Chinese Chemical Letters  2014, Vol.25 Issue (06):929-932   PDF    
Activated carbon produced from paulownia sawdust for high-performance CO2 sorbents
Xiao-Li Zhua,b, Pei-Yu Wangaa, Chao Penga, Juan Yanga, Xing-Bin Yana     
* Corresponding authors at:a Laboratory of Clean Energy Chemistry and Materials, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, China;
b University of Chinese Academy of Sciences, Beijing 100039, China
Abstract: In this paper, activated carbons (ACs) with high specific surface areas were successfully synthesized by simple one-step carbonization-activation from paulownia sawdust biomass, and the effects of the synthetic conditions on their CO2 capture capacity were investigated as well. The results show that, when the mass ratio between activator and biomass is 4, the activation temperature is 700℃ and the activation time is 1 h, as-made AC provides the most micropores for CO2 adsorption. As a consequence, the maximum CO2 uptake of 8.0 mmol/g is obtained at 0℃ and 1 bar.
Key words: Biomass     Activated carbon     CO2 absorption     KOH activation    

1. Introduction

Carbon dioxide is a major green house gas emitted by the combustion of fossil fuels (coal,oil and natural gas) in industrial plants [1, 2]. Therefore the control of anthropogenic CO2 emissions is a crucial matter for its significant role in global climate change. In recent years,much research has been directed toward the development of new technologies for CO2 capture and its storage [3]. For the capture of CO2,traditional technologies mainly include absorption and adsorption-coupled membrane separation. Adsorption processes using novel solid sorbents capable of reversibly capturing CO2 have many potential advantages,such as reduced energy for regeneration,great capacity,excellent selectivity,ease operability,etc. [4]. Up to now,several kinds of solid adsorbents have been investigated,which include carbonaceous adsorbents [5, 6],zeolites and mesoporous silicates [7, 8],organic solids [9],and metal-organic frameworks [10, 11]. Among them,carbonaceous adsorbents,especially porous activated carbons (ACs),have already been highlighted owing to their high specific surface area, excellent thermal and chemical stability,easy-to-design pore structure,and low energy requirements for regeneration [12, 13, 14]. It is well known that ACs can be prepared by pyrolysis from a wide range of different carbon-containing source materials [15]. So far, many biomass-derived ACs,such as from bamboo [14],sawdust [16],algae [17],wood [18],celtuce leaves [19],carpet [20], anthracites [21],etc.,have been prepared by carbonization and activation for CO2 adsorption. However,it should be noted that the preparation of many of the biomass-derived ACs involves two-step processes consisting of carbonization and subsequent activation, and they still display the unsatisfied CO2 absorption capacity (normally lower than 6.0 mmol/g) at 0 ℃ [16].

In this work,porous ACs with high specific surface areas and abundant micropores were prepared using porous paulownia sawdust as the carbon precursor via a simple one-step carbonization- activation. Also,the effects of the preparation parameters (the mass ratio between KOH activator and paulownia sawdust,the activation temperature and the activation time) on the porous structures of as-made ACs and the corresponding CO2 absorption capacity were investigated. The results show that the AC prepared at the optimized conditions (the mass ratio is 4,the activation temperature is 700 ℃ at the activation time is 1 h) has the most micropores,enabling the maximum CO2 uptake of 8.0 mmol/g at 0 ℃ and 1 bar. However,the highest CO2 adsorption capacities reported for activated carbon under ambient conditions is 8.9 mmol g at 0 ℃ [22].

2. Experimental

The paulownia sawdust,collected from a local sawmill of Sichuan Province,firstly underwent pre-oxidation in a muffle furnace at 250 ℃ for 2 h. Afterwards,the pre-oxidized paulownia sawdust was impregnated in a KOH aqueous solution with the predetermined KOH/C mass ratios,and the mixture was dried at 80 ℃ for 12 h. The dry mixture was placed in a tubular furnace, followed by heating up to the predetermined activation temperature at a ramp of 5 ℃/min,then held at this activation temperature for a different time. The heating process was conducted under N2 flow protection. Finally,the cooled AC powder was washed with 1 mol/L HCl solution and ultrapure water until its pH reached 7. In this study,the KOH ratio was set at 2:1,4:1,and 6:1 and the activation temperature was set at 600,700,800,and 900 ℃,and the activation time was 0.5,1,and 2 h. The paulownia sawdustderived ACs is denoted as AS-X-Y-Z,where X represents the KOH/C mass ratio,Y denotes the activation temperature,and Z denotes the activation time.

The morphologies of the activated paulownia sawdust biomass and the as-made ACs were investigated using a field emission scanning electron microscope (FESEM; JSM 6701F). To investigate the structure and composition of the samples,powder X-ray diffraction (XRD; Rigaku D/Max-2400) was performed using Cu Ka radiation. To measure the porous structure of as-made ACs,the standard nitrogen adsorption-desorption isotherm measurements were performed on a Micromeritics ASAP 2020 M volumetric adsorption analyzer at -196 ℃. Prior to each measurement,the sample was degassed at 200 ℃ for 6 h in vacuum to clean its surface. The specific surface area was calculated with the Brunauer-Emmett-Teller (BET) equation from N2 adsorption data in the relative pressure (P/P0) (where P is the measured pressure and P0 is the saturated vapor pressure of the adsorbed fluid) range of 0.01-0.3. The pore-size distribution was determined by a nonlocal density functional method using the nitrogen adsorption data,and assuming a slit pore model. The total pore volume was estimated from the amount adsorbed at a relative pressure of P/P0 = 0.99. Micropore volume (pore size <2 nm) was obtained via a t-plot analysis.

To evaluate the CO2 capture ability,the CO2 adsorption isotherms of the samples were measured using a Micromeritics ASAP 2020 static volumetric analyzer at 0 ℃ and 25 ℃. Prior to each adsorption experiment,the sample was degassed for 4 h at 200 ℃ to ensure that the residual pressure was below 1 × 10-3 mbar. After the sample was cooled down to 0 ℃ or 25 ℃,CO2 was introduced into the system. The CO2 adsorption capacity in terms of the adsorbed volume under standard temperature and pressure was then recorded. The recycling adsorption test of CO2 was performed with a simple regeneration by evacuating at 200 ℃ for 4 h under a pressure of 1 × 10-3 mbar.

3. Results and discussion 3.1. Characterizations of ACs

Paulownia sawdust is an ideal precursor for preparing porous AC because of its low price,great availability,and high biomass yield. As shown in Fig. 1(a),the pristine paulownia sawdust exhibits a bundle structure and large pores. When it was converted into AC (AS-4-700-1),the as-obtained product has a cellular texture and a highly porous structure. As shown in Fig. 1(b),after the chemical activation,irregular pores are formed in the AC retaining no memory of the structure of the pristine paulownia sawdust. The natural properties of sawdust and soft components which are easily etched by KOH resulted in a large number of pores [19]. In addition,the XRD pattern of AS-4-700-1 just shows two broad diffraction peaks at 228 and 438,which correspond to the (0 0 2) and (1 0 0) diffraction patterns (Fig. 1(c)). It is a typical amorphous graphitic carbon.

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Fig. 1.(a) SEM images of pristine paulownia sawdust,and the characterization results of paulownia sawdust-derived AC (sample AS-4-700-1): (b) SEM image,(c) XRD pattern and (d) the N2 sorption isotherms and the corresponding pore size distribution.

The pore textural properties of the activated carbons were analyzed by means of nitrogen physisorption. Fig. 1(d) presents the N2 sorption isotherm of AS-4-700-1 and its corresponding pore size distribution (inset). The N2 sorption isotherm the adsorbed volume significantly increased at low relative pressure. This indicates that the activation conditions developed micropores,as previously reported [14, 20]. The ratio of micropore volume to total volume is about 73%. The characters of pore distribution clearly show that the main peak is centered at around 0.8 nm and most pores are less than 3 nm.

3.2. Influence of activation parameters on CO2 adsorption

In our synthesis,paulomnia sawdust was treated at 80 ℃ for 12 h in an oven to remove excess moisture in the raw materials before activation. As we know,the KOH/C mass ratio,activation temperature,and activation time have significant effects on ACs for CO2 adsorption [14]. In this study,when the activation ratio was fixed at 4 and the activation time was fixed at 1 h,the activated carbon prepared at 700 ℃ (denoted as AC-4-700-1) had the highest adsorption capacity (up to 8.0 mmol/g,asshownin Fig. 2) compared with ACs prepared with other temperatures (600,800 and 900 ℃).

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Fig. 2.CO2 adsorption isotherms at 0 ℃ and 1 bar for ACs prepared at different activation temperatures. Inset shows the relationship between the activation temperature and the maximum CO2 adsorption capacity.

Afterwards,the effect of activation time on the CO2 adsorption capacity of ACs prepared with the fixed KOH/C mass ratio of 4 and the fixed activation temperature of 700 ℃ was studied. As shown in Fig. 3,AC prepared with the activation time of 1 h shows the largest micropore volume and the maximum value of CO2 adsorption compared to the other two ACs.

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Fig. 3.CO2 adsorption isotherms at 0 ℃ and 1 bar for ACs prepared with different activation times. Inset shows the relationship between the activation time and the maximum CO2 adsorption capacity.

Similarly,the effect of the mass ratio between KOH activator and biomass (dry paulownia sawdust) on the CO2 adsorption capacity was investigated as well. As shown in Fig. 4,when the activation temperature and time were fixed at 700 ℃ and 1 h,the amount of CO2 adsorbed on as-prepared ACs first increases and then decreases with the increase of the mass ratio,and the maximum value is achieved at the mass ratio of 4.

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Fig. 4.CO2 adsorption isotherms at 0 ℃ and 1 bar for ACs prepared with different

As shown in Table 1,all activated carbons show high surface areas in the range of~782-2435 m2/g and large total pore volumes in the range of ~0.414-1.181 cm3/g. Among them,the sample of AS-4-700-1 has the highest micropore volume (~0.625 cm3/g) which occupies ~73% of the total pore volume (~0.857 cm3/g). Thus,we believe that the highest capture capacity for the AS-4- 700-1 adsorbent might be owing to its having the most micropore volume.

Table 1
The preparation parameters,porous characteristics and CO2 adsorption capacity of as-made ACs.

The CO2 adsorption isotherms of the AS-4-700-1 sample at 0 and 25 ℃ are shown in Fig. 5. As the curves show,at ambient pressure,the CO2 adsorption capacity is 8.0 mmol/g at 0 ℃ and 4.8 mmol/g at 25 ℃. This maximum value of CO2 adsorption capacity (8.0 mmol/g) is higher than other biomass-derived ACs, such as bamboo [14],sawdust [16],algae [17],wood [18],celtuce leaves [19],carpet [20],and anthracites [21].

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Fig. 5.CO2 adsorption isotherms of the AS-4-700-1 sample at 0 ℃ and 25 ℃ and at 1 bar.

4. Conclusion

In summary,ACs with high surface areas and abundant micropores have been successfully produced from paulownia sawdust through high-temperature carbonization-activation with the KOH. Among them,the AS-4-700-1 product,which was prepared with the optimal mass ratio between KOH and paulownia sawdust (4:1),activation temperature (700 ℃),and activation time (1 h),exhibits the highest CO2 capture capacity of up to 8.0 mmol/g at 0 ℃ and 1 bar. Considering the recyclability of paulownia sawdust and the excellent CO2 capture performance of the asprepared ACs,we believe that our approach can prompt the use of paulownia sawdust for synthesizing high-performance ACs for CO2 capture.

Acknowledgments

The authors acknowledge the support from the Top Hundred Talents Program of Chinese Academy of Sciences and the National Natural Science Foundation of China (No. 51002161).

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