Abstract
To acquire activated carbon fiber from phenol-liquefied wood (PLWACF) with better developed pore structure and a high proportion of mesoporosity, the porosity evolution of PLWACF activated at temperatures from 850 to 950 °C by water steam was detected by the physical adsorption of N2 at -196 °C. Results showed that the pore structure was well developed by prolonging the activation time at 850 to 910 °C, and it was easy to obtain PLWACF having exceptionally high surface area (larger than 2560 m2 g-1). However, PLWACF with a specific surface area larger than 3000 m2 g-1 could only be obtained in the late activation stages from 850 to 880 °C. Using this activation process, the mesoporosity was remarkably developed. The mesopore proportion drastically increased with an increase in activation temperature or time, reaching a maximum of 49.5%. The pore size distribution widened as the activation time increased and appeared to accelerate with the use of a higher activation temperature. The mesopore size distribution was enlarged from 2.8 to 5.8 nm.
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Porosity Evolution of Activated Carbon Fiber Prepared from Liquefied Wood. Part II: Water Steam Activation from 850 to 950 °C
Zhi Jin and Guangjie Zhao*
To acquire activated carbon fiber from phenol-liquefied wood (PLWACF) with better developed pore structure and a high proportion of mesoporosity, the porosity evolution of PLWACF activated at temperatures from 850 to 950 °C by water steam was detected by the physical adsorption of N2 at -196 °C. Results showed that the pore structure was well developed by prolonging the activation time at 850 to 910 °C, and it was easy to obtain PLWACF having exceptionally high surface area (larger than 2560 m2 g-1). However, PLWACF with a specific surface area larger than 3000 m2 g-1 could only be obtained in the late activation stages from 850 to 880 °C. Using this activation process, the mesoporosity was remarkably developed. The mesopore proportion drastically increased with an increase in activation temperature or time, reaching a maximum of 49.5%. The pore size distribution widened as the activation time increased and appeared to accelerate with the use of a higher activation temperature. The mesopore size distribution was enlarged from 2.8 to 5.8 nm.
Keywords: Liquefied wood; Activated carbon fibers; Activation temperature; Porosity evolution; Mesoporosity
Contact information: College of Material Science and Technology, Beijing Forestry University, Beijing 100083, China; *Corresponding author: zhaows@bjfu.edu.cn
INTRODUCTION
Activated carbon fiber (ACF) has attracted the attention of chemists and material scientists due to a commercial interest in applications of chemical separation and heterogeneous catalysis; there is also interest in its use as an energy storage material because of its well-developed pore structure (Suzuki 1994; Huang et al. 2008; Im et al. 2009; Xu et al. 2010). The importance of mesoporosity in ACF has also been recognized. ACFs that have a high amount of mesopores are believed to be superior for constructing electric double layer capacitors (Xu et al. 2010) and can also aid in the adsorption of large molecules such as hydrogen sulfide (Bashkova et al. 2007). Several approaches have been examined to enhance the mesopore region in ACF, e.g., catalysts or polymer additives that are added to the carbon source can increase the pore size distribution (PSD) in ACF (Oya et al. 1995; Ozaki et al. 1997). However, there are some disadvantages, such as complex processes, costly additives, and ionic impurity.
In the ACF preparation procedure, the activation process plays an important role in determining the porosity of ACF. The partial gasification of the precursor fiber with oxidant gases (such as water steam, carbon dioxide, oxygen, or air) is known as the gas activation method. During the gas activation process, the activation temperature greatly affects the gasification rate, and this influences the porosity development in ACF (Rodrı́guez-Reinoso et al. 2000). Generally, an increase in the activation temperature produces a larger specific surface area and enhances the total pore volume (Carrott et al.2001; Park and Kim 2001). For example, by carbonization and water steam activation, the Brunauer–Emmett–Teller (BET) specific surface area (SBET) and total pore volume (Vt) of phenolic resin-based ACFs were increased from 628 to 1740 m2 g-1 and 0.346 to 0.937 cm3 g-1, respectively, with an increase in activation temperature from 750 to 900 °C at an activation time of 30 min. For viscose rayon-based ACF, these quantities were enhanced from 1310 to 2280 m2 g-1 and from 0.694 to 1.40 cm3 g-1, respectively, by increasing the activation temperature from 850 to 900 °C, with an activation time of 60 min (Naik et al. 2011). Conclusively, activating ACF at a higher activation temperature can lead to a reduction in the microporosity and a higher proportion of wide micropores and mesopores, which contributes to the surface area, thus compensating for the decrease in surface area caused by the loss of narrow microporosity (Berger et al. 1976; Yang and Yu 1998).
Recently, ACF from PLW has been prepared (Liu and Zhao 2012; Jin and Zhao 2014; Ma et al. 2014). Part I of this work has shown that the enhancements in the SBET and Vt of PLWACF activated with water steam is more dependent on the activation time as the activation temperature increases from 650 to 800 °C. The development of the mesoporosity becomes noteworthy only by activation at 800 °C for longer than 100 min, with the mesopore volume reaching a maximum of 0.545 cm3 g-1 (Jin and Zhao 2014). It is thought that PLWACF with larger SBET and Vt as well as a higher proportion of mesoporosity can be obtained by a series of activation processes with water steam at a temperature higher than 800 °C.
To acquire PLWACF with better and more developed pore structure as well as a higher proportion of mesoporosity by only controlling the activation temperature and the activation time, we attempted to study the porosity evolution of PLWACF with water steam at activation temperatures ranging from 850 to 950 °C in this study.
EXPERIMENTAL
Materials
The preparation of phenol-liquefied wood (PLW), synthesis of the PLW spinning solution, and preparation of PLW precursor fibers (the dried fiber after curing and washing with deionized water, which was ready to be carbonized and then activated) proceeded as mentioned in Part I of this work (Jin and Zhao 2014). Aliquots of approximately 5 g of the PLW precursor fiber were each heated at a constant rate in a nitrogen stream to 850, 880, 910, and 950 °C and then activated at the correspondingactivation temperature for different activation times by introducing water steam mixed with a nitrogen stream. Finally, they were cooled to room temperature in a nitrogen stream. The PLWACF samples were labeled as “activation temperature (°C)-activation time (min)”. For example, 850-20 corresponds to PLWACF prepared by an activation stage at 850 °C for 20 min.
Methods
The burn-off, which is defined as the difference between the mass before and after activation, was calculated as follows
Burn-off = (w0–w1)/w0 × 100% (1)
where w0 and w1 are the mass of the carbonized fiber before and after activation, respectively.
Nitrogen adsorption/desorption isotherms were collected at -196 °C by an automatic adsorption system (Autosorb-iQ, Quantachrome, USA) to study the porous characteristics of the prepared PLWACF. Before each measurement, the samples were degassed at 300 °C for 3 h. SBET was then calculated using the BET method (Brunauer et al. 1938). The micropore specific surface area was determined using the t-plot micropore analysis method (Deboer et al. 1966). Vt was calculated from the nitrogen volume adsorbed at the maximum value of relative pressure, P/P0. The micropore volume (Vmi) and mesopore volume (Vme) were determined by using quenched solid density functional theory (QSDFT) method (Lastoskie et al. 1993). The PSDs in the micropore and mesopore region were determined using Horvath–Kawazoe (HK) method and the Barrett-Joyner-Halenda (BJH) method, respectively. All samples were examined three times, and the obtained results were within the accepted error range of 5%.
RESULTS AND DISCUSSION
Changes in Burn-off
Figure 1 shows the variations in burn-off during the activation at 850 to 950 °C as a function of activation time. As can be seen, the burn-off value of PLWACF continuously increased with prolonged activation time or increased activation temperature, ranging from 15% to 95%. The burn-off values for samples 850-220, 880-140, 910-100, and 950-60 were all almost equal, reaching a maximum of almost 95%. Volatile matter emitted when the carbon source is subject to attacks from the activator (water steam) is more easily released from the carbon matrix with the augment of activation temperature from 850 to 950 °C (Carrott et al. 2001).
Fig. 1. Changes in burn-off of PLWACF
Pore Structure Evolution
Figure 2 describes the typical N2-adsorption/desorption isotherms for PLWACF. All of the adsorption isotherms were of type-I according to the IUPAC classification, which is consistent with micropore absorption. When activated at temperatures from 850 to 950 °C, the knees of the isotherms become wider with increasing activation time, which suggests the formation of larger pores (Lee et al. 2003). The nitrogen adsorption capacity increases with an increase in activation time; this shows the development of porosity. Furthermore, a higher activation temperature appears to promote an increase in nitrogen adsorption capacity with increasing activation time.
Fig. 2. The nitrogen adsorption/desorption isotherms at -196 °C for the prepared PLWACF
The pore characteristics of all the prepared PLWACF are listed in Table 1. As shown, the SBET and Vtall increased with increasing activation temperature or activation time. As the activation temperature increased, the SBET and Vt progressively increased from 1647 to 2833 m2 g-1 and from 0.775 to 1.589 cm3 g-1, respectively, when activated for 60 min. With activation at 880 °C for 140 min, the SBET and Vt reached maximum at 3461 m2 g-1 and 1.925 cm3 g-1, respectively. Typically, ACFs with SBETlarger than 2560 m2 g-1 are regarded as exceptionally high-surface area materials, so-called “super ACFs”, which are beneficial to applications such as gas storage and supercapacitors (Pandolfo and Hollenkamp 2006; Wang et al. 2008). Compared to the results from Part I of this work, water steam activation from 850 to 950 °C more effectively enhanced the SBET than that at 650 to 800 °C, by which the maximum SBET only reached 2410 m2 g-1 (Jin and Zhao 2014). Moreover, it is interesting to see that when compared with 910-100 and 950-60, 850-140, and 880-100 showed similar SBET values but possessed lower burn-off values. Meanwhile, in the case of 850-180, 850-220, and 880-140, they exhibited high SBET and large Vt values, even though their burn-off values were consistent with those of 910-100 and 950-60. When activated at 850 to 880 °C, the gasification of the carbon matrix and the diffusion of the activator to the inside of the carbon matrix proceed at a suitable rate, making the extent of the activation process more uniform, while activations at temperatures that are too high (910 to 950 °C) lead to a sintering effect, which causes a recession in the pore structure development (Okada et al.2003).
The VHK and VBJH were all enhanced with the increase in the activation temperature or time. The VHKincreased gradually with increasing activation temperature at the short activation time of 20 min from 0.478 to 0.862 cm3 g-1. When the activation time was prolonged to more than 60 min, lower activation temperatures at 850 to 880 °C favored the enhancement in VHK, reaching the maximum at 1.306 cm3g-1 in the case of sample 880-140. The increase in VBJH with activation time appeared to accelerate at a higher activation temperature. As an increase of activation time from 20 min to 60 min, the value of VBJH was increased by 1.53 times at 950 °C compared with 22% at 850 °C. The maximum of VBJHwas 0.350 cm3 g-1 in the case of sample 850-220. Thus, it is sure that the pore formation of the prepared PLWACF is dependent on the development in the microporosity as well as mesoporosity simultaneously.
Table 1. Pore Structure Parameters of All the Prepared PLWACF
VHK: micropore volume calculated by HK method;
VBJH: mesopore volume calculated by BJH method
It should be noted that with the increase of activation temperature or time, the values of Vme/Vt and Vme/Vmi all were drastically increased, while the value of Vmi/Vt was decreased, exhibiting an opposite trend (Fig. 3). Thus, there was clearly a more remarkable development of mesoporosity than of microporosity during the activation process, which was probably a result of the formation of mesopores by the burn-off of walls between adjacent micropores. At the late stage of these four activation processes (with the burn-off larger than 90% as shown in Fig. 1), Smi decreased and the values of Vme/Vmi exceeded 1. Eventually, Vme/Vt reached the maximum of 49.5% compared with the Vmi/Vt of 42.7%, which confirms the predominant role of mesoporosity in PLWACF.
Fig. 3. The evolution of ratios of Vmi/Vt, Vme/Vt, and Vme /Vmi of the prepared PLWACF
Micropore size distribution evolution of PLWACF obtained by means of HK method is shown in Fig. 4.
Fig. 4. PSDs obtained by HK method for the prepared PLWACFs
It can be seen that the micropore size distribution was widened in the whole micropore range (0.3 to 2.0 nm) as the activation was proceeding. The PSD maxima calculated by HK method (DHK) are plotted in Fig. 5.
Fig. 5. The evolution of PSD maximum calculated by HK method of the prepared PLWACF
Fig. 6. PSDs obtained by BJH method for the prepared PLWACFs
The value of DHK was progressively increased with the increase of activation temperature or time, showing that the development in the micropore size distribution took place by pore widening in a gradual way as the activation time increased. When activated for 20 min, the DHK was widened from 0.462 nm to 0.552 nm as the activation temperature increased. Moreover, the increase in the DHKbecame smooth with increasing activation time, and the maximum DHK approached to around 0.580 nm at the late stage of activation at 850 to 950 °C. From this it may be concluded that the enlargement in the micropore size distribution tends to stabilize.
Figure 6 depicts the mesopore size distribution of PLWACF by applying the BJH method, which showed a gradual enlargement of the mesoporosity in the 2.8 to 5.8 nm interval. In the case of samples 850-220, 880-140, 910-100, and 950-60, significant enlargements were observed. However, their DHKvalues were almost identical (approximately 0.580 nm as shown in Fig. 5). Consequently, these widened mesopores primarily came from the corrosion of the existing smaller pores, which are apparently independent of the development of new pores. It is possible that significant surface erosion and carbon structure shrinkage at 850 to 950 °C made the fiber too thin to sustain the formation of new pores with increasing activation time.
CONCLUSIONS
The porosity evolution of phenol liquefied wood-based activated carbon fiber (PLWACF) with water steam activation at 850 to 950 °C was examined.
- SBET and Vt increase with increasing activation time at temperatures ranging from 850 to 950 °C. PLWACFs with SBET greater than 3000 m2 g-1 can only be obtained by activation at 850 °C for 180 min and at 880 °C for 140 min, possibly because the gasification of the carbon matrix and the diffusion of the activator proceeded at a suitable rate.
- The pore formation of the prepared PLWACF is dependent on the development in the microporosity and mesoporosity simultaneously. The amount of microporosity development is relatively mild, and activations at lower temperatures (850 to 880 °C) for longer than 60 min are in favor of larger micropore volume. The mesopore ratio drastically increases as the activation time or activation temperature is increased, showing a remarkable amount of mesoporosity development. At the late stage of activation (with the burn-off larger than 90%), the mesoporosity plays a predominant role, which is evidenced by a significant decrease in the micropore specific surface area and a mesopore volume that exceeds the micropore volume.
- For all activation temperatures, the widening of the pore size distribution progressively takes place as the activation time increases, which appears to accelerate with the enhancement of the activation temperature. Typically, the mesopore size distribution is enlarged from 2.8 to 5.8 nm. Widened mesopores form during the whole activation process, primarily owing to the corrosion of existing smaller.
ACKNOWLEDGMENTS
The authors are grateful for the support from the Specialized Research Fund for the Doctoral Program of Higher Education, Grant. No. 20130014130001.
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Article submitted: May 4, 2014; Peer review completed: August 1, 2014; Revised version received and accepted: August 21, 2014; Published: September 26, 2014.