Liu Qi, Xu Mengdie, Zhao Jing, Wang Yudong, Qi Chenze, Zeng Minfeng, Xia Rui, Cao Xingzhong, Wang Baoyi
Zhejiang Key Laboratory of Alternative Technologies for Fine Chemicals Process, Shaoxing University Shaoxing 312000 China
Institute of High Energy Physics, The Chinese Academy of Science Beijing 100049 China
RSC Adv. 2018 Jan 17;8(6):3225-3236. doi: 10.1039/c7ra12407d. eCollection 2018 Jan 12.
In this study, the catalytic performances of palladium supported on chitosan (Pd@CS) membrane heterogeneous catalysts have been studied from the aspects of free volume by positron annihilation lifetime spectroscopy (PALS). The results showed that the variation in free volume hole size of the Pd@CS membrane catalyst was closely associated with microstructure evolutions, such as increase of Pd content, valence transition of Pd by reduction treatment, solvent swelling, physical aging during catalyst recycling, and so on. The PALS results showed that both the mean free volume hole size of the Pd@CS membrane in the dry or swollen state (analyzed by the LT program) and its distribution (analyzed by the MELT program) are smaller than the molecule size of the reactants and products in the catalysis reaction. However, the results showed that the Pd@CS membrane catalyst has excellent catalytic activity for the Heck coupling reaction of all the reactants with different molecule size. It was revealed that the molecule transport channels of the Pd@CS membrane catalyst in the reaction at high temperature was through a number of instantaneously connected free volume holes rather than a single free volume hole. This hypothesis was powerfully supported by the catalytic activity assessment results of the CS layer sealed Pd@CS membrane catalyst. Meanwhile, it was confirmed that the leaching of Pd nanoparticles of the reused Pd@CS membrane catalyst during the recycling process was also through such instantaneously connected free volume holes.
在本研究中，通过正电子湮没寿命谱（PALS）从自由体积方面研究了负载在壳聚糖上的钯（Pd@CS）膜多相催化剂的催化性能。结果表明，Pd@CS膜催化剂的自由体积孔尺寸变化与微观结构演变密切相关，如钯含量的增加、还原处理导致的钯价态转变、溶剂溶胀、催化剂循环过程中的物理老化等。PALS结果表明，Pd@CS膜在干燥或溶胀状态下的平均自由体积孔尺寸（由LT程序分析）及其分布（由MELT程序分析）均小于催化反应中反应物和产物的分子尺寸。然而，结果表明，Pd@CS膜催化剂对所有不同分子尺寸的反应物的Heck偶联反应均具有优异的催化活性。结果表明，Pd@CS膜催化剂在高温反应中的分子传输通道是通过许多瞬时连接的自由体积孔，而不是单个自由体积孔。CS层密封的Pd@CS膜催化剂的催化活性评估结果有力地支持了这一假设。同时，证实了再利用的Pd@CS膜催化剂在循环过程中钯纳米颗粒的浸出也是通过这种瞬时连接的自由体积孔。
