Development of microporous carbons for CO2 capture by KOH activation of African palm shells
Graphical abstract
Introduction
The growing interest in the capture and/or conversion of greenhouse gases requires an intensification of research on the development of inexpensive porous materials, including highly microporous activated carbons. Among technologies proposed for reduction of CO2 emissions, adsorption is considered as a very promising process for CO2 capture [1], [2], [3], [4], [5], [6]. The effectiveness of this process is enhanced by developing adsorbents with high surface area and properly developed microporosity. There are numerous reports on the use of carbon-based adsorbents for CO2 capture [7], [8], [9], [10], [11], [12]. Among these materials, palm shells attracted a noticeable attention because the Ivory Coast is the fifth largest producer of palm oil, which generates a large amount of waste. The use of palm shells to produce activated carbons is of great interest because of availability of palm shells source, which is an abundant byproduct of palm oil production. These carbons can be easily obtained by carbonization of palm shells followed by physical or chemical activation. Usually physical activation is carried out using carbon dioxide, steam, air or their mixture [13]. Chemical activation involves various agents such as zinc chloride [11], [14], acids [11] and bases [8], [9], [10], [12]. Adsorption performance of activated carbons depends on the pore structure and surface properties. Activated carbons obtained by chemical activation possess often high surface area and well developed micropores, which make them attractive materials for CO2 adsorption [15], [16]. Three kinds of coconut shells-based activated carbons have been investigated so far [17]. It was reported that these activated carbons exhibited high CO2 adsorption capacity, ∼2.55 mmol/g at 200 kPa. Plaza et al. [10] used coffee ground to produce activated carbon, which at 101 kPa reached the maximum CO2 uptake of about 3.0 and 4.7 mmol/g at 25 and 0 °C, respectively. Fungi-based carbon obtained by KOH activation showed CO2 uptake of 5.5 mmol/g at 0 °C [8]. Vargas et al. [11] reported monolithic carbons from African palm stones, activated with H2PO4, ZnCl2, and CaCl2, which adsorbed 2.59 mmol/g (114 mg/g) and 5.7 mmol/g (254 mg/g) of CO2 at atmospheric pressure and 0 °C. Sevilla and Fuertes [9] obtained porous carbon by hydrothermal KOH activation of polysaccharides and biomass; the resulting carbon achieved CO2 uptake of 4.8 mmol/g at 25 °C and 6 mmol/g at 0 °C. Activated carbon fibers have been investigated in order to find the optimal pore size for CO2 adsorption [18]. The highest CO2 adsorption capacity was 5.6 mmol/g (250 mg/g) at 25 °C and 1 bar. Potassium hydroxide (KOH) is one of the most efficient activation agents for preparation of highly porous carbons. The high surface area and micropore volume are essential for capture of CO2. The aim of this study is to develop high surface area microporous carbons by controlled KOH activation of African palm shells from Ivory Coast and test them as adsorbents for CO2 at ambient conditions.
Section snippets
Materials and methods
A series of carbon materials was prepared from Africa palm shells by KOH chemical activation. Palm shells waste was dried in an oven at 120 °C for 24 h to remove moisture. A two-step method was applied: carbonization and activation. In carbonization, the dried palm shells were placed in a tubular furnace and heated at different temperatures, 600 °C and 700 °C in N2 atmosphere for 1 h. The carbonized char was mixed with a desired amount of KOH in a ceramic boat to achieve the weight ratio of KOH to
Results and discussion
Fig. 1 shows the pyrolysis of dried African palm shells in flowing N2 using a heating rate of 10 °C/min. The TG and DTG profiles are presented up to 800 °C. These profiles show two major weight losses about 230 and 550 °C, corresponding to the removal of volatile compounds from thermally decomposed char. According to Cagnon et al. [21] several thermal events were reported for carbonization of biomass; the first weight loss at about 230 °C was attributed to the decomposition of hemicellulose, and
Conclusions
Carbons with high volume of ultramicropores were obtained by carbonization and KOH activation of African palm shells. The resulting materials featured high specific surface area (up to 1890 m2/g) and large pore volume (up to 0.90 cm3/g), which can be tailored by varying the KOH/char ratio during activation process. The aforementioned activation significantly improves adsorption properties of these carbons, especially the volume of ultramicropores, which is essential for enlarging the CO2 uptake
Acknowledgements
The financial support for this research was provided by the Islamic Bank of Development. ASE thanks the Department of Chemistry and Biochemistry at the Kent State University for accepting me as a visiting scientist.
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