焙烧温度对Mg(Al)O复合物结构和表面酸碱性质影响的研究

分别在673、873和1073K分解Mg/Al摩尔比为3的水滑石生成Mg(Al)O复合物.XRD测定均显示MgO物相.873K以上焙烧的样品同时存在MgAl2O4尖晶石物相.微量吸附量热测定其表面酸碱中心的强度和密度,发现碱中心强度和密度顺序为:MAO-2＞MAO-3≈MAO-1,而酸中心强度和密度顺序是:MAO-1＞MAO-3≈MAO-2.红外光谱表明,Mg(Al)O复合物表面酸碱中心主要存在L酸,碱中心是氧阴离子.焙烧温度对表面碱中心类型几乎没有影响.固态MAS27Al核磁共振结果展示,这些样品呈现出显著的Al3+配位情况的变化.

Exploration on Influence of Calcination Temperature on Structure and Surface Acidity and Basicity of Mg (Al)O Composite Oxide

Mg(A1)O composite oxides obtained from hydrotalcite compounds upon calcination at 673、 873 and 1073K, respectively, show the pattems of MgO by X-ray difffraction measurements. The samples calcined over 873K also show the patterns of MgA12O4 spinel.Microcalorimetric measurements of NH3 and CO2 adsorptions were used to determine the strength and density of these sites. It is discovered that the basicity sequence is MAO-2＞MAO-3≈MAO-1, while acidity order is MAO-1＞MAO-3≈MAO-2. NH3 and CO2 molecules were used as the probing molecutles to investigate the nature of acid and base sites on Magnesium-Aluminium composite oxides. Infrared spectroscopic studies of these adsorped molecules indicated that the acid sites were mainly Lewis acid sites, whereas the base sites were surface oxygen anions. The results showed that the calcination temperature has little influence on the nature of surface base sites. Results of solid-state magic-angel-spinning 27Al NMR revealed that significant transitions of coordinate state for Al3+occur in the samples upon calcination at 673、873 and 1073K, respectively.