碱式碳酸镁焙烧法制备多孔氧化镁晶体

Preparation of porous MgO crystals with hydromagnesite roasting method

【目的】实现多孔MgO晶体的可控制备。【方法】以菱镁矿为镁源,采用水化碳化-低温水溶液法,热解Mg(HCO_(3))_(2)溶液合成平均直径为10.0μm、平均长度为50.0μm的多孔棒状碱式碳酸镁(4MgCO_(3)·Mg(OH)_(2)·4H_(2)O);通过焙烧法制备多孔MgO晶体,分别探讨焙烧温度、时间对前驱体4MgCO_(3)·Mg(OH)_(2)·4H_(2)O分解率、MgO物相组成和形貌的影响,探究4MgCO_(3)·Mg(OH)_(2)·4H_(2)O热分解机制。【结果】在焙烧温度为700℃、时间为3.0 h时,制得平均直径为20.0μm、平均长度为50.0μm、比表面积为76.12 m^(2)/g的介孔棒状MgO晶体;在4MgCO_(3)·Mg(OH)_(2)·4H_(2)O分解过程中,随着温度升高,结晶水失去,—OH的分离和C—O键断裂,4MgCO_(3)·Mg(OH)_(2)·4H_(2)O结构彻底崩塌,生成的MgO纳米片在高温下自组装成2种形貌的多孔棒状MgO,一种是纳米片全部覆盖的多孔棒,另一种是一端由纳米片覆盖,另一端光滑的多孔棒,此过程中晶格常数减小,晶粒直径由51.92 nm减小为11.28 nm。【结论】以菱镁矿为镁源可以实现多孔MgO晶体的可控制备。

Objective To improve the efficient utilization of mineral resources and achieve the controlled preparation of porous magnesium oxide(MgO) crystals.Methods In this paper, magnesite was used as raw material and calcined at 750 ℃ for 3.0 h to produce light burned magnesium powder, and then the light burned magnesium powder was sifted to obtain powder with a particle size less than 75 μm. The powder was mixed with deionized water in a mass ratio of 1∶40, and heavy magnesium water was obtained by hydration carbonization method. The porous rod-like hydromagnesite(4MgCO_(3)·Mg(OH)_(2)·4H_(2)O) with an average length of 50 μm and an average diameter of 10.0 μm were successfully prepared by direct pyrolysis of heavy magnesium water using low-temperature aqueous solution without the incorporation of additional additives. Then using hydromagnesite as precursor, porous magnesium oxide crystals were prepared by direct roasting method. The effect of calcination time and temperature on the decomposition rate of the precursor hydromagnesite, as well as the resulting phase composition and microstructure of the magnesium oxide final product, was discussed. At the same time, the thermal decomposition mechanism of the precursor hydromagnesite during its transformation into magnesium oxide crystals was investigated.Results and Discussion The results show that: Neither the duration nor temperature of roasting has significant effects on the decomposition rate of precursor hydromagnesite, nor did they impact the phase composition and micro-morphology of the magnesium oxide crystals. However, two morphologies appeared roasted at of 700 ℃ for 3.0 h. The obtained magnesium oxide features mainly porous rod-like structure with an average diameter of 20.0 μm. A minor fraction comprises rod-like crystal, distinguished by a smooth surface at one end and a porous structure covering the other end. The mean diameter of the part with a smooth surface is 10.0 μm, while the mean diameter of the part covered by a porous structure is 20.0 μm. The average length of the rod-like crystals with different morphologies at both ends is 50.0 μm. Sustaining calcination temperature while extending the calcination time to 4.0 h resulted in a product presenting the same morphology as that observed after 3.0 h of calcination. The average diameter of the porous part and the average diameter of the smooth part was still 20.0, 10.0 μm, respectively. However, the average length of the rod is reduced to 40.0 μm. Under such conditions of the optimal roasting temperature of 700 ℃ and the roasting time of 3.0 h, the decomposition rate of the precursor hydromagnesite reaches a maximum of 99.98%. The pore size of the obtained porous rod-like magnesium oxide crystals is mainly distributed in the range of 2~15 nm, with mesoporous properties. The specific surface area is 76.12 cm^(2)/g, and the total pore volume is 0.21 cm^(3)/g. Based on the thermal analysis data of the precursor hydromagnesite and the simulation of the crystal structure of the precursor hydromagnesite, it can be concluded that the main structure of hydromagnesite is composed of the [MgO_6] octahedron. Because of the bivalent nature of Mg^(2+) ions, the escaped water molecules originate from two distinct coordination environments. One arises from the escape of crystal water, and the other results from the break of hydroxyl bonds. The thermal decomposition reaction of hydromagnesite is characterized as an endothermic reaction and divided into two stages. In these two stages, the molecular structure of hydromagnesite will be adjusted rapidly. When all water molecules escape, the product exists in the form of amorphous magnesium carbonate. The resulting CO_(2) escapes, magnesium oxide forms, and the hydromagnesite structure collapses. As the temperature gradually rises, the generated magnesium oxide nanosheets begin to self-assemble and generate porous rod-like crystals covered by nanosheets. Due to the low concentration of magnesium oxide nanosheets on the surface of some rod-like magnesium oxide, the surface of the region is smooth and there is no nanosheet coverage. Based on the XRD data of the precursor hydromagnesite and magnesium oxide crystals, the cell data of the two crystals were obtained by using Jade software, and the crystal plane spacing of the two crystals was calculated by Scherer formula, so as to obtain the average grain diameter of the two crystals. It is found that during the transformation from hydromagnesite to magnesium oxide, the lattice constant and cell volume of the crystal decrease, signifying a more compact crystal structure in magnesium oxides. Additionally, the grain diameter decreases from 51.92 to 11.28 nm. The decrease of grain diameter, with the increase of specific surface area and the increase of active sites, imparts greater surface energy to magnesium oxide crystals. This enhanced surface energy facilitates the adsorption of nearby atoms and molecules onto magnesium oxide crystals, thus increasing the diameter of magnesium oxide crystals.Conclusion Using magnesite as the initial raw material and without any additional additives, calcination hydration carbonation direct pyrolysis of Mg(HCO_(3))_(2) solution is used to obtain an average diameter of 10.0 μm. The average length is 50 μm porous rod-shaped basic magnesium carbonate was used as a precursor. At a calcination temperature of 700 ℃ and a calcination time of 3.0 h, a maximum decomposition rate of 99.98% is obtained, and a mesoporous rod-shaped magnesium oxide with a specific surface area of 76.12 m^(2)/g is obtained. At the same time, the pyrolysis mechanism of basic magnesium carbonate is analyzed as follows: the loss of crystalline water, separation of —OH groups, and breakage of C—O bonds collectively cause the complete collapse of the basic magnesium carbonate structure. The newly generated magnesium oxide nanosheets undergo self-assembly at high temperatures, leading to a decrease in the lattice constant and the grain diameter from 51.92 to 11.28 nm. Using this process to prepare porous magnesium oxide crystals, raw materials offers advantages such as readily available, cost-effective raw materials, environmental and human-friendly characteristics. This approachis expected to increase the added value of magnesite. however, how to further improve the specific surface area of porous magnesium oxide crystals deserves further consideration and research.