## 4 Conclusions  
We presented two synthesis methods for mesoporous magnesium carbonates from CO\({}_{2}\) that avoid the use of organic cosolvents, which may be useful in green synthesis routes. Moreover, the capture of CO\({}_{2}\) achieved through these routes may potentially contribute to reduce atmospheric CO\({}_{2}\) concentration by mineralization. We used solid CO\({}_{2}\) for convenience, but supercritical conditions also could be reached by syphoning liquid CO\({}_{2}\) into the reactor, which should not change the results and would be more environmentally friendly. Due to the relatively high surface area and small pore sizes of these materials, we expect that they may be useful as adsorbents and in other applications, including capillary transport and in pharmaceutical and drug delivery applications, if their surface area could be increased further by fine tuning the synthesis parameters. We found conditions for aqueous synthesis that allow isolating the morphology called dumbbell shaped aggregates which showed a large specific surface area and mesopores according to BET measurements. These crystals appear to be an intermediate step of growth of spherulitic crystals and were formed by a combination of carbonate and sulfate according to XRD and Raman characterization. Carbonate synthesized in supercritical CO\({}_{2}\) and oven dried had a larger specific surface area and smaller pores than the DSA material. This material had a usequehonite crystal structure, according to XRD, and was composed solely of magnesium carbonate. Carbonates synthesized in scCO\({}_{2}\) and hot plate dried resulted in a micro-mesoporous material with a smaller surface area. Both carbonates synthesized in scCO\({}_{2}\) had a larger area compared with commercial magnesium carbonate and they provide a simple synthesis method that could be implemented in existing CCS facilities during the process where the CO\({}_{2}\) is compressed to obtain scCO\({}_{2}\) for transportation to geological storage. Figure 7: (**a**) BET adsorption isotherms (adsorbed amount cm\({}^{3}\) g\({}^{-1}\) vs. relative pressure) of the DSA product, scCO\({}_{2}\)-hpd, and scCO\({}_{2}\)-od. (**b**) Pore volume cm\({}^{3}\) g\({}^{-1}\) vs. pore width (nm) of DSA and usequehonite product (scCO\({}_{2}\)-od) calculated by density functional theory (DFT). The apparent pores of \(\approx\)50 μm are considered a result of experimental noise, since no pores of these sizes were observed in the SEM (**c**) Cumulative specific pore volume cm\({}^{3}\)g\({}^{-1}\) vs. pore width (nm) of the scCO\({}_{2}\)-hpd product. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **Specific Surface Area (m\({}^{2}\)g\({}^{-1}\))** & **Average Pore Size (nm)** \\ \hline scCO\({}_{2}\)-od usequhonite & 70.4 & 1.02 \\ DSA & 39.1 & 3.78 \\ MgCl\({}_{2}\) + NaHCO\({}_{3}\) + KOH & 11.8 & 50 \\ Commercial carbonate & 18.1 & 4.89 \\ scCO\({}_{2}\) -hpd & 19.8 & 5.48 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Specific surface areas and average pore sizes for different magnesium carbonates. **Author Contributions:** Conceptualization, Y.I.V.-C. and F.J.R.-M.; methodology, Y.I.V.-C., F.J.R.-M., J.L.L.-S., A.J.G.-C.; validation, J.E.O.-C., E.L.-L., C.A.G.-P.; formal analysis, J.E.O.-C., O.C.-E., E.L.-L., C.A.G.-P.; investigation, J.E.O.-C., O.C.-E., E.L.-L., C.A.G.-P.; resources, [PERSON]-C., F.J.R.-M., [PERSON]-S., A.J.G.-C.; data curation, J.E.O.-C., E.L.-L., C.A.G.-P., Y.I.V.-C. and F.J.R.-M.; writing--original draft preparation, F.J.R.-M., J.E.O.-C.; writing--review and editing, [PERSON]-C., F.J.R.-M., [PERSON]-S., [PERSON]-C., E.L.-L., C.A.G.-P.; visualization, F.J.R.-M., [PERSON]-C., E.L.-L., C.A.G.-P.; supervision, Y.I.V. [PERSON] and [PERSON]; project administration, [PERSON] and [PERSON]; funding acquisition, [PERSON], [PERSON], [PERSON], [PERSON]. All authors have read and agreed to the published version of the manuscript.