Review pubs.acs.org/CR
Development of Biochar-Based Functional Materials: Toward a Sustainable Platform Carbon Material Wu-Jun Liu, Hong Jiang,* and Han-Qing Yu* CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science & Technology of China, Hefei 230026, China 1. INTRODUCTION Among the serious global issues facing mankind at present are climate change and environmental pollution. Directly linked to these is the increasing demand to discover economically viable and environmentally friendly energy sources that will undeniably help to create a sustainable future. The energy crisis, environmental pollution, and global warming are serious problems that are of great concern throughout the world. A diverse range of people are driven to find facile, ecofriendly, and cost-effective routes with the aim of resolving these problems. One important aspect of such research is to synthesize a range of functional materials that can be used resolve many of the challenges associated with both current and future strategies. For example, materials with catalytic functionalities can be developed to convert renewable resources. Biomass can be converted into biofuels for use as alternative energy sources.1−3 Materials with high storage capacities can be produced for the storage of low-cost, clean renewable solar, wind, and biomass energy.4 To address environmental pollution and globalwarming issues, adsorbent or catalytic materials can be developed to remove pollutants or capture CO2. Due to their potential applications in energy storage, catalysis, adsorption, and gas separation and storage,5−8 carbon materials are considered as ideal candidates for resolving many of the practical issues encountered (e.g., environmental pollution and global warming). With respect to the synthesis of crystalline carbon nanotubes/nanofibers and graphene, as well as amorphous carbon, activated carbon, and carbon black materials with controllable properties and functionalities, several routes, such as chemical vapor deposition,9 arc discharge synthesis, 10 and carbonization of synthetic or natural polymers11, have been reported. However, these methods usually require tedious synthetic methods as well as organic solvents and electrochemical treatment.12 In addition, they often rely on rather expensive fossil-fuel-based precursors, the use of metal catalysts,13 and complicated apparatus involving high processing temperatures, none of which are environmentally and economically sustainable. These drawbacks limit the large-scale production and commercialization of such carbon materials. Alternatively, lignocellulosic biomass obtainable from rice husk, bean straw, corn stalk, and sawdust is a naturally abundant resource that has great potential as a raw carbon material for synthesizing various functional materials.3,14 For example, pyrolysis of lignocellulosic biomass under anoxic conditions at different heating rates has been intensively studied
CONTENTS 1. Introduction 2. Biochar from Biomass Pyrolysis: Characteristics, Production and Mechanism 2.1. Production of Biochar 2.2. Mechanism of Biochar Formation in the Biomass Pyrolysis Process 2.3. Characteristics of Biochar 2.3.1. Structure of Biochar 2.3.2. Surface Chemistry of Biochar 2.3.3. Bulk Composition and Inorganic Fraction of Biochar 3. Functionalization of Biochar Materials 3.1. Tuning of Surface Properties 3.1.1. Surface Doping and Modification 3.1.2. Surface Recombination 3.2. Pore Structure Tailoring 3.2.1. In Situ Pore Structure Tailoring 3.2.2. Pore Structure Tailoring through Postactivation 4. Applications of Biochar-Based Functional Materials 4.1. Catalytic Applications 4.1.1. Catalysis of Biochar Surface Functional Groups 4.1.2. Catalysis Using Biochar-Supported Nanostructures 4.2. Energy Storage and Environmental Protection Applications 5. Conclusions and Future Perspectives Author Information Corresponding Authors Notes Biographies Acknowledgments References
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Received: April 1, 2015
© XXXX American Chemical Society
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Figure 1. A conventional flow sheet for bio-oil upgrading to biofuels and fine chemicals feedstock. Reprinted with permissions from ref 16. Copyright 2008 American Association for the Advancement of Science.
and commercially used in recent years.1,3 In a typical thermochemical decomposition pyrolysis process, in addition to the renewable liquid fuel (bio-oil), a solid residue called biochar is also produced. The bio-oil, with an average heating value of 17 MJ/kg, can be directly used as a low-grade fuel for boiler systems, diesel engines, gas turbines, and Stirling engines.15 It can also be converted into biofuels and subsequently used as a replacement for fossil fuels. It has also reportedly been used for the extraction of valuable chemicals,15 via a series of complex processes. As shown in Figure 1,16 biooil is obtained through a single step thermochemical conversion of almost all types of lignocellulosic biomass into a chemically complex liquid product that includes acids, alcohols, and ketones, as well as hetrocyclic compounds. These compounds can undergo transformation using catalytic esterification, hydrotreatment, cracking, and re-forming and be converted into light hydrocarbons and aromatics, as well as H2, all of which can be used for alternative fuels or fine chemical feedstock.17 For more detailed information on various bio-oil upgrading processes, which fall beyond the scope of this paper, the reader can refer to the excellent reviews and book chapters by Zhou et al.,1 Xiao et al.,18 Graça et al.,19 and Wang et al.,20 and references therein. Biochar is defined as a carbon-rich, porous solid produced by the thermal decomposition of biomass in a reactor with little or no available air and at moderate temperatures (e.g., 350−700 °C).21 In more technical terms, biochar is produced by socalled pyrolysis of biomass under a limited supply of O2 at relatively low temperatures.22 This process often mirrors the production of charcoal, one of the most ancient industrial technologies developed by mankind.23 From the viewpoint of its use as chemical feedstock, biochar is considered a specific type of biocarbon. This is defined as a wide range of carbon materials derived from all types of biological resources, such as plant, animal, and microbial. It is produced by various methods, such as carbonization, pyrolysis, hydrothermal treatment, gasification, among others.24−26 On the other hand, biochar is
obtained from the pyrolysis of lignocellulosic plant biomass or its derivants, which means that biochar has a narrower scope than biocarbon. This definition distinguishes biochar from hydrochar, which results from the hydrothermal carbonization of biomass or biomass-derived organic compounds, such as carbohydrates and lignin, at relatively low temperatures (130− 250 °C) and high pressure (0.3−4.0 MPa).13 It also distinguishes biochar from activated carbon and carbon black, two typical amorphous carbon materials. As shown in Table 1, there are several distinctions among these three types of materials: principal source, carbon content, structure, and preparation method. From a structural viewpoint, there is no fundamental distinction between biochar and activated carbon, because both of them are amorphous carbon with abundant porosity. Even activated carbon derived from biomass can be considered as a certain type of activated biochar. However, unlike activated carbon, biochar usually has abundant surface functional groups (C−O, CO, COOH, and OH, etc.), which being highly modifiable act as a platform for the synthesis of various functionalized carbon materials. Biochar can be produced on a scale ranging from large industrial facilities to individual farms, since it is a solid residue formed in the pyrolysis of biomass.27 In a typical process, approximately 700 kg of biochar can be produced when 2000 kg of biomass is pyrolyzed (yield 30%−40%),28 making it applicable to a variety of research and practical situations. The physical and chemical characterization of biochar by a range of techniques indicates that it has a relatively low porosity and surface area but contains abundant surface functional groups, as well as minerals such as N, P, S, Ca, Mg, and K.29 These properties allow biochar to be directly applied as an adsorbent, catalyst, and catalyst support.30,31 More importantly, the easily tuned surface functionality and porosity make biochar a promising platform for the synthesis of many other functional materials. A number of functional materials synthesized through the functionalization of the key platform carbon material biochar are shown in Scheme 1. Indeed, although biochar B
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microcrystal or amorphous carbon particles combustion of petroleum, coal tar, or asphalt under air-poor conditions
functionalization is still at the infant stage, applications of biochar-based functional materials have been found in the fields of catalysis,32 energy storage,33 pollutant removal,34 and CO2 capture.35 More importantly, the large-scale application of biocharbased functional materials is considered to be a sustainable process, because anthropogenic CO2 emissions can be mitigated when waste biomass is converted into biochar.13,36 The sustainable concepts of biochar production and applications are summarized in Figure 2.37 CO2 is first removed from the atmosphere via the photosynthesis of green plants; then, through the pyrolysis process, the CO2absorbed by the plant biomass via photosynthesis is no longer liberated but bound to the final carbonaceous structure of the biochar, representing an efficient way of removing CO2 from the carbon cycle and thus helping to diminish the results of climate change, remitting the threat of global warming. For example, it is estimated that storing carbon in biochar avoids the emission of 0.1−0.3 billion tons of CO2 annually.38−41 In addition, the biooil produced in the pyrolysis process can be used as a biofuel and hence may also offset fossil fuel CO2 emissions. As predicted, the cumulative avoided emissions for biochar are substantial at 48−91 billion tons of CO2-C in for this century.37 In the past few years, the great application potential of biochar from the pyrolysis of biomass has received increasing attention, and there is a growing awareness that biochar can provide a versatile and efficient platform for the synthesis of functionalized carbon materials. This awareness has led to a considerable shift in research interest to address various potential applications of biochar in catalysis, energy storage, and environmental protection. Since 2008, there has been rapid growth in publications related to both the preparation and applications of biochar. For example, the technical, economic, and climate-related aspects of biochar production technologies and their application in soil remediation were summarized by Manyà,33 Meyer et al.,34 and Laird et al.23 Current progress and innovation in biochar production and utilization for improving environmental quality have been reviewed by Hyland and Sarmah.35 However, these reviews focused on only some specific aspects of biochar, mainly on biochar production and applications for soil remediation and pollutant removal. To date, no comprehensive and critical review on the biochar formation mechanism, its characteristics, or the functionalization of biochar materials for catalysis and energy storage applications is available. In particular, a systematic evaluation of the merits and demerits of biochar-based functional materials, which play a critical role in the application of these materials, is overdue. We believe that it is timely to comprehensively review the research advances, challenges, and future opportunities of biochar-based functional materials. In particular, there is a need to critically evaluate their application in catalysis, energy storage, and environmental protection. In this review, we discuss the mechanism of biochar formation in the biomass pyrolysis process, the characteristics of biochar, the tuning of its surface properties and functionality, the applications of functionalized biochar, and some perspectives on the future of the functionalization of biochar-based materials. Perceptions regarding how biochar-based functional materials will develop, especially in those fields in which the use of functionalized biochar could be expanded, are discussed throughout the review.
preparation method
amorphous carbon with abundant surface functional groups, amorphous carbon with abundant porosity nanostructures, or porosity pyrolysis of the biomass at medium temperature (400−600 °C) and then carbonization of the coal, asphalt, or biomass at high temperature (700−1000 °C) under functionalization with physical or chemical methods physical or chemical activation
petroleum, coal tar, asphalt, etc. >95% coal, asphalt, biomass, etc. 80%−95%
main source carbon content structure
biomass 40%−90%
biochar
Table 1. Main Distinctions among Biochar, Activated Carbon, and Carbon Black
activated carbon
carbon black
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Scheme 1. Biochar as a Platform Carbon Material for the Synthesis of Various Functional Materials and Their Potential Applications
Figure 2. Schematic illustration of sustainable concepts regarding biochar production, applications, and impact on global climate. (The plants uptake the CO2 from the atmosphere, producing large amounts of biomass, which can be converted into bio-oil and biochar. The bio-oil, following upgrading treatments, is converted to various biofuels and used as an alternative to fossil fuels. It should be noted that the emission of CO2 from biofuels can be fixed by the plant again. Biochar is suitable for soil remediation and with some functionalization can be converted into functional materials, finding applications in catalysis, energy storage and conversion, and environmental protection. Meanwhile, as a recalcitrant form of carbon, the biochar itself can be regarded as a carrier for long-term carbon storage.
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2. BIOCHAR FROM BIOMASS PYROLYSIS: CHARACTERISTICS, PRODUCTION AND MECHANISM Since the principal source of biochar is from biomass pyrolysis, before considering biochar production, mechanism, and its structural/compositional characterization, it is pertinent that the pyrolysis process itself should be defined first. Pyrolysis can be primarily divided into fast pyrolysis and slow pyrolysis; this is based on the difference in heating rate. Fast pyrolysis is generally described as the thermal decomposition of the lowenergy-density biomass (e.g., heating value ∼11 MJ kg−1) in the absence of O2 at medium temperatures (e.g., 400−600 °C) with a very high heating rate (e.g., >300 °C/min) and a short vapor residence time (e.g., 0.5−10 s). This process yields a high-energy-density liquid known as bio-oil (heating value ∼17 MJ kg−1), a relatively low-energy-density gas known as syngas (heating value ∼6 MJ kg−1), and the biochar (heating value ∼18 MJ kg−1). Slow pyrolysis is defined as carbonization of the biomass over a wider temperature range (300−800 °C), with a low heating rate (5−7 °C/min) and a long vapor residence time (usually more than 1 h). In this process, biochar is the main product. During the pyrolysis process, the biomass is heated to a temperature above 300 °C in the absence of oxygen; the organic components are thermally decomposed to release a vapor phase, while the residual solid phase biochar remains. The vapor phase is then cooled to produce the bio-oil, in which polar and high molecular weight compounds are condensed, while the low molecular weight volatile compounds (e.g., CO, H2, CH4, and C2H2) remain in the gas phase.23,42 The physical process and chemical reactions occurring in pyrolysis are very complex and depend on the reactor conditions, heating rate, and the nature of the biomass.43
Table 2. Characteristics of Different Pyrolysis Processes and Typical Yields of Biochar in These Processes
heating rate (°C/min) temp (°C) vapor residence time typical reactor main product biochar yield (wt %)
slow pyrolysis
fast pyrolysis
flash pyrolysis
pyrolytic gasification
5−7
300−800
∼1000
−
300−800 >1 h
400−600 0.5−10 s
400−1000
