K_2SiO_3_C颗粒催化大豆油酯交换制备生物柴油_英文(2)

王建勋 等:K2SiO3/C 颗粒催化大豆油酯交换制备生物柴油 1593

the catalyst during the process also generates large volumes of wastewater. These problems have led to the search for stable and more environmentally friendly solid catalysts. A literature survey indicated that alkali earth oxides, such as CaO, SrO, and MgO, are the main solid catalysts used for the transesterification reaction [2–4]. Alkali metals or alkali earth salts loaded on metal oxide such as KOH/Al2O3 [5], KF/MgO [6], KI/MCM-41 [7], Ca(NO3)2/ Al2O3 [8], and calcined Mg-Al hydrotalcites [9,10] have also been used in recent years.

In recent years, microwave (MW) technology has attracted the attention of researchers due to its unique molecular level heating to give rapid thermal reactions [11]. Many studies on the application of MW dielectric heating have been reported with homogeneous and heterogeneous catalysts in biodiesel production [12,13]. In this study, a solid catalyst (K2SiO3/C) was evaluated as a nonconven- tional basic solid that can absorb microwave irradiation resulting in energy absorption. It is well known that carbon materials can strongly absorb microwave energy. Due to its non-uniformity, “hot spots” can be generated on the surface of carbon materials where the temperatures (above 1200 °C) are higher than at other places, and where chemical reactions can easily take place [14].

Alkali-silicate binders have been known for a long time and the mechanism of solidification in the sol-gel technique of materials preparation has been gradually understood [15]. A K2SiO3 solution used to supply basic sites supported on carbon particles for the transesterification reaction has not been reported. The catalytic performance of the K2SiO3/C catalyst was studied to provide a solid base catalyst for the production of biodiesel.

ml flat-bottom flask equipped with a reflux condenser and a magnetic stirrer. The transesterification reaction of soybean oil (Great Wall Enterprise Co.) and methanol (ACS grade, ECHO Chemical Co.) was carried out in the liquid phase under atmospheric pressure at 65 °C while stirring at 600 r/min for 0.5–3 h. The microwave reactions were carried out in a microwave synthesis reactor (CEM, MARS) working at 2.45 GHz and rated at 150 W. The temperature of the reac-tion mixture was maintained by a fiber optic temperature sensor (Model: Discover Fiberoptic, CEM; range: –50 to 250 °C). For the conventional heating method, a hot plate was used for heating the mixture in the flask. The heating power of the hot plate was 700 W.

After the transesterification reaction, DI water was added into the reaction mixture to stop the reaction. The super-natant was filtrated through a filter paper, and excess methanol and water were evaporated before the analysis of the FAMEs.

1.3 Characterization of the catalyst

The basic strength of the samples (H_) was determined by Hammett indicators [6]. The Hammett indicators for basic site strength used were: bromthymol blue (H_ = 7.2), phenolphthalein (H_ = 9.8), 2,4-dinitroaniline (H_ = 15.0), and 4-nitroaniline (H_ = 18.4). About 1 g of sample was shaken with 10 ml methanol solution of the Hammett indi-cator and left for 1 h to achieve equilibration. The total number of basic sites was determined by titration with ben-zoic acid in methanol using phenolphthalein as the indicator [6]. The X-ray diffraction (XRD) characterization of the catalysts was performed on a MAC MXP18 powder X-ray diffractometer using Cu Kα radiation over a 2θ range from 20° to 80° with a step size of 0.04° at a scanning speed of 3°/min. The microstructures of the K2SiO3/C catalysts were observed by a field emission scanning electron microscope (SEM, JEOL JSM-7401F). The FAME concentration, used to express the biodiesel purity of the product, was deter-mined by a gas chromatograph (Thermo trace GC ultra) equipped with a flame ionization detector and a capillary column (Tr-biodiesel (F), Thermo, 30 m × 0.25 mm × 0.25 μm). Nitrogen was used as the carrier gas. The amount of FAME was calculated by the internal standard (methyl hep-tadecanoate) method according to Chinese National Stan-dards 15051. In order to quantitatively evaluate leaching of the solid base catalyst under the reaction conditions, some parts of the samples taken from the reactor were carefully filtered, and the residual methanol was evaporated in a ro-tary evaporator so that the FAME and glycerol were left as a separate phase. After the evaporation, the dry fraction was treated with 0.1 mol/L hydrochloric acid [16]. The resulting solution was analyzed by inductively coupled plasma opti-

1 Experimental

1.1 Preparation of the catalyst

The K2SiO3/C catalysts were prepared by an impregna-tion method. Typically, a required amount of K2SiO3 solu-tion (reagent grade, 66.2%, Shimakyu’s Pure Chemicals) was diluted with 200 ml deionized (DI) water at ambient temperature. The carbon particles (Taiwan Active Carbon Industry Co., dried at 80 °C for 3 h before being used), which was an irregular type of particle size 1–3.5 mm, was then added into the solution followed by vigorous mixing. The amount of K2SiO3 solution/carbon was varied from 10 wt% to 30 wt%. After equilibrating the mixture for 1 h, the resulting solution was dried in an oven at 120 °C for 24 h. 1.2 Transesterification reaction procedure

The transesterification reaction was performed in a 250