Novel Solid Base Catalysts for the Production of Biodiesel from Lipids

Abstract

The primary commercial biodiesel production processes use homogeneous base catalysts which cause separation and wastewater discharge problems. Solid base catalysts can overcome these drawbacks. However, a solid base catalyst with high activity and stability for biodiesel production still remains a challenge. This dissertation is dedicated to developing novel solid base catalysts and applying them to an environmentally beneficial reactive distillation process for transesterification of vegetable oils with methanol into biodiesel. Novel solid base catalysts included commercial nanopowder calcium oxides, synthesized Al2O3-, SiO2- and zeolite Y-supported base catalysts. For the first time, nanopowder calcium oxides were utilized in biodiesel production at elevated temperatures in this dissertation. Nanopowder CaOs possess a relatively large BET surface area associated with nano-sized particles. The larger surface area of nanopowder CaOs provides more accessible active sites leading to a much higher catalytic activity than laboratory-grade CaO. NanoScale-CaO exhibited a higher activity than nano-CaO due to its larger surface area. Recycling experiments showed that nanopowder CaOs could be used without a significant yield drop for 10 cycles. The loss of BET surface area caused by aggregation of nano-sized particles could be the main reason for the slight yield drop. The reaction microkinetics study was performed guided by experimental data obtained. A reaction mechanism based on Langmuir-Hinshelwood model has been proposed. A mathematical kinetics model was developed and the limiting-step was determined based on this proposed mechanism. The effective reaction constants, effective activation energies and pre-exponential factors have been calculated. Thiele modulus showed that internal mass transfer did not limit the overall reaction rate due to nanosized catalyst particles. Novel mesoporous Al2O3-, SiO2-supported solid base catalysts containing Ca, K as active elements were synthesized by a single-step sol-gel method. The synthesized catalysts possess a large BET surface area in the range of 180-400m2/g and a mesoporous pore size in the range of 60-120Å. The basic sites density can be adjusted to targeted values by changing the Metal/Al or Si molar ratio. Nanosized metal particles were evenly and highly dispersed over pores of supports which resulted in a very high catalytic activity. A 100% yield was obtained in 30min when 1wt% K/Al-0.6 or Ca/Al-4.0 catalyst was used. Ca-loaded catalysts exhibited a higher stability than K-loaded catalysts. The amount of Ca leaching was reduced significantly with the Ca/Al or Si molar ratio. Al2O3-, SiO2- and zeolite Y-supported solid base catalysts were also synthesized by the incipient-wetness impregnation method. The BET surface area of synthesized catalysts was less than that of parent supports because some K2O or CaO particles clogged the pores in parent supports. Though synthesized catalysts present high activity, they lack enough stability in recycling experiments. K leaching was believed to be the main reason for catalyst deactivation. An intensified reactive distillation (RD) system for biodiesel production using both homogeneous and heterogeneous catalysts was developed. It is demonstrated that RD system intensified the transesterification reaction efficiently. A shorter reaction time and a less amount of methanol were needed compared to the conventional batch reactor. The process simulation of the RD system was performed using ASPEN Plus 11.1 software based on the reaction microkinetics data obtained in Chapter 3. A significant enhancement of efficiency was obtained in the RD system catalyzed by the highly active heterogeneous solid base catalyst. Ca leaching was estimated to be reduced due to a much shorter residence time.