Syntheses of Nanostructured Magnesium Carbonate Powders with Mesoporous Structures from Carbon Dioxide

applied sciences

Article Syntheses of Nanostructured Magnesium Carbonate Powders with Mesoporous Structures from Carbon Dioxide

Fernando J. Rodríguez-Macías 1,2 , José E. Ortiz-Castillo 1 , Erika López-Lara 1, Alejandro J. García-Cuéllar 1 , José L. López-Salinas 1 ,César A. García-Pérez 1, Orlando Castilleja-Escobedo 1 and Yadira I. Vega-Cantú 1,2,*

1 Tecnologico de Monterrey, Escuela de Ingeniería y Ciencias, Ave. Eugenio Garza Sada 2501, Monterrey 64849, N.L., Mexico; [email protected] (F.J.R.-M.); [email protected] (J.E.O.-C.); [email protected] (E.L.-L.); [email protected] (A.J.G.-C.); [email protected] (J.L.L.-S.); [email protected] (C.A.G.-P.); [email protected] (O.C.-E.) 2 Pós-Graduação em Ciência de Materiais, Universidade Federal de Pernambuco, Av. Prof. Moraes Rego 1235, Cidade Universitária, Recife 50.670-901, PE, Brazil * Correspondence: [email protected]

Abstract: In this work, we present the results of two synthesis approaches for mesoporous magnesium carbonates, that result in mineralization of carbon dioxide, producing carbonate materials without the use of cosolvents, which makes them more environmentally friendly. In one of our synthesis methods, we found that we could obtain nonequilibrium crystal structures, with acicular crystals branching bidirectionally from a denser core. Both Raman spectroscopy and X-ray diffraction showed these crystals to be a mixture of sulfate and hydrated carbonates. We attribute the nonequilibrium morphology to coprecipitation of two salts and short synthesis time (25 min). Other aqueous synthesis

conditions produced mixtures of carbonates with different morphologies, which changed depending on drying temperature (40 or 100 ◦C). In addition to aqueous solution, we used supercritical carbon

Citation: Rodríguez-Macías, F.J.; dioxide for synthesis, producing a hydrated magnesium carbonate, with a nesquehonite structure, Ortiz-Castillo, J.E.; López-Lara, E.; according to X-ray diffraction. This second material has smaller pores (1.01 nm) and high surface area. García-Cuéllar, A.J.; López-Salinas, Due to their high surface area, these materials could be used for adsorbents and capillary transport, J.L.; García-Pérez, C.A.; in addition to their potential use for carbon capture and sequestration. Castilleja-Escobedo, O.; Vega-Cantú, Y.I. Syntheses of Nanostructured Keywords: porous materials; supercritical carbon dioxide; magnesium carbonate; carbon sequestration Magnesium Carbonate Powders with Mesoporous Structures from Carbon Dioxide. Appl. Sci. 2021, 11, 1141. https://doi.org/10.3390/app11031141 1. Introduction Magnesium carbonates are some of several minerals proposed for “carbon mineraliza- Received: 18 December 2020 Accepted: 10 January 2021 tion”, one of several approaches to Carbon Capture and Sequestration (CCS) [1]. In situ Published: 26 January 2021 mineral carbonation is one of the options, but the need for transport to suitable geological sites and other factors make it costly, ex situ mineral carbonation, precipitating magnesium Publisher’s Note: MDPI stays neu- carbonates from solutions containing magnesium ions is another option [2]. Magnesium tral with regard to jurisdictional clai- carbonates exist in different structures with different thermodynamic stability and different ms in published maps and institutio- degrees of hydration. In general, all magnesium carbonates are of interest in CCS since nal afﬁliations. they have stable and long-lasting forms [3], the most attractive one for CCS is MgCO3, which has a 1:1 molar ratio of magnesium to CO2, but the hydrated forms are favored during precipitation [4]. This makes the study of the precipitation of magnesium carbonates an important ﬁeld of study. Even if the most desirable forms of anhydrous MgCO3 were Copyright: © 2021 by the authors. Li- not obtained, hydrated magnesium carbonates such as nesquehonite (MgCO3·3H2O) have censee MDPI, Basel, Switzerland. been reported to be useful for construction materials [5], a potential application that may This article is an open access article sequester carbon for long periods of time. distributed under the terms and con- The synthesis of magnesium carbonates by mineral carbonation (i.e., formation of ditions of the Creative Commons At- tribution (CC BY) license (https:// stable carbonates with minerals) has been studied widely for CCS [2]. creativecommons.org/licenses/by/ Magnesium carbonate is a valuable product by itself, with a price of USD 100–1000/t, 4.0/). depending on purity, which could offset the cost of carbon capture by ex situ mineralization,

Appl. Sci. 2021, 11, 1141. https://doi.org/10.3390/app11031141 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 1141 2 of 10

estimated to be USD 50–300 per tCO2 sequestered [1]. An option to make magnesium carbonates more valuable would be to create a mesoporous (pore diameters of 2–50 nm) material [6]. The applications of nanostructured mesoporous magnesium carbonate include pharmaceutical ones such as drug stabilizer [7,8] and anticoagulant [9]. Additionally, a mesoporous structure could make these materials suitable as adsorbents for pollutants and other substances. An interesting example of this is a report on mesoporous magnesium carbonate, synthesized with methanol, and treated with additives, which showed good 2 adsorption of CO2, due to the high surface area (over 500 m /g), with the inorganic additives increasing CO2 uptake and selectivity [10]. Another potential use of high surface area carbonates is the adsorption of contam- 2+ 2+ inants. Shahwan et al. report that MgCO3 is a better adsorbent of Pb and Zn ions than clay minerals [11]. Mesoporous calcium carbonates have also been reported as good adsorbents for heavy metal ions (Pb2+ and Cd2+); the removal efficiency depended on the crystal structure [12]. Shan et al. showed that synthetic nesquehonite is a highly efficient agent for removing Cu2+, which was precipitated as a mixture of hydroxides and carbonates [13]. A novel application of hydromagnesite is in the directional displacement of fluids in porous media [14]. This shows the importance of studying the synthesis of magnesium carbonates. Many of the synthesis methods for magnesium carbonate, including those using supercritical carbon dioxide (scCO2), with ethanol as a cosolvent [15] and mineral carbonation in aqueous solution [16] usually do not yield mesoporous materials (pores of 2–50 nm). Another option for cosolvent is methanol, which can increase the solvation power of the main solvent, to control the morphology of carbonates and has been used as a control factor of the pore size according to traditional synthesis works but can result in amorphous carbonates [17]. Reaction temperature is also an important factor that affects pore size [6,7]. However, organic solvents can contribute to emission of pollutants [18]. There are some examples of works exploring mineral carbonation with magnesium, such as that of Yoo et al. [19] in which magnesium comes from seawater desalination waste and is precipitated as carbonates using alkanolamines. In CCS, CO2 is usually delivered and transported in supercritical state for geological storage [20]. Therefore, it is important to study mineral carbonation by reaction with scCO2, which is by itself a solvent as well as a reactant, therefore, these reactions may not require the use of organic solvents such as methanol and ethanol, making it a greener method. Here we compare scCO2 and aqueous synthesis methods with the objective of producing mesoporous magnesium carbonates, via environmentally friendly methods.

2. Materials and Methods

Aqueous synthesis by CO2 sequestration was based on the method of de Vito et al. [16]. We bubbled CO2 through a Pasteur pipette into magnesium solutions in deionized water Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 11 in a stoppered ﬂask, with a ﬂow rate equivalent to 1.1 g/min (Figure1a).

Figure 1. Schematic diagrams showing the experimental method for aqueous synthesis, (a) bub- Figure 1. Schematic diagrams showing the experimental method for aqueous synthesis, (a) bubbling bling CO2 gas, and (b) supercritical fluid (SCF) closed reactor (kept at constant temperature in a COhot2 watergas, andbath, ( bnot) supercritical shown for clarity). ﬂuid (SCF) closed reactor (kept at constant temperature in a hot water bath, not shown for clarity). Three precursors salts (all reactive grade, CTR Scientific, Monterrey, México) were used for both routes: MgCl2·6H2O (10 mL, 6.07 M), Mg(NO3)2·6H2O (10 mL, 6.07 M), MgSO4·7H2O (20 mL, 3.036 M). The pH was adjusted to 8–9 with NH4OH and reactions were carried out for 25 and 60 min at 5, 21, and 70 °C, as shown in Table 1.

Table 1. Design of experiments.

Experimental Precursors Salts Reaction Time Reaction Temperature Drying Temperature Method MgCl2·6H2O Aqueous Syn- Mg(NO3)2·6H2O 25 and 60 min 5, 21, and 70 °C 40 °C thesis MgSO4·7H2O 40 °C scCO2 Synthesis Mg(NO3)2·6H2O 12 h 35–40 °C 100 °C

In the supercritical synthesis, we added 60 g of CO2 (enough to reach a pressure >74 bar) in a custom-made reactor, with a volume of 240 mL (Figure 1b), containing 10 mL of 6.07 M Mg(NO3)2·6H2O (pH adjusted to 8–9 with NH4OH). The reactor was hermetically closed and placed in a water bath at 35–40 °C to reach and maintain supercritical conditions, monitored by observation of the phase change through a window. After 12 h of reaction, the reactor was depressurized. Solid products were collected and washed five times with 5 mL of distilled water, then oven-dried for 12 h at 100 °C (scCO2-od). To analyze the effect of the drying temperature on the material structure, the same synthesis procedure was carried out, and the sample was dried over a hot plate at 40 °C for 12 h (scCO2-hpd). Morphology was observed with a Zeiss EVO MA scanning electron microscope (SEM), from Carl Zeiss AG (Oberkoche, Germany). Some samples were sputter coated with 5 nm of Au, to prevent charging. Diffraction patterns of selected products were obtained with a Siemens/Bruker D5000 diffractometer from Bruker AXS (Karlsruhe, Germany), using Cu Kα radiation (λ = 1.54184 Å), 2θ angle from 5° to 70° with a scan rate of 5 °/min. Profex, an open source program by Nicola Döbelin (www.profex-xrd.org), was used for Rietveld refinement. Raman spectroscopy for the samples scCO2-od and aqueous route was performed in an OceanOptics (Or- lando, FL, USA) QE65000 equipment (785.0 nm laser, 5 s acquisition time). Raman spectroscopy for the scCO2-hpd samples was performed in a Jasco NRS-5100 equipment (785.0 nm laser, 60 s acquisition time), from JASCO Corporation (Hachioji, Japan). BET analysis was performed in a Quantachrome Autosorb iQ®, from Anton Paar Instrumentsn (Boynton Beach, FL, USA), in a 6 mm cell, samples were previously degassed for 2 h at 80 °C followed by 10 h at 100 °C. Appl. Sci. 2021, 11, 1141 3 of 10

Three precursors salts (all reactive grade, CTR Scientiﬁc, Monterrey, México) were used for both routes: MgCl2·6H2O (10 mL, 6.07 M), Mg(NO3)2·6H2O (10 mL, 6.07 M), MgSO4·7H2O (20 mL, 3.036 M). The pH was adjusted to 8–9 with NH4OH and reactions were carried out for 25 and 60 min at 5, 21, and 70 ◦C, as shown in Table1.

Table 1. Design of experiments.

Experimental Reaction Drying Precursors Salts Reaction Time Method Temperature Temperature

MgCl2·6H2O Aqueous Mg(NO ) ·6H O 25 and 60 min 5, 21, and 70 ◦C 40 ◦C Synthesis 3 2 2 MgSO4·7H2O 40 ◦C ◦ scCO2 Synthesis Mg(NO3)2·6H2O 12 h 35–40 C 100 ◦C

In the supercritical synthesis, we added 60 g of CO2 (enough to reach a pressure >74 bar) in a custom-made reactor, with a volume of 240 mL (Figure1b), containing 10 mL of 6.07 M Mg(NO3)2·6H2O (pH adjusted to 8–9 with NH4OH). The reactor was hermetically closed and placed in a water bath at 35–40 ◦C to reach and maintain supercritical conditions, monitored by observation of the phase change through a window. After 12 h of reaction, the reactor was depressurized. Solid products were collected and washed five times with ◦ 5 mL of distilled water, then oven-dried for 12 h at 100 C (scCO2-od). To analyze the effect of the drying temperature on the material structure, the same synthesis procedure was ◦ carried out, and the sample was dried over a hot plate at 40 C for 12 h (scCO2-hpd). Morphology was observed with a Zeiss EVO MA scanning electron microscope (SEM), from Carl Zeiss AG (Oberkoche, Germany). Some samples were sputter coated with 5 nm of Au, to prevent charging. Diffraction patterns of selected products were obtained with a Siemens/Bruker D5000 diffractometer from Bruker AXS (Karlsruhe, Germany), using Cu Kα radiation (λ = 1.54184 Å), 2θ angle from 5◦ to 70◦ with a scan rate of 5 ◦/min. Profex, an open source program by Nicola Döbelin (www.profex-xrd.org), was used for Rietveld refinement. Raman spectroscopy for the samples scCO2-od and aqueous route was performed in an OceanOptics (Orlando, FL, USA) QE65000 equipment (785.0 nm laser, 5 s acquisition time). Raman spectroscopy for the scCO2-hpd samples was performed in a Jasco NRS-5100 equipment (785.0 nm laser, 60 s acquisition time), from JASCO Corporation (Hachioji, Japan). BET analysis was performed in a Quantachrome Autosorb iQ®, from Anton Paar Instrumentsn (Boynton Beach, FL, USA), in a 6 mm cell, samples were previously degassed for 2 h at 80 ◦C followed by 10 h at 100 ◦C. For comparison, another magnesium carbonate sample was prepared by aqueous route by reaction of magnesium chloride, MgCl2·6H2O [Productos Químicos Monterrey S.A. de C.V., Monterrey, México, ACS grade], with potassium hydroxide [KOH, Produc- tos Químicos Monterrey S.A. de C.V., ACS grade] and sodium bicarbonate [NaHCO3, CTR Scientific, Monterrey, México, ACS grade], by dissolving 0.1 mole of each in 100 mL of water. A commercial magnesium carbonate (Gym Chalk, Gibson Gym Chalk, Gibson Athletic, Denver, CO, USA, purchased at Amazon.com) was also characterized by XRD and BET for comparison.

3. Results The net chemical reaction for the synthesis of magnesium carbonate is shown in Equations (1) and (2): a Mg2+ precursor (MgX, X represents the anion) reacts in an alkaline Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 11

For comparison, another magnesium carbonate sample was prepared by aqueous route by reaction of magnesium chloride, MgCl2·6H2O [Productos Químicos Monterrey S.A. de C.V., Monterrey, México, ACS grade], with potassium hydroxide [KOH, Produc- tos Químicos Monterrey S.A. de C.V., ACS grade] and sodium bicarbonate [NaHCO3, CTR Scientific, Monterrey, México, ACS grade], by dissolving 0.1 mole of each in 100 mL of water. A commercial magnesium carbonate (Gym Chalk, Gibson Gym Chalk, Gibson Ath- letic, Denver, CO, USA, purchased at Amazon.com) was also characterized by XRD and Appl. Sci. 2021, 11, 1141 4 of 10 BET for comparison.

3. Results

mediumThe to net form chemical Mg(OH) reaction2, followed for the by synthesis a carbonation of magnesium reaction producing carbonate MgCOis shown3. At in the lowestEquations temperature (1) and (2 (5): a◦ C),Mg2+ due precursor to slow (MgX, kinetics, X represents no precipitate the anion) was reacts formed. in an alkaline medium to form Mg(OH)2, followed by a carbonation reaction producing MgCO3. At the − lowest temperature (5 °C), dueMgX to +slow2OH kinetics,(aq. )no→ precipitateMg(OH) was2 formed. (1) MgX +− 2OH (aq. ) → Mg(OH) − (1) Mg(OH)2 + HCO3 (aq.) → MgCO3 + H2O + OH (aq.) (2) Mg(OH) + HCO (aq. ) → MgCO + HO + OH (aq. ) (2) Different MgCO3 structures were obtained, including hydromagnesite (Mg5(CODifferent3)4(OH) 2 MgCO·4H2O),3 dypingitestructures (Mg were5(CO 3) obtained,4(OH)2·5H 2 includingO), lansfordite hydromagnesite (MgCO3·5H2 O), magnesite(Mg₅(CO₃)₄(OH)₂·4H₂O), (MgCO3), and nesquehonite dypingite (Mg (MgCO5(CO3)43(OH)·3H22O)·5H according2O), lansfordite to XRD (MgCO analysis.3·5H The2O), den- sitymagnesite of these (MgCO hydrated3), and magnesium nesquehonite carbonates (MgCO depends3·3H2O) onaccording their structure, to XRD analyshydromagnesiteis. The usuallydensity is of 2–2.5 these timeshydrated denser magnesium than dypingite carbonates [3], depends therefore, on ittheir would structure, present hydromag- less speciﬁc surfacenesite area.usually is 2–2.5 times denser than dypingite [3], therefore, it would present less ◦ specificFor thesurface reaction area. with Mg(NO3)2·6H2O as precursor at 70 C, spherical nanoparticles were observedFor the reaction (Figure with2a), Mg(NO these are3)2·6H hydromagnesite2O as precursor accordingat 70 °C, spherical to XRD nanoparticles results (shown below).were observed Unluer et(Figure al. [3] have2a), these reported are hydromagnesite similar hydromagnesite according spherical to XRD structures.results (shown The car- bonatebelow). yields Unluer at et 70 al.◦C [3] were have higher reported than similar at 21 hydromagnesite◦C, which is expected, spherical since structures. the solubility The carbonate yields at 70 °C were higher than at 21 °C, which is expected, since the solubility of CO2 decreases at higher temperatures, and the formation of hydromagnesite is not thermodynamicallyof CO2 decreases at higher favorable temperatures, below 55 ◦andC[ 21the]. formation of hydromagnesite is not ther- modynamically favorable below 55 °C [21].

FigureFigure 2. Scanning 2. Scanning electron electron micrographs micrographs of of carbonate carbonate products products (acquired (acquired in in variablevariable pressurepressure mode), showing representa- representative tive morphologies from aqueous syntheses (a–c), from scCO2-od (d) scCO2-hpd (e). morphologies from aqueous syntheses (a–c), from scCO2-od (d) scCO2-hpd (e).

With MgSO4 as precursor (25 min reaction, 21 °C), we◦ obtained a product with a mor- With MgSO4 as precursor (25 min reaction, 21 C), we obtained a product with phology of rods of ca. 20 µm, conjoined and extending from a compact center (Figure 2b). a morphology of rods of ca. 20 µm, conjoined and extending from a compact center (Figure2b). This morphology is usually known as dumbbell-shaped aggregates (DSA) [22]. This type of morphology can appear as an intermediate state of spherulitic growth of many minerals. However, we should remark that in our material we did not observe spherulitic crystals, which may indicate that this nonequilibrium morphology could have been favored under our synthesis conditions. One way this might have happened is by the appearance of a large number of nuclei at the beginning, which then grow and deplete precursors before crystals can achieve the spherulitic morphology. Using magnesium chloride and nitrate as precursors, at the same reaction temperature, we obtained products with two different morphologies (Figure2c), containing magnesium hydroxide [23] in addition to the carbonate. The synthesis with scCO2 generated acicular crystals exclusively, ca 10 µm long (Figure2d). Our results show that carbonate crystal sizes changed with synthesis temperature, in a matter consistent with the reported trend [23] that smaller crystals are favored at higher temperature. Additionally, the drying process Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 11

This morphology is usually known as dumbbell-shaped aggregates (DSA) [22]. This type of morphology can appear as an intermediate state of spherulitic growth of many minerals. However, we should remark that in our material we did not observe spherulitic crystals, which may indicate that this nonequilibrium morphology could have been favored under our synthesis conditions. One way this might have happened is by the appearance of a large number of nuclei at the beginning, which then grow and deplete precursors before crystals can achieve the spherulitic morphology. Using magnesium chloride and nitrate as precursors, at the same reaction tempera- Appl. Sci. 2021, 11, 1141 5 of 10 ture, we obtained products with two different morphologies (Figure 2c), containing magnesium hydroxide [23] in addition to the carbonate. The synthesis with scCO2 generated acicular crystals exclusively, ca 10 µm long (Figure 2d). Our results show that carbonate crystal appliedsizes changed to the sampleswith synthesis inﬂuenced temperature, the surface in a appearance, matter consistent given with that the samplereported dried trend over a[23] hot that plate smaller has a crystals rougher are surface favored (Figureat higher2e). temperature. Considering Additionally, the morphology the drying observed process by SEM,applied the to DSA the samples product influenced and the supercritical the surface appearance, products were given characterized that the sample further. dried over a hot plate has a rougher surface (Figure 2e). Considering the morphology observed by SEM, The diffractogram of the scCO -od carbonate product (Figure3a) matches the diffrac- the DSA product and the supercritical2 products were characterized further. tion pattern of nesquehonite, (MgCO3·3H2O). These crystals look similar to those found The diffractogram of the scCO2-od carbonate product (Figure 3a) matches the diffrac- by Hänchen et al. [24] using CO2 at a pressure of 1 bar, using sodium carbonate as the tion pattern of nesquehonite, (MgCO3·3H2O). These crystals look similar to those found◦ by base, and CO2 at temperatures and pressures above the critical point (120 C, 100 bar) Hänchen et al. [24] using CO2 at a pressure of 1 bar, using sodium carbonate as the base, and but they found precipitation of hydromagnesite and magnesite exclusively. The latter CO2 at temperatures and pressures above the critical point (120 °C, 100 bar) but they found was favored by longer reaction times. By contrast, Yoo et al. found mostly nesquehonite precipitation of hydromagnesite and magnesite exclusively. The◦ latter was favored by whenlonger using reaction supercritical times. By contrast, CO2 at moderateYoo et al. found temperatures mostly nesquehonite (≤60 C), and when when using using super- short chaincritical alkanolamines CO2 at moderate as temperatures the base, with (≤60 hydromagnesite °C), and when using favored short atchain higher alkanolamines temperatures ◦ (70as theC) base, [19]. with hydromagnesite favored at higher temperatures (70 °C) [19].

FigureFigure 3. 3.X-ray X-ray diffractiondiffraction patternspatterns of (a) DSA (dumbbell-shaped(dumbbell-shaped aggregates) aggregates) product, product, (b (b) )magne- magnesium carbonate,sium carbonate, from supercriticalfrom supercritical synthesis, synthesis, hot hot plate plate dried, dried, scCO scCO2-hpd,2-hpd, and and ( c()c) magnesium magnesium car- carbonate 2 frombonate supercritical from supercritical synthesis, synthesis, oven dried, oven scCOdried,2 -od.scCO Reference-od. Reference peaks peaks of crystalline of crystalline structures struc- with the highesttures with correlation the highest are correlation shown as determinedare shown as by determined Rietveld reﬁnement.by Rietveld refinement.

XRDXRD ofof commercialcommercial carbonate carbonate showed showed that that it itwas was hydromagnesite, hydromagnesite, which which is typically is typically thethe more more stablestable hydratedhydrated phase phase of of magnesium magnesium carbonate carbonate [25]. [25 ]. TheThe diffractogramdiffractogram of the the DSA DSA material material (Figure (Figure 3b3)b) corresponds corresponds to toa mixture a mixture of crystalline of crystalline structures.structures. Rietveld Rietveld refinement refinement was was used used to correlate to correlate those thosepeaks peaksto four tostructures: four structures: magnesite mag- (MgCO3), nesquehonite (MgCO3·3H2O), hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O), and lans- nesite (MgCO3), nesquehonite (MgCO3·3H2O), hydromagnesite (Mg5(CO3)4(OH)2·4H2O), fordite (MgCO3·5H2O), with a good match found only for the last three. For the carbonate ob- and lansfordite (MgCO3·5H2O), with a good match found only for the last three. For the tained with the MgSO4·7H2O precursor, the DSA morphology with elongated crystals was carbonate obtained with the MgSO ·7H O precursor, the DSA morphology with elongated favored over regular spherulitic growth4 as2 previously reported [3]. crystals was favored over regular spherulitic growth as previously reported [3]. The diffractogram of the scCO2 carbonate product with hot plate drying (Figure3c) shows a mixture of crystalline structures of hydrated magnesium carbonate. According to the Rietveld reﬁnement the identiﬁed structures correspond to magnesite, hydromagnesite, nesquehonite, and lansfordite. The proportion of each of the phases is shown in Table2. The difference between the composition of both scCO2 products was expected since magnesium carbonate is sen- sitive to temperature and dehydrates when heated at moderately high temperatures [3]. The scCO2-hpd product contains a higher percentage of a more hydrated phase, lansfordite and hydromagnesite, compared with the scCO2-od for which the main component is nesquehonite. Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 11 Appl. Sci. 2021, 11, 1141 6 of 10

Table 2. Magnesium carbonates phase composition determined by Rietveld refinement for prod- Table 2. Magnesium carbonates phase composition determined by Rietveld reﬁnement for products ucts from scCO2 synthesis (oven dried and hot plate dried), and dumbbell-shaped structures from aqueousfrom scCO synthesis2 synthesis. (oven dried and hot plate dried), and dumbbell-shaped structures from aqueous synthesis. Phase scCO2-od scCO2-hpd DSA MagnesitePhase scCO20-od% scCO202-hpd% DSA0% NesquehoniteMagnesite 0%65% 20%0% 0%54% Nesquehonite 65% 0% 54% Lansfordite 27% 60% 40% Lansfordite 27% 60% 40% HydromagnesiteHydromagnesite 8%8% 20%20% 6%6%

The structures of the carbonates produced by the aqueous route changed depending The structures of the carbonates produced by the aqueous route changed depending onon the the precursor. precursor. For For nitrate, nitrate, the the dried dried product product showed showed a mixture a mixture of ofdypingite dypingite and and hydromagnesitehydromagnesite (Figure (Figure 4a).4 Witha). With sulfate sulfate as precursor, as precursor, in samples in samples that that did did not not show show the the DSA morphology,DSA morphology, the peaks the peaks correspond correspond to ato mixture a mixture of of nesquehonite nesquehonite and and hydromagnesite hydromagnesite (Figure(Figure 4b),4b), without peaks peaks for for magnesium magnesium sulfates. sulfates. We We note note that that since since we we dried dried these these productsproducts at at relatively relatively high high temperatures temperatures there there may may have have been been partial partial dehydration dehydration of ofthe precipitate.the precipitate.

FigureFigure 4. 4.X-X-rayray diffraction diffraction patterns patterns (blue (blue lines) lines) of of (a ()a magnesium) magnesium carbonate, carbonate, from from aqueous aqueous route route synthesis synthesis with with nitrate nitrate as precursor,as precursor, showing showing a mixture a mixture of dypingite of dypingite and andhydromagnesite, hydromagnesite, according according to Rietveld to Rietveld refinement. reﬁnement. (b) (Mb)agnesium Magnesium carbonate,carbonate, from from aqueous aqueous route route synthesis synthesis with with sulfate sulfate as asprecursor, precursor, showing showing a amixture mixture of of nesquehonite nesquehonite andand hydromagnesite,hydromagnesite, accordingaccording to to Rietveld Rietveld refinement. reﬁnement.

RamanRaman spectra spectra of of the the DSA DSA product product and and scCO scCO2-od2 nesquehonite-od nesquehonite (Figure (Figure5) show 5) peaks show 2− –1 –1 peakscharacteristic characteristic of the of carbonate the carbonate ion (CO ion3 )(CO at 1107) at and 1107 1400–1500 and 1400 cm–1500[26 cm]. For [26]. the For DSA the DSAproduct, product, the widethe wide Raman Raman peak peak at 1014 at 1014 cm–1 cmindicates–1 indicates that that it is it formed is formed by a by mixture a mixture of ofsulfate sulfate and and carbonate. carbonate. Magnesium Magnesium sulfates sulfates present present Raman Raman peaks thatpeaks depend that depend on the degree on the –1 degreeof hydration, of hydration, going fromgoing a from peak ata peak 983.6 at cm 983.6, for cm hexahydrite,–1, for hexahydrite, and changing and changing to higher to –1 higherwavenumbers wavenumbers as the degreeas the ofdegree hydration of hydration decreases, decreases, up to 1046.1 up cmto 1046.1for the cm monohydrate–1 for the mon- (kieserite, MgSO ·H O) [27]. The observed peak is centered between those corresponding ohydrate (kieserite,4 MgSO2 4·H2O) [27]. The observed peak is centered between those cor- to the tetrahydrate (MgSO ·4H O, Starkeyite, 1000.3 cm–1) and the trihydrate (1023.8 cm–1), responding to the tetrahydrate4 2 (MgSO4·4H2O, Starkeyite, 1000.3 cm–1) and the trihydrate suggesting that these two components are present in the DSA product, in addition to (1023.8 cm–1), suggesting that these two components are present in the DSA product, in hexahydrate. The sulfate does not show in the corresponding diffractogram (Figure3) but addition to hexahydrate. The sulfate does not show in the corresponding diffractogram we suspect that low intensity sulfate peaks may be among the several broad peaks observed. (Figure 3) but we suspect that low intensity sulfate peaks may be among the several broad peaks observed. Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 11

Appl. Sci. 2021, 11, 1141 7 of 10 Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 11

Figure 5. Raman spectra of the DSA product (aqueous route, blue curve, above) and scCO2-od FigureFigure 5.5. Raman spectra spectra of of the the DSA DSA product product (aqueous (aqueous route, route, blue blue curve, curve, above) above) and and scCO scCO2-2od-od (nesquehonite,(nesquehonite, supercritical supercritic, supercritical route, alroute, route, red red curve), red curve), curve), acquired acquired acquired with with a 785 witha 785 nm anm laser, 785 laser, nm 5 s acquisition 5laser, s acquisition 5 s acquisition time. time. time.

ForForFor comparison, comparison, comparison, FigureFigure Figure6 shows 6 shows 6 shows the the Raman the Raman Raman spectrum spectrum spectrum of the of the scCO of scCO the2-hpd 2- scCOhpd material, material,2-hpd material, whichwhich onlyonly only showsshows shows the the the characteristic characteristic characteristic peak peak peak of of the the of symmetric symmetricthe symmetric stretching stretching stretching of theof the carbonate carbonateof the carbonate ion ion ion −1 atat 110011001100 cmcm cm−1− [1[2 [2662].6]. ].

Figure 6. Raman spectrum of the scCO2-hpd, acquired with a 785 nm laser, 60 s acquisition time.

FigureFigure 6. 6.Raman Raman spectrum spectrum of the of scCOthe scCO2-hpd,2- acquiredhpd, acquired with a 785with nm a laser,785 nm 60 slaser, acquisition 60 s acquisition time. time. BET isotherms (measured with adsorption of N2 at 77.35 K) are shown in Figure 7a, theseBET results isotherms show specific (measured surface with areas adsorption of 39.1 of m N2g−at1 for 77.35 the K) DSA are pro shownduct, in 70.4Figure m27ga,−1 for BET isotherms (measured with adsorption2 of N2 at 77.35 K) are shown in Figure 7a, the scCO2-od nesquehonite product, and a lower2 −1 value of 19.8m2g−1 for the 2scCO−1 2-hpd these results show speciﬁc surface areas of 39.1 m g for2 the−1 DSA product, 70.4 m g for 2 −1 these results show specific surface areas of 39.1 m g for 2the−1 DSA product, 70.4 m g for theproduct. scCO 2The-od average nesquehonite pore sizes product, were and 1.02 a nm lower for valuenesquehonite, of 19.8m 3.78g nmfor thefor the scCO DSA2-hpd prod- 2 2 −1 2 product.uctthe, scCOand 5.48 The-od nm average nesquehonite for the pore scCO sizes 2product product were 1.02, driedand nm aon for lower a nesquehonite, hot valueplate, accordingof 3.78 19.8m nm tog for d forensity the the DSA f unc-scCO -hpd product,tionalproduct. theory and The 5.48(DFT) average nm modeling for pore the sizes scCO (Figure 2wereproduct 7b, 1.02c) and dried nm in for onboth nesquehonite, a hotcases plate, the pore according 3.78 size nm distribution to for density the DSA is prod- functionalrelativelyuct, and 5.48 theorywide nm. These (DFT) for modelingresultsthe scCO are (Figure 2better product7 b,c)than anddried the in surface both on casesa hotarea the plate, of pore carbonate according size distribution made to by d isensityreac- func- relativelytiontional with theory wide.bicarbonate, (DFT) These resultsmodeling and that are betterof ( Figurecom thanmercial 7b, thec) surface carbonate, and in area both ofas carbonate showncases thein madeTable pore by 3 size. reactionAccording distribution is withtorelatively the bicarbonate, IUPAC wide classification.and These that results of of commercial adsorption are better carbonate, isotherms than the as [28,29 shown surface], inthe Tablearea scCO 3of.2 According -carbonatehpd carbonate to made the iso- by reac- IUPACthermtion with shows classiﬁcation bicarbonate, an H3 ofbehavior adsorption and thatwhich isothermsof iscom characterizedmercial [28,29], carbonate, the by scCO slit-shaped2 -hpd as shown carbonate pores, in additionally Table isotherm 3. According it shows an H3 behavior which is characterized by slit-shaped pores, additionally it behaves behavesto the IUPAC as type classification IV isotherm w ofhich adsorption corresponds isotherms to a micro [28,29-mesoporous], the scCO material2 -hpd withcarbonate an iso- asaverage type IV pore isotherm size of which 5.48 nm. corresponds We should to awarn micro-mesoporous the reader that material the heat with treatment an average at 80 °C therm shows an H3 behavior which is characterized by slit-shaped pores,◦ additionally it poreprior size to BET of 5.48 measurements nm. We should may warn have the altered reader the that hydration the heat treatmentstate of the at carbonates, 80 C prior tohow- BETbehaves measurements as type IV may isotherm have altered which the corresponds hydration state to ofa themicro carbonates,-mesoporous however material this with an ever this is a necessary step to ensure complete degassing, which otherwise would give isaverage a necessary pore step size to of ensure 5.48 completenm. We should degassing, warn which the otherwise reader that would the give heat inaccurate treatment at 80 °C inaccurate readings. By using the same pretreatment for the oven-dried and hot-plate readings. By using the same pretreatment for the oven-dried and hot-plate dried materials driedprior materialsto BET measurements we aimed to make may both have sets altered of results the comparable.hydration state of the carbonates, how- weever aimed this tois makea necessary both sets s oftep results to ensure comparable. complete degassing, which otherwise would give inaccurate readings. By using the same pretreatment for the oven-dried and hot-plate dried materials we aimed to make both sets of results comparable. Appl. Sci. 2021, 11, 1141 8 of 10 Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 11

33 −11 FigureFigure 7. ( a7.) ( BETa) BET adsorption adsorption isotherms isotherms (adsorbed (adsorbed amount amount cmcm g vs.vs. relative relative pressure) pressure) of ofthe the DSA DSA product, product, scCO scCO2-hpd2-hpd,, and scCO2-od. (b) Pore volume cm3 3 g–1–1 vs. pore width (nm) of DSA and nesquehonite product (scCO2-od) calculated by and scCO2-od. (b) Pore volume cm g vs. pore width (nm) of DSA and nesquehonite product (scCO2-od) calculated by density functional theory (DFT). The apparent pores of ≈50 µm are considered a result of experimental noise, since no density functional theory (DFT). The apparent pores of ≈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 cm3g−1 vs. pore width (nm) of the pores of these sizes were observed in the SEM (c) Cumulative specific pore volume cm3g−1 vs. pore width (nm) of the scCO2-hpd product. scCO2-hpd product. Table 3. Specific surface areas and average pore sizes for different magnesium carbonates. Table 3. Specific surface areas and average pore sizes for different magnesium carbonates. Sample Specific Surface Area (m2g−1) Average Pore Size (nm) scCO2-odSample nesquehonite Specific70.4 Surface Area (m2g−1) Average1.02 Pore Size (nm) DSA 39.1 3.78 scCO2-od nesquehonite 70.4 1.02 MgCl2 + NaHCODSA3 + KOH 11.8 39.150 3.78 CommercialMgCl2 + NaHCO carbonate3 + KOH 18.1 11.84.89 50 CommercialscCO2 -hpd carbonate 19.8 18.15.48 4.89 scCO2 -hpd 19.8 5.48 We should note that nesquehonite has been reported to form in a matter of minutes [30], suggesting that our supercritical synthesis route may not require reaction times as We should note that nesquehonite has been reported to form in a matter of minutes [30], long as 12 h, and we intend to explore what is the shortest time for full precipitation. suggesting that our supercritical synthesis route may not require reaction times as long as 12 h, and we intend to explore what is the shortest time for full precipitation. 4. Conclusions 4. ConclusionsWe presented two synthesis methods for mesoporous magnesium carbonates from CO2 that avoid the use of organic cosolvents, which may be useful in green synthesis We presented two synthesis methods for mesoporous magnesium carbonates from routes. Moreover, the capture of CO2 achieved through these routes may potentially con- CO2 that avoid the use of organic cosolvents, which may be useful in green synthesis tribute to reduce atmospheric CO2 concentration by mineralization. We used solid CO2 for routes. Moreover, the capture of CO2 achieved through these routes may potentially convenience, but supercritical conditions also could be reached by syphoning liquid CO2 contributeinto the reactor, to reduce which atmospheric should not change CO2 concentration the results and bywould mineralization. be more environmentally We used solid COfriendly.2 for convenience, but supercritical conditions also could be reached by syphoning liquidDue CO 2tointo the therelatively reactor, high which surface should area and not small change pore the sizes results of these and materials, would be we more environmentallyexpect that they may friendly. be useful as adsorbents and in other applications, including capillary transportDue to and the in relatively pharmaceutical high surface and drug area delivery and small applications, pore sizes if their of these surface materials, area could we ex- pectbe increased that they further may be by useful fine tuning as adsorbents the synthesis and parameters. in other applications, including capillary transportWe and found in pharmaceutical conditions for aqueous and drug synthesis delivery that applications, allow isolating if their the surface morphology area could becalle increasedd dumbbell further shaped by fine aggregates tuning thewhich synthesis showed parameters. a large specific surface area and mes- oporesWe foundaccording conditions to BET measurements. for aqueous synthesis These crystals that allow appear isolating to be an the intermediate morphology step called dumbbellof growth shaped of spherulitic aggregates crystals which and were showed formed a large by a specificcombination surface of carbonate area and and mesopores sul- accordingfate according to BET to XRD measurements. and Raman characterization. These crystals appear to be an intermediate step of growthCarbonate of spherulitic synthesized crystals in and supercritical were formed CO2 and by a oven combination dried had of a carbonatelarger specific and sur- sulfate accordingface area and to XRD smaller and pores Raman than characterization. the DSA material. This material had a nesquehonite crystal structure,Carbonate according synthesized to XRD, in and supercritical was composed CO2 solelyand oven of magnesium dried had carbonate. a larger Car- specific surfacebonates area synthesized and smaller in scCO pores2 and than hot theplate DSA dried material. resulted Thisin a micro material-mesoporous had a nesquehonite material crystalwith a structure,smaller surface according area. Both to XRD, carbonates and was synthesized composed in solely scCO2 ofhad magnesium a larger area carbonate. compared with commercial magnesium carbonate and they provide a simple synthesis Carbonates synthesized in scCO2 and hot plate dried resulted in a micro-mesoporous method that could be implemented in existing CCS facilities during the process where the material with a smaller surface area. Both carbonates synthesized in scCO2 had a larger areaCO compared2 is compressed with to commercial obtain scCO magnesium2 for transportation carbonate toand geological they provide storage. a simple synthesis method that could be implemented in existing CCS facilities during the process where the CO2 is compressed to obtain scCO2 for transportation to geological storage. Appl. Sci. 2021, 11, 1141 9 of 10

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, Y.I.V.-C., F.J.R.-M., J.L.L.-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, Y.I.V.-C., F.J.R.-M., J.L.L.-S., A.J.G.-C., E.L.-L., C.A.G.-P.; visualization, F.J.R.-M., J.E.O.-C., E.L.-L., C.A.G.-P.; supervision, Y.I.V.-C. and F.J.R.-M.; project administration, Y.I.V.-C. and F.J.R.-M.; funding acquisition, Y.I.V.-C., F.J.R.-M., J.L.L.-S., A.J.G.-C. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by Tecnologico de Monterrey, through the research groups of Nanotechnology for Device Design (F.J.R.-M.), Energy and Climate Change (A.J.G.-C., J.L.L.-S.), and Advanced Manufacturing (Y.I.V.-C.). O.C.-E. thanks CONACYT for financial support. FJRM acknowledges funding by CONACYT through the “Fondo Sectorial de Investigación para la Educa- cioón” (Grant A1-S-43933). C.A.G.-P. acknowledges support from CONACYT through the “Programa Nacional de Posgrados de Calidad (PNPC) SEP-CONACYT”. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the article. Original data sets are available on request from the corresponding author. Acknowledgments: The authors are grateful to Centro del Agua for BET facilities, to the Department of Science (Chemistry and Nanotechnology) and Innovation Center for Design and Technology of Tecnologico de Monterrey for access to laboratories and characterization equipment, and to Regina Vargas-Mejia for support on SEM characterization. We also thank Adolfo Caballero-Quintero director of Servicios Periciales de Nuevo León for his help with Raman characterization on the Jasco NRS-5100 equipment. Conflicts of Interest: The authors declare no conflict of interest.

References 1. National Academies of Sciences, Engineering, and Medicine. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda; The National Academies Press: Washington, DC, USA, 2019. [CrossRef] 2. Sanna, A.; Uibu, M.; Caramanna, G.; Kuusik, R.; Maroto-Valer, M.M. A review of mineral carbonation technologies to sequester CO2. Chem. Soc. Rev. 2014, 43, 8049–8080. [PubMed] 3. Unluer, C.; Al-Tabbaa, A. Characterization of Light and Heavy Hydrated Magnesium Carbonates Using Thermal Analysis. J. Therm. Anal. Calorim. 2014, 115, 595–607. 4. Farhang, F.; Oliver, T.K.; Rayson, M.; Brent, G.; Stockenhuber, M.; Kennedy, E. Experimental study on the precipitation of magnesite from thermally activated serpentine for CO2 sequestration. Chem. Eng. J. 2016, 303, 439–449. 5. Glasser, F.P.; Jauffret, G.; Morrison, J.; Galvez-Martos, J.-L.; Patterson, N.; Imbabi, M.S.-E. Sequestering CO2 by Mineralization into Useful Nesquehonite-Based Products. Front. Energy Res. 2013, 4, 3. 6. Frykstrand, S.; Forsgren, J.; Mihranyan, A.; Strømme, M. On the pore forming mechanism of Upsalite, a micro- and mesoporous magnesium carbonate. Microporous Mesoporous Mater. 2014, 190, 99–104. 7. Yang, J.; Alvebratt, C.; Lu, X.; Bergström, C.A.S.; Strömme, M.; Welch, K. Amorphous magnesium carbonate nanoparticles with strong stabilizing capability for amorphous ibuprofen. Int. J. Pharm. 2018, 548, 515–521. 8. Cheung, O.; Zhang, P.; Frykstrand, S.; Zheng, H.; Yang, T.; Sommariva, M.; Zou, X.; Strömme, M. Nanostructure and pore size control of template-free synthesised mesoporous magnesium carbonate. RSC Adv. 2016, 6, 74241–74249. 9. Frykstrand, S.; Forsgren, J.; Cheung, O.; Zhang, P.; Hong, J.; Strømme, M.; Ferraz, N. Study of mesoporous magnesium carbonate in contact with whole human blood. RSC Adv. 2016, 6, 52810–52816. 10. Vall, M.; Hultberg, J.; Strömme, M.; Cheung, O. Carbon dioxide adsorption on mesoporous magnesium carbonate. Energy Procedia 2019, 158, 4671–4676. 11. Shahwan, T.; Zünbül, B.; Ero˘glu,A.E.; Yılmaz, S. Effect of magnesium carbonate on the uptake of aqueous zinc and lead ions by natural kaolinite and clinoptilolite. Appl. Clay Sci. 2005, 30, 209–218. 12. Wang, P.; Shen, T.; Li, X.; Tang, Y.; Li, Y. Magnetic Mesoporous Calcium Carbonate-Based Nanocomposites for the Removal of Toxic Pb(II) and Cd(II) Ions from Water. ACS Appl. Nano Mater. 2020, 3, 1272–1281. 13. Shan, Q.; Zhang, Y.; Xue, X. Removal of copper from wastewater by using the synthetic nesquehonite. Environ. Prog. Sustain. Energy 2013, 3, 543–546. 14. Castilleja-Escobedo, O.; Sánchez-García, R.E.; Nigama, K.D.P.; López-Salinas, J.L. Directional displacement of non-aqueous ﬂuids through spontaneous aqueous imbibition in porous structures. Chem. Eng. Sci. 2020, 228, 115959. Appl. Sci. 2021, 11, 1141 10 of 10

15. Kim, K.D.; Kim, Y.D.; Kim, S.W. Synthesis of Porous Magnesium Oxide Cubes with Nano-Grain Structure in Supercritical CO2/Ethanol Solution. J. Nanosci. Nanotechnol. 2002, 11, 5723–5728. 16. de Vito, C.; Ferrini, V.; Mignardi, S.; Cagnetti, M.; Leccese, F. Progress in carbon dioxide sequestration via carbonation of aqueous saline wastes. Period. Mineral. 2012, 81, 333–344. 17. Nakashima, Y.; Takai, C.; Razavi-Khosroshahi, H.; Suthabanditpong, W.; Fuji, M. Synthesis of ultra-small hollow silica nanoparticles using the prepared amorphous calcium carbonate in one-pot process. Adv. Powder Technol. 2018, 29, 904–908. 18. DeSimone, J.M. Practical Approaches to Green Solvents. Science 2002, 297, 799–803. 2+ 19. Yoo, Y.; Kang, D.; Choi, E.; Park, J.; Hugh, I. Morphology control of magnesium carbonate for CO2 utilization using Mg ions in industrial wastewater depending on length of alkyl chain of primary alkanolamine, reaction temperature, CO2 concentration, and Mg2+/Na+ ratio. Chem. Eng. J. 2019, 370, 237–250. 20. Sim, S.; Cole, I.S.; Choi, Y.S.; Birbilis, N. A review of the protection strategies against internal corrosion for the safe transport of supercritical CO2 via steel pipelines for CCS purposes. Int. J. Greenh. Gas Control 2014, 29, 185–199. 21. Langmuir, D. Stability of Carbonates in the system MgO-CO2-H2O. J. Geol. 2015, 73, 730–754. 22. Perry, C.T.; Salter, M.A.; Harborne, A.R.; Crowley, S.F.; Jelks, H.L.; Wilson, R.W. Fish as major carbonate mud producers and missing components of the tropical carbonate factory. Proc. Natl. Acad. Sci. USA 2011, 108, 3865–3869. [CrossRef][PubMed] 23. Ding, W.; Ouyang, J.; Yang, H. Synthesis and characterization of nesquehonite (MgCO3·3H2O) powders from natural talc. Powder Technol. 2016, 292, 169–175. [CrossRef] 24. Hänchen, M.; Prigiobbe, V.; Baciocchi, R.; Mazzotti, M. Precipitation in the Mg-carbonate system—effects of temperature and CO2 pressure. Chem. Eng. Sci. 2008, 63, 1012–1028. [CrossRef] 25. Botha, A. DTA and FT-IR analysis of the rehydration of basic magnesium carbonate. J. Therm. Anal. Calorim. 2003, 71, 987–996. [CrossRef] 26. Forsgren, J.; Frykstrand, S.; Grandﬁeld, K.; Mihranyan, A.; Strömme, M. A Template-Free, Ultra-Adsorbing, High Surface Area Carbonate Nanostructure. PLoS ONE 2013, 8, e68486. [CrossRef][PubMed] 27. Wang, A.; Freeman, J.J.; Jolliff, B.L.; Chou, I. Sulfates on Mars: A systematic Raman spectroscopic study of hydration states of magnesium sulfates. Geochim. Cosmochim. Acta 2006, 70, 6118–6135. [CrossRef] 28. Alothman, Z.A. A review: Fundamental aspects of silicate mesoporous materials. Materials (Basel) 2012, 5, 2874–2902. [CrossRef] 29. Sing, K.S.; Everett, D.H.; Haul, R.A.W.; Moscou, L.; Pierotti, R.A.; Rouquerol, J.; Siemieniewska, T. Reporting Physisorption Data for Gas/Solid Systems with Special Reference to the Determination of Surface Area and Porosity. Pure Appl. Chem. 1985, 57, 603–619. [CrossRef] 30. Ferrini, V.; de Vito, C.; Mignardi, S. Synthesis of nesquehonite by reaction of gaseous CO2 with Mg chloride solution: Its potential role in the sequestration of carbon dioxide. J. Hazard. Mater. 2009, 168, 832–837. [CrossRef]