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Supramolecular Catalysis for Chemofixation and Electroreduction of CO2

Periodic Reporting for period 1 - SupraFixCO2 (Supramolecular Catalysis for Chemofixation and Electroreduction of CO2)

Reporting period: 2019-04-15 to 2021-04-14

Global warming has become one of the global concerns which is threatening all life on our planet. There is no doubt that human influence, especially the emission of greenhouse gases (e.g. CO2), is one of the major causes of global warming. The proposed project for the Marie Curie Fellowship aims to construct new systems of supramolecular catalysis for CO2 utilization by making full use of latest advances in host-guest chemistry.
Cucurbit[n]urils (CB[n]s) are a family of water-soluble macrocyclic hosts, capable of binding different kinds of small molecules, macromolecules and even gases. Among them, CB[5] and CB[6] with their relatively small-sized cavities in the CB[n] family are able to encapsulate gases, e.g. N2, O2, Ar, CO, CO2, etc. Recently, CB[8] has been shown capable of binding a range of gas molecules with the aid of first guest incorporation into its nanocavity; additionally, CB[6]-modified Au surfaces have been found to efficiently promote and modulate CO2 electroreduction.
It can be envisioned that CB[n], acting as a gas nanoreactor, could be employed to develop new supramolecular catalysts to promote gas-involved covalent reactions. In general, there are at least two different ways of encapsulating CO2 into CB[n]’s cavity: one is to use an organic reactant as the first guest molecule, and the other is to directly insert an appropriate catalyst. Following the above strategies, two kinds of CO2-involved catalytic reactions, chemofixation of CO2 into epoxides to cyclic carbonates and electroreduction of CO2 to form CO, have been explored using CB-based host-guest interactions.
After a two-year investigation, it is shown that indeed host-guest complexations between CB macrocycles and organic reactants/catalyst did work as expected; unfortunately, no significant improvement on the catalytic conversion of CO2 could be observed. The main reason behind this is that the energy barrier to break the covalent bonds within CO2 remains at extremely high levels even within a nanoscale reaction cavity. More in-depth considerations and rational designs need to be taken into account for such aqueous supramolecular catalysis. It is hoped that this work may provide some initial information on the limitation of CO2 bond activation in aqueous media and attract more research interest from chemists into the areas of supramolecular chemistry and CO2 utilisation.
In the past two years two different kinds of CB[n]-based supramolecular catalysts for chemofixation and electroreduction of CO2 were designed, fabricated, and evaluated. As for CO2 chemofixation, CB[6], CB[7], and CB[8] were employed as nanoreactors to encapsulate organic reactants in aqueous media, and styrene oxide was used as model reactant. The main idea was to use the CB host to stabilise the epoxide in water and promote its ring-opening addition reaction to form cyclic carbonates in the presence of CO2 (1 bar) and Lewis acid co-catalyst. Extensive conditions were tests including CB catalyst loading (10%-100%), type of Lewis acid co-catalyst (ZnCl2, ZnBr2, FeCl2, FeCl3, CuCl2), reaction temperature (20-80 oC), reaction time (16-88 h), and type of epoxide reactants (styrene oxide, cyclohexene oxide, cyclopentene oxide, hexene oxide, isobutylene oxide). Using in situ 1H NMR to monitor this catalytic conversion, it was found that although the epoxide was stabilised within the CB cavity in water without any hydrolysis, no cyclic carbonate was successfully formed in any of these conditions.
As for CO2 electroreduction, CB[8] was utilised to encapsulate an electrocatalyst, Nicyclam. The main purpose was to use this host-guest complexation to enhance CO2 binding to the catalytic Ni centre within CB[8]’s hydrophobic cavity. Through 1H NMR and ITC characterisation, it was found that Nicyclam can be incorporated into CB[8]’s host cavity exhibiting strong binding affinity with a Ka value close to 107 M-1, which is in line with the proposed research plan. On this basis, subsequent CO2 binding into the supramolecular electrocatalyst, Nicyclam-CB[8], was tested by 1H NMR in the presence of CO2 (1 bar); however, no significant enhancement of CO2 binding was observed. Although the initial result did not show enhanced binding, the catalytic performance of this electrocatalyst was further evaluated; however, compared to pure Nicyclam, no improvement on electroreduction to form CO was shown in the case of Nicyclam-CB[8]. Unfortunately, both of these supramolecular catalysts do not work as expected; thus, we were unable to investigate the catalytic mechanism through nanoparticle-on-mirror techniques.
During close collaboration with colleagues in the CO2 electroreduction project, another promising strategy of CO2 utilisation was designed and studied using quantum dot (QD)-based photocatalysts. Through supramolecular ligand modification on QD surfaces, a new photocatalyst evolved up to 2.4 mmol CO g/QDs after 10 h of visible light irradiation with a CO-selectivity up to 20%. Compared to unmodified QDs, this exhibits a four-fold improvement of CO yield and 13-fold increase in CO-selectivity, which highlights the efficiency of a supramolecular strategy of surface engineering on colloidal catalysts. This work was summarised as an article and has just been recently accepted in Chem. Sci., 2021, DOI: 10.1039/D1SC01310F.
To date, two different ways of encapsulating active organic reactants and catalysts into CB[n] macrocycles have been shown to be feasible as planned in the proposed project, which laid down the molecular basis for supramolecular catalysts of CO2 utilisation. Unfortunately, despite these advancements, no significant improvement on the subsequent CO2-involved catalytic reactions (chemofixation or electroreduction) was realised. This is likely on account of the high energy barrier that still remains within the host nanoreactor. It is hoped that this initial information may provide some useful clues for developing new kinds of supramolecular catalysts for CO2 utilisation in the near future.
Instead of in-cavity catalysis, an alternative strategy of supramolecular surface modification has been shown to be effective to promote the CO2 photoreduction to CO. A supramolecular ligand was quantitatively introduced onto QD photocatalysts to stabilise the reaction intermediate, thus increasing the product yield (4-fold) and selectivity (13-fold) of CO in the process of CO2 photoreduction. This research provides a generic approach to promote colloidal photocatalysis for CO2 utilisation to form value-added fuels in an effective and selective manner.
It can be envisioned that this line of research may yield broad impacts to the scientific community and attract more attention into addressing the challenge of global warming. It is anticipated that the scientific progress, financially supported by the Marie Curie Fellowship in the past two years, will inspire more academic and industrial collaboration at the interdisciplinary frontiers of supramolecular chemistry, CO2 utilisation, and colloidal catalysis.
Summary of the project