Engineering Carbon Materials from the Hydrothermal Carbonization Process of Biomass

Engineering Carbon Materials from the Hydrothermal Carbonization Process of Biomass

(Parte 1 de 5)

Engineering Carbon Materials from the Hydrothermal Carbonization Process of Biomass

By Bo Hu, Kan Wang, Liheng Wu, Shu-Hong Yu,* Markus Antonietti, and Maria-Magdalena Titirici*

Synthesis and application of carbon materials have a long history and carbon black, fabricated from fuel-rich partial combustion, has been used for ink, pigments, and tattoos for more than 3000 years.[1] Starting with the discovery of fullerenes[2] and carbon nanotubes,[3] the material science related to valuable carbon materials has become a hot area, motivated by its potential applications in carbon fixation, catalyst supports, adsorbents, gas storage, electrode, carbon fuel cells and cell biology.[4–8] Many synthetic methods, such as carbonization, high-voltage-arc electricity, laser ablation, or hydrothermal carbonization have been reported for the preparation of amorphous, carbonaceous, porous, or crystalline carbon materials with different size, shape, and chemical compositions.[9–13] In this Review, we will focus on a more sustainable approach, which relies on low specific energy input and replaces fossil-fuel-based starting products with biomass.

Biomass is a qualified carbon raw material for the synthesis of valuable carbon materials because it is available in high quality (e.g., as pure saccharose) and huge amount, and is a environmental friendly renewable resource. An illustration of its potential is the production of bioethanol, which has emerged as a new fuel for vehicles (usually by mixing gasoline with alcohol). In the United States, more than 7 billion gallons bioalcohol were produced in 2007.In Brazil,almost all the light automobilesare runningon the blend of gasoline and bioalcohol, and similar scales can be easily envisagedformaterials,appropriatecarbonproductsassumed.Even more abundant, waste biomass derived from agricultural resides and forest byproducts has drawn little attention as a raw material, since only simple combustionhas been used to elevatethe valueof waste biomass. Carbon materials fabricated from waste biomass haveshownpromisingapplicationsas sorptionmaterials,hydrogen storage,biochemicals,and others.[14–17] The problem is that there is still no general and satisfactory process for the production of valuable carbon materials from crude biomass to date.

In this respect, a hydrothermal carbonization (HTC) process might have the opportunity to turn into a powerful technique for the synthesis of valuable carbon materials from biomass, especially crude biomass (Scheme 1). According to different experimental conditions and reaction mechanisms, two HTC processes can be classified. Based on the pyrolysis of biomass, a high-temperature HTC process is apt to synthesize carbon nanotubes, graphite, and activated carbon materials under high temperature and high pressure.[18,19] A low-temperature HTC process is carried out up to 2508C, employing several chemical transformation cascades, and is a more environmentally friendly route.[20,21] Various carbonaceous materials with different sizes, shapes, and surface functional groups have been synthesized by this process. Furthermore, these w.MaterialsViews.com w.advmat.de

Division of Nanomaterials and Chemistry Hefei National Laboratory for Physical Sciences at Microscale Department of Chemistry University of Science and Technology of China Hefei, Anhui 230026 (P. R. China) E-mail: shyu@ustc.edu.cn

Dr. M.-M. Titirici, Prof. Dr. M. Antonietti Department of Colloid Chemistry Max Planck Institute of Colloids and Interfaces MPI Research Campus Golm 14424 Potsdam (Germany) E-mail: magdalena.titirici@mpikg.mpg.de

DOI: 10.1002/adma.200902812

Energy shortage, environmental crisis, and developing customer demands have driven people to find facile, low-cost, environmentally friendly, and nontoxic routes to produce novel functional materials that can be commercialized in the near future. Amongst various techniques, the hydrothermal carbonization (HTC) process of biomass (either of isolated carbohydrates or crude plants) is a promising candidate for the synthesis of novel carbon-based materials with a wide variety of potential applications. In this Review, we will discuss various synthetic routes towards such novel carbon-based materials or composites via the HTC process of biomass. Furthermore, factors that influence the carbonization process will be analyzed and the special chemical/ physical properties of the final products will be discussed. Despite the lack of a clear mechanism, these novel carbonaceous materials have already shown promising applications in many fields such as carbon fixation, water puri- fication, fuel cell catalysis, energy storage, CO2 sequestration, bioimaging, drug delivery, and gas sensors. Some of the most promising examples will also be discussed here, demonstrating that the HTC process can rationally design a rich family of carbonaceous and hybrid functional carbon materials with important applications in a sustainable fashion.

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carbonaceous materials can combine with other components, such as noble metal nanoparticles to form composites with special chemical and/or physical properties.

In this article, we will review the concept and history of the

HTC process at both high and low temperature. The promising potential of the HTC process for the preparation of carbon materials from biomass will be demonstrated. Finally, we will briefly present some examples of the application of carbonaceous materials from the HTC process in fields such as environment, catalysis, energy storage, biology, and sensors.

2. Hydrothermal Carbonization: A New Way Towards Carbon Materials

2.1. Concept and History

Hydrothermal conditions, i.e., application of an aqueous medium over 1008C and 0.1 MPa, are widely found in nature, because many minerals form under these circumstances.[2] Since the pioneering work from 1960s to 1980s, the hydrothermal process has been widely used for the synthesis of a vast range of solid-state compounds such as oxides, sulfides, halides,[2–25] molecular zeolites, and other microporous phases.[25] Nowadays, the hydrothermal process has become an important technique for the synthesis of various kinds of inorganic materials, such as functional oxide[26] and non-oxide nanomaterials[27] with specific shapes and sizes, as well as for the synthesis of new solids.[28,29]

For the synthesis of valuable carbon materials, the educts of the

HTC process usually include carbohydrates, organic molecules, and waste biomass. The treatment of carbon materials under hydrothermal conditions increases or changes solubility, melts crystalline parts, accelerates the physical and chemical interaction between reagents and the solvent, facilitates ionic and acid/base reactions, and finally leads to the precipitation/formation of the carbonaceous structures.

Although there is rarely a clear classification, the HTC process could be classified into two main parts by applied temperature.

Shu-Hong Yu received his B.Sc. degree at Hefei University of Technology and his Ph.D. from the University of Science and Technology of China (USTC). He joined Prof. Masahiro Yoshimura’s laboratory, Tokyo Institute of Technology, as a postdoctoral Fellow. Afterwards, he was as an Alexander-von-Humboldt Research Fellow in the Max Planck Institute of Colloids and

Interfaces, Germany, working with Prof. Markus Antonietti and PD Dr.h abil.H elmutC olfen. He joined the Department of Chemistry USTC as a full Professor in 2002, and was appointed the Cheung Kong Professorship in 2006 by the Chinese Ministry of Education. He is now leading the Division of Nanomaterials and Chemistry at the Hefei National Laboratory for Physical Sciences at Microscale, USTC. His research focuses on hydrothermal carbon, bioinspired self-assembly of new nanostructured materials, and hybrids with high performances.

Maria-Magdalena Titirici received her basic academic education in organic chemistry and material physics in Bucharest, Romania. After Ph.D. work with B. Selergren in Mainz and Dortmund, she joined the Max Planck Institute of Colloids and Interfaces where she is currently head of the research group ‘‘Sustainable Functional Carbonaceous and Polymeric Materials’’. Her current projects and interests include self-assembly of nanostructured materials, molecular electronics, energy and hydrogen storage,

CO2 sequestration agents, chromatographic stationary phases, and drug-delivery systems.

Bo Hu received his B.S. degree from Hefei University of Technology in 2002 and his Ph. D in Inorganic Chemistry from University of Science and Technology of China in 2008 under the supervision of Prof. Shu-Hong Yu. He is interested in the carbonaceous nanostructured materials and other inorganic nanoparticles as well as the self-assembly process for nanodevices.

Scheme 1. Schematic illustration of the hydrothermal carbonization (HTC) process as a powerful technique for the synthesis of valuable carbon materials from biomass and the potential applications of the as-produced carbon materials.

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The high-temperature HTC process proceeds between 300 and 8008C and is therefore clearly beyond the stability of standard organic compounds. Reactive gases and carbon fragments are to be expected from ‘‘thermolysis’’, which enable the synthesis of carbon nanotubes, graphitic carbon materials, and activated carbon materials.[19,30] The low-temperature HTC process performs below 3008C, and functional carbonaceous materials can be produced according to dehydration and polymerization schemes known from ordinary organic chemistry. For the coalification of biomass, the low-temperature HTC process is presumably close to natural coalification[6] but, of course, at highly accelerated speed, decreasing the reaction time from some hundred million years to the time-scale of hours. In addition, it is a spontaneous, exothermic process, with the vast majority of the carbon of the starting products also found in the final product (the ‘‘carbon efficiency’’, a sustainability issue, is close to 1).

HTC in material synthesis is a 100-year-old technique, with increasing interest originating from the charcoal formation.[6] Bergiusfirst describedthehydrothermal transformation of cellulose into coal-like materials in 1913.[31] Then, detailed investigations were focused on the biomass source,[32] the formation process,[3] and the identification of the final coal composition.[34]

Since the discovery of carbon nanotubes in 1991,[3] the high-temperature HTC process has been developed quickly. At the beginning of the new century, a renaissance in the low-temperature HTC process appeared with the reports on the synthesis of uniform carbonaceous particles from sugar or glucose.[20,21] In the past few years, lots of functional carbonaceous materials from biomass have been produced via the HTC process and these materials have shown great potential in many fields.[5] Nowadays, with the gradual acknowledgement of hydrothermal process and carbonization mechanism,[35] the HTC has been widely used to smartly design novel carbon and carbonaceous materials from biomass with important applications.[5,6]

2.2. Hydrothermal Carbonization at High Temperature

2.2.1. Carbon Nanotubes

The HTC process at high temperature is a powerful method for the fabrication of well-crystallized multi-walled carbon nanotubes (MWNTs). Some novel synthesis routes have been reported for the production of MWNTs.[36–39] In particular, Yoshimura co-workers reported a hydrothermal processing for the synthesis of high-quality MWNTs from amorphous carbon without the use of a metal catalyst at a temperature of 8008C and a pressure of 100 MPa (Fig. 1a).[40] The stability and evolution of single-walled carbon nanotubes (SWNTs) has been investigated during hydrothermal treatment at temperature between 200 and 8008C and a pressure of 100 MPa.[41,42] The SWNTs were stable even after mild and short-term treatment, and could transform to short MWNTs and graphitic nanoparticles in a high-temperture and high-pressure water system. The HTC process could also effectively modify the surface of MWNTs, such as the production of hydroxyl-group modified MWNTs.[43]

2.2.2. Three-Dimensional Carbon Structures

The HTC process at high temperature could be used to prepare carbon films and materials with high flexibility.[4,45] A nice case was the formation of carbon films on carbides under hydrothermal conditions at 300–8008C.[4] This simple route enabled to coat the surface of SiC fibers, powders, platelets, and single crystals with carbon films with controllable thickness from nanometer to micrometer. The mechanism is that the surface of the substrate transformed into carbon films as follow:

SiCxOy þnH2O ! SiO2 þxCþnH2

Monodispersed carbon microspheres,[46–49] ellipsoidal carbon microparticles,[50] olivary carbon particles,[51] nanocells,[52] and graphite tubes[53] have been successfully synthesized from carbohydrates[47,49] and organic molecules[46,50,51,52,53] via the high-temperature HTC process (Fig. 1b–d). For the synthesis of uniform, pure, paramagnetic carbon particles (Fig. 1c), Pol and Thiyagarajan[54] have carefully studied the process by measuring the in situ autogenuous pressure and dissociated chemical species as a functions of temperature during the thermolysis of mesitylene. These two parameters are important for the production scale-up of these spherical carbon particles.

Further, the high-temperature HTC process was an effective technique for the fabrication of activated carbon materials.[18,30,5,56] For instance, Salvador et al. have reported the production of activated carbon materials from oak wood and anthracite by the high-temperature HTC process with a broad distribution of micropores and some mesopores.[19] Compared to steam activation, the HTC process has higher gasification rate and better penetration power into the pore structure of the char. The HTC process can quickly and greatly change the porosity of the oak char, widening the micropore structure homogeneously (Fig. 2). It was also shown that this must be carbon from biomass. For anthracite char, the HTC process has shown only little effect on the pore development.

Figure 1. a) Transmission electron microscopy (TEM) image of MWNTs. Reproduced with permission from [40]. Copyright 2001, American Chemical Society. b) TEM image of a chain of connected carbon cells. Reproduced with permission from [52]. Copyright 2001, Elsevier Publishing Group. c) Scanning electron microscopy (SEM) images of spherical carbon particles. Reproduced with permission from [54]. Copyright 2009, American Chemical Society. d) SEM images of olivary carbon particles. Reproduced with permission from [51]. Copyright 2005, American Chemical Society.

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2.3. Hydrothermal Carbonization at Low Temperature: Synthesis of Highly Reactive Carbonaceous Nanostructures

2.3.1. Morphology-Controlled Synthesis of Carbonaceous Nanostructures

The HTC process at low temperature is apt to generate monodispersed colloidal carbonaceous spheres, as shown in Figure 3, from the carbohydrate sources such as sugar,[20] glucose,[21] cyclodextrins,[57] fructose,[58] sucrose,[59] cellulose,[60] and starch.[59] The formation of these materials includes the processes of dehydration, condensation, polymerization, and aromatization.[61] Compared with other routes, the HTC process has a couple of advantages, including low toxicological impact of materials and processes, the use of renewable resources, facile instrumentation and techniques, and a high energy and atom economy.[20,21,35] Catalysts, especially metal ions,[4,62,63,64] can effectively accelerate the HTC process of carbohydrates and direct the synthesis of various carbonaceous materials. Yu and co-workers.[63] have reported that iron ions and iron oxide nanoparticles could accelerate the HTC process of forming the hollow carbonaceous spheres from starch (Fig. 3d). When the controlled-dehydration products of carbohydrates are partially replaced by organic monomers, a new type of hybrid between carbon and polymer latex can be produced by copolymerization and cycloaddition reaction. These latexes have not only the surface properties of the polymers, but also the structural, mechanical, thermal, and electric properties of the carbon framework.[65] Titirici and co-workers have reported the production of carboxylate-rich carbonaceous materials in the presence of acrylic acid by the one-step HTC process of glucose (Fig. 3c).[65]

In the HTC process of carbohydrates, the formation process and the final material structures are rather complicated and a clear scheme has not been reported. This is mainly due to the formation of a multitude of furan-type dehydrated intermediates from carbohydrates, the complexity of the chemistry, and the lack of a satisfactory technique for the final structure discrimination, which could allow the identification of all-carbon sites with higher resolution.[35,58] For example, the dehydration and fragmentation of glucose can give rise to different soluble products, such as furfural-like compounds (5-hydroxymethylfurfural, furfural, 5-methylfurfural), organic acids and aldehydes (acetic, lactic, propenoic, levulinic, and formic acids), and phenols.[61,6–68] Then, polymerization or condensation reactions do occur forming the final forming the final carbonaceous material,[61,69] which have been identified to occure at least along three lines simultaneously, namely aldol-condensation, cycloaddition reactions, and a hydroxymethyl-mediated furan resin condensation.[70]

Among the HTC process of diverse biomass (glucose, xylose, maltose, sucrose, amylopectin, starch), hexose-based carbon sources mainly produce 5-hydroxymethylfurfural (HMF) as the reaction-driving dehydration products,while pentosemainly works via the (more reactive) furfural.[35] A LaMer model[71,72] has been proposed for the explanation of the growth of the carbonaceous materials. This assumes that the final carbonaceous materials display a type of core–shell structure composed of a hydrophobic core and a stabilizing hydrophilic shell that is less dehydrated and contains a large number of reactive oxygen functional groups (hydroxyl/phenolic, carbonyl, or carboxylic).[21,61]

The as-synthesized carbonaceous materials usually have intrinsic porous structures with controllable morphology and

Figure 2. SEM images of oak wood and oak char. a) Oak char gasified with supercritical water at different burnoffs and b) oak char gasified with steam at different burnoffs. The scale bar was 20mm. Reproduced with permission from [19]. Copyright 2005, American Chemical Society.

Figure 3. a) SEM image of monodispersed hard carbon spherules. Reproduced with permission from [20]. Copyright 2001, Elsevier Publishing Group. b) TEM image of carbon spheres. Reproduced from [21]. c) SEM images of carbonaceous materials. Reproduced with permission from [65]. Copyright 2009, American Chemical Society. d) TEM image of hollow spheres. Reproduced from [63].

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surface functionality. For example, the monodispersed carbonaceous spheres after heat treatment (at 2508C) have uniform nanoporous structures and specific Brunauer–Emmett–Teller (BET) surface area of 400m2 g 1. Surface modification of these porous carbonaceous materials (e.g., a high concentration of hydroxyl/phenolic, carbonyl, and carboxylic groups[61]) leads to a high reactivity, which broaden their application in environment, catalyst, electrochemistry, and drug delivery.[73] It is worthy to note that carbonaceous materials with different functionality could be produced by a general route of mixing the carbohydrates with other small organic monomers via the HTC process.[65]

Coupling either hard- or soft-templating effects with the HTC process has shown powerful capability in controlling the synthesis of various carbonaceous nanostructures with special morphology. For example, using ultrathin and ultralong Te nanowires as templates, well-defined ultralong carbonaceous nanofibers can be synthesized from glucose by the HTC process (Fig. 4a).[74] Adjusting the reaction time or the ratio of the tellurium and glucose can effectively control the diameters of carbonaceous nanofibers. Hollow carbonaceous spheres with porous walls could be produced from glucose by the HTC process using appropriately functionalized porous silica particles as templates (Fig. 4b).[75] The efficient deposition of a glucose-derived carbon precursor could be advanced by the electrostatic attraction between the positively charged silica and negatively charged carbon precursors.[75] Soft templates, such as sodium dodecyl benzenesulfonate (SDBS), could induce the template synthesis and assembly process to generate carbonaceous nanowires.[76] Using the anionic surfactant sodium dodecyl sulfate (SDS) as a soft template, the HTC process has been developed for preparing hollow carbonaceous capsules with a reactive surface layer and tunable void size and shell thickness.[7,78]

(Parte 1 de 5)

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