Carbon materials can be used as supports for anchoring metal complexes (heterogenization of homogeneous catalysts), for the preparation of supported metal catalysts and as catalysts on their own (metal-free catalysis). These applications depend mostly on the adequate tuning of the carbon surface properties by functionalization or doping.

Carbon-Supported Metal Catalysts

The performance of supported metal catalysts depends on the metal loading and distribution in the support, and the metal dispersion (particle size). These parameters are mainly determined by the precursor–support interactions, and so proper control of the surface chemistry of the carbon support is of paramount importance. On the other hand, the textural properties of the support are also relevant, as they condition the accessibility of the reactants to the active sites. We have been addressing these issues working both with nanosized carbons and mesoporous materials.

HAADF image of Au nanoparticles (as bright spots) on CNTs prepared by the colloidal method

HRTEM image of Au nanoparticles (as dark spots) on carbon xerogels prepared by the colloidal method

Pt nanoparticles on carbon nanospheres

Au nanoparticle supported on nanodiamonds

Heterogenisation of Homogeneous Catalysts on Carbon Materials

Metal complexes with catalytic properties (homogeneous catalysts) can be immobilized onto carbon materials and used as “hybrid” catalysts for reactions of industrial importance. The hybrid catalysts have been found to be more active than their homogeneous counterparts, allowing higher yields to be obtained at much lower temperatures. Moreover, the hybrid materials show further advantages concerning the possibility of recycling (up to several cycles without appreciable loss of activity). This topic involves collaborations with FCUP, IST, ISEL and the University of Coimbra.

Oxidation of secondary alcohols to ketones with a C-scorpionate iron complex@CNT

Anchoring aniline derivatives onto CNTs

BINOL derivatives@CNT for diethyl zinc and titanium isopropoxide catalyzed alkylation of benzaldehyde

Acetophenone yield obtained by MW-assisted oxidation of 1-phenylethanol with V complexes

Carbon as Catalyst

Carbon materials can replace metal or metal oxide catalysts in many reactions. This versatility is strictly related with the ability of the graphitic structure, which allows functionalization via the unsaturated carbon atoms at the edges of the graphene layers or basal plane defects. Such functional groups can act as active sites. In addition, heteroatom doping allows control of the electronic properties, by introducing electron acceptors or donors. Proper identification of the active sites is required in order to establish correlations of catalytic activity, allowing for the correct benchmarking of the novel carbon catalysts. Examples of this strategy include the oxidative dehydrogenation (ODH) of alkanes on carbons functionalized with carbonyl (quinone) groups, and acid catalysis on materials functionalized with sulfonic acid groups.

Oxidative dehydrogenation mechanism involving the quinone-hydroquinone redox cycle

Activity correlations in the ODH of isobutane with oxidised carbon xerogel catalysts

Activity correlations in the esterification of acetic acid on sulfonic acid functionalized carbon xerogels