Preparation and characterization of activated carbon fibers from liquefied poplar bark

Materials Letters 112 (2013) 26–28

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Preparation and characterization of activated carbon fibers from liquefied poplar bark Jiahui Zhang a, Wenbo Zhang a,n a

College of Material Science and Technology, Beijing Forestry University, Beijing 100083, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 24 March 2013 Accepted 25 August 2013 Available online 31 August 2013

In this paper a novel way was developed to prepare activated carbon fibers from phenol liquefied poplar bark (ACFs-WB) with direct activation method named “one step method”. Liquefied product was spun, stabilized and finally steam-activated immediately as target temperature of activation reached. Surface morphologies and pore structure characteristics were investigated by means of SEM and low temperature nitrogen adsorption. The results showed that micropore development was dominated in ACFs-WB. Also, micropore size distribution in diameter was mainly concentrated in the range of 0.4–0.8 nm. A particularly interesting feature on pore structure was observed at activation temperature of 900 1C, which micropore volume and specific surface area approximately dupled though fiber morphologies in diameter were similar. The results suggested that micropores development occurred intensely and mainly depended on communications or ruptures of intrawalls at activation temperature of 900 1C. & 2013 Elsevier B.V. All rights reserved.

Keywords: Fibre technology Carbon materials Liquefaction Pore structure Biomaterials

1. Introduction Wood liquefaction is one of the techniques using to prepare carbon fibers which had a better property in biodegradability [1]. Carbon fibers from liquefied wood with a tensile strength of 1.7 GPa were successfully prepared in our laboratory [2]. However, comparing to commercial carbon fibers the tensile strength was low. Activated carbon fibers (ACFs) might be better to be developed as an absorbent to emphasize its adsorption capability rather than mechanical application in strength. Generally, the primal preparation of ACFs mainly relied on fossil materials [3,4]. In recent years several researches on preparation of biomass-based ACFs have been seen [5,6]. Wood bark has almost not been used effectively. Furthermore, wood bark contains higher amounts of lignin in its chemical composition than wood. Lignin is considered to be more easily liquefied as it has a higher biological activity to change its structure. In our previous study, foam was prepared from phenol liquefied bark [7]. In order to develop advanced carbon materials, a novel ACFs’ preparation method named “one step method” is exploited. ACFs having excellent adsorption capacities have attracted a lot of interests. More and more researches are focusing on ACFs preparation from biomasss materials on the bases of recycling and sustainable point of view. Compared to fossil materials, biomass-based ACFs have more wide raw material sources. In present study ACFs were prepared by liquefied wood bark. Morphology and

n

Corresponding author. Tel.: þ 86 10 6233 8358; fax: þ 86 10 6233 6061. E-mail address: [email protected] (W. Zhang).

0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.08.103

pore structure of prepared wood bark based-ACFs (ACFs-WB) were investigated.

2. Materials and methods Oven-dried poplar (Populus euramevicana cv. ‘I-214’) bark was ground and screened to size of 60–80 meshes. Bark powder was mixed with phenol containing concentrated sulphuric acid solution (5%) as a reaction catalyst. The ratio of the bark/phenol was 1:4 by weight. The mixture was liquefied in a 500 mL three neck glass flask and refluxed by holding at 160 1C for 2 h with continuous stirring. Liquefied bark was put into the feed box of a self-made apparatus with hexamethylenetetramine (HMTA, 5%, based on the weight of liquefied product).The mixture was heated from room temperature up to 120 1C (75 1C) within 40 min with a stirring speed of 80 rpm. The temperature was held for 10 min for the liquefied bark spinning solution. The spun filaments were continuously wind on a bobbin at the speed of 0.36 m/min. Then the resultant fibers were cured by soaking in a solution containing hydrochloric acid and formaldehyde (volume ratio 30:37) as main components at 90 1C for 2 h at a heating rate of 15 1C/h. Precursor fibers were obtained after the process of washing (distilled water) and drying (85 1C72 1C) for 40 min. Thereafter, precursor fibers were steam-activated with a steam flow of 4.84 g/ min at 700 1C, 800 1C and 900 1C, respectively, under the protection of nitrogen with a heating rate of 4 1C/min from room temperature to activation temperature. Whole activation process was continued to 60 min. After activation yields of ACFs-WB samples were

J. Zhang, W. Zhang / Materials Letters 112 (2013) 26–28

calculated by the formula below: Yieldð%Þ ¼ ½weight of ACFWBðgÞ=weight of precursor fiberðgÞ  100%

ð1Þ Scanning electron microscopy (SEM, S-3400N) was used to observe the morphologies of precursor fibers and ACFs-WB. The surface area, pore volume and pore size distributions (PSDs) calculated using the conventional Brunauer–Emmett–Teller (BET) method and Horvath–Kawazoe (HK) method, respectively, were determined with a specific surface area/pore size distribution detector (Autosorb iQ) from nitrogen adsorption and desorption isotherms at 77 K over a wide relative pressure, p/p0, ranging from 10  6 to 1. Before the measurement all samples were outgassed at 300 1C for 3 h with high purity nitrogen (99.99%).

3. Results and discussion Adsorption and desorption isotherms are shown in Fig. 1. It can be seen that all the isotherms with a slight slope belong to type I

27

according to BDDT (Brunauer–Deming–Deming–Teller) classification. The isotherms were also reversible, indicated the characteristics of micropore adsorption [8]. At activation temperature below 800 1C the adsorbed volume of nitrogen was around 200 cm3/g, showing that a small amount of micropores developed. The adsorbed volume at 900 1C was around twice than that of activated temperature at 700 1C and 800 1C, showing that micropores markedly increased at the activated temperature range of 800–900 1C. Additionally, a slight inclination of hysteresis loop was observed from the figure in the flat curve area around the relative pressure of 0.4, which was considered to be the multilayer adsorption phenomenon on the surface of adsorbent and possibly capillary condensation in mesopores. Also, noticeable differences could be observed in the opening of the knee of isotherms. The knee was more open at 900 1C, indicating a more noticeable contribution of meso- and micropores to the total adsorbed volume and adsorption energy [9]. Surface morphologies of precursor fibers and ACFs-WB are shown in Fig. 2. All fibers showed circular in cross section with a diameter around 25 μm from the SEM observation. With increasing activation temperature the diameters of fibers have almost not

600 1.5

Differential pore volume (cm3/g·nm-1)

Adsorption Desorption

Volume adsorbed (cm3/g)

ACF-900WB 400

Hysteresis loop Knee

ACF-800WB

200

0 0.0

ACF-700WB

0.2

0.4

0.6

0.8

Relative Pressure (P/Po) Fig. 1. N2 adsorption–desorption isotherms of ACFs-WB.

1.0

ACF-900WB

1.0

ACF-800WB

0.5

ACF-700WB 0.0 0.0

0.4

0.8

1.2

1.6

2.0

Pore diameter (nm) Fig. 3. Pore size distribution of ACFs-WB at different temperatures.

Fig. 2. SEM photographs of precursor fibers and ACFs-WB; (a) precursor fiber; (b) ACF-700WB; (c) ACF-800WB; (d) ACF-900WB.

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J. Zhang, W. Zhang / Materials Letters 112 (2013) 26–28

4. Conclusions

Table 1 Characteristics of ACFs-WB at different activation temperatures. Sample

Yield (%)

SBET (m2/g)

Vt (cm3/g)

Davg (nm)

ACF-700WB ACF-800WB ACF-900WB

43.45 41.41 14.37

716.41 815.69 1482.88

0.38 0.40 0.70

2.10 1.92 1.88

SBET, specific surface area; Vt, total pore volume; Davg, average pore diameter.

changed, showing that shrinkage of pore wall did not occur in the range of activation temperature. On the other hand, the surface of precursor fiber in Fig. 2 (a) was smoother than that of ACFs-WB showed in Fig. 2(b–d). Furthermore, sponge-like structures were clearly observed in Fig. 2(d), showing the surface features of ACFsWB were sculptured markedly. Pore size distributions (PSDs) obtained by means of HK method are shown in Fig.3. It can be seen that the micropores were mainly distributed in the range of 0.4–0.8 nm in diameter, which exhibited a distinct property of molecular sieve. The porous developments obtained at activation temperature of 700 1C and 800 1C were very uniform, which may be concluded that pores developments tended to stabilize. As activation temperature was below 800 1C submicron sized pits fairly uniformly distributed were gradually formed with increasing activation temperature. As the activation temperature increases up to 900 1C, which the peak of PSD was much wider, a more developed pore structure was formed. The specific surface areas calculated with the BET method and pore structure characteristics are given in Table 1. The yield was 14.37% at 900 1C, was far less than those of at 700 1C and 800 1C. As activation degree increased, the yield decreased with an increase in BET specific surface area (SBET). As important index of absorbent, the SBET which was approximately duple at activation temperature of 900 1C than those of at 700 1C and 800 1C, had a maximum of 1482.88 m2/g for ACF-900WB. The SBET values were larger than such as pitch-based ACFs or phenol-formaldehyde resin-based ACFs at the similar preparation conditions [10,11]. It can be also seen in Table 1 that the total pore volume (Vt) was 0.70 cm3/g for ACF900WB, which was nearly twice greater than ACF-700WB and ACF800WB, showing the similar trend with the SBET. The Vt and SBET increased significantly on activation process. Activation temperature over 800 1C was found to have significant effects on the development of the pore structure for preparation of liquefied wood barkbased ACFs. The result might be considered to be remarkable decrease of hydrogen and oxygen composition of liquefied product at the activation temperature range of 800–900 1C [12]. A particularly interesting feature on pore structure was observed with fibers activated at 900 1C, which SBET and Vt were approximately dupled though all of the fibers morphologies were similar in diameter. The results suggested that micropores development occurred intensely and mainly depended on communications or ruptures of intrawalls of micropore members at the activation temperature interval of 800–900 1C.

In this study, a novel way was developed to produce activated carbon fibers by using liquefied wood bark with a steam activation temperature range of 700–900 1C. The key activation temperature should be over 800 1C as a parameter during preparation process of ACFs-WB. The resultant activated carbon fibers exhibited a maximum specific surface area of 1482.88 m2/g with a total pore volume of 0.70 cm3/g. The pore structure characteristics revealed that ACFs-WB were highly porous. Micropore development was dominated with a pore size distribution mainly ranging from 0.4 nm to 0.8 nm in diameter, which showed the potential for application as a carbon molecular sieve.

Acknowledgement The authors thank the state forestry administration of People′s Republic of China for financial support through the 948 project (No. 2013-4-04).

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