Investigation of Non-Thermodynamical CO2 Adsorption Behavior for Amine-Modified Nanoporous Silica

Non-thermodynamical CO2 adsorption behavior for amine-modified nanoporous silica is clarified by evaluating the mobility of organic functional group inside mesopores by using pulsed NMR technique. CO2 adsorption behavior of nanoporous silica modified with amino-propyl silane (AP) changes significantly depending on the amount of AP loaded. A low AP loaded sample shows normal adsorption behavior; the amount of CO2 adsorbed decreases with increasing temperature. In contrast, a high AP loaded sample possesses non-thermodynamic CO2 adsorption behavior in which the amount of CO2 adsorbed increases with increasing temperature in within a certain temperature range. To address the mechanism, a pulsed NMR technique was employed to clarify the mobility of AP molecules, and it was found that the mobility of mobile components in a high AP loaded sample increased drastically with increasing temperature while the mobility in a low AP loaded sample remained unchanged. It is understood that the enhancement of the diffusion of CO2 inside nanopores leads to the non-thermodynamic adsorption behavior.


Introduction
Global warming caused by the increase of the amount of CO 2 emission is a serious problem to be solved immediately. The methods using CO 2 absorbing solution are useful and already commercialized in some coal-fired power plants or factories. However, in the method, heating of absorbing solution, usually water, is required to recover CO 2 from the solution, which consumes a lot of energy. To decrease energy requirements in a CO 2 separation system, the development of high performance CO 2 adsorbent is desired [1] [2].
Emission gases of factories or power supply facilities contain a lot of water vapor. If the selectivity for water vapor is higher than that for CO 2 , it is required to remove water vapor from the emission. This process consumes a lot of energy, and developing the adsorbent with higher CO 2 selectivity is crucial.
Generally, adsorption of a gas follows Clausius-Clapeyron equation. At higher temperature, an adsorption isotherm shifts to the higher pressure side and the amount of adsorbed gas at the same pressure decreases. The tendency is the same for water vapor, leading to the decrease of adsorbed amount at higher temperature. Opposite CO 2 adsorption behavior was observed for polyethyleneimine-impregnated materials [26] and amino-modified porous silicas [27] [28]. The amount of CO 2 adsorbed increased upon the increase of the temperature. A porous material with this non-thermodynamic adsorption behavior is expected to be a high CO 2 selection adsorbent for the emission gases of factories.
Amino-propyl (AP) modified nanoporous silicas with different AP amounts have been synthesized by a co-condensation method, and the effect of AP amount on CO 2 adsorption behavior has been evaluated. The non-thermodynamic adsorption behavior is observed for a high AP loaded material. In contrast, the amount of CO 2 adsorbed decreases with increasing temperature for a low AP loaded material. To understand this behavior difference, a pulsed NMR technique is adopted to clarify the mobility of AP molecules in AP loaded nanoporous silicas at different temperature. As a result, a distinct difference is observed in the mobility.

Materials
3-Aminopropyltrimethoxysilane (AP-TMS) was purchased from Aldrich. Hexadecyltrimethylammonium chloride (C 16 TMACl) and tetramethoxysilane (TMOS) were purchased from Tokyo Kasei (Japan). Methanol and 1N sodium hydroxide solution were purchased from Wako Pure Chemical Co (Japan). All materials were used as received.

Synthesis of Nanoporous Silica Spheres
Amino propy-modified nanoporous silica particles were obtained according to the literature [22]. In a typical synthesis, 3.52 g of C 16 TMACl and 2.28 mL of 1 M sodium hydroxide solution were dissolved in 800 g of methanol/ water (50/50 = w/w) solution. A mixture of 1.19 g (7.81 mmol) of TMOS and 0.16 g (0.87 mmol) of AP-TMS (AP-TMS/(TMOS + AP-TMS) = 10 mol%) were then added to the solution with vigorous stirring at 298 K. After the addition of the TMOS and APTMS, the clear solution suddenly turned opaque and resulted in a white precipitate. After 8 h of continuous stirring, the mixture was aged overnight. The white powder was filtered and washed with distilled water at least three times, and then dried at 45˚C for 72 h. The powder obtained was heated in a 60 ml of ethanol solution containing a 1 ml of concentrated hydrochloric acid at 333 K for 3 h to remove the surfactant. Then, the powder was filtered, washed several times with ethanol, and dried at 318 K. Then, the amino propyl modified material was treated with an ammonia solution to remove any residual Cl − ions and to neutralize the protonated amines in the sample [22]. A 0.35 g of the modified sample was suspended in a 20 ml of methanol solution containing a 1 ml of ammonia solution (28%) at room temperature for 8 h. The solid was recovered by filtration, washed with methanol, and finally dried in a vacuum at 423 K for 12 h.

Characterization
Scanning electron micrographs (SEMs) were obtained with a SU3500 (Hitachi). Powder X-ray diffraction measurement was carried out with a Rigaku Rint-2200 X-ray diffractometer using Cu-Kα radiation. Nitrogen adsorption/ desorption isotherms were measured using a BELSORP-mini II (Bell-Japan) at 77 K. Samples were evacuated at 423 K under 0.13 Pa before the measurement. Pore volume was estimated from the amount of adsorbed nitrogen at the relative pressure of 0.95. Pore diameter was calculated by the BJH method for the adsorption branch. CO 2 adsorption isotherms were measured using a BELSORP-MAX-12-N-T-HL at different temperatures. N elemental analyses (EA) were carried out on an Elementer varioEL elemental analyzer.

Pulsed NMR Measurements
The mobility of amino moieties was evaluated by pulsed NMR measurements that measure relaxation time of 1 H which is one of the main constituent elements of amino moieties. The pulsed NMR measurements were performed with JEOL-JNM-MU25A spectrometer operating at 25 MHz for protons ( 1 H) in the phase-sensitive detection mode. Spin-spin relaxation time (T 2 ) was measured for ca. 1 g of sample in a 10 mmφ sample tube [29]. Samples were evacuated at 423K under 0.13 Pa before the measurement to remove adsorbed water. The pulse sequences for T 2 measurements were the solid echo pulse sequences (90˚xτ90˚y: 90˚ pulse width = 2 μs, τ = 8 μs, pulse repetition time = 4 -10 s, cumulated number = 64 -128). The measurements were conducted from 30˚C to 130˚C with 20˚C interval. The data for the decay process of the traverse magnetization decay signal M(t) is analyzed as the sum of the one Gaussian component for short T 2 and one single exponential component for long T 2 as expressed by Eq. (1): where A S and A L are the amplitude for the short and long component. T 2S and T 2L are the corresponding T 2 values. The fraction of short and long component, f S and f L are determined from A S and A L values.

Effect of Amount of AP on CO 2 Adsorption Behovior
Nanoporous silica samples obtained by co-condensation using 10, 30, and 50 mol % of AP-TMS are denoted as AP10, AP30, and AP50, respectively, according to the AP-TMS ratio in the synthesis. The content of nitrogen increases with increasing the ratio of AP-TMS, and results are listed in Table 1. Nitrogen adsorption-desorption isotherms of samples are shown in Figure 1. A steep increase between P/P 0 of 0.2 and 0.3 indicates the existence of mesopores in AP10. However, AP30 and AP50 are found to adsorb much less nitrogen than AP10. Physical properties of samples are also summarized in Table 1. Specific surface area, pore size and pore volume of AP10 are comparable to that in ref 20. Those values are much smaller for AP30 and AP50. Powder XRD measurements were conducted and corresponding XRD patterns are shown in Figure 2. A sharp peak around 2˚ ascribed to d 100 periodicity of mesopores is observed for AP10. The pattern is typical of mesoporous silica. No peak is observed for AP30 and AP50. This indicates that no regular pore structure exists in AP30 and AP50.     temperature. This tendency is quite normal. In contrast, the amount of CO 2 adsorbed increases with increasing temperature for AP30 and AP50 in some temperature range. The amount is the largest at 60˚C and decreases at higher temperature for AP30. This behavior is non-thermodynamic and does not follow the Clausius-Clapeyron equation. As for AP30, the pores are mostly fulfilled with amino moieties, and a few spaces exist at lower temperature. In this case, only a limited amount of CO 2 is captured. However, once amino moieties are more mobile and active at higher temperature, CO 2 molecules could easily penetrate into inside pores to be captured. The optimum temperature for AP50 is 80˚C which is higher than the temperature for AP30. Since much more amount of AP molecules are contained in pores of AP50, higher temperature would be needed for amino-moieties to be mobile.
It is found that CO 2 adsorption behavior changes drastically by adjusting the amount of amino moieties in nanoporous silica. AP10 adsorbs more CO 2 at lower temperature while AP30 and AP50 adsorb more CO 2 at higher temperature. It is interesting that the CO 2 adsorption behavior is highly affected by the ratio of AP loaded.

Mobility of Amino Moieties
It is mentioned that amino-moieties of AP30 or AP50 are more mobile at higher temperature. To investigate the effect of temperature on the mobility of amino moieties in samples, the pulsed NMR measurements were conducted for AP10 and AP30 that show different temperature dependence in CO 2 adsorption.
Normalized T 2 decay curves at different temperatures are shown in Figure 5. A curve is deconvoluted to a short and long component according to Equation (1). Table 2 summarizes spin-spin relaxation time for short (T 2S ) and long (T 2L ) component and corresponding fractions (f S , f L ) for AP10 and AP30. A short relaxation time means that molecules are rigid, and long relaxation time reveals molecules are more mobile.
As for AP10, T 2S and T 2L are almost the same value against temperature, indicating that the mobility of the AP molecules unchanged. However, the fraction of rigid component, f S , decreases and the more mobile component, f L , increases with increasing temperature. The behavior can easily be understood by plotting the data ( Figure 6). F S is 52% at 30˚C and the value decreases to 38% at 130˚C.
The exact opposite tendency is observed for AP30. F S and f L remain unchanged against temperature, and T 2L increases drastically with increasing temperature. The f S :f L ratio is almost 74:26 and the ratio remains unchanged upon temperature increase. The f S of AP30 is much larger than that of AP10, implying that the amount of rigid AP molecules is much larger for AP30. T 2S slightly increases from 11 μs to 16 μs whereas T 2L increases from 32 μs to 97 μs. The increase of the relaxation time for the mobile component is significant. T 2S for the rigid component of AP30 is much smaller than that of AP10, suggesting that many of AP molecules in AP30 are confined more tightly. This could be caused   by the incorporation of larger amount of AP molecules into nanopores. Conversely, T 2L of AP30 at higher temperature is bigger than that of AP10 although the values are lower at temperatures lower than 90˚C. Therefore, the increase of the amount of CO 2 adsorbed at higher temperature could be related to the increase of the mobility of AP molecules. Although the optimum temperature for CO 2 adsorption is different from the result obtained in the NMR experiments, the difference could be caused by the difference in atmosphere, with or without CO 2 , or the length of time for the measurement.
Since the fraction and mobility of soft components in AP10 changed little, the diffusion of CO 2 was not affected by the temperature. As a result, CO 2 adsorption occurred thermodynamically. In contrast, the mobility of soft components in AP30 increased drastically with increasing temperature, leading to non-thermodynamic adsorption behovior.

Conclusion
By increasing the amount of amino propyl moieties (AP) incorporated into nanoporous silica, CO 2 adsorption behovior changes drastically. A low AP loaded sample adsorbs more CO 2 at lower temperature while higher AP loaded samples adsorb more CO 2 at higher temperature. To understand the mechanism, a pulsed NMR technique was employed. It was found that the mobility of mobile component in a high AP loaded sample increased drastically with increasing temperature while the mobility in a low AP loaded sample remained unchanged. The increase in the mobility in the high AP loaded sample could enhance the diffusion of CO 2 inside nanopores leading to the non-thermodynamic adsorption behovior.