Direct Air Capture and Integrated Conversion of Carbon Dioxide into Cyclic Carbonates with Basic Organic Salts

Direct air capture and integrated conversion is a very attractive strategy to reduce CO2 concentration in the atmosphere. However, the existing capturing processes are technologically challenging due to the costs of the processes and the low concentration of CO2. The efficient valorization of the CO2 captured could help overcome many techno-economic limitations. Here, we present a novel economical methodology for direct air capture and conversion that is able to efficiently convert CO2 from the air into cyclic carbonates. The new approach employs commercially available basic ionic liquids, works without the need for sophisticated and expensive co-catalysts or sorbents and under mild reaction conditions. The CO2 from atmospheric air was efficiently captured by IL solution (0.98 molCO2/molIL) and, subsequently, completely converted into cyclic carbonates using epoxides or halohydrins potentially derived from biomass as substrates. A mechanism of conversion was evaluated, which helped to identify relevant reaction intermediates based on halohydrins, and consequently, a 100% selectivity was obtained using the new methodology.

13 C NMR quantification: Quantitative 13 C NMR spectra can be obtained using the inverse gated 1 H decoupled experiment with a correct relaxation delay that ensures full relaxation of the 13 C nuclei, as have been demonstrated previously by us for CO 2 quantification. 1 13 C NMR inverse gated 1 H decoupled spectra were acquired using an inversion recovery experiment (zgig) with a relaxation delay of 60 seconds, set to 5*T 1 according to the previously determined CO 2 T 1 .
Typically, in each experiment 128 transients with 64K data points were collected corresponding to an average duration of 2 h.

CO 2 capture experiments (step 1)
Sorption experiments: The samples for CO 2 capture were prepared using a mixture of solvents DMSO-d 6 (0.5 mL) and correspond amount of organic salts (0.5 -1.5 mmols). All the sorption experiments were performed by bubbling the gas (CO 2 or air) in 5 mm NMR glass tubes with a septum at room temperature for 15 min (CO 2 ) or 16h (air). For the CO 2 sorption quantification, we have previously established this NMR methodology for CO 2 quantification in ILs.
First of all the effect of cation was evaluated (Table S1) and secondly the effect of solvent (Table  S2).

S10
Once captured by forming HCO 3 -, the CO 2 can't be desorbed even heating at 120 o C. However, the bicarbonate can be use as CO 2 source in order to perform cycloaddition reaction as observed in the subsequential section (section 3).

Control experiments
General procedure for the cycloaddition reaction: Methodology using balloon (method 1): ILs (5-30 mol%), solvent (0.5 mL) and the correspondent epoxide (5 mmol) were charged in glass vial connected to a CO 2 balloon. The reaction was performed at 40 o C -70 o C under 1-16 h under magnetic stirring. The product was analysed by 1 H NMR spectroscopy to determine the conversion and selectivity of cyclic carbonates.

Methodology using gas flow (method 2):
Step 1) ILs (0.5 -1 mmol) and solvent (0.5 -1.0 mL) were charged in glass vial where pre-sorption experiments were performed by bubbling (flow rate 75 mL/min) CO 2 for 15 min or atmospheric air for 16 h.
Step 2) In the same vial the correspondent substrate (0.5 -5 mmol) was added. The vials were closed with septum and reaction were performed at 40-70 o C under 1-16 h under magnetic stirring. The product was analysed by 1 H NMR spectroscopy to determine the conversion and selectivity of cyclic carbonates.
Control experiments using balloon of CO 2 were performed to comparative purpose. 2-12 The conversion using CO 2 balloon (method 1) was investigate for BMI.HCO 3 and TBAB as catalyst (Table S2, entries 1-4). High conversions (75-99%) of CO 2 into cyclic carbonate was observed. Using the method 2, even with previous bubbling CO 2 , no conversion can be observed using just TBAB as catalyst (Table S1 entries [4][5], since the bromide IL are not good CO 2 sorbents. In order to evaluate the influence of CO 2 amount available, different concentrations of BMI.HCO 3 were used to the CO 2 cycloaddition reaction (entries 7-9). The CO 2 amount was confirmed by 13 C quantitative NMR ( Figure S12), as expected IL containing HCO 3as anion present 1mol CO 2 /molIL. Considering the CO 2 as the limiting reactant, maximum conversion was observed for all the experiments, confirming the CO 2 mass balance. The increases in the IL concentration from 10 to 30 mol%, reduce the conversion into EC and increase the formation of glycidol carbonate (GC).

IL screening and conversion mechanism evaluation
Test using different bicarbonate-based salts to convert CO 2 into cyclic carbonate using epoxide as substrate without the addition of CO 2 was performed as a proof-of-concept .

Recycle experiments
A recharge experiment was performed bubbling more CO 2 until complete conversion of substrate, and a relation of 2 Eq of substrate to 1 Eq of IL were observed. The formed products present a relation of 1 Eq of GC and 1Eq of EC. With the increases of substrate amount the reaction do not occur anymore, and subproducts started to be formed ( Figure S29)