新型Lewis固体酸Ce~(3+)-Ti~(4+)-SO_4~(2-)/MWCNTs制备及催化酯化反应合成生物柴油性能研究

采用高温浸渍法,通过Ce~(3+)、Ti~(4+)和浓硫酸磺化反应对多壁纳米碳管进行了改性处理,制备了Lewis酸型固体酸催化剂Ce~(3+)-Ti~(4+)-SO_4~(2-)/MWCNTs,并采用透射电镜、拉曼光谱、X射线光电子能谱、吡啶吸附红外光谱、X射线荧光光谱、X射线衍射光谱和NH_3程序升温脱附等多种测试技术对催化剂的物理化学特性和结构特征进行了表征。以Ce~(3+)-Ti~(4+)-SO_4~(2-)/MWCNTs为油酸与甲醇经酯化反应合成生物柴油的催化剂,对其催化性能进行了研究。结果表明,当醇油物质的量比为12∶1,催化剂与反应物质量比为1%,反应温度为65℃,反应5 h,油酸转化率为93.4%。催化剂Ce~(3+)-Ti~(4+)-SO_4~(2-)/MWCNTs在重复使用八次后,油酸的转化率仍为80.8%,由此表明其具有较高的催化活性和稳定性。高催化活性和稳定性是因为,纳米碳管的C 1s结合能较一般炭材料低,使得电子在其管状结构中的流动和逃逸非常容易,从而有助于负载于纳米碳管之上的活性组分之间发生强烈的相互作用,最终促使Ce~(3+)和Ti~(4+)分别与SO_4~(2-)形成稳定的配位键,增大催化剂的晶化程度,并使SO_4~(2-)与纳米碳管结合的更加牢固,增强了催化剂的稳定性,减少了催化剂中活性组分的流失。最后,由于SO_4~(2-)与Ce~(3+)的强相互作用,在不增加纳米碳管表面缺陷的情况下,改变了Ti~(4+)-SO_4~(2-)中表面原子的化学状态,使得S~(6+)离子和Ti~(4+)离子的吸电子能力增加,使催化剂以Lewis酸性活性位为主,避免了SO_4~(2-)/MWCNTs因为以Brnsted酸位为主,而在富含水的反应介质中,由于水合反应而降低其催化活性的现象发生。

Preparation of a novel solid Lewis acid Ce3+-Ti4+-SO42-/MWCNTs and its application in the synthesis of biodiesel from esterification reaction

The Ce³⁺-Ti⁴⁺-SO₄^2-/MWCNTs catalyst was prepared from the modification treatment of multi walled carbon nanotubes by concentrated sulfuric acid, Ce³⁺ and Ti⁴⁺ through the employing of high temperature impregnation method. Physicochemical properties and structural characteristics of the obtained-catalysts were characterized by means of transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, pyridine adsorption FT-IR spectra, X-ray fluorescence spectroscopy, X-ray diffraction and NH₃ temperature programmed desorption. The catalytic activity of Ce³⁺-Ti⁴⁺-SO₄^2-/MWCNTs for the synthesis of biodiesel from the esterification of methanol and oleic acid was investigated. The influence of SO₄^2-/MWCNTs, which was resulted from the addition of Ce³⁺ and Ti⁴⁺, on the structure and catalytic activity was cleared based on the above structure characterization and catalytic activity investigation. The results showed that the conversion of oleic acid reached 93.4% after 5 h reaction at 65℃, when the catalyst/reactants was 1% and the molar ratio of methanol/oleic acid was 12:1. The conversion of oleic acid was 80.8% after the Ce³⁺-Ti⁴⁺-SO₄^2-/MWCNTs were cycled for eight times. Therefore, it can be concluded that this catalyst has high catalytic activity and stability. The high catalytic activity and stability can be explained as follows:the C 1s binding energy of carbon nanotube is much lower than other carbon materials, resulting in easy flow and escape of the electrons in the tubular structure. Thus, the strong interactions will occur among the active groups that have been loaded on the carbon nanotube, which impels Ce³⁺ and Ti⁴⁺ to respectively form stable coordination bonds with SO₄^2-, increases the crystallization degree of the Ce³⁺-Ti⁴⁺-SO₄^2-/MWCNTs catalyst and the active acid sites without increasing the surface defects, and the combination of SO₄^2- and MWCNTs was more stable. In addition, the chemical state of surface atom on the Ti-SO₄^2- was changed due to the strong interaction between SO₄^2- and Ce³⁺, which strengthened the electron withdrawing ability of S⁶⁺ and Ti⁴⁺ ions and enhanced the acidity strength of Lewis acid with changing of the acid type. Hence, the Ce³⁺-Ti⁴⁺-SO₄^2-/MWCNTs will be composed by Lewis acid sites mainly, which is favorable for avoiding the occurrence of hydration of acid active sites for the SO₄^2-/MWCNTs catalyst because it was composed of Brönsted acid sites mainly.