热处理Mg/Al-LDH结构演化和矿物纳米孔材料制备

Structural evolution of heating treatment of Mg/Al-LDH and preparation of mineral mesoporous materials

层状双氢氧化物(LDH)是自然界不常见的矿物,然而却是易于合成的重要材料.本文利用X射线衍射和透射电镜技术研究了Mg/Al-LDH热处理结构演化.结果表明,对于层间阴离子主要为碳酸根的Mg/Al-LDH,在400～800℃之间形成镁铝固溶体似方镁石结构氧化物.在层状双氢氧化物脱出结构水形成氧化物的过程中,产生2～3nm的纳米孔隙,但仍保留原来LDH片状晶体假象形貌,并继承原来的晶体结构取向.煅烧形成的具有似方镁石结构的氧化物可以重新水化形成新生LDH,但重新水化形成的LDH结晶度比原来的LDH结晶度低,这一过程可以导致微米和亚微米颗粒LDH趋于纳米化,并产生纳米粒间孔隙.温度高于1000℃时似方镁石结构氧化物进一步相变为尖晶石结构氧化物和方镁石复合物相,其中的方镁石可以水化为水镁石并且可以酸溶去除,相转变形成的尖晶石呈现纳米多晶并存在纳米粒间孔隙.这一发现为利用矿物相变原理制备廉价似方镁石结构纳米孔材料、LDH结构纳米孔材料和尖晶石结构纳米材料提供了新的思路.

Although hydrotalcite or layered double hydroxides （LDHs） is not a common mineral, it is an important material that can be easily synthesized in laboratory. In this study, structural evolution and BET surface area changes of heat treated Mg/Al-LDH was evaluated by XRD, TEM and N2-BET analyses. The results indicate that for magnesium-aluminum LDH with carbonate as interlayer anion, periclase-like oxide was formed at the temperature of 400-800℃.Meanwhile, 2-3 nanometer mesopores were formed during the decomposition of LDH. However, the heat treated samples still preserve the morphology of original LDH plates. Periclase-like ox- ide formed from LDH heat treatment may re-hydrolyze and recover the structure of LDH. However, crystallinity of the recovered LDH is lower than that of original LDH. Such a heat treatment will result in the formation of （Mg, Al）-oxide nano-crystals and nanopores among the nano-crystals. When the heating temperature exceeds 1 000℃, the periclase-like （Mg, Al）-oxide was trans- formed into a composite with periclase （MgO） and spinel phases. The periclase can be re-hydrolyzed and dissolved in HCl solution. After acid treatment, the sample with high surface area is composed of spinel nano-crystals and nanopores among them. The results of this study will provide a new and economical way to synthesize mesoporous materials through pathways of phase transformation of precursor materials with different compositions.