Desorption Behavior of an Ionic Liquid

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Preparation and SO2 Sorption/Desorption Behavior of an Ionic Liquid Supported on Porous Silica Particles Zhengmin Zhang,† Linbo Wu,*,† Jie Dong,† Bo-Geng Li,*,† and Shiping Zhu*,‡ State Key Laboratory of Chemical Engineering, Department of Chemical and Biochemical Engineering, Zhejiang UniVersity, Hangzhou 310027, China, and Department of Chemical Engineering and Department of Materials Science and Engineering, McMaster UniVersity, Hamilton, Ontario, Canada L8S 4L7

The ionic liquid 1,1,3,3-tetramethylguanidinium lactate (TMGL) was supported onto porous silica particles via a facile impregnation-vaporization method. The TMGL-supported particles gave high porosity and large specific surface area. The SO2 sorption/desorption properties of the silica-supported TMGL (TMGL-SiO2) were evaluated, and high SO2 sorption capacity and rate were achieved. Its capacity reached 0.6 g SO2/g TMGL in 15-30 min with pure SO2 gas and 0.15 g SO2/g TMGL in 17 h with a N2/SO2 mixture gas that contained 2160 ppm SO2. The SO2 concentration was reduced to 12.6 ppm after sorption. The TMGL-SiO2 system could be reused for many sorption/desorption cycles without change in its capacity. It was also characterized by good mechanical strength and thermal stability at temperature up to 130 °C. The SO2 sorbent system appears to be useful in gas desulfurization. Introduction Ionic liquids (ILs) have attracted increasing attention from both academic researchers and industrial practitioners in the recent decade. They are promising materials for green media and catalysts in separation and reaction. It has also been extensively reported that ILs are able to dissolve or absorb some gases that include CO2,1-7 ethylene,5,6 ethane,6 SO2,7 etc. and thus provide new opportunities for developing applications in gas separation and purification. Among these reports, the separation and purification of SO2 and CO2 are of particular interest because the gas emission from various combustion processes has become the major source for air pollution and greenhouse effect. Han et al. first reported that 1,1,3,3-tetramethylguanidinium lactate (TMGL), a new ionic liquid, is able to absorb SO2 from a gas mixture of SO2 and N2 with high absorption capacity and high selectivity.7 Since then, numerous ILs with SO2 sorption ability have been reported,8-13 and their sorption mechanisms have been studied.8,14 In addition to high capacity and selectivity, the low volatility of ILs is another advantage for their applications in SO2 sorption as compared to traditional liquid organic amine SO2 sorbents. However, from the viewpoint of industrial practice, the high viscosity of ILs could be an operational drawback as well as a cause for slow sorption rate because of relatively small gas-liquid interface and slow diffusion rate of gas molecules in viscous media. In addition, a large amount of IL is required if it is directly used as liquid in an absorption process. However if it is immobilized and used as solid in an adsorption process, it significantly reduces the required IL amount and increases the rate of sorption owing to enhanced gas-liquid interface; and as a result, it lowers costs and improves efficiency. In our previous studies, we solidified some ILs via free radical solution and inverse suspension polymerization.15,16 Such polymerized ILs, including linear poly(1,1,3,3-tetramethylguani* To whom correspondence should be addressed. E-mail: wulinbo@ zju.edu.cn (L.W.); [email protected] (B.-G.L.); [email protected] (S.Z.). † Zhejiang University. ‡ McMaster University.

dinium acrylate) (PTMGA) and cross-linked poly(1,1,3,3tetramethylguanidinium acrylate-co-N,N-methylene bisacrylamide) P(TMGA-co-MBA) porous particles, were used as solid sorbents for dry desulfurization. It has been found that PTMGA and P(TMGA-co-MBA) sorbed SO2 with high selectivity, capacity, and rate and could be reused in SO2 sorption/desorption cycles. However, the PTMGA powder collapsed after SO2 sorption, possibly because of a plasticization effect resulting from the sorbed SO2 content. In contrast, the cross-linked P(TMGA-coMBA) particles remained their macroscopic shapes after SO2 sorption, but lost some internal pore structures. The internal pores almost disappeared after several cycles, and the particles adhered to each other to a certain extent. To obtain more practical solid IL-containing SO2 sorbents, we aim at immobilizing TMGL to porous silica particles in this work. By this strategy, the IL spread on the inner surface of particle pores. The porous structure, large specific surface area, and mechanical properties of the support can be maintained. This approach has an advantage for preparation of solid SO2 sorbents for dry desulfurization. Similar strategy has been applied in immobilized amines and ILs for CO2 adsorption.17-19 Here we report the immobilization of TMGL to porous silica particles, characterization of the porous particles, and SO2 sorption/desorption properties of the immobilized IL system, TMGL-SiO2. Experimental Part Materials. 1,1,3,3-Tetramethylguanidinium (TMG, g99.0%, Zibo Senjie Chemical Auxiliary Co., Ltd., China), L-lactic acid (86%, optical purity 98%, Jiangxi Musashino Bio-Chem Co., Ltd., China) and silica particles (SiO2, SI 1700, Grace Company) are all used without further purification. Sulfur dioxide (SO2, g98.0%), nitrogen (N2, g99.5%), and N2/SO2 mixture gas with 2160 ppm SO2 were all purchased from Hangzhou Jinggong Special Gas Co. Ltd., China and used as received. The ionic liquid, 1,1,3,3-tetramethylguanidinium lactate (TMGL), was prepared via direct neutralization of TMG and L-lactic acid as previously reported.7 Immobilization of TMGL. Two methods were adopted in the IL immobilization-impregnation-extraction method and

10.1021/ie801165u CCC: $40.75  2009 American Chemical Society Published on Web 01/15/2009

Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 2143 Scheme 1. Experimental Setup for Measuring the SO2 Concentration before and after SO2 Sorptiona

Figure 1. Particle size distribution of silica SI-1700.

valve 3 was closed and valves 4 and 5 were turned on at the same time to let the gas pass through the sorbent. The change in the SO2 concentration was recorded with time. a (1) Mixture gas cylinder; (2-5) valves; (6) paraffin bath; (7) fuel gas analyzer; (8) gas outlet (adsorbed by NaOH solution before venting).

impregnation-vaporization method. In a typical impregnationextraction experiment, 1 g SI-1700 was added to excess amount of TMGL and stirred. The mixture was then placed in a Soxhlet extractor that contained ethanol and was extracted for 48 h. The resulting particles were dried and stored prior to use. In a typical impregnation-vaporization experiment, 1.5 g TMGL and 10 mL ethanol were mixed. A 3 g portion of SI-1700 silica gel was then added to the solution and the mixture was stirred for 50 min. The ethanol in the mixture was removed by drying at room temperature for 1 h, at 45 °C for 4 h and at 80 °C for 12 h. The resulting TMGL-containing porous silica particles (denoted as TMGL-SiO2) were stored in a desiccator before particle characterization and SO2 sorption/desorption experiments. The weight ratio of immobilized TMGL to silica support (TMGL/SiO2) was controlled in the range from 0.25/1 to 2/1 by adjusting the feed ratio. Characterization. The silica particle size distribution before TMGL immobilization was determined with LS-230 Coulter particle size analyzer using water as medium. The pore size distributions and porosities of SiO2 and TMGL-SiO2 particles were measured with a mercury intrusion method (PoreMaster60, Quantachrome Co.). Thermogravimetric analysis (TGA) of TMGL and TMGL-SiO2 was conducted on a Perkin-Elmer Instrument Pyris 1 TGA under nitrogen with a heating rat of 10 °C/min. SO2 Sorption and Desorption. Both pure SO2 gas and N2/ SO2 mixture containing 2160 ppm SO2 were used in the sorption experiments. For selectivity characterization, pure N2, CO2 and H2 were also employed. The experiment setup and method were the same as previously described15,16 except that the gas rate was about 100 mL/min. The SO2 sorption and desorption were determined gravimetrically. The experiments of SO2 sorption and desorption were repeated to give an estimate of the experimental errors involved in the method. The error was at a level of (5%. For determination of SO2 concentration in the mixture gas after sorption, a fuel gas analyzer (Rose-Mount NGA 2000) was connected to a glass U-tube that was filled with the sorbent (TMGL-SiO2 1/1, 4.2250 g), as shown in Scheme 1. The measurement was carried out with a gas rate of 500 mL/min at 20 °C. The N2/SO2 gas first passed through valve 3 (with valves 4 and 5 closed). When the gas flow was stabilized and the SO2 concentration measured by the analyzer reached a constant level,

Results and Discussion Immobilization and Characterization. To immobilize an IL for SO2 sorption, the selections of support type and immobilization method are important. Chemical immobilization is often preferred for volatile compounds18 because it prevents loss of the immobilized species, but it often bears a high cost. However, in immobilizing an IL for gas-solid applications,19 a relatively weak physical interaction between support and the IL may be adequate owing to IL’s very low volatility. Therefore, a simple and inexpensive physical immobilization method was chosen in this study. In addition to large porosity, large specific surface area, and good mechanical strength, other important requirements in selecting support materials include its affinity to IL (i.e., wettability resulted from close polarity and H-bond interactions between IL and the support) and resistance to acids. An appropriate level of affinity facilitates immobilization and enhances resistance to washing-away effect. Acid resistance is essential because SO2 and other acidic gases such as SO3, CO2, and NO2 in flue gases can acidify and erode support materials. In this work, porous silica was selected as the support. The Si-O structure provides good mechanical strength and acid resistance. The Si-OH group on the particle surface may form hydrogen bond with carbonyl and/or hydroxyl group in TMGL. The particle size distribution of silica SI-1700 is shown in Figure 1. It is a multipeak distribution and a majority of particles have diameters of tens of micrometers. Its porous morphology was illustrated in Figure 2A. Through the mercury intrusion measurement, the specific surface area and porosity were estimated to be 354 m2/g and 85% (see Table 1), respectively. TMGL was immobilized on SI-1700 through the impregnation-extraction process or the impregnation-vaporization process. In brief, in the impregnation-extraction process, the silica particles were first added into TMGL and were then extracted the extra TMGL using a Soxhlet extractor with ethanol. In the impregnation-vaporization process the silica particles were first impregnated with an IL solution, and the silica particles with immobilized TMGL (denoted as TMGL-SiO2) were then obtained after solvent evaporation. The amount of solvent was adjusted according to the TMGL/SiO2 feed ratio to ensure that the particles were fully immerged in the IL solution. Solvent evaporation was assisted with stirring. In the impregnation-evaporation process, the weight ratio of TMGL/SiO2 in the TMGL-SiO2 system was determined by their feed ratio that ranged from 0.25/1 to 2/1. Figure 3 shows the TGA curve of the immobilized TMGL-SiO2 via the

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Figure 2. SEM image of the external morphology of SI-1700 silica particles before (A) and after immobilization (B, TMGL/SiO2 1/1) and the internal morphology after immobilization (C and D). Table 1. Pore Characteristics of Original SiO2 and TMGL-SiO2 Samples Prepared by the Impregnation-Vaporization Process with Various TMGL/SiO2 Ratiosa TMGL/ SiO2

φv,TMGL (%)b

Vp (cm3/g)

Ap (m2/g)

rp (nm)

Fa (g/cm3)

φvp (%)

0/1 0.25/1 0.5/1 0.75/1 1/1 1.5/1 2/1

0 8.85/8.33 17.7/22.0 26.6 /41.3 35.4 /51.1 53.1/67.0 70.8 /70.5

2.64 2.42 2.06 1.55 1.29 0.87 0.78

354 241 161 91.2 59.7 35.3 32.4

14.9 20.2 25.6 34.0 43.3 49.2 48.1

1.33 1.30 1.45 1.48 1.46 1.47 1.45

85.2 75.9 75.0 69.7 65.3 56.1 53.0

a φv,TMGL, Vp, Ap, rp, Fa, φvp are volume fraction of TMGL in pores, total intrusion volume, specific surface area, average pore radius, apparent density, and porosity, respectively. b The first φv,TMGL value is calculated from x/FTMGL/2.64, and the second, from (1 - Vp/2.64), where x is the TMGL/SiO2 weight ratio, FTMGL is the density of TMGL, 1.07 g/cm3, and 2.64 is the Vp of SiO2.

Figure 3. TGA curves of the TMGL-SiO2 samples prepared by the impregnation-extraction process and by the impregnation-vaporization process at a 1/1 TMGL/SiO2 feed ratio, as well as that of the pure TMGL.

impregnation-extraction process and that of TMGL-SiO2 prepared with a 1/1 TMGL/SiO2 feed ratio via the impregnationvaporization process, together with that of the unsupported

TMGL. TMGL started to decompose at 130 °C, and it decomposed almost completely at 300 °C, leaving 1.4% char. TMGL-SiO2 showed the same thermal stability as TMGL, being stable until 130 °C. The initial weight loss of about 2% in both samples was resulted from absorbed water as TMGL is very hygroscopic. Taking into account the initial weight loss and remained char, we calculated the TMGL/SiO2 ratio in the sample by the impregnation-extraction process to be 8.1% and by the impregnation-vaporization process to be 48.2%. The latter is in accordance with the feed ratio. Although the silica gel contained extra amount of TMGL in the impregnationextraction process, little TMGL remained after ethanol extraction. However, in the impregnation-vaporization process, little TMGL was lost. The immobilization ratio can be well controlled by the feed ratio. Therefore, we provided a systematic study on the characterization and SO2 sorption/desorption evaluation of the TMGL-SiO2 samples prepared by the impregnationvaporization process in this work. The TMGL/SiO2 feed ratio was directly used to denote the TMGL/SiO2 ratio in the samples and the TMGL-SiO2 x/1 was used to denote TMGL-SiO2 with an x/1 TMGL/SiO2 ratio. After TMGL immobilization, the silica particles maintained white color. The TMGL-SiO2 samples with a TMGL/SiO2 ratio less than 1.5/1 appeared to be dry powdery, and those with the TMGL/SiO2 ratios equal to or higher than 2/1 appeared to be wet on their surfaces. The particle morphological images of TMGL-SiO2 1/1 are shown in Figure 2B. The porous morphology of SiO2 was basically maintained after the TMGL immobilization, but the pore density on the particle surface reduced to a certain degree. However, an examination of the internal pores by cutting the particles (Figure 2C and D) revealed unchanged porous morphology. The pore characteristics of TMGL-SiO2 are summarized in Table 1. Increasing the amount of immobilized TMGL decreased the intrusion volume (Vp), specific surface area (Ap), and porosity (φvp), but increased the average pore radius (rp) and apparent

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Figure 4. Pore size distributions of the original SiO2 (0/1) and TMGL-SiO2 samples with various TMGL/SiO2 ratios (0.25/1-2/1).

Figure 6. SO2 sorption/desorption curves of blank SiO2 and TMGL-SiO2 samples with various TMGL/SiO2 ratios. The SO2 sorption was carried out at 30 °C and under 0.1 MPa, and the desorption was carried out at 90 °C and under 80 mmHg pressure. The SO2 sorption of pure SiO2 was calculated in grams SO2 per gram SiO2.

Figure 5. Sorption behaviors of TMGL and TMGL-SiO2 1/1 for SO2, CO2, N2, and H2.

density (Fa). While the decrease in intrusion volume, specific surface area, and porosity, as well as the increase in apparent density were anticipated with the addition of TMGL materials, the increase in average pore size needs to be explained. Figure 4 shows the change in the particle pore size distribution. There were both nanoscale and microscale pores in the original silica particles. After TMGL immobilization, the micropores remained but the nanoscale pores diminished gradually and disappeared finally with further increase in the TMGL/SiO2 ratio. Therefore, the average pore size increased with increasing TMGL/SiO2 ratio. The differences in the pore characteristics led to different SO2 sorption behaviors. SO2 Sorption and Desorption Behaviors. The SO2 sorption experiments were carried out at room temperature and under atmospheric pressure, and the desorption experiments were conducted at high temperature and under reduced pressure. After SO2 sorption, the morphological properties of TMGL-SiO2 particles remained unchanged except that their color changed from white to light yellow. No color change was observed for the blank SiO2 itself after SO2 sorption. The change of TMGL-SiO2 color was not as significant as that reported for poly(TMGA) and poly(TMGA-co-MBA).16,17 This is because the color appeared weaker in the presence of bright silica particles. The particles maintained their mechanical strength and no adhesion or agglomeration phenomenon was observed in the samples with a TMGL/SiO2 ratio below 2/1. Figure 5 shows the sorption of TMGL and TMGL-SiO2 1/1 for SO2, CO2, N2, and H2 at 20 °C and under atmospheric pressure. The sorption capacity for SO2 of pure TMGL reached 0.88 g/g TMGL (2.8 mol/mol TMGL), but those for CO2, N2, and H2 were less than 0.0185, 0.0394, and 0.0099 g/g TMGL (or 0.086, 0.288, and 1.01 mol/mol TMGL), respectively. After immobilization, the sorption capacity for SO2 was maintained

Figure 7. SO2 sorption/desorption curves of pure TMGL and TMGL-SiO2 samples with 1/1 and 0.5/1 TMGL/SiO2. The SO2 sorption was carried out at 20 °C and under 0.1 MPa, and the desorption was carried out at 90 °C and under 80 mmHg pressure.

(0.87 g/g TMGL) but those for CO2, N2, and H2 decreased to 0.0081, 0.0078, and 0.0086 g/g TMGL (or 0.038, 0.057, and 0.88 mol/mol TMGL), respectively. Although experimental errors could be larger due to the lower sorption capacities of CO2, N2, and H2, these results suggest that both TMGL and the immobilized TMGL-SiO2 have good selectivity for SO2 sorption. Figure 6 shows the SO2 sorption/desorption curves of blank SiO2 and TMGL-SiO2 samples from 0.25/1 to 2/1 ratio. The pure porous silica adsorbed SO2 with a capacity of 0.15 g SO2/g SiO2. However, its sorption capacity increased greatly after TMGL immobilization, reaching 0.55-0.7 gSO2/gTMGL. The SO2 adsorbed on SiO2 was desorbed completely, but some residual SO2 adsorbed on TMGL-SiO2 remained after desorption for a long period of time. It suggests that the interaction between SO2 and TMGL is stronger than that between SO2 and SiO2. Similar incomplete desorption has been also observed in the ionic liquid TMGA and IL polymers such as poly(TMGA) and poly(TMGA-co-MBA).16,17 It is also evident in Figure 6 that the SO2 sorption rates of TMGL-SiO2 were very high, reaching their equilibrium sorption capacities in about 15 min at 30 °C. The sorption rates were also very high at 20 °C and were much higher than that of pure TMGL, as shown in Figure 7. The rapid sorption is attributed to the large surface area of SiO2 particles on which the IL spread as a thin film. Figure 6 also shows that the SO2 sorption capacity (g SO2/g TMGL) depended on the TMGL/SiO2 ratio. When the TMGL/ SiO2 ratio was less than 1, the sorption capacity decreased with

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Figure 8. Adsorption/desorption cycles of TMGL-SiO2 1/1.

increasing the ratio, but it leveled off when the ratio exceeded 1. Figure 7 also shows that TMGL-SiO2 1/1 exhibited almost the same sorption capacity in comparison to pure TMGL, but TMGL-SiO2 0.5/1 gave a higher sorption capacity. This trend may be attributed to the change in specific surface that was changed in a similar manner (see Table 1). The sorption capacity increased with decreasing sorption temperature. It increased from 0.61 g SO2/g TMGL at 30 °C to about 0.88 g SO2/g TMGL at 20 °C for TMGL-SiO2 1/1, and from 0.67 to 0.95 for TMGL-SiO2 0.5/1. Wu et al. reported an absorption capacity of 1.7 mol SO2 per mol TMGL (equal to 0.53 g SO2/g TMGL) for TMGL ionic liquid at 40 °C and 1.2 bar.7 A comparison of our work to theirs also suggests the significant effect of temperature on the sorption capacity. The effect of the TMGL/SiO2 ratio on the sorption capacity can be explained by the different pore characteristics (especially nanopores) shown in Table 1 and Figure 4, though the SO2 sorption mechanism is complex.7,8 According to our previous study, the physical and chemical adsorptions at surface and the absorption in bulk all contributed to the SO2 sorption in polyILs.17 The specific surface area of the immobilized IL was larger at the smaller TMGL/SiO2 ratios. When the TMGL/SiO2 ratio was less than 0.75/1, the nanopore structure remained in TMGL-SiO2. The specific surface area was of 90-350 m2/g at a TMGL/SiO2 ratio less than 0.75/1, but it became less than 50 m2/g when the TMGL/SiO2 ratio was higher than 1/1 at which the nanopores almost vanished. In addition, SO2 could also be liquefied via capillary agglomeration in the nanopores which might enhance the sorption capacity. The nanopore structure played an important role in the SO2 sorption. The immobilized IL materials were used in multiple sorption/ desorption cycles. Figure 8 shows a typical set of sorption/ desorption cycle data for TMGL-SiO2 1/1. The materials were stable and maintained their sorption and desorption properties in the cycles. In a single cycle, about 0.54 g SO2 was separated. This value is equivalent to 1.73 mol SO2 per mole TMGL. Since TMGL is very hygroscopic, water sorption becomes unavoidable during preparation, storage, and application of TMGL-SiO2. The following experiments were performed to assess the effect of water sorption on SO2 sorption/desorption properties. TMGL-SiO2 1/1 was preserved with a cup of water in an airtight space for various periods of time. The samples thus contained different amount of sorbed water (based on the mass of TMGL). These samples were then used in SO2 sorption/desorption experiments. For comparison, TMGLs containing the same amounts of water were prepared by adding water directly into TMGL and mixing them homogenously and were also used in SO2 sorption/desorption. The results are illustrated in Figure 9.

Figure 9. SO2 sorption/desorption curves of TMGL (A) and TMGL/SiO2 1/1 (B) containing various amount of water sorbed.

Clearly, the water content had a negative effect on the SO2 sorption of both TMGL and TMGL-SiO2. The more water the sample contained, the less SO2 it sorbed, suggesting there is a competition of sorption between water and SO2. Fortunately, the reduction of SO2 sorption capacity of TMGL-SiO2 is relatively lower than that of TMGL at the same water content. The difference may result from that only partial water was dissolved in TMGL since another part was adsorbed on the bald SiO2 surface that had not been covered with TMGL. The sorption rate of TMGL accelerated with increasing the water amount in TMGL, probably because the viscosity of IL was decreased when diluted with water. But for TMGL-SiO2, the sorption rate remained almost unchanged after water was sorbed. This indicates that after immobilization, the contribution of the large specific surface area of the support make the viscosity of IL no longer be a crucial factor that limits the sorption rate. In the desorption process, water and SO2 were removed together. Their total amount of desorption is shown in Figure 9. It is recommended that attention should be paid to avoid exposure to water and moisture in the immobilized IL preparation and preservation. Finally, the SO2 sorption behavior of TMGL-SiO2 1/1 from a simulated fuel gas, a mixture gas of nitrogen and SO2 containing 2160 ppm SO2, was examined. The result is shown in Figure 10. The SO2 sorption selectivity maintained, but the sorption rate and capacity were not as good as those for pure SO2 gas. The sorption capacity reached an equilibrium value of 0.15 g SO2/g TMGL after 17 h. It was simply because the concentration of SO2 in the mixture gas was lower. However, the rate and capacity values were higher than those of poly(TMGA) and poly(TMGA-co-MBA). In addition, it was challenging to remove the sorbed SO2 from TMGL-SiO2 (1/1), as shown in Figure 10. The operation conditions need to be screened to improve the cycle capacity.

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from 2160 to 12.6 ppm. The TMGL-SiO2 having larger specific surface area and more nanoscale pores showed higher sorption capacity. Such immobilized IL materials could be reused in many sorption/desorption cycles, with their sorption capacity and rate remaining unchanged. This work demonstrated the potential application of this type of immobilized ILs as SO2 sorbent in gas desulfurization. However, it should be pointed out that there are other challenges to be tackled, including sorption ability and chemical stability with other fuel gas components such as NOx, HCN, HF, and CO, as well as deleterious effect of fine dust. Figure 10. Sorption behaviors of TMGL-SiO2 1/1 and SiO2 for N2 and N2/SO2 mixture gas containing 2160 ppm SO2.

Acknowledgment We thank 863 Project of China (2008AA062302), NSF of China (20304012, 20428605, and 20674067), and NSF of Zhejiang Province (Y404084 and Y407038) for financial supports as well as Prof. Kunzan Qiu for kind assistance in measuring the final concentration of SO2 in the down stream of the mixture gas after sorption with the fuel gas analyzer. S.Z. also thanks the JB program of NSFC and the CJC Program of Chinese Ministry of Education as well as Zhejiang University for supporting the collaboration. Literature Cited

Figure 11. SO2 concentration in the down stream of a mixture gas of N2 and SO2 containing 2160 ppm SO2 during SO2 sorption experiment: TMGL-SiO2 1/1 (4.2550 g), gas rate 500 mL/min, 20 °C.

A fuel gas analyzer was used to determine the final concentration of SO2 in the down stream of the mixture gas after sorption. The measurement was carried out with a gas rate of 500 mL/min (superficial gas velocity ca. 0.1 m/s) at 20 °C, using 4.225 g TMGL-SiO2 1/1. Figure 11 shows the result. The SO2 concentration in the mixture gas was 2160 ppm, which was measured from the bypass (see Scheme 1). After switching the gas flow to the TMGL-SiO2 U-tube, the SO2 concentration decreased rapidly and dramatically, reaching 36.5 ppm in 20 min and 12.6 ppm in 54 min. The SO2 concentration then increased gradually and recovered its initial concentration. The sorption efficiency was 99.4%, which is better than the conventional SO2 sorption processes.20 From the downstream SO2 concentration and gas rate data, the SO2 sorption in TMGL-SiO2 was estimated and is also shown in Figure 11. The SO2 sorption reached 0.13 g/g TMGL in 130 min and followed the same trend as shown in Figure 10. The shorter sorption time was clearly resulted from the much higher gas rate (from 100 to 500 mL/min). Conclusions TMGL, a TMG-based IL, was immobilized into porous silica particles by a simple impregnation-vaporization method. The pore characteristics and SO2 sorption/desorption properties of the immobilized IL (TMGL-SiO2) samples were investigated. TMGL-SiO2 maintained its porous morphology, having high porosity, large specific surface area, and good mechanical properties. The porosity, specific surface area, and pore size distribution depended on the TMGL/SiO2 ratio. TMGL-SiO2 gave high sorption rate and SO2 capacity from pure SO2 gas, as well as simulated fuel gas. The SO2 concentration in a simulated fuel gas was readily reduced

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ReceiVed for reView July 29, 2008 ReVised manuscript receiVed November 30, 2008 Accepted December 4, 2008 IE801165U