CNFs in Catalytic Applications

Our research group successfully employed fullerene C₆₀ as a molecular platform to anchor twelve 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) moieties (Figure 1a). The obtained C₆₀-derivative has been used as organocatalyst for the oxidation of primary and secondary alcohols to aldehydes and ketones under Anelli’s conditions. The investigated reaction showed a wide applicability to different alcohols, and the catalyst was easily recovered and recycled for at least seven cycles without loss of its catalytic activity. The main advantage of such approach arises from the high functionalization degree of the C₆₀ molecular platform that results in a highly active homogeneous catalyst that can be recovered by a simple and fast work-up procedure involving the use of a short silica pad (Figures 1b-d), combining the benefits of homogeneous catalysis with recyclability and cost abatement.

We have also designed another convenient catalytic system formed by a novel fullerene [5:1]hexakisadduct bearing two 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) radicals and ten 1-propyl-3-methylimidazolium bromide moieties. The synthesized C₆₀IL₁₀TEMPO₂ hybrid has been successfully employed as a catalyst in the selective oxidation of a wide range of alcohols with low catalytic loading (0.1 mol%). Recovering and recycling of such catalytic system involved adsorption onto a multi-layered covalently-linked supported ionic liquid phase (mlc-SILP) through a “release and catch” approach (Figure 2) and its reusing for up to 12 cycles without loss in efficiency. A catalytic synergism between TEMPO and imidazolium bromide moieties combined in the same hybrid has been clearly shown.

A convergent strategy allowed to prepare two single-walled carbon nanotube–polyamidoamine dendrimers hybrids (SWCNT−PAMAM). Pristine SWCNTs were directly functionalized by the reaction with cystamine-based PAMAM dendrimers (generations 2.5 and 3.0) in refluxing toluene. The following immobilization and reduction of [PdCl₄]²⁻ has given rise to catalysts SWCNT-PAMAM-PdNPs (Scheme 1) having highly dispersed small palladium nanoparticles homogeneously confined throughout the nanotubes length. One of these catalytic materials proved to be an efficient heterogeneous catalyst in the Suzuki and Heck reactions. Very low catalytic loadings down to 0.002 mol% have been used reaching a turnover number (TON) of 48,000 and a turnover frequency (TOF) of 566,000 h⁻¹. No loss of catalytic activity has been revealed and it has been possible to use the catalyst for 6 cycles. Evidences of a “release and catch” mechanism operating during the C–C coupling reactions have been proved, although during Heck reaction small catalytically active soluble Pd species are also present.