Langmuir 2007, 23, 3095-3102
3095
Effect of Pore Structure in Mesoporous Silicas on VOC Dynamic Adsorption/Desorption Performance Katsunori Kosuge,* Shiori Kubo, Nobuyuki Kikukawa, and Makoto Takemori Research Institute for EnVironmental Management Technology, National Institute of AdVanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki, 305-8569 Japan ReceiVed September 7, 2006 The dynamic adsorption/desorption behavior of volatile organic compounds (VOCs) such as toluene (C7H8) and benzene (C6H6) was evaluated for three kinds of mesoporous silicas of SBA-15, all having almost the same mesopore size of ca. 5.7 nm, and a MCM-41 silica with a smaller pore size of 2.1 nm using a continuous three-step test. The fiberlike SBA-15 silica exhibited exceptionally good breakthrough behavior, a higher VOC capacity, and easier desorption. The fiberlike silica was composed through the catenation of rodlike particles. The rodlike silicas, by comparison, were proven to be less useful in dynamic adsorption processes because of lower dynamic VOC capacities despite having comparative porous parameters with the fiberlike silica. The large dynamic VOC capacity of the fiberlike silica was attributed to the presence of a bimodal pore system consisting of longer, one-dimensional mesopore channels connected by complementary micropores.
Introduction The removal of volatile organic compounds (VOCs) prior to their emission into the atmosphere is a serious challenge in many industrial processes.1-4 Sorption is a proven and reliable chemical engineering method that provides the additional benefit of recovering valuable VOCs for reuse. The most common concentration range for VOCs emitted in most industrial processes is in the low-pressure region below 2000 ppm, in which microporous adsorbents with a pore size of less than 2 nm are desirable for their practical dynamic adsorption properties. To these ends, activated carbons are the most widely used materials. However, activated carbons have the drawbacks of added fire risk, pore clogging, hygroscopicity, and a lack of regenerative ability. For these reasons, it is important to investigate the adsorption/desorption performance of other types of microporous adsorbents such as zeolites,2,3,5-7 silica gels,8,9 and mesoporous silicas.10-20 * To whom correspondence should be addressed. Phone: +81-29-8618179. Fax: +81-29-861-8459. E-mail:
[email protected]. (1) Foster, K. L.; Fuerman, R. G.; Economy, J.; Larson, S. M.; Rood, M. J. Chem. Mater. 1992, 4, 1068. (2) Brosillon, S.; Manero, M.-H.; Foussard, J.-N. EnViron. Sci. Technol. 2001, 35, 3571. (3) Pires, J.; Carvalho, A.; Veloso, P.; de Carvalho, M. B. J. Mater. Chem. 2002, 12, 3100. (4) Ghoshal, A. K.; Manjare, S. D. J. Loss PreV. Process Ind. 2002, 15, 413. (5) Brihi, T. E.; Jaubert, J.-N.; Barth, D.; Perrin, L. J. Chem. Eng. Data 2002, 47, 1553. (6) Baek, S.-W.; Kim, J.-R.; Ihm, S.-K. Catal. Today 2004, 93-95, 575. (7) Hernandez, M. A.; Corona, L.; Gonzalez, A. I.; Rojas, F.; Lara, V. H.; Silva, F. Ind. Eng. Chem. Res. 2005; 44; 2908. (8) (a) Suzuki, T.; Tamon, H.; Okazaki, M. In Proceedings of the Fifth International Conference on Fundamentals of Adsorption; Le Van, M. D., Ed.; Kluwer Academic Publishers; Boston, MA, 1996; pp 897-904. (b) Suzuki, T.; Tamon, H.; Okazaki, M. In Proceedings of the Fifth International Conference on Fundamentals of Adsorption; Le Van, M. D., Ed.; Kluwer Academic Publishers; Boston, MA, 1996; pp 905-912. (9) (a) Herna´ndez, M. A.; Velasco, J. A.; Asomoza, M.; Solı´s, S.; Rojas, F.; Lara, V. H.; Portillo, R.; Salgado, M. A. Energy Fuels 2003, 17, 262. (b) Herna´ndez, M. A.; Velasco, J. A.; Asomoza, M.; Solı´s, S.; Rojas, F.; Lara, V. H. Ind. Eng. Chem. Res. 2004, 43, 1779. (10) (a) Zhao, X. S.; Ma, Q.; Lu, G. Q. Energy Fuels 1998, 12, 1051. (b) Zhao, X. S.; Lu, G. Q.; Hu, X. Colloids Surf. A 2001, 179, 261. (11) Choudhary, V. R.; Mantri, K. Langmuir 2000, 16, 7031. (12) Qiao, S. Z.; Bhatia, S. K.; Nicholson, D. Langmuir 2004, 20, 389. (13) Serrano, D. P.; Calleja, G.; Botas, J. A.; Gutierrez, F. J. Ind. Eng. Chem. Res. 2004; 43, 7010.
Mesoporous silicas with high surface areas have attracted a great deal of attention due to their wide range of application as adsorbents for environmentally hazardous chemicals, reaction catalysts, catalyst supports, chemical sensors, and electrical and optical devices.21 Mesoporous silicas have great potential for application as adsorbents for the removal of VOCs given their uniform pore size, open pore structure, and in particular, reliable desorption performance. However, studies on the dynamic adsorption/desorption performance of mesoporous silicas in regards to VOCs have been very limited in comparison to those on their adsorption equilibria.14,20 Although the saturation adsorption capacity for mesoporous silicas is high, the adsorption capacity is considerably lower in the low-pressure range. It is well known that mesoporous silicas obtained through triblock copolymer templating such as SBA-15 have complementary micropores in the silica walls connecting the primary one-dimensional (1D) mesopore channels to form a well-ordered 2D hexagonal array.22 These coexisting pore structures of mesopores and micropores have been called a “bimodal pore system”.22a,23 SBA-type mesoporous materials with this bimodal pore system are expected to be ideal as catalysts and adsorbents. Previous work from other research groups has indicated that SBA-15-type mesoporous silicas have a high affinity for various (14) Lee, J. W.; Shim, W. G.; Moon, H. Microporous Mesopororous Mater. 2004, 73, 109. (15) Fox, J. P.; Bates, S. P. Langmuir 2005, 21, 4746. (16) (a) Hartmann; M.; Bischof, C. J. Phys. Chem. B 1999, 103, 6230. (b) Shim, W. G.; Lee, J. W.; Moon, H. Microporous Mesopororous Mater. 2005, 88, 112. (17) (a) Newalkar, B. L.; Choudary, N. V.; Kumar, P.; Komarneni, S.; Bhat, T. S. G. Chem. Mater. 2002, 14, 304. (b) Newalkar, B. L.; Choudary, N. V.; Turaga, U. T.; Vijayalakshmi, R. P.; Kumar, P.; Komarneni, S.; Bhat, T. S. G. Chem. Mater. 2003, 15, 1474. (18) Hartmann; M.; Vinu, A. Langmuir 2002, 18, 8010. (19) Ueno, Y.; Tate, A.; Niwa, O.; Zhou, H-S.; Yamada, T.; Honma, I. Chem. Commun. 2004, 746. (20) Hu, X.; Qiao, S.; Zhao, X. S.; Lu, G. Q. Ind. Eng. Chem. Res. 2001, 40, 862. (21) Stein, A. AdV. Mater. 2003, 15, 763. (22) (a) Kruk, M.; Jaroniec, M.; Ko, C. H.; Ryoo, R. Chem. Mater. 2000, 12, 1961. (b) Clere, M. I.; Davidson, P.; Davidson, A. J. Am. Chem. Soc. 2000, 122, 11925. (23) (a) Liu, J.; Zhang, X.; Han, Y.; Xiao, F.-S. Chem. Mater. 2002, 14, 2536. (b) Yang, C.-M.; Zibrowius, B.; Schmidt, W.; Schu¨th, F. Chem. Mater. 2004, 16, 2918.
10.1021/la062616t CCC: $37.00 © 2007 American Chemical Society Published on Web 02/01/2007
3096 Langmuir, Vol. 23, No. 6, 2007
Kosuge et al.
Table 1. Porous Properties and VOCs’ Adsorption/Desorption Performance of Various Adsorbentsa volumetric adsorption properties
dynamic adsorption properties
porous parameters by N2 sorption isotherms
sample name Fiber A Rod B Rod C MCM-41 Q3 HY AC
C6H6
C7H8
volumetric volumetric amount adsorption desorbed desorption adsorption C7H8 C6H6 amount desorption Vmicro Dpore capacity SBET capacity capacity Vtotal by TPD ratio by capacity He desorbed by ratio by (m2/g) (mL/g) (mL/g) (nm) (mL/g) (mL/g) He (mL/g) (mL/g) purge (%) (mL/g) TPD (mL/g) purge (%) 730 757 430 1154 725 732 1146
0.63 0.65 0.46 0.75 0.40 0.36 0.47
0.12 0.11 0.02 0 0.36 0.47
5.62 5.84 5.46 2.1 0.8 1.86
0.54 0.52 0.42 0.70 0.36 0.33 0.45
0.52 0.48 0.44 0.66 0.39 0.33 0.46
0.042 0.022 0.017 0.015 0.029 0.058 0.110
0.013 0.001 0.003
