K_2SiO_3_C颗粒催化大豆油酯交换制备生物柴油_英文(4)

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.