Active species and deactivation behavior of Al2O3 supported KNO3 catalyst in the synthesis of biodiesel via transesterification of soybean oil
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摘要: 制备了KNO3/Al2O3负载型固体碱催化剂，通过XRD、DRIFT、低温氮吸附、ICP和碱度滴定等手段对催化剂表面性质进行了表征，研究了其对大豆油与甲醇酯交换制备生物柴油的催化性能，剖析了在反应过程中该催化剂的催化本质以及失活原因。结果表明，高温焙烧后Al2O3表面KNO3完全分解，形成了大量偏铝酸钾分散在载体表面；在酯交换反应时Al2O3表面的偏铝酸盐等活性组分不断溶出并参与反应，这是该催化剂表现出高活性的主要原因。在反应过程中生成的产物生物柴油和甘油对催化剂的活性有很大影响，其中，生物柴油与活性物种发生的皂化反应是造成催化剂失活的主要原因。Abstract: Al2O3 supported KNO3 (KNO3/Al2O3) catalyst was prepared and characterized by XRD, DRIFT, nitrogen adsorption, ICP and basicity titration. The catalytic performance of KNO3/Al2O3 in the transesterification of soybean oil with methanol for biodiesel production was investigated; especially, the nature of active species and deactivation behavior during the reaction was considered. The results illustrate that KNO3 on the surface of Al2O3 is completely decomposed after calcination at 873 K, forming potassium aluminate. During the transesterification reaction, aluminate as the active species in KNO3/Al2O3 is gradually dissolved out, which is responsible for the high catalytic activity of KNO3/Al2O3. The biodiesel and glycerol products formed have a great influence on the catalyst activity and the catalyst deactivation is mainly ascribed to the saponification of biodiesel with the active species.
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Key words:
- potassium nitrate /
- supported solid base /
- biodiesel /
- dissolution /
- deactivation
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图 1 KNO3负载及焙烧前后36KNA样品的XRD谱图
■: γ-Al2O3; ●: KNO3
Figure 1 XRD patters of γ-Al2O3(a), 36KNA-u (b) and 36KNA(c)
图 2 KNO3负载及焙烧前后36KNA的红外光谱谱图
Figure 2 DRIFT spectra of γ-Al2O3(a), 36KNA-u (b) and 36KNA (c)
图 3 36KNA在不同温度下催化大豆油酯交换反应的生物柴油收率随时间的变化
(catalyst/oil=1.5%, methanol/oil=9:1)
Figure 3 Yield of fatty acid methyl ester (FAME) as function of reaction time on 36KNA for the transesterification of soybean oil with methanol at various temperatures
图 4 36KNA样品经甲醇处理后固相及液相催化大豆油酯交换反应的收率随时间的变化
(catalyst/oil=1.5%, methanol/oil=9:1, T=343 K)
Figure 4 Yield of FAME as function of reaction time on 36KNA (■), liquid phase (●), solid phase (▲) and both liquid and solid phases (▼) of 36KNA after immersion of methanol at 343 K for the transesterification of soybean oil with methanol
图 5 不同溶剂富集36KNA样品表面可溶性固体的XRD谱图
Figure 5 XRD patters of 36KNA-H2O-393K(a), 36KNA-CH3OH-393K(b), 36KNA-CH3OH-343K(c) and 36KNA-CH3OH-343K-393K(d)
图 6 氢氧化钾、偏铝酸钠与35SAA催化大豆油酯交换反应的收率随时间的变化
Figure 6 Yield of FAME as function of reaction time on (a) KOH and (b) NaAlO2 and (c) 35SAA for the transesterification of soybean oil at various temperatures (catalyst/oil=1.5%, methanol/oil = 9:1)
图 7 经甲醇、大豆油、甘油及生物柴油预处理后的氢氧化钾、偏铝酸钠及36KNA催化酯交换反应的收率
(■：KOH; □：treated KOH; ●：NaAlO2; ○: treated NaAlO2; ▲: 36KNA; △：treated 36KNA catalyst/oil = 1.5%, methanol/oil = 9:1, T=343 K)
Figure 7 Yield of FAME as function of reaction time on various catalysts pretreated with (a) methanol, (b) soybean oil, (c) glycerin and (d) biodiesel at 343 K for the transesterification of soybean oil
表 1 样品的BET比表面积和碱量
Table 1 BET specific surface area and basicity of various catalyst samples
Sample Basicity /(mmol·g-1) Surface area ABET/(m2·g-1) γ-Al2O3 0 143 36KNA-u 0 15 36KNA 2.43 49 Liquid phase of 36KNA with methanola 1.51 - Liquid phase of 36KNA with methanol after reactiona 0.65 - Solid phase of 36KNA with methanola 0.92 - Solid phase of 36KNA with methanol after reactiona 0.61 - a: methanol treatment or transesterification reaction was carried out at 343 K 
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