High-pressure cation-exchange treatment of a ZSM-5 zeolite
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Yu Qiaoa) Department of Structural Engineering, University of California at San Diego, La Jolla, California 92093-0085 (Received 11 February 2009; accepted 20 April 2009)
Under ambient pressure, an aqueous solution may not enter the nanopores of a hydrophobic ZSM-5 zeolite, which imposes difficulties to cation-exchange treatment. In the current study, a high-pressure cation-exchange technique is developed. With a relatively short treatment time, the degree of hydrophobicity is significantly increased. I. INTRODUCTION
ZSM-5 zeolite is an aluminosilicate that usually has a high silica-to-alumina ratio.1–5 It contains two sets of nanopores. One set of nanopores is straight, with the ˚ ; the other set is Scross-sectional size of 5.3 5.6 A ˚ .6 An shaped, with the cross-sectional size of 5.2 5.5 A A1 atom at the center of an AlO4 tetrahedron connects to adjacent SiO4 tetrahedron by sharing an O atom and thus generates a negative framework charge counterbalanced by extra-framework cations, such as alkaline or alkalineearth cations. If the alumina content is relatively high, the nanopore surface can be acidic.7–9 Such ZSM-5 zeolites are chemically reactive and/or catalytically active, having been widely applied for hydrocarbon interconversion, catalysis, etc. Often, the as-synthesized ZSM-5 zeolites need to be further modified to control the properties of inner surfaces of nanopores, such as the degree of hydrophobicity. For instance, for active liquid spring or programmable catalysis, the liquid infiltration needs to be controlled by an external pressure, and the working pressure must be adjusted in an appropriate range.10–14 One commonly used technique is cation-exchange.15–17 As H-form zeolite crystals are immersed in an aqueous solution of electrolyte, cations can diffuse into the network. The framework defect sites can be effectively deactivated. Consequently, the nanopore walls become more nonpolar and less wettable to water; i.e., the effective degree of hydrophobicity increases. For a highly hydrophobic zeolite, however, the ordinary cation-exchange treatment can be relatively inefficient because the electrolyte solution does not enter the nanopores spontaneously, and thus the cation-exchange only takes place at positions close to the pore opening and the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0285
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 7, Jul 2009 Downloaded: 15 Feb 2015
process can be relatively slow. A promising alternative for introducing transition metal cations into extra-framework positions of zeolites is the solid-state ion-exchange technique.18–20 However, the process usually demands high vacuum or with inert gas protection.21,22 II. EXPERIMENTAL
In the current study, a ZSM-5 zeolite with a silica-toalumina ratio of 280 was investigated. The synthesis mixture was prepared by adding to deionized water at room temperature: tetraethoxysilane [(TEOS) 98%; SigmaAldrich, St. Louis, MO], aluminum isopropoxide (98%; Sigma-Aldri
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