OxyCat_CO2 aims to develop heterogeneous epoxidation and cycloaddition catalysts operating simultaneously, under mild conditions, to convert alkenes into cyclic carbonates from CO2 and O2 in air. This approach is atom-efficient and contributes to the valorisation of CO2.
Global warming is one of the greatest challenges facing humanity and it is now essential to find solutions to limit the concentration of greenhouse gases, including CO2, in the atmosphere. To achieve this, chemistry provides solutions that allow CO2 to be regarded not as a waste product, but as a source of high value-added compounds. However, its conversion is not an easy task. Effective catalysts are required. Among the various targets, the formation of biodegradable cyclic carbonates, obtained by reacting CO2 with epoxides, is currently attracting growing interest. Indeed, these compounds have a wide range of applications as monomers in plastics or as solvents for paints, batteries or degreasers. However, it seems even more interesting to start with organic precursors that are more abundant and less expensive than epoxides, in this case alkenes such as styrene, which are produced industrially in large quantities. The objective of this project is to develop a global reaction for the synthesis of styrene carbonate from styrene in the presence of CO2 and a clean oxidant: O2.
In order to carry out simultaneously epoxidation and cycloaddition of CO2 onto epoxides, corresponding catalysts able to work at the same temperatures and pressures must be found. In general, the most efficient cycloaddition reactions operate, using tetraalkylammonium halides, above 100°C and 10 bar of CO2, while aerobic epoxidation reactions, based on redox catalysis and sacrificial electron donors, are optimal at rather low temperatures (< 80°C). The challenge here was therefore to design multi-functional catalyst systems aimed at lowering the conditions on the cycloaddition side, through the use of suitable Lewis acids (LA). Catalysts involving Schiff bases (pyrrole or salophen) and various metal ions (Zn2+, Mn3+, Ni2+, Cr3+) were thus prepared. Among these, those based on Mn3+ or Cr3+ have the advantage of potentially participating in the catalysis of both reactions. First, these combinations were tested in solution, then some were transposed onto a support, once it was ensured that the different active phases did not interfere with each other.
On the cycloaddition side, it was shown that 1) salophen-chromium complexes, especially those carrying an amine group, allowed the quantitative formation of styrene carbonate (SC) at lower temperatures and that 2) the heterogenisation of tetralkylammonium salts on mesoporous silica clearly increased their efficiency in this reaction by synergy effect with the support. Finally, we managed to obtain a maximum yield of 31% for the overall reaction even though the catalysts had to be introduced sequentially. For the epoxidation, surprisingly, the metal complex appeared not to be non-essential.
The simultaneous use of CO2 and O2 reduces the risk of explosive mixtures between O2 and organic compounds. It also avoids the prior energy-consuming separation of CO2 from the air by exploiting the oxidising potential still present in the flue gases, which is a problem during absorption (cf ANR Dalmatien, ANR-11-SEED-0006). We have shown here the feasibility of the global conversion of alkenes into cyclic carbonates. A simplified catalytic system based on a quaternary ammonium salt, a non-redox active metal complex and a sacrificial aldehyde could be more efficient.
This project has so far resulted in 6 publications in international journals. Several other manuscripts are being written, including a literature review (Partners 1 and 2). Despite the health crisis, the results could be presented orally at conferences or seminars (18) or via posters (6). In addition, the project participants were involved in various scientific popularisation activities related to CO2 chemistry.
The aim of the OxCyCat-CO2 project is to use CO2 as both a reagent and a solvent (supercritical conditions) to prepare cyclic carbonates of interest starting from the corresponding olefins via a one-pot reaction with hydrogen peroxide or molecular oxygen. To achieve this, new multifunctional heterogeneous catalysts will be developed by combining, on the same mesoporous support, active phases that are able to catalyze the epoxidation of alkenes and the further cycloaddition of CO2 under the same experimental conditions.
Two ligand libraries, lacunary polyoxometalates (W-POMs) and nitrogen Schiff bases (BS-4Ns) are available within the consortium. W-POM species will be used to catalyze the epoxidation of alkenes in the presence of H2O2 and will be compared to Mn(III) complexes involving BS-4N ligands also able to catalyze epoxidations in the presence of dioxygen and of an aldehyde. The same nitrogen Schiff bases associated with zinc (II) will also be exploited for the cycloaddition reaction of CO2 onto epoxides. These different active phases will then be combined, thus allowing the definition of two classes of bifunctional catalytic systems according to whether they involve i) both polyoxometalates and complexes with nitrogen Schiff bases ("Mix POM / BS-4N ") intended for operations with H2O2 / CO2, or (ii) only complexes derived from Schiff bases (" all BS-4Ns ") intended for operations with O2 / CO2.
In a first approach, these combinations will be tested in solution. In particular, we shall ensure that the two active phases can work in synergy under comparable solvent, temperature and pressure conditions without interfering with one another. An important challenge of the project is to lower the optimum temperature of the CO2 cycloaddition reaction via the design of suitable BS-4N ligands with the help of theoretical chemistry colleagues involved in the consortium. The reactions will first be carried out in organic solvent, a priori in acetonitrile, and then transposed into supercritical CO2. This last solvent will be preferred, especially for combinations of CO2 with O2 in order to limit the risks of explosiveness. Moreover, this association suggests the possibility of using CO2 emissions that are not completely purified. Some terminal and internal alkenes representative of industrial requirements will be considered. Chiral versions of the catalytic "all BS-4Ns" system will be developed and then tested in the context of the enantioselective epoxidation of alkenes and then in the overall process.
Polyoxometalates will then be grafted by amide bonds to the surface of mesoporous silicas and complexes with BS-4N ligands by Si-O-Si bonds. For the sake of simplification, and ultimately, in the case of "all BS-4Ns", the use of a single metal / BS-4N complex to perform both reactions (epoxidation and cycloaddition) will be preferred. In the case of "Mix POM / BS-4N", the pre-association of the two entities in one edifice that will be grafted later through amide linkages will be encouraged.
Monsieur Franck LAUNAY (Laboratoire de Réactivité de Surface)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
ISA - CNRS Institut des Sciences Analytiques
LRS Laboratoire de Réactivité de Surface
IRCELYON Institut de Recherches sur la Catalyse et l'Environnement de Lyon
IPCM Institut Parisien de Chimie Moléculaire
Help of the ANR 508,872 euros
Beginning and duration of the scientific project: September 2017 - 48 Months