Article pubs.acs.org/EF
Enhanced Tolerance to Flue Gas Contaminants on Carbon Dioxide Capture Using Amine-Functionalized Multiwalled Carbon Nanotubes Qing Liu,† Bitao Xiong,‡ Junjie Shi,† Mengna Tao,† Yi He,† and Yao Shi*,† †
Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China ‡ Department of Applied Physics, School of Science, Zhejiang University of Science and Technology, Hangzhou, 310023, China ABSTRACT: The adsorption behavior of adsorbents for carbon dioxide can be significantly affected by flue gas contaminants. In this work, we examined the performance of tetraethylenepentamine (TEPA) impregnated industrial grade multiwalled carbon nanotubes (IG-MWCNTs) in trace amounts of flue gas contaminants such as H2O, NO, and SO2. It was observed that H2O and NO had a minimal impact on CO2 adsorption capacity, while the effect of SO2 on CO2 adsorption was influenced by adsorption temperature and SO2 concentration. Compared with silica-based adsorbents, i.e., TEPA-impregnated MCM-41, aminefunctionalized IG-MWCNTs show significantly better tolerance to H2O and SO2. In addition, we examined the variation of CO2 adsorption with and without SO2 with various experimental methods (N2 adsorption/desorption isotherms, X-ray diffraction, and differential scanning calorimetry analysis) and molecular simulation. Experimental results show that irreversible sulfate/sulphite species deposited into the adsorbent contributes to the decrease on CO2 adsorption, while the results from simulation studies reveal that the enthalpy difference between the isolated TEPA with SO2 and TEPA···SO2 (ΔH(TEPA···SO2)) is larger than that of CO2 (ΔH(TEPA···CO2)), indicating that SO2 has a stronger reaction activity with TEPA than CO2. The increase of the ratio of ΔH(TEPA···SO2)/ΔH(TEPA···CO2) with increasing temperature illustrates that the difference of CO2 adsorption capacity with and without SO2 increases with elevated temperatures.
1. INTRODUCTION Despite continuous warnings about the detrimental influences of carbon dioxide (CO2) on global warming and climate change,1 fossil fuels remain a primary source of energy in the foreseeable future.2 One alternative approach to reduce CO2 emission is carbon capture and sequestration (CCS), which has relatively small impacts on the existing energy infrastructure.3 Aqueous amine based processes represent currently available and practically applied technology for CO2 capture.4 An energy penalty of approximately 30% on top of the power generation of the plant is involved in such processes.4 One approach to reduce the energy penalty is to use solid sorbents as their lower heat capacities compared with aqueous amine.5 Among them, amine-functionalized solid sorbents have gained increasing attention for CO2 capture from flue gas due to their intrinsic high adsorption capacity and selectivity.6 The selection of solid supports is critical for the performance of amine-functionalized adsorbents for CO2 adsorption. A variety of microporous/mesoporous supports loaded with organic amine have been extensively investigated, including activated carbonaceous materials,7−9 zeolite-based sorbents,10,11 silica-supported sorbents,12−14 polymer-based sorbents,15,16 and metal−organic frameworks,17−19 etc. Su et al. employed multiwalled carbon nanotubes as a solid sorbent for 3aminopropyltriethoxysilane functionalization. The CO2 adsorption capacity reaches a maximum of 2.45 mmol/g at a water vapor of 2.2%. The adsorbent displays a desirable stability during a prolonged cyclic operation, as the adsorption capacity and physicochemical properties are preserved after 100 adsorption/desorption cycles.20 The CO2 adsorption behavior of magadiite (MAG) impregnated with branched polyethyleni© 2014 American Chemical Society
mine (PEI) was conducted by Vieira and Pastore. MAG-PEI25 reached a maximum adsorption capacity of 6.11 mmol/g at 343 K for 3 h of adsorption and showed a kinetic desorption of around 15 min at 423 K.21 Lin et al. reported the CO2 adsorption behavior of PEI incorporated MIL-101 adsorbents with different PEI loadings. The CO2 adsorption capacity is enhanced dramatically after modification and reaches the maximum of 4.2 mmol/g at 298 K. The selectivity of CO2/ N2 is up to 1200 at 323 K.18 Flue gas may contain trace amounts of water vapor, NOx, and SOx, which may poison amine-functionalized adsorbents during CO2 adsorption processes. Typical concentrations of SOx and NOx in coal-fired power plants (before sulfur scrubbing and/or selective catalytic reaction units) are approximately 0.2−0.25 and 0.15−0.2 vol %, respectively.22 Therefore, a thorough investigation of amine-functionalized adsorbents for CO2 capture requires the consideration of the influence of gas impurities. Khatri et al.22 studied the adsorption and desorption of CO2 and SO2 on amine-grafted SBA-15 sorbent. The rate of adsorption of SO2 is slower than that of CO2, but the adsorbed S surface species is capable of blocking the active amine sites for CO2 adsorption. Adsorbents constructed using poly and three different silane coupling agents with primary, secondary, and tertiary amines were evaluated by Rezaei and Jones.23 All materials treated with NO2 and SO2 show a dramatic reduction in CO2 capacity. Secondary amines exhibit higher affinity to SO2, while their CO2 capacity loss after exposure to SO2 is Received: July 10, 2014 Revised: September 15, 2014 Published: September 16, 2014 6494
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K/min. The crystal phase was characterized by a powder X-ray diffractometer (XRD, Rigaku D/Max 2550/PC, Rigaku Co., Ltd., Japan) using Cu Kα radiation (40 kV, 30 mA). The mass of sample each measured was almost the same. Fourier transform infrared (FTIR, NICOLET 6700, thermal scientific, USA) using KBr pellets was performed in the range from 400 to 4000 cm−1 by accumulation of 32 scans. 2.3. Adsorption Experiments. The experimental setup for CO2 capture is presented in Figure 1.5 Adsorption was conducted in a fixed
lower than that of primary amines, indicating that more SO2 desorbs from secondary amines during the regeneration step. In another study, conducted by Stevens et al.,24 the diamine modified montmorillonite was studied for CO2 adsorption. The material is stable in pure CO2 and 15% CO2 in N2, in the presence of SO2; however, the adsorption capacity drops dramatically. Hallenbeck and Kitchin3 studied the effect of O2 and SO2 on the capture capacity of a primary-amine based polymeric CO2 sorbent. The resin maintains its CO2 capture capacity of 1.31 mmol/g over 17 cycles in the presence of O2, while it decreases rapidly under exposure to SO2 by an amount of 1.3 mmol/g over nine cycles. In our previous work, industrial-grade multiwalled carbon nanotubes impregnated 50 wt % tetraethylenepentamine (TEPA) (IG-MWCNTs-50) exhibited desirable features including CO2 adsorption capacity, adsorption rate, and recyclability.25,26 The maximum adsorption capacity is 3.088 mmo/g at 343 K when CO2 concentration was kept at 10 vol %, and the flow rate was 50 cm3/min.25 A sharp breakthrough curve indicates a fast adsorption kinetics of IG-MWCNTs-50,25 and the kinetic constant was also calculated according to various kinetic models. The performance of IG-MWCNTs-50 is fairly stable, with only a 1.58% drop in adsorption capacity after five adsorption/regeneration cycles.25 In addition, carbon-based materials have attributes of high chemical, thermal, and mechanical stability required to operate in realistic flue gas streams, which contain CO2, N2 and O2, NOx, SOx, steam, and dust.27 Therefore, in this work, we investigated the influence of moisture, NO, and SO2 on CO2 adsorption performance. To further explore the poisoning mechanism in the presence of SO2, the adsorbents were characterized with various experiment methods including N2 adsorption/desorption isotherms, X-ray diffraction (XRD), and differential scanning calorimetry analysis (DSC), as well as molecular simulation. The TEPAfunctionalized MCM-41 (the most common type of silica) was selected as the comparative study during CO2 adsorption in the presence of contaminants.
Figure 1. Schematic for the experimental system. (1) Nitrogen; (2) carbon dioxide; (3) sulfur dioxide; (4) nitric oxide; (5) mass flow meters; (6) mixing tank; (7) saturator; (8) adsorber; (9) tubular furnace; (10) temperature controller; (11) gas chromatograph; (12) data recording system; (13) vacuum pump.5
bed, which is composed of an adsorption column and a temperaturecontrolled tubular furnace. The adsorption column is made of quartz glass, which is 14 cm in length with an inner diameter of 1.5 cm.28 The adsorption column was placed into a temperature-controlled tubular furnace and filled with 2 g of IG-MWCNTs-50 or MCM-41-50. Adsorbents were treated under a N2 flow at 423 K for 90 min and then cooled to the test temperature. The flow was then switched to a desired simulated flue gas. To investigate the effect of moisture on the CO2 adsorption, water vapor was introduced into the gas stream through a water saturator. SO2 and NO were also mixed with N2 at a predetermined composition. In this work, the concentration of CO2 was kept at 10 vol % at atmospheric pressure, and the flow rate was 50 cm3/min. The concentration of CO2 was measured by a gas chromatograph (GC). The adsorption capacity of CO2 on adsorbents at a given time is calculated by eq 1
2. EXPERIMENTAL SECTION 2.1. Preparation of Adsorbents. TEPA (90%, Sinopharm Chemical Reagent Co., Ltd., China) was incorporated into the IGMWCNTs (TNIM8, Organic Chemical Co., Ltd., China) or MCM-41 (Nanjing XFNANO Materials Tech Co. Ltd.) supports by wet impregnation.25 In detail, 2 g of TEPA was dissolved in 50 g of ethanol (99.7%, Sinopharm Chemical Reagent Co., Ltd., China), and the solution was stirred for 30 min at room temperature. Then, 2 g of IGMWCNTs or MCM-41 supports was added into the ethanol solution of TEPA. After being stirred for 3 h, the mixture was evaporated at 353 K and subsequently dried at 373 K in open air for 1 h. Finally, the sample was ground into powder and sealed in a vial. These samples were denoted as IG-MWCNTs-50 or MCM-41-50, where 50 represents the 50 wt % of TEPA in the composites. 2.2. Characterization of Adsorbents. The surface area and pore volume were determined with a static volume adsorption system (Model-ASAP 2020, Micromeritics Inc., USA) by obtaining the N2 adsorption/desorption isotherms at 77.4 K. The adsorbents were outgassed at 423 K for 6 h prior to measurement. The N2 adsorption/ desorption data were recorded at the liquid nitrogen temperature (77 K) and then employed to determine the surface areas using the Brunuer−Emmett−Teller (BET) equation. The total pore volume was calculated from the amount of adsorbed N2 at P/P0 = 0.99. Thermogravimetric analysis (TGA) and differential scanning calorimetry analysis (DSC) were carried out with a thermogravimetric analyzer (SDT Q600, TA Instruments, Inc., New Castle, DE) under a dynamic N2 atmosphere from 300 to 800 K, with a heating rate of 10
q=
1⎡ ⎢ M⎣
∫0
t
Q
c 0− c ⎤ T0 1 dt ⎥ 1 − c ⎦ T Vm
(1)
where q is the adsorption capacity of CO2 (mmol/g), M is the mass of adsorbent (g), Q is the gas flow rate (cm3/min), c0 and c are influent and effluent CO2 concentrations (vol %), t denotes time (min), T0 is 273 K, T is the gas temperature (K), and Vm is 22.4 mL/mmol. 2.4. Simulation Details. Although there have been a few experimental research studies on trace SO2 poisoning aminefunctionalized adsorbent, the poisoning mechanism has not been reported from theoretical calculations according to our best knowledge. Amine was chosen as the research object because of the chemisorption of CO2 or SO2 onto amine-functionalized adsorbent. All of the theoretical calculations were performed by density functional theory (DFT) using the 6-311G basis set. The calculation process includes geometry optimizations and energy calculation. For TEPA, gas molecule and TEPA coordination with gas molecules (TEPA···CO2/SO2), full optimization, and frequency analysis were performed at the B3LYP/6-31G level. On the basis of the optimized geometry, the enthalpy (H) and Gibbs free energy (G) were calculated using the 6-311G basis set. ΔH and ΔG of were calculated using the following formula: 6495
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bicarbonate or carbonate reacted between CO2, H2O, and TEPA may result in the blockage of the pores; (iii) with the water vapor content increased, the occurrence of the capillary condensation phenomenon led to the blockage of micropores. The effect of NO on CO2 adsorption is found to be hardly noticeable in the scope of the testing concentration (Figure 2b). Similar results were also observed on TEPA impregnated KIT-6 in our previous study28 and aminosilica adsorbent studied by Rezaei et al.23 3.2. Effect of SO2 on CO2 Adsorption. 3.2.1. Effect of SO2 Concentration. MCM-41 has been considered the most common type of silica and studied by many researchers.30,31 The mesoporous nature of the support permits good diffusion of organic amine into the pore space and, following functionalization, good mass diffusion of CO2 gas molecules into and out of the structure.32 Therefore, we chose aminefunctionalized MCM-41 as a reference for comparison in the following study. Figure 2c,d shows the CO2 adsorption capacity of adsorbents under various SO2 concentrations (100, 200, 500, 1000, and 2000 ppm) at 323 K. With the increase of SO2 concentration, the CO2 adsorption capacity of IG-MWCNTs50/MCM-41-50 slightly decreases from 2.765/1.747 to 2.642/ 1.668 mmol/g. The adsorption capacity drop rate of both adsorbents is comparable. 3.2.2. Effect of Adsorption Temperature. To verify the effect of temperature on CO2 adsorption capacity of IGMWCNTs-50 in the existence of SO2, the measurements were performed at various temperatures under the SO2 concentration of 1000 ppm. (Figure 3) With the increasing
ΔH(TEPA···CO2 ) = H(TEPA···CO2 ) − H(TEPA) − H(CO2 ) (2) ΔH(TEPA···SO2 ) = H(TEPA···SO2 ) − H(TEPA) − H(SO2 ) (3) ΔG(TEPA···CO2 ) = G(TEPA···CO2 ) − G(TEPA) − G(CO2 ) (4) ΔG(TEPA···SO2 ) = G(TEPA···SO2 ) − G(TEPA) − G(SO2 ) (5) where ΔH(TEPA···CO2) and ΔH(TEPA···SO2) are the enthalpy difference between the isolated TEPA with CO2/SO2 and TEPA··· CO2/SO2; ΔG(TEPA···CO2) and ΔG(TEPA···SO2) are the Gibbs free energy difference between the isolated TEPA with CO2/SO2 and TEPA···CO2/SO2. All calculations were performed using the Gaussian 09 software on the server of DELL Linux.
3. RESULTS AND DISCUSSION 3.1. Effect of H2O and NO on CO2 Adsorption. Water vapor, NO, and SO2 were chosen as the representative flue gas contaminants as they are the important constituents which may affect CO2 capture. CO2 adsorption capacity under various concentrations of moisture and NO was tested at 323 K (Figure 2). The adsorption capacity of IG-MWCNTs-50 slightly
Figure 3. Effect of temperature on CO2 adsorption capacity on IGMWCNTs-50 in the presence of SO2.25
temperature, the CO2 adsorption capacity increases from 2.109 mmol/g at 293 K to 2.732 mmol/g at 333 K, and decreases to 2.543 mmol/g at 353 K. The difference in adsorption capacities measured with and without the existence of SO2 gets larger with increased temperature, from 0.009 mmol/g at 293 K to 0.542 mmol/g at 353 K. In other words, the CO2 adsorption capacity drops more significantly in the presence of SO2 at high temperature, while it is disadvantageous to CO2 adsorption capacity and kinetics for aminefunctionalized adsorbent at the low temperature. Therefore, it becomes meaningful to determine the adsorption temperature under SO2 concentration of working flue gas conditions. It is verified by the adsorption behavior of flue gas temperature (313, 323, 333, and 343 K) at various SO2 concentrations (100,
Figure 2. Effects of (a) moisture, (b) NO, and (c, d) SO2 on CO2 adsorption capacity on IG-MWCNTs-50 or MCM-41-50 at 323 K.
decreases from 2.765 to 2.579 mmol/g with the relative humidity (RH) increasing from 0 to 100% (Figure 2a). However, water vapor was proven to be beneficial for CO2 adsorption, as a consequence of the partial formation of ammonium bicarbonate with a CO2/N stoichiometric ratio of 1, in place of carbamate with CO2/N = 0.5.29 It can be explained as follows: (i) the water vapor formed into water film on the adsorbent, leading to CO2 mass transfer resistance; (ii) 6496
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adsorption sites in IG-MWCNTs-50 on account of prior loading of SO2.23 In addition, sulfate/sulphite species reacted between SO2 and amine may result in the blockage of the pores, thus hindering the CO2 diffusion into the active sites. The accumulation of indecomposable sulfate/sulphite species will cause the decrease of CO2 adsorption capacity after several adsorption/desorption processes. The only way to maintain the CO2 adsorption capacity is to remove SO2 before the following adsorption process. The CO2 adsorption behavior was tested for five repetitive adsorption/desorption cycles under an SO2 concentration of 100, 200, and 1000 ppm at 333 K. (Figure 6) The CO2
200, 500, and 2000 ppm) in Figure 4. The CO2 adsorption capacity declines by
