Enhancement of CO2 adsorption on phenolic resin-based mesoporous carbons by KOH activation
Introduction
Nowadays the scientific community faces a great research challenge in controlling the greenhouse effect. A gradual reduction of CO2 emission, improvement of energy efficiency, and development of renewable and/or nuclear energy sources are ones of the most important research goals. CO2 is one of the greenhouse gases and hence, it is directly responsible for global warming [1], [2]. The International Panel on Climate Change (IPCC) estimated the CO2 levels would range from 10 Gt to ∼250 Gt of CO2 by year 2100 [3]; this estimation is based on the predicted population, production, and energy use. Currently, fossil fuels serve as the main energy source worldwide because these fuels are convenient due to low cost, high energy density, availability, and feasible technology and infrastructure for energy production [4]; however, their usage accounts for ca. 75% of the current anthropogenic CO2 emissions [5]. The IPCC [6] recommended carbon capture and storage (CCS) as a three-stage strategy for reducing CO2 emissions, which involves: (i) separation, (ii) transportation, and (iii) storage of CO2. The two latter steps have been already developed; however, CO2 capture is still unsolved issue. This step accounts for ca. two thirds of the total cost, which prevents large-scale application of CCS [7]. Thus, intensive studies have been carried out worldwide to improve the existing technologies and/or develop new ones for CO2 capture.
Several techniques for CO2 separation have been developed, for instance, adsorption on solids, absorption in liquid solutions, cryogenic separation, and membrane separation [8]. Among them, adsorption on solids is one of the most promising processes because it assures high adsorption capacity, low regeneration energy (can be accomplished by pressure or temperature-programmed desorption), low cost, straightforward design, operation, and scale up. Various porous materials such as zeolites [9], metal–organic frameworks [10], functionalized mesoporous materials [11], and mesoporous silica/carbon composites [12] have been widely studied for CO2 capture and showed acceptable adsorption capacities. Among them microporous carbons were shown to have high specific surface area, well-developed porosity, tunable pore structures, and remarkable thermal and mechanical stability. For instance, Sevilla and Fuertes examined activated carbons, obtained by hard templating, as CO2 adsorbents and reported adsorption capacities of 3.1 and 5.6 mmol g−1 at 25 and 0 °C, respectively [13]. Choma et al. [14] reported that carbons with uniformly distributed mesopores and additional micropores are well suited for adsorption of small molecules. While micropores enlarge the specific surface area and enhance interactions of gas molecules with the carbon surface, mesopores facilitate diffusion and mass transfer processes; also, they are well suited for adsorption of larger molecules [15], [16].
In this work, we report CO2 adsorption on KOH-activated phenolic resin-based mesoporous carbons with high surface area and large volume of ultramicropores (pores below 0.7 nm). Also, this work shows the effect of phenolic resin precursor, resorcinol or phloroglucinol, on the susceptibility of carbonized phenolic resins on KOH activation. Importantly, by adjusting the KOH activation conditions it was possible to obtain carbons with large fraction of ultramicropores and to enlarge the CO2 uptake. Note that so far the main interest was in explaining the effect of high surface area and micropore volume on the CO2 uptake; however this study shows the importance of ultramicropores in CO2 adsorption at ambient conditions. First, mesoporous carbons were synthesized by soft-templating method using triblock copolymer Pluronic F127 as a structure directing agent and formaldehyde and either phloroglucinol or resorcinol as carbon precursors. Subsequently, post-synthesis activation of the resulting carbons was used to develop additional microporosity. Since these carbons possessed a well-developed mesoporosity, they were more susceptible to KOH activation. The activated carbons showed 2–3-fold increase in the specific surface area and 3–5-fold increase in the volume of micropores. The aforementioned activation of the mesoporous carbons studied resulted in enhanced CO2 adsorption at 760 mmHg pressure: 4.4 mmol g−1 at 25 °C, and 7 mmol g−1 at 0 °C. High CO2 uptake and good cyclability (without noticeable loss in the CO2 uptake after 5 runs) render these carbons as efficient CO2 adsorbents.
Section snippets
Chemicals
Pluronic F127 triblock copolymer (EO106 PO70 EO106, Mw = 12600 Da) was provided by BASF Corp. (Florham Park, New Jersey, USA). Resorcinol (C6H4(OH)2, 98%), phloroglucinol (C6H3(OH)3, 99%), and formaldehyde (HCHO, 37 wt% solution in water, stabilized with 10–15% methanol) were purchased from Acros Organics (Geel, New Jersey, USA). Hydrochloric acid (HCl, 35–38 wt% in water) was purchased from Fischer Scientific (Fair Lawn, New Jersey, USA). Technical grade ethanol (95%), ACS grade potassium hydroxide
Nitrogen adsorption
Low-temperature nitrogen physisorption was used to characterize the specific surface area and porosity of the carbons studied. Fig. 1 shows nitrogen adsorption–desorption isotherms and incremental DFT pore size distributions for the carbon materials prepared using phloroglucinol and resorcinol as precursors. Although the type of the carbon precursor used had a minor effect on nitrogen adsorption properties (see Fig. 1), the phloroglucinol-based samples seem to be more susceptible to
Conclusions
In summary, carbons materials synthesized by using either phloroglucinol or resorcinol as carbon precursors show analogous adsorption properties, although resorcinol-based carbons are more susceptible to KOH activation. The latter, if performed under controlled conditions, represents an efficient way for increasing the amount of ultramicropores in phenolic resin-based carbons, which are primary responsible for CO2 uptake at low pressures. The carbon materials studied show potential to be reused
Acknowledgments
This work was supported by CAPES Foundation, Ministry of Education of Brazil, fellowship PDSE-Sandwich – Process n° 1506-12-9.
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