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.
