Preparation of diethylene glycol monomethyl ether monolaurate catalyzed by active carbon supported KF/CaO
© Lou et al. 2015
Received: 10 September 2015
Accepted: 29 October 2015
Published: 10 November 2015
Diethylene glycol monomethyl ether monolaurate (DGMEML) was synthesized via the reaction of diethylene glycol monomethyl ether (DGME) with methyl laurate (ML) by a new solid base catalyst of KF/CaO/AC, which was prepared by impregnation method using active carbon as carrier. The catalysts were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), nitrogen physisorption-desorption and Hammett indicator methods; the effect of the mole ratio of KF to CaO, DGME to ML molar ratio, amount of catalyst, reaction time and temperature on the yield of DGMEML were studied; and the relationship between the structure of the catalyst and the yield of DGMEML was investigated. The formed KCaF3 and K2O were acting as the main active components in the catalytic transesterification; the highest yield of 96.3 % was obtained as KF-to-CaO molar ratio of 2.0, DGME to ML molar ratio of 4.0, catalyst amount of 5 wt%, and reaction time of 30 min at 75 °C; and the catalyst displayed good stability in the transesterification.
Biodiesel originated from the transesterification of vegetable oils or animal fats with short chain alcohols has been attracted more attention for the biodegradable, nontoxic, comparable calorific value and relative lower emission of NO x and CO2 to the petroleum diesel; and the production of biodiesel increased rapidly around the world (Shahir et al. 2015; Avhad and Marchetti 2015; Gopinath et al. 2015). In recent years, some studies tried successfully to introduce one ether group or more into biodiesel molecules to further reduce smoke emissions, and a novel biodiesel synthesized via the transesterification of fatty acids methyl esters with short chain glycol ethers, such as ethylene glycol monobutyl ether palm oil monoester (Jiang and Yun 2012), ethylene glycol n-propyl ether palm oil monoester (Gao et al. 2011), ethylene glycol monoethyl ether soybean oil monoester (Zhang et al. 2006), and ethylene glycol monomethyl ether palm oil monoester (Jiang et al. 2011; Guo et al. 2015a) et al. have been developed. Compared with the traditional biodiesel, novel biodiesel with higher oxygen content for introducing an ether group can effectively improve the combustion and emission performance (Guo et al. 2013; Guo et al. 2015b; Chen et al. 2014a). At present, novel biodiesel is mainly prepared by homogeneous base catalyst, such as sodium alcoholate (Zhang et al. 2012; Guo et al. 2015c) and KOH (Li et al. 2012; Jiang 2012). The usage of homogeneous catalysts produced large amounts of caustic wastewater giving rise to serious environmental pollution and post-processing was complex (Deshmane and Adewuyi 2013). Recently, Na2SiO3 (Fan et al. 2013), KF/HTL (Chen et al. 2014b) and KF/CaO/Kaolinite (Guo et al. 2015c) acting as solid base catalysts have been respectively used in the production of novel biodiesel of ethylene glycol monomethyl ether soybean oil ester and ethylene glycol monomethyl ether monolaurate. The results demonstrates the heterogeneous solid bases show good catalytic performance, and the catalytic processes have fewer unit operations. Moreover, the simple methods of filtration, centrifugation can be easily used to separate the solid catalyst from the reaction system. It has becoming a promising route for the production of novel biodiesel.
Diethylene glycol monomethyl ether-based biodiesel which contains two ether groups have higher oxygen content. It was found that the density, kinematic viscosity, smoke point, and cetane number of diethylene glycol monomethyl ether-based biodiesel increased obviously compared with that of traditional biodiesel and ethylene glycol monomethyl ether-based biodiesel (Guo et al. 2013, 2015a). Few reports about the solid base catalyst used in production of diethylene glycol monomethyl ether-based biodiesel via diethylene glycol monomethyl ether with fatty acid methyl ester has been revealed.
KF/CaO catalysts showed higher catalytic activity in the manufacture of biodiesel, but it is not easy to separate (Hu et al. 2012; Fan et al. 2014; Jia et al. 2015). Activated carbon with a large surface area as supporter is widely used in the production of biodiesel for the dispersion of active sites effectively, and the surface characteristic of activated carbon does not change at high temperature or pressure (Naranjo et al. 2010; Baroutian et al. 2010; Buasri et al. 2012; Li et al. 2013; Malins et al. 2015; Tao et al. 2015). Inspired by the previous reports, an effective and separable solid bases of active carbon supported KF/CaO was prepared by impregnation method, and tried to use as a catalyst in the transesterification of diethylene glycol monomethyl ether (DGME) and methyl laurate (ML) to produce diethylene glycol monomethyl ether monolaurate (DGMEML). X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), Hammett indicator and nitrogen physisorption-desorption were performed to characterize the structure of the catalysts, in an attempt to explain the correlation between structure and activity of the catalyst. In addition, the effect of mole ratio of KF to CaO and main reaction parameters on the yield of DGMEML was investigated. The catalyst showed an excellent catalytic activity, and could be easily to separate from the system.
Materials and methods
All of the chemicals used in the present work were of A.R. grade and purchased from Aladdin, China. DGMEML sample was synthesized by our laboratory reference to the literature (Jiang 2012), and the structure was confirmed by 1H NMR, 13C NMR and FT-IR spectrum, which are shown in SI.
The active carbon-based catalyst was prepared by wet impregnation method. In order to remove any soluble alkaline and acidic impurities from the AC, a pretreatment was first performed by washing the support with 0.1 M NaOH solution, followed by the second treatment with 0.1 M HCl (Badday et al. 2014). Available active carbon obtained after drying at 80 °C for 24 h. 0.56 g (0.01 mol) CaO powder and 1.00 g active carbon were immersed in 10 mL distilled water, and stirred for 4 h at 80 °C. After the water was evaporated, the solid CaO-active carbon was immersed in KF·2H2O solution with different amounts of KF for 1 h, and then stirred for 4 h at 80 °C, and followed by calcination in muffle furnace at 500 °C for 5 h. The prepared catalyst was sealed by plastic membrane for further use. The resultant composites were named as KCC-n, where ‘n’ stands for the molar ratio of KF to CaO.
The basic strength of the sample (H -) was determined using Hammett indicator. About 50.0 mg of the sample was shaken with 5.00 mL cyclohexane and two or three drops of Hammett indicators-benzene solution (0.1 %, w/w) and then left to equilibrate for 2 h when no further color changes were observed. The Hammett indicators used and the corresponding H − values are as follows: 4-nitroaniline (H − = 18.4), 2, 4-dinitroaniline (H − = 15.0), phenolphthalein (H − = 9.8) (Boz et al. 2009; Yan et al. 2009). Alkaline determination was measured by benzoic acid titration method, using 0.02 mol/L benzoic acid-anhydrous ethanol solution as titrant, until the basic color of indicator adsorbed on the surface of solid alkali just disappeared. Powder XRD diffraction was recorded on a Bruker D8 Advance (Germany) diffractometer, using Cu Kα radiation (λ = 1.5418° A) at 40 kV and 50 mA. The scanning speed was 3° min−1 and scanned area ranged 2θ = 10–80°. Morphology of the samples was observed by SEM using a Rigaku S-4700 spectrometer (Japan). The voltage was 20 kV and the vacuum degree of the sample room was better than 10−4. The BET surface areas and pore sizes were measured using a NOVA 2000e operating on the basis of the physical adsorption of liquid nitrogen performed at a temperature of 77 K by the Brunauer-Emmett-Teller (BET) method. FT-IR spectra were recorded on an AVATAR370 FT-IR spectrophotometer both over the wave number range from 400 to 4000 cm−1 at a resolution of 4 cm−1 and by averaging over 16 scans. Samples were prepared by mixing the powdered solids with potassium bromide.
Results and discussion
Basic strength and basicity of KCC-n catalysts
Basic strength (H-)
7.2 < H − < 9.8
7.2 < H − < 9.8
9.8 < H − < 15.0
9.8 < H − < 15.0
9.8 < H − < 15.0
9.8 < H − < 15.0
Catalytic activity of KCC
Effect of KF-to-CaO molar ratio
The effect of KF-to-CaO molar ratio of KCC-n on catalytic performance was investigated as shown in Table 1. With the increase of n from 0.5 to 2.0, DGMEML yield increased from 23.0 to 96.3 %. The catalytic activity increased significantly which lined with the basicity of the catalysts. Combined with the XRD patterns, the raising of DGMEML yield could be mainly related to the amount increase of KCaF3 and K2O phases. However, further increased n to 3.0, led to the less increase in the resulting basicity of the composite and thus caused a slightly increase in the DGMEML yield as seen in Table 1. Similar phenomenon was observed in literature (Algoufi et al. 2014). On the basis of the results, 2.0 was chosen as the optimum KF-to-CaO molar ratio.
Infuence of the reaction parameters
According to the above-mentioned experiments, the high yield of DGMEML was obtained under DGME/ML molar ratio of 4.0, catalyst amount of 5 wt% with respect to ML, reaction duration of 30 min at 75 °C.
Production of novel biodiesel of diethylene glycol monomethyl ether soybean oil monoester
Based on the optimized conditions of the transesterification of ML with DGME, KCC-2.0 was used as a catalyst in the production of a novel biodiesel of diethylene glycol monomethyl ether soybean oil monoester from the raw material of methyl soybean oil ester biodiesel and DGME (Additional file 1). As expected, the novel biodiesel yield of 90.0 % was obtained when the soybean biodiesel/DGME molar ratio was 1:8, the reaction temperature was 75 °C, the mass amount of catalyst was 5 %, and the reaction time was 30 min. It can be concluded that KCC-2.0 is an efficacious catalyst for the synthetic of soybean oil-base novel biodiesel.
The stability of the catalyst
Diethylene glycol monomethyl ether monolaurate (DGMEML) was successfully synthesized via the transesterification of diethylene glycol monomethyl ether (DGME) with methyl laurate (ML) by an effective solid base catalyst of active carbon supported KF/CaO. Analyses by various modern instruments revealed that K2O and KCaF3 were active groups. The highest yield of DGMEML of 96.3 % was obtained as KF/CaO molar ratio of 2.0, DGME/ML molar ratio of 4.0, catalyst amount of 5 wt%, and reaction time of 30 min at 75 °C; and the yield of DGMEML was satisfied as the catalyst used in the next round. Furthermore, a desirable yield of 90.0 % of novel biodiesel of diethylene glycol methyl ether soybean oil monoester was obtained with KCC-2.0 as catalyst.
SL and LJ designed the study and wrote the manuscript, PW and LG contributed to the data collection, JW and XG analyzed the data. All the authors read and approved the final manuscript.
This work was supported by the Science Research Project of the Ministry of Education of Heilongjiang Province of China (2012TD012, 12511Z030, 12521594, JX201203), and the Graduate Innovation Fund of Heilongjiang Province of China (YJSCX2013-014X).
The authors declare that they have no competing interests.
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