Functionalized Nanotubes DOI: 10.1002/anie.200901658 Defect-Mediated Functionalization of Carbon Nanotubes as a Route to Design Single-Site Basic Heterogeneous Catalysts for Biomass Conversion** Jean-Philippe Tessonnier,* Alberto Villa, Olivier Majoulet, DangSheng Su, and Robert Schlögl The use of biomass will play a key role in the production of chemicals over the next few decades. [1–3] It is estimated that 25 % of the chemicals in the US will be produced from biomass by 2030, [2] and it is therefore clear that billions of tons of biomass-based chemicals will have to be synthesized. This goal constitutes a real challenge for heterogeneous catalysis as its focus will have to move from the well-known C ÀC bond chemistry (cracking, isomerization, and alkylation) to C ÀO bond chemistry (e.g., dehydration and deoxygenation). Therefore, the chemistry of carbohydrates needs to be explored and developed. [4] In this context, basic heteroge- neous catalysts will play a major role in, for example, dehydration, hydrolysis, (trans)esterification, aldol condensa- tion, alkylation, or isomerization reactions. [1, 3] At present, basic heterogeneous catalysts usually consist of alkaline oxides, alkaline-earth oxides, clays, and modified zeolites. [1, 5] However, they present different drawbacks such as the low number of accessible active sites, important diffusion and mass transfer problems, or partial dissolution in the reaction medium for applications in liquid-phase biomass conver- sion. [6] It is therefore necessary to develop new basic heterogeneous catalysts that are highly selective, stable, and easy to tailor on a nano and macro level. Multiwall carbon nanotube (MWCNT) based catalysts fit each of these criteria. They are chemically stable in most media, present a high surface area, and can be directly grown in self-standing blocks by controlling the entanglement of the nanotubes. [7] In addition, many of the reactions known in organic chemistry can be employed to chemically modify MWCNTs, for example, by grafting specific functional groups onto their surface. [8–11] In this way, we believed it should be possible to tailor single-site MWCNT-based catalysts that are highly selective. Herein, we report a novel approach to the synthesis of MWCNT-grafted amino groups by modifying existing structural defects of the MWCNTs. The catalysts were tested in the synthesis of biodiesel as a model reaction for applications in C ÀO chemistry, that is, biomass conversion reactions. The stability of the basic functional groups during the catalytic reaction as a function of the synthetic route employed for their grafting is discussed. The most common synthetic route to functionalized MWCNTs is based on the reactivity of surface carboxyl groups. Single-wall or multiwall carbon nanotubes are first oxidized with nitric acid to create carboxylic acid moieties on their surfaces. Coupling and anchoring of the desired molecule is then performed by, for example, amidation (Scheme 1), esterification, or Friedel–Crafts reactions. [8] Although this oxidation–coupling technique is very popular, it suffers from several drawbacks. It was shown that MWCNTs are shortened during the oxidation step, which could make recovery of the catalyst after reaction more difficult. In addition, the oxidation step also creates a variety of other acidic oxygen-containing groups (see the Supporting Information), [12] which remain in the final sample and are prone to adsorb and react with biomass constituents during the catalytic reaction. These side-reactions lower the selec- tivity for the desired products and can even irreversibly poison the catalyst. In order to circumvent these problems, we developed a more elegant and more efficient synthetic route based on the direct covalent grafting of the desired basic functional groups onto existing structural defects of the MWCNTs. Contrary to the case of graphite, structural defects are common for nanotubes, especially for MWCNTs grown by catalytic chemical vapor deposition (CCVD). Ideal carbon nanotubes are straight along their main axis. [13] However, scanning electron microscopy (SEM; see the Supporting Information) and transmission electron microscopy (TEM) images clearly show that MWCNTs are curved, which implies that they contain many topological defects where one or several hexagons have been replaced by pentagons and heptagons. [13] The intensity of the D band and the D/G ratio in the Raman spectra of the MWCNTs (where D and G are the Scheme 1. Functionalization of MWCNTs by oxidation–amidation. [*] Dr. J.-P. Tessonnier, Dr. A. Villa, O. Majoulet, Dr. D.S. Su, Prof. R. Schlögl Department of Inorganic Chemistry Fritz Haber Institute of the Max Planck Society Faradayweg 4–6, Berlin (Germany) Fax: (+ 49) 30-8413-4401 http://www.fhi-berlin.mpg.de E-mail: tesso@fhi-berlin.mpg.de [**] We acknowledge Süd Chemie as well as the Federal Ministry of Education and Research (Carboscale) for their support. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200901658. Angewandte Chemie 6543 Angew. Chem. Int. Ed. 2009, 48, 6543 –6546  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim