添加Li+对MgO催化性能的影响

本文以IR、TPD、丁烯异构化及直接脱氢反应为手段,对不同Li⁺添加量的MgO催化剂进行了研究。结果表明,表面低配位氧集团是催化剂的主要活性物种,起碱中心作用;表面金属离子起L酸中心作用。酸、碱中心的数目、强度随Li⁺添加量不同而呈规律性变化。这种变化影响了酸、碱中心的协同作用,从而影响其催化活性。

Effect of Addition of Li~+ on Properties of MgO Catalyst

The influence of Li⁺ amount on acidity-basicity of Li⁺/MgOcatalysts and their performance for isomerizatjon and dehydrogenation of 1-butene was investigated by means of IR, TPD-MSand reaction evaluations in a closed circulation apparatus.The IRresults show that a small amount (2.0 mmol) of Li⁺ added on MgO(l mol), the peak of 3400 cm^-1 corresponding to hydrated hydrox-yl groups became more stable while the peak of 3700 cm^-1 ascribing to free hydroxyl groups disappeared, but a large amount of Li⁺ was added, free hydroxyl groups appeared again and both the two hydroxyl groups became unstable.It is suggested that Li⁺ addition could form another hydroxyl groups which combined With Li⁺.TPDprofiles of adsorbed carbon dioxide on the catalysts display that two type basic sites(Ⅰ_Band Ⅱ_B) were found at 445 Kand 525 Kon MgO(H₂O).With addition of Li⁺ increasing, Ⅰ_Bfinally diminished, Ⅱ_Bshifted to a higher temperature and the third type basic site Bappeared at the much higher temperature.TPDresults of absorbed s-butylamine show that two type acid sites (Ⅰ_A:weak acid site, Ⅱ_A:strong acid site) were increased with a small amount of Li⁺ added; but when a large amount of Li⁺ was added, the desorption temperature of I Adecreased to ca.391 Kand the area decreased very rapidly, and Ⅰ_A deaded out.All above results indicate that a small of Li⁺ addition enhanced the basicity and acidity of Li⁺-dopped MgOcatalysts, a large amount of Li⁺ addition covers the weaker basic sites and acid sites while it promotes continuously stronger basic sites of catalysts.It has been found that there is a options in the activity for isomerization and dehydrogenation of 1-butene with respect to the amount of added Li⁺.The maximum activity of isomerization and dehydrogenation of 1-butene are respectively in 25.0 mmol and 2.0 mmol Li⁺ addition.From these observations we conclude that the number and strength of surface basic sites and acid sites can be changed with the amount of added Li⁺, and the Li⁺-dopped MgOcatalysts have a better catalytic activity when they are cooperation of acid and basic sites with reasonably equal strength and number.