Microwave-assisted synthesis of bifunctional magnetic solid acid for hydrolyzing cellulose to prepare nanocellulose
Graphical abstract
Introduction
Due to the exhaustion of fossil fuels, it is very urgent to find new energy substitutes (Jeremy et al., 2014; Lei et al., 2015; Serrano-Ruiz et al., 2011). Lignocellulose is a renewable resource stored in nature with large quantities (van Dam et al., 2005; Yang et al., 2006), providing a great opportunity to solve the depletion of fossil-based energy (Balat and Balat, 2009; Claassen et al., 1999). Cellulose is an inexhaustible renewable natural polymer in nature synthesized by photosynthesis of plants and stacked in lignocellulose with other major components, hemicellulose, and lignin. In recent years, the selective conversion of cellulose to various high-value chemicals and high-quality fuels has become one of the most interesting and attractive topics in the biorefinery field (Lei et al., 2015; Menon and Rao, 2012). As a reinforcing agent in polymer matrixes, nanocellulose warrants a tremendous level of attention in the materials community because of its availability, abundance, low cost, and the related physicochemical characteristics such as unique optical, rheological and mechanical properties, large surface area and aspect ratio, and favorable thermal conductivity compared to other commercial fibers (Abdul et al., 2016; Bidgoli et al., 2019; Nakagaito and Yano, 2004). However, the use of liquid acid in the conventional processes raises difficulty in separation and reusability and causes environmental pollution, which is not in line with principles of resource optimization.
Solid acid is an emerging catalyst applied in producing nanometer-scale cellulosic materials, which could solve the problem of acid recycling with a perfect match of the green chemistry concept. The properties and functions of solid acids vary according to the treatment conditions of different carriers and acids (Chen et al., 2018a; Chen et al., 2018b) including cellulose saccharification (bio-alcohol production and biorefining) (Mai et al., 2006) and glycerol etherification (diesel additive) (Galhardo et al., 2013). Research shows that these demonstrate ideal catalytic features such as high mechanical, thermal and chemical stability, water tolerance, activity, and low material cost. Although there have been many reports on the preparation and applications of those sulfonic acid functionalized materials, the solid acid obtained by this preparation still has some defects such as poor reusability and separation is not easy. Sulfonated metal oxides such as sulfonated TiO2 and ZrO2 and their complexes are important compounds (Fan et al., 2019; Gombos et al., 2020). Although their synthesis is simple and their chemical properties are stable, their morphology is particularly diverse and they are considered to be important features affecting the catalytic performance of solid acids. However, they are expensive because they are noble metals. And it is not in line with the concept of renewable and sustainable. Studies on the preparation of biochar-based catalysts using microwave-assisted carbonization have also been reported, but they are all used for soil improvement (Cao et al., 2018a; Xiong et al., 2017), hydrolysis to produce monosaccharides (Cao et al., 2018b), and pyrolysis to produce bio-oil (Dai et al., 2017). Carbon-based solid acids have attracted much attention due to their unique adsorption activities and reducing power (De et al., 2015). Based on the research, the preparation of a magnetic biochar-based solid acid requires to synthesize magnetic biochar first and magnetite (Fe3O4) was further introduced to enhance its recovery ability. Zhou et al. (2019) and Shan et al. (2016) reported the synthesis and preparation of magnetic biochar making the structure of the charcoal largely damaged caused by a heating method that required conventional heating, high energy consumption, and long-time heating process. This process caused the structure of the charcoal to undergo a large change. The nano-scale cellulose catalyzed by magnetic bio-based solid acids is easy to separate and could produce high-quality and uniform nanoparticles for application (Bidgoli et al., 2019; Nakagaito and Yano, 2004).
Cellulose with a nanometer-scale is called nanocellulose (Dufresne, 2013; Thomas et al., 2018). Nanocellulose is a new type of functional polymer material with a unique structure and excellent properties (Abdul et al., 2016; Dufresne, 2013; Thomas et al., 2018; Yan et al., 2017). Nanocellulose mainly has the following four characteristics: the surface effect, small size effect, quantum size effect, and the macroscopic quantum tunneling effect (Ma et al., 2017; Rebouillat and Pla, 2013). Nanocellulose has a wide range of applications, not only widely used in polymer materials, but also used in the purification and filtration of chemical and pharmaceutical products. There are many studies on the preparation of nanocellulose. Liu et al. (2016) and Niu et al. (2017) used a high concentration of sulfuric acid (i.e. 64 wt%) to hydrolyze cellulose to prepare nanocellulose. However, sulfuric acid is a strong acid that is difficult to remove after the preparation of nanocellulose. It also has a great negative influence on the quality of nanocellulose. Hu et al. (2015) and Kastner et al. (2012) used a solid acid to hydrolyze cellulose to prepare nanocellulose. The yield of nanocellulose due to solid acid hydrolysis of cellulose is not as high as that of direct hydrolysis of inorganic acids such as sulfuric acid (Lv et al., 2019). And the cost of preparation was also an important factor hindering the further development of the technology. However, the solid acid is generally tolerant, and the separation requires a certain number of meshes, and the use is subject to certain limitations.
In this study, the preparation of a magnetic biochar-based solid acid catalyst by microwave-assisted synthesis was investigated for the first time. The catalyst had three distinguished characteristic points. First, the catalyst was magnetic, and it facilitated the separation of the catalyst from the reactants. Second, it had a stable acidity. It still maintain a high acidity after repeated uses. Third, it had strong selectivity, and can effectively convert cellulose to nanocellulose. The rapid synthesis of highly magnetic, highly acidic, and highly selective solid acid catalysts is the main purpose of this study. By utilizing the absorptivity of the magnetic biochar-based solid acid and the strong acid sites, the cellulose can be partially hydrolyzed to form short-chain cellulose in the nanoscale. Compared to other nano-cellulose preparation methods, the magnetic biochar-based solid acid (MBC-SA) catalyst in this study was prepared by microwave-assisted uniformed heating, which solved the problem that the catalyst cannot be easily separated during the cellulose nano crystallization process. The nano-sized cellulose prepared by using the catalyst had a relatively uniform size, thereby greatly improving the quality and application prospect of the nanocellulose. In this work, the basic structure and functional groups of magnetic biochar-based solid acids were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), Scanning Electron Microscope (SEM) and other means to provide a theoretical basis for the mechanism of hydrolysis of cellulose. The structural changes of the nanocellulose preparation process were studied by employing transmission electron microscopy (TEM).
Section snippets
Materials
Corncob (1.35 mm in diameter) was purchased from Home Depot (Richland, WA, USA). Ferroferric oxide (CAS number) was purchased from Sigma-Aldrich (St. Louis, MO) in the form of microcrystalline powder. The microcrystalline cellulose (CAS number: 9004-34-6) used in this work had an average particle size of 50 μm. The sulfuric acid (Concentration is 96 wt%) aqueous solution was purchased from Alfa-Aesar (USA). Distilled water was used throughout the experiment except when stated otherwise.
Preparation of solid acids
Corncob
XRD analysis of MBC-SA catalysts
The crystal structure and metal elements of the catalyst were characterized by using XRD. As shown in Fig. S1, MBC-SA1, and MBC-SA2 had a unique amorphous carbon structure. It can also be seen that the two solid acids exhibited weak diffraction peaks in the range of 10°-30°, indicating that carbon existed in a common state observed in biochar. At the same time, a characteristic crystal form peak of the substance was observed due to the addition of magnetic iron oxide. The peak of iron oxide
Conclusions
Two different ways to synthesize magnetic carbon-based solid acids (MBC-SA) for cellulose hydrolysis were explored. The microwave-assisted synthesis of MBC-SA had good catalytic activity, easy separation, and high stability. Both MBC-SA synthesized with microwave-assisted carbonization had a good performance in microcrystalline cellulose decomposition, however, with their own advantages and limitations. The highest yield of MBC-SA hydrolyzed cellulose to prepare nanocellulose was 57.68%.
CRediT authorship contribution statement
Yunfeng Zhao: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft. Hanwu Lei: Conceptualization, Funding acquisition, Writing - review & editing, Supervision. Yuhuan Liu: Conceptualization, Funding acquisition, Writing - review & editing, Supervision. Roger Ruan: Conceptualization, Funding acquisition, Writing - review & editing, Supervision. Moriko Qian: Validation, Writing - review & editing. Erguang Huo: Validation, Writing - review & editing. Qingfa
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was supported by the Agriculture and Food Research Initiative Competitive Grant no. 2016-67021-24533 and 2018-67009-27904 from the National Institute of Food and Agriculture, United States Department of Agriculture.
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