Biochar as a Catalyst

Renewable and Sustainable Energy Reviews 77 (2017) 70–79

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Biochar as a Catalyst a

MARK b,⁎

a,⁎

Jechan Lee , Ki-Hyun Kim , Eilhann E. Kwon a b

Department of Environment and Energy, Sejong University, Seoul 05006, Republic of Korea Department of Civil and Environmental Engineering, Hanyang University, Seoul 04763, Republic of Korea

A R T I C L E I N F O

A BS T RAC T

Keywords: Biochar Carbon material Catalyst Thermo-chemical process Pyrolysis Gasification

Biochar is pyrogenic carbon rich material generated from carbon neutral sources (i.e., biomass). Being an environmentally benign means for soil amendment, it also offers principle strategies for carbon capture and storage (CCS). In addition, recent recognition of biochar as versatile media for catalytic applications has brought forth initial research exploring the catalytic capacity of biochar and mechanistic practices in various routes. Thus, to provide comprehensive information on the catalytic applications of biochar in the field of catalysis, this review focuses on the catalytic challenges and practices of biochar, e.g., biodiesel production, tar reduction in bio-oil and syngas (synthetic gas: H2 and CO), enhanced syngas production, conversion of biomass into chemicals and biofuels, deNOx reactions, and microbial fuel cell electrodes. This review also provides an indepth assessment on the catalytic properties of biochar with respect to production recipes at the fundamental level. Lastly, the performance of various biochar catalysts is also evaluated in this review.

1. Introduction Biomass is the only carbon neutral alternative to petro-derived fuels and chemicals due to its ubiquitous nature and economic feasibility [1– 4]. The thermo-chemical processes of pyrolysis and gasification have been envisioned as promising technologies for the conversion of biomass into renewable fuels and chemicals since they have been evaluated as being suitable for mass production for energy recovery [5– 7]. Pyrolysis is one of the thermolytic techniques for biomass in the absence of oxidizing agents, while gasification is defined as transferring most of the heating value from the solid phase of carbonaceous materials into combustible gases known as syngas (i.e., H2 and CO) [8,9]. Both techniques can be applied to the production of pyrolytic oil (biocrude), syngas, and biochar. As biochar material is carbonized via dehydrogenation during the thermal degradation of biomass, the physical properties of biochar are highly porous and carbon-rich [10,11]. Fig. 1 illustrates a generation of biochar through thermo-chemical conversion of biomass. Invulnerability of biochar associated with physico-chemical aspects shows versatile applications for environmental, geological, and agricultural practices such as soil amendment and water contaminant removal [12,13]. Thus, a great deal of research has reported on the environmental benefits of biochar as an adsorbent for contaminants in soil and water [14–22]. It can also significantly increase crop yield while reducing the loss of soil nutrients [23,24]. Biochar has also been used as adsorbents for organic dyes (e.g., methylene blue and indigo

⁎

carmine dye) [25,26], inorganic metals (e.g., Pb and Hg) [27], and ions (e.g., NH4+, Cr3+, Sb3+, Cd2+, and Zn2+) [28–31]. In addition, biochar has gained remarkable public and scientific attention as a newfashioned strategy for carbon capture and storage (CCS) [32–34] due to its invulnerability as a carbon sink to remain in the soil for more than 100 years [35]. Fig. 2 represents utilization of biochar. Catalysts have played a crucial role in developing technologies to convert not only conventional carbonaceous feedstocks (e.g., coal, natural gas, and petroleum), but also renewable feedstocks (e.g., biomass) into value-added products such as fuels and chemicals [36]. The global catalyst market is estimated to be about $15 billon [37], more than 35% of the global GDP was associated with catalytic technologies [38], and 95% of industrial products were estimated to be produced via catalytic processes [37]. Carbon-based materials have long been used in heterogeneous catalysis reactions due to their desired properties for catalyst support and carbon-based materials act as direct catalysts in many industrial applications [39]. Along with the use of biochar as produced, researchers have currently devised various modification approaches to further expand its activation capacities [40,41]. Given that biochar is a highly porous and carbon-rich material, biochar is a promising alternative to replace conventional solid carbon-based catalysts with some known demerits (e.g., expensiveness and environmentally unfriendliness). In addition, physico-chemical properties of biochar can be modified via acid/base treatment or carbonization [42,43]; thus, biochar is an excellent candidate for catalytic applications. Nonetheless, there have

Corresponding authors. E-mail addresses: [email protected] (K.-H. Kim), [email protected] (E.E. Kwon).

http://dx.doi.org/10.1016/j.rser.2017.04.002 Received 2 August 2016; Received in revised form 8 February 2017; Accepted 3 April 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

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from bulk gas or liquid film to the external surface of the catalyst; 2) diffusion of reactant to the catalyst internal surface through catalyst pores; 3) adsorption of reactant on the catalyst surface; 4) reactions occurring on the catalytic active sites on the catalyst surface; 5) desorption of product from the catalyst surface; 6) diffusion of product to the internal surface of the catalyst through catalyst pores; and 7) diffusion of product from the external catalyst surface to bulk gas or liquid film [36,44,45]. Thus, catalytic activity is highly contingent on accessibility to catalytic active sites dispersed throughout internal pores [36]. Although biochar is a porous material [46], the morphology and porosity of biochar without activation exhibits very poor catalytic properties [47]. In order to overcome these poor properties, many researches have been directed to modify biochar morphology and porosity via various treatments (Table 1). The essential properties (surface area, pore volume, pore size, and acidity) that affect catalytic ability are also summarized in Table 1. The data in Table 1 suggests that surface area, pore volume, pore size, acidity, and surface functionality are highly contingent on many variables such as biomass origin and thermolysis conditions (e.g., temperature, gaseous composition, heating rate, and heating time). For example, total acidity of Karanja kernel-biochar (generated at 300 °C in N2 for 4 h) increased two-fold at 500 °C [48]. For a poplar woodbiochar generated in the presence of steam at 750 °C, surface area increased from 429 to 621 m2 g−1 with an extended gasification residence time from 0.5 to 1 h [49]. The surface area of biochar produced from poplar wood gasification under 10% CO2/90% N2 atmospheric conditions increased from 435 to 687 m2 g−1 with a temperature rise from 750 to 920 °C [49]. While biochar produced via gasification in 10% CO2/90% N2 developed more micropores in the biochar alone, a biochar produced via gasification in 90% H2O/10% N2 did not develop micropores [49]. The pore volume of the biochar also increased from 0.18 to 0.3 cm3 g−1 with the increase in temperature from 750 to 920 °C due to an enhanced sintering effect at 900 °C [49]. Similarly, biochar was generated from the pyrolysis of lignocellulose and saccharide by treating with H2SO4 for 48 h at three different temperatures (180, 280, and 380 °C) [50]. Accordingly, the resulting products indicated that the pore volume and surface area were proportional to the pyrolytic temperature. However, total acidity was inversely proportional to pyrolytic temperature [48]. Properties of biochar are also variable with the origin of biomass. For instance, a pine-biochar exhibited 50% higher surface area than a peanut hullbiochar under the same pyrolytic conditions [51] and rice straw-

Fig. 1. Generation of biochar via thermal degradation of biomass.

Fig. 2. Utilization of biochar.

been scanty attempts to use biochar as catalysts. Thus, it is highly desirable to properly evaluate the potent role of biochar as catalytic materials. In this regard, this review was organized to elucidate versatile applicabilities of biochar as catalysts. Moreover, it was further intended to improve its modification approaches by providing relevant information on recent studies focusing on biochar catalysts and the reactions to which the catalysts were applied. 2. Generation of biochar and its properties Mass transport phenomena in heterogeneous catalysis can be classified with seven steps, as depicted in Fig. 3: 1) diffusion of reactant

Fig. 3. Schematic of the seven steps involved in heterogeneous catalysis on a porous material. Reproduced from Ref [45] with permission of The Royal Society of Chemistry.

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Table 1 Biochars and their properties. Generation of biochar

Treatment of the generated biochar

Properties of the treated biochar

Ref.

Surface area (m2 g-1)

Total pore volume (cm3 g1 )

Average pore size (nm)

Total acidity (mmol g-1)

-SO3H density (mmol g-1)

None 7 M KOH; 25 °C; 2 h 98% H2SO4; 150 °C; 24 h 20% SO3; 150 °C; 15 h ➀ 7 M KOH; 25 °C; 2h ➁ 20% SO3 in N2; 150 °C; 15 h ➀ 7 M KOH; 25 °C; 2h ➁ 450, 675, or 875 °C; N2; 2 h ➂ 20% SO3 in N2; 150 °C; 15 h None 99% H2SO4; 100 °C; 18 h 99% SO3; 25 °C; 144 h ➀ 7 M KOH; 25 °C; 2h ➁ 675 °C; N2; 2 h ➂ 20% SO3 in N2; 150 °C; 15 h 95–98% H2SO4; 90 °C; 0.5 h 800 °C; air; 2 h

0.13 207 2.7

N/A N/A N/A

N/A N/A N/A

N/A N/A 0.04

N/A N/A 0.6

[54] [54] [54]

2.6

N/A

N/A

3.2

1.0

[54]

1.9

N/A

N/A

2.6

0.9

[54]

2 (450 °C) 640 (675 °C) 1411 (875 °C)

0.01 (450 °C) 0.35 (675 °C) 0.71 (875 °C)

N/A (450 °C) 2.17 (675 °C) 2.20 (875 °C)

2.6 (450 °C) 1.2 (675 °C) 0.4 (875 °C)

0.84 (450 °C) 0.41 (675 °C) 0.38 (875 °C)

[61]

1.1 338

0.001 0.18

1.2 1.1

0.6 5.9

0 0.45

[53] [53]

1.2

0.001

0.8

N/A

0.86

[53]

838–949

0.9

3.2–3.5

1.7–2.0

N/A

[125]

4

N/A

7.7

2.5

0.9

[71]

4

N/A

N/A

N/A

[72,73]

300, 400, or 500 °C; N2; 4 h

None

N/A

[48]

None

0.02 (300 °C) 0.02 (400 °C) 0.03 (500 °C) N/A

N/A

750 °C; H2O/N2 (9:1); 0.5 or 1 h 750 or 920 °C; CO2/ N2 (1:9); 0.5 h 750 °C; H2O/N2 (9:1); 0.5 h

N/A

N/A

[49]

N/A

N/A

[49]

N/A

N/A

N/A

[55]

Poplar wood

750 °C; H2O/N2 (9:1); 0.5 h

280, 340, or 400 °C; 10% O2 in N2; 2 h

N/A

N/A

N/A

[55]

Pine bark Rice husk Switchgrass

950 °C; N2; 2 h 700 °C; N2 N/A

1–2 N/A 1.6

N/A N/A N/A

N/A N/A N/A

[88,91] [89] [126]

Peanut hull or pine

400 °C; N2; 1 h

None None ➀ KOH; 25 °C; 12 h ➁ 200 °C; N2; 1 h ➂ 700 °C; N2; 2 h 99% H2SO4; 100 °C; 18 h

0.18 (750 °C) 0.30 (920 °C) 0.216 (2 h) 0.207 (4 h) 0.204 (8 h) 0.216 (280 °C) 0.301 (340 °C) 0.323 (400 °C) 0.17 N/A 0.09

> 0.8

280 °C; 10% O2 in N2; 2, 4, or 8 h

13 (300 °C) 14 (400 °C) 16 (500 °C) 429 (0.5 h) 621 (1 h) 435 (750 °C) 687 (920 °C) 531 (2 h) 475 (4 h) 464 (8 h) 531 (280 °C) 627 (340 °C) 667 (400 °C) 310–330 117 64

0.5 (total basicity) 8.0 (300 °C) 6.3 (400 °C) 3.9 (500 °C) N/A

1.1 (peanut hull) 1.1 (pine) N/A

5.7 (peanut hull) 3.7 (pine) 0.95

[51]

400–500 °C; N2; 45 min

0.13 (peanut hull) 0.20 (pine) 0.05

0.6 (peanut hull) 0.7 (pine)

Corn stover

242 (peanut hull) 365 (pine) 10

0.70

[101]

Lignocellulose and saccharide

180, 280, or 380 °C; N2; 48 h

[50]

2.6 1.8

4.3 (180 °C) 2.6 (280 °C) 2.2 (380 °C) N/A N/A

N/A

500 °C; N2; 1 h 500 °C; N2; 1 h

5.4 (180 °C) 29.6 (280 °C) 37.2 (380 °C) 0.09 0.16

N/A

Rice straw Rice straw

5.3 (180 °C) 18.7 (280 °C) 23.4 (380 °C) 140 363

N/A N/A

[52] [52]

Rice straw

500 °C; N2; 1 h

772

0.42

2.2

N/A

N/A

[52]

Sewage sludge Sewage sludge

500 °C; N2; 1 h 500 °C; N2; 1 h

18 64

0.02 0.04

4.1 2.5

N/A N/A

N/A N/A

[52] [52]

Sewage sludge

500 °C; N2; 1 h

783

0.61

3.1

N/A

N/A

[52]

Pine wood Activated carbon (commercial)

1000 °C; air

183 1944

0.09 1.2

N/A 1.2

N/A 0.23

N/A 0

[120] [53]

Source

Conditions

Hardwood Hardwood Hardwood

N/A N/A N/A

Hardwood

N/A

Hardwood

N/A

Wood mixture (wood waste, white wood, bark, and shavings)

N/A

Peanut hull Peanut hull

400 °C; N2; 1 h 400 °C; N2; 1 h

Peanut hull

400 °C; N2; 1 h

Biochar (commercial)

N/A

Rice husk

510 °C; 4 s

Palm kernel shell

N/A

Karanja kernel

Poplar wood Poplar wood Poplar wood

None

➀ H2SO4; 105 °C; 24 h ➁ 150 °C; 1 h 0.5 M H2SO4; 25 °C; 24 h None 40% H2O in N2; 700 °C; 1 h ➀ KOH; 60 °C; 2 h ➁ 40% H2O in N2; 700 °C; 1 h None 40% H2O in N2; 700 °C; 1 h ➀ KOH; 60 °C; 2 h ➁ 40% H2O in N2; 700 °C; 1 h 3 M KOH; 80 °C; 2 h

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hydroxyls from alcohol ( > 3000 cm−1) are not observed on sulfonated biochar. Interestingly, the selected temperature for heat treatment significantly affects the number of functional groups on biochar. For a sulfonated biochar, biochar treated at 450 °C yields more aromatic groups than that treated at 650 and 850 °C [61]. As such, activation of biochar also affects biochar functionality. Biochar activated with KOH developed more C–O and C˭O groups relative to hydrothermal activation [52]. These data confirm that the physical properties (surface area and pore volume) of biochar can be modified to a desirable level similar to conventional catalysts (e.g., sulfated ZrO2) [63]. In addition, chemical properties of the biochar catalysts (e.g., functionality) can also be tailored by various factors such as carbonization temperature, gas environment, and chemicals. Fig. 5 illustrates a model suggesting porous biochars containing various chemical functionalities with an image of a porous biochar.

biochar had a 7.5-fold larger surface area and a 4.5-fold larger pore volume than sewage sludge-biochar under a pyrolytic temperature of 500 °C for 1 h [52]. As discussed, the surface area of biochar without pre/post treatment is inferior to that of activated carbon [53]. Thus, employing biochar for the purpose of practical use is not persuasive [48,52–54]. To enhance surface area to the desirable extent, biochar is commonly activated with KOH. For instance, a surface area of a hardwood-biochar increased from 0.13 to 207 m2 g−1 after pre-treatment with KOH at ambient temperature (25 °C) [54]. Another common way to increase biochar porosity is hydrothermal treatment or oxygenation. For instance, the surface area of rice straw-biochar increased from 140 to 363 m2 g−1 at 700 °C in the presence of H2O for 1 h [52]. Ducousso et al. oxygenated a poplar wood biochar (surface area: 574 m2 g−1 and pore volume: 0.219 cm3 g−1) at different temperatures and oxygenation times [55]. At optimal conditions (i.e., 400 °C and 2 h), the largest surface area (667 m2 g−1) and pore volume (0.323 cm3 g−1) were achieved [55]. Co-activation, which is a combination of KOH pretreatment and hydrothermal treatment, can enhance the physical properties of biochar. For example, the surface area of a rice straw-biochar significantly increased from 140 to 772 m2 g−1 with the changes in pore volume from 0.09 to 0.42 cm3 g−1 after KOH treatment at 60 °C was accompanied with heating under H2O at 700 °C [52]. For a sewage sludge-biochar, surface area was increased from 18 to 783 m2 g−1, while pore volume increased from 0.02 to 0.61 cm3 g−1 with the co-activation [52]. Certain catalytic reactions such as transesterification and hydrolysis require strong acid sites. To this end, biochar was sulfonated (i.e., introducing a sulfonate group (–SO3H) onto biochar) using a general procedure for liquid sulfonation via the addition of concentrated H2SO4 ( > 95%) to biochar. The mixture is stirred at temperatures between 90 and 150 °C followed by washing until the solution is neutral without sulfate ion (SO42–) and then dried [56–59]. Gaseous sulfonation includes an exposure of biochar to gaseous SO3 ( > 20%) at temperatures between 25 and 150 °C. It is immediately washed until no SO42– is detected, and then subjected to drying [53,54,60,61]. Liquid sulfonation facilitates the expansion of surface area relative to gaseous sulfonation. Nonetheless, the latter provides more density for –SO3H than the former [53,54]. Biochars with or without chemical pretreatments (Table 1) exhibit different functional groups on the surface. Table 2 summarizes the FTIR adsorption regions for the identification of functional groups on different biochars. Aromatic groups (e.g., phenol), C–O–C, and – COOH exist both on biochar and on sulfonated biochar. It is known that hydroxyl (–OH), C–O, and C˭O groups interact with phenol group through hydrogen bonding, as shown in Fig. 4 [62]. Note that sulfonic groups (–SO3H and S˭O) are only found on sulfonated biochar whereas

3. Application of biochar to heterogeneous catalyst 3.1. Biodiesel production on biochar catalysts Biodiesel is a renewable substitute for conventional petro-derived diesel [64–66]. Heterogeneous acid and base catalysts for biodiesel production (e.g., Al2O3, TiO2, SiO2, ZrO2, CaO, Amberlyst resins, and zeolite) through the esterification and/or transesterification of vegetable oils have been investigated intensively to replace conventional homogeneous acid and base catalysts (e.g., H2SO4, NaOH, and KOH). Note that conventional transesterification suffers, as it inevitably generates environmentally unfriendly acidic and basic waste [67,68]. Fig. 6 shows biodiesel production through the transesterification of triglyceride. A WO3/ZrO2/Al2O3 catalyst showed catalytic performance with a 90% yield of esters (i.e., biodiesel) for 100 h time-on-stream without deactivation at 250 °C [69]. Also, a TiO2/ZrO2 and an Al2O3/ZrO2 catalyst had 94% yield of esters for transesterification of soybean oil with methanol at 175 °C [70]. However, such catalysts require expensive metal precursors for synthesis. Thus, sulfonated biochars have been used as inexpensive heterogeneous catalysts for the production of biodiesel (i.e., esterification and/or transesterification of lipid (triglycerides: TGs) and free fatty acids: FFAs). Thus, the experimental results associated with FAME conversion with biochar and conventional heterogeneous (solid) catalysts were compared in Table 3. The highest yield (88%) of biodiesel products (i.e., esters) from cooking oil was achieved by sulfonated biochar [71]. Here, the simultaneous esterification of FFAs and transesterification of TGs were conducted with a 20:1 M ratio of methanol to cooking oil at 100 °C for 15 h. The yield of methyl esters decreased from 88% to 80% after five reuses of the catalyst, which can be explained by leaching of –SO3H functional groups [71]. McKay and co-authors prepared a palm kernel shell-derived solid base biochar catalyst for transesterification of sunflower oil [72,73]. The yield of methyl esters with a molar ratio of methanol/oil (9:1) was 99% at 65 °C when 3 wt% catalyst was loaded [73]. The solid acid/base biochar catalysts described above yielded high production of biodiesel from various edible oils. However, both catalysts tended to suffer from deactivation after several reuses. The base catalyst was poisoned by undesirable byproducts produced through reactions between CaO and the feed oil during transesterification [74,75]. As summarized in Table 3, biochar catalysts are comparable to non-biochar catalysts (i.e., conventional heterogeneous (solid) catalysts) in terms of ester yield from TGs and FFAs. However, for the practical use of biochar as catalysts for biodiesel production, the stability of biochar catalysts should be improved to avoid the post separation steps for removing S or Ca.

Table 2 FT-IR characterization of biochar surface functional groups. Functionality

Wavenumber (cm-1)

Ref.

Sulfonated biochar –SO3H Aromatic C–H bending Aromatic C˭C bending C–O–C stretching C˭O stretching C–O stretching S˭O stretching

1706–1717 731, 903 1554–1580 1130, 1150 1700–1706 1215 1032–1100

[54,61,71] [61] [61,125] [61] [61,71,125] [61] [53,61]

Non-sulfonated biochar Aromatic C–H bending Aromatic C˭C bending C–O–C stretching C–O stretching C–H bending C˭O stretching –OH

740–756, 873 1581–1585 1130–1150 1110, 1200–1215 1043–1083 1612, 1698–1700 3000–3500

[48,61,125] [48,61,125] [48,61] [48,52,61,125] [48,61] [48,52,61,125] [48,52,71,125]

3.2. Tar removal on biochar catalysts Syngas, a mixture of H2 and CO, can be used as an initial feedstock 73

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Fig. 4. Interaction between phenol group and other functional groups on the surface of biochar. Reprinted from Ref [62], Copyright (2011), with permission from Elsevier.

Fig. 5. A model of porous biochar containing different functional groups.

and poisoning [84]. There have been many efforts to decompose tars in a secondary reactor with noble metal catalysts (e.g., Pt, Pd, and Rh) [85]; however, the recovery of catalyst nonetheless remains a challenging task for the application of this approach. Thus, it is desirable to use an inexpensive catalyst for tar decomposition. In this respect, biochar relative to the conventional catalysts was employed as a catalyst to reduce tars [86]. Klinghoffer et al. found that inorganics (e.g., Fe, Ca, K, P, and Mg) played an important role in methane decomposition, which is a surrogate for the tar decomposing reaction (i.e., C–C and C–H bonds cleavage) [49,87]. Carbon was found to work not only as an active site for methane cracking, but also as a catalytic support where inorganics were dispersed [87]. Carboxylic acid and lactone functional groups on the biochar were not involved in the reaction because they desorbed at the reaction temperature [87]. Biochar and non-biochar catalyst tar removal efficiency as reported

for synthetic natural gas, ammonia, and methanol. It can not only be used to generate heat and power through combustion in gas turbines but also can be converted into fuels and chemicals through FischerTropsch synthesis [36]. Gasification of biomass is a promising renewable route as it can facilitate the mass production of syngas. However, condensable hydrocarbons (i.e., tar) are generated during its production as an inevitable byproduct; the major constituents of tar are aromatic hydrocarbons including toluene, naphthalene, styrene, phenol, and other polycyclic aromatic hydrocarbons (PAHs) [76]. In general, tars can deposit in pipelines of the system, possibly blocking downstream processes [77]. Thus, the removal and/or mitigation of tar is a critical step to commercialize the biomass gasification process for syngas production [78–82]. In practice, catalytic tar cracking was conducted at temperatures between 550 and 900 °C using dolomite, olivine, and base metals such as Ni [77,83]. However, these traditional tar cracking catalysts suffered from deactivation arising from coking

Fig. 6. Transesterification of triglyceride to biodiesel (fatty acid methyl esters: FAMEs).

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Table 3 Biodiesel production on biochar and non-biochar catalysts. Catalyst (Entry no. in Table 1)

Feedstock

Reaction conditions

Ester yield (%)

Ref.

Biochar catalysts Hardwood-biochar (3) Hardwood-biochar (4)

Waste vegetable oil Canola oil

60 °C; EtOH/oil (28:1); 3 h 65 °C; MeOH/oil (15:1); 24 h

[54] [54]

Wood mixture-biochar (6)

Canola oil

Peanut hull-biochar (8)

70

[53]

Commercial biochar (10)

Mixture of palmitic, stearic acids, and soybean oil Mixture of oleic acid and canola oil

48

[125]

Rice husk-biochar (11) Palm kernel shell-biochar (12)

Waste cooking oil Sunflower oil

150 °C; 1.52 MPa N2; MeOH/oil (15:1); 3 h 60 °C; MeOH/oil (20:1); 4 wt% catalyst; 6 h 150 °C; 1.52 MPa N2; MeOH/oil (10:1); 3 h 110 °C; MeOH/oil (20:1); 15 h 60 °C; MeOH/oil (9:1); 5 wt% catalyst; 6h

88% (oil conversion) 85.1 g L−1 (FAME concentration) 44

88 99

[71] [72]

Non-biochar catalysts Aluminum hydrogen sulfate (Al(HSO4)3)

Waste vegetable oils

81

[127]

Zeolite Beta Sulfated zirconia (SO42-/ZrO2)

Waste cooking oils Waste cooking oils

25 44

[128] [129]

Niobic acid (Nb2O5·nH2O)

Waste cooking oils

16

[129]

Tungstophosphoric acid/hydrous zirconia (H3PW12O40/ZrO2·nH2O) Amberlyst−15

Canola oil

90

[130]

45

[71]

220 °C; MeOH/oil (16:1); 0.5 wt% catalyst; 50 min 80 °C; MeOH/oil (3:1); 15 h 80 °C; MeOH/oil (20:1); 10 wt% catalyst; 14 h 80 °C; MeOH/oil (20:1); 10 wt% catalyst; 14 h 200 °C; 4.14 MPa; MeOH/oil (9:1); 3 wt% catalyst; 10 h 110 °C; MeOH/oil (20:1); 15 h

Waste cooking oils

[61]

catalyst) is not yet effective at the low temperatures [91]. Therefore, future efforts need to focus on overcoming these limitations and to expand the applicability of biochar as a catalyst.

in the literature is compared and summarized in Table 4. Most studies have used model reactions of tar decomposition using toluene, naphthalene, and phenol. Overall, the biochars in Table 4 are less active than conventional and commercial catalysts such as Ni. However, the biochar-supported base metal (e.g., Ni and Fe) catalysts exhibited better tar removal performance than the conventional mineral catalysts (e.g., olivine and dolomite) [88–90]. For instance, a catalyst composed of a physical mixture of NiO and wood-biochar removed 97% of real tars produced during sawdust gasification, which led to an increase in syngas production due to catalytic reforming of the tars [90]. Ni-Fe bimetallic catalysts supported on rice husk-biochar produced 7-fold less tars during pyrolysis of biomass than raw biochar and monometallic catalysts (Fig. 7) [89]. The NiO/biochar mixture catalyst was stable for 8 h time-on-stream. The addition of Fe to biochar lowered the activation energy (Ea) of toluene decomposition (48.4 kJ mol−1), compared to 90.6 kJ mol−1 (Ea for biochar without Fe addition) [88,91]. All of the experimental reports summarized in Table 4 indicate that biochar is a promising alternative to remove tar in the gasification processes. One of drawbacks associated with biochar and metal/biochar catalysts for tar removal is the reaction temperature, as its removal proceeds only at temperatures above 700 °C. However, tar removal can be initiated at lower temperatures (e.g., 560 °C) with the conventional Ni catalyst [91,92], while biochar (as a

3.3. Syngas production on biochar catalysts As discussed above, syngas is a versatile intermediate and/or initial feedstock for synthesizing fuels and chemicals. In light of this observation, a number of research groups have explored the possible role of biochar catalysts in syngas production. For example, Ren et al. claimed that a biochar catalyst increased syngas yield through biomass pyrolysis [93]. Syngas yield was recorded to increase from 15 (without catalyst) to 46 wt% (with the biochar catalyst) at 480 °C [93]. The hydrogen fraction in the syngas also increased with the biochar catalyst (27 vol%) compared to that without catalyst (0 vol%) [93,94]. A recent study reported a potential application of biochar as a dry reforming catalyst [95]. A pine wood-biochar was treated with HNO3 (0.1 M) followed by washing and drying. The treated biochar was impregnated with an ammonium tungstate [(NH4)10H2(W2O7)6] solution and the tungsten-promoted biochar catalyst was then thermally treated at 1000 °C under N2 purging. This resulted in a tungsten carbide (WC) supported on the biochar through several reaction steps (WO3 → WO2 → W → W2C → WC). Fig. 8 shows the morphology of the

Table 4 Tar removal on biochar and non-biochar catalysts. Catalyst (Entry no. in Table 1)

Feedstock

Reaction conditions

Biochar catalysts Pine bark-biochar (18) Fe/pine bark-biochar (18) Switchgrass-biochar (20) Commercial biochar Pinewood-biochar A mixture of 15 wt% NiO and wood-biochar Ni-Fe/rice husk-biochar

Toluene Toluene Toluene Phenol Naphthalene Sawdust-tar Rice husk-tar

900 °C; 800 °C; 800 °C 700 °C; 900 °C; 800 °C; 800 °C

Non-biochar catalysts Ni/Olivine Olivine Dolomite

Toluene Naphthalene Phenol

560–850 °C; H2O 900 °C; H2O and CO2 700 °C; H2O and CO2

75

H2O H2O H2O and CO2 H2O and CO2 0.3 s

Tar removal efficiency (%)

Ref.

94 100 81 82 94 97 92

[91] [88] [126] [131] [131] [90] [89]

30–100 55 90

[92] [131] [131]

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selective than commercialized and conventional catalysts. Ormsby et al. prepared biochar from pine wood chips and peanut hulls sulfonated with 99% H2SO4 [51]. For hydrolysis of xylan, the sulfonated pine chip-biochar catalyst showed 85% conversion in 2 h at 120 °C (Fig. 9). In contrast, a commercial activated carbon showed only 57% conversion in 24 h despite its larger surface area (1391 m2 g−1) relative to the biochar catalyst (365 m2 g−1) [51]. Moreover, the biochar catalysts had higher initial reaction rates of cellobiose and xylan hydrolysis than activated carbon and Amberlyst-15 catalysts [51]. A corn stover-biochar was used for hydrolysis of corn stover, switch grass, and prairie cordgrass biomass [101]. The catalyst had higher selectivity toward glucose and xylose (a main unit of cellulose and hemicellulose, respectively) than conventional homogeneous H2SO4 catalyst, demonstrating its enhanced performance over biomass hydrolysis. A Ni catalyst supported on microalgae-biochar that was produced via pyrolysis at 400 °C was prepared by impregnating the biochar with Ni(NO3)2 solutions; it was then tested for hydrotreatment of microalgal bio-oil [102]. The upgraded bio-oil contained ~80% of hydrocarbons and ~12% of oxygenated and nitrogenated compounds, showing a good performance of the catalyst for hydrodeoxygenation and hydrodenitrogenation of biocrude [102].

Fig. 7. Tar formation during pyrolysis of biomass over biochar supported nickel and iron catalysts. Reprinted from Ref [89], Copyright (2014), with permission from Elsevier.

WC/biochar catalyst [95]. Dry reforming of CH4 (CH4 + CO2 → 2H2 + CO, ΔH =247 kJ mol−1) [96] was performed on the WC/biochar catalyst. CH4 conversion decreased but CO2 conversion increased as the CH4/CO2 ratio increased. More H2 was produced with an increase in CH4/CO2 ratio and temperature and the WC/biochar catalyst was stable for 500 h time-on-stream. Apart from syngas production, a biochar can also be used as a carbon platform as a Fischer-Tropsch synthesis catalyst. Yan et al. showed the synthesis of pine wood-biochar-encapsulated Fe particles (10–50 nm) [97]. The core shell-structured Fe catalyst was prepared by a combination of Fe nitrate impregnation on the biochar and thermal treatment at 1000 °C. The catalyst was tested for Fischer-Tropsch synthesis of a wood-derived syngas consisting of 18% H2, 20% CO, 12% CO2, 2% CH4, and 48% N2. The 68% selectivity toward liquid hydrocarbons (C5 to C13) at 95% of CO conversion was reached with the engineered biochar catalyst at 310 °C and 6.7 MPa for 1500 h timeon-stream without significant deactivation. The performance of biochar-catalyst for Fischer-Tropsch synthesis is comparable to conventional catalysts [98–100]. For instance, a Co/SiO2 catalyst had 85% selectivity toward liquid hydrocarbons with an 82% CO conversion at 230 °C and 1.5 MPa [98].

4. Application of biochar to environmental catalyst and microbial fuel cell electrode 4.1. Biochar as deNOx catalysts Nitrogen oxides (NOx) emitted from automobiles or stationary power sources trigger photochemical smog, acid rain, and ozone destruction [103]. Selective catalytic reduction (SCR) is a cost-effective technology to reduce NOx emissions [104]. Activated carbon has widely been used as a low temperature SCR catalyst support due to its low fabrication cost and high NOx removal efficiency [105,106]. Because biochar has similar characteristics to activated carbon, a few studies reported the use of biochar as a catalyst to support deNOx reactions [52,105,107]. A low temperature SCR of NO with NH3 (4NO +4NH3 + O2 → 4N2 +6H2O) was conducted using MnOx-supported biochar derived from rice straw [52]. At 250 °C, the MnOx/rice straw-biochar showed high NOx removal efficiency (85%). In addition, the MnOx/rice strawbiochar had 84% removal efficiency of NOx even at 50 °C. According to Shen et al., improved activity of low temperature SCR was ascribed to its advantageous properties such as high surface area, acidity, Mn4+/ Mn3+ ratio, and oxygen functionality [105]. However, the NOx removal efficiency achieved with biochar-catalysts is still less than commercial conventional SCR catalysts [107] such as Mn/γ-Al2O3 (98% at 200 °C) [108] and V2O5-CeO2/TiO2 (99% at 165 °C) [109].

3.4. Biomass hydrolysis and bio-oil upgrading on biochar catalysts It is known that some biochar catalysts are more active and

Fig. 8. Scanning electron microscopy (SEM) images of pine wood-biochar supported tungsten carbide catalyst under different magnifications. Reproduced from Ref [95] with permission of The Royal Society of Chemistry.

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Fig. 9. Hydrolysis of hemicellulose using biochar catalysts [50,51].

biochar were assessed further in line with its production and activation procedures. The morphology and surface functionality of biochar can be finely tuned via various physical and/or chemical treatments. In this respect, biochar has a high potential to replace expensive and nonrenewable conventional catalysts. The potent role of biochar-derived catalysts has been demonstrated to be successful in various reactions such as the production of biodiesel, removal of tar in bio-oil and syngas, deNOx, syngas production, and biomass hydrolysis. Moreover, biochar is found to have potential as an inexpensive electrode of MFC systems. Properties of biochar catalysts (e.g., surface functionality, surface area, porosity, and acidity) are, however, highly dependent on the origin of biomass, biochar generation conditions, and pre/posttreatments. Thus, to maximize and optimize catalytic properties of biochar for intended reactions, more effort should be put into control of the combined effects of key variables such as biomass type, production conditions (e.g., temperature, time, and reagent gas), and post-treatment conditions (e.g., physical, chemical, or co-activation). However, little information is available regarding exquisite control on the properties of biochar for its catalytic applications. Therefore, further investigations into developing the catalytic properties of biochar will be important to design active, selective, and stable biochar catalysts. Furthermore, in order for biochar to be a viable substitute for industrial heterogeneous catalysts, it is highly desirable to build up a system to allow the production of biochar at industrial scale. In addition, securing the stable sources of supply for the raw biochar materials is also challenging to maintain constant properties for the large-scale production. However, if all these challenges are to be fulfilled, the real world application of biochar catalysts will be stimulated and facilitated to replace expensive and non-environmental benign catalysts, which have been used to date for many purposes.

Fig. 10. Fabrication of a sewage sludge-biochar anode for a microbial fuel cell. Reproduced from Ref [119] with permission of The Royal Society of Chemistry.

4.2. Biochar as microbial fuel cell electrodes Microbial fuel cells (MFCs) are one of the cutting-edge technologies to convert chemical energy to electrical energy by microbial catalysis [110–114]. MFCs are also being considered as possible technical candidates to treat wastewater or soil [115–117], and biochar has been tested as a catalytic electrode material for MFCs [118–122]. Yuan et al. prepared a sewage sludge-biochar at 900 °C and tested it for oxygen reduction reaction (ORR) catalyst activity in MFCs [118,119]. Fig. 10 shows a fabrication process of the sewage sludge-biochar anode for a MFC system [119]. ORR is a rate-determining reaction for the performance of MFCs that occurs through a one-step reaction or twostep reaction [123]. The former involves four electrons while the latter involves two electrons and a hydrogen peroxide (H2O2) intermediate [123,124]. One-step reaction: O2 + 4e- + 4H+ → 2H2O (E°= 1·23 V) Two-step reaction: O2 + 2e- + 2H+ → H2O2 (E°= 0.68 V) H2O2 + 2e- + 2H+ → 2H2O (E°= 1·77 V)

Acknowledgements

There have been some efforts to use biochar as MFC electrodes because of its low cost ($0.02) compared to commercial carbon electrodes ($60) [120]. The performance of biochar catalysts coated on an air cathode was compared to that of a Pt/C-coated cathode. Maximum power density (Pmax) of the former (500 mW m−2) was lower than the latter (625 mW m−2) [118]. The Pmax of a bifunctional biochar-MFC system with anode (biochar) and cathode (biochar) reached 969 mW m−2, which was doubled relative to a MFC system with anode (graphite) and cathode (Pt) [119]. The bifunctional biochar-MFC system lost ~7% of voltage after a 2-month run, confirming its considerably high stability. In recent years, a sewage sludge-biochar co-doped with inorganics (N, Fe, and S) was reported to have a superior ORR activity in acidic and basic conditions as well as oxygen evolution reaction (OER) activity in basic condition [122].

This work was supported by a National Research Foundation of Korea (NRF) Grant funded by the Korean Government (Ministry of Education, Science and Technology (MEST)) (No. NRF2016R1D1A1B03933027). K.H.K. acknowledges support made in part by grants from the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2016R1E1A1A01940995). This research was also supported partially by the R & D Center for Green Patrol Technologies through the R & D for Global Top Environmental Technologies funded by the Ministry of Environment (MOE), Republic of Korea. References [1] Carroll A, Somerville C. Cellulosic biofuels. Annu Rev Plant Biol 2009;60:165–82. [2] Regalbuto JR. Cellulosic biofuels - got gasoline?. Science 2009;325:822–4. [3] Brethauer S, Wyman CE. Review: continuous hydrolysis and fermentation for cellulosic ethanol production. Bioresour Technol 2010;101:4862–74. [4] Chundawat SPS, Beckham GT, Himmel ME, Dale BE. Deconstruction of lignocellulosic biomass to fuels and chemicals. Annu Rev Chem Biomol Eng

5. Conclusions and prospects In this review, the performance of biochar catalysts was evaluated for diverse fields of applications. In addition, the catalytic properties of 77

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