Different Strategies to Anchor Organotin Methoxides on Silica and Their (Re)Use as Heterogeneous Catalysts for Transesterification Reactions

Catalysts were prepared by the heterogenization of the tributyltin(IV) methoxide (n-Bu3Sn(OMe)) and dibutyltin(IV) dimethoxide (n-Bu2Sn(OMe)2) on silica by two strategies: (i) anchoring of the organotin(IV) compounds on silica and (ii) anchoring of the organotin(IV) compounds on a silica gel functionalized with mercaptopropyl groups (SiO2–SH). The characterization results showed that, regardless of the method used to anchor the organotin species, the solids contained approximately 7-10% of Sn. The decrease or disappearance of the O–H and S–H stretching signals in the SiO2 and SiO2–SH spectra after the treatment with the organotin(IV) methoxide derivatives suggest the immobilization via O–Sn or S–Sn covalent bonds. The catalytic performance was evaluated in methanolysis of ethyl acetate, in comparison to the analogous homogeneous systems, and the order of reactivity is n-Bu2Sn(OMe)2 > n-Bu3Sn(OMe) > SiO2–SnBu2 > SiO2–S–SnBu2 > SiO2–SnBu3 > SiO2–S–SnBu3. The solids on which the organotin species were directly immobilized on activated SiO2 showed catalytic efficiencies slightly higher than those immobilized on thiol-functionalized silica. All of the solid catalysts were recovered and reused for recycling studies and characterized again after the reuse tests. A better interaction between n-Bu3Sn(OMe) and both supports was observed, as verified by the low leaching of Sn after the first catalytic reaction.

本研究采用两种策略，将甲氧基三丁基锡(IV)（tributyltin(IV) methoxide，n-Bu₃Sn(OMe)）与甲氧基二丁基锡(IV)（dibutyltin(IV) dimethoxide，n-Bu₂Sn(OMe)₂）多相化负载于二氧化硅载体上，制备得到系列固体催化剂：（i）将有机锡(IV)化合物直接锚定在纯二氧化硅上；（ii）将有机锡(IV)化合物锚定在巯基丙基官能化硅胶（SiO₂–SH）上。
表征结果显示，无论采用何种锚定方式，所得固体催化剂的锡元素质量分数均约为7%~10%。纯二氧化硅与巯基丙基官能化硅胶的红外光谱中，O–H与S–H伸缩振动峰在经有机锡(IV)甲氧基衍生物处理后出现强度降低或完全消失的现象，表明有机锡物种通过O–Sn或S–Sn共价键实现了固定化。
以乙酸乙酯甲醇解反应为模型反应，对比均相催化体系评估了催化剂的催化性能，所得反应活性顺序为：n-Bu₂Sn(OMe)₂ > n-Bu₃Sn(OMe) > SiO₂–SnBu₂ > SiO₂–S–SnBu₂ > SiO₂–SnBu₃ > SiO₂–S–SnBu₃。
直接锚定在活化纯二氧化硅上的有机锡催化剂，其催化效率略高于锚定在巯基官能化硅胶上的同类催化剂。所有固体催化剂均完成了回收与循环使用实验，并在循环测试结束后再次进行了表征分析。
首次催化反应后锡的浸出量较低，证明甲氧基三丁基锡(IV)与两种载体之间的相互作用更为优异。