含碱氧化铝吸附分离丙二醇中的乙二醇

Isolation of ethylene glycol in propylene glycol via adsorption with alkali-containing alumina

乙二醇是生物质基1,2-丙二醇的主要杂质。通过浸渍法将质量分数为20%的NaOH溶液分散于介孔Al_(2)O_(3)中,真空干燥后制得吸附剂,通过吸附方法分离、纯化含质量分数为10%乙二醇的1,2-丙二醇。发现碱性位点对乙二醇有优先吸附的特性,且多孔Al_(2)O_(3)负载的NaOH具有较高的吸附容量。在负载过程中,Al_(2)O_(3)的比表面积大幅下降,且因NaOH沉积而形成新的介孔。CO_(2)-TPD测试结果表明:氧化铝载体的碱量远低于其负载的NaOH的碱量,因此,24.7NaOH-Al_(2)O_(3)对乙二醇的强吸附能力应归因于NaOH而非氧化铝载体。在NaOH负载质量分数为24.7%、吸附剂质量分数为20%、吸附温度为20℃、吸附时间为4 min的较优条件下,1,2-丙二醇的质量分数从90%提高到92%,乙二醇吸附量高达69.2 mg/g。乙二醇移除率约为丙二醇损失率的10倍,表现出较高的分离选择性。Gaussian 03分子模拟软件的计算结果表明:乙二醇的羟基氢原子带有明显的正电性,而1,2-丙二醇的羟基氢原子显示负电性或电中性,故只有乙二醇的羟基有显著的Lewis酸性,这种独特的酸-碱相互作用是分离选择性的根本原因。

Ethylene glycol is the primary impurity in bio-based propylene glycol.In this paper,20%sodium hydroxide was dispersed on mesoporous alumina by impregnation to prepare an adsorbent for the separation of 10%ethylene glycol in 1,2-propanediol.Ethylene glycol could be selectively adsorbed on the basic sites,and sodium hydroxide dispersed on porous alumina had relatively high adsorption capacity.During the loading procedure,the specific surface area of alumina support was decreased to a large extent,and new mesopores were generated.The number of basic sites in the alumina support measured by CO_(2)-TPD was much lower than the contribution from NaOH.Therefore,the high adsorption capacity of 24.7NaOH-Al_(2)O_(3) adsorbent for ethylene glycol should be attributed to NaOH rather than the Al_(2)O_(3) support.Under the preferable conditions including NaOH loading of 24.7%,adsorbent dosage of 20%,adsorption temperature of 20℃,and adsorption time of 4 min,the content of 1,2-propanediol was increased from 90%to 92%.The adsorption capacity for ethylene glycol reached 69.2 mg/g.The removal rate of ethylene glycol was approximately 10 times the loss of propylene glycol,exhibiting high separation selectivity.The calculation results of Gaussian 03 molecular simulation software indicated that the hydrogen atoms of hydroxyl groups in ethylene glycol were significantly positively charged,while those in 1,2-propanediol were negatively charged or neutral.Therefore,only the hydroxyl groups of ethylene glycol exhibited obvious Lewis acidity,and this unique acid-base interaction is the rationale of separation selectivity.