Catalysis research of relevance to carbon management: progress, challenges, and opportunities

Turning carbon dioxide into fuel

Our present dependence on fossil fuels means that, as our demand for energy inevitably increases, so do emissions of greenhouse gases, most notably carbon dioxide (CO2). To avoid the obvious consequences on climate change, the concentration of such greenhouse gases in the atmosphere must be stabilized. But, as populations grow and economies develop, future demands now ensure that energy will be one of the defining issues of this century. This unique set of (coupled) challenges also means that science and engineering have a unique opportunity-and a burgeoning challenge-to apply their understanding to provide sustainable energy solutions. Integrated carbon capture and subsequent sequestration is generally advanced as the most promising option to tackle greenhouse gases in the short to medium term. Here, we provide a brief overview of an alternative mid- to long-term option, namely, the capture and conversion of CO2, to produce sustainable, synthetic hydrocarbon or carbonaceous fuels, most notably for transportation purposes. Basically, the approach centres on the concept of the large-scale re-use of CO2 released by human activity to produce synthetic fuels, and how this challenging approach could assume an important role in tackling the issue of global CO2 emissions. We highlight three possible strategies involving CO2 conversion by physico-chemical approaches: sustainable (or renewable) synthetic methanol, syngas production derived from flue gases from coal-, gas- or oil-fired electric power stations, and photochemical production of synthetic fuels. The use of CO2 to synthesize commodity chemicals is covered elsewhere (Arakawa et al. 2001 Chem. Rev. 101, 953-996); this review is focused on the possibilities for the conversion of CO2 to fuels. Although these three prototypical areas differ in their ultimate applications, the underpinning thermodynamic considerations centre on the conversion-and hence the utilization-of CO2. Here, we hope to illustrate that advances in the science and engineering of materials are critical for these new energy technologies, and specific examples are given for all three examples. With sufficient advances, and institutional and political support, such scientific and technological innovations could help to regulate/stabilize the CO2 levels in the atmosphere and thereby extend the use of fossil-fuel-derived feedstocks.

Oxidation of methane by a biological dicopper centre

Vast world reserves of methane gas are underutilized as a feedstock for the production of liquid fuels and chemicals owing to the lack of economical and sustainable strategies for the selective oxidation of methane to methanol. Current processes to activate the strong C-H bond (104 kcal mol(-1)) in methane require high temperatures, are costly and inefficient, and produce waste. In nature, methanotrophic bacteria perform this reaction under ambient conditions using metalloenzymes called methane monooxygenases (MMOs). MMOs thus provide the optimal model for an efficient, environmentally sound catalyst. There are two types of MMO. Soluble MMO (sMMO) is expressed by several strains of methanotroph under copper-limited conditions and oxidizes methane with a well-characterized catalytic di-iron centre. Particulate MMO (pMMO) is an integral membrane metalloenzyme produced by all methanotrophs and is composed of three subunits, pmoA, pmoB and pmoC, arranged in a trimeric alpha(3)beta(3)gamma(3) complex. Despite 20 years of research and the availability of two crystal structures, the metal composition and location of the pMMO metal active site are not known. Here we show that pMMO activity is dependent on copper, not iron, and that the copper active site is located in the soluble domains of the pmoB subunit rather than within the membrane. Recombinant soluble fragments of pmoB (spmoB) bind copper and have propylene and methane oxidation activities. Disruption of each copper centre in spmoB by mutagenesis indicates that the active site is a dicopper centre. These findings help resolve the pMMO controversy and provide a promising new approach to developing environmentally friendly C-H oxidation catalysts.

A redox non-innocent ligand controls the life time of a reactive quartet excited state - an MCSCF study of [Ni(H)(OH)](+)

The electronic structures of the low and high-spin states of the cationic complex [Ni(H)(OH)](+) that was previously found to be highly reactive toward CH(4) and O(2) were examined. Earlier computational work suggested that the low-spin doublet state D(0) of the Ni(III)-d(7) system is significantly lower in energy than its high-spin quartet analogue Q(1). Recent DFT-studies indicated, however, that Q(1) is the reactive species requiring Q(1) to have a sufficiently long lifetime for undergoing thermal reactions with the small molecule reactants under single collision conditions in the gas phase. These observations raise the question as to why Q(1) does not spontaneously undergo intersystem crossing. Our work based on DFT, coupled-cluster and MCSCF calculations suggests that the hydroxyl ligand behaves as a redox noninnocent ligand and becomes oxidized to formally afford an electronic structure that is consistent with a Ni(II)-(OH)* species. As a result, the doublet and quartet ground states are not related by a single electron spin flip and the intersystem crossing becomes inhibited, as indicated by unexpectedly small spin-orbit coupling constants. After extensive sampling of the potential energy surfaces, we concluded that there is no direct way of converting Q(1) to the ground state doublet D(0). Alternative multistep pathways for the Q(1) --> D(0) decay involving doublet excited states were also evaluated and found to be energetically not accessible under the experimental conditions.

Synthesis, characterization, and photoinduced electron transfer processes of orthogonal ruthenium phthalocyanine-fullerene assemblies

The convergent synthesis, electrochemical characterization, and photophysical studies of phthalocyanine-fullerene hybrids 3-5 bearing an orthogonal geometry (Chart ) are reported. These donor-acceptor arrays have been assembled through metal coordination of linear fullerene mono- and bispyridyl ligands to ruthenium(II) phthalocyanines. The hybrid [Ru(CO)(C(60)Py)Pc] (3) and the triad [Ru(2)(CO)(2)(C(60)Py(2))Pc(2)] (5) were prepared by treatment of the phthalocyanine 6 with the mono- and hexakis-substituted C(60)-pyridyl ligands 1 and 2, respectively. The triad [Ru(C(60)Py)(2)Pc] (4) was prepared in a similar manner from the monosubstituted C(60)-pyridyl ligand 1 and the phthalocyanine precursor 7. The simplicity of this versatile synthetic approach allows to determine the influence of the donor and acceptor ratio in the radical ion pair state lifetime. The chemical, electrochemical, and photophysical characterization of the phthalocyanine-fullerene hybrids 3-5 was conducted using (1)H and (13)C NMR, UV/vis, and IR spectroscopies, as well as mass spectrometry, cyclic voltammetry, femtosecond transient absorption studies, and nanosecond laser flash photolysis experiments. Arrays 3-5 exhibit electronic coupling between the two electroactive components in the ground state, which is modulated by the axial CO and 4-pyridylfulleropyrrolidine ligands. With respect to the excited state, we have demonstrated that RuPc/C(60) electron donor-acceptor hybrids are a versatile platform to fine-tune the outcome and dynamics of charge transfer processes. The use of ruthenium(II) phthalocyanines instead of the corresponding zinc(II) complexes allows the suppression of energy wasting and unwanted charge recombination, affording radical ion pair state lifetimes on the order of hundreds of nanoseconds for the C(60)-monoadduct-based complexes 3 and 4. For the hexakis-substituted C(60) unit 2, the reduction potential is shifted cathodically, thus raising the radical ion pair state energy. However, the location of the RuPc triplet excited state is not high enough, and still offers a rapid deactivation of the radical ion pair state.

Biocatalytic carboxylation

Dwindling petroleum feedstocks and increased CO(2)-concentrations in the atmosphere currently open the concept of using CO(2) as raw material for the synthesis of well-defined organic compounds. In parallel to recent advances in the chemical CO(2)-fixation, enzymatic (biocatalytic) carboxylation is currently being investigated at an increased pace. On the one hand, this critical review provides a concise overview on highly specific biosynthetic pathways for CO(2)-fixation and, on the other hand, a summary of biodegradation (detoxification) processes involving enzymes which possess relaxed substrate specificities, which allow their application for the regioselective carboxylation of organic substrates to furnish the corresponding carboxylic acids (145 references).

Mechanism of cyclic carbonate synthesis from epoxides and CO2

Three interconnected catalytic cycles account for the title reaction catalyzed by a bimetallic aluminum(salen) complex and Bu(4)NBr. In the first, Bu(4)NBr acts as a nucleophile to activate the epoxide. In the second, Bu(3)N generated in situ serves to activate CO(2). In the third, the aluminum(salen) complex brings the two activated species together so that the key bonds can be formed intramolecularly.

Conversion of carbon dioxide into methanol with silanes over N-heterocyclic carbene catalysts

Activate and reduce: Carbon dioxide was reduced with silane using a stable N-heterocyclic carbene organocatalyst to provide methanol under very mild conditions. Dry air can serve as the feedstock, and the organocatalyst is much more efficient than transition-metal catalysts for this reaction. This approach offers a very promising protocol for chemical CO(2) activation and fixation.