环境友好型离子液体催化环氧丙烷反应合成丙二醇醚

丙二醇醚类化合物是性能优良的精细化学品,也是环保型高级溶剂.该类化合物具有两个强溶解性功能基团—醚键和羟基,前者具有亲油性,可溶解疏水性物质,后者具有亲水性,可溶解亲水性物质,因而丙二醇醚具有很强的溶解能力,素有"万能溶剂"之称,可广泛应用于涂料、油墨、油漆、印刷、电子化学品、染料、净洗和纺织等行业.丙二醇醚类化合物目前主要由环氧丙烷和低级脂肪醇反应合成,然而,由于环氧丙烷的位阻效应,使其在酸或碱的条件下开环的位置会不同,从而得到不同的醇醚产物.由于碱催化的醇醚产物更加环境友好,因而越来越被人们所关注.工业上丙二醇醚合成多采用传统的强碱性催化剂醇钠以及氢氧化钠,腐蚀性强,产生的废液量大.本文采用环境友好的非卤素离子液体作为催化剂,研究了其催化环氧丙烷醚化合成丙二醇醚的反应特性.本文采用两步法合成了一系列环境友好的醋酸类碱性功能化离子液体,并在温和的条件下将其用于催化环氧丙烷与醇反应合成丙二醇醚.结果表明,该类离子液体可以高效催化该反应的进行.利用紫外-可见光谱测定Hammett指数来表征实验中所用离子液体的碱强度,并构建了离子液体碱性与催化活性之间的关系.结果表明,离子液体的催化性能和其碱性密切相关,随着离子液体碱性的增加,催化活性增强,其中咪唑醋酸类离子液体碱性强于季胺类,表现出优异的催化性能.离子液体的碱性明显弱于NaOH,但却呈现出更优异的催化性能.相同反应条件下,EmimOAc离子液体作为催化剂,PO的转化率分别较NaOH高出20%–30%,选择性略高于NaOH,这可能是由于二者催化机理不同造成的.传统NaOH催化机理的关键步骤是醇在碱性催化剂的作用下去质子化形成电子供体烷氧根离子,促进环氧丙烷的开环加成.而本文提出了离子液体亲电亲核双活化作用机理,即离子液体在阴阳离子之间的氢键和电荷相互作用的共同作用下,促进环氧丙烷开环和醇的去质子化,形成相应的反应中间体.通过电喷雾质谱分析手段检测到了阴阳离子通过协同作用亲电亲核催化过程中的反应中间体,证明了该假设机理的可行性.此外,还考察了催化剂浓度、醇比、反应温度以及醇的空间位阻效应对反应的影响.以EmimOAc催化合成丙二醇丁醚为例,反应的转化率随催化剂浓度的增加而增大,在催化剂添加量1%(催化剂与PO的摩尔比)时,PO转化率达到最大值为98.2%,1-丁氧基-2-丙醇的选择性为86.4%.当正丁醇与环氧丙烷的摩尔比为3时,转化率最高为88.6%,选择性高达94%.该反应为放热反应,最适反应温度约为140 oC,此时转化率高达96.5%.在环氧丙烷和不同的低碳醇合成丙二醇醚的反应中,反应物醇的碳链越短,支链越少,催化反应效率越高.

Synthesis of propylene glycol ethers from propylene oxide catalyzed by environmentally friendly ionic liquids

A series of acetate ionic liquids were synthesized using a typical two-step method. The ionic liquids were used as environmentally benign catalysts in the production of propylene glycol ethers from propylene oxide and alcohols under mild conditions. The basic strengths of the ionic liquids were evaluated by determination of their Hammett functions, obtained using ultraviolet-visible spec-troscopy, and the relationship between their catalytic activities and basicities was established. The catalytic efficiencies of the ionic liquids were higher than that of the traditional basic catalyst NaOH. This can be attributed to the involvement of a novel reaction mechanism when these ionic liquids are used. A possible electrophilic-nucleophilic dual activation mechanism was proposed and con-firmed using electrospray ionization quadrupole time-of-flight mass spectrometry. In addition, the effects of significant reaction parameters such as concentration of catalyst, molar ratio of alcohol to propylene oxide, reaction temperature, and steric hindrance of the alcohol were investigated in detail.