Mechanism of selectivity control for zeolites modified with organic monolayers
Mechanism of selectivity control for zeolites modified with organic monolayers
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DOI：
10.1016/j.micromeso.2022.111913
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发表时间：
2022-04
影响因子：
5.2
通讯作者：
Xinpei Zhou;J. Falconer;J. Medlin
中科院分区：
文献类型：
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作者：
Xinpei Zhou;J. Falconer;J. Medlin
由于两种分子的分子大小相似，使用分子筛吸附分离 C3H6 和 C3H8 气体是一个具有挑战性的过程。在这项工作中，我们报告了沸石上的有机膦酸（PA）单层通过改变基于烷基尾部性质的扩散机制，显着增强了 C3H6/C3H8 吸附的理想选择性。在 5A 沸石上涂覆十八烷基膦酸 (ODPA) 时，C3H6/C3H8 的动力学选择性在 25 °C 时最初>8，而对于未涂覆的 5A，其动力学选择性仅限于 ∼1.2。动力学模型表明，在 ODPA 涂层的 5A 中，C3H6 和 C3H8 的扩散受到沸石外表面 PA 单层的限制。相反，对于未涂覆的 5A，它是由孔道控制的，因此 5A-ODPA 增强的动力学选择性与不同的限制传输机制有关。 5A-ODPA 的动力学选择性在 25–150 °C 范围内对温度不敏感，因为 C3H6 和 C3H8 的扩散活化能都很小。用其他 PA 修饰 5A 也提高了动力学选择性。表面上由于其烷基尾部较短，正丁基膦酸涂层的动力学选择性低于 ODPA。用叔丁基膦酸（一种空间大的配体）涂覆进一步降低了动力学选择性。然而，部分渗透沸石近表面区域的甲基膦酸严重降低了扩散速率。有机薄膜的使用可以基于在孔入口处提供气体特定阻力来合理设计选择性吸附剂。
The adsorptive separation of C3H6and C3H8gases using molecular sieves is a challenging process due to the similarity in molecular sizes for the two molecules. In this work, we report that organic phosphonic acid (PA) monolayers on zeolites significantly enhanced the ideal selectivity of C3H6/C3H8adsorption by changing the diffusion mechanism based on the properties of the alkyl tail. With ann-octadecylphosphonic acid (ODPA) coating on zeolite 5A, the kinetic selectivity of C3H6/C3H8was initially >8 at 25 °C, whereas for uncoated 5A, it was limited to ∼1.2. Kinetic modeling showed that in ODPA-coated 5A, the diffusion of C3H6and C3H8was limited by the PA monolayer at the external surface of the zeolite. In contrast, for uncoated 5A, it was controlled by the pore channels, so that the enhanced kinetic selectivity from 5A-ODPA was related to a different limiting transport mechanism. The kinetic selectivity was not temperature sensitive in the range of 25–150 °C in 5A-ODPA as the diffusion activation energies of C3H6and C3H8were both small. Modification of 5A with other PAs also increased the kinetic selectivity. Coating withn-butylphosphonic acid yielded lower kinetic selectivity than ODPA, ostensibly due to its shorter alkyl tail. Coating withtert-butylphosphonic acid, a sterically bulky ligand, decreased kinetic selectivity still further. However, methylphosphonic acid, which partially penetrated the near-surface region of the zeolite, severely lowered the diffusion rates. The use of organic films may enable rational design of selective adsorbents based on providing gas-specific resistance at the pore entrance.