Anhydrous MgCO3: Controllable synthesis of various morphology based on hydrothermal carbonization

A novel, simple and efficient anhydrous MgCO3 (AMC) synthesis method using ascorbic acid (ASA) has been developed based on the hydrothermal carbonization. In this process, ASA acts as both a CO2 source and a crystal modifier to regulate and control the crystallization of AMC. Furan derivatives, aldehydes and ketones from ascorbic acid play an unexpected role as a structure-directing agent. The effect of pH values of ASA, reaction time and the concentrations and types of Mg2+ were systematically investigated. Pure AMC with different morphology were successfully prepared.


Introduction
Magnesium carbonate is a class of materials, including anhydrous magnesite (MgCO 3 ), nesquehonite (MgCO 3 ·3H 2 O), lansfordite (MgCO 3 ·5H 2 O) and hydromagnesite (Mg 5 (CO 3 )4(OH) 2 ·4H 2 O). So far, except anhydrous MgCO 3 (AMC), the latter three have been widely utilized in various industrial applications (e.g., pharmaceuticals, rubber industry, lithographing inks, and as precursors for other magnesium-based chemicals [1]). Up to now, immense research interests have focused on the production of these hydrated magnesium carbonate with various morphologies (e.g., needles [2], rods [3], flakes [4] and microspheres [5]) by precipitation, carbonization or hydrothermal methods [6,7]. Actually, AMC possess more advantages compared with hydrated ones owing to their higher CO 2 storage capability and thermodynamic stability. In addition, since AMC inherently have characteristics such as good thermal absorption, excellent fire extinguishing property via releasing a great deal of CO 2 and pollution-free, they are the attractive candidates as a new kind of inorganic flame retardant [8]. However, almost no ready-made AMC products with high quality and purity are available in the current market. One of the major reasons is that it is difficult to be properly prepared under mild conditions. Further, the natural mineral magnesite always accomplished with certain of impurities like Fe 2+ and Mn 2+ . Herein, it is urgent to explore the techniques for AMC synthesis.
However, only a few reports are correlated with the synthesis of AMC, which are typically prepared under harsh conditions such as high CO 2 partial pressure (∼10 MPa), high temperatures (>150°C), and/or long reaction time (>12h), with the products lacking size, morphology or structure homogeneity [9,10]. Liang et al [11] successfully synthesized the AMC under 3GPa and 800°C for 1h based on the MgCO 3 ·3H 2 O, whereas Lou et al [12] prepared the single crystal of AMC under 500°C for 20 h by mixing the MgCO 3 powder, metallic sodium and carbon tetrachloride,. Swanson et al [13] investigated the effect of seed particles on fabricating AMC under 80°C-150°C and 3.0 MPa CO 2 partial pressure. Moreover, only irregular shape and the rhombohedral morphology with size between 1-30 um of AMC have been reported in these works. It is known that multifunctional inorganic materials extensively applied in diverse fields usually possess well controlled morphologies [14,15], and the properties of particles are often associated with their shapes [16,17]. Therefore, it is crucial to design and fabricate inorganic materials with desirable shapes and sizes for practical applications. In our previous study, three different morphologic AMC particles have been prepared and their effect on the Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
properties of PVC have been investigated [18]. In order to extend their application, AMC particles with more special morphologies are necessary to be developed. In the current study, preparation conditions are systematically investigated, and the results demonstrated that uniform AMC with different morphology could be obtained without additional additives and CO 2 source under mild conditions. As shown in figure 1, different AMC can be prepared by hydrothermal carbonization.

Preparation of anhydrous MgCO 3
First, 7.5 g of L(+)-ascorbic acid was dissolved in 50 ml of deionized water, and then 5 mol l −1 NaOH was utilized to adjust pH values before adding a certain of MgSO 4 or MgCl 2 . Then the mixture was added into a Teflon-lined autoclave and hydrothermally treated at 180°C for several hours and the synthesis conditions for AMC are summarized in table S1. Finally, the obtained solid products were separated by centrifugation, washed several times with deionized water, and finally dried at 100°C for 24 h.

Effects of pH values
A series of AMC particles have been synthesized under different conditions, and the summary of conditions is shown in table S1. To investigate the effect of pH values of ASA on the morphology of products, the experiments were performed at 180°C for 3 h. The results are shown in figure 2 for MgSO 4 as Mg 2+ source and figure 4 for MgCl 2 as Mg 2+ source, respectively.
In figure 2, AMC particles show different morphologies at different pH values. When the pH value is relatively low, the AMC are irregular blocks as shown in figures 2(A)-(C). A dramatic shape change occurs when the pH value increases to 10.5, which produces string-like crystals with a length of 0.5-0.8 um accumulated by small lamella (figure 2(D)). Remarkably, as the previous reported [18], well-defined monodisperse dumbbelllike AMC with a length of 1.0-1.5 um assembled by small polyhedrons (figure 2(E)) are obtained when pH value is 12.5. Further increasing the pH value to 13.5, the morphology of AMC shows like hydrangea consisting of sheet-like structures with the diameters in the range of 0.8-1.0 um [18]. Figure 3 show the particle size distribution of AMC, which is in accordance with the SEM results.
Similarly, the pH values of ASA exert substantial influence on the morphology of AMC particles obtained from MgCl 2 system as shown in figure 4. Spindle-like AMC with size in the range of 8.0-20.0 um (figure 4(a)) are produced when the pH value is 5.5, whereas when the pH value reaches 7.0, crystals with spindle shape attach to each other and form the irregular particles just as displayed in figure 4(b). At pH 8.5, microspheres assembled by many polyhedrons (figure 4(c)) are produced. Obviously, the size of the crystals shows a tendency to decrease with the increase of the pH in comparison between figures 4(a)-(c) and (d)-(f). In figure 4(d), AMC particles with size between 0.5-1.0 um exhibit irregular shape. And rod-like AMC with rough surface (figure 4(e)) are obtained when pH value increases to 12.5, while hydrangea-like AMC (figure 4(f)) similar to figure 2(F) are produced under pH value of 13.5. Figure 5 show the particle size distribution of AMC, which is in agreement with the SEM results. All products are identified as pure AMC both using MgSO 4 and MgCl 2 as magnesium source on condition that the pH values are higher than or equal to 7.0. Moreover, the pH value is higher, the crystallization peak is stronger, indicating the crystallinity is better.
When the pH value of ASA is relatively low (7.0), HCO 3 − is the major ion in the system while CO 3 2− mainly exists in the high pH conditions [17]. Consequently, only limited Mg 2+ and CO 3 -2 can combine with each other to form MgCO 3 particles, and then they will aggregate to meet the rule of lowest energy and grow  ). At the high pH condition, the quick dissolution of CO 2 results in a high supersaturation, which leads to a rapid formation of small MgCO 3 precipitates [17].
Because of the high surface energy of these small MgCO 3 particles, they prefer to aggregate together to form irregular polyhedron via specific hydrogen bonding and favorable adsorption interactions [19,20]. Then under the driving of crystal facet selective adsorption and similar orientation attachment modes [21], these irregular polyhedrons can further assemble into different morphological MgCO 3 as shown in figures 2(C)-(F) and 4(c)-(f). In other words, organic molecules (furan derivatives, aldehydes and ketones decomposed from ASA) can adhere to the surface of small MgCO 3 particles via electrostatic interaction and then lead to  the reduction of surface energy and the inhibition of the growth of surfaces in this direction, and finally contributes to the formation of various morphologies.

Effects of Mg 2+ concentration
As shown in figure 7, the morphology and the size of the products are affected by the MgSO 4 concentration. When the concentration is 0.6 mol l −1 , monodisperse corncob-like AMC with the length of 0.8-1.2 um (figure 7(A)) are obtained. However, when the concentration increases to 1.0 mol l −1 , crystals display dumbbell shape as described before in figure 2(E). Furthermore, short rod-like AMC assembled by small lamella are produced under concentration of 1.4 mol l −1 . Increasing the concentration to 2.2 mol l −1 , homogeneous cubes accumulated by layers are observed in figure 7(D) [18]. The particle size distribution of AMC is shown in figure 8. Figure 9 shows the influence of MgCl 2 concentration on the morphology of AMC, clearly, which is not as remarkable as that of MgSO 4 . In comparison among figures 9(a)-(d), all crystals exhibit rod-like shape with    In comparison between figures 7 and 9, it can be seen that under the same reaction conditions, AMC prepared in MgSO 4 and MgCl 2 system almost shows different morphologies, which is mainly associated with influence of Cl − and SO 4 2− on the interaction with crystal face [22]. On the other hand, when the concentration of Mg 2+ is same, the concentration of Cl − in the MgCl 2 system is twice higher than SO 4 2− in the MgSO 4 system.
The large number of Cl − might put obstacles on the collision between organic molecules (aldehydes, ketones and furan derivatives) decomposed from ASA and MgCO 3 crystals, resulting in the limited shape-controlled effect. Obviously, the concentrations of Mg 2+ also can significantly modify the morphology of AMC especially  for MgSO 4 system. The higher the concentration of Mg 2+ in the reaction system, the greater collision frequencies among different ions and formation of more aggregates. With this reaction, organic molecules absorbed on aggregates will reduce their surface energy, which is conducive to the further aggregation and mineralization into different morphology. Figures 12 and 14 show the SEM images of AMC synthesized at 1.0 mol lmol l −1 and 180°C for 2 h, 3 h, 6 h and 9 h. According to the SEM images, corncob-like AMC, as the major phase, are observed with a mixture of a few dumbbell-like particles in figure 12(A). As the reaction time extends to 3 h, complete dumbbell-like AMC ( figure 12(B)) are obtained; however, when the reaction time further extends to 6h and 9h, the shape of AMC changes into double-hemisphere. The corresponding XRD results are shown in figure 16(A), indicating that all products are pure AMC. Figure 13 shows the corresponding particle size distribution, which is consistent with the SEM results. Figure 14 shows the impact of reaction time on the morphology of AMC based on the MgCl 2 as magnesium source. When the reaction time is 2 h, there is just some uniform blocks. And the corresponding XRD pattern shown in figure 10(B) indicates that these particles are mainly Mg 3 O(CO 3 ) 2 . When the reaction time extends to 3, 6 and 9 h, the morphology of products shows like short rod and the longer the reaction time is, the smoother the surface of the crystals is as shown in figures 12(b)-(d). Also, the XRD results ( figure 16(B)) demonstrate that when the reaction time is longer than 2 h, pure AMC can be prepared. Corresponding particle size distribution is shown in figure 15.

Effects of reaction time
The reaction time plays an active role in regulating the morphology of AMC. When the reaction time is in the range of 2-3 h, crystals possess rough surfaces and 0nonequilibrium shapes as shown in figures 12(A)-(B) and 14(a)-(b). With the reaction time extending, these crystals have the possibility to rearrange into a more stable phase. In comparison between the AMC obtained at 6 h and 9 h both for MgSO 4 and MgCl 2 system, similar morphology and size are observed, indicating they are the stable phase in that conditions.

Possible crystallization process
In this study, the pH value of ASA, types and concentrations of Mg 2+ and the reaction time act as crucial roles in regulating and controlling the morphologies of the AMC particles. In this process (figure 17), CO 2 from the ASA can be converted into CO 3 2− by OH − , and then the remaining organic molecules (aldehydes, ketones and furan   derivatives) decomposed from ASA can sequester Mg 2+ due to the high local concentration of Mg 2+ , resulting in the reduction of the collision frequency among different ions but also the formation of small MgCO 3 aggregates. Simultaneously, these organic molecules serve as agglomerant or structure-directing agent to arrange MgCO 3 aggregates to assemble into different morphological MgCO 3 particles [23].

Conclusion
In this study, a novel, green, and economical MgCO 3 synthesis method was developed based on hydrothermal carbonization approach, in which ASA served as both the CO 2 source and the crystal modifier. Then the effect of reaction time, pH values of ASA and the concentrations and types of Mg 2+ were systematically investigated, and  the resulting products were fully characterized. Pure anhydrous MgCO 3 with different morphologies can be obtained under different conditions. Significantly, this method provides the possibility to realize the industrial production of anhydrous MgCO 3 by using different available magnesium source.