Preparation Of Novel Solid Acid And Base Catalysts And Their Application In Biodiesel Synthesis

As fossil fuel reserves are diminishing, energy security and environmental pollution have become the issues of global concern. Achieving sustainable development will require taking the environmental protection into account during the course of economic development. Utilization of renewable, low-pollution, widely distributed and abundant reserves of biomass has become the world's major topic. Biodiesel appears to be one of promising energy sources because of similar physical and chemical properties to fossil diesel. Compared with conventional fossil diesel, biodiesel is technically and economically more competitive due to its biodegradability, renewability, low emission profiles. In this dissertation, two novel types of solid catalysts were developed, i.e. the biomass-derived carbonaceous solid acid and calcined sodium silicate, to achieve a green process of biodiesel production. Low-cost materials were chosen for developing solid catalysts with high efficient catalytic activity through a simple preparation. The elemental composition and structure of above two types of catalysts were studied systematically by using methods of 13C MAS NMR,XRD,FT-IR and elemental analyzer. Their mechanisms of esterification and transesterification were proposed respectively. The application of novel solid catalysts in catalyzing esterification and transesterification of low-cost feedstock was also investigated.1. Biomass-derived carbonaceous solid acids were prepared from waste sawdust by method of sulphonation following carbonization. Such kind of catalyst has many merits, such as low cost, good stability, high efficiency, easy recycling and reusability. The optimum catalyst was obtained under the following conditions:carbonization at 420℃for 1 h and sulphonation at 150℃for 1 h. It catalyzed the esterification of oleic acid and ethanol with an esterification rate of 80% under the following conditions:solid catalyst of 7.0 wt%, a molar ration of ethanol/oil of 10.0:1, reaction temperature of 80℃and stirring speed of 200 rpm. The structural characterization showed that the prepared biomass-derived carbonaceous catalysts consist of amorphous carbon composed of aromatic carbon sheets oriented in a considerably random fashion. Compare with the traditional solid acid, such as SO42-/Fe2O3, SO42-/TiO2 and SO42-/Al2O3, the biomass-derived carbonaceous catalysts have distinct advantages in costs, preparation process, initial reaction rate and esterification rate.2. Lignin-derived carbonaceous catalyst was developed by direct sulphonation of residue from hull of Xanthoceras sorbifolia Bunge hydrolyzed by cellulase after pretreatment of diluted sulfuric acid. The effect of components of carrier materials on the catalytic activity was investigated. The characterizations confirmed that the catalysts prepared by direct sulphonation have similar textural structure and activity to catalysts prepared by sulphonation following carbonization. The esterification of 96% was achieved with 5.0 wt% of solid catalyst, a molar ratio of ethanol/oil of 9:1, reaction temperature of 70℃and reaction time of 5 h. The preparation process of direct sulfonation can replace the two-step process due to a simple procedure for catalyst preparation, mild reaction conditions.3 Normal silicate, such as Na2SiO3·9H2O, was firstly used to catalyze the transesterification of soybean oil and methanol. But the sodium silicate (Na2SiO3) could be hydrolyzed in the presence of high amounts of water, which lead to the instability in catalytic activity. To resolve this problem, a simple calcination process was proposed to obtain solid base at 400℃for 2 h. Calcined sodium silicate catalyzed the transesterification of soybean oil to biodiesel with a yield of more than 95% under reaction temperature of 60℃for 60 min. The oil containing 4.0 wt% water or 2.5 wt% FFA could also be transesterified by using this catalyst. The catalyst can be reused for at least 5 cycles without loss of activity. As a low-cost solid base catalyst, the calcined sodium silicate can be used as adsorbent to adsorb FFA and purify biodiesel, as well as in the transesterification of low-quality oils containing a certain amount fo FFA and water. Furthermore, the deactivated catalyst can be used as building materials.4. There exists an ion-exchange between sodium silicate and methanol, which resulted in the loss of Na+ and the decreased stability. However, the activity of deactivated catalyst can be recovered by the reaction of deactivated calcined sodium silicate with high concentration of NaOH. This simple process compensated the shortcomings of instability. Based on the theory of ion-exchange, a mechanism of transesterification catalyzed by calcined sodium silicate was proposed, In order to compare their abilities to provide protons, the geometry optimization of methanol and NaHSiO3 was performed using density functional theory (DFT) with Becke's three-parameter hybrid exchange function, Lee-Yang-Parr gradient-corrected correlation function (B3-LYP function) and 6-31+G(d,p) basis set. There exist different donors supplying protons for diglyceride ions. One is provided by Si-O-H on the surface of solid catalyst, and the other is provided by methanol in liquid phase, which improved the catalystic mechanism of such kind of catalyst. Furthermore, to improve the structural stability of calcined sodium silicate, the complex solid base Na2O-SiO2-Al2O3 prepared. Although the structural stability was improved, the catalyst performed low activity.5. The biomass-derived carbonaceous catalyst and calcined sodium silicate were used to catalyze the low-quality oil to produce biodiesel. Firstly, acidified oil from soybean soapstock was esterified by biomass-derived carbonaceous catalyst. Under the optimal esterification conditions, the acidic value was reduced from 112.4 mg KOH/g to 3.12 mg KOH/g. Compared to homogeneous acid-catalyzed method, this method can simplify the separation process and greatly reduce the cost of biodiesel. Secondly, calcined sodium silicate was used to catalyze the transesterification of crude cottonseed oil with methanol for biodiesel preparation. Furthermore, the effects of catalyst amount, molar ratio of oil to methanol, reaction temperature and stirring speed were optimized by the response surface method. The optimum transesterification reaction conditions were obtained as following:2.48 wt% of catalyst amount,7.6:1 methanol/oil of molar ratio,59℃and 225 r/m of stirring speed. Under those conditions, the biodiesel yield can reach to 95.1%. Compared with NaOH and CaO, the transesterification catalyzed by calcined sodium silicate has advantages of higher conversion, no soap formation and easy recovery. The calcined sodium silicate is suitable for the production of biodiesel from crude oil. Finally, a two-step method was proposed to produce biodiesel from the bone oil containing high content of fatty acid, in which the first esterification step was catalyzed by solid acid and the second transesterification step was catalyzed by solid base. Compared with combination of concentrated H2SO4-NaOH or concentrated H2SO4-CaO, the combination of biomass-derived carbonaceous catalyst with calcined sodium silicate could obtain higher biodiesel yield, more simple process of biodiesel preparation and more stable crude product.