Preparation Of Solid-Acid And Base Catalysts For Transesterification Reaction To Produce Biodiesel

In recent years, energy and environment problem have become increasingly serious, this stimulated recent interst in altrnative sources for petroleum-based fuels. An alternative fuel must be renewable, clean and environmentally acceptable. Biodiesel, as a renewable source of energy, not only has similar combustion and power performance with petroleum diesel, but also has environmentally friendly features. There are four methods for the synthesis of biodiesel, namely direct mixing method, micro-emulsification, pyrolysis method and the transesterification reaction. Direct mixing and micro-emulsification are physical methods, pyrolysis and transesterification are chemical methods. Using physical methods can reduce the viscosity of oil, however, some problems such as coke and oil pollution are still difficult to resolve; using pyrolysis method will produce a large number of small molecule compounds, the main product is bio-fuel. In contrast, the transesterification of animal or vegetable oils with methanol is simple and effective.Conventionally, this reaction is performed using homogeneous acid and alkaline agents, such as H2SO4,HCl, H3PO4, Na or K alkoxides or hydroxides. However, removal of the soluble catalyst after the reaction is a major problem, which leads to increased costs and a series of environmental concerns. The use of heterogeneous catalysts may offer many process advantages, such as being easier to separate from liquid products.Different solid acid or base catalysts, including solid superacid, heteropoly acid, ion exchange resins, molecular sieves, supported alkali metals and alkaline earth metal catalysts, supported organic base, mixed metal oxides and hydrotalcites. However, most of these catalyst have low activity or easy to leaching from the solid catalyst and acting as a homogeneous catalyst in the reaction, so preparation of an efficient and stable catalyst is meaningful. In this thesis, the transesterification reaction has been investigated over some different kinds of solid acid and base. Combined with different characterization, we studied the structure and surface properties of a variety of acid and base materials, and focused on the effect of the support to the active site. In addition, the catalytic reaction dynamics were also studied to determine the main kinetic parameters including the reaction rate constant, apparent activation energy, pre-exponential factor and so on. The main experimental results and conclusions are as follows:1. Titania-silica mixed oxides for the transesterification of triacetin with methanolTitania-silica mixed oxide with high Ti contents are prepared by sol-gel route in the presence of citric acid, their catalytic activities are evaluated through the transesterification reaction, the results are compared with TiO2-SiO2 prepared by traditional sol-gel method. It's found that TiO2-SiO2 prepared by sol-gel route in the presence of citric acid shows good catalytic activities and stability, it can largely maintain its overall catalytic activity for three consecutive reaction cycles. It's found that Ti species is highly dispersed in TiO2-SiO2 through the characterization of XRD, UV, TPD and IR.In order to investigate the relationship of the existing state of Ti species and the catalytic activities, TiO2-SiO2 is treated by H2SO4 and (NH4)2SO4 respectively. It's found that big TiO2 crystalline particles have already formed in the sample, which lead to the decrease of catalytic activity. The results show that titanium species should be mainly present as polymeric TiOx state, rather than isolated Ti species or TiO2 crystalline, Ti species which highly dispersed on the surface of silicon matrix is the active site for the transesterization reaction.2. CaO supported on porous carbon as heterogeneous catalysts for transesterification of triacetin with methanolThe carbon-supported CaO catalysts are prepared by the wet impregnation method using carbon molecular sieves (CMS), activated carbon (AC), nanoporous carbon (NC-2) and CMK-3 as support, their catalytic activities are also evaluated through the transesterification reaction. The catalysts using CMS and CMK-3 as support, which has only a small amount of oxygen-containing functional groups on the surface of these materials, the corresponding catalysts show good activity but poor stability. When using NC-2 and AC as the support, which has a large amount of oxygen-containing functional groups on the surface of these materials, the corresponding catalysts show an excellent activity and stability.For the sample of CaO/NC-2, the effect of the CaO loading, the amount of catalyst, the calcination temperature and reaction temperature to the catalytic activity were studied. Optimize the reaction conditions, while identifying a number of kinetic parameters. The results show that no mass transfer effects influence the reaction, the reaction follow first order kinetics model, and the apparent activation energy is 44.9 kJ/mol.Combination the results of characterization, we conclude that the relatively high catalytic activity of carbon supported CaO catalyst should be mainly attributed to the present of a large number of CaO species which show strong base character. The surface properties of the support, especially the presence of oxygen-containing functional groups play a key role in the dispersion of CaO species and stability of the resultant catalyst. Therefore, we may propose here that there are at least two types of "CaO" phases in the active and stable carbon-supported CaO catalysts. The type I phase is less stacked, containing some linkages with the carbon support related to the strong interaction between CaO and surface oxygen-containing functional groups of carbon supports, thus having weaker basicity and lower activity. The type II phase is highly stacked exhibiting relatively weak interaction with supports, thus showing stronger basicity and higher activity. Both types CaO species could be regarded as a whole unit, like small CaO cluster or particle, which is highly dispersed on the surface of carbon support.3. Graphite oxide as a novel host material of catalytically active CaO nanoparticles for the transesterification of soybean oil with methanol to produce biodieselGraphite oxide (GO) supported CaO catalyst was prepared and successfully applied to the transesterification of soybean oil with methanol to produce biodiesel. For comparison, GO was heat-treated to reduce the oxygenated functional groups to obtain GO-T and followed by impregnation of CaO, yielding GO-T-supported CaO catalyst. CaO/GO shows an excellent activity and stability, while, CaO/GO-T shows good activity but poor stability.On the basis of the catalytic data and characterization results, we supposed that the surface functionalities may act as anchoring centers for the active phase, thus resulting in the formation of a stable linkage between CaO species and the support material, finally resulting in the formation of a uniform dispersion of CaO species on the surface of GO support.Furthermore, we find an advantage that for the GO-supported CaO catalysts the heat-treated temperature of the recycled ones are lower than NC-2-support catalysts, we suppose that the unique surface properties, layered sturcture and extraordinary electronic transporting properties of the GO support may play an important role for this property.In summary, this work studied several solid acid and base catalysts, which is titania-silica mixed oxide, porous carbon supported CaO catalyst and graphite oxide supported CaO catalyst for the transesterification reaction to produce biodiesel. We obtained several excellent solid catalysts, and make a profound understanding for the nature of the active site, reaction mechanism and reaction kinetics, hoping to provide a reference for the solid acid and base catalyst to produce biodiesel.