多级孔Sn-beta分子筛催化高浓乳酸一步法合成丙交酯

Direct synthesis of lactide from concentrated lactic acid catalyzed by hierarchical Sn-beta zeolite

高浓乳酸(≥90 wt%)一步法合成丙交酯是一种节能高效的丙交酯制备方式,对实现生物可降解材料聚乳酸的低成本生产具有重要的意义.高浓乳酸中存在大量乳酸低聚体,难以高产率合成丙交酯.本文研究了多级孔Snbeta分子筛(Sn-beta-M)催化97%乳酸合成丙交酯,反应3 h,产率达86.5%,反应速率R^(av)_(20 min)达0.252 mmol min^(−1) gcat^(−1).N_(2)吸附/脱附表征证明,Sn-beta-M分子筛中存在丰富的介孔结构,缩短了丙交酯在微孔中的扩散路径,减少了丙交酯开环聚合的副反应;介孔中存在的活性位点促进了L_(3)A和L_(n)A转化为丙交酯.固体紫外漫反射(DR UV-vis)及吡啶吸附红外光谱(Py FTIR)表征证明Sn以四配位的形式存在于Sn-beta-M分子筛骨架中,表现出强Lewis酸性,为丙交酯合成的活性中心.推测Sn-beta-M中Sn和周围的O作为强Lewis酸位点和弱碱位点协同催化L_(2)A及L_(m)A(m≥3)环化生成丙交酯.

Direct synthesis of lactide from concentrated lactic acid(LA)is an energy-efficient method,which is of great significance to reduce the cost of polylactic acid production.It is difficult to produce lactide with high yield from commercial concentrated LA due to the great quantity of LA oligomers.Herein,the hierarchical Sn-beta(Sn-beta-M)zeolite was prepared as a catalyst for the conversion of 97 wt%LA to lactide with yield of 86.5%in 3 h.The mesoporosity in Sn-beta-M characterized by N_(2) adsorption/desorption test shortened the diffusion path way in the micropores,which facilitated the fast escape of lacide avoiding decyclization and oligomerization.L_(3)A and L_(n)A’s conversion to lactide may be attributed to the Sn sites present in mesopores.The Sn^(4+) coordinated in Sn-beta-M framework,exhibited strong Lewis acidity and could act as the active center for the synthesis of lactide.We speculated that the Sn and the surrounding O in Sn-beta-M acted as strong Lewis acid sites and weak base sites respectively in mediating C–O bond formation of lactide from L_(2)A and L_(m)A(m≥3).