ARTICLE pubs.acs.org/est
Gravimetric Analysis of the Adsorption and Desorption of CO2 on Amine-Functionalized Mesoporous Silica Mounted on a Microcantilever Array Dongkyu Lee,† Yusung Jin,† Namchul Jung,† Jaehyuk Lee,† Jinwoo Lee,† Yong Shik Jeong,‡ and Sangmin Jeon*,† †
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), San 31 Namgu Hyojadong, Pohang, Korea ‡ Environment and Energy Planning Department, POSCO, Seoul, Korea
bS Supporting Information ABSTRACT: The kinetics of CO2 adsorption and desorption over amine-functionalized mesoporous silica were investigated using silicon microcantilever arrays. Three types of mesoporous silica with different pore sizes were synthesized and functionalized with a variety of amine molecules. After depositing the silica sorbents onto the free end of each cantilever in an array, mass changes due to the adsorption and desorption of CO2 were determined in situ with picogram sensitivity by measuring variations in the cantilever frequencies. The adsorption and desorption kinetics were found to be diffusion-controlled, and the kinetics were accelerated by increasing the temperature and pore size. The activation energies for adsorption and desorption of CO2 were determined from Arrhenius plots.
’ INTRODUCTION Carbon dioxide is a major greenhouse gas and contributes to undesired global climate changes. The amount of CO2 produced worldwide in 2010 was estimated to be 30 GT. Unless proper actions are taken, this amount is expected to increase to 60 GT in 2050, which will increase the global temperature and cause irreversible climate disasters.1 Although international attempts have been made to reduce CO2 generation by promoting the use of renewable energies, including solar energy and wind power, the meaningful effects of replacing conventional carbon-based energy with renewable energy will only be realized over time. Therefore, it is essential to develop efficient carbon capture and storage (CCS) methods to manage atmospheric CO2 concentrations in the short term. Among the various CCS technologies under development, including membrane-based separation, chemical conversion, and liquid/solid sorption,26 CO2 capture systems that use solid sorbents have many advantages. They are economical, noncorrosive, and eco-friendly.79 Solid sorbents may be classified into two groups according to the CO2 adsorption mechanism: physical adsorption and chemical adsorption. Most solid sorbents, including activated carbons, zeolites, and metal organic frameworks (MOFs), rely on the physical adsorption of CO2, and research has focused on increasing the surface area of the sorbents to enhance the adsorption capacity. However, physical sorbents present serious drawbacks in their general lack of selectivity for CO2. To overcome this problem, chemical r 2011 American Chemical Society
sorbents such as metal oxides and chemically modified silica that selectively bind CO2 have been developed.1012 Amine-functionalized mesoporous silica has attracted much attention as a promising chemical sorbent due to the ease with which pore size and surface chemistry may be controlled.1315 In addition, CO2 binds to amine groups by forming carbamate or carbamic acids,15,1721 resulting in selective adsorption of CO2. However, the impregnation of amines induces the reduction of the surface area and pore volume of the sorbents, affecting CO2 adsorption and desorption kinetics significantly. Although fast CO2 adsorptiondesorption kinetics is an important factor to improve the overall performance of CCS processes,16 previous studies using amine-functionalized sorbents showed that the adsorption half-time (T0.5) was a few minutes and the time to reach 80% of the adsorption capacity (T0.8) was ∼30 min6,10,17,18 Therefore, it is needed to develop a chemical sorbent with fast adsorptiondesorption kinetics as well as high CO2 adsorption capacity. The performance of chemical sorbents is usually evaluated using gas chromatography (GC), mass spectrometry (MS), or thermogravimetric analysis (TGA).2226 However, these methods are limited in that GC and MS may not be used to measure Received: March 1, 2011 Accepted: June 1, 2011 Revised: May 30, 2011 Published: June 15, 2011 5704
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Table 1. Physical Properties of the Synthesized Silica Sorbents surface area pore size pore volume amine loading (mmol sorbent
(m2/g)
(nm) 36 31
(cm3/g)
N/g silica)
1.97 1.21
2.4
MSUF-NA MSUF-MA
484 215
MSUF-DA
188
30
1.14
4.3
MSUF-TA
72
25
0.45
7.6
SBA-NA
821
7.8
0.99
-
SBA-MA
196
6.6
0.21
2.8
SBA-DA
45
5.6
0.1
4.6
-
0.02
5.8
0.68 0.36
1.8
SBA-TA
2.5
MCM-NA MCM-MA
827 670
3.3 2.0
MCM-DA
53
1.8
0.06
3.3
MCM-TA
15
-
0.06
4.8
the in situ adsorption kinetics of CO2 and TGA due to insufficient mass sensitivity. Ma and Wu et al. used a quartz crystal microbalance (QCM) to overcome this problem, but the results were too sensitive to the thickness and roughness of the sorbent coatings on the quartz crystals.27,28 In contrast, the mass sensitivity of the cantilever during the adsorption and desorption was found to be far superior to that of conventional gravimetric sensors, such as TGA and QCM. In addition, the arrayed structure of the cantilevers permitted investigation of multiple samples and noise compensation. Despite these advantages, the microcantilever sensors have mainly been used so far for the detection of gases and biomolecules, and just a few studies have utilized nanoporous material-coated cantilevers as simple gas sensors.29,30 This study presents, to our knowledge, the first use of a microcantilever sensor array as a screening tool for a variety of amine-functionalized mesoporous silica sorbents. The adsorption capacity increased as the amount of amine immobilized on the sorbents increased, but the adsorption kinetics slowed due to the decrease in pore size. We solved this problem by using a verylarge-pore silica sorbent (MSU-F, a mesoporous material developed at Michigan State University). Compared to the other mesoporous silica such as SBA-15 and MCM-41, MSU-F possesses much larger pores. Sorbents with larger pores were found to be more suitable for amine functionalization, despite their smaller surface area. The CO2 adsorption and desorption kinetics sped up upon heating, but the adsorption capacity reached a maximum near 45 °C, due to the competition between the diffusion kinetics and the exothermic adsorption reaction.
’ EXPERIMENTAL SECTION Materials. Sodium silicate (10.6% Na2O, 26.5% SiO2), cetyltrimethylammonium bromide (CTAB, 99%), H2SO4 (48%), Pluronic 123 (P123 Mn. 5800), tetraethyl orthosilicate (TEOS, 98%), 3-amino propyltrimethoxysilane (97%) (APTMS), N(3-(trimethoxysilyl)propyl) ethane-1,2-amine (97%) (APAETMS), 3-[2-(2-aminoethylamino) ethylamino] propyltrimethoxysilane (APAEAETMS), mesitylene, ethanol (99.9%), HCl (35%), anhydrous toluene (99.8%), and NaOH (98%) were purchased from Aldrich (Saint Louis, MO) and used without further purification. Arrays of eight rectangular silicon cantilevers
were obtained from Micromotive (Mainz, Germany). As stated by the manufacturer, each cantilever in the array was 450 μm long, 90 μm wide, and 5 μm thick, with a spring constant of 3.8 N/m. The variation in the resonance frequency across 8 cantilevers is less than 0.3%. Preparation of Amine-Functionalized Mesoporous Silica. Three different mesoporous silica sorbents, MCM-41, SBA-15, and MSU-F, were synthesized as described elsewhere. In brief, MCM-41 was synthesized by CTAB and sodium silicate. The pH of the mixture was adjusted to 10 by addition of H2SO4 followed by heating at 100 °C for 24 h.31 The resulting solid products were filtered, washed, and calcined at 600 °C for 4 h in air. For the synthesis of SBA-15, a mixture of TEOS and P123 was reacted at 100 °C for 8 h. The product was filtered, dried without washing, and calcined at 550 °C for 4 h.32 MSU-F was synthesized from sodium silicate using P123 and mesitylene as pore expenders. A white product was obtained by filtering. The filtrate was washed by stirring in a solution containing ethanol and HCl (40:1) for 3 h, followed by calcination at 550 °C for 4 h.33 The surfaces of the mesoporous silica were modified with a variety of amine molecules, such as monoamine (MA, APTMS), diamine (DA, APAETMS), and triamine (TA, APAEAETMS), using silane chemistry. Excess amine precursors (2 g) were added to 2 g of mesoporus silica in anhydrous toluene. The mixture was allowed to stir for 24 h at room temperature under argon environment. The resulting solid was filtered, washed with toluene, and dried under vacuum at 50 °C overnight. Depending on the combination of silicas and amines, 12 different sorbents were prepared: MCM-NA (no amine), MCM-MA, MCM-DA, MCM-TA, SBANA, SBA-MA, SBA-DA, SBA-TA, MSUF-NA, MSUF-MA, MSUF-DA, and MSUF-TA. Characterization of the Silica Sorbents. The quantity of amine molecules functionalized on each silica material was determined by elemental analysis (C, H, N). Nitrogen adsorption measurements were performed at 77 K using a Micromeritics ASAP 2020 automated volumetric instrument. Prior to each analysis, the materials were degassed at 423 K under vacuum. The specific surface areas (SBET) of the silicas were determined using BET methods in the relative pressure range 0.050.2, and the pore size distribution (PSD) was calculated using the KrukJaroniecSayari (KJS) method. The average pore diameter, pore volume, and surface area of the silica sorbents before and after amine functionalization are summarized in Table 1. Because functionalization of MCM and SBA with TA resulted in almost complete blocking of the pores, MCMTA and SBA-TA were not tested in the cantilever experiments. SBADA and MCM-DA were compared with MSUF-DA to investigate the effect of pore size on CO2 adsorption. Measurements of CO2 Adsorption and Desorption. Dry nitrogen was used as a carrier gas, and the concentration of CO2 used in the adsorption measurements was fixed at 30% by controlling the flow rates of N2 and CO2 using mass flow controllers (Brooks Instrument). The total gas flow rate was fixed at 100 mL/min. For experiments, the sorbent-coated cantilever array was exposed to the gas mixture for 2 h and then rinsed with dry nitrogen for 3 h to remove the adsorbed gases. To investigate CO2 adsorption and desorption, each cantilever was coated with a different sorbent using microcapillary tubes. The capillary tubes were filled with suspensions of the sorbents in ethanol (1 mg/mL), and each cantilever in the array was immersed in the tubes for 1 min. The size distribution of the sorbents in the capillary tube was measured using optical microscopy and the mean size was found to be 25 μm. The sorbents 5705
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Figure 1. Optical microscopy image of the sorbent-coated microcantilever array.
coated the free ends of the cantilevers to avoid stress-induced moduli changes in the cantilever, as shown in Figure 1. The uncoated cantilever 1 was used as a reference cantilever to compensate for any frequency change due to the density and viscosity differences between CO2 and N2. After coating, the cantilever array was dried at 110 °C under reduced pressure (50 mbar) for 1 h. The cantilever was placed inside a home-built flow cell (2 cm diameter and 0.5 cm height), and the temperature was controlled using a resistance heater with a programmable temperature controller (Hanyoung, Inchon, Korea). The flow cell was mounted on a piezoelectric actuator, and the resonance frequency of the vibrating cantilever was measured using optical beam techniques. A laser beam was focused onto the free end of the cantilever (next to the coated position), which reflected the beam onto a position-sensitive detector. The voltage changes due to the vibrations of the cantilever were converted to resonance peaks using a fast Fourier transform (FFT). Multiplexing measurements were conducted by fixing the position of the laser beam and modulating the position of the cantilever array using a motorized stage. The spatial resolution of the motorized stage was stated by the manufacturer to be 50 nm, and the measurement time was 1 s for each cantilever. The change in resonance frequency of the cantilever may be related to the mass change of the cantilever (Δm) using ! k 1 1 ð1Þ Δm ¼ 2 2 2 4π f1 f0 where k is the spring constant, f0 is the initial resonance frequency, and f1 is the resonance frequency after the mass change. The mass of sorbent coated onto each cantilever was calculated from eq 1 to vary in the range 90200 ng. The additional mass change due to CO2 adsorption on the sorbent can be also calculated using eq 1 by measuring the frequency changes of the sorbent-coated cantilever before and after CO2 adsorption.
’ RESULTS AND DISCUSSION Figure 2a shows the changes in mass during the adsorption and desorption of CO2 over various MSUFs at 30 °C and 1 atm. The mass change was divided by the mass of the sorbent coating on the cantilever. The right side of the y-axis indicates the mmol CO2/g sorbent. The measured CO2 adsorption capacities of
Figure 2. Variations in the absolute mass (a) and normalized mass (b) of MSUF-coated cantilevers during the adsorption and desorption of CO2 (black: MSUF-NA, red: MSUF-MA, green: MSUF-DA, blue: MSUF-TA). (c) The normalized masses of DA-functionalized silicacoated cantilevers (black: MCM-DA, red: SBA-DA, green: MSUF-DA).
MSUF-NA, MSUF-MA, MSUF-DA, and MSUF-TA were 4.8, 13.9, 35.7, and 62.5 pg/ng sorbent, respectively, indicating that the CO2 adsorption capacity increased with the amount of amine present in the MSUF. Considering that the surface area of an MSUF decreased with increasing amine functionalization, this result demonstrated that the binding affinity of CO2 to amines was greater than the binding affinity of CO2 to silica. A control experiment using gas chromatography showed that almost pure CO2 adsorption was observed for the amine-functionalized silica sorbents whereas substantial N2 adsorption was observed for bare silica sorbents. Note, however, that the adsorption of CO2 on MSUF-NA induced immediate changes in the mass, whereas the adsorption of CO2 on the amine-functionalized MSUFs induced gradual changes in the mass. The kinetics of CO2 adsorption on various MSUFs was compared by normalizing the curves in Figure 2a, as plotted in Figure 2b. The normalized curves were obtained by dividing the mass changes in Figure 2a by the maximum value of each curve. Note that the CO2 adsorption on amine-functionalized MSUFs occurs very fast in the beginning of the adsorption. T0.5 and T0.8 of MSUF-TA, which exhibits the slowest adsorption kinetics among the MSUF’s, were just 0.4 and 3.7 min, respectively. The 5706
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Table 2. Time Constants for the Adsorption of CO2 on a Variety of Silica Sorbents at 30°C time constant (s) MSUF-MA MSUF-DA MSUF-TA SBA-DA MCM-DA τ1
15
15
15
15
15
τ2
1190
1562
2128
1786
1923
adsorption rate decreased with the quantity of amines functionalized on the MSUFs (MSUF-NA > MSUF-MA > MSUF-DA > MSUF-TA). This trend was attributed to slowed diffusion due to the decrease in pore size. A control experiment was conducted to investigate the effect of pore size on the adsorption of CO2. Figure 2c compares the normalized changes in mass during CO2 adsorption on SBA-DA, MCM-DA and MSUF-DA. Although the same amine was used to functionalize each of the three sorbents, the adsorption rate was found to increase with the pore size, MCM-DA < SBA-DA < MSUF-DA, confirming that the pore size was the most important factor for the diffusion of CO2 through the amine-functionalized silica sorbents. T0.8 was measured to be 0.7, 18, and 34 min for MSUF-DA, SBA-DA, and MCM-DA, respectively. The time-dependent CO2 adsorption curves (Cad(t)) in Figures 2b and 2c were best fit to double exponential functions Cad ðtÞ ¼ aexpð t=τ1 Þ + bexpð t=τ2 Þ + c
Figure 3. Normalized masses of MSUF-TA-coated cantilevers during the adsorption and desorption of CO2 at various temperatures (black: 30 °C, red: 45 °C, green: 60 °C, blue: 75 °C, sky blue: 90 °C). The inset shows Arrhenius plots for the adsorption (red) and desorption (blue) of CO2.
ð2Þ
where a, b, and c are the fitting parameters, and τ1 and τ2 are time constants. The calculated time constants are listed in Table 2. The goodness-of-fit (R2) in the analysis was ∼0.99, indicating that the curves were well fit by eq 2. τ1 was relatively constant, regardless of the sorbent, whereas τ2 increased with the quantity of amine on the MSUFs. Apparently, two different processes were involved in the adsorption of CO2 on amine-functionalized MSUFs. The binding rate of CO2 to the amines was not significantly affected by the type of amines. However, the diffusion of CO2 through the MSUFs was substantially affected by the pore size. Therefore, the fast and slow processes were related to the binding of CO2 to the amine molecules and the diffusion of CO2 through MSUFs, respectively. τ2 was 2 orders of magnitude larger than τ1, suggesting that diffusion was the ratedetermining step for CO2 adsorption. Similar results were observed during desorption of CO2 from the MSUFs, as shown in Figure 2b. Note that the adsorbed CO2 was completely desorbed from MSUF-NA, whereas partial desorption of CO2 was observed for the amine-functionalized MSUFs due to the strong interactions between CO2 and amines. To facilitate the desorption of CO2 from the amine-functionalized sorbents, similar experiments were carried out at higher temperatures. Figure 3 shows the normalized changes in mass during the adsorption and desorption of CO2 over MSUF-TA as a function of temperature. The rates of adsorption and desorption increased with temperature, and complete desorption was observed above 60 °C. The adsorption and desorption curves were best fit to double exponential functions, from which the average time constants were calculated to obtain the activation energy for CO2 adsorption and desorption. The inset of Figure 3 shows the Arrhenius plots for both the adsorption and desorption processes. The apparent activation energies of CO2 adsorption and desorption were calculated from the slopes to be 24 and 53 kJ/mol, respectively. Assuming that the activation energy for adsorption was mainly associated with CO2 diffusion, the difference between the adsorption and desorption activation
Figure 4. CO2 adsorption capacities of various MSUFs as a function of temperature (black: MSUF-NA, red: MSUF-MA, green: MSUF-DA, blue: MSUF-TA).
energies corresponded to the binding energy of CO2 to the amines, which was comparable to the energy of a hydrogen bonded interaction. The temperature dependence of the maximum CO2 adsorbed on each MSUF is presented in Figure 4. Note that the CO2 adsorption capacity of MSUF-NA monotonically decreased with temperature (although this is not clear in the figure due to the choice of scale). In contrast, the CO2 adsorption capacity of amine-functionalized MSUFs increased with temperature up to 45 °C and decreased at higher temperatures because of the competition between the diffusion-limited process and the exothermic adsorption reaction. The amount of CO2 adsorbed on amine-functionalized MSUF increased with the amount of amine present in the MSUF. The amine efficiencies (mmol CO2/ mmol amines) of MSUF-MA, MSUF-DA, and MSUF-TA at 45 °C are calculated to be 0.14, 0.23, and 0.21, respectively, indicating that the diamine and triamine were more efficient than monoamine for CO2 capture due to intramolecular acidbase balance with secondary amines.20,21 The experiments were repeated 5 times and the variation was found to be smaller than (0.06 mmol CO2/g sorbent, indicating that these measurements were highly reproducible. A control experiment was conducted to investigate the reproducibility of the measurements depending on the amount of sorbents loaded on the cantilever. Figure 5a shows the variation in the CO2 adsorption capacity as a function of the mass of MSUF-TA at 30 °C. Although the mass of MSUF-TA ranged 5707
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Figure 5. Variations in the capacity (a) and kinetics (b) of MSUF-TAcoated microcantilevers as a function of MSUF-TA at 30 °C during the adsorption and desorption of CO2. The mass of MSUF-TA ranges from 40 to 170 ng.
from 40 to 170 ng, the CO2 adsorption capacity was almost constant, indicating that the loading amount does not affect the adsorption capacity. In addition, the variations in the normalized CO2 adsorption curves exhibit similar responses regardless of the mass of MSUF-TA as shown in Figure 5b. When the curves were best fit with double exponential functions, the variations in the time constants were 18 ( 3 s for τ1 and 2096 ( 143 s for τ2, confirming that the CO2 adsorption kinetics is independent of the sorbent loading.
’ ASSOCIATED CONTENT
bS
Supporting Information. Size distribution of sorbents coated on the microcantilever array, pore volume and size of MSUF as a function of amine loading, and the selective CO2 adsorption on amine-functionalized sorbents. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: 82-54-279-2392;
[email protected].
Fax:
82-54-279-5528;
E-mail.
’ ACKNOWLEDGMENT This work was supported by Posco’s R&D project (2009Y264). ’ REFERENCES (1) Metz, B.; Davidson, O.; de Coninck, H.; Loos, M.; Meyer, L. IPCC Special Report: Carbon Dioxide Capture and Storage; Cambridge University Press: New York, 2005.
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