- Nano Express
- Open Access
Furfuralcohol Co-Polymerized Urea Formaldehyde Resin-derived N-Doped Microporous Carbon for CO2 Capture
© Liu et al. 2015
- Received: 21 June 2015
- Accepted: 27 July 2015
- Published: 21 August 2015
Carbon-based adsorbent is considered to be one of the most promising adsorbents for CO2 capture form flue gases. In this study, a series of N-doped microporous carbon materials were synthesized from low cost and widely available urea formaldehyde resin co-polymerized with furfuralcohol. These N-doped microporous carbons showed tunable surface area in the range of 416–2273 m2 g−1 with narrow pore size distribution within less than 1 nm and a high density of the basic N functional groups (2.93–13.92 %). Compared with the carbon obtained from urea resin, the addition of furfuralcohol apparently changed the surface chemical composition and pore size distribution, especially ultramicropores as can be deduced from the X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR), and pore size distribution measurements and led to remarkable improvement on CO2 adsorption capacity. At 1 atm, N-doped carbons activated at 600 °C with KOH/UFFC weight ratio of 2 (UFFA-2-600) showed the highest CO2 uptake of 3.76 and 1.57 mmol g−1 at 25 and 75 °C, respectively.
- Microporous carbon
- High N-containing
- Fine micropores
- CO2 capture
Carbon dioxide as the main contributor to global warming has attracted extensive attention worldwide. The reduction or elimination of CO2 emission into the atmosphere is extremely urgent. Therefore, carbon capture and storage (CCS) has been a research hotspot in recent years. Among various technologies for CCS, adsorption is considered to be one of the most promising techniques in practical application due to low energy consumption and mild operating conditions compared to solvent absorption, membrane systems, and cryogenic fractionation . It is possible to reach the goal of the US Department of Energy (DOE) to develop a fossil fuel conversion system capable of capturing 90 % of the produced CO2 that increases the total costs by less than 10 % . However, high adsorption capacity, high CO2/N2 selectivity, and outstanding cycling performance are the key essentials for pressure-swing adsorption or temperature-swing adsorption technology . In this context, zeolites , porous carbons , porous silicas , BCN graphene analogues , porous organic polymers , MOFs [9, 10], and COFs [11, 12] have been studied for CO2 adsorption in recent years. Compared with other materials, carbonaceous adsorbent possesses unique superiority in terms of capture capacity and selectivity due to its hydrophobic property, high thermal and chemical stability, tunable pore structure, low cost, and physical adsorption mechanism .
Recently, researchers [14–16] have shown that the fine micropores and heteroatom incorporation are the decisive factors, which affect adsorption properties. N-doped carbon material with proper pore structure, owing to the promotion of the interaction between CO2 molecules and carbon surface, is considered to be more efficient for CO2 adsorption than traditional carbon such as commercial activated carbon with low capacity of 0.89 mmol g−1 and low adsorptive selectivity . The CO2 uptake of N-doped carbon material can be enhanced effectively so that it can be close to the minimum working capacity required to match the efficiency of a conventional liquid-phase amine-based system . For example, Lu et al.  synthesized N-doped porous carbon monolith with the copolymer of resorcinol, formaldehyde, and lysine as precursor which has shown high CO2 capacity of 3.13 mmol g−1 at 25 °C, 1 atm.
Besides the effect of nitrogen incorporation, the presence of ultramicropores is considered to be another decisive factor. Recently, Presser et al.  studied the CO2 uptake on microporous carbon in relation to the pore size and found that the micropores smaller than 1 nm are responsible for high CO2 adsorption at 1 bar. Jaroniec and Wickramaratne [20, 21] obtained phenolic resin-based carbon with unprecedented amount of CO2 (4.6 mmol g−1 at 23 °C), and their study also showed that a higher capacity resulted from a higher pore volume of fine micropores (<0.8 nm). Zhao et al.  prepared N-doped carbon material using p-diaminobenzene as precursor and found that furfuralcohol can improve the adsorption effectively, but the mechanism is not elaborated clearly. From our point of view, we are trying to find a method for the fabrication of N-doped carbon material with high CO2 capture capacity from widely provided low-cost raw chemicals. In our previous work, we found that urea formaldehyde resin had good ability to produce carbon material with high CO2 capture capacity . Then, we expanded our strategy to urea furfural resin polymerized from urea and furfural, which also showed high CO2 adsorption capacity . However, the carbon source of furfural is not very widely provided as formaldehyde. Here, in this study, for the purpose of further improving the CO2 adsorption capacity, furfuralcohol was chosen to form a copolymer with urea and formaldehyde and treated by the same carbonization-activation process to make porous carbon. The samples co-polymerized with furfuralcohol showed remarkable improvement of CO2 adsorption capacity mainly due to the development of fine micropores, even though they possess comparable specific surface area (about 1000 m2/g) and lower nitrogen content, suggesting that the fine micropores, especially ultramicropores, play a more important role in enhancing the CO2 adsorption capacity of adsorbents.
The samples were prepared by a two-step method. For a typical preparation, 3.0-g urea was added to 4.1 g formaldehyde solution and stirred for 0.5 h, then a certain amount of furfuralcohol (furfuralcohol/urea at a weight of 0, 1, 1.5, and 2) was added to the solution and stirred for 1 h, and then polymerized at 373 K for 24 h. The obtained furfural resin (UFF) was carbonized at 873 K for 4 h under a N2 flow of 50 ml min−1. The carbonized sample (denoted as UFFC) was chemically activated with KOH (KOH/UFFC at a weight ratio of 1 to 4) at several temperatures in the range of 500–800 °C with a ramping rate of 5 °C min−1 under N2 flow of 50 ml min−1 and maintained for 1 h. The obtained material was washed with excess amounts of 1 M HCl aqueous solution followed by deionized water till a neutral pH. Finally, the samples were dried in air at 100 °C for 12 h. The activated carbon was denoted as UFFAa-x-y, where a represents the furfuralcohol/urea weight ratio and be omitted when furfuralcohol/urea weight ratio is 1.5, and x and y represent the KOH/UFFC weight ratio and activation temperature, respectively.
The morphologies of the materials were examined by scanning electron microcopy (SEM) using a HITACHI S-4800 and transmission electron microscopy (TEM) using a JEOL JEM-2100UHR transmission electron microscope. X-ray diffraction patterns were recorded in the range of 2θ = 10°–80° on a PANalytical X’Pert PRO MPD, using a Cu-Kα monochromated radiation source and a Ni filter. The nitrogen adsorption-desorption isotherms were measured at −196 °C using a Micromeritics Tristar 3000 apparatus. Prior to measurement, the samples were degassed in vacuum at 300 °C for at least 3 h. The total specific surface area was calculated using the Brunauer-Emmett-Teller method (p/p o = 0.05–0.25), and the microporous surface area was calculated by the t-Plot method. Narrow micropore distribution was analyzed on Micromeritics ASAP 2020. The micropore distributions were calculated via the Horvath-Kawazoe (HK) method. Fourier transform infrared (FT-IR) spectra were obtained on a Nicolet Fourier spectrophotometer, using the KBr pellet technique. The contents of C, H, and N were determined by using a Vario EL III CHNS/O elemental analyzer. X-ray photoelectron spectroscopy (XPS) measurements were carried out in a UHV system using a monochromated Al Kα radiation (hν = 1486.6 eV) and an Omicron Sphera II hemispherical electron energy analyzer.
CO2 Capture Measurements
CO2 adsorption was performed on Micromeritics Tristar 3000 apparatus at different temperatures (25 and 75 °C). Prior to the analysis, the samples were degassed at 300 °C in vacuum for 4 h. The adsorption capacity was investigated by exposing quantitative amount of CO2 with a concentration of 99.99 % and calculated in terms of adsorbed amount per gram of sample in the pressure range between 0.001 and 1 atm.
Textual properties and chemical composition of various samples
Chemical composition by CHN
V Total a
V Meso b
The narrow pore size distribution of selected samples was further investigated by N2 adsorption at low relative pressure of below 0.03, which is shown in Fig. 2c. It should be noted that the samples carbonized with furfuralcohol and activated in mild conditions generated much more micropores centered at around 0.6 nm than UFA-600. Additionally, the pore size is highly dependent on activated conditions with an obvious enlargement of the pore size. The sample activated at 800 °C possessed enlarged micropores of about 0.9 nm and a broadening of the pore size distribution. This pore enlargement is probably due to the over-activation at higher temperature and is consistent with aforementioned results , as well as the literature reported .
The chemical compositions of the obtained samples are listed in Table 1, and it can be seen that higher nitrogen content was still retained even after activating under severe conditions. In contrast to UFA, the nitrogen content of the UFFA samples with furfuralcohol addition was decreased due to higher proportion of carbon source of the precursor. Interestingly, an increase in the nitrogen content is observed with the increase of the dosage of furfuralcohol, from 7.29 to 8.46 %. One possibility is that KOH prefers etching the fragments carbonized from furfuralcohol to that from urea resin, and the nitrogen atoms are protected during the activation process resulting from different surface charge densities . What is noteworthy is that the insertion of O atoms into the framework occurs during the activation process resulting in the increase of the oxygen content. It is well known that the increase of the activation temperature or the dosage of KOH leads to the release of nitrogen in the form of molecular N2, HCN, or other gases , which is responsible for the decreased of nitrogen content. However, the release of nitrogen reduces the affinity of the samples to CO2 molecule and this can be confirmed by the CO2 adsorption capacity and the isosteric heat of adsorption, which will be elaborated latter.
N-containing groups on the surface of the prepared carbons
-C═NH PhNH2 (%)
CO2 Adsorption Performance
CO2 capture capacities of various samples at 1 atm and different temperatures
CO2 uptake (mmol g−1)
Normalized CO2 adsorption capacities by narrow micropore volume and nitrogen content
Normalized CO2 uptake at 25 °C, 1 atm
Uptake per micropore volume (mmol CO2 cm−3)
Uptake per N content (mmol CO2 (mmol N)−1)
In summary, we have demonstrated a feasible and very simple method for the synthesis of N-doped microporous carbon via a carbonization-activation process by using urea formaldehyde resin co-polymerized with furfuralcohol, which showed tunable surface area and high nitrogen content. N-doped microporous carbon with a high surface area of about 1093 m2 g−1 and a high density of N functional groups (8.03 %) was obtained. Furfuralcohol plays an important role in improving the pore structure of the adsorbent. More ultramicropores of about 0.6 nm were formed in the above microporous carbon, which is a decisive factor for the CO2 adsorption uptake. We also found that much more nitrogen species in the carbonized samples are in the form of nitrogen-containing heterocyclic compounds. The obtained samples showed excellent CO2 adsorption uptake in the range of 1.8–3.76 mmol g−1 and 1.00–1.57 mmol g−1 at 25 and 75 °C, respectively, confirming the combination of physical and chemical adsorption mechanism on CO2 adsorption capacity improvement. In addition, this reaction system can be further improved using other carbon sources.
This work was supported by the National Science Foundation China (U1362202 and 21206196), Innovation Foundation of CNPC (2013D-5006-0404 and 2014D-5006-0404), and China Scholarship Council (CSC No. 201406455026).
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