Ind. Eng. Chem. Res. 2010, 49, 8379–8387
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Kinetics Study and Characteristics of Silica Nanoparticles Produced from Biomass-Based Material Tzong-Horng Liou* and Shao-Jung Wu Department of Chemical Engineering, Ming Chi UniVersity of Technology, 84 Gungjuan Rd., Taipei 24301, Taiwan
Rice husk, a form of biomass material, is a potential source of renewable energy. Recovering silica from rice husk is an important element in manufacturing high value-added products and ensures efficient utilization of a bioresource. This paper describes the kinetics and characteristics of silica prepared from rice husk in an oxidizing atmosphere. Experiments were conducted with rice husk powder under nonisothermal conditions with various gaseous flow rates, sample loadings, reactant grain sizes, oxygen concentrations, and heating rates. Samples were characterized by ICP-MS, XRD, FTIR, HT-DSC, FE-SEM, TEM, and N2-adsorption analyzer. Experimental results show that the silica yield can be increased by decreasing the grain size of raw husk. Acid-pretreatment of rice husk increases the purity, uniformity, surface area, and porosity of the final silica products. The average size of the silica particles was approximately 60 nm. Two distinct reaction stages were observed for total thermal degradation of the rice husk. This study determines the kinetic parameters of rice husk’s combustion reaction, including the activation energy, pre-exponential factor, and reaction order, using thermo-analytical techniques. This study also proposed a mechanism to illustrate the thermal decomposition of the rice husk, which agrees well with the experimental results. 1. Introduction Biomass is an important source of renewable energy; some estimates predict that biomass will account for 15% of the world’s total energy supply and 38% of the primary energy consumption in developing countries.1 Rice husk is a form of agricultural biomass that provides highly efficient thermal energy and is an abundant source of lignocelluloses used in bio-oil preparation.2 A high proportion of ash (mainly silica) remains after converting rice husk into thermal energy or biofuel. Recovering the silica from this rice husk for reuse as a bioresource is therefore an important environmental issue.3 Due to their small-diameter particles, nanostructured silica materials have many technological applications, including filtration films, insulators, and composite fillers, etc.4-6 The silica residue in rice husk ash has been considered for application in producing solar grade silicon,7 microporous or mesoporous molecular sieves (ZSM, MCM, and SBA series),8-10 mesoporous carbons,11 ceramic glazes and pigments,12,13 silicon carbide and silicon nitride ceramics,14,15 and the preparation of honeycomb monoliths and photocatalytic catalysts.16,17 Rice husk, which constitutes 20% of a rice kernel grain’s weight, is a rice-milling industry byproduct. It consists primarily of organic matter and ash. The organic matter consists of cellulose, hemicellulose, and lignin.18 Burning rice husk in air can produce approximately one-fifth of the ash. The resulting ash is rich in silica, with small quantities of metallic elements. Developing countries around the world produce 500 million tons of rice annually.19 Rice husk production should increase in the future; currently, efficiently utilized rice husks make up only a small portion of total production. This creates a growing disposal problem, as the amount of rice husk continues to increase each year. Because the ash from burning rice husk is a fine powder, it does not require further grinding.20 For these reasons, preparing an economically beneficial byproduct of silica nanoparticles from rice husk is an interesting topic. * To whom correspondence should be addressed. Tel.: +886-229089899, ext 4617. Fax: +886-2-29083072. E-mail: thliou@ mail.mcut.edu.tw.
At present, silica nanoparticle materials are prepared using several methods, including vapor-phase reaction, sol-gel and coprecipitation techniques.21-23 However, the high cost of these preparation methods has limited their application. In contrast, silica recovery from rice husk combustion is relatively inexpensive. The major constituents of rice husk are organic materials and hydrated silicon. Because the silicon atoms in the rice husk are naturally and uniformly dispersed by molecular units, silica powder with a very fine particle size, very high purity, and high surface area can be prepared under controlled combustion conditions. In addition, the method of preparing silica nanoparticles proposed here is easier than conventional techniques. The results of this study have the benefit of not only producing valuable silica powder but also reducing disposal and pollution problems. It is possible to recover silica from burned rice husk using several procedures.24-26 The thermal treatment (isothermal or nonisothermal) method, heating rate, reaction temperature, type of biomass used, and kind of gas used for thermal reaction all significantly affect the thermal decomposition rate. Real et al.27 obtained nanometric silica particles with a homogeneous size distribution by burning rice husk at 600-800 °C in a pure oxygen atmosphere. Della et al.28 observed that active silica with a high specific area could be produced from rice husk ash after heat-treating at 700 °C in air. Zhang et al.29 used a hydrolysis and acid pretreatment method followed by an incineration step. Rice husk can also be processed into a superfine silica powder. Some recent studies show that silica nanoparticles can be produced from rice husk by a biotransform method using the Fusarium oxysporum fungus30 and or through a biodigestion process using worms.31 Many studies focus on the application of silica prepared from rice husk. However, the process of converting rice husk into thermal energy requires specific knowledge of thermal decomposition kinetics. As a result, relatively few studies examine the characteristic analysis of silica residue. In addition, the literature lacks information about silica prepared from rice husk using grind-pretreatment and acid-leaching processes in various combustion conditions. This gap in the literature is the primary
10.1021/ie100050t 2010 American Chemical Society Published on Web 08/11/2010
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Ind. Eng. Chem. Res., Vol. 49, No. 18, 2010
Table 1. Proximate Analysis of Rice Huska component (wt %) volatile matter ash fixed carbon moisture a
65.05 7.74 21.26 5.95
Table 2. Surface Area and Silica Yield of Rice Husk
elemental analysis (wt %) C H O N
42.95 6.01 40.62 2.68
Sample was water-washed and unburnt.
motivation for the present study. This study’s primary objective is to examine the thermochemical characteristics of rice husk to produce nanostructured silica. Our previous study examines some elementary properties of the products obtained from the thermal treatment of rice husk.32,33 This present study is a continuation of the previous study. Specifically, this study investigates the characteristics of silica prepared by heating rice husk at an appropriate gaseous flow rate with different sample loadings, powder grain sizes, oxygen concentrations, and heating rates. This study also examines the effect of acid leaching on the surface characteristics of silica products. The physical and chemical analysis methods used in this study include ICP-MS, XRD, FTIR, HT-DSC, FE-SEM, TEM, and a N2-adsorption analyzer. Thermogravimetry (TG) further evaluates the kinetic parameters of rice husk’s combustion reaction. A theoretical mechanism is also developed to explain the experimental results. The method of silica preparation proposed in this study is not complicated and is well suited for mass production.
particle size (µm)
BETa (m2/g)
silica yieldb (wt %)
