柠檬酸钠辅助三水碳酸镁合成无水碳酸镁晶体及其机制

Sodium citrate-assisted synthesis of anhydrous magnesium carbonate crystals from magnesium carbonate trihydrate and its mechanism

【目的】探究柠檬酸钠在三水碳酸镁合成无水碳酸镁晶体过程中的影响。【方法】以轻烧氧化镁为初始原料,通过水化碳化法获得长径比为30的棒状三水碳酸镁。以此为前驱体,柠檬酸钠为添加剂,采用水热法制备无水碳酸镁,探究水热温度、水热时间、添加剂含量对产物物相组成和微观形貌的影响及其形成机制。【结果】水热温度为190℃,水热时间为13 h,柠檬酸钠添加量(质量分数)为10%~30%,可获得物相均一、表面光滑、形状均匀、平均直径为3~5μm的棱柱状无水碳酸镁晶体;在水热反应中,柠檬酸根离子对镁离子具有较强的络合作用,钠离子较镁离子拥有更高的水合能力,导致碱式碳酸镁的形成受到抑制,得到另一种中间产物碳酸氧化镁,无水碳酸镁晶体的生长方式与形貌发生改变。【结论】三水碳酸镁在柠檬酸钠的影响下制备得到棱柱状无水碳酸镁。

Objective Researchers typically use solid precursors such as nesquehonite,magnesium oxide,and magnesium oxalate to prepare MgCO_(3).However,these methods require stringent experimental conditions and specialized equipment,limiting their practicality.This paper investigated the preparation of nesquehonite from lightly burned magnesium oxide and the subsequent synthesis of anhydrous magnesium carbonate.The effects of sodium citrate concentration,citric acid concentration,hydrothermal tem perature,and hydrothermal time on the microscopic morphology and phase composition of the product were discussed.The formation mechanism of prismatic anhydrous magnesium carbonate in a sodium citrate-magnesium trihydrate system was explored.Methods Lightly burned magnesium oxide was first mixed with water at a mass ratio of 1∶30 and hydrated in a water bath at 90℃for 2 h to obtain Mg(OH)2.Carbon dioxide gas was then introduced at a fixed rate for 1h to produce heavy magnesium water(Mg(HCO_(3))2).The obtained water was subsequently pyrolyzed at 55℃for 2 h.After suction filtration,the filter cake was dried at 50℃ for 20 h to obtain white powder nesquehonite(MgCO_(3)·3H_(2)O).Finally,MgCO_(3)·3H_(2)O was mixed with deionized water at a mass ratio of 1∶30,and sodium citrate was added at concentrations ranging from 5%~50%.The mixture was stirred evenly,placed in a polytetrafluoroethylene liner,and reacted at temperatures within 110~190℃for 3~13 h to obtain MgCO_(3).Results and Discussion When the hydrothermal time was extended to 13 h,prismatic anhydrous magnesium carbonate crystals with smooth surface,good dispersion,and an average diameter of 2μm were produced.Increasing the hydrothermal temperature to 190℃ resulted in diamond-shaped anhydrous magnesium carbonate crystals with uniform morphology,consistent size,and average particle sizes of 3~5μm.With the increase of sodium citrate concentration,the particle size of the obtained anhydrous magnesium carbonate decreased.With a hydrothermal temperature of 190℃,hydrothermal time of 13 h,and sodium citrate concentration of 10%~30%,diamond-shaped anhydrous magnesium carbonate crystals with uniform phase,shape,and average particle sizes of 3~5μm could be obtained.During the hydrothermal reaction,strong electrostatic interactions between water molecules and metal ions created a barrier around the ions,forming hydrated metal ions.Water molecules easily integrated into the carbonate structure.Therefore,when carbonate ions combined with magnesium ions,the layer of water molecules around the magnesium ions remained.Under this condition,magnesium carbonate compounds containing crystalline water,such as hydrated magnesium carbonate(MgCO_(3)·3H_(2)O)and basic magnesium carbonate(4MgCO_(3)·Mg(OH)_(2)·4H_(2)O),could be formed at room temperature.Among alkaline earth metal ions,the interaction between magnesium ions and water molecules was nearly the strongest,leading to basic magnesium carbonate often being an intermediate product in most hydrothermal systems.When sodium citrate was introduced into the hydrothermal system,the strong complexation reactions of citrate ions with Mg^(2+)weakened the binding effect of Mg^(2+)and H_(2)O,inhibiting the formation of basic magnesium carbonate.Additionally,since the ionic radius of Na^(+)was greater than that of Mg^(2+),the ionic hydration energy of Na^(+)was less than that of Mg^(2+),which further slowed down the hydration of Mg^(2+)and formed a new intermediate product,magnesium carbonate oxide(Mg_(3)O(CO_(3))_(2)).The Mg3O(CO_(3))_(2) unit cell is a body-centered cubic structure with amorphous morphology,while the basic magnesium carbonate unit cell is a monoclinic structure(P21/C)with a lamellar morphology.When the intermediate phase was basic magnesium carbonate,the anhydrous magnesium carbonate grains stacked on the sheet-like planes,growing in a stepped manner.Under the influence of sodium citrate,magnesium ions were complexed by citrate ions,forming a cubic structure of magnesium carbonate oxide.Without a plane similar to basic magnesium carbonate for support,the original cubic structure served as the matrix,growing in the fixed crystal directions,forming prismatic crystals.Conclusion In the magnesium carbonate trihydrate-sodium citrate hydrothermal system,with a hydrothermal temperature of 190℃,hydrothermal time of 13 h,and sodium citrate concentration of 10%~30%,prismatic anhydrous magnesium carbonate crystals with uniform phase,shape,and average particle sizes of 3~5μm can be obtained.During the hydrothermal reaction,the complexation of magnesium ions by citrate ions and the strong hydration of sodium ions inhibit the hydration ability of magnesium ions.This results in the intermediate product changing from flake-like basic magnesium carbonate crystals to bulk magnesium carbonate oxide.The different morphologies of the intermediate products correspond to different growth modes,transitioning from“stacked”two-dimensional growth to“stepped”three-dimensional growth,which causes the changes in morphology of the final product.