K_2SiO_3_C颗粒催化大豆油酯交换制备生物柴油_英文_

2011

文章编号: 0253-9837(2011)10-1592-05

Chinese Journal of Catalysis

国际版DOI: 10.1016/S1872-2067(10)60265-3

Vol. 32 No. 10

研究论文: 1592~1596

K2SiO3/C 颗粒催化大豆油酯交换制备生物柴油

王建勋 1, 陈洸艟 2, 陈锦章 1,*

1

台中教育大学科学应用与推广学系, 台湾台中 40306

2

明新科技大学自然科学教学中心, 台湾新竹 30401

摘要: 采用浸渍法将钾水玻璃负载于碳颗粒 (粒径 1~3.5 mm) 上, 经 120 °C 烘干后制得 K2SiO3/C 催化剂. 以 X 射线衍射、扫瞄电镜-能量色散 X 射线与 Hammett 指示剂等方法对样品进行了表征. 以 K2SiO3/C 为催化剂, 大豆油为原料, 进行酯交换反应制备生物柴油, 考察了催化剂用量、醇/油摩尔比、反应时间 (微波与传统加热) 等因素对大豆油酯交换率的影响. 当醇/油摩尔比 30:1, 催化剂用量 24 wt% 时, 传统加热反应于 2.5 h 达到 96.5% 酯交换率, 而相同条件下微波加热反应于 1.5 h 达到 96.7% 酯交换率.

关键词: 生物柴油; 固体催化剂; 钾水玻璃; 碳颗粒; 微波吸收 中图分类号: O643 文献标识码: A 收稿日期: 2011-05-27. 接受日期: 2011-07-26.

*通讯联系人. 电话: (886)-4-22183406; 传真: (886)-4-22183560; 电子信箱: ccchen@ms3.ntcu.edu.tw 基金来源: 台湾”国家”科学委员会 (NSC99-2622-M-42-001-CC1).

本文的英文电子版(国际版)由Elsevier出版社在ScienceDirect上出版(/science/journal/18722067).

Biodiesel Production from Soybean Oil Catalyzed by K2SiO3/C

WANG Jianxun1, CHEN Kungtung2, CHEN Chiingchang1,*

1

Department of Science Application and Dissemination, National Taichung University of Education, Taichung 40306, Taiwan, China

2

The Teaching Center of Natural Science, Minghsin University of Science and Technology, Hsinchu 30401, Taiwan, China

Abstract: A solid base catalyst (K2SiO3/C) capable of microwave absorption was used for the transesterification of soybean oil under mi-crowave radiation. The K2SiO3/C catalyst was prepared by an impregnation method that loaded K2SiO3 on carbon particles (1–3.5 mm di-ameter) followed by drying at 120 °C. The catalysts were characterized by X-ray diffraction, scanning electron microscopy- energy disper-sive spectrometry, and the Hammett indicator method. K2SiO3 was well distributed on the support. The effects of reaction variables such as catalyst loading, molar ratio of methanol to oil, and reaction time (under microwave radiation and conventional heating) were studied. When the conventionally heated reaction was carried out at 65 °C with a methanol/oil molar ratio of 30:1 and a catalyst concentration of 24 wt%, the biodiesel conversion was 96.5% after 2.5 h reaction time. The same reaction reached equilibrium after 1.5 h under microwave radiation, and the conversion of biodiesel was 96.7%.

Key words: biodiesel; solid catalyst; potash water glass; carbon particle; microwave absorption

Received 27 May 2011. Accepted 26 July 2011.

*Corresponding author. Tel: +886-4-22183406; Fax: +886-4-22183560; E-mail: ccchen@ms3.ntcu.edu.tw This work was supported by the “National” Science Council of Taiwan (NSC99-2622-M-42-001-CC1).

English edition available online at Elsevier ScienceDirect (/science/journal/18722067).

Global warming due to the heavy consumption of fossil resources and the depletion of natural resources is of in-creasing concern, and for sustainable development, bio-diesel is of increasing attention as a source of renewable energy. The most common way to produce biodiesel is by transesterification. Although transesterification is relatively fast and has high conversions in homogeneous catalyst sys-tems, these have some serious drawbacks [1] such as that the catalyst cannot be recovered and must be neutralized, and the separation of fatty acid methyl esters (FAME) from

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

1594 催 化 学 报 Chin. J. Catal., 2011, 32: 1592–1596

cal emission spectroscopy (ICP-OES, Spectro Genesis) to determine the K concentration.

2 Results and discussion

The catalytic activities of carbon particles with different K2SiO3 loadings were measured. For comparison, the same reaction conditions shown in Table 1 were employed in all experiments. The reaction conditions were not optimized for the highest reaction conversion. Non- loaded carbon parti-cles exhibited no activity. When K2SiO3 was loaded on the carbon particles, the supported catalysts showed catalytic activity. It can be concluded that the observed activities of the carbon supported catalysts were due to their basicity, i.e., a higher basicity result in a higher conversion.

Figure 1 shows the XRD patterns of ground original car-bon particles and catalyst particles (loading of 30 wt% K2SiO3 solution). The carbon particles and prepared catalyst particles had the same XRD patterns which was identical to that of C (PDF 261077). No characteristic peak due to K2SiO3 or any new species such as KOH and SiO2 was ob-served, indicating the high dispersion of K2SiO3 on the car-bon support [17].

Intensity

K2SiO3/CCarbon particle

203040

502 /( o )

607080

Fig. 1. XRD patterns obtained from ground original carbon particle and catalyst particle.

Figure 2 shows typical SEM and EDS mapping images of the K2SiO3/C catalyst (30% K2SiO3 solution loading) and a used K2SiO3/C catalyst. These showed that the morphology was irregular and diverse, with smooth surfaces, large pores, and convex surfaces. The distribution of K2SiO3 on the support surface was homogeneous even on the convex sur-faces and in the large pores. K2SiO3 was found to be very effectively distributed on the surface of the support. In order

Table 1 Properties of K2SiO3/C particles used as solid base catalyst for the transesterification of soybean oil with refluxing methanol

Catalyst K2SiO3 solution loading (wt%)

Basic strength

Basicity H_> 9.8 (mmol/g)

0 0 0.018 0.043 0.053 0.052

Conversion* (%) No reaction 23.3 42.5 63.3 82.0 81.2

Carbon 0 H_ < 7.2

K2SiO3/C 10 7.2 < H_ < 9.8 9.8 < H_ < 15.0 K2SiO3/C 15

9.8 < H_ < 15.0 K2SiO3/C 20

9.8 < H_ < 15.0 K2SiO3/C 25

9.8 < H_ < 15.0 K2SiO3/C 30 with conventional heating.

*Reaction conditions: 12.5 g soybean oil, methanol/oil molar ratio 24:1, catalyst amount 2 g, reaction time 3 h, at the methanol re ux temperature

1 μm

Si

OK

1 μm

SiOK

1 μm

SiOK

Fig. 2. SEM and EDS mapping of samples. (a) and (b) Fresh K2SiO3/C catalyst; (c) Used K2SiO3/C catalyst.

王建勋 等:K2SiO3/C 颗粒催化大豆油酯交换制备生物柴油 1595

to remove adsorbed species from the catalyst surface, the used K2SiO3/C catalysts were washed with anhydrous methanol and dried at 120 °C for 24 h before examination by SEM. K2SiO3 was found to be well distributed on the surface of the support even after the transesterification reac-tion.

In order to compare the microwave absorption ability of carbon particles (30% K2SiO3 solution loading), CaO pow-der and K2SiO3 powder (K2SiO3 solution dried at 120 °C for 24 h and then well ground), a 25 g sample was placed in a glass reactor with an i.d. of 57 mm that was heated at 2.45 GHz with 150 W. The catalyst temperature was measured by a fiber optic temperature sensor located in the sample. In Fig. 3, it is clearly shown that K2SiO3/C is much more ef-fective in absorbing microwave than K2SiO3 and CaO since

250200Temper

ature (oC)

150100500

10080Conversion (%)

6040200Catayst amount (g)

Fig. 4. Influence of catalyst amount on the conversion using conven-tional heating. Reaction conditions: 12.5 g soybean oil, methanol/oil molar ratio 24:1, reaction time 3 h, reaction temperature 65 °C.

Time (min)

Fig. 3. Temperature profiles of K2SiO3/C, CaO, and K2SiO3 under 150 W microwave heating of 25 g sample.

The variables investigated included the catalyst amount, alcohol/oil ratio, and reaction time for the K2SiO3/C catalyst (30% K2SiO3 loading). The catalyst amount was varied in the range of 1–6 g. As shown in Fig. 4, the conversion in-creased as the catalyst amount was increased to 1–3 g. The conversion reached a plateau value for the catalyst mass of 4–6 g.

In heterogeneous catalysis, mass transfer and reactant adsorption on the catalyst are very important; thus, a molar ratio higher than the stoichiometric molar ratio of methanol is needed to shift the equilibrium of the reaction. As shown in Fig. 5, when the methanol loading increased, the conver-sion increased considerably. The maximum conversion was 96.6% at the methanol/oil molar ratio of 36:1.

To study the effect of microwave radiation and conven-tional heating on the conversion, experiments were carried out using a K2SiO3/C catalyst. As shown in Fig. 6, the reac-tion reached equilibrium after 1.5 h under microwave radia-

tion. The conversion of biodiesel was 96.7%. However, the reaction did not reach equilibrium when conventional heat-ing was used for 2.5 h. The conversion of biodiesel was 96.5%. That is, a shorter time was needed under microwave radiation compared to conventional heating.

10080Conversion (%)

6040200

Reaction time (h)

Fig. 6. Comparison of microwave radiation and conventional heating on the conversion. Reaction conditions: 12.5 g soybean oil, metha-nol/oil molar ratio 30:1, catalyst amount 3 g.

1596 催 化 学 报 Chin. J. Catal., 2011, 32: 1592–1596

In order to study the stability of the K2SiO3/C catalyst, the samples were recovered by simple decantation. The remaining catalyst in the reactor was used to catalyze the next batch of soybean oil. A decline was observed in the conversion to methyl esters from 96.6% to 57.9%, indicat-ing the decrease of catalytic activity. This lowered activity may be explained by the dissolution of K species in the het-erogeneous K2SiO3/C catalyst. Glycerol covering of the surface of catalyst was also considered as the reason for the activity loss. The stability of the K2SiO3/C catalyst was bet-ter than that of a KOH/NaX [18] catalyst and KOH/MgO catalyst [19]. After a recycling experiment, it was found that the catalytic activity of the KOH/NaX catalyst and KOH/MgO catalyst decreased to 48.7% biodiesel conver-sion and 26.45% biodiesel yield, respectively.

The solubility of the catalyst was also a very important factor that should be taken into account to correctly interpret the performance and assess its practical use. This was a very important issue since neutralization and washing steps of the biodiesel produced would be necessary if any metal was found in it. Then, the possible advantage of using the het-erogeneous catalyst for this process would be nullified. The results showed that only small quantities (free K+ concen-tration was 3.1 ppm) existed in the biodiesel. The results obtained proved that the K2SiO3/C catalyst prepared has good potential for the biodiesel production from soybean oil.

high energy efficiency to give a faster reaction rate to shorten the reaction time.

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3 Conclusions

The K2SiO3/C catalyst was prepared by an impregnation method. K2SiO3 was well distributed on the surface of the carbon particle and it was an effective base for the transesteri cation reaction. Furthermore, the K2SiO3/C catalyst has good microwave absorption ability and showed

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