gain in EGMEML yield was obtained after removing the catalyst, and the yield of EGMEML increased obviously with the catalyst. This reveals that 30%-KF/NaAlO₂ has good stability, and the nature of the transesterification catalyzed by KF/NaAlO₂ is heterogeneous.⁴⁰

Fig. 10 Leaching test for 30%-KF/NaAlO₂ catalyst.

3.7 Koros–Nowak test

To investigate the effect of diffusion limitations on the catalytic activity, the Koros–Nowak criterion test was designed and conducted.⁴¹ In the present study, different catalyst dosages of 30%-KF/NaAlO₂ were employed with a ML/EGME ratio of 1 : 3 at 120 °C. EGMEML formed during the course of the reaction was quantified with a time gap of 15 min using the GC technique. The results in Table 7 showed that the TOFs were almost similar with the increase of catalyst amount at the same EGMEML yield. Hence, it denotes that the reaction of ML with EGME obeyed the Koros–Nowak criterion and reaction rates were not masked by the rates of transport.⁴²

Table 7 Koros–Nowak test for heat- and mass-transfer limitations

a Reaction conditions: molar ratio of EGME/ML of 3.0, catalyst amount of 5.0%, reaction time of 4 h, reaction temperature of 120 °C and the yield of 40%.
Catalyst amount (wt%)	3.5	4.0	4.5	5.0
TOF^a (mol min^−1 mol^−1)	0.56	0.55	0.55	0.57

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3.8 Kinetics and thermodynamics study of transesterification reaction

With the purpose of discovering the kinetics of the transesterification of ML with EGME over 30%-KF/NaAlO₂, the reaction was explored and described as below:

ML (A) + EGME (B) → products

The reaction rate equation can be written¹⁷ as eqn (3)

−dC_A/dt = k[C_A]^a[C_B]^b (3)

When EGME was excessive, namely C_A0 ≪ C_B0, as shown in Fig. 11A, the plot of −lg(dC_A/dt) vs. −lg(C_A) shows a good linear relationship, and the value of the slope is found to be 0.97. In a similar way, when ML was excessive, namely C_A0 ≫ C_B0, the plot of −lg(dC_B/dt) vs. −lg(C_B) also shows good linear relationship and the value of the slope is found to be 1.10. This indicates that the grading numbers a and b of reactants A and B were 1, respectively. The rate formula of the reaction follows second-order kinetics. To calculate the activation energy, reactions were carried out in the temperature range of 105–120 °C. The activation energy (E_a) could be calculated⁴¹ based on the Arrhenius equation following eqn (4):

ln k = ln A − E_a/RT (4)

where E_a is the activation energy (kJ mol⁻¹), A is the pre exponential factor (h⁻¹), R is the gas constant (8.314 × 10⁻³ kJ K⁻¹ mol⁻¹) and T is the reaction temperature (K).

Fig. 11 Transesterification reaction kinetics analysis of ML with EGME over 30%-KF/NaAlO₂: (A) plots of −lg(dC_A/dt) vs. −lg(C_A) (■) and −lg(dC_B/dt) vs. −lg(C_B) (●) (B); (B) plot of ln k vs. 1/T.

The relationship of ln k vs. 1/T was discussed, and the scatter plot is shown in Fig. 11B. The values of E_a and A from the plot were found to be 56.54 kJ mol⁻¹ and 3.28 × 10⁶ h⁻¹. The value of E_a was greater than 25 kJ mol⁻¹, and the reactions were governed by a chemical step. The observed E_a value in the present study (56.54 kJ mol⁻¹) was found to be within the range of the reported values (26–82 kJ mol⁻¹) for the transesterification reaction catalyzed by heterogeneous catalysts.⁴³

For the sake of explaining the behavior of transesterification reactions, thermodynamic analysis was addressed for evaluating the enthalpy of activation (ΔH^‡), entropy of activation (ΔS^‡), and the Gibbs free energy of activation (ΔG^‡) based on the transition state theory (activation complex theory), which was developed by Eyring in 1935 (ref. 44) to evaluate the thermodynamic parameters of activation from temperature-dependent rate constants. The parameters were calculated from the Eyring–Polanyi equation.⁴⁵ The Eyring–Polanyi equation (eqn (5)) is analogous to the Arrhenius equation.

Taking the natural logarithm of eqn (5) and replacing ΔG^‡ = ΔH^‡ − TΔS^‡, eqn (6) can be written as:

where R, k_B and h are the universal gas (8.314 J mol⁻¹ K⁻¹), Boltzmann (1.38 × 10⁻²³ J K⁻¹) and Planck (6.63 × 10⁻³⁴ J s) constants respectively. Eqn (6) describes the mathematical relationship between the enthalpy and entropy of activation with the rate constant. Thus, the values of ΔG^‡, ΔH^‡ and ΔS^‡ could be calculated from the slope and the intercept of the Erying plot between 1/T and ln k/T as shown in Fig. 12. The values of ΔH^‡ and ΔS^‡ were found to be 41.57 kJ mol⁻¹ and −160.87 J mol⁻¹ K⁻¹ respectively. Further, the value of ΔG^‡ was calculated as 104.77 kJ mol⁻¹ at 393 K. A positive value of the enthalpy of activation (ΔH^‡) indicates that raising the energy level and transforming the reactants to their transition state required energy input (heat) from an external source.⁴⁰ A negative value of entropy of activation (ΔS^‡) suggests that reactant species have joined together to form a more ordered transition state.⁴⁶ A positive value of Gibbs free energy of activation (ΔG^‡) indicates that the reaction was unspontaneous and endergonic in nature.⁴⁵ Based on the data, it could be concluded that the reaction of ML with EGME over 30%-KF/NaAlO₂ is unspontaneous, endothermic and endergonic in nature.

Fig. 12 Eyring plot of 30%-KF/NaAlO₂ catalyzed transesterification of ML with EGME.

3.9 Properties of EGMEML

The properties of the obtained EGMEML and those of the standards of diesel and biodiesel are listed in Table 8. In short, the properties of the obtained EGMEML present many similarities with those of diesel and biodiesel, and therefore, given the properties of the obtained EGMEML it can be deemed as an alternative to diesel.

Table 8 Properties of the obtained EGMEML and the standards of diesel and biodiesel in Europe

Specification	Density/g cm^−3 (15 °C)	Kinematic viscosity/mm^2 s^−1 (40 °C)	Flash point/°C	Heating value/kJ kg^−1	Reference
Diesel	0.838	4.50	64	42.9	47
Biodiesel	0.860–0.900	3.5–5.0	>120	35	48(EN14214)
EGMEPM	0.898	5.98	190	38.2	10
EGMEML	0.905	3.60	133	38.4	Experimental data

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4 Conclusions

The heterogeneous catalyst KF/NaAlO₂ can be used as a solid base catalyst for the preparation of novel biodiesel via transesterification. Analyses by different techniques reveal that KF reacted with NaAlO₂ and generated fluoroaluminates as the dominant active sites for the transesterification. The catalysts show excellent catalytic activity and good stability, and under the optimum conditions the highest EGMEML yield of 91% was obtained and a satisfactory yield of EGMEML was obtained when KF/NaAlO₂ was used for three consecutive rounds without reactivation. The nature of the reaction is unspontaneous and endergonic; the reaction appeared to be a good fit with the second order reaction kinetics, and the activation energy was to be 56.54 kJ mol⁻¹; the reaction rate is free from the heat and mass transfer. KF/NaAlO₂ as a solid basic catalyst is a promising candidate catalyst for the synthesis of novel biodiesel via transesterification.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21176125), the Science Research Project of the Ministry of Education of Heilongjiang Province of China (2012TD012, 12511Z030, 12521594, JX201210), and the Natural Science Foundation of Heilongjiang province of China (B201313).

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