High-Performance Materials from Bio-based Feedstocks. Группа авторов

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СКАЧАТЬ the biochar surface bringing about the transformation of tar into the gas products. When comparing the reforming gases, the treated biochar catalyzing the tar steam reforming was more effective than that in the CO₂ reforming [95]. The steam fed during the reforming reaction could produce additional oxygenated functional groups and aromatic C–O structures [92], which resulted in a gradual increase in catalytic performance. Therefore, steam was a promising agent for boosting and preserving the catalytic activity of biochar in tar reforming.

As discussed previously, raw biochar had slower rates of tar elimination than metal‐supported catalysts. Doping an active metal such as Ni and Fe onto the biochar surface is another favored approach for the development of catalytic biochar [96]. Kastner et al. prepared biochar by pyrolysis of pine bark, which was then impregnated with Fe [97]. Compared to the regular biochar, the Fe‐modified biochar catalyst possessed a lower surface area and pore volume as the Fe particles could hinder and shrink the biochar pores. Although the physical properties of the Fe‐modified biochar catalyst were not remarkable, this catalyst showed that the tar decomposition rate was increased and the activation energy was decreased by 47%. The tar conversion was significantly altered by the metal loading and the tar decomposition increased with higher metal content on the biochar. At a decomposition temperature of 900 °C, the biochar modified with 13% Fe had a tar conversion of 100%. The addition of alkali along with the metal impregnation on biochar was developed in order to further enhance the catalytic activity. The raw biomass material was impregnated with potassium ferrate prior to carbonization at 900 °C for two hours to obtain a K–Fe bimetallic catalyst‐supported biochar [98]. The K₂CO₃ was formed as an active site of this catalyst and showed a superior catalytic activity in cracking of biomass pyrolysis tar at a relatively low reaction temperature (600–700 °C). The active metal sites and pore structure were still maintained after the reaction. This catalyst also showed a high stability as no significant change in tar conversion was observed after five cycles.

       2.5.2.2 Biodiesel Production Processes

Biodiesel, a fuel derived from renewable sources such as vegetable oils and animal fats, has received much attention due to the continuous reduction in petroleum reserves and environmental issues. Biodiesel production via transesterification (Figure 2.5), also known as alcoholysis, is currently the most attractive approach, and can be divided into non‐catalytic, biocatalytic, and chemical catalytic processes. The non‐catalytic process or supercritical alcohol process is carried out in conditions above critical temperature and pressure of the reaction mixture determined from the critical properties of alcohols and triglycerides. This process requires a relatively high reaction temperature of 230–450 °C and high pressure of 19–60 MPa, as well as excess methanol (molar ratio of oil to methanol approximately 1 : 40). These requirements are a limitation to the non‐catalytic process for biodiesel production on an industrial scale [99]. A biocatalytic or enzymatic process produces biofuel with a low environmental impact and can be performed at mild temperature and pressure. Such processes are also not sensitive to the free fatty acid and water content in the feedstock [100, 101]. However, enzyme stability, enzyme reuse, and the high cost of enzyme immobilization are the major drawbacks of this process. In conventional biodiesel production, chemical catalysts (both acidic and basic) are usually used. Solid catalysts have been widely applied in biodiesel production owing to their ease of separation from products and excess reactants. A number of research studies have been conducted on biodiesel production using various types of acidic and basic solid catalysts. Most common is the solid base catalyst or alkali catalyst that can catalyze transesterification reaction in even milder reaction conditions and shorter reaction time than acidic solid catalysts. Calcium oxide (CaO) solid base catalyst can be derived from calcium carbonate‐rich materials such as horn shell and eggshell. With the basic catalyst produced from calcined eggshell, the biodiesel yield reached 97% in transesterification of waste cooking oil and methanol at the ratio of 1 : 6 [102]. Nevertheless, the preparation of a solid base catalyst from calcium carbonate‐rich feedstocks requires high temperatures of 800–900 °C. In addition, the free fatty acid and moisture contents in the oil feedstock should be considered for alkali catalysts. Water molecules in the feedstock can hydrolyze triglycerides into diglycerides and monoglycerides, which yield a greater amount of free fatty acids. The alkali catalyst is able to convert free fatty acids into soap via the saponification side‐reaction. The free fatty acid in the feedstock should be lower than 2% for transesterification.

Figure 2.5 Transesterification of triglyceride.

For feedstocks containing high free fatty acid content, the use of an acid solid catalyst is an alternative to produce biodiesel via esterification reaction (Figure 2.6) [103]. Several solid acids such as metal oxides (particularly ZrO₂, TiO₂, and SnO₂), sulfonic acid‐modified materials, and heteropolyacids have been used in esterification [104–106]. Considering the economic aspect of the biodiesel production process, the price of raw materials and the catalyst are two major determinants of the overall biodiesel production cost [107]. Therefore, much attention has been paid to the utilization of low‐cost bio‐based carbonaceous solids in biodiesel production because of their inexpensive carbon sources, convenient preparation, and easily tunable physical and chemical properties [108]. Furthermore, covalent attachment of active groups such as SO₃H, COOH, and phenolic OH groups on the surface of biochar improve the biochar characteristics. Of the various functionalized catalysts, the sulfonated carbon‐based catalyst is among the most competent for biodiesel production [109]. Effects of the different forms of sulfonating precursors modified on raw biochar support were investigated. Biochar sulfonated with fuming sulfuric acid had a superior acid density over that sulfonated with concentrated sulfuric acid, resulting in a higher activity in the transesterification of vegetable oil. Further modification of biochar with KOH before sulfonating with fuming sulfuric acid could maximize the catalytic activity for transesterification as KOH ascribed to an enhanced porosity, surface area, and acid density of the catalyst [6]. Carbonization of biomass followed by sulfonation resulted in catalysts containing SO₃H, COOH, and OH groups, in which the number of SO₃H groups significantly influence the catalytic activity for biodiesel production [110]. Leaching of SO₃H groups caused the deactivation of the catalyst. The efficiency of esterification between oleic acid and ethanol calculated from the reduction of the reactant acidity reached 98.40% when the time to introduce the SO₃H groups into the bamboo‐derived catalyst was increased from 60 to 120 minutes [111]. Thus, not only the type of sulfonating precursor but also the sulfonating conditions affect the catalytic efficiency.

More information on biodiesel yield produced in the presence of various heterogeneous catalysts is summarized in Table 2.4. In comparison to non‐biomass‐derived catalysts mentioned in this discussion, not only the catalysts produced from carbon‐rich biomass but also from calcium carbonate‐rich biomass have the potential to be competitive with commercial alkali and metal oxide catalysts in biodiesel production. Even though biomass‐derived catalysts showed a slightly lower biodiesel yield, paying more attention to research in biodiesel production with bio‐based catalysts will shortly achieve better results in terms of product yield, catalyst stability, environmental friendliness and cost‐efficiency.

      2.5.3 Biomass‐Derived Activated Carbon

Biomass‐derived activated carbons are conventionally produced from diverse renewable sources via carbonization and activation. The sequence of processes in activated carbon production from biomass causes a considerable increase in the porosity and specific surface area. Other unique properties of activated carbon comprise thermal resistance, stability in both acidic and basic environment, and СКАЧАТЬ