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Highly hydrophobic mesoporous materials as matrix for gas chromatography separation of water-alcohols mixtures
Progress in in Mesostructured Materials Recent Progress S. Qiu, Y. Tang and C. Yu (Editors) (Editors) D. Zhao, S. © 2007 2007 Elsevier Elsevier B.V. B.V. All All rights rights reserved. reserved.
885 885
Highly hydrophobic mesoporous materials as matrix for gas chromatography separation of water-alcohols mixtures Lianxiu Guan^, Junping Lia, Dongjiang Yangab, Xiuzhi Wanga, Ning Zhaoa, Wei Weia and Yuhan Suna* " State Key Laboratory of Coal Coversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, P. R. China; Graduate School of the Chinese Academy of Sciences, Beijing, 100039, China
1. Introduction Since the first mesoporous silica material with regular pore channels was reported in 1992 [1], the application of porous materials with tailored properties is one of the most attractive areas of materials science [2, 3]. Their extremely high surface areas, accessible pores and selective adsorption, which may enhance superior chromatographic performance to class silica-based columns by providing higher and more homogeneous molecular diffusivity, make these mesoporous materials potentially useful for chromatography separation. And it was found that the mesoporous materials were also proposed as possible stationary phases for size-exclusion chromatography [4], normal-phase HPLC [5], reversed-phase HPLC [6], capillary gas chromatography [7], and enantioselective HPLC [8]. However, the purely inorganic mesoporous materials are limited in applications by their lack of organic functional groups. The organic/inorganic hybrid mesoporous organic silicas obtained using a structure-directing template pathway were also limited in their chromatographic separation application because of their low thermal stability. Hence there is still a need to develop the tailored stationary phases with better extraction, enrichment, and separation behavior. Recently, the chloropropyl-modified MCM-41 was found to be the gas chromatography matrix for the separation of the water-alcohols [9], but the separation of the propanol isomers was still the problem, although there were some papers for the separation of alcohols [10,11]. Thus, a silica-based super hydrophobic materials were prepared using
886
nonsurfactant route [12,13] and firstly investigated as the gas chromatography matrix for the separation of complicated water-alcohols systems. 2. Experimental Section 2.1. Materials The materials were synthesized as in the reference [13]. In a typical synthesis, 1.5 g polymethylhydrosiloxane (PMHS) were dripped into a flasks containing 60 ml ethanol. The formed liquids were further stirred for 48 h at room temperature to allow PMHS to react with a part of ethanol and release hydrogen in the presence of NaOH as catalyst. Then 4.6 g of tetraethyl orthosilicate (TEOS) and determined deionized water were introduced to the systems with vigorous stirring for 3 h. The resultant sols were statically aged for 2 d to turn into gels. The obtained gels were heated in a 60°C vacuum oven to remove the ethanol. 2.2. Characterization Transmission Electron Microscope (TEM) images were recorded using a JEOL 100CX microscope with a CeB6 filament and an accelerating voltage of 200 kV. Nitrogen adsorption /desorption isotherms were obtained at -196°C on a Tristar 3000 Soptometer, using static adsorption procedures. Samples were degassed at 150°C for a minimum of 12 h under vacuum (10-6 Torr) prior to measurement. Surface areas were measured using the BET method and pore size distributions were calculated using the modified BJH method. 2.3. Chromatographic Tests In a typical column preparation, a stainless steel pump ( l m long, 3 mm id) was filled with 0.9 g materials (particle diameter of 0.15 ~ 0.18 mm). And then the packed column was aged for 24 h at 250°C. The separation performances of the samples were investigated on the GC 950 gas chromatograph equipped with a thermal conductivity detector. Hydrogen was used as the carrier gas and was driven at the inlet pressure of 0.2 Mpa.The separation experiments were carried out under the conditions: carried gas: H2; flow rate: 15 ml/min; temperature: 3. Results and Discussion Fig.la shows the representative TEM image of the sample. Obviously, it depicted a direct image of the 3D wormhole-like pore frameworks. The obtained hybrids were grinded and tabletted to measure the hydrophobicity. As
887
shown in Fig. lb, the water droplet with ~3 mm diameter on the tablets looked like a round ball, and the contact angle exceeded 150°, indicating the super hydrophobic nature of the materials.
Fig. 1 (a) HRTEM image and (b) Photographs of a water droplet on the tablets of the samples.
methanol
b
watel ll 1 lethanol
III niso propanol
Jl
0 ICPJ
50
ff-piopanol
"7 100
150
200
250
300
350
timelrT unutel
Fig. 2 (a) N 2 adsorption/desorption isotherm of the samples; (b)The hydrophobic mesoporous materials as matrix for the gas chromatographic separation of water-alcohols mixtures.
The N2 adsorption/desorption isotherms clearly illustrated that the sample exhibited type IV isotherm with type H2 hysteresis loop (see Fig. 2a), which further confirmed the mesoporous sturcture of the samples. The results were consistent with Yang's reports [13]. The BET surface area was 758.76 m2g"' with the pore volume of 0.62 cm3g'' and the pore diameter of 3.27 nm, respectively. The hydrophobic materials exhibited the high resolution and efficiency for the wa ter-alcohols mixtures (see Fig. 2a). The separation order was highly consistent with the polarity of alcohols and water. With the decrease of the polarity, the attraction between the stationary phase and these alcohols increased gradually, extending the retention time. Interestingly, the hydrophobic mesoporous material column could achieve the baseline separation of the propanol isomers, which was hardly achieved by the chloropropyl-modfied MCM-41 [9]. The selective separation of the propanol isomers on the hydrophobic column was the result of their different van der Waals interaction with the mesoporous walls. Futhermore, the nanochannels of mesoporous
888
materials were accessible to the linear chains, but not to the branched parts of the iso-propanol. As a result, the retention property of the linear n-propanol was stronger than the branched iso-propanol. At trie same time, the large amount of the hydrophobic methyl groups enlarged the difference of the polarity of the isomers, which also directly enhanced the different retention property. 4. Conclusion The hydrophobic mesoporous materials was synthesized and investigated as GC stationary phase for the separation of complicated water-alcohols systems. The hydrophobic mesoporous materials showed high resolution and efficiency for the tested systems. Importantly, the matrix achieved base-line separation of the propanol isomers via their different Van Der Waals interactions with functionalised surfaces in nanochannel. 5. References [1] C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. C. Beck, Nature, 359 (1992) 710. [2] A. Corma, L. Nemeth, M. Renz and S. Valencia, Nature, 412 (2001) 423. [3] A. Corma, M. J. Diaz-Cabanas, J. Martinez-Triguero, F. Rey and J. Rius, Nature, 418 (2002)514 [4] A. Kurganov, K. Ungerand T. Issaeva, J. Chromatogr. A., 753 (1996) 177. [5] M. Grun, A. A. Kunganow, S. Schacht, F. Schuth and K. K. Knger, J. Chromatogr. A., 740 (1996) 1. [6] T. Martin, A. Galarneau, F. D. Renzo, D. Brunei and F. Fajula, Chem. Mater., 16 (2004) 1725. [7] M. Raimondo, G. Perez, M. Sinibaldi, A. D. Stefanis and A. A. Tomlinson, Chem. Commun., (1997) 1343. [8] C. Thoelen, K. V. D. Walle, I. F. J. Vankelecom and P. A. Jacobs, Chem.Commun., (1999) 1841. [9] L. X. Guan, J. P. Li, H. Cao, N. Zhao, X. Z. Wang, W. Wei and Y. H. Sun, Chem. Letter., 35(2006)516. [10] D. Guillarme, S. Heinisch, J. Y. Gauvrit, P. Lanteri and J. L. Rocca, J. Chromatogr. A., 1078(2005)22. [11] C. L. Hsueh, J. F. Kuo, Y. H. Huang, C. C. Wang and C. Y. Chen, Sep. Purif. Technol, 41 (2005) 39. [12] D. J. Yang, S. R. Zhai, Y. Xu, J. L. Zheng, D. Wu, Y. H. Sun and F. Deng, Stud. Surf. Sci. Catal. 156(2005)473. [13] D. J. Yang, Y. Xu, S. R. Zhai, J. L. Zheng, J. P. Li, D. Wu and Y. H. Sun, Chem. Letter. 34 (2005)1138.
885 885
Highly hydrophobic mesoporous materials as matrix for gas chromatography separation of water-alcohols mixtures Lianxiu Guan^, Junping Lia, Dongjiang Yangab, Xiuzhi Wanga, Ning Zhaoa, Wei Weia and Yuhan Suna* " State Key Laboratory of Coal Coversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, P. R. China; Graduate School of the Chinese Academy of Sciences, Beijing, 100039, China
1. Introduction Since the first mesoporous silica material with regular pore channels was reported in 1992 [1], the application of porous materials with tailored properties is one of the most attractive areas of materials science [2, 3]. Their extremely high surface areas, accessible pores and selective adsorption, which may enhance superior chromatographic performance to class silica-based columns by providing higher and more homogeneous molecular diffusivity, make these mesoporous materials potentially useful for chromatography separation. And it was found that the mesoporous materials were also proposed as possible stationary phases for size-exclusion chromatography [4], normal-phase HPLC [5], reversed-phase HPLC [6], capillary gas chromatography [7], and enantioselective HPLC [8]. However, the purely inorganic mesoporous materials are limited in applications by their lack of organic functional groups. The organic/inorganic hybrid mesoporous organic silicas obtained using a structure-directing template pathway were also limited in their chromatographic separation application because of their low thermal stability. Hence there is still a need to develop the tailored stationary phases with better extraction, enrichment, and separation behavior. Recently, the chloropropyl-modified MCM-41 was found to be the gas chromatography matrix for the separation of the water-alcohols [9], but the separation of the propanol isomers was still the problem, although there were some papers for the separation of alcohols [10,11]. Thus, a silica-based super hydrophobic materials were prepared using
886
nonsurfactant route [12,13] and firstly investigated as the gas chromatography matrix for the separation of complicated water-alcohols systems. 2. Experimental Section 2.1. Materials The materials were synthesized as in the reference [13]. In a typical synthesis, 1.5 g polymethylhydrosiloxane (PMHS) were dripped into a flasks containing 60 ml ethanol. The formed liquids were further stirred for 48 h at room temperature to allow PMHS to react with a part of ethanol and release hydrogen in the presence of NaOH as catalyst. Then 4.6 g of tetraethyl orthosilicate (TEOS) and determined deionized water were introduced to the systems with vigorous stirring for 3 h. The resultant sols were statically aged for 2 d to turn into gels. The obtained gels were heated in a 60°C vacuum oven to remove the ethanol. 2.2. Characterization Transmission Electron Microscope (TEM) images were recorded using a JEOL 100CX microscope with a CeB6 filament and an accelerating voltage of 200 kV. Nitrogen adsorption /desorption isotherms were obtained at -196°C on a Tristar 3000 Soptometer, using static adsorption procedures. Samples were degassed at 150°C for a minimum of 12 h under vacuum (10-6 Torr) prior to measurement. Surface areas were measured using the BET method and pore size distributions were calculated using the modified BJH method. 2.3. Chromatographic Tests In a typical column preparation, a stainless steel pump ( l m long, 3 mm id) was filled with 0.9 g materials (particle diameter of 0.15 ~ 0.18 mm). And then the packed column was aged for 24 h at 250°C. The separation performances of the samples were investigated on the GC 950 gas chromatograph equipped with a thermal conductivity detector. Hydrogen was used as the carrier gas and was driven at the inlet pressure of 0.2 Mpa.The separation experiments were carried out under the conditions: carried gas: H2; flow rate: 15 ml/min; temperature: 3. Results and Discussion Fig.la shows the representative TEM image of the sample. Obviously, it depicted a direct image of the 3D wormhole-like pore frameworks. The obtained hybrids were grinded and tabletted to measure the hydrophobicity. As
887
shown in Fig. lb, the water droplet with ~3 mm diameter on the tablets looked like a round ball, and the contact angle exceeded 150°, indicating the super hydrophobic nature of the materials.
Fig. 1 (a) HRTEM image and (b) Photographs of a water droplet on the tablets of the samples.
methanol
b
watel ll 1 lethanol
III niso propanol
Jl
0 ICPJ
50
ff-piopanol
"7 100
150
200
250
300
350
timelrT unutel
Fig. 2 (a) N 2 adsorption/desorption isotherm of the samples; (b)The hydrophobic mesoporous materials as matrix for the gas chromatographic separation of water-alcohols mixtures.
The N2 adsorption/desorption isotherms clearly illustrated that the sample exhibited type IV isotherm with type H2 hysteresis loop (see Fig. 2a), which further confirmed the mesoporous sturcture of the samples. The results were consistent with Yang's reports [13]. The BET surface area was 758.76 m2g"' with the pore volume of 0.62 cm3g'' and the pore diameter of 3.27 nm, respectively. The hydrophobic materials exhibited the high resolution and efficiency for the wa ter-alcohols mixtures (see Fig. 2a). The separation order was highly consistent with the polarity of alcohols and water. With the decrease of the polarity, the attraction between the stationary phase and these alcohols increased gradually, extending the retention time. Interestingly, the hydrophobic mesoporous material column could achieve the baseline separation of the propanol isomers, which was hardly achieved by the chloropropyl-modfied MCM-41 [9]. The selective separation of the propanol isomers on the hydrophobic column was the result of their different van der Waals interaction with the mesoporous walls. Futhermore, the nanochannels of mesoporous
888
materials were accessible to the linear chains, but not to the branched parts of the iso-propanol. As a result, the retention property of the linear n-propanol was stronger than the branched iso-propanol. At trie same time, the large amount of the hydrophobic methyl groups enlarged the difference of the polarity of the isomers, which also directly enhanced the different retention property. 4. Conclusion The hydrophobic mesoporous materials was synthesized and investigated as GC stationary phase for the separation of complicated water-alcohols systems. The hydrophobic mesoporous materials showed high resolution and efficiency for the tested systems. Importantly, the matrix achieved base-line separation of the propanol isomers via their different Van Der Waals interactions with functionalised surfaces in nanochannel. 5. References [1] C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. C. Beck, Nature, 359 (1992) 710. [2] A. Corma, L. Nemeth, M. Renz and S. Valencia, Nature, 412 (2001) 423. [3] A. Corma, M. J. Diaz-Cabanas, J. Martinez-Triguero, F. Rey and J. Rius, Nature, 418 (2002)514 [4] A. Kurganov, K. Ungerand T. Issaeva, J. Chromatogr. A., 753 (1996) 177. [5] M. Grun, A. A. Kunganow, S. Schacht, F. Schuth and K. K. Knger, J. Chromatogr. A., 740 (1996) 1. [6] T. Martin, A. Galarneau, F. D. Renzo, D. Brunei and F. Fajula, Chem. Mater., 16 (2004) 1725. [7] M. Raimondo, G. Perez, M. Sinibaldi, A. D. Stefanis and A. A. Tomlinson, Chem. Commun., (1997) 1343. [8] C. Thoelen, K. V. D. Walle, I. F. J. Vankelecom and P. A. Jacobs, Chem.Commun., (1999) 1841. [9] L. X. Guan, J. P. Li, H. Cao, N. Zhao, X. Z. Wang, W. Wei and Y. H. Sun, Chem. Letter., 35(2006)516. [10] D. Guillarme, S. Heinisch, J. Y. Gauvrit, P. Lanteri and J. L. Rocca, J. Chromatogr. A., 1078(2005)22. [11] C. L. Hsueh, J. F. Kuo, Y. H. Huang, C. C. Wang and C. Y. Chen, Sep. Purif. Technol, 41 (2005) 39. [12] D. J. Yang, S. R. Zhai, Y. Xu, J. L. Zheng, D. Wu, Y. H. Sun and F. Deng, Stud. Surf. Sci. Catal. 156(2005)473. [13] D. J. Yang, Y. Xu, S. R. Zhai, J. L. Zheng, J. P. Li, D. Wu and Y. H. Sun, Chem. Letter. 34 (2005)1138.