b Department of Environmental Engineering, Daegu University, Gyeongsan 38452, Republic of Korea;
c Department of Bioenvironmental & Chemical Engineering, Chosun College of Science & Technology, Gwangju 61453, Republic of Korea
Biomass has attracted considerable attention because of its potential to replace fossil fuel as a renewable and sustainable resource [1-3]. Biomass conversion processes, such as trans-esterification, hydrolysis, gasification, and pyrolysis, have been applied to produce biofuels in the forms of gases and liquids together with solid byproducts [4-6]. Bio-oil and syngas can be used as biofuels and chemical feedstock and their quality can be upgraded by applying catalytic processes [7, 8].
Studies of biochar, the typical solid byproduct of biomass thermal conversion technologies with a high carbon content, have been performed extensively in recent decades because it is renewable and can be produced using simple and inexpensive processes [9, 10]. Biochar is used widely as a soil amendment, sorbent, and energy storage material and can be a good candidate catalyst for biorefineries and environmental applications [11, 12]. Liu et al.  reviewed the formation mechanism of biochar and its use as catalyst, energy storage material, and so on. Recently, intensive research on the use of biochar as the catalysts for biorefinery and the removal of environmental contaminants has been performed. In this paper, the use of biochar as a catalyst for biorefinery and environmental applications was reviewed along with the production and activation technologies of biochar.2. Production of biochar
Biochar is generally produced by the conversion of biomass using thermal technologies (pyrolysis and gasification), hydro-thermal carbonization, and microwaves [14, 15]. Hydrothermal carbonization (HTC) of biomass, is an environmentally friendly method that can produce hydrochar at relatively low temperatures between 180 ℃ and 350 ℃ in the presence of water under high pressures (2-10 MPa) [16, 17]. Microwave technology produces biochar using microwave energy, which can provide energy to the biomass skeleton directly at the molecular level, which is in contrast to pyrolysis which produces biochar using high tempera-ture heat [18, 19].3. Activation of biochar
Despite the many advantages of biochar, its direct use as a catalyst is difficult because of its low surface area, porosity, and functionality. Therefore, modification technologies of biochar, such as physical and chemical activation, functionalization, and nano-composite synthesis, are necessary [14, 20, 21].
Physical activation using steam or CO2 is normally applied to the biochar of coconut shells, oak wood waste, corn hulls, corn stovers, and refuse-derived fuel (RDF) at temperatures higher than their carbonization temperatures [22-24]. Chemical activation has been applied intensively to produce highly porous carbon using KOH, H3PO4, ZnCl, and sodium/potassium carbonate . Other biochar activation methods using potassium hydrogen phthalate (KHP) as a chemical activator can provide autocatalytic activity by K+ and the activated biochar showed improved surface properties . The co-pyrolysis of lignin with high density polyethylene (HDPE) produced a biochar with improved surface properties and high carbon density with an aromatic structure . The co-pyrolysis of lignin and red mud also produced highly functionalized biochar under a CO2 atmosphere . Microwave steam activation was also applied to the production of waste palm shell-derived activated carbon with high porosity in a short process time with high energy efficiency and product yield . Recently, Fu et al.  synthesized graphitized hierarchical porous biochar and MnFe2O4 magnetic composites and applied them to organic pollutant degradation. The combined strategy of biomass carbonization, activation, graphitization, and solvothermal processes was also applied to the production of graphitized hierarchical porous magnetic biochar composites. The activated and graphitized biochar contained thinner and larger micropores in the carbon sheets than the raw biochar4. Use of biochar as biorefinery catalyst
Biochar produced from a thermochemical conversion process could be applied as a catalyst for the production of biodiesel and bio-sugar and to the catalytic pyrolysis/gasification via a esterifi-cation/transesterification, hydrolysis, and catalytic upgrading reaction [31, 32].
For biodiesel production, sulfonated biochar was used as a heterogeneous catalyst for the esterification and/or transesterification of lipids and free fatty acids. Biochar derived from various biomass, such as peanut hull, rice husk, empty fruit bunches, undaria pinnatifida, and palm kernel shell, have been used as a catalyst for biodiesel production from sunflower oil, waste cooking oil, palm fatty acid distillate, and soybean oil [33-35]. Biochar obtained from the pyrolysis of chicken manure under CO2 was also applied effectively in biodiesel production from waste cooking oil .
The hydrolysis of biomass for the production of monosacchar-ides or oligosaccharides is an essential process to produce a range of chemicals and fuel, such as HMF, furfural, and ethanol. The hydrolysis reaction of biomass was catalyzed by acid catalysts, such as Amberlyst-15, Nafion NR50, and H2SO4. Biochar derived from various biomass and activated by sulfonic acid (biochar-SO3H) and/or phosphoric acid can also be used as a solid acid catalyst for hydrolysis. Until now, the catalytic hydrolysis of cellulose and corn crops was reported using sulfonated biochars obtained from bamboo, cotton, starch, and corn crops as catalysts. The sulfonated biochars showed higher efficiency than commercial catalysts because of the presence of multifunctional reaction sites, such as Bronsted acid (SO3H, COOH) and adsorption sites with phenolic OH groups in their structure [13, 37]. Recently, Cao et al.  reported the production of glucose and 5-HMF from starch-rich food waste, such as bread, rice, and spaghetti, using phosphoric acid-activated wood biochar as a catalyst. Biochar activated by phosphoric acid catalyzed starch hydrolysis and fructose dehydration and achieved a high yield of HMF (30.2 C-mol% from a reaction at 180 ℃ for 20 min) and glucose (86.5 C-mol% from the reaction at 150 ℃ for 20 min) from the catalytic reaction of bread waste and rice, respectively. Liu et al.  reported that the use of sulfonated magnetic porous carbonaceous catalyst derived from waste sawdust is effective on the catalytic reaction due to its high esterification, dehydration and hydrolysis efficiency.
Thermochemical conversion methods, such as pyrolysis and gasification, produce liquid and gas as the main product, respectively. A catalytic upgrading is essential process to increase the yield of high quality pyrolysis oil and syngas (CO and H2) [40-43]. For the efficient upgrading of pyrolysis oil, biochar produced from corn stover, lignin and birch bark, were applied as catalysts to catalytic pyrolysis and catalytic gasification . To improve the syngas yield, various kinds of biochar, which were obtained from pine bark, switchgrass, pinewood, and rice husk, were used for additional tar cracking and syngas production [40, 44]. The biochar catalyst was also applied to the catalytic pyrolysis of plastic waste and their catalytic copyrolysis. Areeprasert et al.  reported the catalytic pyrolysis of ABS (Acrylonitrile Butadiene Styrene)/PC (Polycarbonate) and PCB (Printed Circuit Board) using Fe-loaded biochar and electronic waste char as catalysts. Chen et al.  performed catalytic copyrolysis of bamboo waste with microalgae (Spirulina platensis and Nannochloropsis sp.) using a biochar catalyst derived from bamboo and reported that the O-containing functional groups of biochar induced efficient deoxygenation. Recently, Chen et al.  and Kim et al.  applied biochar as a catalyst in gasification. Chen et al.  reported that biochar, which was derived from rice husk and activated by heat activation, KOH alkalization, HNO3 acidification, and KOH alkalization coupled with HNO3 acidification, were used as the supports for a nickel (Ni) catalyst and Ni/biochars were used for hydrogen production via the steam reforming of acetic acid as catalysts. Among the Ni/biochars, Ni/KOH-HNO3-activated biochar showed the highest catalytic performance. Kim et al.  also applied Ni/activated lignin biochar as a catalyst to the gasification of yellow poplar. The Ni/KOH activated lignin biochar revealed a higher syngas yield than the other commercial activated carbon and γ-Al2O3, as shown in Fig. 1.
|Fig. 1. Effects of Ni loaded catalysts on H2/CO ratio. Reproduced with permission . Copyright 2019, Elsevier.|
5. Use of biochar as environmental catalyst
The increase in atmospheric pollutants, such as nitrogen oxides (NOx) and volatile organic compounds (VOCs) emitted from vehicles, factories, and stationary power sources, has become an urgent worldwide issue because they can cause smog, acid rain, ozone destruction, and indoor air pollution [33, 34]. To remove atmospheric pollutants, the catalytic reduction process is commonly applied and activated carbon is used widely as a catalyst for this purpose because of its low cost, large surface area, and porosity.
Many studies have extensively studied the use of biochar as lower cost catalysts than commercial catalysts and activated carbon . The chemically and physically activated biochars of various biomasses, such as porphyra tenera, Geodae-Uksae, palm kernel shell, and municipal solid waste, were applied to the removal of atmospheric pollutants, such as nitrogen oxide, acetaldehyde, and formaldehyde [50-53].
Kim et al.  reported that the KOH treatment of Kraft lignin biochar increases the surface area and number of O- and N-containing functional groups of Kraft lignin biochar efficiently. They impregnated this KOH-treated Kraft lignin biochar with Mn and found that the Mn/KOH Kraft lignin biochar has higher efficiency on the catalytic acetaldehyde removal than commercial activated carbon regardless of plasma use (Fig. 2). Lee et al.  also applied the biochar produced from the pyrolysis of empty fruit bunches (EFB) as a catalyst for the removal of odorous substances, hydrogen sulfide (H2S), acetaldehyde, and NOx. They reported that steam and KOH-treated EFB biochars have higher catalytic efficiency for the removal of H2S, acetaldehyde, and NOx than commercial activated carbon (Fig. 3). Das et al.  added the biochar of Spruce (Picea sp.) to a biofilter filled with soft wood bark, garden residues, and organic fertilizers and found that the addition of biochar (25%) to a biofilter can increase the hydrogen sulfide removal efficiency of the biofilter.
|Fig. 2. Effects of acetaldehyde removal using the Mn loaded KC, KKC and AC. Reproduced with permission . Copyright 2019, Elsevier.|
|Fig. 3. The effect of Cu amount in KEFB biochars on NO removal. Reproduced with permission . Copyright 2018, Korean Carbon Society.|
Several researchers also used biochar to remove ions in water. Ma et al.  used polyethylenimine modified biochar having abundant amino group for the removal of chromium ion in aqueous solution. Ling et al.  synthesized magnesium oxide embedded nitrogen doped biochar to remove lead ion in water.6. Summary and perspectives
Recent studies on biochar production, activation, functionalization, and their application as a catalyst on biorefineries and the removal of environmental contaminants were reviewed. Biochar obtained from thermochemical conversion processes is difficult to use directly as a catalyst. Therefore, various activation processes, such as physical, chemical, and microwave treatments, have been applied widely to increase the surface area, porosity, and functional groups of biochar. The activated biochar and metal impregnation to these biochars have been applied widely to biodiesel production, hydrolysis, additional catalytic upgrading of pyrolysis/gasification, and the removal of atmospheric and aqueous pollutants. Some activated and modified biochar revealed higher catalytic activity than commercial activated carbons, highlighting the potential of activated biochar as a replacement for expensive and non-environmentally friendly catalysts. How-ever, multiple and complicated steps for the preparation of activated and functionalized biochar are being considered as the limitation on the actual use of biochar derived catalysts. To overcome this limitation, more simplified and economical process for the production of biochar catalyst need to be developed. Despite the potential applications of biochar catalysts obtained from a range of technologies, such as carbonization, activation, and modification methods, more research on catalyst deactivation and regeneration is also needed before they can be commercialized. The development of stable biochar based catalysts which can provide the longer life time will be able to replace the high cost synthetic catalysts used in various kinds of catalytic conversion process. In addition, the use of biochar in environmental application will be expanded if the more efficient regeneration process of deactivated biochar can be applied.Acknowledgments
This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT (No. 2017M1A2A2086839). Also, this study was supported by Nano-Material Technology Development Program through the National Research Foundation of Korean (NRF) funded by the Ministry of Science, ICT and Future Planning (No. NRF-2015M3A7B4049714).
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