The discovery of carbon nanotubes (CNTs) has excited worldwide research interest for the prospect of developing novel carbon-based nanomaterials due to their unique structure-dependent electronic and mechanical properties 1-4. In order to optimize the use of nanotubes in various applications, such as catalyst supports in heterogeneous catalysis 5, field emitters 6, high-strength engineering fibers 3, sensors 7, tips for scanning probe microscopy 8, gas storage media 9, and as molecular wires for the next generation of electronic devices 10, it is necessary to attach functional groups or other nanostructures on the surfaces because the physical and chemical properties of carbon nanotubes can be dramatically influenced by surface modification with organic and inorganic species. Recently, there has been great interest in developing the potential of carbon nanotubes for use as a catalyst support that can offer unparalleled flexibility in tailoring catalyst properties 5. Chemical modification is a common method and is essential for the deposition of catalysts and other species onto nanotube surfaces for nanocatalytic and sensor applications 11, For example, carbon nanotubes were modified with H_2SO_4-HNO_3 12 to generate carboxylic acid groups 13, in which the density and strength of the acid group sites attached on the surface of CNTs is still lower (0.2-0.5 at. percent). Therefore, further developments in the modification of carbon nanotubes are still required. In this letter, sulfonated carbon nanotubes with a high density of sulfonic acid groups were first prepared by reacting concentrated sulfuric acid with multiwall carbon nanotubes at high temperature (250 deg C). It should be noteworthy that CNT are not oxidized, but are sulfonated, although concentrated H_2SO_4 is a strongly oxidizing agent at high temperature. A novel strong protonic acid catalyst is formed with high catalytic activity in esterification with a potential to replace unrecyclable and difficult to separate "liquid acid" catalysts.
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