基于生物柴油制备的纳米碳管改性固体酸催化剂的设计和性能调控

It is very important for the synthesis of a catalyst with high catalytic activity and stability in the entire reaction process when waste animal fats and vegetable oils were used as raw materials for the production of biodiesel due to these raw materials were commonly composed by complex compositions. In this project, a solid Lewis acid catalyst with the employing of multi-walled carbon nanotubes as support will be prepared, and which can be expected to achieve the challenging requirement of catalyst. Some exploring will be done as follows: the type of catalyst, template and arrangement of catalyst on the template will be investigated to know the possible influences to the structure of multi-walled carbon nanotubes when the chemical vapor deposition method was applied to produce multi-walled carbon nanotubes. The intrinsic modulation mechanism for the conversion of Brönsted acid sites to the Lewis acid sites will be studied in detail when the multi-walled carbon nanotubes was sulfonated by concentrated sulfuric acid at high temperature and modified from the introduction of positive ion with strong electron withdrawing (or negative ion with strong negative effect) at the same time. The influences on the surface acidity and thermal stability of the obtained catalyst will be studied since the different coordination type, quantity and bond energy will be formed between the different rare earth metal ions and the multi-walled carbon nanotubes with the modulation of acid site to the Lewis type. The catalytic mechanism, thermodynamic and reaction kinetics will be studied when the solid Lewis acid catalyst with the employing of multi-walled carbon nanotubes as support was applied to catalyze waste animal fats and vegetable oils for the production of biodiesel. The goal of preparing of biodiesel from waste animal fats and vegetable oils can be expected to realize based on these above investigations.

废弃动植物油脂的的成分非常复杂，当以其为原料制备生物柴油时，一种能够在反应过程中保持高催化活性和稳定性的催化剂非常关键。本项目拟合成一种Lewis酸型多壁纳米碳管改性固体酸来满足对催化剂的苛刻要求，拟进行以下探索：研究经化学气相沉积法合成多壁纳米碳管时，催化剂、模板和模板上催化剂的排列方式对合成的多壁纳米碳管结构的影响；研究对多壁纳米碳管进行浓硫酸高温磺化处理的同时，引入具有强吸电子作用的阳离子或强电负性的阴离子进行改性后，对Brönsted酸位转变为Lewis酸位的内在调变机理；研究不同稀土金属离子与经过Lewis酸位调变后的多壁纳米碳管进行键相配位的类型、数量与键能的差异对催化剂的表面酸性和热稳定性所带来的影响；研究Lewis酸型多壁纳米碳管改性固体酸催化转化废弃动植物油脂为生物柴油的催化机理、热力学和动力学行为。通过以上研究，实现利用废弃动植物油脂为原料制备生物柴油的目标。

基于碳基固体酸通常是一种以质子酸性活性位(Brönsted酸位)为主的固体酸，大多数属于硫酸盐型碳基固体酸，随着大量亲水性极性基团-SO3H与炭层发生键合作用进入其骨架中，将进一步提高催化剂的亲水性，这将导致一种硫酸盐型碳基固体酸催化剂的Brönsted酸位更易与水分子发生水合作用，而减弱甚至失去其催化活性。因此，很难同时兼顾高催化活性和稳定性的优势。本研究通过溶胶凝胶法，使用单壁纳米碳管(Single-walled carbon nanotube, SWCNTs)经侧壁羟基功能化作用后得到的SWCNTs-OH和稀土La3+对H3PW12O40进行了协同改性作用，合成了一种以强Lewis酸位为主的固体酸催化剂La-PW-SiO2/SWCNTs，并通过油酸和甲醇的酯化反应研究了合成得到的催化剂的催化活性和稳定性。结果表明，当甲醇与油酸的摩尔比为15:1，反应温度为65°C，催化剂与反应物的质量比为1.5 %，反应8 h后，油酸的转化率为93.1 wt%。经过6次循环使用，油酸的转化率仍然高达88.7 wt%。以上可归因于(≡Si-OH2+)(H2PW12O40-)络合物与SWCNTs-OH中的-OH和La3+键合形成活性组分(La-PW-SiO2)。由于La3+具有强吸电子效应，可与SWCNTs中含有的高度离域化的大π键发生强相互作用，使大量e-移动到SWCNT表面并分散在苯状环结构中。因此，La-PW-SiO2/SWCNTs催化剂的系统电荷不平衡趋势加剧，并导致其中的正电荷过量，该作用有助于Lewis酸位的形成而增加其含量。以F--SO42-/MWCNTs为催化剂，对油酸与甲醇酯化反应进行了动力学研究。基于拟均相二级反应动力学模型，研究了反应温度、甲醇与油酸摩尔比和催化剂负载量对油酸转化率的影响。结果显示模拟和实验结果之间的一致性特别好。通过动力学研究，获得了正逆反应的指前因子(A)和活化能(Ea)，正向反应的A和Ea值分别为1.88 L•mol-1•h-1和32.73 kJ•mol-1，反向反应的A和Ea值分别为1.90 L•mol-1•h-1和32.61 kJ•mol-1。较低的活化能表明F--SO42-/MWCNTs催化剂在酯化反应中具有较高的催化性能。尝试通过螺旋藻回收催化活性下降后的用于催化合成生物柴油催化剂中的稀土金属离子，实现稀土的循环利用。