钙镁铝固体碱催化剂的制备及表征

Preparation and characterization of calcium magnesium aluminum solid base catalyst

采用氢氧化钠、硝酸钙、硝酸镁制得钙镁氢氧化物沉淀,采用氨水、乙酸铝制得氢氧化铝沉淀。两种沉淀依次经陈化、抽滤、洗涤、干燥后,混合焙烧即得钙镁铝固体碱催化剂。以蓖麻油甲醇醇解反应为模型反应,蓖麻油转化率为活性评价指标,采用正交实验考察了催化剂制备条件对其催化活性的影响;采用Hammett指示剂滴定法、TG、XRD、BET及SEM技术对催化剂及其前驱体进行了表征。结果表明:制备催化剂的优化条件为n(Ca)∶n(Mg)∶n(Al)=1.5∶1.5∶1、陈化时间18 h、焙烧温度400℃、焙烧时间6 h,对应的蓖麻油转化率平均可达88.3%,且其催化活性较稳定;催化剂前驱体在300400℃有1个明显的失重台阶,在800℃以后质量基本不随温度变化;钙镁铝固体碱催化剂的碱强度在7.2~15.0之间,总碱位量为12.262 mmol/g,主要由CaO、MgO两种晶体构成,其比表面积为83.83 m2/g、孔容为0.173 2 cm3/g,其颗粒大小在2~20μm之间。

Calcium magnesium hydroxide precipitate was prepared with sodium hydroxide, calcium nitrate and magnesium nitrate. Aluminum hydroxide precipitate was prepared with ammonia and aluminum ace tate. Then calcium magnesium aluminum solid base catalyst was made by mixing and calcinating the two kinds of precipitates in turn by aging, suction filtration, washing and drying. The effects of catalyst prepa- ration conditions on the catalytic activity were investigated by orthogonal experiment, with methanolysis reaction of castor oil as model reaction and castor oil conversion rate as the activity evaluation index. The catalyst and its precursor were characterized by Hammett indicator method, TG, XRD, BET and SEM. The results showed that the optimal preparation conditions of the catalyst were obtained as follows ： n （Ca） ： n （Mg） ： n （Al） = 1.5 ： 1.5： 1, aging time 18 h, calcination temperature 400 ℃ and calcination time 6 h. Under these conditions, the corresponding castor oil conversion rate was up to 88.3% with stable catalytic activity. The catalyst precursor had a significant mass loss step at 300 -400 ℃, and the mass did not change with temperature above 800 ℃. The alkali strength of the solid base catalyst was 7.2 - 15.0, and the total alkali amount was 12. 262 mmol/g. It was mainly composed of CaO and MgO crystals, and the specific surface area, pore volume and particle size were 83.83 m2/g, 0. 173 2 cm3/g and 2-20 p,m.